Refrigeration

Chemical industry, utilizing refrigeration to achieve gas separation and gas cooling; Solidify one substance in the mixture to separate it from other substances; Low temperature storage of liquids; Provide appropriate temperature and pressure for the chemical synthesis process, remove reaction heat, etc. Petroleum cracking, synthetic rubber, synthetic plastic grease, fuel production, and fertilizer production require refrigeration; Natural gas liquefaction, dehydration, storage and transportation also require refrigeration. In mechanical manufacturing, low temperature treatment (- 70 ℃~- 90 ℃) on steel can change its metallographic structure, make austenite become martensite, and improve the hardness and strength of steel. In the steel industry, low-temperature dehumidification is required for blast furnace blast to reduce the coking ratio of molten iron and ensure its quality. During the assembly process of the machine, the interference fit between the parts can be easily carried out by using low temperature. Low temperature crushing technology utilizes the cold brittleness of materials at low temperatures to crush them. The main characteristics and applications of this technology are as follows: it can process high elastic materials that cannot be crushed at room temperature, such as recycling rubber from steel structure tires; Developing fine particles of food and Chinese herbal medicine; Processing and producing nanomaterials with the characteristics of green production.

In the construction industry, using the frozen soil method to excavate earthwork, build bridge foundations, and underground engineering can improve construction efficiency and ensure construction safety. Refrigeration is also applied to cool giant concrete blocks, eliminating the chemical reaction heat released during concrete solidification to avoid thermal expansion and concrete stress. 

Agriculture and animal husbandry, refrigeration is used for low-temperature treatment of crop seeds; Building artificial climate cultivation rooms; Preserve and handle excellent species and livestock breeds. During the freeze-drying process of certain foods, vegetables, etc., sublimation is used to remove moisture, facilitating storage and transportation. Some instant coffee is produced using this freeze-drying process. Low temperature and vacuum, low temperature is one of the most effective means to obtain vacuum or high vacuum. By utilizing the condensation of gas on the low-temperature surface, low-temperature adsorption, and the trapping effect of cold condensation frost,<10% can be obtained_ A very high vacuum of 12Pa. Vacuum technology has such applications in space research and the electronics industry.

1. Low temperature biomedical technology

Refrigeration plays an increasingly important role in low-temperature biomedicine. Using vacuum freeze-drying method to prepare drugs, cryopreservation of plasma, vaccines, cell tissues, certain drugs, and biological samples; Cryotherapy is a reliable, safe, and effective treatment method, especially for treating malignant tumors; Local freezing combined with surgery has good therapeutic effects, such as low-temperature surgical procedures for the heart, tumors, cataracts, tonsils, and transplantation of skin and eyeballs. Many modern medical devices, therapeutic devices, and diagnostic devices also use refrigeration technology.

2. Scientific experimental research

With the increasing demand for energy, refrigeration technology has played an increasingly important role in the development and rational use of existing energy, exploration of alternative fuels and new energy, improvement of energy structure, and improvement of environmental conditions. Such as the exploitation, storage and transportation of natural gas, the development and utilization of nuclear fusion, the successful operation of maglev high-speed trains, low-temperature superconducting technology, and the production and utilization of hydrogen energy. 

The application of low temperature in the aviation and aerospace fields involves life support systems, ground research facilities, and propulsion systems for supersonic flight at the edge of space. The ground test device needs a large volume chamber to simulate deep space conditions, the high vacuum space environment needs to be produced by a liquid nitrogen and liquid helium cooled cryopump, and the production of solid or liquid fuel for vehicles. Cryogenic technology has become one of the key parts of the space program. 

Low temperature technology is also used for cooling instruments, mainframe computer and infrared devices. Infrared astronomical satellites use instruments cooled with 4K liquid helium and 1.8K superfluid helium to detect wideband infrared radiation, while infrared detectors use sublimation (or radiation cooling technology) of solid refrigerants (such as hydrogen, neon, and methane) for cooling. 

In recent years, magnetic resonance imaging (MRI) technology has been adopted by many hospitals. The technology of using superconducting quantum interferometers to measure the human body's magnetocardiogram and magnetoencephalography will also be applied. These devices can detect tissue lesions in the human body without electrode contact or the need for any surgery, and greatly improve accuracy.

General properties and requirements of refrigerants

1、 The thermodynamic properties of refrigerants In refrigeration systems, refrigerants are often in a two-phase region or near saturation state.

