I am Veena Parthan, completed my master's in Thermal Engineering. I am working as a Solar Operation and Maintenance Engineer for the UK Solar sector. I have more than 5 years’ experience in the field of Energy and Utilities. I have a profound interest in renewable energy and their optimization. I have published an article in AIP conference proceedings which is based on Cummins Genset and its flow optimization.
During my free hours, I engage in freelance technical writing and would love to offer my expertise on LambdaGeeks platform. Apart from that, I spend my free hours reading, engaging in some sport activities and trying to evolve into a better person.
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A Scotch Marine boiler is an example of a fire tube boiler wherein the flue gases flow through tubes that are placed in a tank of water. The Scotch Marine boiler has adapted its working principle from the Lancashire boilers which consist of numerous furnaces to increase the heating capacity. Two main differences set Scotch Marine Boiler apart from Lancashire boilers and they are
Scotch Marine boilers are made up of multiple fire tubes to increasing the heating capacity per cross-sectional area
Scotch Marine boilers are half the size of Lancashire boilers as the path for the flue gas to flow has been packed in the available location.
A Scotch Marine boiler is a fire tube boiler that is used in ships. The boiler is shaped like a horizontal cylinder with furnaces at the lower portion of the boiler. Above the furnace, there are many fire tubes. The heat and smoke from the furnace flow to the boiler. The fire tubes have a capping at the end which is known as a smoke box on the outer surface of the boiler shell.
The Scotch Marine Boiler consists of the following parts which are
A Furnace: The space below the boiler is where the furnace is located. A boiler with a single end is usually equipped with four furnaces. The furnaces are corrugated for strength, and they have separate combustion chambers for each furnace.
A Combustion Chamber: This is the area below the shell of the boiler where the fuel is burnt to generate steam from the water. This chamber is made up of four layers of plates which are top plate, backplate, tube plate, and two-sided plates.
A Smoke Box: This unit has several tubes packed together horizontally and connects the combustion chamber and the chimney. The flue gases from the combustion chamber pass through these tubes.
A Chimney: It is used for releasing the flue gases from the combustion chamber into the environment.
A Boiler Shell: This part of the boiler consists of cylindrical plates which are welded together to contain sufficient water and steam. The layout also helps in protecting the inner parts of the boiler. The fitting of the boiler is usually attached to the boiler shell.
Scotch Marine boilers can be single-ended or double-ended. A single-ended boiler is usually composed of somewhere between one to four furnaces while a double-ended boiler has furnaced on either end of the boiler with somewhere between two to four boilers. It is equipped with a horizontally placed drum which is about 2.5 to 3.5 meters in diameter.
The water that passes through the tubes is heated by the flue that is burned in the combustion chamber of the furnace. The flue gases which are produced from the burning of fuel passes on the outer surface of the tubes to the chimney. This adds additional heat to the tubes thereby providing faster steam generation and better quality of steam. The flue gases are then released into the environment through the furnace
The working principle of a scotch marine boiler is simple. The fuel that is used to heat the water will be burnt in a combustion chamber. The fuel is fed to the combustion chamber through a fire hole. Through the convection heat transfer process, the heat produced by burning the fuel is transferred to the water in the chamber surrounding the combustion process.
The water is then converted into steam and supplied to the steam turbine. The flue gases that were produced during the combustion process are released into the environment via the smoke tube into the boiler chimney. Here, water carried away the heat in the exhaust gases passing through the smoke tubes
2. What are the types of boilers?
Several types of boilers are used in the industries. A list of the commonly used boilers are jotted down below:
Shell and Tube boiler
Lancashire boiler
Locomotive boiler
Wet Back Boilers
Dry Back Boilers
Cornish Boiler
Scotch Marine Boiler
Packaged Boiler
Reversal Chamber
Two-Pass Boiler
3. Which is better fire tube boiler or water tube boiler? | Between fire tube and water tube boilers which has more advantages and why?
Among, the two types of boilers i.e., the fire tube boiler and the water tube boiler, the water tube boiler is more efficient than the latter due to the following reasons
The amount of water that is used in the water tube boiler is comparatively lesser than the amount used in a fire tube boiler which in turn results in the quicker generation of steam and less fuel requirement in the case of the water tube boiler.
Since they require less amount of water for the boiling process, the design of the boiler is compact and environmentally friendly.
They respond quickly to changes in load i.e., need of steam. Units that are configured in terms of modules can be charged up and down depending on the required amount of steam.
Their increased efficiency and excellent performance are also attributed to their ability to last longer in comparison to their counterpart.
It is safer to operate a water tube boiler in comparison to a fire tube boiler as they are internally fired.
They occupy small floor space and are usually used in large power plants because of their efficiency and higher steam production.
4. What improvements did the Yarrow Water Tube Boiler have over the Scotch Marine Boiler?
The Yarrow Water Tube Boiler was built with a different concept of working than the Scotch Marine Boiler. A Scotch Marine boiler produces large volumes of steam which is at low pressure. On the other hand, a Yarrow Water Tube Boiler was built to produce fewer volumes of high-pressure steam. It is difficult to compare the merits and demerits of these two types of boilers.
A Yarrow Water Tube boiler is usually used for marine applications with a newly installed turbine not because of the high pressure it produces but also because of its compact sizing and limited maintenance that is required for it to work. While Scotch Marine boilers are often installed in equipment with piston technology which had to undergo timely repair and involves high maintenance.
5. Can anyone explain to me about marine boiler and types of marine boiler
Marine boilers are used for marine applications, whereby the heat produced by the fuel is used for running the ship. The working principle of a marine boiler is to change the state of fluid from liquid to vapor. The temperature of the fluid in the boiler is transformed from liquid to vapor in an enclosed vessel to avoid loss of energy into the surrounding. The heating is carried out in furnaces to ensure that heat is transferred to the operating fluid.
There are two main categories of marine boilers which are
Water Tube Boilers
Fire Tube Boilers
6. What is the function of a pressure gauge in a boiler?
The primary function of a pressure gauge in any type of boiler is to indicate the pressure build-up inside the drum of the boiler which is often represented as kN/m2.
7. What are stay tubes in a boiler?
In a fire tube or water tube boiler, stay tubes provide support and stability to the endplates and the normal tubes. The stay tubes have a greater wall thickness and strong construction to withstand the high surrounding temperature. They have a greater diameter and are welded to the plates on both the upper well as the lower region. The steam is collected in a region made up of shell-like structures and an internal cone.
8. Why are fire tube boilers not suitable for high pressures?
Fire-tube boilers require large volumes of water due to which the steam pressure is produced after a long time. Since both steam and water are contained in a single vessel, the pressure of the steam produced by this type of boiler is not very high and not very dry.
9. What are the different types of water tube boilers and fire tube boilers?
The different types of water tube boilers used in industries are as follows:
Sterling boiler
Simple Vertical Boiler
Babcock and Wilcox Boiler
The different types of fire tube boilers used are as given below:
Immersion Boiler
Scotch Marine Boiler
Cochran Fire Tube Boiler
Lancashire Boiler
Cornish Fire Tube Boiler
Locomotive Boiler
10. Why are economizers used in boilers?
Economizers are devices used in boilers that use the heat from exhaust gases to preheat the cold water entering the boiler. They are heat exchangers aimed at increasing the temperature of the fluid beyond its boiling point and thereby reducing energy used.
Desuperheater is used for carrying out the desuperheating process which is to reduce the temperature of the superheat and to bring back the vapor into a saturated state.A desuperheater performs the role contrary to that of a superheater. In most of the desuperheaters, the temperature of the exit fluid is within 3degrees of the saturation temperature. There are also cases where the discharge temperature is more than 3 degrees of saturation temperature.
In power generation plants, the role of superheat is significant and hence superheaters are highly recommended. When the temperature of the steam is higher than the saturation temperature, then the state of the steam is referred to as superheated. In this state, the liquid and the vapor are not in equilibrium and can be analyzed from the equilibrium charts.
Superheated steam is preferred during the transfer of heat from one source to another because it acts as an insulator while saturated steam is required for heat transfer processes. In power generation processes, there is a need for both heat insulation and heat transfer, and this is respectively carried out using superheating and desuperheating procedures using superheaters and desuperheaters.
The temperature of the superheated steam is lowered using a heat exchanger that uses a coolant to lower the temperature of the superheated steam and is termed as a desuperheater. In most of the desuperheaters, the fluid that is used for lowering the temperature of the superheated steam is the same as it that of the vapor. Water is the fluid used as a coolant in the case of superheated steam.
Desuperheaters are mainly of two types i.e., a direct contact type superheater and an indirect contact superheater which are explained in detail below:
1. Indirect contact desuperheater:In this type of desuperheater, the coolant does not come in direct contact with the superheated vapor. Here the coolant employed will be a liquid or a gas which is allowed to flow through one side of the heat exchanger while the superheated steams pass through the other side. The heat from the superheated steam passes into the coolant through the heat exchanger.
An example of this type of process is the heat exchange between air which is used as a coolant and hot fluid that is passing through the coils where the air does not come in direct contact with the superheated fluid, but the heat is transferred from the fluid to the air through indirect contact or convection mode of heat exchange.
In these types of desuperheaters, the coolant flowrate or the inlet pressure of the superheated steam can be used for controlling the temperature of the desuperheated steam. It is not feasible to control the flow of superheated steam in these types of processes.
2. Direct contact desuperheater:In this type of superheater, the superheated steam comes in direct contact with the coolant. Usually, the coolant that is used for lowering the temperature of the superheated steam is the liquid form of the vapor. Water is used in most cases as a liquid coolant for superheated steam.
In a direct superheater, a measured quantity of coolant is added to the superheater utilizing the mixing process wherein the coolant mixes with the steam. Once it passes through the desuperheater, the coolant leaves or evaporates from the mixture by absorbing heat from the superheated vapor. In this way, the temperature of the superheated steam is lowered.
The amount of coolant to be added to the process is calculated depending on the steam temperature flowing out of the desuperheater. The desuperheater steam temperature would be set above 3 degrees of the saturation temperature. It is essential in such cases, to keep the superheated steam pressure constant.
Desuperheater coils especially the pack less type has a tube-to-tube design. In this type of design, water flows through the inner tube which has a double wall and the refrigerant flows through the annulus between the tube-to-tube walls. The convoluted structure of the inner tube promotes enhanced heat transfer per unit length and unit area. Further, the convolutions that are offered by the coils promote turbulence which also contributes to the increased thermal efficiency. The rate of heat transfer is enhanced with water and refrigerant in a counterflow arrangement.
In residential apartments or homes, a desuperheater buffer tank is a tank in which the water from the pipeline flows into it enters the water heater. The water is preheated by the desuperheater connected to the buffer tank before it is sent to the water heater. Thereby reducing the load on the water heater.
Desuperheater or Steam Desuperheater works on the principle of evaporative cooling whereby the liquid water that is sprayed on the superheated steams results in its cooling. On the other hand, the heat absorbed by the liquid coolant helps it in the evaporation process. The heat is obtained from the superheated steam via convection heat transfer. As a result of this process, the steam that comes out from the desuperheater is at a lower temperature.
In a powerplant with a desuperheater, the accumulation of water near the sides of the equipment can occur due to its continuous operation. A hot water spray can be used to remove the water that is accumulated. The hot water spray is maintained at a temperature close to the steam saturation temperature at the exit of the equipment.
The steam superheater design and sizing are dependent on several requirements with a few being less severe while others having a greater impact on the proper functioning of the desuperheater. To ensure that the desuperheater is performing at an optimal level, the following factors need to be addressed carefully:
1. Ensure that an appropriate amount of cooling is available i.e. ΔTsteam
2. Measure the accurate flow of spray water that is required (Fspray/ Fsteam)
3. Ensure the narrow difference between the steam and saturation temperature (Tsteam – Tsaturation)
4. Fixed range of superheated steam flow rates
5. Fixed range of coolant or water spray flow rates
6. Pressure head of the coolant spray
7. Factors affecting the installation of the desuperheater
These requirements are usually met in applications such as reheat attemperator, bypass process in turbines, and while processing steam for the export. A physical model needs to be in place for the spraying, evaporation, and atomization process of desuperheating. The important rules to be followed for sizing and selection of desuperheater are as follows:
1. It should be ensured that the droplet size is within 250 microns at all operating conditions.
2. The penetration of the spray droplets should be in the range of 15 to 85 percent of the tube diameter. This is to avoid the impingement that can occur. It is a result of cold water hitting the surface of hot bodies or metals or surfaces.
A desuperheater spray nozzle helps in controlling the superheat by regulating the cooling water that will be sprayed through the nozzles in the design. It usually consists of a water control valve which helps in attaining a controlled desuperheated flow temperature and negligible pressure drop. The Kv / Cv value and the number of nozzles which is about 6 to 9 will be calculated according to the process conditions.
Desuperheater is used for carrying out the desuperheating process which is to reduce the temperature of the superheat and to bring back the vapor into a saturated state. A desuperheater control valve helps in controlling the temperature and pressure by adjusting the valve openings depending on the saturation temperature.
In a refrigeration system, the energy from the condensation process of a refrigeration system is left to the ambient environment or discharged to a heat sink. This energy could be used in an effective way for water heating or room heating. To recover the waste heat, the installation of a desuperheater is highly recommended whereby the waste loss can be minimized.
The location of a desuperheater in a refrigeration system is between the compressor and condenser to make use of the energy of the superheated refrigerant. For utilizing the waste heat, a separate heat exchanger should be installed wherein water can be heated using the energy from the superheated gas.
The temperature difference between the discharge from the compressor and the refrigerant condensing temperature will give the available amount of superheat. In case, there is no need for hot water, then this system can be bypassed, and the condenser should have the required condensing power or capability.
Since water is the common fluid that is used in desuperheaters, there are high chances for scaling to take place because as the temperature increases it is difficult to dissolve limestone or calcium carbonate which is the main component of scaling. The allowable temperature of water to limit scaling would be in the range of 65-700C. Further, the use of hard water also increases the chances of scaling. In such cases, it is recommended to use co-current flow to avoid high-temperature risks.
