Heat exchangers are pieces of equipment used to transfer heat from a hot fluid to a cold fluid. They are essential as they carry out the task of changing the temperature of the more valuable fluid which is used later on in the process. The fluids may be of different phases, which may be transformed to another phase. They may be separated by a conductive wall or have direct contact.
Heat exchangers operate using the same set of thermodynamic principles. Thermal energy is always conserved in heat exchangers. The direction of heat flow is always from the fluid with higher temperature to lower temperature.
The mechanism of heat transfer in a heat exchanger is a combination of conduction and convection.
Flow configuration of heat exchangers is countercurrent, co-current or parallel flow, cross flow, and hybrid flow.
The two main classes of heat exchangers are recuperative and regenerative heat exchangers. Recuperative heat exchangers are designed to have separate flow paths for the two fluids. Regenerative heat exchangers utilize a heat storage medium that has direct contact on both fluids.
Recuperative heat exchangers are divided into two groups, the indirect contact, and direct contact heat exchangers. Indirect contact heat exchangers utilize a conductive wall to separate the two fluids. Examples are double-pipe heat exchangers, shell and tube heat exchangers, and plate heat exchangers. Direct contact heat exchangers do not have a partition and heat transfer relies on the direct contact of both fluids.
Regenerative heat exchangers are also divided into two groups, static and dynamic regenerators. Static regenerators have a fixed bed of heat storage medium in which the hot and cold fluids flow intermittently. Dynamic regenerators have a rotating element that contains the heat storage medium.