A large facility that requires several chillers to satisfy the peak building load usually has chillers that have different capacities and efficiencies. As the building load increases from zero to the peak daily use, there’s an optimum sequence of operation that uses the available chillers to best advantage.
The smallest chiller can handle the lowest loads, up to its capacity. Then a second chiller of higher capacity is brought online, replacing the small unit. It carries the load up to its maximum capacity, at which time the smaller chiller is brought back on-line and operates at part load until the building peak passes. As the load decreases, the opposite sequence occurs, at all times using the chillers in combination or alone to provide chilled water at the most efficient rate.
There are many combinations of this arrangement. They can all be optimally controlled by a central EMS. This system applies to boilers and cooling-tower cells as well. They can be sequenced for maximum efficiency by the supervisory energy-management system. The demand is closely matched by the equipment online, in an exact relation that is programmed into the computer. The arrangement is predicated by the supply and return temperatures to the respective pieces of equipment.
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Excerpt from Energy Conservation Projects.