1. Definition and Function:
- The metering device controls the amount of liquid refrigerant entering the evaporator.
- It supplies refrigerant in a regulated amount to enhance the system's cooling effect.
- Lowers the pressure and temperature of the refrigerant to its boiling point as it enters the evaporator coil.
2. Common Types:
- Thermostatic Expansion Valve (TXV or TEV):
- Senses suction temperature via a bulb at the evaporator coil outlet.
- Opens or closes the valve based on superheat, maintaining balance.
- Includes a tube and external equalizer to measure evaporator outlet pressure.
- Electric/Electronic Expansion Valve (EEV):
- Electronically controlled based on outside inputs.
- Despite higher initial costs, improves system efficiency and saves energy.
- Allows lower superheat settings without risking liquid floodback.
3. Criteria for Proper Functioning:
- Correct valve size.
- Good thermal contact of the sensing bulb with the suction line.
- Full column of liquid sent to the valve with no flashing.
4. Common Failures and Troubleshooting:
- Thermostatic Expansion Valve (TXV):
- Rarely fails, with most malfunctions being electrical.
- Possible issues include broken sensing bulb or transmission tube.
- Test by holding the sensing bulb in hand; if it doesn't open the valve, it's malfunctioning.
- Improper or loose placement of the bulb can cause problems.
- Contamination or moisture in the system can cause the valve to stick (e.g., ice freezing in the valve).
- Failure in the open position causes overfeeding (flooded evaporator).
- Failure in the closed position causes starvation (starved evaporator).
- Overheating or loss of charge from the powerhead can occur.
- Misdiagnosis is common; other components may be the actual issue.
5. Importance:
- Regulates the flow of refrigerant within the refrigeration cycle.
- Proper installation and maintenance are crucial for system efficiency.
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