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Re: [EnergyPlus_Support] Re: Purchased Air vs. PTAC
Other things to consider . . .
Is the PTAC outside air flow rate >0? The purchased air load will not include
outside ventilation air loads, but the PTAC DX coil will.
The PTAC DX coil load will include fan heat which is not present in the
purchased air load.
Try comparing the Zone/Sys Sensible Cooling Rate for both cases, this is the
actual sensible cooling delivered to the zone. This should compare very
closely.
Mike
On 15 Jan 2008 at 18:22, anarui.oliveira wrote:
> Indeed.
> In the zone node the difference is not very noticeable, but it is in
> the supply air nodes, as you suggested.
> Thanks.
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Richard Raustad
> <RRaustad@...> wrote:
> >
> > If the zone temperature is held at the same temperature in both
> cases,
> > compare the zone humidity levels. You will probably find that the
> DX
> > Coil maintains a lower zone humidity level which would probably
> account
> > for the difference in cooling energy..You could also compare the
> > humidity ratio of the supply air for each system. Does the DX Coil
> > provide a lower humidity ratio? You could also report the supply
> air
> > mass flow rate and enthalpy and calculate the delivered cooling
> energy
> > as a check.
> >
> > anarui.oliveira wrote:
> > >
> > > Hello all.
> > >
> > > If (for cooling only), maintaining all other parameters in a
> model:
> > >
> > > - a PTAC is fully set to autosize, and a simulation is run,
> > > - and then it is substituted by a simple Purchased Air
> configuration,
> > > for the exact same zone and thermostat,
> > >
> > > why is the "DX Coil Total Cooling Energy" between 1.5 to 4 times
> higher
> > > than the "Purchased Air Total Cooling Energy"?
> > >
> > > I know "Purchased Air" is a simplification, but since we're
> looking
> > > directly at the thermal loads, and not any efficiency-dependent
> > > consumption variable, where does this difference come from?
> > >
> > > Regards,
> > > Ana Rui.
> > >
> > >
> >
> > --
> > Richard A. Raustad
> > Senior Research Engineer
> > Florida Solar Energy Center
> > University of Central Florida
> > 1679 Clearlake Road
> > Cocoa, FL 32922-5703
> > Phone: (321) 638-1454
> > Fax: (321) 638-1439 or 1010
> > Visit our web site at: http://www.fsec.ucf.edu
> >
> > UCF - From Promise to Prominence: Celebrating 40 Years
> >
>
>
>
>
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>
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>
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>
>
>
>
>
========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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