Classification of automotive air conditioning and refrigeration systems

At present, single-stage compressed steam refrigeration cycle system is mainly used,There are four main types. The common point is that they can prevent frosting of the evaporator:
1. Thermostatic expansion valve suction throttle system:
2. Reservoir valve combination system: reduce the exhaust volume by throttling
3. Clutch thermostatic expansion valve system: controlled by pressure switch or thermal switch
4. Clutch throttle pipe system.
(1) Thermostatic expansion valve suction throttle system
The suction throttle valve is between the evaporator outlet and the compressor inlet to control the evaporation pressure within the set value range.
working principle:
When the system works, the pressure at the evaporator outlet acts on the piston of the suction throttle valve, and then passes through the piston
A small hole in the acts under the diaphragm.
When the pressure is high enough to overcome the spring pressure, open the piston of the valve, the evaporation pressure decreases, and the spring pressure moves the piston to the closed position. The piston keeps opening and closing until the evaporation pressure and spring pressure balance. The spring preload here can be adjusted by adjusting the screw.
(2) Reservoir valve combination system
As shown in the figure, it is an integral component composed of suction throttle valve, thermal expansion valve and liquid receiver / dryer,
The accumulator valve assembly is connected with the inlet and outlet of the evaporator.
Working principle: when the system works, the liquid refrigerant flows from the condenser to the evaporator through the accumulator valve assembly. The liquid refrigerant flowing into the accumulator valve assembly falls to the bottom of the accumulator, removes water through the desiccant, enters the expansion valve for throttling and pressure reduction, and then flows to the evaporator. The refrigerant vapor returned from the evaporator flows to the suction throttle valve and enters the compressor. The suction throttle valve can control the evaporation pressure to stabilize within the specified value range.
(3) Clutch throttle pipe system
The difference between clutch throttle pipe system and clutch thermostatic expansion valve system is that it is replaced by throttle pipe
The expansion valve is installed. The throttle tube has simple structure and is not easy to be damaged. However, it can only play the role of throttling and depressurization, can not control the liquid supply of evaporator, and can not ensure the stability of evaporation pressure.
Working principle: when the vehicle speed increases, the compressor speed increases, the evaporation pressure decreases, and the evaporator surface frosts. At this time, the pressure switch or thermal switch cuts off the power supply of the clutch and stops the compressor. After the frost layer melts, the pressure switch or thermal switch is automatically connected and the compressor starts to run. The vapor compression refrigeration system is mainly composed of compressor, condenser, liquid expansion device and evaporator.
When the refrigeration system works, the refrigerant circulates in this closed system in different states. The refrigeration cycle flow of the automobile air conditioning system is shown in the figure
Refrigeration cycle is composed of four processes: compression, heat release, throttling and heat absorption.
1. Compression process
The compressor sucks the low-temperature and low-pressure refrigerant gas at the outlet of the evaporator, compresses it into high-temperature and high-pressure gas, and then sends it to the condenser. The main function of this process is compression and pressurization, so that the gas is easy to liquefy. During the compression process, the refrigerant state does not change, but the temperature and pressure rise continuously to form superheated gas.
2. Exothermic process
The superheated refrigerant gas with high temperature and high pressure enters the condenser (radiator) for heat exchange with the atmosphere. Due to the decrease of pressure and temperature, the refrigerant gas condenses into liquid and releases a lot of heat. This process is used for heat removal and condensation. The characteristic of the condensation process is that the state of the refrigerant changes, that is, it gradually changes from gaseous state to liquid state under the condition of constant pressure and temperature. The condensed refrigerant liquid is a high-pressure and high-temperature liquid. When the refrigerant liquid is supercooled, the greater the degree of supercooling, the greater the evaporation and heat absorption capacity in the evaporation process, and the better the refrigeration effect, that is, the cooling capacity increases accordingly.
3. Throttling process
The high-pressure and high-temperature refrigerant liquid is throttled, cooled and depressurized by the expansion valve, and discharged from the expansion device in the form of mist (small droplets).
The function of this process is to reduce the temperature and pressure of the refrigerant, and quickly change from high-temperature and high-pressure liquid to low-temperature and low-pressure liquid, so as to facilitate heat absorption, control the refrigeration capacity and maintain the normal operation of the refrigeration system.
4. Endothermic process
The mist refrigerant liquid cooled and depressurized by the expansion valve enters the evaporator. Because the boiling point of the refrigerant is far lower than the temperature in the evaporator, the refrigerant liquid evaporates and boils into gas in the evaporator. Absorb a large amount of surrounding heat during evaporation
Reduce the temperature in the car. Then, the refrigerant gas with low temperature and low pressure flows out of the evaporator and waits for the compressor to suck in again. The characteristic of endothermic process is that the refrigerant state changes from liquid state to gaseous state, and the pressure remains unchanged at this time, that is, the change of this state is carried out in the process of constant pressure.
If the above process is carried out repeatedly, the temperature in the car can reach and maintain a given state.