Introduction to basic knowledge of refrigeration and air conditioning, must be read by beginners~

February 21, 2024

             Introduction to basic knowledge of refrigeration and air conditioning, must be read by beginners~ 


There are many methods of refrigeration, including gas expansion refrigeration, liquid gasification refrigeration, vortex tube refrigeration, and thermoelectric refrigeration. Among them, liquid gasification refrigeration is the most widely used, and its working principle is to use the heat absorption during liquid gasification to achieve refrigeration.

1. The working principle of air conditioning (mainly refrigeration, opposite heating):

The high-temperature and high-pressure refrigerant discharged by the compressor passes through a four-way valve and passes through a heat exchanger on the outside of the room. It is then transferred to the outdoor air for heat dissipation through heating, and the refrigerant changes from a gaseous state to a liquid state. It is then throttled and depressurized through a capillary tube (or solenoid valve). The refrigerant then passes through a one-way valve, a filter, a two-way valve (outdoor high-pressure pipe), and enters the indoor heat exchanger to absorb heat (achieve refrigeration). The refrigerant then passes through an outdoor three-way valve (outdoor low-pressure pipe) ——Four way valve - compressor. Complete a single loop refrigeration cycle.

2. Types of components in refrigeration systems:

The air conditioning refrigeration system mainly consists of four major components and some auxiliary equipment in the refrigeration system.

The four major components are:



Throttling mechanism

Evaporator composition.


Auxiliary equipment:

Oil separator;

Drying filter;

Liquid storage tank;

Electromagnetic four-way directional valve;

Electromagnetic valve; Check valve.

3. The working principle of each component

Compressor: As the "heart" of refrigeration systems, it has a significant impact on the operational performance, noise, energy consumption, vibration, and service life of air conditioning. According to thermodynamic principles, it can be divided into volume type and velocity type.

Volume type: Inhale a certain volume of gas into the cylinder and change the volume inside the cylinder to achieve gas compression and forced discharge.

Speed type: to achieve a certain speed for the inhaled gas, then slow down to convert kinetic energy into pressure energy and discharge it. The growth of gas is derived from the conversion of speed.


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Vortex compressor: composed of two vortex bodies meshing with the stator and rotor.

Rotary compressor: composed of a spindle with an eccentric wheel, cylinder, sleeve, air inlet, sliding plate, spring, exhaust valve, and exhaust port.

Condenser: The condenser transfers the cooling capacity of the evaporator in the refrigeration system, along with the power consumption of the compressor, to the environmental medium (cooling water or air). According to cooling, it can be divided into air-cooled, water-cooled, and evaporative. During refrigeration, heat is mainly dissipated to the outside, while during heating, heat is absorbed to the outside.

Evaporator: The main function of the evaporator is to boil and evaporate the low-temperature and low-pressure refrigerant liquid that has been throttled in the evaporator, absorbing the heat of the cooling medium, and then becoming low-pressure saturated (usually slightly overheated) vapor, which is continuously sucked away by the compressor. In other words, an evaporator is a heat exchange device that absorbs heat (i.e. outputs cooling).

Throttling element: Throttling element is one of the four main components in the refrigeration system. Its main function is to throttle and reduce pressure on high-pressure liquid refrigerant, ensuring the pressure difference between the condenser and evaporator, so that the refrigerant entering the evaporator absorbs heat and evaporates at the required low pressure, thereby achieving the purpose of refrigeration and cooling.


Adjust the refrigerant flow rate entering the evaporator to adapt to the load changes of the evaporator and ensure the normal operation of the system.

Capillary tube: It is a very thin tube with an inner diameter of usually 0.5-2.5mm and a length of 1-5m. When the refrigerant flows inside the pipe, it needs to overcome the resistance inside the pipe, which naturally generates a certain pressure drop.

Capillary characteristics: Due to being in a normally open state, when the compressor stops, the high and low pressures can quickly balance, which is conducive to the compressor starting again. Simple structure, convenient production, and affordable price. For the heat pump system, the refrigerant throttling process in the capillary tube, [Source: Refrigeration Encyclopedia official account], does not have import and export problems. The operating performance of the refrigeration system using capillary tubes is highly sensitive to the amount of refrigerant charged.


When the operating conditions of the refrigeration system change, the capillary tube's ability to regulate the refrigerant flow rate is poor. Due to the small inner diameter of the capillary tube and the long length of the pipeline, it is easy to be blocked. Therefore, attention should be paid to the dryness and cleanliness inside the system. Generally, a drying filter is installed in front of the capillary tube to prevent blockage.

Thermal expansion valve: a regulating valve that controls the liquid supply to the evaporator and is also a throttle valve for refrigeration devices. The thermal expansion valve regulates the flow rate by utilizing the change in superheat of the refrigerant at the outlet of the evaporator. According to the different structures, it can be divided into internal balance type and external balance type.

Disadvantages of thermal expansion valves: low control accuracy and limited adjustment range. The diaphragm deformation of the valve body is limited, resulting in a smaller range of valve needle opening changes, thus limiting the adjustment range of flow. Unable to meet the requirements for large flow regulation. The high-temperature gas at the location where the signal feedback lags behind the evaporator needs to first heat the outer shell of the temperature sensing package. The outer shell of the temperature sensing package has a large thermal inertia, which leads to reaction lag.


