Document r63GRyX4dr7Bm8JJM3xykBX1E

648 CHAPTER 38 1965 Guide And Data Boole must be considered in the selection of refrigerants for cen trifugal compression cycles: 1. The pressure ratio required between condenser pressure and evaporator pressure. It a preferable to maintain the lowest possible pressure ratio, in order to achieve the highest pomible compressor efficiency, and to obtain the greatest.possible useful range from; the compressor. Normal practice in the refrigeration industry has been to use pressure ratios up to 2.5 per stage for multi-stage machines, and up to 4.5 for aingle-etage machines. 2. The sonic velocity of the refrigerant. The peripheral velocity' neceeaary at the impeller tip to obtain the required measure ratio is related to the speed of sound in the refrigerant. This relation ship (Ut/at) is known as the impeller tip peripheral Mach number (also known as the rotational Maih number). Sonic velocities.of; the common refrigerants are relatively low, and the peripheral speeds for these applications are usually dictated by Mach.Dum ber considerations. With gases such as air and helium', where sonic' velocities are much higher, limits on peripheral speed are often? established by. the mechanical strength considerations. of the impeller. If rotational speed is fixed, a relatively high sonic, velocity will require a larger impeller diameter, and-this will result in a larger overall machine size. 3. The ratio of specific volume ofthe suction vapor to the refrigara-' Hon effect.' Together, items I and 2 establish the-relationship between rotational speed and diameter, in order to obtain the, required pressure ratio. This item establishes the limits.of di-_ ameter and rotational speed for which there will be reasonable' compressor efficiency at a given refrigeration load. When rotaA tional speed is also fixed, as it is in 3600 rpm (2 pole) direct-drive hermetic compressors, the first two items establish the impeller. tip diameter and item 3 establishes the capacity range for a given refrigerant In tbe early development of centrifugal compressors for refrigeration, the preceding factors led to the use of methylene chloride as a refrigerant. However, a wide variety of refriger ants became available with, the development of other halor, carbon compounds. These make possible the selection.of a refrigerant having suitable properties for virtually any specific use or type of equipment. Included in this group are the more, common-refrigerants: 11, 12, 22, 113, and 114. Of these, Refrigerant 11 has been used most extensively to date for,air conditioning with centrifugal refrigeration. Besides having, properties well suited to centrifugal compressors, it has.a' low operating pressure, high basic efficiency, and low cost. Refrigerants 12, 13, 500, and 22 have been used in high tonnage and low temperature applications. Refrigerant 114 has been utilized in intermediate capacity applications.: During recent years, the advent of .direct-drive hermetic, machines created a requirement for refrigerants with high' specific volume in order to obtain lower capacity ratings at the fixed rotational speed of 3600 rpm. Direct-drive,- 60-cycle, herraetics, therefore, employ Refrigerant 113 to obtain lower, capacities than those which could be efficiently obtained with Refrigerant 11. Currently, however, this same load range is bang covered by higher speed single-stage Refrigerant 11 com pressors. In tire industrial and chemical fields, ammonia, methylene chloride, isobutane, methyl chloride, ethylene, butane, propane, sulfur dioxide, dichloroethylene, and water vapor have all been used as refrigerants in centrifugal refrigeration systems. COMPRESSOR TYPES The centrifugal compressor has been used extensively in compression cycle refrigeration, because of its simplicity, its early development, and the fact that its optimum size falls within the useful capacity range. The centifugal compressor is a relatively high speed machine, in which a continuous stream of gas is compressed by a combination of centrifugal force and the conversion of kinetic energy into pressure by diffusion. Since the motion is one of pure rotation, it is possible to obtain smooth vibration-free mechanical operation. Open Type Compressors All early centrifugal machines, were of the open type. The success of these marhmfts is dependent upon the shaftseaL ' The seal design for centrifugal machines is particularly critical because it must perform several functions: 1. Seal against leakage of air into the system when the com-, pressor is operating below atmospheric pressure. 2. Beal against outward leakage of refrigerant when the'com pressor is operating above atmospheric pressure. 1 3. Seal against'oil entering the system. - --: 4. Perform the above functions, both during operation.and, when themachine fa shut down. ,-.j A variety of **! have been developed to meet these re quirements. While' individual designs vary, there are several points common to .all centrifugal compressors used for re frigeration: L A rotatingseal ring is fixed to the shaft and rotates at com pressor epeed/This ring has & highly polished surface, in a plane, perpendicular to the'shaft.., ' 2. A stationary seal ring is fixed to the compressor caring and: does notrotate. It is free to move along the'axis of the compressor* shaft when- displaced, due to-thrust.forces.