Keeping enclosures conditioned

Maintaining temperatures inside enclosures protects electrical and electronic equipment. However, equipment heating and cooling options have their pros and cons.

10/02/2015


Figure 1: Heat exchangers, such as the gray unit on the side of the enclosure in the photo, can be used only for installations where the highest ambient temperature is cooler than the maximum desired enclosure temperature. Courtesy: AutomationDirectWhen automation system enclosures are installed in industrial facilities and plants, cooling and/or heating thermal management systems are often required. These systems protect the automation components installed therein from temperature extremes and moisture buildup. Consistent temperatures are required to ensure optimal operating conditions. Otherwise, cyclic broad temperature fluctuations may significantly reduce the life expectancy of electronic components.

Enclosure thermal management systems provide the required operating conditions through the use of heaters, fans, heat exchangers, air conditioners, vortex coolers, thermoelectric coolers, thermostats, hygrostats, and other components. 

Enclosure heating

Enclosure heating systems are simple compared to their cooling counterparts. Enclosure heating is needed to protect components from moisture buildup caused by condensation, which may cause component failure due to changing contact resistances, flashovers, creepage currents, reduced insulation properties, and corrosion. While the greatest risk lies with equipment exposed to high humidity or extreme variations in temperature—as may be found in outdoor installations and poorly insulated enclosures—moisture buildup can also occur in highly protected and well-sealed enclosures under certain climatic conditions.

Enclosure heating systems usually consist of an electric heater installed at or near the bottom of the enclosure. Larger enclosures with heating requirements of more than 100 W may employ a heater with an integral fan to circulate the heat throughout the enclosure.

To prevent overheating of the electronic components, heaters are typically controlled by a normally closed thermostat or hygrostat. The control device may be integral to the heater or a separate device. Thermostats for heaters are available in fixed temperature (preset on/off temperatures) and adjustable temperature models.

While only some enclosures require heating, many more need cooling because the components housed therein often generate significant amounts of heat. 

Problems with excessive heat

Excessive heat inside an enclosure can have many destructive consequences including:

  • Decreased life expectancy of expensive control equipment, such as programmable logic controllers (PLCs), human-machine interfaces (HMIs), and ac drives
  • Nuisance faults in electrical and electronic components, such as overloads tripping unexpectedly
  • Erratic performance of circuit breakers and fuses, which can cause entire systems to shut down unexpectedly.

The same components that can be damaged by heat may also be the source of the heat. These components include, but aren't limited to, power supplies, ac drives/inverters, transformers, communication devices, battery backup systems, soft starters, PLCs, and HMIs. The heat from these internal sources may be augmented by external heat sources, such as high ambient temperatures, solar heat gain, and external heat-producing equipment, such as nearby ovens and furnaces.

There are many options for enclosure cooling systems. Choosing the right one depends on a proper understanding of the enclosure protection requirements, the equipment inside the enclosure, and the surrounding environment. Fortunately, there are a number of options for dealing with internal and external heat sources. 

Convection cooling

"Convection" is the term for heat transfer occurring between a moving fluid or gas (typically air in this article) and a solid surface where there is a temperature difference between the fluid and the surface. In everyday terms, convection is why it feels colder on windy days than when the air is calm.

Convection is complex to quantify, but there are a few basic principles to consider. These principles are intuitive; we know how convection feels. But they are worth mentioning because they help explain the relative merits of different cooling methods:

  • Heat (thermal energy) always moves from the higher temperature material to the lower temperature material, such that the hotter material becomes cooler, and the colder material becomes warmer. Simply stated, this means you can't cool something with hot air, and you can't warm something with cold air.
  • The heat transfer rate is proportional to the temperature differential between the two materials. If you are cooling something with air, using colder air will cool it faster.
  • The heat transfer rate increases as the flow rate of the fluid across the surface of the solid increases. If you are cooling something with air, increasing the air flow rate will cool it faster.

The simplest method of enclosure cooling is by natural convection through the surface of the enclosure. Natural convection is the airflow created when warm air rises and is replaced by heavier cool air. Natural convection inside an enclosure occurs when the air around a heat source is warmed and expands, making it less dense than the surrounding air. This warmer air rises to the top of the enclosure and is replaced by cooler air.

This constant exchange of air around heat sources induces air flow within the enclosure that can be used to remove heat. Louvers can be added near the top of the enclosure to allow the hot air to escape, while grilles can be added near the bottom to allow cooler outside air to be drawn in.

However, natural convection has several limitations:

  • The air flow rates induced by natural convection tend to be small, which limits the heat load that natural convection is capable of cooling.
  • The maximum ambient temperature must be colder than the maximum allowable enclosure temperature.
  • The enclosure protection rating must allow it to be open to the surrounding atmosphere without risk of component failure due to dust or water intrusion. This is often an issue as the louvers and grilles that allow natural convection can frequently permit airborne contaminants and/or spraying water to enter. Filters can be added to prevent dust intrusion where there are low concentrations of airborne dust, but using high density filters will impede air flow and reduce cooling capacity.

Because of these limitations, louvers and grilles are typically only effective for NEMA 1 enclosures with small heat loads located in climate-controlled spaces. These spaces must also be generally clean and dry.

If the heat loads are too high to be cooled by natural convection, forced convection can be used to cool the enclosure.


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