Re: Question on insulation

On Nov 26, 3:09 pm, skg <gskil...@xxxxxx> wrote:
Hello everybody

I have a question about cryogenic insulation. Imagine a sandwich of
aluminum foil and rockwool like that:

--------------------- Al Foil
############ Rockwool
--------------------- Al Foil
############ Rockwool
--------------------- Al Foil
############ Rockwool
--------------------- Al Foil
############ Rockwool
--------------------- Al Foil
############ Rockwool
--------------------- Al Foil
############ Rockwool
--------------------- Al Foil

We used to call that "superinsulation" at the Helium Research
Laboratory. Our large Dewars were lined with it. There were scores
of layers and about every 20th layer of aluminum was replaced with
copper foil for mechanical strength, IIRC.

Evacuated this sandwich is used all the time to insulate cryogenic
tanks against radiative and convective heat losses. What will happen
if there is no vacuum? Heat conduction will increase, but by how much
(aluminum is highly reflective, rockwool thickness of 5 mm lambda of
0.04 [W/(mK)])

That will, of course, depend on how many layers there are and how bad
your vacuum has gotten, as well as the size and shape of the container
and the working temperature differential:

http://en.wikipedia.org/wiki/Heat_conduction#Newton.27s_law_of_cooling
In the equation:

Q = h * A * (T0 - Ta)

the To is the temperature on one side of the thermal barrier, and Ta
the temperature on the other side, A is a function of the geometry of
the vessel (presumably constant if the only thing changing is the
vacuum), and h accommodates all the properties of the insulating
barrier.

Any idea on how to get a good estimate?

Empirically...

How well would
such
an insulation work at diminishing temperature difference, ie not
DT=200K but DT=50 or 20 or 10 K?

Newton's Law of Cooling chows that the heat flow Q in watts (energy
per unit time) will be directly proportional to the temperature
difference, all other things being equal.

Tom Davidson
Richmond, VA
Regards, George

.

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