What Is Vapor-Compression Refrigeration?

The boiling temperature (saturation temperature) of a liquid changes with the pressure of the liquid. The lower the pressure, the lower the saturation temperature of the liquid; for example, the boiling temperature of 1Kg liquid R22 at a pressure of 0.584Mpa is 5 . Amount) is 201.246KJ / Kg; the boiling temperature at a pressure of 0.64MPa is 8 ° C, and the heat absorption (refrigeration capacity) is 198.695 KJ / Kg.

Absorbs heat as liquid evaporates

Compressor, condenser, expansion valve and evaporator are composed of four main parts. The refrigerant is used to change the physical state (gaseous state, liquid state) of the refrigerant in a closed system to achieve a continuous and stable set of refrigeration equipment.

Various parameters of the refrigeration cycle: (refrigerant R22)

Refrigerant working parameters in evaporator: gaseous state: pressure 0.64 Mpa; temperature 8 ;

Compressor outlet: Gaseous state: pressure 1.5 Mpa; temperature 85 ° C;

Parameters in the condenser: Liquid: pressure 1.5 Mpa; temperature 37 ° C;

Cooling water temperature: outlet temperature: 37 ; inlet temperature: 32 ;

Freezing water temperature: outlet temperature: 8 ; inlet temperature: 13 .

Due to different compressor models, the above parameters are also different.

1) Compressor:

Compressor classification: Piston compressor, screw compressor, centrifugal compressor, scroll compressor, etc.

2) Condenser and evaporator:

Generally it is a horizontal shell and tube type; plate heat exchangers developed in the 1990s have been adopted by some manufacturers on small refrigeration units.

3) Throttle expansion valve:

1) Function: reduce pressure and temperature; regulate flow.

2) Type:

A. Manual expansion valve

B. Thermal expansion valve: It consists of a temperature sensor bag and a diaphragm.

C. Float valve: keep the liquid level constant during evaporation.

D. Electronic expansion valve.

The low-pressure, low-temperature refrigerant vapor flowing from the evaporator is sucked by the refrigeration compressor, and compressed into high-pressure, high-temperature vapor by the compressor and discharged, so that the refrigerant vapor is divided into a high-pressure region and a low-pressure region. The high pressure zone from the discharge port of the compressor to the inlet end of the throttling element is called the high pressure or condensing pressure, and the temperature is called the condensing temperature. The low pressure zone from the outlet of the throttling element to the suction port of the compressor is called the low pressure or evaporation pressure, and the temperature is called the evaporation temperature. It is the pressure difference between the high pressure and the low pressure caused by the compressor that makes the refrigerant continuously flow in the system. Once the pressure difference between high and low pressure disappears, which is one of the balance between high and low pressure, the refrigerant stops flowing. The pressure difference between the high pressure region and the low pressure region and the magnitude of the pressure difference are completely the result of the compressor compressing the vapor. Once the compressor is capable of compressing the vapor, the pressure difference formed is very small, and the refrigeration cycle does not exist. The continuous operation of the compressor is achieved by consuming electrical or mechanical energy.

The vapor compression refrigeration cycle can be summarized into four processes:

Evaporation process of vapor compression refrigeration cycle

After the liquid refrigerant flows into the evaporator through the throttling element, it starts to boil and vaporize due to the decrease in pressure, and its vaporization (evaporation) temperature is related to pressure. In the process of liquid vaporization, the heat of surrounding media, such as water, air, or objects, is absorbed, and the temperature of these media is reduced due to the push of heat, which achieves the purpose of cooling. The vaporization of liquid is a gradual process, and finally all the liquid becomes dry saturated vapor, and then flows into the suction port of the compressor.

Compression process of vapor compression refrigeration cycle

In order to maintain a certain evaporation temperature, the refrigerant vapor must be continuously drawn out of the evaporator. The refrigerant vapor from the evaporator is sucked into the compressor and compressed into a high-pressure gas. During the compression process, the compressor consumes a certain amount of mechanical energy. The mechanical energy is converted into thermal energy in this process, so the temperature of the refrigerant vapor is increased, and the refrigerant vapor is overheated.

Condensing process of vapor compression refrigeration cycle

The high-pressure refrigerant vapor discharged from the refrigeration compressor releases heat in the condenser and transfers the heat to the surrounding medium-water or air, thereby gradually condensing the refrigerant vapor into a liquid. In the condenser, there are two basic conditions for the refrigerant vapor to emit heat to the medium: first, the temperature of the refrigerant vapor must be higher than the temperature of the surrounding medium, and the proper temperature difference must be maintained; For the amount of refrigerant vapor in the condenser, the condenser must have an appropriate tube length and area to ensure that the refrigerant vapor can be fully condensed in the condenser. ^[1]

Throttle process of vapor compression refrigeration cycle

The refrigerating liquid coming out of the condenser is decompressed to the evaporation pressure through a pressure reducing device (such as a water saving valve, an expansion valve, etc.). After throttling, the temperature of the refrigerant also drops to the evaporation temperature, and a part of flashing vapor is generated. The throttled airflow mixture enters the evaporator for the evaporation process.