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Re: AW: [EnergyPlus_Support] IR Hemispherical Emissivity

Perhaps I was not clear when I said that the absorption of infrared 
energy within the *thickness* of the glass is controlled by "IR 
Transmittance at Normal Incidence".  Setting this value to zero 
implies high absorption, therefore no transmittance.  The 
interactions of the various surface properties is quite complex. 
Please see the "Window Calculations Module(s)" section of the 
Engineering Reference starting on p. 76 for a detailed explanation of 
how each property is used.

If I understand correctly, you are seeking to control the *surface* 
absorptivity for the purpose of radiant exchange.  This is controlled 
by "IR Hemispherical Emissivity" and it is equivalent to thermal 
emissivity/absorptivity.  Setting this to a very small number (zero 
is not allowed) reduce the radiant exchange to near zero.


On 22 Oct 2002, at 8:34, Alexander Beisteiner wrote:

> Dear Mr. Witte
> Thank you for your explanation, but the term "IR Hemispherical
> Emissivity" is still unclear to me. As you mentioned, if glass does
> not absorb energy, the field "IR Transmittance at Normal Incidence" is
> set to zero. So, if energy is not absorbed by the material, heat
> exchange (longwave radiant exchange) cannot happen. What role does the
> field "IR Hemispherical Emissivity" play in this consideration?
> Furthermore, I want to produce opaque windows which do not allow
> visible transmittance and do not absorb energy: is it true to set the
> fields "Solar Transmittance at Normal Incidence" and "Visible
> Transmittance at Normal Incidence" to zero and the fields "Solar
> Reflectance at Normal Incidence" and "Visible Reflectance at Normal
> Incidence" to one? I would appreciate your help. Regards 
> -----Ursprüngliche Nachricht-----
> Von: Michael J. Witte [mailto:mjwitte@xxxxxxxx] 
> Gesendet: Montag, 21. Oktober 2002 19:06
> An: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Betreff: Re: [EnergyPlus_Support] IR Hemispherical Emissivity
> No.  "IR Hemispherical Emissivity" is a surface property which 
> applies to longwave (infrared, IR) radiant exchange.  If I understand
> your question correctly, you are trying to set absorption of energy
> within the glass thickness to zero.  For longwave infrared, this
> property is controlled by "IR Transmittance at Normal Incidence".
> Mike
> On 21 Oct 2002, at 15:01, Alexander Beisteiner wrote:
> > Dear Madam/Sir
> > With reference to the field "IR Hemispherical Emissivity" in the
> > object Material:WindowGlass, may I assume that emissivity equals
> > absorbtion. If I want to set absorption coefficients for glass zero,
> > may I set the fields for front and back side zero?
> > 
> > Alexander Beisteiner
> > __________________________________
> > alexander.beisteiner@xxxxxxxxxxxxxxxxxx
> > Forschungs- und Technologietransfer
> > Pinkafeld GmbH
> > Steinamangerstrasse 21
> > 7423 Pinkafeld
> > 
> > http://www.fh-pinkafeld.ac.at <http://www.fh-pinkafeld.ac.at/> Tel.:
> > 0043/3357/45370-1324 Fax: 0043/3357/45370/1010
> > 
> > 
> ========================================================
> EnergyPlus Testing and Support      
> EnergyPlus-Support@xxxxxxxx

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