The Vapor-Compression Cycle

W to the system. Since At/ of the working fluid is zero for the cycle, tire first law is written  

The measure of the effectiveness of a refrigerator is its coefficient of performance a , defined as  

On the basis of a unit mass of fluid, the equations for the heat absorbed in the evaporator and the heat rejected in the condenser are  

These equations follow from Eq. (2.3 2) when the small changes m potential and kinetic energy are neglected. The work of compression is simply  

To design the evaporator, compressor, condenser,and auxiliary equipment one must know tire rate of circulation of refrigerant/w. This is determined from the rate of heat absorption in tire evaporator by the equation  

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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 incorporating an expansion valve is shown in Fig. 9.1h, where line A- 1 represents the constant-enthalpy throttling process. Line 2 + 3, representing an actual compression process, slopes in the direction of increasing entropy, reflecting the irreversibility inherent in the process. The dashed line 2 - 3 is the path of isentropic compression (see Fig. 7.6). For this cycle, the coefficient of performance is simply... 

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. 

Figure 4. (a) Schematic of the vapor-compression cycle with an oil-lubricated compressor, (b) The vapor-compression cycle shown on a temperature entropy diagram operating between a low pressure Pi and a high pressure Ph and between a low temperature Tc and ambient temperature To. 

The refrigeration techniques required to reach cryogenic temperatures are different than those of conventional vapor-compression refrigeration, which is used for most cooling applications closer to ambient temperatures. Most domestic refrigerators and air conditioners use the vapor-compression cycle. Figure 4a shows a schematic of the vapor-compression cycle, and Figure 4b shows the path of... 

Refrigeration cycles can also be implemented in microsystems. The vapor compression cycle, which is the inverse of the Rankine cycle micro steam turbine), consists of compressing a gas and... 

For the combined system, the absorption cycle, serve to ensure the condensation of the refrigerant for the vapor compression cycle. The latter can operate between temperatures of condensation and evaporation desired. 

The vapor-compression cycle was first used by French engineer Nicolas Leonard Sadi Carnot in 1824. Then in 1832, American inventor Jacob Perkins was the first to demonstrate a compression cooling technology that used ether as a refrigerant. But it was in 1852 that Scottish engineer William Thomson, also known as Lord Kelvin, conceptualized the first heat pump system, dubbed the heat multiplier. ... 

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