Heat is both a byproduct and one of the greatest enemies of electrical and electronic components. If not dissipated, heat has the potential to cause early failures

and malfunctions.

Components commonly packaged in electrical enclosures, computer server racks and other product compartments are the vital controls for the drives and displays used in many industries. The primary purpose of these electrical enclosures is to protect and secure the components they house. If an enclosure is cooled properly, the components within can have a long, useful life. Without proper cooling, however, the components in these enclosures can be subject to damaging heat, shortening their longevity and reliability.

Although manufacturers’ specifications vary, most electrical distribution and control equipment is designed to operate properly and achieve normal life expectancy under ambient air conditions of 104 to 122°F (40 to 50°C). Reducing the operating temperatures within electrical enclosures is an effective way to increase life expectancy and system reliability. If an enclosure is properly cooled, the cost associated with cooling can be recovered over the life of the equipment (figure 1).

Heat Sources in Electrical Enclosures

A number of heat sources can cause the internal temperature of an enclosure to rise to an unacceptable level if heat is not removed. Among them are the working components themselves, ambient air and solar load.

Working Components. The primary source of heat production in electrical enclosures is from the working components. Devices that transmit motive power have voltage drop or efficiency losses that are converted into heat. In the case of electronics or microprocessors, nearly all of their power is converted into heat. The means for calculating and estimating heat generation is available from enclosure cooling manufacturers in the form of spreadsheets or calculators. Heat gain or loss is expressed in watts or BTUs.

Ambient Air. Air outside the enclosure also can be a source of heat gain. In some installations, the ambient air may be cool enough to allow the enclosure to dissipate heat. In many cases, however, ambient air temperature is high, so it adds to the heat load.

Solar Load. When enclosures located outdoors are exposed to direct sunlight, heat is transferred inside the enclosure in a process known as solar load or solar gain. (A commonly experienced example of solar load occurs inside an enclosed automobile parked outdoors on a sunny day.)

The effects of solar load can be significant. Thermal insulation, white reflective paint finishes and a roof or sunshield often can help offset solar load. Double-walled enclosures can be used to mitigate the effects of solar load, but this tends to be a costly process.

Humidity and Air Infiltration. Ambient air entering an electrical enclosure carries both heat and humidity. High relative humidity in the ambient air potentially increases the heat content. Therefore, in most cases, it is best to seal the enclosure to limit this effect. Condensing water vapor or dew, particularly in outdoor enclosures, will damage the electrical and electronic components in an enclosure. It is best to seal up enclosures and feeding conduits completely to avoid any humidity effects or this type of heat gain.

Removing the Heat from Electrical Enclosures

Heat transfer by natural convection is the simplest, most common method of cooling electronics. Relying entirely on hot air rising, however, generally is not sufficient to safely cool sensitive electronics and electrical power-transmitting components. Among the devices used to dissipate heat are heat sinks, blowers and fans, heat exchangers and air conditioners.

Heat Sinks. Often, natural convection is used in conjunction with a heat sink to keep electronics cool. An electronic component mounted on a heat sink helps cool the component by dissipating heat into the air. Such passive thermal management solutions are found in consumer electronics, appliances and systems where enclosures or compartments are not subject to sufficient heat gain to cause a significant heat buildup.

Environmental testing is performed for agency-listed products and mass-produced products to be sure passive heat dissipation will be effective.

Blowers, Fans, Motorized Impellers and Fan Trays. Open-loop cooling systems such as blowers, fans, motorized impellers and fan trays can be used when the surrounding air can be passed over the heat-producing components and exhausted from the enclosure. Fans usually have some type of air filtration. Even applications in the cleanest environments should have minimal protection to prevent airborne particulate from being drawn into the enclosure. Packaged blowers can be used to cool in enclosures with high static-pressure conditions, and motorized impellers and fan trays can be used to cool hot spots, either alone or in conjunction with other cooling systems.

Two airflow options are available when open-loop cooling is used on an enclosure:

• Pressurize the enclosure with positive airflow out of the enclosure.
• Create a vacuum with negative pressure to pull air into the enclosure.

