CA1043286A - Self-cleansing diffusion pump - Google Patents

Abstract

ABSTRACT
A diffusion pump capable of separating impurities from its pump fluid during operation is disclosed. Skimming drains are provided in the pump's boiler for periodically skimming the evaporative surface of the working fluid. This eliminates nearly all contaminants of higher molecular weight than the pump fluid.
In the foreline of the pump, a series of peripheral gutters are provided for trapping, separating and draining off condensates.
The gutters facilitate the removal of impurities of lower molecular weight than that of the pump fluid. Means are also provided for further removing trace quantities of residual volatile impurities which tend to backstream up the diffusion pump barrel. The highly purified pump fluid allows for a more vigorously working evapora-tive surface, thereby increasing the throughput of the diffusion pump. Together with the elimination of volatile impurities from the pump barrel, this facilitates the attainment of significantly higher ultimate chamber vacuum. The withdrawn condensates and skimmed residues also form the basis for use of the apparatus as a device for obtaining a high degree of separation between liquids of very close vapor pressures.

Description

~0~1L32~36 The invention relates to diffusion pumps for producing high vacuum and more particularly to an improved diffusion pump by which the pump fluid may be separated from its impurities during operation.
Diffus~on type pumps for producing high vacuum are well known. The original effort of Gaede around 1914 was supple-mented by Langmuir in the development of the vertical jet diffu-sion pump disclosed in U. S. Patent No. 1,393,350, and the single inverted jet or mushroom pump covered by U. S. Patent No. 1,320,874.
A multiple jet modification of the mushroom pump is described in - U. S. Patent No. 1,367,865. Such pumps operate on the principle ;
that a liquid having relatively heavy molecules is vaporized in the pump by raising i~s temperature. The vapor comprising heavy . .~ .
molecules is directed by suitable nozzles in a direction away - -from the region to be evacuated, towards a mechanical forepump.
The accelërated molecules of vapor compress against molecules ;
ahead of the nozzle, forcing them toward the mechanical forepump and thereby reducing the pxessure within the evacuated region.
The vapors are recondensed on a cool wall of the pump where the .~ , .
, 20 liquid is permitted to return to the bottom of the pump to be : ,:
reheated and vaporized.
' ~ Originally mercury was employed as the workhng fluid in these diffusion pumps, but later various organic oils and silicone fluids were developed as pump fluid, and today~these `-fluids~have almost completely replaced mercury in diffusion ;~
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pumps;.~ In particular, the silicone oils, of which DC~705 ,l (pentaphenyl trimethyl trisiloxane) manufactured by ~ow Corning Corp.,~ LS an example, are in ~ide use today.

Performance of diffusion pumps has been erratic and ;~ 30 ~subj~ect to certain limitations. It was long ago ~bserved that the evaporative surface of a studied pump fluid in a~pot still, !
when rapidly evaporatlng, seems to separate into two different , !

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areas of turbulence, resulting in a "schizoid" evaporative sur-face. In one area of the surface, termed the "working" area, very rapid evaporation of the fluid takes place, while in the other area, known as the "torpid" area, very little evaporation takes place. ~ discussion of this phenomena is found in the article "Torpid Phenomena and Pump Oils", K.C.D. Hickman, The Journal of Vacuum Science and Technology, Volume 9, No. 2, and "Surface Behavior in the Pot Still", K.C.D. Hickman, Industrial and En~neering Chemistry, Volume 44, No. 8. Since the torpid , areas of the evaporatlve surface within the diffusion pump boiler ` release vapors at a very low rate, the diffusion pump speed, throughput and ultimate vacuum attainable are limited to the extent that the evaporative surface is affected by torpidity.
Various remedies have been suggested to alleviate or overcome the problem of torpidity in diffusion pump boilers. See, , ; for example, the above article by Hickman, Hickman U. S. Patent No. 2,080,421, and "A New Type of Diffusion Pump Boiler for Ultrahigh Vacuum Use", H. Okamoto and Y. Murakami, Vacuum, Volume 17, No. 2. The suggested solutions include the use of 20 a central purge sump within the boiler for segregating certain -impurities which overflow therein during boiling; the use of a ` boiler heater designed to induce tremendous turbulence in the -pump fluid (Stevenson Flash Boiler~; and various means for cir- ;
culating the pump fluid in the boiler. The latter means include stirring or otherwise rotating the fluid mechanically, and posi-tioning the applied heat so as to induce circulation (~-boiler ; of Murakami). Numerous diffusion pump boiler modifications are shown and described at pages 974-976 of the first above-referenced -Hickman article.
While then suggested solutions have reportedly increased molecular throughput somewhat, they do not have the capability to purify the pump fluid within the boiler thereby eliminating the 'f : .
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~L0~3Z1~i causes of torpidity, as discussed below. The exception is the purge sump, which does remove a llmited quantity of impurities from the surface of the evaporating li~uid. As long as torpid areas of the evaporative surface prevail, molecular throughput and attainable vacuum remain drastically limited.
Another limitation in diffusion pumps on ultimate attainable vacuum is imposed by a phenomenon known as "backstream-ing". Backstreaming, also known as "reverse fractionation", constitutes a back migration of some molecules from the jets ,: :
back into the vacuum chamber and is inherent in a diffusion pumping process. As pressure in the chamber being evacuated decreases, the rate of backstreaming increases, and when it ~
equals the throughput of gas, no further decrease in chamber -pressure occurs. The phenomenon of backstreaming, and various suggested remedies therefor, are discussed in Hickman Patents Nos~ 3,034,700 and 2,080,421, Scatchard Patent No. 2,905,374, Nelson Patent No. 2,291,054, Bachler Patent No. 3,317,122 and -Hayashi Patent No. 3,171,584. For example, in Hickman Patent No.
3,034,700 and in Bachler Patent No. 3,317,122 it is suggested -20 that backstreaming can be reduced by cooling the diffusion pump -~
barrel only behind the jet or adjacent the upper stages of a ~` multi-stage jet assembly, with the lower portions of the diffu-sion pump barreI being maintained warm. This reportedly main~
tains~a long column of forwardly moving pump fluid vapor, ~iving ~the molecules less chance~to diffuse backstream. Another often employed way of reducing backstreaming is the use of one or more cryogenically cooled baffles between the vacuum chamber and the pump. The baffle primarily attempts to condense and trap con-1'i ', taminant~gases from the chamber and to prevent diffusion pump vapors~from backstreaming into the chamber. Many of the chamber gases criginate from materials in~the chamber which have "out-; gassed" under the influence of ~igh vacuum. While cooled baffles , ; .

- ~)432~6 have been helpful in trapping chamber gases and reducing back-streaming, they have not been able to trap all passing gases, and once they are filled with condensate, they lose their effective-ness. If the baffle is warmed, the condensables drip into the boiler and cannot be removed.
Although molecules of the pump fluid itself exiting the diffusion pump jets have a tendency to backstream to a slight degree, it is primarily molecules of "light ends" which backstream through the diffusion pump barrel toward the lower pressure - 10 vacuum chamber, thus severely limiting the degree of vacuum attainable. "Light ends" are those contaminants present with , -~ the pump fluid which are of lower molecular weight than the pump fluid itself, and may include broken away fractions of molecules of the pump fluid itself, which is, in the case of the pump fluid DC-705, a pentamer. One system for partially removing light end contaminants from the pump fluid, thereby reducing backstreaming, is shown in ~lickman Patent No. 3,034,700, Fig. 1, and in the first above-referenced Hickman article, Fig. 25 and page 976. ~-This sy~tem;consists of collecting condensed dis~illate in annular alembics defined in the foreline of the diffusion pump.
The distillates comprise light end substances which have escaped the diffusion pump barrel and passed along with vacuum chamber gases into the foreline.
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~`, In Hickman Patent ~o. 2,080,421, wherein total pump fluid constituents, including impurities, are identified by . ~ :
/ lettere A through z from the lightest light ends through the -~ heaviest ends, diffusion pump apparatus is disclosed~wherein ::^
certain impurities were isolated within intern-al compartments.

However, only contaminating components A, B and Z are disclosed as having been successfully isolated.

