What Does an Expansion Valve Do?

Figure 1: An expansion valve.

Expansion valve is an important part of refrigeration system, which is generally installed between liquid storage tank and evaporator. Expansion valve plays an important role in daily life and is one of the key components of air conditioning. The air conditioner will not be able to absorb enough heat from the home without this valve.

1. Function

The expansion valve is installed at the inlet of the evaporator, also known as the thermal expansion valve, which has two main functions:

1.1 Throttling

The liquid refrigerant with high temperature and high pressure passes through the throttle orifice of the expansion valve. And then it becomes a low temperature and low pressure atomized hydraulic refrigerant creating conditions for the evaporation of the refrigerant.

1.2 Controlling the Flow of Refrigerant

The refrigerant evaporates from liquid to gaseous after the liquid refrigerant passing through the evaporator. Then it absorbs heat and reduces temperature. The expansion valve controls the flow of refrigerant to ensure that the outlet of evaporator is completely gaseous refrigerant. If the flow is too large, liquid refrigerant left at the outlet may enter the compressor to produce liquid hammer; If the refrigerant flow is too small and the evaporation is completed in advance, it may cause insufficient refrigeration.

2. The function of AC Electronic Expansion Valve

Figure 2: An AC electronic expansion valve.

2.1 Adjusting Flow

The AC electronic expansion valve controls the opening of the valve by sensing the change of refrigerant superheat at the outlet of the evaporator through the sensing bulb. So, it can adjust the refrigerant flow into the evaporator and make the refrigerant flow in the copper pipe matching the heat load of the evaporator.

When the heat load of the evaporator increases, the opening of the AC electronic expansion valve will also increase, then the refrigerant flow will also increase. On the contrary, the refrigerant flow will decrease.

2.2 Controlling Superheat

The AC electronic expansion valve has the function of controlling the superheat of refrigerant at the outlet of evaporator. It can not only ensure the full utilization of the heat transfer area of the evaporator, but also prevent the compressor from being damaged by liquid hammer during suction. So, the central air conditioning compressor will have a longer service life.

2.3 Throttling and Depressurizing

The AC electronic expansion valve can change the refrigerant saturated liquid at normal temperature and high pressure into refrigerant liquid at low temperature and low pressure. And it can also produce a little flash gas. The purpose of absorbing heat from the outside is realized after the pressure is reduced, and the heat absorbed in the room can be accurately measured.

2.4 Controlling Evaporation Level

The AC electronic expansion valve has the function of controlling the liquid level of evaporator. It can not only ensure that the heat transfer area of the evaporator can be fully utilized, but also prevent liquid carrying during suction and reduce the superheat of suction.

3. Function of the Expansion Valve in Automotive Air Conditioning System

Expansion valve is a key component of automotive air conditioning system. At present, there are two types of expansion valve (F-type expansion valve and H-type expansion valve) used in automotive air conditioning.

Figure 3: F-type expansion valve and H-type expansion valve.

The working process is the same, whether it is F-type expansion valve or H-type expansion valve. The expansion valve of automobile air conditioning is generally installed at the inlet of evaporator, which is the dividing point of high pressure and low pressure of automobile air conditioning refrigeration system.

3.1 How Does an Automotive Expansion Valve Work

After the supercooled liquid with high-temperature and high-pressure from the condenser enters the expansion valve, the throttling will achieve because of the reduction of the internal section of the expansion valve. And the refrigerant pressure will decrease. After leaving the expansion valve, the refrigerant fluid channel suddenly becomes larger, and the saturated liquid expands rapidly into low-temperature and low-pressure unsaturated liquid. It becomes a gas-liquid refrigerant to make the evaporator work in the best state.

3.2 3 Main Functions of Automotive Expansion Valve

1. Due to the inlet size of the expansion valve of automobile air conditioner is small, it can reduce the refrigerant pressure and has the function of throttling and reducing pressure.

2. The automotive expansion valve can automatically adjust the refrigerant flow according to the inlet pressure, and then the stability of temperature in the vehicle and the comfort of the whole vehicle will be improved.

3.Control refrigerant flow and prevent water hammer and abnormal overheating. The expansion valve uses the temperature sensing bag as the temperature sensing element to control the flow and ensure a certain degree of superheat at the outlet of the evaporator. This prevents the liquid refrigerant from hitting the compressor.

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