Heat Exchanger

Classification of Heat Exchanger

Heat exchangers are classified based on two different categories. The first considers the flow configuration within the heat exchanger, while the second is based on equipment type classification, primarily by construction.

The classification of heat exchangers are as follows:

Based on Flow configuration

1. Counter-flow heat exchanger: Fluids enter the exchanger from opposite ends in counter-flow heat exchangers and flow parallel to each other in opposite directions. It is the most efficient design because it allows the maximum heat transfer per unit mass. 

2. Parallel flow heat exchanger: Both fluids enter the exchanger from the same end and flow parallel to each other in the same direction. Although less efficient than countercurrent flow, it provides more uniform wall temperatures.

3. Cross-flow heat exchanger: Both fluids flow perpendicularly to each other in this type of heat exchanger. They are somewhere in the middle of counterflow and parallel flow exchangers in terms of efficiency.

Based on construction design

1. Regenerative heat exchanger: The path flow in the regenerative heat exchanger consists of a matrix through which the hot fluid passes, first giving up its heat, and then the cold fluid passes through the same channel picking up the heat stored. Regenerators are primarily utilised in power plants and other energy-intensive industries for gas/gas heat recovery. Static and dynamic regenerators are the two primary types of regenerators. But these are not very common because they are typically used in specialised applications only. 

2. Recuperative heat exchangers: Typically, a recuperator captures heat that would otherwise be lost. There are different kinds of recuperative heat exchangers classified into indirect contact, direct contact, and specials. 

Heat Exchanger Types

The heat exchanger types are as follows:

Indirect Contact Heat Exchangers

Indirect contact heat exchangers use tubes or plates to keep the fluids exchanging heat separately. The types of the indirect heat exchangers are as follows:

• Shell and tube heat exchanger: It consists of a cylindrical shell containing several tubes. One fluid passes across a bunch of metal tubes, while the other circulates via the surrounding steel shell. The fluid enters through the front end and leaves the exchanger through the rear end.

• Plate heat exchanger: Plates are used to separate the fluids while exchanging heat in this. These heat exchangers are thin, slightly separated plates with large surface areas and small fluid flow passages for heat transfer.

Direct Heat Exchangers

Some examples of direct heat exchangers are steam injection used for heating fluids in pipelines with the help of condensation, Direct contact condensers, which are used as an alternative to tubular condensers because of the low cost, and direct heating, which can be used by passing hot air streams. 

Working Principles of Heat Exchanger

Heat exchanger functions by transferring heat from higher to lower temperatures. Heat can thus be transferred from the hot fluid to the cold fluid if a hot fluid and a cold fluid are separated by a heat-conducting surface.

The operation of a heat exchanger is governed by thermodynamics. Heat can be transferred with the help of conduction, convection, or radiation. Conduction is the transfer of thermal energy from one material to another through the motion of a fluid such as heated air or water.

Convection is the transfer of thermal energy from one surface to another through the motion of a fluid such as heated air or water, and thermal radiation is a heat energy transfer mechanism characterised by the emission of electromagnetic waves from a heated surface or object.

The laws of thermodynamics are the fundamental concepts that underpin heat exchangers.

1. The Zeroth Law of Thermodynamics states that in thermal equilibrium, thermodynamic systems have the same temperature. If two systems are in thermal equilibrium with a third system, the two former systems must also be in thermal equilibrium with one another; hence, all three systems are at the same temperature.

2. The First Law of Thermodynamics states that energy cannot be created or destroyed, but it can be transmitted from one medium to another, such as heat.

3. The Second Law of Thermodynamics establishes entropy (S) as an additional property of thermodynamic systems, which describes a closed thermodynamic system’s natural invariable tendency to increase in entropy over time.

Conclusion

These principles collectively govern the working of the heat exchanger; the Zeroth Law establishes temperature as a measurable property of thermodynamic systems, the First Law describes the inverse relationship between a system’s internal energy and its surrounding environment, and the Second Law expresses the tendency for two interacting systems to move towards thermal equilibrium.

So, in the heat exchanger, a higher-temperature fluid (T1) interacts with a lower-temperature fluid (T2), directly or indirectly, allowing heat to transfer from T1 to T2 and move towards equilibrium. After the heat transfer, there is a decrease in T1 temperature and an increase in T2 temperature. Therefore, heat exchangers can be used to either heat a fluid or cool down the fluid.