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TitleExperiment Studies on the Wet Vacuum Pump (1st Report-Effects of the

Supplement of Piston Water)

Author(s) Torii, Osamu

Editor(s)

CitationBulletin of University of Osaka Prefecture. Series A, Engineering and nat

ural sciences. 1963, 11(2), p.21-29

Issue Date 1963-03-20

URL http://hdl.handle.net/10466/8015

Rights

21

Experimental Studies on the Wet Vacuum Pump

' (lst Report-Effects of the Supplement of Piston Water)

Osamu ToRII*

(Received Nov. 30, 1962)

The present paper is the first of the series in which the experimental studies on

a wet vacuum pump with a double suction and exhaust type i.e. "Nash type" casing are reported. The article deals primarily with the effects of the supplement of water

(or "pis.ton water"), which acts as piston against gas in the pump.

These subjects, which have not been investigated so well up to the present except for a few matters,i)2) are very important factors for the grasp of the characteristics

of this pump. In this experiment they will be fairly explained.

1. Introduction

As a runner revolves in a pump casing containing a certain amount of piston water

and centrifugal force acts on this water, a hollow space is formed in the central part of

the pump, which is surrounded by the blades, the hub of the runner and the free surface

of the piston water. And the volume of this hollow space becornes larger or smaller

according as the runner revolves on account of the eccentric form of the casing.

Wet vacuum pump named Nash type or Elmo type puts these phenomena to practical

-tuse, and has long been used in many branches of the industry.

But its characteristics have not been so well known. Perhaps this is because both

the piston water and the gas exist together, affecting each other, and furthermore the

water is in the state of unsteady and non-uniform flow in the interior part of the pump.

Therefore the phenomena are very confused and any theoretical analysis to be satisfied

with has not yet been established.3)4)

And few experimental data, which afford a sound basis for the theoretical analysis,

have been published.5) The reason is that the correct and reliable measurements are very

difucult. The diMculty of taking the exact measurements is thought to be caused by the

fact that, not only the relationship between the capacity rate of gas flow and the compres-

sion ratio (degree of vacuum), but the infiuence of the piston water or its supplement

participate, and interact each other, consequently they produce the three dimensional

characteristics.

From this point of view, the author designed an accurate controlling system of the

piston water and performed the experiments of this pump as follows.

2. Apparatus and Experimental Methods

The dimensions of the casing and the runner adopted in this work are summarized as

follows.

* Department of Mechanical Engineering, College of Engineering.

(Casing)

Radius of curvature at eccentric part:

Effective width : Eccentricity : Radial clearance on minor axis i

(Runner)

Outside radius : Effective width : Hub ratio : Blades (straight and radial) :

Suction and exhaust of the gas are conducted at

this pump belongs in "Port plate type".), and the

by a side cover of the casing made of cast iron or

purpose of observation of the piston water flow.).

exhaust ports.

For the measurements of suction and exhaust

pressure the mercury manometers are used. For

the porpose of the gas fiow measurement an

orifice flow meter of 16.4 mm diameter, which

has been calibrated, is set at the upstream side

of the suction valve.

The degree of vacuum is regulated by the

suction valve, and the exhaust valve is kept full-

open during this experiment.

The piston water is supplied just before the

suction port from the feed tank of 2.2m head

with a over flow system, and its quantity is

adjusted by a needle valve set at the middle of

the feed pipe. In order to take an exact water

measurement, the pressure difference at an orifice

of 6.7 mm diameter, which is set at the upstream

side of the needle valve, is measured by a mano-

meter using acetylene tetrabromide. (r=2975

kglm3)

In brief, the needle valve supplying

the momentary rate of its flow. In this '

to be constant irrespective of the degree of

The foregoing test pump is driven

input is measured by a wire strain gauge

pump and the motor.

r.=100 mmbe =23.3 mm

e!r.=:O.200

ai=o.7s mm

ra=100 mmba == 21.5 mm

ri!rd =O.600

16 Ieavesx5 mm thickness

only one side of the runner

other side･ of the runner is

transparent synthetic resin

Fig. 1 shows the suction

(namely

enclosed

(for the

and the

the piston water

experlment,

vacuum.directly by the variable

torquemeter

Fig. 1. Details of the Suction and

Exhaust ports.

is regulated in response to checking

the rate of supply water is arranged

speed motor, and the pump

which has been set between the

Eoperimental Studies on the I)Vbt VZicuum 1hrmP

's 3. Experimental Results and its Considerations

Fig. 2 represents the characteristic curves of this pump in case of

a concrete example that the pump performance is changed owing to

water. But in this figure the individual measured value is omitted by

cateness of the expression.

