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No pump pressure control example, so far, takes advantage
of both the variable speed pump and a digital control system.
The digital control system VSD control algorithm adjusts the
differential pressure setpoint based on the demands of all the
valves (Fig. 81) and all loads are satisfied with significant
savings over any of the three fixed setpoint options. Since, when
using valve position load reset there is no difference in
performance between the three locations, DP-1 is preferred
because of initial cost. Valve position load reset provides
adequate control performance whether the sensor is only
proportional or is only a static pressure sensor as compared to
a differential pressure sensor.
Specification:
Anytime any AHU chilled water valve is greater than 15%
open for greater than one minute, the secondary pump shall be
started under EPID control at 20% speed and with a ramp
duration of 120 seconds. The pump VSD shall be controlled by
a differential pressure sensor located between the supply line
leaving the plant room, as far from the pump as practical to
avoid hydronic noise that may be present at the immediate pump
discharge, and the system return line. At start-up the differential
pressure setpoint shall be 90 kPa (See Note 1). Anytime any
load control valve is greater than 95% open, the differential
pressure setpoint shall be incremented at the rate of 1.5 kPa
every minute up to a maximum value of 114 kPa. Anytime all
load control valves are less than 80% open, the differential
pressure setpoint shall be decremented at the same rate down
to a minimum of 21 kPa. After 12 minutes, the increment/
decrement rate shall be changed from one minute to three
minutes (See Note 2). All values shall be user adjustable.
NOTES:
1. From Figure 81, pump pressure is 108 kPa if all AHUs
require full flow, therefore, the 90 kPa value is an arbitrary
compromise.
2. This relaxes the control demands for smooth stability after
the response to the initial load.
Figure 81 shows the ideal performance of the load reset
setpoint control with each AHU demanding one-third flow. All
control valves are full open and the differential pressure adjusted
to produce a speed of 525 RPM. If the coil loading is non-
symmetrical to the point that AHUs 1 and 2 are fully loaded
while the others are off, the operating point for 1/3 system flow
is the same as shown in Figure 80 for 1/3 system flow, since
AHU 1 requires a differential pressure of 84 kPa for full flow.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
355
144
126
108
90
OPERATING POINT LINE AS FLOW
PER AHU VARIES FROM 12.5 L/s TO
ZERO L/s AND THE DIFFERENTIAL
PRESSURE IS RESET DOWN AS
72
REQUIRED TO KEEP AHU VALVE
WITH THE GREATEST DEMAND FULL
OPEN
54
PUMP CURVE
@ 525 RPM
VALVE POSITION LOAD
RESET AND
36
PROPORTIONAL PLUS
INTEGRAL CONTROL
18
0'
0
12.5
25
Fig. 81. Variable Setpoint, Variable Speed
Pumping Control (Ideal Curve).
Pump Speed Valve Position Load Reset
A pump speed valve position load reset program with over 20
valves can become cumbersome. Also, if any one valve, for
whatever reason, stays open most of the time, then the load reset
program becomes ineffective. Figure 82 shows an example of the
valve position load reset program concept applied to a
multibuilding facility with varying differential pressures entering
each building, due to varying distances from the pumping plant.
The example address two issues, differential pressure control
within each building to relieve control valves from extremely
high differential pressures and pump speed load reset.
FULL LOAD
OPERATING
POINT
PUMP CURVE
@ 1750 RPM
EACH CONTROL VALVE
FULL OPEN AT 12.5 L/s &
24 kPa DROP
PUMP REQUIRED:
75 L/s @ 108 kPa
SYSTEM CURVE FOR STATIC
ELEMENTS OF SYSTEM (CHILLER,
PIPING, FITTINGS, BALANCING
COCKS, COILS, STRAINERS, ETC.)
37.5
50
62.5
75
FLOW IN L/s
M15291
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