Expansion Valve Introduction

28-Nov-2025 4Comments

Expansion Valve Introduction

Thermal expansion valves are crucial automatic control components in refrigeration systems, typically installed between the receiver and evaporator. They utilize temperature changes in the gas chamber head (temperature sensor) as a signal to adjust the valve opening, altering the refrigerant flow rate. This allows the medium-temperature, high-pressure refrigerant to pass through the valve, becoming low-temperature, low-pressure wet vapor. The refrigerant then absorbs heat in the evaporator, achieving a cooling effect.

Expansion Valve Components and Functions

1. Valve Body: Provides the refrigerant flow path, the connection interface to the system, and the mounting location for internal components. It is made of aluminum or brass.

2. Diaphragm: Functions as a diaphragm. Transmits the force from the air box head to the transmission plate with minimal resistance. Its characteristics are determined by its material, condition, thickness, and corrugated shape, and it is generally made of stainless steel or copper alloy.

3. Air Box Cover: Together with the air box seat, diaphragm, temperature sensor, and capillary tube, it forms the air box head. It is made of stainless steel or brass.

4. Temperature Sensor: Senses the external temperature and transmits the pressure converted from the temperature to the air box head. It is made of copper.

5. Capillary Tube: Transmits the pressure from the temperature sensor or provides a charging channel. It is made of copper.

6. Transmission Plate: Transmits the force from the diaphragm to the transmission rod and provides axial and radial limiting. It is made of aluminum or brass.

7. Air Box Seat: Connects to the valve body, houses the transmission plate, and together with the air box cover, diaphragm, temperature sensor, and capillary tube, forms the air box head. It is made of stainless steel.

8. Transmission Rod: Transmits the force from the diaphragm to the valve core. The valve consists of an upper drive rod and a lower drive rod. Some products have a steel ball spot-welded onto the lower drive rod.

9. O-ring: Seals the drive rod and valve body without creating excessive frictional resistance.

10. Valve core: Controls the flow area of the valve port under the action of the air box head and spring force. Material: stainless steel or brass.

11. Adjusting spring: Provides a certain preload according to the static assembly overheat requirements and provides a force to balance with the air box head. Note that the spring stiffness, pitch, and parallelism of the two end faces have a significant impact on the product. Material: stainless steel or spring steel.

12. Adjusting screw: Adjusts the preload of the spring.

13. O-ring: Seals the adjusting screw and valve body without affecting the adjustment performance.

The Role of Expansion Valves in Refrigeration Systems:

① Throttling and Pressure Reduction: The expansion valve throttles and reduces the pressure of the high-temperature, high-pressure refrigerant after condensation in the condenser, transforming it into a low-temperature, low-pressure vapor-liquid mixture that easily evaporates. This mixture then enters the evaporator to evaporate and absorb heat from the surrounding environment.

② Flow Regulation: Based on the temperature signal obtained from the temperature sensor or gas chamber head, the expansion valve automatically adjusts the refrigerant flow rate into the evaporator to adapt to constantly changing refrigeration loads.

③ Maintaining a Certain Superheat and Preventing Liquid Slugging and Abnormal Overheating: By regulating the flow rate, the expansion valve ensures a certain degree of superheat in the evaporator, guaranteeing the effective utilization of the evaporator's total volume and preventing liquid refrigerant from entering the compressor and causing liquid slugging. Simultaneously, it controls the superheat within a certain range to prevent abnormal overheating.

General Selection Methods for Thermostatic Expansion Valves:

1) Determine the refrigerant type of the system.

2) Determine the evaporation temperature, condensation temperature, and refrigeration capacity of the evaporator.

3) The pressure difference between the inlet and outlet of the thermostatic expansion valve.

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