Document NNkJG9pKX47y3Y385B8d7JV0p

766 CHAPTER 39 1949 Guide' to cool with even moderately low outside temperatures, 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. - Both water and air are practical media to which the condenser heat may be rejected, but the generation of steam requires too high a temperature. Practical operation therefore dictates that the heat pump be used in con junction with either an air or water heating system. Since development of a heat pump to date includes the use of air, water,' and earth as heat sources and air and water as heat sinks, there are 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. A typical arrangement of a heat pump system with air as the source of heat is shown in Fig. 9. If the air seldom drops below freezing, heating is often. required in the morning and cooling during the afternoon in order to main-. tain comfortable conditions in such a system. The arrangement as shown lends itself to changing over automatically as required. 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 in the reciprocating, rotary or gear types of compressors; centrif ugal force may be applied as in the centrifugal compressor; an ejector may be used as in the steam jet refrigeration 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 utilized. A detailed discussion of the equipment required for each of these types of systems is beyond the scope of this chapter. For a'more comprehensive, treatment, reference may be made to the bibliography. The present discussion is limited to' positive displacement reciprocating compressors, rotary compressors arid centrifugal compressors. Reciprocating Compressors Reciprocating compressors may be classified according to (a) cylinder design, (b) compressor drive, (c) valves, and (d) lubrication and cooling. Cylinder Design. . Cylinder design may vary as to number, arrangement, and action (i:e. single-acting or double-acting). Single-acting compressors usually have their cylinders arranged vertically, radially, or in a V or W shaped arrangement. Double-acting compressors with.refrigerant gas drawn in and compressed on both the head and crank ends of the cylinder are usually arranged horizontally. Reciprocat ing 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 speeds with a low displacement per cylinder together with an in crease in the number of cylinders. Whereas the original reciprocating compressors were slowspeed (50 to 55 rpm) steam driven devices, modern electric motor driven compressors range up to 3500 rpm. Cylinder heads are usually bolted tight to the cylinders, but in some large compressors where there is danger of wet compression 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 as to the source of motive power and a^ .to whether they, are open or hermetic. Practically all Refrigeration 767; modern compressors are electric motor driven although a few large, steam-driven compressors are still being installed where steam forms the most economical source of energy. In a few cases, as with truck transportation, the compressor may; be internal combustion engine driven. 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 external source with one end of the compressor crankshaft extending through the crankcase and usually V-belt driven. The point of emergence of the shaft from the crankcase forms a weak point of refrigerant leakage and is most frequently sealed: with a bellows type crankshaft seal.' Horizontal double-acting compressors operate with a sliding piston rod, moving back and forth through a stuffing box. If the motor is direct drive and enclosed within the compressor housing, the compressor is classi fied as closed or hermetic. This eliminates the necessity of any shaft seal and not only prevents refrigerant leakage at this point but reduces operating noise. . One disadvantage is the inaccessibility of moving partB for repairs, but lubrication is greatly simplified since both the motor and compressor operate in a sealed space with the lubricating oil. Compressor Valves. All refrigeration compressor valves are dependent for their operation upon a difference in pressure between the inside of the cylinder and the Fig. 9. Schematic Operation of Reversed Cycle Conditioning System suction or discharge line. Although mechanically operated valves might have some advantage, they have proved unsatisfactory because each change in the evaporator orcondenser operating pressures requires a change of valve setting. The pressure differentials required for operation of the valves depend upon the valve design andthe compressor speed. The suction and discharge valves may be arranged with both located in the compressor head or with the suction valve on the top of the piston and the discharge valve in the compressor head .(uniflow arrangement). The valves them selves are usually classified as either poppet, ring-plate or flexing. Lubrication and Cooling. Rubrication of modern compressors is accomplished by either splash lubrication or forced lubrication.: The latter is used on large com pressors, while simple splash lubrication is used in the smaller units. Large compressors are usually water cooled with the water jacket either cooling the cylinder walls or both the cylinder walls and the compressor head. Small com-, pressors are either water cooled or air cooled with extended finned surfaces cast on the exterior of the cylinder. In a few cases small compressors may be found in which, there is no attempt to add any purposive cooling other than through non-finned sur faces to the lower temperature air. Water cooling is more effective than air cooling,* but even under the beat conditions cylinder cooling removes only a-portion of thesuperheat in the refrigerant gas. This removal'of heat from the cylinder results in some decrease in the work of compression as well as reduction in condenser load. '