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Rotary pump back-migration
Rotary pump back-migration received 3 March 1978
N S Harris, Edwards High Vacuum, Crawley, W Sussex
A paper in our Education Series: The Theory and Practice of Vacuum Science and Technology in Schools and Colleges.
This review deals exclusively with vacuum system contamination which can be attributed to back-migration of rotary pump working fluid. It discusses the origins of this type of contamination and survey methods used to prevent and control it.
1. Introduction
The rotary pump is a positive displacement pump, i.e. a mechanical pump which transports the gas, from the vacuum system, from the inlet side of the pump with the aid of vanes etc. by compressing it and expelling it to atmospheric pressure. The moving parts inside are sealed and lubricated by oil. In its own right the rotary pump has many applications, for example it is used in the manufacture of electric lamps and thermos flasks and in distillation, drying and sputtering. Rotary pumps develop their full speed from atmospheric pressure down to about 1 mbar, the speed then decreasing to zero at their ultimate pressure. They can be used in conjunction with other pumps which are more economical for obtaining high pumping speeds below 10 mbar. Diffusion pumps which operate in the molecular flow region (below 10 -3 mbar) need to be "backed' by a backing p u m p and the rotary pump is the one most widely used. The attainable ultimate vacuum with such a combination can be influenced by the hydrocarbons emanating from the rotary pump oil (i.e. back-migration). Even with two stage rotary pumps, a small amount of back-migration of molecules from the pump interior cannot be avoided. 2. Mode of operation Several forms of mechanical rotary pump exist. Two of the most popular types will be described here. 2.1. Rotary vane pump. Originally designed by Gaede ~ in 1905, the pump is shown diagrammatically in Figure 1. The mechanism consists of a cylindrical housing (the stator), in which an eccentrically positioned rotor fits tightly against a cylindrical seat machined into the stator. The rotor contains spring-loaded vanes which slide in diametrically opposed slots. Thus as the rotor turns the blades are held in contact with the stator wall at all times. During operation gas molecules entering the inlet of the pump pass into the volume created by the eccentric mounting of the rotor in the stator. The crescent-shaped gas volume is then compressed, forcing the outlet valve open and permitting gas discharge into the surrounding atmosphere. In order to produce a seal which will combat the very high pressure difference between suction and discharge compartments the gap between the rotor and stator is required to be of a very small
5TATOR
Figure 1, Rotary vane pump.
order. To complete the seal, and to reduce friction and wear, a thin oil film is continuously maintained between the compartments, by oil circulating into the pump interior through variously arranged oil ways. The seals necessary between the blade ends and stator are made in the same way. The circulating oil is ejected back into the reservoir through the outlet valve together with the pumped gas. The ultimate vacuum is limited by vapour/gas solution-dissolution across the top seal, between suction and discharge compartments, and to outgassing of the lubricating oil. This limitation can be overcome by providing a second pump in series with the first; or as it is known a two stage, instead of a single stage pump (see Figure 2). The first or high vacuum stage is 'backed' by the second or low vacuum stage via an internal transmission port, the discharge valve from the first stage being eliminated. Due to this elimination, the pressure at the discharge port of the first stage is very considerably less than atmospheric pressure. Thus the pressure difference across the top seal in the first stage is much reduced, enabling lower pressures to be achieved, before oil outgassing begins to affect the performance. Oil for lubricating and sealing the first stage is outgassed by the second stage before being passed to the first stage. This together with the sharing of corn-
Vacuum/Volume 28/number 6/7. 0042-207X/78/0701-0261S02.00/0 © Pergamon Press Ltd/Printed in Great Britain
261
N S Harris: Rotary pump back-migration INLET I
HIGN
VACUUM OR
OR SECOND
DISCHARGE
l
STAGE
Figure 2. Two stage rotary vane pump. pression between two stages gives very much lower pressure than does a single stage pump. Frequently the first stage is considerably larger than the second largely for reasons of economy. When this is the case a blow-off valve is provided in the interstage to discharge gas when the throughput of the first stage exceeds that of the second stage. 2.2. Rotary piston pump. The rotary vane pump is the generally adopted design for small pumps (up to about 60 m 3 h 1). Larger capacity pumps tend to be of a different design, the rotary piston mechanism tends to be used more frequently. This is partly due to tradition where a limit on the peripheral speed of steel blades exists due to friction and consequent oil breakdown. With modern blade materials such as 'asbestos reinforced plastic', peripheral speeds can be increased by a factor of four over steel. Figure 3 shows a rotary piston type of pump
ROTATING CAM
~-
--
HOLLOW CYLINDER- -
SHA;~T~
~HOLLOW TONGUE
--
========================= : ' "
".",i.''
5;
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Figure 3. Rotary piston pump. where the piston (a hollow cylinder with a hollow tongue attached) is made to precess around inside a circular stator by means of the rotating cam within it. The operating cycle, i.e. increase volume, isolate and discharge are similar to the basic m o v e m e n t s described in the rotary vane pump. Pumps with speeds up to about 1000 m 3 h-1 are available. A l t h o u g h today the rotary vane type is mainly used for pumps of low and medium pumping speed whilst the rotary piston is preferred for higher pumping speeds, clear cut advantages or disadvantages cannot be formulated for either type. A fundamental difference in design between the two types can be stated as follows: the vane type pump is a force-sealed structure whilst the rotary piston is geometrically sealed. This has consequences as regards manufacture and function of the most important p u m p elements, those responsible for providing a 262
seal between the pressure and suction spaces. The rotary piston pump requires a very careful adjustment of many relative fits in order to obtain the close tolerances required for sealing. The force sealed vane pump automatically adjusts these clearances to the smallest possible amount, a fact which naturally presents manufacturing advantages. One advantage of the rotary vane design is the comparatively small out-of-balance forces consequent upon rotation. The only eccentrically rotating masses are the blades, the centre of mass of a blade pair describing an oval path twice per rotor revolution. With the use of various plastic blades, which are somewhat lighter than steel, this imbalance becomes insignificant. The inevitable out-of-balance forces which exist with rotary piston pumps can be partially balanced by a counter weight on the driving pulley on the shaft end at the expense of introducing some out-of-balance couple or 'rocking' force. Another way of achieving good balance is to mount two pumps on a c o m m o n shaft together in the same casing 2. A small counter weight on the pulley may also be used and substantial freedom from both reciprocating and 'rocking' forces obtained. If the inlets and outlets of the two pumps are connected in parallel a more even gas flow into the pump is also achieved. Many balanced pumps now in manufacture employ three stages served by a c o m m o n shaft.
