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SECTION 26 - VARIABLE GEOMETRY DIFFUSER
FORM 160.54-M1
ISSUE DATE: 10/25/2019
JOHNSON CONTROLS
JCI COMPANY CONFIDENTIAL
26
Surge Waiting
The VGD is held in last position for a wait time, de-
fined by the Probe Wait Time setpoint (0.5 to 15 min-
utes; default 10). During this period, the controls will
look for a second surge. If a second surge occurs, nor-
mal Hot gas bypass Control takes over and Hot gas
bypass Override state is entered. If there are no other
surges detected during this period, the VGD returns to
Probing.
The time remaining in the PROBE WAIT time period
is displayed on the VARIABLE GEOMETRY DIF-
FUSER Screen as TIME REMAINING.
With software version C.OPT.01.22.307 (or earlier),
any stall conditions are ignored in this state. With soft-
ware version C.OPT.01.23.307 (and later), if a stall is
detected while in Surge Waiting, the control transitions
to Stall Reacting, where the VGD is driven closed until
stall is no longer detected. Then if the Surge Waiting
Timer is still active, it transitions back to Surge Wait-
ing, otherwise it transitions out of Stall Reacting as it
normally does.
Hot gas bypass Override
Applies for units with optional Hot gas bypass. When-
ever the Hot gas bypass Valve position is greater than
0%, the VGD mechanism will be kept at its last position.
This keeps the VGD at a position of similar surge stabil-
ity when the system head is later lowered and the hot gas
valve closes. This state remains in effect until the Hot gas
valve position returns to 0% (or if equipped with software
version C.OPT.01.21.307 and later, a stall is detected)
whereupon it returns to the Stall Reacting State.
Hot gas bypass operates as described in
of this manual. Except that on surge
response, the Hot gas valve will not be opened until a
second surge is detected. The Hot gas Minimum Load
Override function is not affected by VGD operation.
PRV-VGD Inhibit
If the Pre-rotation Vanes position exceeds the PRV-
VGD Inhibit setpoint (40% to 100%), the VGD is
pulsed open per the Open Pulse setpoint. While this
is in effect, Control Status displays PRV POSITION
OVERRIDE.
STALL SENSOR VALIDATION
(Software version C.MLM.01.14.xxx (and later) or
C.OPT.01.14.306 (and later)).
This feature verifies the operation of the Stall Pressure
Transducer by comparing its voltage output to the volt-
age output of the Condenser Pressure Transducer. Since
both transducers are measuring essentially the same
pressure, both outputs should be within an acceptable
range of each other. If the Stall Transducer is not reading
accurately, a warning message is displayed and the Vari-
able Geometry Diffuser operation is disabled.
While the chiller is running, the outputs of the Stall
Transducer and Condenser Transducer are compared. If
the difference between them exceeds 0.28 VDC for 3
continuous minutes, the VGD is driven to the full open
position and WARNING – CONDENSER OR VGD
SENSOR FAILURE is displayed. The VGD is held in
the open position until the warning is manually cleared.
It can be cleared when the voltages are within the accept-
able range of each other and the WARNING RESET key
is pressed in SERVICE access level. When the warning
is cleared, the VGD returns to normal operation.
The Stall Pressure Transducer unprocessed DC voltage
output passes through the Stall Detector Board J2-2 and
is connected to the microboard at J8-1. This value is
then used for comparison to the condenser transducer.
EXTREME STALL MONITOR
(Software version C.MLM.01.14.xxx (and later) or
C.OPT.01.14.306 (and later))
The Variable Geometry Diffuser ring can be damaged
by operating in extreme stall for long periods of time.
To prevent damage, the VGD is disabled during ex-
treme stall conditions and a warning message is dis-
played.
While the chiller is running, if the Stall Detector Voltage
(output of the Stall Detector Board) exceeds twice the
High Limit setpoint, for the duration programmed as the
Extreme Stall Duration setpoint (10 to 20 minutes; de-
fault 10), the VGD is driven to the full open position and
WARNING – CONDITIONS OVERRIDE VGD is dis-
played. The VGD is held in the open position until the
warning is manually cleared. It can be cleared after the
Stall Detector Voltage returns to less than two times the