JOHNSON CONTROLS
40
FORM 160.67-O2
ISSUE DATE: 10/9/2020
SECTION 2 - PRE-STARTUP AND SYSTEM OPERATING PROCEDURES
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.
To provide protection for retrofitted chillers that do not
have the ability to monitor turbine first stage pressure,
the logic monitors the governor valve actuator output
from the chiller panel as done on previous chillers.
The Governor Position Controller is inactive when the
chiller is stopped with its’ outputs set to 100%.
When the chiller is running at or above the minimum
rated speed, if the integrated speed control PID at-
tempts to open the governor valve more than a preset
value (determined by field testing at start up), the out-
put of the Governor Position Controller will decrease.
LSR2 and LSR1 will select the lower signal to over
-
ride the Leaving Chilled Liquid Temperature Control-
ler output and unload the chiller as described above.
With the load reduced, the turbine will begin to speed
up and the speed control PID will begin to close the
governor valve, thus limiting the torque output of the
turbine. While the Governor Position Controller is ac
-
tive, the primary Leaving Chilled Liquid Temperature
Controller is inactivated and its output is set to equal
the appropriate tieback, thus providing a bumpless
transfer to normal leaving chilled liquid temperature
control. After the load is reduced sufficiently to return
the governor valve to a safe position for 5 seconds, the
Leaving Chilled Liquid Temperature Controller is re-
activated and its’ PID algorithm begins controlling the
output at the tieback value.
Turbine Horsepower Limiting
All new YST chillers are provided with a transmitter to
monitor the turbine first stage pressure. This pressure
is used along with the steam inlet pressure and tem-
perature and the turbine exhaust pressure to calculate
the turbines power output. A separate PID controller is
provided to unload the chiller when the turbine power
approaches the maximum that the compressor bearings
are rated for or 115% of the turbine design horsepower,
whichever is lower. The logic will calculate the actual
horsepower based on the steam inlet temperature and
pressure so that the override controller automatically
adapts to fluctuations in the quality of the steam sup-
plied to the turbine inlet.
If the calculated horsepower increases above the set
point, the output of the Turbine Horsepower Limit
Controller will decrease. LSR2 and LSR1 will select
the lower signal to override the Leaving Chilled Liquid
Temperature Controller output and unload the chiller
as described above. With the load reduced, the turbine
will begin to speed up and the speed control PID will
begin to close the governor valve, thus reducing the
horsepower output of the turbine. While the Turbine
Horsepower Limit 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
turbine horsepower has not exceeded the set point for
5 seconds, the Leaving Chilled Liquid Temperature
Controller is reactivated and its’ PID algorithm begins
controlling the output at the tieback value.
VARIABLE SPEED CHILLER CAPACITY
CONTROLS OPERATION
During operation, the speed, vanes and hot gas valve
are controlled by the output of a single PID control-
ler that responds to changes in the leaving chilled liq-
uid temperature. The Capacity Ratchet Mode Selector
directs the controllers output to the appropriate con-
trolled device depending on system load and system
pressure differential (condenser minus evaporator
pressure). During off design operation, the outputs of
the high condenser pressure, low evaporator pressure
and torque limiting override PID controllers are also
directed to the appropriate device to maintain stable
operation and prevent the chiller from shutting down.
The tuning of the primary Leaving Chilled Liquid Tem-
perature, Evaporator Pressure and Condenser Pressure
PID controllers is automatically adjusted to match the
response time of the process depending on the device
being controlled. To achieve maximum efficiency at
part load conditions, the unloading sequence is speed
reduction, vane closure, and hot gas valve opening.
Refer to the YST OptiView™ Control
Logic Diagram (Capacity Control 1,
Capacity Control 2, Capacity Control
3, and Capacity Ratchet Mode Selector
Diagrams) along with the following para-
graphs for a more detailed description of
the operation. If any of the speeds, ramp
rates, or time delays described in the fol-
lowing paragraphs are adjustable, the
factory default values are show for clarity.
Refer to the OptiView™ Control Center
Operation Manual Form 160.67-O1 for
details on adjustable setpoints.