Ask A Scientist© Environmental Earth Science Archive

name         Dave
status       other

Question -   Hi, I work on heating and air conditioning equipment, I am
doing some work in a plant where they make products out of plastic,and
what I was
wondering about or need to know is that they use a machine that use
compressed air at 110 psi inlet and they send it thru a air dryer to get
the dew point
down then thru this machine that has a compressor in it,  that reduces the
air temp going
out to -79 degrees. Is there any info about the principle of this
operation and if so where
can I read about it..It is fascinating to me how it works also a little
knowledge is good.


You can find out about this by reading the thermodynamics section of an
introductory physics text, or by looking for "air conditioner" in the book
"The Way Things Work" or in an encyclopedia.

The basic idea is that air molecules attract each other, so it takes energy
to separate them.  If you pack them together at high pressure, you get the
energy it took to separate them back in the form of kinetic energy (i.e., the
molecules move faster, and this means they have a higher temperature).  The
compressor you're talking about does the same thing, but in reverse.

Tim Mooney

=========================================================

All the information I can find on the Web is fairly technical, and couched
in the useful but rather impenetrable language of thermodynamics.  So I'll
try to make it brief.  You can just skip to the last paragraph if you want
the REALLY short answer.

What you are observing is the "Joule-Thompson effect."  When a gas is
compressed (or the reverse process, is expanded), it can either increase its
temperature, decrease its temperature, or keep its temperature the same.
Temperature, it turns out, is directly proportional to the kinetic energy of
the gas molecules.  A change in temperature means that energy is being
transferred, one way or another, between the molecules' kinetic energy and
some other store.  In compression or expansion of a gas, that "other store"
is the "intermolecular" potential energy between different gas molecules.  A
compressed gas flowing out of a container undergoes much what is called
"free expansion," in which the gas doesn't transfer mechenical energy by
pushing against anything.  So, there is approximately no energy transfer in
your situation other than that between the molecules' kinetic energy and
intemolecular potential energy.

Exactly what the temperature does when a gas expands depends on the
interactions between the gas molecules themselves. There are three
possibilities:

1.  If the molecules completely ignore each other, and basically don't care
how close any other gas molecules are (an "ideal" gas), then expansion won't
change any intermolecular energies.  If they don't change, the kinetic
energies of the gas molecules won't change either.  The speeds of the gas
molecules remain constant, and the temperature of the gas does too.
2.  If the gas molecules were so close together that moving apart would give
them a lower potential energy, then the molecules repel each other.
Expansion lowers the intermolecular potential energy.  This means that
expansion releases energy from the intermolecular potential, and that energy
is picked up by the molecules' kinetic energy.  In other words, the gas
molecules speed up and the temperature increases.
3.  If the gas molecules were feeling lonely before expending, that is, if
moving apart gives the gas molecules higher potential energy, then the gas
molecules attract each other.  Expansion raises the intermolecular potential
energy, lowering the molecules' kinetic energy.  The molecules slow down and
the temperature decreases.

What is happening in your situation?  Upon expansion, the gas cools.  That
means that the gas molecules are attracted to each other.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

==============================================================================