I am building a device that has two pipes, one inside the other. The
outer pipe is 4" in diameter; the inner pipe is 3.5". There is a 1/4"
space between the two pipes on all sides. The inner-pipe is constantly
exposed to heat, which is ok. However, the outer pipe heats up as well
and that's unacceptable. I am looking for some sort of a material or a
compound to fill the area between the two pipes which would absorb all
the heat from the inner pipe.

The pipes are about a foot long, 4" and 3.5" in diameter, about 5mm
thick and made of aluminum. The max temperature the inner pipe is
exposed to is about 150C or 300F. Any suggestions? I am looking for
something ready-made and commercially available.

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Clarification of Question by dedavai-ga on 25 Jun 2005 14:16 PDT

Thank you for the response. Liquid would be ideal, however that's not
an option because the area between the pipes is not air- (and liquid-)
tight.

What kind of a fiberglass would have to used? And where can I purchase some?

Would it be possible to run liquid through the tubes like a
condensation tube in a chemistry laboratory?  The aluminum specific
heat is relatively high when compared with other metals.  If this is a
viable option, water could be used to cool the pipes if pressure
variances could be dealt with.  But a pump would have to be installed
along with another condenser to keep the water from boiling away.  If
you can build a moving fluid coolant system Dow Chemical Company
offers an enormous amount of different industrial coolants.  I would
recommend the Inhibited Ethylene Glycol-based solution sold under the
title Dowtherm 4000 since you can use it as a single fluid coolant
with a small amount of extra cycling.  Water would be the cheapest
fluid; however, the condensing columns would probably negate the money
save in fluid costs.  Solids are often difficult to find because they
are not mobile and typically have a relatively low heat capacity and
would transfer a large amount of heat to the second pipe.  A cheap
alternative to liquid cooling would be to put fiberglass insulation in
the space to absorb the heat at a better rate the normal air that has
a terrible heat capacity.  Liquid cooling would be the better option
but would require more work and expense.