Home/ Tools/ ε-NTU/ ERV/HRV Eff./ Sizing

Air-to-Air Effectiveness Calculator (ε-NTU)

Use the ε-NTU method to find heat exchanger effectiveness from NTU and the capacity ratio, for counterflow, parallel-flow or crossflow arrangements.

Flow arrangement

NTU (UA / C_min)

Capacity ratio C_r = C_min/C_max

1.0 when both streams equal

Min stream mass flow

kg/s (air)

Inlet temperature difference ΔT

°C

Effectiveness ε

0 %

Recovered heat

0 kW

Outlet approach

0 °C

UA required

0 kW/K

The ε-NTU formulas

Counterflow: ε = [1 − e^{−NTU(1−C_r)}] / [1 − C_r e^{−NTU(1−C_r)}] (at C_r=1, ε=NTU/(1+NTU)).
Parallel: ε = [1 − e^{−NTU(1+C_r)}] / (1+C_r).
Crossflow (both unmixed): ε = 1 − e^{(C_r⁻¹ NTU^0.22 (e^{−C_r NTU^0.78} − 1))}.

Frequently asked questions

Why is counterflow always higher?: In counterflow the cold outlet meets the hot inlet, so the cold stream can approach the hot inlet temperature. Crossflow and parallel-flow cap out lower for the same NTU.
How do I get NTU?: NTU = UA / C_min, where U is overall heat-transfer coefficient (W/m²K), A the core area, and C_min the smaller of the two stream heat capacities (kg/s × cp). Our sizing tool works the other way round.
What is a typical effectiveness?: Well-built plate exchangers reach 80–90% (counterflow); crossflow 60–70%. If measured effectiveness is far below the design NTU prediction, suspect fouling or bypass.

Preliminary estimate only. Zibo Qiyu designs plate, rotary and cross-flow air-to-air heat exchangers — request a quote.