Effects of Port Geometry, Dimensions and Position on the Performance of a Rotary Compressor

Transcription

1 Purdue University Purdue e-pubs International Compressor Engineering Conference School of Mechanical Engineering 1992 Effects of Port Geometry, Dimensions and Position on the Performance of a Rotary Compressor A. B. Tramschek University of Strathclyde K. T. Ooi Nanyang Technological University Follo this and additional orks at: Tramschek, A. B. and Ooi, K. T., "Effects of Port Geometry, Dimensions and Position on the Performance of a Rotary Compressor" (1992). International Compressor Engineering Conference. Paper This document has been made available through Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at Herrick/Events/orderlit.html

2 EFFECTS OF PORT GEOMETRY. DU(ENSIONS AND POSITION ON THE PERFORMANCE OF A ROTARY COMPRESSOR. Dr A.B. Tramschek, or K.T. Ool Senior Lecturer & Dean. of, Engineedng school of Mechanical and Production Engineering, Glasgo. Scotland. Nanyang Technological University, Singapore University of Strathclyde, ABSTRACT Rotary slldlng vane compressors are positive displacement compressors ith built-in volume ratios. Their suction and discharge processes are continuous and thus they are normally valveless machines. The suction and discharge port dimensions and positions thus critically affect the performance of the machine. The paper first presents the effects or the suction port geometry on the performance or the machine. It then follos by examining the infiuence or both suction and discharge port positions on the machine performance. The analysis as carried out using a theoretical model describing a circular rotor-stator configuration ith radially disposed vanes, here the orking tluid is air. The resulting effects on the pressure volume diagram and the instantaneous mass-angle or rotation history are shon. INTRODUCTION Rotary sliding vane compressors are machines ith built-in volume compression ratios. Hence tor given machine dimensions. the value ot the pressure at the end or the compression process Is predetermined by the suction pressure and the actual volume compression ratio. Matching the suction and discharge port positions ith the built-in volume ratio is lmporu.nt In order to optimise machine performance. An e.llperimental Investigation by Kruse Ill suggested that this matching Is the key to Improving the machine performance. The analysts that follos examines the effects of suction port geometry and/or port positioning on the machine performance using a theoretical model [21 (31. PORT DESIGN CONSIDERATIONS The appropriate choice or the suction and the discharge port positions tor compressors ith built-in volume ratios depends very much on the variation of the cell volume. Figure 1 shos schematically a typical rotary compressor ith eight vanes and table 1 shos Its nominal operation conditions and its capacity. The variation of an tnd!vtduai cell volume is shon in Figure 2 trom hich it may be seen that the maotimum cell volume occurs at a leading vane angular position of about 0 degrees hilst the minimum cell volume occurs at 18 degrees. These positions correspond to mid-cell angles ot degrees and degrees respectively. Although a s.ymmetrtcal rotor-stator arrangement Is being considered, deviations of the above angular positions from mid-cell angles of 180 degrees and 360 degrees are caused by the Inclusion or the leading vane and its slot as part of the cell volume. For a correct port design, the ma.llimum cell volume position should correspond to the suction port closing angle hereas the minimum volume position should correspond to the dischatge port closing angle. The opening of the suction port is based on the value ot the suction pressure. It is best located at the angular position here the cell pressure immediately before the suction port opens is just belo the nominal suction pressure. The discharge port opening angle should correspond to the position here the cell pressure reaches a value just above the nolllinal discharge pre11sure. The latter position Is alays delayed tor a re degrees 1t the operational discharge pressure ot the compressor Is to be varied (especially above the design pressure) as over compression losses are generally smaller than the under-compression losses tn terms ot their contribution to the Indicated po er. 1177

