Specifications

55
to the sum of HGBP ON DELTA T plus HGBP OFF DELTA T
See Fig. 26.
The HGBP ON DELTA T must be set to a value larger
than the minimum delta T to which the chiller can un-
load.
Surge Protection (Fixed Speed Chiller) —
The
PIC II monitors surge, which results in a fluctuation on the
compressor motor amperage. Each time the fluctuation in am-
perage exceeds an operator-specified limit (SURGE DELTA %
AMPS) plus a load correction factor, both SURGE COUNTS
and SURGE PROTECTION COUNTS are incremented by
one. If more than 4 SURGE PROTECTION COUNTS occur
within an operator-specified time (SURGE TIME PERIOD),
the PIC II declares an Excessive Compressor Surge Alarm
(238) and the chiller is shut down. Both SURGE COUNTS
and SURGE PROTECTION COUNTS are decreased by one if
no surges occur within the SURGE TIME PERIOD.
With version 10 and higher software — If a surge occurs,
the guide vane position will be reduced by 10%. The guide
vanes will be prevented from increasing position until either
the Surge Time Period expires causing the SURGE PROTEC-
TION COUNT to return to zero or the Entering Condenser
Water temperature decreases by 1 degree or the leaving con-
denser water temperature increases by 1 degree.
If the machine has a split ring diffuser a correction its posi-
tion will be made simulatneously.
The threshold at which a current fluctuation is interpreted as
a surge can be adjusted from the OPTIONS screen. Scroll to
the SURGE DELTA % AMPS parameter and use the IN-
CREASE or DECREASE softkey to adjust the surge threshold.
The default setting is 10 %. The SURGE TIME PERIOD can
be adjusted from the OPTIONS screen. Scroll to the SURGE
TIME PERIOD parameter and use the INCREASE or DE-
CREASE softkey to adjust the time duration. The default set-
ting is 8 minutes.
SURGE PROTECTION COUNTS are displayed in the
COMPRESS screen. Both SURGE PROTECTION COUNTS
and SURGE COUNTS are displayed in the SURGPREV
screen.
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 saturated
temperature 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. With a VFD the lift capability and the position
of the surge line also vary with ACTUAL VFD SPEED.
The surge line constructed from the SURGE/HGBP DELTA
T and SURGE/HGBP DELTA P points is based on full load
conditions and 100% compressor speed. As ACTUAL VFD
SPEED is reduced, the SURGE/HGBP DELTA P values are
automatically reduced so that the surge line duplicates the com-
pressor lift capability at the reduced speed. If the actual operat-
ing point (lift vs. load) goes above the surge prevention line
then the controls enter SURGE PREVENTION mode.
Changing the VFD SURGE LINE GAIN adjusts the rate at
which the surge line is adjusted in response to changes in AC-
TUAL VFD SPEED. Increasing VFD SURGE LINE GAIN re-
duces the size of the SURGE PREVENTION “on” area. This
area is illustrated in Fig. 25.
When the controls enter SURGE PREVENTION mode the
first response is to increase ACTUAL VFD SPEED and in-
crease the lift capability of the compressor while preventing the
guide vanes from opening further. Should the compressor reach
100% ACTUAL VFD SPEED and still be in the surge preven-
tion region, the controls will energize the HOT GAS BYPASS
RELAY and if the SURGE LIMIT/HGBP OPTION is config-
ured for Hot Gas Bypass).
If load decreases while the chiller is in SURGE PREVEN-
TION mode the ACTUAL GUIDE VANE POSITION will close
but the ACTUAL VFD SPEED will not decrease.
ACTIVE REGION as found on the SURGPREV screen is
based on how far into the surge prevention area that the load
point has moved. This is used to determine the size of the speed
boost to the VFD.
NOTE: If upon ramp-up, a chiller with VFD tends to go to full
speed before guide vanes open fully, it is an indication that the
lift at low load is excessive, and the operating point moved
directly into the surge prevention region. In this case, investi-
gate the ability of the condenser cooling means (e.g., cooling
tower) to provide cooling water in accordance with the design
load/entering condenser water temperature schedule.
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.
SURGE PREVENTION — VERSION 8 SOFTWARE
AND EARLIER — The surge prevention algorithm first de-
termines if corrective action is necessary. The algorithm checks
four sets of operator-configured data points, the lower surge
point (MIN. LOAD POINT [T1,P1]), the low surge point
(LOW LOAD POINT [T2,P2]), the middle surge point (MID
LOAD POINT [T3,P3]), and the upper surge point (MAX
LOAD POINT [T4,P4]). 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. 25. The two sets of load points on
HGBP
On
HGBP Off As ACTIVE
DELTA T Decreases
HGBP On As ACTIVE
DELTA T Increases
∆P
∆T
HGBP ON
DELTA T
HGBP OFF
DELTA T
HGBP
Off
LEGEND
P = (Condenser Pressure) – (Cooler Pressure)
T = (ECW) – (LCW)
Fig. 26 — 19XR Hot Gas Bypass/Surge
Prevention (Option 2)
ECW — Entering Chilled Water
HGBP — Hot Gas Bypass
LCW — Leaving Chilled Water
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