Technical question: PCP vs CO2

[DT Fletcher]

We all know about how a CO2 gun will get very cold and even freeze-up due to the liquid co2 absorbing heat when changing states from liquid to gas. The question I have, does the same thing happen with PCP air guns? I have never owned a PCP gun so have no experience. From the physics, the compressed air is going to absorb heat when released but does it ever get to the same freezing point as CO2 guns?

[Mr. Gen]

In simple terms no - because the air is only compressed and not liquid like CO2.........it simply expands rather than expand into the gaseous state.

[Vic Thompson]

I don't have a very detailed answer, but when I've emptied my HW100 cylinder rapidly the cylinder cools down but not by very much. I'd estimate that it ends up at a few degs C when initially at 18 to 20 degs C. I presume that when you refer to freezing you mean that the cylinder cools enough to allow for a build up of frost on the cylinder (from water in the atmosphere). I reckon that under certain circumstance this could be achieved with a metal air cylinder dependent on internal volume and initial pressure, surface area, and physical properties of the material forming the cylinder.

HTH, Vic Thompson.

[Adam77K]

You get a cooling effect from expansion, just as you get heating on compression. What you don't get is the latent heat from the change of phase (liquid to gas) which is a much bigger cooling effect.

[DT Fletcher]

You get a cooling effect from expansion, just as you get heating on compression. What you don't get is the latent heat from the change of phase (liquid to gas) which is a much bigger cooling effect.

That's what I was thinking, but, not having any personal experience, figured it would be best to check. The question comes from an article written in 1980 about a Girandoni air gun; the writer makes the statement that the valve could freeze up, a la CO2 guns. Back in 1980, there weren't many or any PCP guns on the market, so, it seems that the writer just jumped to the conclusion that compressed air guns would have the same property of potentially freezing up with multiple shots like CO2.