Payne, J. W.

Weinbrecht, J. F.

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsInternational Compressor Engineering Conference School of Mechanical Engineering2002Reflux Gas Densification Technology AnInnovative Dry Compression ProcessJ. W. PayneJ. W. IndustriesJ. F. WeinbrechtJ. W. IndustriesFollow 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.htmlPayne, J. W. and Weinbrecht, J. F., " Reflux Gas Densification Technology An Innovative Dry Compression Process " (2002).International Compressor Engineering Conference. Paper 1517.https://docs.lib.purdue.edu/icec/1517C6-1REFLUX GAS DENSIFICATION TECHNOLOGYAN INNOVATIVE DRY COMPRESSION PROCESSJim W. Payne, President, J, W, Industries, Inc., 601 Paragon Road SEAlbuquerque, NM 87105 P: (505) 873-1784 F: (505) 873-3854 E: jw@jwabq.com*John F. Weinbrecht, Development Engineer, 12405 Sierra Grande NEAlbuquerque, NM 87112 P: (505) 294-3692 F: (505) 299-4117 E: JFWeinb@aol.comABSTRACTAll positive displacement gas compressors that gain pressure increase by volumetric reduction of afixed mass are characterized by heat co-generation, often called “the heat of compression”. This co-generated heat penalizes efficiency, and is the cause of many design problems arising from thermaldistortion and from high temperature levels in mechanical components.In contrast, Reflux Gas Densification is based on a constant volume, variable mass cycle that is closerto isothermal in thermodynamic nature. The process is governed more by Dalton’s Law of AdditivePressures and by Newton’s Laws of Motion. Thermal effects in the flow stream are those primarilyassociated with gas flow dynamics. Co-generated heat has been reduced to a point where temperatureincrease in the working fluid does not cause design or performance limitations.INTRODUCTIONWhen compressing gaseous fluids by volume reduction, co-generation of heat is intrinsic to theprocess. Efforts to mitigate it include cooling the process gas as it passes through the compressor, and theuse of multistage compression with cooling between stages. The initial concept for reflux gasdensification was based on inserting a heat exchanger into the flow stream after discharge, and refluxingback into constant volume displacement cavities.The first attempt to demonstrate the concept failed to show any improvement in process fluiddischarge temperature level. After much study, it became evident that refluxing could only be done whiledisplacement cavity volume remained constant. Subsequent design and development effort has beendirected toward methods of refluxing while maintaining constant displacement volume, maximizingcavity pressure at refill closure, and minimizing reflux flow loop pressure drop.The change from a variable volume to a constant volume process brought about a major change inthermodynamic nature. Disregarding any kinetic energy effects, heat co-generation for a variable volumecompression process is determined by the specific heat ratio of the process fluid. The process requires themaximum amount of frictionless work for a given pressure ratio. The reflux gas densification process istheoretically isothermal, and there is no intrinsic co-generated heat. The process requires the minimumamount of frictionless work for a given pressure ratio.BACKGROUNDThe concept for Reflux Densification came from a study of the Roots blower carried out at the LosAlamos National Laboratory in support of a research program requiring non-contaminating gascompression in an experimental flow system. The study also covered modified versions incorporatingcooled reflux to reduce the temperature level at discharge. Upon evaluation, performance of these fell farshort of expectations. A concept for improving the reflux arrangement was identified from the study.The concept was disclosed to the Laboratory. However, further development fell beyond the needs ofthe supported program, and the invention was released by the Department of Energy through a waiver ofpatent rights. It is the subject of four utility patents issued by the United States Patent Office. Thetechnology has had little relevant operational history or analogy to draw on, and development activity hasbeen directed toward proof-of-principle demonstration and the collection of supporting data to provide thebasis for engineering and design.DESCRIPTIONThe densifier design consists of a pair of involutely lobed rotors driven in opposing directions througha pair of timing gears. These feed intake pressure process fluid into an integrated reflux flow loopoperating at discharge pressure. Reflux is directed into displacement cavities after intake completion, andis closed off immediately prior to discharge. By this means the pressure differential at discharge subjectto compression by volume reduction is held to a very low level.Constant volume displacement is maintained from intake through to discharge, a key design featureadopted from the Roots blower. Slippage or backfill is limited by design features, and by sonic choking.There are no valves and no contacting or rubbing parts in the flow stream to contaminate the processfluid. The physical design can accommodate high discharge pressure and high input shaft power.The present design does not include a means for heat removal during the process. The arbitrarily setperformance goal for the development effort was to obtain a pressure increase ratio of 10:1 within atemperature level increase of 100 degrees F (55.6 degrees C) from intake to output for an air cycle. Fromexperimental evidence and from math model projections, that goal appears to be attainable.Figure 1 displays a cross section of a Roots type lobed rotor compressor in simplified form, and liststhe pros and cons of the process as well as design features essential for reflux densification. The basicdesign has remained unchanged since it was first introduced to industry in the mid 1850s. The transverseflow arrangement and constant volume displacement from intake through to discharge are design featuresthat make