Types of Heat Exchangers
Primarily, what is the purpose of a heat exchanger? And what are the different types of heat exchangers?
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So, after understanding the mechanism of thermodynamics and heat transfer in detail, we switch to an application based topic for this time - Heat exchanger. Recalling the three modes of heat transfer, conduction, convection and radiation, we move next to another of the favourite question asked by an interviewer in an interview for a mechanical engineering position - What are the different types of heat exchangers. To get to know the types of heat exchangers, first, let us understand the mechanism of the heat exchanger and why heat exchangers are needed.
Any device which allows heat to be exchanged between two or more fluids maintained at different temperatures without allowing the fluids to mix can be classified as a heat exchanger. Heat exchangers are an essential component of the process industry and power production plants. Apart from the industrial applications, heat exchangers also find their applications in household applications such as refrigerators, air-conditioners and heaters. The common modes of heat transfer that take place in heat exchangers are conduction and convection.
Types of Heat exchangers based on flow direction
It is worthwhile to know typically the heat exchanger design would be an attempt to meet the heat transfer requirement. A typical heat exchanger is designed with two concentric tubes of different diameters, popularly referred to as a double-pipe heat exchanger. Based on the direction of flow in the tubes, the classification is given as :
Cross-flow or counter-flow
Parallel flow heat exchanger
In the parallel flow heat exchanger, the direction of the fluid flow in both the tube is the same. The inlets for the hot and the cold fluid are at one end while the outlets for the hot and the cold fluid are at the other end. Thus the hot fluid and the cold fluid flows in the same direction.
Crossflow heat exchanger
While for the cross-flow heat exchanger, the direction of the fluid flow in both the tube is opposite. The inlet for the hot fluid and the outlet for the cold fluid is at one end. And the outlet for the hot fluid and inlet for the cold fluid is at the other end. Thus the hot fluid and the cold fluid flows in the opposite direction.
Compact heat exchanger
Compact heat exchangers cater to the problem of large surface area. When the ratio of the exposed heat transfer surface area to its volume ranges greater than 700, such compact heat exchangers are useful. Common examples of compact exchangers are lungs, radiators in cars and regenerators of the Sterling engine. These special types of heat exchangers typically serve the purpose where the weight and volume are critical to the design. One of the methods to expose a larger surface area is by placing numerous fins close to each other on the surface. Based on the purpose of fins, these heat exchangers are further classified as:
Unmixed crossflow compact heat exchanger
When the fins are placed in such a way, that the fins allow external fluid flow only between the interfins and no interaction takes place between the fluid flowing through different interfins
Mixed crossflow compact heat exchanger
The fins here allows the mixing of external fluid flow between different fins.
Shell and tube heat exchanger
Shell and tube types of heat exchangers are quite commonly found in industries. Numerous tubes are placed in the shell such that the axes of the shell and tubes align parallel to each other. The heat exchange happens when one fluid medium flows through the tubes while the other fluid flows through the shell. These kinds of heat exchangers are often bulky thereby needing larger accommodation space. Quite often, some barriers such as baffles are placed inside the shell so that better heat exchange takes place. Such type of heat exchangers is usually classified based on the number of shells and tube roundabouts.
One shell and two tube pass: The tube is bend to construct one U-turn and thus the tube makes only one pass in the shell
Two shells and four tube pass: The tube is bent to make four tube passes thereby implying two entries into the shell.
Plate and frame heat exchanger
In this type of heat exchanger, usually, multiple plates with ridged flow passages are provided. The flow passages are so arranged that the hot and cold fluids flow alternatively. Thus, each hot fluid stream flows between two cold streams and vice-versa thereby enhancing the rate of heat transfer. Such types of heat exchangers are very effective when both the mediums are liquid.
Classification of heat exchanger based on application
Heat exchangers are built to serve the purpose of heat transfer specific to the need. Thus, based on the applications, they can be further categorized as:
In this type of heat exchanger, the cooling fluid is to be cooled till it condenses while flowing in the heat exchanger
In this type of heat exchanger, we observe the opposite of what happens in the condenser. The hot liquid is to be heated till it vaporizes while flowing in the heat exchanger.
In this type of heat exchanger, the mode of heat transfer is radiation between the hot fluid and the surrounding.
Heat transfer process inside the heat exchanger
It is commonly observed that many people think there is only one mode and one way of heat transfer happening inside the heat exchanger. This is not true. Usually, the two flowing fluids will be separated by a physical boundary, a wall of the enclosure. So, in reality, primarily heat transfer takes place within the first fluid through convection, then as the heat reaches the wall, conduction mode of heat transfer takes place in the wall and outside the wall, the heat is transferred through convection again. Thus, for the calculations process of thermal resistance, the inner fluid, the outer fluid as well as the wall needs to be considered.
The design of the first plate heat exchanger was submitted by Albrecht Dracke belonging to Germany in the year 1878 and he was granted the patent for the same. However, the successful and commercial development of heat exchangers is associated with Dr Richard Seligman. To cater for the processing needs of dairy, Dr Seligman in 1910 founded the Aluminium Plant & Vessel Company Limited. Even today, his name resonates in the process industry due to his excellent work on the design of heat exchangers. In honour of his contribution, the Society of Dairy industry awarded him the first-ever gold medal.
We discussed what is the purpose of the heat exchanger
Alongside, we tried to classify the types of heat exchangers in detail.