US 3283479 A
Description (OCR text may contain errors)
NOV. 8, 1966 ER ETAL 3,283,479
COMBINATION TRAP PUMP I Filed July 9, 1964 2 Sheets-Sheet l INVENTORS THOMAS H. BATZER ROBERT H. MCFARLAND ATTORNEY Nov. 8, 1966 ER ET AL 3,283,479
COMBINATION TRAP PUMP Filed July 9, 1964 2 Sheets-Sheet 2 INVENTORS THOMAS H. BATZER ROBERT H. MCFARLAND ATTORNEY United States Patent 3,283,479 COMBINATION TRAP PUMP Thomas H. Batzer and Robert H. McFarland, Livermore, Califi, assignors to The United States of America as represented by the United States Atomic Energy Commission Filed July 9, 1964, Ser. No. 381,603 13 Claims. (Cl. 55316) The invention described herein was made in the course of, or under, Contract W-7405ENG48 with the United States Atomic Energy Commission.
This invention relates to traps and pumps as utilized in creating very high vacuums. More particularly, it relates to a combination cold trap and sorption pump which has come to be known by the term trump.
The creation of vacuum in the range of to 10- Torr necessitated by recent advances in various technologies involves requirements that are much more exacting a vacuum of lesser degree as employed previously. The pumps and associated machinery of such a vacuum producing system must meet high standards of performance and they must be so designed that contaminants do not reach the vacuum chamber. The system must be capable of continuous operation without violation of the integrity of the vacuum within the system. The process for producing the vacuum and the machinery employed must be simple to reduce the possibilities for mechanical failure.
One of the most universal methods of obtaining high vacuums involves the use a diffusion pump. A pump of this type does not have the ability to remove gas molecules from the vacuum system by itself, but concentrates them for removal by a second pump. This second pump, called a backing pump, is usually of the mechanical or sorption variety, .both of which have certain disadvantages which will hereinafter become apparent.
A basic problem in producing ultra-high vacuum is the occurrence of large pressure fluctuations called bursts. It is thought that the bursts, which register as fast rising excursions of the pressure gauges, arise from mechanical pump oil molecules within the diffusion pump boiler cracking into lighter fractions due to the high boiler temperature and low thermal stability of the oil molecules. This presupposes that the system is tight. Pressure bursts sometimes appear when O-ring seals are employed. It is further theorized that these mechanical pump oil molecules invade the diffusion pump boiler by backstreaming from mechanical forepressure pumps used to back the diffusion pump.
One conventional way of eliminating pressure bursts is to employ two diffusion pumps in series backed by a mechanical pump. Then the bulk of the back-streaming mechanical oil cracks in the intermediate stage backing diffusion pump so that the primary diffusion pump, working on the vacuum chamber, only encounters the lighter cracked oil fractions. These cracked fractions are in the form of non-condensible gases, not oil vapor. Therefore, no further cracking occurs and the gas cannot penetrate the primary diffusion pump jets. Other bulky multicomponent complicated apparatus arrangements have also been utilized in attempts to eliminate bursts. However, there exists a pressing need for a compact simplified apparatus to obtain high vacuum reliably Without bursts.
The present invention provides a combination trap and pump, or trump in a form which eliminates offending and unnecessary machinery from .a vacuum pumping system. Such a trap-pump or trump is comprised of a gastight outer casing provided with gas inlet and outlet means which are spatially intercommunicated by the volume within the interior of said casing so that gas can pass Patented Nov. 8, 1966 therebetween. A large gas sorptive capacity means is disposed in the volume for contacting gas passing therethrough. This sorptive means has the capacity for sorbing large quantities of gas in relation to the volume of the outer casing. An oil molecule sorptive metal surface means is also disposed in the volume to intercept oil molecules entering said inlet means and is in spatial intercommunication with said gas sorptive means. This combination trap-pump provides a new and valuable device which solves many of the problems encountered heretofore in producing high vacuums. For example, it is a simple, eifective, inexpensive device virtually free of sources of possible mechanical failure. It can be regenerated without removal from the pumping system and performs very eflfectively in either capacity as a trap 'and/ or a pump. It has no moving parts to wear out and is also noiseless and vibrationless, very unlike the mechanical pumps it replaces. eliminates the need for both the mechanical backing pump and the intermediate stage diffusion pump, thereby reducing the necessary machinery from two expensive complicated vacuum pumps to one relatively inexpensive simple reliable machine. In addition, the trump can be used by itself to effect high vacuum where smaller vacuum chambers are involved.
