US3644051A

US3644051A - Turbomolecular and stator pump having improved rotor construction

Info

Publication number: US3644051A
Application number: US869848A
Authority: US
Inventors: Ascher H Shapiro
Original assignee: Sargent Welch Scientific Co
Current assignee: Sargent Welch Scientific Co
Priority date: 1969-10-27
Filing date: 1969-10-27
Publication date: 1972-02-22
Anticipated expiration: 1989-02-22
Also published as: NL7014593A; DE2046693A1; FR2065419A5; CA954487A; GB1332004A; BE757354A; CH532199A

Abstract

An axial turbo-type vacuum pump operating in the free molecule flow pressure range and characterized by a number of multistage groups, each group being operable principally in a different pressure range, and each stage of each group comprising a rotor element and an associated stator element. Each rotor includes a hub portion and a row of blades extending radially outwardly therefrom, with the blades having leading and trailing edges, and front and rear blade face portions in respect to the direction of travel on the rotor element. Each stator is of approximately mirror-image configuration in relation to its associated rotor. The relation between blade thickness blade chord, interblade spacing, radial blade span and blade angle are different for the elements of each group, and these and other characteristics of the rotors and stators, including the presence of trailing edgeleading edge overlap, are determined for each group to provide optimum performance characteristics for the entire turbopump.

Description

United States Patent Shapiro 1 Feb. 22, 1972 [54] TURBOMOLECULAR AND STATOR PUMP HAVING IMPROVED ROTOR FOREIGN PATENTS OR APPLICATIONS Switzerland ..415/193 Primary Examiner--Henry F. Raduazo Attorney-Greist, Lockwood, Greenawalt & Dewey [57] ABSTRACT An axial turbo-type vacuum pump operating in the free molecule flow pressure range and characterized by a number of multistage groups, each group being operable principally in a different pressure range, and each stage of each group comprising a rotor element and an associated stator element. Each rotor includes a hub portion and a row of blades extending radially outwardly therefrom, with the blades having leading and trailing edges, and front and rear blade face portions in respect to the direction of travel on the rotor element. Each stator is of approximately mirror-image configuration in relation to its associated rotor. The relation between blade thickness blade chord, interblade spacing, radial blade span and blade angle are different for the elements of each group, and these and other characteristics of the rotors and stators, including the presence of trailing edge-leading edge overlap, are determined for each group to provide optimum performance characteristics for the entire turbopump.

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TURBOMOLECULAR AND STATOR PUMP HAVING IMPROVED ROTOR CONSTRUCTION BACKGROUND AND DESCRIPTION OF THE PRESENT lNVENTlON The present invention relates to vacuum pumps, and more particularly to pumps known as turbomolecular pumps, so called because of their use of one or more axial-flow turbo stages acting as compressors, and because of the fact that they are effective when gas flow therein in relation to the important blading dimensions of the pump is in a free molecule flow state occasioned by very low pressures. At one time, even though the axial flow, multistage compressor principle was known to be effective for use in gas turbine engines and other fluid flow devices, it was believed that compressors of this type would not be suited for use in producing high vacuums on a practical basis, since the pressure rise across each stage was of such a low order in continuum fluid flow that far too many stages were believed to be required to produce a desirably low vacuum.

Recently, however, it has been discovered that turbo-type pumps, if operated under correct conditions, could produce sufficient pressure rise per stage to make it practical to obtain satisfactory vacuum levels with a reasonable number of stages. Normally, pumps of this type comprise an impeller with a plurality of rotors thereon, disposed within a cylindrical casing having a center inlet and an outlet at either axial end thereof, with the vacuum being provided by rotating the centrally disposed impeller shaft so as to move the rotors between adjacent stators at high speed, the rotor and stator pairs being of opposite hand angular disposition on either side of the center inlet. Thus, a typical pump comprises a multigroup, to stage, left-hand compressor assembly and an axially oppositely disposed right-hand compressor assembly with the left unit adapted, upon appropriate rotation of the impeller shaft, to move gas molecules to the leftand the right-hand assembly being adapted to move molecules to the right under the same direction of rotation. In keeping with known practices, the stators are ordinarily disposed in a substantially mirror-image relation to the rotors with which they are associated, although this is not strictly necessary.