3 EFFECTS OP SUCTION PORT CONFIGURATION This section discusses the effects of the suction port configuration on the performance of a given compressor. To different suction port geometries. termed the 'old' port configuration (see Figure 3(a)) and the 'ne' port configuration (Figure 3(b)) ere Investigated, and their infiuence on the performance or the compressor ere compared. The 'old' suction port (Figure 3(a)) as sited on the end face of the rotor and as located beteen the crescent shaped gap formed beteen the rotor and stator. The size or the suction port is thus restricted by the dimensions of the rotor. stator and the offset beteen the to centres. This particular shape of the suction port produces a particular variation In the port area ith respect to the variation of cell volume and influences the breathing characteristic of the compressor. Figure 3(cl shos variation or the active port areas ith leading vane angular position. This shape or suction port as found to have certain disadvantages as follos:- 1. The port cross sectional area Is small in the region close to the suction port opening position and hence causes a slo recovery in cell pressure during the initial suction stage. This contributes to poer losses as may be seen from the pressure volume diagram In Figure 4(e). 2. The maximum idth of the suction port area Is limited by the dimensions of the rotor, the stator and the offset beteen their centres. a. In the simulation study, it as assumed that the cell pressure Is uniformly distributed throughout the hole volume of the cell. This assumption may be questionable during the suction process because air needs to flo axially through the end race of the compressor here the suction port Is sited and pass along the hole length of the cell before filling the cell volume. During this process some form of axial pressure distribution ould be expected and the situation ould be exaggerated by a long axial flo path. A simplified simulation ot the suction process may be Inaccurate. In an attempt to overcome all these problems a ne port configuration as Introduced. The change envisaged rectangular-like holes distributed along the stator all. see Figure 3(b). Predicted results for the to different suction port geometries are shon in Figures 4(a) to 4(!). Figure 4(a) shos the variation or the active suction port area for the old and the ne suction ports. It also shos the effects or dltterent axial port leneth on the variation or the port area for the ne port hen axial port length varies from 6 mm to 30 mm. It may be seen that the suction port area Increases directly as the axial length of the port Increases. The extra port area during the initial suction stage causes the ceil air mass to increase rapidly (see Figure 4(b) during the initial suction process. This effect results in rapid pressure recovery as shon in Figure 4(c) and hence improves the breathing characteristics or the compressor as reflected In pressure-volume diagram in Figure 4(e). The resulting variation on cell air temperature is also shon, Figure 4(d). Figures 4(c), 4(e) sho that a further reduction In the suction loss Is achieved by having a longer ajtlal port length. Figure 4(!) compares the ettects of the old and ne suction port configuration on the performance of the compressor. Some Increase In the free air delivery is achieved by using a ne port configuration. The Indicated and shaft Input poer decrease sllghtly as the axial length or the ne port Increases and as a result, the specific tree air delivery (Utre/kWs) increases. Hoever, the benefits diminish' hen the axial length of the ne port exceeded 25 mm. The overall advantages and the 1mprovements stemming from the modified suction port may be summarised as follos:- a) The machine shos a better breathing characteristic:- rapid pressure recovery during the suction process. A reduction in the indicated poer as predicted. b) The size or the suction port is no longer limited by the radll of the rotor and stator as it as before. Speaking In terms of suction port design, this feature provides greater tlextblllty in the selection of dimensions. 1178

