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Refrigeration
Chemical industry, the use of refrigeration to achieve gas separation, gas suspicion; so that a mixture of material coagulation, and thus separate from other substances; liquid storage at low temperatures; chemical synthesis process to provide the right temperature and pressure, remove the heat of reaction. Oil cracking, synthetic rubber, synthetic plastics, fuel production, fertilizer production needs refrigeration; natural gas liquefaction, dehydration, storage and transportation also need refrigeration. Mechanical manufacturing, low temperature treatment of steel (-70 ℃ ~ -90 ℃) can change its microstructure, the austenite into martensite, improve the hardness and strength of steel. In the steel industry, it is necessary to dehumidify the blast furnace blast at low temperatures to reduce the coking ratio of hot metal and ensure the quality of molten iron. In the machine's assembly process, the use of low temperature for easy interference fit between parts. Low temperature crushing, low temperature crushing technology is the use of materials at low temperature cold brittle properties of the material crushed. The main features and uses of the technology are as follows: It can process highly elastic materials that can not be crushed at room temperature, such as recovering rubber from tire of steel structure, developing fine particles of food and Chinese herbal medicine, processing and producing nanometer material, and has the characteristics of green production.

Construction industry, excavation of earthwork with frozen soil method, building bridge foundation, underground engineering, etc. can improve construction efficiency and ensure construction safety. Refrigeration is also used to cool giant concrete blocks, eliminating the heat of chemical reaction that is released when the concrete cures, to avoid thermal expansion and concrete stress.

Agriculture, animal husbandry and refrigeration are used for low-temperature treatment of crop seeds; construction of artificial climate training rooms; and preservation and handling of fine species and livestock species. Some foods, vegetables and other freeze-drying process, the use of sublimation to remove moisture, easy to store and transport, some instant coffee is the use of this freeze-drying process for the production. Low temperature and vacuum, low temperature is one of the most effective means of obtaining vacuum or high vacuum. The use of gas condensation at low temperature surface, low temperature adsorption and condensation frost capture effect, can obtain very high vacuum <10_12Pa. Vacuum technology has such uses in the space research and electronics industries.

Low temperature biomedical technology

Refrigeration plays an increasingly important role in cryogenic biomedicine. Use of vacuum freeze-drying method for the preparation of drugs, cryogenic preservation of plasma, vaccine, cell tissue, certain drugs and biological samples; frozen medical treatment is reliable, safe, effective and easy treatment, especially for the treatment of malignant tumors; Surgery has a good therapeutic effect, such as heart, tumor, cataracts, tonsil and other low-temperature surgery, skin, eye transplant surgery. Many modern medical devices, therapeutic apparatus, diagnostic apparatus also uses the cooling technology.

2. Scientific experimental research

With the increasing demand for energy, refrigeration technology has played an increasingly important role in developing and rationalizing existing energy resources, exploring alternative fuels and new energy sources, improving energy structure and improving environmental conditions. Such as natural gas exploitation, storage and transportation, development and utilization of fusion, successful operation of high-speed maglev train, low-temperature superconducting technology, hydrogen production and utilization.

Cryogenic applications in aerospace and aeronautics include life support systems, ground research facilities, and propulsion systems that fly ultra-high-speed sound waves at the edge of space. Ground test equipment requires a large chamber to simulate deep space conditions, the high vacuum space environment to use liquid nitrogen and liquid helium cryogenic pump to produce, vehicle solid or liquid fuel production, low temperature technology has become space One of the key parts of the plan.

Cryogenic technology is also used to cool instrumentation, mainframe computers and infrared devices. Infrared astronomy satellite uses 4K liquid helium and 1.8K superfluid helium cooled instruments to detect broadband infrared radiation. Infrared detectors utilize sublimation (or radiant cooling) of solid refrigerants (hydrogen, neon, methane, etc.) To cool.

In recent years, magnetic resonance imaging (MRI) has been adopted by many hospitals. The use of superconducting quantum interferometer measurement of human magnetocardiography and magnetoencephalography technology will also be applied. These devices in the human body without touching the electrode or without any surgery case, the detection of human tissue lesions, and to greatly improve the accuracy.

The general nature and requirements of refrigerants

First, the thermal properties of refrigerants Refrigerants in refrigeration systems are often in the two-phase region or close to saturation.

