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Overall Heat Transfer Coefficients

Reference data and engineering information about overall heat transfer coefficients for heat transfer applications.

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Overview

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

Key Formulas

Fourier's Law

q=−k∇Tq = -k \nabla T

Heat flux proportional to temperature gradient.

Convective Heat Transfer

Q=hA(Ts−T∞)Q = hA(T_s - T_\infty)

Heat transfer between surface and fluid.

Stefan-Boltzmann Law

q=εσT4q = \varepsilon \sigma T^4

Radiative heat flux from a surface.

Thermal Resistance

Rth=LkAR_{th} = \frac{L}{kA}

Resistance to heat conduction.

Variables

Symbol Description Unit
qq Heat flux W/m²
kk Thermal conductivity W/(m·K)
hh Convection coefficient W/(m²·K)
TT Temperature K
ε\varepsilon Emissivity —
σ\sigma Stefan-Boltzmann constant 5.67×10⁻⁸ W/(m²·K⁴)

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  Btuft2⋅hr⋅°F=5.678  Wm2⋅K=4.882  kcalhr⋅m2⋅°C1 \; \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  W/(m2⋅K)U = 13.1 \; \text{W/(m}^2\text{·K)}:

q=U⋅ΔT=13.1  Wm2⋅K×(80°C−20°C)=786  Wm2≈750–800  Wm2q = 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