Zimmern, B.

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Purdue UniversityPurdue e-PubsInternational Compressor Engineering Conference School of Mechanical Engineering1984From Water to Refrigerant: Twenty Years toDevelop the Oil Injection-Free Single ScrewCompressorB. ZimmernFollow this and additional works at: https://docs.lib.purdue.edu/icecThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlZimmern, B., "From Water to Refrigerant: Twenty Years to Develop the Oil Injection-Free Single Screw Compressor " (1984).International Compressor Engineering Conference. Paper 500.https://docs.lib.purdue.edu/icec/500FROM WATER TO REFRIGERANT: TWENTY YEARS TO DEVELOP THE OIL INJECTION-FREE SINGLE SCREW COMPRESSOR Bernard Zimmern Single Screw Compressor,Inc. Norwalk, CT ABSTRACT 1982 saw the introduction on the market of the oil injection-free single screw compressor, simultaneously for oil-free air applications and for air conditioning. Although the two technologies are far apart since one deals with the compres-sion of air, the other with halocarbons, those achievements matured simultaneously not by accident but because they are both based on the same unique features inher-ent in the basic structure of the single screw· This paper presents an historical review: of the findings and tests which led very early to the consideration of oil~njection­free applications; of the theoretical pro-blems which were solved partly by logic, partly by accident and luck; and of the practical problems which had to be over-come to achieve industrial products; basically corrosion in the case of air compression, efficiency in the case of air conditioning and heat pumps. I. INTRODUCTION In 1962 we started building our first sin-gle screw compressor; it was an oil free machine. But it is only just now, twenty years later, that the first oil injection- free single screw machines are beginning to appear on the market -- simultaneously as air compressors and as refrigeration and air conditioning compressors. Why has it taken so long? Let us review the main problems which were encountered and the solutions which had to be found to progress from a proto-type to a marketable product. 513 But first, for the sake of clarity, let us recall briefly what a single screw com-pressor is and more particularly an oil in-jection-free single screw compressor. An sse An SSC (Single Screw Compressor) is made with one screw whose grooves are swept by the teeth of pinions, also called gate-rotors. There are usually two gaterotors to double the swept volume and balance the thrusts. SSC's are built according to various con-figurations described in the literature (see ref.1,2) and schematically presented in Fig.l to 4. There can be basically two types of screws, cylindrical or planar (sometimes conical), and two types of gaterotors, also cylindri-cal or planar. This results in four combinations; CP, PP, CC and PC. The basic advantages of all four are their compactness, cost, balanced compression and low noise which have helped them to capture around half of the Japanese screw air compressor market and are now enabling them to expand quickly in other countries. An Oil Injection- Free SSC An oi~injection free sse is basically an sse in which the oil injection associated with any screw for lubricating, sealing and cooling has been eliminated. This does not mean that it is an oil-free machine -- since the bearing can still be oil lubricated -- but it can be. Oil injection-free screw compressors al-ready exist. Twin screws running at high speed and not touching each other because they are synchr6nized by timing gears, are well known. Such a system would be extremely difficult to apply to a single screw machine since the shafts of the rotating parts are transverse and synchronizing them with precision would be a difficult task. The oil injection-free SSe is only possi-ble because of a peculiarity of the sse, that is the gaterotors are idlers. So, little or no power is needed to turn them, which means that little or no thrust ex-ists at the contact points between the screw and gaterotors and there is there-fore no need for good lubrication. Water, whisky, champagne or any liquid can do the job. All that is required of the liquid inject-ion in an sse is to cool and, if possible, to seal. Hence an obvious idea is to use, as an injection fluid, a liquid already existing in the gas to be compressed: for air, water since air contains moist-ure for refrigerant, the condensed gas This prevents contamination by a foreign substance like oil. The 1962 Prototype This idea led to the first design in 1962 which was a 10 HP compressor; the screw was made of bronze, the casing of a cast iron sleeve in an aluminum case and the gaterotors of a plate of phenolic sup-ported by a stainless steel gaterotor support. Thi3 machine ran with water injection con-talning some molybdenum bisulfide (which later proved to be unnecessary); the bear-ings were grease bearings protected by mechanical seals. The result was not a ~ad machine since it achieved some 9.2HP/m /min at 7 bars pres-sure (26BHP/100 cfm) i.e. some 70% isen-tropic efficiency and passed a 1000 hours endurance test in our first licensee's lab. Why,then,20 years before a marketable product? 