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 most suitable for high pressure applications. They are most popularly used in oil refineries and other units involving large chemical processes.

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 sheets.

Two different fluids run through the device; one through the tubes and the other outside the bundle, but 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 the one at a lower temperature. This heat exchange takes place through the tube walls.

The fluids can either be liquids or gases. Shell and tube heat exchangers with either only liquids or gases, are classified as one phase or single phase. Two phase types involve transfer of heat between two fluids of different phases, that is a liquid and a gas.

Boilers are two phase systems, 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. These devices can be of 1, 2, or 4 pass variety. The number of passes indicate the number of times that the fluid pass through the tubes.

U-Tube: In the U-tube heat exchangers, the tubes in the bundle, bend to form a U. The ends of these 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 through the outlet plenum.

Two phase U-tube heat exchangers are commonly used in nuclear power plants called pressurized water reactors. Using these, 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: As the name suggests, these have straight tubes. The ends open into two separate 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, 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.