RICHARD
S. MORSE
N A T I O N A L RESEARCH CORPORATION, CAMBRIDGE 42, MASS
P
RIOlt to 1940, high vaeuuni (that is, the ust' of pi ui'i'h htIOU-0.1 mm. of mercury) wab with minor exceptions liniitcvl
to laboratory use. The electronics industry commonly eshaustcvl .various tubes t o 10-7 mm., but the volumes involved wverc small and only noncondensable gases were usually pumped. I n the, chemical industry the distillation of fatty acids was common at slightly higher pressures. The vacuum distillation of fats an(l oils was done experimentally in Europe (4,23) and no doubt reached its greatest development on a commercial scale iri thiz country under Hickman (10,11) andassociates, who denionstratrd the commercial possibility of producing vitamin concrntratw from fish oils by distillation in the micron range of prtmures ( I micron or l p = 10-3 mm.). During t>hewar, high vacuuni techniques were developed to a remarkable degree. Many processes have been commercializrd, and it is fair to state that high vaeuum is now at the stat? of ad-
\ m w i w i i t \vhtw thv pwbleni? of vacuum per se are u s u a l i ~ ondary to ot,her questions of the process. 50longer need thtb t~riginec~r assume that a process is economically or technically uri-ound merely because it involves the use of free air pressures beI(JW 100 microns, even in conjunction with valves and vacuumtight shafts, or temperatures ranging from -100" to +1000" C. The most recent factor in the advancement of modern vacuum ic*c.hnologyhas been the atomic energy program. The design and c.onstruction of both the gas diffusion and electromagnetic separation process plants for the preparation of U235were achieved i n \IENT
Design problems of the vacuum engineer are many anti var? greatly, depending on the process and pressures involved. TTith present day methods of construction and with pumping spcwl. available for both condensable and noncondensable gases, thc~w appears to be no limit to thp size of vacuum systems n-hi1.11cmt now be constructed. Leakage problems usually fall in t w o categories: ( n 1 Small waled-off systems are common in the electronics industry nhiiri, pressures belo\v 10-7 nini. must be maintained for years. It’ any leakage or degassing occurs, sonir form of “pump” in the guise of a “gc>tter” (barium, magnesium, err.] must be included in the e v a ~ uated space to react n-ith and remove any subsequently evolvrd gases. (15)I n most processes a ciertain amount of noncondenaahli~ gay leakage, outgassing, or gas or vapor forniation may be to1t.ra t t d , if adequate pumping s p e d is provided. Leakage may IIC cxpressed as a rate of leak or rise in pressure-i.e., in niicron c.ut)ir i‘c’ot per hour or microns per hour-for a given apparatu,*. I I I studying materials of construction, furnace bricks, etc., from thr. point of view of outgassing, it is sometimes more convenient t i l c.oiiipare products in terms of the time constant for a l i n o ~ ~ ri.1. -n i i i pu’ssure after closing off the system. Special valves are usually required at pressurrh beloiv 1.0 mni. because of leakage problems. --it the Ion-or pressures ao-ralltd packless valves are commonly used, the stem movement being i r t msed in a fli3xihle bellom. A somelvhat simpler design involve,. rhe use of sliding-type stem gasket3 (-34 1. Corisiderablr attcBrition must also be paid t o the design of valve seats; the iniprdailc*c, introduced t o gas flow must be kept at a minimum. Rotating shafts may lie introduced into a vacuum system ill itiimcrous ways (Figure 1). The use of flexible bellow, for ex:irnplc. offers an i n t o r ~ c ~ t ii iri iia~i t r i \ of transmitting limited motion
1065
T h e present status of high vacuiinl technology is retiewed, w i t h pictures of recently developed equipment arid process plants which enlplo) pressures in the micron range. Discussion is limited to industrial applications of tacuunl techniques at 10 5 t o 10-1 m m . in such fields as dehydration, metallurgy, distillation, metal etaporatiun, etr. Operating data o n large diffusion piimps, gages,
and steani ejectors are gilen w i t h respectile limitations. Large scale operations are mentioned from the point of liew of design arid (*onstruction to illustrate the fact that, in so far a? technical problenib of high \ac.uum pc’r se are van-
c,errird. operations of an] scale are no\+ fea.ihle.
