Refrigerant Feed Components; Electronic Expansion Valve (Exv); Moisture-Liquid Indicator; Filter Drier - Carrier Air Conditioner Operation And Service Manual

Refrigerant Feed Components —
all necessary refrigerant controls.
ELECTRONIC EXPANSION VALVE (EXV) — A cut-
away view of valve is shown in Fig. 30.
High-pressure liquid refrigerant enters valve through bot-
tom. A series of calibrated slots have been machined in side of
orifice assembly. As refrigerant passes through orifice, pressure
drops and refrigerant changes to a 2-phase condition (liquid
and vapor). To control refrigerant flow for different operating
conditions, a sleeve moves up and down over orifice and mod-
ulates orifice size. A sleeve is moved by a linear stepper motor.
Stepper motor moves in increments and is controlled directly
by EXV module. As stepper motor rotates, motion is trans-
ferred into linear movement by lead screw. Through stepper
motor and lead screw, 1500 discrete steps of motion are ob-
tained. The large number of steps and long stroke results in
very accurate control of refrigerant flow. The minimum posi-
tion for operation is 120 steps.
The EXV module controls the valve. The lead compressor
in each circuit has a thermistor located in the suction manifold
after the compressor motor and a thermistor located in a well
where the refrigerant enters the cooler. The thermistors mea-
sure the temperature of the superheated gas entering the com-
pressor cylinders and the temperature of the refrigerant enter-
ing the cooler. The difference between the temperature of the
superheated gas and the cooler suction temperature is the su-
perheat. The EXV module controls the position of the electron-
ic expansion valve stepper motor to maintain superheat set
point.
The superheat leaving cooler is approximately 3° to 5° F
(2° to 3° C), or less.
Because EXV status is communicated to the Main Base
Board (MBB) and is controlled by the EXV modules (see
Fig. 31), it is possible to track the valve position. By this
means, head pressure is controlled and unit is protected against
loss of charge and a faulty valve. During initial start-up, EXV
is fully closed. After initialization period, valve position is
tracked by the EXV module by constantly monitoring amount
of valve movement.
The EXV is also used to limit cooler saturated suction tem-
perature to 50 F (10 C). This makes it possible for the chiller to
start at higher cooler fluid temperatures without overloading
the compressor. This is commonly referred to as MOP (maxi-
mum operating pressure).
If it appears that EXV is not properly controlling circuit op-
eration to maintain correct superheat, there are a number of
checks that can be made using test functions and initialization
features built into the microprocessor control. See Service Test
section on page 29 to test EXVs.
NOTE: The EXV orifice is a screw-in type and may be
removed for inspection and cleaning. Once the motor canister
is removed the orifice can be removed by using the orifice
removal tool (part no. TS429). A slot has been cut in the top of
the orifice to facilitate removal. Turn orifice counterclockwise
to remove. A large screwdriver may also be used.
When cleaning or reinstalling orifice assembly be careful
not to damage orifice assembly seals. The bottom seal acts as a
liquid shut-off, replacing a liquid line solenoid valve. If the bot-
tom seal should become damaged it can be replaced. Remove
the orifice. Remove the old seal. Using the orifice as a guide,
add a small amount of O-ring grease, to the underside of the or-
ifice. Be careful not to plug the vent holes. Carefully set the
seal with the O-ring into the orifice. The O-ring grease will
hold the seal in place. If the O-ring grease is not used, the seal
O-ring will twist and bind when the orifice is screwed into the
EXV base. Install the orifice and seal assembly. Remove the
orifice to verify that the seal is properly positioned. Clean any
O-ring grease from the bottom of the orifice. Reinstall the ori-
fice and tighten to 100 in.-lb (11 N-m). Apply a small amount
of O-ring grease to the housing seal O-ring before installing the
Each circuit has
motor canister. Reinstall the motor canister assembly. Tighten
the motor nut to 15 to 25 ft-lb (20 to 34 N-m).
Service Test section on page 29.
MOISTURE-LIQUID INDICATOR — Clear flow of liquid
refrigerant indicates sufficient charge in system. Bubbles in the
sight glass indicate undercharged system or presence of non-
condensables. Moisture in system measured in parts per mil-
lion (ppm), changes color of indicator:
Change filter drier at first sign of moisture in system.
FILTER DRIER — Whenever
shows presence of moisture, replace filter drier(s). There is one
filter drier on each circuit. Refer to Carrier Standard Service
Techniques Manual, Chapter 1, Refrigerants, for details on ser-
vicing filter driers.
LIQUID LINE SOLENOID VALVE — All TXV units have
a liquid line solenoid valve to prevent liquid refrigerant migra-
tion to low side of system during the off cycle.
LIQUID LINE SERVICE VALVE — This valve is located
immediately ahead of filter drier, and has a
connection for field charging. In combination with compressor
discharge service valve, each circuit can be pumped down into
the high side for servicing.
58
Check EXV operation using test functions described in the
Green — moisture is below 45 ppm;
Yellow-green (chartreuse) — 45 to 130 ppm (caution);
Yellow (wet) — above 130 ppm.
IMPORTANT: Unit must be in operation at least
12 hours before moisture indicator can give an accurate
reading. With unit running, indicating element must be
in contact with liquid refrigerant to give true reading.
STEPPER
MOTOR (12 VDC)
ORIFICE ASSEMBLY
(INSIDE PISTON SLEEVE)
Fig. 30 — Electronic Expansion Valve (EXV)
PL-EXVB
1
1 BRN
2
2 WHT
3
3
J7 RED
4
4 BLK
5
5 GRN
PL-EXVA
1
1 BRN
2
2 WHT
3
J6 RED
3
4
4
BLK
5
5 GRN
ELECTRONIC EXPANSION VALVES (EXVs)
Fig. 31 — Printed Circuit Board Connector
moisture-liquid indicator
1
/ -in. Schrader
4
LEAD SCREW
PISTON SLEEVE
A
E
D
EXV-B
B
C
A
E
D
EXV-A
B
C