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
Message:
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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
again.

I hope this qualitative discussion has been of some help.

R. Bersin....

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