3. Rotary pump back-migration With a well designed diffusion pumped vacuum system employing good liquid nitrogen traps above the diffusion pump, there can still occur a certain degree of hydrocarbon contamination. The source of this contamination is usually attributed to the diffusion pump, but in most circumstances it is certainly traceable to the mechanical pump oil. Consider the changes in gas flow taking place in the pipeline between the diffusion pump and rotary pump as the rotary pump operates from atmospheric pressure down to its ultimate pressure. The high density of the flowing gas stream in the pipeline, which prevails almost exclusively during initial exhaust (down to about 1 mbar), prevents the mechanical pump oil vapour from diffusing towards the diffusion pump. However, as the pressure falls and the flow changes to 'transitional', the mean free path of the vapour molecules increases, thus molecular collision is reduced and oil vapour may begin to backmigrate from the mechanical pump to the diffusion pump. If a rotary pump in a valveless rotary/diffusion pump system is left running while the diffusion pump is not working, entry of back-migrating oil into the diffusion pump is unhindered. Direct contamination of the vacuum system during roughing through a by-pass line is also another problem. The likelihood of entry into the diffusion pump while it is running is thought to be very small, this obviously depends on whether the pump has an ejector stage working into the backing spout. Holland 3 has suggested that the provision of a small leak into the backing line will help prevention of back-migration into the diffusion pump by keeping the flow in the transitional region. This is probably not an easy practical proposition, care must be taken to ensure that the backing-line pressure does not rise above that of the critical backing pressure of the diffusion pump, otherwise the pumping action of the diffusion pump jets will be destroyed and gross back-streaming will occur. Any contaminating oil which is successful in entering the boiler can enhance the back-migration rate of the diffusion pump. The extent of this enhancement is
N S Harris: Rotary pump back-migration
dependent on the pump design, i.e. whether it is fractionating, non-fractionating, self-purifying or has an ejector stage. Oils used in rotary pumps are usually hydrocarbon mineral oils of much higher average vapour pressure at room temperature than that of diffusion pump fluids. They are not pure compounds but contain molecules with a wide mass range. Molecules with the lightest weight form the most volatile group and evaporate preferentially. Also, as will be discussed, the oils undergo degradation in the rotary pump releasing components of light molecular weight. Thus, the ultimate pressure of a rotary pump is not simply the saturated vapour pressure of the oil at the pump temperature. The use of low-vapour pressure diffusion pump oils in rotary pumps has not proved satisfactory. Fulker e t a l 4 measured the back-migration rates for several different fluids (see Table 1)
Table 1. Back-migration rates from a two stage mechanical pump Oil or fluid
Back-migration rate (p.g cm- 2 min- ~)
Observations
further experimental evidence, for when they measured the electrical resistance of the lubricating film between the pump blade and wall during pump operation, low resistance values indicated direct contact between these surfaces. Further studies have been made using quarts crystal microbalances and high resolution mass spectrometers which have established without doubt that oil molecules are degraded when exposed to rubbing metal surfaces. Baker and Laurenson 6 measured the back-migration rates of rotary pumps using a quartz crystal microbalance. They found that a single stage pump gave a back-migration rate which was higher during the first half-hour of running (10 tzg cm -1 min-1), which then settled down to a steady value of 3 t,g cm -z rain -~, whilst a two stage pump using the same oil gave a steady value of half this rate. They felt this reduction was due to 'stripping' of the oil before entry into the second stage. Baker e t al v used gas liquid chromatography and found that there were no fractions of molecular weight higher than 250 in the back-migration products although the hydrocarbon oil used had an average molecular weight of 530. This suggested that: (a) oil cracking takes place during operation,
Normal uninhibited mineral oil 18 mineral oil + 1% MoS2 4.7 mineral oil + 1% Oleic acid 6.5 Silicone fluid 10 Polyphenyl Ether
13
Apiezon C oil
15
(b) only the light fractions have high enough vapour pressures to contribute to the back-migration.
Pump seized after ½h of operation Difficult to start pump due to high viscosity
using a quartz crystal microbalance. The table shows that a reduction in back-migration is gained by the addition of boundary lubricants (i.e, molybdenum disulphide and oleic acid); however, the results are not conclusive since the addition caused a drop in the running temperature of the pump which will have a corresponding effect on the vapour pressure of the fluid. The lubricating properties of silicone fluid were not sufficient to prevent seizure of the pump, and Apiezon C and polyphenyl ether fluids are limited because of their thermal decomposition into lighter fractions by friction of the rubbing surface within the pump. A zeolite sorption pump cooled with liquid nitrogen could be used as an alternative to back the diffusion pump. Although this avoids the problem of rotary pump back-migration, a sorption pump, unlike a rotary pump, does not have consistent volumetric speeds for different gases and its use on a continual cycle basis is limited. Hence, several authors have studied the source and nature of back-migration in rotary pumps so that suitable counter means could be devised to overcome the problem. Hockley and Bull 5 measured the ultimate pressure of a two stage rotary pump and showed that the ultimate increased as the operating temperature of the pump was raised by a heater, Their results suggested that this pressure rise was not solely due to the presence of oil vapour but that it was due to the appearance of lighter fractions produced by thermal degradation of the oil brought about by local heating of the pump surfaces resulting from friction of the pump blades. This was supported by
Fulker 8 examined the back-migration vapours using a differentially pumped 60 degree mass spectrometer. He confirmed that only the lighter fractions were present up to mass number 161. Although higher fractions could be present they were possibly being cracked in the spectrometer. Olejniczak 9 went one step further with combined gas-liquid chromatographic and mass-spectrometric analysis and identified some organic vapours from heated rotary pump oil and found they mainly contained the C8 to C20 hydrocarbon component. Fulker e t a l 4 constructed an apparatus to simulate sliding conditions in a rotary pump, using a steel rotor with a force of 1 kg, moving at speeds of 5 m s- =. They studied the degradations of various fluids, and observed the production of light fractions. They point out that the frictional rubbing of stainless steel surfaces not only provides heat to produce cracking but can also possibly encourage catalysis. From this wealth of experimental results it can be concluded that the source of back-migration from oil filled rotary pumps is due not only to the vapour pressure of the pump fluid, but that there is an important contribution from thermal degradation. The main factors influencing back-migration rates appear to be interrelated and difficult to separate, i.e. (a) the size of the pump: in general the larger the pump the higher the back-migration rate but correlation is complicated by variations in running temperature; (b) the effect of pump blade velocity: the higher the blade velocity the higher the back-migration rate, however, increased velocity is accompanied by higher pump operating temperature. This is illustrated in Figure 4 taken from a paper by Laurenson e t a l to which shows that the backmigration rate rises with temperature which is raised as the rotary pump speed is increased. Pump operating temperature thus appears to be the dominating factor. Unpublished results of Baker 11 (shown in Figure 5) indicate an approximation to linearity with a log plot of back-migration against 263
N S Harris: Rotary pump back-migration taJ
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200 RPM
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Figure 4. Back-migration rate from a two-stage rotary pump.
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TEMPERATURE (,°C)
Figure 6. Vapour pressure vs temperature relationship of rotary pump fluids.
20
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60
80
TEMPERAT (°U C RE )
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Figure 5. Back-migration vs pump temperature. a linear plot of temperature, for two different sizes of rotary pump. A modified vapour pressure relationship is thus suggested.
4. Rotary pump fluid Various liquids have been used as working fluids for rotary pumps. Selection has been dependent on various criteria, they should have good lubrication properties, low vapour pressure at room temperature, good thermal stability, non-toxicity, chemical inertness, high flashpoint, reasonable viscosity at room temperature and low cost. Since there is a wide choice of pump fluids available it is usually best practice to use the fluid recommended in the pump manufacturer's catalogue. The types of oil employed for oil-sealed rotary pumps have vapour pressures at least as low as 10 -4 mbar at room temperature. They consist basically of mineral oils which often contain light fractions. Rotary pump oils supplied by various manufacturers are very similar in that they are good quality oils offering low vapour pressures even at high temperatures. Figure 6 shows the vapour pressure vs temperature dependence of typical rotary pump oils available. Numbers 8A, 15 and 16 are high quality straight mineral oils, covering a wide range o f viscosities. Number 8A is a low viscosity oil, 15 a medium 264
viscosity and 16 a high viscosity oil, all having a comparatively low vapour pressure. Numbers 17 and 18 are highly refined oils containing additives that impart detergent, anti-oxidant, antiwear and protective properties of a high order. The presence of inhibitors increases the vapour pressures so that the ultimate vacua obtainable are slightly worse than those obtained with the plain oils. Fulker e t a l 4 reported that the replacement of mineral oils by other lubricant fluids gave disappointing results in terms of the improved performance against the extra cost involved. For example diffusion pump fluids such as Apiezon C gave a backmigration rate and ultimate pressure that tended to rise to the values of the oils normally used, and the polyphenyl ethers (Santovac 5) were found to degrade readily when exposed to rubbing friction. Holland 12 has reported on the use of perfluoro polyethers ('Fomblin'), as a lubricant in a rotary pump. Baker e t a l la found that back-migration rates of 'Fomblin' were only marginally better than a mineral oil; however, the fluid has the valuable property of not polymerizing under electron bombardment. 'Fomblin' fluids are compounds made solely from carbon, fluorine and oxygen and as they do not contain hydrogen or silicon they cannot react to form, respectively, either hydrocarbon or silicaceous surface contaminants when their vapours are degraded. 'Fomblin' fluids are inert to most reactive chemicals and can be used in direct contact with materials such as UF6, fluorine, oxygen, ozone, etc. The fluid is however relatively expensive and is perhaps best used in pumps requiring small oil levels, e.g. rotary pumps having built in oil circulation devices. A grade of 'Fomblin' suitable for rotary pump use is now commercially available. A suitable grade is also available for use in diffusion pumps.