A desuperheater which is also termed a water furnace desuperheater or a geothermal desuperheater helps in reducing the costs of water heating and room heating. The excess amount of heat that is absorbed during the summers is used for heating the water. During winter, the heat that is available via a desuperheater is at a much lower cost than a standard domestic water heater.
The heat that is rejected is made use of an in desuperheater hot water superheater. It is recommended to have a buffer tank or a pre-tank which would help in preheating the water.
In residential or domestic water heating using desuperheaters, the heat during the summers is used for heating the water. It is essential to have a desuperheater pump that would help in pumping the water to the buffer tanks before it is available for the desuperheating process. During winter, the heat that is available via a desuperheater is at a much lower cost than a standard domestic water heater.
It is essential to note if the sizing of the pump is appropriate for heating purposes. The desuperheater uses the heat energy that is being removed while its main purpose is to cool the room.
The desuperheater cost which can be installed for residential purposes is very much affordable and costs about $1350 approximately. For installing a desuperheater, it is essential to have a heat pump which is included in the total cost that is mentioned. A heat pump with a coefficient of performance of value 4 would help in saving 75% which is a great investment when it comes to the residential or domestic water heater.
A desuperheater is used for removing the heat that is present in the superheat thereby reducing the temperature of the superheat close to saturation temperature or below. An attemperator is used for regulating the steam temperature of the boiler. A desuperheater is usually located downstream from the boiler where saturated steam would be useful. While an attemperator is allocated close to the boiler where high temperatures could have an impact on the walls or surfaces which would, in turn, have an impact on the process operation.
Venturi desuperheaters or annual desuperheaters help in reducing the temperature of the superheated steam by bringing it in direct contact with water. Here evaporative cooling takes place. They can be used in different environmental conditions and can be vertically or horizontally installed. When they are vertically installed, there is a substantial increase in the turn-down ratio.
These types of superheaters prevent the accumulation of water, which is not vaporized, which is a major drawback in most of the desuperheaters. Here the droplets of water that fail to vaporize will be sent back to the high-temperature region where they will be completely vaporized.
The advantage of using Venturi desuperheater is that they can be installed either vertical or horizontal. Further, they are built of heavy materials and do not have any moving parts which could interfere with their proper functioning. They are generally used in controlling temperatures of fluid that are sent to the evaporator or used in heat exchangers especially at the entrance to reduce the dimensions and cost.
In a propane refrigeration system, water is used for condensation of the propane after the compression stage. It is recommended to use two propane desuperheaters which work on the same principle that is to reduce the temperature of the superheated steam. Such a system should also be equipped with 6 propane condensers in parallel orientation. Shell and tube heat exchangers are usually used in this type of system.
1. How does a desuperheater work in a boiler? | Function of desuperheater in a boiler
Desuperheaters are used in boilers to reduce the temperature of the superheated steam that is produced in the superheater for electricity generation. The desuperheater helps in lowering the high temperature of the steam to low temperatures that will help in safely carryout the other process operation. The temperature of the superheated steam is controlled by bringing the steam in direct or indirect contact with a coolant. The injected water is then allowed to evaporate.
The two main reasons for lower the steam temperature are as follows:
1. The downstream equipment is designed to handle lower temperatures hence it is essential to lower the temperature of the steam.
2. To ensure that a controlled temperature is maintained for processes that required a specific temperature.
A steam desuperheater is used for lowering the temperature of superheat by bringing the superheat in direct or indirect contact with a coolant.
The superheated steam loses some of its heat in the turbine though not all of it. The remaining superheat which when exposed to a lower pressure results in entrained droplets of water flashing into steam which causes water hammer and other conditions.
The job is completed using the surface condenser which removes all the steam from the entry point and below the saturation so that the steam is condensed can be used for other purposes which include recycling to the boiler or other load extraction processes.
3. How is desuperheating of steam in superheaters and reheaters in a steam power plant considered a loss inefficiency?
In a desuperheater, the heat from the steam is not being used and contributes as waste heat which needs to be recovered through integrated systems. Further, the steam temperature at the outlet of the desuperheater is lower than before. Hence, this results in a loss of efficiency.
For systems with reheating, the heat that is obtained from coal or any other fuel is always less than the heat that is available for the steam. A reheater can never attain 100% efficiency. As a result, the available efficiency will be multiplied by the actual efficiency and this will lower the efficiency value.
4. How much water is required to desuperheat steam?
The amount of water required in a desuperheater depends on the amount of superheat or degrees of temperature that need to be lowered and depends on the pressure of the steam header. It can be calculated using an enthalpy balance whereby the summation of the enthalpy of steam and water is equal to the heat that is present in the exit stream. For carrying out this calculation, a steam chart would be handy.
Since the heat capacity of steam and the heat of vaporization is noted to be 0.5BTU/lbf and 1000 BTU/lbf respectively, the amount of water that is required for desuperheating would be less than the amount that one would guess. The water that is used for desuperheating should be demineralized to avoid solid build-up in the desuperheater.
In short, the amount of water required for desuperheating superheated steam depends on the temperature of the steam and the degrees of temperature to be lowered.
5. How does a pressure-reducing desuperheating system work in a thermal power plant?
In a pressure-reducing desuperheating system which is also known as a PRDS system, the required steam quality of specific quantity, temperature and pressure is released. The steam that is used in this system is either fresh steam or steam that is bled. This process is carried out using attemperating water that is obtained from the condensate water. The two fluids are mixed at controlled measures to obtain the steam at specific pressure and temperature.
6. What keeps a superheater from being damaged by heat before a boiler makes steam?
The reason why the superheater is not affected by the heat is that the steam that flows through the superheater cools the metal surfaces and other parts thereby reducing damages to the superheater.
7. What is the maximum velocity of water through the spray nozzle for the desuperheater?
The maximum velocity of water through the nozzle is about 46 to 76 meters per second. The turbulence is noted to be low when the minimum velocity of water is low, such that droplets of water get suspended from the steam and fall out.
8. Desuperheater Energy Balance
It can be calculated using an enthalpy balance whereby the summation of the enthalpy of steam and water is equal to the heat that is present in the exit stream. For carrying out this calculation, a steam chart would be handy.
Hsteam + Hwater = Qexit stream
9. what is the use of a desuperheater in a superheater?
Desuperheaters are used in boilers to reduce the temperature of the superheated steam that is produced in the superheater for electricity generation. The desuperheater helps in lowering the high temperature of the steam to low temperatures that will help in safely carryout the other process operation.
10. Turn off desuperheater in winter
It is recommended to turn off the desuperheater during the winter because there are chances of absorbing heat from the pipeline carrying hot water, thereby reducing the efficiency of the system to heat the house during the winters.
To have a better understanding of Desuperheaters, it is recommended to read on Superheaters
In industry, heat transfer problems are usually resolved for composite materials or systems with different layers which involve different modes of heat transfer such as conduction, convection, and radiation.The thermal resistance that is offered by the different layers in a system is referred to as the Overall Heat Transfer Coefficient. It is also known as the U-factor.
The U-factor that is used in calculating overall heat transfer is analogous to the convection heat transfer coefficient used in Newton’s law of cooling.The overall heat transfer coefficient is dependant on the geometry of the object or surface. For example, in a wall, we can observe different modes of heat transfer, the outer surface of the wall experiences convection heat transfer while the space between the walls undergoes conduction mode of heat transfer.
The overall heat transfer coefficient of the wall is taken to be a sum of the convective heat transfer coefficient and the conductive heat transfer coefficient. In short, the overall heat transfer coefficient is the summation of the individual heat transfer coefficient. Further explanation on the derivation of the overall heat transfer coefficient and using it for composite heat transfer problems are explained below.
In industrial applications, it is essential to know the overall heat transfer coefficient, especially in cases where the heat transfer rate needs to be optimized for better performance of a system. To calculate the heat transfer rate Q(dot) for any system with different fluids or different layers, it is essential to know the overall heat transfer coefficient.
From the value of the overall heat transfer coefficient and the rate of heat transfer, it is possible to calculate the individual heat transfer coefficient. This would help in modifying a particular portion of the thermal system for better performance as per the requirements.
Under steady-state conditions, the rate of heat transfer from a fluid at bulk temperature T1 to solid at bulk temperature T2 over an incremental area dA is given by the rate of heat transfer dQ(dot) i.e.
dQ(dot) = U*(T2 – T1)*A
Here the overall heat transfer coefficient is represented by the letter U.
The formula for the Overall Heat Transfer coefficient is given by
Qdot = U*(T1 + T2)*A
Derivation for the Overall Heat Transfer coefficient for Wall given below
Consider a composite wall that is exposed to the external environment at temperature T1, and the conduction coefficient is noted to be H1. The ambient temperature inside the room is T2 and the convection coefficient is H2. Here the heat transfer is using conduction and convection. Either side of the wall experiences heat transfer using convection at different magnitudes.
The temperature inside the wall varies and is a value between T1 and T2 if there is no source of heat generation from within the wall. The conduction coefficient of the wall is taken to be K in this case unless the wall is made up of different layers which is the usual case. In real life scenario, the wall is made up of different layers such as plastering, bricks, cement, etc. In such cases, it is essential to take into consideration the thermal resistance offered by each layer of the wall.
The overall heat transfer coefficient for the above system is as given below:
And the rate of heat transfer Q(dot) = UAΔT
It is evident that U is not a thermophysical property and depends on the flow, velocity, and also on the material through which the heat transfer takes place.
Fouling is a usual problem that is encountered in heat exchangers. It is an additional layer that is formed on the inner surface of the heat exchanger. Several factors contribute to the fouling of the surfaces of heat exchangers. The rate of heat transfer is reduced because of fouling which in turn affects the heat transfer efficiency.
The decrease in heat transfer efficiency is accounted for in calculations using the fouling factor. It is often referred to as the dirt factor. The fouling factor is dependent on the fluid on either side of the heat exchanger.
The overall heat transfer coefficient with fouling is given by
In the above equation,
U represents the overall heat transfer coefficient
h0 is the heat transfer coefficient on the shell side
hi is the heat transfer coefficient on the tube side
Rdo is the fouling factor on the shell side
Rdi is the fouling factor on the tube side
OD is the outer diameter of the tube
ID is the inner diameter of the tube
A0 is the outer area of the tube
Ai is the inner area of the tube
Kw is the value of resistance offered by the tube wall
From the equation, it is evident that the value of the overall heat transfer coefficient decreases with an increase in either or both values of fouling factor (i.e., tube side or shell side). This decrease in the overall heat transfer coefficient will in turn reduce the rate of heat transfer.
The S.I. unit of overall heat transfer coefficient is W/m2 K. Another unit that is used for representing the overall heat transfer coefficient is Btu/(hr.ft2 0F).
The unit conversion from SI unit to English units is follows:
The flow rate has an impact on the overall heat transfer coefficient. It is noted that there is a 10% decrease in heat transfer coefficient when the mass flow rate increases by three times. This estimation of the heat transfer coefficient is based on the Dittus-Boelter correlation.
While keeping the area constant, it is observed that the heat transfer coefficient increases by increasing the mass flow rate. A 90% increase in heat transfer coefficient is expected by doubling the mass flow rate. With this increase, there is an expected increase of pressure drop which is proportional to the mass flow rate.
For cases where the velocity is constant, the pressure drop decreases and is inversely proportional to the mass flow rate. The positive aspects that are attained from a higher heat transfer coefficient are lost due to the increased pressure drop when the area is kept constant.
The table below provides the overall heat transfer coefficient for a few equipment that are very often used in the industry. The range is provided because the overall heat transfer coefficient is dependent on the fluid that is used in the equipment. For gases, the value of the heat transfer coefficient is very low and that of liquids is much higher.
Equipment
U (W/m2)
Heat Exchanger
5-1500
Coolers
5-1200
Heaters
20-4000
Condensers
200-1500
Air Cooled Heat Exchangers
50-600
Table 1: Overall Coefficient of Heat Transfer for different Equipment
In heat transfer problems which consist of two different fluids which could be water and alcohol at two different temperatures, in such cases the average of the temperatures of the two fluids is used for solving the heat transfer problem which is termed as the average overall heat transfer coefficient.
Let’s take Q to be the heat flowing through the surface at an average temperature ΔTavg, and the area across which the heat transfer takes place is taken to be A. The average overall heat transfer coefficient for this heat flow is as given below
For heat exchangers, the overall heat transfer coefficient can be based on either the inside area or on the outside area
When the overall heat transfer coefficient is calculated based on the inside area, the convection coefficient at the inside is taken to be 1/hi, while the conduction coefficient at the interface is taken to be 1/ln(r0/ri)/2πkL and the convection coefficient on the outer surface of the heat exchanger is taken to be 1/h0.
Therefore, the overall heat transfer coefficient based on the inside area is given as
When the overall heat transfer coefficient is calculated based on the outside area, the convection coefficient at the inside is taken to be 1/hi, while the conduction coefficient at the interface is taken to be 1/ln(r0/ri)/2πkL and the convection coefficient on the outer surface of the heat exchanger is taken to be 1/h0.
Therefore, the overall heat transfer coefficient based on the inside area is given as
The significant difference between the two-equation is in the area, when the overall heat transfer coefficient is based on the inside area, the inner area of the heat exchanger is used in the equation. While when the overall heat transfer coefficient is based on the outside area, the outer area is taken in the equation.
When heat is flowing through a composite material, the thermal resistance offered by different layers of the material which can be due to heat conduction or convection is referred to as the overall heat transfer coefficient. The overall heat transfer coefficient is the summation of the individual heat transfer coefficient. The thermal resistance is analogous to the electrical resistance in a circuit. Here the heat transfer coefficient is dependent on the material in series or parallel arrangement.
It is of great interest to determine the individual heat transfer coefficient from the overall heat transfer coefficient. For example, for a heat exchanger, the overall heat transfer coefficient can be measured experimentally, from this overall coefficient, extracting the thermal resistance offered by the hot and cold fluid individually is the problem to be solved.
