There is no dispute that power densities have increased as enclosure volumes have gotten smaller. Packing components more densely reduces the circuit size and increases speed, but it leaves little room for heat dissipation. At the same time, manufacturers involved in food, chemical, water and wastewater processing, oil refining and petrochemical processing, among others, have become more dependent on sophisticated microprocessors, programmable logic controllers (PLCs) and variable-frequency drives (VFDs). As a result, the need for proper heat dissipation has become crucial to keep controls protected. Tightly packed enclosures and panels restrict airflow, resulting in rapidly rising internal temperatures, thermal runaway and increasing control failures.
Thermal testing has proven that natural convection cooling is not adequate for today’s smaller, high power-density enclosures. Heat dissipation by forced convection (fan cooling) is the most frequently used method of cooling. Forced-air cooling systems can provide heat transfer rates that are 10 times greater than those achievable with natural convection and radiation. Even those rates, however, are not adequate to cool electronic components when they are located in industrial plant environments, where ambient temperatures often can exceed 90°F (32°C).
To reduce enclosure temperatures and prevent failure of high density controls, the internal enclosure temperature must be lowered to below the room temperature. Research by control-system manufacturers has shown that for each 18°F (10°C) increase in temperature, online production shutdowns occur twice as often, increasing the failure rate of electronics by 40 percent. Most manufacturers of electronic components specify maximum operating conditions of 104°F (40°C) and 90 percent humidity for proper operation.
Enclosure Cooling Methods
The never-ending drive to reduce the cost and size of electronics while increasing speed and complexity has created a significant design dilemma. For enclosures located in ordinary locations (NEMA 12, 4 and 4X environments, for example), the designers select forced-air fan cooling or refrigerant-based air-conditioning.
Fans often are selected because they are relatively inexpensive and easy to install. Unfortunately, the factory air pulled into the enclosure by fans often contains just enough nearly invisible oil aerosols or other contaminants to coat the surfaces of sensitive, expensive electronic boards in control enclosures.
Drawbacks of refrigerant-based air-conditioning include limits on maximum ambient temperature, large physical size, maintenance and high initial cost.
For enclosures located in hazardous locations, cooling solutions are limited to just a few types of technologies. While refrigerant-based models can be built for hazardous location (HAZLOC) environments, they are expensive to purchase and costly to maintain.
Vortex cooling offers safe and reliable alternatives to these conventional cooling methods.
Enclosure Cooling with Vortex Coolers
A vortex enclosure cooler uses a vortex tube to convert a filtered, compressed air supply into refrigerated air without the use of electricity, ammonia or other refrigerants. The vortex tube creates cold and hot air by forcing compressed air through a generation chamber that spins the air centrifugally along the inner walls of the tube at a high rate of speed (1,000,000 rpm) toward a control valve. A small percentage of the hot, high speed air is permitted to exit at the control valve. The remainder of the — now slower — airstream is forced to counterflow through the center of the high speed airstream. The counterflow airstream gives up heat as it travels through the center of the generation chamber before it finally exits through the opposite end as cold air. There are no moving parts in a vortex tube, so the systems are reliable, inherently safe and have low maintenance requirements (figure 1).
The cold air produced is discharged at low pressure and low velocity into the enclosure. The hot air in the enclosure is vented outside of the enclosure box through an integral relief valve. The relief valve, baffling and cooler-to-enclosure seal maintain the integrity of NEMA-rated boxes in ordinary locations.
Vortex cooling also is suitable for cooling enclosures located in hazardous locations because they are inherently safe when used in areas with temperature classifications of T4 or higher. There are no electrical requirements and no moving parts to generate electrical charges. The only potential ignition source is the hot surface at the hot exhaust. When supplied with compressed air that does not exceed 120°F (49°C), vortex coolers are approved for Class I, II and III Division 2 or Zones 2 and 22 locations. They can be used in ambient temperatures up to 175°F (80°C) when used with an approved purge system.
The cooled air that is introduced into the enclosure is filtered and dried to 5 µm before it enters the vortex cooler. This creates a clean, cool and controlled environment inside the enclosure and helps keep controlled processes up and running. An added benefit is that the vortex cooler produces a slight positive pressure inside the enclosure to keep out dust and dirt. Hazardous-location models rely on a purge system to maintain safe enclosure pressures when the vortex cooler is not operating. An integral check valve keeps the enclosure sealed when the unit is not cooling, so the purge system maintains enclosure pressure (figure 2).
In conclusion, compact, multifunction electronic controls, VFDs, servos and PLCs are extremely sensitive to heat and contamination. Excessive heat causes components to “cook,” digital displays to misread, controls to drift and breakers to trip below their rated loads. The result often is lost productivity from machines and production line shutdowns.
Vortex coolers offer a solution. By using an internal vortex tube to convert factory compressed air into a clean, dry, low pressure, cold airstream that is distributed throughout the enclosure, these systems provide efficient, safe and reliable enclosure protection from heat- and dirt-related problems for electronics in ordinary and hazardous locations. PC