Heat exchangers are typically classified according to flow arrangement and type of construction. The simplest heat exchanger is one for which the hot and cold fluids move in the same or opposite directions, and this heat exchanger consists of two concentric pipes of different diameters.
- parallel-flow arrangement. In the parallel-flow arrangement, the hot and cold fluids enter at the same end, flow in the same direction, and leave at the same end.
- counter-flow arrangement. In the counter-flow arrangement, the fluids enter at opposite ends, flow in opposite directions, and leave at opposite ends.
The figure represents the directions of fluid flow in the parallel and counter-flow exchangers. Under comparable conditions, more heat is transferred in a counter-flow arrangement than in a parallel flow heat exchanger. The temperature profiles of the two heat exchangers indicate two major disadvantages in the parallel-flow design.
- The large temperature difference at the ends causes large thermal stresses.
- The temperature of the cold fluid exiting the heat exchanger never exceeds the lowest temperature of the hot fluid.
The heat transfer surface in heat exchangers can be arranged in several forms. Heat exchangers are therefore also classified as:
- Double pipe heat exchangers. Double pipe heat exchangers are cheap for design and maintenance, making them a good choice for small industries. One fluid flows inside the tube in these exchangers, and the other fluid flows outside. Although they are simple and cheap, their low efficiency coupled with the high space occupied in large scales has led modern industries to use more efficient heat exchangers like shells and tubes.
- Shell and tube heat exchangers. In their various construction modifications, shell and tube heat exchangers are probably the most widespread and commonly used basic heat exchanger configuration in the industry. Shell-and-tube heat exchangers are classified according to the number of shell and tube passes involved. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260 °C). The shell and tube heat exchangers can withstand high pressures due to their shape. In this type of heat exchanger, several small bore pipes are fitted between two tube plates, and primary fluid flows through these tubes. The tube bundle is placed inside a shell, and the secondary fluid flows through the shell and over the surface of the tubes. In nuclear engineering, this design of heat exchangers is widely used, as in the case of steam generators, which are used to convert feedwater into steam from heat produced in a nuclear reactor core. The heat exchange surface must be maximized to increase the amount of heat transferred and the power generated. This is obtained by using tubes. Each steam generator can contain anywhere from 3,000 to 16,000 tubes, about 19mm in diameter.
- Plate heat exchangers. A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This arrangement is popular with heat exchangers using air or gas as well as lower velocity fluid flow. The classic example of a heat exchanger is found in an internal combustion engine in which an engine coolant flows through radiator coils, and air flows past the coils, which cools the coolant and heats the incoming air. Compared to shell and tube exchangers, the stacked-plate arrangement typically has lower volume and cost. Another difference between the two is that plate exchangers typically serve low to medium pressure fluids, compared to medium and high pressures of shell and tube.