Effects of the release of pressurized heated air

[curious sir]

When pressurized heated air is released does the temperature drop down below ambient temperature, or simply finds temperature equilibrium with ambient air?

 

[russ_watters]

Welcome to PF!

The answer depends on the starting conditions, so we can't answer without them.

 

[curious sir]

Ok. Begin with a tank with ambient air at ambient temperature and pressure, (we'll assume sea level for the purpose of this question). I paint it black. Then I seal it, (in other words,the released valve is closed). Then I place it in the sun which heats up the air molecules and pressure air builds inside the tank. At say, 120 psi, I release, (open the release valve), the pressurized hot air from the tank. The air from the tank begins to expand and therefore its temperature drops. The question is how much does it drop. Does it drop below ambient air temperature or does it seek temperature equilibrium with ambient air?

 

[russ_watters]

If the pressurization is caused by heating, releasing the pressure will not drop the temperature back below ambient.

 

[curious sir]

Thanks. I was thinking maybe that in the scenario I presented the thermodynamics might work similar to a heat engine whereby it is heated on one end, then the hot pressurized air released on the other end would turn so cold it is used in cryogenics. Different specific parameters, I guess.

 

[russ_watters]

Not quite. To liquefy air, you compress it, then COOL it, then expand it (repeat if necessary).

 

[Drakkith]

Does this temperature drop only occur for a phase change, like in the expansion valve of an AC unit or the can of compressed air I use to clean my PC of dust?

 

[russ_watters]

No, any fluid flowing through any nozzle/throttling valve will cool...though there is also an energy loss associated with the throttling.

 

[Drakkith]

No, any fluid flowing through any nozzle/throttling valve will cool...though there is also an energy loss associated with the throttling.

So in my compressed air can, is the bottle getting cold mostly because of the evaporation of the compressed liquid as it turns to gas while the pressure drops? Or is it that the liquid and gas inside the can is only because of the phase change and the gas exiting the nozzle is cooled from the act of moving through the nozzle?

 

[russ_watters]

Its mostly due to the evaporation. The pressure in the can doesn't drop much when you first pull the trigger - it takes a few seconds for the can to get cold.

 

[Drakkith]

Yep, that's what I thought. Thanks Russ.

 

[russ_watters]

I explained it weakly, but I was just typing a short message on my Droid...

The pressure in the can can't drop much due to the release of the gas, since it is being replenished continuously by the evaporation of the liquid - it can only drop until it reaches the saturation pressure of the gas at room temperature, at which point it gets replenished very quickly by the boiling liquid. Then the boiling liquid starts to cool itself and the gas, reducing the temperature until an equilibrium is reached between the gas loss and boiling rate at whatever temperature and pressure that is.