Heat exchangers are very important devices in engineering. We are using them in different kinds of applications. Because of their different capabilities, the use of these devices is very common in the industry and daily. Here you can find detailed information about heat exchangers. The general topics that this article covers;
Table of Contents ;
- What is a heat exchanger? How does it work?
- A general explanation of the calculations of these systems.
- What are their types of them?
- What are the advantages and disadvantages of heat exchangers?
- What are the applications?
- How to select a proper one for different applications?
What is the Heat Exchanger?
Heat exchangers are the devices that we use in the heat transfer between the fluids without mixing them. So they find themselves places in different kinds of engineering applications. Because of it, they are very common around the world.
In a typical heat exchanger, heat transfer takes place between the fluids in convection and conduction mechanisms. Convection takes place between the separating walls and the fluids inside them. Also, conduction takes place through the separating walls of tubes. So, in the calculations, we need to take care of these two parameters.
To take care of these parameters, they created special calculation methods for heat exchangers. By calculating them, we can obtain the correct results. In the preceding sections, you can find detailed information about these calculations.
Types of Heat Exchangers
As we stated above, heat exchangers are very common in lots of kinds of applications. So, they developed and improved different types of them. These types serve best in the proper applications for them. Around the types;
- Double-pipe types,
- Parallel flow,
- Compact heat exchangers,
- Printed circuit heat exchangers,
- Crossflow types
- Shell and tube configurations,
- Plate and frame configurations,
- Regenerative types.
As you see above, the general types of these systems depend on the type of configurations. Let’s check them in detail. Before starting these types, there are other purposes that they use systems like heat transfers.
There are also boiler systems where the fluid takes the heat and vaporizes. So, unlike in conventional systems, there is vaporization of the fluid.
Also, we use condensers to condense the flowing fluid inside them.
This configuration is very common around the other ones. If we take a look at the general configuration of double-pipe heat exchangers, we can say that there are two pipes inside the system. So, through one pipe a cold fluid flows. Also in the other pipe, hot fluid flows. And heat transfer between these fluids takes place between these pipes.
The pipe configuration is concentric in these configurations. One pipe is bigger than the other one. And the smaller pipe is inside the bigger pipe. So, heat transfer takes place in this configuration.
Also, there are two types of double-pipe configurations;
Counter Flow Heat Exchangers
This classification is made according to the flowing directions of the fluids inside the heat exchangers. In counter-flow ones, the hot and cold fluids are flowing in inverse directions. Because of this reason, we call these types of heat exchangers counter-flow ones.
Parallel Flow Heat Exchangers
Also as you understand that the fluid flow in these types of heat exchangers is in a parallel direction. Because of this reason, we call these types parallel flow ones.
Compact Heat Exchangers
In heat exchangers, the total heat transfer area is a very important parameter. With the increasing heat transfer area, the total heat transfer also increases. So, the general efficiency increases. Engineers are trying to enhance the total area of these systems to increase the heat transfer.
An important indicator of the heat transfer area of heat exchangers is area density. The unit of the area density is m2/m3. So, this indicates the total heat transfer area of a specific volume of heat transfers. And, it is very important to have high area densities.
If the area density of the heat exchanger is bigger than 700 m2/m3, we call these systems compact heat exchangers. The most important example of compact configurations is the car radiators which have around 1000 m2/m3 area densities. Also, the human lungs have 20.000 m2/m3 which is very high for making the oxygen transfer between the blood.
Compact heat exchangers are very useful in gas-liquid heat transfers. Because the convection capacities of gases are very low. To compensate for this gap, they increase the total area in that conduction heat transfer takes place.
Also, these types are very useful where the area for these systems is limited. So, we can use these systems in these areas.
And also, if we take a look at the general structure of these systems, there are corrugated fins that separate the fluids. These lots of corrugated fins prove very high area for heat transfer.
