Page 287 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

284 The HVAC World

The preceding examples demonstrate the proper and improper use
that can be made of the pump affinity laws in the field when trying to
change the pump diameter for a specific application. Similarly, the
calculation of pump energy must use the preceding equations, not just
the affinity laws themselves. Figure 10.7b includes the true pump
horsepower for the same system head curve. At 50 percent flow, the
pump horsepower is 35 percent, not 12.5 percent, of full flow as would
appear from the affinity law curves.

10.5 Computing Pump Performance
Equation 10.1 is one of the most important equations in this book,
as it is the basic calculation for determining variable-speed pump
performance. By its use, a computer program can be developed to
determine rapidly the pump efficiency and speed at any system
flow and head. A single point was developed in Fig. 10.4. Now we
should show this procedure for an entire water system, from mini-
mum to maximum flow. Assume a 3000 gal/min chilled water sys-
tem with a pump head of 100 ft served by two 1500 gal/min pumps.
At maximum flow of 1500 gal/min, the pump fitting loss is 10 ft, the
system friction loss at 3000 gal/min is 70 ft, and the differential
pressure maintained for the coil loss is 20 ft for a total of 100 ft of
pump head.
The results of the calculations are shown in Table 10.1. They have
been rounded up, so there are slight discrepancies between the table
and Fig. 10.8.
Figure 10.8 includes the flow-head curves for single pump and two
pump operation with the pump efficiencies included. The system head
curves with one and two pumps running are also shown. The system
head curve points, Q – h , are connected to the head-flow points, Q – h ,
2 2 1 2
on the pump curves. This demonstrates graphically how the equiva-
2
lent points are developed. The value, Q h , is included in the calcu-
2 2
lations to demonstrate the procedure for calculating the equivalent
points.
This is also the procedure used to determine the point of transition
from single pump operation to two pump operation. In this case, this
transition is made at 1100 gal/min flow as shown in Fig. 10.9 for the
kW input to the pump motors. This procedure can be used for any
number of pumps operating in parallel. It is the easiest when all
pumps are of the same size; it can be done with pumps of different
capacities.

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