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<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Greetings,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>I am modeling the energy savings associated with various
energy conservation measures and would like to have the chilled water system
components autosize.š I have left most fields in their default settings –
44œF CHW supply, 10œF CHW and CW temperature differentials, VSD on the CHW
pump, nothing in the capacity fields for pumps, chillers, or cooling towers.š I
have consistent CHW temperature differential requirement of 10œF among cooling
coils, the chiller loop attachment sheet, and the chilled water loop sheet.š
Here’s the kicker – when I look at hourly reports of supply and
return chilled water temperatures, the temperature differential is greater than
20œF and the CHW flowrates are less than half of what they should be.šš The
total loop load and chilled water supply temperature are what I would expect,
but the pumping energy is being underestimated.š <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Has anyone run into this one yet?š You would only see this
incongruity if you were to look at hourly reports for chilled water loops.š The
problem appears to lie in the autosizing of the cooling coils.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>I fiddled with the cooling coil sizing ratio, and I achieved
my 10œF temperature differential when the sizing ratio was set at 2.60.š
Unfortunately this also increased the pump size, which raises the minimum
flowrate through the variable primary flow chiller.š I tried to limit the
maximum flow of the pump using size ratio fields, but these need to be ™1.0.š 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.š 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.š Given the number of
separate runs I need to do, I’d like to not have to do this.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>To complicate things, this model also has free cooling on
the 10œF DT system and an entirely separate system with a 16œF DT and two
centrifugal chillers in series (45%/55% capacity).š I am having the same
cooling coil autosizing/pump energy issue on the 16œF system.š 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?).<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Thanks for your time,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Eric<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><u><font size=2 face="Times New Roman"><span
style='font-size:10.0pt'>
</span></font></u><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
10.0pt'>Eric Studer, PE, LEED AP<br>
DMI<br>
<st1:Street w:st="on"><st1:address w:st="on">35 Walnut Street</st1:address></st1:Street><br>
<st1:place w:st="on"><st1:City w:st="on">Wellesley</st1:City>, <st1:State
w:st="on">MA</st1:State> <st1:PostalCode w:st="on">02481</st1:PostalCode></st1:place><br>
Ph: (781) 431-1100 ext 12<br>
Fax: (781) 431-1109<br>
e-mail: estuder@dmiinc.com</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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