Metering Devices : Fixed Metering Devices

 Fixed Orifice Metering Devices:

• Functionality:

  • These metering devices control refrigerant flow based on the pressure difference across the device, specifically between the condensing and evaporator pressures.
  • They are less expensive than expansion valves but cannot maintain superheat, making them most effective on systems with constant loads.

• Types of Fixed Orifice Metering Devices:

  1. Accurator (Piston Type) Metering Device:

     • This device uses a small piston with a drilled hole to regulate the flow of refrigerant.
     • Operation: Liquid refrigerant enters, the pressure is reduced, and low-pressure liquid with some flash gas moves to the evaporator.

  2. Capillary Tube:

     • Regulates refrigerant flow by the pressure drop across the tube, which depends on the length and internal diameter of the tube.
     • Construction: The longer the tube and the smaller its internal diameter, the greater the pressure drop.
     • Example: Capillary tubes have small inside diameters and must be handled carefully to avoid bending or constricting them.

Handling and Maintenance:

• Common Issues:

  • Capillary tubes can become restricted by contaminants like desiccant beads or debris.
  • Solution: If blocked, cut off a small portion (1-2 inches) of the capillary tube, replace the filter dryer, evacuate, dehydrate the system to 500 microns, and then recharge it according to the manufacturer’s specified weight.

• Critical Charge Systems:

  • Fixed orifice systems operate with a critical refrigerant charge, meaning the charge must be precisely weighed using a digital scale.
  • The exact charge weight is listed on the unit's data plate.

• Clearing Blockages:

  • Sometimes blockages in capillary tubes can be cleared using a wire or specialized cleaning kits that include wires of various diameters to match the capillary tube size.
  • Special capillary tube cutters are available to cut and open the tube without causing damage.

• Capillary Tube Sizing:

  • Correct sizing is crucial for proper system performance.
  • Sizing charts are available to match the tube size to the system’s specifications.
  • For example, an R22 system with a 20-degree evaporator may require a TC 36 capillary tube with a specific inside diameter.

Special Tools:

• Capillary Tube Cutter: A tool designed to cut capillary tubes without causing burrs or damage.
• Pin Gauge: A tool used to measure the inside and outside diameters of capillary tubes, ensuring the correct size is used for the application.