Metering Devices: Superheat and Multi-Circuited Evaporators

 Superheat Overview:

• Definition: Superheat is the difference between the suction line temperature and the evaporating temperature.
• Importance:
  • High Superheat: Indicates the evaporator is starving of refrigerant, leading to inefficient operation.
  • Low/No Superheat: Indicates the evaporator is flooding, which can lead to liquid floodback and potential compressor damage.

Example of Superheat Measurement:

• System: Freezer evaporator using R-404A refrigerant.
• Steps:
  1. Measure suction line temperature at the TXV bulb location (without touching the bulb directly).
  2. Attach a pressure gauge to a suction line fitting and measure suction pressure.
  3. Convert the suction pressure to the corresponding evaporating temperature using a temperature-pressure chart.
  4. Subtract the evaporating temperature from the suction line temperature to determine superheat.
  • Example Calculation: Suction line temperature = 18°F, Suction pressure = 36 psig (corresponding to a 3°F evaporating temperature), Superheat = 18°F - 3°F = 15°F.

Adjusting Superheat:

• To Lower Superheat:

  • Goal: Decrease superheat from 10°F to 6°F.
  • Method: Decrease spring pressure by turning the adjustment stem counterclockwise (backing out) one-quarter turn at a time, waiting 10-15 minutes between adjustments.
  • Result: Lower suction line temperature, indicating increased refrigerant flow.

• To Increase Superheat:

  • Goal: Increase superheat from 6°F to 14°F.
  • Method: Increase spring pressure by turning the adjustment stem clockwise (inward) one-quarter turn at a time, waiting 10-15 minutes between adjustments.
  • Result: Higher suction line temperature, indicating decreased refrigerant flow.

External Equalization in TXVs:

• Purpose: Necessary for multi-circuit evaporators where pressure drops occur within the evaporator.
• Function: An externally equalized TXV senses pressure at the evaporator outlet, accounting for pressure drops and ensuring proper refrigerant flow.
• Comparison:
  • Internally Equalized TXVs: Sense pressure at the evaporator inlet, leading to potential starving of the evaporator if pressure drops are significant.
  • Externally Equalized TXVs: Sense pressure at the evaporator outlet, providing accurate control of refrigerant flow.

Example of External Equalization:

• Misapplied Internal Equalization:
  • Example system with a 40°F evaporator using an internally equalized TXV shows significant superheat (38°F) due to pressure drop, indicating the evaporator is starving.
• Proper External Equalization:
  • Using an externally equalized TXV, pressure is accurately sensed at the evaporator outlet, resulting in proper superheat (10°F) and efficient operation.