While the diffusion pump structures discussed and ref-erenced above aid in the reduction of torpidity on the fluid's : ~ , :,. ; , ~, ~L0~3Z86 evaporative surface within the boiler and in the reduction of backstreaming by light end substances into the vacuum chamber, thereby increasing ultimate ~; attainable vacuum,the suggested structures cannot produce a 100% continuously ~ working evaporative surface, nor reduce backstreaming and achieve high fluid - separation to the extent of the present invention described below. ~;
- The disclosed embodiment of the present invention provides a dif-fusion pump, including modifications to existing large diffusion pumps, capable - of continually cleansing the working fluid during operation, thereby producing a continuous, 100% working evaporative surfaceJ as well as drastically reducing backstreaming, which heretofore has severely limited ultimate attainable vacuum.The diffusion pump apparatus of the invention is also capable of achieving a ` high degree of separation among fluids of slightly different volatility, and thus has utility in the field of refining and other arts involving high purification and separation of fluids.
It has been found that the torpidity phenomena, which has been the subject of study for some years as discussed above, is due almost entirely -: . . - . .
to the presence of "heavy end" contaminating substances present in the pump fluid. The heavy ends comprise substances of molecular weight higher than -that of the pump fluid, and include to a large extend polymerized molecules of lighter substances. These heavy ends tend to collect into non-evaporating , "islands" on the evaporative surface, restraining the release af motive fluid molecules in these areas. Thus, only the working "holes" in the schizoid } evaporative surface release fluid molecules at an appreciable rate. ;
; The present invention provides in a method for separating fractions of a liquid, which method includes the steps of heating a body of the liquid :
to a temperature sufficiently high that the liquid has a vapor pressure at ``
j least substantailly as high as the pressure to which the body of the liquid is :i , .
! - subjected, condensing various fractions of the liquid at various levels above ; the sur~ace of the heated bod~ of liquid, and collecting various liquid frac-tions at t~eir ~espective condensation levels, the improvement of withdrawing from the heated body of l~iquid a portion of the upper surface thereof, and discharging such withdrawn portion to a collection vessel exterior of the body ~ ~;

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~ 32~il6 of liquid.
From another aspect, the invention provides a fluid refining system including a boiler, a fractionating column with its upstream end in communica-; tion with such boiler, a series of distillation plates at various levels with-in the fractionating column, a conduit providing communication between the interior of the boiler at the level of the surface of liquid therein and an , external source of low0r pressure than that of the boiler, a valve in said conduit, and means for withdrawing liquids from the surface of fluid in at least one distillation plate in the fractionating column.
The diffusion pump apparatus of the present invention is capable of almost totally eliminating heavy end substances from the pump fluid during operation of the pump, thereby removing the principal cause of torpidity on ~
the evaporative surface. This helps provide for a vigorous, 100% working ;-surface, resulting in a much greater molecular throughput and greatly improved pump efficiency. The elimination of heavy ends is accomplished in part b~ the ~ -periodic skimming of the evaporative surface of the pump fluid in the boiler , during operation. Skimming outlets are located around the periphery of the -boiler and also toward the center of the boiler in large pumps having concen- ;
tric annular boiler channels. The skimming openings are provided at various levels on the boiler and are connected, when skimming valves are opened, to a I collection v0ssel existing at lower internal pressure than that of the boiler.
`l The diffusion pump apparatus of the invention preferably also includes means for withdrawing condensed distillate from the forepressure line of the ,~
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pump. Alembics are provided in the foreline for catching condensates flowing `
down the internal walls thereof. Valved lines connect ~he alembics with a collection vessel of lower internal pressure than that of the foreline. Con- -densates within the foreline contain high concentrations of 'llight end" sub-:~i :, .
stances, and the removal of these light ends prevents th0ir return to the boiler for~revaporization and possible polymerization into heavy end molecules.
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In this ~ay, backs-treaming is nearly~ eliminated, since most light ends are `; ' prevented from re~ent0ring the flo~ of vapors through the diffusion pump jets.
~ The presence of light ends in the diffu~ion pump barrel is the primary cause of " ,~
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backstreaming, which greatly reduces ultimate attainable vacuum in a vacuum chamber-diffusion pump system. As indicated above the prevention of light ends from returing to the boiler also reduces torpidity by eliminating poly-merized molecules therefrom.
Light end contaminants cannot be completely eliminated from the system via the foreline. Usually there is present a very small quantity of light ends of molecular weigh~ only slightly above that of the pump fluid itself.
For maximum pump performance, these substances must be re~oved. They are difficult to isolate ~;

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2~6 in the foreline, where pressure is relatively high and pressure differences are only slight, and where the concentration of such heavy light ends is generally not as great as it is in the pump barrel. The heavyl71ighttends, not being as volatile as the ~`
lighter light ends which may be isolated in the foreline, con-dense on the diffusion pump barrel walls nearly as readily as does the motive fluid. Therefore, molecules of the heavy light ends are continually present in the diffusion pump barrel and avail-::; : ;
able for backstreaming toward the vacuum chamber and for combining - 10 by polymerization to form heavy end contaminants which cause ... .. .
torpidity in the boiler. Being somewhat more volatile than the pump fluid itself, the uncombined heavy light end molecules tend to backstream more readily than those of the pump fluid, toward the lower pressure of the vacuum chamber. If after separation and removal of most light light ends from the foreline, pressure could be made sufficiently low, with a sharper gradient, and temperature sufficiently high in the foreline, the heavy light ends could be drawn in greater concentration into the foreline before condensation, condensed therein and trapped within the alembics. The substances would thus be present in high concen-trations within the condensate and could be-drawn off from the alembics. However, to achieve such separation would require an extremely strong roughlng or backing pump connected ~o the foreline, and extremely high temperatures within the foreline. In most situations, such a large vacuum pump would be required to achieve ~-the separation as to be impracticable and economically unfeasible. -~
Also, any lighter light ends present at the time under such con-ditions~would necessarily be drawn away through the roughing pump and~exhausted. These light light ends would thus be lost and unavailable for separation and analysis or salvage.
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~ ~ A solution to the problem of the trace quantities of `~' heavy llght ends present in the diffusion pump barrel is provided .: ~
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. ' 1 ~0~ 86 in the system herein described, On the internal surface o$ the diffusion pump barrel where pressure is low and a strong pressure gradient exists, are a series of alembic-like plates or gutters which are positioned so as to trap a major portion of the heavy light end substances which have condensed on the barrel surface, before these condensates have a chance to revaporize and back-stream toward ~he vacuum chamber. Valved draw off lines lead from each of the gutters to a collsction vessel of lower pressure than the barrel pressure at the level of that particular gutter. Certain of the gutters are provided with - ' draw off lines at mor0 than one level, so that motive fluid condensate, which may be present in the bottom of certain gutters, can be returned to the boiler, while condensates containing heav~ light end contaminants can be drawn off from , upper levels of the gutters into a collection vessel.
In a vacuum system including a number of dlffusion pumps, one pump having all of the above-discussed modifications can act as a ~'slave" pump to the remaining pumps having only boiler and foreline modifications. Boiler fluid would be exchanged between the pumps to that the fluid of each pump would ~1 reach a high degree of purification.
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As discussed above, the apparatus of the present invention also has ' ' ' utilit~ as a precis~on fluid separation device. Apparatus described below for ' ' ' .
boiler surface skimming and for collecting and separating condensates in the ' ' ~ foreline and on the barrel of a diffusion pump can ~e utilized in other separa~

; tion arts apart from diffusion pumps, such as those relating to the separation ~, of petroleum fractions, uranium isotopes, metals, and other organic and ;'1 inorganic chemi:cals. ' ~' .` .
The diffusion pump apparatus aDd the method of this invention is ~-' illustrated and described herein in connection with~an umbrella or mushroom ; ' ' ~ ~ type inverted-nozzle pump of the type shown in Un~ted'States : -, . ,,: .
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~43Z86 Patents Nos. 2,206,093, 2,386,298, 2,436,849, 2,905,374, :~ 3,251,537, 3,317,122 and 3,536,420. A model diffusion pump embod~ing some of the improvements of the invention was constructed :
and operated, the results of such operation being discussed in National Aeronautics and Space Administration Technical Memo-randum X-68272, published November 12, 1973 and presented at the Seventh Space Simulation Conference, Los Angeles, California, November 12~1~, 1973. Remaining improvements according to the .; invention are discussed in NASA Technical Memorandum X-2932, to be published in or prior to January, 1975.
: Figure 1 is a schematic elevational view showing a ~:~ vacuum chamber and connected model diffusi~n pump including apparatus according to the present invention;
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il Figure 2 is an enlarged view of the model diffusion .. . .
pump;
Figure 3 is a sectional elevational view of a typical prior art large mushroom-type diffusion pump;
Figure 4 is a sectioned perspective view of the boiler of a typical prior art large diffusion pump7. .
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`~ 20 Figure 5 is a sectional ele~ational view showing modi~
fications to a prior art diffusion pump according to the present ;: :
invention;
. Figure 6 is a sectional pla~view taken along the ::~
, line 6~6 of Figure 5; and . .
: Figure 7 is an enlarged sectional:elevational view :, :~, : ..
taken:along the line 7-7 of Figure 6 with parts removed for clarity:~
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A. LABORATORY MODEL DIFFUSION PUMæ

Figure I of the drawings shows:a vacuum chamber 10 :.-.

