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Spray Booth Safety Consideration (Extraction & Explosivity)

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J_Ashley

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I’m looking at setting up/purchasing a spray booth, but, not having much experience with them, I want to make sure what I decide upon meets both performance and safety requirements.

It will be used for spraying a variety of finishes, of which shellac probably raises the most complications. I have read that the potential for explosion with rising concentrations of alcohol mean that explosion proof equipment (fan/electrics) becomes necessary. The spray area will be about 3’ x 4’, and so I was aiming for a fan rated at around 1200 CFM (2000m³/h).

I currently have a particular spray booth in mind where the motor sits outside of the airstream, the electric light is sealed, and the switches are all on the outside of the unit. So whilst not advertising itself as explosion proof, it seems at least safety considerations have been factored into the design.

I’ve read about Lower/Upper Explosive Limits, but I’m unsure of the reality of the situation. Should the above setup be enough to keep concentrations down, and thus be sufficiently safe?
 

guineafowl21

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From vague conversations about setting up a distillery, I think the thing you need to research is ATEX ratings. I can’t be much more help than that, but if the spray booth is manufactured for that purpose and installed correctly, I can’t see too many problems.
 

Sideways

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If it blew up and you hadn't taken proper advice, would your insurance pay up ?
If that doesn't matter, then for what it's worth, it sounds like the booth you're looking at is on the right lines.
I would do some math just to see if you are anywhere near the explosive atmosphere concentrations and need to worry or not. I'm not saying this is right, but I find making estimates and doing rough calculations helps me understand unfamiliar situations.
Make some worst case guesses like
What % of your coating is flammables / solvent ?
Worst case, 100%.
How fast can you ever dump it into the air just pulling the trigger ?
Maybe 1 litre in 4 or 5 minutes ?
How much air will the fan drag through your booth in those 4-5 minutes - assume it only achieves half or quarter the advertised airflow as filters clog up, manufacturers exagerate and ducting slows everything down.
(say) 2000 m3/hr * 1/4 for real world airflow * 5min/60 min = 40 cubic metres
Is 1/40 litre (about 25ml or maybe 25grams) flammables per cubic metre of air an explosive atmosphere ?

If you load the atmosphere with solvent without turning the extractor on first, all bets are off ! :D
 

J_Ashley

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Resurrecting a previous post of mine……

I’ve decided to go with an 18” Atex rated fan, as that should cover a range of potentially hazardous sprayable materials and also provide sufficient CFM given the planned booth opening.

With regards to setup, I wondered whether anyone with experience of spray booths could offer any comment on the attached design (not my own). I am also considering moving the fan and blue filter to the bottom, but essentially preserving the same layout. However, I’m unsure whether I’ve overlooked something and the fan placement is not ideal.

Booth.JPG
An 18” fan should provide sufficient CFM that the drop off caused by the filtration means air movement is still great enough.
 

scholar

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I cannot comment on the explosion proof considerations - for this reason, I only spray water-based finishes in my spray-booth. (I did a write—up of my knock-down set-up years ago here).
Just one point occurs to me from my own experience - you look like you are fitting the concertina pleated type filter - I use this, but soon found that quite a lot of fine dust does find its way through into the back of the box and out through the exhaust airstream - I fitted a secondary box filter behind the main pleated filter and this makes a big difference.

looking forward to seeing the completed setup

cheers
 

scholar

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I don’t really follow your picture btw (as regards airflow, particularly).

Anyway, as mentioned, my spray booth was designed on different, non-explosion-proof, principles, but for your info, here is the original write-up I did years ago.


It has served me well and there is not much I would do differently. I subsequently added a big snake of spiral reinforced plastic ducting (like play tunnel) the diameter of the fan outlet - this can then be directed out of the room/through a window.

cheers
 
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Sideways

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Interesting.
He's charging money for plans but seems to have done nothing about specifying the fan used, specifying the design airflow, the air resistance losses due to the cabinet design and the clean / fully loaded air filter. There isn't any indication that he even attempted to measure the airflow of the completed unit.
That's money for nothing in my book. Anyone can design a plywood box ...
 

scholar

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I have no basis to knock the design and I am no expert on spray booth technology, but a couple of observations:

- the cupboard idea looks neat, but be prepared for the cupboard doors to get coated with overspray, making it impractical.

