SpecMate: Simple Steps to Keep Enclosures Cool

July 23, 2003
For every 18o F operating temperature rise above ambient, the life expectancy of the electronics is halved

The new machine control system design is complete. The talented but inexperienced designer packed more electronics into the cabinet than you thought possible. She's been able to make great use of compact drives, motion controllers, a robust operator interface, and high-density compact I/O modules on DIN-rails.

You look it over, equally impressed, knowing that a reduced footprint for both machine and control system was a customer-mandated imperative. You also say, "Looks like you have a lot of generated heat to deal with in that cabinet. Are you sure you're not going to toast the transistors?"

Now, if the young engineer hadn't considered heat in the design, satisfaction might quickly turn to despair. But, no, enclosure heat has been accounted for and satisfactorily dealt with. Smiles all around.

What if heat generation hadn't been reckoned with? Well, unless there are extraordinary components and an extraordinary operating environment, it's still easy to deal with. In fact, it's only when the cabinet heat load has been decided that the appropriate cooling solution can be determined.

The general rule of thumb used by Rittal (

www.rittal-corp.com) for most cabinet components indicates that the operating temperature should not exceed 110-115? F, and should be targeted at 90-95? F.

Hoffman Enclosures (www.hoffmanonline.com) says research has shown that for every 18? F operating temperature rise above 72-75? F, the life expectancy of the electronics is halved.

The first step is to predict the temperature rise in the enclosure. Total up the watts dissipated by the components, as listed in their specs. The engineers at Hoffman say it's just not possible to approximate heat input merely from enclosure size--heat generation of components varies greatly.

Enclosure surface area will determine its ability to dissipate the generated heat. Hoffman indicates a typical 48x36x16-in. gasketed, non-ventilated, painted steel cabinet housing components that generate 300 W (1,023 BTU/hr) of heat has 7.1 W/ft.2 of input power. For this cabinet, the expected internal temperature rise is 30? F. Under normal operating environments, some degree of heat removal is warranted. Aluminum or stainless steel enclosures dissipate heat poorly compared with painted steel or non-metallic materials of construction, so add an additional 50% heat load to the calculated example

To determine the airflow that must be provided through the cabinet to limit temperature rise, Bud Industries (

www.budind.com) says divide the internal heat load generated by the acceptable temperature rise (compared to ambient) to arrive at a airflow requirement. To limit temperature rise to 10? F for the above heat load, about 95 cfm is advised. This correlates nicely with Hoffman's graphical representation of internal heat load vs. airflow at various temperature rise levels. Both suggest applying an additional 25% safety margin.

Once the airflow requirement is determined, sizing the appropriate fan and filter is a matter of reviewing vendor specs for various size fans. Our example yields a 5 or 6-in. fan and filter combination. Filter media can be as fine as 3 um, but may restrict flow. Select a filter media/fan combination that specifies flow through the media.

Placement of the fan requires a decision about whether the fan exhausts the cabinet or acts as a blower. In general, blowing in air at or near the bottom of the enclosure has several advantages. Fan life will be extended, since cooling air is being drawn through the fan, rather than hot air being pulled out through it. It provides a positive enclosure pressure, helping to keep out dust, dirt, and other contaminants. This configuration also produces more internal turbulence, reducing the chance for hot spots and improves the overall contribution of radiational dissipation through enclosure walls.

The inlet and outlet should be located as far apart as possible in order to minimize short-circuit air flows. Hoffman also recommends that if the cabinet configuration dictates using two fans in parallel, they should be identical in size and performance.

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