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[EnergyPlus_Support] DX Cooling Coil Sizing in EnergyPlus with high outdoor air
Dear E+ Forum, I have lately wrestled with autosizing feature in
EnergyPlus, especially for DX coils with, say, more than 20% outdoor air
(OA). The extended story goes something like this: ? I want to
"autosize" primarily for LEED / ASHRAE 90.1 simulations of a
baseline system. ? Often, I manually size the minimum outdoor air
amount to match the Proposed system's outdoor air flow. ? This
has unpredictable results. When the OA amount is low, say less than 15%
of the total air flow, everything works smoothly. ? When the OA
flow is higher: o E+ calculates the correct coil entering air condition
for the higher OA flow o It then checks to see if the airflow rate per
watt capacity is within its limits of .00004027 ? 00006041 m3/s / W.
When doing so, autosized airflow is considered = Rated airflow. o More
outdoor air means that you may well violate the low limit because
airflow is calculated based on the zone loads and coil capacity is
greater due to the high OA amount. When this happens, E+ cuts the DX
coil capacity until it falls within the limits. o That means the DX coil
is smaller than needed to maintain a specified discharge temperature. o
That results in zone unmet loads. o That causes modeler headaches that
do not readily disappear. ï?§ I was privileged to get a new
personal record of 200,000,000 warnings about the flow / watt limits
being exceeded recently. I was sure that would inspire great confidence
in me with whomever was reviewing the LEED submittals! ï?§ Zone
sizing factors may need to exceed 175% for a majority of zones before it
capacity becomes acceptable. It, of course, makes the Proposed system
look more favorable, but is artificial. For actual coils in Proposed
systems, the problem plays out somewhat differently, but has many
similarities and headaches. There are a few other factors that I have
not mentioned because this post is already too long! The short story is:
I am wondering what others do to properly size DX coils with high OA
amounts so that warnings and unmet loads are minimum? Thanks in advance.
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