Some questions on building a DIY spray booth fan

[Saki 4]

Hi, I want to build a spray booth in my basement studio. The design I have in mind would have a spray area 36" x 36" x 36" max, with a filter at the back and a fan behind the filter. The fan would then connect to an exhaust hose and vent outside through a basement window directly above and behind the booth.  I have some questions about the fan:

(1) What is the best type of fan for this setup? (I was thinking of an inline duct fan?)

(2) What is the minimum CPM I should look for in the fan, especially if the air passes from the fan, through an exhaust duct, then outside?

(3) As an alternative, I am considering a "waterfall" style booth, where water flows down the back and sides of the booth in lieu of a filter. In this case, where would be the best place to locate the fan — underneath the work area? Also, in a waterfall style booth, does the fan still need to vent outside? 

Any suggestions or feedback would be great.

Thanks.

[liambesaw 1,707]

Either centrifugal or squirrel cage fans for pulling air through a filter.   An inline duct fan not gonna do much if it's an axial fan.

[Saki 4]

Ah, that makes sense. Thanks.

By any chance, do you have any suggestions on the power/CFM necessary?

[liambesaw 1,707]

I don't, I've only built filters like that for painting and venting industrial fumes.  It shouldn't take much.  Little 4 inch ones from Amazon should be fine I'd think, they'd cycle air through a 3x3 box pretty quick

Edited March 4, 2019 by liambesaw

[Bill Kielb 652]

so ..............paint spray exhaust quantity. The generally accepted rule is 100 feet per minute face velocity which means 3’ X 3’ =. 9 sq, ft and 9sq ft. X 100 ft/min = 900 cfm. That would be free air though so by regulation you would need to account for the filter pressure drop and discharge duct losses. OSHA considers this standard for employees and based on maintaining overspray under peremissable lower exposure levels.

wow what a mouthful! Most that I measure end up to be in the 50 to 100 feet per minute face velocity which then results in about a 500 - 1000 cfm fan for small booths. Spraying glaze is a bit different in that it is not volatile, is water based and often is not the primary purpose of the surrounding area.

having said all that, rarely do I ever measure a home booth that is compliant but at 500 cfm only the majotiy of the ejected spray is that which is deflected back by the pot. Most folk wisely use their masks to spray, use low pressure or HVLP and leave the fan running for some time after they are done spraying which is helpful.

next important point is providing makeup air from outside else you end up sucking all the fumes back in from any gas appliances.

axial fans are fine as long as rated, but these fans are not duct booster type when we are talking 500-1000 cfm.

I have attached an industrial standard that likely applies as OSHA applies where a business has an employee.

we always encourage folks to be safe and compliant which means air velocity, fire safety of large exhaust fans as well as electrical safety.

good luck, and be as safe as practical,  just so you know a bathroom type fan or small exhaust is not nearly compliant not near 500- 1000 cfm. How big is 1000 cfm? If your house has a 3 ton air conditioner then your furnace high fan speed is likely 1200- 1500 cfm for your whole house.

 

 

Edited March 4, 2019 by Bill Kielb

[Bill Kielb 652]

31 minutes ago, liambesaw said:

I don't, I've only built filters like that for painting and venting industrial fumes.  It shouldn't take much.  Little 4 inch ones from Amazon should be fine I'd think, they'd cycle air through a 3x3 box pretty quick

Yeah, technically these are probably not compliant and provide about 100-150 cfm on a good day. Tough problem to comply actually for most homeowners. 

Edited March 4, 2019 by Bill Kielb

[liambesaw 1,707]

Bump it up to 6 inch to get closer to 500cfm, 7 cycles per minute then

[Bill Kielb 652]

5 minutes ago, liambesaw said:

Bump it up to 6 inch to get closer to 500cfm, 7 cycles per minute then

Yeah, technically compliant is probably 1000 but I do see many at about 500 and a six inch axial will get you probably 600 free air so the choice of many. Axial not the greatest for volatile vapor though as it needs,to be explosion proof rated, so if ya ever start spraying regular paint in the booth ......... I like to stress the makeup air though because often folks don’t think about it and end up with a dangerous CO surprise.

