Document jZyq6eZM1oJj8erRx1N9r1KR

Refrigeration 867 installing a smaller refrigeration plant, augmented by a storage system, and by operating it for longer periods. The Heat Pump It has been almost 100 years since Prof. William Thomson (Lord Kelvin) first proposed the use of a compressor as a, "warming engine" and as a means of heating buildings to replace equipment for direct burning of fuels. Several early working models were constructed, but the device has remained essen tially of laboratory interest until the last 20 years. Although frequently referred to incorrectly as the reverse cycle system, the heat pump cycle is identical with the ordinary refrigeration cycle, and differs only in the sense that the desired effect is rejection of the heat from the condenser rather than absorption of heat in the evaporator. A discus sion of the coefficient of performance for the heat pump is found earlier in this chapter. The first actual residential heat pump installation was probably made in Scotland in 1927 and since that time, a number of commercial and residen tial systems have been made in this country. Both progress and growth of interest have been particularly rapid in recent years and, consequently, at the present time there are several hundred residential installations and probably a greater number of commercial systems. However, much re search is needed before the residential heat pump installation can success fully emerge to compete economically and with equal reliability with the more common forms of heating and fuels. From an analysis of the equation for the coefficient of performance, it is evident that the economical adaption of the heat pump as a practical means of heating, requires that the temperature of the source from which the heat is extracted be as high as possible, and that the temperature of the sink to which the heat is rejected for heating purposes, be as low as possible. Thus, with a small temperature spread between the evaporator and the con denser, six or more times as much heat may be obtained theoretically (and three to five times practically) as the heat equivalent of the work necessary to operate the system. There are a number of limitations, however, the most serious of which is the lack of ready availability of a practical source of heat. One of the major problems in the development of the heat pump involves research on, and the compilation of reliable design data for, the various heat sources and sinks available. ' The four principal potential sources of heat are air, water, earth, and solar energy. Of these, the first three are primary sources of heat which may be used alone. The fourth, solar energy, while of tremendous potentiality, will probably be developed in most localities as auxiliary to the other three. In addition, there are other minor sources such as process waste heat, sewage, etc., which may be used under special circumstance. There are also a number of industrial applications of heat pumps, for purposes other than space heating, which are practical largely through economic considerations of the particular process involved. Table 4 pre sents a summary of the advantages and disadvantages of each of these major heat sources. By reference to Table 4, it will be seen that, to date, the most satisfactory heat sources are air, water, and earth, and that air and water are the most satisfactory heat sinks. There are, therefore, six possible combinations of source and sink in application: air to air, air to water, water to air, water to water, earth to air, and earth to water. In addition, it should be recog-