N S Harris: Rotary pump back-migration
5. Back-migration prevention and control
The problem of back-migrating oil vapours from rotary pumps is commonly solved by installing traps in the backing line. These may operate either by condensation on liquid nitrogen cooled surfaces or by adsorption on surface-active materials. One manufacturer used an ingenious trap based on a cold cathode discharge (ion-baffle). This device together with other more commonly used devices will be discussed. 5.1. The ion-baffle foreline trap. The ion-baffle was described by HaefeP 4 and is shown diagrammatically in Figure 7(a). It is a device, simple in operation and maintenance, normally mounted directly above the rotary pump inlet, any back-migrating vapours encounter a water-cooled region containing a baffle plate which deflects the vapours against the cylindrical baffle body. This body constitutes the earthed cathode and the central rod the anode. A cold cathode discharge is set up by the application of a few kV. Provision of external magnets ensures that the electrons emitted from the cathode travel in long spiral paths before reaching the anode. Residual gas ionization is thus very efficient and the cathode is bombarded with ions. The back-migrating oil molecules in contact with the inner surface of the cathode are broken down by ion and electron bombardment and their residues and polymerized compounds deposited on the body wall. When first operated hydrogen and low weight hydrocarbons CH~, C2H, and C3H,, are copiously produced and the measured pressure rises. Over several hours of operation the pressure slowly decreases as the fragmented products are trapped. Figure 7(b) shows mass spectra of the ultimate vacuum of a two stage pump with and without the ion-baffle. It thus appears to be efficient in reducing oil contamination. 5.2. Adsorption traps. Adsorption trapping is a convenient method of reducing oil vapour back-migration from a rotary pump. Oil molecules are trapped as they encounter the physical and chemical forces at the large active surface of an adsorbent in a trap. Baker and Laurenson 6 assessed the effectiveness of different sorbents for back-migration reduction. Table 2 gives a summary of their results for both zeolites (molecular sieves, synthetic dehydrated crystalline alumino silicates) and activated materials such as alumina and charcoal. The greatest reduction was obtained using activated alumina. Activated charcoal also reduced back-migration by 99% but because of the small particle size in which this material was available it presented a low conductance path through the trap and the pumping speed was thus reduced by 95 %. Fulker s using a 60 degree magnetic deflection mass spectrometer showed that activated alumina could remove 99.7% of back-migration vapours from a two
Table 2. Reduction in back-migration by sorbents. Tests carried out using a 9 m 3 h- 1 rotary pump filled with uninhibited oil
Method of reduction
Effect on pump performance
Open tray trap P2Os Zeolite 13X (~ in. pellets) Zeolite 10X (~ in. pellets) Packed column trap (h = 5 in.) Zeolite 3A (~ in. pellets) Zeolite 10X (~- in. pellets) Zeolite 13X (~ in. pellets) Activated alumina (~ in.-~ in. granules) Activated alumina (½ column) Activated charcoal ( ~ in.-~ in. granules)
None
50 %
None
30
Approximately 40 ~ reduction in speed
/ I
65 % 70 % 90700
20 % reduction in speed
99 %
10 700reduction in speed
99 %
95 % reduction in speed
99%
WITI4OgTION BAFFLE TOTALPR,19x lO'3rnba~
=
~.
'~L~I'-'~ ("0~ ~"
Slow pump down required 50%
stage rotary pump, even when partially saturated with water. A 13X molecular sieve trap although effective when dry (removing 99.8% of back-migrating vapours), deteriorated when wet and did in fact release fractions as masses 85 and 86, thought to originate from previously sorbed contaminants displaced by water vapour. Figure 8 shows mass spectra taken when sampling the back-migration vapour and gas from a pump with and without an alumina trap. As can be seen almost all organic molecules with a molecular weight over 80 have been removed. Sorbed water vapour can be driven off from the sorbent material by heating to about 300°C. This is best done in an oven outside the system. Unfortunately the organic molecules remain and consequent periodic replacement of the alumina is thus necessary. Where moist gas has to be handled frequently, it is advisable to by-pass the trap during the rough pumping period. A commercially available automatic by-pass valve is illustrated in Figure 9. As shown the valve is held in the by-pass position at atmospheric pressure by a compression spring, with the vacuum system port communicating directly to the mechanical roughing
1420 C2H... CsH... _ 5kV
Efficiency (percentage decrease in backstreaming)
z
COOLING WATER ~_ 4~3Z
f,,,,I 13 15 17 ZG28 ,I ~,1
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171 Z61Za t z7 (b)
WITH iON BAFFLE TOTALPR.I x lO'*mk~.: IVt/P.
Figure 7. (a) Diagrammatic arrangement of the ion-baffle. (b) Mass spectra of two-stage pump and without the ion-baffle. 265
N S Harris." Rotary pump back-migration 10-5
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0 !lrll!
1
(b) WITHOUTBY-PASSVALVE
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I0 20 30 40 50 60 70 80 90 I(30II0 120 130 I#0 J
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Figure 10. Ultimate vacuum after a number of pump-down cycles.
MASS NUMBER
Figure 8. Mass spectra of back-migration vapour. (a) Without trap.
(b) With trap.
~HAFT
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tRAP
VALVE PLATE---_
~LVE SEAT
~LVE BODY FROM VACUU~
TO MECHANICAL ROUGHIN6 PUMP
Figure 9. Automatic by-pass valve. pump port, i.e. the foreline trap is by-passed. Changeover occurs progressively and is completed when the system pressure reaches 40 m b a r (approximately). The decreased pressure is sensed by the inner (lower) face of the diaphragm which will actuate the valve to isolate the mechanical roughing p u m p and divert the flow through the foreline trap. The effectiveness of such a trap and by-pass combination in reducing the water adsorption has been described by Dennis 15. Figure 10 shows the ultimate vacuum attainable after a number of pump-down cycles on a 115 l. volume system pumped by a two-stage 266
20 m 3 h - 1 rotary pump. Without a by-pass valve, the ultimate starts at 10 - z m b a r and rises to above 10-1 m b a r by the 14th cycle. With a by-pass valve the initial vacuum is less than 10 -2 mbar and rises to a little above 10-2 mbar after the same number of operations. This value had risen to 6 l0 -z m b a r at the end of 100 cycles after which no further increase was apparent. Baker e t a l ~3 have suggested a method for the preferential chemisorption of hydrocarbons on large surface area active metals. It is believed that the trap would avoid or reduce water vapour physi-sorption. The active metal sorbent (e.g. nickel) is chemically deposited onto alumina which has a large surface area, giving an active area of several hundred m2g -t . When used for rotary pump trapping the active material is raised in temperature to enhance chemisorption of oil vapour and reduce the physi-sorption of water vapour. Preliminary tests suggest that this type of trapping was better than a comparative trap filled with activated alumina. 5.3. Liquid nitrogen cooled traps. This type of trapping is in principle the same as that used for the reduction of backstreaming from diffusion pumps. Investigations by Baker and Laurenson 6 on the efficiency of a low conductance liquid nitrogen trap above a rotary pump, indicate a better than 97 ~/ reduction in measured back-migration with no effect on p u m p performance. (N.B. about 15 times worse than the adsorption trap i.e. the adsorption trap using 13X activated alumina allows 0,2 % of back-migrating vapour through the trap compared with 3 % for the liquid nitrogen trap just described.) One drawback is that a means of automatically topping up the trap with liquid nitrogen must be provided. It was shown that if the trap was allowed to warm up at r o o m temperature re-emission of trapped vapours became a great problem. In fact if the warming up was accelerated, e.g. by the insertion of an air-line in the trap, the evolution rate was so high as to be able to create a pressure above that at which an oil diffusion p u m p ca1~ be backed (i.e. the diffusion pump will stall). It is worth noting that temperatures lower than 100°C seem to be necessary to trap effectively rotary pump back-migration, since light fractions from degraded oil e.g. CH2 cannot be condensed unless a refrigerated trap operates at liquid nitrogen temperature.