4 c) In pn. lce, a shorter tlo path for the air induced during the suction process should result In less suction heating and hence Improve the volumetric errlclency of a machine. This effects as not included hoever in the theoretical model. d) This port configuration suits better the assumption made In the simulation model that the cell pressure Is alays distributed uniformly In a ceil Cells may be tilled quickly by the air induced because or the axially distributed and shorter flo path. EFFECTS OF PORT POSITIONING The streets of the suction and discharge port positions on the pressure volume diagram and the cell air mass versus leading vane angle diagram are illustrated in Figures 5 to 12. Figures 5(a) and 5(b) sho the pressure volume diagram and the air mass versus leading vane angle diagram under the correct port positioning. Correct port positioning means that the suction port opens at the position here the cell pressure equals the nominal suction pressure (In reality hoever, a modest level of partial vacuum must exist inside the cell, to promote air flo Into the cell from the suction chamber). the suction port closes as soon as the cell volume attains its maximum value. The discharge port opens as soon as the cell pressure reaches the nominal discharge pressure and closes at the position hen the cell volume attains the minimum volume. In the discussion that follos various symbols are defined: P. nominal suction pressure P nominal discharge pressure M, normal residual air mass level before suction process M2 normal air mass level at the end of suction process The Influence or each port-position is Ulustrated as follos:- Suction port openlpg ancle (@,) al. If the suction port Is opened before the cell pressure reaches the nominal suction pressure, under-expansion results. This is illustrated In Figure 6(a). Losses due to this street are shon by the shaded area in the Figure. The air mass in the cell Increases gradually shortly after the commencement or the suction process. See Figure 6(b). b) If the port it; opened after the cell pressure reaches the nominal suction pressure, over e)lpanslon occurs. The cell air mass increases very rapidly in the early stage or the suction process, because or a high pressure dlccerential across the suction port during this period. The etrect Is shon in Figures 7(a) and 7(b). SUctlpn pprt clpslng angle!b,j a) If the suction port is closed before the cell volume reaches the maximum cell volume position, a reduction In the cell pressure to a level belo that of the nominal suction pressure occurs as the cell volume Is stu! increasing. This premature suction port closing results In a situation here less air is induced into the machine. These etrects are illustrated in Figures 8(a) and B(b). Premature closure of the suction port results In cell pressures less than the nominal discharge pressure hen the discharge port opens and backflo occurs through discharge port. b) It the suction port is closed after the cell reaches the maximum cell volume position, air Inside the cell flos out or the cell through the suction port. This etrect is caused by the reduction In the cell volume after the maximum volume position attempting to produce a rise in the cell pressure. This 1179

5 pressul'e rise is small and air flos out through the suction port at a constant nominal suction pressure. A delay In port closure results in an under compression condition and may be explained as follos. consider ( v ) P,- V P, here P, is the nominal suction pressure and n is a compression index. As Vt reduces. and since Vz Is fixed for a given discharge port location the value of Po reduces to a value less than the nominal discharge pressure. P ; under compression occurs. The effects are shon in Figures 9(a) and 9(b). Discharge port opening angle (B.) a) It the discharge port is opened before the pressure in the cell reaches the nominal discharge pressure, under compression results. Reverse flo occurs in the initial stage of the discharge process. This Is shon in figures lo(a) and lo(b). b) rr the discharge port ls opened after the cell pressure reaches the nominal discharge pressure. over compression results. Air flos out of the cell during the Initial stage or disch!ll"ge process ith a higher velocity than that or the nonnal condition. This Is shon by the steep slope of the cell air mass-angle curve during that period. It Is Illustrated In Figures ll(a) and ll!bl. PlKharge nort closing angle!b.) a) If the discharge port Is closed prematurely, more residual air stays in the cell at the end or discharge process. At the same time since the cell volume is st111 decreasing, the cell pressure rises. Less air lll be delivered and less air ill be induced folloing the re-expansion to the suction pressure. This is shon in figures 12(a) and 12(b). b) If the discharge port is closed after the cell in question reaches the minimum cell volume position and if the cell is ithin the sealing arc region then this ill. not alter either the pressure volume diagram or,the cell mass-angle diagram. CONCLUSION The effect of the port geometry and port positioning on the perronnance of a machine is significant. The analysis shos that, to optimise the machine perfonnance, It is important to match the built-in volume ratio ith the operational pressure ratio. rr a machine has to operate under a ide range of operating pressure ratios, it Is suggested that the discharge valve should be used, as mentioned In (11. NOJIENCLATURE FAD (1/s) Pt(kW) Pt(kW) Rr(m) Ra(m) TDC e(mm),( ).< > :t > 13.< > Free air delivery Indicated poer Shaft poer Rotor radius Stator radius Top dead centre (angular reference position, o ) Eccentricity beteen stator and rotor centres Suction port opening angle.r.t. TDC Suction port closing angle.r.t. TDC Discharge port opening angle.r.t. TDC Discharge port closing angle.r. t. TDC 1180

