7
NOTE: The external connector of the EXV must be cleaned and
coated with silicone grease (Part No. 397 EE) to keep out
condensation and prevent corrosion.
3.3.2 - The head pressure controls
The controller can deal with the following:
• in the case of air-cooled units, for each circuit, fan stages
together with, if necessary, a variable speed fan (controlled
by an 4xAI-2xAO board)
• in the case of water-cooled units, a water valve. This valve
is controlled by an 4xAI-2xAO board which can deliver a
0-10 V d.c. or 4-20 mA signal, depending on the
configuration.
3.3.3 - The evaporator pump
In appropriate cases the controller can also regulate an
evaporator pump. This facility does not require an additional
board.
3.3.4 - The condenser pump
In appropriate cases the controller can also regulate a
condenser pump (for water-cooled units). This control does not
require an additional board.
3.3.5 - The evaporator heater
The evaporator heater can be regulated by the unit control on
air-cooled units to protect the evaporator against frost. This
control does not require an additional board. If this control is
used, evaporator pump control must also be via the unit
control.
3.3.6 - Pressure sensors
These are used to measure the following pressures in each
circuit:
• Discharge gas pressure
• Suction pressure
• Oil pressure
• Economizer pressure
These electronic sensors deliver 0 to 5 V d.c. to either the basic
board or a 4xAI-2xAO slave board. Two types of sensors are
used. One is calibrated for the high pressure side and oil
pressure and the other for the low pressure side and economizer
pressure.
Discharge pressure sensors
These are on the high pressure side of the lead compressor in
each circuit. They replace the usual discharge gas pressure
gauges and are used as appropriate to control head pressure or
by the high pressure load shedding option.
Oil pressure sensors
These sensors, located at the oil pressure port of each
compressor, measure the oil pressure to the compressors. The
economizer pressure is subtracted from this value to arrive at
the differential oil pressure.
Suction pressure sensors
These are used to measure the low pressure side of each circuit.
They are located in the high pressure side of the evaporator.
NOTE: When connecting the power supply for the boards,
maintain polarity.
In the event of a power supply interrupt, the unit restarts
automatically without the need for an external command.
However, any faults active when the supply is interrupted are
saved and may in certain cases prevent a circuit or unit from
restarting.
3.2.7 - Light emitting diodes on boards
All boards continuously check and indicate the proper
operation of their electronic circuits. A light emitting diode
(LED) lights on each board when it is operating properly.
MAIN red LED - basic and slave boards
• The MAIN red LED flashes at about 2 second intervals to
show that the module is working properly.
• If this LED is permanently unlit, the power supply must
be checked.
• On slave boards, if this LED is permanently lit there is a
problem requiring the board to be changed.
• On the basic board, if this red LED is permanently lit or
flashes in turns strongly then weakly, there is either a
faulty basic board or a poorly positioned EPROM.
SIO green LED - basic and slave boards
(item SIO on the board)
• This LED flashes continuously to show that the board is
communicating correctly over its internal bus.
• If this LED is not flashing, check the wiring of the SIO
bus and the address of the board (slave board only). If the
basic board is not linked to any slave boards, this LED
should not flash.
• If all slave boards indicate a communication fault, check
the SIO bus connection on the basic board. If this
connection is correct and the fault persists, replace the
basic board.
CCN green LED - basic board
(item CCN on the board)
• This LED flashes to show that the basic board is
communicating over its CCN bus.
3.3 - The controls
3.3.1 - Electronic expansion valve (EXV)
The EXV is used to adjust the refrigerant flow to changes in
the operating conditions of the machine. For this purpose, a
series of calibrated orifices are machined into the wall of the
refrigerant inlet port. As the refrigerant passes through these
orifices, it expands and becomes a bi-phase mixture (liquid and
gas).
To adjust the refrigerant flow to changes in operating
conditions, a piston moves constantly up or down to vary the
cross-section of the refrigerant path. This piston is driven by an
electronically controlled linear stepper motor. The high degree
of accuracy with which the piston is positioned ensures that the
flow of refrigerant is precisely controlled.
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