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A Reader Writes:
We specify coolers for enclosures that end up in a wide range of ambient temperatures and humidities. My boss says we should simplify operations by using the same refrigeration unit, but this means in some cases the unit will have much more than the calculated minimum capacity. What problems could this cause? How could we avoid them?
--from March 2003 CONTROL
Stop at the Worst-Case Scenario
When calculating cooling needs, it is essential that the worst-case scenario be used to determine the size of the cooling unit. Electrical and electronic equipment must be protected from severe high temperatures (even for brief periods of time) and dramatic temperature changes. This may only occur at certain times or when certain pieces of equipment are running. This inevitably leads to a cooling unit that appears to be oversized for the application.
...Standardizing the AC unit across various applications may result in a very large cooling unit on a low cooling-need cabinet. The result is a cooling unit that would run for very short periods of time. The short run time may not be enough for the unit to remove humidity. Humidity is as much a problem as high temperatures and can damage equipment.
...Cycling a cooling device more often (turning it off and on frequently) leads to premature failure and increased maintenance. Having an oversized or undersized cooling unit leads to wasted power (added power costs), decreased cooling performance, reduced equipment life, and additional maintenance needs.
...Each application has its challenges and must be researched to provide the most economical cooling solution. Sizing cooling equipment often requires trade-offs (cabinet size, position from external heat sources, etc.) Cooling equipment is based on worst-case scenarios to prevent elevated temperature from occurring.
...Too large a cooling unit leads to inadequate dehumidifying and decreased cooling equipment life. But too small a unit will stress the cooling equipment and possibly not be able to handle worst-case cooling needs. Reserve capacity is often added to the cooling needs as insurance and to provide for added future additions of heat-producing equipment. Though standardization may prove to be economical up-front, longterm the costs are much higher.
Brian Mordick, Product Manager, Thermal Products
Hoffman Enclosure Co., www.hoffmanonline.com
Beware Overtemperatures and Alarm Trips
For best results, be careful not to oversize the unit. Be certain that both the evaporator and condenser air flow paths cannot short circuit or are impeded. Be cautious of adding protective covers to the outside of the unit that may reduce air flow and unit thermal performance. Seal the electrical enclosure to prevent humidity and outside air from entering. Closed-loop enclosure cooling is the goal.
...This said, it is inevitable that one size (capacity) air conditioner unit may be called upon to handle all of the applications for a user's product line.
...The factory low-temperature cut-out setting (the temperature at which the compressor will shut off) is typically 75° F. Overcooling the electrical components can become a problem in some circumstances. The immediate danger is of cycling the compressor on too many times per hour. This will shorten the compressor life and result in a sudden failure.
...A compressor short cycling time-delay relay is an option that is available from enclosure air conditioner manufacturers. This will delay the restarting of the compressor in the cases where the enclosure air has been cooled down rapidly and then reheats quickly due to the internal and external heat loads.
...Unfortunately, large swings in the enclosure temperature can result as a unit cools down the air within the enclosure. Sunlight, ambient, and internal heat sources will quickly raise the temperature of the relatively small amount of recirculated air within the enclosure. Although the thermostat has a differential setting that will allow the enclosure temperature to rise 10-15° F before it restarts the compressor, this can occur rapidly. The time delay relay will prevent this but allow the internal temperature to rise even further. At times, systems with alarms can have nuisance trips as the temperature can spike several times an hour.
...Also, critical electronic components will be exposed to wide swings in temperature and relative humidity. In cases where outside air leaks into a cabinet, condensation could form when surfaces are cooled below the dewpoint of the outside air that is leaking inwards.
...Sizing should be limited to a reasonable factor of safety. This is recommended to be not more than 25% above the estimated worst-case loads taking solar, internal, and ambient temperature heat gain into account. The enclosure surface area and thermal insulation (if used) also should be a part of the load calculation. Calculation assistance can be found on most manufacturers web sites and in catalogs.
Bruce K. Kreeley, Director of Engineering
Many Aspects Can Be Standardized
If you only use one refrigeration unit, you will end up with too much capacity on some enclosures. Take air conditioners as an example. If you select one air conditioner for all of your enclosure cooling needs, you will have to size the unit for your worst case, which is the hottest ambient temperatures and the highest heat loads.
...For one application, this might be a great solution, for another application the unit may be significantly larger than required. From a performance point of view, an oversized unit will perform in the application. The unit will cool rapidly, but will be idle for long periods of time between cycles. Some potential issues for oversizing an air conditioner are:
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