- the booth is really quite limited in size and only suitable for the sort of very small parts he shows himself spraying

- an 18” fan as the OP proposes sounds a good beefy size, but is much bigger than the little fan the youtuber shows. [Also the youtube says his is just a brushless fan - that is not necessarily explosion-proof, which requires ATEX rating as others have said].

- the fan in the design redirects the airflow back to the work area - this is something I would aim to avoid as it risks (guarantees) airborne dust being drawn back onto the workpiece/drying area.

- the youtuber shows him sanding into the spray booth - that is a ridiculous idea and what dust extractors are for.
 

Ozi

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As important as the amount of air flow is the effectiveness of it, it is very easy to get high flow but leave dead spaces and re-circulation, a few years ago I used to inspect items like these and fume cupboards etc. That doesn't make me a designer but I have seen some very good and very bad systems, in professional use. Unless you are always working on small items you will want the whole cabinet getting consistent air changes so it can be good to avoid a single point of extraction, one set-up that can work well is to extract from behind a slightly restricting screen such as a perforated plate. You will probably have seen steel sheet that's drilled with a pattern of holes. That will reduce your total flow but even it out. Any dead spots will show up as they stay clean. Also where the air comes in and is extracted to matters. When you first set-up get some baby powder and puff it around the cabinet. Ask someone else to watch while you stand in the position you will be in while spraying as you will affect the air flow. If you use a strong light with the rest of the area a bit dimmer than you would normally have a workshop they should see if any of the dust is coming back past you or collecting in pockets in the corners of the cabinet. Final thing the dusty flammable mist and vapor will be going somewhere. The worst examples I have seen include the shed where extracted saw dust collected, a canteen, the corridor outside a chemistry class room, don't add your washing line or next doors car to that list.
 

martin.pearson

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Something else that you may or may not have thought about. Where is this spray booth going to be situated & will your insurance cover you if by some chance something goes wrong? Don't just phone & ask, make sure you have something in writing from the insurance company.
 

Sideways

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Accepting this box for what it is, a common minimum specification that is seen in commercial spray booths is an airflow equivalent to about 2.5 miles per hour (if I remember correctly) away from the operator towards the part. This can be towards a spray back or a downdraught towards a ducted floor.
Partly this is to carry particles and fumes away from the operator.
Partly it is driven by another specification - achieving a minimum number of air changes per hour inside the spray booth (in a spray booth, the back is normally large - say 1/3 the area of the entire end wall or more)

A small spray back in a cabinet is not going to achieve anywhere near the desired number of air changes in a room, but I would expect a credible design to achieve at least the 2.5mph airflow across the full surface area of the filer, with the filter loaded up with paint from several spray sessions.

Paint filter manufacturers do quote a pressure drop for their filters when new and when fully loaded with a given weight of paint residue per square metre. More expensive filters will take a greater paint load and create less pressure drop. Pleated cardboard with holes on the front helps the filters last longer but adds to the pressure drop.
An extra factor (+25%, +50% ?) should be added for pressure drop inside the cabinet as the air changes direction, scrubs against the surfaces and is pulled through apertures. If a fan faces a hard surface within one diameter of it's inlet or outlet (again if I remember aright) this has a big impact on the airflow and really should be avoided. Get too close and the airflow is absolutely crippled.
2.5mph x the filter area will give a number for the cubic metres/ min airflow needed at that pressure drop
Characteristic curves will be available from any reputable fan manufacturer to show what airflow it can move at what pressure drop. It's then just a case of looking up on the graph to see if the fan is up to the job....

A few numbers would make the whole thing a lot more credible :)
 
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J_Ashley

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A few more details……

Space is moderately restricted (although can’t exceed about 2.10m including ducting), as I don’t want to sacrifice a significant volume of the workshop over the spray booth. Also, it will only be used occasionally, hence I like the ‘wardrobe’ style as it can essentially be closed up and forgotten about when not in use.

I was hoping to mount the fan vertically (fans are typically around 45-50cm deep), either above or below the spray chamber, to save on the overall depth of the booth. However, compared to having it positioned horizontally directly behind the filters, I was unsure whether this would be detrimental to airflow.

On paper an 18” fan seems too big, but I’m assuming that the fan will likely not rate as high as advertised, and I will end up losing significant airflow to filters and other resistance. I have also heard that too greater airflow is also problematic, as the finishing material is pulled away at too fast a rate.

