Refrigeration and CryogenicsMaciej Chorowski

Faculty of Mechanical and Power Engineering

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

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.

Single stage vapour compression cycle

Lennard-Jones potential

COEFFICIENT OF PERFORMANCE of Single stage vapour compression cycle

Single stage vapour compression cycle

MULTISTAGE VAPOR COMPRESSION SYSTEMS

Two-stage compound system with a flash cooler: (a) schematic diagram; (b) refrigeration cycle.

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.)

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.

MULTISTAGE VAPOR COMPRESSION SYSTEMS – Coefficient of Performance

THREE-STAGE COMPOUND SYSTEM WITHA TWO-STAGE FLASH COOLER

THREE-STAGE COMPOUND SYSTEM WITHA TWO-STAGE FLASH COOLER - COP

TWO-STAGE COMPOUND SYSTEM WITHA VERTICAL INTERCOOLER (ammonia)

Cascade system (different refrigerants, equipment, oil)

Cascade system