Document v69yJ171j1j4e75NM1ydB6rNY

710 CHAPTER 39 1948 Guide ance, is greater than that of a refrigeration cycle operating between the same temperature limits. In heating by the reverse refrigeration cycle energy is absorbed in an evaporator from some-available source of heat, pumped to a higher tem perature and delivered to a condenser *. The heat from the condenser is used for heating purposes. The compressor acts as a heat pump whose fundamental function is to raise the potential of the heat. The theoretical ratio of the heat delivered to the work,of compression,1 the coefficient of performance, is given in Equation 10. where (cop) = =-Tc iC -- l8 cop - coefficient of performance. r,, = absolute temperature of evaporator. Tc = absolute temperature of condenser. (10) Thus, with a small spread of temperature between the evaporator and the condenser, 6 or 8 times as much heat may be obtained theoretically, and 3 to 5.times practically, as the work introduced. There are a number of limitations, however, die most serious of which is the lack of ready availability of a practical source of heat. 1. Well water is the most desirable since its temperature is higher than other sources even in the winter, and thus a large amount of beat may be removed in relation to the weight of water handled. 2. Air may be used but its specific heat is low and its temperature uncertain. When the most heat is needed, the temperature of the air is lowest, thus resulting in the least favorable temperature combination. 3. It has been proposed to obtain heat by freezing water but this is still in the experi mental stage. Some of the other factors which act as limitations are: the large tem perature spread when using air as a source of heat arid when attempting to cool with even moderately low outside tempera&res, the frequent disparity between the size of the cooling load and heating load requiring extra equipment for a complete heating load, and the relatively high initial cost of equipment as compared to that at present available for heating by conventional means. Because of these limitations, the present application of the system is largely limited to temperate climates, such as Florida and Southern California, or to heating only for intermediate seasons, or to other locali ties which have peculiar advantages as, for instance, the ready availability of well water. In these locations it is frequently possible to do all of the heating necessary with the refrigeration equipment so that the extra cost is only that of reversing the functions of the condenser arid evaporator. There are a number of reversed systems now in operation, particularly among utility companies, using well water as the source of heat. These systems range in size up to 320 hp. In the case of the largest system in operation at present, the cost of the electrical energy would have to be approximately 0.7 cents per kilowatthour in order to compete with oil at 6 cents per gallon. _ A typical arrangement of a reversed cycle conditioning system where air is used as a source of heat is shown in Fig. 5. If the air seldom drops Refrigeration 711 below freezing, heat is often required in the morning and cooling during the afternoon in order to maintain comfortable conditions in such a system. The arrangement as shown lends itself to automatically changing over as required. REFRIGERATION EQUIPMENT AND ARRANGEMENTS Types of Compressors There are many different types of compressors, using various refrig erants. Each type has its advantages for its particular application, and those generally used for air conditioning are of the following types: 1. Reciprocating compressors (commonly referred to as piston type). 2. Centrifugal compressors. Reciprocating compressors are available in a wide range of sizes and types. Any of a number of refrigerants, including dichlorodifluoromethane (F-12), methyl chloride, ammonia, carbon dioxide, and sulphur dioxide may be used in reciprocating machines. The first of these is used exten sively in direct expansion systems of comfort air conditioning. Compressors may be classified into two general types, (a) open type, (b) enclosed type. If the driving mechanism is external to the compressor, then the shaft must be brought out through the crankcase and a shaft seal or stuffing box must be used to prevent escape of the refrigerant. This type of compressor is known as an open-type compressor. When the driving mechanism is located within the crankcase of the compressor in such a way as to avoid the necessity of a shaft seal, the compressor is known as the completely enclosed or hermetically sealed type. Open-type compressors may be further classified as belt driven and directly connected. A great number of direct-driven units are now being used which generally operate at higher rotational speeds than the beltdriven type. The present tendency is toward forced lubrication of the bearings of compressors by means of an oil pump driven from the crankshaft, although there are many splash lubricated compressors on the market. The chief