Pston Rng Assembly for a New Natural Gas Vehcle Symmetrcal Multstage Wobble-Plate Compressor Andrl Arafat, Zar Asrar Ahmad, Ardyansyah Syahrom, Nor Ilham Mohd Anon, Md. Nor Musa, Anullotf Abdul-Latf, Wan Al Wan Mat Faculty of Mechancal Engneerng, Unverst Teknolog Malaysa, 81300 Skuda, Johor, Malaysa Tel: +607-5534878, Fax: +607-5566159 Abstract: Natural gas s an alternatve fuel of choce n the market today due to the ncrease n the prce of petroleum, as well as out of envronmental concerns. Pressure requrement for a natural gas vehcle (NGV) storage tank s 3000 psg (06 bars). Thus, at NGV refuelng facltes, the natural gas need to be stored at a hgher pressure n order to refuel the NGV at the pressure requred. Compressors are needed n the compresson process at the refuelng facltes. A new compressor desgn for natural gas refuelng applance has been developed whch s the symmetrcal multstage wobble-plate compressor. Ths compressor desgn s the newest varaton of the axal recprocatng pston compressor. The success of the compressor desgn n compressng gas depends on the pston rng assembly desgn. Through ths paper, the process of desgnng the pston rng assembly and consderatons taken for ths new compressor desgn were explaned. The results presented are those from prelmnary tests usng ar on the workng flud. Real tests on natural gas are to be organsed utlsng all the experence and lesson learnt from that on ar. Keyword: Compressor, wobble-plate, NGV, pston rngs, ol-free, leakage 1. Introducton An alternatve to usng gasolne and desel as vehcle fuel s the use of natural gas. Compressed Natural Gas (CNG) offers fuel cost savngs to the vehcle owner, as well as greater effcency of energy resource utlzaton, clean burnng fuel and lower emssons than gasolne or desel fuel. The total energy use for Natural Gas Vehcles (NGV) ncludes not only drect vehcle consumpton but also extracton, processng, transportaton, dstrbuton and compresson of the gaseous fuel. As more publc refuelng statons are constructed the cost of fuelng a natural gas powered vehcle wll become more economcal and convenent. In lne wth ths, more natural gas vehcle refuelng facltes are beng bult by Petronas (Malaysan natonal petroleum company) to serve ths demand. Currently there are 39 NGV refuelng statons natonwde. The government s targetng for 94 refuelng statons by the year 009 to serve a total of 57,000 NGVs. However, usng natural gas as the vehcle fuel requres dfferent refuelng and storage technology as t nvolves compressng the gas to a hgh workng pressure up to 10 bars to ensure compact storage. The recprocatng wobble-plate compressor s one of the axal pston compressor varatons. The wobblng moton from the wobble plate s transferred to pstons va connectng rods. A typcal exstng wobble-plate compressor for ar-condtonng systems uses one common pston sze for sngle stage compresson (Fgure 1). The pston sze s adequate to accommodate connectng rods wth ball jont connectons made usng caulkng process. A new symmetrcal multstage wobble-plate compressor beng developed at UTM for natural gas compresson applcaton utlzes fve-stage compresson to acheve 10 bars [1,,5,6] (Fgure ). Ths leads to fve dfferent pston szes, wth the sze gettng smaller for hgher pressures. The NGV refuelng compressor desgned s for a medum capacty of 10 Nm 3 /hr, and could be used n mn statons for housng blocks, offce clusters, vehcle fleet depots or shoppng malls. It s not ntended for fast refuelng such as n full-fledged commercal statons wth hgh throughput, but t would provde a faster refuelng than overnght home refuelng applances. The man components of a new multstage symmetrcal wobble-plate compressor for such a mn staton would cover the compresson system, the drver system, and dspenser and controls. The refuelng unt s generally a self-contaned, ol-free outdoor applance that wll fll the vehcle gas storage cylnder at a pressure of 10 bars (3600 psg) wthn 0.5-1 hours, gvng an average mass flow rate of 0.67 kg/mn. Wth cascadng storage tanks at the mn statons beng constantly pressurzed, NGVs would not have to wat long to refuel. The specfcaton data of a new multstage symmetrcal wobble plate compressor are gven Table.1 Mechancal frcton losses n pston assembly amount to approxmately 40% n the pston-rng-lner trbologcal system. The hgh frcton n pston rngs makes t very mportant for proper desgn to mnmse these losses. Tradtonally, pston rngs for recprocatng moton were made from cast ron. In ths case of usng cast ron wth steel counterface, lubrcatng wth ol becomes necessary to reduce frcton and as part of the coolng system. Lubrcaton ol flm also functons to prevent leakage between pston rng and cylnder lner. However, n the case of compressng natural gas nto vehcle tank, t s preferable not to have ol lubrcaton so as to avod ol contamnaton of the gas that would n turn affect fuel performance.