30 connected to the low pressure side of a model sel~f-cleansing :'-.~ diffusion pump assembly according to the inventi~n, generally ':

:: indicated by the reference number 11. The assembly 11 includes . ~ . . .

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~0432~6 a diffusion pump barrel 12, a chimney and nozzle assembly 13, a boiler 14, a forepressure line 16 including alembics 17 and valved draw off lines 18, a line 19 leading through a foreline cold trap 21 to a mechanical backing pump 22, vessels 23 and 24 for collecting contaminants removed from the boiler 14 and the foreline 16 respectively, a foreline pressure gauge 26, a blo~er 27 ~ox co~ling the diffusion pump barrel 12, and a boiler con- :.
taminant adding device 28. Included on the vacuum chamber are a pressure gauge 29, a chamber contaminant adding device 31 in-cluding a valve 32, an auxiliary bleed valve 33, and a chamber baffle 34 which may be chilled to provide a refrigerated con-densing trap for gaseous substances travelling toward the diffu- . .
sion pump barrel 12. An additional baffle 37 and valve 38 may be provided between the valve 36 and the diffusion pump barrel 12.
The contaminant collection vessels 23 and 24 are connected by valved lines 39 and 41 to a source of lower pressure than that : :
existing within the diffusion pump boiler 14 and foreline 16 so that the vessels 23 and 24 will draw liquids from the boiler 14 and the alembics 17, respectively,~when the appropriate valves ~ :
are opened. The vacuum source may be the mechanical backing pump 22 itself or some other euitable source, since the pressure existing within the boiler 14 and foreline 16 during operation of the pump assembly 11 are not extremely low, as is the pressure .~
within the upper end of the diffusion pump barrel 12. Contaminant ~: , .
.1 : drain valves 35 and 36 are provided~on the vessels 23 and 24 for .:~ -~l removing cont~amlnants:when the vessels 23 and 24 are appropriately .: :
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solated:from the source of vacuum~a~a~from the boiler and fore-line.~Atmospheric bleed valves~:40 and 45 are also provided on :.
the vessels 23 and 24 for the same~ purpose.
:30 ~ ~ portion of the diffusion pump assembly 11 is shown ,: in greater detail in Figure 2. The boiler 14 contains motive ~ .:
fluid~42~up to the level of a skimming drain line 43 which i5 open - :

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~3286 to the interior of the boiler. The line 43 can be opened by a valve 44 to a llne 46 leading to the collection vessel 23. On the opposite side of the boiler 14 is the contaminant adding device 28 which includes an open-ended graduated tube 47 and a valve 48 for admitting liquids from the tube 47 into the interior of the boiler 14~ Such contaminating liquids may be added to - determine their effect on pump performance and the ability of the purification apparatus of the model pump to separate the liquids out of the pump fluid, as discussed below. The device 28 10 may also be used to replenish pump fluid in the boiler 14 as -heavy and light end constituents are skimmed and drawn off.
The fluid level shown in Figure 2 should be maintained through : . ~
`~ all skimming operations, since skimming is provided only at one level in the model pump. The chimney and nozzle assembly 13 ;
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includes noæzles or jets 49a, 49b and 49c which are angled down-wardly to create, when pump fluid passes therethrough, a low :,, , pressure on the high vacuum side 51 of the diffusion pump. At the bottom of the diffusion pump barrel 12, several openings 52 are provided around the periphery of the chimney and nozzle ` 20 assembly 13 so that any fluid condensing on the lower interior of the chimney and nozzle assembly 13 returns to the boiler 14 ; by passing into the foreline entrance and thence down through a conduit 53, through the line 43 and into the boiler 14. Simi- ;-j larly, condensate from the surface of the barrel 12 returns to -~i the boiler via the lines 53 and 43.
~ ~ The foreline l~ of the diffusion pump includes a pair `~i of peripheral alembics 17a and 17b, and lines 18à, 18b and 18c are provided to conduct aondensed fluid away from the foreline 16~ Each of the lines 18 may be opened by a valve to a light end collection vessel 24 of lower pressure than that of the foreline 16, as shown in Figure 1. The lower line 53, which remains open to the interior of the boiler 14, returns to the ..
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boiler those condensates which have not been removed through the lines 18. Electric heating tape 54 is wrapped around the foreline 16 from just above -the line 53 to just below the backing pump line 19, so that temperature within the foreline 16 can be main-tained at a predetermined, nearly consistent level along the length of the foreline. This allows condensable fluids entering the fore-- line in the gaseous state to travel farther up the foreline 16 before condensing on the walls thereof, and with temperature nearly constant along the foreline's length, light end substances o~ differing volatilities are allowed to condense at different -~ levels under the influence of subtle variation in pressure along the length of the foreline 16. The highest pressure in the fore- -; line is found adjacent the backing pump line 19, and the lowest ~ -pressure is found closest to the diffusion pump barrel 12.
; As Figure 2 indicates, a valved bottom drain line 56 is provided in the boiler 14 for draining the plImp fluid 42 out ' of the boiler when the diffusion pump is not operating. The glass walls of the boiler 14 include thermocouple inserts 57 and 58 for monitoring the temperature at various positions within , 20 the boiler 14. The~the~mocouple inserts 58 extend from the back of the boiler. The foreline 16 also includes a thermocouple ` insert 59 for monitoring temperature at the position shown.
Foreline pressure is monitored via the pressure gauge 26 shown in Figure 1, while chamber pressure is monitored by the gauge , 29, also shown in Figure 1.
In a particular run of the model self-cleansing diffu-sion~pump assembly 11, five year old DC-705 silicone pump fluid moleaular weight 546) taken from the cyclotron diffusion pump at Michigan State University was used in the boiler 14 of the ;-,~ 30 model pump. The oil had been ~scolored by long use to a dark ` brown color. In an initial standardizing test conducted with a ~-standard G-4 single stage glass diffusion pump, the five year .~ ~ .:.
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~ 12-old oil had attained a maximum vacuum, or minimum pressure, of 2.5 x 10 5 torr (~m. of mercury) in four hours of pump operating time. New DC-705 oil had achieved a minimum pressure of 1.0 x 10-6 torr in three and one-half hours operation in the G-4 test pump.
The boiler size and evaporative surface area of a G-4 single stage pump are approximately the same as those of the model pump. Each pump has a boiler capacity of about S5 milliliters.
After one day of operation in the model diffusion pump shown in Figures 1 and 2, the five year old DC-705 oil produced a maximum test chamber vacuum of 5 x 10 5 torr. The reason for the difference in ultimate vacuum between the G-4 test pump and the model pump illustrated in the figures, using the same five year old pump oil, is primarily that the model pump including a large number of joints between the diffusion pump barrel 12 and the vacuum chamber 10 (some of which are seen in Figure 1). These joints were not perfect but allowed a small amount of gas leakage into the system, consistently raising m~nimum attainable pressures in the system in all tests with the pump. Outgassing of gasket materials around joints also contributed to the gas load in the system.
During the first day of operation of the five year old oil in the model diffusion pump, chamber pressure often varied from about 10-4 torr to 5 x 10-5 torr. A torpid evaporati~e ,, ~ .
surface appeared approximately 80% of the time. For brief per~

iods the evaporative surface would become 100% working. Between ,: .
torpLd a~nd working periods, a surface previously described as schizoid would appear briefly, with numerous small working "holes"
present in the~;otherwise-torpid surface. Improved chamber pressure always'accompanied~he~lQO~orki~g periods~, often(l~owerin~-<~ahamber ., .
pressure~by 0.5 decade (five times~. The 100% working evapora-tive surface appeared about 20% of the time during the first day of operation.
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3~i When removal of light end and heavy end contaminants from the pump fluid commenced, marked increases in pump perform-ance immediately became apparent. Within eight hours from the first &eparation of such impurities, chamber pressure was lowered by more than one decade to a level of 1 x 10-6 to 5 x 10-6 torr.
During approximately the second day of pump operation, the follow-ing quantities of fluid containing the indicated contaminants were removed, in the indicated order:

