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RE: [EnergyPlus_Support] Re: air density in the heat balance

Fred:
 
We ran into this issue as well when completing RP865...where we saw different methods used
between BLAST and DOE-2...makes a small, but non-trivial difference. See the papers below for 
more discussion:
 

 Yuill, G., Haberl, J. 2006. "Accuracy Tests for Simulations for Single Zone, Two Pipe, Four Pipe and Induction Air Handling Systems " ASHRAE Transactions-Research, Vol. 112, Pt. 1, (January).

 

 Yuill, G., Haberl, J. 2005. "Accuracy Tests for Simulations of Constant Volume, Dual Duct and Variable Volume Air Handling Systems  (4796)," ASHRAE Transactions-Research, Vol. 111, Pt. 2, pp. 137-153, (June).  

 

 
Jeff
 
8=! 8=) :=) 8=) ;=) 8=) 8=( 8=) :=') 8=) 8=) 8=?
Jeff S. Haberl, Ph.D., P.E.............................jhaberl@xxxxxxxxxxxx
Professor......................................................Office Ph: 979-845-6507
Department of Architecture.......................Lab Ph: 979-845-6065 
Energy Systems Laboratory.......................FAX: 979-862-2457 
Texas A&M University..............................77843-3581
College Station, Texas, USA.......................URL: www-esl.tamu.edu
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________________________________

From: EnergyPlus_Support@xxxxxxxxxxxxxxx on behalf of Fred Buhl
Sent: Fri 2/6/2009 2:30 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Re: air density in the heat balance



I think the statements in the Eng Ref are a little misleading.

1) In the HVAC calculation, the design volumetric flow rates are changed to
design mass flow rates using a standard air density:

   StdBaroPress=101.325d0*(1.d0-2.25577D-05*Elevation)**5.2559d0
    StdBaroPress=StdBaroPress*1000.d0
    StdRhoAir=PsyRhoAirFnPbTdbW(StdBaroPress,constant_twenty,constant_zero)

So the conditions are: a) elevation corrected standard barometric pressure; b) 20 C air temperature
c) dry air (no moisture content)

If Elevation = 0.0, then StdRhoAir = 1.294.

Then all calculations are done in terms of mass flow rate.

2) in the envelope calculation, the density of air is calculated using the outside barometric
pressure and the relevant air temperature and humidity ratio. For instance, in calculating
interior surface convection coefficients, the current zone temperature and humidity ratio
are used.

Fred Buhl

direzionecontraria wrote: 

	Cz is on page 5. 
	1.204 kg/m3 is on pages 573 and 603.
	Regards
	Francesco
	
	--- In EnergyPlus_Support@xxxxxxxxxxxxxxx <mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx> , Fred Buhl <wfbuhl@...> <mailto:wfbuhl@...>  
	wrote:
	  

		 Could you tell us what section or page of the Engineering 
		    

	Reference you
	  

		 are looking at?
		
		Fred Buhl
		
		direzionecontraria wrote:
		    

			Hello!
			I refer to "Engineering Reference" 
			
			      

	http://apps1.eere.energy.gov/buildings/energyplus/pdfs/engineeringref <http://apps1.eere.energy.gov/buildings/energyplus/pdfs/engineeringref> 
	ere
	  

			nce.pdf
			In the equations of the heat balance on the zone, for example in 
			      

	the 
	  

			equation (2), there is the term Cz, which is the air capacitance.
			Cz = (air density )*(volume of the zone)
			I read that the standard air density is 1.204 kg/m3.
			What are the standard conditions ? How is the air density 
			      

	calculate in 
	  

			different conditions from the standard conditions?
			With Regards
			Francesco
			
			
			
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