This figure clearly illustrates the following

generalities.

1) In case of a small rate of 'suplly water,

the pump characteristics sometimes come to

the discontinuity at the vicinity of the no-

discharge.

2) Under the condition of zero compression

ratio (named "free-discharge"), .the capacity

icate of gas flow (Qo) decreases, and the ppwer

input (LBo) increaseswith increasing the rate

of supply water (a).

3) When the vacuum is varied, both the

inclination of the capacity rate of gas flow

(AQIAHk) and the power input (ALB/AHls)

grow larger with decreasing the rate of supply

water.

And now it is especially noticeable that

such transition of the pump performance is

sharply influenced by the slight differential of

the rate of supply water. While the pump

characteristics may be thought to be influenced

by the temperature of the piston water or the

the experiment, proved to be not so potent. The

a little more fully under the following items.

Maximum Vacuum Under the state of the{ncreases with shutting of the suction valve,

suddenly when the vacuum amounts to a

driving condition of the pump.

According to the observation of the '

(as in Fig. 7), these phenomena are thought to

surface of the piston water and the gas swells

runner.

Then if･ such discOntinuous characteristics

charge, the degree of vacuum

after all the vacuum at that time is regarded

the condition of the pump.

tu qg.g･fi

.tsBopt

.fi

s>sce

Bo=enst'issg

ta

.8g8

Fig.

23

N==1500 r.p.m. as

the rate of supply

reason'of its intri-

1,6

1.4

'M

1.2

--"-

.e't

1.0.8

--e-Rateofsu4±emes.tei'

1.i1e-!Oco'sec,

.i'7tilf,sec,o3bicc,lis'ec.

O.6'Itt-ttltt-tttL-/ l

i

O.5

O.4

O.3

O.2

i'

t

O.1 IxiOO.2O.4O,6

2.wet

Degree of vacuum %

Chara¢teristic curves of the

vacuum pump.

gas. But this effect is, as the result of

above-mentioned results will be explained

pump in motion, the degree of vacuum

but the suction sometimes grows worse

certain value, which is determined by each

piston water flow in the'interior part of the pump

be caused by the fact that the boundary

out of the circumference of the revolving

' come about at the vicinity of the no-dis-

decreqses contrary even if the suction valve is shut Ipore, and

as the maximum which can be got under

O. ToRII Fig. 3 shows that the maximum vacuum (Hk.m...) are plotted as an ordinate with

the rate of supply water (q) as an abscissa. But when the capacity rate of gas flow (Q)

is less than O.05 m31min., the discontinuous point cannot be confirmed due to the restriction

i'n the apparatus. (i.e. when Q<O.05 m31min.-too near to the no-discharge-, the reading of

'the gas flow meter becomes uncertain, and moreover the momentary pressure charge. of

the vacuum cannot be measured by the mercury manometer.) Therefore in such case,

the no-discharge vacuum (Hls.shut) is substituted for the maximum vacuum (His.ua.), but

judging from Fig. 2 such substitution may be admitted.

Fig.3 reveals the following. The '

+curve (Hls.vuam.-q) becomeg discontinuous

.at a certain rate of supply water (qc),

and its rate, which is named "the critical 6oo

rate of supply water" by the author, abdecreases with increasing the revolving g E 4oOspeed of the pump. In the case of q>qc dithe maximum vacuum shows a tendency

to increase a little, but it is the rate of 200no account. On the other hand the maxi-

mum vacuum grows worse in the case

of q<qc, the more the revolving speed O 10 20 30 40increases, the larger this tendency be- qcc/sec･comes. Fig･ 3･ Maximum vacuum vs. Supply water. The critical rate of supply water are Solid symbols: Maximum vacuum. Opensymbols: No-discharge vacuum.varided also by the effect of eccentricity Parameter : Revolving speed of theef the casing, which wiil be reported runner.minutely in the next paper.

On account of the fact mentioned above, it can be said that the discussion on the

maximum vacuum is insignificant without the consideration on the supplement of piston

water.