Printed Circuit Heat Exchangers
This is a very different application of heat exchangers. The internal structures of these types are made by the chemical etching processes same as the manufacturing of PCBs. With this chemical etching process, 1 to 3 millimeters wide metal channels are obtained. And they stack these channels to each other with diffusion bonding processes.
So with these very thin internal structures, they obtain very high area densities such as 2500 m2/m3. The general materials are stainless steel, titanium, copper, and nickel.
The most important advantage of these printed circuit heat exchangers is the very high heat transfer coefficients. So, the heat transfer between the flowing fluid in these channels is very high.
Also, we can use these systems in a very wide range of temperature applications. The general temperature range is between -200 to 1000 Celsius degrees.
And also, another important advantage of these printed circuit heat exchangers, they are very compact in structure. We can find very small sizes of them around the market.
Like their advantages, there are also very serious disadvantages to these systems. The most important disadvantage is the pressure drops of the flowing fluids. The pressure drop causes a drop in the efficiency of the piping system. So, we need additional pumping power for these systems.
Another important disadvantage is, that we need to use very pure liquids and gases. Because the channels are very thin and prone to be clogged by impurities. And also, cleaning these channels is a somewhat hard thing.
Also, these configurations are very common in the compact heat exchanger market. In these systems, the flow of the cold fluid and the hot fluid is perpendicular to each other. Also, some fins separate the cross-flow of the fluids to obtain more even heat transfers. The most important application is the car radiators.
Shell-Tube Heat Exchangers
This class of heat exchangers is the industrial class of heat exchangers that we use in big applications. In the construction of these types, there are tubings inside the heat exchanger shells that fluid passes inside it. And between these tubes and the shell, the outer fluid flows. So, the tube walls are the main interfaces where heat transfer between two fluids takes place.
Inside these types, tubes are very long in structure and there are lots of tubes inside the shell. This is because to increase the heat transfer rate between the fluids. Two fluid flows take place parallel to the shell structure.
According to the number of tube passes, we can classify the shell-and-tube heat exchangers as one-shell-pass and two tube passes. In these systems, there is only one U-turn of the pipe inside the shell. Also if there are two U-turns, we call these systems two shell pass and four tube pass systems.
These heat exchangers are very big in structure and we use them in industrial applications in general. So, we can not use them in aerospace and automotive applications.
Plate and Frame Types
These are also very common types of heat exchangers. The general working principle of the plate and frame types is that hot and cold fluids are flowing inside the minimal pipes on plates. So, there is a very effective heat transfer takes place. Also, we can increase the number of plates that fluid flows inside them.
We use these systems in the liquid to liquid heat transfer applications which are big such as industrial applications.
Regenerative Heat Exchangers
The working principles of these systems are very different. Instead of two hot and cold fluids at the same time, there is an alternate flow of the hot and cold fluids inside them. There are two types of regenerative heat exchangers; Static types and dynamic types.
In static types, there is a ceramic-based porous mass inside the heat exchanger. Once the hot fluid flows over this mass, the heat of the hot fluid is transferred to this mass. After the hot fluid, cold fluid passes over this mass to take the stored heat from it. This is the general working principle of these systems.
Also in dynamic systems, there is a continuous flow of hot and cold liquids. Also, there is a dynamic mass inside these heat exchangers that rotates inside the chamber. And with this rotation, it takes the heat from the hot fluid and leaves this heat in the cold fluid.
Calculations of Heat Exchangers
In the design of heat exchangers, we need to make some engineering calculations to reach the potential solutions. Generally, we use general heat transfer analyses to find out the efficiency of the heat transfer.
In tubing and piping systems of heat exchangers, the heat transfer between the two flowing fluids takes place through convection and conduction. The two convection mechanisms take place between two flowing fluids. Also, the conduction takes place at the tube wall. So, we need to make our calculations according to this fact.
Overall Heat Transfer Coefficient
This heat transfer system is the same for all the portions of heat exchangers. So, if we calculate the overall heat transfer coefficients, it will be very easy to make further calculations.
