Purdue University

Purdue e-Pubs International Compressor Engineering Conference

School of Mechanical Engineering

1988

Valve Losses in Reciprocating Compressors Friedrich Bauer Hoerbiger Ventilwerke AG

Follow this and additional works at: http://docs.lib.purdue.edu/icec Bauer, Friedrich, "Valve Losses in Reciprocating Compressors" (1988). International Compressor Engineering Conference. Paper 631. http://docs.lib.purdue.edu/icec/631

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VALVE LOSSES IN RECIPROCATING COMPRESSOR S Fried rich Bauer Hoerb iger venti lwerk e AG Vienn a, Austr ia

ABSTRACT Over all flow losse s of comp ressor valve s are influe nced not only by the valve geom etry but also are ampli fied by valve pocke t losse s. The magn itude of the pocke t losse s was measu red for four typic al pocke t shape s. Key varia ble param eters that influe nce the losse s were estab lishe d. The resul t is shown in "Pock et-Fa ctor" diagra ms which shall perm it optim izatio n of the pocke t shape at the conce ptual stage of cylin der desig n.

INTRODUCTION The effor t to reduc e energ y losse s as much as possi ble in all techn ical proce sses requi res funda menta l and detai led knowl edge of the reaso ns for thesea losse respe ctive share of loss contr ibuto rs. s as well as of the The basic losse s in recip rocat ing comp ressor s are, by now, knowl edge. The press ure and subse quent common and well defin ed energ y losse s cause d by gas flow in and out of the cylin der have gener ally been refer red to as "valv e losse s". Becau se of these exist ing valve losse s, it has been a conti nuous effor t throu ghout the indus try to desig n more effic ient autom atic plate valve s for comp resso rs. Hoerb iger has been worki ng in this area for a long minim ize the flow resis tance and lossetime and is striv ing to s of the valve . Aside from the valve itsel f, the imme diate surro undin gs of the valve also contr ibute a signi fican losse s as the gas enter s and exits the t amoun t of the press ure cylin der.

p

Flg.1 : PV Diagr am at

263

v a ~:ompresslan cylin der

e diagra m with the Figur e 1 shows a typic al press ure volumarge ·line press ures disch cylin der press ure (P), the sucti on and as the press ures (from press ure gauge s Ps and P0 ) as welland PKD) durin g a full (PK~ measu red in the valve chamb ers comp ressio n cycle . the vario us press ure In the diagra m the areas comp rised by in a comp ressio n cycle . curve s repre sent the work perfo rmed losse s, requi res This work, in conju nctio n with the valve close r scrut iny: and PKD - Pp) The lightl y-sha ded loss areas (P 5 s.- PKs valve the to uted attrib canno t be en the valve chamb er They are cause d by restr ictio ns betwe 1 recei ver, line arge disch or line on sucti and the P~ have been and Ps ures press the where ly, respe ctive outsi de the tions varia ure press ic dynam by and measu red der. cylin sent the losse s The heavi ly-sh aded areas in Figur e 1 repre valve pocke ts. their in ed mount s valve the by d cause t and cagin g are Besid es the valve itsel f, the valve pocke rs. ibuto contr loss l antia subst other itsel f is The press ure loss of the comp ressor lvalve with the valve tunne wind a in ally iment exper ined determ ndicu lar to the moun ted on a level surfa ce locat ed tsperpe produ ce well- defin ed direc tion of flow. These measu remen press ure loss throu gh the coeff icien ts of flow with which the lation exclu des the effec ts valve can be calcu lated . This calcu der. of the rest of the flow system in a cylin ure losse s in the As furth er tests have shown , the press at times , be great er by a valve s mount ed in valve pocke ts can, s determ ined throu gh the magn itude of two or more than the losse . afore menti oned tests PROCEDURE

a given mass flow lf the ratio of the press ure drop t;, P at ure drop at the same press the to valve nted t-mou pocke a throu gh ed as the "Pock et mass flow throu gh the valve alone is defin the valve insta lled in of loss ure press the then (PF), r" Facto area) can be calcu lated the cylin der pocke t (the heavi ly-sh aded g the press ure loss of at norma l condi tions by simpl y multir plyin the valve alone by the pocke t facto PF. 2 ) ¢valv e alone A P valve + pocke t PF ( ¢valv e + pocke t P valve alone A

wher e¢ "'

area, A/{:f and repre sents the equiv alent valve A is the free lift area and 5 is the flow coeff icien t.

By integ ratio n it follow s that LOST WORK valve + pocke t PF

::;,~

LOST WORK valve alone

264

As the losses at the valve and pocket can be reduced by "tuning" the shape of the pocket to the valve, measureme nts of four different commonly used valve pocket configura tions (see Figure 2) have been made to determine "pocket factors" as a function of the most important geometry parameter s.

1.)Valve pocket with round passage

2.lVolve pocket with slot-shape d passage

lxw

3.lValve pocket axis perpendic ular to cylinder axis, semicircle passagE

f tat

~.JValve

pocket oxis p or a l l e l to cylinder axis, moonshape d passage

sur face

cylinder cylinder

F I g • 2- A l tern at i v e

con

axis

cylinder

dia

dla

t i g u rat i on s o t c y L i n de r

pockets

This research shows that the pocket factors for suction and discharge valves with the same pocket configura tion differ due to the opposite directions or flow. Extensive data were accumulate d for each of the pocket configura tions and the results were compiled into various diagrams shown in Figures 3 through 6. All plotted values are based on actual tests.

