Thermodynamics

612 | ThermodynamicsP34Q LQ H12W inFIGURE 11–5The P-h diagram of an idealvapor-compression refrigeration cycle.hAnother diagram frequently used in the analysis of vapor-compressionrefrigeration cycles is the P-h diagram, as shown in Fig. 11–5. On this diagram,three of the four processes appear as straight lines, and the heat transferin the condenser and the evaporator is proportional to the lengths of thecorresponding process curves.Notice that unlike the ideal cycles discussed before, the ideal vaporcompressionrefrigeration cycle is not an internally reversible cycle since itinvolves an irreversible (throttling) process. This process is maintained inthe cycle to make it a more realistic model for the actual vapor-compressionrefrigeration cycle. If the throttling device were replaced by an isentropicturbine, the refrigerant would enter the evaporator at state 4 instead of state4. As a result, the refrigeration capacity would increase (by the area underprocess curve 4-4 in Fig. 11–3) and the net work input would decrease (bythe amount of work output of the turbine). Replacing the expansion valveby a turbine is not practical, however, since the added benefits cannot justifythe added cost and complexity.All four components associated with the vapor-compression refrigerationcycle are steady-flow devices, and thus all four processes that make up thecycle can be analyzed as steady-flow processes. The kinetic and potentialenergy changes of the refrigerant are usually small relative to the work andheat transfer terms, and therefore they can be neglected. Then the steadyflowenergy equation on a unit–mass basis reduces to1q in q out 2 1w in w out 2 h e h i(11–6)The condenser and the evaporator do not involve any work, and the compressorcan be approximated as adiabatic. Then the COPs of refrigeratorsand heat pumps operating on the vapor-compression refrigeration cycle canbe expressed asandCOP R q Lw net,in h 1 h 4h 2 h 1(11–7)COP HP q H h 2 h 3(11–8)w net,in h 2 h 1where h 1 h g @ P1and h 3 h f @ P3for the ideal case.Vapor-compression refrigeration dates back to 1834 when the EnglishmanJacob Perkins received a patent for a closed-cycle ice machine using etheror other volatile fluids as refrigerants. A working model of this machine wasbuilt, but it was never produced commercially. In 1850, Alexander Twiningbegan to design and build vapor-compression ice machines using ethylether, which is a commercially used refrigerant in vapor-compression systems.Initially, vapor-compression refrigeration systems were large and weremainly used for ice making, brewing, and cold storage. They lacked automaticcontrols and were steam-engine driven. In the 1890s, electric motordrivensmaller machines equipped with automatic controls started to replacethe older units, and refrigeration systems began to appear in butcher shopsand households. By 1930, the continued improvements made it possible tohave vapor-compression refrigeration systems that were relatively efficient,reliable, small, and inexpensive.