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