[BLDG-SIM] eQuest: Auto-sizing of Cooling Coils and Pumps and its Impact on Temperature Differential

Eric Studer estuder at dmiinc.com
Wed Oct 24 13:39:55 PDT 2007


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
DMI
35 Walnut Street
Wellesley, MA  02481
Ph:   (781) 431-1100 ext 12
Fax:  (781) 431-1109
e-mail:  estuder at dmiinc.com

 



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