57
configured threshold. The threshold is calculated from a com-
bination of GUIDE VANE POSITION and the difference be-
tween CONDENSER PRESSURE and EVAPORATOR
PRESSURE. The operator configured data points are the MIN-
IMUM and MAXIMUM SATURATED TEMPERATURE
DIFFERENCE (Surge/HGP Delta Tsmin and Surge/HGBP
DeltaTsmax), the MAXIMUM AND MINIMUM GUIDE
VANE POSITIONS (Surge/HGBP IGVmax and Surge/HGBP
IGVmin) printed on a label fixed to the bottom of interior face
of the control panel. A line is calculated between these points.
Whenever the ACTUAL GUIDE VANE POSITION is on the
left side of the plotted line, the algorithm will energize the hot
gas bypass valve to falsely load the compressor. If the ACTU-
AL GUIDE VANE POSITION falls to the right side of the plot-
ted line by more than the SURGE/HGBP DEADBAND, the hot
gas bypass valve is deenergized.
If the chiller is used for other than its original design condi-
tions, instructions to adjust the Surge / Hot Gas Bypass config-
urations are found on page 79.
HGBP OPTION = 1 (VFD OPTION ENABLED) — The
HGBP is only energized if a SURGE PROTECTION COUNT
is registered, the ACTIVE DELTA T falls below the SURGE/
HGBP DELTA T or the VFD LOAD FACTOR approaches 1.0
and it is not possible to increase VFD speed. The VFD speed
cannot increase when the VFD TARGET SPEED is equal to
VFD MAXIMUM SPEED or if the VFD TARGET SPEED is
FORCED to a fixed value.
HGBP OPTION = 2 — This option energizes the HOT GAS
BYPASS RELAY solely based on the ACTIVE DELTA T (actu-
al temperature difference between the LEAVING CHILLED
WATER and ENTERING CHILLED WATER). Evaluation of
the ACTIVE DELTA T begins at the completion of ramp load-
ing. The hot gas bypass valve is energized if the ACTIVE DEL-
TA T is less than the HGBP ON DELTA T. The hot gas bypass
relay will be turned off when the ACTIVE DELTA T is greater
than or equal to the sum of HGBP ON DELTA T plus HGBP
OFF DELTA T See Fig. 27.
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.
If a surge occurs, the guide vane position will be reduced by
10%. The guide vanes will be prevented from increasing posi-
tion until either the Surge Time Period expires causing the
SURGE PROTECTION COUNT to return to zero or the En-
tering Condenser Water Temperature decreases by 1 degree or
the Leaving Chilled 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.
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
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. 27 — 19XR Hot Gas Bypass/Surge
Prevention (Option 2)
ECW — Entering Chilled Water
HGBP — Hot Gas Bypass
LCW — Leaving Chilled Water
a19-1864
To Next Page
Loading...