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10 ml light ends 3 ml light ends 8 ml heavy ends 2 ml light ends 2 ml light ends Following the removal of these contaminants, a chamber pressure ~
varying between 8 x 10-7 and 1 x 10-6 torr was attained. The ~ -corresponding boiler evaporative surface behavior associated with this pump efficiency was 100% working 90~ of the time.
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After seven dayseof system operation, chamber vacuum reached improved levels of between 5 x 10 7 and 1 x 10 6 torr.
The 100~ working evaporative surface now appeared approximately ` 99~ of the time.
During the eighth day, a further 2 ml of pump fluid ~ ;
` containing light ends was removed from the foreline. Following ~ -~
this final purification, a continuous 100~ working evaporative , . .
surffice~appeared. Chamber pressure remained between 10 7 and 10 6 torr. ~ ! :
During this run of the model diffusion pump, when chamber pressure was varying from about 5 x 10-7 and 1 x 10 6 ;i ~ torr, a test to determine the effectiveness of the chamber baffle ~ 30 34 (see Figure 1) was conducted The chamber baffle 34 had been ',,! maintained at temperatures varying between -40F and -20F. When the baffle was warmed to a temperature of ~40F, chamber pressure , ., .. ,.. ... ,. , . ,.. , , ;, . ~ . . - . - . , ,. - . - . . . . ,- ,. ~ , . . . . . .. . :

32~6 rose to about 5 x 10-3 torr, due to vaporization of warmed con-densates which had been detained by the chamber baffle. However, the system later recovered from this pressure rise, and in fact when chamber baffle temperature exceeded 60F, vacuums of around 10 6 torr were again attained and maintained. These results con-firm that when diffusion pump oil is maintained in an extremely high purity condition, high vacuum can be attained merely with the use of a water cooled chamber baffle. Water cooled baffles could save enormous expense at space performance testing facili-ties, for example, where each of a large numher of diffusion pumps usually employs a chamber baffle cooled with costly cryogens such as liquid nitrogen or liquid helium.
During the above test of the self-cleansing model pump, foreline pressure gradually and steadily decreased as chamber ~ ;
pressure decreased and as the continuous 100~ working evaporative surface condition was approached, from about 4 x 10-3 torr to . r about 2 x 10 3 torr. During this period, foreline temperature, controlled by the electric heating tape 54 shown in Figure 2, remained at about 260F and did not vary significantly. The i 20 gradual lowering of pressure in the foreline thus steadily changed the conditions of equilibrium within the foreline, such that a substance of a particular vapor pressure would cond~nse at a continually rising level in the foreline aslpressure decreased.
Thus a particular substance which may have condensed near the entry to the foreline after only several hours or a day of pump operation (prior to purifica~ion of the pump fluid) may later condense just above the lower alembic 17b, being trapped therein.
Still later, the same substance may reach an even higher position of condensation within the foreline 16, thereby flowing into the : j :
~ 30 upper alembic 17a for collection.
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Because of this steady change of conditions, or equi~i-brium parameters, within the foreline, the lightest, most volatile ,~ '.
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light ends are the first light end contaminants to be collected ` in the forearm alembics 17a and 17b. The alembics are drained of these lightest light end substances (the drain line 18c may also be opened), and pressure in the foreline progresses down-wardly. The next cut of light ends from the foreline occurs when pressure is somewhat lower, 50 that contaminants contained in the fluid removed from the alembics now comprise somewhat heavier, less volatile light light ends. If any of the lightest, most volatile light ends are still present within the system, they would be likely to be passed completely out of the foreline, through the line 19 toward the mechanical backing pump, under the new conditions~ Thus, the initial light end cut should be suffi-., , -: .
cient to remove nearly all of these most volatile substances, particularly if separation and salvage of the substances is desired. ~-~
As the opieration of the system progresses, light end contaminants continue to be removed. Ultimately, the heaviest, least volatile ;l light end substances can be largely removed from the foreline.
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~1 As previously discussed, under any set of equilibrium conditions existing in the foreline at a given time, there is a slight varia- ~ - -j 20 tion in pressure along the length of the foreline 16. The pres-¦ sure varies from lowest at the upstream end of the foreline to -~
the highest at the downstream end ad~acent the backing pump line 19. Light end substances of varying volatilities can thus be drawn ~rom the~lines 18a, 18b and 18c during operation of the pump.
`~ ~ After the initial drawing off of light ends and substan-~tial stabilization of foreline pressure, further light end removal 3l ~ may be accomplished by draining only the upper alembic 17a through its draw off line 18a. This results from the fact that once fore- ~ ;
line pressure has reached a minimum, primarily the heavier light ends are~ being trapped by the alembics 17a and 17b. Most of the light end condensates within the lower alembics 17b will revaporize from time to time, travelling farther up the forearm to be eventually -', ,.,.' .~

~43Z86 recondensed higher along the walls of the foreline 16, adjacent the backing pump line 19 where temperature is somewhat lower. Thus, most of such condensates eventually are trapped by the upper alembic 17a.
The model self-cleansing diffusion pump system shown in Figures 1 and 2 was also tested using new DC-705 oil. After three days of operation, the vacuum chamber pressure gauge 29 indicated chamber pressure to be a consistent 5 x 10 5 torr. A 100~ working evaporative surface existed about 10% of the time. When 6 ml of fluid containing light end impurities were removed from the fore-line, 2 ml from ea¢h of the drain lines 18a, 18b and 18c, chamber pressure fell by 0.6 decade. The evaporative surface became 100%
working 25~ of the time. On the fifth day, the removal of appro-ximately 7 ml of boiler fluid containing heavy end fractions, by skimming the evaporative surface, increased the 100% working sur-face condition to 50% of the time, with a slight reduction in chamber pressure. Foll~wing an additional removal of 3 ml of , ':'J
~` light ends containing condensate from the top alembic, on the sixth day, chamber pressure decreased, reaching an ultimate level oE from 7 x 10 7 to 2 x 10-6 torr. Eventually a continuous 100%
working evaporative surface was achieved, following a further , removal of 4 ml fluid from the upper alembic in the foreline. ~;
The foregoing test provided convincing evidence that even new unused DC-705 silicone oil contained some light end and some heavy end impurities~ and that the model pump 11 is capable of a ~ery high degree~ of separation and purification. Minimum 't ~ ~ press~ure attainable in the test chamber was reduced from about ' ~ ~ 5 x 10-5 torr to about 1 x 10 6 toxr, or about 1.5 decades, by the~removal of light end and heavy end substances during operation of the~pump.
other tests~were conducted using the model self-cleansing :
difusion pump assembly illustrated in Figures 1 and 2. In one :: . . ,:: . -" ~: , :; : , ~ 17 `

~0~3;~t~ : ~
test, new DC-705 oil which had been further purified in the model pump as described above, was deliberately contaminated to deter-mine the effect of certain contaminants on pump operation, and the ability of the model pump to eliminate them. This test was actually a continuation of the above run of the pump using new DC-705 oil, beginning with the 13~h day of operation. The contaminants added were three common phthalate plasticizers which have been found to outgas from many of the articles present in a space simulation chamber. The three phthalate contaminants added were: di-isooctyl phthalate (DIOP-390 molecular weight); di-isodecyl phthalate (DIDP- ' 446 M.W.); and di-octyl phthalate/di-2-ethylhexyl phthalate (DOP/
DEHP-390 M.W.). 2 ml of each contam~nant were added directly into :~ . . ..
the pump fluid through the boiler contaminant adding device 28.
:^ ..: .
The effect was an immediate, sharp rise in chamber pressure from ~`1 about 10 6 torr to about 2 x 10-3 torr, with chamber pressure stabilizing at about 7 x 10 torr. With the removal of 4 ml fluid from the top alembic of the foreline, chamber pressure dropped to :.- .
2.5 x 10-5 within one hour. The insertion into the boiler of an `, additional 2 ml of phthalate contaminant mixture (containing the above three components), the prèvious pressure fluctuation was duplicated. 10 ml of new DC-705 was added through the device 28 -~
to prevent depletion of the boiler fluid supply during the pending cleansing period. To purify the fluid, 2 ml of fluid containing light end fractions were removed from the upper alembic 17a of the forearm 16. This opera~tion was repeated again;within 30 minutes. Within one hour, chamber pressure was reduced 3 decades, ;~
from 3 x 10-3 torr to 2.5~x 10-6 torr. 15 hours later, a chamber ;;
,..................................................................... .. .
pressure~of 8.5 x 10 7 torr was achieved. -~' The operation of the model diffusion pump assembly , 30 described above established that a high level of motlve fluid purity can be attained and maintained by utilization of the con- ~ -taminant removal apparatus shown and described. The model pump ~
';, .' ,. :