- It has a highly: pnltahftd surface which contacts,the rotating seal ring. This is the point at which the rotating element is sealed from the stationary, structure of the compressor. , 3. The seal surfaces are lubricated and cooled by oiL 4. Oil which may leak-into the compressor past the seal mem bers is .then returned directly to the oil reeervoir, usually by; gravity drainage. 5. Oil leaking outward past the seal b collected in an oil return' chamber and returned either manually or automatically to the oil reservoir: Variations in seal design involve the physical structure and arrangement of-the Stationary and,rotating rings, lubri cation and cooling provisions, and the method of seal oil return. One <***1 used for low pressure refrigerants uses polished metal contact surfaces on rotating and stationary seal rings.' These surfaces are separated when running, and the seal is maintained by a stream of oil flowing between the seal, faces. During shutdown, tiie' sealing faces are held together by spring pressure. Another type of seal employs one or more additional seal members between stationary and rotating rings. One or more carbon rings are held between the rotating and stationary caal rings by, spring pressure, oil pressure, or a combination' of the two. The seal faces and,carbon ring are lubricated and' cooled by oil, and are held in a fixed relationship to each other while operating and.when shut.down. The carbon ring is free to rotate between the rotating and stationary seal rings. Many additional seal arrangements are in use that are as effective as those described. Those listed are merely intended ' to show the diversified .designs available to meet the basicrequirements for centrifugal compression duty.. Open type mmbini>B may be provided with either an inboard j or outboard bearing design. With the inboard design, the, hearing chambers are vented to the refrigerant passages and a ginglo shaft seal is employed. With the outboard design,-the hpAiing chambers and oil reservoir are vented.to atmosphere, ftnH shaft b*a1 are provided on both ends of the shaft, be tween the gas pansag*1** and the bearings. `With the latter oil leakage outward drains directly to the oil reservoir. Inward oil leakage may have provisions for automatic .return to the reservoir, or may be limited to such minute amounts that oil return is not deemed necessary. J Open type m^hinog have been built with from one or two etagoo (for water shilling applications) to nine stages (for use with ammonia). Three and four stages are often used for low temperature applications requiring `a -high' pressure ratio. Compressors 1 649 Special drwe4hrough compressor arrangements with as many as twelve stages have been built, for very high compression ratios. While hermetic compressors are now used for most standard electrically driven water chilling applications, the open type compressor is still used with `steam turbine drives and other suitable prime movers for special industrial, and phofnitrflj applications. Some open type machines use variable inlet-guide vanes as a capacity control, but many use variable qywd or suction throttling, or both. Mast open type compresor eft-rings are made of a high grade castiron and are horizontally split for easy assembly, inspection, and' service. Impellers are either fabricated of carbon steel (which is often lend coated for corrosion protection), manufactured of stain less material,' or are precision cast of aluminum. Brazed steel and brazed forged aluminum are also used. Barrel-type,casing designs are used with high pressure re frigerants. The internal design is similar'tothatof the hori zontally split casing machines, but the entire unit is wrapped in a fabricated steel or forged steel barrel with' end-flange type construction.` The bearings and seals are mounted on the end flangwa. This design offers' considerable reduction in - casing or shell) thickness, and hence'reduces machine size and weight. Hermetic Compressors .The-hermetic compressor is basically similar to the open type, except.for the inclusion of the motor within the refrig erant space, which eliminates the shaft seal. However, certain limitations and innovations have characterized the hermetic machines. They have been limited to either one or two stages, utilizing variable inlet guide vanes for capacity control The two-stage models use, the guide vanes on either one stage' or on both stages. Hermetic compressors have either shrouded or unshrouded impellers. They .are generally single-purpose machines used for water chilling applications. Some two- stage models consist of two single-stages mounted on opposite ends of the motor, with a connecting crossover pipe between the first stage volute and the second stage inlet. Other models have two stages mounted axially on the same end of the motbfj with a conventional crossover