Machine Design
Sizing Fans for Electrical Enclosures

Sizing Fans for Electrical Enclosures

Simply knowing the cubic-feet-per-minute requirements is not enough when choosing fans.

Several factors come into play when estimating the size of cooling fans needed to keep electronics cool inside enclosures. Here’s a look at a step-by-step process for roughly sizing fans based on the task they must perform.

Start with the volumetric flow or cubic-feet-per-minute requirement, the CFM. Then establish a maximum temperature (Tm) and target temperature (Ta), and find the difference, the delta T, (ΔT= Tm - Ta)

For calculations, use the number of watts supplied by the enclosure’s power supplies as the maximum total watts going to the enclosure’s electronics. Assume there are two 60-W power supplies, so the total number of watts is 120. This provides a working estimate of watts that may be on the high side, but using it prevents having to cut sheet metal to install a larger fan later.

First, convert watts to Btu/hr, then solve for the CFM requirement for the equipment fan(s).

To convert watts to Btu, use the conversion factor:
1 W = 3.415 Btu/hr, so 120 W = approximately 410 btu/hr, which is the amount of cooling needed to offset that heat.

And the equation for determining the CFM is:

CFM = (BTU/hr)/{(Specific heat) х (Specific weight х 60) х ΔT}

In our example, Tm is 192°F; Ta is 70°F, and ∆T is 122°F and watts required are 120 or 410 Btu/hr.

Specific heat of air, Cp, varies from 0.2936 Btu/lb-°F at 40°F to .2403 Btu/lb-°F at 140°F. For a constant value, use, .240 Btu/lb-°F.

The Btu/hr, ∆T, and Cp have been established, so the only variable left is the Specific Weight of Air or Density. As the density of air changes, it will change the air mass flow. Temperature and barometric pressure are a few of the things that affect air and air mass flow.

Solving for Volumetric Flow (CFM):

Rearranging the heat transfer formula:

M = Q / (Density of Air x 60 min.) x (Cp) x (∆T)

Now solve for my CFM using the requirements for a perfect day under ideal conditions at sea level. For a perfect day:

Temperature: 59°F

Pressure: 29.921 in. Hg

Relative humidity: 60%

Dew point at 59°F at sea level with pressure at 29.92 in. Hg: 45°F

Density at sea level at 59°F; 0.002378 lb-sec2/ft3, and it’s 0.07634 lb/ft3 at 70°F.

Specific weight of air at 59°F a sea level: 0.07651 lb/ft3

M = 410 Btu/hr / (.07634 lb/ft3 x 60 min) x (.240 Btu/lb-°F) x (122°F)

M = 3.057 ft3 / min

So the enclosure needs a fan or combination of fans that move a little over 3 cubic ft of air per minute?

The main purpose here is to get a general idea what size fan is needed to keep the electronics inside this enclosure cool. If there are hot spots inside the enclosure, baffles or plenums can direct air flow on to them.

One piece of valuable advice I gleaned from a catalog stated that once the initial or approximate size requirement for CFM has been established, double the size needed and make the exhaust openings 1.5 times larger than the inlet opening to reduce static pressure. This can be accomplished by vents having the same or greater area than the inlet opening. Then run the CFD analysis and prototype for validation to  determine if your selection meets the requirement.

To finalize assessing the fan requirement estimate, all parameters must be taken into account. It is not enough to base your calculations solely on normal conditions without factoring other influences such as temperature and altitude. When they are introduced into the equation, they have an effect on the fan’s ability to keep components inside the enclosure cool.

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