Frictionless Compressor Technology
A SEMINAR REPORT ON “Frictionless Compressor Technology” In the partial fulfillment for award the degree of Bachelor of Engineering From University of Rajasthan, Jaipur Guided By:Mr. Kuldeep Sharma Lect. Department of Mechanical Engineering Submitted By:Akshat Yadav IV Year, Mechanical Engineering DEPARTMENT OF MECHANICAL ENGINEERING JAIPUR ENGINEERING COLLEGE AND RESEARCH CENTRE Session 2007-2008 JAIPUR ENGINEERING COLLEGE AND RESEARCH CENTRE CERTIFICATE This is to certify that Akshat Yadav, student of VIII Semester, B. E. , Mechanical Engineering, has completed the work of seminar and compiled the report entitled “ the submission.
Frictionless Compressor Technology” under my guidance and supervision. Report has been found satisfactory and approved for Date: Mr. Kuldeep Sharma Lect. Department of Mechanical Engineering ACKNOWLEDGEMENT I wish to express my sincere gratitude to Mr. Pankaj Sharma, Asst. Professor & Head, Department of Mechanical Engineering, who not only inspired me to work on this seminar but also guided me at each and every step so as to bring out this report in the present form. I would also like to thank s Mr. Kuldeep Sharma Lect. Department of Mechanical Engineering for their valuable advices at crucial times.
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I would also like to thank my friends who provided me with a proper atmosphere of study. Finally I would like to thank the almighty for his blessings without which this work could not have been accomplished. Akshat Yadav IV Year, Mechanical Engineering Table of Contents 1. Types of compressors 1. 1 Centrifugal compressors 5 1. 2 Diagonal or mixed-flow compressors 1. 3 Axial-flow compressors 1. 4 Reciprocating compressors 1. 5 Rotary screw compressors 1. 6 Rotary vane compressors 1. 7 Scroll compressors 1. 8 Diaphragm compressors 2. The “Emerging Technology in Centrifugal Compressor 3.
Frictionless compressor 4. Mechanicalcomponents11 5. Advantages 6. Applications 7. References 24 25 26 10 9 1. Gas compressor A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible, so the main action of a pump is to transport liquids. Types of compressors The main types of gas compressors are illustrated and discussed below:
Centrifugal compressors Centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants.  Their application can be from 100 hp (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa).
Many large snow-making operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines. Diagonal or mixed-flow compressors Diagonal or mixed-flow compressors are similar to centrifugal compressors, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to the axial direction. The diagonal compressor has a lower diameter diffuser than the equivalent centrifugal compressor.
Axial-flow compressors Axial-flow compressors are dynamic rotating compressors that use arrays of fan-like aerofoil to progressively compress the working fluid. They are used where there is a requirement for a high flows or a compact design. The arrays of aerofoil are set in rows, usually as pairs: one rotating and one stationary. The rotating aerofoil’s, also known as blades or rotors decelerate and pressurize the fluid. The stationary aerofoil’s, also known as a stators or vanes, turn and decelerate the fluid; preparing and redirecting the flow for the rotor blades of the next stage. 1] Axial compressors are almost always multi-staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial Mach number. Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is normally used to improve operation. Axial compressors can have high efficiencies; around 90% polytrophic at their design conditions. However, they are relatively expensive, requiring a large number of components, tight tolerances and high quality materials.
Axial-flow compressors can be found in medium to large gas turbine engines, in natural gas pumping stations, and within certain chemical plants. Reciprocating compressors A motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders. Reciprocating compressors use pistons driven by a crankshaft. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Small reciprocating compressors from 5 to 30 horsepower (hp) are commonly seen in automotive applications and are typically for intermittent duty.
Larger reciprocating compressors up to 1000 hp are still commonly found in large industrial applications, but their numbers are declining as they are replaced by various other types of compressors. Discharge pressures can range from low pressure to very high pressure (>5000 psi or 35 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, noisier, and more costly than comparable rotary units.