i -18-~ 43Z1516 further demonstrated that once heavy end contaminants have been initially skimmed from the evaporative surface in the pump boiler, heavy and light end contaminants both can be controlled to a large extent merely by periodic withdrawal of condensates from the foreline. This is due to the fact that after an initial skimming most recurring heavy ends in the boiler comprise polymerized molecules made up of light end molecules or light end molecules in combination with molecules of the basic motive fluid.
As discussed above, the model diffusion pump was subject to a number of limitations. Among these limitations were the large ~ -number of joints employed in the vicinity of the dif~usion pump barrel and the foreline, and the~size of the model dif~usion pump.
Nonetheless, the separation and purification capab~ities of the model pump demonstrated that greatly improved pump performance ., . . ~ .
~` can be attained employing the two types of modifications shown.
.j , .
An even higher degree of purification and contaminan~ separation can be attained with a third diffusion pump modification described below in connection with large existing pumps. The third modifi-cation, which could not be incorporated in the glass model pump, 20 relates to apparatus within the diffusion pump barrel itself for separating out the extremely small quantities of light end sub-~,j :'' ..
1 stances~ which are only slightly below the molecular weight and $ slightly above the vapor pressure of the motive fluid itself.
`;~! The separation of this type impurity cannot be completely accom-plished~-within the diffusion pump foreline.
B. MODIFICAT}ON OP LARGE DIFFUSION PUMPS
A typical large~diffusion pump 65 of the mushroom or n~erte~-nozzle type is ~shown in Figure 3. The pump 65 includes a ;~ concentric channel boiler 66~, disposed at the bottom of a chimney 30 and nozzle assembly 67`. The boiler 65 includes electrical resis- -;
tance~he-ating~elements 68 disposed~between concentric boller channels for providing a large heating area, and a metal dome 69 receives :: . ~ ::
j : i .,; , :~ ~ ',"' '-~'.
'' ~ : lg ~)43Z86 conductive heat from the electrical heating elements 68 for assuring the vaporization of pump fluid striking it. The boiler includes a valve drain line 71. Surrounding the chimney and nozzle assembly 67 is a barrel 72 about which a cooling jacket 73 containing cooling lines 74 is usually wrapped. The pump in-; cludes a foreline elbow pipe 75 leading through a foreline (not shown) to a mechanical backing pump (not shown).
Referring to Figure 4, the boiler 66 of the typical prior art diffusion pump 65 of Figure 3 is shown in greater - 10 detail. The boiler 66 includes three separate concentric annular - :.
boiler channe~Z~i 78a, 78b and 78c. The electrical resistance h heating elements 68 are enclosed within conductive metal heating rings 79a and 79b, and 78b and 78c. Communication is provided among all the boiler channels 78 by a break in the heating rings 79a and 79b. As seen at the left in Figure 4, the rings 79a and 79b terminate in the vicinity of the drain line 71 and thus are C-Zshaped. Anlannular flange 81 extends into the boiler from the chimney and nozzle assembly 67 above to prevent the travel of vaporized pump fluid directly into the diffusion pump barrel 72 rather than through the chimney 57 (see Figure 3). The flange 81 does not reach the boiler bottom. The boiler 66 is designed to ; maximize contact between the motive fluid and heated surfaces.
Some`boilers include add1tional heating fins extending through several of the boiler channels 78 in zigzag fashion for addition- : -~
ally increased surface contact.
.. ,, .. . ~ .
~ Figure 5 shows a diffusion pump assembly 85 similar to , i the prior art diffusion pump 65 shown in Figures 3 and 4 but j' including self-purification and separation modifications according ! to the~invention. The modified pump 85 includes a boiler 86 with electrical resistance heating element rings 87, a dome 88, ~` ~a filllng and drain line 89, a chimney and nozzle assembly 90, a barrel 91 encircled by cooling jackets 92 and a foreline 93 ~ ;

~,: : :.,' , , -20- ;

~ ~343Z~6 :
connected through an opening 94 to a mechanical backing pump (not shown). Heating means is provided about the foreline 93, and may comprise electric heating tape 95 wrapped around the foreline and covered by an insolating jacket 95a.
As indicated in Figure 5, the boiler 86 of the diffusion pump 85 is modified to provide evaporative surface skimming means from inner and outer positions. On the inner side of the boi~er 86 within the dome 88 are skimming tubes 96a, 96b, 97a, 98a and 99a, communicating through openings in the base of the dome 88 - 10 with the interior of the boiler at preferably four different levels. Opposite skimming tubes 97b, 98b and 99b at the levels of 97a, 98a and 99a, respectively, are seen in Figure 6. All of the skimming tubes are connected through valves 100 with a heavy end collection vessel 101. The vessel 101 is connected through a valve line 102 with a source of lower pressure than that within ' the boiler 86. A valved drain line 103 and a valved atmospheric -bleed line 104 are also provided for periodically draining the ~; liquid from the collec~ion vessel 101. Several outer skimming lines 106a, 106b, 107a and 108a are also seen in Figure 5. These outer skimming lines are connected to a separate heavy end collec-tion vessel 109. The vessel 109, like the vessel 101, has valved :, .
lines 111, 112 and 113 connected to a vacuum source, a drain sump and the atmosphere, respectively. The r ason for separat~ collec-tion vessels 101 and 109 for inner and outer boiler-skimmed ~i residueg is that heavier heavy ends, which largely comprise ;~ ~ polymerized light ends, tend to be higher in concentration toward ~the boiler~s periphery, where temp~rature is somewhat lower due ,. ... .
;; ~ to the presence of returned condensates from the barrel wall. - -Lighter heavy ends remain toward the boiler center. Thusy the 3a skimming apparatus herein described provides for separation -~

between the two ranges of heavy ends, if desired.
., , '' ', :~ ,' '.:' - -21- ` ~

~3;2~36 Figures~6 and 7 indicate the various positions of the ~
boiler skimming lines. An additional valved outer llne 114a, -not seen in Figure 5, connects with the back side of the boiler 86, with a fron~al line 114b at the same level. Lines 107b and 108b extend from the front of the boiler 86 at the level of lines 107a and 108a, respectively. The inner and outer skimming lines provide for boiler surface skimming at preferab-~y four different levels, as indicated in Figure 7. Inner~lines 96a-~an~:l96b and outer lines 106a and 106b are preferably positioned at the level of full boiler capacity, which is the four gallon level in many typical diffusion pumps. An opening 105 is provided in the chimney flange 110 (similar to flange gl of Ft~gur~ 4 pump) of ;
the pump 85 to provide for su~face communication throughout the boiler 86, as seen in Figure 6. At preferably about 5 milli-meters below the boiler capacity level are the inner skimming tubes 97a and 97b and ~he outer skimming tubes 114a and 114b.
The tubes 98a, 98b, 107a and 107b are preferably about 10 milli- -; ;
. , -I meters below the capacity level, while the tubes 99a, 99b, 108a ` and 108b are at about 15 millimeters below the capacity boiler ;
level. The multiple skimming levels provide for surface skimming when the pump fluid is at various levels below full, as well as at the ~ull level. This allows for the loss of portions of the :,: . . :
pump fluid after various stages of contaminant removal. Means ~not shown) can be provided for automatically opening the proper ~ sklmming lines in cor~elation with the boiler fluid level, which `~ can be ascertained by a sensor. Means tnot shown) can also be pro~ided for determining when skimming is required, by sensing ; -~
evaporative su~ace behavior and/or vacuum chamber pressure.
~Referring againsto Figure 5, the foreline 93 of the ;~ 30 modified diffusion pump 85 is shown with purification and separa- i tion modifications. The interior surface of the foreline 93 ;' ~ includes~a sexies of alembic-like gutters or troughs 116, 117, `