Second, the chemical, safety and environmental properties of refrigerants

2.1, the thermal stability of the refrigerant

Under certain conditions, the heating temperature of the refrigerant will decompose, but in the normal operating conditions of refrigeration, the refrigerant is thermally stable because its operating temperature is lower than its decomposition temperature. The maximum operating temperature of the refrigerant actually controlled by the refrigeration system is also limited by factors such as the refrigeration condition, the type of lubricant and the material of the compressor.

2.2, the dissolution of the refrigerant and water

Different refrigerants have different water solubility. Ammonia is readily soluble in water, and the resultant aqueous solution has a solidification temperature below 0 ° C, so the refrigeration system will not clog the refrigerant line due to icing but will corrode the metal material in contact with it. Halogenated hydrocarbons and hydrocarbon refrigerants difficult to dissolve in water, when the water content in the refrigerant exceeds the solubility, there will be free state of water; when the cooling temperature is below 0 ℃, the free water will be blocked due to freezing throttle mechanism aisle. Hydrolysis of water occurs after the refrigerant hydrolysis phenomenon, the formation of acidic substances, corrosion of metal materials, reducing the electrical insulation properties of the winding. Therefore, the free cooling water system does not allow the existence of water, the general set dryer in the system.

2.3, refrigerant and lubricating oil solubility

Different refrigerant liquid and lubricating oil solubility is different, the same refrigerant and different lubricating oil solubility is also different, and some completely miscible, and some almost insoluble, and some dissolved. In the refrigeration temperature range, R717 and R744 almost insoluble in mineral oil; R22, R152a, R502 partially compatible with mineral oil, they completely miscible with the lubricating oil at high temperature, stratification at low temperatures, a layer of oil more, A layer of less oil. R11, R12, R21, R500 completely miscible with mineral oil to form a homogeneous solution. R134a and polyol esters (referred to as POE) synthetic lubricating oil is miscible, and mineral oil is insoluble.

NOTE: The lubricant in the refrigeration system is in a liquid state. When the refrigerant and the lubricant are immiscible, the advantage is that the evaporating temperature is relatively stable, while the refrigerant and the lubricant are separated into two layers in the refrigeration equipment, so that it is easy to separate ; Disadvantage is the heat transfer surface of the heat exchanger, will form a film to curb heat transfer. When the refrigerant and oil miscible, the heat transfer surface will not form an oil film. Lubricant with the refrigerant can penetrate to the compressor with the various components, forming a good lubrication conditions. However, care should be taken that the viscosity of the lubricant dissolving the refrigerant will be reduced and the evaporation temperature will increase under the same pressure

2.4, the role of refrigerants on metals and non-metals

Ammonia has no corrosive effect on steel and slightly corrodes copper, aluminum or copper alloy. However, if the ammonia in water, then the copper and copper alloys (except phosphor bronze) have a strong corrosive effect. Halocarbons do not corrode almost any metal, only corrosion of magnesium and aluminum alloys containing more than 2% magnesium. Halogenated hydrocarbons in the case of water will be hydrolyzed into acidic substances, have a corrosive effect on the metal. Therefore, a mixture of an aqueous refrigerant and a lubricating oil can dissolve copper. Halogenated hydrocarbon refrigerants is a good organic solvent, it is easy to dissolve natural rubber and resin, make the polymer material soften. Inflated or blistered. Therefore, the choice of chiller sealing materials and electrical insulation materials, do not use natural rubber, resin compounds, but to use halogenated hydrocarbon corrosion chloroprene, neoprene, nylon, plastic and other materials. Hydrocarbon refrigerants have no corrosion on metal materials.

Note: Plated steel phenomenon: When the refrigerant in the system with copper or copper alloy contact, the copper will be dissolved in the mixture, and then deposited on the higher temperature of the steel parts to form a copper film, which is the so-called plating Steel phenomenon. Copper phenomenon in the compressor crankshaft bearing surface, suction, exhaust valve and other smooth surface particularly obvious. It affects the clearance of the moving parts of the compressor and the sealing of the suction and discharge valves, making the compressor unusable in severe cases.

2.5, the electrical insulation of refrigerant In a hermetic compressor, the motor coil and the direct contact with the refrigerator, requiring refrigerant should have good electrical insulation properties. Electrical breakdown strength indicates an indicator of refrigerant electrical insulation properties. It is worth noting that impurities, the presence of lubricating oil will reduce the electrical insulation strength of the refrigerant.

2.6, refrigerant safety Safety, including toxicity and flammability.

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