Comparison of Holweck- and Gaede-pumping stages

Comparison of Holweck- and Gaede-pumping stages

Vacuum~volume44/numbers 5-7/pages 681 to 684/1993 0042-207X/93$6 00+ 00 © 1993 Pergamon Press Ltd Printed m Great Britain !Comparison of Holweck- a...

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Vacuum~volume44/numbers 5-7/pages 681 to 684/1993

0042-207X/93$6 00+ 00 © 1993 Pergamon Press Ltd

Printed m Great Britain

!Comparison of Holweck- and Gaede-pumping ,stages A Conrad and O G a n s c h o w , Arthur Pfeiffer Vakuumtechmk Wetzlar GmbH--Em Unternehmen der Balzers-

Gruppc

Emmehusstrasse 33, D W-6334 Asslar, Germany

Wtde-range turbomolecular pumps extend the forehne pressure range of conventtonal turbo pumps from typtcally 0 1 mbar up to 10 mbar, whtch allows the use of dry dtaphragm pumps with < 10 mbar fmalpressure and relattvely low pumpmg speed (5-30 1mm- I) for backing This can be attained by different types of molecular pumps, tn harness with turbo stage rotors with typical pumping channel sizes and rotor-stator gaps of I mm and 0 2 - 0 4 ram, respectively The present work compares two types of these molecular pumps with respect to their pumping charactermtlcs and technical risks due to the specific construction parameters. The results of all investigations show slgmhcant benehts for the Holweck-pumptng stage in all items compared. Even the high compression ratio per axial rotor length of the Gaede stage can be substantially exceeded by interlocked Holweck-stages of different diameters.

Introduction Since the end of the 1980s wide-range turbopumps have been used in a great variety of vacuum applications But what are the decisive reasons for this9 What are the advantages for the customer and what makes these pumps so attractiveq Comparing a typical, ordinary pumping line to one involving a wide-range turbo, the following advantages become apparent l Due to the increased maximum tolerable outlet-pressure of the wide-range turbo from 0 l mbar up to typically 5-10 mbar, the forehne pumping-speed can be decreased and so design sizes and costs are reduced 2 The foreline pressure range ~>5 mbar can be provided by simple, dry forepumps, e g diaphragm pumps, instead of ordinary, oil-sealed and lubricated rotary vane pumps Well priced pumps are available on the market 3 BackdlffUSlOn of hydrocarbons to the high vacuum side along the turbopump becomes markedly suppressed even after venting because of the--especmlly m case of the H o l w e c k - s t a g ~ long and interlocked connections, as well as the small clearances and pumping channel dimensions Now the question arises, how to bring up the maximum tolerable exhaust pressure of the turbopumpq That can be realized by means of molecular pumps speofically designed for this purpose One can choose from several types of molecular pumps to solve the problem Starting with the first, it was the well known Gaede cyhnder pump (1913) I, then the Holweck thread pump (1923)2 and last but not least the Slegbahn spiral groove pump (1926) 3 with all their modifications Combination of axial-, radml- and circumferential-pumping effects supplies the axial gas dehvery In physical terms, the pumping effect is generated by the momentum interchange of molecules between moving and standing parts with suitable shape Presently the Holweck-stage is most frequently used by nearly all widerange turbo manufacturers, except one, who uses the Gaede-type

Both types will be explained In terms of their mare properties by means of both test-pumps used An explanation of measured characteristics of performance data and a valuation comparison regarding performance data and rehablhty will follow

