Operating Controls - Carrier 17DA Start-Up, Operation And Maintenance Instructions Manual

use the same sensors as the operating functions. Per customer
specifications, these may be supplemented by mechanical
switches which directly measure pressures or temperatures.
These switches are connected to discrete inputs of the PLC and
the PLC controls the machine.
Starters and VFDs contain their own safeties that will shut
down the starter/drive and then report the trip to the PLC via
the communications line. It may be necessary to go to the drive
control panel to read the actual trip code. On the PLC, the dis-
play may only show that the trip was initiated by the drive.
STANDARD SAFETY FUNCTIONS — The following
safety functions are standard:
• low chilled water flow
• low condenser water flow
• high discharge gas temperature
• chilled water low temperature
• condenser high pressure
• cooler low pressure
• seal oil pressure
• bearing oil pressure
• thrust bearing high temperature
• seal movement switch
• excess shaft movement switch
• gear and turbine oil pressure switches
JOB SPECIFIC SAFETY FUNCTIONS — The settings for
job specific safety functions will be found in the custom oper-
ating manuals provided with the machine. Examples of job
specific safety functions are:
• vibration monitoring
• optional RTD bearing temperature sensors for the compres-
sor and the drive components
• turbine overspeed
• failure of a synchronous motor to synchronize
• TEWAC (totally enclosed, water cooled) motor cooling coil
leak
Operating Controls —
representations of the basic control system governing water
temperature, power demand limit, and condenser liquid level
(liquid flow control). A comprehensive description of the com-
plete control logic specific to each machine is provided in the
custom operating manual. A brief description of the basic sche-
matic follows.
The primary control devices are the compressor guide
vanes, the hot gas bypass valve (optional) and the condensed
liquid valve. Compressor speed may also be varied on some
jobs. That will not be covered here.
Compressor inlet guide vanes control the amount of refrig-
erant gas that the compressor pulls from the cooler. By varying
this capacity, the control system maintains correct leaving wa-
ter temperature. The correct temperature is set on the PLC and
can be also be set remotely. The PLC directs the position of the
guide vanes based on the leaving water temperature. The guide
vanes can also be controlled manually from the PLC.
The guide vane position signal is sent to an actuator that
moves the guide vanes in proportion to the control signal. The
standard actuator is pneumatic and the signal from the PLC is
converted to a 3 to 15 psi instrument signal to the actuator
Figures 10 and 11 are schematic
positioner. A solenoid valve in the instrument line prevents the
actuator from opening the guide vanes until the start-up se-
quence is complete. The solenoid will also cause the guide
vanes to close immediately upon machine shut down. This pre-
vents the pressure equalization gas flow from the condenser to
the cooler from back spinning the compressor and driver.
In case of supply air failure, the reserve air tank has suffi-
cient volume to close the vanes.
The hot gas bypass valve works together with the guide
vanes. At low loads, the guide vane opening will be small. The
hot gas valve will start to open as the guide vanes approach the
minimum opening. The purpose of the hot gas bypass is to pro-
vide enough gas flow to the compressor to allow it to operate in
a stable way at low chiller loads.
The guide vane position as determined by water tempera-
ture can be overridden by other functions. Functions can over-
ride only to close the vanes and reduce the machines capacity.
No override can increase the guide vane position.
Override functions are:
• refrigerant temperature too low
• condenser pressure too high
• motor current too high, based on the demand limit set point.
Inside the compressor, the guide vane actuator shaft also
moves a diffuser throttle ring. Its position is determined by
cams. During low loads, typically below 50%, the guide vanes
and throttle ring move together. The diffuser throttling ring pro-
vides optimum performance down to 10% load. At higher
loads the throttle ring is fully open and capacity is controlled by
the guide vanes alone or in conjunction with variable compres-
sor speed.
Variable frequency drives (VFDs) can be used along with
the guide vanes to control machine capacity. When a variable
speed drive or turbine is used, refer to the custom operating
manual for specific details of the control sequence.
Figure 12 shows a typical turbine control which determines
the required compressor speed using the difference between
leaving chilled water and entering condenser water. This is not
the only way to determine speed and sometimes the speed and
guide vane position are varied together to optimize power con-
sumption. As long as the compressor speed required to main-
tain the desired leaving chilled water temperature is sufficient
to maintain the required condenser pressure, it is most econom-
ical to control capacity by varying speed alone. If higher com-
pressor rpm is required to maintain condenser pressure, the
guide vanes will temporarily take control of capacity. The se-
quence is chosen based on individual job requirements.
Condenser refrigerant level is set when the machine is com-
missioned. The level ensures that the subcooler tubes are cov-
ered with liquid and the liquid level does not reach the con-
denser tubes. A level control on the condenser operates the
control valve in the line connecting the condenser drain with
the cooler liquid inlet.
The level control function can reside in the PLC. In that
case a level transmitter is mounted on the condenser in place of
a level controller. The control still operates the liquid line
valve.
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