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3. Radiation

When two objects at different temperatures come within proximity of each other, heat is exchanged. This occurs through electromagnetic radiation emitted from one object and absorbed by the other. The hot object will experience a net heat loss and the cold object a net heat gain because of the temperature difference. This is called thermal radiation.

Radiation heat loads are usually considered insignificant when the system is operated in a gaseous environment, since the other passive heat loads are typically much greater in magnitude. Radiation loading is usually significant in systems with small active loads and large temperature differences, especially when operating in a vacuum environment.

The fundamental equation for radiation loading is: Qrad = F e s A (Tamb4 - Tc4)

Where:
Qrad = radiation heat load (W)
F = shape factor (worst case value = 1)
e = emissivity (worst case value = 1)
s = Stefan-Boltzmann constant (5.667 X 10-8W/m2K4)
A = area of cooled surface (m2)
Tamb = Ambient temperature (K)
Tc = TEC cold ceramic temperature (K)

Example calculation: A Charge Coupled Device is being cooled from an ambient temperature of 27°C (300 K) to -50°C (223 K).

The detector surface area (includes 4 edges + top surface) is
8.54 X 10 -4 m2 and has an emissivity of 1. Assume the shape factor = 1

From the equation above:
Qrad = (1)(1) (5.66X10-8 W/m2K4) (8.54 X 10-4 m2) [(300 K)4 - (223 K)4]
= 0.272 W