IM 830-6
34 www.DaikinApplied.com
Steam Modulating Valve Selection
The steam modulating control valve is expected to vary
the quantity of steam through the coil. Any movement
of the valve stem should produce some change in the
steam flow rate. To select a modulating steam valve:
1.
Obtain the supply steam inlet pressure.
2.
Determine the actual heat requirement of the space
to be heated.
Table 29: Modulating 2-Way, Normally Open, Steam Valves – Pressure Drop
Pressure Drop Across the Valve
2-Way CCV Part
No.
Cv
Maximum
Rating
Connection
Size
2 PSI
3 PSI
4 PSI
5 PSI
10 PSI
15 PSI
B215HT073
0.73
1/2"
10.99
13.71
16.11
18.33
28.03
36.74
B215HT186
1.86
22.34
34.93
41.06
46.70
71.42
93.60
B215HT455
4.55
54.65
85.44
100.43
114.24
174.72
228.97
B220HT731
7.31
3/4 inch
110.02
137.27
161.36
183.54
280.70
367.86
2-Way and 3-Way Hot Water and Chilled
Water Modulating Valve Selection
The unit ventilator control valve is expected to be able to vary
the quantity of water that flows through the coil in a modulating
fashion. Any movement of the valve stem should produce
some change in the amount of water that flows through the coil.
Oversized control valves cannot do this. For example, assume
that when the control valve is fully open, the pressure drop
through the coil is twice as great as the drop through the valve.
In this case, the control valve must travel to approximately 50%
closed before it can begin to have any influence on the water flow
through the coil. The control system, no matter how sophisticated,
cannot overcome this. Oversized control valves can also result in
“hunting” which will shorten the life of the valve and actuator and
possibly damage the coil.
To correctly select the proper Chilled Water Modulating Valve:
1.
Determine the flow of water and the corresponding
pressure drop through the coil.
2.
Obtain the pressure difference between the supply
and return mains.
3.
,
on the basis of taking 50%
of the available pressure difference (at design flow)
between the supply and return mains at the valve
location. The valve should have a pressure drop
greater than that of the coil.
Figure 58: Formula Equation to Calculate Cv
Q = Capacity in gallons per minute
Cv = Valve sizing coefficient determied experimentally for
each syle and size of valve, using water at standard
conditions as the test fluid
∆P = Pressure differential in psi
G = Specific gravity of fluid (water at 60°F = 1.0000)
Cv = Q
�
G
∆P
CAUTION
Care must be taken with modulating valves to provide proper
water flow. In freezing conditions, water flow must be maintained
through the heating coil or a suitable freeze-prevention solution
employed to prevent freeze-up. Similarly, the cooling coil must be
drained or a suitable freeze-prevention solution employed.
Figure 59: Actuator Wiring
WHT
(Input 2 to 10 VDC)
RED
(24VAC Supply)
ORG
(Output 2 to 10 VDC)
BLK
(Common)
BLK
WHT
ORG
RED
Locating Rib
Note:
The actuator spring returns the valve to the open position when
the actuator is de-energized (off).
