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638 CHAPTER 38 1965 Guide And Data Book Wrist pin bushings in cast iron or forged rods are usually be used in a compressor. Speeds up to 1200 fpm as determined made of bronze with loadings up to 3000 psi- The finish here by Equation 8, have been applied. The limiting factor is gas as well as the aluminum or bronze rods should be 16 microinch. velocity through the valves. It is, therefore, the ratio between Clearances of 0.0002 to 0.0004 have been used successfully. piston area and valve area in a design that limits the piston Pressure lubrication to these areas isn't necessary but splash holes and open oil grooves must be placed in the unloaded zone anH some grooving is often carried into the loaded zone. Clearances on the crankpin end of the rod should be 0.00075 to 0.001 per inch of diameter. When the eccentric strap is used some means of retaining it on the crankpin must be made. This can be accomplished by the use of washer type springs between the strap at the wrist pin end and the piston pin bosses, to obtain a close fit between the strap and piston pin bosses or shields attached to the eccentric of the shaft. Bearing load on the eccentric di ameter will be very low due to the large size of the eye, but Piston speed: fpm =* 2S(rpm)/12 (8) where S -- stroke, inches. The wristpin usually is made of steel, case hardened to 50-60 Rockwell C and ground to an 8 microinch finish. It can be fitted tightly in the piston bosses and held in place with spring clips or can be made floating but restrained from striking the cylinder walls by similar clips. Pin diameter is determined by the size of connecting-rod bearing needed. the rubbing speed must be held below 3000 fpm.In small hermetic compressors in sizes below ) hp, the lubrication scotch yoke type piston-connecting rod arrangement is used Lubrication systems range from the simplest splash system advantageously. It is an economical assembly and-readily to the most elaborate forced feed systems with filters, vents formed by hydrogen brazing steel components. and equalizers. The type of lubrication required depends largely on bearing loads and application. Piston, Piston Ring and Wrist Pin For low to medium bearing loads and for factory assembled Pistons are usually made of cast iron or aluminum. Cast iron is used for tight fits to keep leakage to a minimum without nging rings. Aluminum is used where weight is of importance and one or more rings are always provided. Clearances between cylinder and piston vary from 0.0003 in. per inch diameter for small cast iron pistons to 0.003 in. systems where cleanliness can be controlled the splash system gives excellent service. Bearing clearances must be larger, however, otherwise oil does not enter the bearing readily. Thus, combined with the splashing effect of the dippers in the oil and the freer bearings, there is a tendency for the com pressor to operate somewhat noisily. Further, tire splash at high speed encourages frothing and oil pumping which, how per inch (or more) for large aluminum pistons. The minimum rWranr*. is determined by the theoretical expansion of the ever, is no problem in package type equipment but might prove so in remote systems where gas lines are long. t. materials, but the actual clearance used must be determined by tests because temperature differences between various points of the piston may cause distortions which greatly exceeds the expansion at uniform temperatures. Tapered or stepped pistons may be used to compensate for heat distortion without sacrificing the sealing effect of the piston skirt. Piston rings are used extensively, but care should be taken to determine if the second or third ring is improving or re A flooded system is so called to cover those arrangements which include discs, screws, grooves, oil-ring gears, or other devices which lift the oil to the shaft or bearing level. These devices flood tire bearing and are not much superior to splash systems except that the oil is not agitated as violently, and quieter operation results. Since little or no pressure can be developed by this, method, it is not correct to consider it as forced feed. ducing .the performance. It will often be found that the addition of a second or third piston ring will increase, the motor watts without increasing the capacity. Piston rings in a compressor will normally scrape oil'from the cylinder walls in the wrong direction, that is, into the cylinder space. If this is a problem, an oil scraping ring may be used. It must be provided with reliefs in the form of drain Forced feed lubrication develops pressure by means of a -pump, either gear, vane, or plunger, which forces oil under pressure into the bearing. Smaller bearing clearances can be used because there is adequate pressure to feed oil in sufficient quantity for proper bearing cooling. As a result, the com pressor is