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c. A manual balancing valve may be installed in the
boiler bypass to equalize the boiler drop and the
bypass drop.
WATER VALVE SIZING EXAMPLES
EXAMPLE 1:
A two-way linear valve is needed to control flow of 45F
chilled water to a cooling coil. The coil manufacturer has
specified an eight-row coil having a water flow pressure drop
of 3.16 psi. Further, specifications say that the coil will pro-
duce 55F leaving air with a water flow of 14.6 gpm. Supply
main is maintained at 40 psig, return is at 30 psig. Select
required capacity index (C
v
) of the valve.
Use the water valve C
v
formula to determine capacity index
for Valve V1 as follows:
Where:
Q = Flow of fluid in gallons per minute required
is 14.6 gpm.
G = Specific gravity of water is 1.
h = Pressure drop across the valve. The
difference between the supply and return
is 10 psi. 50% to 70% x 10 psi = 5 to 7 psi.
Use 6 psi for the correct valve pressure
drop. Note that 6 psi is also greater than
the coil pressure drop of 3.16 psi.
Substituting:
Select a linear valve providing close control with a capacity
index of 6 and meeting the required pressure and temperature
ratings.
EXAMPLE 2:
A bypass valve is required to prevent flow through the chiller
from dropping below 90 percent of design flow. When sizing
valves for pump or chiller bypass applications (Fig. 16), sys-
tem conditions that cause the valve to open or close com-
pletely must be considered before a pressure drop can be
selected.
Assume the following:
System flow at design, 1000 gpm
Pump head at design, 48 ft
Pump head at 90 percent flow, 50 ft
Pressure across mains at AHU 1 at design flow, 28 ft
Chiller pressure drop, 12 ft
Chiller piping loop design pressure drop, 8 ft
With full system flow, Valve V5 is closed. Pressure drop
across V5 equals the pump head minus the friction drops to
V5. Pressure drop across Valve V5 is then 48 ft – 12 ft
(chiller drop) – 4 ft (supply drop) – 4 ft (return drop) or 28 ft.
With system flow at 90 percent, the pump head rises to
50 ft, while the friction drops fall to the lower values shown in
Figure 16. For additional information on chiller bypass opera-
tion see Chiller, Boiler, and Distribution System Applications
section. Pressure drop across V5 equals the pump head
minus the friction drops to V5. Pressure drop across Valve
V5 is then 50 ft – 9.6 ft (chiller drop) – 3.2 ft (supply drop) –
3.2 ft (return drop) or 34 ft. Converting ft to psi, 34 ft x 0.4335
psi/ft = 14.7 psi.
Substituting the flow of water, specific gravity of water, and
pressure drop in the C
v
formula shows that the Valve V5
should have a C
v
of 235.
C
v
QG
h
-------------=
C
v
14.6 1
6
------------------6==
C
v
900 1
14.7
---------------- 235==
Appendix A: Valve Selection and Sizing