Cabinet electronics: Thermal considerations
Industrial enclosures, or electronics cabinets, house the components used to control automation and instrumentation equipment in a manufacturing facility. As designers have been able to reduce the size of the electronic components while still retaining their power, the control cabinet also has shrunk, which saves space on an oftentimes crowded factory floor.
However, the smaller components still generate the same amount of heat—in a much tinier space. Properly cooling the electronics, which have varying levels of temperature sensitivity, boosts system longevity and reliability, and reduces the risk for failure. In a smaller cabinet, where there is less surface area to dissipate the heat, there is a greater need for a thermal management solution.
Convection, fans, cold plates, heat exchangers
Depending on the application, many solutions are available—including natural convection, fans, cold plates, air-to-liquid heat exchangers, and air-to-air heat exchangers—to thermally control the electronic components in the cabinet.
If the enclosure contains just a few heat-generating components or if it’s very large, natural convection from the walls of the cabinet may be enough to keep it cool. When it doesn’t work, it is commonplace to see many cabinet doors propped open to let ambient air cool the electronics, but this dangerous practice can make the components more susceptible to dust, vermin, and debris that invariably clutter factory floors, making them more prone to failure. Exposing the components by leaving the door open also is a safety hazard.
If the cabinet is located in a sterile environment, such as a clean room, a simple solution could be a fan blowing air through the cabinet. The cool, external air would typically be drawn through louvers low on the cabinet walls and exhausted from the top of the enclosure.
Another thermal management option is liquid cooling, including air-to-liquid heat exchangers and cold plates. In air-to-liquid heat exchangers, the heat is absorbed by a coolant that circulates through the system. However, this may not be a good alternative for electronics cabinets, because humidity in the enclosure can lead to moisture condensing on the cold heat exchanger. The fans in the enclosure can inadvertently distribute the droplets throughout the cabinet, putting the electronics at risk. A cold plate removes heat from electronics with liquid coolants, including water, glycol/water mixtures, or other chemicals. Using a cold plate that’s positioned directly on the electronics poses the same problem: condensate can form from the coolant used in the plate being below the dew point of the air in the enclosure and can cause the electronic components to get wet and malfunction.
The solutions above are not ideally suited to the unique requirements of most control cabinets, which are commonly found in a factory environment, which contains steel and other materials that could cause the electronics in the cabinet to short out. These factory environments, as well as outdoor applications like cell phone towers, are more likely to attract bugs and vermin that can get into the cabinet and chew on the wires or short out high-voltage components, as well as dust and moisture that can adversely affect the performance of the components. In a factory environment, the control cabinet is typically sealed and adheres to National Electronics Manufacturers Association (NEMA) standards. NEMA 12 cabinets are dust-proof; NEMA 4 enclosures are protected from water and dust. Sealed cabinets are more likely to require the use of heat exchangers to keep electronic components cool.
Heat exchangers are compact, efficient thermal management solutions for sealed electronic enclosures. Different kinds of heat exchangers are available, depending on the size and requirements of the cabinet. Air-to-air heat exchangers feature passive two-phase heat pipe technology or folded fin impingement cores combined with energy-efficient fans to reliably cool electronics. One kind of heat exchanger is installed through the cabinet wall, with half the system inside the cabinet (typically in an air duct that returns the cooled air to the bottom of the enclosure) and the other half located outside. Air circulates in each half of the exchanger, which transfers the heat from inside the enclosure to outside the cabinet. The enclosure remains sealed through the use of a gasket between the heat exchanger and the enclosure wall.
Another option is an air-to-air heat exchanger that is mounted on the exterior of the cabinet wall. Heated air from the cabinet travels through a hole in the enclosure wall and into the heat exchanger. This kind of heat exchanger is often used as a last resort for those customers who did not consider thermal management when designing the control cabinet, and therefore the only room left for a heat exchanger is outside the cabinet. Air-to-air heat exchangers that are mounted on the exterior of the cabinet may take up valuable floor space. Also, because the air flow path is more tortuous, more powerful, and more costly, fans will be required to move the same amount of air through a flush mount heat exchanger.
