Stepless Variable Capacity Control

7/28/2019 Stepless Variable Capacity Control

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FIG. 2: Part ia l ly regulated valve plate

The maximum piston speed (Fig. 3) determines the l imit for th is type of capacityregulat ion, since an increase of the forceacting on the valve plate keeps the valveopen through the whole -discharge stroke.Depending on the cylinder end (head- orcrankend), the maximum pis ton speed isreached before or af te r mid-stroke.20.

w 105" 120 IW 5lJ' INJ"FIG. 3: Instantaneous compressor-piston velocityfo r a given crank position having a 5:1 connectingrod to crank throw and a 12-in. s t roke, at 360RPM

The stepless capacity regulation can therefore be activated between the beginning ofthe discharge stroke and the point of maximum piston speed. The intake volume whichis smaller than the fu l l piston stroke bythe volumetric eff ic iency, ca n be reducedby more than SO percent. The regulationrange i s therefore:

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R SOEvFor example: Ev 8 R so

--:lr62.5%

Hence, i t is theoret ical ly possible t cvarythe capacity steplessly from 100% - 37.S%.On medium and high speed compressors thisspan is greater since the dynamic pressurecurve is out of phase with the pis ton speedcurve. On double-act ing cyl inders theregulation range can be increased by unloading the crankend completely a t SO percent capacity, the headend is then regulated again over i t s fu l l range, which is :

RD = SO KD"iJV:"7

KD is a correct ion factor from Fig. 4,which compensates for the ra t io of crankthrow to the connecting rod length. Figs.5 and 6 show a theoret ical and an actualpressure-volume diagram, respect ively, ofa cylinder end equipped with a steplessreverse flow regulat ion. The enclosed areaof a PV-diagram is proportional to theenergy consumed for compression work,therefore , these diagrams also demonstratethe savings of horsepower.

I.......

~~1~~ 1-lC:1jj

7

6

5 ---------4

3

2

4248

PisfOfl Posifion 8DfmDX.Pisfon S p . ~

o,30,7

0/tD,6

5 Plsfon P1slli0ffh =Total Slro/tLo=Sigmo

45 Cl'ank EndD,5 HeadEnd

FIG. 4: Correction factor fo r ra t io of connectingrod length to crank throw (A.= 5, KD 1.11)


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' - - - - ~ - V h ~ - - - - - ~ FIG . 5: Theoretic al PV,diagram for rev erse flowre gul atio n; Pl = suctio n pr es su re, P2 = discharg eVH pi st on dis plac em ent, V1n0x actualcapacit y, Vmm min. capacity with rev er se flowVo cle ar an ce; 1 = suction va lv e opens, 2 = valv e closesat full capacity , 2' an d 2" = suction va lv e cl oses at partia l ca pacit y

FI G. 6: Actual PV-diagram for rev erseflow re gul at io n

Stepless Regulation from 100% - 0%By adding a fixed clearance volume to lowerthe volumetric eff ic iency of the headendto Ev = .S , a s tep less variable capacity control from 100% - 0% can be accomplished.Fig. 7 shows the PV-diagram for th is ar-rangement.

v, ~ - - - - - - - - - - - v h - - - - - - - - - - ~ ~ FIG . 7: PV"d iag ra m for EV = .5Yo cl ea rance volu meVz = cle aran ce pocket volume

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FIG . 10: Sucti on val ve wit h unloade rand actu at or for steple ss reverse flowon non- lube compressors

The necessary unloading force i s generatedin a diaphragm actuator by a variab le , hutsteady, pneumatic pressure. The unloadingpressure determines the closing time ofthe suction valve and has to be varied according to the capacity required. Fig. 8shaHS the necessary unloader force (1,11icbis proportional to the a ir pressure on the diaphragm) in rd a t io n to the capacity ofthe compressor.For a capacity range of - 60%, th is diagram shows su ff ic ien t l inea r i ty to assure adequate performance of the system.If the regulation span i s expanded to 100% - 40%, a spec ial pressure l im ite r hasto be in sta l led which stre tches the portionfrom 60% - 40 '; so i t can he used on the actua tor (Pig. 9).


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FIG . 13: Actual indic ator diagram of an ad dit ional clearan ce volume reg ulatio n with limite dre gula ti ng ra ng e

Fig. 14 shows an in te resting poss ib ili ty ofaccommodating the required clearance volumefor a single-ac ting cylinder direc tly in tothe suction duct. The regular suctionvalve is used as the regulating (clearancepocket) valve, and the clearance pocket is defined by the volume between the checkvalve and the suction valve.

tlr=r-

FIG . 14: Additi onal cle arance vo lume regulati onwith dir ect manual opera tion on the machine andcleara nce pocket being part of th e su ct ion duct.a additi onal cle arance volume, b regulatin gvalve , d n unloader pis to n, e spr in g, f = sp in -dle , g = handwheel, i suc ti on valve , k E dis-charge va lv e

Required Size of the Clearance PocketThe size of the clearance pocket is determined by the range of capacity regulation,fixed clearance volume, compression ra t io ,and the type of gas compre ssed.

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RVminvS'oVoVHVz

R 1 - Vmin-v -fo Vo

VHVz == VH R ( -

regulation rangeminimum capacity maximum capacityfixed clearance volume in %fixed clearance volumepiston displacementrequired clearance volumef'z can be found in Fig. 15, whereby mexponent of polytrope.For constant pressure ra tio s the clearancevolume can also be determined by :

I f the pressure ra tio changes during theregula ting process, the following fo rmulahas to be used:

Yz == VH

I t can be seen tha t on large cylinders withsmall pressure ra tios (below 2) and a wideregulation range, the required volume becomes ra ther large. On large singleacting cylinders i t is therefore advisableto use the so lution as shown in Fig. 14.

'i''z=(m;pe/pa)

\5 2,0 2,5 3Jl 3,5 1.,0 1.,5 5p- - - -p e /pa

FI G . 15: Diag ram for siz in g of cle arance pocket


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FIG . 16: Clearance volume regula tio n with se rvo cylin der and addit ional sc rew sp ind le for manualope rat ion: Cle arance pocket in the cyli nder cover1 re gula ti ng va lve with un lo ade r, 2 c se rvo cyli nder wit h pist on, 3 a screw spi ndl e and han d wheel

REFERENCESOTT, J . , "Pneumati sche Regelung vonKolbenverdichtern", Pneumatik Digest,Volumes 2, 3, &4, 1973BAUER F . and MOLNAR, K. G., "M ethods ofStepless and Loss-free Capacity Controlfor Reciprocati ng Type Air Compres sors",Fluid Power In ternational, January 1968

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Stepless Variable Capacity Control