JOHNSON CONTROLS
41
SECTION 2 - PRE-STARTUP AND SYSTEM OPERATING PROCEDURES
FORM 160.67-O2
ISSUE DATE: 10/9/2020
2
VARIABLE SPEED CHILLER LOADING
SEQUENCE
Hot Gas Mode
The chiller is started with the Leaving Chilled Liquid
Temperature Controller inactive and the hot gas con-
trol signal at 100%. When the ramp-to-rated speed is
initiated, the vane output signal is set to the minimum
start-up value (PRVM). The chiller is first ramped up
to the higher fixed rated speed set point to allow the
speed control PID time to stabilize the speed prior to
loading the chiller. This is done to avoid cycling the
speed down into the critical speed range of the turbine
as the governor is throttled during ramp up.
After the stabilization time delay has elapsed, the speed
set point is ramped down to the minimum speed de-
termined by the Minimum Speed Calculation to suite
the measured pressure differential input. The Capacity
Ratchet Mode Selector sets the Leaving Chilled Liquid
Temperature Controller to the hot gas mode, the tun-
ing parameters for hot gas control are loaded, and its
output is set to the current hot gas control signal. The
hot gas valve is ramped closed. As the system pres-
sure differential (condenser minus evaporator pres-
sure) increases, the outputs of the Minimum Speed and
Minimum Vane Position Calculation will increase. The
speed set point and vane control signals are immedi-
ately set to the higher values to prevent surging.
If the load is light and the leaving chilled liquid tem-
perature decreases to within 2°F of the Leaving Chilled
Liquid Temperature Controller set point, the controller
becomes active, and its’ PID algorithm begins modu-
lating the hot gas valve to prevent overshooting of the
set point as the chilled liquid loop is pulled down to
design temperature.
PRV Mode
If the leaving chilled liquid temperature remains more
than 2° F above the set point, the Leaving Chilled Liq-
uid Temperature Controller will remain inactive and
the hot gas valve will continue to be ramped closed.
When the Capacity Ratchet Mode Selector detects
a 0% HGV control signal to the actuator output, the
tuning parameters for PRV control are loaded into
the Leaving Chilled Liquid Temperature Controller
and its’ output is set to the current PRV control signal
which will be equal to the higher of the minimum start-
up value (PRVM) or the Minimum Vane Position Cal-
culation output. The PRV will begin ramping open. As
the system pressure differential increases, the output of
the Minimum Speed Calculation will increase and the
speed set point signal is immediately set to the higher
value to prevent surging.
If the load is light and the leaving chilled liquid temper-
ature decreases to within 2 °F of the Leaving Chilled
Liquid Temperature Controller set point, the controller
becomes active, and its’ PID algorithm begins modu-
lating the PRV to prevent overshooting of the set point
as the chilled liquid loop is pulled down to design tem-
perature.
Speed Mode
If the leaving chilled liquid temperature remains more
than 2° F above the set point, the Leaving Chilled Liq-
uid Temperature Controller will remain inactive and
the PRV will continue to be ramped open. When the
Capacity Ratchet Mode Selector detects a 97% PRV
control signal, the tuning parameters for Speed con-
trol are loaded into the Leaving Chilled Liquid Tem-
perature Controller and its’ output is set to the current
speed set point in % which will be equal to the higher
of the minimum rated speed in % or the Minimum
Speed Calculation output in %. The Leaving Chilled
Liquid Temperature Controller becomes active, and its’
PID algorithm begins modulating the speed set point to
maintain the leaving chilled liquid temperature at set
point.
VARIABLE SPEED CHILLER UNLOADING
SEQUENCE
Speed Mode
If the load decreases, the output of the Leaving Chilled
Liquid Temperature Controller will decrease. For the
greatest efficiency, the speed must be reduced as much
as possible prior to closing the compressor pre-rota-
tion vanes. As the system pressure differential falls,
the compressor is capable of stable operation with less
refrigerant gas flow (lower minimum speeds). The
Minimum Speed Calculation output signal to HSR4
provides a minimum speed to suite the measured pres-
sure differential input. The Capacity Ratchet logic
constantly monitors the Minimum Speed Calculation
output signal to ensure that the chiller is always operat-
ing at the minimum speed possible while maintaining
system stability.
To Next Page
Loading...
