Radiation Heat Transfer in Electronic Equipment
Radiation cooling of electronic components and boxes is not usually a concern to thermal engineers. The three factors that affect cooling by radiation are: the temperature difference between an object and its surroundings, the surface characteristics of t
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Radiation Heat Transfer in Electronic Equipment
5.1 Introduction Radiation cooling of electronic components and boxes is not usually a concern to thermal engineers . The three factors that affect cooling by radiation are: the temperature difference between an object and its surroundings, the surface characteristics of the object and its surroundings, and the view that the object has of its surroundings. Typically , the temperature difference is the delta between the device case and an outer chassis, or an outer chassis and the walls of a room. Since radiation heat transfer is based on this temperature difference, when the delta between a component and its surroundings becomes high enough for radiation cooling to matter, the device is most likely already above its maximum junction temperature . At room temperature , the total emissive power of a perfect emitter is about 450 W/m 2 (142.7 Btu/h ft 2). This is less than 10% of the heat that convection in air can transfer. The characteri stics of the surface that covers the device or the chassis in question are another important variable. Materials used in electronics are normally opaque to radiation . Therefore , the surface characteri stics are important to a depth of only about 2.5 X 10- 6 m (0.0001 in.) for metals and about 0.5 X 10- 3 m (0.02 in.) for nonmetals. In the range of temperatures used for electronics, the color of the surface does not affect the radiation emittance . The third variable is the view factor. This is the fraction of the radiation that leaves one surface and is intercepted by another surface. This can be as high as 1.0 for the case of a sphere inside a larger sphere , or quite low, such as two plates at an angle approaching 180°. Algebraic equations can calculate the view factor, and are usually done with the aid of a computer.
396 R. Remsburg, Advance Thermal Design of Electronic Equipment © Springer Science+Business Media Dordrecht 1998
5.1 Introduction
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Although radiation is not usually a concern when trying to cool an electronic package, the package may absorb radiation heat by being near a high temperature source. Such a case may occur in the engine compartment of a car, where electronic modules are exposed to the radiative heat of hot engine components, and exhaust manifolds. Although the color of an object is not important in radiative cooling, color is important when the object may absorb heat energy from a wide band radiation source. This may occur when we expose an electronic package to the sun.
5.1.1 The Electromagnetic Spectrum Radiation heat transfer occurs by electromagnetic waves, traveling at the speed of light, about 3 X 10 8 mls (9.8 X 108 ftls). These waves may travel through a vacuum or through a gas. Some gases absorb radiation and this must be considered in exact calculations. Examples of gases that affect the radiation transfer are water vapor, carbon dioxide, and air containing a large quantity of particulate matter. Standard air does not affect the results enough to be considered in the power range and tempe
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