SECTION 4 - GENERAL OPERATING SEQUENCE
JOHNSON CONTROLS
125
FORM 160.72-O1 (810)
of the pre-rotation vanes to suite the measured
HEAD pressure input This output also provides
one of the input signals for the hot gas valve con-
trol to use in calculating the output to the hot gas
valve.
The display will indicate if the chiller is currently
in this mode of operation.
Hot Gas Control
7.
This control modulates the hot gas valve when
the capacity control signal reaches the minimum
PRV limit established by the ANTI-SURGE cal-
culation. As long as the capacity signal from the
TEMPERATURE CONTROL is above the ANTI-
SURGE output signal, the hot gas valve remains
closed.
When the capacity signal falls below the anti-
surge output (minimum vane position) signal, the
output of this control decreases, which starts to
open the hot gas bypass valve. Thus at low loads,
the capacity is controlled by modulating the hot
gas bypass valve. A ratio calculation compares
the anti-surge calculation to the capacity control
signal output When the capacity drive signal de-
creases below the anti-surge value the vanes no
longer are able to close to control temperature.
The hot gas ratio calculation opens the hot gas by-
pass valve as a scaled ratio of the available signal
below the vane anti-surge value. This way the hot
gas bypass valve operates completely automati-
cally to maintain the reduced capacity demands of
the chilled water system.
The control software reverses the signal so that
0%=Closed and 100%=Open for Man/Auto Sta-
tions and displays on the screen.
The display will indicate that the hot gas control is
currently modulating the hot gas valve.
Subcooler Level Control
8.
This programmed control function uses an input
signal from a level transmitter installed on the
condenser. It uses proportional plus reset con-
trol to provide an output signal which increases
as the level goes above the setpoint to open the
liquid level control valve and maintain the level
at setpoint
The control output signal is selected by signal
selector SS five minutes after start-up when the
ramped valve control signal has decreased to
equal the PID Instructions output signal. At shut-
down, the output signal drives the valve to 100%
to open the liquid level control valve.
raMp-up of CapaCiTY ConTrols
The ramp-up feature is used to provide a gradual in-
crease in chiller capacity during start-up.
Refer to the CAPACITY CONTROL DIAGRAM
along with the following description to best understand
the operation:
The display will indicate when the leaving chilled
water TEMPERATURE CONTROL is in the manual
mode.
This occurs when the unit is shut down, during ramp-
up and when the DEMAND LIMITER or the EVAP
LOW PRESS. OVERRIDE or the HIGH DISCHARGE
PRESS. OVERRIDE control signal is being used to
control the capacity of the chiller.
With the unit stopped, the TEMPERATURE CON-
TROL is in manual, its setpoint is equal to the leav-
ing chilled water temperature and its output is equal to
the PRV output signal. (When in manual, the controller
drives its output to the same as the tieback value, which
would be 0% with unit off.)
After the compressor motor starts, the PRV signal goes
to the PRE-ROTATION VANES MINIMUM START-
UP POSITION value. This value determines the posi-
tion of the compressor pre-rotation vanes at the instant
the main motor starts. It is adjusted to keep the starting
load on the motor low but also allow enough gas flow
through the compressor to prevent surging at start-up.
The HOT GAS RATIO CALCULATION output be-
gins to decrease at this time to start closing the hot gas
bypass valve.
After the motor power drops below the DEMAND
LIMIT setpoint, its output signal increases rapidly to
100%. The Selector Switch (SS) and Low Selector Re-
lay (LSR) will then select the lower PRV RAMP CON-
TROL output signal.
When the output signal from the PRV ramp rises above
the PRV Minimum Startup Position, the selector switch
(SS) will change position to control the PRV based on
the temperature control or demand limit by the PRV
ramp rate. The PRV signal will then track the primary
LSR output signal until the next chiller start.
4
Содержание YORK OM Titan
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