Router speed control for shop-vac?


I don't know where else to ask this, and it seems like this group would probably have some expertise in this area.
I want to use an old shop-vac as a heavy-duty exhaust fan. I have an application where I need to pull hot air out of an equipment area. I made a serious mistake and built 3" flexiduct into the walls to the exhaust fan area. The 3" flexi is a very thin straw, and normal fans can't suck enough air through them. (I've tried 5 different fans of varying types and they can't hack it.) I can't change the flexi without ripping out several walls -- not gonna happen. Hence the shop-vac.
The shop-vac can pull LOTS of air through the 3" duct -- TOO much. I need to reduce the amount of air it pulls. I could just introduce a "leak" in the duct but that seems like a very bad solution.
I'd much rather slow the fan down to just the right level I need, which will also reduce the noise level.
Is a router speed control like this one OK to use on the kinds of motors used in shop-vacs? http://www.harborfreight.com/cpi/ctaf/displayitem.taf?ItemnumberC060 It's rated for 15A, and the shop-vac only pulls 5.3A.
BTW the vac will run for 2-4 hours at a time. That's probably more than the recommended duty cycle for a shop-vac but hopefully it will be OK. Especially if I slow it down.
Thanks, Gary
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Hm, I should have dug deeper before I posted. I found a thread where this has been discussed before:
http://groups.google.com/group/rec.woodworking/browse_thread/thread/f0ff527 21d001e33/420088a1a53dbd88
The consensus there seems to be that shop vacs (and any other "noisy" tool) always use "universal" motors, which should have no trouble being controlled by the Harbor Freight speed control.
So unless someone has a good reason not to, I'll go make a run to Harbor Freight. Or if someone has a suggestion for a fan that really can pull a lot of air through a 3" duct -- for cheap -- I'd like to hear that too. (Anybody want to part out their old dust-collector fan/motor?? :-)
Maybe I should try an inexpensive blower/vac, e.g. http://store.yahoo.com/sterlingtime/power-tools-heat-guns-blowers-ele - bwr.html ? I'd still need to slow that one WAY down. They claim 100cfm, and I need to cut it down to 50-60 or so. So I'd still need the speed control.
BTW I'm quite concerned about the safety issues of this idea, since the vac will be stashed off in a closet and not immediately visible. I definitely don't want it to overheat and start a fire.
Gary
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Wed, Feb 1, 2006, 6:07pm (EST+5) snipped-for-privacy@xxxfrii.com (Gary Fritz) doth admit amazement: Hm, I should have dug deeper before I posted. I found a thread where this has been discussed before: <snip>
The real trick would be finding a subject that "hasn't" been discussed before.
JOAT Shhh... that's the sound of nobody caring what you think.
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to just the right level I need, which

I seriously doubt the motor would last using it that many hours at a time and I seriously doubt that you will reduce the motor noise very much either. I would either adapt a low end dust collector to it or visit a local Air Condition/Furnace dealer and see if they have any old furnace blower motor/fans. I have seen these at garage sales for $5-$10. I think an induction motor is going to serve you better.
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The noise I can live with. Motor not lasting forever wouldn't kill me either, as this is a freebie scrounged vac anyway. Just so it doesn't die in a burst of flames.

Suggestions for low-end dust collector? I can't find much of anything for less than $150 or so. I'm trying to make this as close to zero-cost as possible.
Furnace blowers: I haven't seen anything that can move enough air through a 3" duct with high static pressure. They are all designed to move air through large, smooth ducts, and their CFM drops to near zero with a high- resistance load. If you know of a fan or class of fans that would work, I'm all ears.
Thanks, Gary
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Instead of sucking thru the 3" flexi try blowing thru it. High velocity A/C systems blow lots of air volume thru small flex ducts.
Art

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Might work for the airflow, but I don't want to blow air into the hot area. I need to remove heat FROM the hot area. I really need to suck the air out.

I tried a Nutone 696N fan, nearly identical to the 695 fan you linked to. It's rated at 50-75 cfm. It barely pulled 25cfm through my duct. The simple cheap Nutone fans simply do not have the oomph it takes to pull sufficient air through this duct.
Well, I went to Harbor Freight and got the speed control. It appears to be working like a charm. I can tune the airflow to exactly what I need. The shopvac is **considerably** quieter at the slower speed.
Thanks for the help! Gary
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It's a matter of perspective. Blow the air in through the ducts and vent the hot air out through another one.
Gary Fritz wrote:

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I didn't mean to reverse the direction of the air flow. Put the blower in the hot area pushing the air out. Most blowers will move a lot more air into a restriction than when trying to suck thru the same restriction.
Art
wrote:

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Not gonna work. Here's the deal: the "hot area" is an enclosed sound- absorbing box for a noisy and hot Home Theater projector. The projector puts out about 800-900 watts of heat. It will heat the room on its own quite handily, and since it's mounted on the ceiling, it ends up baking in its own hot air. Plus the people watching the movie each put about 100W of body heat into the room. (The first night in the theater, before I built the sound box and the exhaust system, the projector and the 8 people watching generated so much heat that the projector overheated and shut down! I was afraid I'd fried it, but fortunately there was no permanent damage.)
I don't WANT to push all that hot air into the room, and try to eventually vent it out. I want the hot air OUT of there.

Ah. Yes, and the flexiduct would inflate and expand, instead of shrinking and further restricting the airflow. That would work better, IF it was an option to put the fan in the hot area. But it's not -- there's no room for a fan in the projector box, and I can't have the noisy fan/vac in the theater room. It needs to be several walls away, on the other end of the duct.
Gary
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Sounds like your solution works. Hopefully it will last for a long time, but please let us know if it dies.
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wrote:

A pool contractor I know uses the shop vac blower motors in his buisness. The vacs runs continuously sucking air from behind the vinyl liners as the pool fills up. This can take the best part of a day on on the average pool. I know the vacs don't last forever like this( I have the extra hoses to prove it) but they do last longer than one might expect, and four hour run time is no big deal. Good luck
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Why don't you just use a cheap $15 bath exhaust fan? They are meant to push 50-60 cfm through a 3 or 4" duct. Here's an example. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item `16844088&category188
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