Plot the point of operation for each pump on the seriesed curve, Fig. 16. The point of
operation of each pump is on the single-pump curve when both pumps are operating.
Each pump supplies half the total required head.
When a single pump is running, the point of operation will be at the intersection of the
system-head curve and the single-pump characteristic curve, Fig. 16. At this point both
the flow and the hp (kW) input of the single pump decrease. Series pumping, Fig. 16, re-
quires the input motor hp (kW) for both pumps; this is the point of maximum power in-
put.
- Compute the possible savings
If the system requires a constant flow of 45 gal/min (2.84 L/s) at 26-ft (7.9-m) head the
two-pump series installation saves (0.75 hp - 2 x 0.25 hp) = 0.25 hp (0.19 kW) for every
hour the pumps run. For every 1000 hours of operation, the system saves 190 kWh. Since
2000 hours are generally equal to one shift of operation per year, the saving is 380 kWh
per shift per year.
If the load is frequently less than peak, one-pump operation delivers 32.5 gal/min (2.1
L/s). This value, which is some 72 percent of nail load, corresponds to doubling the sav-
ing.
Related Calculations. Series operation of pumps can be used in a variety of designs
for industrial, commercial, residential, chemical, power, marine, and similar plants. A se-
ries connection of pumps is especially suitable when full-load demand is small; i.e., just a
few hours a week, month, or year. With such a demand, one pump can serve the plant's
needs most of the time, thereby reducing the power bill. When full-load operation is re-
quired, the second pump is started. If there is a need for maintenance of the first pump,
the second unit is available for service.
This procedure is the work of Jerome F. Mueller, P.E., of Mueller Engineering Corp.
PARALLEL PUMPING ECONOMICS
A system requires a flow of 80 gal/min (7.4 L/s) of 20O^0 F (92.5^0 C) water at a 2O^0 F (36^0 C)
temperature drop and a 13-ft (3.96-m) system head. The required system flow can be han-
dled by two pumps, one an operating unit and one a spare unit. Each pump will have an
0.5-hp (0.37-kW) drive motor. Could there be any appreciable energy saving using some
other arrangement? The system requires 50 hours of constant pump operation and 40
hours of partial pump operation per week.
Calculation Procedure:- Plot characteristic curves for the proposed system
Figure 17 shows the proposed pump selection. Looking at the values of the pump head
and capacity in Fig. 17, it can be seen that if the peak load of 80 gal/min (7.4 L/s) were
carried by two pumps, then each would have to pump only 40 gal/min (3.7 L/s) in a paral-
lel arrangement. - Plot a characteristic curve for the pumps in parallel
Construct the paralleled-pump curve by doubling the flow of a single pump at any given
head, using data from the pump manufacturer. At 13-ft head (3.96-m) one pump produces
40 gal/min (3.7 L/s); two pumps 80 gal/min (7.4 L/s). The resulting curve is shown in Fig.