Document 3xExEX58XzX36n7eGeOj9yVE

712 CHAPTER 39 1948 Guide - advantages of the forced lubricated compressor are that the lubrication system requires less energy for its operation than the splash type; the oil can be easily filtered before it enters the bearings, and less^oil is usually , required. ' . The compressor capacity must be selected for and matched to the maximum load for the installation on which it is-to be used. Air-con ditioning loads, however, vary over a wide range, and a wide fluctuation in air conditions may result during periods of light load if on-and-6ff control of full compressor capacity is used. To prevent such undesirable fluctuation, several methods are employed to vary the capacity of reciprocating compressors, such as: 1. By-passing one or more cylinders, of a multi-cylinder compressor, from discharge to suction. 2. Rendering the suction valves of one or more cylinders of a multi-cylinder compressor inoperative. This is usually accomplished by depressing the suction valves. Refrigeration_____________________ . - ,,'______________________ 7J3 The evaporator is usually constructed as an integral part of the.centrifugal type condensing unit, to chill water which is then circulated to the air conditioning system. This is done because it would not be economical to pipe.these large volumes of refrigerant any distance. "Centrifugal compressors like reciprocating compressors can be:divided into two general types, open and enclosed. In general, the open type compressor is geared to the driving mechanism, and operates at higher speed, than the driving motor or turbine. A modern, completely enclosed, direct-driven centrifugal compressor is illustrated in Fig. 6. 3. Varying the speed of the compressor, usually by using variable speed or two-speed electric motors. 4. Using clearance pockets to control the quantity of refrigerant pumped. 5. Restricting the suction inlet to one or more of the cylinders of a multi-cylinder compressor either by an automatic modulating valve or by an on and off valve. All of these methods, with the exception of variable speed, result in a slightly lower over-all compressor efficiency when in use, since the mechanical losses remain constant whereas the Quantity of refrigerant pumped is lower. Centrifugal compressors are used with very low pressure refrigerants; usually both evaporator and condenser work below atmospheric pressure. Water and monofluorotrichloromethane (F-ll) are the refrigerants com monly used in centrifugal machines. , Compression of the refrigerant is accomplished by means of centrifugal force; therefore, this type of compressor is inherently suitable for large volumes of refrigerant at low pressure differentials. Two or more stages are usually required and high speeds are necessary to obtain good efficiency. 25 ' 30 35 . 40 45 50 EVAPORATOR TEMPERATURE, DEG FAHR 55 Fig. 7. Performance Characteristics of Compression Refrigeration Machines at Constant Speed The compressor capacity can be varied by controlling the condensing pressure. This , is accomplished by regulating the quantity and tern-; perature of the condenser cooling water. The capacity falls off with increasing condensing pressure. Centrifugal compressors are seldom built for less than 50 tons capacity, since it is not practical to. make impellers which pump much less than the volume of refrigerant required for this tonnage. Characteristics of Compression Systems The various types of compression systems have quite different charac teristics of capacity and power with varying evaporator and condenser temperatures, as may be noted from curves in Figs. 7 and 8. ; From Fig. 7 it may be observed that power requirements for the centri fugal compressor increase much more rapidly than for the reciprocating