Thermodynamics

618 | Thermodynamicshalogenated CFCs (such as R-11, R-12, and R-115) do the most damage tothe ozone layer. The nonfully halogenated refrigerants such as R-22 haveabout 5 percent of the ozone-depleting capability of R-12. Refrigerants thatare friendly to the ozone layer that protects the earth from harmful ultravioletrays have been developed. The once popular refrigerant R-12 has largelybeen replaced by the recently developed chlorine-free R-134a.Two important parameters that need to be considered in the selection of arefrigerant are the temperatures of the two media (the refrigerated space andthe environment) with which the refrigerant exchanges heat.To have heat transfer at a reasonable rate, a temperature difference of 5 to10°C should be maintained between the refrigerant and the medium withwhich it is exchanging heat. If a refrigerated space is to be maintained at10°C, for example, the temperature of the refrigerant should remain atabout 20°C while it absorbs heat in the evaporator. The lowest pressure in arefrigeration cycle occurs in the evaporator, and this pressure should be aboveatmospheric pressure to prevent any air leakage into the refrigeration system.Therefore, a refrigerant should have a saturation pressure of 1 atm or higher at20°C in this particular case. Ammonia and R-134a are two such substances.The temperature (and thus the pressure) of the refrigerant on the condenserside depends on the medium to which heat is rejected. Lower temperaturesin the condenser (thus higher COPs) can be maintained if therefrigerant is cooled by liquid water instead of air. The use of water coolingcannot be justified economically, however, except in large industrial refrigerationsystems. The temperature of the refrigerant in the condenser cannotfall below the temperature of the cooling medium (about 20°C for a householdrefrigerator), and the saturation pressure of the refrigerant at this temperatureshould be well below its critical pressure if the heat rejectionprocess is to be approximately isothermal. If no single refrigerant can meetthe temperature requirements, then two or more refrigeration cycles withdifferent refrigerants can be used in series. Such a refrigeration system iscalled a cascade system and is discussed later in this chapter.Other desirable characteristics of a refrigerant include being nontoxic,noncorrosive, nonflammable, and chemically stable; having a high enthalpyof vaporization (minimizes the mass flow rate); and, of course, being availableat low cost.In the case of heat pumps, the minimum temperature (and pressure) forthe refrigerant may be considerably higher since heat is usually extractedfrom media that are well above the temperatures encountered in refrigerationsystems.11–6 ■ HEAT PUMP SYSTEMSHeat pumps are generally more expensive to purchase and install than otherheating systems, but they save money in the long run in some areas becausethey lower the heating bills. Despite their relatively higher initial costs, thepopularity of heat pumps is increasing. About one-third of all single-familyhomes built in the United States in the last decade are heated by heat pumps.The most common energy source for heat pumps is atmospheric air (airto-airsystems), although water and soil are also used. The major problemwith air-source systems is frosting, which occurs in humid climates whenthe temperature falls below 2 to 5°C. The frost accumulation on the evapo-