Fgure 1 Exstng Wobble-plate Compressor (Sngle Sde Compresson) Fgure The New Mult-stage Symmetrcal Wobble-plate Compressor Table.1 Specfcaton data of a new multstage symmetrcal wobble plate compressor. Materal Selecton for Pston Rngs Assembly The pston rng assembly are the most crtcal parts to be desgned to ensure good sealng wth lower frcton, low leak rate, long lfe and hgh effcency. In these hgh pressure and temperature condtons, materal selecton s very crtcal for the pston rng and lner. The polytetrafluoroethylene (PTFE) based materal wth addtve fllers was adopted for ths compressor. The PTFE based materal s very sutable for ol-free/unlubrcated sldngs because ths materal has low frcton and wear, low thermal expanson, low shear strength to ensure rapd transfer to the counterface and low frcton, leadng to longer mantenance ntervals, and wde range performance sutable for many operatng condtons f flled wth fllers.e. carbon, graphte, bronze, molybdenum dsulphde (MoS ), fber glass, etc [3]. For successful operaton, the cylnder lner as the counterface to the pston rngs must be desgned sutable to the pston rng materal. The lner was desgned as smooth as possble wth surface roughness ra 0.088 and Vckers Hardness of 14 MPa. 3. Gas Pressure and Forces Actng on the Pston Rngs Fgure 3 llustrates the smplest form of pston rng/seal (a), and a cross-sectonal vew of the rng, nstalled n the groove of a pston, located n sealng poston (b), and n a neutral poston (c), but n contact wth the cylnder lner. When the rng s n the operatng poston the external face of the rng presses aganst the wall of the cylnder due to ts bult-n tenson, whch forces the open ends of the rng to stretch outwards, thus establshng the so-called prmary seal contact. Once pressure s appled the rng s pushed aganst the sde wall of the opposte sde of the pston groove where t reaches ts secondary contact poston on the downstream sde. Forces actng durng compresson for each pston or stage are shown below (Fgure 4):
Force Dstrbuton Durng Compresson Cycle 3000 500 000 Force (N) 1500 Stage-1 Stage- Stage-3 Stage-4 Stage-5 1000 500 0 0 40 80 10 160 00 40 80 30 360 Shaft Angle ( 0 ) Fgure 3 Pston rng pressure and gas flow. (a) Pston square rng (b) Rng n sealng poston (c) Rng n neutral poston Fgure 4 Force Dstrbuton Durng Compresson 4. Gas Leakage of Pston Rngs The desgn of relable pston rngs for ol-free applcatons wth good characterstcs as regards frcton and leakage s presented here. Bascally the man functon of pston rngs s to prevent gas leakage under compresson wth low frcton and wear. Usually, each pston has a mnmum of two pston rngs nstalled. To mnmze sde force effects and to take the weght of the pston, rder rngs are also ncorporated n the whole assembly, preventng rubbng between the pston and the cylnder lner. The performance of pston rngs depends on many parameters nvolved durng sldng. Some of the major varables can be dentfed as: load or mean effectve pressure, pston velocty, pston and lner materal, gas composton, operatng temperature, and surface fnsh. Predctng gas leakage durng operaton s very complcated and dffcult to measure. Theoretcal estmatons may be used for some of the solutons [4]. In an ol-free gas compressor there are three possble paths of gas leakage through the rngs (Fgure 5): (a) Between the rngs and the surface of the cylnder lner, (b) Between the rngs and the groove of the pston, and (c) Through the gaps of rngs. b a a c b c Fgure 5. Three possble paths of gas leakage through the rngs
