In a large proportion of the heat exchangers encountered in practice, each fluid stream leaves in the same phase state as that in which it entered (a gas leaves as a gas, a
liquid as a liquid). The result is that, as heat is transferred from the hotter fluid to the cooler one, the former temperature diminishes while the latter rises.
In other heat exchanger types, however, especially in boilers and condensers, the essential function is to change the phase of one of the streams. In such cases, the temperature change in that stream is often small enough to be neglected In the case of the boiler, the phase-changing fluid may enter as a sub cooled liquid, and subsequently rise in temperature to its boiling point; then, throughout the two-phase region, the temperature will vary only because of the small difference of pressure that may prevail; thereafter, before leaving the heat exchanger, the fluid may become superheated. Temperature changes therefore do occur in the phase-changing fluid within a boiler, but are often disregarded by the designer who wants to employ an analytical formula that is valid only for the case of zero change.