First of all, we need to calculate the two convection and one conduction thermal resistances of heat transfers. But in general assumptions, the conductive resistances of thin walls of tubes are very small if we compare with the convection heat transfer resistances. So, we can only consider the convective heat transfers in heat exchangers. And the heat transfer coefficient becomes;
C is the overall heat transfer coefficient over the heat exchanger which has units of W/(m2°C) or Btu/(hr-ft2°F) in English units.
Rtotal is the total thermal resistance of the heat transfer in the heat exchanger which has the unit of °C/W. or F.hr/BTU.
hi is the internal flow convective heat transfer coefficient? And he is the external flow heat transfer coefficient.
Ai and Ae are the areas of the tube and the internal and external flows. They are nearly the same and have units of m2 or ft2. So, the general equation to find the overall heat transfer coefficient for heat exchangers reduces to;
So, if you know the heat transfer coefficients of internal fluid flow and the external fluid flows around the tubes, you can calculate the overall heat transfer coefficients of heat exchangers.
If you look at this formula, you can say that the overall heat transfer coefficient is directly related to the types of fluids that flow inside the heat exchangers.
Effect of Fins on Total Area
If we are using fins in heat exchanger tubes, we need to calculate the total finned area and add this area to the total area of the tubes. Also, we need to consider the fin efficiency which depends on the temperature drop along the finds. So the total area becomes,
Fouling Problem in Heat Exchangers
Fouling is a common problem in heat exchangers that we need to consider in the design phase. This is the calcification of the inner and outer surfaces of tubes because of the minerals in water or fluid that we use. In time, it is visible to see the fouling in tubes.
Also, there are different types of fouling. The second one is chemical fouling which occurs with the corrosion of the tubes and pipes because of the chemical features of the fluids. So, generally, heat exchanger designers are making glass coating on the surfaces of the tubes to prevent chemical attacks.
Another kind of fouling is the algae formation in warm waters. Also, there are different prevention methods to remove the algae formation fouling.
In the heat transfer calculations, we need to consider the fouling effects of the inner and outer surfaces of the tubes of heat exchangers. We can consider them by adding their thermal resistances of them to the total thermal resistance of the heat transfer calculations. So, in this case, we calculate the worst-case scenario for the heat exchanger that we design.
Heat Analysis of Heat Exchangers
According to the thermodynamic’s First Law, we can make simple total heat transfer calculations for the fluids that flow along the pipes and tubes of heat exchangers. So, it is very important to have a heat analysis for these systems.
First of all, we need to make assumptions that, there is no heat leak through the walls of the heat exchangers. So, the only heat transfer takes place between two fluids. Also, in general use, the mass flow rates of the fluids are the same over the time that we use these systems. And also, there is no velocity and elevation change of the fluid through these systems.
So, we can build a very simple heat energy relation between these two fluids;
In this basic heat transfer equation;
- mh and mc are the total mass flow rates of the hot and cold fluids that flow through the heat exchanger system. The general units of them are kg/s or lbf/s.
- Ch and Cs are the specific heat capacities of hot and cold fluids. The unit of it is J/(kg °C) or BTU/(s.lbf.F).
- Th-out and Tc-out are the exit temperatures of the flowing hot and cold fluids in the system. The units are Kelvin, Celsius, or Fahrenheit.
- Th-in and Tc-in are the inlet temperatures of the flowing hot and cold fluids.
According to these equations, the temperature differences between the fluids are the most important factor in a heat exchanger system. Also, the heat capacities have an important effect on the total heat changes of the flowing fluids.
Furthermore, we can state that if the heat capacity of the fluid increases, the total temperature change of this fluid decreases.
