Diagnosing A/C Systems : Refrigerant Restriction

 Overview:

• A refrigerant restriction typically occurs in the liquid line after the liquid receiver and condenser, often at the metering device or a blocked filter dryer. Such restrictions can significantly impact system performance.

Effects of a Refrigerant Restriction:

1. Evaporator Temperature:

   • Condition: Lower than normal.
   • Reason: Reduced refrigerant flow into the evaporator decreases pressure and temperature.

2. Superheat:

   • Condition: Higher than normal.
   • Reason: Limited refrigerant causes it to boil off quickly, increasing superheat.

3. Condensing Temperature:

   • Condition: Lower than normal.
   • Reason: Less refrigerant enters the evaporator, absorbing less heat, leading to lower condensing temperature and pressure.

4. Subcooling:

   • Condition: Relatively normal but may increase slightly.
   • Reason: Liquid refrigerant backs up in the condenser, increasing contact time and subcooling.

Example: Partially Restricted Metering Device 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.

• Restricted Conditions:

  • Evaporator Side:

    • Low side pressure decreases to 33 PSIG → 10°F evaporating temperature (too low for proper air conditioning).
    • Evaporator temperature difference (TD) increases to 70°F (80°F return air - 10°F evaporator temperature).
    • Superheat increases to 40°F due to reduced refrigerant flow, leading to quick boiling within the evaporator.

  • Condensing Side:

    • High side pressure decreases to 246 PSIG → 116°F condensing temperature.
    • Condenser split decreases to 16°F (116°F condensing temperature - 100°F ambient air).
    • Subcooling increases to 21°F (116°F condensing temperature - 95°F liquid line temperature) due to liquid refrigerant backing up in the condenser.

Key Takeaways:

• Low Evaporator Temperature and High Superheat: Indicators of refrigerant restriction, causing the evaporator to starve and overheat.
• Low Condensing Temperature and Pressure: Result from reduced heat absorption in the evaporator, lowering the high side pressure.
• Increased Subcooling: Liquid refrigerant backs up in the condenser, leading to more contact time and higher subcooling.

Prevention and Best Practices:

• Service Practices: Ensure proper evacuation and dehydration of the system using a micron gauge to reach a minimum of 500 microns.
• Filter Dryer Maintenance: Regularly check and replace filter dryers to prevent obstructions.
• Purge Hoses: When adding refrigerant, always purge hoses to prevent introducing air into the system.