Now, the maximum vacuum (Hg.ma..) is plotted as a logarithmic ordinate with the

revolving speeq (N) as a logarithmic abscissa, as shown in Fig. 4. But in this case Hls.rm.

are ･adopted on' the critical points, which answer to each qc, in Fig. 3. The other results,

which are taken in the experiment on the various casing, are plotted too in Fig.4 for

reference.

Fig. 4 reveals that a trasition point (Hk.max.t, IV}) exists on the curve (Hli･.mats.-N),

which has not yet been reported up to the present.

Hg.ma.. increases with increasing of AT but in the case of N>ATI the grade of its

increase becomes very slowly. ' Qo, LBo vg･ Speed PIotting In"order to investigate the similarity of this pump,

the dimensionless capacity rate of gas flow (Qo/2Uh.6s.b,) are plotted as an ordinate with

the revolving speed (N) as an abscissa, as shown in Fig. 5. Where, Ult is the tip velocity

of the runner, and as is the depth of the crescent shaped passage of the casing (See Fig.

'l

t(N1'

ttr.p.m.y18001500

/

uT i' t 1200Ii,l/1('q,,)

1

/j.

i1

.]II

---- - !1050

oA

I

o

:b

900

-iA l

!11

A

ch=e8/I-

di

Ealperimental Studies on the l)Vizt VZieuum R"mp

600

b-

)il'

1e/r.==O,245lttt''letrttr/.-eelr.--=-O.161

t-y;t,!'.tg.-op

t/n8.

/k-..

300

7r･o

500

400

200

100

e/r.=-O.125

,,i

/L,P''i-ll/Q&ltl

l'plefr,,･--Q.200

l[{

.

1tts'1tit'

eI

6009.0012001500 1800

25

N r,p,m. ' ' Fig. 4. Maximum vacuum vs. Revolving speed fo the runner.

' (Solid symbols is shown as transition point.)

1). The parameter is the rate of supply water.

Fig. 5 clearly represents the following facts. The dimensionless capacity rate Qf gas

flow decreases with decreasing the speed of the runner, the more the rate of supply water

increases the larger its tendency becornes. The main reason is that the form of the

boundary surface of the piston water and the gas changes always even in the state of the

free-discharge, and this fact will be approved by Fig. 8.

In Fig. 8 it is worth paying attention that, the shut-off position of the suction port

26 O ToRII

10

08

"ts

u.? 06

$to-Qo4

02

Fig

2100 Nrptn5 Dimensionless capacity rate of gas flowvs Revolvmg speed m the case of free dis-charge (parameter. rate of supply water)

08

07

06

05

04

03:-

ec

=:

'e". 02

Fig

Ol

400

6 Drivmgthe case of

(parameter

800 1200 1600 2000 Nrpm torque vs Revolvmg speed infree discharge rate of supply water)

megeg.,"g,,,. ,i

,te :.I'ge""s,

,"s

Xys

me tslx

ptS?pss

Wa･,

wsee

lilS'i'li$t gg

Hls-100mmhg Q---O362m3/min 4-8cc/sec

Hls--485mmhg Q-O.05m3/min. q-8cc/sec.Fig 7 Moment photograph of the piston water fiow.

eewwee

ew

*,es

sc

Eecee

-tny'eeastw

w",e

.,i,l:l

tiisl

W,

yifts

Hsi-=300mmeg Q-O207m'/mm q-8cc/sec.

twee,ee/ee,es/ex･ag ewee

eeeej･.eeesme wa faee ee

Hls =- 445 mmhg Q=O q-8 cc/sec

N-1200r.pm. (const ) (Shutter speed' 10-5sec.)

EJqiberimental Studtes on the Wbt VZzcuuJJv Pbemp

eeeemessx

}1va

S31#re

Wit

elS

ee

t:

sme pa

27

sma

kgeige;, :.

/,,mgts, as meilillll

N-1800r/m Q--O.735m3/min q-12cc/sec N-1500r/m Q-O597m3/mm q-10cc/sec

K,,ss,ueda.ma,igtw ee,eswime･#･ee eeee

s

IV-1200r/m Q-0450m3/mm q-8cc/sec N-900r/m Q--O300m3/mm q-6cc/sec Fig 8 Moment photograph of the piston water flow H=O (free-discharge)

(Shutter speed' 10r5 sec)

is located near to the maJor axis of the elliptical casing (0ssL-- 800), nevertheless the radial

distance between the boundary surface and the center of the runner does not always be-

come the maximum at this position, and the maximum point moves with the change of

the revolving speed.

