The Vapor-Compression Refrigeration Cycle

Refrigeration systems are important in industrial and home use when temperatures less than the ambient environment are required. Of the several types of refrigeration systems, the most widely used is the vapor-compression refrigeration cycle. It is essentially a Rankine cycle operated in reverse, where heat is absorbed from a cold reservoir and rejected to a hot reservoir. Due to the constraints of the second law, this process can be accomplished only with a concomitant consumption of power. 

It emerges in state 2, where it is a vapor. The heat transferred is given by  

The high-pressure vapor is then condensed at Th, expeUing heat ( h to the hot reservoir. This process occurs in the condenser where the fluid exits at state 4. 

The high-pressure liquid is then expanded in a valve back to state 1 so the cycle can be repeated. A valve is used instead of the turbine that was used in the Rankine cycle. The amount of work that would be produced by a turbine is small, so we replace it with a valve to reduce the complexity. This step is represented by a throttling process, where  

The coefficient of performance, COP, of a refrigeration cycle measures its performance. It is defined as the ratio of the heat absorbed from the cold reservoir (the refrigeration effect) to the work required  

---

Consider the vapor-compression refrigeration cycle of Fig. 9.1ft with Freon-12 as refrigerant the evaporation temperature is 10(°F), show the effect of condensation temperature on the coeffi of performance by making calculations for condensation temperatures of 60, 80, and 100(°F). 

Figure 5.2-4 (a) The vapor-compression refrigeration cycle. (/ ) The vapor-compressron refrigeration cycle on T-S and P-H diagrams. 

The coefficient of performance (C.O.P.) for the vapor-compression refrigeration cycle is - ... 

Vapor-Compression Cycles The most widely used refrigeration principle is vapor compression. Isothermal processes are realized through isobaric evaporation and condensation in the tubes. Standard vapor compression refrigeration cycle (counterclockwise Ranldne cycle) is marked in Fig. ll-72<7) by I, 2, 3, 4. 

Does the ideal vapor compression refrigeration cycle involve any internal irreversibility ... 

An actual vapor compression refrigeration cycle operates at steady state with R-134a as the working fluid. Saturated vapor enters the compressor at 263 K. Superheated vapor enters the condenser at 311K. Saturated liquid leaves the condenser at 301 K. The mass flow rate of refrigerant is 0.1 kg/sec. Determine... 

R-134a enters the compressor of a steady-flow vapor compression refrigeration cycle as superheated vapor at 0.14 MPa and — 10°C at a rate of 0.04 kg/sec, and it leaves at 0.7 MPa and 50°C. The refrigerant is cooled in the condenser to 24°C and saturated liquid. Determine (a) the compressor power required, (b) the rate of heat absorbed from the refrigerated space, (c) the compressor efficiency, and (d) the COP. 

In comparing the performance of a real cycle with that of a Carnot cycle, one has in principle a choice of temperatures to use for the Carnot calculation. Consider a vapor-compression refrigeration cycle in which tlie average fluid temperatures m the condenser and evaporator are Th aiid Tc, respectively. Corresponding to Th and Tc, tlie heat transfer occurs with respect to surroundings at temperature T , and Which provides the more conservative estimate of j(Camot) a calculation based on Th and Tc, or one based on Ts and... 

This is like Illustration 3.7-2 except that the Rankine rather than vapor compression refrigeration cycle is used. Only properties of point 2, and path from 1 — 2 changes. 

DRY COMPRESSION - The compression of vapor, in a vapor-liquid vapor-compression refrigeration cycle. 

The net effect of this vapor compression refrigeration cycle, and its incorporation into a vapor degreaser, is to transfer heat from the hot solvent vapor above the boiling and rinsing sumps to ambient air adjacent to the cleaning machine. 

Figure 3.9 The ideal vapor-compression refrigeration cycle. The four unit processes are sketched on the left, while the path on a Ts diagram is shown on the right.

Estimation of the COPof a Vapor-Compression Refrigeration Cycle... 

Although the T-s diagram is veiy useful for thermodynamic analysis, the pressure enthalpy diagram is used much more in refrigeration practice due to the fact that both evaporation and condensation are isobaric processes so that heat exchanged is equal to enthalpy difference A( = Ah. For the ideal, isentropic compression, the work could be also presented as enthalpy difference AW = Ah. The vapor compression cycle (Ranldne) is presented in Fig. H-73 in p-h coordinates. 

The vapor-compression cycle of Fig. 9.1 ft is shown on a PH diagram in Fig. 9.2. Such diagrams are more commonly used in refrigeration work than TS... 

The effectiveness of a refrigeration cycle is measured by its coefficient of performance. For given values of Tc and TH, the highest possible value is attained by the Carnot refrigerator. The vapor-compression cycle with reversible compression and expansion approaches this upper limit. A vapor-compression cycle with expansion in a throttle valve has a somewhat lower value, and this is reduced further when compression is not isentropic. The following example provides an indication of the magnitudes of coefficients of performance. 

---