Diagnosing A/C Systems : Inefficient Compressor

 Overview:

• An inefficient compressor refers to a compressor that functions electrically but has mechanical issues, such as leaking valves or worn rings, which prevent it from properly compressing refrigerant. This issue affects both thermostatic expansion valve (TEV) systems and fixed bore metering devices.

Key Indicators of an Inefficient Compressor:

1. Condensing Temperature and High Side Pressure:

   • Condition: Lower than normal.
   • Reason: The compressor cannot properly increase discharge pressure due to internal mechanical failures, leading to lower condensing temperatures and pressures.

2. Subcooling:

   • Condition: Remains relatively normal or slightly lower.
   • Reason: Most refrigerant remains in the condenser, so subcooling is not a primary indicator for this issue.

3. Evaporator Temperature and Pressure:

   • Condition: Higher than normal.
   • Reason: Leaking discharge pressure flows back into the suction side through faulty valves, increasing suction pressure and evaporator temperature.

4. Superheat:

   • Condition: Higher than normal.
   • Reason: The evaporator is not receiving enough refrigerant, causing the remaining refrigerant to boil off quickly, raising superheat.

Example: Inefficient Compressor on a 10-SEER Air Conditioning System with R22:

• Normal Operating Conditions:

  • Low side pressure: 69 PSIG → 40°F evaporating temperature.
  • High side pressure: 278 PSIG → 125°F condensing temperature.
  • Ambient air: 95°F, resulting in a 30°F condenser split.
  • Subcooling: Approximately 10°F.

• Inefficient Compressor Conditions:

  • Condensing Side:

    • High side pressure decreases to 196 PSIG → 100°F condensing temperature.
    • Condenser split decreases to 5°F (100°F condensing temperature - 95°F ambient air).
    • Subcooling decreases to 5°F, though this is not typically used as an indicator for diagnosing compressor issues.

  • Evaporator Side:

    • Low side pressure increases to 144 PSIG → 80°F evaporating temperature.
    • Evaporator temperature difference (TD) drops to 0°F (80°F return air - 80°F evaporator temperature), resulting in no heat transfer.
    • Superheat increases to 25°F, indicating insufficient refrigerant is reaching the evaporator.

Unique Characteristics of an Inefficient Compressor:

• High Suction Pressure and Low Discharge Pressure: This is the only malfunction where you will observe both high suction pressure and low discharge pressure simultaneously, due to internal leaks in the compressor.

Diagnostic Tools:

• Amperage Measurement: Use a clamp-on ammeter to measure the compressor's amperage. An inefficient compressor typically shows lower amperage than expected because the compressor is not working as hard. For example, if a Copeland compressor should draw 29 amps under specific conditions (130°F condensing temperature, 20°F evaporating temperature), an inefficient compressor might draw significantly less.

• Factory Performance Charts: Compare actual operating conditions and compressor amperage against factory performance charts to verify the diagnosis. These charts provide expected values based on condensing and evaporating temperatures and can help confirm if the compressor is underperforming.

• Consult Factory Representatives: If needed, reach out to the manufacturer or factory representative for additional guidance on diagnosing and verifying an inefficient compressor.