Heat inside electronic enclosures is a hazard that is often underestimated. Air-conditioning electronic enclosures can ensure a constant level of operational reliability and prolong service life for electronic systems under difficult ambient conditions.

Figure 1. Figure 1 maps the service life of electronic enclosure components relative to temperature. This logarithmic function is governed by the operating temperature.
Many control systems in modern electronic equipment are based on semiconductor components and computer technology. These components are being packed inside enclosures in higher and higher densities, resulting in higher rates of heat dissipation and less natural dissipation through the enclosure's surface area. Studies have shown that as the temperature stress on these components increases, their life expectancy decreases (figure 1).

Figure 2. In any enclosure, air temperature near the top of the space is higher than that around the base. External heat sources can magnify internal temperature.
An electronic enclosure is nothing more than an enclosed space. As inside a room, inside an enclosure, the air temperature near the top of the enclosed space is higher than the air temperature around the base (figure 2). Because of this temperature difference, a slight airflow - called natural convection - occurs. As heat-generating components such as transformers are installed in the enclosure, the air temperature will rise. Due to the limited heat-transmission capabilities of the enclosure's surfaces, it may not be possible for natural convection alone to extract the heat within the cabinet. So, the temperature of the air inside rises.

Hot spots are areas inside the electronic enclosure that are not involved in natural convection. Formed mainly around the top of the electronic enclosure, hot spots occur due to undercuts or installations that prevent an exchange of air. A rise in ambient temperature can intensify the severity of hot spots.

In addition, external heat sources located near the enclosure can cause problems inside the enclosure. An open area, large orifices or a missing enclosure panel can produce avenues for external heat, dust and humidity to enter an enclosure. External heat sources also can magnify internal temperature.

If the temperature in the area surrounding the component rises to its maximum operating temperature, critical point is reached. If a component inside an enclosure reaches critical point, the only recourse is to open the enclosure. This is only a short-term solution. Active cooling via a closed-loop air conditioner, however, can prevent this type of crisis.

Figure 3. Humidity must be extracted from the enclosure or it may condense on the components.

Humidity Inside the Enclosure

Humidity is another enemy of electronic equipment. As the humidity levels inside an enclosure increase, the life expectancy of the components inside the enclosure decreases (figure 3). An air conditioner reduces the humidity level by converting the warm air inside the enclosure into cold air. This encourages the accumulation of condensation inside the air conditioner and dehumidifies the air inside the electronic enclosure.

At the beginning of the cooling process, the air conditioner produces a relatively high amount of condensation, which is collected and burned off into the atmosphere or released through a condensate drain line. Condensate must be extracted from the unit or it may cause damage to the electronic equipment. After a few hours of operation, the humidity inside the electronic enclosure adjusts to a constant level, lower than that outside the electronic enclosure.

Figure 4. When the air temperature inside an enclosure drops below the dewpoint, humidity can condense on the inside surfaces of the cabinet.
If wide fluctuations in ambient temperature occur, condensation can accumulate (figure 4). Dewpoint is a term used to express the precipitation of water from the relative humidity. If temperature drops below dewpoint, humidity can condense on the inside surface of the cabinet. This is one reason why the temperature setting inside an electronic enclosure is set relatively high - at approximately 95°F (35°C).

Operational Issues

An electronic enclosure air conditioner incorporates two airflow circuits: internal and external. The internal circuit cools the inside of the electronic enclosure. (figure 5). Thermal energy is extracted from the air inside the electronic enclosure by the coolant in the refrigerant circuit. With the aid of the condenser, the coolant transports the thermal energy into the external airflow circuit, where the coolant is cooled to release the thermal energy.

Correctly sizing an air conditioner requires precise information about the installed power components' heat dissipation, how the components are mounted and the type of enclosure to be used. Finally, the ambient conditions, including the dust or emission content in the ambient air, relative humidity level and known fluctuations in ambient temperatures, should be described.

Figure 5. Whether side- or top-mounted, an air conditioner is used to extract thermal energy. A refrigeration circuit is used to transport the heat to an external airflow circuit.
In addition, it is important to ensure satisfactory airflow in each enclosure compartment. Installations should not be placed within 8" of air inlet and outlet orifices. Also, consider the built-in cooling supplied on individual components. If the airflow volume of components with large, built-in fans exceeds that of the cooling system, or if the airflow circuits counteract each other during operation, refrigeration capacity is reduced and heat pockets can form.