The number of refrigerants used at the time of publication has reached 70-80 and is constantly increasing. But there are only over ten types used in the food industry and air conditioning refrigeration. The only widely adopted ones are the following: [2]
ammonia
(Code: R717)
Ammonia is the most widely used medium pressure and temperature refrigerant. The solidification temperature of ammonia is -77.7 ℃, the standard evaporation temperature is -33.3 ℃, and the condensation pressure at room temperature is generally 1.1~1.3MPa. Even when the cooling water temperature reaches 30 ℃ in summer, it is impossible to exceed 1.5MPa. The unit standard volume refrigeration capacity of ammonia is approximately 520 kcal/m3.
Ammonia has good water absorption, and even at low temperatures, water will not precipitate from the ammonia solution and freeze, so there will be no "ice blockage" phenomenon in the system. Ammonia does not have a corrosive effect on steel, but after containing water in the ammonia solution, it has a corrosive effect on copper and copper alloys, and slightly increases the evaporation temperature. Therefore, copper and copper alloy materials cannot be used in ammonia refrigeration devices, and it is stipulated that the water content in ammonia should not exceed 0.2%.
The critical temperature of ammonia is relatively high (tkr=132 ℃), the latent heat of vaporization is high, and it is 1164KJ/Kg at atmospheric pressure. Under standard operating conditions, the refrigeration capacity per unit volume is also large, and the size of the ammonia compressor can be smaller.
Pure ammonia has no adverse effect on lubricating oil, but when there is moisture, it will reduce the lubricating effect of the refrigeration oil. Ammonia is not easily soluble in lubricating oil, so an oil separator should be installed in the device to reduce the entry of lubricating oil into the condenser and evaporator, and to prevent the heat exchange surface from being contaminated by oil and reducing the heat transfer performance.
Pure ammonia has no corrosive effect on steel, but when ammonia contains moisture, it will corrode copper and copper alloys (excluding phosphorus bronze). Therefore, copper and copper alloys are not used for pipelines and valves in ammonia refrigeration systems.
Liquid ammonia is transparent and colorless, ammonia vapor is colorless, and has a strong irritating odor. Ammonia has significant toxicity to the human body, and when ammonia splashes onto the skin, it can cause frostbite. When the volume of ammonia vapor in the air reaches 0.5-0.6%, it can cause an explosion. Therefore, the concentration of ammonia in the air inside the computer room should not exceed 0.02mg/L.
Ammonia is not easily combustible at room temperature. When heated to 350 ℃, it decomposes into nitrogen and hydrogen, which can explode when mixed with oxygen in the air. When mixed with air in a volume fraction of 11% -14%, it can burn. It may explode when exposed to an open flame between 16% and 25%. At 0.5% -0.6%, a person who stays in it for half an hour will be poisoned.
Ammonia is extremely soluble in water, and can dissolve 130 liters of ammonia per liter of water at 0 ℃. Generally, the water content in liquid ammonia is less than 0.2%.
Ammonia has a small specific gravity and viscosity, high heat release coefficient, low price, and is easy to obtain. However, ammonia has strong toxicity and flammability. If measured by volume, when the ammonia content in the air reaches 0.5% to 0.6%, a person can be poisoned by staying in it for half an hour, ignite when it reaches 11% to 13%, and explode when it meets an open flame when it reaches 16%. Therefore, it is necessary to pay attention to ventilation and exhaust in the ammonia refrigeration room, and regularly remove air and other non condensable gases from the system.
In summary, the advantages of ammonia as a refrigerant are: easy to obtain, low price, moderate pressure, large unit refrigeration capacity, high heat release coefficient, almost insoluble in oil, low flow resistance, and easy to detect leaks. Its disadvantages are: it has a pungent odor, is toxic, can burn and explode, and has a corrosive effect on copper and copper alloys.
