223
FORM 160.54-M1
ISSUE DATE: 10/25/2019
JOHNSON CONTROLS
JCI COMPANY CONFIDENTIAL
26
Certain YORK compressors are equipped with a Vari-
able Geometry Diffuser (VGD). It is used to reduce ro-
tating stall conditions and associated stall noise. Stall
may occur at low load conditions with high head. A
mechanical ring, located in the diffuser passage af-
ter the impeller discharge, is mechanically operated
through linkages via an electric actuator like that used
to operate the pre-rotation vanes. It is closed (extended)
to narrow the diffuser gap. It is opened (retracted) to
open the diffuser gap. An internal actuator end switch
prevents travel beyond the fully open or closed posi-
tions. The VGD limit switch indicates when the VGD
is fully closed. The switch closes when the VGD is in
the fully closed position. The switch status is displayed
on the VGD screen and VGD SETPOINTS Screen
(Software version C.MLM.01.14.xxx (and later) or
C.OPT.01.14.306 (and later)). It’s displayed as Closed
when the VGD is in the full closed position Otherwise,
it is displayed as Open. In response to a stall condition,
the ring is closed as much as necessary to eliminate the
stall. Since stall is caused by reduced gas flow through
the compressor, narrowing the diffuser gap reduces the
cross sectional area through which the gas flows, there-
by increasing the gas velocity through the compressor.
The diffuser ring is also closed in response to surge
conditions as described below.
Software version C.MLM.01.10.xxx (and later) or
C.OPT.01.10.302 (and later) is required for this feature.
The VARIABLE GEOMETRY DIFFUSER screen
displays all parameters relevant to this feature. All
setpoints are maintained on the VARIABLE GEOM-
ETRY DIFFUSER SETPOINTS screen.
STALL DETECTION
Stall noise in the compressor discharge is detected
as gas pressure pulsations. (See
A Stall Pressure Transducer (025-39464-000
or 025-40088-000), mounted in the discharge scroll
of the compressor, detects the pressure pulsations and
outputs DC voltage pulsations to the
Stall Detector
Board (031-02418-000 or 031-2418-001). This board
converts the voltage pulsations into an analog voltage
that represents the magnitude of the stall noise. This
analog voltage is displayed on the VARIABLE GE-
OMETRY DIFFUSER Screen as STALL DETECTOR
VOLTAGE and is input to the microboard where it is
compared to the Low Limit and High Limit setpoint
thresholds to determine if the stall noise is acceptable
or unacceptable. In a typical YK chiller, a signal below
0.5 VDC indicates little or no stall. A signal above 0.8
VDC indicates stall noise is present.
SURGE DETECTION
Surge events are sensed as described in
AUTOMATIC OPERATION
The VGD operation is illustrated in the State Diagram
illustrated in
shown in circles. They are Stall Waiting, Stall React-
ing, Probing, Surge Reacting, Surge Waiting and Hot
gas Override. Arrows on the diagram indicate actions
that move the control from one “State” to another. For
clarity, different arrow line styles are used for Stall
(__), Surge (__ . __) and Hot gas (_ _ _) response ac-
tions. During chiller operation, the current state is dis-
played as the CONTROL STATUS on the VARIABLE
GEOMETRY DIFFUSER Screen.
In general, the VGD is maintained in the most open po-
sition possible that does not allow stall. It is fully open
when the chiller is started. If stall occurs, it is driven to-
ward the close position until the stall is eliminated. After
a wait period, it is again pulsed open until either a stall
occurs or the fully open position is reached, whereupon
the cycle repeats. In response to a surge, it is driven to-
ward the close position for a selectable period of time.
After a wait period, it is again pulsed open unless anoth-
er surge occurs, whereupon Hot gas operation overrides
VGD operation and the VGD is held in place.
The Diffuser Gap Open and Diffuser Gap Close LED’s
on the VARIABLE GEOMETRY DIFFUSER Screen
illuminate when the program is initiating the respective
output. The following sections describes the operating
states:
Chiller Off, Pre-lube or Coastdown
The VGD diffuser gap is driven fully open.
Stall Waiting
This state is entered on startup, after a Stall Reaction,
or Hot gas valve closure. The VGD is held in last posi-
tion until one of the following occurs:
SECTION 26 - VARIABLE GEOMETRY DIFFUSER
