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Electronic
Expansion
Fig. 15) —
Standard units are equipped with a bottom seal
EXV. This device eliminates the use of the liquid line solenoid
pumpdown at unit shutdown. An O-ring has been added to bot-
tom of orifice assembly to complete a seal in the valve on shut-
down. This is not a mechanical shut-off. When service is
required, use the liquid line service valve to pump down the
system.
High pressure refrigerant enters bottom of valve where it
passes through a group of machined slots in side of orifice as-
sembly. As refrigerant passes through the orifice, it drops in
pressure. To control flow of refrigerant, the sleeve slides up and
down along orifice assembly, modulating the size of orifice.
The sleeve is moved by a linear stepper motor that moves in in-
crements controlled directly by the processor. As stepper motor
rotates, the motion is translated into linear movement of lead
screw. There are 1500 discrete steps with this combination. The
valve orifice begins to be exposed at 320 steps. Since there is
not a tight seal with the orifice and the sleeve, the minimum po-
sition for operation is 120 steps.
Two thermistors are used to determine suction superheat.
One thermistor is located in the cooler and the other is located
in the cylinder end of the compressor after refrigerant has
passed over the motor. The difference between the 2 ther-
mistors is the suction superheat. These machines are set up to
provide approximately 5 to 7 F (2.8 to 3.9 C) superheat leaving
the cooler. Motor cooling accounts for approximately 22 F
(12.2 C) on 30GTN,R units and 16 F (8.9 C) on 30GUN,R
units, resulting in a superheat entering compressor cylinders of
approximately 29 F (16.1 C) for 30GTN,R units and 23 F
(12.8 C) for 30GUN,R units.
Because the valves are controlled by the EXV module, it is
possible to track the position of the valve. Valve position can be
used to control head pressure and system refrigerant charge.
During initial start-up, the EXV module will drive each
valve fully closed. After initialization period, valve position is
controlled by the EXV module and the MBB.
The EXV is used to limit the maximum cooler saturated
suction temperature to 55 F (12.8 C). This makes it possible for
the chiller to start at high cooler fluid temperatures without
overloading the compressor.
Energy Management Module (Fig. 16) —
factory-installed option or field-installed accessory is used for
the following types of temperature reset, demand limit, and/or
ice features:
• 4 to 20 mA leaving fluid temperature reset (requires
field-supplied 4 to 20 mA generator)
• 4 to 20 mA cooling set point reset (requires field-
supplied 4 to 20 mA generator)
• Discrete inputs for 2-step demand limit (requires field-
supplied dry contacts capable of handling a 5 vdc, 1 to
20 mA load)
• 4 to 20 mA demand limit (requires field-supplied 4 to
20 mA generator)
• Discrete input for Ice Done switch (requires field-
supplied dry contacts capable of handling a 5 vdc, 1 to
20 mA load)
See Demand Limit and Temperature Reset sections on
pages 46 and 43 for further details.
Capacity Control —
The control system cycles com-
pressors, unloaders, and hot gas bypass solenoids to maintain
the user-configured leaving chilled fluid temperature set point.
Entering fluid temperature is used by the Main Base Board
(MBB) to determine the temperature drop across the cooler and
is used in determining the optimum time to add or subtract ca-
pacity stages. The chilled fluid temperature set point can be au-
tomatically reset by the return temperature reset or space and
outdoor-air temperature reset features. It can also be reset from
Valve
(EXV)
(See
This
STEPPER
MOTOR (12 VDC)
ORIFICE ASSEMBLY
(INSIDE PISTON SLEEVE)
Fig. 15 — Electronic Expansion Valve (EXV)
an external 4 to 20 mA signal (requires Energy Management
Module FIOP/accessory).
With the automatic lead-lag feature in the unit, the control
determines which circuit will start first, A or B. At the first call
for cooling, the lead compressor crankcase heater will be deen-
ergized, a condenser fan will start, and the compressor will start
unloaded.
NOTE: The automatic lead-lag feature is only operative when
an even number of unloaders is present. The 040-070 units
require an accessory unloader to be installed on the B1 com-
pressor for the lead-lag feature to be in effect.
If the circuit has been off for 15 minutes, and the unit is a
TXV unit, liquid line solenoid will remain closed during start-
up of each circuit for 15 seconds while the cooler and suction
lines are purged of any liquid refrigerant. For units with EXVs,
the lead compressor will be signaled to start. The EXV will re-
main at minimum position for 10 seconds before it is allowed
to modulate.
After the purge period, the EXV will begin to meter the re-
frigerant, or the liquid line solenoid will open allowing the
TXV to meter the refrigerant to the cooler. If the off-time is less
than 15 minutes, the EXV will be opened as soon as the com-
pressor starts.
The EXVs will open gradually to provide a controlled start-
up to prevent liquid flood-back to the compressor. During start-
up, the oil pressure switch is bypassed for 2 minutes to allow
for the transient changes during start-up. As additional stages
of compression are required, the processor control will add
them. See Tables 5A and 5B.
If a circuit is to be stopped, the control will first start to close
the EXV or close the liquid line solenoid valve.
For units with TXVs, the lag compressor(s) will be shut
down and the lead compressor will continue to operate for
10 seconds to purge the cooler of any refrigerant.
For units with EXVs, the lag compressor(s) will be shut
down and the lead compressor will continue to run. After the
lag compressor(s) has shut down, the EXV is signaled to close.
The lead compressor will remain on for 10 seconds after the
EXV is closed.
During both algorithms (TXV and EXV), all diagnostic
conditions will be honored. If a safety trip or alarm condition is
detected before pumpdown is complete, the circuit will be shut
down.
16
LEAD SCREW
PISTON SLEEVE
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