Rotary screw compressors Rotary screw compressors use two meshed rotating positive-displacement helical screws to force the gas into a smaller space. These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Their application can be from 3 hp (2. 24 kW) to over 500 hp (375 kW) and from low pressure to very high pressure (>1200 psi or 8. 3 MPa). They are commonly seen with roadside repair crews powering air-tools.
This type is also used for many automobile engine superchargers because it is easily matched to the induction capacity of a piston Engine Rotary vane compressors Rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor. The rotor is mounted offset in a larger housing which can be circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing.  Thus, a series of decreasing volumes is created by the rotating blades. Rotary Vane compressors are, with piston compressors one of the oldest of compressor technologies.
With suitable port connections, the devices may be either a compressor or a vacuum pump. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Dry vane machines are used at relatively low pressures (e. g. , 2 bars) for bulk material movement whilst oil-injected machines have the necessary volumetric efficiency to achieve pressures up to about 13 bars in a single stage. A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor.
Scroll compressors A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases. The vane geometry may be involutes, Archimedean spiral, or hybrid curves. They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range. Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid or gas between the scrolls.
Diaphragm compressors A diaphragm compressor (also known as a membrane compressor) is a variant of the conventional reciprocating compressor. The compression of gas occurs by the movement of a flexible membrane, instead of an intake element. The back and forth movement of the membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in touch with the gas being compressed. Diaphragm compressors are used for hydrogen and compressed natural gas (CNG) as well as in a number of other applications. The “Emerging Technology in Centrifugal Compressor • Refrigerant Compressors • • •
Primarily used for commercial and industrial comfort and process cooling Extremely efficient operation Oil free design The Frictionless compressor is the world’s first totally Oil-Free compressor specifically designed for the Heating, Ventilation, Air Conditioning and Refrigeration (HVACR) industry. The convergence of aerospace and industrially proven magnetic bearings, variable-speed centrifugal compression and digital electronic technologies enables the frictionless compressors (nominal 60-150 ton capacity range) to achieve the highest compressor efficiencies, cost effectively, for middle-market, water-cooled, vaporativecooled and air-cooled HVACR applications. The well-proven energy performance advantages of variable-speed centrifugal Compressors are now brought to mainstream middle-market applications through the use of High-speed, two-stage centrifugal compression with integral variable-speed drive. Compressor speed is reduced as the condensing temperature and/or heat load reduces, Optimizing energy performance through the entire operating range from 100% to 20% or Below of rated capacity.
Operations to near zero loads are achievable via an optional, digitally controlled, load balancing valve. Centrifugal compressors tend to be more efficient than screw or scroll compressors, and take advantage of speed control more effectively, but they are usually only available in larger sizes. By using the smaller shaft, they are able to take advantage of the centrifugal compressor technology in a smaller size than is normally available. F r i c t i o n- l e s s Te c h ngyo lo Compressor
A new compressor technology introduced during the 2003 International Air-Conditioning, Heating, Refrigerating Exposition (AHR Expo), held l January in Chicago, may have a significant effect on the future of mid-range chillers and rooftop applications in watercooled, evaporative cooled, and air-cooled chilled water and direct-expansion (DX) systems. Designed and optimized to take full advantage of magnetic-bearing technology, the compressor was awarded the first AHR Expo Innovation Award in the energy category, as well as Canada’s Energy Efficiency Award for its potential to reduce utility-generated greenhouse-gas emissions.
The compressor is key to a new water cooled centrifugal-chiller design, with Air-Conditioning and Refrigeration Institute (ARI) tests indicating integrated part-load values (IPLVs) not normally seen with conventional chillers in this tonnage range. This article describes this new compressor technology and its first use in an ARI-certified chiller design. Problem with Traditional Technology 1. Oil-lubricated equivalent wastes more on friction and irreversible loss. 2. Oil fouling costs more due to higher ? T. 3. On / off control costs more. 4. Part-load inefficiency costs more for traditional chillers.
