Thermodynamics

620 | Thermodynamicsa result of this modification, the condenser of the heat pump (locatedindoors) functions as the evaporator of the air conditioner in summer. Also,the evaporator of the heat pump (located outdoors) serves as the condenserof the air conditioner. This feature increases the competitiveness of the heatpump. Such dual-purpose units are commonly used in motels.Heat pumps are most competitive in areas that have a large cooling loadduring the cooling season and a relatively small heating load during theheating season, such as in the southern parts of the United States. In theseareas, the heat pump can meet the entire cooling and heating needs of residentialor commercial buildings. The heat pump is least competitive in areaswhere the heating load is very large and the cooling load is small, such as inthe northern parts of the United States.11–7 ■ INNOVATIVE VAPOR-COMPRESSIONREFRIGERATION SYSTEMSThe simple vapor-compression refrigeration cycle discussed above is themost widely used refrigeration cycle, and it is adequate for most refrigerationapplications. The ordinary vapor-compression refrigeration systems aresimple, inexpensive, reliable, and practically maintenance-free (when wasthe last time you serviced your household refrigerator?). However, for largeindustrial applications efficiency, not simplicity, is the major concern. Also,for some applications the simple vapor-compression refrigeration cycle isinadequate and needs to be modified. We now discuss a few such modificationsand refinements.Cascade Refrigeration SystemsSome industrial applications require moderately low temperatures, and thetemperature range they involve may be too large for a single vaporcompressionrefrigeration cycle to be practical. A large temperature rangealso means a large pressure range in the cycle and a poor performance for areciprocating compressor. One way of dealing with such situations is to performthe refrigeration process in stages, that is, to have two or more refrigerationcycles that operate in series. Such refrigeration cycles are calledcascade refrigeration cycles.A two-stage cascade refrigeration cycle is shown in Fig. 11–10. The twocycles are connected through the heat exchanger in the middle, which servesas the evaporator for the topping cycle (cycle A) and the condenser for thebottoming cycle (cycle B). Assuming the heat exchanger is well insulatedand the kinetic and potential energies are negligible, the heat transfer fromthe fluid in the bottoming cycle should be equal to the heat transfer to thefluid in the topping cycle. Thus, the ratio of mass flow rates through eachcycle should beAlso,m # A 1h 5 h 8 2 m # B 1h 2 h 3 2 ¡ m# Am # h 2 h 3B h 5 h 8COP R,cascade Q# LW # m # B 1h 1 h 4 2net,inm # A 1h 6 h 5 2 m # B 1h 2 h 1 2(11–9)(11–10)