The Log Mean Temperature Difference (LMTD) is the effective driving force for heat transfer across a heat exchanger. It accounts for the varying temperature difference between hot and cold streams along the exchanger length. LMTD = (ΔT1 − ΔT2) / ln(ΔT1 / ΔT2).

What is the overall heat transfer coefficient U?

U combines the thermal resistances of the hot-side film, tube wall, and cold-side film into one value (W/m²·K). Typical values range from 100–500 W/m²·K for liquid-liquid exchangers and 20–200 W/m²·K for gas-liquid exchangers.

When is counterflow better than parallel-flow?

Counterflow always achieves a higher LMTD for the same terminal temperatures, so it requires less surface area for the same duty. It also allows the cold outlet temperature to exceed the hot outlet temperature, which parallel-flow cannot do.

What is NTU and effectiveness?

NTU (Number of Transfer Units) = U × A / C_min, where C_min is the smaller of the two fluid capacity rates (mass flow × Cp). Effectiveness ε = actual heat transfer / maximum possible heat transfer. These dimensionless values are useful when outlet temperatures are unknown.

How do I choose the U value?

U depends on fluid types, velocities, and fouling. For water-water exchangers use 800–1500 W/m²·K; for oil-water use 100–400 W/m²·K; for steam condensers use 1000–3000 W/m²·K. Consult TEMA standards or process data sheets for precise values.

Heat Exchanger Calculator

Enter fluid inlet/outlet temperatures, flow rates, and fluid properties to calculate heat duty, LMTD, and required surface area.

Flow Configuration

Hot Fluid

Inlet T_h,in (°C)

Outlet T_h,out (°C)

Mass flow ṁ_h (kg/s)

Cp_h (J/kg·K)

Cold Fluid

Inlet T_c,in (°C)

Outlet T_c,out (°C)

Mass flow ṁ_c (kg/s)

Cp_c (J/kg·K)

Overall Heat Transfer Coefficient U (W/m²·K)

Typical: water-water 800–1500 · oil-water 100–400 · gas-liquid 20–200

Fill in the inputs and click Calculate.

Summary

This heat exchanger calculator applies the LMTD method to determine heat duty (Q), Log Mean Temperature Difference, and the required heat transfer surface area for counterflow or parallel-flow configurations. Enter the four terminal temperatures, the overall heat transfer coefficient U, and both fluids' capacity rates to get instant results. NTU and effectiveness are also reported for cross-checking.

How it works

Select the heat exchanger flow configuration: counterflow or parallel-flow.
Enter the hot fluid inlet and outlet temperatures.
Enter the cold fluid inlet and outlet temperatures.
Input the overall heat transfer coefficient U, along with the mass flow rate and specific heat for both fluids.
The calculator computes heat duty Q, LMTD, and required surface area A = Q / (U × LMTD).
Review results including NTU and heat exchanger effectiveness.

Use cases

Sizing shell-and-tube heat exchangers for process plant design.
Checking whether an existing heat exchanger meets a new duty.
Comparing counterflow vs parallel-flow configurations for the same duty.
Quick feasibility estimates during early-stage engineering studies.
Teaching and verifying manual LMTD calculation examples.
Determining required surface area when U and temperatures are known.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu

Heat Exchanger Calculator

Energy Balance Check

Log Mean Temperature Difference

Heat Exchanger Results

Temperature Profile

Summary

How it works

Use cases

Frequently Asked Questions

Heat Exchanger Calculator

Use the Heat Exchanger Calculator

Energy Balance Check

Log Mean Temperature Difference

Heat Exchanger Results

Temperature Profile

Summary

How it works

Use cases

Frequently Asked Questions