Solution with Frictionless Compressor 1. Frictionless oil-free using magnetic bearings. 2. Soft start and Ramp up. 3. Power Management. 4. No COP deterioration with time. 5. Low noise and smooth Operation. 6. Low maintenance costs as there are no wearing parts. The First Intelligent Compressor Mechanicalcomponents 1 Magnetic bearings and bearing sensors • Composed of both permanent and electromagnets • Enables precisely controlled frictionless compressor shaft rotation on a levitated magnetic cushion • Bearing sensors located at each magnetic bearing, , feedback rotor orbit and thrust/axial information in real time to Bearing control Cross-section of axial bearing 2 Permanent-magnet synchronous motor • Powered by PWM (pulse width modulated) voltage supply • High-speed variable frequency operation affords high efficiency, compactnes and soft start capability s 2 3 Touchdown bearings • Carbon-lined radially and axially located bearings support the rotor when the compressor is not energized • Prevents contact between the rotor and other metallic surfaces 4 Shaft and impellers • Only one major moving compressor component • Acts as rotor for permanent-magne synch t ronous motor • Impellers are keyed directly to the motor rotor
Cross-section of radial bearing 4 3 3 1 5 2 1 6 4 5 Compressor cooling • Liquid refrigerant flow is controlled electronically, cooling electronic, mechanica and electromechanical l compressor component to assure maximum efficiency s and safe operation 6 Inlet guide vane assembly • Trims compressor capacity and is digitally integrated With the variable-spee control, to optimize energy d Efficiency and compressor performance 6 1 5 2 3 6 4 6 8 7 9 10 11 Electrical Components 1 Soft start module • Significantl reduces high in-rush current at startup y • The startup inrush current is only 2 amps vs. ypically up to 500-60 amps 0 Experienced by traditional screw compressor in this tonnage range – truly s redefining soft starts 2 Variable frequency drive • IGBT (Insulate Gate Bipolar Transistor is an inverter d ) That converts a DC voltage into an adjustable three-phas AC voltage e • Signal from the motor/bearing controller determine the inverter output s frequency, voltage and phase, thereby regulating the motor speed converts mechanica energy back into electrical energy.
In case of power failure, l This patented control scheme allows for a normal deLevitation and shutdown 3 Three-phase terminal block • Connection point for primary power supply 4 Rectifier • Converts AC line power into a high-voltage DC power source for motor, bearings and control operations 5 Capacitors • Energy storage and filter for smooth DC voltage • Provide power to the magnetic bearings, along with motor rotation, to ensure rotor shaft evitation through Compressor coast down in the event of an external power loss 6 DC-DC converters • Supplies and electrically isolates the high and low DC voltages required for the control circuits Controls connection Network connection for external control and monitoring Bearing sensor feed through • Hermeticall sealed connection enabling the transfer y s of power to the electromagnetic bearings and shaft position and rotation signals to the control modules 7 8 1 2 3 9 10
Driver Board/EXV Control Compressor and bearing controller • Central processor of the compressor system • Continuousl updated with critical data from the y motor/bearing and external sensor that indicate the s compressor and chiller/rooftop package Operating status • Software enabled, it responds to changing conditions and requirements to ensure optimum system performance • Computes the required shaft position signalsthat control the magnetic bearings • Processe motor current information s to control motor speed 11 PWM amplifier • Supplies power to the electromagnetic bearings 8 11 THE BEARINGS Traditional centrifugal compressors use roller bearings and hydrodynamic bearings, both of which consume power and require oil and a lubrication system. Recently, ceramic roller bearings, which avoid issues related to oil and reduce power consumption, were introduced to the HVAC industry. The lubrication of these bearings is provided by the refrigerant itself. The TT300 compressor’s onboard digital electronics manage operation while providing external control and Web-enabled access to a full array of performance and reliability information.
