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APPENDIX
Fig. 16. Chiller Bypass Application.
EXAMPLE 3:
Sizing water valves for heating coils is especially critical. In
Figure 17, a valve with a C
v
of 12 will have 30 percent of the
available pressure drop when full open, while a valve with a
C
v
of 5 will have 70 percent of the available pressure drop.
As shown in Figure 18, the valve with 70 percent of the avail-
able pressure drop essentially provides the equal percent-
age water flow control, resulting in linear coil heat transfer
and stable temperature control. The valve with only 30 per-
cent of the available pressure drop has a more linear flow
control which results in nonlinear coil heat transfer. See
EQUAL PERCENTAGE VALVE section for further informa-
tion.
Fig. 17. Equal Percentage Valve Hot Water Application.
Fig 18. Effect of Pressure Drop in Hot Water Valve Sizing.
EXAMPLE 4:
A three-way mixing valve is needed for a heat exchanger
application with a bypass line. Water flow is specified at the
rate of 70 gpm. Manufacturer data for the exchanger indi-
cates a pressure drop of 1.41 ft of water through the
exchanger coils.
RETURN
SYSTEM STRAINER
HEAT/
COOL
COIL
1
V1
B1
HEAT/
COOL
COIL
2
V2
B2
HEAT/
COOL
COIL
3
V3
B3
HEAT/
COOL
COIL
4
V4
B4
V5
PUMP
SUPPLY
DP SETPOINT = 34'
180 GPM
PER
AHU COIL
DP
M10605
CHILLER
12
9.6
36
40.4
48
50
PD
4
3.2
4
3.2
PD
4
3.2
PD
ZERO REFERENCE
0
32
37.2
NUMBERS IN CIRCLES = GAGE PRESSURES
PUMP INLET = ZERO FOR SIMPLICITY
TOP NUMBERS = FULL FLOW
BOTTOM NUMBER = 90% FLOW
PD = PRESSURE DROP
VALVE VI
C2339A
180F
HOT WATER
SUPPLY
40 PSI
HOT WATER
RETURN
HEATING
COIL
LOCAL
PIPING
2.2 PSI
DROP
20 GPM AT DESIGN,
4.3 PSI DROP
30% PRESSURE DROP, Cv = 12
70% PRESSURE DROP, Cv = 5
CASE B: 62 PSI
CASE A: 50 PSI
100%
0% 100%
STEM TRAVEL
FLOW AT CONSTANT
PRESSURE DROP
C2340
70%
PRESSURE
DROP
30%
PRESSURE
DROP
IDEAL EQUAL
PERCENTAGE
VALVE
CHARACTERISTIC
Appendix A: Valve Selection and Sizing