|
Greetings, I am modeling the energy savings associated with various
energy conservation measures and would like to have the chilled water system
components autosize. I have left most fields in their default settings –
44°F CHW supply, 10°F CHW and CW temperature differentials, VSD on the CHW
pump, nothing in the capacity fields for pumps, chillers, or cooling towers. I
have consistent CHW temperature differential requirement of 10°F among cooling
coils, the chiller loop attachment sheet, and the chilled water loop sheet.
Here’s the kicker – when I look at hourly reports of supply and
return chilled water temperatures, the temperature differential is greater than
20°F and the CHW flowrates are less than half of what they should be. The
total loop load and chilled water supply temperature are what I would expect,
but the pumping energy is being underestimated. Has anyone run into this one yet? You would only see this
incongruity if you were to look at hourly reports for chilled water loops. The
problem appears to lie in the autosizing of the cooling coils. I fiddled with the cooling coil sizing ratio, and I achieved
my 10°F temperature differential when the sizing ratio was set at 2.60.
Unfortunately this also increased the pump size, which raises the minimum
flowrate through the variable primary flow chiller. I tried to limit the
maximum flow of the pump using size ratio fields, but these need to be ≥1.0. I
can enter the selection flowrate of the pump and I get the right temperature differential,
but the minimum flow through the chiller appears to be based on the flow the
loop would calculate when the pump is autosized. There is a brute-force method
of doing this, which is to specify the minimum flow ratio based on the flows
listed in the error message and to specify the pump size. Given the number of
separate runs I need to do, I’d like to not have to do this. To complicate things, this model also has free cooling on
the 10°F DT system and an entirely separate system with a 16°F DT and two
centrifugal chillers in series (45%/55% capacity). I am having the same
cooling coil autosizing/pump energy issue on the 16°F system. It seems
unlikely that the use of load management and equipment control techniques
necessary for free cooling and series chillers would result in this error
(right?). Thanks for your time, Eric
Eric Studer, PE, LEED AP =====================================================You received this e-mail because you are subscribed to the BLDG-SIM@xxxxxxxx mailing list. To unsubscribe from this mailing list send a blank message to BLDG-SIM-UNSUBSCRIBE@xxxxxxxx |