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Re: [EnergyPlus_Support] cfm/ton limits
The standard EnergyPlus sequence for sizing equipment is
1. Determine the cooling required by the zone - this is reported in the eio
output file
2. Determine the required zone supply air flow rate based on the specified
cooling supply temperature in the ZONE SIZING object and other limits (such as
min OA requirement) that are also specified in the ZONE SIZING object. This is
reported as the calculated supply air flow rate in the eio output file.
3. If the user has specified a flow/zone instead of design day method, then
the user supply air flow rate overrides this with the user-specified flow,
reported as the user value in the eio output.
The reported values will include sizing factors, also.
4. From that point forward, the supply air flow rate is fixed. The PTAC unit
will take that flow rate and size the DX coils to match, within the m3/s/W
(cfm/ton) limits.
So the first question is - is the supply air flow rate adequate to meet the
zone load? For an hour where the zone is not meeting setpoint, check the flow
rate and supply air temperature. If it is at the autosized flow rate and near
14C, something is wrong with steps 1-3 - check the day type on the cooling
design day and the schedules for internal gains and thermostats on that day
type.
Another possiblity is that the outside air fraction is very high, exceeding the
capabilities of a standard DX coil. But that would result in a coil size at
the low end of the flow/capacity limits, so I don't think that's the issue
here.
If you can't resolve this, please provide an input file.
Mike
On 28 Apr 2007 at 0:08, asheaffe wrote:
> I am having a problem getting PTAC cooling coils to size correctly.
> The coil cooling capacities are sizing smaller than they should
> (considerably smaller - by a factor of almost 6). The eio file
> reports that the coils are sizing right up at the limit of 450 cfm/ton
> (6.04E-05 m3/W). Sooo - I am taking this to mean that the low tonnage
> is correspondingly limiting the air flow. However, I need more
> tonnage to supply zone temps - so why is the sizing routing limiting
> my tonnage?
>
> How do I move them off of this limit? The PTAC air flows are all
> autosized. My zone sizing design cooling temp is 14C - which I think
> is fairly resonable.
>
> Thanks for the help,
>
> Andrew
>
>
>
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>
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>
> Attachments are not allowed -- please post any files to the appropriate folder
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>
> EnergyPlus Documentation is searchable. Open EPlusMainMenu.pdf under the
> Documentation link 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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The group web site is:
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