Carrier Air Conditioner specifications Refrigerant Feed Components - Each circuit has

Refrigerant Feed Components — Each circuit has

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

of O-ring grease to the housing seal O-ring before installing the motor canister. Reinstall the motor canister assembly. Tighten the motor nut to 15 to 25 ft-lb (20 to 34 N-m).

Check EXV operation using test functions described in the 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:

Green — moisture is below 45 ppm;

Yellow-green (chartreuse) — 45 to 130 ppm (caution); Yellow (wet) — above 130 ppm.

Change filter drier at first sign of moisture in system.

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.

FILTER DRIER — Whenever moisture-liquid indicator 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 1/4-in. Schrader connection for field charging. In combination with compressor discharge service valve, each circuit can be pumped down into the high side for servicing.

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.

STEPPER

MOTOR (12 VDC)

ORIFICE ASSEMBLY (INSIDE PISTON SLEEVE)

LEAD SCREW

PISTON SLEEVE

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

Fig. 30 — Electronic Expansion Valve (EXV)

		PL-EXVB
	1	BRN	A
	1
	2	WHT	E
	2
	3		D	EXV-B
J7	3	RED
	4	BLK	B
	4
	5	GRN	C
	5
		PL-EXVA
	1		A
	1	BRN
	2	WHT	E
	2
J6	3	RED	D	EXV-A
	3
	4	BLK	B
	4
	5	GRN	C
	5

ELECTRONIC EXPANSION VALVES (EXVs)

Fig. 31 — Printed Circuit Board Connector

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