265

Configu~ation

Pocke t

1

PF

PF

SUCTION POCKET

DISCHARGE POCKET

1+-----~-----+----~~----~

2

1

Flg,3-Po~ket

3

4-

fnc:to r

for

confi gura tion

s (PF) vs. the Figure 3 shows graph s of the pocke t facto~ uratio n at a config flow passag e geome try (FI¢vl for the first ing va~iables were suctio n and a discha rge pocke t. The followation: consid ered essen tial in the overa ll evalu

F

=

the area of pocke t passag e the ratio of this area to the equiv alent valve area

c/D

the

~atio

defini ng the valve distan ce

icient s of It is evide nt from these graph s that theofcoeff the fi~st tions flow fo~ the va~ious geomet~ic condi valve pocke ts and for confi gurat ion are differ ent fo~ suctio n s and pocke ts have the disch arge valve pocke ts even if the valve pocke t in this case is same dimen sions. The PF for the suctio n rge pocke t. discha the fo~ PF the than r somew hat highe

266

Pocket Configuratio n 2

.':1+ (1-J..~ )tan ex

E D

D

2

D

A

.6 /

.4-

v

v

./

/

.2 0

"~·-·

•2

.4-

.6

.8

1 w/l

PF

Flg.4--Pack et

factor

for configurat ion 2

A comparison of configuratio ns 1 and 2 shows that at an equal ratio of F/~ and equal c/D a slot-shaped passage is better than a round-shaped passage. The same observation holds for the discharge pocket. The PF is smaller for the discharge pocket than for the suction pocket.

267

Pocket Configuratio n 3

PF

1

.5+-----~-------.------~-----,.-~

.02

.06

.1

.18

• 1 4-

.2 c/D

Ffg.S-Pock et

factor

for

configurat ion

3

In configuratio n 3 the eenterline of the valve pocket coincides with the bottom wall of the cylinder end. Half of the valve is in the cylinder head which prc;>vides a .sloped relief towards the cylinder. The only variable ia this case is the angle of the valve pocket re,cess. Subsequently , there is only one curve shown for the suction pocket and one curve for the discharge pocket. The pocket factors are smaller in this configuratio n than in the previous two configuratio ns. In this case, the PF for the suction pocket is smaller than the PF for the discharge pocket.

268

Pocket Configura tion 4

ql .8~~.--.--,---·~-,

1.2

c=o+

PF

1.4. 0/d

CD+2e-dlt ono:

5

PF SUCTION POCKET

5

Flg.6-Poc ket

foetor

for configura tion 4-

Configura tion 4 gives a variety of results also. A drawing of the overlappin g of the cylinder and the valve is provided to help determine the valve exposure F = D". 'f. The value 'f is determined by the relationsh ip of ene valve diameter to the cylinder diameter and by the eccentric ity of the valve to the cylinder. Figure 6 shows the previously taken approach plotting PF dependent of F/¢v and c/D for the sickle-sha ped portion of the pocket. Pocket factors for practical valve exposures in Figure 6 are comparable to results found in pocket configura tions 1 and 2. PF for discharge valves exceed those for suction valves.

269

CONCLUSIONS

g In designin g the passage s and valve pocket shapes the followin observa tions should be noted: During the short periods when the valve is open, the gas flows through a number of passage s (i.e., pipes, flanges on cylinde rs, cylinde r passage s, valve cages, valves, valve pockets) until it reaches the cylinde r itself. All these differe nt areas impose changes in velocity and directio n which result in pressur e losses. Therefo re, when speaking of "valve losses" , it has to be kept in mind that the lightly- shaded areas in Figure 1 are caused by element s outside of the valve chamber and that the heavily- shaded areas correspo nd to the real valve losses multipl ied by the pocket factor (PF) which depends on the shape of the valve pockets . The valve contain s the only cross-s ection in the whole system which can be defined as a "dynami c" cross-s ection. Its cycle goes from being totally closed to totally open to totally closed during every revolut ion. The valve motion must be of a nature that no adverse , prematu re failure occurs. If this function must be perform ed exactly for a prolong ed period of time, it require s conside rable design and technol ogical efforts . The valve area, therefo re, can be regarded as the most expensiv e area in relation ship to the valve surroun ding non-var iable areas. For instanc e, one square inch of ing increase d valve area can be obtaine d only by develop rather complic ated designs , by opting for a low valve life, or by utilizin g expensi ve materia ls. Optimiz ing thE: flow area around the valve is often attainab le with little added cost and, once properly designe d, these flow passage s are not affected by dynamic conditio ns. Econom ically and technic ally, it would be wrong to size the " inexpen sive areas" the same or only slightly larger are than thE> valve area, conditio ns which, unfortu nately, too common. The effort must be focused on increasi ng the "inexpe nsive areas" as much as possibl e in order to achieve minimum overall losses. When designin g new cylinde rs, three points have to be conside red concern ing the valve pockets : l) 2) 3)

Clearan ce volume Total flow resistan ce Economi cal manufac turing

capacity The influenc e of the clearanc e volume· on the cylinde r permit us can easily be calcula ted. The diagram s of this study a to determin e approxim ately the total flow resistan ce of of cylinde r at the early design stage. This is certain lyce when advanta ge as energy losses are of increas ing importan analyzin g compres sion work.

270