Heat Exchanger Calculator
Sensible Heat Duty Is the Starting Point, Not the Whole Design
Before an engineer sizes a heat exchanger's physical area, tube count, or plate arrangement, there's a simpler upstream question: how much heat actually needs to move from one fluid stream to another per unit time? That number — the heat duty — is what every downstream sizing calculation is built on top of.
The Formula
This is the sensible heat equation: Q is the heat duty, m is the mass flow rate of the fluid, Cp is its specific heat capacity, and ΔT is the temperature change the fluid undergoes. In metric units (kg/s, kJ/(kg·K), °C or K), Q comes out directly in kilowatts; in imperial units (lb/hr, Btu/(lb·°F), °F), Q comes out in Btu per hour.
Where This Calculation Matters
- Process heating and cooling — any system that heats or cools a fluid stream, from a building's hot water loop to an industrial process cooler, starts its sizing from this duty calculation.
- HVAC coil sizing — air handling coils are specified by the heat duty they need to deliver, which is worked out the same way using the air stream's mass flow, specific heat, and temperature rise or drop.
- Preliminary exchanger selection — before running a full LMTD or effectiveness-NTU analysis, engineers use the basic duty figure to shortlist exchanger types and sizes from a manufacturer's catalog.
Worked Example
Heating a water stream flowing at 2 kg/s (specific heat 4.186 kJ/(kg·K)) through a 20°C temperature rise:
| Variable | Value |
|---|---|
| Mass flow rate (m) | 2 kg/s |
| Specific heat (Cp) | 4.186 kJ/(kg·K) |
| Temperature change (ΔT) | 20°C |
| Heat duty (Q) | 167.44 kW |
This is a basic sensible-heat duty estimate. It does not size the physical heat transfer area (no LMTD or overall U-value calculation) and does not account for phase change such as boiling or condensing.
How to Use This Calculator
- Select Metric (kg/s, kJ/kg·K, °C) or Imperial (lb/hr, Btu/lb·°F, °F) units.
- Enter the Mass Flow Rate of the fluid.
- Enter the fluid's Specific Heat Cp.
- Enter the Temperature Change ΔT the fluid undergoes.
- Select Calculate to see the resulting heat duty.
Related Calculations
Sizing the structural elements around mechanical equipment? The Structural Beam Calculator and I-Beam Calculator cover the load-bearing side of a mechanical room design.