Carrier HS070-160 manual Electronic Expansion Valve

The EXV test can be repeated r-rs required by pressing

may be due to out-of-calibration thcrmistorfs), or inter- mittent connections between processor board tcrn?inals

a n d EXV plug. Recheck all wiring connections ;tnd voltage signals.

Other possible causes of improper refrigerant flow control could be restrictions in liquid line. Check i‘or plugged filter drier(s), stuck liquid line solenoid valve(s) or restricted metering slots in the EXV. Formation of ice or frost on lower body of electronic expansion valve is one symptom of restricted metering slots. Clean or replace valve if necessary.

NOTE: Frosting of valve is normal during compressor Quick Test steps and at initial start-up. Frost should dissipate after 5 to 10 minutes operation of a system that is operating properly. If valve is to be replaced, wrap valve with a wet cloth to prevent excessive heat from damaging internal components.

EXV OPERATION - These valves control the flow of liquid refrigerant into the cooler. They are operated by

the processor to maintain 20 degrees F of superheat between the cooler entering refrigerant thermistor and the lead compressor entering gas thermistor (located between the compressor motor and the cylinders). There is one EXV per circuit. A cutaway drawing of valve is shown in Fig. IO.

4 -STEPPER MOTOR

3-LEAD SCREW 2 -PISTON

1 -ORIFICE ASSEMBLY

Fig. 10 - Electronic Expansion Valve

High-pressure liquid refrigerant enters valve through bottom. 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 refrlg- erant flow for different operating conditions, piston moves up and down over orifice, thereby changing orifice size. Piston is moved by a linear stepper motor. Stepper motor moves in increments and is controlled directly by processor module. As stepper motor rotates, motion is transferred into linear movement by lead screw. Through

stepper motor and lead screws, 760 discrete steps of n~otjc‘rn arc obtained. The large number of steps and long st~‘cdkt’ r*csults in very itccurate control of refrigerant flow.

B e c a u s e tht: EXVs art: wntmEItd hy rtw processor module, it is possible to track vaIvc pr>sition. During initial start-up, EXV is fully cloxcd. After start-up, v;rIkc position is tracked by processor by constantly thsct-ving amount of valve movement.

The processor keeps track of the EXV position by counting the number of open and closed steps it has sent to each valve. II has no direct phvsical feedback of valve position. Whenever the unit is switched from STANDBY to RUIN, both valves will be initialized. This means the processor will send enough closing pulses to the \,alve to move it from fully open to fully closed and then reset the position counter to zero.

The EXV open Quick Tests will send enough pulses to the valve to drive it from fully closed to fully open. The position of the valve at the start of the test has no effect on the number of puI$cs sent.

In the same manner, the EXV close Quick Tests will send enough pulses tr, rhe valve to drive it from fully open to fully closed,

When the EXV opens, the metering slots are not uncovered until step 160. This is fully closed position when the circuit is operating. The fully open position is

760steps.

T h e m STATr]subfunction shows the EXV valve

positions. They should ch,ange constantly while the unit operates. If a vrxlve should stop moving for any reason (mechanical or electrical) other than a processor or thermistor failure. the processor will continue to attempt to open <)r close the valve to correct the superheat. Once the calculated valve position reaches 160 (fully closed) or 760 (fully open) it will remain there. If the EXV posi- tion reading remains at 160 or 760 and the cooler and compressor refrigerant thermistor displays are reading the measured temperature correctly, the EXV is not moving. Follow the EXV checkout procedure to deter- mine the cause.

The EXV is also used to limit cooler suction tem- perature to 55 F (13 C). This makes it possible for the

chiller to start at higher cooler water temperatures with- out overloading compressor. This is commonly referred

to as MOP (maximum operating pressure).

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