Think About Cabinet Cooling Early
Cooling needs should be evaluated early in the design process of any system. Nearly all systems require some degree of forced cooling. Early estimates of the location of components in the cabinet, the heat to be dissipated and the amount of space needed for the cooling device itself will save you time, trouble and expense.
Keep it simple. If ambient air is cool and clean enough, use it. It's free. If the ambient is too hot, dirty or corrosive, then a closed-loop system is needed. A heat exchanger is usually a lower-cost choice than an air-conditioner. See if it will do the job. Remember not to overcool. While you don't want to oversize the cooling equipment, you don't want to undersize it, either.
Propeller fans are designed to move large volumes of air at low static pressure. Blowers are used in higher static-pressure applications and are at maximum efficiency when operating near their peak static pressure.
Pressurize, Don't VacuumizePressurizing the cabinet is far more desirable than drawing out the air. Plan to pump filtered air into the cabinet. This lets you gain the advantage of using cracks between panels and around doors or other small openings as part of the exhaust area rather than as sources for the intake of dust and dirt. If pressurization is impossible and a fan or blower must be used to exhaust the enclosure, a filter at the air inlet is recommended.
Nothing is more important than clean filters. Clogged filters restrict airflow and cause motors, compressors and other components to work harder and fail prematurely. Timely filter servicing is vital to your system.
Be sure that all the exhaust area is located downstream, as far as possible from the air inlet and beyond all heat-producing components. An open-base cabinet sitting only one-quarter inch off the floor can waste a substantial percentage of cooling air even if the air is directed upward initially. A properly planned air path avoids all "short circuits" or losses by forcing the cool supply air to pass through the components that are to be cooled before reaching the exhaust area. This allows for maximum cooling efficiency.
Let Nature HelpCooling air should enter the enclosure from as low as possible and leave the enclosure from above the highest hot component. Thus, the forced air flows upward through the heat-producing components and adds to the natural buoyancy of the heated air.
A booster fan located downstream or at the outlet can draw added cooling power through densely packed components. It could permit the use of a smaller, quieter packaged blower than originally indicated, allowing more panel space for other uses.
Entering air needs exit room. The cross-section area of the airstream throughout the flow path in the cabinet should be at least equal to the effective area of the air intake. If the ratio is less, "choking" of the delivered air may result. On the basis of the average grill material, with 65 percent open or effective area, table 1 gives the typical effective area that should be allowed for discharge.
Use ducts for even cooling. If maintaining an even temperature from top to bottom of the enclosure is important, ducts along the sides of the enclosure offer a solution. Multiple duct outlets allow precise control of the location and quantity of air delivered.
Where possible, locate heat-sensitive electrical components toward the bottom of the enclosure because the warmest air temperatures will be at the top. Maintain adequate spacing between components within the enclosure to minimize airflow restriction.
Sometimes baffles work wonders. At times, an excessively hot component or an isolated area in the enclosure presents a problem in an otherwise well-cooled system. A baffle to channel air across the location may be the best solution.
Neoprene vibration isolators minimize the possibility of trouble associated with vibration. All portions of a system will respond to periodic forces in varying degrees. This excitation can occur regardless of the balance or design of the air-moving equipment, because any given construction could be in harmony with any of the driving forces in the blower motor.
This article was contributed by Kooltronic Inc., Pennington, N.J., a manufacturer forced ventilation and closed-loop cooling for enclosures containing electronic, industrial and other heat-producing equipment. For more information from Kooltronic, call (609) 466-3400 or visit www.kooltronic.com.