Spraying Acrylic with Alcohol - Will I Blow up the Garage?

kanuck wrote:

I'm not sure where you get 2.2 for LEL.  My OSHA and NIOSH references both say 2.0%.

Characteristics of a product will vary from manufacturer to manufacturer.  While OSHA and NIOSH do list a more conservative LEL, some of the other research data I have shows IPA at around 2.2. 

kanuck wrote:

Correct and interesting but not an issue since I will have the fan running.

Sorry, sometimes I don't articulate very well. I think that was intended to provide a simple starting point by showing vapor concentration under static conditions.  In actuality, I believe it does become relevant unless one leaves a booth running 24/7.  In inefficient booth design, vapors can pool inside ductwork and accumulate during spraying operations.  After the booth is turned off, concentrations increase as a result of continued flash-off from both the object sprayed and overspray on the booth walls and filters.  This is basic ventilation theory, and is demonstrated regularly in industry as fan belts loosen, blades accumulate debris and/or corrode, and as filters begin to clog.  Been there, seen it, have the t-shirt.

But is it realistic for modelers? The theory remains the same, and I think the practical considerations remain relevant; to the extent they are prioritized and implemented depends upon a host of variables, answerable only by the individual modeler and their requirements.

kanuck wrote:

I think I can safely conclude that a 180 cfm range hood is rated at that with the range hood filter in place and nominal ducting as would be the case for a kitchen installation.  Agree?

No. Fans are selected based on the requirements of the system, and are chosen based on their flow rate and their pressure requirements (fan static pressure, or fan total pressure).  Flow rates are expressed as actual cfm at the fan inlet.  Filtration and ducting are factors that have to be considered in fan selection.

kanuck wrote:

On this point you are absolutely dead-wrong.  My intent was to start with some assumptions, do the calcs, and see if a range-hood was feasible.  I didn't not start with a conclusion that I would use a range hood and back-calculate the answer that supports an un-informed choice.

Sorry, didn't mean to pre-judge.  Unfortunately, in the time I've been involved with this stuff, I have seen many facilities-related issues where my statement regarding "blind" justification was precisely what was intended by the personnel I had to deal with.  Often, these justifications were predicated on what happened to be on hand at the moment, coupled with a "can-we-just-get-away-with-this-for-awhile" mentality.

kanuck wrote:

Correct that I will use whatever I desire, but hopefully founded on some reasonable science and conservative assumptions.  Just like I wouldn't put in a dust-collection system in my garage to saw a 2x4, I won't put in an elaborate spray booth if the risks are already ALARP.  Reasonably practicable is of course in the eye of the beholder.

Keith, I think you understand better than most.  "Reasonably practicable" is one of the factors that makes industrial hygiene an interesting, and sometimes complicated, field of work.  After all the data is collected and risk is ascertained, professional judgement still plays a vital role in balancing process improvement with protecting personnel.  Reasonably practicable in the eye of the beholder really depends on whose eye you're talking about.  Obviously we try to work alongside a client to give them a solution that works for them.  At other times a governing standard is the deciding factor, whether the client likes it or not.  Of course, MONEY is always the overriding issue...

styrene wrote:

If you are suggesting that I've just been educated, and choosing to ignore the advice is akin to driving drunk (even though I have jumped through some analytical hoops to arrive at a conclusion), I'd have to agree its a bad comparison.

No matter how you slice it, behavioral modification, or getting folks to change the way they think about an issue, is still one of the best ways to approach a healthy work environment.  Removing the human element from a process can only be so effective.  If my advice on these threads has helped someone reduce a potential hazard in their home, then I feel as if I've accomplished something. 

Gip

Excellent note!  My thoughts follow...

 

styrene wrote:

Okay, let's see: 1.  LEL of Isopropanol (IPA) = 2.2%. Since 1% = 10,000ppm, then the LEL=22,000ppm. So 10% of the LEL = 2,200ppm.

I'm not sure where you get 2.2 for LEL.  My OSHA and NIOSH references both say 2.0%. 

styrene wrote:

2.  Let's say I instantaneously vaporize just 2ml of 100% (anyhdrous) IPA into a spray booth measuring 2' X 2' X 2'.  Using 1 atm (760mm Hg) and 20 degrees C, and given completely static conditions, concentrations inside the booth = approx. 2,880ppm, 600 ppm greater than 10% of the LEL.

