Handbook of air conditioning and refrigeration / Shan K

7.50 CHAPTER SEVEN
Use of Balancing Valves
Equal-percentage two-way control valves are widely used to modulate the flow rate of chilled water
flowing through the coils during part-load operation. Because of the lower installation cost and
since often there is only limited space available inside the ceiling plenum, the direct-return piping
system is often the best choice for the chilled water system in a multistory building. For a variableflow
building loop using a direct-return piping arrangement, the argument concerns whether a balance
valve is necessary for each branch pipe to balance the water flow according to its requirement,
such as for branches CK, DJ, EH, and FG, as shown in Fig. 7.20a. If there are no balancing valves
installed in the branch pipes, after the control valve balances the water flow at design load, can it
still effectively adjust the amount of chilled water entering the coil as required in part-load operation.
This depends mainly on the type of control valve, the control mode adopted, the variation in
pressure drop between various branches, and the difference in main pipe pressure drop between the
farthest and the nearest branch regardless of whether equal-percentage two-way control valve with
modulation control (such as proportional plus integral control ) or two-way control valve with twoposition
on/off control (including pulse-width-adjusted two-position control) is used.
For an equal-percentage two-way control valve with modulation control for many AHUs, consider
a plant-building loop in a chilled water system, as shown in Fig. 7.20a. At the design load, the
chilled water volume flow rate through branch FG is 80 gpm (0.30 m 3 /min), the corresponding
pressure drop of its fully opened equal-percentage two-way valve is 7.5 ft WC (3.3 psi or 2.3 m
WC), and the pressure drop across the farthest branch FG is 20 ft WC (8.7 psi or 6.1 m WC). From
Eq. (5.8), the flow coefficient
C v � V˙
Usually, the difference between the pressure drop across the farthest branch from the building pump
FG and the pressure drop across the nearest branch CK is often within 60 ft WC (26 psi or 18 m
WC). At design load, for a fully opened two-way control valve in branch CK, even if all this 26 psi
(18 m head loss) has been added, the chilled water volume flow rate is then
From Fig. 5.16, for a typical equal-percentage two-way valve, when the percentage of the valve
stem travel lies between 80 and 100 percent, the relationship between the percentage of full-range
travel of valve stem z and the percentage of water flow rate when the valve is fully opened is
where k � proportional constant
K � flow parameter affected by size of valve
Since 80/238 � 0.336, substituting into Eq. (7.17) gives
0.336 � 0.004e5.5z V˙ v
√�p vv
� 80
√3.3
� 44
V˙ � C v √�p vv � 44√3.3 � 26 � 238 gpm (15 L / s or 0.90 m 3 / min)
� Ke kz � 0.004e 5.5 (7.17)
And the percentage of full-range travel of valve stem z � 0.805. That is, an equal-percentage twoway
control valve in branch CK will close its opening from 100 percent fully open to 80.5 percent
for water flow balance at design load. Avery et al. (1990) emphasized that “If properly selected
valves (those with equal percentage ports and with the correct actuators) are used, 20 percent or less
of the stroke will be used to balance the flow. The rest of the stroke will still be available to modulate
the flow within the design limits.” Rishel (1997) also stated that manual balance valves and automatic
balance valves should not be used on variable-volume, direct-return, modulating type, coil
control valve, chilled water systems.
Therefore, an equal-percentage, two-way control valve, direct-return VAV system can balance
the water flow in a direct-return chilled water system, and at least 80 percent of its stroke is still
V˙ v