What are Heat Exchangers?

Heat exchangers are the types of equipment that are primarily employed to transfer the thermal energy from one fluid to another, provided that one of the fluids should be at a higher thermal energy content than the other fluid.

In the field of engineering, focused on heat and mass transfer, heat exchangers are employed to serve the purpose of heating and cooling fluids. Heat transfer between two fluids can only be achieved if the two fluids have some temperature differences. For both the cooling and heating processes, the purpose can be served by the use of heat exchangers. The heat exchangers are primarily used in industries to transfer the thermal energy of a fluid by heating the fluid to a certain temperature and transferring it to the working fluid (usually a cold fluid), for the purpose to bring it to a high temperature. The most important configuration of heat exchanger using liquid for the heat exchange process is that both the liquids can be separated by a wall where there is no physical liquid-to-liquid contact, or simply the two liquids can be allowed to mix inside the heat exchanger (which causes heating of the cold fluid) to obtain the required heat transfer.

Heat exchanger based on the flow configuration

Based on the flow configurations of the two fluids, heat exchangers can be classified into three categories.

Counter-flow heat exchanger

The flow arrangement of the two fluids in the heat exchanger is in the opposite direction in this type of heat exchanger. Hot and cold fluids are usually the working fluids for such kinds of heat exchangers. This kind of heat exchanger gives the maximum heat transfer required.

Parallel flow heat exchanger

When both the working fluids of the heat exchanger flows along the same direction or parallel. Both the fluids enter the device from one end and leave through the other end.

Cross-flow heat exchanger

When the two fluids flow in such a way that they cross each other and have multiple points of intersection, such heat exchangers are known as cross-flow heat exchangers.

Heat exchanger based on construction features

Based on the constructional features, the heat exchangers can be classified as follows:

Shell-and-tube heat exchangers

The shell and tube heat exchanger is a tubular heat exchanger. In the shell-and-tube heat exchanger, one of the fluids flows through a series of parallel tubes and is enclosed in a cylindrical cavity, the other fluid is forced to flow through the cavity shell, such kind of heat exchangers are well known as shell and tube heat exchangers. The shell and tube heat exchanger is used in the oil refining industries.

Multiple shell and tube pass heat exchangers

These kinds of heat exchangers employ several shells shaped like a U-tube into the cavity. The required heat transfer between the fluids can be achieved by allowing the fluid to flow through this number of tube arrangements repeatedly. The fluid is generally re-routed in such kinds of heat exchangers.

Compact heat exchangers

These are large contact heat exchangers that generally have large heat transfer surfaces. These are generally used when one of the fluid's convective heat transfer coefficients is less than the other.

Heat exchanger based on type of the heat exchange process

Based on the type of heat exchange process, these heat exchangers can be classified into the following categories:

Regenerative heat exchangers

In this kind of heat exchanger, the hot fluid and cold fluid pass alternately through a mesh that holds suspended solid particles. Due to the passage of hot fluid, the solid matrix becomes hot. When the cold fluid passes through this matrix, the heat is transferred from the solid matrix to the cold fluid.

Recuperative heat exchangers

In these kinds of heat exchangers, the fluids that take part in the heat transfer process are divided by a separating contour. These heat exchangers are extensively used when there is a restriction on direct contact between the fluids.

Efficiency of heat exchanger

An ideal heat exchanger can be termed as the one where there is a total energy transfer between the fluids, or there is total heat transfer between the fluids without any losses. Practically a total heat transfer is not achievable. Hence a comparison is always made with the ideal heat exchanger to analyze the ability of the heat exchanger in exchanging the heat. This comparison can be established in terms of percentage known as efficiency. If the amount of heat exchange provided by a practice heat exchanger can be taken as a ratio with an ideal heat exchanger, then the ratio value is called the efficiency of the heat exchanger.

It can be written in the form of an expression as,

$\in =\frac{Amount of heat transfered by the practical heat exchanger}{Amount of heat transfer that can be acheived by the ideal heat exchanger}$

Where the symbol, $\in$ denotes the efficiency of the heat exchanger.

Context and Applications

The topic finds its extensive application in the chemical and oil-gas industries. Besides, the topic is extensively taught in the following courses:

• Masters in Science (Physics)
• Bachelors in Science (Physics)
• Bachelors in Technology (Mechanical and Chemical Engineering)

Practice Problems

1. Which of the following heat exchangers provides the maximum heat transfer?

1. Counterflow heat exchanger
2. Frame heat exchangers
3. Tube-in-tube heat exchangers
4. All of the above

Correct option- a

Explanation: The counter flow heat exchanger provides the maximum amount of heat exchanger.

2. Which of the following heat exchangers provides heat exchange between the fluids through U-tubes?

1. Shell-and-tube heat exchangers
2. Tube heat exchangers
3. Tubular heat exchangers
4. Multiple shells and tube heat exchangers

Correct option- d

Explanation: In the multiple shell and tube heat exchangers, tubes as U tube-shaped configurations are used.

3. Which of the following classification of heat exchangers is based on flow configuration?

1. Tube exchanger
2. Parallel-flow heat exchanger
3. Regenerators
4. None of these

Correct option- b

Explanation: The parallel-flow heat exchangers are based on the flow configurations of the fluid flow. Here, both the fluids flow in the same direction.

4. Which of the following heat exchangers employs a shell and a cylindrical cavity?

1. Recuperators
2. Regenerators
3. Shell and tube heat exchangers
4. Heaters

Correct option- c

Explanation: In the shell and tube heat exchangers, the hot fluid flows through the tubular arrangements and the cold fluid flows through the shell cavity. In heaters, a boiler is employed rather than a heat exchanger.

5. In which of the following heat exchangers thermal energy is transferred due to multiple intersections of fluids occurring?

1. Parallel flow heat exchangers
2. Thermal reservoir matrix
3. Cross flow heat exchangers
4. Cooling radiators

Correct option- c

Explanation: The flowing fluids move to cross each other in the cross-flow kind of heat exchangers. Cooling radiators usually employ counterflow heat exchangers.