JOHNSON CONTROLS
39
SECTION 2 - PRE-STARTUP AND SYSTEM OPERATING PROCEDURES
FORM 160.67-O2
ISSUE DATE: 10/9/2020
2
FIXED SPEED CHILLER UNLOADING SEQUENCE
PRV Control
If the load decreases, the output of the Leaving Chilled
Liquid Temperature Controller will decrease to close
the vanes to the minimum position established by the
Minimum Vane Position Calculation.
As the system pressure differential falls, the compres-
sor is capable of stable operation with less refrigerant
gas flow (lower minimum compressor pre-rotation
vane positions). For the greatest efficiency, the com-
pressor pre-rotation vanes must be used for capacity
control rather than hot gas bypass whenever possible.
The Minimum Vane Position Calculation output signal
to HSR5 provides a minimum closure of the pre-rota
-
tion vanes to suite the measured pressure differential
input. This output also provides one of the input signals
for the Hot Gas Ratio calculation which to use in con
-
trolling the output to the hot gas valve.
Hot Gas Ratio Control
The Hot Gas Ratio calculation controls the hot gas by
-
pass valve at a scaled ratio to the percentage of avail-
able Leaving Chilled Liquid Temperature Controller
signal below the Minimum Vane Position Calculation
output, thus providing a 0-100% control signal for the
hot gas bypass valve.
To provide greater stability at very low system pressure
differentials when the minimum vane position calcula-
tion would normally be 0%, a minimum vane position
may be entered into the HGVRAT set point. HSR1 will
select this higher value to give the Hot Gas Ratio cal
-
culation a greater range of control for the hot gas valve.
As long as the capacity signal from the Leaving Chilled
Liquid Temperature Controller is above the Minimum
Vane Position Calculation output (or the HGVRAT)
signal, the hot gas valve remains closed. When the
capacity signal falls below the Minimum Vane Posi-
tion Calculation output, the tuning parameters for hot
gas control are loaded into the Leaving Chilled Liquid
Temperature Controller. The output of the Hot Gas Ra
-
tio calculation increases, which starts to open the hot
gas bypass valve. Thus at low loads, the capacity is
controlled by modulating the hot gas bypass valve.
HGV Temperature Controller
In some cases, it may be desirable to allow the vanes
to go completely closed which would leave no signal
for the Hot Gas Ratio calculation to control the hot gas
valve. A separate PID controller is provided which con-
trols at a leaving chilled liquid temperature set point
that is slightly below the main Leaving Chilled Liquid
Temperature Controller set point. If the load continues
to decrease with the vanes fully closed, the leaving
chilled water temperature will decrease below the set
point of the HGV Temperature Controller causing its
output to increase. HSR6 will select this higher signal
and begin opening the hot gas valve to maintain the
leaving chilled liquid temperature at the lower HGV
Temperature Controller set point.
FIXED 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. LSR2
and LSR1 will select the lower signal to override the
Leaving Chilled Liquid Temperature Controller output
and close the compressor PRV as described above, thus
preventing unsafe operation and an unnecessary shut-
down.
If the override signal falls below the Minimum Vane
Position Calculation output, the tuning parameters for
hot gas control are loaded into the override controller.
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.