Consider a wall of thickness 5cm is made of bricks which has a thermal conductivity K=20 W/m K. The inner surface of the wall is exposed to room temperature of 250C while the external surface is exposed to the hot atmospheric temperature of 400C. What is the overall heat transfer coefficient, given the convection coefficient of air 25 W/m2K?
From the above problem, we can conclude that the system is exposed to convection on either side of the wall and conduction heat transfer within the wall. The thermal conductivity of the wall is given to be 20W/mK while the convection coefficient of air is noted to be 25 W/m2K.
3. overall heat transfer coefficient formula for cylinder
The overall heat transfer coefficient for a cylinder is given by the formula below which experiences both conduction and convection mode of heat transfer
4. overall heat transfer coefficient for evaporator
Forced circulation – steam flowing outside and liquid flowing inside
900-3000
Table 2: Overall Heat Transfer Coefficient for Evaporators
5. Overall heat transfer coefficient shell and tube | overall heat transfer coefficient for shell and tube heat exchanger | how to calculate overall heat transfer coefficient for heat exchanger | How do you calculate the overall heat transfer coefficient of an evaporator?
The overall heat transfer coefficient for any heat exchanger can be calculated using the below equation the method used might vary. One can choose the LMTD method as well
6. Graphite heat exchanger overall heat transfer coefficient
The overall heat transfer coefficient for heat exchangers which are molded graphite to graphite is about 1000W/m2K while the overall heat transfer coefficient for graphite to air is observed to be 12 W/m2K
7. Aluminium overall heat transfer coefficient
The overall heat transfer coefficient for aluminum is noted to be 200W/m2K
8. Air to air heat exchanger overall heat transfer coefficient
The overall heat transfer coefficient of air-to-air heat transfer coefficient is noted to be between 350 to 500 W/m2K.
9. Area of the heat exchanger from overall heat transfer coefficient
The area of a heat exchanger can be calculated from the overall heat transfer coefficient using the following formula
10. In which heat exchange process the value of the overall heat transfer coefficient will be highest?
The overall heat transfer coefficient is the highest for tubular heat exchangers used for evaporation with steam flowing outside the tubes and liquid flowing inside. They are noted to have an overall heat transfer coefficient in the range between 900 to 3000 W/m2K.
11. Can the overall heat transfer coefficient be negative?
In cases where the reference temperature is taken as the adiabatic wall temperature, the overall heat transfer coefficient will be negative which indicates that the heat flux is in the opposite direction with a definite temperature gradient.
12. Does the overall heat transfer coefficient change with temperature?
Overall heat transfer coefficient is dependent on the temperature gradient; therefore, temperature changes can result in changes in a temperature gradient. So, yes overall heat transfer coefficient changes with temperature.
13. What is the overall heat transfer coefficient and its application?
The thermal resistance that is offered by the different layers in a system is referred to as the Overall Heat Transfer Coefficient. It is also known as the U-factor. It is used in extracting the individual heat transfer coefficient of the different layers of a system.
The overall heat transfer coefficient of a system can be measured but the individual heat transfer coefficient of a system is difficult to obtain. In such situations, the overall heat transfer coefficient along with the rate of heat transfer will help in determining the individual heat transfer coefficient
14. What are the factors affecting the overall heat transfer coefficient?
The factors affecting the overall heat transfer coefficient are thermophysical properties such as the density, viscosity, and thermal conductivity of the fluid. Further, it is affected by the geometry and area across which the heat transfer is taking place. The velocity of fluids affects the overall heat transfer coefficient to a large extend. In heat exchanges, the type of flow also has a significant impact on the overall heat transfer coefficient.
15. What is the overall heat transfer coefficient in round tubes? | overall heat transfer coefficient pipe
A fluid flowing through a round tube experiences convective heat transfer between the fluid flowing on the outside and the outer surface of the tube, and also between the fluid flowing in the inside and the inner surface of the tube. There is conduction heat transfer between the outer surface and inner surface of the tube. Hence the overall heat transfer coefficient is given as follow:
A thermostatic expansion valve is a component that is used in the refrigeration system or air conditioning system that helps to control the amount of refrigerant that is released into the evaporator. Hence a thermostatic expansion valve ensures that the superheat from the evaporator coils is released at a steady rate. Although it is termed a ‘thermostatic’ valve, it is not capable of controlling the temperature of the evaporator coils. The temperature in the evaporator depends on the pressure which is often controlled by adjusting the capacity of the compressor.
Thermostatic expansion valves are also known as metering devices though other devices might be referred to with a similar name such as a capillary tube. In abbreviated form, TX or TXV is used to refer to as the Thermostatic expansion valve.
The function of a TXV is to regulate the flow of refrigerant into the evaporator coils depending on the superheat required. The TXV consists ofa sensory bulb filled with gas that senses the evaporator pressure. A spring beneath the diaphragm of the valve also exerts pressure. Further, the lower section of the diaphragm exerts another pressure. If the pressure of the gas in the sensing bulb is higher than the combined pressures around the diaphragm; the valve opens.
Thermostatic expansion valve responds to changes in pressure. Though, three main forces are usually considered in the study of valve opening. Another force determines the opening and closing of the valves which the force exerted by the refrigerant.
There are several designs of thermostatic expansion valve that are available in the market but the main components inside a TEV are the following
The main structure that holds the different components together is the valve body which is composed of an inbuilt orifice that restricts the refrigerant flow.
A thin flexible material which is made up of metal is the diaphragm which flexes to apply pressure on the pin.
The size of the orifice opening is adjusted using a pin or needle which controls the flow of refrigerant.
It consists of a spring that has a counter effect to the action of the pin.
It consists of a sensing bulb and a capillary line installed at the exit section of the evaporator which causes the valve to open and close.
The thermostatic expansion valve specifications vary from one design to another and depending on the refrigeration or air conditioning system. For example, in the Emerson series of thermostatic expansion valves itself, there is variation in the port valve design, the sizing, and the ranges of evaporation temperature.
Specification for Emerson TX7 series of Thermostatic expansion valve is tabulated below:
The valve remains open during the normal functioning of the refrigeration system. The working of a thermostatic expansion is explained below:
When the cooling load on the refrigeration system is high, the evaporator temperature increases which is senses by the sensory bulb of the TEV. This indicates that more refrigerant needs to be provided for the refrigeration load. The gas in the sensory bulb increases and the spring of the TEV experience an increase in pressure P1. As a result of this, the diaphragm bends downward allowing more refrigerant to flow through the valve opening into the evaporator
It is noted that the pressure below the diaphragm P2 also increases with the increasing superheat in the evaporator coils of the refrigeration system. This increase in pressure closes the valve opening of the TEV. Another pressure P3 is exerted by the spring below the diaphragm which opposes the closure of the valve. The valve will open if P1 is much greater than P2 and P3 thereby allowing the entry of refrigerant.
When the cooling load reduces in the HVAC system, the pressure P1 is less than P2 and P3 which results in the closing of the valve partially allowing an only a limited amount of refrigerant to flow into the evaporator coils of the refrigeration system. In this way, the TEV helps in maintaining the flow of refrigerant into the evaporator coils based on the superheat which is senses by the sensory bulb located on the TEV.
The thermostatic expansion valve is located between the evaporator and condenser region of the refrigeration cycle. The main body of the valve is often made from brass and consists of an inlet and outlet valve. The inlet opening is at the bottom of the device while the outlet valve is situated at the lateral side of the valve. A removable cap at the adjacent side helps in adjusting the superheat of the refrigerant.
The steps to be followed during the installation of a Thermostatic expansion valve are given bellow: –
It is recommended to clean any dust or soldering particles in the valve fittings or any other parts that might interfere with the normal functioning of the refrigeration system.
It is essential to protect the TEV by wrapping the body of the valve with a wet cloth to protect thermal agents and it is recommended to keep the soldering torch away from the valve body. Further, it should be ensured that no excess solder should be used as there are chances that it might enter the valve and interfere with the refrigeration process.
The senor bulb of a TEV that is attached to the suction line controls the valve and keeps check of the system temperature. Further, the TEV is usually installed close to the coils of the evaporator. In case the TEV comprises an equalizing pressure system, then the suction line and pressure line should be connected and should be located after the sensor bulb of the valve.
The sensing bulb is usually located on the top of the suction line, especially in a small line. For systems with sensor bulbs outside the refrigeration system, special protection against ambient conditions is required. Further, the suction line should be insulated to one foot on both sides.
For HVAC systems having lines with large diameters, the TEV bulb is positioned at 5 or 7’ o clock direction at the lower portion of the suction line. It is recommended to install the bulb on a horizontal platform of a suction line.
The TEV bulb can be attached to the vertical or horizontal region of the suction line but should never be located on the elbow which could interfere with the proper functioning of the bulb in sensing temperatures.
TEVs are never located on the lower side of the cooling line as the oil flowing through the line acts as an insulator thereby interfering with the normal operation of the sensor bulb.
In a system with multi-evaporators installed with multiple TEVs; the TEVs should not be located at the common suction line. Instead, it should be clamped onto the suction line of each evaporator to obtain a clear indication of each evaporator’s operating condition.
While adjusting TEV, it should be ensured that there is 20 minutes gap between each adjustment. TEVs are used for adjusting the flow of refrigerant into the evaporator coils. The valve consists of a pin or a needle that allows setting the coolant flow. The needle turned to a quarter is accounted to be one degree. Moreover, the needle should be adjusted only after every 20 minutes, as it is very sensitive. The steps to be followed while adjusting a TEV are as follows: –
Have a clear picture of whether the temperature reading should be increased or decreased in the TEV.
Locate the position of the needle/pin.
The needle should be turned one-quarter clockwise for every degree increase in temperature and vice-versa for every degree decrease in the temperature.
There are not particular means of calibrating the Thermostatic Expansion Valve, but it can be adjusted as it is a valve with modulating options. On turning the stem of the valve clockwise, the built-in pressure increases will result in a higher superheat.
While turning the stem anti-clockwise, the pressure in the spring decreases which reduces the superheat. The TXV loses its charge in the powerhead when the refrigeration system is turned off, but there is no chance that the valve is out of adjustment. It is recommended not to re-adjust a faulty valve; instead, it should be replaced. The new valve which will be replaced should be protected from overheating due to brazing.
There are two different types of Thermostatic expansion which are
Internally Equalized Thermostatic expansion valve
Externally Equalized Thermostatic expansion valve
An internally equalized Thermostatic expansion valve is used when the inlet pressure of the evaporator forces the valve to close. When an internally equalized TEV is used in a system with a large pressure drop across the evaporator, the pressure below the diaphragm is greater than the pressure exerted by the gas in the sensory bulb causing the valve to close and results in a superheat which is higher than that is required. This results in a starving condition.
An externally equalized TEV functions with the outlet evaporator pressure and flows to the same location as the valve temperature sensory bulb. It compensates for the pressure drop that occurs across the evaporator or refrigerant distributor. An externally equalized TEV is usually used on an evaporator with multiple circuits of refrigerant and distributor.
An internally equalized Thermostatic expansion valve is used when the inlet pressure of the evaporator forces the valve to close. When an internally equalized TEV is used in a system with a large pressure drop across the evaporator, the pressure below the diaphragm is greater than the pressure exerted by the gas in the sensory bulb causing the valve to close and results in a superheat which is higher than that is required. This results in a starving condition.
The internally equalized TEVs are usually used on large systems with a capacity greater than 1 ton and on any system that uses a distributor. It should be noted that an internally equalized TEV cab be replaced with an externally equalized TEV but not vice-versa.
An externally equalized TEV functions with the outlet evaporator pressure and flows to the same location as the valve temperature sensory bulb. It compensates for the pressure drop that occurs across the evaporator or refrigerant distributor. An externally equalized TEV is usually used on an evaporator with multiple circuits of refrigerant and distributor. For an evaporator without a distributor if the pressure drop across the evaporator is noted to be greater than 3 psi, then an externally equalized TEV needs to be used.
In a refrigeration system, if the evaporator coils are composed of extremely long tubes or tubes with narrow internal diameter then there are higher chances for greater pressure drop between the inlet and the outlet. In case the pressure drop is too high, then the saturation temperature of the refrigerant at the evaporator outlet will be lower than the saturation temperature of the refrigerant at the evaporator inlet.This calls for the need increased amount of superheat to create a condition of equilibrium around the diaphragm or TXV. To offset the effects of this high pressure, drop across the evaporator, and externally equalized TEV needs to be installed.
This line connects the lower portion of the diaphragm to the evaporator outlet; thereby ensuring that the measured superheat is related to the saturation conditions at the evaporator exit. The externally equalizing line is not capable of reducing the pressure drop but ensures that the evaporator coil area is effectively used for evaporation thereby increasing the efficiency and performance of the refrigeration system.
The advantages of a thermostatic expansion valve are as follows:
The TEV can change its valve opening depending on the superheat condition in the coils of the evaporator.
It can maintain a varying refrigerant charge to adjust varying ambient conditions.
Its capability to adjust the valve opening by sensing the pressure increase which benefits the refrigeration system in increasing its performance and preventing damage to the compressor due to flooding.
Unless the need of the device is to provide fixed release of refrigerant or coolant, a thermostatic expansion valve is the device that is largely preferred over the other options in an HVAC system.
The major disadvantage of using a thermostatic expansion valve is that if the pressure difference between the P1 (TEV sensing bulb) and combined pressures P2 (below the diaphragm) and P3 (the spring exerts a pressure (are not significant then the opening and closing of the valve will not work properly which will interfere with the proper release of the refrigerant as per the need of the heat loading. In such cases, it is recommended to install a balanced port or electronic expansion valve to cope up with the varying needs and limitations that may come up.
Thermostatic Expansion valves are largely used in the HVAC system especially in air-conditioning and refrigeration units. They are usually installed in units with larger capacities. Few areas where the thermostatic expansion valves are used are
Split AC
Refrigeration units used in industries
Central AC
Packaged Air conditioners
There are many more applications wherein the thermostatic expansion valve can be installed in the future depending on the requirements to be met.