Known turbopumps constructed as set forth above, have presented a number of shortcomings and have heretofore commonly lacked performance of which, with proper design, they might be capable. It is believed that a principal reason for this has been that, until now factors influencing turbopump construction and performance have not been properly understood, with the result that there has been sufficient room for improvement in the design of vacuum pump components, such as in the area of blade design and location, and in particular. to the shape and angle of the blades, their positioning with respect to other blades forming a part of the same rotor or stator, and in relation to blades on more or less adjacently disposed rotor-stator pairs. In the prior art, it has been taught that it is desirable to arrange the blades on a rotor and its adjacent stator with relatively great overlap, that is, so that a trailing edge portion of one blade overlaps the leading edge portion of the following blade. This teaching is a corollary of the premise on which many prior art pumps have been based, namely. that the flat rotor edges parallel to the plane of rotation should present large areas for facing similarly large areas on oppositely directed portions of associated adjacent stators, so that there will be a large interfacial area therebetween, and so that there will always be at least a portion of these faces of the stator surface and the rotor surface overlapping each other, even when a blade is axially opposite in interblade opening on an opposed member. As a result, blade thickness in relation to interblade spacing, according to this prior art theory, would necessarily be relatively large, and in no case less than unity. Accordingly, rotors and stators made in keeping with this teaching are normally characterized in that there is no unobstructed line of sight between blades when the rotor is viewed parallel to the rotational axis thereof. It has also been taught in the prior art that the blades should occupy only the portion of the rotor lying relatively closely adjacent the outer periphery thereof.

With the above set forth teachings being typical of those in the prior art, and with prior art vacuum pumps having been built generally along the lines set forth, so called turbomolecular vacuum pumps have not thus far exhibited the characteristics of which, with proper design, they might be capable. Accordingly, an object of the present invention is to provide an improved turbomolecular vacuum pump.

A further object of the invention is to provide an easily constructed, reliable vacuum pump in which vacuums of the order of 10' to l0" torr. and better may be achieved at the inlet end of the pump.

Another object is to provide a vacuum pump capable of producing very high-volume flows or pumping speeds.

Another object is to achieve high-vacuum levels and high pumping speeds while still utilizing only a relatively small number of stages.

Another object is to produce high pumping speeds in a turbopump of small cross-sectional diameter.

A still further object of the invention is to provide turbomolecular pump components, particularly rotors and stators, in which only a few basic blade configurations provide optimum performance for a number of stages of which the components form a part.

A still further object is to provide a pump having rotor and stator elements characterized by blades which are thin in relation to the distances they are spaced apart from one another, especially for stages nearer the inlet end of the pump.

Still another object of the invention is to provide a turbomolecular pump having a pair of principal compressor assemblies, each comprised of a number of groups differing from one another in the configuration and disposition of the rotors and stators forming individual stages within the groups.

A further object of the invention is to provide a vacuum pump in which stages most closely adjacent the high-vacuum region include rotor and stator rows comprised of blades having the trailing edges of each blade thereof spaced apart, in a circumferential direction parallel to the plane in which the stator or rotor lies, from the leading edge portions of each following blade, at least in the radially outer portions of the blades.

Another object of the invention is to provide a vacuum pump in which rotors and stators in successive groups or stages are characterized by the absence of trailing edge-leading edge overlap in the group of rotors and stators near the inlet end of the pump, and are characterized by a certain amount of such overlap in the intermediate group, and even more overlap in the group adjacent the outlet end, as well as rotor and stator blades having decreasing angles between the blades and the plane occupied by the rotors and stators as such rotors or stators are disposed in groups lying more closely adjacent the outlet end of the pump.

Another object is to provide a turbomolecular pump having one or more of the characteristics set forth above and characterized by a pair of substantially identical compressor assemblies, with blade angles set at mirror-image values, disposed on oppositely axial ends of a common rotatable shaft.

