Editing Heat exchanger (section)

==Flow arrangement==
[[Image:Delta T 1.svg|thumb|upright|left|Countercurrent (A) and cocurrent (B)]]

There are three primary classifications of heat exchangers according to their [[flow arrangement]]. In ''parallel-flow'' heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In ''counter-flow'' heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is the most efficient, in that it can transfer the most heat from the heat (transfer) medium per unit mass due to the fact that the average temperature difference along any unit length is ''higher''. See [[countercurrent exchange]]. In a ''cross-flow'' heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.

<gallery style="float:right; margin:0.5em 0 0.5em 1em">
Image:Heat_exc_1-1.svg|Fig. 1: [[Shell and tube heat exchanger]], single pass (1–1 parallel flow)
Image:Heat_exc_2-1.png|Fig. 2: Shell and tube heat exchanger, 2-pass tube side (1–2 crossflow)
Image:Heat_exc_2-2.png|Fig. 3: Shell and tube heat exchanger, 2-pass shell side, 2-pass tube side (2-2 countercurrent)
</gallery>
For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence.

The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the "[[log mean temperature difference]]" (LMTD). Sometimes direct knowledge of the LMTD is not available and the [[NTU method]] is used.