FORM 160.72-O1 (810)
JOHNSON CONTROLS
122
SECTION 4 - GENERAL OPERATING SEQUENCE
ANTI-RECYCLE OPERATION
All start counters and timers in the anti-recycle opera-
tion described below are adjustable. The default values
are used below as examples.
Cold Starts1.
If the motor has been shutdown for A. Cold
Start Downtime hours, the cool down (anti-
recycle) time after the first start will be Cold
Start Recycle Time (1st Start) minutes. If
the chiller fails to start on the second try, the
cool down time will be Cold Start Recycle
Time (2nd Start) minutes.
Any additional starts would be considered B.
hot starts.
Hot Starts2.
If the chiller has been running less than A. Cold
Start Uptime minutes, the cool down time
will be the difference between Cold Start
Uptime minutes and the Last Start Run-
time. For example:
Cold Start Uptime = 30 minutes
Last Start Runtime =10 minutes
Cool Down Time - 20 minutes
If the chiller has been running for B. Cold Start
Uptime minutes or longer, the cool down
time will be the Hot Start Maximum Re-
cycle Time.
Excessive Starts3.
When the motor is started, a 24-hour counter is
enabled. During the next 24-hour period, the
maximum number of starts allowed will be the
Allowed Starts Per Day. Additional starts at-
tempted in any 24-hour period will be locked out
for Excessive Start Lockout Time hours.
CAPACITY CONTROL OPERATION
Refer to CAPACITY CONTROL DIAGRAM and the
description for the applicable screens GRAPHIC DIS-
PLAY DESCRIPTION, along with the following de-
scriptions to best understand the operation.
Major capacity control devices
Compressor Pre-rotation Vanes1.
The compressor pre-rotation vanes (PRVs) are in-
ternal guide vanes in the suction flow path to the
first stage impeller wheel. The PRVs are used to
throttle the refrigerant flow through the system as
a means of controlling capacity in response to the
leaving chilled water temperature. If the leaving
chilled water temperature falls below the setpoint,
the PRVs are partially closed until the net cooling
is reduced and the leaving chilled water returns to
setpoint.
In the event of high motor power, the capacity
control signal is over-ridden and the compressor
PRVs are closed to keep the motor power down.
On start-up, the PRVs are closed to reduce the
starting load torque of the compressor.
The compressor pre-rotation vanes are closed on
shutdown to reduce backflow of high pressure gas
from the condenser, which might otherwise cause
the compressor to spin backwards at a high rate
of speed.
The mechanical pre-rotation vane linkage at the
compressor is operated by a Conoflow piston op-
erator, which has a pneumatic 3-15 psig / 0.2-1.0
bar direct acting positioner (increasing air signal
opens the PRVs). The capacity control signal from
the control panel is converted from 4-20 mA DC
to a 3-15 psig / 0.2-1.0 bar pneumatic signal by an
I/P transducer. The vanes will be fully closed at 4
mA DC, fully open at 20 mA DC.
Hot Gas Bypass Valve2.
The hot gas bypass valve is used primarily at low
loads to maintain a minimum suction gas flow re-
quired by the compressor for stability. When the
compressor has reduced capacity to its minimum
flow (via pre-rotation vane throttling), further ca-
pacity reductions are accomplished by opening
the hot gas bypass valve. This maintains the flow
to the compressor by bypassing the discharge gas
back to the compressor suction.
However, the hot gas flow replaces the useful
evaporation in the cooler since the compressor
flow is at minimum. Thus, the net chilling capac-
ity is reduced (albeit not efficiently).
The minimum suction flow or minimum com-
pressor PRV position will vary. As the differential
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