wit 11 zero lrakage. Various methods for desigriing self-pumping ~11tl externally pumped shaft seals have lieen developed for use at high speed, n-hich permit transmission of even large amounts of poiver into any vacuum system, rrgarcilpss of the pressure or leakage specifications. T h e flowof gases and vapors a t pressures below 1 mni. must be given special consideration (15j. Adequate engineering data are n o w available (3’) to permit the design of any size equipment, b u t to date the chemical industry has in many instances failed to realize the fundamental problems involved. It it; common practice t o employ a combination of the Fanning equation (16)and friction graph for Reynolds numbers above 2100, or l’oiseuille’s law for Reynolds numbers below about 1000. I n the micron range, liow, Poiseuille’s lavi does not apply, and the effective viscosity may tie 1r.ss than 17 of the gas viscosity as given i n tlir handlxnb
B
Grease Cup
N
[Vacuum Side ROTARY BELLOWS SEAL Figure 1 .
ROTATING SHAFT BELLOWS TYPE VACUUM. S E A L
Vacuum Side
GREASE T Y P E S L I D I N G SEAL
Three Hasic \lethods Which 3Iay H e Used in the Design of Variium-Tight Shafts
7 --
7000 -_I_-
CHARACTERIST ICs OF VACUUM PUMPS
----i--i TYPICAL HIGH
_---
___
r-
tl Figure 3 . Speed 1's. Pressure Data for Various Comrnerc+al I'11tnps
I. 50-cuhic-frlot-prr-nlirlu t r compound mechanical pump (2 Iiorscpowrr) II. 100-cul,ic-foot-i,irm i n u t e single-stage m e rhanidnl pump ( 5 horsrpower) C'. 2-inch three-stag- oil rliffuaion p u n ~ p(200 w a t t s ) I ) . 6-inch one-stage ejertor-type oil diffusion pump (6000 w a t t s ) E . 6-inch tr-o-stage IIOOSt P r diffusion p u m p (JO(H) watts) F. &inch four-stage oil diffusion p u m p (22.5 w a t t s ) 10-inch two-stagbooster diffusion pump (6000 W a t t R ) It. 10-inch three-stnpe oil d i f f u s i o n p u m p (2250 watts) I . 16-inch thrre-stapr oil d i f f u s i o n pump'(2250 watts)
(;.
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1947
1067
10 . .i pottion of the gas \$ithin the -\stem under test is passed through a niass spectrometer tube designed for niaYnnuni response to helium ;is a helium probe is passed over the leaking portion of the iiquipment, extremel) wi-itive and rapid response is obtained in the output amplifier of the .peetiornetel tube. This is the iiiost -1 nqitive leah detection method non L n o ~ t nindicating ~ the presence of 1 pait o r lit~lium in 250,000 of pa< at 111111
L)ik FLSIO\ PUZIPS
\\ i t h the exception of niolecular diag
are used in -ome foreign Inhoiatoiies, diffusion pumps are non uiii\ ersally eniployed for the attain.01 .IO 10 10 100 ment of preibuic- belon 10-3 mni on a TOTAL PRESSURE- m m Hg h g e scale Recent trend- h a r e demu n ~ t t a t e d this p o s s i b i h t ~ of improving - the foieuicssuie chaiacteiistics of pumps through the use of high boiler prcassures and improved jet design. I t is non' apparent that t h e limit o n gas handling capacitj- of a diff'usioii lopinerit~in vacuum t e c h i q u c hy the t:lccttolipump helon lo-! niiii. is in reality only a funct,ion of the "pumpnot been radical but have, ccntcred about. thix development of automatic machinery for mass production. FoI ing speed" of the entrance to the mouth of the pump (Figure 3). Oil diffusion punips. 32 inches in diameter with a capacity of pressures belon- 1 micron, small oil-diffusion pumps without, cold ovt'i' 30,000 cubic feet per minute a t 10 -6 inm.. have been built iii traps are now c~>nimonly employed except, wherc the gn>at nuniher,~. Although this is a large volurnc at t h e p w s n r e e good stabiliry of niercury as a pumping medium i* :I n i iii~-oivcd,the actual rfficienry in trrnis of poiver consumption or cause of the exhaust cycle. n-hrti one considers t h a t Irss t h a n 0,0004 i i i ~ i ~ ~ t n i eisi iridiculous t The development of largr: niagnctrons and ut hili,high