Question-and-Answer Resource for the Building Energy Modeling Community
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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.
