Formula & Calculator
Overall Heat Transfer Coefficient
Combines individual film and fouling resistances into a single coefficient that governs heat transfer rate through a wall.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| U | Overall heat transfer coefficient | W/m2.K |
| hi | Inside film coefficient | W/m2.K |
| ho | Outside film coefficient | W/m2.K |
| Rf | Combined fouling/wall resistance | m2.K/W |
What it means
The overall heat transfer coefficient U is a measure of the total thermal conductance of a heat exchanger, accounting for all resistances to heat transfer. It is defined by the equation 1/U = 1/h_i + (t/k) + 1/h_o + R_f_i + R_f_o, where h_i and h_o are convective heat transfer coefficients on the inside and outside, t is the wall thickness, k is its thermal conductivity, and R_f are fouling resistances (due to deposits). This combined coefficient is used in the equation Q = U A ΔT to compute the heat transfer rate. U is essential for sizing heat exchangers: a higher U means more heat transfer per area, leading to a more compact design. However, U is not constant; it depends on flow conditions, temperatures, and fouling. In practice, U is determined from correlations or measured data. The overall heat transfer coefficient is a central concept in thermal system design, including boilers, condensers, radiators, and refrigeration systems.
Worked example
Overall Heat Transfer Coefficient – Two Examples
Real‑World| Parameter | Value |
|---|---|
| hi | 2000 W/m²K |
| ho | 1500 W/m²K |
| Rf | 0.0002 m²K/W |
| Parameter | Value |
|---|---|
| hi | 1000 |
| ho | 1000 |
| Rf | 0.0005 |
Common mistakes
- Thermal resistances: 1/h_i and 1/h_o are convective resistances; R_f is fouling resistance. Ensure all are in the same units (K·m²/W).
- Heat transfer coefficients: h_i and h_o depend on flow conditions, fluid properties, and geometry – use appropriate correlations.
- Overall coefficient based on area: U is usually based on the outside area; if you use inside area, the equation must be adjusted.
- Fouling factors: These are often given in tables; do not ignore them for design.
- Units: U in W/(m²·K); all resistances must be in K·m²/W.
Applications
The overall heat transfer coefficient U accounts for the total thermal resistance between two fluids in a heat exchanger. Its inverse, 1/U = 1/h_i + 1/h_o + R_f, includes the convective resistances on both sides and any fouling resistance. Engineers use U to size and rate heat exchangers, as it combines all heat transfer contributions into a single parameter. The coefficient depends on flow regimes, fluid properties, and exchanger geometry. Accurate estimation of U is critical for ensuring that the exchanger meets thermal duty while minimising area and cost. Fouling resistance accounts for deposit buildup over time, so designers often add a margin. By understanding U, engineers can predict heat exchanger performance under varying operating conditions and schedule cleaning intervals.
- Design and rating of heat exchangers (shell‑and‑tube, plate, spiral)
- Performance monitoring and fouling diagnosis
- Thermal design of boilers, condensers, and evaporators
- Selection of heat transfer fluids and flow arrangements
- Energy auditing and efficiency improvement projects