Magnetic-bearing technology is significantly different. A digitally controlled magnetic-bearing system, consisting of both permanent magnets and electromagnets, replaces conventional lubricated bearings. The frictionless compressor shaft is the compressor’s only moving component. It rotates on a levitated magnetic cushion (Figure 1). Magnetic bearings—two radial and one axial— hold the shaft in position (Figure 2). When the magnetic bearings are energized, the motor and impellers, which are keyed directly to the magnetic shaft, levitate.
Permanent-magnetic bearings do the primary work, while digitally controlled electromagnets provide the fine positioning. Four positioning signals per bearing hold the levitated assembly to a tolerance of 0. 00002 in. As the levitated assembly moves from the center point, the electromagnets’ intensity is adjusted to correct the position. These adjustments Occur 6 million times a minute. The software has been designed to automatically compensate for any out-of-balance condition in the levitated assembly. FIGURE 1. Electromagnetic cushions continually change in field strength to keep the rotor shaft centrally positioned.
FIGURE 2. A digitally controlled magnetic-bearing system consisting of two radial and one axial bearing levitate the compressor’s rotor shaft and impellers during rotation. SHUTDOWNS AND POWER FAILURES When the compressor is not running, the shaft assembly rests on graphite-lined, radially located touchdown bearings. The magnetic bearings normally position the rotor in the proper location, preventing contact between the rotor and other metallic surfaces. If the magnetic bearings fail, the touchdown bearings (also known as backup bearings) are used to prevent a compressor failure.
The compressor uses capacitors to smooth ripples in the DC link in the motor drive. Instantaneously after a power failure, the motor becomes a “generator,” using its angular momentum to create electricity (sometimes known as back EMF) and keeping the capacitors Charged during the brief coast down period. The capacitors, in turn, provide enough power to maintain levitation during coast down, allowing the motor rotor to stop and delevitate. This feature allows the compressor to see a power outage as a normal shutdown. OIL-FREE DESIGN
Oil management, particularly as it pertains to the lubrication of compressor bearings, is a critical issue in refrigeration system design. But with magnetic bearings, this issue is avoided. Only a very small amount of oil is required to lubricate other system components, such as seals and valves; often, however, experience shows that even this small amount of oil is not needed. Avoiding oil-management systems means avoiding the capital cost of oil pumps, sumps, heaters, Coolers, and oil separators, as well as the labor and time required to perform oil related Services.
Reports indicate that for many installations, compressor-maintenance costs have been cut by more than 50 percent. Most air-cooled products (including chillers, rooftop units, and condensing units) use DX evaporators. Most DX systems allow oil to travel through the refrigeration circuit and back to the compressor oil sump. Great care must be taken during design to provide oil return, particularly at part load, when refrigerant flow rates are reduced. Water-cooled chillers often use flooded evaporators.
In a flooded evaporator, even small amounts of oil can coat evaporator tubes and significantly diminish chiller performance. This can lead to an elaborate oil-recovery system. Magnetic bearings eliminate the need for these systems and oil management in general. In fact, the only required regular maintenance of the compressor is the quarterly tightening of the terminal screws, the annual blowing off of dust and cleaning of the boards, and the changing of the capacitors every five years.
Complete service agreements and extended maintenance contracts can be provided by the manufacturer. THE MOTOR Most hermetic compressors use induction motors cooled by either liquid or suction-gas refrigerant. Induction motors have copper windings that, when alternating current is run through them, create the magnetic fields that cause the motor to turn. These copper windings are bulky, adding size and weight to the compressor. Two-pole, 60-Hz induction motors operate at approximately 3,600 rpm. A higher number of revolutions per minute can be obtained by increasing the frequency.
Compressors that require higher shaft speeds tend to use gears. While gears are a proven technology, they create noise and vibration, consume power, and require lubrication. The magnet-bearing compressor features a synchronous permanent-magnet brushless DC motor with a completely integrated variable-frequency drive (VFD). The stator windings found conventional induction motors are replaced with a permanent-magnet rotor. Alternating current from the inverter energizes the armature windings. The stator (excitation) and rotor (armature) change places.