Both the TEV and Capillary Tube work towards a common goal of controlling the flow of refrigerant into the evaporator coils but the way it functions varies. The difference between the functioning of the capillary tube and thermostatic expansion valve are tabulated below:
Thermostatic Expansion Valve
Capillary Tube
The valve opening is adjusted according to the superheat which is sensed by the sensory bulb of the TEV
It does not respond to the heat load changes and the valve opening is fixed.
It provides better efficiency as the refrigerant flow is adjusted according to the heat load
Lower efficiency as the refrigerant flow is not controlled by the heat load.
It is capable of functioning at a broader range of ambient temperatures. As the temperature is higher, the TEV will release more refrigerant. A shortcoming of this capability is slugging which can damage the compressor coils.
When the ambient temperature increases, the system must work harder to provide the required cooling
This type of valve can adjust itself to varying need of refrigerant charge thereby contributing to increased performance
It cannot accommodate varying needs of refrigerant charge thereby impacting the overall performance of the refrigeration system.
This type of expansion valve is usually used in gas cookers. This expansion valve works on the principle that liquid expands when heated. It consists of a PHIAL usually made of copper which is filled with liquid. The PHIAL is connected to a bellow using a capillary tube. This valve is connected to the bellow. When the liquid expands due to the increased temperature, the bellow pushes the valve into its position. In this way, gas flow is stopped to the burner.
The liquid expansion thermostatic valve is adjusted by using a temperature adjustment bar which moves the valve either closer or away from its position. In this way, a higher or lower temperature is obtained before achieving the bypass rate.
There are 4 types of forces that are exerted on thermostatic expansion valve which are
Pressure in the sensory bulb which an opening force.
Pressure in the evaporator or the pressure exerted by the external equalizer i.e., a closing force.
The spring below the diaphragm exerts a closing force.
The refrigerant that flows through the needle exerts an opening force.
When the pressure exerted by the refrigerant is higher than the usual norm, the force exerted by this force will be greater which will result in an inflow of more refrigerant through the coil.
While when the liquid pressure is lower, this will result in less flow through the coil. These fluctuations in superheat will be unacceptable especially for systems with accurate feeding requirements for the evaporator.
A balanced TXV is a solution for this pressure fluctuation that is experienced due to the pressure exerted by the refrigerant. Here the pressure of the refrigerant is used for balancing the top and bottom part of the needle. The liquid pressure in this type of TXV is used as a balancing force which neither contributes to the closing or opening of the valve.
When a thermostatic expansion valve is installed on a split system with two TXVs and two check valves. This unit is referred to as Bidirectional TXV It is recommended to install the Bi-directional TXV on the condensing unit and the tubing between the valve and the heat exchanger placed indoors needs to be insulated. To reduce the pressure, drop, it is essential to increase the insulation diameter.
The function of an electronic thermostatic expansion valve is like that of an ordinary thermostatic expansion valve. But using an electronic TEV ensures that the refrigerant flows in controlled in precise ratios or levels. The overheating that is required is calculated using a temperature sensor that is clamped onto the expansion valve and another one on the evaporator outlet.
The installation and control of the electronic expansion valve are simple and highly reliable. The valve is controlled using a centralized unit to controls the refrigerant flow through the entire system. It can improve the performance of the refrigeration system even at low condensing pressures. The plus point of electronic TEV is that it can enhance compressor performance without considering the evaporator load.
This type of TEV can improve the performance of the evaporation system and increasing the refrigeration capacity by around 15%. There are several designs of TEVs that are available in the market while most of the electronic TEVs are composed of a permanent magnet and copper coil inside the motor body to create an electromagnetic field. The motor is attached to the shaft which is linked to a thread. When the system is switched on, the shaft exerts pressure on the thread and thereby on the needle which is then pushed to its position. In this way, the electronic expansion valve functions.
The major difference between an electronic expansion valve and thermostatic expansion valves is that in a thermostatic expansion valve the opening is dependent on the pressure exerted while an electronic expansion valve operates using temperature sensors that calculated the required overheating. The electronic expansion valves enhance the performance of the refrigeration system to a greater extend when compared to that of an ordinary TXV due to the precise measurements
These types of TXVs are also referred to as constant pressure expansion valves as the pressure of the refrigerant is controlled in the refrigeration unit. It sends the refrigerant into the evaporator in a controlled and metered manner so that the pressure that is required to change the refrigerant from liquid to vapor is attained.
The valve body is made up of metal with a diaphragm inside the body. On the upper portion of the diaphragm, a spring is located which is always acted upon by pressure and is controlled by an adjustable screw. There is a seat beneath the diaphragm which is controlled by a needle linked to the diaphragm. The needle moves according to the diaphragm. Hence when the diaphragm moves down, the needle also moves down resulting in the opening of the valve.
The major difference between an automatic expansion valve and a thermostatic expansion valve is that the thermostatic expansion valve regulates the refrigerant flow depending on the headload that is exerted on the evaporator. While an automatic expansion valve functions according to outlet pressure; it releases the refrigerant into the evaporator coils based on the constant evaporator pressure.
A TXV can be used in varying ambient conditions, unlike AEV which can be used only in controlled conditions where the pressure in the evaporator is constant which is a limitation. This results in lower performance of refrigeration system installed with AEV in comparison to a refrigeration system that has TXV as a metering device of the refrigerant flow to the evaporator coils.
1. Why is electronic thermostatic expansion valve preferred over the ordinary TEV?
An electronic TEV is superior to that of an ordinary TEV by releasing precise and accurate amounts of refrigerant into the system by calculating the overheating. But in ordinary TXV, the refrigerant release is carried out by sensing the pressure. The electronic expansion valves enhance the performance of the refrigeration system to a greater extend when compared to that of an ordinary TXV due to the precise measurements.
2. How does a TEV maintain the refrigerant flow in an HVAC system?
The function of a TXV is to regulate the flow of refrigerant into the evaporator coils depending on the superheat required. The TXV consists of a sensory bulb filled with gas which senses the evaporator pressure. A spring beneath the diaphragm of the valve also exerts pressure.
Further, the lower section of the diaphragm exerts another pressure. If the pressure of the gas in the sensing bulb is higher than the combined pressures around the diaphragm; the valve opens.
Thermostatic expansion valve responds to changes in pressure. Though, three main forces are usually considered in the study of valve opening. Another force determines the opening and closing of the valves which the force exerted by the refrigerant.
1. In a refrigeration system that uses Thermostatic expansion valve for regulating the release of refrigerant. The pressure exerted on the valve are as follows
Pressure P1 in the sensory bulb – 5 psi
Pressure P2 below the diaphragm – 2 psi
Pressure P3 by the spring below the diaphragm – 2 psi
Based on the above information, is it expected for the TEV to open or close.
From the above information we know that
P1>P1+P2
5 psi > 4 psi (i.e., 2+2 psi)
i.e., the pressure in the evaporator is much higher than combined pressure exerted by the spring and the pressure below the diaphragm which concludes that more refrigerant is required for handling the heat load. Therefore, the TEV will open allowing the refrigerant to be released into the evaporator coils.
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Superheat in HVAC system is the heat that the refrigerant in the evaporator coils can handle whereby the liquid refrigerant boils to form a vapor. It is a known fact that water will vaporize into steam when the temperature is increased after a certain point. The same principle is used in a refrigeration system where the fluid will be a refrigerant and not just water.
Suppose we left the water to boil beyond a certain limit, then it is obvious that the steam would get hotter and hotter. When the temperature of the fluid increases, the pressure is also expected to increase, and the water will evaporate like steam.
Similarly, the refrigerant in the evaporator will also start to boil with the additional heat that is added to it. The heat absorption process does not stop and continues. The heat absorbed by the refrigerant as it changes from liquid to vapor over a given temperature is referred to be superheated.
Superheating in physics is also defined as heating a fluid beyond the boiling temperature where the fluid is expected to be in a metastable state wherein the internal effects can result in boiling of the fluid at any time.
Superheat for an HVAC system is calculated while starting up a refrigeration unit or while resolving an issue with the operating system. Further, the system should be operating for more than 15 minutes to achieve a steady state to take an accurate reading. The reading that is taken is compared to the industry standards.
The Superheat for an HVAC system is calculated as the temperature difference between the saturation temperature of the fluid and the actual temperature of the gas. The refrigerants which are used in the HVAC system often boil at temperatures lower than that of water. Suppose a refrigerant’s boiling temperature is -200C and it is heated to -100C, then the refrigerant is superheated by 10 degrees although the temperature is in negative value.
Superheat = Current Temperature – Boiling Temperature
A lower superheat suggests that the refrigerant is more than there isn’t sufficient heat load which might result in liquid refrigerant entering the compressor coils resulting in their damage. While a high superheat suggests that there is a limited amount of refrigerant for the heat load which can result in overheating and the efficiency of the refrigeration system is compromised.
By calculating the superheat, an HVAC engineer can tell how much of the liquid is entering the evaporator coils or how far the refrigerant is moving through the coils.
To measure superheat in HVAC, the following steps need to be followed which are
It is essential to measure the pressure at the lower side of the system using a pressure gauge.
The measured pressure should be used for determining the temperature using an HVAC chart.
In the next step, it is essential to measure the temperature of the suction line leaving the condenser but should be 4 to 6 inches away from the compressor.
These measurements can help one in determining the superheat or achieving the target superheat. Suppose the measurement of temperature at the suction line gives a value of 55 degrees and the conversion of the suction pressure to respective temperature gives 40 degrees as the value then the difference between the two values will give the superheat which is 15 degrees in this example.
It is essential for an HVAC engineer to know how to calculate, measure, or find the target superheat for an HVAC system. It also makes life easy for an HVAC engineer to troubleshoot issues with the refrigeration system.
The refrigerant that enters the coils of an evaporator vaporizes completely before approaching the exit of the evaporator. The vapor becomes cold as it evaporated entirely. As the cold vapor again enters the coils of the evaporator, it starts absorbing heat from the surroundings and then becomes superheated. As the vapor becomes superheated, it absorbs only the sensible heat in the evaporator coils. This process increases the efficiency of the system
Effect of Superheating
Superheating occurs at invariable pressure and a temperature higher than the saturation temperature. When the vapor undergoes sensible heating, that is when the process is termed superheating. The efficiency of the refrigeration process increases with superheating but the vapor density decreases as it exits the evaporator and enters the compressor. Further, the amount of vapor that enters the compressor is subsequently reduced.
From this, we can conclude that the capacity of the refrigeration process increases with an increase in superheat and decreases with a decreased density of the superheated vapor. Hence the possible outcome from these opposite trends can be established based on the amount of superheat that is available.
What is Subcooling?
Subcooling is the process whereby the refrigerant is cooled to a temperature lower than the saturation temperature of the refrigerant at corresponding condenser pressure. The refrigerant that is being cooled will be in a liquid state. The refrigerant can be subcooled in two different ways which are
By bringing about modifications in the condenser such that the subcooling process can be attained
Upgrading the system with internal and external heat exchangers would enhance the subcooling process.
Effects of Subcooling
The capacity of the refrigeration process is enhanced when a refrigerant is subcooled using some source of the coolant. It is observed that the efficiency of the refrigeration system can be improved by 1% for every 2 degrees of subcooling. There are new condenser designs in the market that can enhance the subcooling process thereby increasing the efficiency of the refrigeration process.
Flash gas production is minimal during the expansion process and higher latitude can be attained which makes it easier to manage the piping and evaporator location.
Importance of Subcool, Superheat and Temperature difference
To ensure that there is proper refrigerant charge in an HVAC system, it is essential to calculate the superheat, subcooling and to know the temperature gradient across the coil. The importance or advantages of knowing the subcool, superheat and temperature difference are given below
1. It notifies an HVAC engineer to have appropriate refrigerant levels to achieve high refrigeration efficiency and capacity.
2. Helps in proper diagnosis and repair of the respective problem. i.e., avoids diagnosing and repairing the evaporator when the issue is with the compressor. This could turn out to be an expensive mistake.
3. If the superheat is observed below, the possible issue should be that there is too much refrigerant in the evaporator.
4. If the superheat is observed to be too high, this indicates that the amount of refrigerant is too low for the available heat load. The possible reasons for the high superheat could be due to plugged evaporator coils or defective metering unit.
An HVAC system is said to be running with high superheat or low subcool when there is a limited amount of refrigerant in both the evaporator coils and in the compressor. The possible reason for the high superheat and low subcool could be due
1. Restriction in the liquid line
2. Faulty metering system
3. Excessive airflow through the evaporator coils.
In an HVAC system, converting a refrigerant from liquid to vapor involves adding heat to the system at boiling temperature. Heat added above boiling temperature is referred to as superheat.
To find superheat in the suction line, it is essential to know the suction pressure and boiling temperature in the evaporator at any given pressure. This method of finding the superheat from the pressure and temperature is often referred to as temperature- pressure method for finding superheat.
As the evaporator coils more and more heat, the liquid refrigerant starts boiling and at some point, only vapor can be found in the coils. There might be some vapor left behind which is still cold.
The cold vapor passes through the evaporator coils and absorbs heat, after a point; all the available vapor will be heated to a temperature above the saturation temperature. After all the liquid boils off, the additional heat that is added to the vapor is referred to as the Suction Superheat.
Example: A refrigerant is saturated state enters the evaporator coils at 45F and this temperature is obtained from the suction pressure at 120 PSIG for R-410 A. The temperature probe that is placed at the suction line reads 55F. From the temperature reading at the suction line, it is evident that the refrigerant is superheated by 10 degrees.
After the state of the refrigerant has changed and the process has stopped, the cooling of the refrigerant ceases. The temperature of the cool vapor rises rapidly. The heating of the refrigerant vapor ensures that no liquid will enter the compressor coils and thereby reducing the chances of compressor damage.
