Document MMg2zX9xnmB49EyvY9EoRM4pa

942 CHAPTER 66 1965 Guide And Data' Book that of the heat-power thermodynamic cycle governing the* conversion between heat energy *r>H mtyVnifvd work. It is derived from the Second Law of Thermodynamics (see Chap* ter 67, Terminology). The operating efficiency, or coefficaeht of performance (CP), of an elementary perfect refrigeration system for cooling and heating effects is given by Equations 1 and 2 respectively.' xohere T, " evaporator temperature, Fahrenheit, absolute.-' , T, " condenser temperature, Fahrenheit, absolute. - With the ideal Carnot cycle operating as a heat pump, the co efficient of performance is In actual systems, the coefficients'of performance are much lower and are defined in the following terms:.'- The heating coefficient of performance, (CP)*, of an'in stalled heat pump may- be defined as the ratio of the total instantaneous useful heating effect produced by.;the- heatpump system at stated conditions, to the heat equivalent of the total energy input rate required to drive or operate the system. If total energy input of all auxiliaries such'as. fans and pumps is not included, it should be so stated. . The cooling coefficient of performance, (CP)e, of an- in stalled heat-pump may be defined as the ratio of the in stantaneous useful refrigeration effect produced by-the heatpump system at stated conditions, to the. heat equivalent of the total'energy input rate required to drive or operate the system: The term performance factor, (PF), is similar*to coeffi cient.of performance, but is used when* referring to values based on an extended period of time, such as a day, month) or. season. The .period of time covered should be given when using this term. If supplemental heat is involved,. its effect should also be specified. Heat Pump Types Heat pumps for air-conditioning service may be'classified according to (a), type of heat source and Rink, (6) heating and cooling.distribution fluid, (c) type of thermodynamic cycle, (d) type of building structure, and (e) size, and con figuration. The more common types are shown'in Table' 1. The air-to-air heat pump is the most common type. It is particularly suitable for factory-built unitary heat pumps, and has been widely used for residential and commercial applications. The first diagram in Table 1 is typical of the refrigeration circuit employed. In smaller unitary heat pumps, it is common to replace the expansion and check valves with a capillary tube for refrigerant control. A few installations have been made in which the forced-convection indoor Vat transfer surface has been replaced by a radiant panel. In air-to-air heat-pump systems, as shown in- the second diagram of Table 1, the air circuits may be interchanged by means of.dampers (motor-driven or manually operated),.to obtain either heated-or cooled air for the conditioned With this.system one heatrexchanger coil is always iy evaporator and the other is,.always.the condenser. The conditioned air will pass over the 'evaporator during the cooling cycle and the outdoor air will pass over the.condensef. The change from cooling to heating -is..accomplished, by positioning the dampers. . . A xoater-to-odr heat pump uses water as a heat source and sink, and uses air to transmit heat to or. from the conditioned space. Air-UywaUr heat pumps are commonly used ifi large build ings where zone control is necessary, and ere also sometimes' employed for the production of hot or cold water in industrial applications. Earth-io-air heat pumps may employ direct expansion of the refrigerant in an embedded coil, as illustrated in Table i, or they may be of the indirect type, described under the water-: to-airtype. A vxUer-to-vxiter beat pump uses water, as the heat source and sink for both cooling and heating operation. Heating- cooling changeover' may be accomplished in the (refrigerant circuit, but, in many cases, it is more convenient to perform the switching in the water circuits as illustrated in Table 1. *' An carth-to-xoater beat pump (not shown in Table 1) may be like the earth-to-air type shown, except for the substitution of a refrigerant-water heat erchangyr for. .the ,finned, coil shown on the indoor side. It may take-a form similar to the water-to-water system shown, when a' secondary-fluid ground coU is used... , Some heat pumps which use.earth as the heat source, and sink are essentially .of .the water-to-air .type. An-antifreeze solution is pumped through a circuit consisting of the chiller- condenser and a pipe coil embedded in the earth. Other types of heat pumps in addition to those listed in Table 1 are possible. An example is one which utilizes solar energy as a source of heat; its refrigerant, circuit may re semble the water-to-air, air-to-air, or other, types, depending on the form of solar collector and the,means .of. heating and cooling distribution employed. Another variation is the use of more than one-Vat.source. Some heat pumps have * utilized air as the primary heat source, but. are changed over-to extract heat.from-,water, (e.g., from a well or storage tank) during , periods of peak load. The use of solar energy requires anntyr heat source during periods of insufficient solar radiation.. - Other Refrigeration Cycles . .;' Any thermodynamic cycle that is capable of producing a cooling effect may theoretically be used as- a heat pump: Other than the ordinary vapor-compression cycle, possible cycles include: (a) the heat-operated absorption cycle,' (b)' the ejector-cycle, (c) gas cycles, both open closed,* and (d) the thermoelectric cycle.1* - ' 1 1 *'1-* It is not currently practiaable-to use any of these cycles as heat pumps, due to,limitations in efficiency, cost, or rise. rteai P^P5' - ; ^ HEAT.SOURCES' AND ..SINKS TabJe'2:8hows`-thc principal media' being-used' with heat pmjs"aa~a;heaf source for-heating and as a heat sihk-for cooling: The'most practical choice for a particular-applies 943 tion will be influenced primarily by geographic location, climatic conditions, initial cost, availability, and type of structure. Various factors to be considered for each source are given in Table '2. A more' detailed discussion of design and selection factors for each source and sink follows.