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Walk-in cooler two-stage compression and cascade refrigeration cycle
author: supermarket refrigerated showcase category: walk-in display coolers Views: 560Time: 2016-08-22
walk-in cooler two-stage compression and cascade refrigeration cycle
In the walk-in cooler vapor compression refrigeration cycle, when the refrigerant is selected, its condensation pressure , The evaporation pressure is determined by the condensation temperature and the evaporation temperature. The condensation temperature is limited by the temperature of the ambient medium (water or air); the evaporation temperature is determined by the purpose of the refrigeration device. When the condensing temperature increases or the evaporation temperature decreases, the pressure ratio of the compressor will increase. Due to the existence of the compressor clearance volume, after the pressure ratio is increased to a certain value, the volume coefficient of the compressor becomes zero, the compressor no longer sucks air, and although the refrigerator is running continuously, the cooling capacity becomes zero.
The minimum evaporating temperature of single-stage compression is not only limited by the volume coefficient of zero, as the pressure ratio increases, in addition to causing a decrease in cooling capacity, an increase in power consumption, a decrease in performance coefficient, and a decrease in economy, the exhaust temperature is also limited Another important reason for choosing compressor stages. If the exhaust temperature is too high, it will make the lubricating oil thinner, worsen the lubrication conditions, and even cause the lubricating oil to carbonize and cylinder pulling.
For ammonia refrigerants, due to the large adiabatic index and high exhaust temperature, the pressure ratio of ammonia single-stage compression is generally not expected to exceed 8; the adiabatic process index of Freon refrigerant is relatively small, but from the perspective of economy The pressure ratio of their single-stage compression is generally not expected to exceed 10. Under this condition, the lowest evaporation temperature that can be achieved by single-stage compression at different condensation temperatures.
For centrifugal compressors, to ensure a certain efficiency, the enthalpy increment of each stage of compression cannot exceed a certain range. When the pressure ratio is so large that the enthalpy increase exceeds this range, two or more stages of compression are required.
For rotary compressors, the volumetric efficiency does not decrease significantly as the pressure ratio increases, but the air temperature will increase. The use of multi-stage compression, intermediate cooling, and multi-stage throttling can not only reduce the exhaust temperature, but also improve the cycle performance.
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