Honeywell AUTOMATIC CONTROL Engineering Manual page 454

VALVE SELECTION AND SIZING
36
40.4
CHILLER
12
PD
9.6
DP SETPOINT = 34'
PUMP
48
50
0
ZERO REFERENCE
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
operation 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 (see General
Engineering Data section).
ENGINEERING MANUAL OF AUTOMATIC CONTROL
4
PD
SUPPLY
3.2
DP
V5
4
PD
3.2 RETURN
SYSTEM STRAINER
Fig. 16. Chiller Bypass Application.
32
37.2
HEAT/ HEAT/ HEAT/
COOL
COOL COOL
COIL
COIL COIL
1 2 3
V1 V2
B1 B2 B3
4
NUMBERS IN CIRCLES = GAGE PRESSURES
3.2
PUMP INLET
TOP NUMBERS
BOTTOM NUMBER
PD
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.
900 1
C v =
14.7
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 available pressure drop essentially provides the equal
percentage water flow control, resulting in linear coil heat
transfer and stable temperature control. The valve with only
30 percent 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
information.
444
180 GPM
HEAT/
PER
COOL
AHU COIL
COIL
4
V3
V4
B4
= ZERO FOR SIMPLICITY
= FULL FLOW
= 90% FLOW
= PRESSURE DROP
M10605
= 235