Page 233 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

230 The HVAC World

the three systems using different coil and control-valve arrange-
ments. Similar energy evaluations will be made for condenser water
in Chap. 11.
Three-way valve system. Figure 8.6a describes a secondary system
with 10 air-handling units, each with a requirement of 200 gal/min,
for a total design flow of 2000 gal / min. Three-way valves are
installed on the air handlers so that the flow is constant regardless
of the load. The coil, control-valve, and piping loss is 26 ft, the pump
fitting loss is 8 ft, and the header losses are 50 ft, for a total pump
head of 84 ft. If the pumps have an efficiency of 86 percent and the
motors have an efficiency of 92 percent, the total electrical consump-
tion at any load is 40 kW. Equations 6.12, 6.13, and 6.14 are used for
this calculation.
Two-way valve system with coil circulators. This system is similar to
the three-way valve system above and is described in Fig. 8.3b. Each coil
1
is fitted with a 1 ⁄ 2 -hp circulator that has a pump duty of 200 gal/min at
16 ft. The head of 16 ft results from a coil loss of 10 ft, pump fittings
of 2 ft, balance valve of 2 ft, and pipe friction and fittings of 2 ft. The
pump has an efficiency of 67 percent, and the pump motor has an effi-
ciency of 84 percent if it is of the high-efficiency type. This pump runs
continuously whenever the coil is in operation, and its energy con-
sumption is 1.1 kW.
The secondary pumps have a total head of 72 ft, consisting of 8 ft of
loss in the pump fittings, 50 ft in the system headers, and 14 ft of dif-
ferential pressure across the coil, and its control valve and piping.
Each pump has an energy consumption of 17.5 kW at design flow
with a pump efficiency of 86 percent and a wire-to-shaft efficiency of
90 percent. Adding the circulator pump kilowatts to those of the sec-
ondary pumps produces a total kilowatts of 10
1.1 2
17.5 for a
total energy consumption of 46.0 kW at full load with a system flow of
2000 gal/min.
Contemporary two-way valve system. The same chilled water distribu-
tion system when equipped with two-way coil control valves would
utilize variable/primary pumping (see Fig 8.6c). It is no longer neces-
sary to use primary/secondary pumping since chillers can now accept
variable flow in their evaporators. This will be explained in detail in
Chap. 9. Without the friction loss in the secondary pump fittings, the
friction loss of the chilled water system consists of 26 ft across the
coils, control valves, and piping with 50 ft loss in the supply and return
headers for a total of 76 ft. The total energy consumption at 2000
gal/min is 37 kW with a pump efficiency of 86 percent and a wire-to-
shaft efficiency of 90 percent for the variable-speed drive and motor.

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