Evaporators: Superheat and Delta T

Temperature Difference (TD) vs. Delta T:

• Temperature Difference (TD): The difference between the air temperature entering the evaporator and the refrigerant boiling temperature inside the evaporator.
  • Usage: Mostly in commercial refrigeration applications.
• Delta T: The difference between the air temperature entering the evaporator and the air temperature leaving the evaporator.
  • Usage: Mostly in air conditioning applications.

Examples:

• Air Conditioning Evaporator:
  • TD Example: 75°F return air, 40°F refrigerant boiling temperature = 35°F TD.
  • Delta T Example: 75°F air entering, 55°F air leaving = 20°F Delta T.

Measuring TD and Delta T:

• Evaporator TD Measurement:
  • Measure suction pressure (e.g., 72 psig).
  • Convert to evaporating temperature using a pressure-temperature chart (e.g., 42°F).
  • Subtract the evaporating temperature from the return air temperature (e.g., 75°F - 42°F = 33°F TD).
• Delta T Measurement:
  • Drill small holes in the supply and return boxes.
  • Insert temperature probes and allow the unit to run.
  • Measure air temperature in return and supply (e.g., 75°F return air, 56°F supply air = 19°F Delta T).

Superheat:

• Definition: The measurable heat above the saturation temperature or the amount of sensible heat added to the vapor after evaporation.
• Importance: Verifies full evaporation, preventing floodback or liquid flooding that could damage the compressor.
• Superheat Locations:
  • Evaporator Superheat: Measured at the evaporator outlet.
  • Compressor (Total) Superheat: Measured at the compressor inlet.

Superheat Calculation Example:

• Evaporator Superheat:

  • Measure low-side pressure (e.g., 69 psig for R-22).
  • Convert to evaporating temperature (e.g., 40°F).
  • Place a thermometer on the suction line at the evaporator outlet (e.g., 50°F).
  • Subtract the evaporating temperature from the suction line temperature (e.g., 50°F - 40°F = 10°F evaporator superheat).

• Compressor Superheat:

  • Measure temperature at the compressor inlet (e.g., 60°F).
  • Total superheat is the difference between the evaporating temperature and compressor inlet temperature (e.g., 60°F - 40°F = 20°F compressor superheat).

Superheat Guidelines:

• Rule of Thumb: Average evaporator superheat is 10°F, with a range of 5-20°F depending on the application.
• High Superheat (Above 20°F): Indicates the evaporator is starving for refrigerant (boiling off too soon).
• Low Superheat: Indicates the evaporator is flooding, risking liquid refrigerant entering the suction line and damaging the compressor.