Of the two options, pressurizing via positive airflow is preferred because it utilizes all louvers, cracks and openings in the enclosure as part of the exhaust flow. Creating a vacuum pulls air and possibly dirt in through those same openings.

Open-loop systems are limited in that they can cool only to a certain point above the ambient temperature. In applications where the ambient temperature is too high, or the airflow necessary to provide the required amount of cooling becomes too high, an air conditioner, water-to-air heat exchanger or air-to-air heat exchanger is necessary.

Air Conditioners. Special-purpose air conditioners are recommended where high heat transfer and closed-loop cooling are required. Unlike their comfort cooling counterparts, special-purpose air conditioners are closed-loop systems designed for use in higher ambient conditions. Typical air conditioners have refrigerant-charged compressors that are controlled by a thermostat to limit electrical enclosure temperatures. Initial investment and operating costs are higher than the other enclosure cooling systems mentioned.

Air conditioners should be sized for a maximum allowable temperature from which the BTU/hr rating is determined (figure 2). The maximum enclosure air temperature should be limited to the lowest maximum operating temperature for each specific device. Typically, the most susceptible devices are variable-frequency drives and computers.

Heat Exchangers. Air-to-air heat exchangers use closed-loop cooling, which means they will cool an enclosure without introducing outside air. Heat exchangers are a good choice when it is necessary to remove heat from an enclosure while keeping ambient air out. Air-to-air heat exchangers operate by means of two separate airflows that pass through a convoluted metal-foil element. Enclosure heat is transferred from one side to the other through the element and exhausted to the outside. Heat exchangers have a lower initial cost and generally are less expensive to maintain than air conditioners. However, heat exchangers share the same limitations as open-loop cooling systems.

Water-to-air heat exchangers provide cooling in a closed-loop system when a reliable source of clean, cool or chilled water is available. Models designed primarily for use in harsh environments are available, and they can offer cooling capacities in excess of air-to-air heat exchangers. Water-to-air heat exchangers are particularly useful in highly contaminated environments that would require frequent filter cleaning, filter replacement or cleaning of the heat exchanger core. Water-to-air heat exchangers provide greater heat transfer performance than air-to-air heat exchangers in a compact package.

Enclosure Cooling Additions

Depending on your application, add-ons such as internal heaters and NEMA-rated enclosures can improve the operation of your enclosure cooling setup.

Avoiding Condensation. To avoid condensation buildup, internal heaters are an important accessory to consider when designing a cooling system for an application. In outdoor applications, during the night or off-peak hours, the internal enclosure temperature may drop below the dewpoint. This causes condensation to accumulate on sensitive electronics and electrical contact surfaces, leading to corrosion and, ultimately, failure. To prevent this problem, an internal heater can be used to maintain the temperature of the enclosure. Commonly, heaters are incorporated into air conditioners and offered as individual devices.

NEMA Enclosure Ratings. NEMA enclosure ratings are placed on enclosure cooling units and electrical enclosures to designate the environmental hazard from which the contents are being protected. NEMA defines the standards for different levels of protection of electronics enclosures. Typical examples of NEMA ratings include:

• NEMA 12 for indoor use to provide protection from dust and dripping liquids.
• NEMA 3R for outdoor use and rain-proof applications.
• NEMA 4X for indoor/outdoor use to provide protection from washdown and corrosive environments.

The NEMA rating on an air conditioner should be matched to the NEMA rating on an enclosure being cooled.

In conclusion, the cost of installing an enclosure cooling system designed for a specific application is low when compared to the cost of the overall system. By contrast, the economic and safety-related consequences of improper heat dissipation for systems used in critical infrastructure as well as other possible service interruptions should be considered at the design stage. Lost revenue due to heat-related failures can quickly justify the expense for enclosure cooling.

 It is important to think about cooling in the early stages of the design process, whether designing a new system or retrofitting an existing system. If you configure your enclosure to run cool and dry from the beginning, you can enjoy reliable system operation and avoid the cost and frustration of system failure.  

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