Often the manufacturers of HVAC systems provide pressure-temperature charts that make the technicians’ life easier. This chart helps a technician to charge an HVAC system with an appropriate amount of refrigerant. These charts are often provided near the condensing unit of the HVAC unit. The charge of refrigerant is based on factors such as ambient temperature and the load capability of the system.
Most of the condensers in HVAC systems are already charged with refrigerant. The refrigerant charge in the condenser and the line set up will depend upon the manufacturer. In this way, the installation process becomes much easier for an HVAC engineer. The charge adjustments can be made as per the length of the line set up.
This method of charging units with refrigerant works well with refrigeration systems that come as a pack wherein the loop requires repair while the charge must be recovered. The refrigerant must be charged as recommended by the manufacturer in terms of an ounce. There are means of charging an HVAC system without using an appropriate superheat or subcooling method.
When an HVAC engineer is charging an HVAC unit, the technician needs to get the exact temperature difference from where the fluid changed its state. If the superheat is high, the system will be undercharged and if the superheat is low, the system will be overcharged. This method of charging the system is called superheat method and is not used while charging a heat pump or an air conditioner.
But if an air conditioner was equipped with a thermostatic expansion valve, then the system needs to be charged using the superheat method or the subcooling method.
It is of great importance for an HVAC engineer to understand superheat and subcooling as it is closely tied to the diagnosis of an HVAC unit. For an apprentice or a fresher in the HVAC department, it is essential to know how to deduct the superheat capacity of an HVAC system. Further, one should also develop skills in reading the pressure-temperature charts that are provided by the manufacturer as these days most units are provided with these charts.
It is recommended to understand the basic laws associated with HVAC systems such as Boyles Law, Sensible Heat, etc which would make lives easy for an HVAC engineer. Also important concepts on High Superheat, Low Superheat, and Superheater would be beneficial for a mechanical engineer or a technician.
1. What is superheat and subcooling in an HVAC system
An HVAC system is said to be running with high superheat or low subcool when there is a limited amount of refrigerant in both the evaporator coils and in the compressor.
2. What are the possible reasons for high superheat in a refrigeration unit?
The possible reason for the high superheat could be due to the following reasons
1. Restriction in the liquid line
2. Faulty metering system
3. Excessive airflow through the evaporator coils.
4. Plugged compressor coils
5. Limited airflow through the evaporator coils
3. How to calculate the superheat for a refrigerant at a temperature of 58.500C?
Superheat is calculated as the difference between boiling temperature and current temperature
Boiling Temperature of refrigerant = 48.500C
Superheat = Current Temperature – Boiling Temperature
Superheat = 58.50 – 48.50
= 100C
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The concept of superheating is crucial in understanding the functioning of steam power plants and boilers. A superheater is a device that increases the temperature of steam above its saturation point, resulting in dry and high-temperature steam. This process enhances the efficiency and performance of the steam power system. Superheaters are typically located in the flue gas path of a boiler and are classified into two types: radiant superheaters and convection superheaters. Radiant superheaters are placed in the furnace area, while convection superheaters are positioned in the convective pass of the boiler. Superheaters play a vital role in preventing condensation, improving heat transfer, and ensuring the reliability of steam turbines.
Key Takeaways
Superheater Type
Location in Boiler
Radiant Superheater
Furnace area
Convection Superheater
Convective pass
Understanding Superheaters
What is a Superheater in a Boiler?
A superheater is an essential component of a boiler that plays a crucial role in steam generation. It is responsible for increasing the temperature of the steam produced by the boiler, resulting in superheated steam. Superheated steam refers to steam that has been heated to a temperature higher than its saturation point at a given pressure. This process of superheating the steam has several significant benefits in terms of boiler efficiency and overall performance.
The Concept of Superheat
To understand the concept of superheat, we need to delve into the basics of steam generation and heat transfer in a boiler. When water is heated in a boiler, it undergoes a phase change from a liquid state to a gaseous state, resulting in the formation of steam. This steam initially exists as saturated steam, which is a mixture of water vapor and liquid water droplets.
The superheater, located in the flue gas path of the boiler, is designed to further heat the saturated steam by absorbing heat from the flue gases. This additional heat transfer raises the temperature of the steam above its saturation point, converting it into superheated steam. The superheater achieves this by utilizing different types of superheaters, such as radiant and convective superheaters, which are strategically placed within the boiler system.
Superheated Steam and its Significance
Superheated steam offers several advantages in various applications, particularly in thermal power plants and industrial processes. The increased temperature of superheated steam allows for more efficient energy conversion and power generation. It enhances the performance of steam turbines by increasing their efficiency and reducing the risk of blade erosion caused by wet steam.
Moreover, superheated steam provides better control over steam temperature, ensuring consistent and precise heat transfer in heat exchangers. This is crucial in industries where temperature control is critical for maintaining product quality and process efficiency.
It is important to note that the design, operation, and maintenance of superheaters are crucial for their optimal performance and longevity. The choice of superheater materials, steam quality, boiler pressure, and temperature control play a significant role in preventing superheater failure and ensuring safe and efficient boiler operation.
In conclusion, superheaters are integral components of boilers that increase the temperature of steam, resulting in superheated steam. This process has numerous benefits, including improved boiler efficiency, enhanced heat transfer, and better control over steam temperature. Understanding the role and significance of superheaters is essential for optimizing the performance of thermal power plants and industrial processes.
Types of Superheaters
Radiant Superheaters
Radiant superheaters are a type of superheater used in steam generation systems. They are designed to increase the temperature of the steam by transferring heat through radiation. This type of superheater is typically located in the hottest part of the boiler’s flue gas path, where it can absorb the maximum amount of heat. Radiant superheaters are commonly used in thermal power plants to improve boiler efficiency and steam temperature control.
Convective Superheaters
Convective superheaters are another type of superheater that increases the temperature of the steam by transferring heat through convection. Unlike radiant superheaters, convective superheaters are located in the cooler part of the flue gas path. They are designed to extract heat from the flue gas and transfer it to the steam. Convective superheaters are commonly used in boilers to increase the steam temperature and improve overall heat transfer efficiency.
Separately Fired Superheater
A separately fired superheater is a type of superheater that is independent of the main boiler. It has its own combustion system and is used to superheat steam separately from the main boiler. This type of superheater is often used in large thermal power plants where the main boiler may not be able to provide sufficient superheating capacity. Separately fired superheaters allow for better control over steam temperature and can be used to increase the overall efficiency of the power plant.
Electric Steam Superheater
Electric steam superheaters are a type of superheater that use electricity to generate heat and superheat the steam. They are commonly used in applications where a clean and reliable source of heat is required. Electric steam superheaters are often used in industries such as food processing, pharmaceuticals, and laboratories. They offer precise temperature control and are easy to install and maintain.
Geothermal Superheater
Geothermal superheaters are a type of superheater that utilize the heat from geothermal sources to superheat steam. Geothermal energy is a renewable and sustainable source of heat that can be harnessed for power generation. Geothermal superheaters are commonly used in geothermal power plants to increase the temperature of the steam produced by the geothermal wells. This type of superheater helps to improve the efficiency of the power plant and maximize the energy conversion from heat to electricity.
In summary, superheaters play a crucial role in steam generation and boiler efficiency. They are designed to increase the temperature of the steam, improve heat transfer, and enhance the overall performance of thermal power plants. The different types of superheaters, such as radiant superheaters, convective superheaters, separately fired superheaters, electric steam superheaters, and geothermal superheaters, offer various advantages and are used in different applications based on their design and operating principles. Proper selection, operation, and maintenance of superheaters are essential for ensuring safe and efficient power generation.
Superheater Design and Components
The superheater is an essential component in steam generation systems, particularly in thermal power plants. Its main function is to increase the temperature of the steam produced by the boiler, improving boiler efficiency and enhancing the overall performance of the power generation process.
Boiler Superheater Coil Material
The choice of material for the boiler superheater coil is crucial to ensure its durability and efficiency. The superheater coil is subjected to high temperatures and pressures, as well as corrosive flue gases. Therefore, it is commonly made from high-quality alloy steels that can withstand these harsh conditions. These materials offer excellent heat transfer properties and resistance to corrosion, ensuring the longevity and reliability of the superheater.
Boiler Superheater Design
The design of the boiler superheater plays a vital role in achieving optimal steam temperature control and maximizing energy conversion. Superheaters are typically positioned in the flue gas path, where they absorb heat from the flue gases and transfer it to the steam. There are two main types of superheaters: radiant superheaters and convective superheaters.
Radiant superheaters are located in the radiant heat zone of the boiler, where they absorb heat directly from the combustion process. They are typically used in boilers with high steam temperatures and are effective in achieving rapid steam temperature increase.
Convective superheaters, on the other hand, are positioned in the convective heat zone of the boiler. They absorb heat from the flue gases after they have passed through the radiant heat zone. Convective superheaters are commonly used in boilers with lower steam temperatures and provide a more gradual increase in steam temperature.
Superheater Header
The superheater header is an integral part of the superheater system. It acts as a distribution manifold, collecting the superheated steam from the individual superheater tubes and delivering it to the steam turbine. The design of the superheater header ensures uniform steam distribution and minimizes pressure drop, optimizing the performance of the steam turbine.
Superheater Attemperator
To maintain the desired steam temperature, superheater attemperators are employed. These devices control the temperature of the superheated steam by injecting a controlled amount of water or steam into the superheater outlet. By adjusting the amount of water or steam injected, the attemperator can regulate the steam temperature and prevent overheating. This is particularly important in situations where the load on the boiler fluctuates, ensuring the safe and efficient operation of the superheater.
In conclusion, the design and components of the superheater are crucial for achieving efficient heat transfer, steam temperature control, and overall boiler performance. The selection of appropriate superheater materials, along with the careful design of the superheater coils, headers, and attemperators, contribute to the safe and reliable operation of the thermal power plant and the optimization of power generation.
Superheater Operations
A superheater is an essential component in steam generation and plays a crucial role in enhancing boiler efficiency and power generation in thermal power plants. It is responsible for increasing the temperature of the steam beyond its saturation point, resulting in superheated steam.
Primary Superheater and Secondary Superheater
Superheaters are typically classified into two types: primary superheaters and secondary superheaters. The primary superheater is located in the flue gas path, where it absorbs heat from the flue gases and transfers it to the steam. On the other hand, the secondary superheater is positioned in the convective section of the boiler, where it further increases the steam temperature.
The primary superheater, also known as the radiant superheater, is designed to withstand high temperatures and is constructed using materials that can withstand intense heat. It is responsible for heating the steam to a certain temperature before it enters the secondary superheater.
The secondary superheater, also known as the convective superheater, continues the process of increasing the steam temperature. It utilizes the heat transfer from the flue gases to further superheat the steam. The convective superheater is designed to maximize heat transfer efficiency and ensure the steam reaches the desired temperature.
Superheater Efficiency
The efficiency of a superheater is crucial for optimal steam generation and energy conversion. A well-designed superheater ensures that the steam temperature is precisely controlled, allowing for efficient power generation. It also contributes to the overall boiler efficiency by maximizing heat transfer and minimizing energy losses.
To achieve high superheater efficiency, it is essential to consider factors such as the type of superheater, steam quality, boiler pressure, and the materials used in its construction. Proper maintenance of the superheater is also crucial to prevent any potential failures that could impact its performance and overall boiler safety.
Use of Superheaters
Superheaters are widely used in various industries where high-temperature steam is required. They play a vital role in processes such as power generation, heat exchangers, and steam turbine efficiency. By increasing the steam temperature, superheaters enable more efficient heat transfer and enhance the overall performance of the system.
The use of superheaters allows for better control over the steam temperature, which is essential in applications where precise temperature control is required. Superheated steam also offers advantages such as increased energy transfer, improved turbine efficiency, and reduced condensation in the steam distribution system.
In conclusion, superheaters are integral to the operation of boilers in thermal power plants. They increase the temperature of steam beyond its saturation point, enhancing efficiency and enabling various industrial processes. Proper design, maintenance, and utilization of superheaters contribute to improved heat transfer, energy conversion, and overall system performance.
Comparisons and Distinctions
Difference Between Radiant and Convective Superheaters
In the realm of steam generation and boiler efficiency, the design and operation of superheaters play a crucial role. Superheaters are heat exchangers that increase the temperature of steam, enhancing its energy content and improving the efficiency of thermal power plants. There are two main types of superheaters: radiant superheaters and convective superheaters. Let’s explore the differences between these two types.
Radiant Superheaters:
Radiant superheaters are located in the radiant section of the boiler’s flue gas path. They are exposed to the highest temperatures and heat transfer rates. These superheaters are typically made of high-temperature materials that can withstand the intense heat. Radiant superheaters utilize radiant heat transfer to raise the temperature of steam. They are positioned in the path of the hot flue gases, allowing direct heat transfer from the combustion process to the steam.
The key characteristics of radiant superheaters are:
They operate at high temperatures, typically above 1000°C.
They are designed to handle high heat fluxes.
They are effective in achieving high steam temperatures.
They are commonly used in boilers with high-pressure and high-temperature conditions.
They contribute to the overall efficiency of the boiler by increasing the steam temperature.
Convective Superheaters:
Convective superheaters, on the other hand, are located in the convective section of the boiler’s flue gas path. They are exposed to lower temperatures compared to radiant superheaters. Convective superheaters utilize convective heat transfer to raise the temperature of steam. They are positioned in the path of the flue gases after they have passed through the radiant superheaters. This allows for further heat transfer from the flue gases to the steam.
The key characteristics of convective superheaters are:
They operate at lower temperatures compared to radiant superheaters.
They are designed to handle lower heat fluxes.
They are effective in achieving moderate steam temperatures.
They are commonly used in boilers with medium to low-pressure conditions.
They contribute to the overall efficiency of the boiler by further increasing the steam temperature.
Difference Between Superheater, Reheater, and Air Preheater
In the context of steam generation and heat transfer, it is important to understand the distinctions between a superheater, reheater, and air preheater. Each of these components serves a specific purpose in the thermal power plant.