frcqwlicy tubes poet1 IIIW problems for the vacuuni c'nginiw. The ncu w b i c foot per niinutc, o r gaq i. hoing hantlled at atniospliei,ic conditious. high valuc~soi voltagc' and power which have iwcn reached pl:u>t, U-ith nc\vei' iajei.toi, typc oii-diftu>ionpuiiipa (131greatpr cpcetic tie\v requirements on m a t e ~ i a l sof construction to. ensure niaint i s a r c 1 1 0 ' 1 ~poabible in thc: higher niicron range. JTith minor (>simpiianrc' of high vacuum over a period of 'timc anti undcr advi~i~se Iioiw the use of mercury in diffusinn pumps in rhie coiir1ti'~-is no\v operntiiig conditions. Osygcn- or gas-free (~opper1i:w pri~vt>il liniiti~dt o lahot,atory apparatus 01' clectronic a p p l i i ~ t i n r ~\r-lic,i,e . c,sst,ntial l o the construction of larger nirtal tubes, and grc~lt ic~rc~pi~essurt, opwatioii above 1nim. i> essential. (.niphasis must be placed on probli>msof outgassing and of tight130th chloi~iiiated compounds, such as the so-callctl -11 0 ~ 1 0 1 ~ ne56 of glass-to-metal arid other forms of si,:tls. In somo cas(>> ,sc,i,ie.. and r(7fiiiittl hydrocarbon pump oils h a r e betti usid with ,terns h a w been c~iiiploycd. continuously operated pimping huciws> in iiiaiiy large installations during the war, pat~ricularl!. Piniilar prohlems are cncouiitered in thv opcration of c.j-clotr~~its 1)t~:iusc of g1~catc.rstability in t h r presence of n-atrr r a p o r and bc~ t n dothcr equipment n.hic+ depend upon the use of Ion- gas pi~i~sr a u w of lon-c,i,cost. The usc I J ~silicone type oils ha. i~cveritly ,surcs to permit the formation of rlcctron or ion hranir. In t he t w n advancwl. and the i,esulting iinprored stability ~ i t ~ x(~xposi. latter i n ~ t a n c e ,considerably higher putnpir1.q .poe& ( 1000 t o III'~~ro Nii. a t high temperature is a useful factor (21. H r c a u w of 10.000 liters per scconti 1 at p r e w i r e ~bi~low . lo--;niin. at'(' c i ~ n i n i o ~ ~ . i a o i i t ;~niinationproblems at the rstrc~iiielyIon- prrwures employed. t l i c ~t'lt'Cti'lJliiP iiiiItistr>-i n ni:tny in-tancc till prc.fcri the rjryaiiic ~ v ~ w o ~ . ~OF ~ I\Irri~s i c n AND ~ A L T S ~ I I I I I ~nQhich
aes involving condensable vapors, .steam ejector s>-,steiiis ai'? reconiiiir~ndriiunder most conditions f o i , all total gas prr-wre,. a b o w 500 microns (Figure 1). (3-
:I- 10
Y A C U U M PROCESSES IS ELECTROSICS r .
1 lie nianufacture of alinost all electronic devices-pliotocells,
i,ctifiers, radio tubes, x-ray tubes, etc.-involves operations at \.i'ry low pressures (i.e., less than lo.-' mm.) to prevent ionization. I
1'1~i~sw1,es above 1 niicron are used on a large scale for the product ion of light bulbs, fluorescent lights, neon lights, small receiving
I'acuuni evaporatioii techniques have heen ptwtieed i t i t l i ~ laboratory for man>-years. Metals ~ r s t t l t havo t ~ l ~ ~ c ~ t ~ heatcd icnlly i n a small boat or filanicnt, evaporated or SUIilinictl at air pressui'ej beloiv 10-4 inni., ant1 cwndeiised in thc foi,iii o f a thin filni on the surface to be treated. I n this nianner front .siii,face inirrors ui' aluminum or chromiuiii c.an be fornieti, quat't z ri>plic.:isniailc,, etc. (??,a. During the ivai the leu> coating industry WLL> griurly c~xpanllt~tl iiecause of interest in loiv reflcctiori surfaccs for optical devii*tTn. It is now common practice t o form quartc usually of inagnesiuni fluoride, by vacuuni evaporation. By such a procedure the light reflection per optical ,iurfacil is ni:ttcrially reduced and the transmission correspondingly increased. It is not uncommon t o increase the light, tran.~rni~:sion of inore coniplicated optical systems as much as ZOOC,. Considerable interest has recently bccn shown in the coating of plastic parts with various decorative metals such as gold, alurninuni, copper, etc. Larger scale equipment 11:~s l ~ e ndevelolir~l for such work on a protlurti~~ii ..Cali> (1 1 .
