Increased volumetric capacity of axial flow compressors used in turbomolecular vacuum pumps
A turbo-molecular vacuum pump of increased pumping capacity has parallel access toward two initial impeller-rotors. An additional annular space is provided around the periphery of the first rotor and the conventional first stator disc is omitted, thus creating an accelerated annual gas flow entering directly into the second rotor without an accumulation of pressure.
The invention relates to axial flow stages of rarefied gas compressors commonly used in turbo-molecular vacuum pumps and, more particularly, to modifications of rotor/stator arrangements at the inlet of such pumps. The purpose of the invention is to increase the volumetric flow rate, as related to the property called ‘pumping speed’ in high-vacuum technology. The basic idea may also be of use for very high altitude aircraft for increasing the mass flow of rarefied air into the engine.
BACKGROUND OF THE INVENTIONAxial flow pumping stages used in turbo-molecular vacuum pumps are essentially rarefied gas compressors that pump and compress the gas by a displacement process, resulting from sweeping the gas through angled blades attached to a rotating shaft. The peripheral velocity of the blades must be very high because the displacement process is non-positive and back flow must be prevented as much as possible. In vacuum pumps, usually operating in molecular flow conditions, this velocity must be commeasurable to the normal thermal molecular velocities of the pumped gases. This is necessary for effective capture probability of molecules entering into the space between passing blades.
It becomes immediately apparent that the maximal pumping rate (pumping speed) will be limited by the maximum peripheral velocity of the pumping blades. This in turn is limited by the strength of the material of the blades. Peripheral velocity is proportional to the product of the diameter of the rotor (the distance between outer edges of opposite blades) and the rotational speed (RPM). To obtain highest possible pumping speed, the first bladed rotor is placed near the inlet plane of the pump. To prevent backflow, the inner diameter of the pump body is made to be very close to the tip of the blades. Thus, the pumping speed is limited for a given rotor diameter and given RPM.
A secondary limitation arises from lower peripheral velocity at the base of the impeller blades because the diameter (or radius) is smaller at that location.
Thus, for consideration of maximizing pumping speed, the blades cannot be made too long, and the effectiveness of molecular capture will depend on the average peripheral velocity of blade surfaces. The pumping speed will depend on this average and on the annular area of the inlet plane traversed by the pumping blades.
Accordingly, there is a need for improving the pumping speed without substantially changing the overall size, cost and power consumption of the pump.
SUMMARY OF THE INVENTIONAccording to the first aspect of the invention, a vacuum pump is provided with multiple axial flow stages in series, consisting of rotor/stator pairs placed at the inlet of the pump. Typically, the angles of the blades in the stator disks are opposite to the blade angles of the rotors. Regardless of the shape and size of the pump inlet structure, the limitation of the pumping speed at the inlet plane of the first stage is the property that the invention desires to increase. Thus, additional annular space is provided at the periphery of the inlet rotor through which gas molecules can have access to the second stage rotor. This additional flow, under molecular flow conditions, does not interfere with the flow arriving from the exit side of the first rotor, because the molecules do not collide and are “unaware” of the presence of others. In prior art operation mode, the presence of the first stator will increase the pressure (or density), due to compression, which may result in backflow and negate the desired effect. Therefore, it is proposed to remove the first stator disk entirely and thus use the first rotor only for the pumping speed effect and free it from producing compression. This renders the first rotor to become an auxiliary capture mechanism which sends an annular molecular beam into the second rotor. The first rotor can be confined at its periphery by a thin annular guard which will prevent radial spreading of the exit flow. The pumping speed of the second rotor is increased by receiving an additional flow from the added peripheral access space. This flow is only limited by the width of the annular space and the distance of the second rotor to the inlet plane of the pump, i.e. by conductance of that passage.
Thus, the basic invention provides an arrangement of sharing the initial gas flow capture among the first two rotors. Depending on detailed design, as much as 50% improvement in pumping speed can be achieved. Conversely, the blades can be made shorter to increase their average peripheral velocity, which is desirable for efficient pumping of gases having a low molecular weight. It is not effective to extend the new arrangement to the third stage rotor because of an additional loss of compression due to the omission of stator discs and due to the resultant longer distance to the inlet plane of the pump. Conductance of annular cylindrical conduits rapidly diminishes with the growing ratio of length to width of the duct.
