Case Study

A client needed to size vents on the front of the system enclosure. A colleague had suggested totaling up the area of all the fans inside the box, and using that for the vent area. Trouble was, that total was so large that it was incompatible with even the most basic industrial design requirement (like the need for a front panel in the first place!

The approach that we chose for this client involved taking a closer look at the  details of the fans. Instead of totaling up the area occupied by the diameter of the fans, we used the effect of any front panel vent restriction on the flow rate driven by the fans to figure out what the vent area should be. This entailed the following steps:

• Estimate the total volume flow rate. Because very little detailed information about the contents was available, I chose half of the free-delivery flow rate for each fan, and added them all up.
• Likewise, estimate the pressure drop for the fans at about half the shutoff pressure (given on the fan curve). Reason that since the fan curve is roughly linear around the middle of the range (away  from the knee!), a small increase of, say, 5% in pressure drop translates to about 5% reduction in volume flow rate.
• Identify a likely free-area-ratio for the front panel vents, given the requirements of the system. In this case, ID wanted a particular view-blocking mesh. Most meshes (and EMI screens, for that matter) aren't as porous as I would like, and entail non-negligible pressure losses.
• From the free-area-ratio, estimate the pressure loss coefficient. Or use the data sheet, if there is one.
• From the loss coefficient and the total volume flow rate estimate, back out the overall area needed to produce a pressure drop around 5% of the fan operating pressure. (I chose 5% figuring that this level of change in the total volume flow rate would be barely noticeable.) The pressure drop through the panel Dp = 1/2*r*V^2, where r is the air density (about 1.2 kg/m^3) and V is the average approaching air velocity, which you get from V=Q/A. Q is the total volume flow rate and A is the overall front panel vent area (without mesh).

It turned out that the vent area requirement, calculated this way, met the industrial design and manufacturability goals for the front panel. The designer was able to come up with a design to keep everyone happy