Drawing No. EH–TH–032 // Thermal Engineering & HVAC
NTU-Effectiveness Calculator
Reviewed August 2026
Find heat exchanger effectiveness from NTU and the heat-capacity-rate ratio, and use it to predict heat duty and outlet temperatures without needing to guess an outlet condition first.
What problem does this solve?
The effectiveness-NTU method solves heat exchanger problems directly from inlet conditions and exchanger size (UA), without needing to guess an outlet temperature first the way the LMTD method requires. This is especially useful when UA is known from a rating or prior test, and you want the resulting duty and outlet temperatures for new inlet conditions.
Inputs
Results
Background
NTU = UA / Cmin, where Cmin is the smaller of the two stream heat capacity rates (Ch = mass flow × specific heat). Cr = Cmin/Cmax, ranging from 0 (one stream's temperature barely changes, e.g. a condensing or evaporating fluid) to 1 (both streams have equal capacity rates).
Counterflow: ε = [1 − exp(−NTU(1−Cr))] / [1 − Cr·exp(−NTU(1−Cr))], with ε = NTU/(1+NTU) in the special case Cr=1. Parallel flow: ε = [1 − exp(−NTU(1+Cr))] / (1+Cr).
Qmax = Cmin × (Th,in − Tc,in) is the thermodynamic upper limit on heat transfer. Actual duty Q = ε × Qmax. Outlet temperatures follow from energy balance: Th,out = Th,in − Q/Ch, and Tc,out = Tc,in + Q/Cc.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
For the same NTU and capacity ratio, counterflow effectiveness is always equal to or greater than parallel flow, because counterflow can theoretically approach ε=1 (full temperature crossover) as NTU grows large, while parallel flow is capped at ε = 1/(1+Cr) no matter how large NTU gets — the two streams can never do better than reaching a common outlet temperature in parallel flow.
Cr = 0 means one stream's heat capacity rate is effectively infinite compared to the other — the classic case is a condenser or evaporator, where one side is changing phase at constant temperature and absorbs or releases heat without its own temperature changing. In this case, counterflow and parallel-flow effectiveness formulas both reduce to ε = 1 − exp(−NTU).
Both methods describe the same physical exchanger and give identical results when applied to the same problem — the difference is which inputs you start with. LMTD needs all four terminal temperatures (or requires guessing outlet conditions iteratively); effectiveness-NTU only needs the inlet conditions and UA, making it more direct when outlet temperatures are unknown.