Superheater:
A superheater is a heat exchanger that increases the temperature of steam above its saturation point. It is located in the flue gas path of the boiler and utilizes heat transfer to raise the steam temperature. The superheater plays a crucial role in improving the efficiency of the power plant by increasing the energy content of the steam. It ensures that the steam leaving the boiler is superheated, which is essential for various industrial processes and power generation.
Reheater:
A reheater is another heat exchanger that is positioned in the flue gas path after the high-pressure turbine. Its primary function is to reheat the steam that has passed through the high-pressure turbine. By raising the temperature of the steam, the reheater improves the efficiency of the power plant by allowing for additional expansion in the low-pressure turbine. This increases the overall energy conversion and power generation capabilities of the plant.
Air Preheater:
An air preheater is a heat exchanger that is responsible for heating the combustion air before it enters the boiler. It utilizes the heat from the flue gases to raise the temperature of the incoming air. By preheating the combustion air, the air preheater improves the thermal efficiency of the boiler. This results in better fuel combustion and reduced fuel consumption, contributing to the overall efficiency and sustainability of the power plant.
In summary, while the superheater increases the temperature of steam, the reheater reheat the steam after the high-pressure turbine, and the air preheater preheats the combustion air before it enters the boiler. Each of these components plays a vital role in optimizing the efficiency of the thermal power plant and ensuring effective heat transfer throughout the system.
The Science Behind Superheating
Is Superheat Latent Heat?
Superheating is a fascinating phenomenon that occurs in the world of thermodynamics and heat transfer. It involves raising the temperature of a substance, such as steam, above its boiling point without changing its phase from a gas to a liquid. But is superheat considered latent heat? Let’s find out.
In simple terms, latent heat refers to the heat energy required to change the phase of a substance without changing its temperature. For example, when water boils and turns into steam, it absorbs latent heat. However, superheating is different. It involves adding additional heat energy to a substance that is already in the gaseous state, increasing its temperature beyond the boiling point.
Superheating is achieved by passing the steam through a superheater, a component in a boiler system. The superheater is designed to absorb heat from the flue gas path and transfer it to the steam, increasing its temperature. This process allows for precise control of the steam temperature, which is crucial in various applications, such as thermal power plants and steam turbines.
Why is Superheating Desirable?
Superheating offers several advantages in steam generation and boiler efficiency. By increasing the temperature of the steam, the energy conversion efficiency in power generation can be significantly improved. Superheated steam has higher enthalpy, which translates to increased work output in steam turbines.
Moreover, superheating enhances the heat transfer process. The higher temperature of the steam allows for more efficient heat transfer in heat exchangers, resulting in improved overall system performance. It also reduces the risk of condensation and corrosion in the steam distribution system, ensuring the delivery of high-quality steam to various industrial processes.
Superheating and Supercooling Phenomenon
Superheating is not the only phenomenon that occurs in the world of thermodynamics. Supercooling, on the other hand, involves cooling a substance below its freezing point without it solidifying. While superheating is desirable in many applications, supercooling is often an unwanted occurrence.
The superheating and supercooling phenomena are influenced by various factors, including the type of superheater used, the materials used in its construction, and the control of steam pressure and temperature. There are different types of superheaters, such as radiant superheaters and convective superheaters, each with its own advantages and limitations.
To ensure the efficient operation of superheaters, regular maintenance is essential. Proper inspection and cleaning of superheater tubes are necessary to prevent fouling and corrosion, which can lead to reduced heat transfer efficiency and potential superheater failure.
In conclusion, superheating plays a crucial role in various industries, including power generation and heat transfer. By increasing the temperature of steam, superheaters improve boiler efficiency, enhance heat transfer, and ensure the delivery of high-quality steam. Understanding the science behind superheating is vital for optimizing energy conversion and maintaining the safety and performance of boiler systems.
Practical Applications and Considerations
Superheat is a crucial aspect of HVAC systems, boilers, and thermal power plants. It plays a significant role in ensuring efficient heat transfer and steam generation. Let’s explore some practical applications and considerations related to superheat.
Superheat in HVAC Systems
In HVAC systems, superheat refers to the process of increasing the temperature of the refrigerant vapor above its saturation point. This is achieved by removing any remaining liquid content from the vapor. Superheated vapor is then used to cool the air, providing comfortable indoor temperatures.
When HVAC systems utilize superheat, it offers several benefits such as:
Improved Efficiency: Superheating the refrigerant vapor allows the system to operate at higher efficiency levels, reducing energy consumption and costs.
Better Temperature Control: Superheated vapor provides more precise temperature control, ensuring optimal comfort levels in different areas of a building.
Preventing Liquid Refrigerant Damage: By removing any liquid content from the vapor, the risk of liquid refrigerant entering the compressor and causing damage is minimized.
When is Superheat Charging Method Used?
The superheat charging method is commonly used during the installation and maintenance of HVAC systems. It involves adjusting the refrigerant charge to achieve the desired superheat value. This method is typically employed when:
Troubleshooting: Superheat charging can help diagnose issues related to refrigerant flow, system performance, or component malfunction.
System Optimization: By fine-tuning the superheat value, the system can be optimized for maximum efficiency and performance.
Retrofitting: When retrofitting an existing HVAC system, the superheat charging method ensures compatibility and proper functioning of the new components.
Benefits of Superheaters in Boilers
Superheaters are essential components in boilers used in thermal power plants and industrial processes. They increase the temperature of the steam produced, offering several benefits:
Enhanced Efficiency: Superheating the steam improves the overall efficiency of the boiler, resulting in better fuel utilization and reduced operating costs.
Increased Power Generation: Superheated steam has higher energy content, allowing for increased power generation in steam turbines.
Improved Heat Transfer: Superheaters optimize heat transfer by maintaining a uniform and controlled steam temperature throughout the system.
Boiler Safety: Superheaters play a crucial role in preventing boiler tube overheating and potential failures, ensuring safe and reliable operation.
Superheaters can be classified into two main types: radiant superheaters and convective superheaters. They are designed using specific materials to withstand high temperatures and pressures.
In conclusion, superheat has practical applications in HVAC systems, where it improves efficiency and temperature control. In boilers, superheaters enhance power generation, heat transfer, and overall system safety. Understanding the considerations and benefits of superheating is essential for optimizing the performance of these systems.
Frequently Asked Questions
Interview Questions and Answers on Superheaters
Superheaters play a crucial role in steam generation and boiler efficiency. They are responsible for increasing the temperature of the steam, ensuring optimal heat transfer and enhancing the overall performance of thermal power plants. Here are some commonly asked questions about superheaters:
Q: What is the purpose of a superheater in a boiler?
A: The main purpose of a superheater is to increase the temperature of the steam produced by the boiler. By raising the steam temperature above its saturation point, the superheater improves the energy conversion process and enhances power generation efficiency.
Q: How does a superheater work?
A: Superheaters are typically located in the flue gas path of a boiler. They consist of tubes through which the steam passes after it has been heated in the boiler. The superheater absorbs heat from the flue gases, increasing the steam temperature before it enters the steam turbine.
Q: What are the different types of superheaters?
A: There are two main types of superheaters: radiant superheaters and convective superheaters. Radiant superheaters are located in the radiant heat zone of the boiler, while convective superheaters are positioned in the convective heat zone. Each type has its own advantages and is used based on the specific requirements of the boiler.
Q: What materials are used in superheater construction?
A: Superheaters are typically made from high-temperature resistant materials such as alloy steels, stainless steels, and nickel alloys. These materials can withstand the high temperatures and pressures encountered in the superheater section of the boiler.
Q: How does a superheater affect steam quality?
A: A well-designed superheater ensures that the steam leaving the boiler is dry and of high quality. By increasing the steam temperature, the superheater reduces the moisture content and improves the steam’s thermal properties, making it more suitable for various industrial processes.
Q: What factors can lead to superheater failure?
A: Superheater failure can occur due to various factors, including high temperatures, thermal stress, corrosion, and mechanical wear. Regular maintenance and monitoring of superheaters are essential to prevent failures and ensure safe and efficient boiler operation.
Q: How does a superheater contribute to boiler safety?
A: Superheaters play a crucial role in maintaining safe boiler operation. By controlling the steam temperature, they prevent the formation of wet steam, which can cause damage to the turbine blades and other components. Proper superheater operation ensures the safe and reliable functioning of the boiler.
Q: What is the impact of superheater design on steam turbine efficiency?
A: The design of the superheater has a direct impact on steam turbine efficiency. A well-designed superheater ensures that the steam supplied to the turbine is at the desired temperature, maximizing the energy conversion process and improving overall turbine performance.
Remember, understanding the role of superheaters in steam generation, boiler efficiency, and heat transfer is crucial for anyone working in the field of thermal power plants and energy conversion.
In conclusion, understanding the important concepts of a superheater is crucial for anyone involved in the field of steam power generation. We have explored the purpose of a superheater, which is to increase the temperature of steam beyond its saturation point, resulting in more efficient energy transfer and improved turbine performance. We have also discussed the different types of superheaters, including radiant, convection, and combination superheaters. Additionally, we have examined the factors that affect superheater performance, such as steam flow rate, temperature, and pressure. By grasping these key concepts, engineers and operators can optimize the operation of superheaters and enhance the overall efficiency of steam power plants.
Frequently Asked Questions
What is the content of a superheater in thermal power plants?
The content of a superheater in thermal power plants primarily includes steam that has been heated above its boiling point to increase its thermal energy and prevent condensation during the process of power generation. The superheater also consists of various components such as tubes for steam flow, headers, and supporting elements.
What is the difference between a radiant superheater and a convective superheater?
The main difference between a radiant superheater and a convective superheater lies in their heat transfer methods. A radiant superheater absorbs heat by radiation from the combustion process, while a convective superheater absorbs heat through the convection of flue gases.
What is the role of an attemperator in a superheater?
An attemperator in a superheater is used to control the steam temperature. It does this by injecting water into the steam to reduce its temperature when it exceeds the desired limit, thereby preventing damage to the downstream equipment.
What is the difference between a superheater and a reheater?
The difference between a superheater and a reheater lies in their function within a boiler system. A superheater heats saturated steam to a superheated state, increasing its thermal energy and preventing condensation. On the other hand, a reheater heats up the partially expanded steam coming from the high-pressure turbine to increase its thermal energy before it enters the low-pressure turbine.
What is the transparency content in a superheater?
The transparency content in a superheater refers to the clarity of the operational and maintenance procedures, safety guidelines, and performance data. This transparency is crucial for efficient operation, maintenance planning, and safety compliance.
What is the difference between a convective superheater and an electric superheater?
A convective superheater absorbs heat through the convection of flue gases, while an electric superheater uses electric coils to heat the steam. The choice between the two depends on factors such as the available power source, operational efficiency, and cost considerations.
What is a geothermal superheater?
A geothermal superheater is a type of superheater used in geothermal power plants. It uses the heat from the earth’s core to superheat the steam, which is then used to turn the turbines and generate electricity.
What is superheat and why is it important in steam generation?
Superheat is the process of heating steam above its boiling point to increase its thermal energy and prevent condensation. This is important in steam generation as it improves the efficiency of the steam turbine and prevents damage to the turbine blades due to condensation.
When is the superheat charging method used in HVAC systems?
The superheat charging method is used in HVAC systems when the system operates with a fixed orifice or capillary tube. It ensures the correct amount of refrigerant charge to maintain the desired level of superheat, thereby optimizing system performance and efficiency.
Why does superheating and supercooling occur in a boiler system?
Superheating occurs in a boiler system to increase the thermal energy of the steam and prevent condensation, which can damage the turbine blades. Supercooling, on the other hand, occurs when the steam or liquid is cooled below its boiling or freezing point without it changing its state, which can help in certain cooling or refrigeration processes.
When there is an excess amount of refrigerant in the coils of the evaporator in comparison to the heat load. This condition is termed as low superheat. The reason for low superheat could be due to insufficient heat load or due to excessive amounts of refrigerant entering the evaporator.
There may be some amount of liquid refrigerant in the suction line which might enter the compressor and cause compressor damage. The reasons for low superheat are explained below:
REFRIGERATION SYSTEM WITH TXV (CREDITS: Wikipedia),Image Attribution : Carlo Viso
1. Excess amount of Refrigerant
When there is an excess amount of refrigerant that is flowing through the evaporator coils, enough heat will not be absorbed by the evaporator to vaporize the liquid refrigerant. As a result, we have a low superheat and as the refrigerant can absorb enough heat in the suction line; there is a high possibility that it might enter the compressor and damage that unit.
2. Overfeeding in the metering unit
A metering unit that allows more than the needed amount of refrigerant to the evaporator coils will cause flooding. In case the sensing bulb of the thermal expansion valve is not insulated properly then there is a high possibility of the valve being flooded or overfed. When the device overfeeds, there are high chances for both the suction pressure and the discharge pressure to increase.
3. Reduced airflow through the evaporator
One of the most common reasons for low superheat is due to reduced airflow. With reduced airflow, there isn’t enough warm air to vaporize the refrigerant. As a result, there will be a reduced amount of refrigerant vapor and there is a high possibility for the liquid refrigerant to enter the compressor and cause damage to the unit. In this case, both suction and discharge pressures will be lower than usual levels.
It is recommended to clean dirty filters, coil, and motors to allow more air to enter through the evaporator.
4. Reduced airflow through the condenser
When the amount of air entering the condenser is low, there is a high possibility for higher pressure and temperature in the condenser and the condenser coils, the refrigerant is available to the metering device at higher pressure.
With an increased pressure drop across the metering device, more refrigerant enters the flow. As more refrigerant enters the flow, the suction and discharge pressure increase; also results in subcooling. The main reason for low airflow through the condenser is due to poor motor bearings or obstructions in the unit.
5. Large Sized Equipment
When the system or equipment is too large, but the load is not enough that is enough heat is not available to vaporize the liquid refrigerant into vapor, then it will result in low superheat. With oversized equipment, the indoor relative humidity is expected higher than usual.
