I am converting a 12' x 20' shed into a spray booth. My plan is to put a 24", 4,100 cfm fan in one end and build a filter wall in front of it. The filter wall will have a 5' x 8' grid of filters. The air inlet side will have a connecting tunnel to my shop with squirrel cage fans to provide positive pressure in the spray booth. This will also draw heated/cooled air from my shop. Attached is a picture of my plan...
Does this make sense? I believe 4,100 cfm will give me 100 fpm air flow. Does anyone have experience with this size room and fan? I am worried that the fan is overkill.
Much larger fan and your intake should be 2/3 larger than exhaust. Exhaust should be able to pull air through easily so you don't create a wind tunnel and exhaust should be on south side of building if possible.
There are several major problems with your plan. Suggest you find an HVAC pro and they can advise on the basics. According to your plan you'll end up very negative in both rooms and where the inlet is needs to encourage air flow in a straight line to your exhaust wall. If you open those double doors wide open you'll be a lot closer to supplying adequate air for that fan. I would cancel any plan to draw air from your conditioned space- It will only cause another set of problems esp if you have a forced air system running there.
you show on the drawing that the ht of the exhaust filter area is 5 ft high,i would recommend this, if your room height is 12ft clear i would go with a 10ft high plenum as you want a good laminar flow, also what size ductwork are you using from the main bldg to the spray area, you need to look at the pressure drop going thru the supply ductwork.
Use a three phase fan motor and control the speed with a VFD (variable frequency drive). Run it at full speed when your spraying and then slow it down when curing, this way you won't waste heat in the winter and AC in the summer.
You want a nice smooth/gentle flow not a lot of turbulence. I'd ditch the little 24" fan (probably noisy as well) for at least 30" and possibly a 36" belt driven multispeed fan. I would also not have air coming from the side bouncing off the far wall before exhausting. I'd screen off those double doors with filters and you should be good to go and clean.
Your fan with 4100 is way to small, like Herb said you need 100fpm at the face of the booth, example for a 10 x7 open face booth would require a 7000cfm fan.
I am not sure where you are located, but the one and only way to set up a spray booth if you are in a climate where there is colder winters is to have a make up air unit. For your situation separate the heating for the shed and the shop. Install a make up air unit to feed the shed/spray area. I know it costs a lot, but it is the only way. Sorry for the bad news.
Thanks for all the comments...I've learned a lot and have a lot to think about. I would like one piece of clarification. From the comments and from reading previous forum entries I understand the size of the fan is determined by the width x height x 100 fpm. However, is this the size of the total wall (in my case 11' wide x 7' high) or the size of the filter opening in the wall (in my case 8' wide x 5' high)? This would dictate either a 7,700 cfm or a 4,000 cfm fan.
From what I can find air requirement is based on the size of the filter wall. Also note that in the case of large booths 50CFMs is allowable. So technically your 4100cfm fan would do the job 8x5x100=4000. I would still go with the larger/quiter multispeed belt drive fan. Then you can adjust air flow to meet your situation. Also it's not just about safety but getting good finish results
"To calculate the amount of exhaust air needed for the booth, a simple calculation is used. CFM = Booth Face Area in square feet multiplied by the required flow (typically 100 FPM)
For example, and 8 X 10 filter bank (80 square feet) would require an exhaust of 8000 CFM (100 X 80) to achieve the required 100 FPM velocity.
To calculate the existing velocity knowing the exhaust volume the following formula is used:
Velocity = Exhaust in CFM divided by the booth filter area.
For example, a fan that exhausts 9000 CFM with a 10 X 10-filter bank would have a velocity of 90 FPM.
You're getting on track now. As drawn you would have 100fpm but only right at the filter wall. 5' in front of the filter wall would be significantly less air movement.
You could build out sidewalls and ceiling from the filter wall that match the dimension of your filters but I'm sure you probably can't work with 5' ceiling ht. BUT within that confined space and at the very front of it where the walls stop you would technically be capable of 100 fpm.
This is why you see open face booths usually have walls and a ceiling that basically match the filter wall- you constrict the larger space down so you increase the air flow to what it needs to be within the booth. As soon as you get outside the booth your airflow drops.
I stress "on paper" and "technically speaking" because there are other issues that affect proper air flow that have to be factored in so the math is not as simple as it might seem.
I encourage professional help with a booth installation.
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