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5.12 - Slave compressor start-up sequence
Lag compressors are started and stopped in a sequence
designed to equalise their number of start-ups (value weighted
by their operating time).
5.13 - Controlling the EXV
EXVs, if used, control the flow of refrigerant in the evaporator.
They are controlled in order to maintain a constant level of
superheat at the thermistor for the lead compressor inlet gas
(located between the compressor motor and the cylinders).
A thermistor and a pressure sensor, in the lead compressor of
each circuit, are used to measure this superheat. The thermistor
measures the temperature of the superheated gas entering the
cylinders. The pressure sensor measures the suction gas
pressure. The controller converts this value into a saturated
temperature. The difference between the superheated gas
temperature and the saturated temperature is the superheat. The
control system sets the position of the EXV to hold this
superheat at the setpoint configured for the unit.
Since the EXVs are driven by the controller their positions are
always known. During start-up of a circuit its EXV is fully closed
to ensure pump down. After pump down, the system continuously
controls and monitors the valve position. Similarly, on shutdown
of a circuit the EXV is closed again to ensure pumping down.
EXVs are also used to restrict suction temperature, making it
possible to start the unit at higher water and suction tempera-
tures without overloading the compressors. This procedure
controls what is known as Maximum Operating Pressure (MOP).
5.14 - Head pressure control on air-cooled units
There are two methods, configurable only by Carrier Service,
for controlling the condenser fans:
• Controlling condensation by reference to a setpoint
(default): The saturated head pressure is controlled by
reference to a fixed setpoint (user-definable in the Setpoints
menu). This temperature is maintained by cycling fans on
and off, as well as by varying the speed of a fan in
appropriate cases.
• Controlling condensation by reference to the position of
the EXV: The saturated head pressure is controlled by
reference to the position of the EXV and the superheat, by
cycling fans on and off as well as by varying the speed of a
fan in appropriate cases. The control system seeks to keep
the EXVs as wide open as possible whilst maintaining the
correct level of superheat with the fans. When the second
condensing setpoint is selected (reclaim setting), the control
system will automatically revert to controlling by reference
to a setpoint even if control by reference to the EXV has
been selected. The original configuration is reinitialised
when the first condensing setpoint is re-selected.
NOTE
Certain units can have up to 8 fan stages, of which one per
circuit is a variable-speed fan, depending on their configura-
tion and wiring.
5.15 - Head pressure control on water-cooled units
There are two configurable methods for controlling the conden-
ser water valves (optional):
• Controlling head pressure by reference to a setpoint:
Saturated head pressure is controlled by reference to a user-
definable fixed setpoint. This temperature is maintained by
using the valves to control the flow of water in each conden-
ser circuit.
• Controlling head pressure by reference to the position of
the EXV: Saturated head pressure is controlled by reference
to the position of the EXV and the superheat, by adjusting
the flow of water in each condenser circuit. The valve control
system seeks to keep the EXVs as wide open as possible
whilst maintaining the correct level of superheat. When the
second head pressure setpoint is selected (reclaim setting),
the control system will automatically revert to controlling by
reference to a setpoint even if control by reference to the
EXV has been selected. The original configuration is
reinitialised when the first head pressure setpoint is re-
selected.
5.16 - Active setpoint selection
Two setpoints can be selected as active in cooling mode, and
one setpoint in heating mode. Usually, the second cooling
setpoint is used for unoccupied periods or for ice storage (brine
unit). Depending on the current operating mode, the active
setpoint can be selected either by choosing the item in the
Information menu. or with the user volt-free contacts, or with
network commands, or by the setpoint time schedule program
(Schedule 2).
The following tables show a summary of the possible selctions
as a function of the control typpes (local, remote or network),
and the parameters below:
• Local setpoint selection: item No. 11 of the Information
Menu permits selection of the active setpoint, if the unit is
in the local operating type.
• Heating/cooling operating mode.
• Control contacts: status of the remote heating and
cooling control contact. This contact is only active, if the
unit is under remote operating control. See section 3.6.6.
• Schedule 2 program status: schedule program for
setpoint selection. See section 4.3.11.6.