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Re: [EnergyPlus_Support] total cooling load

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] total cooling load
From: "Michael J. Witte" <mjwitte@xxxxxxxx>
Date: Mon, 23 Aug 2004 14:29:43 -0500
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We have several threads going now on basically the same topic: the calculating 
and reporting of latent loads using PURCHASED AIR.  It is critical that users 
understand the limitations of the PURCHASED AIR model.  The description of the 
model and its outputs beings on p. 520 of the Input Output Reference (pdf p. 
548).  The following excerpts are pertinent here:

-------------

The purchased air component can be thought of as an ideal unit that mixes air 
at the zone return condition with the specified amount of outside air and then 
adds or removes heat & moisture at 100% efficiency in order to produce a supply 
air stream at the specified conditions.  

Note: Every zone served by a purchased air component should have a return air 
node, even though this node is not connected to anything. 

Note: The outside air flow rate is never allowed to be greater than the supply 
air flow rate.  

Purchased Air Sensible Cooling Energy[J] 
The energy removed from the mixed outside and return air stream in order to 
lower its temperature to the specified temperature of the supply air stream. 
Any energy needed for moisture addition or removal is ignored. 

Purchased Air Total Cooling Energy[J]
The total energy (sensible and latent) removed from the mixed outside and 
return air stream in order to bring the mixed air stream to the specified 
temperature and humidity ratio of the supply air stream.

---------------------

Remember that PURCHASED AIR does not control the zone humidity, but it does 
supply air at a user-specified humidity ratio.  So the purchased air latent 
load is whatever is required to take the zone air (or mixed air if outside air 
is specified) and condition it to the user-specified supply conditions.  
PURCHASED AIR is a great tool for sensible loads.  With outside air, it is 
reliable for peak loads, but not for annual loads.  The latent loads from 
PURCHASED AIR must be used with caution with a clear understanding of what they 
represent.

Mike


On 23 Aug 2004, at 8:02, Rahul Chillar wrote:

> Tek
> if you have set the humidity level to some value for your zones then purchased
> total would be different from purchase sensible ( which i believe you have done
> in your case, now), in this case purchase air will tell you the sensible and
> total energy. The Difference of the Two will give you the latent energy removed
> from your zone.
> 
> Thanks
> Rahul Chillar 
> 
> teklay Weldeabzgi <teklay1979@xxxxxxxxx> wrote:
> if that is correct why is the Purchased Air Total
> Cooling Energy[J] greater than Purchased Air Sensible
> Cooling Energy[J]. 
> 
> --- Rahul Chillar <chillarrahul@xxxxxxxxx> wrote:
> 
> > 
> > Hello
> > To determine the actual latent loads you will have
> > to simulate an hvac system, i think purchased air
> > will give only the sensible total loads and not
> > latent because it is not a system which can control
> > the humdity levels etc of your zones. 
> > Purchased air is designed as a wonderful concept to
> > get the user in the ball park without having to
> > simulate a system.
> > Reporting the below variables should be sufficent
> > for your case where you only need to heat the
> > building.
> > HVAC,Average,Purchased Air Total Cooling Rate[W] 
> > HVAC,Sum,Purchased Air Total Cooling Energy[J]
> > thanks
> > Rahul Chillar 
> > 
> > teklay Weldeabzgi <teklay1979@xxxxxxxxx> wrote:
> > 
> > i am simulating buildings to know the cooling load
> > reduction using different building design
> > parameters.
> > i am using a purchased air system. we i need to see
> > the total cooling energy of the zones is the
> > AIR:Purchased Air Total Cooling Energy give me the
> > result. the ONE:Zone/Sys Sensible Cooling Energy
> > gives
> > the sensible energy only. is AIR:Purchased Air Total
> > Cooling Energy the sum of the latent heat gain and
> > the
> > sensible heat gain. to make cealr my question how do
> > i
> > get the cooling load of the building?
> > tek
> > 
> > 
> >             
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> 
> Thanks 
> Rahul Chillar 
> Grad Student 
> Building Systems lab
> Univ Of Illinois Urbana Champaign
> 
> 
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> 
> 
> 



========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx




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References:
- Re: [EnergyPlus_Support] total cooling load
  - From: teklay Weldeabzgi
- Re: [EnergyPlus_Support] total cooling load
  - From: Rahul Chillar

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