514 II. THE WATER FLOODED AIR SSC In retrospect, it appears that we benefited from many lucky breaks but it took us a long time to identify them properly. The Floating Gaterotors Because we had only $6,000 available to build that 1962 prototype, we used a sim-ple method to make the gaterotors; we cut them from a thin plate of phenolic backed up by a steel support; the phenolic pro-vided the seal, the steel took the pressure load. We reduced the roots of the teeth to give them angular flexibility and to comp-ensate for possible angular dividing inac-curacies since our manufacturing means wer~ quite limited. When our first licensee, Peugeot, the car manufacturer, started its development, it very logically reduced the cost of the gaterotors by molding a plastic material over a metal support. And this was the beginning of our troubles because no water-flooded compressor had an acceptable life. In despair, Peugeot decided to use oil which is also a logical move for car engin-eers. Oil helped but did not completely cure the problem, as we discovered a few years later when L started hearing about sudden gaterotor failures happening in an erratic way. We started investigating this problem our-selves and it occurred to us that we were probably encountering chattering or back-lash resonance as often happens with a pair of gears when one is an idler; we learned later that the same thing also hap-pens to twin screw machines when the torque on the driven screw is too small to damp the vibrations. We realized that by making the teeth flexi-ble on our 1962 prototype we had probably avoided this resonance phenomenon, To overcome the problem, we hurriedly de-veloped an improved gaterotor, now called the floating gaterotor; this solution is similar to the 1962 solution except that instead of having each tooth independently flexible, the whole plastic gaterotor can have a slight angular displacement on its metal support; the connection being made for instance by a pin and an 0-ring (see ~igures 5 and 6). This is far better because the damping ef-fect can be better controlled, the gate-rotor precision can be much higher,etc ... In such floating gaterotors, if for any reason, such as a manufacturing mistake, the gaterotor is subjected to an acceler-ation or deceleration, only the plastic part accelerates or decelerates and, having very little inertia, this does not create wear, while the heavy part, the support, continues to rotate at constant speed. The validity of the principle wa,s:dramatic-ally demonstrated in 1972 when our first Japanese licensee, to convince themselves of the advantages of the single screw, ran endurance tests on two European made air-ends equipped with plain molded gate-rotors and on two identical compressors equipped with floating gaterotors. The European ones, after running fine for awhile, suddenly failed, one at around 400 hours and the second at 800. The two floating gaterotor machines had not shown any sign of loss of capacity when the en-durance test was stopped at 4,000 hours. Water Absorption By Plastics Phenolics are excellent materials to run with water lubrication but unfortunately they absorb enough water to swell and this creates a lot of headaches in mass pro-duction because it is hard to control their accuracy. Fortunately, the develop-ment of the sse coincided with the dis-covery of many high performance technical plastics and around 1974 - 75 we were able to find compounds which met the many requirements we had formulated. The many thousands of air compressors operating in Japan with a failure rate from all causes (package components included), during the first two years of warranty, of much less than 1% are good proof thereof. And fortunately some of those plastics do not swell in water. Corrosion But the most baffling problem has been corrosion. On the 1962 prototype, we did not notice it because the machine was running nearly all the time. And indeed as long as the compressor is running, there is no prob-lem. The problem starts when the com-pressor is idle; the corrosion develops particularly between the crests of the screw thread and the casing so that after one or two weeks, it becomes impossible to turn the shaft. 515 It is of course possible to use materials such as bronze; and this is the approach pursued by the U.S.Navy which has proven ihe validity of a water flooded sse, part-icularly the possibility of using water lubricated bearings, thereby dispensing with the costly and troublesome mechanical seals. But bronze is too expensive for the in-dustrial market, so only two years ago one of our Japanese licensees found what seem-ingly looks like the right answer: coating the cast iron with nickel so that the air-end can be made of the same inexpensive material as an oil flooded sse. i.e. cast iron. Bearings are still protected by mechanical seals because the warranty is for 20,000 hours, but there is hope that water lubricated bearings will soon be possible. III. THE OIL INJECTION-FREE REFRIGERATION AND AIR CONDITIONING SSG The development of such a machine followed a parallel but later path. The original concept of injecting condensed gas into the compressor to cool it came around 1969 - 70 when it became