Reflux Gas Densification possible.AdvantagesSimple and rugged designNo valves and no contacting or rubbingparts.High volumetric inputDisadvantagesHigh “heat of compression” levelsLimited pressure ratio inputVery noise pollutingPoor volumetric efficiencyRequisite Design FeaturesTransverse FlowConstant volume displacementNo process fluid contaminationFigure 1ROOT’S BLOWERFigure 2 displays a cross section of a six-lobe reflux gas densifier in simplified form. A refluxflow loop has been added to reduce the pressure differential at discharge. Rotor lobes have beenadded to make reflux more effective and to improve volumetric efficiency.Figure 2REFLUX DENSIFIERFigure 3 displays the densifier flow paths and terminology used in math modeling, design study, andcycle sequence description.Figure 3AdvantagesSimple and rugged designNo valves and no contacting or rubbing parts.High volumetric inputLow heat co-generation. Near isothermalcycle.High pressure ratio-capacity.DisadvantagesHigh volumetric efficiency difficult to obtainfor small displacements.Requisite Design FeaturesTransverse flowConstant volume displacementNo process fluid contaminationFLOW PATHS AND TERMINOLOGYLow impedance is a prevailing consideration throughout the flow loop design. Flowvelocities of Mach 0,05 or lower are maintained from the discharge port through the distributionmanifold and the reflux conduits. Conduit width is equal to rotor length. Conduit depth andreflux flow area are uniform up to the refill port, where the depth converges to form a linearnozzle,For pressure ratios above critical, the refill nozzle throat will flow at sonic velocity until cavitypressure rises to the critical level, then fall off to slightly above rotor tip speed at refill closure. The useof linear nozzles serves to keep volumetric change to a minimum as displacement cavities pass by refillports.CYCLE SEQUENCEBackfillBackfill or slippage is fluid mass freely expanding back from discharge into the intake region throughrotor mesh and rotor end-to-housing clearances. It acts as a partial pressure already within thedisplacement cavity when it is open to the intake port. Backfill is limited by design features and by sonicchoking, and varies with pressure ratio.IntakeFluid mass entering through the intake port is the primary source of initial cavity charge. It acts as anadditive pressure to that of backfill, and brings cavity pressure up to suction line level. An intake adapterconverts suction line geometry to a rotor length equivalent area passage.LowfillThe lowfill state is established when backfill and intake fluids are mixed together and come to thermalequilibrium at intake pressure. The ratio of backfill to intake mass determines volumetric efficiency.RefillThe refill port is part of the reflux system flow loop. As a displacement cavity opens to the port,additional mass circulated back from discharge flows in through the refill port and freely expands into thecavity. At refill completion, total cavity pressure is due to the pressure from mass already containedwithin the cavity and the additive pressure component from reflux.HighfillThe highfill state is established when the reflux fluid is mixed together with the fluid already containedwithin the cavity and comes to thermal equilibrium. Dynamic effects associated with flow loopimpedance, refill port configuration, non-ideal process fluid properties, and a fixed rotor geometry allcontribute to the amount of pressure drop from the discharge to the highfill state.DischargeAfter refill closure, displacement cavities open into discharge. The contained process fluid iscompressed from highfill to discharge pressure, with an associated reduction in volume and rise intemperature. Discharge over intake temperature increase is a product of the temperature differencebetween highfill and discharge and the pressure ratio of discharge to intake pressure.Discharge DistributionFrom the discharge region, backfill mass flows back to the intake region through rotor-to-rotor androtor-to-housing clearances. Reflux and output mass flows through the discharge port into thedistribution manifold. Reflux mass is directed back through the reflux flow loop conduits, and outputmass exits the densifier through the outlet port.DESIGN EVOLUTIONIncluding the original concept, there have been four major design iterations to this point. The first wasbased on a four lobed rotor configuration, with an integral heat exchanger placed in the flow loop. Thesecond iteration adopted a six lobed rotor configuration to improve volumetric efficiency. The thirditeration incorporated linear nozzle refill ports to better maintain constant volumetric displacement, andeliminated the integral heat exchanger.The fourth iteration moved the primary refill ports forward toward intake and added secondary refillports to extend the time available for refluxing. Turning vanes were also added in the reflux loop.Figure 4 displays a cross section of the reflux densifier with secondary refill ports added.Figure 4EXTENDED REFLUX DENSIFIERPERFORMANCEFigures 5 and 6 display graphs of performance comparisons between volumetric compression andreflux densification. Theoretical graphs are based on an ideal frictionless process fluid with a 1.40specific heat ratio, 100% volumetric efficiency, no heat transfer in or out, and no mechanical losses.Actual or realistic graphs are based on projected performance of a reference compressor or densifierdesign having mechanical and volumetric efficiency losses but no heat transfer.The Figure 5 graphs show the theoretical and actual discharge temperatures for both cases from aninlet temperature of 72 deg. F. (22 deg. C). Densifier performance projections are based on a 95%displacement cavity fill completion. Volumetric compression projections are based on the same unit,without reflux. They do not take into account performance limitations imposed by the temperatureincrease.The Figure 6 graphs display theoretical and actual net efficiencies for both cases, referenced to