A conventional sorption pump can be utilizedto back a diffusion pump, but the problem there is that they cannot be pumped through, i.e., into and out of, and they must be degassed at atmospheric pressure. The present invention is also a sorption pump, but one which can be regenerated conveniently in working position, at low pressure, unlike most sorption pumps, and in addition, performs effectively as a trap to prevent backstreaming mechanical pump oil molecules from penetrating further in the vacuum pumping system.
Accordingly, it is an object of the present invention to provide a sorption pump which will independently back a difiusion pump.
It is another object of the invention to provide an efficient sorption pump which can be regenerated without removing it from a pumping system.
It is afurther object of the invention to provide a sorption pump which also functions effectively as a trap.
It is yet another object of the invention to provide an eflicient combination trap-pump of simple yet effective design to prevent the possibility of contaminating the vacu- It is still a further object of the invention to provide a combination trap-pump, economical in cost, which will perform effectively for extended periods of time without failure.
A general advance in the art and other objects will appear hereinafter are also contemplated.
The combination trap-pump will be described with reference to the accompanying drawings, of which:
FIGURE 1 is asectional view of a generic first embodiment;
FIGURE 2 is a sectional view of a second and preferred embodiment; and
FIGURES 3 and 4 are schematic diagrams of the device of the invention arranged in a pumping system.
Referring to FIGURE l, shown therein is the simplified, although highly successful, working embodiment 10 of the trap-pump of the invention, also termed trump hereinafter. An external or outer casing 11 is provided as a cylindrical tube. This configuration was chosen hecause of its structural rigidity and ability to withstand pressure when the interior is evacuated. Any configuration, preferably self-sustaining, of outer casing w il-l sudiice so long as it is gas tight and will withstand atmospheric pressure and is constructed of vacuum service material The present invention adapted for immersion in cryogenic. coolant. Type 304 stainless steel has successfully been used for the outer casing, but any material could be employed which is not porous, cleans up easily, does not out-gas significantly, and can withstand extreme temperature cycling. End cover 12 is mated to the outer casing with a vacuumtight seal and has tubes 13 and 14 projecting therethrough in sealed relation. Tubes 13, 14 are open at both ends and the exterior ends which terminate outside the trump casing are formed for coupling by means of vacuum tight seals to a vacuum pumping system. The interior end of tube 13 extends into the trump casing 11 to approach vacuum tight end cover 16 sealing the other end of easing l 11. The interior end of the tube 13 is out (man angle for purposes of assembly. Tube 14 extends only shortly into the trump casing past the end cover 12. These different lengths of inlet and outlet tubes expose the gas to the greatest portion of the contents of the trump by forcing it to traverse, in effect, from one end of the casing i to the other. End cover 16 is removable for initial packing and .to enable the contents of the trump to be easily I replaced. 1 Inside the outer casing 11 a tightly packed column of copper turnings 17, commonly called copper wool, extends along an arcuate or circuitous path from inside the internally projecting end of tube 14 to end cover 16 and then into. the internally projecting end of tube 13. :A-lso, a cross layer of copper wool 18 is located inside the outer casing 11 approximately midway betweenthe lend covers 12 landl6 to provide a thermal conductivity :path which spans the cross section. This thermal path serves to distribute and reduce thermal gradients within 1 the trump and aids in reducing the cool down and bakeout times. 1 Other wool or tunings 17 than copper could be used ;such as stainlesssteel, bronze, aluminum, etc., but copper is used with preference sincein a clean condition it has nan unusual or high aflinity for oil molecules. One of ithe, main values of the present invention is its ability to ,trap backstreaming oil vapor from mechanical vacuum pumps by using a large surface area of clean copper to capture the oil molecules. The location of the trump in the pumping system, and its internal design, ensures conitact between the ba-ckstreaming oil molecules and the ,clean copper. This causes the effective trapping of oil molecules, and yet permits, and does not hinder, reverse flow of the :gas molecules being evacuated. Molecular isieve material 19, in bead form, is disposed in the remaining space within casing 11. The copper wool column 17 3 packed tightly into the ends of the tubes 13, 14 prevents jthe beads of molecular sieve material 19 from entering therein. The copper wool also keeps the sieve material from being blown out of the trump through the tubes 13, 5214 if pressure is quickly let into the trump. The copper wool 17, in addition to trapping backstreaming oil vapor from the mechanical vacuum pump, provides for conduct- ;ance of heat to and from the sieves 19 since the con- 'ductance' of the copper wool is greater than that of ,the sieve material.