Other details:
  • Primary use would be to spray guitars. A 5ft x 3ft booth opening should allow for a little manoeuvrability, and suspending the guitar from the booth ceiling on a rotatable bracket will improve access further.
  • Assuming 100CFM per square foot is required (I’ve seen this quoted in various sources), then I need circa 1500 CFM. However, I realise this needs to be after the drop off caused by filters and any other resistance to airflow.
  • I didn’t include anything on the above diagram, but the fan will be ducted directly outside (into my garden). It will not be directed towards any building or obvious obstacle.
  • The fan will definitely not be used for other extraction purposes. Shop vac, chip extractor and air filter are covering other bases.
Got to be honest, I appreciate all the advice, but it shows how many considerations and complications there are in doing things like this properly. Providing a spray booth is setup correctly, obviously that permits more experimentation with a greater number of finish types, but maybe it’s just not worth the hassle. Makes me think I should just stick to French polishing with a rubber or spirit varnishing with a brush!
 

Sideways

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Think qualitatively about it.
2.5mph is a slow walking pace. Just enough to be able to take the fumes and droplets away from you when you spray. You barely feel that airflow when you walk.

2.5mph across a 5x3' spray back is 3300 cfm, which must be achieved at a reasonable suction to overcome the air resistance.
So your numbers look a factor of two too low to me.

You will need a centrifugal type fan to create enough suction. Simple plate fans as designed to mount in walls don't generate enough suction to work in this application. Their airflow drops dramatically as soon as they encounter any resistance.
You are doing small projects. Keep your filters clean and your airflow will be high enough that you never approach the explosive limits so no need for ATEX fans.
Cheapest may be one of those hamster wheel type centrifugal fans. They generate better suction than many, are a commodity item, and as nothing of note should get through your filter, the fine blades shouldn't get damaged or gummed up.
How I would do it anyway :)
 
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Henniep

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I’m looking at setting up/purchasing a spray booth, but, not having much experience with them, I want to make sure what I decide upon meets both performance and safety requirements.

It will be used for spraying a variety of finishes, of which shellac probably raises the most complications. I have read that the potential for explosion with rising concentrations of alcohol mean that explosion proof equipment (fan/electrics) becomes necessary. The spray area will be about 3’ x 4’, and so I was aiming for a fan rated at around 1200 CFM (2000m³/h).

I currently have a particular spray booth in mind where the motor sits outside of the airstream, the electric light is sealed, and the switches are all on the outside of the unit. So whilst not advertising itself as explosion proof, it seems at least safety considerations have been factored into the design.

I’ve read about Lower/Upper Explosive Limits, but I’m unsure of the reality of the situation. Should the above setup be enough to keep concentrations down, and thus be sufficiently safe?
The design and use of spray booths using flammable substances pivots around electrical/intrinsic safety. Not being from your neck of the woods, I can't quote specific literature but suggest that there must be safety legislation or an ISO standard that provides the standards/info that you are looking for. Best of luck mate.
 

scholar

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I assume something along these lines should suffice:

Airbox 5000 m³ per hour
Just with regards to the airflow capacity, I have no doubt that fan would provide sufficient for your design, and probably well over sufficient. You would need a speed controller (which helps to dial down the noise to the minimum necessary to achieve the desired airflow); as you have already commented, too much airflow doesn’t help.

I originally worked on a gross airflow guide of 6000m3/hour but that is way more than I find is needed for my spray booth setup, which looks to be a bit larger area than your plan

I have no idea if the unit you show is ATEX rated (unless i missed something).

cheers
 

scholar

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PS I didn’t follow the comment that the fan needed to be a centrifugal type (rather than Axial). However, I do know that beyond the basics, extractor specs get very technical, so there may be some issue I am not aware of.

As far as I know, my spray booth fan is not ATEX rated, but I do sometimes wonder if I would like to spray solvent based material, so am following this discussion with interest

Cheers
 

J_Ashley

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However, I do know that beyond the basics, extractor specs get very technical, so there may be some issue I am not aware of.
I am certainly finding this.

Based on the advice above I've tried to think more about the numbers involved. Although I hope to spray some shellac (i.e. potentially an explosive hazard) the amount sprayed in one session would be minimal. Therefore, by concentrating on good airflow and decent (and regularly replaced for clean) filters, the concentration of solvents should never reach anywhere near the Lower Explosive Limit. However, there’s still the nagging doubt that I’m being naïve.

ATEX rated fans that achieve sufficient airflow look to be prohibitively expensive though. Plus then you're into different lighting/electrics too.
 

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