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Re: [EnergyPlus_Support] A few problems with controls
I do not see a way to autosize this type of system.
1. Regarding the sizing of the PTACs. EnergyPlus autosizing does not know how
to deal with more than one system at a time. So, you will not be able to
autosize both of these. The autosizing also does not know about the
contributions from other systems, so there is no straightforward way to
autosize the PTAC units. The best way I can think of to get the values for
these PTAC units would be to create a dummy zone that is controlled to 20C with
purchased air and use MIXING objects to push the desired volume of 20C air into
the affected zones. Run this with the PTACs autosized, and then take the
values from the eio outputs and put them back in as hard sizes. Tedious, I
know.
2. Regarding control of the OA supply DX cooling coil. Your controls are
correct. The DX coil is not large enough to handle the load. If you report
"1E DX OUTLET:System Node Setpoint Temp[C]" and "1E DX:DXSystem Part-Load
Ratio" you will see that the DX coil is running at 100% most of the cooling
design day, yet it is unable to satisfy the setpoint. There are a few hours in
the morning where the setpoint is satisfied (03:00 to 06:30). Why is the coil
undersized? Autosizing of DX coils is limited to keep the coil within the
models limits of 300 to 450 cfm/ton (see Engineering Ref. pp. 298-299, pdf pp.
332-333). If you look at the autosized outputs in the eio file, the ratio of
flow/capacity is 4.026963e-5 m3/s/W which is 300 cfm/ton.
The reality here is that a DX coil capable of treating 100% outside air in
Chicago to 12.8C is going to run at less than 300 cfm/ton which can then have
problems with freezing up the coil. It is likely that the actual equipment to
serve such a load will have two or more DX coils in series. You can model this
easily in EnergyPlus by DXSystem:AirLoop objects in series. The second one
should run only when the first one cannot meet the load. Of course, the
EnergyPlus autosizing will size them both to be 300 cfm/ton of system flow
rate. You will probably need to hand size these as well.
Mike
On 12 Dec 2006 at 0:58, asheaffe wrote:
> Boy am I confused. I have several problems with the 1EOnly file.
>
> I want to have a ventilation system that supplies 100% OA. The
> ventilation systems needs to have a constant 12.8 C supply temp from
> the cooling coil, and a constant 20 C supply temp from the heating
> coil, at a constant flow rate of 0.092m3/s. Then I want a PTAC to
> take over the supplemental heating and cooling loads throughout the
> year (no OA flow through the PTAC, zone recirculation only).
>
> I can't seem to get the cooling coil to supply a constant 12.8,
> regardless of the setpoint manager that I have installed.
>
> Then, when I specify a flow rate for that main system, it is
> automatically applied (against my wishes) to the PTAC. How do I
> make it so the flow through the PTAC is autosized based on the
> supplemental heating and cooling needs?
>
> Also, for a system like this where I have two different flows being
> supplied at different temperatures to the room, what should I use
> for the heating and cooling design temp fields in the zone sizing?
>
> Thanks,
>
> Andrew
>
>
>
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
>
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
>
> Attachments are not allowed -- please post any files to the appropriate folder
> in the Files area of the Support Web Site.
>
> EnergyPlus Documentation is searchable. Open EPlusMainMenu.idf and press the
> "search" button.
>
> Yahoo! Groups Links
>
>
>
>
>
========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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