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<div class=Section1>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I too cannot claim to be capable of tearing a chiller apart and
putting it back together, but I can affirm Aaron’s fundamentals appear
on-cue to my understanding. Less cooling </span><span style='font-size:
11.0pt;font-family:Wingdings;color:#1F497D'>à</span><span
style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> less
compressor work </span><span style='font-size:11.0pt;font-family:Wingdings;
color:#1F497D'>à</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> $$$ savings.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The attached email exchange from a few months back may be of
interest to those following/participating in this discussion, though it is a
bit lengthy. In it, I attempt to “reverse engineer” the three
default curves found in eQuest/DOE2 which together define behavior of a centrifugal
chiller, observing and discussing the isolated effects of all the variables
that are taken into account on an hourly basis… One following along can
see I was a bit puzzled along the way, but arrived at an “ah-ha”
moment when I followed through and applied the curves against each other to
observe their net effects on efficiency under set conditions.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>My hope then and now is that sharing this thought process may
help others arrive at their own “eureka” moments =).<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>~Nick<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><img width=119 height=37 id="Picture_x0020_1"
src="cid:image001.jpg@01CBE8AC.43DD0BD0" alt="cid:489575314@22072009-0ABB"></span><b><span
style='font-size:11.0pt;font-family:"Stylus BT","sans-serif";color:#2D4D5E'><o:p></o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Stylus BT","sans-serif";
color:#2D4D5E'><o:p> </o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Stylus BT","sans-serif";
color:#2D4D5E'>NICK CATON, E.I.T.</span></b><b><span style='font-family:"Stylus BT","sans-serif";
color:#2D4D5E'><o:p></o:p></span></b></p>
<p class=MsoNormal><span style='font-size:7.5pt;font-family:"Calibri","sans-serif";
color:#CC9900'>PROJECT ENGINEER<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Calibri","sans-serif";
color:#2D4D5E'>Smith & Boucher Engineers</span><span style='font-size:7.5pt;
color:#CC9900'><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Calibri","sans-serif";
color:#2D4D5E'>25501 west valley parkway<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Calibri","sans-serif";
color:#2D4D5E'>olathe ks 66061<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Calibri","sans-serif";
color:#2D4D5E'>direct 913 344.0036<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Calibri","sans-serif";
color:#2D4D5E'>fax 913 345.0617<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><a href="www.smithboucher.com"
title="blocked::www.smithboucher.com"><span style='font-size:10.0pt'>www.smithboucher.com</span></a></span><u><span
style='font-size:10.0pt;font-family:"Calibri","sans-serif";color:blue'> </span></u><span
style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
bldg-sim-bounces@lists.onebuilding.org
[mailto:bldg-sim-bounces@lists.onebuilding.org] <b>On Behalf Of </b>Aaron
Powers<br>
<b>Sent:</b> Tuesday, March 22, 2011 2:41 PM<br>
<b>To:</b> Deepak Tewari<br>
<b>Cc:</b> bldg-sim@lists.onebuilding.org<br>
<b>Subject:</b> Re: [Bldg-sim] Difference in chiller energy for VAV and CAV
system<o:p></o:p></span></p>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='margin-bottom:12.0pt'>Bill, you're right about
that. A central CAV system with reheat is limited by the most loaded
zone. My statements before were simply for one air path and one zone,
which is probably rare. The degree to which this hurts a CAV system
depends on the diversity of the loads. If all zone loads remain similar,
then the supply air temperature off the CAV coils will start to float upwards
at reduced loads, and you'll see an increased chilled water delta-T.<br>
<br>
As for the increased chiller efficiency, here's my thinking. At a
constant load with no mixing valves, an increased delta T across the cooling
coils allows for a lower gpm. Slowing the water flow rate and raising the
entering water temperature increases the effectiveness of the evaporator barrel
