Carrier 30XA080-500 Controls, Start-Up, Operation, Service And Troubleshooting Instructions page 56

SERVICE
Economizer Assembly -- Each circuit on 30XA090-
500 units have an economizer assembly. The 30XA080 unit is
non-economized and has one main electronic expansion valve.
Tile 30XA080 unit is controlled the same way as units with a
separate economizer assembly. See Fig. 30.
Electronic Expansion Valve (EXV) -- See Fig. 31
for a cutaway view of the EXV. High-pressure liquid refriger-
ant enters valve through the top. As refi'igerant passes through
the orifice, pressure drops and refrigerant changes to a 2-phase
condition (liquid and vapor). To control refrigerant flow for dif-
ferent operating conditions, an actuator moves up and down
over the orifice and modulates the orifice size. A sleeve is
moved by a linear stepper motol: The stepper motor moves in
increments and is controlled directly by the EXV module. As
the stepper motor rotates, motion is transferred into linear
movement by a the lead screw. Tile large number of steps and
long stroke results in veU accurate control of the refiigerant
flow. The stepper motor has either 3690 (main) or 2785 (econ-
omizer) steps.
MAIN EXV CONTROL -- Each circuit has a thermistor lo-
cated in a well in the discharge line of the compressor (DGF)
and another one located in the compressor motor cavity (SGT).
Each circuit also has discharge and suction pressure tmnsducel:
Discharge and suction pressure as measured by the tmnsducel_
am converted to saturated temperatures. The main control logic
for the EXV uses dischmge superheat to control the position of
the EXV. The difference between the temperature of the
discharge gas and the saturated dischm'ge temperature is the
superheat. The EXV module controls the position of the elec-
tronic expansion valve stepper motor to maintain the discharge
superheat set point.
Tile EXV control logic has several ovemdes, which am also
used to control the position of the EXV.
• Approach between SST and LWT
• Maximum Operating Pressure (MOP)
_roach -- If the approach (pinch), which is the difference
between leaving fluid temperature and saturated suction tem-
perature, is equal to or less than the pinch set point then the
EXV will not open any further even though discharge super-
heat set point is not met. Pinch set point is calculated using
suction superheat, discharge supeflleat and pinch offset. Pinch
offset is used to adjust calculated pinch set point do to accuracy
of transducers and thermistors.
MOP -- The EXV is also used to limit cooler saturated suction
temperature to 55 F (12.8 C). This makes it possible for the
chiller to stmt at higher cooler fluid temperatures without over-
loading the compressol: This is commonly referred to as MOP
(maximum operating pressure). If the SST is equal to or greater
than the MOP set point then the MBB will try to control the
EXV position to maintain the MOP set point which will result
in dischguge supeflleat to meet the set point.
Tile dischguge supeflleat leaving the compressor is main-
tained between approximately 18° and 25 ° F (10 ° and 14 ° C),
or less. Because EXV status is communicated to the Main Base
Bomd (MBB) and is controlled by the EXV modules, it is pos-
sible to track the valve position. The unit is then protected
against loss of charge and a faulty valve. During initial start-up,
the EXV is fully closed. After an initialization period, valve
position is tracked by the EXV module by constantly monitor-
ing the amount of valve movement.
ECONOMIZER
RELIEF
ECONOMIZER
COMPRESSOR
ECONOMIZER
RELIEF
MAIN LINE
.SHUT OFF
BRAZE PLATE
HEAT EXCHANGER
FILTER DRIER
MAIN EXV
*SIGHT GLASS
*Economizer EXV has sight glass
on opposite side of valve shown.
MAIN FLOW
TO COOLER
Fig. 30 -- Economizer Assembly
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