What Is an Expansion Valve?

Expansion valve is an important part of the refrigeration system and is usually installed between the liquid storage tank and the evaporator. The expansion valve makes the medium-temperature and high-pressure liquid refrigerant throttling into low-temperature and low-pressure wet steam through its throttling, and then the refrigerant absorbs heat in the evaporator to achieve the cooling effect. The expansion valve controls the valve flow through the superheat change at the end of the evaporator to prevent it The evaporator area is underutilized and knocks the cylinder.

Chinese name: Expansion valve
Make up: Valve body, temperature sensor, balance tube

Features: Throttling, reducing pressure and regulating flow
Application: Cooling System

Expansion valve composition

Simply put, the expansion valve is composed of three parts: the valve body, the temperature sensor package, and the balance tube.

Expansion valve temperature package

The temperature-sensing bag is filled with refrigerant in a gas-liquid equilibrium saturation state. This part of the refrigerant is not connected with the refrigerant in the system. It is usually tied to the outlet pipe of the evaporator and is in close contact with the tube to sense the superheated steam temperature at the outlet of the evaporator. Since the refrigerant inside it is saturated, the pressure of the saturated state at the temperature is transmitted to the valve body.

Expansion valve balance pipe

One end of the balance tube is connected to the outlet of the evaporator slightly away from the temperature sensing bag, and is directly connected to the valve body through a capillary tube. The role is to transfer the actual pressure at the outlet of the evaporator to the valve body. There are two diaphragms in the valve body. The diaphragm moves upward under pressure to reduce the flow of refrigerant through the expansion valve, and seeks balance in dynamics.

Quality judgment of expansion valve

The ideal working condition of the expansion valve should be to change the opening degree and control the flow rate in real time as the evaporator load changes. However, in fact, due to the hysteresis in the heat transfer of the temperature felt by the temperature sensor, the response of the expansion valve is always half a beat. If we draw a time flow chart of an expansion valve, we will find that it is not a smooth curve, but a wave line. The quality of the expansion valve is reflected in the amplitude of the twists and turns. The larger the amplitude, the slower the response and the worse the quality of the valve.

Working principle of expansion valve

Structure and working principle of expansion valve ^[1]

1 The role of thermal expansion valve:
The thermal expansion valve is installed at the inlet of the evaporator, often called an expansion valve, and has two main functions:
1) Throttling effect: After the high-temperature and high-pressure liquid refrigerant is throttled through the orifice of the expansion valve, it becomes a low-temperature and low-pressure misty hydraulic refrigerant, creating conditions for the evaporation of the refrigerant;
2) Control the flow of the refrigerant: the liquid refrigerant entering the evaporator, after passing through the evaporator, the refrigerant evaporates from the liquid state to the gaseous state, absorbing heat and lowering the temperature inside the car. The expansion valve controls the flow of refrigerant to ensure that the outlet of the evaporator is completely gaseous refrigerant. If the flow is too large, the outlet contains liquid refrigerant, which may enter the compressor and cause liquid hammer. If the flow of the refrigerant is too small, the evaporation will be completed in advance, causing Insufficient cooling

2 Types of thermal expansion valves:
According to different balance methods, thermal expansion valves are divided into internal balance type and external balance type; external balance type thermal expansion valves are divided into F-type and H-type two structural types.
1) Structure and working principle of internal balanced expansion valve:

Structural drawing of internal balanced F-type thermal expansion valve. The temperature-sensing bag is filled with refrigerant and placed on the outlet pipe of the evaporator. The temperature-sensing bag is connected to the upper part of the diaphragm through a capillary tube, and the temperature of the refrigerant at the outlet of the evaporator is sensed. The pressure of the evaporator is felt under the diaphragm. If the air conditioning load increases and the evaporation of the hydraulic refrigerant in the evaporator is completed in advance, the temperature of the refrigerant at the outlet of the evaporator will increase, the pressure on the diaphragm will increase, and the valve stem will be pushed to increase the opening of the expansion valve and enter the evaporator. The increase of refrigerant flow increases the cooling capacity; if the load of the air conditioner decreases, the temperature of the refrigerant at the outlet of the evaporator decreases, and the valve opening degree is reduced according to the same principle, thereby controlling the flow of the refrigerant.

2) Structure and working principle of external balanced expansion valve:

The principle of the external balanced expansion valve is basically the same as that of the internal balanced expansion valve, with the following differences: The evaporator inlet pressure is felt under the diaphragm of the internal balanced expansion valve; the evaporator is felt under the diaphragm of the external balanced expansion valve. Outlet pressure.

3) H-type expansion valve

The H-type thermal expansion valve has four interfaces to connect to the refrigeration system. Two of them are the same as ordinary thermal expansion valves. One is connected to the liquid storage dryer and the other is connected to the evaporator inlet. The other two interfaces are connected to the evaporator outlet and one. Connect the compressor inlet. The temperature-sensing bag is directly in the refrigerant airflow at the evaporator outlet. The expansion valve eliminates the temperature-sensing bag, capillary tube and external balance connection in the F-type thermal expansion valve, which improves the adjustment sensitivity, has a compact structure, and is reliable in vibration resistance.

1 from filter dryer 2 to evaporator 3 from evaporator 4 to compressor 5 measuring hole 6 ball 7 spring 8 movable foot 9 refrigerant 10 film down pressure compensation 11 metal film 12 temperature sensing element

Expansion valve refrigeration system

In the refrigeration system, the expansion valve is one of the four major components of the refrigeration system [compressor, condenser, throttle (expansion valve), evaporator].

Throttle valve can also be called expansion valve or regulating valve.

Expansion valve features

1. The expansion valve adopts advanced dual-flow-balanced orifice.

2. The expansion valve uses a dual-flow thermal expansion valve, which saves the number of expansion valves, check valves and solenoid valves in the refrigeration system.

3. The bidirectional balanced flow port of the expansion valve makes the static superheat degree change with the change of the condensing pressure or the pressure drop through the valve port.

4. The expansion valve has a stable superheat degree, which makes the system run stably.

5. The expansion valve is suitable for various work needs such as refrigeration and air conditioning.

6. Expansion valve evaporation temperature range: -40 to + 10 .

7. The expansion valve is suitable for R12 (Freon), R22 (difluoromonochloromethane), R134a and other media.

Classification of expansion valves

From the balance point of view, the expansion valve can be divided into two types of internal balance and external balance.

The balance pressure of the internal balance type is taken at the inlet of the evaporator, and the balance pressure of the external balance type is taken at the outlet of the evaporator. Internal balance type expansion valve is generally used in domestic air conditioners. The cooling capacity of automobile air conditioners is generally large, and the pressure loss of the refrigerant in the evaporator is also large, so only the external balance type is used.

From the point of view of the superheat regulation structure, the expansion valve is divided into internal adjustment type and external adjustment type.

Internal adjustment can only be adjusted before the installation of the air conditioning system. Its advantages are small size and compact structure, which is suitable for batch shaped products. The external adjustment type can be adjusted at any time on the air conditioning system according to the situation, which has great flexibility, but the increase of parts makes the size and weight larger.