Heat Exchanger Duty Calculator Formula
Understand the math behind the heat exchanger duty calculator. Each variable explained with a worked example.
Formulas Used
Heat Duty (Q)
duty_kw = mass_flow * cp * abs(temp_in - temp_out)Heat Duty
duty_btu = mass_flow * cp * abs(temp_in - temp_out) * 3412.14Variables
| Variable | Description | Default |
|---|---|---|
mass_flow | Mass Flow Rate (m)(kg/s) | 5 |
cp | Specific Heat (Cp)(kJ/(kg·K)) | 4.18 |
temp_in | Inlet Temperature(°C) | 80 |
temp_out | Outlet Temperature(°C) | 40 |
How It Works
Heat Exchanger Duty
The thermal duty Q is the rate of heat transfer between the hot and cold streams. It is calculated from the energy balance on one side of the exchanger.
Formula
Q = m × Cp × T_in - T_out
where m is the mass flow rate, Cp is the specific heat capacity, and the temperature difference is the change experienced by the fluid. At steady state, the duty calculated on the hot side must equal the cold side (energy balance).
Worked Example
Water flowing at 5 kg/s is cooled from 80°C to 40°C.
- 01delta_T = |80 - 40| = 40°C
- 02Q = 5 × 4.18 × 40 = 836 kW
- 03In BTU/h: 836 × 3412.14 = 2,852,549 BTU/h
Frequently Asked Questions
What is a typical heat exchanger duty range?
Small process heaters may handle 10-100 kW, while large industrial exchangers can handle 1-100 MW. Power plant condensers may handle 500+ MW of thermal duty.
How does phase change affect the calculation?
If the fluid undergoes a phase change (boiling or condensation), you must add the latent heat: Q = m × h_fg, where h_fg is the latent heat of vaporization. The sensible heat formula shown here only applies to single-phase fluids.
What if I know the cold side conditions?
The duty must be the same on both sides (energy conservation). Calculate Q from whichever side you have complete data for. You can then find the unknown temperature or flow rate on the other side.
Ready to run the numbers?
Open Heat Exchanger Duty Calculator