Molecular sieve material 19, sometimes called artificial fzeolite pellets or beads, which fills the remaining volume lot the trump, i.e., from end cover 12 to the copper wool layer 18 and from the copper wol layer 18 to the copper wool adjacent end cover 16, is in communication with the I ;!gas stream in the. copper wool. 7 l The molecular sieve material or sieves which are the "commercial product of the Union Carbide Corporation marketed under the Linde trademark are suitable for ipresent purposes; Such sieve materials are alkali metal aluminosilicates quite similar to many natural clays and feldspars. Three types are available: 4A, 5A, and 13X. The generalformula for type 4A is 0.96:0.04 Na O- 1.00 fAl O -l.92i0.09 SiO -H O. TypeSA is produced thorn ,type 4A through ion exchange of about 75% of the sodium ions for calcium ions. These sieves are a new series of commercial absorbents. The crystals are highly .porous with pores only 16-20 billionths of an inchin diameter and of uniform size. Other materials which could satisfactorily be substituted for the zeolite are activated alumina and activated charcoal, the latter of the two being particularly effective.
The whole trump of FIGURE 1 is immersed in a cryogenic liquid, preferably liquid nitrogen during operation as either a trap or a pump. The low'temperat'ure of the liquid increases the gas sorbing ability of the molecular sieves.
Utilizing a trump, instead of a pump of the sorption trump, instead of a mechanical pump as a backing pump for a diffusion pump, is that a trump eleminates the possibility of backstreaming -oil vapor, an undesirable characteristic which often accompanies mechanical pumping. Preferred embodiment 20 of the invention is depicted in FIGURE 2 to which reference is now made..
A central stainless steel tube 21 is used therein cforcontaining a cryogenic liquidfor cooling the molecular sieves. Thetube is made of relatively thin wall construction to reduce consumption of the cooling liquid.
Accordion structure ribs 22 provide structural strength to withstand 'the pressure difierentials involved due to the thin wall construction. An alternative embodiment could have the cryogenic liquid coaxially' surrounding .the
molecular sieves and copper wool with the gas flow through the center, but that arrangement would be less efiicient in the use of the cryogenic =liquid. A copper sheath 23 depends from and coaxially surrounds. the.
central tube in spaced relation forming an annular .void between the. tube 21 and the sheath. The sheath is1attached to or in contact with the, central tube 21 as a.
heat transfer to the sheath 23. The sheath has a large number of perforations 26 through it to permit the passage of gas flow therethrough. A copper screen 27. lines the inside of the sheath 23 to constrain molecular :sieve. material 28 which fills theannular void between the central tube 21 and the sheath 23.
A polished copper skirt 29 depends from the central tube 21jtrom a hard solder joint 31 and coaxial ly surrounds the inner sheath 23 in spaced relation thereto,
thereby permitting the free passage of gas flow up into the skirt 29 to contact the sieves through the holes 26. The 'pohshed copper skirt 29 provides a heat shield reducing the heat load from Warm casing wall 32 vto the zeolite bed.
