Heat Exchanger (NTU)

Use LMTD when you know all four temperatures and want the area. Use effectiveness-NTU when you know the inlets and the hardware but not the outlets — which is the situation you are actually in when rating an exchanger you already own, or predicting what happens when the duty changes.

Set the capacity rate of a boiling or condensing stream very high (Cr → 0) — its temperature does not move, and then every arrangement gives the same effectiveness, which is why a condenser does not care about flow direction. Counterflow is always the best arrangement at a given NTU; balanced parallel flow can never exceed 50 % effectiveness no matter how large you build it. UA is assumed constant and fouling is not included.