For the pston wth several rngs the total leakage mass flow ( m ) through the th rng become: m m + m + m (1) a b c The flow through the gaps of the pston rngs s presumed as a one dmensonal compressble sentropc flow. Therefore the gas leakage through the gap of th rng would be wrtten as: k 1 c A k.p + 1 P + 1 T [1 T + 1 P () m where A αf k R(k 1) But f the flow speed n the gap equals the speed of sound, B P c (3) T m where B αf k R(k + 1) k + 1 k 1 The flow between the pston rngs and the cylnder lner and between the pston rngs and the pston groove are consdered as flow n a thn clearance between two smooth surfaces. Ths problem can be solved usng two dmensonal ncompressble vscous lamnar flow theory. Naver-Stokes equaton was used to predct the leakage between the rngs and the cylnder lner ( m ), and the leakage between the rngs and the pston groove ( m ): 3 a π D a (p p 1 ) m + a (4) 4 µ R T h b 3 π (p p + ) m b 1 b (5) D 1 µ R T ln D b Now and C E πd 4h π D 1ln D b The total leakage through th rng s thus: k 1 k 3 3 A.P + 1 P 1 C a + E b (p p 1 ) T [1 T + + 1 P + + µ R T (6) m Ths formula to predct flow leakage through pston rngs s usually used for butt jont type of pston rngs (Fgure 6). Several types of cut jonts were desgned to mnmse leakage. Scarf jont and step-cut jont were popular shapes for use n hgh-pressure ol-free compressors. These jonts are more easly cut, nstalled and removed compared wth the gastght jont constructon whch costs up to twce as much. In accordance wth the sealng functon requred, pston rng jonts can be shaped to any degree of complexty wthn the bounds determned by the
materal propertes and rng dmenson. To mnmse the leakage between pston rng cylnder lners, rng backup sprngs can attached behnd the rng to apply a slght amount of pressure (-3 ps) and assst the pston rng n establshng the ntal seal. Fgure 6 Varous types of cut jonts for pston rngs 4. Rngless Labyrnth Plunger Pston For Last Stage In case of the last, hgh-pressure stage where pressure was rased to 3000 ps, many pston rngs are needed n order to reduce the load per rng. For ths stage, f pston rngs are to be ncluded, more than sx pston rngs would be needed, thus makng the use of pston rngs not practcal because of the complex structure and lmted space to nstall them (snce the dameter s very small, 10 mm). Therefore, the establshed concept of plunger pstons wthout pston rngs was adopted for the ffth (last) stage nstead. Intally the desgn for ths last stage used the plunger pston concept wth PTFE cylnder lner. The plunger concept requres tght clearance between the pston and the lner [1]. Pston sde force caused scuffng on the lner surface by the plunger pston edges. The lner covered only a porton of the pston durng sldng moton especally at bottom dead centre (BDC) where maxmum moment occurred at the fulcrum pont at pont a between the pston and the pston lner as shown n Fgure 7. Takng the moments about pont a, the force actng on the pston lner at pont b s gven as: L out F b Fcrt sn φ (7) Ln Where F F sn φ + F a crt b The optmum clearance between the pston and the lner s 5-6 µm. As PTFE s softer compared to the pston materal, ths clearance gets bgger as the compressor runs durng ntal testngs as a result of F a and F b forces actng on the nner surface of the cylnder lner. Heat generated due frcton also caused the lner materal to soften. Ths caused more deteroraton due to scuffng by the pston edge and hence enlargng the clearance. Greater clearance ncreases pston degrees of freedom whch n turn lead to more scuffng. Scuffed lner materal was pushed n front of the pston head fllng the clearance volume avalable. Thus, t was crucal to reduce F a and F b forces actng on the cylnder lner. The maxmum values of F a and F b at each stage s gven n Table. Ths scuffng problem has been solved by usng the pston rng concept n combnaton wth hard-chromed cast ron cylnder lner. Cast ron lner s much harder compared to PTFE and could wthstand hgh temperature. Sealng can also be acheved usng pston rngs whch slde along the smooth hard-chromed surface of the lner bore. One alternatve soluton s usng crosshead concept at pston coupler to mnmse the sde force effect. Fgure 7 Load actng on the pston