3;~1~
118, 119 and 123~ In addition, an elbow 122 of the foreline includes traps 123, 124 and 125 for catching condensates as they flow down along the elbow wall toward the boiler. The various levels for condensate removal, from the top to the bottom of the foreline, provide for trapping of condensates of varying volatility, as discussed in connection with the model pump, and separate collection thereof. Two valved lines, referenced by 116a and 116b, 117a and 117b, etc., extend from an upper level and from the bottom of each gutter and trap, respectively. The reason for the bi-level fluid outlets is that the bottom of each gutter or trap, particularly in the traps and lower gutters, will usuall~ contain nearly pure motive fluid. On the other hand, the upper levels of the gutters and traps will contain .;, . . . .
fluid having high concentrations of condensed light end sub-stances. Thus, the lines 116b, 117b, 118b, etc., which lead -, into a collection vessel 127, are used primarily for recovering the basic motive fluid from the gutters and traps. The vessel 127 includes a valved drain line 128 and valved lines 129 and 130 connected to a source of lower pressure than that of the foreline and to the atmosphere, respectively. The valved lines ; 116a, 117a, ll~a, etc., from the upper levels of the gutters and traps may be connected to separate collection vessels (not shown) if separation of ~luids containing light end contaminants of vary- -ing volatilities is desired. ~ach collection vessel would be provided with a drain line, a line leading to the source of lower pressure, and a bleed line to the atm~sphere, as with the ~-vessel 127. If separation is not desired, the upper level gutter and trap line may lead to a common collection vessel.
For maximum fluid separation in the foreline 93, with somewhat lowered throughput and consequently reduced vacuum pro-duction, each of the gutters 116 through 120 can be provided with a condensing baffle above (not shown for clarity). In addition, ~

:
-.
; 23 ~ ~

: . ~ . .. ., . - .. ,. :. .. : . .. . .- . . . . ~ . . . . . . . . . ~

3Z~6 :
packing (not shown) can be provided in each gutter to increase effective condensing area. Such packing might comprise, for example, stainless steel or another inert material in wire mesh or finely sp~n form.
A heating jacket 132 is positioned about the entire -foreline 93 and functions in the same manner as the heating tape described above in connection with the model diffusion pump assembly 11. In addition to this func~tion of bringing light i end substances to high enough levels within the foreline for con-` 10 densation and trapping in appropriate gutter$, the heating jacket 132 also aids in the separation of light ends from the motive fluid within the gutters and traps, and their withdrawal through :`?
the lines 116a, 117a, 118a, etc. At each gutter, the foreline wall is warmer than the gutter itself and other points within ' the interior of the foreline. This causes fluids contacting , the foreline wall to convect upwardly along the foreline wall, ' thus creating a circulation pattern within the gutter. The `
;`, fluid circulation pattern is up along the wall, inward along the surface of the fluid, and then downward and outward along the - ;
bottom of the gutter toward the wall. This circulation pattern :, , .
~ aids in bringing the lighter fluids toward the surface of the .j ...
collected condensate. Since the light end fluid within each gutte~ is at conditions very close to its liquid-vapor equili~
brium conditions, the light end condensate tends to move ~oward the liquid surface for incipient vaporization and actual vapori~
~ ` zation to some extent. Polymerization of these light end frac- ~ ;
`~ tions at gutter surfaces may also play an important part as to -the ease~in w~ich a specific fraction is drawn from the surface , into a vessel of lower pressure.
As discussed in connection with the model self-cleansing diffusion pump assembly 11, at the initial stages of `I pump operation the lightest light end contaminants are trapped ., '~. ~ .
.
::. .

. . .

3Z1~6 within the lower traps 123, 124 and 125. This occurs because pressure within the foreline is relatively high, not having yet been effected by increased pump performance and progressively higher vacuum within the vacuum chamber connected to the low pres-sure side of the diffusion pump barrel 91. These light light ends should be initially removed as completely as poss~ble via the lines 125a, 124a~and~123a.

. . .
As in the operation of the model diffusion pump, pro-~ gressively lowered pressures within the foreline 93 cause progres- -10 sive changes in the average position of condensation for a given light end contaminant. Thus, the light end-containing liquids drawn throuyh the lines 125a, 124a and 123a initially will be higher in vola~ility than those later drawn therethrough, and if each fraction is to be separately collected, the collection vessels r~ for each of the drain lines 125a, 124a and 123a will have to be ~
` drained at intervals. As in the model self-cleansing pump, once ~ -pressure within the foreline 93 has stabilized at minimum values, ~ ~;
withdrawal of nearly all light end contaminants can be accomplished through ~ne or several of the uppermost gutters 116a, 117a and 20 118à~. This is because as maximum foreline vacuum is reached, only the heavier light ends will be condensing in the yutters.
The lighter light ends will have been previously removed, or if any remain, will primarily be passed out of the foreline through the opening 9~ toward the mechanical backing pump. At this ~ -i. .,,, ~ .
` point, many o~ the light ends in the lower gutters 120, 119 and 118 tend to revaporize from time to time and travel ~arther up the foreline, eventually recondensing in a higher level gutter ~ -and enriching the concentration of light end contaminant in that I ~ gutter.~ Thus most light ends remaining at this point can be ,~"! ~ 30 removed through the uppe~gutter or gutters. ~ - -In the initial stages oE operation of the diffusion pump 85 including the above-described foreline modification, the ~ .

, .. ; :. -~ ~ 25 Z8~
preferred procedure for light end contaminant removal is the sequential opening of the "a" skimming lines, beginning with the trap line 125a and ending with the uppermost gutter line 116a.
The withdrawn condensates of each gutter or trap can then be analyzed to determine the distribution pattern of light end contaminants for a given pump fluid and a given set of chamber conditions after a given period of time of pump operation. This operation can then .:
be repeated after various periods of time, and the results can be used to automate the removal of foreline light end contaminants.
According to the results, the valves of the lines 125a, 124a, ; etcl, through 116a can be programmed to be opened at predetermined times and for predetermined time periods for maximum light end con-...
~; taminant removal and minimum withdrawal of the basic motive fluid.

`^ ~fter conditions have been altered by increased pump performance .:~ . . .
and foreline pressure has dropped, as discussed above, fluid -removal from the upper gutter line or lines can be timed to occur periodically according to past performance and analysis of with-drawn foreline condensates, or such removal can be made respon-sive to changes in vacuum chamberL~r~ssure. For example, if chamber pressure rises and is sustained at the higher level, fluid could be removed from the uppermost gutter line 116a for several seconds, since such increased pressure would indicate the presence of volatile contaminants. If a sensor determines tha~ chamber pressure is fluctuating, indicating the probable o~currence of alternate periods of working and torpid evapora-tive surface, the presence of heavy end contaminants within the boiler would be indicated. As discussed above, after an early initial heavy end hoiler skimming operation, recurring heavy ends in the boiler are likely to be primarily made up of polymerized llght ends. Therefore, when such chamber pressure fluctuations are indicated, the foreline skimming valves can again be pro-grammed to remove fluid from the uppermost gutter 116a. If the '' '"'' ~

~ , ~ . . , . , :,.. . . ,. ~

3,'~
pressure fluctuations then persist, a second boiler skimming operation from the appropriate level could be mandated by the automatic system.
During periods when light end fluids are not being removed from the foreline, the series of gutters and traps ~ -creates a waterfall effect within the foreline. Each gutter fills with condensate and overflows into the next gutter or trap, and so on:, eventually flowing through the elbow 122 of the foreline and returning to the boiler. In fact, the water-fall efect prevalls continually, involving all gutters except ;~ those being drained at any given time.
- It should be pointed out that fluids withdrawn from the traps and gutters of the foreline 93 will never be 100% pure light end contaminants. There will always be a significant poxtion of basic motive fluid within such withdrawn fluid. The apparatus described effects the enrichment of a particular light end contaminant in a particular gutter of the forearm, enabling the eventual removal of nearly all of that contaminant. As indicated above, the proper sequencing and timing of gutter fluid removal can result in a minimum remo~a~of motive fluid using the multiple-gutter structure described. If the motive fluid within the w~hdrawn condensates is to be salvaged or if the light end contamlnant from a particular gutter is desired in a more con~en- ;
trated form, the withdrawn condensates can be separated by another means, such as a centriugal molecular still as described in the above-referenced NASA Technical Memorandum X-68272.
- : :
For an additionally high degree of separation and puri-:j ~ . .. ..
fication not attainable with the above-discussed foreline and boiler modifications alone, diffusion pump barrel modifications ;

shown in Figure 5 may also be made. As indicated, the diffusion ; . .. . ..

pump barrel 91 may include around its inner periphery condensate ~ collection gutters 140, 150, 160 and 170. Between the gutters on ,, :

,~ : ', 3Z~6 the outside of the diffusion pump barrel 91 are the cooling jackets 92 which cool the barrel 91 for condensing the motive . . .
fluid and various contamina*ing~flu1~s~thareon. The-cooling jackets 92 are interrupted at the level of each gutter for the provision of a heating band 134 which extends around each gutter and is broken only at the location of draw off lines described below. The bands 134 may comprise electric heating tape 95 employed to heat the foreline 93, around the barrel wall at each ~
.,:, :' ' gutter.
The modified diffusion pump barrel 91 acts as a high purification-separation still, as does the modified foreline 93.
However, the modified barrel 91 is capable of attaining a much higher degree of purification and separation than is the foreline ` 93, after the appropriate separation has been made by fluid with-.... .. .
drawal from the foreline gutters. After such separation, vir--~ tually the only remaining light end contaminants should be those very close in volatility to the basic motive fluid itself. As discussed above, foreline modifications can only remove a portion `
of these heavy light end contaminants, leaving the rest to back-stream within the pump barrel and reduce ultimate attainable chamber vacuum to some extent. Since, as discussed above in connection with the analysis of used five year old DC-705~ oil ;
taken from the cyclotron diffusion pump at Michigan State Univer-sity, the quantity of heavy light ends close in volatility and molecular weight to the motive fluid is quite small, the diffu-sion~pump barrel modifications described herein are designed to ~'~ separate~ out ext~emely minute quantities of contaminants within ~' the~dif~usion pump barrel, possibly in the parts per billion range. Foreline pupi~ cation is conducted first in order that ~;~ 30 substantially only heavy light end contaminants remain in the -barrel along with the motive fluid.
., .

