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

Gary Fritz wrote:
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.

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?

How about getting some silent PC fans, mounting them on the sides of
the projector box, and forgetting the duct entirely?
http://www.overclockers.co.uk/acatal...e_Fans_47.html

"Matt" wrote:
How about getting some silent PC fans, mounting them on the sides of
the projector box, and forgetting the duct entirely?
http://www.overclockers.co.uk/acatal...e_Fans_47.html

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

"Travis Jordan" wrote:
You can't get there from here.
The duct diameter is: 3.00 inches
Flow Rate 75 CFM
Velocity 1530 FPM

Let's see, the duct cross-sectional area is just about 7 in^2. One
cubic foot of 3" duct would be 12^3 in^3 / 7 in^2 = 247" = 20'. Hm.
So the whole 20' length only holds 1 cubic foot!? I hadn't worked
through that before...

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

I'm not concerned about the fan noise. It's 2 walls away. I assume
you meant turbulence noise in the duct? I can tolerate some of that,
just not as much as the shopvac made. The duct is inside the plenum on
top of the projector, and the plenum is made up of 2" thick foam
running around the edges of the projector & jammed between the
projector and the ceiling. That will contain a lot of the noise.

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

No can do. The projector is in the basement. The ceiling over the
projector is the floor of my wife's office. She would not appreciate
the hole in her floor. :-)

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

With a 7.5A draw, this thing is pumping out something close to a
kilowatt of heat. I need to exhaust the heat from the room or the room
gets uncomfortably hot and the projector overheats. It's worse if I
have 6-8 people in there watching a movie.

Gary

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

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.

Gary Fritz wrote:
With a 7.5A draw, this thing is pumping out something close to a
kilowatt of heat. I need to exhaust the heat from the room or the
room gets uncomfortably hot and the projector overheats. It's worse
if I have 6-8 people in there watching a movie.

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.

http://www.epinions.com/pr-Hitachi_R...ir_Conditioner

Hmmm well Im out of ideas man. I'm having a hard time picturing this
projector though.... it's been a long time since I've seen what (I
think) you are describing. Have you looked into getting a smaller
projector? I've got a Sony; about 8"x 8"x 2".... extremely sharp
picture.

I did have one other thought - and it's as worthless as most of my
other ones.... and all of the formulas you guys have been trading back
and forth have already pretty much left me in the dust. But..... if the
projector box is as sealed up with foam as you described, how can the
fan suck any air out of it at all?

Gary Fritz wrote:
Andy Hill wrote:
I gotta agree. Unless the room is extremely small, 900W into the
room would be better handled by the "normal" HVAC system.

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.

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?

"Matt" wrote:
But..... if the projector box is as sealed up with foam as you
described, how can the fan suck any air out of it at all?

The projector is mounted a few inches from the ceiling. The
space between the proj and the ceiling is sealed with foam. The
exhaust duct is inside that sealed area. The top side of the projector
has ventilation holes in it; since those are the only holes in the
sealed area, that's where the air flows. The exhaust fan sucks air
into the bottom of the projector, through the body where the heat gets
generated, and out through the duct. Just not fast enough.

Andy Hill wrote:
probably looking at 2kW in the closed room. Urk....I can see your
problem -- that room's gonna be both hot and stuffy.

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

Is this a raised floor, then (not sure how else you could have a
return on the floor in the basement)?

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....

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?

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

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.

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.

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.

Maybe put an old fashioned paddle fan on the ceiling - most are quiet.
When I need a winter heat boost here in Florida, I flick on the cathedral
ceil paddle fan. Instantly feels warmer.
Also the slight breeze allows you to feel cooler during summer. Move around
a lot of air with those things.

"Gary Fritz" wrote in message
...
Andy Hill wrote:
probably looking at 2kW in the closed room. Urk....I can see your
problem -- that room's gonna be both hot and stuffy.

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

Is this a raised floor, then (not sure how else you could have a
return on the floor in the basement)?

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....

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?

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

Problem: I can't suck enough air because I screwed up on the
installation.

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.

Help??

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.