quieter in operation. holes in the piston or suitable recesses for oil return, otherwise the ring is next to useless. A cheap and often sufficiently Gear pumps are used to a large extent. Spur gears are simple but have a tendency to promote flashing of the refrigerant effective oil scraping effect can be obtained by leaving the dissolved in the oil. This is due to sudden opening of the tooth bottom edge of the piston as sharp as possible,instead of breaking the corner as is usual for other machined parts.. volume as two teeth disengage. This disadvantage is not apparent in internal type eccentric gear or vane pumps where . The finishes of the piston and cylinder wall should be better than 32 microinch. . The shape of the piston is determined to a great extent by - a gradual opening of the suction volume takes place. It will, therefore, be seen that the eccentric gear type pump, the vane pump or the piston pump will give better performance than the space required for the pin and the connecting rod. A one- simple gear pumps when pump is not submerged in the oil to-one ratio between length and diameter, with the'pin mid Oil pumps must be made with minimum clearances to per- way -between top and bottom will always provide an' ample mit pumping a mixture of gas and oil. There should be pro shoe for the connection rod. For economical design, however, vision on tiie discharge ride of the pump to bleed a email the pin may -be placed as close to the piston top as possible 1 quantity of oil into the crankcase. This vents the pump, pre and the skirt should be made no longer than necessary. The vents excessive pressure and assures more prompt priming. A minimtim length of the piston is determined to a large degree steamer should be inserted in the suction line to keep foreign by the cylinder-piston clearance^ Larger clearances will cause substances from the pump and bearings. more pronounced edge loadings'for the same piston length. A The strainer will not catch very- fine particles. If large length to a diameter ratio of 0.80 is considered satisfactory with a 0.001 to 0.002 in. clearance per inch diameter for'a connecting rod to crank arm ratio of 4.5 to 5. quantities of such particles are present and if bearing loadings are high, it may be necessary to use an oil filter in the dis charge side of the pump. There is no well defined limit to the piston speed that can In designing a compressor, some provision must be made Compressors 639 to return oil from the suction gas. The returning oil must flowinto the compressor crankcase. Opposing this oil flow there b a flow of gas from piston leakage,'so the velocity of the leak age gas must be tow to permit oil to separate from the gas. A separating chamber may be built as part of the compressor to help separate oil from the gas. In many designs, a check valve is inserted at the bottom of the 0il return port to prevent a surge of crankcase oil entering the suction. This check valve must be provided with a bypas, which is always open to permit the check valve to open wide after the oil surge has passed. When a separating chamber is used, the oil surge is trapped before it can' enter the suction port, thus making a check valve less essential Hermetic Motors Hermetic motors can be loaded considerably greater than open motors because of the suction gas cooling that is em ployed. Loads of 200-250 percent above nominal have been applied to these motors. The cost of a hermetic motor is normally determined by D*L where D is the outside diameter or frame size, and L is the stack length. It can be seen that on a fixed design D is a constant and L is the only variable which will effect a cost reduction. The D*L relationship can also be used as a parame ter in an attempt to redesign for the next lower frame size. To effectively use a hermetic motor, it is important to design the maximum load as close to. breakdown torque at 85 percent nominal voltage as possible. This is normally 80 percent of breakdown torque on single-phase motors and 85 percent on polyphase. Doing this will afford better operation at the low toad and possibly higher voltage conditions. The limiting factor at high toads is normally the motor tempera ture, while at light loads the limiting factor is the discharge temperature. Overdesign of the motor will result in higher. minimum load conditions because of the larger llR-losses which will reflect an increased actual suction and thus a higher discharge gas temperature. . . . It is therefore important to keepTosses to a minimum if the compressor is to be capable of operating at high compression ratios. The single-phase motor presents more problems in design in this area than does the polyphase,as the relationship between main and auxiliary windings becomes more impor tant and critical along with necessary starting equipment. The locked rotor rate of rise must be be kept low.enough to insure against