Posted to News on 23rd Nov 2016, 00:00
Eliminating thermal runaway within electrical enclosures
As engineers look to downsize panels and enclosures, the need for proper heat dissipation has become crucial to keep controls protected. The experts at Vortec offer some insights.
The increased power densities and downsizing of electrical components such as inverters and PLCs means that enclosures can be much smaller to house a given level of functionality. But with this reduction in volume, there is the risk that tightly packed enclosures and panels will restrict airflow, resulting in rapidly rising internal temperatures, thermal runaway and increased 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 ten times greater than those achievable with natural convection and radiation, but even those rates are not adequate to cool electronic components when they are located in many plant environments, where ambient temperatures can often exceed 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 10C° increase in temperature, online production shutdowns occur twice as often, increasing the failure rate of electronics by 40%. Most manufacturers of electronic components specify maximum operating conditions of 40°C and 90% humidity for proper operation.
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, forced-air fan cooling and refrigerant based air conditioning is usually selected by designers because fans 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 hazloc environments, they are very expensive and costly to maintain.
Vortex cooling offers a safe and reliable alternative to the problems with these conventional cooling methods. 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 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 counter-flow through the center of the high speed airstream, giving 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.
The cold air produced is discharged at low pressure and low velocity into the enclosure, while the hot air in the enclosure is vented outside the enclosure box through an integral relief valve. The relief valve, baffling and cooler to enclosure seal maintains the integrity of high IP rated enclosures in ordinary locations.
Vortex cooling is also an ideal solution 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 49°C, vortex coolers are approved for Class I, II and III Division 2 or Zones 2 and 22 locations and can be used in ambient temperatures up to 80°C when used with an approved purge system.
The cooled air that is introduced into the enclosure is filtered and dried to 5 microns before it enters the vortex cooler, creating a clean, cool and controlled environment inside the enclosure and helping to keep controlled processes up and running. An added benefit is that the vortex cooler pro-duces a slight posi-tive 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.
Vortex coolers are available in cooling capacities up to 5000 btuh (1465 watts). All ordinary locations models are controlled via an adjustable electric or mechanical thermostat. Recently introduced electric models are completely self-contained, are ‘plug and play’ and can be top or side mounted and maintain the IP rating of the enclosure. Hazardous locations models are ATEX approved and are regulated with a fixed mechanical thermostat that maintains enclosure temperatures between 24 to 38°C. The typical high pitch noise that most people associate with vortex tubes has been eliminated in all of the newer models using a variety of sound abatement techniques, reducing sound levels as low as 62 dB.
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.
