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<DIV dir=ltr align=left><SPAN class=562255104-22032011><FONT color=#0000ff
size=2 face=Arial>I have a bit different view of this:</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=562255104-22032011><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=562255104-22032011><FONT color=#0000ff
size=2 face=Arial>The load seen by the chiller is not just the load in the
zones, but the load from the system. The system load is what is required
to cool the supply flow down to the supply air temperature (SAT) setpoint.
The SAT is either fixed, or cold enough to satisfy the zone requiring the most
cooling (the "critical" zone), assuming the zone loads are met. All zones
except for the critical zone don't need as much cooling. In a VAV system,
the cooling is reduced via reduced flow. This reduced flow means less
system load to the chiller plant. In contrast, the CAV system load does
not get reduced because some zones have lower loads. The full fan flow
needs to be cooled to the SAT. In this case, the non-critical zones are
kept from being overcooled by the addition of reheat.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=562255104-22032011><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=562255104-22032011><FONT color=#0000ff
size=2 face=Arial>Regards,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=562255104-22032011><FONT color=#0000ff
size=2 face=Arial>Bill</FONT></SPAN></DIV><BR>
<DIV dir=ltr lang=en-us class=OutlookMessageHeader align=left>
<HR tabIndex=-1>
<FONT size=2 face=Tahoma><B>From:</B> bldg-sim-bounces@lists.onebuilding.org
[mailto:bldg-sim-bounces@lists.onebuilding.org] <B>On Behalf Of </B>Aaron
Powers<BR><B>Sent:</B> Monday, March 21, 2011 8:11 AM<BR><B>To:</B> James V
Dirkes II, PE<BR><B>Cc:</B> Deepak Tewari;
bldg-sim@lists.onebuilding.org<BR><B>Subject:</B> Re: [Bldg-sim] Difference in
chiller energy for VAV and CAV system<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV>There are several things going on here:</DIV>
<DIV> </DIV>
<DIV>- 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.</DIV>
<DIV>- 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.</DIV>
<DIV>- 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.</DIV>
<DIV>- 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.</DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>Aaron<BR></DIV>
<DIV class=gmail_quote>On Mon, Mar 21, 2011 at 8:13 AM, James V Dirkes II, PE
<SPAN dir=ltr><<A href="mailto:jvd2pe@tds.net">jvd2pe@tds.net</A>></SPAN>
wrote:<BR>
<BLOCKQUOTE
style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8ex; PADDING-LEFT: 1ex"
class=gmail_quote>
<DIV lang=EN-US link="blue" vlink="purple">
<DIV>
<P class=MsoNormal><SPAN
style="COLOR: #1f497d; FONT-SIZE: 11pt">Deepak,</SPAN></P>
<P class=MsoNormal><SPAN style="COLOR: #1f497d; FONT-SIZE: 11pt">Here are some
thoughts:</SPAN></P>
<P><SPAN
style="FONT-FAMILY: Symbol; COLOR: #1f497d; FONT-SIZE: 11pt"><SPAN>·<SPAN
style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN></SPAN><SPAN style="COLOR: #1f497d; FONT-SIZE: 11pt">Less fan
energy = less cooling load, since the fan energy is a part of the total
cooling load.</SPAN></P>
<P><SPAN
style="FONT-FAMILY: Symbol; COLOR: #1f497d; FONT-SIZE: 11pt"><SPAN>·<SPAN
style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN></SPAN><SPAN style="COLOR: #1f497d; FONT-SIZE: 11pt">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></P>
<P><SPAN
style="FONT-FAMILY: Symbol; COLOR: #1f497d; FONT-SIZE: 11pt"><SPAN>·<SPAN
style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN></SPAN><SPAN style="COLOR: #1f497d; FONT-SIZE: 11pt">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></P>
<P><SPAN
style="FONT-FAMILY: Symbol; COLOR: #1f497d; FONT-SIZE: 11pt"><SPAN>·<SPAN
style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN></SPAN><SPAN style="COLOR: #1f497d; FONT-SIZE: 11pt">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></P>
<P class=MsoNormal><SPAN
style="COLOR: #1f497d; FONT-SIZE: 11pt"></SPAN> </P>
<DIV>
<P style="TEXT-ALIGN: center" class=MsoNormal align=center><B><SPAN
style="COLOR: #1f497d; FONT-SIZE: 13.5pt">The Building Performance
Team<BR></SPAN></B><B><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><A href="tel:616%20450%208653"
target=_blank>616 450 8653</A></SPAN></P></DIV>
<P class=MsoNormal><SPAN
style="COLOR: #1f497d; FONT-SIZE: 11pt"></SPAN> </P>
<DIV>
<DIV
style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0in; PADDING-LEFT: 0in; PADDING-RIGHT: 0in; BORDER-TOP: #b5c4df 1pt solid; BORDER-RIGHT: medium none; PADDING-TOP: 3pt">
<P style="MARGIN-LEFT: 0.5in" class=MsoNormal><B><SPAN
style="FONT-SIZE: 10pt">From:</SPAN></B><SPAN style="FONT-SIZE: 10pt"> <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></P></DIV></DIV>
<DIV>
<DIV></DIV>
<DIV class=h5>
<P style="MARGIN-LEFT: 0.5in" class=MsoNormal> </P>
<DIV>
<DIV>
<P style="MARGIN-BOTTOM: 12pt; MARGIN-LEFT: 0.5in; MARGIN-RIGHT: 0in"
class=MsoNormal>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<BR><BR></P></DIV></DIV>
<P style="MARGIN-LEFT: 0.5in"
class=MsoNormal> </P></DIV></DIV></DIV></DIV><BR>_______________________________________________<BR>Bldg-sim
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