The following selection process will help determine the size of the fan required for your application.
• Forced air systems can provide much greater heat transfer rates than those available with natural convection and radiation, therefore internal electronic packages have lower hot spot temperatures with forced air systems. The amount of cooling air flowing through an enclosure determines the temperature rise inside the enclosure due to the heat input. The more air that flows through the enclosure, the lower the temperature rise.
• Fans can be used at the exhaust to draw air through an enclosure, or at the inlet to blow air into the enclosure. Generally, a blowing fan at the air inlet is recommended for the following reasons:
1. A fan at the inlet will raise the internal air pressure within the enclosure, which will help to keep dust and dirt out of an enclosure that is unsealed or opened frequently.
2. A blowing fan at the inlet will produce slightly more turbulence, which improves the heat transfer characteristics within the enclosure.
3. Fan life is prolonged since it is located in the path of the entering cooler air.
• The air inlet to the enclosure should be located as far as possible from the air outlet in order to prevent the airstream from short cycling. In a short cycling condition the air leaving the enclosure through the air outlet re-enters the enclosure through the air inlet. This condition results in a reduction in cooling efficiency. In general, it is recommended that the enclosure air inlet be on the side of the enclosure near the bottom and the air outlet be located on the opposite side and near the top.
• Fans should not be located adjacent to an area that restricts the free flow of cooling air. The use of a plenum in front of the fan is a good practice since it improves fan performance. The air velocity must be allowed to develop in order to effectively overcome the flow resistance. When the fan blades are located at the downstream end of the plenum housing, the air has a longer flow path. This improves the air velocity profile and fan performance.
• The enclosure fan system should have an air outlet area at least equal to the air inlet area.
• The system cooling efficiency changes with altitude because of reduced air density. Airflow through an enclosure should be increased when the air density decreases.
• If more than one fan is used in parallel, in the same enclosure, then both fans should be identical.
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