Shilian, Z.

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Purdue UniversityPurdue e-PubsInternational Compressor Engineering Conference School of Mechanical Engineering1986An Analysis of Significant Differences BetweenSuction and Discharge Valves in Reciprocating AirCompressorsZ. ShilianFollow 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.htmlShilian, Z., "An Analysis of Significant Differences Between Suction and Discharge Valves in Reciprocating Air Compressors" (1986).International Compressor Engineering Conference. Paper 559.https://docs.lib.purdue.edu/icec/559AN ANALYSIS OF SIGNIFICANT DIFFERENCES BETWEEN SUCTION AND DISCHARGE VALVES IN RECIPROCATING AIR COMPRESSORS Zhon.g Shilian Department of Power Engineering, Huazhong University of Science and Technology, Wuhan, China ABSTRACT Using the valve pressure loss equation, the suc-tion valve power loss derived is about 64.3% of total valve power loss, while the discharge valve power loss only 35.7% of it. In additioa, differences lie between the structures and operating conditions of suction and discharge valves. Analysis shows that the major requi-rement for suction valve is to reduce power loss and the requirement for discharge valve is higher than that for suction valve in sealing, strength and ser-vice life. Based on the experimental result, synthe-tical economical efficiency being considered, a com-bined plan of valve arrangement ia a range of reci-procating air compressor for pnettmatic power is dis-cussed. INTRODUCTION Fer a rationally-designed and well-made air com-pressor with tae ring type valves, by means of modern design method and experimental technique, the parame-ters of valve are adjusted close to the optimun opera-ting eoaditions, and the compressor is accompanied with lower specific power consumption. The question is whe-the~ the specific p9wer consumption could be further redneed throagh improving the valve again. The aather thought of the straight-flew valve since in this type of valve the pressure loss is the smallest among the valves o~ same diameter. The experimental investiga-tion kaa shown that the suction straight-flow valve substituted for ring type valve can obviously decrease the speoifie power, which depends mainly on Mach nwa-ber ia ring type valve: the greater Mach number is, 647 the more obvious the effect will be. But the dischar-ge straight-flow valve substituted for ring type valve shows almost no effect, in particular, when Mach number is comparatively small, the performance of straight-flow valve is even worse thaa that of ring type valve. This phenomenon shows that some signifi-cant differences lie between the suction and dischar-ge valves, as analysed in the following. In this paper, some macro analyses with regard to these differences are made;and an economical plan of valve arrangement is discussed on the basis of the experimental results. DISTRIBUTION PERCENTAGE OF LOSS WORK IN SUCTION AND DISCHARGE VALVES There are alrea~ new methods for calculating valve loss work and the experimental data obtained by means of modern measurement technique has rather high accuracy, but for general purpose of comparison, in this paper, the values of ratio of loss work in suction and discharge valves to total loss work of the valves are calculated by approximate eqaation suggested in Refereneef1)without using the accurate methods. Two-stage air compressor for pneumatic with 7 bar of discharge pressure is made as a target for calculation since this type of compressor is typical and produced in large amounts. Simplified Assumption (1) The valves open full~ for the complete valve event and close at piston dead centre. (2) According to the statistics data of air compres-sor for pneumatic in oQr national L-type series (doQble-action, water cooling) and V-type series (single-action, air cooling), when 7 bar of dis-charge pressure~ two-stage compressiaa and plate valve(ring typeJ are used, the auction valve ac-tion time is equal to about 140 degree of crank angle, and discharge valve action time is equal to about 70 degree of crank angle in a cyele. (3) The through-flow areas of suction and discharge valves are equal. (4) There is a same pressure ratio in both the first and second stages, Let the pressure ratie =vtr = 2.828. 648 (5) Mach numbers in the suction and discharge valves are calculated respectively under the suction temperature 20'C, and the corresponding discharge temperature, about 120·c. Thus, the values of ratio of loss work in the suctian and discharge valves to total loss work of the valves can be calculated easily by means of pressure loa,s equation. A derivation is given i.n Appendix. Calculating Results (1) In the double-action cylinder the valve loss work is about 64.J% of total loss work of valves and discharge valve loss work about 35.7% of it. (2) For the single-action cylinder (head side) the percentages of loss work of suction and discharge valves are about 57% and 43% respectively. The above calculation is certainly quite approxi-mate since some assumptions are made. In pwaetiee, as