Thermodynamics question... compression ignition possible in a spray gun? - PeachParts Mercedes-Benz Forum

777funk · #1 03-03-2010, 04:20 PM

I know PV=nRT so that would mean that a change in Temp (increase) would also equal an increase proportionally in Pressure.

but here's a practical question...

I've read about warming lacquer in a spray gun before spraying (I build electric guitars for a living). I've read that Gibson back in the 1950s heated the lacquer to around 140 or so without any ill effects but a few people have made me wonder if the pressure from heating lacquer in a sealed container or even in a pressurized container at times (pressure pot) could cause an explosion similar to what happens in a Diesel engine (compression and heat equals BOOM!!)

Curious of this. I know there are always exceptions to rules and strange things like spontaneous combustion (oily rags in a trash bin igniting without an ignition source etc.) that happen at times.

I am curious if there are any rules that would make a highly flammable solvent based material explode under pressure at these temps (110-150 degrees F).

I think in our diesels the compression pressure is more like 300 - 450 psi and with a fast change in pressure resulting in a temp change.

But the last thing I want is a quart or two of lacquer popping a paint gun through the roof or through me

compress ignite · #2 03-03-2010, 05:14 PM

If as the Urban says they were heating "It" ,What were they heating it in?
[The Supreme Commander's (That's the Gentler Sex) Pressure Cooker?]

MS Fowler · #3 03-03-2010, 06:03 PM

If I were to spray heated paint, I would heat it OUTSIDE of the spray gun; not in a sealed container. After its warm, I'd pour it into the gun, and spray a test pattern, and then apply.

turbobenz · #4 03-03-2010, 06:18 PM

No oxygen inside spray cans. Even if there was you'd have to compress the contents extremely quickly. However the can probably will explode at 140F due to the liquefied propellant boiling in the sealed can. The bottom would probably blow out leaving lacquer everywhere but no internal ignition. I suppose its possible for ignition if the propellant is nitrous oxide but usually non food grade products use butane, isobutane, propane, or alcohol-liquified gas mixtures. IF it was Nitrous oxide i suppose it could decompose into oxygen at higher temperatures and cause a powerful explosion but Id image the container would give before those temps were reached.

777funk · #5 03-03-2010, 06:30 PM

Quote:

Originally Posted by turbobenz

No oxygen inside spray cans. Even if there was you'd have to compress the contents extremely quickly. However the can probably will explode at 140F due to the liquefied propellant boiling in the sealed can. The bottom would probably blow out leaving lacquer everywhere but no internal ignition. I suppose its possible for ignition if the propellant is nitrous oxide but usually non food grade products use butane, isobutane, propane, or alcohol-liquified gas mixtures. IF it was Nitrous oxide i suppose it could decompose into oxygen at higher temperatures and cause a powerful explosion but Id image the container would give before those temps were reached.

This is in a spray gun (siphon feed cup gun) and other times it would be in a pressure pot which is pressurized just enough to push the fluid through the lines (approx 5-10 psi).

JHZR2 · #6 03-03-2010, 06:54 PM

Youre balancing two things... heat in (makes the solvent more volatile) with pressurization (makes the gas want to go back to liquid phase). In a diesel, you have mechanical compression - heat is generated because of the work being done on the system. Same in your air compressor. In a gun that is heated, work is being done on the lacquer in the can, kind-of. In reality, energy is being input, and the lacquer is performing pressure-volume work. Volume is constant, so pressure goes up. The higher pressure goes up, the more the vapor wants to go back to liquid, but the more heat that goes in, the more vapor can stay as vapor.

Pressure vessel plus vaporization makes a bomb, more or less, be careful with heating it.

Id be worried to heat it outside of the can though, as youll flash off more solvent, which will change the characteristics of the lacquer.

The question is if you are heating the solvent above its flashpoint, and if so, what is the oxygen content in the can, so that we can define flammability limit. The vapors and oxygen have to be in a specific range to ignite... Plus youll need a spark.

Unless you get up to the autoignition point of the vapor (doubt it), youll be OK, especially if you minimize headspace and heat gently, so you bring the bulk temperature up and minimize vaporization.