N S Harris; Rotary pump back-migration
5.4. Roughing-line purge combined with liquid nitrogen trap. As discussed the liquid nitrogen trap works well provided it is kept cold. If the trap returns to r o o m temperature, oil is free to migrate to the high vacuum side. This problem was overcome by Santeler 16 who provided a deliberate leak of high purity nitrogen gas into the roughing-line of the system. The purge can be arranged to be on whenever the trap is warm. The flow rate is set to maintain viscous flow in the roughing-line. Carbon dioxide, water vapour and other contaminants are thus kept from back-migrating by the flowing gas purge. 5.5. Thermoelectrically cooled foreline trap. When an electric current flows in a closed circuit of two different conductors, heat energy is absorbed at one junction of the couple and given out at the other. This is the well-known Peltier effect. For metallic conductors the temperature difference between the junctions is very small, but with certain semiconductor materials the temperature difference becomes large enough to be of practical use for cooling purposes. A typical cooling module consists of a number of p- and n-type semiconductor couples mounted thermally in parallel and electrically in series. The cold junctions are clamped against the object to be cooled and the hot junctions on the opposite face are maintained in contact with a heat sink, which is cooled by air convection or water circulation. Baker ~v used such a thermoelectrically cooled metal baffle to reduce rotary pump back-migration. At a minimum temperature of - - 4 0 ' C he reported a 75 % drop in the value of back-migration compared to the value measured at ambient temperature. These results he felt were disappointing considering a similar baffle mounted above a diffusion pump could reduce back-streaming by two orders. He ascribed the ineffectiveness of the foreline baffle to the presence of light oil fractions which cannot be condensed at 40:C. Kendall ~8 reported successful tests using a similar baffle whose surfaces were coated with an artificial zeolite. This provides a very large effective surface area to which the oil clings firmly in the form of a film a few molecules thick. Such films have significantly lower vapour pressures than the liquid in bulk. A reversing switch was included so that at intervals of a few weeks the adsorbed vapours could be driven off by reversing the current through the module. 5.6. Dry stage rotary pumps. Obviously if rotary pumps could be used without lubricating oil, decomposition products would not be formed. Wycliffe and Power ~9 described a two stage rotary pump comprising a lubricated low vacuum stage and a 'dry' high vacuum stage. The tendency for oil vapours to backmigrate into the high vacuum system was reduced. Wycliffe 2° has indicated that the back-migration rate from such a dry stage pump was 10 times lower than that of an equivalent conventional two stage rotary pump. The main disadvantage of any dry stage p u m p is that of a considerable loss of 'compression ratio'. A conventional rotary pump can have a compression ratio of 10S:l across a single stage because o f the efficient sealing effect of the lubricating oil. With a dry stage pump, whether it operates with self-lubricating vane materials or as in the pump just described, with a small gap between rotor and stator, it is usual to achieve compression ratios of a much lower order. It is therefore usually necessary to run a dry stage in series with a conventional type rotary pump. Brand 2t has described a rotary p u m p which consists of a dry high vacuum stage using solid lubricating materials backed by a conventional oil sealed
HIGHVACUUM
/ GAS INLET
HIGHVACUUM I •/
DRY STAGE
OIL SEALED STAGE
{NTERSTAGE GAS PURGETO REDUCE j CONTAMINATIONREACHINGDPY, STAGE
DRYSTAGE
TRAP
SEALED STAGE
01L
INTERSTAGETRAPTO REDUCE CONTAMINATIONREACHINGDRY STAGE
Figure 11. Two stage rotary pump with dry high vacuum stage. stage. Interstage gas purge can be used to reduce contamination reaching the dry stage (see Figure 11). Although much has been done to develop self-lubricating materials, their performance generally does not compare with that of fluid lubricated types. Because of the absence of oxygen under vacuum conditions, the metal oxides, which prevent local welding of metal surfaces, cannot form. Substituted polymeric materials are heated by rubbing friction and evaporate and decompose. Graphite, due to the absence of chemisorbed oxygen in vacuum, develops a high coefficient of friction. Composite materials are now available, such as MoS2-filled carbon and plastic materials, and carbon fibre filled PTFE. Figure I I also illustrates a similar pump arrangement to that of Brand (devised by Holland et al22), having an interstage sorbent trap to reduce contamination, back-migration should be reduced to that of the solid lubricant used in the dry stage. Baker et a113 used a liquid nitrogen probe in the high vacuum inlet of the dry stage of this pump, to collect any condensable contaminants emitted. N o n e were detected after several hours of running. It appears that at present it is not possible to match the performance and cost of an oil-sealed rotary pump plus sorbent trap by making pumps with solid lubricants of low desorption rate. Conclusions We have seen that oil-sealed mechanical rotary pumps can produce contamination from the pump fluid. This contamination can be a problem in certain applications requiring clean environments e.g. electron and ion-beam equipment. Means of reducing back-migration have been discussed and it would appear that the best proposition is an adsorption trap, together with a by-pass valve if moist gas is to be frequently handled. We have seen, even with this type of trap, a small amount of rotary pump back-migration occurs (approximately 1 ~o). To prevent its entry into the diffusion pump a certain amount of operational care must be taken. F o r example direct contamination of a vacuum system can occur via the roughingline if on completion of 'roughing' of the system the rotary pump is not switched to the 'backing' mode, and if a rotary pump continuously exhausts an unheated diffusion pump, rotary pump oil can condense on the cold walls of the diffusion pump and flow into the boiler. With proper trapping precautions and provided proper operating procedures are adhered to back-migration from rotary pumps can be reduced to negligible levels. 267
N S Harris: Rotary pump back-migration
Acknowledgements The a u t h o r wishes to express his gratitude to the Directors o f E d w a r d s High Vacuum for permission to publish this review.
References 1 W Gaede, Verhandlungen der Deutschen Physikalischen Gesellschaft VII, Jahrgang, Nr 14/21 (1905). 2 A Lorenz, Trans 5th Nat Vac Symp AVS, p. 79 (1958). 3 L Holland, Vacuum 21, 1971, 45. 4 M J Fulker, M A Baker and L Laurenson, Vacuum 19, 1969, 555. 5 D A Hockly and C S Bull, Vacuum 4, 1954, 40. 6 M A Baker and L Laurenson, Vacuum 16, 1966, 633. 7 M A Baker, L Holland and L Laurenson, Proc 3rd Int Conf Fluid Sealing, p. 45 (1967). s M J Fulker, Vacuum 18, 1968, 445.
268
9 j Olejniczak, Proc 4th Int Vac Congr, p. 290 (1968). lo L Laurenson, L Holland and M A Baker, Syrup Lubrication in Hostile Environments, Inst Mech Engrs, (1969). 11 M A Baker, Private Communication (1973). ~2 L Holland, Vacuum 22, 1972, 234. 13 M A Baker, L Holland and D A G Stanton, J Vac Sci Technol 9, 1972, 412. 14 R A Haefer, Trans 8th Nat Vac Symp, p. 1346 (1961), (also Vakuum-Tech 10, 95). 12 N T M Dennis, Electron Comp 13, 1972, 67. 16 D J Santeler, J Vae Sci TechnolS, 1971, 299. 17 M A Baker, Vacuum 18, 1968, 599. ~8 B R F Kendall, Vacuum 18, 1968, 275. 19 H Wycliffe and B D Power, British Patent 1.248.031 (1971). 2o H Wycliffe, Private Communication (1974). 21 R Brand, British Patent 1.178.265 (1970). 22 L Holland, M A Baker and L Laurenson, British Patent 1.178.203 (1970).
