spray booth filter material - advice needed

[bricol]

Every place I've worked at who have sprayed their own components have had filtration before the extract fan, mostly using simple to change cardboard framed panels. They were changed when they blocked up with overspray and stopped working.

The filters also helped stop paint building up on the fan impellor, as well as stopping the neighbours coming round complaining of contamination on the cars in their car parks (the best complainer being the Readymix depot - you couldn't see paint specs for all the cement and sand dust covering our cars . . ).

The most obvious one had a large assembly of filters - several square metres, and an enormous bifurcated fan to get the motor out of the air stream. Current place they are contained in quite a large assembly around the back of the paint booths and out of sight.

Best automotive paintshop around here showed me his very expensive system - he is surrounded by houses and I couldn't believe he didn't get hassle, based on previous place of employments issues.

 

[AVI_8]

The booths I've used have exhaust filters, usually on the back wall low down some exhaust through the floor otherwise the fan blades will become covered in paint, I'm pretty sure extraction filters are a legal requirement

View attachment 326655 View attachment 326656

I think what Parm was saying is that you don’t need filters on the exhaust, downwind of the fan, they should be upwind of the fan just like your pictures show.

 

[JimmyS]

The filters on the extract side will help protect the fan from paint build up over time. I think normally they are just a cheapish G3 fibre media filter but I'm no expert on spraybooths as normally I try and stay away from them. If a company is wanting one designed they are normally doing it as they dont fancy the quotes received to buy one and are trying to cut corners. The last one was for a company that makes fire rated duct and they wanted one about 10m by 3m from memory but thought they could knock something up themselves. I know its a different kettle of fish for something for DIY use but no point in going to the bother of making something if its not going to work and might be really dangerous in my opinion.

A spraybooth follows the same sort of airflow as something like a low grade cleanroom in as much as normally the air comes in at one side through a filter bank in a laminar flow/ piston flow type pattern and gets drawn through the extract in a similar fashion so you dont stir up duct etc with turbulance. Airflow could be horizontal or vertical over the room but normally it would be cheaper to do it so it moves horizontally. Vertical will be better as any dust etc will get drawn though the floor, helped by the air velocity and gravity buts its more expensive to do. The booth should be slightly negative in pressure to the surroundings to stop fumes escaping but only slightly ideally. The filter bank on the extract takes the heavy paint crap out but obviously not the solvents. It also helps even out the airflow as the pressure drop over the filter bank is fairly constant so the velocity across the whole bank becomes fairly constant. The velocity of the air and its laminar flow helps keep all the paint aerosol moving across the booth to the extract and away.

The extract filter bank and fan are normally sized to give a specific face velocity over the filters. If you havent got enough velocity over the booth then you will struggle to get the paint overspray etc to move towards the extract. If you havent got enough airflow then you will also risk getting high solvent levels in the booth which is not great. With the wee mixed flow fan you have, looking at the fan curves for it I recon you would get about 0.33m3/s or so flowrate. If you work that out to a face velocity over the back 2.4m side of the booth (best case) and say the filters were 2m high then you have an area of 4.8m of filter, with a fan doing about 0.33m3/s the face velocity would be about 0.07m/s - which is bugger all. If you look at commercial spraybooths the face velocities seems to be about 0.7m/s and in the USA it seems to be a minimum of 0.5m/s.

My worry would be that with the arrangement you are thinking about the flowrate would be too low to clear the overspray etc so you end up with a poorly performing booth but worse than that you could end up with a really high solvent concentration - assuming you are working with cellulose thinners. Thinners just seems to be a mix of any low boiling point solvents in various ratios, probably depending on cost/ availability. Typically seems to be a mix of stuff like acetone, Xylene, Tolulene and other similar things. You have the issues with breathing the stuff and paint aerosol if its 2 pack which can be mitigated by using a air fed mask but you also have the explosion risk. If you look at the lower explosive limits for some of these solvents you don't need very high concentrations to get a bang. I think acetone is about 2.5% and I'm pretty sure Xylene and Tolulene are about 1% but I could be wrong. The low flow rate from the fan could cause you to be working in a solvent laiden airstream that creeps up higher until it reaches the lower explosive limit and you get an explosion in the booth. I would think you want either a explosion proof motor on the fan or a bifrucated axial fan mounted outside the booth or something similar.

I hope this makes sense and you get something sorted out.

 

[Nick DV]

So just to clarify, if I had this set up in my garage, would I be able to spray things using 2k paint safely and have no impact on the environment? Obviously I would use the correct PPE inc. an air fed mask The spray booth will only be around 2m x 3m max as only used for spraying components not vehicles.

 

[Baldrick]

well, in the end ive ordered the fibreglass panel filters and the concertina style ones.

im going to start cutting holes in the wall today.
im going with a one metre square hole, with 4 of the 450mm square fibreglass filter panels, and a metre square section of the concertina style behind it.
The intake is slightly smaller, and fan is moving 1360m3/hr, which i will control off a fan controller to have a high enough outflow, and negative pressure in the room.

Thanks for your help.

 

[the snooper]

well, in the end ive ordered the fibreglass panel filters and the concertina style ones.

im going to start cutting holes in the wall today.
im going with a one metre square hole, with 4 of the 450mm square fibreglass filter panels, and a metre square section of the concertina style behind it.
The intake is slightly smaller, and fan is moving 1360m3/hr, which i will control off a fan controller to have a high enough outflow, and negative pressure in the room.

Thanks for your help.

I may have this wrong but I work your fan out to be 612 cfm a spray booth is in the region of 8000 cfm granted yours us smaller then a spray booth you'll just have to see

 

[carbon]

im going with a one metre square hole, with 4 of the 450mm square fibreglass filter panels, and a metre square section of the concertina style behind it.

I’m no expert on this but thought the concertina style filter was to be used on its own without anything in front of it as the air path through it traps the airborne paint particles. There is a high efficiency version that has a thin fibreglass mat on the back.

This is from the Andreae site

Can a prefilter be used in front of an Andreae?

It is not our recommendation. Typically, prefilters are used to increase the life of the filter behind them. Andreae Filters already last 3-5x longer than other media such as fiberglass. If the purpose of adding a prefilter is to increase efficiency, use an Andreae Original as prefilter backed with a high efficiency more expensive polyester filter.

Andreae Filters FAQ

 

[Baldrick]

ok, ill put the concertina in front of the fibreglass. it probably doesnt need the fibreglass panel filters, because the concertina has a additional filter to the back of it.
the fan is almost sucking the room inside out, i might have to turn it down, but will see when its up and running.
ill post some pics when its done...ive boxed in filter panels and making air box outside the room just now. .

its obviously done on a tight wads budget so if it works ill be delighted, its cost me about 60 quid for the filters so far, im even recycling the wood etc ...the tree huggers will be happy (er)

 

[vw1]

ok, ill put the concertina in front of the fibreglass. it probably doesnt need the fibreglass panel filters, because the concertina has a additional filter to the back of it.
the fan is almost sucking the room inside out, i might have to turn it down, but will see when its up and running.
ill post some pics when its done...ive boxed in filter panels and making air box outside the room just now. .

its obviously done on a tight wads budget so if it works ill be delighted, its cost me about 60 quid for the filters so far, im even recycling the wood etc ...the tree huggers will be happy (er)

Sounds very interesting. Can't wait for photos etc.