Evaporators: Effects of Heat Load

 Heat Load Impact:

• High Heat Load:
  • Increases evaporator temperature and suction pressure.
• Low Heat Load:
  • Lowers evaporator temperature and suction pressure (often due to low airflow).
• Superheat:
  • Affected by load, but depends on the load amount and the type of metering device used.

Evaporator Temperature Difference (TD):

• Consistency:
  • TD should remain constant regardless of load, provided there is an adequate supply of saturated refrigerant in the evaporator.
• Example:
  • For an air conditioning evaporator with a TD of 35°F:
    • High Load: 85°F return air = 50°F evaporator temperature (85°F - 35°F).
    • Low Load: 65°F return air = 30°F evaporator temperature (65°F - 35°F).

Example with R-22 Air Conditioning System:

• Startup Condition:
  • High Space Temperature: 85°F return air causes a higher evaporator temperature, quick refrigerant boiling, and early superheat.
  • Result: High superheat (25°F), but TD remains 35°F.
• Reduced Load:
  • Cooling Space: As return air temperature drops to 80°F, more refrigerant fills the evaporator, reducing superheat (15°F). TD remains 35°F.
• Design Condition:
  • Target Space Temperature: 75°F return air, evaporator is fully saturated with refrigerant, and superheat normalizes to 10°F. TD remains constant at 35°F.