[time-nuts] Thermal time constant

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Mar 11 01:53:01 UTC 2010


Depends on the duration of the experiment.
Urethanes arent well known for stability.

The low thermal resistance of close cell urethanes is largely due to 
their better retention of the blowing agent which has lower thermal 
conductivity than air.
Closed cell foam glass (however the sulphur dioxide released when a cell 
is broken may be an issue) is much more stable than plastic foams the 
thermal resistance of which slowly deteriorates due to gas diffusion 
(air diffuse in and blowing agent diffuses out).
The gradual permeation of the plastic foam by water vapour can also have 
a significant effect.

Foam glass is sometimes used to insulate very large cryogen tanks.

Bruce

Bob Camp wrote:
> Hi
>
> Low density (1 or 2 lb / cu ft) urethane foam is going to be a better insulator than styrofoam. I believe it's reasonably opaque at IR.
>
> Bob
>
> On Mar 10, 2010, at 8:23 PM, Bruce Griffiths wrote:
>
>    
>> 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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>>>>>
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