In 1996, an article in this magazine1 sparked debate of the merits of primary/secondary (P/S) vs. primary-only (P-only) pumping of distribution water in central chiller plants. The consensus of numerous articles published since is that P-only pumping has plant kilowatt-demand advantages over P/S pumping, especially if low-load delta-T exists. This article is the first in a series using a chiller-plant model2 to study the effect of the two pumping approaches on total plant kilowatt demand. The model adheres to the fundamental principal of the first law of thermodynamics.3

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