System information

24
SURGE CORRECTION CONTROL — There are two stag-
es for surge correction: envelope control (surge prevention)
and surge protection.
Envelope Control
— A surge condition occurs when the lift
becomes so high that the gas flow across the impeller reverses.
This condition can eventually cause compressor damage. The
surge prevention algorithm notifies the operator that chiller op-
erating conditions are marginal and to take action, such as low-
ering entering condenser water temperature, to help prevent
compressor damage.
If a high sound condition occurs at low guide vane position,
the HGBP valve is used to decrease the sound level. The enve-
lope control algorithm is an operator-configurable feature that
can determine if lift conditions are too high for the compressor
and then take corrective action. High efficiency mode or low
noise mode can be selected. Lift is defined as the difference be-
tween 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 across the impeller
and the size of the wheel.
If Actual Lift is higher than reference lift, a capacity inhibit
signal will be sent. If Actual Lift is higher than reference lift
plus Surge Line High Offset, a capacity decrease signal will be
sent. If Actual Lift is lower than reference lift minus Surge
Deadband, these 2 signals will be canceled. Capacity Control
will respond to these 2 signals and make correction on IGV1
TARGET POSITION, VFD TARGET SPEED, and HGBP ac-
tuator.
Surge Protection
— The Surge Protection algorithm will run
after SURGE DELAY TIME has elapsed when compressor
has been commanded to turn on. It compares the present PER-
CENT LINE CURRENT value with the previous value once
every second. If the difference exceeds the maximum AMPS
change value (SURGE DELTA % AMPS + [PERCENT LINE
CURRENT / 10]), an incidence of surge has occurred, and the
surge protection signal will be sent.
When an incidence of surge determined in this manner has
occurred, the SURGE COUNTS will be incremented by one.
On receiving the surge protection signal, Capacity Control will
make corrections on IGV1 TARGET POSITION, VFD TAR-
GET SPEED, and HGBP actuator. When correction is in ef-
fect, Surge Protection Count will be added every 10 seconds. If
IGV, VFD and HGBP cannot be corrected, chiller will send a
shutdown alarm when the surge count is greater than 4.
HOT GAS BYPASS (HGBP) CONTROL — The hot gas
bypass function is used to artificially load the chiller and keep
it running under low load conditions or to prevent surge condi-
tions. Since this also reduces the performance of the machine,
HGBP Control is a user-selectable option.
Hot gas bypass operation has three different modes when
installed (hgbp_opt = 1) and enabled (hgbp_sel > 0):
• Hot gas bypass for surge correction — Each compressor
has unique lift characteristics that can be plotted to deter-
mine performance. The controller will determine operat-
ing conditions that could result in compressor surge and
activate the hot gas bypass valve to prevent surge until
the chiller operating parameters are in a safe area on the
curve where the valve may be closed again.
• Hot gas bypass for low load operation — In this condi-
tion, the hot gas bypass valve will be opened to prevent a
recycle shutdown from occurring. The hot gas bypass
valve will remain open until this minimal loading condi-
tion has passed and there is no surge condition present.
• Combination surge correction and low load operation —
When this option is selected, both HGBP for surge pre-
vention and HGBP for low load operation will be per-
formed. Surge prevention will take higher priority if both
conditions are satisfied.
GUIDE VANES AT MAX.
MODULATE VFD
GUIDE VANES
VFD AT START-UP SPEED,
MODULATE GUIDE VANES
VFD
STARTUP:
VFD TO START-UP SPEED,
GUIDE VANES CLOSED
MAX.
STA RT
MIN.
VFD % SPEED
0
TRAVEL LIMIT
100
GUIDE VANES % POSITION
Fig. 35 — Guide Vane Position and VFD Speed
a19-2129