A heat exchanger is a device used to provide heat transfer between non-mixing media. In this instance a heat exchanger is a device used in a liquid distribution system to recover energy from one liquid to another liquid (typically water-based). Other types of heat exchangers are dealt with under air-to-air heat recovery devices and coils.
A tube in tube liquid heat exchanger consists of one tube inside another with one fluid flowing in the inner tube and the second flowing in the annulus between the tubes.