Correct and interesting but not an issue since I will have the fan running.

styrene wrote:

3.  Factoring in the fan, however, given 180cfm in an 8 cubic foot booth, provides approx. 22.5 air changes per minute, more than enough to dilute IPA vapors below an explosive concentration.

Yup!

styrene wrote:

4.  HOWEVER, the size of the fan is not big enough in cfm rating to provide adequate exhaust.  The largest booth you can build to give you a MINIMAL 80fpm face velocity with that fan is 1.5' X 1.5'; and even then it's not enough.  Once you add in filtration and ducting you put the fan under a fairly significant load.  Without the appropriate "oomph!" to drive the vapors out the exhaust duct, organic vapors may begin to pool and collect in corners and dead spots within the system and inside ducting.  Leaving your paint job overnight to flash off, and the solvent in overspray pigment in the filters to do the same causes an increase in vapor concentration.  Cleaning your brush afterwards will also cause higher solvent concentrations.  Do you think you'll ever spray with lacquers, or maybe with a spray can? (And please, never say "never."  I know better.)    My concern would not only be what happens when you're spraying, but additionally, what might happen when you turn your booth on.

I think I can safely conclude that a 180 cfm range hood is rated at that with the range hood filter in place and nominal ducting as would be the case for a kitchen installation.  Agree? 

I would of course leave the fan running during cleaning, and for a while after my work is done using a timer on the circuit.  Good point though on what might settle in overnight.  I would hit the corners of my booth, and the duct, with compressed air for a couple of minutes before 'firing-up' (grin) the fan the next time.

Regarding the use of lacquers or spray cans, my current position is that I won't.  That being said, if I ever did, I am fully aware of the limitation of a range-hood booth and would make alternate appropriate spraying arrangements (e.g. outside)

styrene wrote:

I know you're looking for justification to use a range hood.

On this point you are absolutely dead-wrong.  My intent was to start with some assumptions, do the calcs, and see if a range-hood was feasible.  I didn't not start with a conclusion that I would use a range hood and back-calculate the answer that supports an un-informed choice. 

styrene wrote:

The bottom line is that you'll use whatever you desire.  Many of the points I raise are risk factors.  Mathematically and chemically, I can't prove to you that what you have in mind is not the way to go.  I've only seen a couple spray booth fires, and all I can say experientially is that because of their design criteria, the buildings they were in did not burn down, and the fire was completely contained.  Professionally, however, all I know is that the risks far outweigh any perceived benefit.

Correct that I will use whatever I desire, but hopefully founded on some reasonable science and conservative assumptions.  Just like I wouldn't put in a dust-collection system in my garage to saw a 2x4, I won't put in an elaborate spray booth if the risks are already ALARP.  Reasonably practicable is of course in the eye of the beholder. 

styrene wrote:

It's interesting, isn't it?  Educating people on driving safely, not drinking and driving, coming to a complete stop at a red light and stop signs, posted speed limits in crowded neighborhoods, and putting air bags in cars, and still 20 or 30 thousand people a year are killed in auto-related incidents.  Maybe it's a bad comparison, but still...

If you are suggesting that I've just been educated, and choosing to ignore the advice is akin to driving drunk (even though I have jumped through some analytical hoops to arrive at a conclusion), I'd have to agree its a bad comparison. 

styrene wrote:

To you and Bgrigg I say, "Get the proper fans!"

I absolutely promise to get the proper fan, I just can't promise it will be the fan you recommend. 

styrene wrote:

Oh, BTW, if you have to wear a respirator with a spray booth, the booth is designed/performing poorly.

I agree 100%.

styrene wrote:

OK, I've had my say.  Please feel free to flame away.

No flames from me!!!  Your observations are excellent, and the insights you provide this community are invaluable.  I hope you see my reply as nothing but a straight-forward sharing of perspective in the spirit of healthy, respectful and open dialogue.

As for the final choice on the range-hood, the jury is still out while I reflect on your thoughts, risks, costs, etc.

A huge thanks for your very thoughtful reply to my original post!!!  This forum and it's members are fantastic.

Cheers!

Keith

See? I told you!

And he's absolutely right. I'm in the early planning stages of converting my booth to either a downdraft or backdraft design. Early planning equaling talking about it.

And I think the auto-related deaths is over 40,000...