[Dick White 318]

I built a small waterfall booth out of two plastic laundry sinks stacked and screwed together like a clam shell with the front side of the upper one cut away. The waterfall is made from standard small pvc plumbing pipe with small holes drilled in it every several inches, arranged around three sides of the top. The water flows down the sides and out the drain in the bottom of the lower sink and into a tub sitting on the floor underneath. This tub is half full of water and has a standard sump pump in it to recirculate the water back up to the piping at the top. Because there is no need to pull the overspray out via serious fan volume (the waterfall captures it and washes it down), the only need for exhaust is to relieve back pressure in the enclosure from the compressed air of the spray gun (i.e., so that the overspray that didn't make it all the way to the waterfall on the back wall doesn't just blast back out the front opening into your face), I used a high CFM bathroom vent fan (high CFM being relative to that class of device) mounted through the top of the booth. I chose a bathroom vent fan that also had a light to better see what I am spraying. I made this several years ago, but I don't think I spent more than $100USD.

[Magnolia Mud Research 618]

Without equal volume of makeup air, the exhaust fan will NOT produce even the 500 ft/min velocity minimum.  the blades will be moving and just heating the air and overheating the motor.   

A major problem of inline blowers for spray booths - paint or glaze - is clogging the motor cooling system.   

LT

[Bill Kielb 652]

4 minutes ago, Dick White said:

I built a small waterfall booth out of two plastic laundry sinks stacked and screwed together like a clam shell with the front side of the upper one cut away. The waterfall is made from standard small pvc plumbing pipe with small holes drilled in it every several inches, arranged around three sides of the top. The water flows down the sides and out the drain in the bottom of the lower sink and into a tub sitting on the floor underneath. This tub is half full of water and has a standard sump pump in it to recirculate the water back up to the piping at the top. Because there is no need to pull the overspray out via serious fan volume (the waterfall captures it and washes it down), the only need for exhaust is to relieve back pressure in the enclosure from the compressed air of the spray gun (i.e., so that the overspray that didn't make it all the way to the waterfall on the back wall doesn't just blast back out the front opening into your face), I used a high CFM bathroom vent fan (high CFM being relative to that class of device) mounted through the top of the booth. I chose a bathroom vent fan that also had a light to better see what I am spraying. I made this several years ago, but I don't think I spent more than $100USD.

Hard to find real standards on water systems, it’s pretty subjective and without 100 feet per minute  capture velocity its hard to capture the spray particles. So your home made is likely not compliant as far as standards go

[Bill Kielb 652]

34 minutes ago, Magnolia Mud Research said:

Without equal volume of makeup air, the exhaust fan will NOT produce even the 500 ft/min velocity minimum.  the blades will be moving and just heating the air and overheating the motor.   

A major problem of inline blowers for spray booths - paint or glaze - is clogging the motor cooling system.   

LT

100 feet per minute velocity is the general specification which translates usually to 1000 cfm +. ...........   500 cfm is often what we measure on non compliant systems. The velocity is the important part to capture the overspray.

Edited March 4, 2019 by Bill Kielb

[Magnolia Mud Research 618]

Yhea, I used the wrong units.

The point is that the system will adjust the net flow to equalize what goes out to match what comes in.  Makeup air oftentimes becomes the limit and the spray booth does not function as expected.  

[Bill Kielb 652]

15 minutes ago, Magnolia Mud Research said:

Yhea, I used the wrong units.

The point is that the system will adjust the net flow to equalize what goes out to match what comes in.  Makeup air oftentimes becomes the limit and the spray booth does not function as expected.  

Yes, understood this is the return air to the fan so you are right, losses on the suction side of the fan are critical especially with motors cooled by their own airstream. Makeup air though is outside air supplied from outdoors to offset the exhaust. Without it folks tend to suck the combustion gases back down the flue of their water heaters and furnaces. We always try to stress the need for makeup air to offset the exhaust and not depend on infiltration through windows and doors to be a source for this.

your point is absolutely correct in that system losses should be accounted for in the initial design and later maintenance.  Makeup  air from outdoors to offset the amount being discharged by the fan are often overlooked which can have catastrophic health effects mainly excess Carbon Monoxide, carbon clogged heat exchangers and so on. 

Edited March 4, 2019 by Bill Kielb