apparent that in-jecting oil into the screw chamber did not provide much sealing due to dissolved re-frigerant. Like everybody else we tried to find a liquid in which the refrigerant gas would not dissolve but, after a dram-atic failure on an assignment we had from the aerospace industry in 1969, it became obvious that such a liquid either did not exist or would be quite exotic and would impose unrealistic operating constraints. The reasoning was that, since there was no sealing effect, and since from air-water tests, we knew lubrication was not needed, we could just cool by injecting the con-densed liquid. It was my good fortune at that time to meet one of the men whose enlightened in-telligence has most contributed to the successful development of the SSG and who, by supporting the above views, gave us the psychological backing we needed to under-take what looked like a formidable tas~. I am referring to Mr. L.C.CONSTANT, director of engineering at HALL THERMOTANK PRODUCTS LTD. Indeed, when we started testing an R 22 liquid-injected sse around 1975, it gave very poor performance; we were' happy to reach 50% isentropic efficiency at a pres-sure ratio of 3 and a high speed of around 7000 to 8000 rpm. But these were very small machines in the 4HP range. The reason for choosing so small a size was that we knew it would not be much of a problem to achieve reasonabl~ efficien-cies in large compressors above lOOKw but that the interesting market was below lOOKw where the screw had difficulty beat-ing the recip because of its oil circuit, the cost of which makes the system non-competitive, particularly for air con-ditioning. An interesting detail about our development effort is that we theorize that one of the configurations of SSG, the PC tyoe, was the ideal candidate for such refrigeration and heat pump oil-free application; indeed it is the only com-pressor to our knowledge all of whose clearances can be reduced to 0 after as-sembly by a slight displacement of the parts, whereas most screw compressors have built-in clearances that have to be lived with. In 1977 to 1980, we spent many hundreds of thousands of dollars developing machine tools for that pur-pose and making prototypes but all to no avail. The machine never exceeded the 70% isen-tropic efficiency we had set as our mini-mum target, though it just barely reached it. Fortunately, at the same time I had con-vinced one of our licensees to try this oil-free orientation for a large machine of the CP type and the results were much better than forecast. We got the message and at the end of 1980, by means of a three month crash program we designed, manufactured and tested a 30Kw CP compressor which, on the first run, gave us the 75% we had been looking for for so many years along with all the extra feat-ures we needed~ low noise (around 80db), direct drive (3000 or 3600 rpm) capacity modulation continuous or step-by-step from 1 0 0% to 15% etc ... This, plus' successful endurance tests on totally oil-free heat pumps (see reference 4)seem to have helped HALL THERMOTANK to decide that it was time to launch the oil injection-free SSG (see reference 5). It is worth noting that many of the im-provements needed to achieve this result came from solutions developed for the PC but which were transposed to the GP. So our efforts were not wasted. IV. CONCLUSION One question might be raised: Gould the development have been carried out more quickly since 20 years is a long period of time? We are tempted to say "yes" and blame all the adverse conditions which stood in our way. SlG But in all honesty we should say we do not know. Development processes are tortuous paths, not straight forward; there is probably a right time for every idea and it takes time for them to mature and this whole process is not at all under human control. REFERENCES [1] B.Zimmern and Dr.Ganshyam G. Patel, Design and Operating Characteristics of the Zimmern Single Screw Compres-sor, Compressor Technology Conference, Purdue, 1972. [2] E.Kallmann and B.Zimmern, On the Pro-perties of a New Type of Single Screw Compressor, XIV th International Con-gress on Refrigeration, Moscow,l973. [3] Clarke, Hodge, Hundy and Zimmern, A New Generation of Compressors for Re-frigeration, The Institute of Refrig-eration, London, 1976. [4] Degueurce, Pascal and Zimmern, Prob-lems Encountered and Results Obtained in Direct Steam Compression Utilizing an Oil Injection-Free Single Screw Compressor, Compressor Technology Con-ference, Purdue, 1980. [ 5] G.F.Hundy, The Development of the Sin-gle Screw Compressor and Oil Reduced Operation, The Institute of Refriger-ation, London, 1982. Fig. I Fig.Z PC Type CP Type (Planar screw/cylindrical gaterotors) (Cylindrical screw/planar gaterotors) J Fig.3 Fig.4 PP Type CC Type (Planar screw/planar gaterotors) (Cylindrical screw/cylindrical gaterotors) 517 Fig.S Floating gaterotor Fig.6 Cross-sectional view floating gaterotor 518 

From Water to Refrigerant: Twenty Years to Develop the Oil Injection-Free Single Screw Compressor

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From Water to Refrigerant: Twenty Years to Develop the Oil Injection-Free Single Screw Compressor

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