:'' ' , -28-~.. .. .. . , . ,. , . . . ,. . ". .

~0~3286 . The ultra-high purification capability of the modified ~- .
diffusion pump barrel 91 depends in part on the existence therein -: of large variations in pressure at different levels. In the ; foreline 93 only microns or tenths of microns pressure difference exists throughout the foreline length, whereas decades of,pressure difference exists in the barrel 91. For example, in the diffusion pump 85 of Figure 5, pressure above the top gutter 140 after '; . foreline purification may be around 10-8 torr, but around 10-7 torr near the gutter 150, in the 10 6 or 10-5 torr range adjacent ' .
, 10 the next lower gutter 160, and around 10-4 torr near the lowest gutter 170. The high degree of separation discussed can be achieved by utilizing this strong pressure gradient to condense ,~ .
~, heavy lLght end substances at varying levels depending upon vola- . ~
., tili~y, and to separate such contaminants from the motive fluid ,''''' , itself. The operation is very similar to that of the modified ~ foreline 93. ~:, ',, As in the foreline 93 as discussed above, the barrel gutters 140, 150, 160 and 170 can be provided with packing (not ` shown) for increased condensing area. Such packing ~an increase ',~' 20 sèparation capabilities in the pump barrel 91. ', ,l A valved low pressure line 135 extends from the diffu~
'' sion pump barrel 91 at a point above,the highest gutter 140, where ,~ pressure is lower than that existing anywhere below in the diffu-:~ sion pump barrel9 This low pre~sure line 135 extends downward to :~.
: , .: :: ..
. serve a.fluid collec~ion system for each gutter in the man,~er .
described below. In addition, a valved line 136 extends from the ,low~r~end of the line 135, leading to a holding vacuum pump (not '~~
shown)~which may be the same mechanical backing pump that is con~ '`.~, "'~.
.. .. .. .
~'¦ . nected~to the foreline 93. This pump provides preliminary pres~
i. 30 sure reduction for the collection system and in fact can serve ,~ : one or several of the lower gutters by itself, since ~hese gutters x'~:: "''~'' are at higher pressure ranges. A valve 137 in the line 135, which 29~

~32~il6 :;
may be positioned as shown or higher or lower, depending upon pressures existing in the barrel 91, is provided toward this end. ~.
At an upper position in the uppermost gutter 140, a valved fluid draw off iine 141 is positioned to deliver fluid : : -into a collection vessel 142. The line 141, when opened, with-draws condensates only from adjacent the upper surface of the ::
fluid in the gutter 140, for reasons similar to those discussed in connection with the foreline modifications above. Since the collection vessel 142 must be at lower pressure than the pressure ~ . . .
within the barrel 91 adjacent the gutter 140 in order to withdraw fluid from the gutter 140, it is connected ~y a valved line 143 ~ -to the low pressure line 135. The vessel 142, like all low pressure collection vessels previously described, is also pro-vided w~th a valved drain line 144 and an atmospheric bleed line 145, which must be opened in order to drain the vessel 142. Ex-tending from the bottom level of the.gutter 140 is a valved gutter .
drain line 146 which connects into a lower gutter draw off line 151.
: ~ .
To withdraw surface condensate from the gutter 140, the ~ ::
line 143 is first opened to lower the pressure in the vessel 142. - .
Then the line 141 is opened to~draw fluid into the vessel 142.
To drain~the vessel 142, the lines 141 and 143 are:closed, and ~ ~
the lines 144 and 145 are opened. ;~ ;
The fluid collection structure fori:.the next lower -:~
barrel~gutter 150 is~similar to that above, with the reference :~
numbers~lSl-156 describing similar elements. Fluid drained from .
the lower portion of the gutter 140 through the line 146 may be drawn~i~ ~ o~the collection vessel 152 along with fluid from the : .-top of the gutter 150 or drained into the gutter 150 to enrich the concentration therein of the particular fractdon present in the gutter 140 bottom. This can be accomplished by sizing of :
: ,~ ., ,: '"' , ~ 3Z86 ; the line 146 such that liquid is able to drip down therethrough, even though there is a slight pressure differential between the gutters 140 and 150. Alternatively, after the surface condensate in the gutter 140 has been substantially withdrawn, the gutter 140 may be allowed to overflow into the gutter 150, thereby enriching the gutter 150 in the condensates of the gutter bottom 140. The remaining diffusion pump barrel gutters 160 and 170 have fluid collection apparatus similar to that of the gutters 140 and 150 and numbered accordingly. The valved gutter drain line 176 leadi.ng from the lowermost gutter 170 is connected into a collec-tion vessel 182 having a valved line 183 leading to the low pres-sure line 135 and having drain and bleed lines 184 and 185 simi-lar to those above.
Toward the bottom of the diffusion pump barrel 91, a valved auxiliary boiler fluid add line 187 is provided, the pur-` pose of which is described below.
In the initial operation of the diffusion pump assembly85 inc}uding barrel modifications, all valves shown on the right cide of the barrel 91 in Figure,5, from the line 135 through the ~ ~;
20 line 185, are normally closed. The valves all remain closed ;
during foreline condensate removal operations. Prior to any condensate removal from the pump barrel 91, it is estimated that ~
the motive fluid is between 98~ and 99.8% pure. Maximum chamber ~ ;
vacuum attainable by boiler skimming and foreline purification has been~attained, and may range, depending upon tha motive fluid ;-~
employed, from about 10 5 to 10-~ torr. Pump fluid expelled from `~
~ the chimney and nozzle assembly 90 would of course continually `~ be filling up the lower three gutters 150, 160 and 170. The upper gutter 140 is thus the primary location for removing back-streaming heavy light end contaminants. Most of such heavy light ends reach the barrel wall abova tha uppex gutter 140 by back-streaming upward from lower points beneath the jets. To a much , ,,, ' .'" :.
, j .

- -31- ~
~' 1 ,' .