N S Harris, Edwards High Vacuum, Crawley, W Sussex
A paper in our Education Series: The Theory and Practice of Vacuum Science and Technology in Schools and Colleges.
This review deals exclusively with vacuum system contamination which can be attributed to back-migration of rotary pump working fluid. It discusses the origins of this type of contamination and survey methods used to prevent and control it.
1. Introduction
The rotary pump is a positive displacement pump, i.e. a mechanical pump which transports the gas, from the vacuum system, from the inlet side of the pump with the aid of vanes etc. by compressing it and expelling it to atmospheric pressure. The moving parts inside are sealed and lubricated by oil. In its own right the rotary pump has many applications, for example it is used in the manufacture of electric lamps and thermos flasks and in distillation, drying and sputtering. Rotary pumps develop their full speed from atmospheric pressure down to about 1 mbar, the speed then decreasing to zero at their ultimate pressure. They can be used in conjunction with other pumps which are more economical for obtaining high pumping speeds below 10 mbar. Diffusion pumps which operate in the molecular flow region (below 10 -3 mbar) need to be "backed' by a backing p u m p and the rotary pump is the one most widely used. The attainable ultimate vacuum with such a combination can be influenced by the hydrocarbons emanating from the rotary pump oil (i.e. back-migration). Even with two stage rotary pumps, a small amount of back-migration of molecules from the pump interior cannot be avoided. 2. Mode of operation Several forms of mechanical rotary pump exist. Two of the most popular types will be described here. 2.1. Rotary vane pump. Originally designed by Gaede ~ in 1905, the pump is shown diagrammatically in Figure 1. The mechanism consists of a cylindrical housing (the stator), in which an eccentrically positioned rotor fits tightly against a cylindrical seat machined into the stator. The rotor contains spring-loaded vanes which slide in diametrically opposed slots. Thus as the rotor turns the blades are held in contact with the stator wall at all times. During operation gas molecules entering the inlet of the pump pass into the volume created by the eccentric mounting of the rotor in the stator. The crescent-shaped gas volume is then compressed, forcing the outlet valve open and permitting gas discharge into the surrounding atmosphere. In order to produce a seal which will combat the very high pressure difference between suction and discharge compartments the gap between the rotor and stator is required to be of a very small
5TATOR
Figure 1, Rotary vane pump.
order. To complete the seal, and to reduce friction and wear, a thin oil film is continuously maintained between the compartments, by oil circulating into the pump interior through variously arranged oil ways. The seals necessary between the blade ends and stator are made in the same way. The circulating oil is ejected back into the reservoir through the outlet valve together with the pumped gas. The ultimate vacuum is limited by vapour/gas solution-dissolution across the top seal, between suction and discharge compartments, and to outgassing of the lubricating oil. This limitation can be overcome by providing a second pump in series with the first; or as it is known a two stage, instead of a single stage pump (see Figure 2). The first or high vacuum stage is 'backed' by the second or low vacuum stage via an internal transmission port, the discharge valve from the first stage being eliminated. Due to this elimination, the pressure at the discharge port of the first stage is very considerably less than atmospheric pressure. Thus the pressure difference across the top seal in the first stage is much reduced, enabling lower pressures to be achieved, before oil outgassing begins to affect the performance. Oil for lubricating and sealing the first stage is outgassed by the second stage before being passed to the first stage. This together with the sharing of corn-
Vacuum/Volume 28/number 6/7. 0042-207X/78/0701-0261S02.00/0 © Pergamon Press Ltd/Printed in Great Britain
261
N S Harris: Rotary pump back-migration INLET I
HIGN
VACUUM OR
OR SECOND
DISCHARGE
l
STAGE
Figure 2. Two stage rotary vane pump. pression between two stages gives very much lower pressure than does a single stage pump. Frequently the first stage is considerably larger than the second largely for reasons of economy. When this is the case a blow-off valve is provided in the interstage to discharge gas when the throughput of the first stage exceeds that of the second stage. 2.2. Rotary piston pump. The rotary vane pump is the generally adopted design for small pumps (up to about 60 m 3 h 1). Larger capacity pumps tend to be of a different design, the rotary piston mechanism tends to be used more frequently. This is partly due to tradition where a limit on the peripheral speed of steel blades exists due to friction and consequent oil breakdown. With modern blade materials such as 'asbestos reinforced plastic', peripheral speeds can be increased by a factor of four over steel. Figure 3 shows a rotary piston type of pump
ROTATING CAM
~-
--
HOLLOW CYLINDER- -
SHA;~T~
~HOLLOW TONGUE
--
========================= : ' "
".",i.''
5;
5TATOR--
Figure 3. Rotary piston pump. where the piston (a hollow cylinder with a hollow tongue attached) is made to precess around inside a circular stator by means of the rotating cam within it. The operating cycle, i.e. increase volume, isolate and discharge are similar to the basic m o v e m e n t s described in the rotary vane pump. Pumps with speeds up to about 1000 m 3 h-1 are available. A l t h o u g h today the rotary vane type is mainly used for pumps of low and medium pumping speed whilst the rotary piston is preferred for higher pumping speeds, clear cut advantages or disadvantages cannot be formulated for either type. A fundamental difference in design between the two types can be stated as follows: the vane type pump is a force-sealed structure whilst the rotary piston is geometrically sealed. This has consequences as regards manufacture and function of the most important p u m p elements, those responsible for providing a 262
seal between the pressure and suction spaces. The rotary piston pump requires a very careful adjustment of many relative fits in order to obtain the close tolerances required for sealing. The force sealed vane pump automatically adjusts these clearances to the smallest possible amount, a fact which naturally presents manufacturing advantages. One advantage of the rotary vane design is the comparatively small out-of-balance forces consequent upon rotation. The only eccentrically rotating masses are the blades, the centre of mass of a blade pair describing an oval path twice per rotor revolution. With the use of various plastic blades, which are somewhat lighter than steel, this imbalance becomes insignificant. The inevitable out-of-balance forces which exist with rotary piston pumps can be partially balanced by a counter weight on the driving pulley on the shaft end at the expense of introducing some out-of-balance couple or 'rocking' force. Another way of achieving good balance is to mount two pumps on a c o m m o n shaft together in the same casing 2. A small counter weight on the pulley may also be used and substantial freedom from both reciprocating and 'rocking' forces obtained. If the inlets and outlets of the two pumps are connected in parallel a more even gas flow into the pump is also achieved. Many balanced pumps now in manufacture employ three stages served by a c o m m o n shaft.
