Air movers and coolers

Air movers can be generally classified as fans or blowers. Fans are usually axial fans or propellers that produce low-pressure air movement. Blowers, on the other hand, are usually centrifugal devices that deliver air at a higher pressure, usually for a process such as a reaction, as well as cooling. For a discussion of heat exchangers, see When fans are used to cool enclosures, they either draw air through the cabinet, or move it through an air-to-air heat exchanger. Although heat exchangers do not mix inside with outside air, they do pass heat to the environment. However, the technique isolates outside dirt and debris from sensitive components within the heating system. An airtight cabinet without a fan can dissipate heat but may require a large enclosure for a minimum temperature rise. Adding a fan improves the cooling process over convection and radiation alone by up to tenfold. When a fan is required for enclosure cooling, a washable air filter at the incoming air stream is recommended. When using air filters, a plenum must also be used to separate the fan from the filter and to maintain even, effective airflow. If a plenum is not used, the filter is placed against the fan reducing filter efficiency because 90% of the airflow passes through the tips of the fan blade. Carefully positioning the fan in the cabinet can avoid some common cooling pitfalls. First, placing a hot component, such as a transformer, in the incoming airflow may seem good for the transformer but subjects other components to unwanted heat. If possible, hot devices should be located near the exhaust so that their heat dissipates directly to the outside. Also, because axial fans have relatively low static pressure, their exhaust grill is often sized too small. To be safe, the size of the opening