1~;)43Z~6 lesser extent, some of the heavy light end substances reach the gutter 140 as a result of outgassing in the vacuum chamber above.
Heavy light end molecules of course also condense lower in the barrel 91 and are collected in the gutters 150, 160 and 170. Some of these condensates can be recoveréd by surface removal through the lines 151, 161 and 171, such removal being aided by the heating of the barrel adjacent the gutters with the heating bands 134. The surface contaminant removal is thus accomplished similarly to the surface removal operation in the foreline 93 described above.
;As indica~ed above, motive fluid condensate will nor-... I .
mally be found present in very high concentrations in the lower portion of each gutter, particularly the gutters 150, 160 and 170, and their presence is in part the reason for additional valves 157/ 167 and 177 which may be provided in the surface draw off lines 151, 161 and 171. After removal of fluid con-tà~ning light end contaminants from the surface portion of the gutter 140, the drain line 146 may be opened to drain the lower , '~portion of the gutter 140 into the collection vessel 152 (or 20 admitted to the nex~ lower gutter 150 as discussed above, for ~;~
~urther~ enrichment,processing)~ This would be accomplished by opening only the line 153 to lower the pressure within the vessel 152 below that adjaGent the gutter 140, then opening the .. . . .
Iline 151 with the valve 157 closed, and opening ~he line 146.
. :
1~When the~fluid is collected in the vessel 152, the vessel could '.3': then be~drained through the line 154 for analysis by closing the ,~ . .
ilines 151 and 153, opening the bleed line 155 and then opening the line~l54. If this condensate comprises nearly pure motive fluid,~it can be re-admitted to the barrel and the boiler via - ;
the linè 187. Condensates from the lower portions of the gutters ~: :
150 and~ 160 can be drained similarly. The gutter drain line 176 for the gu*ter 170 is of course provided with its own collection -;,1 - , :j ~ .:
:' , : ' ~0~28~
; vessel 182. During periods when the gutters are not being drawn from, the various condensates overflow successive gutters in a waterfall effect.
Since the pressure gradient within the diffusion pump barrel 91 is from lowest at the top to highest at the bottorn, opposite that of the foreline 93, the lightest of the heavy light end substances would be expected to be found in the gutter 170, with the heaviest found in the gutters 140 and 150. However, du~ to the high concentration of motive fluid condensate in the lower gutters, it may be found advantageous to use only the uppermost gutter 140 for the collection of heavy light end substances. In fact, the lower three gutters, particularly the gutters 160 and 170, would also contain very small concentrations of the lightest of the heavy end contaminants in the fluid ~
spectrum, which may escape the boiler skimming process. Heavy end substances of molecular weight and volatility very close ~-to that of the basic motive fluid will be carriied along with the motlve fluid through the chimney and nozzle assembly 90 ~ -into the diffusion pump barrel and will conden~e on the walls ~ 20 of the barrel 91 even more readily than the moti~e fluid itself.
Small concentrations of these substances may be found in fluid drained from the bottom of the gutters 160 and 170. If fluid collected in the vessels 172 and 182 is found to be rich enough in the light heavy end contaminants, these gutters may be drained `
periodically as part of a programmed pump barrel purification -procedure. -The a~tomatic operation of the diffusion pump barrel -~
j~ purification and separation apparatus described above may be pro- ~
;¦ grammed~according to results of initial analysis. Variations in -;
~t~e type of pump fluid used~ in the exact specifications of the ; pump boL1er and foreline,~and in the conditions within the vacuum -` ahamber will dictate drastically different results in the collection ~ -"'.' : ' .',''-'' ,:
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- ~;)4;;~86 gutters 140-170. After such analysis, the opening of the fluid withdrawal lines can be sequenced and duration timed in order to provide maximum withdrawal of heavy light end and light heavy end contaminants with minimum withdrawal of basic motive fluid. Of course, as with the operation of the foreline purification modifi-cations, all fluid withdrawn into the collection vessels 142-182 will contain a large amount of the motive fluid, but the system is designed to remove high concentrations of these contaminants as has never be~ore been possible, so that pump performance can be increased and ultimate attainable chamber vacuum can be addi-tionally lowered by one or more decades.
As indicated above, the diffusion pump assembly 85 including boiler, foreline and barrel modifications can be used ,~
as a "slave" pump to a group of additional diffusion pumps serving ~ `
the same vacuum system. Each of the additional "master" pumps would have boiler and foreline modifications only. The "slave-master" relationship between the pumps would simply be one of boiler fluid exchange. For example, the slave pump shown in Figure 5 would dispense portions of its highly purified pump fluid from its boiler drain line 89 into the boilers of each of the master pumps. Such fluid would be admitted to each master pump through an auxiliary fluid adding line such as the line 187 ;
shown in Figure 5. In turn, fluid from the boilers of the master pumps, very pure but not to the extent of the fluid in the fully ~; modified slave pump 85, would be taken from the boiler drain lines of each of the master pumps and delivered through the auxiliary fluid add line 187 into the slave pump 85 of Figure 5. Such ~ circulation between the pumps could be accomplished slowly and ^' continuously, but would preferably be done periodically, with , the slave pump 85 exchanging fluid with only one other pump at a time. - ~-`' ~

~ 3~043~636 The fluid purification and separation apparatus of the ~ above-described diffusion pump, including boiler, foreline and barrel modifications, can also be applied, in total or in part, purely as means of fluid separation in other disciplines apart from diffusion pumps and high vacuum production. For example, - the surface heavy end skimming modifications of the invention can -be employed in a fractionating column for the refining of petroleum.
~` Surface skimming such as that described herein can be employed at all evaporative surfaces in the refining process--in the boiler and at each distillation plate in the fractionating column (not ishown in the drawings). The employment of such skimming modifications to refining equipment will very significantly increase throughput in the refining process by removal of heavy end fractions at all surfaces, thereby greatly reducing torpidity and providing for . ! :
continuous nearly 100% working evaporative surfaces. In addition, j-~
petroleum fractions wit~drawn and collected through the skimming $ operations would comprise ¢ertain of the fractions desired to be ! ~
recovered. The fractions recovered in this way would of course ~ary from level to level in the column, with the heaviest heavy l ` 20 ends taken from the boiler and the lightest taken from the upper- -! most distillation plate. Efficiency,~productivity~anddoutput~
of a refinery can thus be achieved at a minimum capital expenditure.
The extremely high purification and separation capability of the foreline and barreL modifications described a~ove can be employed, in total or in part, for ~the separation of the isotopes -of uranium. Raw feedstock uranium slurry would be heated in a boLler~s1milar to the boiler 86 of the~ diffusion pump 8S shown in Pigure~5~ In the-forellne and barrel gutters, means would pre-ferably~be provided for increasing condensing area, such as baffles 30~ over the~forelLne gutters and stainless steel packing within the foreline and barrel gutters tnot shown). As mentioned above, such ~ ; b~afles~slow pump thro ~ hput but increase separation capabilities. i :., ~ " ' ~ ~ 35 ;

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Gutter packing also increases separation capahilities. The high vacuum and pressure gradient present in the diffusion pump barrel can be utilized to aid in the separation/enrichment of the -isotopes of uranium.
Separation of metals from one another and from impurities can also be effected using the purification and separation appa-ratus described hereinabove, in total or in part. The methods and ~;
apparatus of the invention are particularly adaptable for the ` separation of metals in the low and intermediate melting point range, such as those metals melting below about 2500F.
. . , Various other embodiments and alterations to these , ~ -preferred embodiments will be apparent to those skilled in the ' art and may be made without departing from the ~spirit and scope of the following claims. , ~ ~ .
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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A fluid refining system including a boiler, a fractionating column with its upstream end in communication with such boiler, a series of distilla-tion plates at various levels within the fractionating column, a conduit pro-viding communication between the interior of the boiler at the level of the surface of liquid therein and an external source of lower pressure than that of the boiler, a valve in said conduit, and means for withdrawing liquids from the surface of fluid in at least one distillation plate in the frac-tionating column.

2. In a method for separating fractions of a liquid, which method in-cludes the steps of heating a body of the liquid to a temperature sufficiently high that the liquid has a vapor pressure at least substantially as high as the pressure to which the body of the liquid is subjected, condensing various fractions of the liquid at various levels above the surface of the heated body of liquid, and collecting various liquid fractions at their respective condensation levels, the improvement of withdrawing from the heated body of liquid a portion of the upper surface thereof, and withdrawing from at least one collected liquid fraction a portion of the upper surface thereof.

3. In a method for separating fractions of a liquid, which method in-cludes the steps of heating a body of the liquid to a temperature sufficiently high that the liquid has a vapor pressure at least substantially as high as the pressure to which the body of the liquid is subjected, condensing various fractions of the liquid at various levels above the surface of the heated body of liquid, and collecting various liquid fractions at their respective condensation levels, the improvement of withdrawing from the heated body of liquid a portion of the upper surface thereof, and discharging such withdrawn portion to a collection vessel exterior of the body of liquid.

4. The method of claim 3 which further includes withdrawing from each collected liquid fraction a portion of the upper surface thereof, and dis-charging each such withdrawn portion to a separate exterior collection vessel.

5. In a fluid refining system including a series of evaporative vessels, each of such vessels adapted to contain a body of liquid defining an evapora-tive surface, a first of such evaporative vessels comprising a boiler for initially heating an incoming feed liquid and the remaining evaporative vessels comprising a series of distillation plates within a fractionating column in communication with the effluent end of the boiler, the improvement comprising at least one conduit providing communication between the interior of at least one of the evaporative vessels at the level of the evaporative surface thereof and an external source of lower pressure than that of said one evaporative vessel, and a valve in said conduit, whereby areas of the evaporative surface which impede evaporation of the liquid may be removed from the evaporative sur-face by periodic opening of said valve.

6. The apparatus of claim 5 wherein at least one said conduit is in-cluded at the evaporative surface of each evaporative vessel.