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Steam Modulating Valve Selection (MicroTech II™)
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.
3.
Select a valve (Cv) from Table 18, which gives the capacity range
Table 18. Modulating 2-Way, Normally Open, Steam Valve – Pressure Drop
Cv
Connection
0.73
1/2" (13mm) FNPT
1.8
1/2" (13mm) FNPT
4.6
1/2" (13mm) FNPT
7.3
3/4" (19mm) FNPT
11
1" (25mm) FNPT
18.5
1 1/4" (32mm) FNPT
Hot Water and Chilled Water Modulating Valve
Selection (MicroTech II)
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 Hot Water or 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. Select a valve size (Cv) from Table 19 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.
4. Select a normally open valve for hot water, or 2-pipe CW/HW
coils. For chilled water coils select a normally closed valve.
Table 19. 2-Way and 3-Way Modulating Valve Pressure Drop (Hot Water and Chilled Water)
Recommended
C
Connection
v
Flow Rates
2 GPM to 3 GPM
0.73
1/2" (13mm)
(.13 L/s) to (.19 L/s)
2 GPM to 7 GPM
1.8
1/2" (13mm)
(.13 L/s) to (.44 L/s)
5 GPM to 16 GPM
4.6
1/2" (13mm)
(.32 L/s) to (1.0 L/s)
9 GPM to 20 GPM
7.3
3/4" (19mm)
(.57 L/s) to (1.3 L/s)
IM 817-4
2 psig
5 psig
Capacity Range (MBh)
11
14
18
27
34
44
68
86
112
109
137
178
164
206
269
275
346
452
2
3
4
Valve
(.13)
(.19)
(.25)
Pressure Drop
O
17.3
38.8
–
WPD Ft of H
2
(kPa)
(51.6) (116)
WPD Ft of H
O
2.8
6.4
11.4
2
(kPa)
(8.5) (19.1) (34.0) (53.1) (76.4) (104)
WPD Ft of H
O
–
–
–
2
(kPa)
O
–
–
–
WPD Ft of H
2
(kPa)
based on a 60% pressure drop at the low end of the range and
100% pressure drop at the high end of the range. For example:
With 2 psig (13.8 kPa) inlet steam pressure, the valve with a Cv
of 4.6, in the full open position, would have a 1.2 psig (8.3 kPa)
pressure drop at 68 MBh (20.1 kW) and a 2psig pressure drop at
86 MBh (25.2 kW). The valve should have a capacity less than
or equal to the space to be heated.
Valve Inlet Pressure
13.8 kPa
Capacity Range (kW)
22
3.2
4.0
53
7.8
9.9
136
20.1
25.2
216
31.8
40.0
325
48.0
60.3
547
80.7
101.4
132.4
5. Select either a 2-way or 3-way modulating valve. The 3-way
valve is generally selected for diverting water back to the return
main where a constant pump head pressure is required.
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 65 - Actuator Wiring
Normally Open (Stem Up) – Push Stem Down to Close
Normally Closed (Stem Up) – Push Stem Down to Open
Note: The actuator spring returns the valve to the stem up posi-
tion when the actuator is de-energized (off)
Water Flow Rates GPM (L/s)
5
6
7
8
9
10
(.32)
(.38)
(.44)
(.51) (.57) (.63)
–
–
–
–
–
–
17.7
25.6
34.8
–
–
–
2.7
3.9
5.3
7.0
8.8
10.9
(8.1)
(11.7) (15.9) (20.8) (26.3) (32.5) (39.3) (46.8) (54.9) (63.7) (73.1) (83.2)
–
–
–
–
3.5
4.3
(10.5) (12.9) (15.6) (18.6) (21.8) (25.3) (29.0) (33.0) (37.3) (41.8) (46.6) (51.6)
34.5 kPa
5.2
6.3
Note: The actuator plug-in wiring
12.9
15.6
for the Steam Valve is the
32.9
39.9
same as the Hot Water and
52.2
63.3
Chilled Water Modulating
78.7
95.4
Valve. (see figure 65 )
160.4
CAUTION
White/Brown (Stem Up)
Yellow (24 VAC Supply)
Brown (Stem Down)
White (Common)
Locating Rib
11
12
13
14
15
(.64)
(.76) (.82) (.88) (.95) (1.01) (1.07) (1.13) (1.20) (1.26)
–
–
–
–
–
–
–
–
–
–
13.2
15.7
18.4
21.3
24.5
27.8
5.2
6.2
7.3
8.5
9.7
11.0
16
17
18
19
20
–
–
–
–
–
–
–
–
–
–
–
–
–
–
12.5
14.0
15.6
17.3
Page 23 of 60
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