Refrigeration and Cryogenics Maciej Chorowski Faculty of Mechanical and Power Engineering.
Refrigeration and Cryogenics Maciej Chorowski
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Transcript of Refrigeration and Cryogenics Maciej Chorowski
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Refrigeration and CryogenicsMaciej Chorowski
Faculty of Mechanical and Power Engineering
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COEFFICIENT OF PERFORMANCEOF REFRIGERATION CYCLE• The coefficient of performance is an index
of performance of a thermodynamic cycle or a thermal system. Because the COP can be greater than 1, COP is used instead of thermal efficiency. The coefficient of performance can be used for the analysis of the following:
• A refrigerator that is used to produce a refrigeration effect only, that is, COPref
• A heat pump in which the heating effect is produced by rejected heat COPhp
• A heat recovery system in which both the refrigeration effect and the heating effect are used at thesame time, COPhr
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Refrigeration processes of a vapor compression refrigerating system• Evaporation. In this process, the refrigerant
evaporates at a lower temperature than that of its surroundings, absorbing its latent heat of vaporization.
• Superheating. Saturated refrigerant vapor is usually superheated to ensure that liquid refrigerant does not flow into the compressor.
• Compression. Refrigerant is compressed to a higher pressure and temperature for condensation.
• Condensation. Gaseous refrigerant is condensed to liquid form by being desuperheated, then condensed, and finally subcooled, transferring its latent heat of condensation to a coolant.
• Throttling and expansion. The higher-pressure liquid refrigerant is throttled to the lower evaporating pressure and is ready for evaporation.
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Single stage vapour compression cycle
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Lennard-Jones potential
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COEFFICIENT OF PERFORMANCE of Single stage vapour compression cycle
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Single stage vapour compression cycle
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MULTISTAGE VAPOR COMPRESSION SYSTEMS
Two-stage compound system with a flash cooler: (a) schematic diagram; (b) refrigeration cycle.
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MULTISTAGE VAPOR COMPRESSION SYSTEMS - interstage pressure
Interstage pressure is usually set so that the compression ratio at each stage is nearly the same for higher COPs. For a two-stage compound system, interstage pressure pi, psia (kPa abs.), can be calculated as:
For a multistage vapor compression system with z stages, the compression ratio Rcom for each stage can be calculated as
where pcon condensing pressure, psia (kPa abs.)pev evaporating pressure, psia (kPa abs.)
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MULTISTAGE VAPOR COMPRESSION SYSTEMS
Heat balance of entering and leaving refrigerants in a flash cooler and at the mixing point: (a) in the flash cooler; (b) at the mixing point 3 before entering the second-stage impeller.
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MULTISTAGE VAPOR COMPRESSION SYSTEMS – Coefficient of Performance
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THREE-STAGE COMPOUND SYSTEM WITHA TWO-STAGE FLASH COOLER
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THREE-STAGE COMPOUND SYSTEM WITHA TWO-STAGE FLASH COOLER - COP
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TWO-STAGE COMPOUND SYSTEM WITHA VERTICAL INTERCOOLER (ammonia)
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Cascade system (different refrigerants, equipment, oil)
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Cascade system