[Bldg-sim] emissivity of air

Chip Barnaby via Bldg-sim bldg-sim at lists.onebuilding.org
Mon Apr 3 08:58:16 PDT 2017

Chris / all --

IES is not alone -- CSE (California Simulation Engine) accounts for this
effect.  CSE performs the simulations for CBECC-Res, the Title 24
residential compliance tool.

The CSE zone model is based on (underappreciated) work by Joe Carroll
from the 1980s. See http://www.ibpsa.org/proceedings/BS2013/p_2487.pdf
and papers referenced therein.  In particular,
Carroll, J. A., 1980. An ‘MRT Method’ of Computing Radiant Energy
Exchange in Rooms, Proc. Second Systems Simulation and Economic Analysis
Conference, San Diego, CA.

At thermal wavelengths, air (especially humid air) is hazy.   Depending
on space size, some/most of the thermal radiant will be absorbed before
it reaches other surfaces.  But it takes a big space (read "SuperDome")
for the effect to be very significant.

CSE creates a virtual surface that participates in the long-wave radiant
network.  For an example 2850 ft2 zone with 24528 ft2 surface area
(including partitions) exposed to the room air, the air "surface" area
is 1784 ft2 or about 7%.   So the effect is modest but not entirely

Common comfort assessment methods (PMV / PPD) use MRT that is generally
calculated directly from surface temperatures (in other words, ignoring
the shielding effect of intervening air).  That method introduces an
error for large spaces but is generally a reasonable approximation.

I don't think there would be much effect on small scales that would be
seen within cooling equipment.

AFAIK, EnergyPlus does not represent this effect, although I may be
wrong.  It certainly could, given that it uses a first-principles heat
balance zone model.

Chip Barnaby


Charles S. "Chip" Barnaby, FIBPSA / FASHRAE

chipbarnaby at gmail.com

879 Bean Rd, Moultonborough, NH 03254  USA

(o) +1 603 253 4593 or +1 603 253 4400   (m) +1 781 883 4593

----- Original message -----

From: "Chris Yates via Bldg-sim" <bldg-sim at lists.onebuilding.org>

To: "bldg-sim at lists.onebuilding.org" <bldg-sim at lists.onebuilding.org>

Subject: [Bldg-sim] emissivity of air

Date: Mon, 3 Apr 2017 13:34:31 +0100

Dear all,

In IES apache 'simulation options' there is a setting called 'Internal
air emissivity model'. This model has the effect of moist air dampening
higher radiant temperatures. Apparently, this effect is more pronounced
in large spaces - though I haven't tested this.

IES seems to be quite unique in implementing the 'emissivity of air'.

The concept seems a little unusual to me. So, I was wondering what
experiences the simulation community has of this phenomenon? Where else
it may be implemented in simulation tools? And, how does it feature for
common comfort temperature assessments involving some combination of
radiant temperatures (e.g. operative)?

Also, might it also demonstate enhanced comfort for systems with
direct adiabatic cooling in dry climates (e.g. spray bar in supply
side of AHU)?

Best regards



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