10 Expansion Valves, Driers

Thermostatic Expansion Valve (TEV/TXV)

The Expansion Valve is the “brain” of the refrigeration cycle, responsible for metering refrigerant flow into the evaporator.

• Primary Function: To reduce the pressure of liquid refrigerant (causing “flash gas”) and maintain a constant superheat at the evaporator outlet.

• Location: Installed at the inlet of the evaporator, between the liquid line and the evaporator coil.

• Key Components: * Power Assembly: Contains the sensing bulb and diaphragm.

  • Sensing Bulb: Attached to the evaporator outlet; senses suction line temperature.

  • Orifice: The narrow opening that creates the pressure drop.

• Working Principle: It balances three pressures:

  1. Bulb Pressure ($P_1$): Acts to open the valve.

  2. Evaporator Pressure ($P_2$): Acts to close the valve.

  3. Spring Pressure ($P_3$): Acts to close the valve (adjustable).

• Superheat: Usually maintained between $5^\circ\text{C}$ to $10^\circ\text{C}$ to ensure no liquid refrigerant returns to the compressor (slugging).

• Types: * Internally Equalized: Used in small systems with low pressure drop.

  • Externally Equalized: Used in large systems or where the evaporator has a high pressure drop.

Filter Drier (Liquid Line Drier)

The Filter Drier is the “guardian” of the system, ensuring the refrigerant remains chemically pure.

• Primary Function: To remove moisture, acid, and solid contaminants (dirt, metal chips) from the refrigerant.

• Location: Installed in the liquid line, usually after the receiver and before the expansion valve.

• Desiccants (Drying Agents):

  • Molecular Sieve: Highly effective at picking up water molecules.

  • Activated Alumina: Best for removing acids caused by motor burnouts or oil breakdown.

  • Silica Gel: Older technology, less common in modern high-pressure systems.

• Structure:

  • Perforated Plates/Screens: Filter out physical debris.

  • Solid Core or Loose Beads: The desiccant material that absorbs moisture.

• Identification of Issues:

  • Temperature Drop: If there is a significant temperature difference between the inlet and outlet of the drier, it is restricted (clogged).

  • Sight Glass: Often placed after the drier; bubbles indicate low charge or a restriction in the drier.

• Types:

  • Bi-directional: Used in Heat Pumps (works in both cooling and heating modes).

  • Suction Line Drier: Larger driers installed after a compressor burnout to protect the new compressor.

Quick Revision Points for RAC Exams