A heat exchanger is a system in which energy is exchanged between two fluid mass without getting mixed with each other or coming into direct contact with each other. The composite wall present between them acts as a thermal resistance element. The direct contact heat exchanger is the exception to this configuration.
Working of A Heat Exchanger
The heat exchanger arrangement consists of a large shell with a tube in it. Hot fluid flows through the tube inside the large shell through which colder fluid flows. These two fluids run in the opposite direction. Heat exchange takes place – the cold fluid warms up and the hot fluid cools down without coming in contact with each other.
Types of Heat Exchangers
On the basis of the configuration, there are three types of heat exchangers.
1. The simple parallel or counterflow configuration
The simple parallel and counterflow configurations are versatile and most commonly used. A counterflow heat exchanger has bent around in a so-called Heliflow compact heat exchanger configuration. In counterflow, the direction of the flow of both fluids is different. In a parallel flow configuration, both the fluids flow in the same direction.
The couterflow is better than the parallel flow because –
- Due to the more uniform temperature difference in counterflow, the thermal stresses are reduced.
- The outlet temperature of the cold fluid can achieve the highest inlet temperature of the hot fluid.
- Due to higher uniform temperature difference, a higher uniform rate of heat transfer is maintained throughout the heat exchanger.
2. The shell-and-tube configuration
The shell and tube configuration is most commonly used for large heat exchangers. Due to its flexibility in design, it is used widely in industries.
It consists of U-tubes of a two-tube pass, with one shell pass exchanger being installed int he supporting baffles.
3. The cross-flow configuration
In this configuration, both the fluids remain unmixed. This configuration allows heat exchange between airstreams in an air handling unit. It is mainly used in ventilation and cooling systems.
Each fluid has a prescribed path through the exchanger and is not allowed to mix to the left or right. This is made of thin aluminium metal panels. The thermal energy is exchanged through these panels.
To minimize heat losses, heat exchangers are called recuperator and regenerator.
A recuperator stores the heat which would otherwise be lost. Its configuration is counter flow and is located within the supply and exhaust air streams of an air handling system. Recuperator recovers the waste heat.
A regenerator works in a similar manner, only the incoming and outgoing fluids move in opposite directions at different time through the same channel. The heat is stored when the hot fluid passes through the channel and when cold fluid passes through the same channel, it picks up the heat stored in the channel.
Materials In Heat Exchanger
Heat exchangers can be made of metals, ceramics composites and plastics. Different materials come with different advantages.
Ceramics are used for higher temperature applications. Also, these can be used with corrosive and abrasive fluids at low or high temperatures.
Plastics are cheaper than metals. Also, it is lightweight and resists corrosion and fouling. Plastics are engineered to have good thermal conductivity. But these aren’t the first choice as they tend to be mechanically weak and are not suitable for high-temperature usage.
Composite is widely used as it posses the best features of their parent materials. It is lightweight, has better conductivity and corrosion resistivity.
The heat exchanger has always been a major part of many industries. In future, more technologies in heat sink will come into existence.