A heat exchanger is a component that allows the transfer of heat from one fluid (liquid or gas)
to another fluid. Reasons for heat transfer include the following:
1. To heat a cooler fluid by means of a hotter fluid
2. To reduce the temperature of a hot fluid by means of a cooler fluid
3. To boil a liquid by means of a hotter fluid
4. To condense a gaseous fluid by means of a cooler fluid
5. To boil a liquid while condensing a hotter gaseous fluid
Regardless of the function the heat exchanger fulfills, in order to transfer heat the fluids involved
must be at different temperatures and they must come into thermal contact. Heat can flow only
from the hotter to the cooler fluid.
In a heat exchanger there is no direct contact between the two fluids. The heat is transferred
from the hot fluid to the metal isolating the two fluids and then to the cooler fluid.
Types of Heat Exchanger Construction
Although heat exchangers come in every shape and size imaginable, the construction of most heat exchangers fall into one of two categories: tube and shell, or plate. As in all mechanical devices, each type has its advantages and disadvantages.
Tube and Shell
The most basic and the most common type of heat exchanger construction is the tube and
shell, as shown in Figure 1. This type of heat exchanger consists of a set of tubes in a
container called a shell. The fluid flowing inside the tubes is called the tube side fluid
and the fluid flowing on the outside of the tubes is the shell side fluid. At the ends of
the tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s).
The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tight
seal. In systems where the two fluids are at vastly different pressures, the higher pressure
fluid is typically directed through the tubes and the lower pressure fluid is circulated on
the shell side. This is due to economy, because the heat exchanger tubes can be made
to withstand higher pressures than the shell of the heat exchanger for a much lower cost.
The support plates shown on Figure 1 also act as baffles to direct the flow of fluid within
the shell back and forth across the tubes.
A plate type heat exchanger, as illustrated in Figure 2, consists of plates instead of tubes
to separate the hot and cold fluids. The hot and cold fluids alternate between each of the
plates. Baffles direct the flow of fluid between plates. Because each of the plates has
a very large surface area, the plates provide each of the fluids with an extremely large
heat transfer area. Therefore a plate type heat exchanger, as compared to a similarly
sized tube and shell heat exchanger, is capable of transferring much more heat. This is
due to the larger area the plates provide over tubes. Due to the high heat transfer
efficiency of the plates, plate type heat exchangers are usually very small when compared
to a tube and shell type heat exchanger with the same heat transfer capacity. Plate type
heat exchangers are not widely used because of the inability to reliably seal the large
gaskets between each of the plates. Because of this problem, plate type heat exchangers
have only been used in small, low pressure applications such as on oil coolers for
engines. However, new improvements in gasket design and overall heat exchanger
design have allowed some large scale applications of the plate type heat exchanger. As
older facilities are upgraded or newly designed facilities are built, large plate type heat
exchangers are replacing tube and shell heat exchangers and becoming more common.