The motor and key electronic components are internally refrigerant cooled, so no specialcooling is required for the VFD or the motor. The use of permanent magnets instead of rotor windings makes the motor smaller and lighter than induction motors. Using magnetic-bearingtechnology, a 75-ton compressor weighs 265 lb —about one-fifth the weight of a conventional compressor. A variable-spee drive (VSD) is d required for the motor to operate. The VSD varies the frequency between 300 and 800 Hz, which provides a compressor-speed range from 18,000 to 48,000 rpm. This avoids a gear set.
The VSD is integrated into the compressor housing, avoiding long leads and allowing key electronic components to be refrigerant-cooled. The VSD also acts as a soft starter; as a result, the compressor has an extrem ely low startup in-rush current: le ss than 2 amps, compared with 500 to 600 amps for a traditional 75-ton, 460-v screw compressor with a cross-the-linestarter. With the integration of the motor, VSD, and magnetic-bearin system, the capacitors required for the g motor and drive can be us ed as a backup power source f or the b e a rin gs in the event of a power outage or emergency shutdown.
C A P A C IT Y D E F F IC IE N C Y AN Among the key parameters affecting performance are capacity (tons) and efficiency (kilowatts per ton). The compressor’s capacity ranges from 60 to 90 tons, depending on the operating conditions. Plans call for that range to be extended to 150 tons water-coole and 115 tons air-cooled by the end d of 2004 with the use of R-134a refrigerant. An R-22 version is planned for retrofit applications. Efficienc improvements stem from a combination of the centrifugal compressor, permanent y magnet motor, and magnetic bearings.
Within the compressor, efficienc is affected by the y Compressor isentropic efficienc (the efficienc of the wheels) the motor, and the bearings. y y , Traditional induction motors of this size typicall are in the 92-percent efficienc range. This y y compressor’s permanent-magnet motor has an efficiencyof 96 to 97 percent. Efficienc is further y enhanced with the use of magnetic bearings, which avoid the friction of rubbing parts associated with traditional oiled bearings. Conventional bearings can use as much as 10,000 w, while magnetic bearings re- quire only 180 w.
That amounts to 500 times less friction loss. Current development projects are expanding the range and duty of the compressor wheels and promise to offer even greater efficiencyfor water-coole and air-cooled duties and different capacities. d CONTROLS The new compressor effecti e ly is a computer. It provides diagnostic and performance information v through Mod- bus to the refrigeration system, which then communicates to the building automation system through Modbus, LonWorks, or BACnet. C H IL L E A P P L IC A T IO N R
The compressor manufacturer and a major chiller manufacturer teamed up to develop a line of ARIcertified water- cooled chillers, which were expected to be introduced in January 2004. The combination offlooded evaporated technology and oil free system has allowed very close approaches and, subsequentl , enhanced performance. The integrated VFD allows excellent part-load y performance as power consumption drops o ff, depending on the head relief, near the cube root of the shaft speed. The compressor includes wheels tuned for water-cooledduty in the dual-compressor format, which further enhances part-load performance.
Tested in accordance with ARI Standard 550/59098, Water Chilling Package Using the Vapor Compressio Cycle, a 150-ton (nominal) chiller has a fulls n load performance of 0. 629 KW per ton (5. 6 COP) and an IPLV of 0. 375 KW per ton (9. 4 COP). A ll IPLVs are weighted for standard operating conditions and the time spent at those conditions. F IG U R E3. A ccording A R I testing,a 150-tonfrictionless to chiller has a full-loadperform ance 0. 6 2 9KW per ton (5. 6 C O P ) of and an IP L Vof 0 . 3 7 5KW per ton (9. 4 C O P ). SO U N DAN D V IB R A T IO N
Because the rotating assembl levitates,there essentiall is no structure-borne vibration. The magnetic y y bearings create an air buffer that prevents the only major moving part the motor rotor from transmitting vibration to the structure Similarly, sound levels are extremely low, primarily because of . refrigerant-gasmovement through the compressor and the rest of the refrigeration system . There are no tonal issues, such as those found with some screw compressors, and the noise occurs in the higher octave bands, where it is easier to attenuate.
