Leak Check and Evacuation) Why Do We Evacuate?

 

1. Importance of Evacuation

• Purpose:
  • Remove Moisture: Prevents acid and sludge formation inside the refrigeration system.
  • Eliminate Non-Condensables: Such as air, which can affect system efficiency and performance.

2. Why Moisture is Bad

• Chemical Reactions:
  • Moisture can react with refrigerant and oil to form acids.
  • These acids can cause corrosion and damage to system components.
• Formation of Sludge:
  • Sludge can clog and obstruct the refrigerant flow, reducing system efficiency.

3. Understanding Non-Condensables

• Definition: Gases that do not condense within the operating temperature and pressure range of the system.
• Example: Air (primarily nitrogen).
  • Condensation of Nitrogen: Only occurs at extremely low temperatures (-320°F at atmospheric pressure), not achievable in typical HVAC systems.
• Impact on System:
  • Non-condensables take up space in the condenser, reducing the area available for refrigerant condensation.
  • This increases head pressure and lowers operating efficiency.

4. Principles of System Evacuation

• Basic Measurements:

  • Sea Level: 0 PSIG equals 29.92 inches of mercury (Hg) or 760,000 microns.
  • Inches of Mercury (Hg): A common measure for vacuum levels.
  • Microns: A finer measurement used for deep vacuums, essential for precise evacuation.

• Lowering Pressure:

  • Reduces the boiling point of water, allowing moisture to evaporate and be removed by the vacuum pump.

5. Evacuation Process and Measurement

• Vacuum Levels:

  • 1 inch Hg: Equals 25,400 microns.
  • 500 microns: The target level for most manufacturers, ensuring effective moisture removal.

• Evacuation Steps:

  • Start at High Pressure: Gradually lower the pressure to avoid boiling the oil in the vacuum pump.
  • Monitor with Micron Gauge: Provides accurate measurement of vacuum levels.

6. Illustrative Example: System Evacuation and Boiling Point

• Scenario:

  • Vacuum Chamber: Contains water at 80°F.
  • At Atmospheric Pressure (29.92 inches Hg): Water boils at 212°F.

• Step-by-Step Process:

  • Pulling Vacuum: Connect vacuum pump and start reducing pressure.
  • 1 inch Hg (25,400 microns): Water boiling point drops to 80°F.
  • 1000 microns: Water boils at 1°F.
  • 500 microns: Water boils at -12°F.
  • 250 microns: Water boils at -24°F, ensuring rapid moisture removal even in cooler environments.

7. Practical Considerations

• Vacuum Pump Oil:

  • Use fresh, dehydrated oil to ensure efficient evacuation.
  • Change oil regularly to prevent moisture saturation.

• Ensuring Complete Evacuation:

  • Monitor Micron Levels: Ensure they stay low after isolating the vacuum pump.
  • Look for Rapid Moisture Boiling: Indicates effective moisture removal.