HVAC question: Sucking through a straw

[Gary Fritz]

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

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

[Travis Jordan]

You can't get there from here.

The duct diameter is: 3.00 inches Flow Rate 75 CFM Velocity 1530 FPM Pressure 14.7 PSI Temperature 85 F Duct length 20 FT Duct Roughness 0.01 FT

At 1530 FPM, any fan is going to make a lot of noise.

How about opening up the ceiling over the projector and just venting it into the attic?

[Matt]

How about getting some silent PC fans, mounting them on the sides of the projector box, and forgetting the duct entirely?

[Andy Hill]

I gotta agree. Unless the room is extremely small, 900W into the room would be better handled by the "normal" HVAC system.

[Gary Fritz]

It's about 14x14x8. The "normal" HVAC thermostat is upstairs so it doesn't notice if we're baking in the theater room. Plus the cold-air return duct is on the floor, so it doesn't pull the heat off the ceiling.

[Travis Jordan]

Even with no heat gain from outside or above, that is 8*200 = 1600 +

3414 BTU plus whatever ancilliary equipment (DVD player, cable box, etc.) you might have.

Time for a small split system.

[Andy Hill]

Hmmm. And in another posting you talk about 6-8 folks in the room. @100 watts / person, that's another 800 watts input into the system. Shoot, with all the other odds and ends you're probably looking at 2kW in the closed room. Urk....I can see your problem -- that room's gonna be both hot and stuffy.

Is this a raised floor, then (not sure how else you could have a return on the floor in the basement)? Any other options for getting house air in and out of the room (not necessarily outdoors), like maybe under the floor into an adjacent room?

[Gary Fritz]

I never anticipated that problem. This is in the *basement* -- it's usually COLD down there!

No, it's carpet on the concrete foundation. The return comes from a plenum in the ceiling and runs down the wall to a vent on the wall right at floor level. In extreme need I suppose I could punch another hole in the wall near the ceiling to tap into that return and draw hot air off the ceiling, but then the floor's likely to get mighty cold. Maybe I just need a quiet fan to circulate air in the room better....

Not really. Sucking it through the exhaust duct would be ideal, except dummy here used a 3" duct...

[Andy Hill]

Would exhausting into the ceiling be a viable option? I'm not talking about punching all the way into your wife's office -- just up into the joist space. Without knowing the layout of the basement, it's hard to tell how much static pressure you'll be pushing against, 'tho. Of course, where to source your fresh air is the other problem.

[m Ransley]

Maybe a large 12 x18 or so vent at the ceiling cut through the wall, some duct work and proper fan to blow the hot air to another area of the basement. You want to just remove the hot air right. Or bath fans through the wall high up, Panasonic has quiet units several might give good flow.

[m Ransley]

Since the 3"pipe wont do the cabinet maybe consider dumping that heat into the room with muffin fans, then tackle getting air in and out of that room. You need an Hvac guy to come out and look and figure what will work without guessing. If the rest of the basement is cooler just moving air in and out may do it.

[bumtracks]

[John Harlow]

Funny. I like the "ain't going to happen" thing. Dime to a dollar you are in upper management.

Here's an idea: disassemble an A/C unit and pipe the condensor next to the projection TV to absorb its heat.