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chillers in series

asked 2017-12-16 21:11:39 -0500

Matt Koch's avatar

updated 2017-12-17 10:49:09 -0500

I am trying to model a single pump/dual chiller Variable Primary Only loop, but with chillers in series, not in parallel. When they are in parallel, the plant auto-sizing correctly determines 120 kg/s for the combined pump that drives both chillers (at their inlet), i.e. 60 kg/s per chiller. When they are in series, the plant-auto-sizing incorrectly determines 60 kg/s for the combined pump, when it still should be 120 kg/s, i.e. 120 kg/s per chiller. The loop design temperature difference is 10 oF. I should think that applies the same whether the chillers are in parallel or series. The demand side of my plant is a simple load profile. Can someone get me unstuck, or can OpenStudio/EnergyPlus not handle chillers in series?

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answered 2017-12-17 15:27:17 -0500

If all of the chiller input fields are the same, and all you are changing is applying a parallel vs. series arrangement for the chillers, then the chiller's are still being sized for the same auto-sized load and loop design temperature difference. Therefore, each chiller will also be sized for the same flow rate of 60 kg/s. When the chillers were in parallel, the pump flow rate was sized for meeting demand in both chillers simultaneously, so 120 kg/s. Now that the chillers are in series, the pump flow rate is sized for 60 kg/s because that "single stream" can flow through both chillers sequentially.

Refer to this post for more details on setting fractions of auto-sized chiller capacity in OpenStudio and EnergyPlus, and this post for a detailed discussion on sizing routines in EnergyPlus.

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Thank you kindly for your reply, Aaron. I have been to the Engineering Reference several times to figure this out, to no avail. At this point, in parallel, I have the design loop delta T at 10 R and the sizing factor at 0.5, and in series, I have the design loop delta T at 5 R and the sizing factor at 1.0. This does seem to give 120 kg/s in both cases, and also roughly 400 ton per chiller in both cases - just what I wanted. It does seem to give plausible results, too. Can you surmise if this is the right approach?

Matt Koch's avatar Matt Koch  ( 2017-12-18 08:39:40 -0500 )edit

I would be careful about changing design loop delta T because the chilled water coils on the demand side have similar rated inlet / outlet air and water temperature fields used for sizing. If you don't adjust the coils input fields to match the new design loop delta T, then the supply and demand equipment will be sized for different conditions.

If your goal is to have the same 120 kg/s rated pump flow for both arrangements, could you hard-size flow rate and capacity by entering specific values that you desire?

Aaron Boranian's avatar Aaron Boranian  ( 2017-12-18 10:10:08 -0500 )edit

Yes, I was playing with that earlier. It turns out that although I specify a 120 kg/s pump and it correctly shows up in the Equipment Summary, during the annual simulation, each chiller in the series configuration still only sees 60 kg/s. This is rather frustrating. It ought to be straight forward, but does not seem to be so.

Matt Koch's avatar Matt Koch  ( 2017-12-18 11:03:35 -0500 )edit

I've read this a few times and throught about and I don't agree that two chillers in series requires 120 kg/s. surely it is 60 kg/s. Does the rest of your answer below take that into account already, or not?

TomB's avatar TomB  ( 2018-03-15 06:08:33 -0500 )edit

answered 2017-12-18 12:55:27 -0500

Matt Koch's avatar

OK, so I was doing a comparison of a water-cooled dual chiller, dual tower Variable Primary loop with Parallel Evaporator/Parallel Condenser, Series Evaporator/Parallel Condenser and Series Evaporator/Series Condenser, respectively.

Parallel Evaporator/Parallel Condenser: image description image description

Series Evaporator/Parallel Condenser: image description image description

Series Evaporator/Series Condenser: image description image description

I did all of this with Auto-Sizing everywhere. I had to use the following sizing factors and loop temperature differences (in red) to not only get plausible equipment sizes but to then also get plausible flow rates and temperature differences for the annual simulation runs.

image description

I find this extremely odd, but then again, I do not really know how the (plant) sizing details work. Also note that the fan flow rate and fan power for the series condenser Case C is highly suspect, too, certainly due to the weird sizing factors and loop temperature differences I forced onto this.

I am just curious whether I am way out in the tall weeds with this, or whether there is some system behind these sizing factors and loop temperature differences? As I pointed out earlier, hard-sizing all of this spits out the right equipment sizes, but not the right flow rates during the annual simulation. Auto-sizing per the above does spit out (seemingly?) plausible results in turn.

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I actually do not believe any of the above is correct other than for the traditional Case A. OpenStudio and/or EnergyPlus seem to have a really hard time with the serial arrangement of chillers on the evaporator and/or condenser side, and neither the Input/Output Reference nor the Engineering Reference nor the Application Guide for Plant Loops offer real clues as to how to go about this. My suspicion is that such serial configurations cannot be handled by OpenStudio and/or EnergyPlus after all, despite the deceptive ease with which such can be arranged in the HVAC tab of OpenStudio?

Matt Koch's avatar Matt Koch  ( 2018-01-11 10:35:53 -0500 )edit

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Asked: 2017-12-16 21:11:39 -0500

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Last updated: Dec 18 '17