JOHNSON CONTROLS
38
FORM 160.67-O2
ISSUE DATE: 10/9/2020
SECTION 2 - PRE-STARTUP AND SYSTEM OPERATING PROCEDURES
The chiller may not be restarted until after the compres-
sor coast down sequence is completed and the message
"COMPRESSOR SHTDN; TURBINE COOLDOWN"
(Pressure Lubrication) or "SYSTEM READY TO
START" (Ring Lubrication) is displayed.
CAPACITY CONTROLS
Overview
Both fixed and variable speed modes of operation are
included. The fixed speed mode would likely only be
used in plants where the condensing cooling liquid is
always at a high temperature so that speed reduction
would not be possible due to the high system pres-
sure differential. The Speed Control Mode is factory
set for Variable (default mode) to provide the greatest
efficiency of operation in plants where the condenser
cooling liquid is allowed to decrease to the minimum
allowed. This allows the turbine speed to be reduced
to the minimum rated speed and the compressor pre-
rotation vanes to be closed completely prior to opening
the hot gas valve. In this mode, the capacity controls
will automatically utilize the full range of the governor,
compressor PRV and hot gas valve capabilities to con-
trol the capacity of the chiller and provide anti-surge
and override control functions to prevent unsafe opera-
tion.
For easy adjustments all operating mode selections and
PID tuning parameters are included on various setpoint
screens on the OptiView™ Control Center graphic dis-
play.
Only a qualified YORK Service Techni
-
cian should make adjustments to capacity
controls.
FIXED SPEED CHILLER CAPACITY
CONTROLS OPERATION
During operation at a fixed speed, the vanes and hot
gas valve are controlled by the output of a single PID
controller that responds to changes in the leaving
chilled liquid temperature. A ratio calculation is used
to position the hot gas valve after the vanes have been
closed to their minimum position based on the sys-
tem pressure differential (condenser minus evaporator
pressure). A separate PID controller is also provided to
control the hot gas valve based on leaving chilled liquid
temperature if operating conditions require the vanes
to be fully closed at low loads. During off design op-
eration, the outputs of the high condenser pressure, low
evaporator pressure and torque limiting override PID
controllers override the output of the primary Leav-
ing Chilled Liquid Temperature Controller to maintain
stable operation and prevent the chiller from shutting
down. The tuning of the primary Leaving Chilled Liq-
uid Temperature, Evaporator Pressure and Condenser
Pressure PID controllers is automatically adjusted to
match the response time of the process when the hot
gas valve is being controlled. To achieve maximum
efficiency at part load conditions, the unloading se-
quence is vane closure and hot gas valve opening.
Refer to the YST OptiView™ Control Logic Diagram
(Capacity Control 1, Capacity Control 2, and Capac
-
ity Control 3 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 de-
scribed in the following 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.
FIXED SPEED CHILLER LOADING
SEQUENCE
The chiller is started with the Leaving Chilled Liquid
Temperature Controller inactive and its output equal
to the fixed speed tieback signal after LSR3. When the
ramp-to-rated speed is initiated, the vane output signal
is set to the minimum start-up value (PRVM). After the
minimum rated speed of 3200 RPM has been achieved,
the vanes are ramped open and the hot gas valve is
ramped closed. As the system pressure differential
(condenser minus evaporator pressure) increases, if the
output of the Minimum Vane Position Calculation in-
creases above the PRV ramp output signal, HSR5 will
select the higher value to open the vanes farther and
prevent surging. The Leaving Chilled Liquid Tempera-
ture Controller becomes active when the vane ramp
signal is at 97% or the leaving chilled liquid temper
-
ature decreases to within 2°F of the Leaving Chilled
Liquid Temperature Controller set point. The Leaving
Chilled Liquid Temperature Controller will adjust the
vanes as required to maintain the leaving chilled liquid
temperature at set point.
For fixed speed operation, TDSP17 – Turbine Stabi-
lization Delay should be set to 0 to allow loading to
begin as soon as the minimum rated speed is achieved.
Refer to the OptiView™ Control Center Operation
Manual Form 160.67-O1
for details on adjustable set-
points.