Logarithmic Mean Temperature Difference Method
In heat exchanger systems, the mean temperature difference of the heat exchangers is a very important phenomenon. But throughout the piping system, the total temperature difference changes. So, we need to have a mean temperature difference regarding the heat exchanger system. To find it, we are using the logarithmic mean temperature difference method formula here,
This formulation will give a real insight into the temperature difference in heat exchangers. ΔT1 is the inlet and outlet temperature difference of the hot fluid. And ΔT2 is the inlet and outlet temperature difference of the cold fluid in the heat exchanger.
Also in general, we use the correction factor in the calculation of the temperature difference for the cross-flow type heat exchangers. You can find these correction factors in product catalogs.
Heat Exchanger Effectiveness
This is another important method to determine the general performances of heat exchangers. We can make effectiveness calculations to see the total effectiveness of these systems. We can calculate the effectiveness;
So, we need to define the maximum possible heat transfer in the heat exchanger. We have shown the calculation of the heat transfer above. In that calculation, we need to use the biggest temperature difference. These are the temperature differences between the hot and cold inlet fluids.
Also, the need to use the smaller heat capacity value fluid in this calculation. And we have the maximum heat transfer rate that this system can have.
By using this effectiveness value, you can calculate the actual total heat transfer in a heat exchanger if we compare it with the theoretical value.
NTU Method for Heat Exchangers
NTU means the number of transfer units that we calculate to see the effectiveness of a heat exchanger. This is a dimensionless value that we calculate with this formula;
In this formula, the U is the overall heat transfer coefficient that we defined above. And As is the total heat transfer surface. Cmin is the minimum heat capacity that fluid has.
How to Select a Heat Exchanger According to the Calculations Above?
These calculations are giving very good insight into the heat exchanger selection. You can make and use these calculations to find the best heat exchanger for an application.
Firstly, you need to start to select a type of heat exchanger for your application. You can find the types of the above. After that, you need to calculate the unknown temperature values of the inlet and outlets of the fluids. So, you need to know all the inlet and outlet temperatures.
After that, you need to calculate the logarithmic mean temperature difference value by considering the correction factor that you have. And then, you need to calculate the overall heat transfer coefficient. Finally, you need to calculate the total heat transfer surface that you need.
Above these calculations, you need to consider the effectiveness or NTU value of the heat exchangers while you are selecting a proper one for your applications.
The effectiveness ranges from 0 to 1. 1 is the best scenario which means 100% effectiveness. But in some catalogs, there is an NTU factor that you need to read effectively.
And also, the smaller NTU values are better for higher effectiveness. NTU values below 1.5 are the best. And NTU values which are bigger than 3 are not good in effectiveness.
In general, the parallel flow heat exchangers have the highest effectiveness and lower NTU values.
Other Important Factors About Heat Exchangers
There are also other kinds of parameters that we need to consider in the selection of proper heat exchangers. From the engineering viewpoint, these are the other factors that we need to consider.
Pumping Power and Pressure Loss
It is very important to consider the power requirements. Because there are lots of internal tubings and channels in these systems that lead to extra pressure losses. With the increasing tubings, the pressure loss increases. Also, with the increasing internal surface area and tubings, the effectiveness increases. To compensate for this pressure loss, we need extra pumping power for the fluid flow.
Around these parameters, we need to select a proper system that will be best for optimum effectiveness and power consumption. Do not forget that with the increasing pumping powers, the total electricity costs are increasing. So, this is a very important parameter in the operational costs.
We generally use heat exchangers as a part of a complete system. And there are initial investment and maintenance costs of these systems. We need to thoroughly consider these costs while we are selecting proper heat exchangers for our applications.
There are different technologies regarding heat exchanger systems. With the changing and improving technologies, the total investment costs and initial values are increasing.
Type of Heat Exchangers
The type of heat transfer is very important for the different kinds of fluids that we use. The type of fluid is a very important parameter in the selection of the type of heat exchanger that we use. So, it is very important to consider different types of heat exchangers for different fluids.
