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Currently there are about 30 kinds of commonly used refrigerants. Commonly used are ammonia (Ammonia) and Freon (Freone). First of all, ammonia, it is used earlier, widely used in cold storage, cold storage and other large refrigeration equipment, its main advantage is the unit volume of cold production, the cost is cheap, not with the metal and refrigeration oil reaction, good thermal stability, but there are toxicity, corrosion of organic fittings of the obvious shortcomings. Secondly, Freon, which is a saturated hydrocarbon halogen derivatives of the general term, in which the fluorine alkanes writing FC, chlorofluorocarbon alkanes writing CFC, hydrogen-containing writing HFC, both writing HCFC. commercial Freon numbering according to the rules of the left to the right the first one is the number of carbon atoms minus one; the second one is the number of hydrogen atoms plus one; the third one is the number of fluorine atoms, the number of chlorine atoms, chlorine atoms are not numbered, and if it contains bromine atoms, the first number according to the above principles, and then add one. First, according to the above principles, then add the letter B and the number of bromine atoms. According to this principle, CBrF3 on the writing of FC-13B1. Freon's application is more than 60 years later than ammonia, but it was introduced to its non-toxic, odorless, non-flammable and non-explosive, good stability, good lubrication of the equipment and become the star of the refrigeration industry, CFC-12 is widely used in the production of refrigerators, and others such as CFC-11, HCFC-22, HCFC-113, HCFC-114 are also widely used.
However, Freon has its fatal shortcomings, it is a "greenhouse gas", the greenhouse effect value than carbon dioxide 1700 times, more dangerous is that it will destroy the ozone in the atmosphere. 80's, the United States of America, California, two scholars took the lead in pointing out that the CFCs (hydrochlorofluorocarbons) in the ultraviolet light under the action of chlorine atoms, chlorine atoms and ozone free-radical chain reaction, a chlorine atom can consume tens of thousands of ozone molecules, and the chlorine atom can consume tens of thousands of ozone molecules. One chlorine atom can consume tens of thousands of ozone molecules, thus affecting the absorption of 250-320 nanometer ultraviolet rays by ozone molecules, so that excessive ultraviolet rays reach the earth's surface, directly affecting the survival of human beings and other organisms. It is particularly important to point out that CFCs are chemically very stable, and the emitted CFCs can steadily reach the stratosphere and stay there for 40-150 years, causing long-lasting damage to the ozone layer. As a result of Freon's destruction of the ozone layer, scientists have even found ozone holes over the Earth's poles. Therefore, the Montreal Agreement of 1990 stipulated that by the end of this century all countries in the world should stop the production and emission of Freon. Countries are now looking for alternatives to Freon, which are called "green refrigerants" because they meet environmental requirements. It is very difficult to find alternative products that meet the environmental requirements and have practical use performance. At present, the possible products are CFC-22, HFC-134a, HFC-152a, etc.
In addition to this, we are also looking for alternative products that meet the environmental requirements and have practical use performance.
In addition, researchers have developed a variety of alternative technologies, including magnetic cooling and adsorption cooling. Magnetic cooling is also called "paramagnetic salt adiabatic cooling". Paramagnetic salt contains iron or rare earth elements, its 3d, 4f layer of electrons are not full, so it has a magnetic, in the excitation and demagnetization process will absorb heat or exothermic, the use of this nature of the development of refrigeration technology with high refrigeration efficiency, low cost, simple structure, etc., the most attractive thing is that it does not pollute the environment, so there are great prospects for development. For example, magnesium cerium nitrate as a refrigerant magnetic cooling machine cooling can be close to OK. adsorption cooling is the use of adsorption - desorption of the nature of the adsorption or exothermic refrigeration, commonly used refrigerant systems include metal hydride - hydrogen, zeolite molecular sieve - H2O, activated carbon - nitrogen, praseodymium oxide (cerium oxide) - oxidation system, and so on. The new refrigeration technology takes full account of the refrigerant and environmental sustainability and sustainable development requirements, is figuratively referred to as "green refrigeration", is a major trend in the future development of refrigeration technology, should become my refrigeration industry to seize the opportunity to meet the challenges of the mainstream.
Liquid nitrogen is generally used in ultra-low temperature freezing.
The boiling point of liquid nitrogen is 77 ° K (-196 ° C), in the normal atmospheric pressure temperature below minus 196 degrees Celsius will form liquid nitrogen, if pressurized, you can get liquid nitrogen at a relatively high temperature.
The following is some knowledge of the use of liquid nitrogen:
1. Physical and chemical constants of the substance:
National Standard No. 22005
CAS No. 7727-37-9
Chinese name Nitrogen
English name netrogen
Molecular formula N2 Appearance and properties Colorless, odorless gas
Molecular weight 28.01 Vapor pressure 1026.42kPa(-173℃)
Melting point -209.8℃ Boiling point:-195.6℃ Solubility Slightly soluble in water, ethanol
Density Relative density (water=1)0.81(-196℃); Relative density (air=1)0.97 Stable Stable
Hazardous labeling 5( Non-flammable gas) Main applications Used in the synthesis of ammonia, nitric acid production, as a substance protection agent, refrigerant
Hazard
Route of invasion: inhalation.
Health hazards: the nitrogen content in the air is too high, so that the partial pressure of oxygen in the inhalation gas decreases, causing hypoxia and asphyxiation. Inhalation of nitrogen concentration is not too high, the patient initially feel chest tightness, shortness of breath, weakness; followed by restlessness, extreme excitement, running, shouting, trance, unsteady gait, called "nitrogen Moetzin", can be entered into a coma or comatose state. Inhalation of high concentrations, the patient can quickly appear coma, respiratory arrest and death
Hazardous properties: If high heat, the container pressure increases, there is a risk of cracking and explosion.
Combustion (decomposition) products: nitrogen.
Be careful when using it, don't let the liquid nitrogen flow to the skin, it will frostbite!
When using it, it is usually placed in a heat preservation container, sealed well, and usually wear gloves when operating.