Balancing of a Water and Air System (PDF

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Pump Circuits
Primary-Secondary
Primary-secondary pump circuits reduce pumping horsepower requirements while
increasing system control quality. Primary-secondary
circuits are designed so that when two circuits are interconnected, flow in one circuit will
not cause flow in the other if the pressure drop is eliminated in the pipe common to both
circuits.
Figure 11.40 shows a primary-secondary pump circuit. The common pipe (A-H), which
may vary in length, has a negligible pressure drop and ensures the isolation of the
secondary circuit from the primary circuit. Therefore, in order to overcome the pressure
loss in the secondary circuit (A-B-C-D-E-F-G-H) and provide design flow, a secondary
pump (Figure 11.41) must be installed.
The primary pump and the secondary pump have no effect on each other. The function of
the primary pump is simply to circulate water around the primary circuit. The secondary
pump then supplies the terminals. The secondary flow may be (1) less than, (2) equal to,
or (3) greater than the primary flow (Figures 11.42, 11.43, and 11.44).
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