Question-and-Answer Resource for the Building Energy Modeling Community
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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!
David
