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Several articles over the last several years have extolled the virtues of primary-only variable flow chilled water pumping systems compared to the conventional primary-secondary pumping system. One article1 even predicts the "demise" of primary-secondary systems. While there is no question that the primary-only system has many advantages, the primary-secondary system may still be preferable for some applications.
System Description This article addresses two pumping configurations applied to variable flow, variable speed chilled water systems: primary-only (p-only) pumping, shown in Figure 1, and conventional primary-secondary (p-s) pumping, shown in Figure 2. There are other pumping schemes, in particular variations on the p-s system using distributed secondary pumps2 that may be preferable to these two schemes in some applications, but they are beyond the scope of this article.
For both options, the distribution system is variable flow using two-way valves at cooling coils. The distribution pumps (primary pumps for the p-only system and secondary pumps for the p-s system) are fitted with variable speed drives (VSDs). VSDs are optional - the pumps could simply "ride their curves" as flow varies - but VSDs significantly reduce pump energy and are cost effective in most variable flow chilled water applications. For both systems, the VSDs are controlled to maintain differential pressure at a remote location in the system at a setpoint determined to be sufficient to deliver the required chilled flow through any coil. The setpoint may be constant or reset downward at part-load conditions.
Two elements of the p-only system in Figure I deserve more detailed discussion: * The chillers are piped in parallel
with automatic isolation valves at each chiller to shut off flow when the chiller is off. The pumps could also be piped individually to each chiller as shown with the p-s system in Figure 2 (see discussion later regarding the advantages and disadvantages of this approach for p-s systems). But with p-only systems, the headered pump design has two key advantages that make it preferable to dedicated pumping:
1. Flow through a chiller that is just starting up can be slowly increased from zero to the minimum rate by controlling the speed at which the isolation valve opens. This slows the rate of change of chilled water flow through other operating chillers, which is...