Shell and Tube Heat Exchangers

Heat exchangers are devices that help in efficient heat exchange from one medium to another. The media are separated from each other with solid material so that they never mix with each other. These devices are widely used in refrigeration, air conditioning, different types of power plants and natural gas processing units. Shell and tube heat exchangers are a class of heat exchanger that is most suitable for high pressure applications. They are most popularly used in oil refineries and other units involving large chemical processes.

Structure

As the name suggests, a shell and tube heat exchanger contains a bundle of tubes inside a large pressure vessel which is referred to as the shell. The tubes on each end are attached to tube sheets. Two different fluids run through the shell and the tube heat exchanger, one through the tubes and the other outside the bundle of tubes within the shell. As the fluids flow through the system, transfer of heat takes place from the fluid that is at a higher temperature to that at a lower temperature. The fluid inside the tubes is called the tube side fluid whereas that which is flowing outside the tubes and within the shell is called the shell side fluid. This heat exchange takes place through the tube walls. The fluid that has to be heated or cooled runs through the tubes. The fluid that runs through the shell outside the tube bundle is heated or cooled depending on whether the fluid inside the tubes have to be cooled or heated. Use of a number of tubes instead of one increases the efficiency of the heat exchanger as circular surfaces of all the tubes increases the net area of transfer of heat.

Single Phase or Double Phase

The fluids can either be liquid or gas. Shell and tube heat exchangers with either only liquid or gas in both the tubes and the shell are called the one phase or single phase heat exchanger. Two phase heat exchangers involve transfer of heat between two fluids of two different phases, that is a liquid and a gas. Boilers are two phase heat exchangers in which liquids are heated to a gas. In condensers, the gas is cooled down into its liquid phase. The change in phase takes place in the shell part. Shell and tube heat exchangers can be 1, 2 or 4 pass heat exchangers. The number of passes indicate the number of times that the fluid pass through the tubes.

Designs

U-Tube Heat Exchanger: In the U-tube heat exchangers, the tubes in the tube bundle bend to form a U. The ends of the tubes open into plenums or water boxes through holes in the tube sheet. There is an inlet plenum and an outlet plenum. The fluid enters through the inlet plenum, runs the entire length of the U-tube and leaves the heat exchanger through the outlet plenum. As the fluid flows inside the U-tube, heat transfer takes place between the fluid in the tube side and that in the shell side.

Two phase U-tube heat exchangers are commonly used in nuclear power plants called pressurized water reactors. Using these heat exchangers, water that has been obtained by condensing steam is heated back to steam that is used to turn steam turbines. As the steam turbines rotate, power is generated.

Straight-Tube Heat Exchanger: As the name suggests, these heat exchangers have straight tubes. The ends open into two separate tube sheets. In case of one pass straight tube heat exchangers, one end has the inlet plenum while the other end has the outlet plenum. In case of a two pass straight tube heat exchanger, both the inlets are on the same side.

Shell and tube heat exchangers have made large-scale cooling of different media in various industries very convenient. It is important to ensure proper cleaning of the heat exchanger tubes. Although traditionally methods of hydro-blasting water was used, now more advanced techniques are being applied.