What causes all that heat? There are basically two sources of heat which cause an electrical enclosure’s internal temperature to rise above the manufacturer’s recommended ratings for the control equipment: internal and external sources.
Internal Sources
The same items which can be damaged by heat may also be a key source of the heat. These include items such as:
• Power supplies
• Servos
• AC Drives/inverters
• Soft starters
• Transformers
• PLC systems
• Communication products
• HMI systems
• Battery back-up systems
External Sources
Sources of heat which can cause the internal temperature of your enclosure to rise above a desired level involve the external environment. These include items such as:
• Industrial ovens
• Solar heat gain
• Foundry equipment
• Blast furnaces
How do you reduce internal temperature of your enclosure?
There are three basic cooling methods to reduce the internal temperature of your enclosure: natural and forced convection, and closed-loop cooling.
Natural Convection Cooling If the ambient temperature outside the enclosure is cooler than the inside of the enclosure, then the heat can be dissipated into the atmosphere by radiating it through the surface of the enclosure and with louvers or grilles with filters. (Image 1)
Forced Convection Cooling If you have clean and cool ambient air outside of the enclosure, then a simple forced-air system may be adequate. A system such as a filter fan and the associated grille with the appropriate filter may be an acceptable option. (Image 2)
Closed Loop Cooling While natural and forced convection cooling are typically less expensive, they are very limited in their effectiveness to reduce the temperature to an acceptable level. A more typical choice, in these cases, is an air conditioning unit that provides closed loop cooling.
Closed-loop cooling is designed to keep the ambient air separate from the internal enclosure air. This system is needed for:
• Harsh environments
• Washdown requirements
• Heavy dust and debris
• The presence of air-borne chemicals
• Ambient temperatures as high as or higher than the desired internal temperature (Image 3)
How do air conditioners work?
Air conditioners, by name, intimate that they are producing cold air, when they are simply transferring the hot air away from the inside of the enclosure. The air being blown into the enclosure has the heat removed; it is colder than the internal air. There are two basic systems in an air conditioner. First there is the compressor, refrigerant, evaporator coil, and condenser coil system which transfers the heat to the outside of the enclosure. 
The second system is an air moving system which is comprised of an evaporator fan plus a condenser fan. The brain of the system is very similar to a typical home unit using a thermostat. The thermostat monitors the temperature inside the enclosure and will start the refrigeration cycle when the temperature reaches the setpoint. Typically the setpoint is 95 degrees Fahrenheit (typically the coolest temperature desired) and normally the thermostat has a differential setting of +5 degrees F, so the refrigeration cycle will begin at 100 degrees F.
The evaporator fan runs continuously to keep the air circulating inside the enclosure, allowing the internal temperature to be monitored by the thermostat. When the temperature reaches the setpoint the compressor will turn on and start the refrigerant flowing. The refrigerant will flow through the evaporator coils and collect heat from inside the enclosure. The condenser fan will blow across the coils to transfer the heat outside the enclosure.
During this cycle, humidity is also removed from inside the enclosure. Most air conditioners have either a drip tube to collect and remove the condensation or a condensate removal system to burn off the condensate outside of the enclosure. (Image 4)
How are air conditioners rated?
Air conditioners are rated by their cooling capacity; this is the maximum amount of heat energy an air conditioner can remove. They are expressed in BTUs per hour in the US and Watts per hour in other parts of the world. A BTU is defined as the amount of heat energy needed to raise the temperature of one pound of water by one degree Fahrenheit in one hour.
Manufacturers typically use different points on the performance charts to declare the BTU rating of their units, but use the maximum outside temperature for which the unit is rated (i.e.125°F). Some manufacturers will use a point on the performance chart of 95 degree F internal temperature and 95 degree F external (ambient) temperature to compare the efficiency of their units to other manufacturers’ air conditioning units.
How do you choose the right air conditioner?
The best way to select the right-sized air conditioner for your enclosure is an online calculator, such as (https://ftp.automationdirect.com/pub/stratus_ac_btuh_calculator.zip) to determine the BTU rating needed; then use the performance charts supplied by the manufacturers to determine what unit would best meet your actual needs.
After you have determined the BTU size needed, determine the type of environment in which it will be located, such as NEMA 12, 4, or 4X; select the appropriate voltage rating and that will lead you to the proper unit.
By Lenny Filipkowski,
AutomationDirect
Product Manager,
Industrial Components
Originally Posted: Sept 1, 2010