5. Prototype Test Results A seres of tests were done after the compressor has been developed and fabrcated. Frst, mechansm tests were done wthout pressure loads, to check whether the mechansms work properly or not. Then pressursed tests wth load onto the system were conducted to check the jonts and ppngs for leakage. Fnally, performance tests at full load, wth 3 bars sucton pressure and varyng operatng speeds were looked nto [7]. Some ssues arsng durng the tests, and measures taken to overcome them, are gven n Table 3. The results of pressure versus tme the latest test are shown below: Fgure 8 Result test at sucton pressure 1 bars and speed 600 rpm Fgure 9 Result test at sucton pressure 3 bars and speed 50 rpm Fgure 10 Results of tests at sucton pressure of 3 bars and speed of 400 rpm Fgure 11 Results of tests at sucton pressure of 3 bars and speed of 700 rpm Table 3 Some ssues arsng after testng the compressor and mprovng the desgn 7. Concluson
The concept of a mult-stage symmetrcal wobble-plate compressor was successfully desgned, fabrcated, and tested. In an ol-free gas compressor there are three possble paths of gas leakage through the rngs; between the rngs and the surface of the cylnder lner; between the rngs and the groove of the pston and through the gaps of rngs. The mathematcal models of gas leakage through pston rngs assembly can be satsfactorly adopted to descrbe the workng processes n a cylnder. However, there were several desgn ssues regardng the structural ntegrty of the pston rngs.e. pston sde forces, manufacturng and tolerance ssues. For the ffth stage the pstons need to be further mproved. In overall, the compressor desgn s promsng for natural gas vehcle refuelng usage. 8. Acknowledgement The authors would lke to thank the Mnstry of Scence, Technology and Innovaton, Malaysa, for the fnancal support for ths work under the IRPA grant, and to Unverst Teknolog Malaysa and Petronas Research and Scentfc Servces for logstcal support provded. 9. References 1. Ahmad, Z.A., Abdul-Latf, A., Syahrom, A., and Musa, M. N. (006). Effect of Sde Force on the Structural Integrty of the New Symmetrcal Mult-Stages Wobble-plate Compressor, Regonal Conference on Computatonal Mechancs & Numercal Analyss - CMNA006. 1-14 May 006. Syah Kuala Unversty, Banda Aceh, Indonesa.. Ahmad, Z.A., Abdul-Latf, A., Syahrom, A., and Musa, M. N. (003). Study on the Stressng of a New Symmetrcal Wobble Plate Compressor: Knematcs and Forces, Malaysan Scence and Technology Congress. 3-5 September 003. Kuala Lumpur: COSTAM. 3. Baumann, H. (1998). Desgn and Development of an Olfree, Hermetc Hgh Pressure Compressor. Internatonal Compressor Engneerng Conference at Purdue Unversty, West Lafayette, IN, USA. 4. Lu, Yong et. al, (1986). Predctng For Sealng Characterstcs of Pston Rngs of a Recprocatng Compressor, Internatonal Compressor Engneerng Conference at Purdue Unversty, West Lafayette, IN, USA. 5. Syahrom, A., Musa, M.N., Abdul-Latf, A. and Abdullah, M.A. (005). Desgn and Development of a Compressor for Natural Gas Vehcle Refuellng, Internatonal Conference on Engneerng Desgn 005. 15-18 August 005. Melbourne: ICED. 6. Syahrom, A., Musa, M.N., and Abdul-Latf, A. (005). Wobble-Plate Compressor for Natural Gas Vehcle Refuelng, Asa Pacfc Natural Gas Vehcles Assocatons Conference & Exhbton. July 6-8, 005. Kuala Lumpur: ANGVA. 7. Syahrom, A., Musa, M.N., Ahmad, Z.A. and Abdul-Latf, A. (006). New Symmetrcal Wobble-Plate Compressor, 18 th Internatonal Compressor Engneerng Conference at Purdue, 17-0 July 006. West Lafayette, IN, USA. 10. Lst of Symbols A Flow area of valve b Wdth or thckness of pston rng C Dametrcal tolerance between the pston and the cylnder lner D Dameter of cylnder f Flow area of gap of pston rng h Heght of pston rng k Isentropc ndex m Mass flow rate P Gas pressure R Gas constant T Gas temperature α Flow coeffcent δ Average clearance of contactng surface µ Gas velocty