JOHNSON CONTROLS
42
FORM 160.67-O2
ISSUE DATE: 10/9/2020
SECTION 2 - PRE-STARTUP AND SYSTEM OPERATING PROCEDURES
PRV Mode
When the Leaving Chilled Liquid Temperature Con-
troller output is equal to the Minimum Speed Calcu-
lation output signal, the Leaving Chilled Liquid Tem-
perature Controller PID calculation is halted while the
tuning parameters for PRV control are loaded and its’
output is set to the current PRV control signal which
would be 100% since the PRV signal would be at 100%
while the speed is being controlled. The controller PID
algorithm then begins modulating the PRV control sig-
nal to maintain the leaving chilled liquid temperature at
set point. As the vanes are closed, the system pressure
differential will decrease resulting in a lower output
from the Minimum Speed Calculation.
The Capacity Ratchet logic will decrease the speed set
point to the lower value after 30 seconds to maintain
maximum efficiency of operation. If the system pres-
sure differential increases, the output of the Minimum
Speed Calculation will increase. The speed set point is
immediately set to the higher value to prevent surging.
For the greatest efficiency, the compressor pre-rotation
vanes must be closed as far as possible prior to opening
the hot gas valve. The Minimum Vane Position Cal-
culation output signal to HSR5 provides a minimum
closure of the pre-rotation vanes to suite the measured
pressure differential input. The Capacity Ratchet logic
constantly monitors the Minimum Vane Position Cal-
culation output signal to ensure that the chiller is al-
ways operating at the lowest PRV signal possible while
maintaining system stability.
Hot Gas Mode
If the load continues to decrease, the output of the
Leaving Chilled Liquid Temperature Controller will
decrease. When the Leaving Chilled Liquid Tempera-
ture Controller output is equal to the Minimum Vane
Position Calculation output signal, the Leaving Chilled
Liquid Temperature Controller PID calculation is halt-
ed while the tuning parameters for HGV control are
loaded and its’ output is set to a value equal to 100%
minus the current HGV control signal. This value
would be 100% since the HGV signal would be 0%
while the vanes are being controlled. The controller
PID algorithm then begins modulating the HGV con-
trol signal to maintain the leaving chilled liquid tem-
perature at set point.
If the load continues to decrease, the Leaving Chilled
Liquid Temperature Controller output will begin de-
creasing from 100%. This signal is subtracted from
100% to provide the increasing signal required to open
the HGV. As the HGV is opened, the system pressure
differential will decrease resulting in lower outputs
from the Minimum Speed Calculation and Minimum
Vane Position Calculation. The Capacity Ratchet logic
will decrease the speed set point and PRV control sig-
nals to the lower values after 30 seconds to maintain
maximum efficiency of operation. If the system pres-
sure differential increases, the outputs of the Minimum
Speed Calculation and Minimum Vane Position Calcu-
lation will increase. The higher values are immediately
moved into the speed set point and PRV control signal
to prevent surging.
VARIABLE SPEED CHILLER OVERRIDE
CONTROLLERS
High and Low Refrigerant Pressure
The Evaporator Pressure and Condenser Pressure con-
trollers are inactive when the chiller is stopped with
their outputs set to 100%. If the monitored parameter
exceeds the set point of the controller during any ab-
normal operation when running at or above the mini-
mum rated speed, the output of the appropriate control-
ler is set to the tieback signal for the device currently
being controlled before the controller is activated, thus
providing a bumpless transfer to the override control.
If the monitored parameter exceeds the set point of the
controller, the controller output will decrease. LSR1
will select the lower signal to override the Leaving
Chilled Liquid Temperature Controller output and un-
load the chiller as described above using the Capacity
Ratchet Mode Selector to direct the controllers output
to the appropriate controlled device depending on sys-
tem load and system pressure differential, thus prevent-
ing unsafe operation and an unnecessary shutdown.
While any override controller is active, the primary
Leaving Chilled Liquid Temperature Controller is in-
activated and its output is set to equal the appropriate
tieback, thus providing a bumpless transfer to normal
leaving chilled liquid temperature control. After the
override controllers monitored parameter has not ex-
ceeded the set point for 5 seconds, the Leaving Chilled
Liquid Temperature Controller is reactivated and its’
PID algorithm begins controlling the output at the tie-
back value.
Turbine Governor Position Power Limiting
In some applications, during high load/pulldown con-
ditions, the turbine may be capable of producing more
horsepower than the compressor bearings are rated for.
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