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HVAC question: Sucking through a straw
I've got an application where I want to suck air out of an enclosed
space. (It's a large CRT front-projector that draws 7.5A and pumps out a LOT of heat. I have a duct sucking from a plenum I built on the top of the projector that draws the air through the projector's body into another space.) Problem: I can't suck enough air because I screwed up on the installation. When I was building the room, I couldn't find any decent flexible duct in anything larger than 3". So I ended up running 20' of 3" plastic flexi-duct (the cheap plastic & spiral-wire dryer stuff) in the wall. Please don't tell me I need to rip out the flexi and replace it with 6" metal ducting or whatever. It ain't gonna happen. The flexi is built into the ceiling, running over a furnace plenum and through a very convoluted path down through a wall, and there's no way to put in new duct now without ripping out several walls. I know (now) that the 3" flexi was a Very Bad Idea, but now I have to work with what I've got. I tried several bathroom-type fans. None of them will pull more than about 20cfm. 70cfm 3", 110cfm 4", etc, none do any better. (Am I correct in assuming that you'd have to derate a 4" fan by 1.77x when feeding it into a 3" duct to account for the 1.77x smaller cross- sectional area of the 3" duct?) Finally I found "bilge blowers," which are high-cfm fans designed to ventilate boat under-deck areas. The motor I got is a 3" (so no derating) and rated for 145cfm. According to the specs (see http://www.attwoodmarine.com/Product...ctions/69255f- english.pdf) it should pull over 75fm through 1.0" of static pressure. I don't know what my static pressure is, but obviously it's way high -- the bilge blower will only pull maybe 30cfm. That's an improvement, and almost enough to be usable, but I'd like 50-70cfm. I thought putting 2 fans "in series" -- output of one connected to the intake of another -- would help. The "downstream" one should see a vastly reduced static pressure due to the "upstream" one blowing air into its intake, so I thought that might put me back into the good operating range of the fan and boost my throughput. No joy; I added another fan and it hardly affected the cfm at all. It *is* possible to suck a lot of air through the thing -- I hooked up a shopvac and it pulled more air than I wanted, along with a thunderous rumble of turbulence. But that's a very noisy expensive solution, and I haven't been able to find a regular fan that can do it. Help?? Gary |
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