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Electronic expansion valve: The application of electronic expansion valve overcomes the shortcomings of thermal expansion valve and provides conditions for the intelligence of refrigeration equipment. The main method is to use the electrical signals generated by adjusting the parameters to control the voltage or current applied to the expansion valve, in order to achieve the purpose of regulation. However, electronic expansion valve control systems are complex and expensive.

Classification of electronic expansion valves:

Electromagnetic electronic expansion valve;

Electric electronic expansion valve.

4. Auxiliary equipment for refrigeration systems

Oil separator: The high-pressure gas (gaseous working fluid and lubricating oil) from the compressor enters the oil separator and separates the lubricating oil from the working fluid gas, flowing down the inner wall of the cylinder. The working gas is led out of the oil separator through a central tube through a porous baffle. The separated lubricating oil is concentrated in the lower part of the oil separator and can be discharged regularly, or the lubricating oil can be automatically returned to the crankcase of the compressor by using a float valve.

Drying filter: The function of a drying filter is to absorb moisture from the refrigeration system, block impurities in the system from passing through, and prevent ice and dirt blockages in the refrigeration system pipelines. Due to the fact that the most easily clogged part of the system is the capillary (or expansion valve), the drying filter is usually installed between the condenser and the capillary (or expansion valve).

Storage tank: During the operation of the refrigeration system, the refrigerant circulation in the system will change due to changes in operating conditions or adjustments to the refrigeration capacity. The liquid storage tank is set to balance and stabilize the refrigerant circulation volume in the system by using the liquid storage capacity of the liquid storage tank. [Source of this article: Refrigeration Encyclopedia official account], so that the refrigeration system is in normal operation. In addition, when a component in the refrigeration system malfunctions and needs to be repaired, certain operating methods can be used to recover the system refrigerant into the storage tank.

The storage tank is generally set between the condenser and the throttling element to allow the refrigerant liquid in the condenser to smoothly enter the storage tank. The position of the storage tank should be lower than the condenser.

Electromagnetic four-way directional valve: After the electromagnetic coil is energized, it changes the direction of action of the valve block in the valve body and changes the direction of refrigerant flow. So as to achieve the purpose of refrigeration or heating.


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Electromagnetic valve: An electromagnetic valve is usually installed in front of the expansion valve to directly control the amount of refrigeration entering the evaporator. It can also be installed before the unloading cylinder of the compressor to control the unloading of the compressor cylinder.

Divided into two types:

Direct opening and closing solenoid valve;

Indirect solenoid valve.

Check valve: A check valve is usually connected in parallel with an auxiliary capillary, and the two are then connected in series with the main capillary. A one-way valve is a tubular component with a movable spherical or hemispherical plug made of stainless steel or nylon material inside the pipe. Combined with capillary tubes, it can adapt to two different operating conditions: refrigeration and heating.

5. Types and states of refrigerants

Air conditioning refrigerants mainly include R22, R410A, and R32.

R22 is a medium temperature refrigerant with a standard solubility of -40.8 degrees Celsius in water. It dissolves with mineral oil and does not burn or explode. It has low toxicity and strong permeability.

R410A is a HFC refrigerant mixed by R32 and R125 at 50% and 50% mass fraction, [source of this article: refrigeration encyclopedia official account]. It is a near azeotropic mixture, and its thermodynamic properties are very close to that of single working medium. Compared with R22, the condensation pressure of R410A increases by 1.6 times, making it a high-pressure refrigerant that requires an increase in system pressure resistance.

R32, abbreviated as difluoromethane, is a coolant with zero ozone depletion potential. It is a gas at room temperature and a colorless transparent liquid under its own pressure. It is soluble in oil but insoluble in water. It is an energy-saving, carbon reducing, environmentally friendly, non-toxic refrigerant, but flammable and explosive!

6. Refrigerant state

Saturated refrigerant: In a saturated refrigerant, the gas-liquid conversion is in a state of equilibrium, and the liquid and gas are in a saturated state.

There is a one-to-one correspondence between saturation temperature and saturation pressure, and saturation pressure increases with the increase of saturation temperature. One of the two changes, and the other will also show corresponding changes. The refrigerant in the evaporator absorbs heat and the refrigerant in the condenser releases heat in the system, where the refrigerant is in a saturated state.

Superheat: The value at which the temperature of superheated steam exceeds the saturation temperature under a certain pressure.

Overheating: The temperature of the refrigerant vapor in the overheated refrigerant is higher than the saturation temperature at the corresponding saturation pressure, and overheating only occurs after the liquid refrigerant evaporates. In the case of overheating, there is no one-to-one correspondence between the temperature and pressure of the refrigerant gas.

Near the outlet of the evaporator, the refrigerant continues to absorb heat after complete evaporation and becomes an overheated refrigerant with a temperature higher than saturation. In the case of overheating, the refrigerant enters the compressor through the suction pipe.

Supercooling: Supercooling occurs when the temperature of the refrigerant liquid in the subcooled refrigerant is lower than the saturation temperature at the corresponding saturation pressure, and there is no steam in the flowing refrigerant. In the case of subcooling, there is no one-to-one correspondence between the temperature and pressure of the refrigerant liquid.

Between the outlet of the condenser and the throttle valve, the refrigerant may condense into a saturated liquid and continue to release heat and cool, becoming an undercooled refrigerant.






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