Calculate cabinet cooling during design
A thermal management solution like an air-to-air heat exchanger helps ensure that electronics are working properly. To get the most efficient performance out of a heat exchanger—as well as the electronics in the cabinet—make sure that the heat exchanger is properly sized by accurately calculating cooling requirements. Designers who fail to acknowledge the heat generated by the components in the enclosure, end up considering an air-to-air heat exchanger at the last minute when testing the electronics and noticing thermal problems.
By calculating heat requirements early in the design process, customers can determine the size of the cabinet required for the electronics and set aside space for a heat exchanger. This is a simple task for a relatively small cabinet with only a few components, but can appear to be an overwhelming challenge for an enclosure that contains dozens of custom circuit boards with hundreds or even thousands of heat-generating components.
An electronics cabinet for an outdoor application, like a cell phone tower, is also heated by the sun. If the enclosure is not insulated, sunlight adds to the heat that is generated by the components inside the cabinet. A heat exchanger for an outdoor enclosure needs to be larger to compensate for the extra solar heat load.
When trying to determine heat loads, many customers may overestimate to allow for future equipment that may be installed in the cabinet and to ensure that the heat exchanger can keep the electronics cool. For example, if the components are going to generate 1,000 W of energy, some customers double the cooling requirement to accommodate future potential growth. However, when the heat load increases, the size of the heat exchanger required to dissipate that extra heat also grows larger, and more space in the electronics cabinet must be devoted to the exchanger. Doubling the heat load approximately doubles the size of the heat exchanger.
Another rule of thumb to consider is that an air-to-air heat exchanger is always going to keep the temperature of the air inside the cabinet warmer than ambient air. An electronics cabinet may function normally if the temperature is 20 degrees above the outside ambient air. But if a customer wants the temperature difference to be 10 degrees so that the other 10 degrees is a safety margin, the size of the heat exchanger doubles to meet that new requirement—and the customer will need to pay a lot more for something that is not really needed.
On rare occasions, the maximum external ambient temperature might be specified as equal to, or higher than, the maximum temperature inside the enclosure. In this case, an air conditioner is required to cool the air inside the enclosure below the external ambient temperature. Since air conditioners typically have condensation drains, compressors, and seals that can leak, they tend to cost more to operate (due to higher electrical usage and higher maintenance costs).
Temperature monitoring, alarm
Finally, heat exchangers should be monitored. Dirt, dust, and cobwebs can block the airflow through the external fins, resulting in reduced performance. Simply blowing compressed air through the fins opposite the normal flow direction can restore the heat exchanger’s original performance. Likewise, fans can fail, causing the heat exchanger’s performance to immediately degrade. It is a good idea to include a simple thermostat or thermal sensor in the enclosure to determine if the enclosure exceeds a certain maximum temperature. The over-temperature alarm could be as simple as a red strobe on the top of the enclosure.
Fans also can be equipped with hull effect sensors to verify that they are turning at the proper speed. A thermostat is often a simpler and more reliable option in a factory environment. In a remote location like a cell phone tower, the electronic monitoring of fan sensors may make more sense.
Compact reliable heat exchange
Ensuring that an electronics cabinet has enough space for a thermal management solution to effectively control the heat generated from the components within the enclosure can help lengthen the life of the equipment and maintain performance. Heat exchangers have emerged as a compact, reliable solution for control cabinets, especially for applications in factory or outdoor environments that require sealed enclosures to prevent dust and moisture from harming the components. By accurately estimating the heat that the components will generate before designing the cabinet, customers can ensure that enough space is allotted for an air-to-air heat exchanger, instead of having to mount a thermal management solution to the exterior of the enclosure.
– W. John Bilski is senior engineer, Thermacore Inc. Edited by Mark T. Hoske, CFE Media, Control Engineering.
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