A re-entrant truncated conical copper partition 33 seals the lower end of the central tube 21. A copper truncated cone shaped skirt 34 forms a nose on outlet. tube; 35
and is disposed in parallel relation to the re-entrant copper partition 33 by copper spacing and heat transfer ribs 37 which are hard soldered to the reentrant partition 33.
Copper wool 38 fills the space between the remnant partition 33,-the flanged end 34, and the ribs 37. The a copper wool eflects a cold condensing surface which in-' tercepts backstreaming oil vapor entering the trumpv through the outlet tube 35 which as usually operated connects with a mechanical fore-pressure pump. The whole unit is enclosed within a gas tight stainless steel'outer casing 32. One end of the casing may-be sealed as at 39 to a flanged removable cover, e.'g., to a flange of a seal 41 as described in US. patent application Serial No.
236,750, filed December '1, 1962, by Thomas H. =Batzer,:
nection of the trump 20 with :a vacuum chamber or a diflusion pump.
'I hree modes of operation employing apparatus of the present invention are contemplated. The following explanation is made with reference to FIGURE 3. Therein is shown a simple vacuum pumping system utilizing a trump. A vacuum chamber is connected to the trump through valve B, and the trump is connected to a mechanical pump through valve A.
In the first mode of operation the trump is closed off from the vacuum chamber by valve B. Then the trump is cleaned up by simultaneous degassing and baking out. This involves opening valve A and using the mechanical pump to evacuate the trump while simultaneously heating it at around 350 C. During cleanup, the trump traps backstreaming oil molecules from the mechanical pump. When the trump reaches about 10 microns or less of pressure at approximately 350 C., the mechanical pump used during cleaning is valved off by closing valve A. The trump is then filled with a cryogenic liquid (the embodiment 10 show by FIGURE 1 is immersed in the cryogenic liquid). The trump is then valved open to the vacuum chamber by opening valve B, and it then pump the gases out of the chamber :by sorption. This mode is usually employed on smaller volumes, in comparison with the trump capacity. The trump in this mode of operation and under these conditions will, by itself, create .a vacuum of about 0.1 micron, but usually the evacuation process is stopped at around 5 or 6 microns. Then the mechanical pump is started and valved in by opening valve A to remove the remaining inert gases, e.-g., helium, through the trump. During this operation the trump acts as a trap and captures backstreaming oil vapor from the mechanical pump. Utilizing this first mode of operation for the trump provides very clean vacuums and produces excellent results such as pressures in the range of l of mercury.
It should be pointed out that it is not necessary to fill the trump with liquid nitrogen for it to operate as a pump. After bake out the mechanical pump could be valved off and the trump opened to the volume to be evacuate-d. During cool down to room temperature, the will pump on a vacuum chamber, and it can be made to pump further by filling it first with water and then with ice Water, or further still by packing it with Dry Ice, or yet further still by successively cooling the trump to further lower temperatures with various cryogenic liquids. *It is therefore not necessary to have liquid nitrogen available to evacuate a volume to moderate vacuums with the trump.
In the second mode of operation the trump is first cleaned up as in the previous modeof operation. The mechanical pump is valved off by closing valve A and the trump is filled with liquid nitrogen. The trump is then valve open thruogh valve B to the vacuum chamber and used to rough out, or partially evacuate the chamber. Then the trump is valved oil" from the chamber by closing valve B and it is cleaned up as before by a second bake out and evacuation. The mechanical pump is then valved off by closing valve A and the trump filled with liquid nitrogen and again opened to the vacuum chamber through valve B. The diffusion pump is started and the trump then acts as a backing pump for the diffusion pump. This is for larger volume evacuations as compared with the capacity of the trump. Using this mode of operation guarantees no b'ackstreaming mechanical pump oil vapor, but it does not necessarily create lower base pressures than obtainable by the third mode of operation.