a result of complicated dynamic behaviour in valves and differences 1.n the parameters and operatiDg co.a-4itions, the measured percentage of loss work is not full¥ consistent with above-calculated value. In par-ticular, when valve dynamic behaviour is not ideal, the inconsistency is shown obviously. But the above· calculatiea at least provides a significant eoaoept, i.e. when through-flow capability in suction valve is equal to that in discharge valve, the suetioA valve power loss will be Qbviously greater than discharge valve power loss. This concept is consistent with the experi•ental results. STRUCTURAL DIFFERENCES As is well known, tb.e clearance volwae has in-fluence on perforaence of compressor, in particular, the first-stage olearanee volume will reduce volwae efficiency of compressor and cause higher specific power consumption of it, hence, a small clearance volume in the valve assembly is desirable. The major structural difference between suction and discharge vaives is that the values of their clearance volume are aot equal, and they are depended on the type of valve, eperating conditions, etc.. · In typical plate valve design, according to the strength requirement, for air compressor for pneumatic 649 with 1 bar of diseharge pressure, the seat height is so small that the clearance volwme of suction valve is greater than that of discharge valve. Take the well-known Hoerbiger type plate valve (8> as an exam-ple, only when the seat height is about 35-40 mm, the two clearance volume values are nearly equal. But in air compressor for pneumatic the seat heigkt usually is about 20mm so that the clearance volttme of dischar-ge valve is about 60% of suction valve clearance. In addition, in our national air compressor aeries with ring type valve, the value of ratio of discharge valve clearance to suction valve clearance is about 0.7-0.85. Just as opposed to the plate valve, in the strai-ght-flow valve, the suction valve clearance volume is much smaller than the discharge valve clearance volume and usually it is about 60% of the latter. DIFFERENCES IN OPERATING CONDITIONS The operating condition of discharge valve is more critical than that of suction valve. Major dif-ferences display in (1) Discharge valve opens·and closes under the extre-mely rapid pressure change so that the impact ve-locity in general is higher than suction valve. There is no doubt that the stress of discharge valve is also higher. (2) The operating temperature of discharge valve is higher. Though it has no direct influence on fa-tigue strength, heat strain and store-up carbon are more aurious than those in suction valve, which will cause more failures in operation. (3) The leakage of discharge valve is greater than that of suction valve because of the following reasons: the acting time of discharge valve in a cycle is shorter than that of suction valve, hence, the leakage time is longer; the average sealing pressure is higher; and the heat strain reduces sealing ability. The statistics data shows that the life of dis-charge valve usually is lower than that of suction valve. And discharge valve meets with more failures than suction valve, in particular, when lower quality material is applied. 650 (1) (2) (J) (4) QUESTION OF INTEREST The obvious differences between the suction and discharge valves determine the distinct require-ments for them. As analyzed above,the major re-quirement for suction valve is to reduce power loss and the requirement for discharge valve is higher than that for suction valve in sealing, strength and service life. However, to have a small clearance is the common requirement. There is a great variety of valve type, however, absolutely evaluating good or bad valve types is not proper. In fact, the features of each valve type determine a suitable range of operating con-dition. Certainly, some of them have a wide range of suitability (e.g., ring type valve) and others have a narrower one le.g., read type valve). The above analyses show that using the same type of suction and discharge valves appears to be imperfect. When the suction valve is of straight-flow type and the discharge valve plate type, not only the power loss in the suction valve is de-creased effectively, but also the total clearance volume of the valves is rather small, hence, in-creasing the volume efficiency of the compressor. The experiment we made in some air compressors with different speeds has, to a certain extent, demonstrated the above tendency. For example, the experimental result obtained in a V-type air com-pressor (capacity 9 M3/min, discharge pressure 6.86 bar, speed 1490 rpm, air cooling) shows that the specific power is reduced from 6.01 to 5.76 kw/(M~/min). The economical plan of valve arrangement is related to the valve power loss, the cost of production, the service life and compressor operating time rate, the magnitude of annual production, and its operating conditions, etc •• For the compressor with high speed and great Mach number, both the suction and discharge valves are arranged by strai-ght-flow type valve, whieh will be advantageous, but when Mach number in the valve is of average or small value, selecting combined plan of strai-ght-flow type and plate valve will be economical, in particular, when the compressor operating time rate is high and annual production is great. In this case, both suction and discharge valves should have a long and the same service life to avoid much maintenance work. Certainly, produc-tion o! the valve assembly must be commercial. 