Sizing a special purpose enclosure air conditioner for an indoor application is often relatively straightforward as the weather or climate is not going to have a major impact on the sizing or selection process. However, outdoor applications present a set of environmental factors that must be addressed before a cooling option can be chosen. Accounting for these conditions is important to make sure a cooling unit is properly sized for the thermal load, and therefore, will prevent the internal components from overheating. Here, thinking outside the box truly matters! Below are special considerations that will have a great impact on selecting the appropriate cooling option in outdoor locations:
Ambient Air
While this problem is not exclusive to outdoor applications, warmer outdoor air surrounding your enclosure can be a potential source of heat gain within an enclosure. Overall, as ambient temperatures rise, the air conditioner’s capacity to dispel heat is diminished. Some examples include: an enclosure located on or near a paved parking lot, the rooftop of a building, or close to other machinery that may be giving off heat. In these areas, the ambient air may be so hot that it adds to the heat load, and therefore, reduces the ability of the cooling device to cool the air inside the enclosure.
Solar Load
When enclosures are located outdoors and exposed to the sun, heat will be transferred to the inside of the enclosure; this is known as solar load or solar gain. The effects of solar load can be significant. For example, when a car is parked outside on a sunny day, one will notice a dramatic rise in temperature inside the vehicle, even after a relatively short period of time. Since solar load has this same effect on an enclosure, thermal insulation, white reflective paint finishes, and a roof or sunshield can be used to help offset increased heat load. Some enclosures are double walled for this purpose, however this can be a very costly feature.
NEMA Rating
The National Electrical Manufacturers Association (NEMA) has set standards, or ratings for various types of electrical enclosures, most commonly used in industrial environments. These applications can be subject to dust, dripping liquids, rain, washdown, and corrosive atmospheres, and would typically utilize an air conditioner specifically designed for these conditions. The minimum rating for an outdoor electrical enclosure would be NEMA type 3R for use in an application that may be subject to falling liquid or light splashing, such as rain. The maximum rating for an outdoor enclosure would be NEMA type 4X to provide protection from dust, washdown and corrosive environments. The NEMA rating of the special purpose air conditioner must be equal to or greater than the NEMA rating of the electrical enclosure to maintain the enclosure NEMA rating.
Special Purpose Air Conditioner Performance
The heat load that must be removed by a special purpose air conditioner varies depending on the ambient air, solar load and the components contained within the enclosure. The external temperature differential surrounding the enclosure and the operational state of the internal components will directly impact the rate of cycling that the air conditioner will undergo. Rapid cycling of the compressor in a special purpose air conditioner can lead to a reduced life expectancy. This has to be taken into account as the enclosure temperature will not be constant when the air conditioner cycles on the thermostat. In order for the air conditioner to perform as intended and not exceed the recommended cycling rate of 6-8 cycles per hour, it is important to make sure the unit is sized properly for the application.
When considering all of the factors that go into determining the correct cooling unit for an application, online sizing programs can be a helpful tool. These all-encompassing programs have numerous inputs which account for the different conditions that may be experienced with the application and generate the best cooling product based on the environmental conditions present. It is important to use the most extreme operating conditions, such as maximum load and maximum ambient temperature when inputting data into the program so the internal components are protected regardless of the environmental conditions the application may experience.