The invention accomplishes these objects and others which are inherent therein by providing a turbomolecular pump having a cylindrical exterior casing, an inlet and at least one outlet, at least one compressor assembly for causing gas flow in the free-molecule flow pressure range in the region of said inlet, in which at least one compressor stage includes a rotor assembly with a plurality of rotor blades thereon, and in which the trailing edge of at least one segment of a given blade is spaced apart from the leading edge of the immediately following blade in the direction of rotation thereof, with succeeding downstream stages including diminished spacing or trailing edge-leading edge overlap in the direction of rotation, with the working faces of the rotor and stator blades being progressively of lesser radial event and lower angle to the plane of rotation as the rotor or stator assembly with which the blades are associated is positioned relatively closer to the outlet of the machine, and in which the blades, particularly those associated with the rotors and stators nearer the inlet end, are of relatively thin cross section in relation to the spaces between them.

The manner in which these objects and advantages, and others which are inherent in the invention are attached will become more clear as the following detailing description of the preferred embodiments of the invention proceeds, and as reference is made to the accompanying drawings forming a part hereof, in which like reference numerals indicate corresponding parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view, partly diagrammatic, of a turbomolecular high-vacuum pump made in accordance with the present invention",

FIG. 2 is an enlarged perspective view of one form of rotor made in accordance with the present invention;

FIG. 3 is a further enlarged fragmentary front elevational view, looking upstream, of a portion of the blade containing portion of the rotor of FIG. 2;

FIG. 4 is a view of some of the rotor blades of FIG. 3, taken looking radially inwardly along lines 4-4 thereof;

FIG. 5 is a sectional view of the blades taken along lines 5 5 of FIG. 3;

FIG. 6 is a sectional view of the blades, taken along lines 6 6 of FIG. 3;

FIG. 7 is a cross-sectional view of a typical blade of the assembly of FIG. 2, taken along lines 77 of FIG. 4;

FIG. 8 is a sectional view of an interblade spaced in the rotor of FIGS. 2 and 3, taken along lines 88 of FIG. 4, that is, perpendicular to an inclined blade face;

FIG. 9 is a fragmentary enlarged elevational view, looking downstream, of a portion of a stator associated in use with the rotor of FIGS. 27;

FIGv I0 is a front elevational view, looking upstream, of a rotor from an intermediate stage of the pump;

FIG. 11 is a front elevational view looking downstream, of a portion of a stator operatively associated with the rotor of FIG.

FIG. 12 is a fragmentary front elevational view, looking upstream, of a rotor adapted for use near the outlet end of the pump:

FIG. 13 is a radially inwardly directed view of a portion of the rotor of FIG. I2 taken along lines I3- I3 thereof;

FIG. I4 is a sectional view of the tip portion of the blade of the rotor of FIG. 13, taken along lines l4l4 thereof;

FIG. I5 is a vertical sectional view of the interblade space in the rotor of FIG. 13, taken along lines 15-15 thereof, that is, perpendicular to the inclined blade face;

FIG. 16 is a schematic projected view of the blade portions of a rotor, showing the points at which some of the dimensions referred to herein are taken; and

FIG. 17 is a diagrammatic view ofa rotor assembly, showing the points at which some of the other measurements referred to herein are taken.

Although it will be understood that the principles of the invention may be practiced with various pump constructions, the invention will be particularly described herein with reference to an embodiment in which a cylindrical casing contains a pair of compressor assemblies, one each on the left hand and right-hand sides, each being adapted to draw gas molecules in the free-molecular flow pressure range from a central portion of the casing toward opposite axial ends of the casing. with the molecules following a generally axially directed flow path.

Referring now to the drawings in greater detail, FIG. I shows a vertical sectional view, partially diagrammatic, of an improved vacuum pump assembly 30. The turbomolecular pump unit itself, designated 32, is adapted to have the inlet thereof directly associated with an evacuable region 34 such Lit as a bell jar or the like (not shown), and adapted to transfer the output thereof to a so -called fore pump 36 of a type known in the art, through one or more conduits schematically indicated at 38 from the output lines 40 of axially oppositely disposed annular collector passages 42.

Referring now to the structure of the pump 32 in greater detail, this unit is shown in include a transition piece 44 which communicates with the evacuable region 34, a large scroll portion 46 surrounding the outer surfaces 48 of a cylindrical casing 50 having a plurality of wide inlet openings 52 defined in part by oppositely axially facing, centrally disposed margin portions 54 of the casing 50. The inner surface 56 of the easing 50 is adapted to receive a plurality of mounting rings 58, adjacent pairs of which axially locate the outer edge portions 60 of a plurality of stators 62. Each stator 62 has a continuous, annularly extending hub portion 64, the inner margin 66 of which define a circular opening therein. Each stator 62 has a plurality of stator blades 67 thereon, the construction ofwhich will be described in further detail herein. The axially disposed, spaced apart array of stator elements 62 constitutes the stator assembly 68.

