Page 261 - HVAC Pump Handbook

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Configuring an HVAC Water System

258 The HVAC World

Figure 9.7b is a typical head variation for many campus buildings.
Most college and university campuses now utilize many energy-wasting
devices such as balance valves, pressure-reducing valves, and
crossover bridges to overcome this friction variation. This system head
area demonstrates how broad are the actual head requirements of a
campus-type system. For example, at 50 percent load on the building,
the pump head can vary from 27 to 74 ft. With constant-speed pumps,
all this head difference must be destroyed by mechanical devices to
maintain adequate flow to all parts of the building. With the correct
pumping arrangement and variable-speed pumps, all this wasted head
can be eliminated. With proper control, the variable-speed pump will
always operate at the needed system flow and head without any over-
pressure or wasted energy. A careful review of this figure will be made
in Chap. 15, as well as how pump performance is affected by such
broad variations in system head.

9.3.3 Pumping system losses
All the foregoing discussion has been for water system flow and head
evaluation. Losses through pumping systems themselves are often
ignored or included with the system head losses. It has been learned
that pumping system losses can be significant and should be analyzed
separately from the water system losses. Figure 9.8a describes an end
view of an actual pumping system consisting of five pumps, each with
a capacity of 700 gal/min, for a total system capacity of 3500 gal/min.
The individual loss for each fitting is shown along with its K factor.
The total loss for the pump fittings is 6.9 ft. Recognizing Hydraulic
Institute’s estimate that the losses in pipe fittings can vary by as
much as 10 to 35 percent, these losses have been multiplied by plus
20 percent to achieve a reasonable loss for the fittings. The losses for
this system would therefore be 8 ft with 700 gal/min flowing through
an individual pump. Figure 9.8b is the pump curve for this system;
each pump could operate at a maximum flow of 1100 gal/min if
improperly controlled. At 1100 gal/min, the pump fitting loss becomes
(1100/700) 1.90 8, or 18.9 ft. A loss of only 8 ft may not raise much
concern, but a loss of close to 19 ft should be of concern to anyone
making a serious energy evaluation of this water system.
This system had a differential pressure of 30 ft across the largest
coil and its control valve and a system loss of 78 ft. With the esti-
mated 8 ft of loss throughout the pump fittings, the total system loss
becomes 116 ft. Figure 9.8c describes the uniform system head curve
for this system, which has been adjusted to recognize the variable
loss through the pump fittings. Again, no longer is the uniform sys-
tem curve a smooth curve, but rather it is one of scalloped shape. It is

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