Vapor-compression cycles

Natural gas Hquids are recovered from natural gas using condensation processes, absorption (qv) processes employing hydrocarbon Hquids similar to gasoline or kerosene as the absorber oil, or soHd-bed adsorption (qv) processes using adsorbants such as siHca, molecular sieves, or activated charcoal. Eor condensation processes, cooling can be provided by refrigeration units which frequently use vapor-compression cycles with propane as the refrigerant or by... 

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. 

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 term refrigeration refers to the gas coming into contact with evaporator coils on a dx vapor-compression cycle, coils on an absorption cycle, vortex... 

Equipment diagram for refrigeration using a basic vapor compression cycle. 

Figure 2, the pressure-enthalpy plot of the standard vapor compression cycle, traces the state of the refrigerant through the refrigeration system. (Enthalpy represents the energy of the refrigerant as... 

The standard vapor compression cycle on the pressure-energy diagram. 

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... 

---