When there is an excess amount of refrigerant but a limited amount of heat load that is available in the evaporator, the condition is referred to as low superheat. This could be caused due to low airflow or due to plugged coils in an evaporator. When there is a limited amount of refrigerant entering the condenser, this could be the result of poor compression, an oversized metering device, or overfeeding.
This condition is referred to as low subcooling. When there is limited heat load in the evaporator and limited refrigerant in the condenser, this condition is referred to as low superheat low subcooling. The superheat will help in identifying if the low suction is a result of limited heat entering the evaporator coils.
Low superheat normal subcooling can indicate that the refrigerant charging is high either due to plugged evaporator coils or due to plugged air filters. The reason for the normal subcooling despite the low superheat is because the refrigeration system is installed with a liquid line receiver. The temperature drop across the liquid line filter or dryer gives a clear indication of the possible cause is due to plugging.
To raise superheat, there should be more heat load that is available for the evaporator coils to handle. While to lower superheat, more refrigerant should be added so that the heat load can be handled by the coils of the evaporator. It is recommended to add refrigerant to lower superheat and recover refrigerant to increase superheat. It should be noted that additional superheat should not be added if the superheat is found to be 5F already.
A low discharge superheat alarm indicates that the compressor is flooding with the refrigerant. This is mostly because the expansion valve is overfeeding to the evaporator or due to a faulty actuator.
A low evaporator superheat is a condition wherein the refrigerant hasn’t been capable of carrying enough heat load to the compressor coils. This will limit the refrigerant from vaporizing, because of which liquid refrigerant will enter the compressor which will cause slugging that damages the compressor units and other components of the refrigeration system.
A suction pressure low superheat condition occurs when the capacity regulator is large because of which it feeds in more refrigerant into the coils of the evaporator as the heat load is not enough for the available refrigerant. Another possible reason for this condition could be the high capacity of the thermal expansion valve.
To maintain the total capacity of the system, it is essential to have an appropriate refrigerant charge in the system so that suction pressure and superheat are kept to the right levels that would help in the proper functioning of the refrigeration system.
A low suction superheat carrier is referred to when there isn’t enough air that flows through the evaporator coils. This limits the heat from being carried to the coils of the evaporator which results in low suction superheat. The possible reasons for low suction superheat could be the dirty of plugged evaporator coil that restricts air from flowing through the coils. It is recommended to add refrigerant to lower the suction superheat and add refrigerant to increase the suction superheat.
In a low-temperature superheater, the steam entering the turbine has a high moisture content which increases the rate of erosion. Further, a decrease in the superheat temperature also causes quenching of the metal surfaces of the equipment it passes through.
There is the possibility of stresses on the surface of superheaters, steam pipes, stop valves, and turbine inlets. A severe vibration is reported in case of sudden chilling of the turbine rotor.
A low suction pressure low superheat is encountered when there is low heat load which could be because of dirty air filters, an insufficient amount of air flowing through the system, or because of the air being too cold. Other possible causes of low suction pressure low superheat are the non-uniform distribution of the refrigerant and could be the result of oil clogged evaporators.
Low superheat indicates that there is an excess amount of refrigerant in the evaporator, or the heat load is not sufficient to vaporize the liquid refrigerant to vapor before it moves to the compressor resulting in compressor damage. Plugging of the evaporator coils can also result in low superheat.
On the other hand, low subcooling indicates that there is an excess amount of refrigerant in the condenser. For refrigeration systems that using a thermostatic expansion valve, it is recommended to be maintained between 100F to 180 F.
Therefore, a low superheat low subcooling TXV is one where the refrigerant is in excess in the evaporator and is limited in the condenser resulting in variations in the subcooling below 100F
0 Degree superheat or low superheat on a low-temperature refrigeration system could indicate that the refrigerant is not carrying enough heat through the coils of the evaporator to vaporize the refrigerant before entering the compressor coils. Even in a low-temperature refrigeration system, it is essential to collect enough heat that is equivalent to the refrigerant charge in the system.
A heat pump that is operating at low superheat does not have enough heat load for the excess amount of refrigerant that is available in the coils of the evaporator resulting in liquid refrigerant entering the compressor valves and causing damage to the compressor and other mechanical components of the refrigeration system.
It is therefore suggested to maintain the superheat of the refrigeration system within certain limits such that the damages to the parts of the refrigeration system are minimized. Further, it is recommended to carrying out timely cleaning of the evaporator coils and the compressor valves to avoid plugging that would reduce the flow of air which could also limit the efficiency of the system.
It indicates that there isn’t enough heat load for refrigerant that is available in the evaporator coils which could result in flooding of the compressor. The compressor is designed to only work with vapors or gases and the entry of liquid will damage the compressor coils and their other components.
A low superheat could also be the result of plugged evaporator coils which is stopping the entry of the heat load. Limited airflow through the system could also result in low superheat because sufficient airflow is required for carrying the heat to vaporize the refrigerant. A faulty metering device or overfeeding of refrigerant can also result in low superheat.
2. If in recovery boiler feed water temp is low What effect of low temp will be in superheated steam or final steam?
The boiler operates with a layer of heat transfer surface which is hot, and water passes over this surface. As the water passes over the hot surface, steam is produced which enters the steam system. The pressure at the heat transfer surface is higher than at the water system because of the heat of the water.
The steam bubbles leaving the heat transfer surface will either be superheated or cooled to the saturation temperature as it rises through the water. The latter can happen. When water is fed to the boiler, it passes in between the heat transfer surface and the boiling water.
Water that is fed into the boiler is usually preheated but is always cooler than the water in the boiler. As the steam rises from the heat transfer surface to this cold-water layer, the steam bubbles condense resulting in two major issues.
The steam bubbles will have some tiny water droplets in them. As a large amount of feedwater enters, the quality of steam is reduced as the boiler reaches isothermal conditions. Secondly, the addition of cool water reduces steam production.
The issues mentioned above can be reduced by using a continuous steam boiler because, in such a boiler, water will be added at low rates because of which the boiler water will be at the isothermal condition and there will be no clouds or mist that will be formed.
3. How to increase low-pressure superheated steam to high pressure?
It is possible to increase the pressure of air using a vapor compressor, but it is not the same when it comes to the increasing pressure of steam as it contains condensate which can damage the compressor. Further, the increasing temperature cannot guarantee an increase in pressure of the superheat instead, the steam might get more superheated without any increase in pressure.
It is possible to increase low pressure superheat to high pressure superheat by combining a low-pressure steam flow with high-pressure steam. But this will result in the backflow of high-pressure steam into a low-pressure pipe. To prevent this backflow, an ejector needs to be installed.
In an ejector, the higher-pressure steam is used as means of pulling the low-pressure steam whereby the high-pressure steam does not backflow into the low-pressure line. This helps in maintaining the high pressure of the superheated steam in the outlet.
Superheated steam at a temperature of 3000C and absolute pressure of 1.013 bar enters a pipe. What is the additional amount of heat that the superheated steam carries in comparison to saturated steam passing the same pipe at the same pressure?
Enthalpy of saturated steam at 1.013 bar is 2676 kJ/kg (retrieved from the steam table)
Enthalpy of superheated steam at 3000C and 1.013 bar is 3075 kJ/kg (retrieved from the steam table)
Enthalpy of the superheat = Enthalpy of superheated steam – Enthalpy of saturated steam
3075 kJ/kg – 2676 kJ/kg = 399 kJ/kg
The specific heat capacity of the superheat can be determined by dividing enthalpy in the superheat by the difference between the saturation and superheat temperatures
Specific Heat Capacity = (Enthaply in Superheat)/(Superheat Temperature-Saturation Temperature) = (399 kJ/kg)/(300-100) = 1.995 kJ/kg 0C
In a refrigeration system, high superheat is a condition when the evaporator coil is not provided with enough refrigerant for the heat load that is present. In short, it means that an insufficient amount of refrigerant is reaching the evaporator coil, or the heat load is too much for the evaporator coil to work on.
If the amount of refrigerant is lower than what is required; it will evaporate soon after a few passes through the coil. Soon after the refrigerant evaporates, the vapor will continue the cycle by carrying away heat from the load while passing through the evaporator coil.
This heat picked up by the vapor will increase the temperature of the vapor to a higher value i.e., the vapor reaches superheat temperatures. When there exists less amount of refrigerant in the system, the pressure at both suction and discharge ends of the cycle is lower than usual.
2. Restriction in the liquid line
When the liquid line of the system is restricted, there will be an inadequate flow of the refrigerant to the evaporator coil. The pressure at the suction and the discharge ends of the cycle would be lower than normal pressure. The symptoms observed due to restriction in the liquid line are like those noted in a refrigeration system with low refrigerant.
There is an observed decrease in temperature at the location of restriction. There are also chances for the moisture in the system to freeze and cause the restriction.
3. Airflow through the evaporator is too high
When there is an excess flow of air through the evaporator coil, the capability of the system to remove moisture is reduced. The vapor picks up more than usual heat which causes the suction pressure to be higher than normal pressure and has a higher superheat.
4. Excessive heat load
With higher loads, there will more than the usual heat content that is passing over the evaporator coil which will be absorbed by the vapor. This increases its superheat. When the ambient temperature inside a room is higher than usual or when there are too many people in a room, there are higher chances for an increase in the superheat.
5. Faulty Metering Unit
There is a possibility of recording a higher superheat when the metering device is not installed correctly or due to faulty in the unit.
Superheat means the amount of refrigerant that is present in the evaporator. High superheat indicates that the amount of refrigerant in the evaporator is low or not sufficient. Subcooling indicates the amount of refrigerant that is available in the condenser. Low subcooling means that there is an insufficient amount of refrigerant in the condenser.
A refrigeration system is said to be running a high superheat and low subcooling condition when there exist insufficient amounts of refrigerant in the evaporator as well as the condenser.
Superheat means the amount of refrigerant that is present in the evaporator. High superheat indicates that the amount of refrigerant in the evaporator is low or not sufficient. Subcooling indicates the amount of refrigerant that is available in the condenser. High subcooling means that there is an excessive amount of refrigerant in the condenser.
A refrigeration system is said to be running a high superheat and high subcooling condition when there exist insufficient amounts of refrigerant in the evaporator and excessive amounts of refrigerant in the condenser. The possible reasons for high subcooling are a faulty metering device, underfeeding, fault in the head pressure control system, especially during low ambient conditions.
High subcooling will reduce the performance of the refrigeration system and ultimately damage the compressor valves. Hence it is recommended to troubleshoot this issue at the earliest as possible.
When the amount of refrigerant in the evaporator is insufficient for the heat load, then the superheat condition is referred to as high superheat. The state of having an insufficient amount of refrigerant in the evaporator and enough refrigerant in the condenser is termed as High Superheat Normal Subcooling. It is rare for this condition to exist because usually when there is high superheat there should be either low subcooling or high subcooling.
As mentioned earlier, when the refrigerant in the condenser is in excess, that condition is referred to as high subcooling. When there is an adequate amount of refrigerant in the evaporator for the heat load, it is referred to as normal superheat. Therefore, a refrigeration system that operates with an adequate amount of refrigerant in the evaporator and with an excess amount of the refrigerant in the condenser is termed as High Subcooling Normal Superheat.
High superheat in a refrigeration system occurs when there is a limited amount of refrigerant in the evaporator for the heat load that is present. High superheat indicates that
A refrigeration system is expected to have a high suction pressure when there is leakage of refrigerant through the discharge valve. Further, the compressor is not capable of providing the evaporator coil with the required refrigerant to handle the heat load. This condition is termed as High superheat High suction pressure or High head pressure High superheat. The possible reasons for high suction pressure are
High discharge pressure superheat is a condition whereby there is air present in the system. When the refrigeration system is exposed to this condition, the best solution is to charge the system with refrigerant. Sometimes, even a clogged condenser can cause high discharge pressure. In such cases, it is advised to clean the condenser. In some cases, a closed discharge valve can also cause high discharge pressure and can be reduced by opening the discharge valve.
No superheat or low superheat is an indication the refrigerant hasn’t picked up enough heat because of which the liquid will not completely boil into vapor. This liquid refrigerant which will be transferred into the compressor will damage the compressor. Along with this if there exist excess amount of refrigerant in the condenser. This condition is referred to as no super heat high subcooling.
When the refrigerant is low in the system, there are high chances for low suction pressure. When the refrigeration system is running with high superheat and low subcooling, the refrigeration charge is usually low. In such a condition, the system is expected to be at high superheat and low suction pressure. Another possible reason for low suction pressure high superheat is the insufficient amount of heat entering the evaporator which could be because of limited airflow or due to a dirty/plugged evaporator.
An accumulator is a vessel that stores refrigerant in a saturated state and stops the liquid refrigerant from entering the compressor. It is used as a protection tank. Bigger accumulators are installed to contain larger volumes of liquid to protect the compressor while the increase in capacity of evaporators is not observed. When the amount of refrigerant is limited with an accumulator installed in the system. It is referred to as an accumulator high superheat
A dirty or plugged evaporator coil will limit the air flowing through the evaporator thereby reducing the amount of heat that enters the evaporator which results in high superheat. It is also a concern if there an excessive flow of air through the evaporator as the system’s capability to remove moisture is limited.
A heat pump acts as a refrigeration system in the cooling mode. The indoor unit functions as an evaporator and the outdoor system function as a condenser. As the refrigerant charge in the evaporator is low, the heat pump will not be able to handle the heat load and this state is referred to as Heat pump High superheat
A high delta T which is above 210F could be a result of limited airflow indoors. If the air movement in the environment around i.e., indoors is limited, the system is not capable to move enough heat from the surroundings to the evaporator of the system. Further, there will a decreased supply air temperature on the system which will, in turn, result in a higher delta T. Hence this condition is termed as High delta T low superheat.
For systems with low delta T, the compressor of the refrigeration system will be a danger as the saturated liquid refrigerant will enter the compressor.
