There's a small problem with my calculation and the published figures
for the thermal resistance of styrofoam.
If the measurements include radiative transfers the thermal resistance
per unit area of a styrofoam slab should reach a limiting value (when
the radiative transfer component dominates) as the thickness is increased.
Possibilities
1) My calculation is incorrect.
2) Infrared absorption in thick styrofoam slabs is significant.
3) The tabulated figures for the thermal resistance of styrofoam are
merely scaled up from the values measured with thin sheets.
This gives misleading values for thick sheets if the tabulated values
include radiative transfer.
Bruce
Bob Camp wrote:
Hi
Since styrofoam is being rated as a building insulation, it's reasonable to
believe that the material on both sides is up around 1. I highly doubt that
somebody tossing styrofoam in walls is going to add a radiation factor ...
In our application we're talking about a metal block inside a metal enclosure,
polishing the surfaces could drop the emissivity by>10X. If the budget allows,
you could gold plate the surfaces in addition to polishing them .....
Bob
On Mar 10, 2010, at 8:01 PM, Bruce Griffiths wrote:
Oops forgot a factor of 4
Radiative heat transfer for surface with an emissivity of 1 at 300K is about
612uW/square cm/degree C
which is equivalent to about 25mm of styrofoam.
Which raises the question what's the emissivity of the isothermal surface used
when measuring the thermal resistance of a slab of styrofoam?
Bruce
Bruce Griffiths wrote:
Radiative heat transfer (for a surface with an emissivity of 1) is around
150uW/square cm /degreeC at 300K.
That's equivalent to about 10cm of styrofoam.
It seems unlikely that the radiative heat transfer component is included in the
thermal resistance rating for Styrofoam.
The radiative component is independent of insulation thickness where the
insulation doesnt absorb in the 10-30um infrared region.
Adding carbon black to the foam appears to increase the thermal resistance of
25mm thick foam by about 10%.
Bruce
Bob Camp wrote:
Hi
That raises the interesting question of weather radiant transfer is already included
in the rated thermal resistance. My guess is that the 8" of foam is enough to
cover any radiation issues and still get you above 20 C / W.
Since you are probably starting with 1 or 2" slabs, including the aluminum foil
would be pretty easy. It can't hurt and it might help.
Bob
On Mar 10, 2010, at 7:29 PM, Neville Michie wrote:
I have been wondering about achieving the rated thermal resistance from plastic
foam,
the problem being that radiant transmission may be very strong through the foam.
What happens if you interleave concentric sheets of foam plastic with aluminium
foil? (taking care to keep the foil
on isothermal surfaces)
Will this stop the radiant transfer and leave only the thermal conduction of
the plastic foam?
cheers, Neville Michie
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