excessive motor temperatures with the motor protection available. The maximum temperature under these jnditions should be held to 300 F. With better, protection, a higher rate of rise can be tolerated, and thus a less expensive motor may be used. The materials employed in these motors must display quali ties of high dielectric strength and. resistance to fluid and mechanical abrasion, as well as being compatible with an atmosphere of Refrigerant 22 or Refrigerant 12, and oil. Hermetic motor selection: ,1 1-Types f motors in use (by application) Refrigerator compressors > Low to medium torque--Split phase or P.S.C. 1 High-torque--C.8.CJL or F.S.C. . Room air-conditioner compressors ! PR.C. or C.S.C.B. u... ,. Central air conditioning and commercial refrigeration/- CR.C.R., 1-phaae, to 6 hp < :'1' o 3-phase,'2 hp to 100 hp - 2. Factors in selection a. Insulating system' ...* b. Efficiency and performance factor 'c. Starting and breakdown torques d. Temperatures and starting currents v..- e. Cost and availability - -: Seals Stationary and rotary type seals have been employed ex tensively on reciprocating compressors. Hie older stationary type usually employed a metallic bellows and hardened shaft for a wearing surface. Its use has dropped off considerably with one of the main reasons being a high first cost. - The rotary seal which is more prominently used is con siderably less expensive and trouble free. A synthetic :seal tightly fitted to the shaft prevents leakage at this point and also seals against the back face of a carbon hose. The front face of this carbon nose seals against a stationary cover plate. This type of design has been used on shafts up, to .4-in. diameter. The rotary.seal should be designed; so that'the carbon nose is never subjected to the full thrust of the shaft. The design should be capable of absorbing movement through the end play of the shaft. The spring should be so designed that a minimum cocking force is applied. Material should be such that a minimum of swelling and shrinking is encountered. Suction and Discharge Valves The limiting factors of piston speed are the valve area that can be accommodated and the gas velocities permissible without excessive throttling losses. The actual gas ve locities vary for different'points of the stroke. For design purposes, gas velocity = bore area X average piston speed /valve area. 'The permissible gas velocity through the re stricted area of the valve will depend upon the loss in volu metric efficiency and the excess power requirements the designer is willing to take. In general, with ammonia, ve locities up to 12,000 fpm and with Refrigerant 12, Refrigerant 22, velocities up to 6,000 fpm result in no material loss in volumetric efficiency or increase in horsepower. The velocity in the restricted valve area is not the only factor; the shape of the valve and adjacent ports, and the number of directional changes of the gas flow affects the efficiency of the conversionof velocity head to pressure head. ~ The ideal valve would meet the following specifications: ' 1. Largest possible restricted area, of the shortest possible length. 2. Straight gas flow, no directional changes. ` 3. Light weight combined with low lift for quick action. 4. Minimum unbalance. ' 5. No harmful clearance. 6. Rugged. 7. Inexpensive. 8. Tight seating. Most valves used today are in one of tire following groups: a. A free floating reed valve with a backing to limit' (he move ment. It seats against a flat surface with elongated ports. It is extremely simple and stresses can be calculated readily. It. is limited to relatively small port areas and multiples' are often used. As more reeds are used the back-stop machining or arrange ment becomes quite expensive. Totally backed with a' curved back-stop, it is probably the only valve that will withstand con tinuous slugging. , ;. ,, b. A reed, clamped at one end and with full back-stop support or a stop at the tip to limit the movement. This cantilever reed is more complicated than the free floating reed because it does not move in a simple way. Even where a curved back-stop is used the reed does not roll up on the curved surface as might be expected, but has a waving, snaking motion when opening. This motion will cause stresses far greater than those calculated from'the curvature of the back-^top. The waving movement must be timed so that the reed tip is not whipped into the valve plate or tip breakage wQl result. _ ' e. Free floating ring valoe. A truly free floating ring is seldom used because it would have only gas forces to return it to the seat after, it had opened. Since this gives unnecessary losses some type of spring return is provided in the form of coil springs or a wavy, flpring ring in bock of the valve. Ring valves are par ticularly adaptable to designs using'cylinder sleeves. - Without sleeves coring and machining become difficult and' expensive..