The third mode of operation is the fastest way to produce very clean ultra-high vacuums and is the method most usually employed. It works well with all size volume vacuum chambers. Valve B is closed and valve A is opened. The trump is evacuated up to valve B with the mechanical pump and then cooled with liquid nitrogen. The vacuum chamber is up to standard air pressure. Valve B is then opened and the mechanical pump is operated to rough the system down to approximately 1 micron, while the trump trap backstreaming oil vapor from the mechanical pump. After the system is roughed, valve B between the trump and the vacuum chamber is closed. The trump is then cleaned up by bake out and evacuation, as described before. The mechanical pump is then valved olf by valve A, the trump is cooled with liquid nitrogen, the diffusion pump is started, and the trump is valved in, by opening B, to back the dilfusion pump and pumping then proceeds as in the second mode.
These three modes of operation provide an extremely clean system and eliminate the mechanical vacuum pump from the ultrahigh vacuum pumping to provide a smooth vacuum pressure without pressure derivation. By backing properly trapped dilfusion pumps with trumps, base pressures of 2 l0 mm. of mercury have been produced. Before the development of the trump these results were very difiicult to achieve and more difficult to maintain. The vtrump is responsible for simplifying the method of producing ultra-high vacuums, and making it less susceptible to mechanical failure.
An enlightened employment for utilizing the trump places two trumps in parallel between the mechanical forepressure vacuum pump and the diffusion pump in the vacuum chamber. FIGURE 4 presents this arrangement schematically. Thus, while one trump is backing the difiusion pump, the other can be undergoing clean up by evacuation and bake out. This is the preferred embodiment in modes 2 and 3. A faster method of evacuation is thereby provided and a way to maintain an ultra-high vacuum indefinitely is achieved.
Although several embodiments of the invention are possible, and alternatives have been described, these are merely illustrative and various modifications can be made Without departing from the spirit and scope thereof. It is to be understood, however, that this invention is not limited to the specific embodimens thereof except as encompassed in the following claims.
What is claimed is:
1. A combination trap-pump for producing vacuums comprising;
(a) a gas tight outer casing having inlet and outlet means spatially intercommunicated by the volume within said casing for passage of gas therebetween,
(b) means providing a large gas sorptive capacity diposed in said volume for contacting gas passing therethrough, and
(c) previous oil molecule sorptive metal surface means disposed in said casing to competely transect the gas passage volume therein in at least the oulet means region thereof for interception of oil molecules entering from said outlet means and being in gas passage intercommunication with said gas sorptive means of paragraph (b).
2. The trap-pump of claim 1 wherein the gas sorptive means is selected from the class consisting of artificial zeolites, activated charcoal, and activated alumina.
3. The trap-pump of claim 1 wherein the metal sorptive means is selected from the class consisting of copper, bronze, stainles steel, and aluminum.
4. The trap-pump of claim 1 wherein;
(a) the gas sorptive means comprises artificial molecular sieve material, and
(b) the sorptive metal surface means comprises clean copper surfaces.
5. A combination trap and pump for producing vacuums comprising;
(a) a gas tight cute-r casing capable of withstanding extreme temperature cycling,
(b) at least first and second open tubes projecting through and terminating inside said outer casing, said tubes comprising means for introducing and exhausting gas from said casing,
(c) copper wool forming a path inside said outer casing between the interior terminations of said first and sec ond tubes, and
(d) molecular sieve material filling the remaining volume of said outer casing.
6. A combination trap-pump for producing vacuums omprising;
(a) a tubular gas tight outer casing capable of withstanding extreme temperature cycles,
(b) first and second end covers for the ends of said casing,
(c) a first open tube projecting in sealed relation through the first of said end covers and terminating inside said outer casing near the second of said end covers,
((1) a second open tube projecting in sealed relation through the first of said end covers into said outer casing and terminating near said first end cover,
(e) copper wool disposed inside said outer casing forming a path extending from inside the end of said first tube to terminate inside the end of said second tube,
(f) a layer of copper wool extending transversely across the interior cross section of said outer casing, and
(g) molecular sieve material filling the remaining volume within said outer casing.
