Spray Booth Electrical Blower Question

Well, I've finished the booth
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=51957

Yep, you'll be underpowered at the lower cfm rating you quote, if you are building a back-drafted or laminar-flow booth (I understand your cross-drafting terminology, now. Typically, however, cross drafting is airflow that is off-angle from the primary airflow direction.)

You are correct; downdraft booths only need a face velocity of about 50 fpm as opposed to the laminar flow booth's 80-100 fpm. As these booths are using gravity, and the solvents' vapor densities to aid in capture, the reduced airflow is very workable.

Gip Winecoff

QUOTE: Originally posted by styrene With a booth face size of 24" X 20" you should be getting a fan that is rated at least 350 cfm. This will give you a face capture velocity of 100 fpm. Even at the lowest recommended capture velocties of 80 fpm, the fan should be pulling about 270 fpm or better. The advertisement stated the 240 fpm is "open flow"; I am assuming this means total cfm without resistance. Once you add a filter and appropriate duct work, the efficiency of the fan is going to decrease--maybe significantly. Since you're already underpowered at 240 cfm, the added resistance makes your fan selection even worse. QUOTE: ...is ignition protected... Maybe I looked at the wrong fan add but I did not see this statement quoted. My apologies if I overlooked it... But the fan motor should not be in-line for exhausting organic vapors. A squirrel cage (centrifugal) fan is one of the better choices for a booth. At the VERY least, the motor should be shaded pole (brushless).

I kind of switched gears on you...my original plan was to use a marine bilge blower @ 240 CFM, but the DC to AC conversion was proving to be more of a challenge than I expected. You can Froogle what I WAS talking about with this link:
http://froogle.google.com/froogle?btnG=Search+Froogle&q=marine+in-line+blower&show=dd&scoring=mrd
These are designed to exhaust fuel vapors from boats, so they are safe as in-line blowers.

I later decided to use one of these:
http://www.growgeek.com/californiahydro/acshpoblexfa.html
..thinking about the 265 CFM @ $84 or the monster 465 CFM @ $97 (thoughts?)

But my concern is with being under-powered @ 265 CFM or over-powered @ 465 CFM

QUOTE: QUOTE: Do you think the 465 CFM would be an overkill for a cross-vent. unit? Sorry, but you just lost me. What is a cross-vent unit, and how does it interact with the booth? If you are talking about a device to provide positive pressure airflow at an angle perpendicular or so to the booth face, don't do it. You'll overcome booth capture velocities, and wind up with vapors concentrated in the very areas you're trying to remove them from. If you mean something else entirely please help me out here... Gip Winecoff

From what I understand, down-draft booths require less face capture velocity than cross-draft booths. See here: http://www.briansmodelcars.com/tutorials/tutorial.asp?TutorialID=23&CurPage=1

After duct-work (about 6 ft roughly w/ 2 or 3 “90 degree” bends to get to the window) and a vent-able plenum, I just want to make sure I’m not being under-powered @ 265 CFM or over-powered @ 465 CFM

I have also been looking to build a spray booth, found that the fan from a car would be good(the one for the interior ventilation). It has the motor out of the way from the airflow.
Works on 12VDC dawing 10-25A depending on model.
The one for a Ford Focus seems to be wide and low so I think I will use that one for a down draft booth.

As for the dimmer, there are different types of dimmers depending on the type of load inteded to be hooked up to it. The cheapest are meant for resistive loads only.

Michael

QUOTE: It pushes 240 CFM and is ignition protected so I don't burn my house down. (Not bad for $30)

With a booth face size of 24" X 20" you should be getting a fan that is rated at least 350 cfm. This will give you a face capture velocity of 100 fpm. Even at the lowest recommended capture velocties of 80 fpm, the fan should be pulling about 270 fpm or better. The advertisement stated the 240 fpm is "open flow"; I am assuming this means total cfm without resistance. Once you add a filter and appropriate duct work, the efficiency of the fan is going to decrease--maybe significantly. Since you're already underpowered at 240 cfm, the added resistance makes your fan selection even worse.

QUOTE: ...is ignition protected...

Maybe I looked at the wrong fan add but I did not see this statement quoted. My apologies if I overlooked it... But the fan motor should not be in-line for exhausting organic vapors. A squirrel cage (centrifugal) fan is one of the better choices for a booth. At the VERY least, the motor should be shaded pole (brushless).

QUOTE: Do you think the 465 CFM would be an overkill for a cross-vent. unit?

Sorry, but you just lost me. What is a cross-vent unit, and how does it interact with the booth? If you are talking about a device to provide positive pressure airflow at an angle perpendicular or so to the booth face, don't do it. You'll overcome booth capture velocities, and wind up with vapors concentrated in the very areas you're trying to remove them from. If you mean something else entirely please help me out here...

Gip Winecoff

QUOTE: Anyone with more electrical know how-- dimmer switch on an AC plug in blower -- good or bad idea?

Sometimes dimmer switches will work with devices containing motors, sometimes they won't. I think it has to do with the inrush current when the motor starts; some dimmer switches just won't allow enough voltamps for the motor to start. Get one designed for a ceiling fan and you should be fine.

It "should" work okay, though I find dimmers eventually just become an on/off switch over time. I think I would try to use a stepped switch, like what fans come with in the first place.

I use an ordinary 2 speed kitchen hood on the slow speed, the high speed just sucks the paint up into the filter! So I AB on low and clean up on high.

