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SECTION 26 - VARIABLE GEOMETRY DIFFUSER
• The PROBE WAIT time period (0.5 to 15 minutes;
default 10; as programmed with the Probe Wait
Time setpoint) expires. When the timer expires,
the Probing state is entered. The time remaining in
the PROBE WAIT time period is displayed on the
VARIABLE GEOMETRY DIFFUSER Screen as
TIME REMAINING.
• A stall is detected (Stall Detector Board output
goes above the High Limit setpoint). Enters Stall
Reacting State.
• A surge is detected and enters Surge Reacting
State.
• The compressor Pre-rotation Vanes (PRV) posi-
tion increases more than the PRV Offset (1 to 5%;
default 3; as programmed with the PRV Offset
setpoint). Enters Probing state. The PRV position
is displayed on the VARIABLE GEOMETRY
DIFFUSER Screen as 0% (fully closed) to 100%
(fully open).
Setting the PRV Offset setpoint to 0%
disables this function. Probing will be ini-
tiated based on PROBE WAIT time only.
Probing
In this state, the VGD mechanism is opened in pulses
to open the diffuser gap. The pulses are initiated every
10 seconds. The duration of the pulses are defined by
the Open Pulse setpoint (1 to 9 seconds; default 2). This
probing continues until the Stall Detector Board output
exceeds the High Limit setpoint (0.5 to 1.2 VDC; default
0.8) to indicate stall is present or a surge is detected. Typi-
cally, at lower loads below 70%, stall will be sensed and
controlled before a surge occurs. At higher loads and very
high heads or lift, a surge can occur while probing which
may be momentary in nature and not evidenced as stall
noise.
The number of times the Stall Detector Board output goes
above the High Limit setpoint threshold is displayed on
the VGD SETPOINTS Screen as the VGD Count. This
count can be reset using an ADMIN access level. The ac-
cumulated time the Stall Detector Board output voltage is
greater than the HIGH LIMIT threshold is displayed on
the VGD Screen as VGD TIME.
224
Stall Reacting
If a stall is detected from the Probing, Stall Waiting or
Surge Waiting (software version C.OPT.01.23.307 and
later) states, the state is changed to Stall Reacting.
In this state, the diffuser gap is closed until the Stall De-
tector Board output drops below the Low Limit setpoint
(0.4 to 0.8 VDC; default 0.6). After Stall Reacting, the
state returns to Stall Waiting, or Surge Waiting (soft-
ware version C.OPT.01.23.307 and later) depending on
the state it was previously in. If it entered Stall React-
ing from Stall Waiting, it returns to Stall Waiting. If
equipped with software version C.OPT.01.23.307 (and
later), it could have entered from Surge Waiting. If this
is the case, it will return to Surge Waiting only if there
is still time remaining on the Surge Waiting Timer. If
no time remaining, it transitions out of Stall Reacting
as it normally does.
If equipped with the optional Hot gas bypass feature
and the Hot gas valve position is greater than 0% when
the stall is detected, the VGD is held in its last position.
It is not driven closed.
Surge Reacting
The VGD is closed for a specific time period, defined
by the Surge React Time setpoint (1 to 30 seconds; de-
fault 5). A surge is detected as detailed in SECTION 23
- HOT GAS BYPASS and SECTION 25 - SURGE PRO-
TECTION of this manual. The Surge Detected LED on
the VARIABLE GEOMETRY DIFFUSER Screen il-
luminates for 5 seconds each time a surge is detected.
The program looks at a change in the condenser minus
evaporator pressures over a short time period to detect a
back flow or surge condition of the compressor. A surge
is generally of more concern and potentially damaging
to the compressor than stall. So, if a surge is detected,
this overrides any of the Stall action states. Opening the
diffuser gap may lower the lift capability of the machine.
Therefore, there are some conditions where the Probing
can actually cause a surge. In this case it is likely that
simply closing the diffuser gap some amount will stabi-
lize the compressor. Therefore, for units with VGD and
Hot gas options, the Hot gas bypass Valve will not be
opened on the first surge or during the time the VGD is
closing.
JCI COMPANY CONFIDENTIAL
FORM 160.54-M1
ISSUE DATE: 10/25/2019
JOHNSON CONTROLS
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