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<br><br><div class="gmail_quote">On Tue, Mar 22, 2011 at 5:30 AM, Deepak Tewari <span dir="ltr"><<a href="mailto:dipaktwri@yahoo.co.in">dipaktwri@yahoo.co.in</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;">Aaron <br>Can you elaborate how the larger chilled water delta-T will decrease chiller lift and increase efficiency?<br>
<div class="hm"><br><br><br></div><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><div class="hm"><font face="Tahoma" size="2"><hr size="1"><b><span style="font-weight: bold;">From:</span></b> Aaron Powers <<a href="mailto:caaronpowers@gmail.com" target="_blank">caaronpowers@gmail.com</a>><br>
<b><span style="font-weight: bold;">To:</span></b> "James V Dirkes II, PE" <<a href="mailto:jvd2pe@tds.net" target="_blank">jvd2pe@tds.net</a>><br><b><span style="font-weight: bold;">Cc:</span></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><span style="font-weight: bold;">Sent:</span></b> Mon, 21 March, 2011 8:41:21 PM<br><b><span style="font-weight: bold;">Subject:</span></b>
Re: [Bldg-sim] Difference in chiller energy for VAV and CAV system<br></font></div><div><div></div><div class="h5"><br><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>
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<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 rel="nofollow" href="mailto:jvd2pe@tds.net" target="_blank">jvd2pe@tds.net</a>></span> wrote:<br>
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<p class="MsoNormal"><span style="color: rgb(31, 73, 125); font-size: 11pt;">Deepak,</span></p>
<p class="MsoNormal"><span style="color: rgb(31, 73, 125); font-size: 11pt;">Here are some thoughts:</span></p>
<p><span style="font-family: Symbol; color: rgb(31, 73, 125); font-size: 11pt;"><span>·<span> </span></span></span><span style="color: rgb(31, 73, 125); 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: rgb(31, 73, 125); font-size: 11pt;"><span>·<span> </span></span></span><span style="color: rgb(31, 73, 125); 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: rgb(31, 73, 125); font-size: 11pt;"><span>·<span> </span></span></span><span style="color: rgb(31, 73, 125); 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: rgb(31, 73, 125); font-size: 11pt;"><span>·<span> </span></span></span><span style="color: rgb(31, 73, 125); 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: rgb(31, 73, 125); font-size: 11pt;"> </span></p>
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<p style="text-align: center;" class="MsoNormal" align="center"><b><span style="color: rgb(31, 73, 125); font-size: 13.5pt;">The Building Performance Team<br></span></b><b><span style="color: rgb(31, 73, 125);">James V. Dirkes II, P.E., LEED AP<br>
</span></b><span style="color: rgb(31, 73, 125);">1631 Acacia Drive NW<br>Grand Rapids, MI 49504<br><a rel="nofollow">616 450 8653</a></span></p></div>
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<p style="margin-left: 0.5in;" class="MsoNormal"><b><span style="font-size: 10pt;">From:</span></b><span style="font-size: 10pt;"> <a rel="nofollow" href="mailto:bldg-sim-bounces@lists.onebuilding.org" target="_blank">bldg-sim-bounces@lists.onebuilding.org</a> [mailto:<a rel="nofollow" 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 rel="nofollow" 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>
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<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>
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