Approach Temperature

Approach temperature is the smallest temperature difference between the two fluid streams in a heat exchanger, typically at the exchanger's outlet. A smaller approach means more heat is recovered but requires more surface area.

In any heat exchanger the hot and cold streams converge in temperature but can never fully meet. The approach temperature is the minimum gap between them, and it sets the practical limit on how much heat can be transferred. Designers trade a tight approach (better energy recovery, larger and costlier equipment) against a wider approach (cheaper, smaller, but more wasted heat). It is a central variable in pinch analysis and in sizing economisers, condensers and recuperators.

Related terms

Pinch Analysis · Log Mean Temperature Difference (LMTD) · Heat Exchanger