1068
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Vol. 39, No. 9
IIachine for Producing Vacuum-ETaporated Coatings on Large Surfaces by a Continuous .kittomatic I'roccsb
One of the largest tierriaiid,?fur e\,aporated I'roiit suriacr ~iii~,ror,. may come from the tclevisiori induatr>- \vlieic lioth front surfaccb planc and spherical mirrors are required for the Schmidt type optical system. Equiprnc.nt has surfaces to be treated (lenson, mirror.3, etc.) may be coiitiriuously introduced and removed froin t hca high va,muni zone \vht,rcx ('[Jilture. This I)roducts ing takes place, n-ithout the ujual l o s ~in p hich arc automat icall? ved t h o u g h succehsive zones of aaing preahure, ranging from atmospheric t u 10-4 nim. I.:ithcxr a metal or salt is evaporated from a continuous type sourc~eof vapor. and the jigs thPn proceed through zone's 01 increasing p r e s s u ~ cuntil ~ remorcd t o the air T h e continuous largcs scale coating of nietals u u cloth, paper, and sheet p1astic.s has b e i ~ nknowri, a t least by sputtering techniques, for aonie tiiiw. Gold leaf, for example, is produced on a commercial scale b>-such a procedure both licre and in England. Recently conaitlcrable interest has been shown in proccwrs for continuous137 evaporating zinc and aluminum on paper for clectrical condcnscxrs, at pressures in tlie iiiieron range. Iinprovenient in weight, sizis, aiid voltage hreakdo\vn can be achieved as compared with thc. usual laniinated foil coiidenser, and products made in this mariner h a w , rcwntly h e n iritroduccd in this country (8). \ \C:C:USl SlE?'ALLUKG\
Prior t u 1940, \vitli tlie cxc.c.ption of woIk by Heraeus i l l ( ;ermany (9),large ,sc.alt. applications o f vacuum metallurgy \vert' virtually unliIioIvn. Thi> .sit uiitioii \vas largel>- the re.zult of a lack of adequate V ~ C U U I ~equipnicrit I . \-acuum may be applied in the ficld of metallurgy as follons: ( n i .1 high vacuum is a truly inert atmosphere, aiid by i t a use nictals may be protected during processing. (h) h t retluccd pressures the boiling points of metals are reduced sometimes as much as 1000" C. This phenomenon makes it possihlr to purify metals by distillation.
lJ 00001 (1 00039
0 00002
U 00012 I) 00045 0 00040
0 00006
0 00230
8 930
8 '32'2
.54,500 20 3 88 100 3
54,500
2430
"340
17.3 76 99.4
September 1917
INDUSTRIAL AND ENGINEERING CHEMISTRY
T h e redueid prcssurc serves t\vo functions in this reaction: I t preveiits oxidation of the magnesium formed, and it lowers the hoiling point of magnesium so that the metal may be removed as rapidly as formed. Finally, the removal of magnesium by distillation forces reduction to proceed a t almost 500" C. beloLv the temperature required for rctduction a t atmospheric pressure. This general type of vacuum reaction can be applied to the preparation of the other alkali and alkaline earth metals. Thcx production of crrtain metals from their salts or by direct redurt'ion of ores appears to he conipetitivr with present electrolytic methods. Somi, of tho rcactionc xhich have h w n invwtigatcil are :
h l l commcwial metals are contaminated with dissolved and combined gases. I n many cases minute quantities of gas are capable of greatly reducing the ductility and toughness of metals, as well as the electrical and thermal conductivities. Dissolvrd gases are also released in solidifying metal; this accounts for porosity in castings under some conditions. I n many cases gases may be reniovrd from metal Iiy simply melting under reduced pressure. H y h o g e n can almost aln-ays tx, removed by such treatment,. Sitrogen can be removed in niaiiy cases aiid occasionally gen may be eliminated. 0. moval is difficult since pas-is generally present a,