7. A combination trap and pump for producing vacuums comprising;
(a) a gas tight outer casing having inlet and outle means spatially intercommunicated by the volume within said casing defining a gas flow passage therebetween and having a chamber for containing a coolant defined by a re-entrant receptable wall portion projecting within said volume,
(b) particulate gas sorptive material arranged in heat exchange relation with said receptacle wall portion and in gas flow communication with said gas flow passage Within said casing, and
(c) pervious sorptive metal surface means disposed in said casing to completely transect said gas flow passage in a region between said outlet means and said particulate gas sorptive means.
8. The trap and pump of claim 7 wherein (a) the gas sorptive means comprises molecular sieves,
(b) the sorptive metal surface means comprises clean copper.
9. A combination trap and pump for producing vacums comprising (a). a gas tight outer casing having inlet and outlet means spatially intercommunicated by the volume within said casing to define a gas flow passage therebetween and provided with a re-entrant partition defining a central chamber for holding a cryogenic liquid,
(b) molecular sieve material surrounding said partition in thermal communication with said central chamber and gas flow communication with said gas flow passage in said casing,
(c) means for containing said molecular sieve in thermal communication around said central chamber and for permitting the passage of gas molecules therethrough, and
(d). clean pervious copper sorptive surface means disposed in said casing to completely intersect said gas flow passage in a region between said outlet means and said molecular sieve material.
10. The trap and pump of claim 9 wherein (a) said seive containing means consists of screen material, and
(b) said clean copper sorptive surface means consists of copper wool.
ums comprising (a) a gas tight outer casing having inlet and oulet means spatially intercommunicated by the volume within said casing and provided with a partition de fining a central'chamber for holding a cryogenic liquid,
(b) molecular sieve material surrounding and in therma1 communication With the said central chamber, (6) copper screen material for containing said sieves in thermal communication around said central chamber and for permitting the passage of gas molecules therethrough,
(d) polished copper sheet contiguous in spaced relation around said screen material in the path of gas fiow, and
(e) copper wool surrounding said outlet means dis:
posed in the path of gas flow and in heat exchange relation with said central chamber. 12. A combination trap and pump for producing vacuums comprising (a) a generally cylindrical cold wall tube having top and bottom ends and partitioned central well portion,
(b) molecular sieve material surrounding said cen-,
tral portion of said tube and constrained therearound by copper screen material, (c) a polished copper skirt depending from said cylindical tube in spaced relation around said constrained 3 molecular sieve,
(d) it re-entrant partition sealing the bottom end of said tu e,
(e) an oulet tube having a flanged ,end formed in a skirt parallel in spaced relation With;said partition,
(f) copper wool disposed between said re-ent-rant partition and said skirt of said outlet tube,
(g) a double ended gas tight outer casing having the first end sealed in gas tight relation around the top end of said cold Wall tube said casing enclosing said topper skirt with the second end sealed in gas tight relation around said outlet tube, and
(h) an inlet tube communicating with the interior of said outer casing.
13. A combination trap and pump for producing .vac-r ums comprising (a) a stainless steel accordioned relatively thin walled cylindrical cold wall partion tube,
(b) a coaxial cylindrical copper sheath depending from and surrounding said partition tube in spaced relation thereto and having multiple perforations therethrough and attached to said tube with a thermal transfer joint,
(c) molecular sieve material, disposed between said sheath and said partition tube, (d) copper screen adjacent to and inside said sheath and constraining said sieve material against said par (i) a double ended gas tight stainless steel outer casing having the first end sealed in gas tight relation around the upper end of said cold wall partition tube, the other end of said casing sealed ingas tight relation around said outlet tube enclosing said copper skirt of said outlet tube, said casing enclosing said copper skirt depending from said partition tube, and
9 10 (j) an inlet tube communicating with the interior of 3,172,745 3/1965 Needham et a1. 55 269 said outer casing. 7 3,200,569 8/ 1965 Wheeler 55208 References Cited by the Examiner OTHER REFERENCES UNITED STATES PATENTS 5 German printed application No. 1,110,356, July 1961.
2,465,229 3/1949 Hipple 55-387 X 2,841,323 7/1958 Lindenblad 230-69 REUBEN FRIEDMAN Exammer- 3,081,068 3/ 1963 Milleron 55269 X J. ADEE, Assistant Examiner.