Design engineers face many challenges with the layout of electrical and electronics enclosures, and one frequently asked question addresses the location of thermal management products. The goal is to position the equipment so as to provide adequate protection from temperature extremes as well as preventing the formation of condensation. Phil Herbert of Stego (UK) explains
Heating and cooling devices in enclosures are designed to protect electrical and electronic components primarily from condensation and also from low and high temperatures. However, even with the appropriate equipment and controls, problems may arise due to incorrect positioning within the enclosure.
As the requirement of heaters for the prevention of condensation formation becomes more widely acknowledged, engineers and design teams must consider the equipment placement in an enclosure along with the devices they are intended to protect. It is not uncommon to find systems added after the fact, fitted into whatever space remained. While this may be the only solution available, it could potentially be the cause of other problems such as creating ‘hot spots' or ‘heat nests' near temperature sensitive electronics.
Ideally, most heaters will perform optimally when mounted near the bottom of an enclosure and used in conjunction with a separate controller such as a thermostat and/or hygrostat. With the controller located in an area of the cabinet that is representative of the average temperature or humidity requirement, the heater should then be placed in a position near the bottom but not directly beneath the controller. This placement will ensure that the controller is not influenced by direct heat from the heater.
For smaller areas, heaters will generally provide adequate heating power to maintain temperature and humidity control. For example, a 900 x 600x 600 wall mounted, insulated stainless steel enclosure with a desired interior temperature of 10°C with an ambient temperature of 0°C will require a 100W heater:
Power (W) = (enclosure surf. area) x (delta T) x (heat transmission coefficient)
= (1.014 m2) x (10 K) x (4.5W/ m2K)
In the case of Fig. 2, with all other parameters remaining the same, the height and width have been increased to 800 and 600mm thereby increasing both the air volume and the surface area. Accordingly, the required heating power has also increased:
Power (W) = (encl. surf. area) x (delta T) x (heat transmission coefficient)
= (1.428m2) x (10 K) x (4.5W/m2K)
For larger enclosures with greater heating power requirements, standard heaters are not a practical solution. As Fig. 2 shows, the most effective heat distribution is accomplished by a fan-assisted heater with greater air circulation to ensure rapid and efficient control of the temperature and/or humidity.
However, as mentioned previously, space for a tall heater is not always available. Packing densities have increased as more equipment is designed into smaller spaces. In the case of an example enclosure, only 100W of heating power is required, but the high packing density limits the available space for a convection heater.
The alternative is a compact fan-assisted heater positioned to provide adequate heat distribution throughout the cabinet. The position of the controller can vary depending on the air flow and temperature gradient, providing that it is not impacted by direct heat.
In any circumstance where a heater is required, the location of all other equipment relative to a heater should be carefully considered. Most heater manufacturers recommend a minimum distance of 50 mm from components inside an enclosure. However, the temperature sensitivity of each component should be assessed along with the heater temperature profile to ensure no damage will occur. (Calculations are available online at http://www.stego.co.uk/)
Enclosure cooling solutions range from filter fans to heat exchangers and high performance air conditioning systems. In all cases, the intent is to remove excess heat from the cabinet interior. Whether naturally or mechanically achieved, the basic principle of heat rising is utilized.
One common and simple method is by using forced air ventilation, which is most effectively achieved with filter fans. Since outside air is introduced into a clean sealed environment, high efficiency filters are required to maintain that integrity.
This arrangement with the filter fan (air intake) at the bottom and the exhaust filter near the top is highly effective by using cooler ambient air to displace the warmer air inside the enclosure. The exhaust filter is typically mounted as close to the top of the cabinet as possible to take advantage of natural convection forces, and should also be located as far as possible from any heat producing components. If designed properly, the air path created by the filter fan system will pass through critical areas that are to be cooled, allowing for maximum cooling efficiency. Ideally, a control thermostat should be located in one of the critical areas where it will turn the fan on and off when temperature set points are reached.
Many other arrangements are possible, even so far as letting ventilation occur naturally. One such system would allow for passive cooling by letting the warmer air escape through a roof-mounted vent. Again, the key is that cooler air is used for displacement, so an intake filter would be required near the bottom of the cabinet.
Designing the layout of cabinets and enclosures that house sensitive electronic components is a challenging task. While it may seem a less important consideration than many other aspects of proper control system design, the suitable placement of heating and/or cooling components can have a major impact on system operations. Following these simple guidelines will help ensure system functionality and long service life.
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