What is Heat exchanger?
Equipment that moves heat from a hot stream to a cold one without mixing them.
It estimates how much otherwise wasted heat is recovered — for example flue gas heating combustion air or water — and the outlet temperatures.
A ventilation unit that warms cold outdoor air with the warm room air is a heat exchanger.
Give one of: UA, the effectiveness, the duty, or an outlet temperature. The outlet states follow from the enthalpy balances of both streams; a specification with a temperature cross (the cold side ending hotter than the hot side) is refused, and so is one that would boil a liquid side, since phase change is not modeled.
Given UA, the duty solves
Q = UA\,\frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)}
(\Delta T_1, \Delta T_2: the temperature differences at the two ends). The effectiveness is \varepsilon = Q / Q_{\max}, with Q_{\max} the smaller duty that brings one stream to the other's inlet temperature.
The log-mean temperature difference uses the end temperatures; the variation of the specific heat inside the exchanger is not resolved. No heat loss to the surroundings.