Improved turbomolecular pump

Abstract

 A turbomolecular pump comprising, in addition to usual axial flow pumping stages consisting of alternated rotors (1,1a) and stators (2,2a), one or more tangential flow pumping stages (10;30) arranged at the exhaust side of the pump, to raise up the compression ratio. Each axial flow stage consists of a plane disk rotor (12;35) encompassed to a coplanar annular stator (13;36), spaced apart from the rotor disk, so that a free annular channel is formed in-between (14;38), in which channel the pumped gases move along a tangential flow. Such channel is closed by a baffle (15;39), and communicates with a suction port (17;40) and a discharge port (18;41). The plane disk rotor may be provided of vanes (37,37a,37b). It is possible to combine tangential flow stages (10) having rotor without vanes (12) and tangential flow stages (30) having rotor with vanes (35); in such a way it is possible to raise the exhaust pressure up to the atmospheric pressure, and to discharge directly in the atmosphere.

Description

[0001] The present invention refers to an improved turbomolecular pump, especially to a turbomolecular pump of increased compression ratio, capable of extending the operating range towards higher pressures. Conventional turbomolecular pumps usually have operating ranges from about 10-⁷ to 10-¹ or 1 Pascal, i.e. they cannot exhaust directly to atmosphere. This means that they need to be teamed up to a forepump which produces the necessary fore vacuum and discharges the pumped gases at atmospheric pressure. However, contamination of the turbomolecular pump with lubrication oil of the forepump may occur, which prevents pumping at the lower operating range. This may be avoided by maintenance at short intervals, which raises the costs of operation, in addition to a higher initial cost of the vacuum system. Moreover, the combination of a turbomolecular pump with a forepump is cumbersome, which is a disadvantage in most applications.

[0002] So-called hybrid turbomolecular pumps have also been developed to reduce the necessity of these backing pumps. U.S. Patent No. 4,732,529, U.S. Patent No. 4,826,393 and U.S.Patent No. 4,797,068 disclose turbomolecular pumps including a compression ratio raising section consisting of rotors formed with spiral grooves, or screw rotors, which guide gas from the high vacuum section to a simpler exhaustion system, e.g. to a membrane pump. Although such hybrid turbomolecular pumps do not need complex exhaustion systems consisting of a number of auxiliary vacuum pumps, they still require a forepump, because they are incapable of discharging gases at atmospheric pressure. A new type of roughing pump which can reach a low ultimate pressure (3.10-² Pascal) has also been developed. As reported in J.Vac.Sci.Technol.,A, Vol.6,No.4, 2518-2521, Jul/Aug 1988 , this pump is a turbo vacuum roughing pump comprising radial flow pumping stages consisting of impellers rotating into channels with grooves which direct radially the flow of the pumped gases, and a peripheral flow pumping stage at the exhaust side, which raises the pressure so that the pump can discharge at atmospheric pressure.

[0003] However, this pump is only a roughing pump that can by no way replace a turbomolecular pump, the ultimate pressure of which is lower of several orders of magnitudes (10-⁷ Pascal) than the ultimate pressure of this roughing pump (10-² Pascal).

[0004] A first object of the present invention is to provide a turbomolecular pump with a high compression ratio. Another object of the present invention is to provide a turbomolecolar pump which is capable of discharging gases at atmospheric pressure, without being combined with forepumps.

[0005] A further object of the present invention is to provide a turbomolecular pump which is relatively not cumbersome in comparison with previous vacuum systems having similar operating range.

[0006] For attaining the foregoing objects, an improved turbomolecular pump according to the invention comprises at the suction side a plurality of pumping stages consisting of alternately arranged rotors and stators provided with inclined blades, the rotor blades being inclined in the inverse direction to the stator blades, for pumping gases along an axial flow through said pumping stages, characterized in that it further comprises at least one pumping stage at the exhaust side, consisting of a rotor and a coplanar stator with a free annular channel defined in-between, along a part of their circumferences, said free annular channel being in communication with a suction port and a discharge port for pumping gases with a flow tangential to said rotor from said suction port to said discharge port.

