The epsilon-NTU Method: Predicting Heat Exchanger Performance

Effectiveness, the practical metric

Effectiveness (epsilon) is the ratio of actual heat transferred to the maximum thermodynamically possible:

epsilon = actual_heat_transfer / (Cmin x (Thot_in - Tcold_in))

It is the number you see quoted as “70% efficient” for a recovery core, and it is directly related to the métriques d'efficacité reported for air-to-air units.

NTU: the size-and-flow term

The Number of Transfer Units scales with core size and inverse with flow:

NTU = (U x A) / (m_dot x cp) where UxA is the overall conductance and m_dot x cp is the smaller heat capacity rate (Cmin).

From NTU to outlet temperatures

For a counter-flow plate core the effectiveness is approximately:

epsilon = (1 - exp(-NTU x (1 - Cr))) / (1 - Cr x exp(-NTU x (1 - Cr)))

with Cr = Cmin / Cmax. Raise NTU (bigger or slower core) and effectiveness rises toward its limit. Cross-flow cores sit a little lower than counter-flow for the same NTU – a design trade-off covered under Perte de charge.

Why engineers use it

Given flow rates, fluid properties and a target effectiveness, the epsilon-NTU relation sizes the required UA – the bridge between a performance spec and a physical échangeur à plaques.

Frequently asked questions

Is higher NTU always better? It raises effectiveness but also core size, material and fan power; designs stop where payback peaks.

What is Cr? The capacity ratio; when both streams are equal air flows, Cr is near 1 and effectiveness is most sensitive to NTU.

Need help specifying the right air-to-air heat recovery for your project? Contact the QIYU engineering team – WhatsApp +86 157 5335 5505, or email kuns913@gmail.com.

Questions fréquemment posées

Questions fréquemment posées

What does effectiveness mean for a heat exchanger?

Effectiveness is actual heat transferred divided by the maximum possible; it tells you what fraction of the available temperature difference you recover, independent of size.

Why use NTU instead of just area?

NTU equals UA divided by the smaller capacitance rate and summarizes how much heat-transfer capacity you have relative to the weaker stream; with the epsilon-NTU method you can predict outlet temperatures from capacity ratio and NTU.