Heat Exchanger Sizing
Learn how to size a heat exchanger for your application. Understand the key parameters, calculation method, and how to ensure adequate capacity with fouling allowance.
The Sizing Equation
Heat exchanger sizing is based on the fundamental equation: Q = U x A x LMTD. To find the required area (A), rearrange: A = Q / (U x LMTD).
Where Q is the required heat transfer rate (W), U is the overall heat transfer coefficient (W per square metre per degree C), and LMTD is the log mean temperature difference (degrees C).
Step-by-Step Sizing
- Determine the required heat duty (Q). Calculate from the process: Q = mass flow x specific heat x temperature change. For phase change (condensation or evaporation), Q = mass flow x latent heat.
- Select the overall heat transfer coefficient (U). Use published tables based on the fluid combination and exchanger type. Add fouling factors for both sides.
- Calculate the LMTD. Use the inlet and outlet temperatures of both fluids. For counter-flow: LMTD = (deltaT1 - deltaT2) / ln(deltaT1 / deltaT2), where deltaT1 and deltaT2 are the temperature differences at each end.
- Calculate the required area: A = Q / (U x LMTD). Add 10 to 20% safety margin.
- Select an exchanger with at least this area. Consider pressure drop, material compatibility, and maintenance access.
Example Calculation
| Parameter | Value |
|---|---|
| Hot fluid inlet | 90 degrees C |
| Hot fluid outlet | 70 degrees C |
| Cold fluid inlet | 20 degrees C |
| Cold fluid outlet | 60 degrees C |
| Heat duty (Q) | 200 kW |
| U (water to water) | 1,500 W/m2/C |
| LMTD (counter-flow) | 39.1 degrees C |
| Required area | 3.4 square metres |
| With 20% margin | 4.1 square metres |
Always check both sides
The heat lost by the hot fluid must equal the heat gained by the cold fluid (minus losses). If these don't match, one of your flow rates or temperatures is wrong. Always verify the energy balance before finalising the sizing.
Tip: For cooling applications, check the approach temperature (the difference between the hot fluid outlet and the cold fluid inlet). A smaller approach requires more area. If your approach is less than 2 degrees C, the exchanger will be very large and expensive. Consider increasing the cold fluid flow to improve the approach.