On Wed, 11 Nov 1998 09:26:24 -0000 Gerry Cox <gcox@DEK.COM> writes:
>I need to cool an object to 4C below ambient. Ambient is in the range
>20C to
>30C. The object does not generate any heat of its own. Cooling does
>not have
>to be rapid. I have sucessfully used peltier devices controlled by a
>PIC16C73 running a closed loop with a temperature sensor and PWM
>controlled
>mosfet drive to the peltier devices. However I have a problem
>dissipating
>the heat from the hot side of the peltier devices in the space
>available
>without increasing the ambient and defeating the object of it all :-(

You won't have to drive the Peltier very hard to get 4C cooling, I think
that it still deserves consideration.  They work most efficiently driven
with DC to the cooling level required.  It does work to PWM all the way
to max and off, but you'll get more heat on the hot side compared to
driving with DC.  A simple switcher type of regulator chip with some
minimal output filter will work fine.

>
>So I am experimenting with the idea of using a compressed air supply,
>which
>is available near the device, to achieve the cooling. I.e. from first
>principles I understand that if I allow the air to expand, then its
>temperature will fall. Does anyone know the maths involved and have
>any
>advice on methods to expand the air and cool the object?

There is a device that does exactly that, I'm not sure what it is called
but I did play with one a long time ago.  It was intended to cool the air
going into a chemical-proof suit like is worn for toxic spill cleanups.
The person inside the suit could then breathe clean cool air (*).  A lot
of air at about 100 psi went into one side, most of it came out warm from
one end, but some was quite cold coming from the other end.  I was more
impressed that anything could expand air and make it warmer at the same
time.    Opening or closing a valve at the warm end that regulates the
output of cold air.

I also remember that Popular Mechanics or Popular Science did an article
expalining how it worked (something about "seperating the cold from warm
air molecules", but those publications aren't exactly hardcore science)
There is a spiraling flow of air inside that makes something happens in
addition to simple expansion.   Also the magazine had plans to build one
in a simple machine shop.   It takes a lot of air, probably 10 cfm.  I
don't know if a much scaled-down unit could be built readily.


>An air
>supply is available at up to 6 Bar with quite high flow rate. The
>temperature of the compressed air before expansion is close to the
>ambient.

(*) Obligatory safety notes:  Unless specially processed, industrial
compressed air has quite a bit of oil in it and should not be used for
breathing.   In this project, it may be necessary to pipe the spent air
out of the box rather than just let it go where it may and deposit water
and oil on everything.  Be sure that nothing can burst if the air outlets
are blocked.


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