Re: Quartz heater: Asorption length of short wavelength IR in air?
James Whitby wrote:
>Dubious Dude wrote:
>>By the way, would you be able to direct me to an online source that indicates
>>the opacity of glass in the 2~3 micron range? Thanks.
>
> http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd060_e.html has curves for some common
> window glass types.
It seems that in the 2-3 micron range, transmission ranges from 20-76%. But
let's try to get more specific. It is an old building and the glass looks
anything but special, so I assume it is not polished plate, not glare reducing,
and it is certainly not colored (though it isn't exactly the cleanest). Hence,
it is probably corresponds to the curved designated "heat absorbing". Right
around 2.5 micron, transmission transitions from 45% to below 25%. So
significantly more than half of the radiant energy is reflected. Assuming that
absorbed IR robs energy from what would otherwise be transmitted an reflected,
let's knock 5-10% off the reflected energy. For double-layer windows (meaning
2 separate sheets of glass, not some special inert gas between them), we can
assume the absorbed energy is converted to heat at the window and lost to the
outside immediately through conduction, since there is such a temperature
gradient to the outside. Just an off-the- cuff simplifciation, which may or
may not be valid. All-in-all, I estimate that 50% or more of the energy is
bounced back inside. Not the greatest efficiency, but who knows how it compares
to convection heating, where the hot air simply disappears out the window.
Thank you, Dr. Whitby
.