Yes, high superheat is bad as it indicates that there isn’t sufficient refrigerant to handle the heat load from surroundings or environment that needs to be cooled. A high superheat could also indicate a restriction in the liquid line which is the reason for the limited flow of refrigerant into the evaporator coil. Further excessive airflow could also result in high superheat as the air will carry an excessive amount of heat which the evaporator coil is not ready to handle causing a high superheat. An incorrect metering unit or feeding device also results in high superheat which should be rectified.
2. How can I reduce superheat?
The superheat in a refrigeration system can be reduced based on the cause. If the cause is due to the limited refrigerant, then recharging of refrigerant in the condenser is the right step. In case the superheat is due to excessive airflow, then a sir release valve should be installed thereby maintaining the amount of superheat that can be handled by the evaporator. Troubleshooting the metering device is also a method of reducing the superheat.
3. What causes high discharge superheat?
The possible reason for high discharge superheat could be leakage of refrigerant. Other possible reasons for high discharge superheat are restriction in liquid line or restriction in the filter. Further, a restriction in the actuator feeding to the evaporator could also result in high discharge superheat. There are cases where the system might face high discharge superheat due to restriction of the airflow to the condenser. In this case, it would be recommended to clean the condenser as it is clogged due to dirt.
4. What is a good superheat for 410a?
A good superheat for 410a would be approximately 10F around the evaporator. The suction pressure and suction temperature are measured. The temperature corresponding to the gauge pressure is taken and the difference between the two temperatures should be 10F for a good superheat. The charging and discharging of the refrigeration system will be based on this value.
5. Why do we have suction accumulator installed?
A suction accumulator is installed to avoid the refrigerant in liquid state from flooding the compressor. An accumulator is usually seen in a heat pump or on any device where liquid refrigerant is a concern.
6. What is meant by subcooling? Is subcooling desirable?
Subcooling can be defined as the condition whereby the liquid refrigerant is at a temperature lower than the saturation temperature. Subcooling is the difference between the liquid refrigerant temperature and the saturation temperature of the refrigerant.
It is desirable to have subcooling as it helps it enhancing the efficiency of the refrigeration system as the amount of heat removed per pound of refrigerant is higher. It also ensures that the liquid refrigerant reaches the expansion valve.
7. Is it necessary to know the superheat of a system. If yes, why?
Yes, it is essential to know the superheat of a system as it gives an indication if the level of refrigerant is too less or too much in the evaporator. If the superheat is high, then the amount of refrigerant is limited thereby reducing the efficiency of the system as more energy is required to operate the system. On the other hand, if the superheat is too low, then there are chances for the liquid to enter the compressor resulting in compressor damage.
Gauge Pressure can be defined as the pressure that is relative to the atmospheric pressure. For pressures that are above atmospheric pressure, gauge pressure is taken to be positive while for pressures below atmospheric pressure, gauge pressure is noted to be negative. Gauge pressure is referenced to be zero at atmospheric pressure.
For example, while filling air in a flat tyre, the air is inside the tyre is filled in terms of gauge pressure and the atmospheric pressure is observed to be zero. This is because the tire gauges are designed to operate at 0 atmospheric pressure.
What is the relationship between Gauge pressure and True pressure?
Gauge Pressure can be formulated as the difference between absolute pressure and atmospheric
Pabs = Pg + Patm
where Absolute Pressure is denoted as Pabs, Atmospheric pressure as Patm and Gauge pressure as Pg
If the tire gauge reading is observed to be 36 psi (pounds per square inch), then the Absolute Pressure will be sum of the atmospheric pressure (which is a constant, i.e., 14.7 psi) and gauge pressure reading
i.e. Pabs = Pg + Patm
= 36 psi + 14.7 psi
= 50.7 psi
What is the gauge pressure of the trapped air?
The gauge pressure of air trapped in a vessel or a tube can be measured using a manometer. A manometer is a U- shaped tube often filled with mercury as a fluid to measure pressure. The difference in the height of fluid (i.e., mercury) is used for measuring the gauge pressure.
For example, the gauge pressure can be measured using a U- tube with one end exposed to the atmosphere and a balloon connected to the other end. The absolute pressure is greater than the atmospheric pressure by an amount hρg which is taken to be the gauge pressure.
Pressure Gauge is a tool used for measuring the pressure exerted by a fluid which can be liquid or gas, per unit area which is expressed in terms of Newton per square meter or pounds per square inch.
Liquid Glycerine is often used in pressure gauges due to its excellent vibration dampening properties at room temperatures. They usually operate in the temperature range between -200 C and + 600C. There are other fluids which are used as liquids in pressure gauges depending on the application, but the most promising liquid is glycerine.
Working principle of Pressure Gauge
Pressure gauges work using principle of Hook’s law which states that the force required to compress or expand a spring depends on the distance i.e., F = kx, where k is the spring constant, x, the distance to which the spring is compressed or expanded, and F is the forced applied.
When pressure is applied on an object, there exists an inner pressure force and an external pressure force. Further, the pressure exerted in a Bourdon tube will be more in the inner surface due to the smaller surface area compared to the outer surface
Pressure Gauge calibration is the comparison of values of the unit that is being tested to the values that are measured from an accurately calibrated device. The pressure gauge is usually used for calibrating and tuning fluid flow machines. The fluid flow machines would be unreliable if not calibrated using a pressure gauge. Pressure gauges are calibrated according to the National Standards (NMISA).
These types of pressure gauges are used for measuring relative pressure in the range of 0.6 to 7000 bar. They belong to the category of mechanically driven pressure measurement devices as they do not require electrical energy to power.
The Bourdon Tube Pressure Gauge has an oval-shaped cross-sectional area with tubes that radially packed. The pressure exerted by the measuring source creates a motion on the other end of the tube which is not clamped. This motion that is created on the other end of the tube is taken to be the pressure which is measured. A C- shaped Bourdon tube can be used for measuring pressures up to 60 bar. Bourdon tube packed with windings of exact angular diameter i.e., helical tubes are used for measuring high pressures that exceed 60 bar.
Bourdon tube pressure gauges are manufactured according to set standard of EN 837-1. There are Bourdon tube pressure gauges which are filled liquid, these types of gauges are used for critical applications where the readings to need to be accurate and precise.
What is Oil Pressure Gauge?
Oil pressure Gauges can be categorised into mechanical gauges and electrical gauges
Mechanical Pressure Gauge
This gauge measures the pressure of oil at the end of pipe connecting the pump and the filter. To measure the pressure, an oil take-off pipe taps on the engine block. The needle movement in the dial indicates the measured pressure.
By tapping of the engine block, oil is sent to the gauge by using a copper or plastic bore. The pipe is arranged in such a way that it will be exposed to minimum damage to prevent leakage of the engine oil. The gauge is composed of a coiled tube which is termed as bulb. The open end of the bulb is connected to outer casing of the gauge.
The oil fed that is fed into the supply pipe is at almost the same pressure as when the oil leaves the engine. Under this pressure, the bulb tries to maintain its position, in doing so the needle in the dial moves and indicates the pressure. The higher the pressure, the larger will be the degree of movement of the needle.
Electrical Pressure Gauge
This gauge measures the pressure of oil at the end of pipe connecting the pump and the filter. To measure the pressure, a screwed sensor taps on the engine block. The needle movement in the dial indicates the measured pressure.
Working Principle
This type of pressure gauge is powered by electric current which is supplied from one of the wires that is present in the dashboard. The current that is supplied through the wire passes through a coil which is wound with a wire in the needle’s pivot. A magnetic field is produced which causes the needle to move within the dial depending on the measured pressure.
The extend to which the needle moves or the reading it shows depends on the current that flows through the gauge. The contributing factor is the resistance offered by the gauge wire that is earthed in the engine block using the sensor. All gauges are illuminated for the ease of reading the measurement at night.
It is an instrument used for measuring pressure as well as pressure differences. This pressure gauge was modelled and developed by Dwyer which has currently set standards for the pressure gauges used in industries. It is primarily used for measuring positive and negative pressure i.e., vacuum.
Working Principle
A magnehelic pressure gauge is composed of a diaphragm that is sensitive to pressure changes. The dial of the pressure gauge responds based on the pressure applied. The appropriate positioning of the instrument is required for proper functioning of this pressure gauge. It should be placed at the right level and in a vertical position or else the diaphragm will give inaccurate readings as it will sag.
Industries and labs had been using conventional analogy tire gauges for measuring pressure since ages. But ever since the discovery of digital instruments has led to the use of digital pressure gauges which provide the most accurate reading. It is easy to operate a digital tire gauge, that is to switch on the gauge and position it on the valve stem to get the corresponding reading.
This type of pressure gauge is used for measuring both positive and negative pressures in a vacuum. Few examples where compound pressure gauge is employed are
for leaking testing in pressure lines,
for measurement of low pressures, and
for pressure measurements in test chamber
Its capable to measure positive and vacuum pressure only for pressures below 200 psi.
Working Principle
The compound pressure gauge consists of a sensor that is capable of measuring both positive as well as negative vacuum pressures. The zero pointer of the instrument is referenced at ambient pressure. The gauge consists of a vent hole which allows to compensate for the changes in atmospheric pressure.
These are pressure gauges that can measure the pressure from fluid and provide direct reading of the pressure measurement unlike Analog pressure gauges which require an operator to manually read the positioning of the needle in the dial for the respective pressure reading.
Psi in a pressure gauge is pounds per square inch which is the unit for the measured pressure. It is the pressure exerted by one pound force over an area of one square inch
A supply pressure gauge helps in determining the amount of air or water or fuel in a tank. Air brake vehicles are usually provided with a supply pressure gauge to measure the amount of air in the tank. For vehicles with dual air brake system, there is a pressure gauge for every half section of the system
Column of water is sometimes used for measuring pressure. A non- SI unit for measuring pressure is inch of water and can be defined as the pressure that a column of water that is 1 inch height exerts under standard conditions.
A pressure canner is a vessel that is fitted with a lid that has a dial or weighted gauge that regulated the steam that builds up inside. The steam that is build up inside the vessel is released when the pressure exceeds the limit the vessel can handle. Further, the steam that is build up inside is hotter than boiling water. Dial gauge regulators are found in older types of pressure canners. The dial displays the exact pressure build up inside the canner.
A pressure canner is a vessel that is fitted with a lid that has a dial or weighted gauge that regulated the steam that builds up inside. The steam that is build up inside the vessel is released when the pressure exceeds the limit the vessel can handle. Further, the steam that is build up inside is hotter than boiling water. Weighted gauge regulators are made up of disc like pieces that must be placed on the vent pipe with preferred choice of weight and like the one-piece regulator, this regulator makes a rocking sound.
This type of pressure gauge is used as a diagnostic tool to ensure that the fuel pressure in the engine is maintained and is running at good performance levels. They also help to prevent any kind of damage that might occur due to pressure build up on the fuel pump or on the injector
This pressure gauge is used for measuring absolute pressure of the fuel-air mixture contained in the intake manifold. The diaphragm in the manifold pressure gauge is used for measuring the absolute pressure. The accurate power configuration and settings for an aircraft engine is obtained using the manifold pressure.
The normal oil pressure gauge reading when an engine is running should be between 25 and 65 psi. When the pressure gauge reading is higher than 80 psi, then there is problem of high oil pressure which needs to be dealt with.
Photohelic pressure gauge is a Magnehelic pressure gauge equipped with a switch to adjust between high and low gas pressures. It is an advanced version of Magnehelic pressure gauge that helps in saving money with reduced usage of compressed air and provides a longer life for the pressure gauge.
High pressure required for waterjet cutting is smoothened out by using pressure gauge snubber. These high-pressure fluctuations are created by reciprocating pumps and controlling these fluctuations help in extending the life of the pressure gauge and reducing the calibration time. These gauges are preferred over a valve due to their small orifice which reduces the cases of clogging.
A pressure gauge snubber consists of a pressure vessel with a capillary that has a small bore. The pressure is accumulated in the gauge and the built-up pressure is smoothened out thereby reducing the fluctuations. The gauge is equipped with a steel filter at the inlet to the capillary to avoid dirt from entering or clogging the bore.
This transducer converts pressure into an electrical signal. The principle behind the working of a strain gauge pressure transducer is piezo resistance i.e., the change in resistance value with respect to the physical deformation or changes caused to the material when exerted by pressure. This transducer is when wired to a Wheatstone bridge can convert small changes in resistance to electrical signals corresponding to the pressure exerted.
The accuracy class in pressure gauges help in determining the permissible percentage of error. The accuracy classes for pressure gauges are 0.1, 0.25. 0.6, 1, 1.6, 2.5 and 4. The gauges with pointer stop are the range of 10 to 100%.
Pressure gauges have removable rings which are termed as bayonet ring. A bayonet is an indentation on the outer surface of the ring. Usually, a bayonet ring has up to five indentations. The rings help in holding a gasket and window. The dial can be found on removing the gasket and window. This ring is mostly seen in pressure gauges where the operator must access the adjustable pointer.
Thin walled cylinder with convolutions and metal as material of construction, are Bellow pressure gauge they are closed at one end while the other end is open and can move about. On applying pressure to the sealed end, the bellows will compress and move upwards. The rod in between the bellows and the transmission system will also move up and initiate the movement of the pointer. They can provide longer stroke length and exert greater forces. These bellows are fabricated using different materials depending on the application.
The deflection that is produced can be expressed as below
In case of high pressure, the entire disc will be blown and break into piece to release the build-up pressure. To protect the gauge from breaking or from being blown up, pressure gauge blows out protection is provided. An advisable design for protecting the pressure gauge from the over pressure is by separating the front and back part of the pressure gauge using a solid wall. Using such a design, the front part will not be affected though the back part will blow out thereby providing protection to the pressure gauge.
A differential pressure gauge helps in measuring the differences in two measured pressure. They are usually used for measuring pressure levels in closed tanks, over pressure in room and for controlling pump stations.
A pressure element divides the two chambers in a differential pressure gauge. If the pressures in the two chambers are the same, then there occurs no difference in the pressure element. On the other hand, if there exists difference in pressure between the two chambers, then the pressure element displaces, and mechanical movement indicates the pressure difference value.