The principle of the invention remains valid whether or not the first stator is entirely removed or kept in its usual place with any blade angle, including tilted backward (i.e. at the same tilt as the rotor blades), or made to be 90 degrees to the plane of rotation. It also remains valid regardless what kind of impellers follow the first two (or three) axial pumping stages, including axial-flow, or molecular drag impellers (with Gaede, Holweck, Siegbahn type pumping channels), spiral or concentric, or the regenerative/centrifugal kind. It also remains valid whether or not the shield around the blades of the first rotor is present. It remains also valid regardless of the shape of the conduit leading from the inlet plane of the pump to the entry into the second rotor; it can be a concentric or non-concentric annular cylinder, or it can have a cross-section of elliptical, ovoid, or polygonal shape.
There are three preferred embodiments apparent. First, for a given size of the inlet flange (or inlet area of the pump), create as great as possible peripheral access from the inlet plane of the pump to the inlet area of the second rotor. Second, use a smaller rotor, rotating at a higher RPM to obtain a pumping speed equivalent to same size pumps made according to the prior art. Third, enlarge the body of the pump near the inlet to provide a higher conductance passage to the vicinity of the second rotor. The use of a smaller rotor assembly reduces the weight with advantages of lower rotational moment of inertia and lower load for the bearings. This arrangement can also be achieved by placing the first rotor (as an auxiliary rotor attached to the main shaft) above the conventional inlet plane of the pump, protruding into the vacuum chamber, as long as sufficient space is available for pumped gas to enter directly into the second rotor.
For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated by reference and in which:
A simplified cross-sectional diagram of a conventional turbo-molecular pump is shown in
The basic embodiment of the invention is shown in
Claims
1. A high-vacuum pump containing an axial flow compressor as used at the inlet of turbo-molecular vacuum pumps, comprising: a housing having an inlet port and an exhaust port; a rotor containing impellers, attached to the same rotating shaft, having inclined blades; one or more additional axial flow stages located within the housing, followed by pumping stages operating at higher pressures, characterized by providing access for the pumped gas toward two inlet rotors operating in parallel, each having inclined blades which provide the pumping mechanism; and a motor to rotate said impellers such that the gas is pumped from said inlet to said exhaust port.
2. A high-vacuum pump as defined in claim 1, wherein the leading inlet rotor impeller, having preferably a 45 degree blade angle, has space around its periphery to provide access for the pumped gas to the second rotor-impeller thereby increasing the molecular capture probability at the inlet of the pump and where the conventional following stator disk is either entirely omitted or has blade angles which do not impede the accelerated gas molecules leaving the first rotor from passing directly toward the second rotor.
3. A high-vacuum pump as defined in claim 2, wherein a thin cylindrical shield, having converging or diverging or curved shape, is placed in close proximity to the first rotor to prevent spreading pumped gas molecules in lateral directions and help direct the gas exiting the first rotor toward the second rotor.
4. A high-vacuum pump as defined in claim 1, wherein the inlet section of the pump is enlarged to accommodate the additional, preferably annular, space for an increased conductance access for pumped gas toward the second impeller.
5. A high-vacuum pump as defined in claim 1, wherein the first inlet rotor is shaped such that the impeller blades are elongated being placed in an arrangement of an inverted truncated cone so as to increase the area of interaction between the pumped gas and the pumping blades.
6. A high-vacuum pump as defined in claim 1, wherein the first rotor can be placed as an auxiliary rotor, attached to the main rotating shaft, above the conventional inlet plane, protruding into the vacuum chamber where the connecting tube between the pump and the chamber has a larger diameter than the pump inlet port.
7. An axial flow compressor as defined in claim 1, used in applications for pumping rarefied gases, such as air in high-altitude aircraft in order to increase the mass flow of air into the engine.
Type: Application
Filed: Mar 18, 2010
Publication Date: Oct 21, 2010
Inventor: Marsbed Hablanian (Wellesley, MA)
Application Number: 12/661,464
International Classification: F04B 25/00 (20060101);