Heat Transfer Rates
As we showed in the calculations, if you know the total heat transfer rate according to the desired fluid mass flows and the fluid temperatures, it will be very easy to select a proper heat exchanger with a proper heat transfer rate. So, you need to have strict calculations in your hand to select a proper heat exchanger.
Weight and Volume
It depends on the type of application where you want to use these heat exchangers. For example, you will use smaller ones for smaller applications such as automotive radiators and so on. Also, for industrial applications, you need to select bigger ones. But the smaller one means better costs.
Also, this is a very important consideration in the different conditions that we have in heat exchangers. The type, pressure, and temperature of fluids will affect the material consideration of the heat exchangers.
For example, if we are using a corrosive fluid, the internal materials must be special to withstand that corrosive environment. If we use very high temperatures of fluids, we also need to consider high-temperature resistant materials. These are the direct costs of the heat exchanger expenses.
And also, if we are working with very high pressures, the material and the structure of the heat exchangers must withstand these values.
So the type of fluid has a very important effect on the material selection. And with the increasing quality of materials, the total investment costs for heat exchangers are increasing.
Applications that We Use Heat Exchangers
The market for heat exchangers is very big. In lots of kinds of applications, we use these systems to increase or decrease the temperatures of the fluids. You can find the general application areas here.
Oil Refinery Systems
Oil refineries are very common all around the world. They are working on the distillation of oil to the useful types of different fuels. So these distilled fuels are stored in different tanks and sent to the customers. So in complex distillation systems, we use shell and tube types of heat exchangers.
Steam Generation Systems
Also, we use heat exchangers in the steam generation systems. Steams are very important in the energy production from water. So, we need to use big sizes of shell and tube types of heat exchangers.
Cryıgenic applications are also very common in the research and development centers. To obtain cryogenic systems and very cold fluids, they use plate-type heat exchangers because of the very efficient heat transfers.
We use furnaces in the industry to melt the materials and also other kinds of heating processes. Sometimes the heat source of furnaces is directly used. And also we use this heta to transfer other fluids. So, with plate types, we can do this kind of thing.
Also in refrigeration systems, the use of heat exchangers is very common. You know that in refrigeration systems, there are working fluids. And heating and cooling of these fluids, the most common type of heat exchanger are the condenser type.
In power plants where we use water to produce electrical energy, we generally use condensers to convert the very hot steam to water. So in these phase changes, the use of heat exchangers is very common.
Old Steam Trains
Maybe in today’s technology, there is no use for steam trains and locomotives. But it is a very important example that we use heat exchangers for a very long history. Boiler-type heat exchangers were very common in steam production. So, there is a phase change again which we use boilers for it.
Also, the use of heat exchangers is very common in engine radiators. In radiator systems, we generally use compact heat exchangers. So, the hot water that takes heat from the engine is cooled down by the air. This air is forced convection and there is a fan application. So, we have a system that one of the fluids is water and the other one is gas.
Cooling of Electronics
We are using these systems in the cooling applications of electronic products. Like the radiators, we need to use compact systems to cool the electroınics. Electronics can create heat because of the electric currents on them. So, we use compact heat exchangers also in these systems.
Aerospace engines also require air cooling for some components. So the use of compact systems is also very common in jet engine applications. Furthermore, aerospace applications require compactness more. So the most compact and smaller solutions are generally used in aerospace applications.
Industrial Paint Systems
In industrial painting systems, they apply the painting applications at a specific temperature for the most efficient coating. So, they need to heat the paint, which is a liquid. This heating system is take place with the shell and tube types of heat exchangers.
Industrial Cooling Systems
Heat is one of the most important problems that people need to deal with in industrial manufacturing processes. So, we need to apply very big and efficient cooling systems in these applications. And we use double pipe heat exchanger systems generally to obtain the optimum cooling systems.
Also heating, ventilation, and air conditioning systems, we use heat exchangers as intermediate elements. In these systems, there are lots of kinds of heating and cooling applications for liquids. So, we use different kinds of heat exchanger systems to do it.
