Honeywell AUTOMATIC CONTROL Engineering Manual page 366

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
C
C VARIABLE
H
H SPEED
I
I DRIVE
L
L
L
L
E
E
R
R
1 2
AHU 1 requires a 28 foot differential pressure (8 feet for the
coil, 8 feet for the valve, and 12 feet for the balancing valve),
for full flow. If 28 feet is available at AHU 1, all other AHUs
will have at least the required design differential pressure.
Controlling the pump with a sensor positioned as shown for
DP-2 set for 28 feet is acceptable.
Locating the sensor (DP-3) at AHU 6 and set for the 16 feet
required by AHU 6 will not work when only AHU 1 is operating.
With these conditions DP-3 maintains a maximum drop of only
16 feet across AHU 1 which needs a 28 ft differential pressure
because of the balancing valve. If the sensor is positioned at
AHU 6, the setpoint must still be 28 feet if the system is to
operate satisfactorily with non-symmetrical loading.
DP-1 located at the variable speed drive (VSD) is the most
convenient place. It requires the DP-2 setpoint plus the friction
losses between the pump and AHU 1.
Figure 80 shows the operating curve of the system with the
differential pressure sensor located in the DP-2 position and
set for 28 feet. With each AHU at one-third flow, the speed is
1412 RPM, which produces a 28 foot differential pressure at
AHU 1 with 400 gpm system flow. When the coils are equally
loaded at one-third flow, each control valve takes a 26 foot
drop. In this configuration the pump never operates much below
the 1400 rpm speed because of the 28 ft head setpoint.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
HEAT/ HEAT/
COOL COOL
COIL COIL
1 2
DP
DP
2
1
V1 V2
B1 B2
Fig. 79. Variable Speed Pump Control
356
HEAT/
HEAT/
COOL
COOL
COIL
COIL
3
4
V3
B3
B4
OPERATING POINT @
400 GPM & 1750 RPM
48
42
OPERATING POINTS LINE AS FLOW
VARIES FROM 1200 GPM TO ZERO
GPM WITH 28 FT HEAD MAINTAINED
36 ACROSS AHU 1
PUMP CURVE
@ 1412 RPM
30
OPERATING POINT
EACH CONTROL
24
WITH EACH
VALVE @ ONE
CONTROL VALVE
THIRD FLOW
AT ONE THIRD
(PRESSURE
FLOW AND PUMP
DROP ACROSS
AT 1412 RPM
18
VALVES 26 FT)
12
6
0
0 200 400
FLOW IN GPM
Fig. 80. Fixed Setpoint with PI,
Variable Speed Pumping Control
HEAT/
HEAT/
COOL COOL
COIL
COIL
6
5
DP
3
V6
V4 V5
B6
B5
M15066
FULL LOAD
OPERATING POINT
PUMP CURVE
@ 1750 RPM
EACH
CONTROL
VALVE FULL
OPEN AT
200 GPM &
8FT DROP
PUMP REQUIRED:
1200 GPM @ 36' HEAD
SYSTEM CURVE FOR STATIC
ELEMENTS OF SYSTEM (CHILLER,
PIPING, FITTINGS, BALANCING
COCKS, COILS, STRAINERS, ETC.)
600 800 1000 1200
M15067