Heat Exchanger LMTD Calculator

Formula

• LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂)
• Q = ṁ_h × Cp_h × (T_hi − T_ho)
• A = Q / (U × LMTD)

About this calculator

Part of our Thermal & Fluids Calculators collection. Heat transfer, pipe flow, Reynolds number, and thermodynamic cycles.

How it works

Log mean temperature difference uses the hot and cold inlet and outlet temperatures for your flow arrangement. Heat duty comes from hot-side mass flow and specific heat. Required area equals heat duty divided by U times LMTD.

Enter your values in the inputs above and click CALCULATE. Results appear on the right without a page reload. No login and no server upload.

Worked example

Counterflow: hot 150→80°C, cold 20→70°C, ṁ=2 kg/s, Cp=4.18, U=500 W/m²·K.

1. ΔT₁ = 150−70 = 80°C, ΔT₂ = 80−20 = 60°C
2. LMTD = (80−60)/ln(80/60) = 69.5°C
3. Q = 2×4.18×70 = 585 kW
4. A = 585000/(500×69.5) = 16.8 m²

Result: LMTD 69.5°C, area 16.8 m².

When to use

• Sizing heat exchanger surface area from temperature profiles
• Comparing counterflow vs parallel flow LMTD
• Checking heat duty against hot-side energy balance

Limitations

• Assumes constant U and no phase change
• Returns NaN if any ΔT ≤ 0 (invalid temperature cross)
• Single hot-side flow rate used for duty; cold side not balanced separately

FAQ

Counterflow vs parallel?

Counterflow gives higher LMTD and smaller required area for the same duty. Parallel flow is used when temperature cross must be avoided.

Why NaN results?

If hot outlet is colder than cold outlet (temperature cross), ΔT becomes negative and LMTD is undefined. Check your temperature inputs.