Hoiweck- and Gaede-pumpstages The characteristic sizes of test-pumps have been selected to be nearly equal in order to guarantee an honest comparison (Figures 1, 2) So the maximum rotor diameter is 50 mm, frequency of rotation 1500 Hz and the critical axial and radial clearances are 0 3 mm The axial length of the Holweck-stage is 50 mm in comparison to 33 mm of the four series connected Gaede-stages The drive- and bearing-system of the TPH 060, one of the smallest turbopumps, has been used In detad these are a brushless dc motor, an oil-lubricated bearing system at exhaust pressure level and a permanent magnet radial bearing at the inlet-side Essential for the pumping effect in both types is the circumferential velocity component due to rotation In the Holweck-stage (Figure 1) the thread-angle of typically 20 ° to the velocity vector leads to an increased absorption rate of one channel s~de wall and consequently to an increased desorption rate perpendicular to this wall area, whose major component is axially directed--in a pumping downstream direction At the same time, dependent on pressure gradient, a backstream against pumping direction in the channel and across the border between the channels appears The difference of both streams is equal to the total throughput (equation 1) The pumping effect in the Gaede-stage (Figure 2) is caused only in the circumferential direction Gas comes in and goes out at the circumference Different shapes with sizes of 3 4 mm form the channel (Figure l, cross-sections) Inlet and outlet are separated by a channel narrowing section, with a small radial clearance The backstream in the channel, across the separator section, along the surfaces both sides of the disc (0 3 mm axial 681

A Conrad and 0 Ganschow Pumping stage companson

Tesr-Gaede-srage -<20mbar

Test-H0!,weck-stage -<20mbar

_

_

0

~X18

0 3turn

°!

8 2Stun, i

@50m,m

t~get ,,xl,~

~P2

L

Figure 2. Gaede-test pump w~th stgnlllcant ~tems smooth, rotatmg disc w~th cross-secUon view of channel and surrounding Inlet and outlct separated by a sectional separator with small clearance

Figure 1. Holweck-test pump with significant items smooth, rotating carbon fibre cylinder and opposite stator with tuner thread-structure

clearance) and along the sealing clearance between stator disc and shaft are influential in determinmg the perfolmance data

Vacuum performance data The properties of a molecular pump including backstream can be generally expressed as

Q =p~So--(p2--p,)Cb

It IS obvious that S I achieves its maximum S(, only if the compression ratio K0 and the backstream Cb, are at a high level and a low level, respectively In the molecular flow range K0 goe~ from linear to exponential dependence on the square root of molecular mass of pumped gas Only pumps approaching the classical conditions of Gaede t and Becker 4, at which channel backstream is dominant, show the exponential rise, whilst in case of outer backstream dominated pumps the rise would be linear Hence, the well known drop in pumping speed for light gases ~s easy to understand When the molecular flow range is maintamed at higher pressure compression ratio drops (backstream conductances increase) drastically (Figure 4), which also results

Q = throughput P~/P2 = i n l e t / o u t l e t - p r e s s u r e

F[owmeter

Cb = effectwe b a c k s t r e a m c o n d u c t a n c e So = p u m p i n g speed at zero pressure d r o p Defining the compression ratio at zero flow

(1) (Q =

0

0),

Test-pump

Ko = P2/P~,equation 1 yields Cb = S o/( K o - 1)

(2)

Further on throughput can be expressed with the aid of the effective pumping speed at the inlet S, and the outlet $2, the forehne pumping speed

Q =p]S1 =p2S2

682

DUO I S *Im ~/h-diaphragm-pump for throughput measuremenls

(3a, b)

U n d e r the use of equations (1), (2), (3a) and (3b) one attains for the inlet pumping speed

s , = s 0 / [ ] + (So~S2 - l)/K0]

@

(4)

1000 mbar atmosphere

Figure 3. Vacuum scheme of experiment

A Conrad and 0 Ganschow Pumping stage comparison

I0 9

_

10 1

I0 3

mbar {

1

test-pump

H.,l!~Tr~ik- L } auemdii

o 1°2 =~
10 o

10 1

10-1

TPH/U ~

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I II

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S

< o 2

10 8

r,[x.

