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【正文】 ssure vapor pression refrigeration is used when several evaporators are needed at various temperatures, such as in a supermarket, or when evaporator temperature bees very low. Low evaporator temperature indicates low evaporator pressure and low refrigerant density into the pressor. Two small pressors in series have a smaller displacement and usually operate more efficiently than one large pressor that covers the entire pressure range from the evaporator to the condenser. This is especially true in ammonia refrigeration systems because of the large amount of superheating that occurs during the pression process. Thermodynamic analysis of multistage cycles is similar to analysis of singlestage cycles, except that mass flow differs through various ponents of the system. A careful mass balance and energy balance on individual ponents or groups of ponents ensures correct application of the first law of thermodynamics. Care must also be used when performing secondlaw calculations. Often, the refrigerating load is prised of more than one evaporator, so the total system capacity is the sum of the loads from all evaporators. Likewise, the total energy input is the sum of the work into all pressors. For multistage cycles, the expression for the coefficient of performance given in Equation (15) should be written as ACTUAL REFRIGERATION SYSTEMS Actual systems operating steadily differ from the ideal cycles considered in the previous sections in many respects. Pressure drops occur everywhere in the system except in the pression process. Heat transfers between the refrigerant and its e
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