MadSci Network: Physics

Re: Why does a gas canister become cold when pressure is released?

Date: Mon Feb 26 14:27:37 2001
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Physics
ID: 982735536.Ph

Dear Professor Hahn:
Let's look at your question on a very basic level.  Assume you have an open 
container, at room temperature and atmospheric pressure.  You pump air into 
the chamber at a pressure up 10 atmospheres and seal it off.  The chamber 
was initially at room temperature and pressure inside and outside equal.  
Pumping the air into the chamber takes energy;   tne molecules are closer 
together and collide with each other much more frequesntly,  to 10X as 
frequently as you stuff them into the chamber.   If this is done rapidly, 
then the chamber walls will heat up because they are being bombarded with 
air molecules at higher and higher frequenccy, and these collisions impart 
energy to the walls and heat them.   The kinetic energy of the molecules 
gained by the "stuffing" process has been converted to heat energy as the 
walls heat up.  Note that the rate of heat conduction away from the walls 
by the air outside is much lower because the collision frequencies of the 
outside air with the walls is much lower, so therefore there is not 
equilibrium and the wall temperature rises.

Now if you leave the chamber around for several hours it will cool down 
because the surrounding gas will gradually conduct away the heat and 
eventually the entire high-pressure chamber will be at room temperature as 
before BUT it is full of gas at 10 atmospheres pressure, which means that 
the kinetic energy expended stuffing the chamber is now sitting there with 
stored potential energy, of an amount somewhat less than the initial 
kinetic energy because some of that was converted to heat which has been 
conducted away by the external gases.  (You never get something for 
nothing!) So now you have the room-temperature gas at high pressure stored 
in the chamber.  Although the gas collision frequency with the inner walls 
is still higher than the outside, all is in thermal equilibrium but you 
have this "bomb" with high potential energy sitting there.

Now you suddenly let the air out of the chamber.   The collision frequency 
of molecules with the inside wall drops very rapidly.   The potential 
energy of the molecules making 10X as many collisions/second is converted 
to kinetic energy of the gas stream leaving the chamber, with the 
consequence that the intercollision frequency goes down as the pressure 
drops and the gas cools below room temperature, since that is where it 
started before venting.  With the body of gas moving at high velocity and 
expanding the temperature drops and heat is conducted from the chamber 
walls to the cooler gas whose presure is falling within the chamber and 
being exhausted.  Potential energy is being "taken away from" the chamber.

The gas does not really "absorb" energy; it has high potential energy which 
is converted into kinetic energy of high mass flow at reduced pressure 
during the exhausing phase.  In principle if the heating of the gas during 
the initial compression were stored by maintaining the chamber at the 
temperature it rises to when pressurized, the approximate net effect of 
venting the chamber would essentially just reduce it to room temperature 

I hope this qualitative discussion has been of some help.

R. Bersin....

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