(i.e. the enthalpy of the refrigerant now has a greater ability to approach the
enthalpy of the higher enthalpy water). Running the compressor as before
would over-cool the water. To maintain a constant chilled water supply
temperature, the mass flow of the refrigerant must be reduced. Depending
on the type of compressor unloading mechanism, this should produce some energy
savings.<br>
<br>
This is my understanding simply based on theory, and I'm sure it's much more
complicated in reality. Again, I know that DOE2 does not account for
varying inlet conditions to the chiller evaporator barrel, but I'm not sure
about ePlus. <br>
<br>
Aaron<o:p></o:p></p>
<div>
<p class=MsoNormal>On Tue, Mar 22, 2011 at 5:30 AM, Deepak Tewari <<a
href="mailto:dipaktwri@yahoo.co.in">dipaktwri@yahoo.co.in</a>> wrote:<o:p></o:p></p>
<div>
<div>
<div>
<p class=MsoNormal>Aaron <br>
Can you elaborate how the larger chilled water delta-T will decrease chiller
lift and increase efficiency?<o:p></o:p></p>
<div>
<p class=MsoNormal style='margin-bottom:12.0pt'><br>
<br>
<o:p></o:p></p>
</div>
<div>
<div>
<div class=MsoNormal align=center style='text-align:center'><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
<hr size=1 width="100%" align=center>
</span></div>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> Aaron Powers <<a
href="mailto:caaronpowers@gmail.com" target="_blank">caaronpowers@gmail.com</a>><br>
<b>To:</b> "James V Dirkes II, PE" <<a href="mailto:jvd2pe@tds.net"
target="_blank">jvd2pe@tds.net</a>><br>
<b>Cc:</b> Deepak Tewari <<a href="mailto:dipaktwri@yahoo.co.in"
target="_blank">dipaktwri@yahoo.co.in</a>>; <a
href="mailto:bldg-sim@lists.onebuilding.org" target="_blank">bldg-sim@lists.onebuilding.org</a><br>
<b>Sent:</b> Mon, 21 March, 2011 8:41:21 PM<br>
<b>Subject:</b> Re: [Bldg-sim] Difference in chiller energy for VAV and CAV
system</span><o:p></o:p></p>
</div>
<div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
<div>
<p class=MsoNormal>There are several things going on here:<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal> <o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>- Yes, the lower fan energy for VAV will add less heat to
the chilled water loop. This will propogate to the pumps, chillers, and
heat rejection.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>- Chilled water coils are complex heat-exchangers, which do
not exibit linear behavior. Reducing the air flow decreases the
water-side coil effectiveness; therefore, at a given load, a CAV system will
have a larger chilled water delta-T.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>- With equivalent pumping schemes, this will result in pump
savings for the CAV system (in the absence of 3-way valves). I'm not sure
about ePlus, but this can be demonstrated in the latest DOE2.2.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>- In reality, the larger chilled water delta-T will decrease
chiller lift and increase its efficiency. Again I'm not sure about ePlus,
but in DOE2.2, chiller curves are a function of a dT parameter which is the
difference between condenser entering and chilled water leaving temperatures.
Its an attempt to account for chiller lift, but it does not give an efficiency
credit for increasing the chilled water delta-T. So, you will not see the
chiller efficiency boost in DOE2 for CAV systems due to a greater chilled water
delta-T.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal> <o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>In my experience, the VAV fan savings (and reduced chilled
water load savings) usually outweigh the pump and chiller savings for
CAV. However, it varies from building to building. For
example, if you had a rare building with a low air-side static pressure drop to
begin with and a long, high head pumping system, then its possible that the CAV
system will be more efficient overall.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal> <o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>Aaron<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>On Mon, Mar 21, 2011 at 8:13 AM, James V Dirkes II, PE <<a
href="mailto:jvd2pe@tds.net" target="_blank">jvd2pe@tds.net</a>> wrote:<o:p></o:p></p>
<div>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'>Deepak,</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'>Here are some thoughts:</span><o:p></o:p></p>