As you see above, the general applications of these systems are very common. You can work on one of these applications where you need a professional application. So, selecting a proper heat exchanger system for each of them is not an easy business.
Advantages and Disadvantages of Heat Exchangers
Like the other kinds of systems, there are advantages and disadvantages of heat exchangers that you need to consider. By knowing these advantages and disadvantages, you will decide if you need to use one of these systems or not. Also, you will be aware of the possible costs.
Advantages of Heat Exchangers
- The most important advantage of them, the heat transfer between the fluids is very efficient. Probably, you can not find any other system that makes this transfer more efficient than these systems.
- You can use a wide variety of fluids in these systems. So, heat transfer between the irrelevant fluids is very effective in heat exchanger systems.
- The fluids are not getting mixed in heat exchanger systems.
- They are very easy to install and very easy to operate systems. Because the general structures of heat exchangers are not complex.
- Heat exchangers are working as a steady system. So, there is no net change in the fluid flows that increase or diminish the power requirements.
- In the market, there are lots of types of heat exchangers that you can use in specific applications. Wide variety of them for nearly all kinds of applications.
Disadvantages of Heat Exchangers
- The most important disadvantage of them is the selection of a proper one is somewhat hard. The heat exchanger producers generally consult you according to the application. But you need to be aware of the general requirements and calculations that we explained above.
- The operational and maintenance costs can be high. Because they are a very dynamic system that there is a constant flow inside them. The calcification problems and other breakdowns will cause extra costs.
- If you are using a heat exchanger in a system, it is very important to consider extra power requirements. Because the complex internal system of the heat exchangers will cause a very big head loss of the fluid flow. So, you need to have an extra powerful pumping system for them.
- In some very high-temperature applications, they can be dangerous. You require very strict audits and maintenance of these systems. Otherwise, you can face very serious accidents.
Conclusion on Heat Exchanger Systems
As you see above, the topic of heat exchangers is very comprehensive. There are various kinds of constructions and types of heat exchanger systems in general. So, the selection of the right one for an application is not an easy business.
There are also heat transfer and efficiency calculations of heat exchangers. You need to make these calculations and know the theoretical requirements before making research about one of these systems. Around these calculations, heat exchanger effectiveness and NTU methods.
The general application areas of these systems are very common. There are lots of kinds of different applications from aerospace to oil refineries. Because they are very important intermediary elements in common engineering systems.
Also, you need to be aware of the advantages and disadvantages of the heat exchanger systems. Like the other engineering systems, these systems have also advantages and disadvantages.
Finally, do not forget to leave your comments and questions below about the heat exchanger systems.
Your precious feedbacks are very important to us.
FAQs About Heat Exchangers
There are different kinds of applications of heat exchangers in general. Around these applications; are HVAC systems, automotive, aerospace, industrial production systems, and other kinds of engineering systems where we need heat transfer between the liquids.
Their working principle of them is very simple. There are internal tubings and plates in these systems where two fluids in different temperatures are coming in contact. So, heat transfer between these fluids takes place. And we obtain a fluid that is at the intended temperature, by using another fluid as a heat source. These are the general working principles of heat exchangers in general.
The efficiency of these systems is generally assessed according to the area density. Are density is a very important thing. With higher area density, there is more contact between the fluids to make heat transfer between them. The compact type of heat exchangers have the highest area densities which makes them the most efficient systems.
In these systems, there is constant fluid flow around minimal pipes. So, the bigger versions of these systems can produce noises. But these noises are not high as an engine.
There are different advantages and disadvantages of heat exchangers. According to the application, you need to consider the downsides and advantages. So, you can assess whether it is worth it or not. But in general, we use these systems as intermediate systems of complete engineering applications. Because of that, they are generally compulsory systems for working on the general systems. Think about a cooling system of an industrial process. Without a heat exchanger, it will not work.
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