3. Rotary pump back-migration With a well designed diffusion pumped vacuum system employing good liquid nitrogen traps above the diffusion pump, there can still occur a certain degree of hydrocarbon contamination. The source of this contamination is usually attributed to the diffusion pump, but in most circumstances it is certainly traceable to the mechanical pump oil. Consider the changes in gas flow taking place in the pipeline between the diffusion pump and rotary pump as the rotary pump operates from atmospheric pressure down to its ultimate pressure. The high density of the flowing gas stream in the pipeline, which prevails almost exclusively during initial exhaust (down to about 1 mbar), prevents the mechanical pump oil vapour from diffusing towards the diffusion pump. However, as the pressure falls and the flow changes to 'transitional', the mean free path of the vapour molecules increases, thus molecular collision is reduced and oil vapour may begin to backmigrate from the mechanical pump to the diffusion pump. If a rotary pump in a valveless rotary/diffusion pump system is left running while the diffusion pump is not working, entry of back-migrating oil into the diffusion pump is unhindered. Direct contamination of the vacuum system during roughing through a by-pass line is also another problem. The likelihood of entry into the diffusion pump while it is running is thought to be very small, this obviously depends on whether the pump has an ejector stage working into the backing spout. Holland 3 has suggested that the provision of a small leak into the backing line will help prevention of back-migration into the diffusion pump by keeping the flow in the transitional region. This is probably not an easy practical proposition, care must be taken to ensure that the backing-line pressure does not rise above that of the critical backing pressure of the diffusion pump, otherwise the pumping action of the diffusion pump jets will be destroyed and gross back-streaming will occur. Any contaminating oil which is successful in entering the boiler can enhance the back-migration rate of the diffusion pump. The extent of this enhancement is
N S Harris: Rotary pump back-migration
dependent on the pump design, i.e. whether it is fractionating, non-fractionating, self-purifying or has an ejector stage. Oils used in rotary pumps are usually hydrocarbon mineral oils of much higher average vapour pressure at room temperature than that of diffusion pump fluids. They are not pure compounds but contain molecules with a wide mass range. Molecules with the lightest weight form the most volatile group and evaporate preferentially. Also, as will be discussed, the oils undergo degradation in the rotary pump releasing components of light molecular weight. Thus, the ultimate pressure of a rotary pump is not simply the saturated vapour pressure of the oil at the pump temperature. The use of low-vapour pressure diffusion pump oils in rotary pumps has not proved satisfactory. Fulker e t a l 4 measured the back-migration rates for several different fluids (see Table 1)
Table 1. Back-migration rates from a two stage mechanical pump Oil or fluid
Back-migration rate (p.g cm- 2 min- ~)
Observations
further experimental evidence, for when they measured the electrical resistance of the lubricating film between the pump blade and wall during pump operation, low resistance values indicated direct contact between these surfaces. Further studies have been made using quarts crystal microbalances and high resolution mass spectrometers which have established without doubt that oil molecules are degraded when exposed to rubbing metal surfaces. Baker and Laurenson 6 measured the back-migration rates of rotary pumps using a quartz crystal microbalance. They found that a single stage pump gave a back-migration rate which was higher during the first half-hour of running (10 tzg cm -1 min-1), which then settled down to a steady value of 3 t,g cm -z rain -~, whilst a two stage pump using the same oil gave a steady value of half this rate. They felt this reduction was due to 'stripping' of the oil before entry into the second stage. Baker e t al v used gas liquid chromatography and found that there were no fractions of molecular weight higher than 250 in the back-migration products although the hydrocarbon oil used had an average molecular weight of 530. This suggested that: (a) oil cracking takes place during operation,
Normal uninhibited mineral oil 18 mineral oil + 1% MoS2 4.7 mineral oil + 1% Oleic acid 6.5 Silicone fluid 10 Polyphenyl Ether
13
Apiezon C oil
15
(b) only the light fractions have high enough vapour pressures to contribute to the back-migration.
Pump seized after ½h of operation Difficult to start pump due to high viscosity
using a quartz crystal microbalance. The table shows that a reduction in back-migration is gained by the addition of boundary lubricants (i.e, molybdenum disulphide and oleic acid); however, the results are not conclusive since the addition caused a drop in the running temperature of the pump which will have a corresponding effect on the vapour pressure of the fluid. The lubricating properties of silicone fluid were not sufficient to prevent seizure of the pump, and Apiezon C and polyphenyl ether fluids are limited because of their thermal decomposition into lighter fractions by friction of the rubbing surface within the pump. A zeolite sorption pump cooled with liquid nitrogen could be used as an alternative to back the diffusion pump. Although this avoids the problem of rotary pump back-migration, a sorption pump, unlike a rotary pump, does not have consistent volumetric speeds for different gases and its use on a continual cycle basis is limited. Hence, several authors have studied the source and nature of back-migration in rotary pumps so that suitable counter means could be devised to overcome the problem. Hockley and Bull 5 measured the ultimate pressure of a two stage rotary pump and showed that the ultimate increased as the operating temperature of the pump was raised by a heater, Their results suggested that this pressure rise was not solely due to the presence of oil vapour but that it was due to the appearance of lighter fractions produced by thermal degradation of the oil brought about by local heating of the pump surfaces resulting from friction of the pump blades. This was supported by
Fulker 8 examined the back-migration vapours using a differentially pumped 60 degree mass spectrometer. He confirmed that only the lighter fractions were present up to mass number 161. Although higher fractions could be present they were possibly being cracked in the spectrometer. Olejniczak 9 went one step further with combined gas-liquid chromatographic and mass-spectrometric analysis and identified some organic vapours from heated rotary pump oil and found they mainly contained the C8 to C20 hydrocarbon component. Fulker e t a l 4 constructed an apparatus to simulate sliding conditions in a rotary pump, using a steel rotor with a force of 1 kg, moving at speeds of 5 m s- =. They studied the degradations of various fluids, and observed the production of light fractions. They point out that the frictional rubbing of stainless steel surfaces not only provides heat to produce cracking but can also possibly encourage catalysis. From this wealth of experimental results it can be concluded that the source of back-migration from oil filled rotary pumps is due not only to the vapour pressure of the pump fluid, but that there is an important contribution from thermal degradation. The main factors influencing back-migration rates appear to be interrelated and difficult to separate, i.e. (a) the size of the pump: in general the larger the pump the higher the back-migration rate but correlation is complicated by variations in running temperature; (b) the effect of pump blade velocity: the higher the blade velocity the higher the back-migration rate, however, increased velocity is accompanied by higher pump operating temperature. This is illustrated in Figure 4 taken from a paper by Laurenson e t a l to which shows that the backmigration rate rises with temperature which is raised as the rotary pump speed is increased. Pump operating temperature thus appears to be the dominating factor. Unpublished results of Baker 11 (shown in Figure 5) indicate an approximation to linearity with a log plot of back-migration against 263
N S Harris: Rotary pump back-migration taJ
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1 600 RPM j_ I000 RPM
200 RPM
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110
TEMPERATURE (,°C)
Figure 6. Vapour pressure vs temperature relationship of rotary pump fluids.
20
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-
I
60
80
TEMPERAT (°U C RE )
I00
Figure 5. Back-migration vs pump temperature. a linear plot of temperature, for two different sizes of rotary pump. A modified vapour pressure relationship is thus suggested.
4. Rotary pump fluid Various liquids have been used as working fluids for rotary pumps. Selection has been dependent on various criteria, they should have good lubrication properties, low vapour pressure at room temperature, good thermal stability, non-toxicity, chemical inertness, high flashpoint, reasonable viscosity at room temperature and low cost. Since there is a wide choice of pump fluids available it is usually best practice to use the fluid recommended in the pump manufacturer's catalogue. The types of oil employed for oil-sealed rotary pumps have vapour pressures at least as low as 10 -4 mbar at room temperature. They consist basically of mineral oils which often contain light fractions. Rotary pump oils supplied by various manufacturers are very similar in that they are good quality oils offering low vapour pressures even at high temperatures. Figure 6 shows the vapour pressure vs temperature dependence of typical rotary pump oils available. Numbers 8A, 15 and 16 are high quality straight mineral oils, covering a wide range o f viscosities. Number 8A is a low viscosity oil, 15 a medium 264
viscosity and 16 a high viscosity oil, all having a comparatively low vapour pressure. Numbers 17 and 18 are highly refined oils containing additives that impart detergent, anti-oxidant, antiwear and protective properties of a high order. The presence of inhibitors increases the vapour pressures so that the ultimate vacua obtainable are slightly worse than those obtained with the plain oils. Fulker e t a l 4 reported that the replacement of mineral oils by other lubricant fluids gave disappointing results in terms of the improved performance against the extra cost involved. For example diffusion pump fluids such as Apiezon C gave a backmigration rate and ultimate pressure that tended to rise to the values of the oils normally used, and the polyphenyl ethers (Santovac 5) were found to degrade readily when exposed to rubbing friction. Holland 12 has reported on the use of perfluoro polyethers ('Fomblin'), as a lubricant in a rotary pump. Baker e t a l la found that back-migration rates of 'Fomblin' were only marginally better than a mineral oil; however, the fluid has the valuable property of not polymerizing under electron bombardment. 'Fomblin' fluids are compounds made solely from carbon, fluorine and oxygen and as they do not contain hydrogen or silicon they cannot react to form, respectively, either hydrocarbon or silicaceous surface contaminants when their vapours are degraded. 'Fomblin' fluids are inert to most reactive chemicals and can be used in direct contact with materials such as UF6, fluorine, oxygen, ozone, etc. The fluid is however relatively expensive and is perhaps best used in pumps requiring small oil levels, e.g. rotary pumps having built in oil circulation devices. A grade of 'Fomblin' suitable for rotary pump use is now commercially available. A suitable grade is also available for use in diffusion pumps.
