Evaporators: Evaporator Temperature and TD

 Evaporator Temperature:

• Definition: The boiling or evaporating temperature of the refrigerant inside the evaporator tubing or coil.
• Measurement:
  • Measure suction pressure with a low-pressure gauge or compound gauge.
  • Refer to the gauge face or use a pressure-temperature chart to find the corresponding evaporator temperature.
• Examples:
  • 69 psig (R-22): Corresponds to a 40°F evaporating temperature.
  • 49 psig (R-22): Corresponds to a 25°F evaporating temperature.

Evaporator Temperature Difference (TD):

• Definition: The difference between the space (conditioned) temperature and the evaporator temperature.
• Importance: Different systems are designed with specific temperature differences. Knowing the design TD helps determine what the evaporator temperature should be.
• Examples:
  • Air Conditioning: Space temperature of 75°F with an evaporator temperature of 40°F results in a 35°F TD.
  • Reach-In Cooler: Space temperature of 40°F with an evaporator temperature of 20°F results in a 20°F TD.
  • Walk-In Cooler: Space temperature of 35°F with an evaporator temperature of 25°F results in a 10°F TD.

Refrigeration and Humidity:

• Dehumidification: All evaporators remove moisture from the refrigerated space.
  • Desirable: In air conditioning for comfort.
  • Undesirable: In refrigeration, excessive moisture removal can lead to product damage (e.g., freezer burn).
• Humidity Levels Based on TD:
  • Air Conditioning (35°F TD): 50% humidity.
  • Reach-In (20°F TD): 65% humidity.
  • Walk-In (10°F TD): 85% humidity.
  • Impact: Lower TD results in higher humidity, retaining moisture in products (e.g., produce). Higher TD removes more moisture, lowering relative humidity, which is beneficial for comfort but can lead to spoilage in refrigeration systems.

Example of Temperature Difference Affecting Humidity:

• Scenario:
  • 20°F TD: 75°F air and 55°F water, less condensation.
  • 35°F TD: 75°F air and 40°F water, more condensation (sweating on the glass).
  • Explanation: Greater temperature difference increases condensation, demonstrating the effect of TD on humidity.