Some central chilled water systems connect the evaporators on multiple chillers in series.  That is, return chilled water flows through the first chiller and then the next chiller, and controls are set so that each chiller covers part of the required cooling range. The idea in this type of connection is that it is better suited to a larger temperature spread across the plant, and the chilled water pump size is generally smaller.  A larger temperature spread would be when the return water coming in to the first chiller is much higher than the desired chilled water supply temperature going out to the building after the final chiller. The series set-up only works well if the chilled water distribution system is configured to produce a larger temperature spread.

Parallel connected evaporators in central plants with multiple chillers are piped so that the chilled water return branches to each chiller evaporator and the supply is similarly branched together into a common header.  These systems handle the same temperature spread across each chiller evaporator, and each chiller requires a water volume suited to the characteristics of the chiller.  This set up will require larger (or more) pumps, and will likely only work with a smaller temperature spread across the chillers.