Table of Contents
Overall Heat Transfer Coefficient
The overall heat transfer coefficient (U-value) combines all thermal resistances in series — convection films on both sides and conduction through the wall — into a single coefficient. It represents the total heat transfer rate per unit area per degree of temperature difference between the two fluids.
In heat exchanger design, the U-value determines the required surface area for a given heat duty. A higher U-value means more efficient heat transfer, requiring less surface area. The U-value is limited by the largest thermal resistance in the system, which is often the gas-side film coefficient.
U-Value Formula
Typical U-Values
| Heat Exchanger Type | U (W/(m²·K)) |
|---|---|
| Gas-to-gas | 10-40 |
| Gas-to-liquid | 20-300 |
| Liquid-to-liquid | 150-1200 |
| Condensing steam-to-water | 1000-6000 |
Frequently Asked Questions
Why is the gas-side coefficient usually the bottleneck?
Gases have much lower thermal conductivity and density than liquids, giving convection coefficients of 5-250 W/(m²·K) compared to 100-10,000 for liquids. Since resistances add in series and U is limited by the largest resistance, improving the gas-side coefficient (with fins, turbulence) has the greatest impact on overall performance.
What about fouling resistance?
In real heat exchangers, deposits build up on surfaces over time, adding thermal resistance called fouling. Design calculations include fouling factors (typically 0.0001-0.001 m²K/W) added to the total resistance. Fouling can reduce heat exchanger performance by 10-30% and must be considered in sizing.