Document V3mqD8EOGV4O8gRqDJaw4a34p

918 CHAPTER 36 1957 Guide will probably be several years before proper development and design in formation will enable the widespread satisfactory application of the heat pump. It is for this reason that no design information is presented at this time in The Guide. A wealth of literature concerning individual installa tions is available, and reference to the many articles which have appeared in current technical magazines should form sufficient information for those interested in the development of this type of heating. BASIC REFRIGERATION EQUIPMENT Compression Refrigeration Machines Compression of the refrigerant gas drawn from the evaporator may be accomplished by one of several means. Positive displacement may be used as ini the reciprocating, centrifugal force may be applied as in the cen trifugal compressor; an ejector may be used as in the steam jet refrigeration Refrigeration 919 to the cylinders, but in some large compressors where there is danger of wet com pression or of foreign materials entering the compressor space, a secondary head known as a safety head, may be seated at the end of the cylinder and held in position with heavy springs; Normally, this head remains stationary, but excessive pressures in the clearance space are relieved by movement of the safety head, and thus prevent damage to the cylinder. Compressor Drives. Reciprocating compressors may be subdivided on the basis of source of motive power, and whether they are open or hermetic. Practically all modern compressors are electric motor-driven, although a few large, steam-driven compressors are still being installed where Bteam forms the most economical source of energy. In a few cases, as with truck transportation, the compressor may be driven by an internal-combustion engine. The division of compressors into open or-closed types is dependent upon whether the motive power is received from an external source, or whether the motor is direct drive and sealed within the housing. In the open type, power is received-from an cycle; or absorption of a low pressure refrigerant gas in a secondary fluid, followed by the absorbent's release upon application of heat, may be uti lized. A detailed discussion of the equipment required for each of these types of systems is beyond the scope of this chapter. For a more compre hensive treatment, reference may be made to the bibliography. The Fig. 11. Diagrammatic View of a Reciprocating Compressor present discussion is limited to positive displacement reciprocating com pressors, and centrifugal compressors. external source with one end of the compressor crankshaft extending through the Reciprocating Compressors Reciprocating compressors (see Fig. 11) may be classified according to rankcase, and usually V-belt driven. The point of emergence of the shaft from the wif v.S^iforms a weak point of refrigerant leakage, and is most frequently sealed tn a bellows type crankshaft'seal. Horizontal double-acting compressors operate witn a sliding piston rod, moving back and forth through a stuffing box. If the motor (a) cylinder design, (b) compressor drive, (c) valves, and (d) lubrication jsaireet-dnve an<i enclosed within the compressor housing, the compressor is classi- ! Si and cooling. as closed or hermetic. This eliminates the necessity of any shaft seal, and not di ,iPrevents re^r'ge.rant 'eakage at this point, but reduces operating noise. One Cylinder Design. Cylinder design may vary as to number, arngraement, and umauvantage is the inaccessibility of moving parts for repairs, but lubrication is t> action (*.e., single-acting or double-acting). Single-acting compressors usually have the luj s.lmP.**e<^.j8*nce both the motor and compressor operate in a sealed space with their cylinders arranged vertically, radially, or in a V or W shaped arrangement- i Double-acting compressors, with refrigerant gas drawn in, and compressed on botn the head and crank ends of the cylinder, are usually arranged horizontally. R" ciprocating units are available with from one to sixteen cylinders with the V, W, or radial arrangements best adapted to the greatest numbers. The present trend is toward higher operating Bpeeds with a low displacement per cylinder, together with an increase in the number of cylinders. Whereas the original reciprocating com pressors were slow speed (50 to 55 rpm) steam driven devices, modern electric motor- driven compressors range up to 3500 rpm. Cylinder heads are usually bolted tight oni"?pre8Sor ^olves. All refrigeration compressor valves are dependent for their sunt-10n uPon a difference in pressure between the inside of the cylinder and the adv ? r discharge line. Although mechanically-operated valves might have some or^ j C' they **ave Proved unsatisfactory because each change in the evaporator diffAndf-n1er'ope^ating Pressure requires a change of valve setting. The pressure tj. erentlals required for operation of the valves depend upon the valve design and loci0??"5!801- sPed- The suction and discharge valves may be arranged with both "ted in the compressor head, or with the suction valve on the top of the piston and