[0007] According to another feature of the invention, to enhance the pumping effect in the viscous flow range, a tangential flow pumping stage may be added in which the rotor consists of a disk provided with blades. Illustrative embodiments of the invention are hereinafter described in conjunction with the drawings, where:

• FIG.1 is a schematic view in axial section of a part of a turbomolecular pump according to the invention;
• FIG.2 is a perspective view of part of the pump of FIG.1, with a partially broken first embodiment of a tangential flow pumping stage;
• FIG.3 is a partially broken plane view of a pumping stage of FIG.2; and
• FIG.4 is a perspective view of of a partially broken second embodiment of a tangential flow pumping stage.

[0008] With reference to FIG.1, a turbomolecular pump according to the invention comprises a certain number of axial flow pumping stages, each consisting of a rotor 1 or 1a, and of a stator 2 or 2a, contained in a cylindrical pump body 3, as known in the art. The pumping stage consisting of rotor 1a and stator 2a is also shown in FIG. 2. Each rotor consists of a disk 5 mounted on a rotatable shaft 6, and carrying at its periphery an array of radially protruding inclined blades 7, 7a, 7b. Each stator consists of a similar disk with a central hole for the shaft 6 of the rotors. Each stator is fixed to the pump body 3, and consists of a disk 8 provided with blades 9, 9a, 9b, which are inclined in a direction that is inverse to the direction of the rotor blades 7, 7a, 7b.

[0009] Gases coming from the suction side, not shown but indicated by arrow A, are pumped by the described stages along directions parallel to the axis of the cylindrical body 3, i.e. an axial flow of gases is produced through the alternate rotors and stators, as indicated by the arrow B of Fig. 1.

[0010] According to the invention, one or more pumping stages of different conception are added downstream the axial flow pumping stages.

[0011] In FIG.1 two of such pumping stages are shown, indicated globally with the reference numerals 10 and 30.

[0012] Each of the pumping stage 10 and 30 still comprises a rotor mounted on shaft 6, and a stator fixed to the the pump body 3. Constructional details of these pumping stages are also illustrated in FIGURES 2 to 4. With reference to FIGURES 1, 2 and 3, pumping stage 10 comprises a rotor consisting of a plane disk 12 secured to shaft 6. Rotor 12 is encompassed by a substantially coplanar stator having the shape of a ring 13 spaced apart from the rotor disk 12, so that a free annular channel 14 is defined between rotor and stator. A baffle 15 closes channel 14 between a suction port 17 and a discharge port 18, provided in an upper closure plate 21 and in a lower closure plate 23, respectively. Closure plates 21 and 23 are joined together by suitable means, e.g. by connection of downwardly extending edge 22 of plate 21, so as to form a closed casing containing the pumping stage. Central holes are provided in both plate 21 and 23, for the passage of the shaft 6. The baffle 15 may be a radial projection of the stator 13, as shown in FIGURES 2 and 3, or a separate element tightly secured to the stator ring 13. The operation of the pumping stage above described is the following.

[0013] Gases pumped by the axial flow pumping stages come to suction hole 17, as indicated by arrow D in FIGURES 1 and 2, and enter into channel 14. Here, the gas molecules strike the rotating disk 12 and keep a speed with a component tangential to the disk 12, as indicated by arrow E. By this process the molecules are transferred within free channel 14 from the suction port 17 to the discharge port 18 according to a tangential flow, and leave channel 14 through discharge port 18, as indicated by arrow F. The flow of gases that is produced in the free channel 14 is referred to as "tangential flow" because it parallel to the direction of the velocity of the rotor, which is a tangent to the rotor.

[0014] This tangential flow pumping stage is effective in a molecular or transient flow pressure range, and permits to raise the outlet pressure from about 1 Pascal, that is the usual outlet pressure of conventional turbomolecular pumps, to 10³ Pascal and even more. At higher pressure ranges, i.e. in the viscous flow range, pumping stages with plane rotor disks are no more effective. It has been found that a different rotor design, such as shown in detail in FIG. 4, can produce a further raise of the outlet pressure, up to the atmospheric pressure.