<p><span style='font-size:11.0pt;font-family:Symbol;color:#1F497D'>·
</span><span style='font-size:11.0pt;color:#1F497D'>Less fan energy =
less cooling load, since the fan energy is a part of the total cooling load.</span><o:p></o:p></p>
<p><span style='font-size:11.0pt;font-family:Symbol;color:#1F497D'>·
</span><span style='font-size:11.0pt;color:#1F497D'>If the pump is variable
volume, the pump energy required for VAV fans will be slightly less due to less
fan heat to cool.</span><o:p></o:p></p>
<p><span style='font-size:11.0pt;font-family:Symbol;color:#1F497D'>·
</span><span style='font-size:11.0pt;color:#1F497D'>If you are using the
identical chiller for each system (VAV, CAV), then the chiller should use less
energy also, due to less fan and pump heat.</span><o:p></o:p></p>
<p><span style='font-size:11.0pt;font-family:Symbol;color:#1F497D'>·
</span><span style='font-size:11.0pt;color:#1F497D'>A more common comparison
would be to contrast a VAV chiller system with a CAV packaged rooftop
system. For that comparison, the part load efficiencies of chiller and
compressor / DX coil will be a major factor. Dehumidification will also be
different for DX vs. chilled water coils.</span><o:p></o:p></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
<div>
<p class=MsoNormal align=center style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;text-align:center'><b><span style='font-size:13.5pt;color:#1F497D'>The
Building Performance Team<br>
</span><span style='color:#1F497D'>James V. Dirkes II, P.E., LEED AP<br>
</span></b><span style='color:#1F497D'>1631 Acacia Drive NW<br>
Grand Rapids, MI 49504<br>
616 450 8653</span><o:p></o:p></p>
</div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-size:11.0pt;color:#1F497D'> </span><o:p></o:p></p>
<div>
<div style='border:none;border-top:solid windowtext 1.0pt;padding:3.0pt 0in 0in 0in;
border-color:-moz-use-text-color -moz-use-text-color'>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
margin-left:.5in'><b><span style='font-size:10.0pt'>From:</span></b><span
style='font-size:10.0pt'> <a
href="mailto:bldg-sim-bounces@lists.onebuilding.org" target="_blank">bldg-sim-bounces@lists.onebuilding.org</a>
[mailto:<a href="mailto:bldg-sim-bounces@lists.onebuilding.org" target="_blank">bldg-sim-bounces@lists.onebuilding.org</a>]
<b>On Behalf Of </b>Deepak Tewari<br>
<b>Sent:</b> Monday, March 21, 2011 2:31 AM<br>
<b>To:</b> <a href="mailto:bldg-sim@lists.onebuilding.org" target="_blank">bldg-sim@lists.onebuilding.org</a><br>
<b>Subject:</b> [Bldg-sim] Difference in chiller energy for VAV and CAV system</span><o:p></o:p></p>
</div>
</div>
<div>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
margin-left:.5in'> <o:p></o:p></p>
<div>
<div>
<p class=MsoNormal style='mso-margin-top-alt:auto;margin-bottom:12.0pt;
margin-left:.5in'>Dear All,<br>
<br>
I am comparing the energy performance of a constant volume and variable volume
system for a composite climate of India (New Delhi) in EnergyPlus. The building
area is 7500 sq m. The chiller capacity is same for both the cases. The chilled
water to the cooling coils is supplied by a constant speed pump. <br>
<br>
The savings in the fan energy is evident due to variable speed of the supply
fan in case of VAV. However i am getting energy saving in cooling energy
(chiller energy) also, in VAV compared to CAV, which i feel is due to higher
delta T (chilled water) across cooling coil for CAV compared to VAV, this in
turns increases the chiller electricity consumption. However while discussions
with some consultant, it is their feeling that the cooling energy would remain
same for both type of systems.<br>
<br>
I want to ask has someone else tried this simulation and would there be any
difference in cooling energy or not? <br>
Thanks in advance.<br>
Deepak<o:p></o:p></p>
</div>
</div>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
margin-left:.5in'> <o:p></o:p></p>
</div>
</div>
</div>
</div>
<p class=MsoNormal style='margin-bottom:12.0pt'><br>
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<p class=MsoNormal><br>
<br clear=all>
<br>
-- <br>
Sent from my DynaTAC 8000x<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<br clear=all>
<br>
-- <br>
Sent from my DynaTAC 8000x<o:p></o:p></p>
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