Overall Heat Transfer Coefficients

Overview

Engineering reference data for Overall Heat Transfer Coefficients in heat transfer.

Key Formulas

Fourier's Law

$q = -k \nabla T$

Heat flux proportional to temperature gradient.

Convective Heat Transfer

$Q = hA(T_s - T_\infty)$

Heat transfer between surface and fluid.

Stefan-Boltzmann Law

$q = \varepsilon \sigma T^4$

Radiative heat flux from a surface.

Thermal Resistance

$R_{th} = \frac{L}{kA}$

Resistance to heat conduction.

Variables

Overall Heat Transfer Coefficient Reference Table

The following table provides rough average values of the overall heat transfer coefficient U for common fluid and material combinations under practically still fluid conditions.

Unit Conversion

The overall heat transfer coefficient can be converted between unit systems using:

$1 \; \frac{\text{Btu}}{\text{ft}^2 \cdot \text{hr} \cdot °\text{F}} = 5.678 \; \frac{\text{W}}{\text{m}^2 \cdot \text{K}} = 4.882 \; \frac{\text{kcal}}{\text{hr} \cdot \text{m}^2 \cdot °\text{C}}$

Practical Example: Water-to-Air Copper Heat Exchanger

Estimate the heat flux for a copper heat exchanger with water at a mean temperature of 80°C on one side and air at 20°C on the other. Using $U = 13.1 \; \text{W/(m}^2\text{·K)}$:

$q = U \cdot \Delta T = 13.1 \; \frac{\text{W}}{\text{m}^2 \cdot \text{K}} \times (80°C - 20°C) = 786 \; \frac{\text{W}}{\text{m}^2} \approx 750\text{–}800 \; \frac{\text{W}}{\text{m}^2}$

Important Considerations

The tabulated values are rough estimates for still or low-velocity fluids. Actual U values depend on:

• Fluid velocities and flow regime (laminar vs. turbulent)
• Fluid viscosities and thermal properties
• Condition of the heating surfaces (fouling, scaling)
• Temperature difference magnitude
• Heat exchanger geometry

For precise engineering calculations, always verify with manufacturer data or detailed thermal analysis.

References

• Original Source