[0015] With reference also to FIG.1, the pumping stage effective in the viscous flow range is indicated with 30. Pumping stage 30 is arranged in series, downstream pumping stage 10. As pumping stage 10, it comprises a closed casing consisting of an upper plate 31 with a downwardly extending edge 32 connected to a lower plate 33. Shaft 6 extends axially in the casing, and carries a rotor disk 35 with peripheral vanes such as 37, 37a, 37b, lying on planes perpendicular to the plane of disk 35. A coplanar stator ring 36 encompasses rotor 35 but is spaced apart from it, so that a free annular channel 38 is defined between the periphery of the vanes of the rotor and the stator. A baffle 39 obstructs the free channel 38 between a suction port 40 made in upper plate 30 and a discharge port 41 made in lower plate 33.

[0016] As shown in FIG. 1, gases discharged from port 18 of pumping stage 10 come to the suction port 40 of the pumping stage 30, as indicated by arrow G, and enter into channel 38 between rotor and stator. Here, gases molecules get kinetic energy by striking the rotor, a circular flow with a tangential speed component is produced in free channel 38, and gases are pumped from suction port 40 to discharge port 41. In this last stage the pressure is raised to about 10⁵ Pascal, so that the pump can exhaust directly to the atmosphere through port 43 in the pump body 3, as indicated by arrow I in FIG. 1.

[0017] The peripheral velocity of the rotor of this turbomolecular pump, including both axial and tangential stages, is usually not less than 250 m/s, preferably from 350 to 400 m/s. For example, in a small pump equipped with a rotor having a diameter of 100 mm (0.01 m), the angular velocity of the rotor is of 50,000 r.p.m. to obtain a peripheral velocity of 260 m/s (v_p=2πr.v_a). For larger diameters of the rotor, the angular velocity may be lower, provided that the peripheral velocity does not drop below about 250 m/s. It is apparent from the above description that the number of both the axial flow and the tangential flow pumping stages, either of the type with plane rotor or of the type with vanes rotor, may be varied according to the specific applcations, without departing from the scope of the invention.

Claims

1. Improved turbomolecular pump comprising at the suction side a plurality of pumping stages consisting of alternately arranged rotors (1,1a) and stators (2,2a) provided with inclined blades, the rotor blades (7,7a,7b) being inclined in the inverse direction to the stator blades (9,9a,9b), for pumping gases along an axial flow through said pumping stages, characterized in that it further comprises at least one pumping stage (10) at the exhaust side, consisting of a rotor (12) and a coplanar stator (13) with a free annular channel (14) defined in-between along a part of their circumferences, said free annular channel being in communication with a suction port (17) and a discharge port (18) for pumping gases with a flow tangential to said rotor (12) from said suction port (17) to said discharge port (18).

2. Improved turbomolecular pump according to claim 1, characterized in that it comprises an additional pumping stage (30) at the exhaust side, arranged downstream said discharge port (18), said additional pumping stage (30) consisting of a rotor (35) with vanes (37,37a,37b) and a coplanar stator (36), a free annular channel (38) being defined between the periphery of said vanes (37,37a,37b) and said stator (36) along a part of the circumferences of said rotor (35) and stator (36), said annular channel being in communication with a suction port (40) and a discharge port (41) for pumping gases with a flow tangential to said rotor (35) from said suction port (40) to said discharge port (41).

3. Improved turbomolecular pump according to claim 2, characterized in that said vanes (37,37a,37b) are perpendicular to the plane of said rotor (35).

4. Improved turbomolecular pump according to any of the preceding claims, characterized in that said free annular channel (14;38) is delimited at its ends by a baffle (15;39) arranged between said discharge port (18;41) and said suction port (17;40).

5. Improved turbomolecular pump according to claim 1, characterized in that said rotors (1,1a) of said pumping stages for pumping gases along an axial flow, and said rotor (12) of said pumping stage (10) for pumping gases with a flow tangential to said rotor (12) are mounted on a same rotatable shaft (6).

6. Improved turbomolecular pump according to claim 1,2 and 5, characterized in that also said rotor (35) of said additional tangential flow pumping stage (30) is mounted on said rotatable shaft (6).

7. Improved turbomolecular pump according to claim 5 or 6, characterized in that said rotors (1,1a;12;35) of said axial and tangential flow pumping stages are rotated, in operation, at a peripheral velocity of at least 250 m/s.

8. Improved turbomolecular pump according to claim 1 or 2, characterized in that each of said tangential flow pumping stages (10,30) is contained in a casing consisting of upper and lower plates (21,23;31,33) provided with suction and discharge ports (17,18;40,41).

Drawing