An apparatus that transfers heat between two or more fluids is called a heat exchanger. Depending on the type of exchanger, the fluids may be in direct contact or separated and may be single or two phase. Despite sharing many design principles, devices utilising energy sources, such as fired heaters or nuclear fuel pins, are not typically classified as heat exchangers.

Some sort of classification is required before we can talk about heat exchangers. There are two common methods that are used. In the first, the heat exchanger’s internal flow configuration is taken into account, whereas in the second, the equipment type is primarily classified by construction. Here, both are taken into account.