Cheers,

Bill 

Okay, let's see:

1.  LEL of Isopropanol (IPA) = 2.2%. Since 1% = 10,000ppm, then the LEL=22,000ppm. So 10% of the LEL = 2,200ppm.

2.  Let's say I instantaneously vaporize just 2ml of 100% (anyhdrous) IPA into a spray booth measuring 2' X 2' X 2'.  Using 1 atm (760mm Hg) and 20 degrees C, and given completely static conditions, concentrations inside the booth = approx. 2,880ppm, 600 ppm greater than 10% of the LEL. 

3.  Factoring in the fan, however, given 180cfm in an 8 cubic foot booth, provides approx. 22.5 air changes per minute, more than enough to dilute IPA vapors below an explosive concentration.

4.  HOWEVER, the size of the fan is not big enough in cfm rating to provide adequate exhaust.  The largest booth you can build to give you a MINIMAL 80fpm face velocity with that fan is 1.5' X 1.5'; and even then it's not enough.  Once you add in filtration and ducting you put the fan under a fairly significant load.  Without the appropriate "oomph!" to drive the vapors out the exhaust duct, organic vapors may begin to pool and collect in corners and dead spots within the system and inside ducting.  Leaving your paint job overnight to flash off, and the solvent in overspray pigment in the filters to do the same causes an increase in vapor concentration.  Cleaning your brush afterwards will also cause higher solvent concentrations.  Do you think you'll ever spray with lacquers, or maybe with a spray can? (And please, never say "never."  I know better.)    My concern would not only be what happens when you're spraying, but additionally, what might happen when you turn your booth on. 

I know you're looking for justification to use a range hood.  The bottom line is that you'll use whatever you desire.  Many of the points I raise are risk factors.  Mathematically and chemically, I can't prove to you that what you have in mind is not the way to go.  I've only seen a couple spray booth fires, and all I can say experientially is that because of their design criteria, the buildings they were in did not burn down, and the fire was completely contained.  Professionally, however, all I know is that the risks far outweigh any perceived benefit.

It's interesting, isn't it?  Educating people on driving safely, not drinking and driving, coming to a complete stop at a red light and stop signs, posted speed limits in crowded neighborhoods, and putting air bags in cars, and still 20 or 30 thousand people a year are killed in auto-related incidents.  Maybe it's a bad comparison, but still...

To you and Bgrigg I say, "Get the proper fans!" 

Oh, BTW, if you have to wear a respirator with a spray booth, the booth is designed/performing poorly.

OK, I've had my say.  Please feel free to flame away.

Gip

 

gjek wrote:

In the case of acrylics, windex doesn't have an explosion point that I am aware of. You didn't say but my assumption is you are exausting the fumes outside.

Excellent points made, and you're right in that I'll use Windex for cleaning.  If it's good enough for the folks in 'My Big Fat Greek Wedding' for everything that ails ya, it's good enough for me.  As for the flash point hazard of Windex, the MSDS says" "it is an aqueous solution containing an alcohol and does not sustain combustion".

And you're correct, I'm venting outside.

Cheers!

  I was trying to think of a worse case scenario, maybe spraying 1/3 oz of lacquer thinner while cleaning your AB after use with enamels. I wouldn't worry about the paint mixed with alcohol because when you are painting you maintain control over the volume of paint. I doubt you would crank the paint out at max rate for 9 seconds without ruining what you intended to paint. That would be 1/6 oz alcohol into about 30 sq ft of air over 9 sec at STP . I think you would be under the explosion point. When cleaning however there is a much higher chance of blasting cleaner at a higher rate. In the case of acrylics, windex dosen't have an explosion point that I am aware of. You didn't say but my assumption is you are exausting the fumes outside.

Gip will suggest that both of us get "proper" fans!

Thanx for the welcome!!

Well, with all the calculations in the world, there's nothing like some emperical experience to tell a story too!!  Glad to hear you've experienced nothing when AB spraying.

I of course didn't mention the required opening to provide some respiratory protection for me.  At 180 cfm, I couldn't have an opening of greater than 1.8 to 2.25 ft2 to achieve 80 - 100 fpm face velocity.

I see that Gip (Styrene) is a great resource on this stuff.  I wonder what his thoughts would be.

Cheers!

Kanuck

Whoa, slow down. I don't understand half of that!

I'm using a 36" Nutone and Tamiya Acrylics thinned with their thinner (which just happens to be Isopropyl Alcohol mixed with Acrylic Retarder) and I have had any explosions yet.

I even went so far as to leave a votive candle burning while spray 99% Isopropyl and still have all my eyebrows! Actually, it blew out the candle without igniting.

So, I don't recommend it to others, but I use one myself.

BTW welcome to the forums!