Number of Transfer Units (NTU)

NTU is an exchanger's size measured in the only currency that matters: conductance compared with the thermal inertia of the fluid flowing through it. UA is how well the machine can transfer; ṁcₚ is how much heat the limiting stream can carry per degree. Their ratio is dimensionless and tells you immediately what class of equipment you are holding: NTU below 0.5 is a trim heater that barely touches the fluid, 1–3 is normal process duty, 3–5 is a close-approach unit, and above 5 you are in regenerator and cryogenic territory where surface becomes very expensive per degree gained.

The effectiveness-NTU method was developed by W. M. Kays and A. L. London for the compact heat exchangers of gas-turbine regenerators and published in Compact Heat Exchangers (1955): a book still on working desks seventy years later. Their motivation was practical: LMTD design demands all four terminal temperatures, but a regenerator problem usually gives you the two inlets and asks what comes out, which drives LMTD into iteration and NTU straight to an answer. Worked example: U = 500 W/(m²·K) on 8 m² against 1.5 kg/s of water gives NTU = 4000/6279 = 0.64, a small unit. Trap: ṁcₚ must be the minimum stream's, not whichever stream you measured first.