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Oct 01, 2007, 11:54 AM
Balsa for me, thanks.
irish_lord99's Avatar
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Discussion

spray booth Filter system


Hey all,

I just recently purchased an airbrush and compressor set-up and now I have to get started building a spray booth for it. My problem is ventilation (or rather, lack thereof). I live in an apartment building, and it wouldn't really work to vent the spray booth out the window, because I'd get paint all over the neighbor's windows. Bad news for me if that happens. So I've been looking at different spray booth set-ups online, and I think I have an idea:

Basically, I want to have a large filter between the work area and the ventilation fan (yes, I know to use fans with brushless or fully enclosed motors ), and then have the fan push the air into another filter system. I read online about a guy who used a water filter (not a water curtain, but an in-line water filter) and he said that it cleaned out about 90% of the paint particles (he failed to include a diagram or picture). I figure after the water filter I'll take a cloth filter for a respirator and hose-clamp it to the exit pipe on the system.

It took me quite a while to figure out exatly what I wanted to do. I figured that a common under-the-sink type water trap wouldn't work because the water would all get blown out of the system too fast. I thought and thought, but the answer was actually close at hand all the time! What I wanted was a nargile! (a hookah, a bong, a water pipe, call it what you want)

Anyways, nargiles are quite abundant here in Turkey, and no, I've never smoked anything but tobacco in one. So I put together a quick idea in solidworks and I'm posting it to get feed back. It may be a while till I get it all together cause the airbrush and compressor are in the states, but it'd be nice to fine tune the design now. If there's anyone out there who has any experience with this, I'd appreciate the input.

Thanks,
Irish
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Oct 01, 2007, 01:14 PM
Quote:
Originally Posted by irish_lord99
Anyways, nargiles are quite abundant here in Turkey, and no, I've never smoked anything but tobacco in one. So I put together a quick idea in solidworks and I'm posting it to get feed back. It may be a while till I get it all together cause the airbrush and compressor are in the states, but it'd be nice to fine tune the design now. If there's anyone out there who has any experience with this, I'd appreciate the input.

I'm wondering how you're gonna get a hundred or so CFM air flow through your nargilebonghookaframmus. Unless you plan to make it 4" or 6" inlet and outlet, that is, and use a squirrel cage blower (ordinary blade type box fans won't make more than a couple inches water column pressure, y'know . . .).

Badger offer a table top airbrush spray booth with fan and filter, BTW.

If nothing else, have a look at a powered air filter for a different approach to the same problem. The air filter in the link is way larger than necessary for an airbrush spray booth, but that's just a matter of scaling things down a bit; paint dust is not a problem for bag filters.
Oct 01, 2007, 04:45 PM
Balsa for me, thanks.
irish_lord99's Avatar
Thread OP
Hey Plumber,

Thanks for the input. The link you provided was a dead end though.
What do you mean by a "squirrel cage blower"? Is that something like a duct blower type fan?

What if I were to keep the inflow pipe the same diameter and instead make the pipe surrounding it smaller? Would that decrease the amount of pressure needed to push air through? I have been planning on using a locally found fan/blower which would be of the ordinary blade box type, but I had no idea how much air they move or how much pressure they would make.

thanks,
~irish
Oct 02, 2007, 09:02 AM
Quote:
Originally Posted by irish_lord99
Hey Plumber,

Thanks for the input. The link you provided was a dead end though.
What do you mean by a "squirrel cage blower"? Is that something like a duct blower type fan?
The link works, I just checked it. If you mean "dead end" as in not suitable for your needs, that's probably true. Such a powered air filter would be major overkill for an airbrush spray booth in an apartment. I only provided the link to maybe give you food for thought.

Squirrel cage blower : http://www.grainger.com/Grainger/items/4C448

Quote:
What if I were to keep the inflow pipe the same diameter and instead make the pipe surrounding it smaller? Would that decrease the amount of pressure needed to push air through? I have been planning on using a locally found fan/blower which would be of the ordinary blade box type, but I had no idea how much air they move or how much pressure they would make.
That's the problem - blade fans, like the sort you might stick in a window on hot days, cannot develop much pressure differential. They're suited for high volume, but have lousy output pressure because of the same dynamics that cause ordinary wings to develop tip vortices - there is nothing to prevent the air flowing radially across the blades and off the tip, and as back pressure on the fan increases the air flow rate through the fan drops accordingly.

A tolerably accurate rule of thumb is that 1 foot of water depth equates to about 1/2 pound of pressure at the bottom.

You need an order of magnitude more delivered air pressure than the inherent flow rate losses (parasitic drag, for example) and water column pressure in the design of your water filter.

If you did use a box window fan, you'd need the piping through the filter to be no less than half the diameter of the fan's box, and the water in the filter (through which the air must flow) could only be a couple of inches deep.

Otherwise the air simply won't flow through the filter because the head pressure of the box fan would be insufficient.

If you examine the tech specs of the referenced squirrel cage blower (above) you'll see that the blower is rated in CFM @ (somenumber) of inches "SP". That means inches of water column "static pressure", or back pressure on the blower. For example, the referenced blower is rated at 465 CFM @ 0.0000 " SP, meaning that in free air (in the middle of the room) the blower will move 465 cubic feet of air per minute. If you increase the static pressure to 2", the air delivery rate drops to 396 CFM, at 4" it is down to 305 CFM, and at 6" the rate is down to 125 CFM. The static pressure numbers are "inches of water column", meaning water depth. If your water filter had a water column of 6", the referenced blower would only push 125 CFM of air through the filter >if there are no other restrictions<, e.g pressure losses due to small pipe size, excessive fittings (bends, elbows, etc.), pressure losses due to long pipe runs, yada, yada, yada.

The other "calculation" you need to fiddle with is air changes per hour in the subject space (the spray booth volume). Plan on not less than 4 air changes per hour, but 8 would be better (you're trying to filter the air here, and ordinary air change rates for air conditioning and ventillation purposes are far too low).

Convert your air change rate per hour to cubic feet per minute and you'll have the nominal CFM rating of the blower size you need, and then add the CFM losses due to restrictions inherent in your filter design and layout to arrive at the real CFM rating of the blower you need to make it work.

I think you'll wind up with a whopping blower size (and noise) AND a whopping water filter size, if the thing is to work with any appreciable effect.

It would be far more expedient and much less costly to simply strap high efficiency household air conditioning filters (95% rating @ 5 microns, or better) to the box fan's intake and be done with it. Use two filters if you like, or even four filters. Ordinary box fans can handle that much filter media and still deliver a respectable air flow rate.

I've got about 40 years engineering under my belt (no, I'm not a plumber, that's just a moniker supplied by a lady friend), and the water filter you're proposing strikes me as a waste of precious model building time, particularly when there are readily available dry air filters at the BORG.
Oct 02, 2007, 09:26 AM
Registered User
I'd build a spray booth and vent it out the window. To trap the paint a hig flow polyester felt (the type that is used in range hoods over the stove) should be sufficient. So your neighbours have only the smell to complain about

Dumped range hoods from rebuilding projects make great spray booths BTW!
Last edited by MarkusN; Oct 04, 2007 at 07:35 AM.
Oct 04, 2007, 06:15 AM
Balsa for me, thanks.
irish_lord99's Avatar
Thread OP
Okay, looks like I'll have to abandon the water filter idea. Thanks guys! I'll look around and see what filter's are available locally.


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