LMTD Heat Exchanger Calculator

Method

For parallel or counterflow, ΔT_lm = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂). The calculator then uses Q̇ = U × A × F × ΔT_lm, where F is the correction factor. For true parallel or counterflow, F is 1.

Worked example

For counterflow water cooling from 120 °C to 80 °C against a stream heated from 20 °C to 60 °C, both terminal differences are 60 K. The LMTD is therefore 60 K. At U = 200 W/(m²·K), A = 10 m² and F = 1, the estimated rate is 120 kW, or 120 kWh over one constant hour.

Important limits

This tool does not determine U, calculate exchanger area, perform a heat balance from flow rates or choose a correction factor. Do not use this direct model for phase change, crossflow or multi-pass arrangements without the appropriate correction factor and thermal-design method.

Frequently asked questions

What equation does the LMTD calculator use?

It calculates LMTD = (ΔT1 − ΔT2) / ln(ΔT1 / ΔT2), then heat-transfer rate Q̇ = U × A × F × LMTD. F is the LMTD correction factor.

When should the LMTD correction factor be 1?

Use F = 1 for true parallel-flow or counterflow exchangers. Crossflow and multi-pass exchangers may need a validated correction factor from the applicable thermal-design method.

Does the energy result predict plant consumption?

No. It multiplies the calculated heat-transfer rate by the duration you enter, assuming all inputs remain constant. It does not model pump power, changing loads, control behavior or utility tariffs.