Sorry, I don't quite follow that - the pressure withing a closed system (gas or liquid) is equal throughout the system.

Compressed air is a propellant in it's own right - it doesn't need another propellant as well.

I have one of those cans of air that I can re-fill with my air compressor... what i don't
have is the propellant.

If you take the same volumn of air, same pressure but add a propellant to one of the cans and not the other, I'm pretty sure one of the cans of air will propel the air out of the can
at a sustained flow rate.... while the one without the propellant will not be able to sustain
the flow rate.

So how do (for example) car spray painters (obviously need a pretty steady flow rate for that) manage just using compressed air? What flow rate do you need when you're only using it as a duster?!?

(Not having a go at you - merely putting forward the argument for the non-need of propellants for a duster for [deity of choice]'s sake...) :-)

Paint spray guns run at lower pressures on regulated lines. I don't remember specific numbers but probably 15 - 25 psi. When the pressure in the tank of the air compressor that is supplying the regulated pressure, drops to 100 psi, the compressor turns on and pumps back up to maybe 150 psi and shuts off. The air supplied to the spray gun is constant.

Greg, I understand how it works - it was a rhetorical question. ;-)

The question was about the propellant being air. The aerosol containers used for "air" sprays are quite small volume and when there is a rapid decrease in pressure (as in when you depress the button), the temperature drops rapidly, which also acts to reduce the pressure further (temporarily) [1 bar/degree K/C]. That's why, if you keep the spray going long enough, it's possible for the can to get so cold that your skin sticks to it.

This can be seen from time to time with scuba cylinders that have accidentally (or otherwise) been left with the valve open - it doesn't take long for the valve to freeze. Not a good idea, as when it gets to that low a temperature, moisture can form inside the cylinder around the neck - which will need to be dried out so as not to rust the steel and ruin the cylinder.

When used in short bursts (which is the best way to use them) it doesn't become such an issue. There is no doubt that other propellants can be added to the air to act as a lubricant for improving constant flow in certain applications but, I would say totally unnecessary for this application - as we don't need a constant flow for a "blow duster".

Let's save the environment huh!?!

I don't know but I would hazard a guess that the air in the can maybe compressed enough to turn it into a liquid form as with most gasses ie: liquid oxygen, propane, butane and thereby not needing a seperate propelant but then again I could be talking a load of B/S any other theories.

Alan

Alan, you're correct. I've used compressed air where a bit of liquid wound up on the surface. For that reason, these things scare me.

Alan, if there's any moisture in the air, it can turn to liquid when the air is compressed but otherwise, no, the air doesn't turn to liquid (not at any pressure we could get it to anyway. Diving air is the cleanest, driest air you'll ever breathe - it doesn't turn to liquid even at 300 bar (4350 psi for the yanks on here) ;-) which is way, way higher than anything you could stick in an aerosol. (a 300bar cylinder is made from 1/4 inch steel!)

OK steve that seems to be my theory out the window I did think that the aerosl can might be a bit flmsy so if any one out there knows come on share your secret.

Alan

Yep, Scuba air is very very dry... thats why when you breath down two scuba tanks
the beer tastes much better than you could ever believe.. :-)

But the bottom line is I would not suggest using a canned air source with propellant in the
mix for any optical glass cleaning.

and yes, I do have an adaptor for using my dry, clean scuba tank air on lots of my equipment... including camera optics...

Heh, heh, heh.... I'm with you all the way my man! :-))

I use the Co2 cartridges and I haven't had anything come out of them but air.

hmmmm.. he, he.... ;-)

I think you might find carbon dioxide comes out of a CO2 cylinder... ;-)

Senior moment?