Re: [EnergyPlus_Support] night radiation

["Michael J. Witte" <mjwitte@xxxxxxxx>]

My apologies for the delayed response to this question.

First, In the Engineering Reference, p. 146 (pdf p. 173) it says:

"The following assumptions are made in deriving the heat balance equations:
 . . .
3) The glass layers are opaque to IR. This is true for most glass products. For
thin plastic suspended films this is not a good assumption, so the heat balance
equations would have to be modified to handle this case."

But in Material:WindowGlass, we have the field "IR Transmittance at Normal
Incidence", with max value of <1.0.  So is this value honored?  For a middle
layer in a window construction, yes.  For the outside layers, no.

After some consultation with experts on the subject, the conclusion is that
this does not matter, because a complete window construction using conventional
materials will always block IR:

"After looking at a number of glass and plastic products in LBNL's OPTIC
library, I'd like to add an observation why the IR transmittance should be set
at 0 for virtually all cases:  the IR transmittance of glass is of course 0;
the only time that the IR transmittance of a plastic product goes significantly
above 0 is when the product is very thin (example: IR transmittance 0.467 for
0.05mm Teflon). However, such products are never used by themselves and always
in conjuction with glass, which drops the IR transmittance again down to 0."

Mike


On 22 Aug 2004, at 7:30, mby wrote:

>
> Dear all,
>
> How to set up the E+ program to take into account
> radiation from a room through a clear window toward
> sky during night? I this program capable to model
> this.
>
> Best regards
>
> Bojic
>
>
>
>
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx




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