Next, in Fig. 6 the drivmg torque (TBo) is plotted as a logarithmic ordmate with the

revolvmg speed (N) as a logarithmic abscissa. Fig. 6 illustrates the fact that the relation-

ship between the torque and the speed approximates to the 1inear Ime and !ts angular

coethcient approximates to 2, with decreasing the rate of supply water. Namely in the

case of q.O, it can be s]id that LBoocN3 approximately. When the rate of supply water

increases the power input (LBo) mcreases more than the value obtained from this relation.

However, in the case of q=O, both the capacity rate of gas fiow and the driving

torque are guessed merely from the curve (Qo-q) and (TBo-q) by means of the extrapo-

lation, and it is difficult to keep the pump m motion under such condition for a long time,

because the temperature of the piston water rises rapidly.

As the result of the foregoing investigation, it can be said that the law of similarity is

not perfectly concluded because of the influence of the supply water in the usual case

(q40).

Isothermal Efficiemcy There are various

the vacuum pump, but it seems to be proper to

case of this pump, because the variation

of the gas temperature by compression or 6oo

expansion is kept constant approximately

owing to the fiow of piston water. Atn. s 400 Fig.9shows an example of the iso- NeMciency curve about Tis. In the range g

of q>q, th, increases with decreasing Of S 2oo

q, but in q<q, the fact above stated is e . >not always true. .

The dotted curve shows the tendency

of the rate of supply water which gives

the best isothermal eMciency at each Fig.degree of vacuum, and this curve suggests

the actual system for supplying the piston

water, that ,is qoc AM", where the power

x varies with the revolving speed of the runner

generally larger than 1.0.

4.

Wlth the intention of the comparative studies

of the pump, the experimental studies on the

the first place. And its results are summarized as

1) The similarity law is not perfectly concluded

The main reason is that the boundary surface of

always by the influence of the supply water, and

ing the speed of the pump.

2) The pump characteristics turn discontinuously

and its value increases with decreasing the

mum vacuum grows much worse in the case vacuum shows a tendency to increase, but it is

3) The tendency of the curve (Hls-q) on which

maximum at every degree of vacuum (Hls) is

tical system for supplying the piston water.

4) The relationship of the maximum vacuum

revealed, and it is illustrated that "the ''

author, exists on the curve (Hls.mats.-N).

It seems that the tendency of the results

scarcely with the experimental results.

methods in

adopt the

calculating

isothermal

the eMciency

ethciency (Ti's)

of

in

1l

, ah

a rt"ez 1

't1 207e

A -･1

2S7e

/:tl ?8el

/･r30re

2Z7r･.s=asr.L-J

t t

tia

tl

i

t

toz4-s1

sl

rtl

'

'=ti::tli' --..J.-..---

o lo (qc) 2o 3o 4o Rate of supply watEer (cc/sec)

9, Isoefficiency curverfabout isothermal

eMciency. (revolving speed of runner: 1200 r.p.m.)

and the eccentricity of the casing but is

Conclusions

on the various structures and sizes etc.

supplement of piston water are performed in

follows:

even in the state of the free-discharge.

the piston water and the gas is changed

its effects become larger with decreas-

at a certain rate of supply water (qc),

revolving speed of the runner. The maxi-

of q<q,, and in the case of q>qc the

the rate of no account.

the isothermal ethciencies are to be

shown, and this curve suggests the prac-

(Hls.mam.) vs. the revolving speed (N) is

transition point (Hlil.ma..t, AIL)" named by the

calculated with Pfleiderer's formula4) agrees'

E vperimental Slrudtes on the Wlat l72icuum Ptzmp 29

Acknow!edgements

The author wishes to express his gratitudes to Emeritus Professor Keizo

admitting this work and several criticisms.

Gratitudes are also due to Professor Yoshihiro Miyai for his helpful

encouragements.

Tabushi for

advises and

1)2)3)

4)5)

M.s.C.

C.

Y.

Shirakawa,

Uchimaru,Pfleiderer,

Pfleiderer,

Senoo and

Reference

Journal of J. S. M. E. 38, No. 222 (1936).

Blower and Compressor, 6th ed., p. 249-260 (1948).

Die Kreiselpumpen, 2nd ed., p. 437-448 (1932).

Die Kreiselpumpen, 4th ed., p. 555-571 (1955).

T. Kasai, Transaction of J. S. M. E., 26, No. 162 (1960).