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Re: [EnergyPlus_Support] Cooling & Heating Design Air Flow Method

Ruchi:

When using specified air flows in zone sizing and the "flow/zone" 
method, the central air handler flow rate will be the sum of the 
specified flows, regardless of whether these are adequate to meet the 
load.  The ratio of the specified air flow rate and the calculated 
air flow rate is then applied as a sizing ratio to the remaining 
components.  See bullet number 3 at the top of p. 251 in the Input 
Output Reference for an explanation.  That is why you are seeing 
changes in sizes.

To check the actual loads against the design loads, compare your peak 
zone/sys sensible heating and cooling rates from the simulation 
period against the reported peak heating and cooling loads in the eio 
sizing report.

Mike

On 9 May 2003, at 18:10, Ruchi Choudhary wrote:

> 
> 
> Thanks very much Mike. Following up on your explaination: I specified
> the flow/zone method in the zone sizing object and noted resulting the
> peak heating and cooling capacity of the system.
> 
> Then, I increased the interior
> space gains in the zone without changing the specified amount of
> air/zone. I expected the capacity of the system to remain the same
> because now it should use the flow/zone to size the system capacity.
> However the peak system capacity also changed.
> 
> So it seems to me that the output in *ssz is based on loads obtained
> from the design day simulation no matter what the airflow method
> specification is. However both from what you say and the text in the
> "Design Objects" section, it should not? Or is it that the flow/zone
> is used for sizing components, but the system capacity will always be
> sized on the basis of the design day simulation? Following are my
> interpretations of the functional dependencies:
> 
> System Capacity=f(sys. supply temperature, total massflow rate, total
> outside air)
> 
> total mass flow rate=f(sum of mass flow rates of all zones)
> 
> mass flow rate of a zone = f(zone supply temp., design load)
> 
> Are any of this "mis"-interpretations? Also, it would be great if you
> could tell me which of these get affected if I choose the flow/zone
> method for specifying airflow.
> 
> also, when I specify the air/zone, the *.eio file outputs both the
> air/zone calculated on the basis of design day simulation as well as
> the value computed on the basis of the design day simulation...
> 
> 
> I also have another question :/
> 
> How can I make sure that the actual energy use in the zone is close
> enough to the design load (so the design load is not grossly
> over-estimated)? Should I check the values of the heating/cooling rate
> against the design load??
> 
> thanks very much for your help!
> ruchi.
> 
> 
> On Fri, 9 May 2003, Michael J. Witte wrote:
> 
> > If you specify a volumetric airflow rate in the zone sizing object,
> > then this overrides the design load calculation for that zone.  The
> > specified rate is then used when sizing the central air handling
> > components (fan, coil, etc.)  This does not override the specified
> > supply air temperature for sizing.  Supply air volumes are sized to
> > meet the sensible load in the zone based on the design day
> > simulations.  For hourly details of these loads, see the *.zsz and
> > *.ssz output files.
> >
> > Mike
> >
> >
> >
> > On 7 May 2003, at 14:32, Ruchi Choudhary wrote:
> >
> > >
> > > Dear All,
> > >
> > > In the Zone Sizing Object there are two fields that allow the user
> > > to either specify the volumetric airflow rate, or compute it on
> > > the basis of the design day simulation.
> > >
> > > My understanding is that the mass flow rates computed by the zone
> > > sizing object are used in conjunction with the system supply
> > > temperature and outside air flow rate to compute the system
> > > capacity.
> > >
> > > So if I specify the volumetric airflow rate for a zone, will it be
> > > used to determine the system capacity rather than the output from
> > > design day simulation? By doing so, I am implicitly also
> > > over-riding the design load computed by the zone-sizing object and
> > > zone supply air temperature specification?
> > >
> > > Also, the *.eio file lists the design volumetric air flow rates
> > > for each zone. I am assuming these are computed on the basis of
> > > the design load and zone-sizing specs. (supply temperature and
> > > humidity)?
> > >
> > > ruchi.
> > >
> > >
> > >
> > >
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