2、 Chemical, safety, and environmental properties of refrigerants

2.1. Thermal stability of refrigerants

Under certain conditions, the refrigerant will decompose when heated, but under normal operating conditions of refrigeration, due to the working temperature of the refrigerant being lower than its decomposition temperature, the refrigerant is thermally stable. The maximum operating temperature of the refrigerant controlled by the refrigeration system is also limited by factors such as refrigeration conditions, type of lubricating oil, and material of the compressor.

2.2. Dissolution of refrigerant and water

The solubility of different refrigerants varies. Ammonia is easily soluble in water, and the solidification temperature of the generated aqueous solution is lower than 0 ℃. Therefore, the ammonia refrigeration system will not block the refrigeration pipeline due to icing, but will corrode the metal materials in contact with it. Haloalkane and hydrocarbon refrigerants are difficult to dissolve in water. When the water content in the refrigerant exceeds the solubility, free water will appear; When the refrigeration temperature is below 0 ℃, free water will block the throttling mechanism channel due to icing. After water dissolves the refrigerant, hydrolysis occurs, generating acidic substances that corrode metal materials and reduce the electrical insulation performance of the winding. Therefore, free water is not allowed to exist in the refrigeration system, and a dryer is generally installed in the system.

2.3. Solubility of refrigerants and lubricating oils

The solubility of different refrigerant liquids and lubricating oils is different, and the solubility of the same refrigerant and different lubricating oils is also different. Some are completely miscible, some are almost insoluble, and some are partially dissolved. Within the refrigeration temperature range, R717 and R744 are almost insoluble in mineral oil; R22, R152a, and R502 are partially miscible with mineral oil, and they are completely miscible with lubricating oil at high temperatures. At low temperatures, they exhibit layering, with one layer containing more oil and the other layer containing less oil. R11, R12, R21, R500 are completely miscible with mineral oil, forming a uniform solution. R134a is miscible with polyol ester (POE) synthetic oil, but insoluble with mineral oil. 

Note: The lubricating oil in the refrigeration system exists in a liquid state. When the refrigerant and lubricating oil are not miscible, its advantage is that the evaporation temperature is relatively stable. At the same time, in the refrigeration equipment, the refrigerant and lubricating oil are divided into two layers, making it easy to separate; The disadvantage is that an oil film will form on the heat transfer surface of the heat exchanger to suppress heat transfer. When the refrigerant and lubricating oil are miscible, no oil film will form on the heat transfer surface. Lubricating oil can penetrate into various components of the compressor together with the refrigerant, forming good lubrication conditions. However, it should be noted that the viscosity of the lubricating oil that dissolves the refrigerant will decrease, and the evaporation temperature will increase at the same pressure

2.4. The effect of refrigerant on metals and non-metals 

Ammonia has no corrosive effect on steel, but has a slight corrosive effect on copper, aluminum, or copper alloys. But if ammonia contains water, it has a strong corrosive effect on copper and copper alloys (except for phosphorus bronze). Haloalkane has no corrosive effect on almost all metals, but only on magnesium and aluminum alloys containing more than 2% magnesium. Haloalkane can be hydrolyzed into acidic substances in the presence of water, which is corrosive to metals. So, a mixture of aqueous refrigerant and lubricating oil can dissolve copper. Haloalkane refrigerant is a kind of good organic solvent, which can easily dissolve natural rubber and resin, and soften polymer materials. Swelling or bubbling. Therefore, when choosing the sealing materials and electrical insulation materials for refrigerators, natural rubber and resin compounds should not be used, but materials such as chloroprene, neoprene, nylon and plastic resistant to haloalkane corrosion should be used. Hydrocarbon refrigerants do not corrode metal materials. 

Attention: Steel plating phenomenon: When the refrigerant comes into contact with copper or copper alloy in the system, copper will dissolve in the mixture and then deposit on high-temperature steel components, forming a layer of copper film, which is called steel plating phenomenon. The phenomenon of copper plating is particularly evident on the bearing surface of the compressor crankshaft, as well as the smooth surfaces of suction and exhaust valves. It will affect the fit clearance of the moving parts of the compressor, as well as the sealing of the suction and exhaust valves, and in severe cases, it will prevent the compressor from working properly.

2.5. Electrical Insulation of Refrigerant In a closed compressor, the coil of the motor is in direct contact with the refrigerator, and it is required that the refrigerant should have good electrical insulation performance. The electrical breakdown strength is an indicator of the electrical insulation performance of refrigerants. It is worth noting that the presence of impurities and lubricating oil can reduce the electrical insulation strength of the refrigerant.

2.6. The safety of refrigerants includes toxicity and flammability.

2.7 Environmental performance and indicators: ozone depletion potential value, global warming potential value, overall greenhouse effect size, atmospheric life, life cycle climate performance.