Metering Devices : Automatic and Electronic Expansion Valves

 Automatic Expansion Valves (AEV):

• Functionality:

  • The AEV maintains evaporator pressure and prevents the evaporator from getting too cold, functioning similarly to a pressure regulator.
  • It stops the pressure from getting too low, making it suitable for systems with fairly constant heat loads, such as small chilled water systems, soft serve ice cream machines, slush drink machines, and some flaked ice machines.

• Operation:

  • AEVs operate in reverse compared to other metering devices.
  • When the heat load increases and pressure rises, the AEV closes to maintain pressure, which can starve the evaporator.
  • When the load decreases, the AEV opens to maintain pressure, potentially flooding the evaporator.
  • This makes AEVs less suitable for applications with varying heat loads.

• Adjustment:

  • AEVs have an adjustable cap that allows for setting the desired evaporator pressure by turning the adjustment knob.

Electronic Expansion Valves (EEV):

• Technology:
  • EEVs are becoming more common due to advances in technology.
  • They are controlled by a computer board that uses an electric stepper motor to open and close the valve based on variable voltage inputs.
• Benefits:
  • EEVs offer high accuracy in metering refrigerant, resulting in extremely efficient evaporator performance.
• Components:
  • EEVs include an electronic stepper motor, a needle and orifice for metering, and typically have a strainer that may or may not be removable for cleaning.
• Controller and Sensors:
  • Controller: Receives input from sensors and provides a variable output voltage to the EEV motor.
  • Sensors:
    • Thermistor (Temperature Sensor): Measures suction line temperature at the evaporator outlet. It is typically an NTC (Negative Temperature Coefficient) thermistor, meaning resistance decreases as temperature increases.
    • Pressure Transducer: Senses low-side pressure and sends this information back to the controller.
• Operation:
  • The controller calculates superheat using the input from the thermistor and pressure transducer and then positions the EEV accordingly.
  • The EEV can also be used for pump-down control, as cutting power to the valve causes it to close, eliminating the need for an additional solenoid valve.

System Diagram:

• The EEV is shown with liquid refrigerant entering at the bottom and metering into the evaporator.
• The thermistor and pressure transducer are placed at the evaporator outlet, feeding information back to the controller.
• The controller then adjusts the EEV to maintain proper superheat and refrigerant flow.