Evaporators: Evaporator Functions

 Development of Evaporators:

• Early Designs:
  • Originally, evaporators were just pipes with fins to increase surface area for heat transfer.
  • Early evaporators relied on natural convection (gravity coils) for air movement.
• Advancements:
  • The addition of fans or blowers significantly improved heat transfer.
  • Circuiting of the coil further increased efficiency.

Circuiting of Evaporators:

1. Single Circuit Evaporator:

   • Design: Low-pressure refrigerant enters from one end and passes through the entire coil in a series loop.
   • Issues: Can cause significant pressure drops, leading to malfunctions such as high compression ratios and low suction pressures.

2. Multi-Circuited Evaporator:

   • Design: High-pressure liquid refrigerant is fed to the metering device (e.g., TXV) and then distributed to multiple circuits within the coil.
   • Advantages: Substantially decreases pressure drop, resulting in better evaporator efficiency.

Function of the Evaporator:

• Primary Function: Absorb heat from the space.
• Operation: Warm air enters the evaporator coil, where refrigerant at reduced pressure and temperature boils and absorbs heat. Cool air exits the coil.

Evaporator Terminology:

1. Latent Heat: Heat added or removed that results in a change of state but no change in temperature (e.g., refrigerant boiling in the evaporator).
2. Sensible Heat: Heat that causes a measurable change in temperature but no change in state (used to measure superheat).
3. Saturated Refrigerant: Refers to liquid and vapor together during the change of state in the evaporator.
   • Saturation Temperature: The temperature at which refrigerant changes state at a specific pressure.
4. Evaporator Temperature: The saturation temperature or boiling point of the refrigerant within the evaporator coil.
5. Suction Temperature: The temperature of the suction line at the outlet of the evaporator or the inlet of the compressor.

Basic Evaporator Function Example:

• Return Air Temperature: 75°F enters the evaporator coil in an air conditioning application.
• Exit Air Temperature: 55°F leaves the coil.
• Refrigerant Saturation Temperature: Starts at 40°F and remains until fully evaporated.