Overall heat-transfer rate
A practical interpretation of the U–A–ΔT relation for preliminary heat-exchanger or wall-transfer estimates.
ENGINEERING PRINCIPLE
The overall coefficient represents combined conduction and convection resistances; heat rate scales with area and driving temperature difference.
Q = UAΔTVariables and units
- Q = heat-transfer rate (W)
- U = overall coefficient (W/m²·K)
- A = transfer area (m²)
- ΔT = selected mean temperature difference (K)
Worked example
With U = 250 W/m²·K, A = 4 m² and ΔT = 30 K, Q = 30,000 W = 30 kW.
Calculation method
- Define the physical heat-transfer surface.
- Obtain U from an appropriate resistance or tested-equipment basis.
- Use a representative mean temperature difference and calculate Q.
Practical applications
- Heat exchangers
- Jacketed vessels
- Insulated walls and process equipment
Review boundary and reference
- Fouling, phase change, temperature profiles and transient operation can change U and ΔT.
- Do not use a single ΔT where an LMTD or correction factor is needed.
Formula and displayed units independently checked for the stated conditions. This is not a code-compliance design method.
Source: F. P. Incropera et al., Fundamentals of Heat and Mass Transfer, overall heat-transfer relation.