thetheoretical frictionless isothermal work requirement.Figure 5DISCHARGE TEMPERATUREFigure 6NET EFFICIENCYAPPLICATION CONSIDERATIONSMany features of the densifier design were carried over from present state lobed rotor compressors. Inthe absence of relevant operational history, arbitrary limits have been placed on rotor tip speeds and onreflux flow loop velocity. The large improvement in pressure ratio capability is largely the result of closetolerance in rotor running clearances made possible by the reduction of co-generated heat.High volumetric efficiency is achieved by sonic choking at pressure ratios above critical (about 2:1),by the addition of barrier lobes to carry perimeter slippage forward into discharge, by limiting rotor meshclearances, and by limiting rotor end face to housing clearances. Small displacement units may requiresacrificial coatings on adjacent housing end faces.Bearing LoadsThe transverse flow arrangement produces a radial load on rotor bearings in the direction of the intakeside. Rotor timing is achieved by straight spur gearing that does not produce thrust, resulting in very lowaxial bearing load components.Oil lubricated spherical roller bearings support the rotors and maintain alignment. The input bearingsare fixed in axial position within the rotor housing to maintain end face clearance. The opposite endbearings are allowed to float axially to adjust for any thermal expansion difference between rotor andhousing. This difference is minimal, since the temperature difference between rotor and housing is low.Single Unit SizesIn general, the upper size limit for densifiers will be determined by tradeoff between the difficulty offabricating large massive units driven at lower speeds, and using smaller units piped in parallel to meethigh volumetric input requirements. The following sizes are based on holding to a range of rotor pitchdiameters from 4 to 16 inches (100 to 400 mm).Single unit displacements can range from 0.10 lpr (liters per revolution) to more than 60 lpr.Single unit pump speeds can range from 0.20 cmm (cubic meters per minute) to more than 75 cmm.Large displacement single units with low intake pressure (i.e.- 20 bar and below) can operate atpressure ratios up to 10:1.Small displacement units with high intake pressure (i.e.- 80 bar and above) can only operate at lowerpressure ratios as determined by allowable bearing loads, (i.e.- possibly not more than 2.5:1).StagingThe transverse flow arrangement and the rugged rotor design with large solid center section permit anin line arrangement for two or three stages, joined together by flexible couplings and driven from a singleower source. By placing the largest unit first in line and the smallest last, reduction in torque is matchedwith diminishing rotor size.This arrangement is suitable for compressing from a suction line pressure of 20 bar up to 300 bar intwo stages, and compressing from a suction line pressure of 2 bar up tp 300 bar in three stages. In eithercase the footprint area is relatively small. Rotor shaft diameters are suitable for transmitting the powerrequired to drive all stages..CLOSING COMMENTSThe Reflux Gas Densification cycle and the Reflux Densifier design have evolved together and areinterdependent. Together they can reduce heat co-generation associated with positive displacementcompressors to a level where it no longer represents a design or performance limitation. The resultingprocess is non-contaminating, near-isothermal, and fundamentally more efficient.The design is scalable over a wide range of operating parameters, and can be applied wherever gasesor vapors must be compressed. Studies of potential applications have included single unit pressure ratiosup to 10:1, single unit displacements of from 0.10 lpr to 60 lpr, single unit pumping speeds of from 0.2cmm to 75 cmm, and discharge pressures from staged units up to 300 bar.The reflux flow loop arrangement in the present design has evolved without the benefit of relevantanalogy or operational history, and without extensive research and development. Information obtainedfrom a broader development effort can be expected to further improve performance as operational resultsare obtained and a strong, empirically derived design base is established.The immediate motivation for developing and deploying this technology is to remove a major barrierto public acceptance for the adoption and use of natural gas as a primary ground-based vehicle fuel. Thephysical arrangement and operational characteristics of the reflux densifier are particularly well suited forthe compression of natural gas for CNG vehicle refueling.Successful development will also provide a key element of technology needed to support and enable agradual transition from hydrocarbon to hydrogen based vehicle fuels. The technology base required fornatural gas infrastructure systems is also fully applicable for gaseous hydrogen systems. While still inabundant supply, natural gas can serve as a bridge fuel during the lengthy development period.REFERENCESPublicationsThe High Ratio Recirculating Compressor, published in the proceedings of the 1988 InternationalCompressor Engineering Conference at Purdue University.The Flow Work Compressor, published in the proceedings of the 1990 International CompressorEngineering Conference at Purdue UniversityPatentsU.S. Patent No. 4,859,158 entitled “High Ratio Recirculating Gas Compressor”.Issued August 22, 1989. (Allowed to expire)U.S. Patent No. 5,090,879 entitled “Recirculating Rotary Gas Compressor”, issued February 25, 1992U.S. Patent No. 5,439,358 entitled “Recirculating Rotary Gas Compressor”, issued August 8, 1995U.S. Patent No. 6,312,240 entitled “Reflux Gas Compressor”, issued November 6, 2001,Filed on May 27, 2000

Reflux Gas Densification Technology An Innovative Dry Compression Process

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Reflux Gas Densification Technology An Innovative Dry Compression Process

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