Be careful all the same.

moon161 · #7 03-03-2010, 08:01 PM

The compression of the diesel engine heats the air to very hot temps, above the autoignition point of D2, I think around 900F, I'd have to bust out my thermo book or FE review book. This compression does a large amount of work on the air sealed in the piston to heat it.

The fuel burns because it's well atomized when injected either onto a hot glow plug, or into air well above the ignition temperature of the fuel.

An airbrush is completely different. You have a cup open to the air on one end, or at most 10 psi and a needle stuck into the confined jet of the air stream at the other, which is at less than atmospheric pressure. Because of adiabatic cooling and conservation of energy, this jet will be below room temperature. The air volume in the cup will heat minimally when it is compressed, but you don't feel the cup get hot, do you?

It sounds like you will be heating the body of the brush somehow, or the cup, where the paint is mostly liquid. If you don't control the temperature you'll start to cook off solvent the vapor pressure increases, and eventually boil it when you reach the boiling temperature of the paint, and then the flash and flame points, or just stop when you exhaust the capacity of your heat source to make it hotter.

There's no intrinsic mechanism here that works like a diesel engine. You're sucking paint into an AIR jet that is below room temperature. If you work within the manufacturer specified temperature range for the paint an solvent, I don't see a real danger from heating it in a controlled fashion that does not involve flames or other ignition sources. If you decide to feed your airbrush with an oxygen bottle, on the other hand, you won't need fancy paint, you could light off an oily rag.

Some hydrocarbons (acetlyene) become unstable over 15 psi and go boom but you're not using welding gas as a solvent. Read the labels on your cans or call the manufacturers for temperature recomendations.

Have you tried adding solvent to thin the paint? That's the usual way AFAIK.

777funk · #8 03-03-2010, 08:28 PM

Quote:

Originally Posted by moon161

Have you tried adding solvent to thin the paint? That's the usual way AFAIK.

I usually add solvent at around 50/50 in my quart cup gun and maybe a little less solvent in my Pressure Pot setup simply because it's a nicer Iwata gun and atomizes better.

But yes, lacquer thinner or another solvent is the usual way to thin the material.

The reason I'm thinking about heat is because it allows use of less solvents which means less to gas off later when drying (faster dry time) and it'll build quicker with more solids/less solvents.

But of course I don't want to blow the roof off of my house. This stuff burns well. I've done some match experiments and the vapors definitely go whooosh pretty good. I keep my spray area well ventilated for certain.

Oh also, the flash point is 0 degrees F

moon161 · #9 03-03-2010, 09:04 PM

Do you guys take care to ground your guns or your work? I know some materials will shed electrons to air like a van de graaf generator. I expect someone would've figured that our the hard way a long time ago.

Short summary of the above- I expect the hottest part of your system will be the paint heater. If the paint is safe in contact w/ that, stay within safe temperature limits.

That being said, the small amount of heat you add to the paint won't make a bit of difference as soon as it comes into contact with the much larger mass of the propellant and room air and your work. I think the temperature of your work will determine the temperature of the paint on it, and the temperature of the room air will determine how much solvent it will bear.

Chas H · #10 03-03-2010, 09:54 PM

Boat building friends of mine always heated the varnish. But they heated it in an open paint cup. A body shop friend heated enamel for quicky paint jobs, again by heating paint in the spray cup. I don't see any advantage in heating lacquer, which is thinned much more than varnish or enamel with volatile and flammable thinners. I strongly recommend against heating anything in a sealed container.
As to diesel, it will light off around 220 deg IIRC.

4x4_Welder · #11 03-04-2010, 12:11 AM

The mixture wouldn't be right, there is too much airborne solvent to allow enough oxygen for ignition.
In a diesel, it's only air being compressed, and then the fuel is shot in consuming the oxygen.
In a gasoline engine, there's a stoichiometric mixture (12:1-15:1, 14.7:1 being ideal) being inhaled into the cylinder, then compressed as a homogenized mixture and it still takes a spark to light it unless there's a problem with the engine. Temperatures are typically 150º-180º at that point, possibly more, and that mixture is designed to burn. I think you're ok, but then if the manufacturer says anything about not heating, then don't do it.