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Refrigeration and CryogenicsMaciej Chorowski

Faculty of Mechanical and Power Engineering

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COEFFICIENT OF PERFORMANCE

OF REFRIGERATION CYCLE The coefficient of performance is an index of

performance of a thermodynamic cycle or athermal system. Because the COP can be

greater than 1, COP is used instead of thermalefficiency. The coefficient of performance canbe used for the analysis of the following:

A refrigerator that is used to produce a refrigerationeffect only, that is, COP

ref

A heat pump in which the heating effect is produced byrejected heat COPhp

A heat recovery system in which both the refrigerationeffect 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 usuallysuperheated to ensure that liquid refrigerant does notflow into the compressor.

Compression. Refrigerant is compressed to a higherpressure and temperature for condensation.

Condensation. Gaseous refrigerant is condensed toliquid form by being desuperheated, then condensed,and finally subcooled, transferring its latent heat ofcondensation to a coolant.

Throttling and expansion. The higher-pressure liquidrefrigerant 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 pressureInterstage 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 p i, psia

(kPa abs.), can be calculated as:

For a multistage vapor compression system with z stages, the

compression ratio Rcomfor 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 WITH

A VERTICAL INTERCOOLER (ammonia)

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Cascade system (different refrigerants,

equipment, oil)

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Cascade system