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40
Capacity Overrides. Both of these corrective actions try to
reduce the lift experienced by the compressor and help prevent
a surge condition.
Surge Prevention Algorithm with VFD —
This is
an operator configurable feature that can determine if lift con-
ditions are too high for the compressor and then take corrective
action. Lift is defined as the difference between the pressure at
the impeller eye and at the impeller discharge. The maximum
lift a particular impeller wheel can perform varies with the gas
flow through the impeller and the diameter of the impeller.
A surge condition occurs when the lift becomes so high the
gas flow across the impeller reverses. This condition can even-
tually cause chiller damage. When enabled, the Surge Preven-
tion Algorithm will adjust either the inlet guide vane (IGV)
position or compressor speed to maintain the compressor at a
safe distance from surge while maintaining machine efficiency.
If the surge condition degrades then the algorithm will move
aggressively away from surge. This condition can be identified
when the SURGE/HGBP ACTIVE? on the HEAT_EX display
screen displays a YES.
The surge prevention algorithm first determines if correc-
tive action is necessary. The algorithm checks two sets of
operator-configured data points, the lower surge point (MIN.
LOAD POINT [T1,P1]) and the upper surge point (FULL
LOAD POINT [T2,P2]). The surge characteristics vary be-
tween different chiller configurations and operating conditions.
The surge characteristics are factory set based on the original
selection with the values displayed inside the control panel of
the chiller. Since operating conditions may affect the surge pre-
vention algorithm, some field adjustments may be necessary.
The surge prevention algorithm function and settings are
graphically displayed on Fig. 21 and 22. The two sets of load
points on the graph (default settings are shown) describe a line
the algorithm uses to determine the maximum lift of the com-
pressor for the particular maximum operating speed. When the
actual differential pressure between the cooler and condenser
and the temperature difference between the entering and leav-
ing chilled water are above the line on the graph (as defined by
the minimum and full load points), the algorithm operates in
Surge Prevention mode. This is determined when the ACTIVE
DELTA T is less than SURGE/HGBP DELTA T minus the
deadband.
When in Surge Prevention mode, with a command to in-
crease capacity, the VFD speed will increase until maximum
VFD speed is reached. At VFD MAXIMUM SPEED, when Ca-
pacity still needs to increase, the IGV’s open. When in Surge
Prevention mode, with a command to decrease capacity only
the IGVs will close.
Surge Protection VFD Units —
The PIC II monitors
surge, which is detected as a fluctuation in compressor motor
amperage. Each time the fluctuation exceeds an operator-
specified limit (SURGE DELTA % AMPS), the PIC II registers
a surge protection count. If more than 5 surges occur within an
operator-specified time (SURGE TIME PERIOD), the PIC II
initiates a surge protection shutdown of the chiller.
On VFD units, if a surge count is registered and if ACTUAL
VFD SPEED is less than VFD MAXIMUM SPEED then motor
speed will be increased by the configured VFD increase step.
While the SURGE PROTECTION COUNTS are >0, a speed
decrease will not be honored.
The surge limit can be adjusted from the OPTIONS screen
(see Table 2). Scroll down to the SURGE DELTA % AMPS
parameter, and use the or softkey
to adjust the percent of surge. The default setting is 10% amps.
The surge time period can also be adjusted from the
OPTIONS screen. Scroll to the SURGE TIME PERIOD
parameter, and use the or softkey
to adjust the amount of time. The default setting is 8 minutes.
Access the display screen (COMPRESS) to monitor the
surge count (SURGE PROTECTION COUNTS).
Surge Protection (Fixed Speed Chiller) —
The
PIC II monitors surge, which is a fluctuation in compressor
motor amperage. Each time the fluctuation exceeds an
operator-specified limit (SURGE DELTA % AMPS), the PIC II
counts the surge. If more than 5 surges occur within an
operator-specified time (SURGE TIME PERIOD), the PIC II
initiates a surge protection shutdown of the chiller.
The surge limit can be adjusted from the OPTIONS screen.
Scroll down to the SURGE DELTA % AMPS parameter, and
use the or softkey to adjust the
percent of surge. The default setting is 10% amps.
The surge time period can also be adjusted from the
OPTIONS screen. Scroll to the SURGE TIME PERIOD
parameter, and use the or softkey
to adjust the amount of time. The default setting is 8 minutes.
Access the display screen (COMPRESS) to monitor the
surge count (SURGE PROTECTION COUNTS).
HEAD PRESSURE REFERENCE OUTPUT (See
Fig. 23) — The PIC II control outputs a 4 to 20 mA signal for
the configurable Delta P (condenser pressure minus evaporator
pressure) reference curve shown in Fig. 23. An output is avail-
able on the ISM module [Terminal J8 (+), J8 (–) labeled spare].
For chillers with Benshaw Inc. solid-state starters terminal strip
labeled J8 (+), J8 (–) located next to the RediStart MICRO™
input/output card is provided. The Delta P at 100% (chiller at
maximum load condition default at 35 psi), DELTA P AT 0%
(chiller at minimum load condition default at 25 psi) and MIN-
IMUM OUTPUT points are configurable in the EQUIPMENT
SERVICE-OPTIONS table. When configuring this output en-
sure that minimum requirements for oil pressure and proper
condenser FLASC orifice performance are maintained. The 4
to 20 mA output can be used as a reference to control a
tower bypass valve, tower speed control, or condenser pump
speed control.
Lead/Lag Control —
The lead/lag control system auto-
matically starts and stops a lag or second chiller in a 2-chiller
water system. A third chiller can be added to the lead/lag sys-
tem as a standby chiller to start up in case the lead or lag chiller
in the system has shut down during an alarm condition and ad-
ditional cooling is required. Refer to Fig. 17 and 18 for menu,
table, and screen selection information.
INCREASE
DECREASE
INCREASE DECREASE
INCREASE DECREASE
INCREASE DECREASE
MINIMUM
REFERENCE
OUTPUT
DELTA P
AT 100%
DELTA P
AT 0%
DELTA P
0 mA 2 mA
4 mA
(0%)
20 mA
(100%)
4 T0 20 mA OUTPUT
Fig. 23 — Head Pressure Reference Output