Specifications

65
SERVICE
Economizer Assembly —
Each circuit on
30XW175,200,250,275,300,350,400 units has an economizer
assembly. The 30XW150,185,225,260,325,375 units do not
have an economizer and have one main electronic expansion
valve. The 30XW150,185,225,260,325,375 units are con-
trolled the same way as units with a separate economizer as-
sembly. See Fig. 48.
Electronic Expansion Valve (EXV) — See Fig. 49
for a cutaway view of the EXV. High-pressure liquid refriger-
ant enters valve through the top. As refrigerant passes through
the orifice, pressure drops and refrigerant changes to a 2-phase
condition (liquid and vapor). The electronic expansion valve
operates through an electronically controlled activation of a
stepper motor. The stepper motor stays in position unless pow-
er pulses initiate the two discrete sets of motor stator windings
for rotation in either direction. The direction depends on the
phase relationship of the power pulses.
The motor directly operates the spindle, which has rotating
movements that are transformed into linear motion by the
transmission in the cage assembly. The valve cone is a V-port
type which includes a positive shut-off when closed.
The large number of steps and long stroke results in very ac-
curate control of the refrigerant flow. The stepper motor has ei-
ther 4260 (main) or 2785 (economizer) steps.
MAIN EXV CONTROL — Each circuit has a thermistor lo-
cated in a well in the discharge line of the compressor (DGT)
and another one located in the compressor motor cavity (SGT).
Each circuit also has discharge and suction pressure transducer.
Discharge and suction pressure as measured by the transducers
are converted to saturated temperatures.
The main control logic for the EXV uses discharge super-
heat to control the position of the EXV. The difference between
the temperature of the discharge gas and the saturated dis-
charge temperature is the superheat. The EXV module controls
the position of the electronic expansion valve stepper motor to
maintain the discharge superheat set point.
The EXV control logic has several overrides, which are also
used to control the position of the EXV.
• Approach between SST and LWT
• Maximum Operating Pressure (MOP)
Approach
— 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 superheat 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 62 F (16.7 C) for standard water-cooled units,
and 55 F (12.8 C) for high condensing units. This makes it pos-
sible for the chiller to start at higher cooler fluid temperatures
without overloading the compressor. This is commonly re-
ferred 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.
The discharge superheat 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
Board (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 EXV CONTROL — The economizer EXV
is controlled by the circuit EXV board. There is an economizer
gas temperature thermistor and economizer pressure transducer
located in the line, which runs from the economizer assembly to
the compressor. The economizer pressure is converted to satu-
rated temperature and is used to calculate economizer superheat.
Economizer superheat equals economizer temperature minus
saturated economizer temperature. The economizer EXV only
operates during normal conditions when the capacity of the
circuit is greater than 70%. Once the capacity of the circuit is
6" MINIMUM
CLEARANCE FOR
THERMISTOR
REMOVAL
1.188 in.
2.315 in.
1/4-18 NPT
Fig. 46 — Dual Leaving Water Thermistor Well
(00PPG000008000A)
a30-4080
Fig. 47 — Typical Remote Space Temperature
Sensor (33ZCT55SPT) Wiring
7
8
TB6
SEN
SEN
SENSOR
a30-4081
O-RING
BRASS NUT 3/8 - 24 FOR
ASSEMBLY ON BRASS WELL
Fig. 45 — 5K Thermistor
(30RB660036 Thermistor Kit)
a30-4079
MAIN EXV CONNECTOR
ECONOMIZER
EXV CONNECTOR
NAME PLATE
1
6
4
5
1
2
3
Fig. 48 — Economizer Assembly
LEGEND
EXV — Electronic Expansion Valve
1—Fuse Plug Adaptor
2—High Flow Access Fitting
3—Filter Drier
4—Main Expansion Valve
5—Economizer Expansion Valve
6—Brazed Plate Heat Exchanger
a30-4840