Another principal component of the pump unit 32 is a rotatable impeller assembly 70 which comprises a left-hand compressor section 72, a right-hand compressor section 74, both sections 72, 74 including a plurality of rotors 78, and a cylindrical center section 76 containing no blades and being spaced inwardly apart from the inlet openings 52 of the casing 50. The left-hand and right-hand compressor portions 72, 74 are mirror images of each other, one being adapted to impart gas molecule flow to the left as shown in FIG. I, for a given direction of impeller rotation and the other being adapted to evacuate gas molecules to the right. Accordingly, only the lefthand compressor 72 will be described in detail, it being understood that its counterpart, the right hand compressor 74 is identical but with blade angles disposed in mirror-image fashion to those of compressor 72.

The left-hand compressor 72 includes a plurality of rotors 78, mounted for rotation on the hub portion 80 ofthe impeller 70. Each rotor unit 78 and an associated, immediately adjacent stator unit 62 forms a single stage, and a plurality of adjacent stages of the same blading shape and size form a multistage working group 82. A plurality of working groups 82, 84, 86 are arranged in axially outwardly arranged relation, with each of the stator-rotor pairs within a particular group having the same configurations, but differing from the construction of the pairs of rotors and stators in an adjacent working group, as will be set forth more fully herein.

The impeller 70 is supported on bearing

units

88, 90 and is driven by a helical gear 92 which may in turn be driven by a much larger diameter, motor driven gear or the like (not shown). The elements of the pump unit 32 shown in FIG. I but not particularly described herein are generally conventional, do not form a necessary part of the invention, and will therefore not be further described in further detail herein.

Before referring in detail to the construction of the various parts of the apparatus shown in FIGS. 2-15, it should be understood that the following description of the particular forms of segmented blade rotor and stator construction illustrated are forms which have been found to be structurally feasible and easy to manufacture, but that other forms of rotors, stators and the like having the characteristics claimed herein would also be advantageously operable with the invention. In this connection, for example, it will be apparent to those skilled in the art that whereas three groups of rotors are illustrated, a turbo pump according to the invention might have only two groups, or might have four or more, and that each group might, but need not, consist of an equal or nearly equal number of stages.

Referring now to FIGS. 2-8, a rotor 78 of the type preferably used in the group 82 nearest the inlet end of the pump is shown to include a row of blades 94, each blade 94 terminating at an inner root portion 96 thereof where it is joined to and forms a part of the hub 98. An opening 100 defined by a radially inwardly directed hub wall portion 102 is disposed in the center of the hub 98, inwardly of the principal body portion 104 of the hub 98.

Each blade 94 in the first or upstream stage rotor is a segmented blade having a tip segment A, an intermediate segment B and an inner segment C. The outer or tip segment A includes a radially outwardly directed face portion 106, and a flat front face portion 108', segment B includes face 110 and segment C includes face 112, all faces 108, 110, 112 lying in a common plane parallel to the direction of blade rotation. Each blade 94 also includes angled working faces 114, 116, 118 on segments A, B, and C, respectively directed generally downstream. The faces 114, 116, 118 are angularly offset from one another in the direction of rotation, so that, for instance, the outermost downstream face 114 is spaced substantially apart from the face 120 preceding it and which, being parallel but on the opposite side of the blade 94, faces downstream. The face 122 lying radially inwardly of face 120 is spaced a lesser distance apart from the downstream face 116 immediately following it than is face 120 from face 114, and the least spacing in the rotational direction occurs between upstream face 124 and the downstream face 118 immediately following it. As will be appreciated, since the faces disposed radially outwardly have the greater linear velocity, the tangential or rotational direction linear spacing should be greatest near the periphery of the rotor 78. By reference to FIG. 7, it can be seen that the

innermost portions

126, 128, 130 of each blade segment A, B, C has a reduced thickness in respect to the

outer portion

132, 134, 136 of the same segment, and that the thickest inner section 130 lies closest to the hub 98 while the thinnest innermost portion 126 is associated with the outermost blade segment A.

