Types of Heat Exchangers
The principal types of heat exchangers used in chemical and allied industries which will be discussed in this chapter, are listed below:
- Double-pipe exchanger: the simplest type, used for cooling and heating.
- Shell and tube exchangers: used for all applications
- Plate and frame exchangers (plate heat exchangers): used for heating and cooling.
- Plate-fin exchangers.
- Spiral heat exchangers.
- Air-cooled: coolers and condensers.
- Direct contact: cooling and quenching.
- Agitated vessels.
- Fired heaters.
Heat exchanger types are also classified according to the direction of flow of the hot and cold fluids with respect to each other, or according to the temperature distribution of the two fluids along the exchanger length. Thus, we may have the following types of heat exchanger:
a) Parallel-flow exchanger
b) Counter-flow exchanger
c) Cross-flow exchanger
d) Condenser or evaporators
Parallel-flow exchangers: The hot fluid and cold fluids flow in the same direction, hence the name parallel-flow. Many devices, such as water heaters, oil heaters and oil coolers, etc., belong to this class.
The temperature difference between hot and cold fluid keeps on decreasing from inlet to exit as shown in following fig.1.2
Counter-flow Exchangers: In this case the fluids flow through exchanger in opposite directions, hence the name counter flow. The temperature distribution in counter-flow exchanger is shown in fig.1.3 Below:
It can be seen that the temperature difference between the two fluids remains more nearly constant as compared to the parallel-flow type. This arrangement gives maximum heat transfer rate for a given surface area. If the fluid flows through the exchanger only once, it is called a single pass heat exchanger. In many designs, one or both fluids may traverse the exchanger more than once. Such exchangers are called multi-pass exchangers.
Cross-flow exchangers: Here the two fluids flow at right angles to each other. Two different arrangements of this exchanger are commonly used. In one case, each of the fluids is unmixed as it flows through the exchanger. As a result, the temperatures of the fluids leaving the exchanger are not uniform. An automobile radiator is an example of this type of exchanger. In other case, one fluid is perfectly mixed while the other is unmixed as it flows through the exchanger.
Condenser: In a condenser the condensing fluid (hot fluid) remains at constant temperature throughout the exchanger while the temperature of the colder fluid gradually increases from inlet to outlet. Similarly in an evaporator the boiling fluid (cold fluid) remains at constant temperature while the hot fluid temperature gradually decreases. The temperature distribution in condenser is shown below. Since the temperature of one of these fluids remains constant, it is immaterial whether the two fluids flow in the same direction of opposite direction.