Magic Smoke

Once the magic smoke comes out, things don't work any more.

John Kasunich
jmkasunich@fastmail.fm
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Tue, 13 Feb 2007

Cooling

A while back I calculated the total power dissipation inside the electronics box (PC, power supplies, Geckos, spindle VFD, etc) to be about 300 watts. That is too much for a sealed box this size to dissipate through the walls, so I need to bring some air inside. There is a simple formula for calculating the amount of air needed to carry away a given amount of heat:

CFM = 1.75 * power / deltaT

CFM is the airflow in cubic feet per minute, power is in watts, and deltaT is in degrees Celsius. I assumed 30C maximum air temperature in my basement. Most electronics are rated to work with 40C air, so that gives me a deltaT of 10C. If the heat load is 300 watts, I need about 50 CFM.

Fifty CFM fan isn't much - an 80mm square PC case fan will usually do that if running with minimal back pressure. But I don't want to bring dust and dirt into the enclosure with the air. That means I need a filter, and I need a fan with enough pressure to overcome the resistance of the filter. Ordinary fans aren't up to that task - their performance drops like a rock as the back pressure builds up.

Fortunately, about a year ago my employer got rid of some old Digital Equipment storage arrays. These were top-of-the-line units: six rackmounted drawers, each holding eight blazing fast (and blazing hot) 10,000 RPM SCSI drives plus redundant power supplies. All mounted in a full height 19" rack weighing at least a couple hundred pounds. This beast provided a whopping 384 gigabytes of storage.... Yep, they were only 8G drives. Now you know why it was in the dumpster.

The only parts of it that were worth grabbing were the cooling blowers. But the blowers are nice. They are what's known as a backward curved motorized impeller. The ones from the storage array were mounted on the back of the drawers, and pulled air through the densly packed drive array, then exhausted it down and out the back. The pics below show the impeller inside its plastic case, and after removing the case and mounting it to a piece of sheet steel.

Compared to an axial fan of the same size and RPM, a backward curved impeller can provide anywhere from 4 to 10 times as much pressure, at a somewhat lower flow rate. The nice thing about a backward curved impeller compared to a centrifigual blower (such as a furnace blower) is that the backward impeller works just fine without a housing around it. I just mounted the impeller on a plate, then spaced the plate off the bottom of the cabinet, with the inlet of the impeller lined up with a hole in the bottom of the cabinet. Like so:

All that's left is the filter. So off I went to the local auto parts store. When a normal person goes looking for an air filter for their car, they look up the make and model of the car in the big book that sits on the shelf, then grab the appropriate box and leave. But I didn't have a car in mind. I was just searching for a certain shape and size. I got a few strange looks as I rummaged through the shelves, opening boxes and measuring filters. Eventually I picked out a Fram CA3902, for the princely sum of $6.99. It is 6" tall and 6" in diameter (more or less), giving lots of surface area for low pressure drop. I just happened to have an aluminum disk that was a perfect fit for the top of the filter, as well as some 8-32 threaded rod and a wingnut. The result:

This setup should give me a nice supply of clean air for the cabinet. As a bonus, since the blower is at the inlet, the entire cabinet is pressurized. That will help keep crud from sneaking in any place that isn't perfectly sealed.

By the way, it turns out a CA3902 air filter fits early 1990's Chevy Cavalier 6-cylinder engines, among others...

(posted: 13 Feb 2007 23:10) (permalink)