By reference to FIG. 8, it can be seen that the spacing taken along lines A-A is the greatest between oppositely directed, leading and following

faces

120, 114 and that there is progressively less perpendicular and tangential space between pairs of

surfaces

122, 116, and 118, 124, respectively. Nevertheless, the spacing between blade segments even at the radially inward portions thereof is still substantial, and is of a type which is opposite from prior art teachings. Likewise, blade thickness is slightly larger near the roots of each segment, as pointed out above, but the blades are still very thin in relation to prior art teachings and prior art constructions. By reference to FIGS. 36, it will be seen that for first group rotors, the included acute angle between an upstream face 120 and the plane of rotation is a relatively steep or high pitch angle, for example, 40 in the illustrated embodiment. The significance of this point will be set forth herein.

Referring now to FIG. 9, a sector or portion of a first group stator 62 is shown to include a plurality of blades 67 extending outwardly from a hollow hub 64 which includes a margin 66 defining a large diameter opening therein. The blades 67 on the stator 62 are also segmented blades having stator blade segments D, E, and F with a

flat face portion

138, 140, 142, associated respectively with segments D, E, and F. Angularly inclined upstream work faces I44, 146, 148 are provided on segments D, E, and F in the same manner as are their

counterparts

114, 116, and 118 on rotor blades 94. By reference to FIGS. 3-9, it will be seen that the rotor 78 and the stator 62 are virtually identical, and therefore, for purposes of illustration, FIG. 3 is taken looking at the rotor assembly 78 from the downstream side looking stream, whereas FIG. 9 is an axial view of the stator taken from the upstream side looking downstream to emphasize the similarity of their configurations, although they are of opposite hand with respect to blade angle when assembled in a working machine. In the stator 62, however, the outer margins or edge portions 60 of each blade 67 extend radially outwardly to a somewhat greater extent than do their counterparts 132 in the rotors 78, since the outer edge portions 60 are adapted to be engaged in locking relation by the mounting rings 58 (FIG. 1). Thus, in the areas where stators and rotors are placed in facing relation, corresponding parts thereof are mirror images of each other.

Referring now to FIG. 10, a rotor unit 78a is shown for inclusion in an intermediate group of the turbo pump. It is similar to the rotor 78 shown in FIGS. 2 and 3, except that the spacings between the working faces 114a, 116a, 11 8a respectively and the faces 120a, 122a, and 124a are, in the direction of rotation thereof, somewhat less so that there is a very slight overlap between a trailing edge of a typical segment A and the leading edge 152 of the segment A immediately following it. The same situation is present in respect to the respective trailing edges 154 and leading edges 156 of blade segments B, as will be noted, and the same is true of the edges of segment C.

It may also be noted by reference to FIG. 10 that the angular inclination of the

faces

114a, 116a and 118a in respect to a rotational plane is somewhat flatter than for a rotor near the inlet of the turbo pump, and in this case, the angle between a face and the plane of rotation is preferably about 20. The

front face portions

114a, 116a, 118a are wider than their counterpart faces 108, '110, 112, in the first group, and accordingly, the entire width, measured parallel to the face, of

faces

114a, 116a, 118a is significantly larger than that of their counterpart faces 114, 116, 118 in the first stage, the axial thickness of the

stators

78 and 78a being the same. However, the thickness of each blade segment A, B, C in the second group, measured perpendicular to a face thereof, is slightly reduced in relation to the same dimension of first stage blades.

Referring now to FIG. 11, a stator 62a is shown which is a substantial mirror image of rotor 78a and, except for the greater radial extent of the outer edge portion 60a of blade segment D, and the inward extent defined by surface 66a, this stator is of exactly the same geometry as its associated rotor 78a.

Referring now to FIGS. 12-15, it is shown that a rotor 78b for a group near the outlet end of the pump includes a blade 94!) having only a single segment G, which is defined by a downstream face 114b, a flat front face portion 10811, a leading edge 152i; and a trailing edge 150b. The radial extent of the working face 114!) is relatively small compared to the overall diameter of the rotor 78!), and the trailing edge-leading edge overlap is substantial, being significantly in excess of that present in the intermediate stages of rotors.