6 ACKNOWLEDGEMENT The authors ish to acknoledge the support received from the Hydrovane compressor Company Limited during the period of Dr Ool's doctoral studies. REFERENCES 1. Kruse, H. "Experimental Investigation on rotary vane compressor." Proceedings of the 1982 Purdue Compressor Technology Conference 2. Tramschek, A.B., Ooi, K.T. "Geometrical optimization of sliding vane compressors." European Conference on Developments in Industrial Compressors.!MechE Headquarters, London Oct Ooi, K.T. "Geometrical optimization of rotary sliding vane air compressors." Ph.D Thesis, University ot Strathclyde, Glasgo, Operating speed rev/min Atmospheric pressure Atmospheric temperature Nominal suction pressure Nominal discharge pressure Capacity bar K bar (ABS) bar (ABS) 1/s Table Operating conditions and capacity or the compressor 1181

7 FIGURE 1. SCHEMATIC ARRANGEMENT OF A ROTARY SLIDING VANE COMPRESSOR 100[ gao l u 0 o----6o------,2o------,so oo----3so LEADING VANE ANGULAR POSITION!DEGREESJ FIGURE 2. VARIATION OF CELL VOLUME. 1182

8 _/SUCTION PORT ENDFACE FIGURE 3a 'OLD' SUCTION PORT CONFIGURATION SUCTION PORT STATOR ENOFACE FIGURE 3b 'NEW' SUCTION PORT CONFIGURATION = 8 \...,.E 10 I 'o I i1j 6 I ----'OLD' SUCTION PORT - ---'NEW'SUCTION PORT, - - DISCHARGE PORT 1 'E ' \ I., I /' :; 2 \ L----s o=---,2o----,ao--24o oo LEADING VANE ANGULAR POSITION <DEGREES) FIGURE 3Cc) VARIATION OF ACTIVE PORT I I I 1183

9 t:r 12..,E 10 < a: 0 c.. :z: 0 4 i= 2 (,.) ::::) en OLD PORT ---NEW (6mm) NEW (9mm) - -NEW(12mm) - -NEW(15mm) NEW (mm) ---NEW C25mm) NEW (30mm) LEADING VANE ANGULAR POSITION!DEGREES) FIGURE 4(a) VARIATION OF ACTIVE SUCTION PORT AREA en en < ::::!!: a: < J 30...J (,.) z ;. a: ;o U) U) a: c.. (,) 0 oo - - -OLD PORT - -- NEW (6mm) NEW (9mm) ---NEW (12mmJ - -NEW (15mm) NEW (mml NEW (25mm) -----NEW (30mml LEADING VANE ANGULAR POSITION (DEGREES! FIGURE 4(b) VARIATION OF CELL AIR MASS OLD PORT --- NEW (6mml NEW (9mml --- NEW (12mml --NEW (15mml NEW (mml ---NEW (25mml -----NEW ( Omml LEADING VANE ANGULAR POSITION (DEGREES> FIGURE 4(c) VARIATION OF CELL PRESSURE

10 lei: ffi 600 D ::: OLD PORT --- NEW (6mm) NEW(9mm) --NEW (15mm) --- NEW(12mm) NEW (mm) --- NEW (25mm) NEW (30mm) o o LEADING VANE ANGULAR POSITION (DEGREES) FIGURE 4(d) VARIATION OF CELL TEMPERATURE OLD PORT --- NEW (6mml NEW (9mm) --- NEW (12mml - - NEW (15mml NEW (2Qmml NEW (25mm) NEW (30mm) CELL VOLUME (10-6 m 3 l FIGURE 4(e) PRESSURE-VOLUME DIAGRAMS -FAD (1/s) -.- Pt (kw) --+- Pi (kw) -- FAD/Pt (1/kWs) a :i 9 0: 6================ ffi a SUCTION PORT AXIAL LENGTH (mm) FIGURE 4(f) VARIATION OF COMPRESSOR PERFORMANCE VARIABLE WITH SUCTION PORT AXIAL LENGTH 1185