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-

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_

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,

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10 ~ 10 2

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,500, \'iz~-rCH/0 060"1,4],x

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10 1 mbar

10 o

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10 2

I IX'V # / I Z I

l

IIII

I

I

II

I

! diaphragm-pump

I

7i

I

_

I/1~'1!1

Figure 4. Compression ratio vs forehne pressure of Holweck- and Gaede-

J/ I IIHI

I

_L_

/I" Iliil

pump for hydrogen H2, helium He and mtrogen N~ For the purpose of comparison compression ratio for N2 of ordinary small and big turbopumps TPH 060 and TPH 1500, respectively, is shown

I II II

10 3 10 -~

I 10 -1

II 10 o inlet-pressure

II 10 ~ mbar

10 z

Figure 5. Throughput vs suction pressure for nitrogen N2 of Holweckand Gaede-test pump with 4 m 3 h- 1 nominal diaphragm forepump in pumping speed reduction This critical pressure range is dependent on the geometric sizes of the pumping stages Reducing the size shifts it to higher pressures (Figure 4) In case of the both test-pumps, mainly dominated by outer backstream, this critical pressure range Pc scales well with the rotor-stator splits # So, analysing a variety of small, multistage wide-range turbopumps from various manufacturers yields-the following relation, which holds for forehne pressures of 1-70 mbar Pc x 9 ~ 6 m b a r m m

(5)

Assuming validity of (5) for our test pump, for which 9 = 0 3 mm, one gets pc ~ 20 m b a r This is confirmed by measurement (Figure 4), which clearly slgmfies the wide-range qualification for both pumps

Measurement of performance data The vacuum schematic (Figure 3) for measuring pumping speed and compression ratio consists of a gas supply on 1000 mbar-

level, dosing valves (D i, D2) and capacitive pressure m e a s u n n g gauges (Pi, P2) for 1 mbar and 10 m b a r at inlet and forehne, respectively The throughput at the inlet is measured by thermal flowmeters, so that the quotient of throughput Q and pressure p, determines the pumping speed S, at the inlet In order to determine maximum pumping speed a rotary vane pump D U O 1 5 has been selected and real wide-range conditions for separate measurement have been provided by a nominal 4 m 3 h-~ diaphragm pump with 1 5 m 3 h - l at 1 rnbar and 4 m b a r final pressure Compression ratio was rumply measured by forehne gas inlet (D2) and variation of p2, so that K0 = P2/P~ Ko curves of a small 60 1 s - t turbopump (TPH 060) and a big 1500 1 sturbopump (TPH 1500) are also shown It can be clearly seen that both pump types are acceptable for wide-range apphcatlons, because the dechnmg part of the curve at 10 m b a r is far above the 0 2-0 4 mbar for the ordinary turbopumps Merely the molecular

Table 1. Summary of performance data and relevant construction sizes

So

Holweck Gaede TPH 060

H2 He N2 H2 He N2 H2 He N2

So with diaphragm pump

141s-~ 0 2 1 s - J 20 05 20 09 0 7 1 s ' 0 3 1 s -1 06 0 45 06 05 451s-' -52 -56 --