By reference to FIGS. 13 and 14, it will be noted that the downstream and upstream blade faces 114b, 1201; are of a relatively flat pitch, in this case about 10. It will also be noted in FIG. 14, as with the other rotor blades, that the innermost or root portion 154 of segment G is of reduced thickness with relation to the end face portion 106b thereof. 7

Referring now particularly to FIG. 15, it will be seen that the spacing between faces 120b, 114b, measured perpendicular to the blade faces, is small in relation to its counterpart in the rotors used with the inlet and intermediate stages, although, because of the low pitch of the blades, the spacing therebetween in the direction of movement is significantly larger. As pointed out above, however, blade chord spacing ratios and blade thickness are still of dimensions and relations contrary to prior art teachings and products.

The stator which is associated with each third group or rela tively higher pressure region rotor is not shown, it being understood that, like the other stators, it is a substantially mirror image of its associated rotor.

Typically, a pump made according to the invention comprises from about two to about seven or more pressure stages, each composed of a rotor and stator of the types illustrated in FIGS. 2-9', a like number of stages consisting of rotor-stator pairs of the type illustrated in FIGS. 10 and 11', and a similar number of stages comprised of rotors such as that shown in FIGS. 12-15, with associated stators. It is not necessary that each multistage group assembly have the same number of rotor-stator pairs, but it is preferred according to the invention that the stages most upstream have the configurations shown in FIGS. 2-9, that the intermediate stages be comprised of rotors and stators such as those illustrated in FIGS. 10 and 11 and that the last stages include rotors of the type shown in FIGS. 12-15. As pointed out above, the number of stages within groups may vary considerably, and the number of groups in each compressor section may vary somewhat. The determination of the number and type of stages and groups is made depending on the requirements which the pump will be required to fitl. More first stages, with more open blades, give greater volume, and more stages of the type illustrated in FIG. I2, for example, give higher pressure rise per stage.

Reference will now be made to FIGS. 16 and 17 which illustrate somewhat diagrammatically the components having the characteristics referred to herein, and which show the points at which the measurements in question are taken. By reference to FIG. 16, for example, it will be noted that the blade chord is designated b, the blade angle is designated and is measured as the included inclined angle between the plane of rotation and the working face of the rotor blade, with blades having higher numerical value angles being referred to as more steeply inclined or higher pitch blades, and those having angles of smaller numerical values being referred to as flatter or lower pitch blades. The tangential spacing between blades is designated s", with the total tangential space between corresponding parts of adjacent blades being designated s,,". The interblade space to blade thickness ratio, s/b, will be referred to further herein. The thickness of a blade is designated 1", and therefore, s will be equal to s plus t divided by the sine of angle a". With a 45 blade angle, for example, and 1 equal to 1, s equals s plus 2; with a" equal to 30, 5,, equals plus 2. The interblade spacing measured perpendicular to a blade working surface is designated w and s is equal to w times the sine of the angle When reference is made herein and in the claims to interblade spacings and blade thicknesses, the measurements intended to be meant are those adjacent the outer ends of the blades, that is, for example, the spaces between segments A or D.

Referring now to FIG. 17, the radius of the hub is designated r,, and the outer radius of the rotor is defined r,, the difference being the radial blade span or length l The length l in relation to interblade spacings sis referred to as the aspect ratio of the section in question. The location of the blades on the hubs and the desired spans or lengths thereof will be discussed in relation to the overall rotor radius r,.

According to the present invention, a number of generalizations involving these angles and dimensions and the relations thereof to one another have been formulated and typical desirable values for these angles and dimensions characterize components illustrated herein which have been found to produce best results. Thus, speaking generally, it will be understood that with gas molecules being enclosed in a highvacuum region, a number of factors relevant to their behavior when acted upon by a pump such as that described herein are pertinent.