11 .. -E 11100!m gj rooo!!j0 P, a ====-- 0o oo c2o CELL VOLUME no-im Y1_ Ia! cooo u 0 : 160 1<1() i c2o "' coo i 10!i 00.., 40 CORRECT PllfMATURE 3o- ----so 90 -CORRECT CELL VOLUME no"' m 3 I PREMATURE C!o LEAOIN(i VANE ANGUlAR POSmON IDEGllfESI FIGLR 5. P-V DIAGRAM AND CELL AIR MASS- ANGlE DIAGRAM FOR COMMSSDR OP RATING AT CORRECT PORT POSITIONING 0 o 60 c2o eo 240 a110 LEADING VANE 1\NGUlAR!'OSmON IOEGREESI FIGURE I. P-V DIAGRAM ANO CELL AlA MASS-ANGlE DIAGRAM COM""RISON BETWEEN CORRECT AND PREMATURE SUCTION POirr opening l..., "' cooo i.oo g 0.s ceo 1<1() C il 100 "' eo S eo <I() CORRECT DELAYED Ia! go CORRECT CELL VOLUME 110" 0 m 3 1 OELAYEO O OL-==60-1--CeG ilq LEOING VANE ANGULR POSITION (DEGREES) FIGURE 7. P-V DIAGRAII AND CEll AIR MlSS-ANGlE DIAGRAM COM""FIISON BETWEEN CORRECT ANO DELAYED SUCTION PORT OPENING (b).....!! c o 1 "' 100 " 10 8 <I().. 00 CORRECT \ I \ \ '-.. PREMolllJE 0 o " GOL---, CELL VOLUME ico-i m 3 1 correct PREMATURE 00 so 1 ceo 2< LEADING VANE ANGUlAR POSmON (DEGREES! FIGURE 8. P-v DIAGRAM lno CELL R MASS-ANGLE DIAGRAM COY""RSON BETWEEN CORRECT AND PREMATURE SUCTION PORT ClOSING Ia! (b) 1186

12 -E lz 400 le... correct DELIYEO ' ' <> 0 ' Ia I e i 400!!l CORRECT PI!ETURE lal [ 140 " 1..: '" 100 i!0 "' "' CELL VOLUE (10" 0 m 3 ) CORRECT DELIYED (b) r1 ;; 100 i 10 =l CEUYOL (10-t m CORRECT PllE.. TURE,, (b) I ! LEADING V"E ANGUlAR POSITION (DEGREES) FIGURE 9 P V DIAGRAM AND CELL AIR SS ANGLE DIAGRAM COMPARISON BETWEEN CORRECT ANO DELAYED SUCTION PORT CLOSING 0 o eo LEADING \line ANGIAAR POSITION!DEGREES! AGIJIE 10. P V DIAGRAM AND CEU AIR MASS-ANGLE DIAGRAII COIIAIAISON BETWEEN CQAAECT AND PREMATIJI'I DISCHAI!Gi PORT OPENNG. 10 -,--CORRECT,, 'I I ----DELAYED (a) CORRECT PREMATURE lal ! ;: 1!0 "' "' so <> 40 0 o ao= o CELL VOLUME (10., m 3 I ---CORRECT ---- DELVED (b) l 100 i 10 i o=------,,:=o ----so ,90:;----.,.,. --COARI!CT CELL VOlUME m m 360 LEADING VANE NGULIR POSmON!DEGREES! AGURE 11. P V DIAGRAM AND CELL AlA MASS ANGLE DIAGRAM COIIPARISON BETWEEN CORRECT AND DELAYED DISCHAilGE POI!T OPENING. 121) LEWNG VANE ANGULAR POSITION!DEGREES) FIGUil12 P V DIAGRAM ANl CELL AlA MASS-ANGLE llagram COIIAASON BETWEEN CORRECT AND Pli<MAtuiiE DISCHARGE PORT CIDSING. 1187