| Forced Air Cooling | ||
| Air has mass and thermal capacity. It transfers energy. It can be compressed. When it's compressed, air transmits hydrostatic pressure. Although it has a rather low specific heat compared to other fluids, air is the most commonly used medium for heat transfer. It is available everywhere on the surface of the planet. Air is usually taken directly from the surrounding atmosphere and returned to it with a revised thermal content. Materials or objects that are cooled or warmed are almost always immersed in air. Even if another medium is used to cool a heat source, the ultimate heat sink is still the atmosphere, and, even in that case, the secondary heat transfer path to the atmosphere is usually a stream of air. |
A standard density figure of 0.075 lb/ft3 is used to rate fan performance. The thermal capacity of air, based on that value, is 0.569 W•min/°C/ft3. For every cubic foot of air per minute in an air stream, 0.569 watts of power dissipated as heat are transferred for an air temperature change of one degree Celsius. The thermal capacity of air, when held at a constant pressure is often expressed as 7.59W•min/°C/lb. |
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| Forced Air Cooling | ||
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An air flow requirement, expressed in CFM (cubic feet per minute) or m3h (cubic meters per hour), is determined by the rate that is needed to maintain the equipment at a certain temperature, not by what is needed to remove a certain heat flux. Equilibrium requires that all power dissipated inside the equipment be removed from it by the heat transfer mechanism, but internal temperatures reached in achieving equilibrium are almost always unknown. The required air flow rate is usually found by experiment, most conveniently by using a source of moving air with variable flow rates. |
This can be as simple as employing a fan with variable speed control in the design location during package development. The fan speed needed for equilibrium and, by implication, the air flow rate, can be found by monitoring temperature at critical locations in equipment during operation. Optimally, determination of air flow requirements follows careful consideration of the thermal aspects of package layout. Placement of components with the greatest need of cooling in locations most favorable for cooling can radically reduce fan size, power and noise. Significant changes in the cooling fan requirements for a system can be made by modifying the air flow circuits and distribution patterns. <more> |
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Forced Air Cooling | |
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The air circuit layout is an important element in determining the pressure drop in air moving through a package. Changes in air flow direction, constriction of the air channel, and use of auxiliary devices such as finger guards, filters and louvers increase the pressure drop in an air stream. |