N S Harris: Rotary pump back-migration
5. Back-migration prevention and control
The problem of back-migrating oil vapours from rotary pumps is commonly solved by installing traps in the backing line. These may operate either by condensation on liquid nitrogen cooled surfaces or by adsorption on surface-active materials. One manufacturer used an ingenious trap based on a cold cathode discharge (ion-baffle). This device together with other more commonly used devices will be discussed. 5.1. The ion-baffle foreline trap. The ion-baffle was described by HaefeP 4 and is shown diagrammatically in Figure 7(a). It is a device, simple in operation and maintenance, normally mounted directly above the rotary pump inlet, any back-migrating vapours encounter a water-cooled region containing a baffle plate which deflects the vapours against the cylindrical baffle body. This body constitutes the earthed cathode and the central rod the anode. A cold cathode discharge is set up by the application of a few kV. Provision of external magnets ensures that the electrons emitted from the cathode travel in long spiral paths before reaching the anode. Residual gas ionization is thus very efficient and the cathode is bombarded with ions. The back-migrating oil molecules in contact with the inner surface of the cathode are broken down by ion and electron bombardment and their residues and polymerized compounds deposited on the body wall. When first operated hydrogen and low weight hydrocarbons CH~, C2H, and C3H,, are copiously produced and the measured pressure rises. Over several hours of operation the pressure slowly decreases as the fragmented products are trapped. Figure 7(b) shows mass spectra of the ultimate vacuum of a two stage pump with and without the ion-baffle. It thus appears to be efficient in reducing oil contamination. 5.2. Adsorption traps. Adsorption trapping is a convenient method of reducing oil vapour back-migration from a rotary pump. Oil molecules are trapped as they encounter the physical and chemical forces at the large active surface of an adsorbent in a trap. Baker and Laurenson 6 assessed the effectiveness of different sorbents for back-migration reduction. Table 2 gives a summary of their results for both zeolites (molecular sieves, synthetic dehydrated crystalline alumino silicates) and activated materials such as alumina and charcoal. The greatest reduction was obtained using activated alumina. Activated charcoal also reduced back-migration by 99% but because of the small particle size in which this material was available it presented a low conductance path through the trap and the pumping speed was thus reduced by 95 %. Fulker s using a 60 degree magnetic deflection mass spectrometer showed that activated alumina could remove 99.7% of back-migration vapours from a two
Table 2. Reduction in back-migration by sorbents. Tests carried out using a 9 m 3 h- 1 rotary pump filled with uninhibited oil
Method of reduction
Effect on pump performance
Open tray trap P2Os Zeolite 13X (~ in. pellets) Zeolite 10X (~ in. pellets) Packed column trap (h = 5 in.) Zeolite 3A (~ in. pellets) Zeolite 10X (~- in. pellets) Zeolite 13X (~ in. pellets) Activated alumina (~ in.-~ in. granules) Activated alumina (½ column) Activated charcoal ( ~ in.-~ in. granules)
None
50 %
None
30
Approximately 40 ~ reduction in speed
/ I
65 % 70 % 90700
20 % reduction in speed
99 %
10 700reduction in speed
99 %
95 % reduction in speed
99%
WITI4OgTION BAFFLE TOTALPR,19x lO'3rnba~
=
~.
'~L~I'-'~ ("0~ ~"
Slow pump down required 50%
stage rotary pump, even when partially saturated with water. A 13X molecular sieve trap although effective when dry (removing 99.8% of back-migrating vapours), deteriorated when wet and did in fact release fractions as masses 85 and 86, thought to originate from previously sorbed contaminants displaced by water vapour. Figure 8 shows mass spectra taken when sampling the back-migration vapour and gas from a pump with and without an alumina trap. As can be seen almost all organic molecules with a molecular weight over 80 have been removed. Sorbed water vapour can be driven off from the sorbent material by heating to about 300°C. This is best done in an oven outside the system. Unfortunately the organic molecules remain and consequent periodic replacement of the alumina is thus necessary. Where moist gas has to be handled frequently, it is advisable to by-pass the trap during the rough pumping period. A commercially available automatic by-pass valve is illustrated in Figure 9. As shown the valve is held in the by-pass position at atmospheric pressure by a compression spring, with the vacuum system port communicating directly to the mechanical roughing
1420 C2H... CsH... _ 5kV
Efficiency (percentage decrease in backstreaming)
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WITH iON BAFFLE TOTALPR.I x lO'*mk~.: IVt/P.
Figure 7. (a) Diagrammatic arrangement of the ion-baffle. (b) Mass spectra of two-stage pump and without the ion-baffle. 265
N S Harris." Rotary pump back-migration 10-5
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0 !lrll!
1
(b) WITHOUTBY-PASSVALVE
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Figure 10. Ultimate vacuum after a number of pump-down cycles.
MASS NUMBER
Figure 8. Mass spectra of back-migration vapour. (a) Without trap.
(b) With trap.
~HAFT
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tRAP
VALVE PLATE---_
~LVE SEAT
~LVE BODY FROM VACUU~
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Figure 9. Automatic by-pass valve. pump port, i.e. the foreline trap is by-passed. Changeover occurs progressively and is completed when the system pressure reaches 40 m b a r (approximately). The decreased pressure is sensed by the inner (lower) face of the diaphragm which will actuate the valve to isolate the mechanical roughing p u m p and divert the flow through the foreline trap. The effectiveness of such a trap and by-pass combination in reducing the water adsorption has been described by Dennis 15. Figure 10 shows the ultimate vacuum attainable after a number of pump-down cycles on a 115 l. volume system pumped by a two-stage 266
20 m 3 h - 1 rotary pump. Without a by-pass valve, the ultimate starts at 10 - z m b a r and rises to above 10-1 m b a r by the 14th cycle. With a by-pass valve the initial vacuum is less than 10 -2 mbar and rises to a little above 10-2 mbar after the same number of operations. This value had risen to 6 l0 -z m b a r at the end of 100 cycles after which no further increase was apparent. Baker e t a l ~3 have suggested a method for the preferential chemisorption of hydrocarbons on large surface area active metals. It is believed that the trap would avoid or reduce water vapour physi-sorption. The active metal sorbent (e.g. nickel) is chemically deposited onto alumina which has a large surface area, giving an active area of several hundred m2g -t . When used for rotary pump trapping the active material is raised in temperature to enhance chemisorption of oil vapour and reduce the physi-sorption of water vapour. Preliminary tests suggest that this type of trapping was better than a comparative trap filled with activated alumina. 5.3. Liquid nitrogen cooled traps. This type of trapping is in principle the same as that used for the reduction of backstreaming from diffusion pumps. Investigations by Baker and Laurenson 6 on the efficiency of a low conductance liquid nitrogen trap above a rotary pump, indicate a better than 97 ~/ reduction in measured back-migration with no effect on p u m p performance. (N.B. about 15 times worse than the adsorption trap i.e. the adsorption trap using 13X activated alumina allows 0,2 % of back-migrating vapour through the trap compared with 3 % for the liquid nitrogen trap just described.) One drawback is that a means of automatically topping up the trap with liquid nitrogen must be provided. It was shown that if the trap was allowed to warm up at r o o m temperature re-emission of trapped vapours became a great problem. In fact if the warming up was accelerated, e.g. by the insertion of an air-line in the trap, the evolution rate was so high as to be able to create a pressure above that at which an oil diffusion p u m p ca1~ be backed (i.e. the diffusion pump will stall). It is worth noting that temperatures lower than 100°C seem to be necessary to trap effectively rotary pump back-migration, since light fractions from degraded oil e.g. CH2 cannot be condensed unless a refrigerated trap operates at liquid nitrogen temperature.