When two magnetic-bearin compressors were g integrated into a chiller, the sound pressure was 77 dBA at 3. 3 ft under ARI Standard 575-94, Method o f Measurin Machine Sound within an Equipment Space g ry . M O D E L E D N E R G Y V IN G S E SA Chiller applications were modeled for Phoenix; Chicago; Tampa, Fla. ; and New York to estimate operating costs and payback times. The program compared an hourly analysis of a 150 ton frictionless chillers with that of a water cooled reciprocating chiller (Phoenix, Chicago and Tampa) and a water cooled centrifugal chiller (New York).
Each city showed an annual energy saving of more than $4,500 and to tree year payback (Table 1) Advantages 1. Easy to work with Designed for HVACR applications by HVACR engineers the compressor is virtually a “plug-and, play” solution. It features the same standard suction, discharge and economizer ports as conventional compressors It mounts in the standard way. It can use the same power wiring with a single control . and monitoring connection. 2. Easy on product cost This frictionless magnetic bearing design needs no oil management system.
And because there’s no oil to coat the heat transfer surfaces the unit’s high efficiency can be maintained over the lifetime of , the product. The outstanding efficiency of the compressor gives equipment manufacturers the option to offer the highest efficiency/lowes emissions cost effective performance in its tonnage range. t , 3. Easy on the ears A sound level less than 70 dBA, with virtually no structure-borne vibration, eliminates the need for expensiveattenuation accessories. 4. Easy to handle 265 pounds (120 kg) is less than 20% of the weight of competitive compressors with an approximate 50% smaller footprint. . Easy refrigeran t choice Since the compressors are optimized for HFC -134a a well known, environmentally responsible, , refrigerant. 6. Easy to control Onboard digital electronics make the compressor “the compressor” with a brain. Inside, the compressor is totally self-correcting and incorporates a system of sophisticated self-diagnostics , monitoring and control. Outside, you can tap into this intelligence by using control outputs in various for including web-enabled monitoring and control. 7. Easy on energy The ompressor enables chiller and rooftop manufacturers to achieve the necessar product efficiency y levels to meet and exceed ASHRA 90. 1 and the California Title 24 requirements for energy E efficiency. Application 1 Water-cooled chiller applications 2 Rooftop and packaged system applications 3 Air-cooled chiller applications 4 Multiple compressor applications References 1. Perry, R. H. and Green, D. W. (Editors) (2007). Perry’s Chemical Engineers’ Handbook, 8th Edition, McGraw Hill. ISBN 0-07-142294-3. 2. Dixon S. L. (1978).
Fluid Mechanics, Thermodynamics of Turbomachinery, Third Edition, Pergamon Press. ISBN 0-08-022722-8. 3. Aungier, Ronald H. (2000). Centrifugal Compressors A Strategy for Aerodynamic design and Analysis. ASME Press. ISBN 0-7918-0093-8. 4. Bloch, H. P. and Hoefner, J. J. (1996). Reciprocating Compressors, Operation and Maintenance. Gulf Professional Publishing. ISBN 0-88415-525-0. 5. Reciprocating Compressor Basics Adam Davis, Noria Corporation, Machinery Lubrication, July 2005 6. Introduction to Industrial Compressed Air Systems 7.
Screw Compressor Describes how screw compressors work and include photographs. 8. Technical Centre Discusses oil-flooded screw compressors including a complete system flow diagram 9. Tischer, J. , Utter, R: “Scroll Machine Using Discharge Pressure For Axial Sealing,” U. S. Patent 4522575, 1985. 10. Caillat, J. , Weatherston, R. , Bush, J: “Scroll-Type Machine with Axially Compliant Mounting,” U. S. Patent 4767293, 1988. 11. Richardson, Jr. , Hubert: “Scroll Compressor With Orbiting Scroll Member Biased By Oil Pressure,” U. S. Patent 4875838, 1989.