Introduction to Commercial Refrigeration Systems : Metering Devices

 Modulating Metering Devices:

1. Thermostatic Expansion Valve (TXV):
   • Operation: Uses a bulb partially filled with refrigerant attached to the suction line at the evaporator's outlet.
   • Function: Senses refrigerant temperature leaving the evaporator and adjusts the valve opening to maintain constant superheat.
   • Design: Specific to refrigerant capacity and application; most are field adjustable, though some manufacturers offer non-adjustable versions to prevent unnecessary adjustments.
2. Automatic Expansion Valve (AXV):
   • Operation: Uses a diaphragm as the sensing element, without a sensing bulb.
   • Function: Opens on a drop in evaporator pressure and closes on an increase, maintaining constant pressure in the evaporator but not constant superheat.
   • Application: Commonly used in systems with a constant load, like lobster tanks and drinking fountains.
3. Electronic Expansion Valve (EEV):
   • Operation: Offers precise control of refrigerant flow using a separate controller and a stepper motor.
   • Function: The motor opens and closes the valve ports, regulating refrigerant flow into the evaporator.

Fixed Bore Metering Devices:

1. Piston Metering Device:

   • Design: Short body with a hole that determines flow at a specific pressure difference.
   • Operation: Susceptible to condenser pressure changes, which can cause low superheat in the evaporator when pressure increases.

2. Capillary Tube:

   • Design: Finely machined copper tubing with diameter and length determining pressure drop and refrigerant flow.
   • Operation: Does not control superheat or pressure, but controls refrigerant flow at a fixed rate.
   • Application: Often used in systems with a constant load or in small equipment.
   • Advantages: No moving parts, low potential for problems, doesn't wear out, and costs less than other metering devices.