[time-nuts] temperature stability basics

J. Forster jfor at quik.com
Sat Nov 27 00:30:57 UTC 2010


> Dont get the idea that radiation is only significant for large
> temperature differences.
> For two parallel surfaces at any distance apart the black body
> radiation between them (around room temperature 300K) is near to 6
> watts per square metre per degree (C*) of temperature difference.

Only for infinitely large plates where the View Factor is 1. If the View
Factor is <1, the plate spacing matters.

> That is an R rating of 0.14 in parallel  with the conduction through
> the insulation that is IR transparent.

If there is not vacuum between the plates, there is the possibility of
convective heat transfer. The issue there is the film coefficients which
vary drasticall with air motion. Film heat transfer is very complicated,
to say the least.

> The rate is proportional to the temperature difference. The fourth
> power law only becomes significant when the temperature difference is
> quite large.
> If the surface is clean and polished the emmissivity of the surface
> can be significantly reduced, hence the silver lining in a dewar.

True.

> With sheet materials, an oil film, corrosion and dust can rapidly
> increase the emmisivity. Gold is good because of its freedom from
> oxidation and discolouring and ease of "flashing" onto any surface.
> If you make a sandwich with N layers of metal foil, the radiation
> transmission is reduced by 1/N.
>
> Cheers, Neville Michie
>

-John

==============




More information about the time-nuts mailing list