display cases Practical refrigerants (1)
ammonia
The standard evaporation temperature of ammonia is 33.4℃, the freezing temperature is 77.7℃, and the safety level is B2. Ammonia has better thermal properties and thermophysical properties. Its pressure is relatively moderate in the range of normal temperature and ordinary low temperature. The cooling capacity per unit volume is large, the viscosity is small, the flow resistance is small, the specific gravity is small, and the heat transfer performance is good. In addition, ammonia is cheap and easy to obtain, so it is the first and still widely used refrigerant for display cases. Ammonia is mainly used in large-scale industrial refrigeration equipment, and domestic large and medium-sized cold storages use ammonia as a refrigerant. Because the final compression temperature of ammonia is high, the compressor cylinder needs to be cooled. Ammonia is a refrigerant that is difficult to dissolve in lubricating oil (solubility does not exceed 1%). Oil film will accumulate on the heat transfer surface inside the pipes of the ammonia refrigerator and the heat exchanger, which will affect the heat transfer effect. In addition, lubricating oil will also accumulate in the lower part of the condenser, accumulator and evaporator. These parts should be drained regularly. The water content in the ammonia system must not exceed 0.2%, because the presence of water makes the ammonia refrigerant impure, and a lot of heat is released during the formation of the ammonia solution, making the ammonia solution have a higher evaporation temperature than pure ammonia; more importantly The hazard is that it is highly corrosive to zinc, copper, bronze and other copper alloys, with the exception of phosphor bronze. Therefore, the ammonia refrigeration system does not allow the use of copper components, and the wear-resistant parts and seals (such as bearing bushes, sealing rings, etc.) are limited to high-tin phosphor bronze materials. Although the safety level of ammonia is B2, it has a long history of use and people have mastered the relevant technology, so it is still widely used in display cases refrigeration.
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