651 APJ?EIWIX CALCULATTIW THE PERCENTAGE OF LOSS WORK IN THE SUCTION .AND DISCHARGE VALVES NOTATION A piston area p a acoustic velocity k ratio of specific heat (air, k=1.4) M average mach nt.Unber in the valve p average pressure in plenum chamber ~P instantaneous pressure loss across valve R gas constant S piston stroke T gas absolute temperature in valve V instantaneous cylinder volume V average equivalent velocity in valve eq Vmv cylinder swept volume X piston displacement from head end centre W loss work in the valve k pressure ratio 9 crank angle measured from head end centre f crank/connecting rod length ratio Subscripts d discharge s suction Abbreviations CESV CEDV E.::SV HEDV crank end suction valve crank end disch22·ge valve head end suction v2lve head end discharge valve Calculating When the average 1f.Jach number in valve is smaller, 652 the instantaneous pressure loss can be calculated according to the following equation ~P= ~t;2p (Si,tt+ ~Sin29i'H.a <I> The loss work in suction and discharge valves can be illustrated using the shaded area of p-v dia-gram in Figure 1. For simplicity, the area::J can be calculated using the following tntegration. W=l~ A#' rJ.v =Ap rXz41> ctx <2> \t Jx, where x1 and x2 are piston displacements corresponding to v 1 and v 2 • X: f(u-cose t ~(I-C.oS29) (j) d. 'X.= t (Sil\& + ~ .sirL2e) de · c4> Substituting (1), (4) into (2), we have w = ~t t>~wM' J.:"<sitt.e + ~ s;,..2e)1J.e c5> where a1 and e2 are crank angles corresponding to x1 and x2 ·: (Sirt8 + ~Si~t28i::; SiJe + 3A.SiiecD58 (6> In equation (6), the terms with A higher than second-order are neglected. Substituting (6) into (5) and integrating itf we have W= 11-uA/' '' .Ma(cSideo!.8 -!.use)!''+( .!}\si ..... e)!e') c7> ,, rs., s 3 e. -+ a, The values of t'f, and fh for integration limit are listed in Table '1 on the basis of assumption (2). Let A = 0.2, substituting the value of 9 in Table 1 into (7), the loss work of the corresponding valve can be calculated, as well listed in Table 1 Table 1. valve al l:h o~a) w names de g. deg. deg. C:J:<;SV 220 360 140 Wc;es= 1.130011 ~,.M; CEDV 110 180 70 Wc&p = o.t9ot P.t ~)¥ M~ . HESV 40 180 140 Wlfis :f.OMB P.s V1wM! HEDV 290 360 70 WHEP= O.l9.29P" '4w M~ 653 For double - action cylinder, the avero.ge loss work in the suction valve and that in the discharge valve are respectively W.s = Wces; WHES : t-1079 ~ VswM: WrA.:. Wen~ W.u = 0.8255~ "wMl For single - uction cylinder (head side) the loss work of H.t:S valve and that of HED vD.lve are res-pectively WHES and \/ILD • For double - action cylinder the percentage of the discharge valve loss work to the total loss work of valves is W" : o.825S M; w.+ w$ o.s2~5 Ml ... ,.,oTlJ 115 (8) .,.a u• where M"- Vl(et _ '"" "- ~ ..tRT,t Ma _ V,~1 _ Vs:. s- a.; -'IllS According to aosmm1tion substituting (9), (10) n.c1.d T into (8), we have I c9> C/0) (3), ~.. = Vse.t , = 393.2, T ~ 293.2 ,s w. - 0·8%55 It ltJ·l w. -t w; - I I :I.Jjl =JJ.J/. cl l o.8Z5t• 3,).2 -+ /.I01f• ~,J.Z then, Using the smne E:c-;;hod as for the ningle-c::.ct ion cylinder, the ~Jerce-,;tuco of suction valve loss work and that of di::clw.rso vol ve loss work arc about 57% and 43%respectivoly REFERENCES (1) FlinkEl.M.E: "IIopmHeBLie KOMrrpeccopLI "Maskez,1960 (2) H. Davis: "Effects of Reciprocati.ng Compressor Valve Design on Performance and Reliability" I. Mech. E(1970) 654 (3) J. F. T. MacLaren and S. V. Kerr "An Analytical and Experimental stud1 of Self-Acting valves in a Reciprocating Air Compressor" I. Mech. E (1970). (4) M.A. Miles "The Increased Utilization of R~cipro­cating compressors by The Diagnosis and Preven-tion of valve Failures" I. Mech. E (1970) (5) J. Boyle 1 A.B. Tramschek 1 J.Brown and J.F.T. Ma-cLaren trThe Apportioning of Port Areas between Suction and Dischar~e Valves in Reciproceting compressors" proc. 6th Compressor Tecll.nology Con-ference, Purdue University, 1982. (6> Derek Woolatt "Estimating Vs.lve Losses whtJn Dyn~­mic Effects A:re Impor1;ant" proc. 6th Compressor Technology Conference, Purdue University, 1982. (7) A.s. kopilivech "Calculating pressure lo~s of the valve in piston compressor" XHMHqecxoe H H~Tsuoe MamauocTpoeHHe" , 1W2. -(8) Hoerbiger valve sample data. v Fig. 1~ Pressure-volume diagram calculating idealized valve loss work 655 

An Analysis of Significant Differences Between Suction and Discharge Valves in Reciprocating Air Compressors

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An Analysis of Significant Differences Between Suction and Discharge Valves in Reciprocating Air Compressors

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