Heat exchangers, as one can figure out from the name, are devices used for exchanging heat between fluids. When heat is transferred from one liquid to another (or one medium to another medium), a heat exchanger is required for the smooth flow of heat. In this article, we will understand the detailed working of different types of heat exchangers and Evaporators and understand how they are relevant in our day-to-day life. 

Working of a heat exchanger and Evaporators: 

The working of heat exchangers and Evaporators is based on thermodynamics. Thermo means heat and dynamics means the study of forces. Together thermodynamics is a branch of science where we are concerned with the flow of heat energy, different types of energy, and their inter-relationship. So, to understand heat exchangers, we will look into thermodynamics. 

We all have read in our junior classes that heat is transferred in three ways: 

1. Conduction: When two materials are in proximity to each other, the heat passes from high-temperature material to low-temperature material. For example, if you put a hot drink in a cup, the cup also becomes hot. 
2. Convection: The hot fluid rises and the colder material sinks due to which the transfer of heat takes place. For example, when you boil water, the hot water that was in proximity to the flame rises and the cold water sinks. As a result, the whole of the water becomes hot. Sometimes, artificial convection is used for transferring heat like in a hydronic heating system. 

• Radiation:  When heat is emitted by a heated object like the sun, the process is called radiation. 

Heat exchangers also use a similar process for transferring heat and are based on different principles of thermodynamics. 

Types of heat exchangers: 

Depending upon the medium of the fluid and the temperature, different types of heat exchangers are used. Besides fluid type and temperature, the classification of heat exchangers is also done based on different characteristics. Certain factors are considered when the classification of heat exchangers is done. These are:  

• Flow arrangement: 

One of the primary factors for classifying heat exchangers and evaporators is the arrangement of flow. It refers to the direction in which the fluids move. Based on the flow, we have four types of heat exchangers: 

1. Cocurrent flow: If fluids move in parallel and in the same directions, it is called cocurrent flow. The benefit of this type of flow is that heat is spread uniformly across the wall of the exchanger. 
2. Countercurrent flow: The fluids are moving parallel but in opposite directions, it is called countercurrent. The benefit of this flow is that the heat transferred is more. 
3. Crossflow: If the direction of fluids is perpendicular, it will be called crossflow. It encompasses the benefits of both cocurrent and countercurrent flow. 
4. Hybrid flow: Hybrid employs multiple flow directions to suit the different temperature and pressure arrangements. 

• Construction method 

The second basis for the classification of heat exchangers is the construction method used. Following are the different types of heat exchangers based on construction method: 

1. Recuperators: A recuperator provides for different flow paths for different fluids and these are widely used in industries. 
2. Regenerators: Regenerators are those heat exchangers which don’t have different paths. Rather the fluids pass alternately from the same passage. 
3. Direct contact heat exchangers: When the two mediums are in direct contact with each other for exchanging heat, such heat exchangers are called direct contact exchangers. Common Examples of this kind of heat exchanger are steam injectors and cooling towers. 
4. Indirect contact heat exchangers: Just opposite the direct contact exchangers, the indirect contact exchangers have different tubes or plates for separating the two fluids. plate type heat exchangers are common examples of indirect contact heat exchangers. 
5. Static heat exchanger: When the components of the exchanger remain static during the transfer of heat, it is called static or fixed bed regenerators. 
6. Dynamic heat exchanger: If the material of the exchanger does not remain static during the transfer of heat between fluids, such heat exchangers are dynamic.                                    Based on design: 

Following are the different types of heat exchangers based on design: 

1. Shell and Tube heat exchangers: When different tube bundles are fitted in a cylindrical shell for the flow of heat, it is called a Shell and Tube heat exchanger. It is used for steam generation, oil cooling, etc. 
2. Double pipe heat exchangers: These heat exchangers are made up of concentric, cylindrical pipes and follow the principle of indirect contact. 
3. Plate type heat exchangers: These are designed by using a bundle of thin corrugated plates. 
4. Other variants: Besides these, condensers, boilers, fan-cooled heat exchangers, etc. are also different variants of heat exchangers.

   Conclusion: 

   Heat exchangers play a vital role in different appliances by aiding the transfer of heat. They are used in several appliances and different forms. Further, these heat exchangers and evaporators come in different types of designs and differ on the basis of construction material, flow direction, temperature, etc. Appropriate heat exchangers are used in different appliances based on the purpose the appliance has to serve.