Multiple chiller installations should operate the chillers to be as fully loaded as possible because chillers are most efficient at full-load conditions. This can be arranged if the chilled-water distribution system is a primary-secondary loop arrangement. The chillers are piped in parallel, and output is to a single loop off of which each secondary building loop draws fluid. A single chiller can provide chilled water to the entire facility with such a setup. Otherwise, consider three chillers serving separate building loops: each is always operating under inefficient part-load conditions since each is sized to handle the peak load of its respective building.
The staged-primary-loop design allows the smallest chiller to handle the load up to its capacity. Then the next larger chiller is used, up to its capacity. Then the small chiller is brought back on-line, while the large chiller operates at full capacity, and so forth. Depending on the capacities and efficiencies of the chillers, the staging can be even more complicated, adding chilled-water reset and other functions. It’s a common controls application that can generate significant savings at a reasonable cost.
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Excerpt from Energy Conservation Projects.