What one needs to know about heat Exchangers

A heat exchanger, as the name implies, is basically a piece of equipment that serves the purpose of transferring heat from one medium to another. This media might be separated by a wall that is solid in nature so as to prevent mixing or they can also be in direct contact. These are used widely for space heating, air conditioning, refrigeration, in power, chemical and petrochemical plants along with in petroleum refineries, sewage treatment and in natural gas processing. A fluid actually does the hard work of carrying heat. The secret of their efficiency lies in two things. Although a little scientific, they are still easy to understand. The wall that separates the heat exchanger tubes needs to have a wide surface area. At the same time, it is equally important to reduce the flow of these fluids while passing through these tubes. Fins can be added in one or both directions to increase the exchanger’s performance manifolds. These fins also increase the surface area and also create possibility of turbulence during fluid flow.  

One can find the classic example of a heat exchanger in the internal combustion engine where a circulating fluid that is known as an engine coolant flows through the radiator coils and the air flows past these coils and cools the coolant while heating the incoming air. There are many kinds of heat exchangers like the shell and tube ones. These consist of a series of tubes. One of the sets of these tubes contains the fluid that is either to be heated or cooled. It is the second fluid that runs over these tubes which are either being heated or cooled in order for it to provide the heat or absorb the heat that is required.

In this kind of a heat exchanger, there is also a set of tubes that is known as a tube bundle which can be made from many different kinds of tubes. For example they can be plain or longitudinally finned. These tube and shell heat exchangers are used typically for high pressure applications that usually have pressures more than the thirsty bar along with temperatures that go above than two hundred and sixty degrees Celsius. The reason behind this is that tube and shell heat exchangers because of their shape are robust.

When designing these tubes in the shell, there are several thermal design features that have to be considered. There can be many different kinds of variations on the tube and shell design. It is typical though that the ends of each of the tubes are connected to plenums that are also known as water boxes some times, through the holes that are there in the tube sheets. These tubes can either be in the shape of a U and called U-bundle and they can also be straight.

These tubes have a small diameter that makes the heat exchanger compact as well as economical. Wider diameter makes the tubes far less economical. Though there are more chances for the heat exchanger to foul up faster and the small size also makes the cleaning of such a fouling much more difficult. So to overcome these problems, tubes of larger diameters can be used as well. So it can be said that in order to determine the diameter of the tube, the cost and fouling nature of the fluids as well as the available space and cost are all taken into consideration. Baffles are also used in the tube and shell heat exchangers so as to direct the fluid across the tube bundle. They prevent the tubes from vibrating too. 

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