I'm not a hundred percent sure this is a good idea or not, but you might be able to install a dimmer switch so you can control the speed of the fan. It might work. It might just wreck the blower.

I think it would be great for a side draft one. I'm building a rear draft one and I'm just using an ordinary household box fan. You don't want any actual wind happening because there are a few processes that can be made more difficult by having fast air movement though.

Anyone with more electrical know how-- dimmer switch on an AC plug in blower -- good or bad idea?

QUOTE: Originally posted by nathaniel Actually that looks like a decent blower for the price. You won't have to mess around with wiring either. Depending on where the switch is (if there's a switch) it might be ideal for a bottom suction based spray booth.

Building a downdraft is tempting, but since this will be a tabletop unit, it would be a little too tall to be comfortable. (the working surface would be at least 6-7 inches above the table top)

Do you think the 465 CFM would be an overkill for a cross-vent. unit? (my booth will be 24 wide by 20 high, at least that is the plan.) I'll be building a box with a vent-able plenum too, so I can control the air flow, somewhat. For 14 bucks more, I'd vote for the stronger fan.

Actually that looks like a decent blower for the price. You won't have to mess around with wiring either. Depending on where the switch is (if there's a switch) it might be ideal for a bottom suction based spray booth.

wow - It might be cheaper and safer to just get a shaded pole blower. I found a good deal here:
http://www.growgeek.com/californiahydro/acshpoblexfa.html
..thinking about the 265 CFM @ $84 or the monster 465 CFM @ $97 (thoughts?)

Anybody find any better deals?

QUOTE: But, I did see a pump online w/ a 5 amp fuse. I assume that If I get a 5 amp (or more) AC/DC converter, then splice a car ciggerrette lighter onto the pump's wiring then plug that into the converter, that might work...no?

Yeah that should work fine. Fuses are typically based on 1.5 times the expected current draw, and for motors that would be the starting current. Constant current draw should be less than 1 amp, but you have to base everything on the starting current for a motor. Large industrial motors have motor control centers for that very purpose. They actually limit the starting current inrush in stages until the motor gets up to operating load.

QUOTE: If not, how would I use the computer power supply? There are 4 leads on the 12v connectors. How would I rig that up? (...but still have the problem that my current comp. power supply doesn't have a freakin on/off switch!)

If your power supply has the on/off leads I mentioned earlier you should be OK. Just put a switch across them. If it is one of the newer styles without the power leads it probably won't work at all. They will not provide voltage to the peripheral connectors until they receive an "OK" signal from the motherboard. That's how the computer is able to shut itself off when it is shutdown; the switching is handled by the motherboard and the physical on/off switch does not go to the power supply.. If you can find the pinout of the motherboard connector and jump that signal wire you can fool the power supply into thinking everything is ready, but I don't know which wire it will be.

You could wire in an on/off switch on the AC cord, or buy a timer unit for lamps. They usually come with an on/off switch...

If your power supply doesn't have an on/off switch, then turn it off at the power point.
If thats a hassle/difficult to get too, then put a switch in line with one of the power leads running to the fan.

Or cut into the power lead from the computer power supply to the wall socket, and place a junction box with a switch mounted in it, and wire your power through the switch.
Just like some desk lamps, with the on/off switch half way down the power lead.

As far as your old computer power supply wiring goes, in the 4 way connector BLACK IS NEGATIVE 12V and RED IS POSITIVE 12V.

An AC/DC convertor will work just fine also, but see if you can get one with a switch on the front.
Also take note of the current ratings, sometimes they slap on a big 10AMP claim, but it actually runs at 6 amps continuous.
Marketing, what a wonderful thing!

I looked up some more specs (I really need to go to the local boat shop and see what the amps are. ) But, I did see a pump online w/ a 5 amp fuse. I assume that If I get a 5 amp (or more) AC/DC converter, then splice a car ciggerrette lighter onto the pump's wiring then plug that into the converter, that might work...no?

If not, how would I use the computer power supply? There are 4 leads on the 12v connectors. How would I rig that up? (...but still have the problem that my current comp. power supply doesn't have a freakin on/off switch!)

If it is an OLD AT-style supply there are two leads for the on/off switch. The newer power supplies are controlled from the motherboard but the old style did have a manual on/off switch. Most of the wire bundles will have plastic multi-pin connectors on them. Two (possibly one double-length) will be different and go to the motherboard. They will have plastic snap-locks on the sides of them. The remainder will be something like 5-pin connectors that fit the peripherals. If it is the old style there should be two single leads (possibly bundled together) that have spade lugs on the ends of the wires. These went to the on/off switch.

These look like 12v / DC units. I have an old AT computer power supply (i know it works) but the stupid thing doesn't have an on/off switch. (...shot down my first idea of using a stash 120mm computer cooling fans I have.) What else can I use as a power supply for this puppy

Nathaniel did give you some good advice. You need to find out the required supply voltage first. It will almost certainly be DC, and I expect it will be 12 volts, but I'm not certain. Then you'll have to find a power supply that will provide that voltage and enough continuous draw and starting current to power the motor. Bottom line is that the need the motor specifications to start with.

Is it powered by an AC or DC source? Most things like this are DC. You may need to get something like an old computer power supply. Any old/dead computer might still have a power supply that works. I'm not sure in anyway if I just gave you good advice or very bad advice though.