I’m working on an electronic design that includes a fan to exhaust warm air, and a filter on the air intake to keep the electronics from getting covered in dust. The finished project will be placed in a somewhat dirty room that is quite remote from where I will be monitoring the equipment. I want to sense whether the filter is dirty enough to restrict air flow. Of course I can (and will be) monitoring temperatures, but I’m looking for something that’s a more direct measure of air flow.
I had an idea: that there should be a difference in fan speed between a clean filter and a dirty filter. The fan speed should go down, I thought, if the filter was clogged up (as it would be working harder, kind of like a car going uphill). So I decided to test this thought out. I connected a small fan (an 80 mm “case fan") to 12 VDC, and monitored its RPM. Sitting on my desk, this fan rotated at just under 4,500 RPM. Then I put a piece of cardboard across the intake side of the fan, expecting the RPM to go down, though I didn’t know by how much. The RPM with the cardboard completely blocking the intake was 4,700 RPM — it went up! This effect was completely repeatable…
Does anyone know the explanation for this?
In the old blog, Clarke Echols said:
ReplyDeleteSimple physics. To move air requires work. When no air is moving, no work is being done, so fan runs faster. Not obvious, but that’s what’s going on.To measure pressure loss through the filter, use a manometer. Get filters rated down to under 1 micron (40 microinches). To reduce dust in general, especially in carpeted homes, use a Dyson cyclone vacuum cleaner. It’s the best thing out there. And keep vacuuming until no more dust is coming into the machine (watch incoming dust as it enters the container).Most bag-type vacuums don’t do the job of keeping air clean. The bags can’t be made dust-tight or they plug up too quickly and you lose suction.