US2944732A

US2944732A - Vacuum pumps

Info

Publication number: US2944732A
Application number: US633854A
Authority: US
Inventors: Lorenz Albert
Original assignee: WC Heraus GmbH and Co KG
Current assignee: WC Heraus GmbH and Co KG
Priority date: 1955-02-24
Filing date: 1957-01-14
Publication date: 1960-07-12
Anticipated expiration: 1977-07-12

Description

July 12, 1960 A, L R N 2,944,732

VACUUM PUMPS OriginaI Filed Feb. 24, 1955 AL BERT L GRENZ A TTOPNE Y5 7 2,944,732 VACUUM PUMPS Albert Lorenz, Hanan (Main), Germany, assignor to W. C. Heraeus, G.n1.b.H., Hanan, Germany Original application Feb. 24, 1955, ser. No.; 490,316. Divided and this application Jan; 14, 1957, Ser. No. 633,854 a 1 Claim. (Cl. 230-141) This invention relates to improved Roots type high vacuum pumps, and is a division of my co-pending application Serial No.. 490,316, filed February 24, 1955, now

abandoned. e v

Roots type pumps (also known as Connersville blowers) have been in use for many years, and are described in technical literature such as Industrial Chemistry, by E. R. Riegel, Reinhold Publishing Corporation, pp. 696-7.

In a typical Roots pump, a, pair of parallel and laterally spaced shafts are journalled in a casing which has an inlet and an outlet. Separate impellers mounted on each shaft mesh as the shafts are rotated and sweep fluid from the inlet'to the outlet. Such pumps have been used in the past principally for moving relatively large volumes of liquid or gas under atmospheric orhigher pressures and at moderate rotational speeds, say, several hundred revolutions per minute. 1

Patented July 12,1960

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sure zone and prevent the pump from obtaining the dey sired low pressures.

In using theRoots type pumps for the higher presssure, ranges, say, atmospheric pressure and higher, itfhas always been general practice to make clearances as small as possible to improve pumping efficiency. However, in using the Roots type pump as a vacuum pump, partial-- larly below about mm. Hg, the clearances between the moving parts of the pump can be considerably in obtained.

creased with little harm to the pumping eflici'ency because of the'relatively large mean free path of the gas molecules. For example, clearances between moving parts can be .1 mm. and larger. This permits the pumps to be operated at exceptionally high rotational speeds, say 2,000 to'3,000 revolutions per minute, which further improves their efiiciency as highyacuurn pumps.

Thus, by providing a Roots type pump for operation in the vacuum range with sufliciently large clearances for high speedv operation and a highly polished surface for the impellers, a pump of improved efliciency'c'an'be The advantages ofa Roots type pump having ran tively large clearances and highly polished interior sur' faces can be further improved by making the impellers of light construction, Whichaids in the high speed operation ofthe pump. Light construction can be achieved,

for instance, by making the impellers hollow instead of Z shafts to form an integralunit;

In the past, it has been considered good practice to provide the Roots type pump impellers with roughened surfaces to reduce the back flow of gas, the thought being that the roughness reduced the clearances between the moving parts and the pump and provided increased impedance to back flow of gas. However, I have discovered that in operating Roots type pumps in the vacuum range, say less than 50 mm. Hg, roughness of the impeller surface and the interior casing surfacenot only fails to improve pumping efliciency, but actually impairs it.

According to this invention, I provide a Rootstype pump for operation in the pressure range of less than 50 mm. Hg in which the surfaces of the impellers are made microscopically smooth, and of an impervious material which does not absorb or adsorb significant quantities of \gas molecules. Preferably, the interior of the casing is also made gas impervious and microscopically smooth. The discovery that a Roots pump of improved efficiency is obtained by making the impellers and casing interior as smooth as possible is an unexpected result in view of the previous practice of the industry either to leave the surfaces of the impellers untreated or else deliberately increase the roughness of the surfaces of thei-mpellers. Even a small degree of roughness in the surfaces of the impellers or pump casing provides minute dead spaces or pockets in which traces of gas can settle and not be readily pumped. As long as Roots pumps are operated at relatively high pressures, say, atmospheric pressure, this phenomenon either does not occur or is not significant. However, in the low pressure ranges, particularly less than a few mm. Hg, even small quantities of gas trapped in the surface pockets seriously impair the efliciency of the pump. For example, as a portion of an impeller with a rough surface moves through a high pressure zone to force lgas toward the pump outlet, countless pockets become full of relatively high pressure gas. As the piston moves into the low pressure zone of the pump, gas trapped in the pockets can escape into the low pres- These and other aspects of the invention will bemore fully understood from the following detailed description taken in conjunction "with the accompanying drawing infwhicha Fig. 1 is a schematic sectional elevation of a Roots type pump having hollow impellers in accordance with this invention; and v Fig. 2 is a schematic perspective view, partially broken away, showing an impeller made of segments in accord; ance with another form of this invention.

