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huge condenser water pump saving because of more chillers

asked 2015-10-02 09:03:11 -0500

I am trying to study the influence of chiller number on building energy consumption and found the most significant difference is the condenser loop pump! Two simple cases were simulated using OS1.7 (link: 1 Chiller, 2 Chillers)

All the plant loops are autosized and all the equipment performances are the same. The only difference is that first case has 1 chiller with sizing factor of 1, the other case has 2 chillers and each of them with sizing factor of 0.5. Results show that the second case pump energy is only 1/3 of the first case which is mainly resulted from condenser water pump. Anyone knows how to explain the results?

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answered 2015-10-03 15:20:47 -0500

The sequence for the chillers may have constant speed condenser water pumps, providing the same amount of condenser water flow any time that the chiller is operating. (Use of constant speed condenser water pumps is common in practice.)

In your case with two chillers, you are actually reducing the flow rate and required energy by half when only one chiller is operating (which may be most of the time) as opposed to two chillers.

Similarly on the chilled water side, if you have primary pumps that are constant speed the same effect will take place. The magnitude should be less than for the condenser water pumps. Secondary pumps should be more or less the same in both cases if the buildings are identical.

Lastly, if the single chiller is oversized, then there could be minimum flow rate requirements that are hit resulting in excess energy use. The case with two smaller machines may not be hitting any limit.

I recommend to review the building load requirements and verify that the 1.0 and the 2 x 0.5 capacities are suitable. The savings by moving to two chillers at 1/2 the load should be less than what you observed if the chillers are sized to equal the load. Your results are understandable if the load can actually be satisfied by one chiller at 0.5 factor.

Verify the capacities of chillers and pumps, don't rely only on the sizing factor to determine the capacities.

I hope this helps!


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Thanks for your reply! I checked the pumps and they are variable volume. As for the chiller sizing factor. If I use size factor of 1 instead of 0.5, it seems that total loop flow rate will be assigned to each chiller and only one chiller is enough. Is that correct?

Yan's avatar Yan  ( 2015-10-16 09:50:02 -0500 )edit

The pumps may be defined as variable speed, but double check that there is some driver from the system that causes them to vary by tracking a variable to show the actual flow rate during the simulation.

A simplified way to describe the sizing - verify from the output reports that the total chiller capacity matches the load reported by the AHUs/coils. Make a manual adjustment if necessary so that you have two chillers that are each sized at one-half the needed coil load.

DancingDavidE's avatar DancingDavidE  ( 2015-10-16 16:38:50 -0500 )edit

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Asked: 2015-10-02 09:03:11 -0500

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Last updated: Oct 02 '15