between the stages. The two7 stage machines are normally, directly connected to ihe motor shaft, and are driven by 2-pole motors. Single-stage units which utilize smaller diameter impellers, driven at higher,rotational speeds are, now available. Most of these designs utilize speed increasing gears for 50/60~cycle electrical power. However, they are available as directs driven units, with high frequency power supplied by special . motor-generator'sets. The early hermetic designs were in the. low tonnage range (100 to 400 tons) employing.Refrigerant 113 or Refrigerant 11. These.were followed by units employing Refrigerant 11, up to a capacity of 1500 tons, and. Refrigerant'.114,' to as high as 2000 tons. The present' trend is toward the' use'of higher pressure refrigerants to reduce compressor size to a minimum. With the smaller compressor casings, it is possible to mount the compressors on the heat exchangers to give a completely preassembled cooling package. This mimimizes installation costs and space requirements. Motor cooling on hermetic centrifugal compressors haw been accomplished by: , cooling. Circulating chilled water or condenser water "rourfi a jacket or cored passages in the motor stator. u* 5oo**n0- Drawing suction, intercooler, or condenser gas through the motor passages. . 3* Lyjnid cooling.' Injection of high pressure liquid refrigerant the motor, at either normal condensing temperature or sub- 000 jV 4 P*ht below condensing temperature, where it is ex panded to suction pressure. SYSTEM COMPONENTS Evaporators and Condensers Evaporators and condensers for centrifugal refrigeration systems are of the horizontal shell-and-tube type. (See Chap ters 42 and 43.) The most common versions have a steel shell and copper tubes of the integral rolled-fin type, with water inside the tubes and refrigerant in the shell, and with'two or more support sheets to support the tubes. Some have fabri cated or cast-iron separable water boxes and others have integral welded water boxes. All have separate end plates (some containing the water nozzles) which can-be removed so that the tubes can be cleaned by mechanical means from either or both ends. The tubes fill the lower part of> the shell in the evaporator, in such a manner that a number of different pass arrange ments can be obtained on the water side. Liquid refrigerant, .toplus the expansion-flashed gas, flows in at the bottom of the partially flooded evaporator. The boiling action serves splash liquid over the full tube surface area. The space used for separating the liquid splashed up by the boiling action is directly above the tubes. Some designs have sufficient space for gravity separation, and others have' eliminators-with gas space above, through which the dry vapor flows'.to the evaporator discharge pipe. The tubes fill the major part of the shell in the condenser, leaving only enough extra space to distribute the gas through out the shell. Just as in the evaporator, the tubes are laid out to permit a number of different pass arrangements. There are normally no segmented baffles to control crossflow, but there is often a plate directly in front of. the. condenser.inlet to deflect the flow from the compressor to either side, thus avoiding direct impingement of the high velocity, compressor discharge gas on. the tubes. There may also be a longitudinal baffle to assist in isolating nonoondensables in selected areas for efficient removal. The load on the condenser of a system consists of the evaporator load plus the work of compression. See Chapters 42 and 43 for heat transfer and other details of of evaporators and condensers. To obtain a more compact design, it is possible to combine the evaporator and condenser in a single shell. Several recent designs have been offered on this basis. The evaporator and condenser sections are separated by a longitudinal baffle which acts to seal the high side (condenser) tube bundle from tiie low side (evaporator) bundle. Other Components Another major part of every refrigeration system is the refrigerant flow control device. In centrifugal refrigeration the most common refrigerant flow control device in use is the float valve, which is especially suited for use with flooded type heat exchangers, and which automatically regulates the flow of refrigerant from the condenser to the evaporator. The float is usually placed on the high pressure side of the valve, thereby maintaining the condenser essentially free of liquid. Many standard centrifugal refrigeration machines today have a lubricating system that is vented to the refrigerant. Some machines have an integral pump, driven directly, or by gearing, from the compressor shaft. Others use a separate motor-driven gear pump, immersed in the oil sump which is usually contained in the compressor base. Journal bearings and collar, or Kingsbury type, thrust bearings, are used almost exclusively; one exception is the special high-speed aircraft refrigeration compressor which uses ball bearings. Centrifugal refrigeration systems, especially those opdating with refrigerants having pressures below atmospheric, require