The low capacity chiller is alternately commanded with a high capacity chiller. Fig. 2 shows an example of a light load / peak load chiller application with 1 low capacity chiller and 2 high capacity chillers.
Fig. 2. Alternate Low and High Capacity Chillers
The total number of stages to control all chillers is equal to
Number of stages = number of high capacity chillers * 2 + 1
The total number of stages must be incremented by 1 when the chiller plant uses free cooling / ice storage.