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4. Convection

When the temperature of a fluid (in this case, a gas) flowing over an object differs from that of the object, heat transfer occurs. The amount of heat transfer varies, depending on the fluid flow rate. Convective heat loads on TECs are generally a result of natural (or free) convection. This is the case when gas flow is not artificially induced as with a fan or pump, but rather occurs naturally from the varying density in the gas caused by the temperature difference between the object being cooled and the gas.

The convective loading on a system is a function of the exposed area and the difference in temperature between this area and the surrounding gas. Convective loading is usually most significant in systems operating in a gaseous environment with small active loads or large temperature differences.

The fundamental equation that describes convective loading is:
Qconv = h A (Tair - Tc)

Qconv = convective heat load (W)
h = convective heat transfer coefficient (W/m2 °C) (typical value for a flat, horizontal plate in air at 1 atm) = 21.7 W/m2 °C
A = exposed surface area (m2)
Tair = temperature of surrounding air (°C)
Tc = temperature of cold surface (°C)

Example calculation: A square plate is being cooled from 25°C to 5°C. The top and four sides are exposed surfaces. The plate is 0.006 meters thick and each side is 0.1 meters long.

From the convection equation:
Qconv = (21.7 W/m2°C) (0.0124 m2)(25°C - 5°C)
= 5.4 W

It is very important to avoid allowing condensation to form when cooling below the dew point. This problem may be avoided by enclosing the cooling system in a dry gas or a vacuum environment.