K0 max

L10

Pk

10 16 65 15 23 60 600 8 x 103 7 × 103

50ram 42 28 28mm 24 19 18mm 13 7

49mbar 76 55 70mbar 12 9 01mbar 0 15 0 10

Total axial length

Spilt ra&al/axlal

50 mm

0 3 mm/00 03mm/03mm

33 mm 10mm/05mm 51 mm

So = maximum attainable pumping speed

Ko(Pk) = Koma,/e Komax = exp (Xax/LlO), Xax = axial length 683

A Conradand 0 Ganschow Pumping stage comparison

flow c o m p r e s s m n ratio 10-65 (H~ N , ) should be higher m o~de) to achieve the m a x i m u m p u m p i n g speed So ( e q u a t m n 4) ot 2 I s ' (Holweck) a n d 0 6 I s ~ (Gaede) The present p u m p s are not able to p r o w d e the m a x i m u m p u m p i n g speed (F~gure 5, Table 1) in c o m b l n a u o n with the d i a p h r a g m p u m p because of its very low p u m p i n g speed near final pressure o f < 0 4 m ~ h ' = 0 I I ' at t h r o u g h p u t s < I m b a r 1 s t Because of anothe~ reason K. should be h~gher too The wide-range turbostage~ would operate at a pressure level which is too high, because 4 mbal final pressurep./K. = 0 08 m b a r = p~ (Figure 5)I According to Figure 4 0 08 is just in the d r o p p i n g range of t u r b o K0 characteristic For h~gh t h r o u g h p u t s > 1 m b a r 1 s '. which c o r r e s p o n d to lngh forellne pressures > 10 mbar, p u m p i n g speed decreases rapidly. due to increasing b a c k s t r e a m

IPH

062

H

Conclusions As m e n U o n e d earher, b o t h p u m p s meet the vacuum requirements with some differences in p u m p i n g speed which is higher fol the H o l w e c k - p u m p But are there o t h e r items, which lead to a quahficatlon '~ Table 1 summarizes all relevant characteristic data o f b o t h types a n d a c o n v e n u o n a l t u r b o p u m p T P H 060 The G a e d e - p u m p provides slight a d v a n t a g e s regarding forehne pressure p~ and reqmred axial length for the same compression latlo It Is 60 70% of the Holweck-length With respect to the rehablhty aspects, slgmficant a d v a n t a g e s for the Holweck p u m p a p p e a r In the radial direction only a small clearance of 0 3 m m scparates rotor- a n d stator-palts, whilst the G a e d e - p u m p is b o u n d e d m the radial and axial du ectmns Therefme, a r o t o r - s t a t o r contact with subsequent d a m a g e reduced by differential thermal expansion is m u c h m o l e p r o b a b l e The design a n d the assembly procedure For the H o l w e c k - p u m p is easier Just screwing the parts together IS sufficient, whilst the G a e d e design m a k e s a stacking process, due to the half-stator parts, necessary The only d i s a d v a n t a g e o( the H o l w e c k - p u m p , a low compression ratio pel axial length, is substantially o v e r c o m p e n s a t e d by Interlocking stages at different d~ameters This causes a mulUphcatlon o f the single compression r a h o s F o r example > 107 for nitrogen is achievable along a 50 mm axml length The evident a d v a n t a g e s for the H o l w e c k - p u m p can be extended by using a c a r b o n fibre remlorced rotating cyhnder less expansion due to centrifugal forces d e t e r m m e d by a lou density to Y o u n g s m o d u l u s relation, non-critical b e h a v l o u r m the case of rotor stator touch just a shght removal o f carbon-fibre, but no d e s h u c t l o n a p p e a l s P u m p remains m operation F~gure 6 shows the c o n s e q u e n t transfer o f all items m e n t i o n e d above represented by the wide-range t u r b o p u m p T P H / U 062 H

684

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1 I,

rtserv()lr ( [ II]ll ~Oflt)( ( I l ( ) t l

IlldL¢131 L 1 (

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Figure 6. SchematJcdiawlng ol the ND 6] ~ldc-range turbopuinp 1 PH I, (162 H

a p p h c a b l e for lngh and ultrahigh vacuurn requirements Ke> data are 45 60 1 s ) for all gases, 106 10 ~ compression r a u o [oi H ~ N : a n d ~>25 m bar forehne pressure compatibility R e c o m l n e n d e d forepumps are "small' d i a p h r a g m p u m p s ~ ltla 1 6 I iron ' p u m p i n g speed and < 15 m b a r final pressure Othel typical wide-range t u r b o p u m p s on the same basis are T P D 020 T P H / U 180 H a n d TPH,'U 450 H

References iWGacdc, 4nn/Ph~s 41(4),337(191z;) -F tlolweck (omptesRendus, 177, 43 (1923) ~() Kellstmm ZPhI~ 41 516(1927) ~W Becket l'a/, Tech 9/10, 215 (I966,)