Referring to Fig. l, a. Roots type pump includes a pump casing 10 in which a pair of Roots type impellers 11 are each mounted on rotatable shafts 12. A pump inlet 13 is on one side of the casing, and an outlet 14 is on the other side of the casing. The outlet is adapted to be connected to a suitable fore-vacuum pump (not shown), for example, a conventional mechanical oil sealed rotary pump, which reduces the back pressure on the outlet of the Roots pump to less than 50 mm. Hg. Each impeller has a pair of longitudinal bores 15 to reduce their weight and improve the operating characteristics of the pump at high rotational speeds.

Referring to Fig. 2, impeller 20 is mounted on a shaft 21. The impeller includes an outer shell 22 which may be of thin sheet metal or of a rigid inorganic plastic,

for example. In either case the surface of the impeller is microscopically smooth and has a highly polished appearance. The interior of the shell includes a series of longitudinally spaced ribs 23 having their outer surfaces shaped to conform with 'the cross section of the impeller. Each rib has a pair of openings 24 on opposite sides of the shaft to reduce the total weight of the impeller, and each rib is spaced fromadjacent ribs by spacer sleeves 25 disposed around the shaft. The ends of the impeller are closed by solid stiffener plates 26 which also have their edges shaped to match the cross section of the impeller surface. Asviewed in Fig. 2, only one stiffener plate is visible. A small opening 27 in one of the stiffener plates permits the gas pressure in the impeller interior to become substantially equal to that of the work- Z-is of extremely light construction and well adapted for highspeed operation.

In accordance with this invention, the impellers of either the type shown in Fig. 1 or the type shown in Fig. 2 Kate provided with .microscogiically -smooth surfaces.

The smooth surfaces are provided by anylofiseveral'welln known techniques. For example, the surfaces (can 'be made extremelysmooth by .verygfine polishing .orby the coating of the impeller surfaces with layers .of .material which have alreadybeempolished. .Forexarnple, sheets of hard organic synthetic plastic with extremely smooth or :pol'ished surfaces can be bonded to the impeller surfaces.

Smooth surfaces can also be provided by applying :to the impellensurfaces suitablecoating materials. ".Forexample, smooth varnish, enamel, metallic, or oxidecoatings'may be applied by dipping, spraying .orbysuitable vacuum coating operations which are well-known to those skilled in the coating art. For example, a highly polished chromium plating provides a corrosionresistant surface, and a surface which improves pumping efliciency;

Smooth layers can also be appliedbycathode spraying or evaporation in vacuum, or by electrolytic :orelectro; chemical processes. Any of the well-known methods for improving the smoothness of surfaces can be utilized in this connection to give the .surfacesaismooth vitreouslike finish.

The interior of the pump casing is also .provided with a smoothsurface by any of the above described methods.

Thus, this invention provides an improved Rootstype pump with lightweight impellers for high vacuum operation in which the pump is adapted to operate at high rotational speeds and has *highly polished or smooth interior surfaces to improve pumping efficiencyin the vacuum range below about 50 mm. Hg. 1

I claim:

A Roots type vacuum pump for operation at very high mechanical speeds above 2000 r.p.m. and at absolute pressures below 50 mm. Hg comprising a casing having an inlet and an outlet, a pair of shafts journaled v.in the casing .forrotation, .andra pair of rotatable impellers mounted on the respective shafts in the casing, the impellers being adapted to intermesh without actually touching in the central region of the casing to drive gas from the casing inlet toward the outlet, the surfaces of the impellers and adjoining interior surfaces of the casing having a microscopically smooth finish with a substantial clearance between the intermeshing surfaces of the:twoimpellers and between the surfaces'of the impellers and the interior of the casing, whereby no sliding frictionis generated byrotation of the impellers, each or" the impellers includinga plurality of longitudinally spaced transverse reinforcing ribs secured to the associated shaft, the ribs having an outline conforming to the desired cross-sectional shape 'of'the'impellen'the ribs having large openings therethrough to decrease their weight, a pair of solid end plates secured to the associated shaft having the same outline shape as the ribs, and .a skin of thin flexible materialformed around and conforming to the outline shape of the ribs and end .plates, the skin .being secured along the edges thereof to the end plates to form a light hollow impeller body.

References Cited .in thefile of this :patent UNITED STATES PATENTS 736,039 Wainwright Aug. 11, 1903 1,078,444 Wilburn Nov. 11, 1913 1,682,565 Hill Aug. 28, 1928 1,785,386 McIntyre Dec. 16,,l930 2,403,455 Phillips July 9, 1946 2,515,956 'Greenberg July 18, 1950 2,519,588 McCulloch Aug. 22, 1950 2,721,694 Van Atta Oct. 25, 1955 FOREIGN PATENTS 251,443 Great Britain May 6, 1926 306,719 Italy Mar. 30, 1933