N S Harris; Rotary pump back-migration
5.4. Roughing-line purge combined with liquid nitrogen trap. As discussed the liquid nitrogen trap works well provided it is kept cold. If the trap returns to r o o m temperature, oil is free to migrate to the high vacuum side. This problem was overcome by Santeler 16 who provided a deliberate leak of high purity nitrogen gas into the roughing-line of the system. The purge can be arranged to be on whenever the trap is warm. The flow rate is set to maintain viscous flow in the roughing-line. Carbon dioxide, water vapour and other contaminants are thus kept from back-migrating by the flowing gas purge. 5.5. Thermoelectrically cooled foreline trap. When an electric current flows in a closed circuit of two different conductors, heat energy is absorbed at one junction of the couple and given out at the other. This is the well-known Peltier effect. For metallic conductors the temperature difference between the junctions is very small, but with certain semiconductor materials the temperature difference becomes large enough to be of practical use for cooling purposes. A typical cooling module consists of a number of p- and n-type semiconductor couples mounted thermally in parallel and electrically in series. The cold junctions are clamped against the object to be cooled and the hot junctions on the opposite face are maintained in contact with a heat sink, which is cooled by air convection or water circulation. Baker ~v used such a thermoelectrically cooled metal baffle to reduce rotary pump back-migration. At a minimum temperature of - - 4 0 ' C he reported a 75 % drop in the value of back-migration compared to the value measured at ambient temperature. These results he felt were disappointing considering a similar baffle mounted above a diffusion pump could reduce back-streaming by two orders. He ascribed the ineffectiveness of the foreline baffle to the presence of light oil fractions which cannot be condensed at 40:C. Kendall ~8 reported successful tests using a similar baffle whose surfaces were coated with an artificial zeolite. This provides a very large effective surface area to which the oil clings firmly in the form of a film a few molecules thick. Such films have significantly lower vapour pressures than the liquid in bulk. A reversing switch was included so that at intervals of a few weeks the adsorbed vapours could be driven off by reversing the current through the module. 5.6. Dry stage rotary pumps. Obviously if rotary pumps could be used without lubricating oil, decomposition products would not be formed. Wycliffe and Power ~9 described a two stage rotary pump comprising a lubricated low vacuum stage and a 'dry' high vacuum stage. The tendency for oil vapours to backmigrate into the high vacuum system was reduced. Wycliffe 2° has indicated that the back-migration rate from such a dry stage pump was 10 times lower than that of an equivalent conventional two stage rotary pump. The main disadvantage of any dry stage p u m p is that of a considerable loss of 'compression ratio'. A conventional rotary pump can have a compression ratio of 10S:l across a single stage because o f the efficient sealing effect of the lubricating oil. With a dry stage pump, whether it operates with self-lubricating vane materials or as in the pump just described, with a small gap between rotor and stator, it is usual to achieve compression ratios of a much lower order. It is therefore usually necessary to run a dry stage in series with a conventional type rotary pump. Brand 2t has described a rotary p u m p which consists of a dry high vacuum stage using solid lubricating materials backed by a conventional oil sealed
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01L
INTERSTAGETRAPTO REDUCE CONTAMINATIONREACHINGDRY STAGE
Figure 11. Two stage rotary pump with dry high vacuum stage. stage. Interstage gas purge can be used to reduce contamination reaching the dry stage (see Figure 11). Although much has been done to develop self-lubricating materials, their performance generally does not compare with that of fluid lubricated types. Because of the absence of oxygen under vacuum conditions, the metal oxides, which prevent local welding of metal surfaces, cannot form. Substituted polymeric materials are heated by rubbing friction and evaporate and decompose. Graphite, due to the absence of chemisorbed oxygen in vacuum, develops a high coefficient of friction. Composite materials are now available, such as MoS2-filled carbon and plastic materials, and carbon fibre filled PTFE. Figure I I also illustrates a similar pump arrangement to that of Brand (devised by Holland et al22), having an interstage sorbent trap to reduce contamination, back-migration should be reduced to that of the solid lubricant used in the dry stage. Baker et a113 used a liquid nitrogen probe in the high vacuum inlet of the dry stage of this pump, to collect any condensable contaminants emitted. N o n e were detected after several hours of running. It appears that at present it is not possible to match the performance and cost of an oil-sealed rotary pump plus sorbent trap by making pumps with solid lubricants of low desorption rate. Conclusions We have seen that oil-sealed mechanical rotary pumps can produce contamination from the pump fluid. This contamination can be a problem in certain applications requiring clean environments e.g. electron and ion-beam equipment. Means of reducing back-migration have been discussed and it would appear that the best proposition is an adsorption trap, together with a by-pass valve if moist gas is to be frequently handled. We have seen, even with this type of trap, a small amount of rotary pump back-migration occurs (approximately 1 ~o). To prevent its entry into the diffusion pump a certain amount of operational care must be taken. F o r example direct contamination of a vacuum system can occur via the roughingline if on completion of 'roughing' of the system the rotary pump is not switched to the 'backing' mode, and if a rotary pump continuously exhausts an unheated diffusion pump, rotary pump oil can condense on the cold walls of the diffusion pump and flow into the boiler. With proper trapping precautions and provided proper operating procedures are adhered to back-migration from rotary pumps can be reduced to negligible levels. 267
N S Harris: Rotary pump back-migration
Acknowledgements The a u t h o r wishes to express his gratitude to the Directors o f E d w a r d s High Vacuum for permission to publish this review.
References 1 W Gaede, Verhandlungen der Deutschen Physikalischen Gesellschaft VII, Jahrgang, Nr 14/21 (1905). 2 A Lorenz, Trans 5th Nat Vac Symp AVS, p. 79 (1958). 3 L Holland, Vacuum 21, 1971, 45. 4 M J Fulker, M A Baker and L Laurenson, Vacuum 19, 1969, 555. 5 D A Hockly and C S Bull, Vacuum 4, 1954, 40. 6 M A Baker and L Laurenson, Vacuum 16, 1966, 633. 7 M A Baker, L Holland and L Laurenson, Proc 3rd Int Conf Fluid Sealing, p. 45 (1967). s M J Fulker, Vacuum 18, 1968, 445.
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9 j Olejniczak, Proc 4th Int Vac Congr, p. 290 (1968). lo L Laurenson, L Holland and M A Baker, Syrup Lubrication in Hostile Environments, Inst Mech Engrs, (1969). 11 M A Baker, Private Communication (1973). ~2 L Holland, Vacuum 22, 1972, 234. 13 M A Baker, L Holland and D A G Stanton, J Vac Sci Technol 9, 1972, 412. 14 R A Haefer, Trans 8th Nat Vac Symp, p. 1346 (1961), (also Vakuum-Tech 10, 95). 12 N T M Dennis, Electron Comp 13, 1972, 67. 16 D J Santeler, J Vae Sci TechnolS, 1971, 299. 17 M A Baker, Vacuum 18, 1968, 599. ~8 B R F Kendall, Vacuum 18, 1968, 275. 19 H Wycliffe and B D Power, British Patent 1.248.031 (1971). 2o H Wycliffe, Private Communication (1974). 21 R Brand, British Patent 1.178.265 (1970). 22 L Holland, M A Baker and L Laurenson, British Patent 1.178.203 (1970).