Help With Cabinet Cooling

--from June 2003 Control Design

Answers

We have switched back to stainless steel enclosures after trying non-metallic enclosures. We experienced severe over-temperature problems. The non-metallic material did not conduct the heat load to the atmosphere as well as the stainless steel enclosure.

...Hoffman has a free software package for determining the size of enclosure based on the heat-producing components in the panel and also takes the environmental and mounting considerations in consideration. We have used this a baseline for our sizing and have not had any difficulties with overheating.

One way to cool your cabinet is to use a Vortec or similar design cooler. These coolers run off compressed air. It is positive ventilation, so it keeps your panel clean. Through some testing, we found filter fans allow contaminants into the panel. The Vortec unit is very low cost to operate, since it does not run off of electricity, but rather low air pressure. To increase the cooling you increase the air pressure.

...Even though it uses air, it does not create moisture. The unit is maintenance-free and does not have filters to replace. It is also easy on the eyes--it does not affect the appearance of the machine. The Vortec unit is small and easy to hide. Installation is also very easy.

The right way to answer the question is by an analysis of the particular system for heat transfer, heat generation, and, if appropriate, air flow. The analysis could potentially include conduction, convection, and radiation considerations. It should include the specifics of geometry, orientation, load, and ambient conditions. It would then give the operating temperatures for the components in the cabinet.

...This type of analysis can be done by finite element analysis and by computational fluid dynamics (FEA and CFD). Creating a general model of the system would allow various case studies to be performed, such as the impact of ambient conditions, the impact of changing the power level of components, the impact of changing materials of construction, and the impact of various fan changes or other cooling methods. The calculations could be benchmarked against existing data and be used to reliably predict performance under other conditions.

If the ambient temperature is as high as 90° F, the opportunity to use fans/filters is limited. We recommend avoiding temperatures above 95° F for electronics (PC boards, micro processors, etc.). Studies have shown product longevity declines and failures rise when operated at or above this temperature.

...With this said and the ambient temperature being 90°, that provides only 5° F (T) to work with. For very light loads, a fan/filter may work. The amount of air that will be needed to remove 5° depends on the amount of heat that is being produced. For a small heat load of 100 W, 66 CFM is needed.

...Air Flow/T/heat load relationship--The greater the T the less air flow you need to remove an equal amount of heat. Lower the heat load and you can lower the airflow for the same T. Lower the T, then you must increase the air flow for the same heat load.

...Fan/filters vs. heat exchangers--Fan/filters and heat exchangers can never cool below the ambient temperature. Both also follow the Air Flow/T/heat load relationship above. There is little difference in performance between the two. Heat exchangers are closed systems--no outside air is introduced to enclosed components.They are useful where ambient air will cause contamination to enclosed electronics, which may result in failure. Heat exchangers are typically 5-10 times more expensive than fan/filters, but they don't need as much regular maintenance.

...Heat exchangers increase the surface area of an enclosure. The air flow over this surface area provides a heat transfer from inside the enclosure to the ambient air. Heat exchangers can meet UL Type 4, 4X, 12, and 3R ratings.

...Fan/filters are open systems--ambient air flow is clean enough to be used as a means to pull heat out of the enclosure. Filters--requiring regular cleaning--are used to ensure the air is clean. Fan/filters are typically rated to UL Type 1 and in some cases Type 12. UL Type 12 fan/filters require even more maintenance, have significantly reduced air-flow performance, and have filters that are not reusable. If filters are not kept clean, air flow is reduced and in some cases eliminated. This ultimately leads to equipment failure.