Handbook of air conditioning and refrigeration / Shan K

variable-flow water system installed with a constant-speed pump, the design system operating
point is preferably located to the left of the region of pump maximum efficiency, because the system
operating point of an oversized pump moves into or nearer to the region of pump maximum
efficiency.
Combination of Pump-Piping Systems
When two pump-piping systems 1 and 2 are connected in series as shown in Fig. 7.15a, the volume
flow rate of the combined pump-piping system, [gpm (m3 /s)] is
V˙ 1
V˙ 2
FIGURE 7.14 Water system curve and system operating point.
V˙ com
V˙ com � V˙ 1 � V˙ 2
(7.12)
where and are the volume flow rate of pump-piping systems 1 and 2, gpm (m3 /s). The total
head lift of the combined system �Hcom [ft WC (m WC)] is
�H com ��H 1 ��H 2
(7.13)
where �H1 and �H2 are the head of pump-piping systems 1 and 2, ft WC (m WC).
It is simpler to use one system curve to represent the whole system, i.e., to use a combined system
curve. The system operating point of the combined pump-piping system is illustrated by point
P with a volume flow of V˙ P and head of �HP. The purpose of connecting pump-piping systems in
series is to increase the system head.
When a pump-piping system has parallel-connected water pumps, its volume flow rate V˙ [gpm
(m 3 /s)] is the sum of the volume flow rates of the constituent pumps , , etc. The head of each
constituent pump and the head of the combined pump-piping system are equal. It is more convenient
to draw a combined pump curve and one system curve to determine their intersection, the
system operating point P, as shown in Fig. 7.15b. The purpose of equipping a water system with
parallel-connected water pumps is to increase its volume flow rate.
V˙ 1
V˙ 2
WATER SYSTEMS 7.35