Specifications

50
quire a measurement of COOLER DELTA T and is thus unaf-
fected by changes in flow. With Variable Primary Flow Surge
Prevention there is no difference in field configuration between
constant and variable flow water systems.
The controls calculate the conditions at which the compres-
sor will surge based on operating conditions and configured
values entered into the OPTIONS screen.
The configurations as used by the controls would plot out
on a graph as a curved line. If the present operating conditions
plot at or above this line, surge prevention is turned on.
The way in which surge prevention functions will differ
with the presence or absence of hot gas bypass.
HGBP OPTION = 1 — 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 in-
crease 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. 23. The HGBP ON DELTA T must be
set to a value larger than the minimum delta T to which the
chiller can unload.
Surge Prevention Algorithm — This is an operator
configurable feature that can determine if lift conditions 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 (if the SURGE LIMIT/HGBP OPTION is configured
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 Protection — The PIC III monitors surge, which
results in a fluctuation on the compressor motor amperage.
Each time the fluctuation in amperage exceeds an operator-
specified limit (SURGE DELTA % AMPS) plus a load correc-
tion factor, both SURGE COUNTS are incremented by one
and the VFD will increase in speed provided that it is not al-
ready operating at VFD MAXIMUM SPEED or that the VFD
TARGET SPEED is forced. If the VFD cannot increase in
speed because the VFD is already at maximum speed of the
target speed is forced then the SURGE PROTECTION
COUNTS are also incremented by one. If more than 4 SURGE
PROTECTION COUNTS occur within an operator-specified
time (SURGE TIME PERIOD) and the ACTUAL VFD
SPEED is greater than 90% then the PIC III declares an Exces-
sive Compressor Surge Alarm (238) and the chiller is shut
down. If more than four SURGE PROTECTION COUNTS
occur within the SURGE TIME PERIOD and the ACTUAL
VFD SPEED is less than 90% then the chiller is shut down on a
Excessive Compressor Surge / Low Speed Alarm (236). Both
SURGE COUNTS and SURGE PROTECTION COUNTS are
decreased by one if no surges occur within the SURGE TIME
PERIOD.
On chillers with VFDs, if a SURGE COUNT is registered
and the ACTUAL VFD SPEED is less than the VFD MAXI-
MUM SPEED, then TARGET VFD SPEED will be increased
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. 23 — Hot Gas Bypass/Surge
Prevention (Option 2)
ECW — Entering Chilled Water
HGBP — Hot Gas Bypass
LCW — Leaving Chilled Water
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