For example, the speed of the molecules of a particular gaseous species will depend on the absolute temperature and on the molecular weight of the molecules thereof. The nature of the gas flow will depend upon the relative frequency of collisions of gas molecules with each other or with a rotor or stator blade. This will be determined by the mean free path of the molecules in relation to the blade and interblade dimensions. Therefore, with a sufficiently large mean free path, compared with the blade and interblade dimensions. the behavior of the molecules is predominantly, and in fact almost exclusively, affected by their collisions with the working parts of the apparatus rather than with one another. Because of the highvacuum conditions present, the temperatures throughout the turbopump are generally substantially uniform, since conduction and radiation of energy from the blades are very large compared with the kinetic energy of molecules striking the blades. One principal factor affecting turbo pump per formance is the ratio of blade speed to mean molecular speed, while other factors, such as blade geometry and blade spacing, influence the likelihood that gas molecules may pass through a rotor or stator more easily in one direction than in the contrary direction. Since the gaseous densities increase downstream by many orders of magnitude, and the volume flows correspondingly decrease, different values for the various.

parameters, namely, a s/h, r,,/r,, and r/w" are selected for different stages in order to obtain optimum performance with respect to high level of vacuum and high volume flow or pumping speed.

Since the likelihood that molecules will pass in the desired direction to produce the desired vacuum level depends on all of these factors named above, and since prior theories of turbopump construction have been embodied in turbopumps which have not been entirely satisfactory, tests were conducted and apparatus made to determine whether a new approach to turbopump design would be successful. As a result of such efforts, turbomolecular pumps which are greatly improved were produced, and it has been shown that some factors previously believed important in pump design may be neglected for practical purposes, while other factors are relatively more important, and some factors considered in the present design were apparently not considered at all in the prior art.

With the above in mind, the results of the experiments conducted indicated that, with rotor-stator configurations such as those shown in FIGS. 2-15, unexpectedly large pressure rises were able to be obtained across those stages comprising the middle and downstream groups of rotors, while excellent flow volume was retained; and unexpectedly large flow volume was obtained, particularly with the rotor-stator pairs making up those stages comprising the group nearest the high-vacuum end of the pump, but also in intermediate group stages. As a result, the overall performance of the pump was outstanding, exceeding by several times the performance of similar sized pumps both in respect of pressure attainable with a given number of stages and flow volume or pumping speed.

To summarize the results of the tests made, it was determined that rotors and stators near the inlet end of the turbo pump should preferably incorporate a relatively steep angle pitch, say about 40 for example, and that the blade thickness I should be as small as possible consistent with requisite strength and manufacturing feasibility, in relation to the spacing s between blades. For rotors and stators near the inlet end, the relation of the hub radius r,, to the total radius r, should be in the range of about 0.4 to about 0.8, and preferably about 0.5 or 0.6. The length or span of a rotor or stator blade near the inlet end, in relation to the interblade spacing 3, should be at least L5 to I. It was discovered, for example, that performance improved somewhat with aspect ratios increasing from about 1.5 up to about 4 or 5, but that after an aspect ratio of about 4 to 5 was reached, there was no significant advantage to a further increase therein. The ratio of interblade spacing s to thickness t for rotors or stators near the inlet end should be at least unity, and preferably in the range of4 to 6 to l or more, limited only by considerations of strength and manufacturing feasibility. Thus, referring to the embodiment of FIG. 3, dimension I (measured perpendicular to the blade face) for segment A would be about 0.055 inches to 0.105 inches, while the interblade spacing s, taken as shown in FIG. 16, would be about 0.430 inches, for example. The ratio is somewhat less with respect to the inner segments B and C, as illustrated, but the ratio is nevertheless still large compared to prior art constructions and prior art teachings.

Another parameter which is significant is the ratio between the interblade spacing s and the blade chord b. It is preferred that this number also exceed unity for rotors and stators near the inlet end, although prior art teachings have tended to indicate the contrary. Thus, for example, measurements of transmission coefficient of first stage blades indicated that as the spacing to chord ration (s/b) increased from about I to 4 up to about 3 to 2, the transmission coefficient more than tripled, increasing from about 0.2 up to somewhat over 0.6. In determining the ultimate dimensions of the units in question, blade speed is an important factor, with tip speeds of about 900 to 1,400 feet per second or more being desired, the limitation being determined from considerations of strength to prevent bursting due to centrifugal forces. In the pumps described herein, set forth merely by way of example and not by way of limitation, rotors of about 6 to 7 inches in diameter were used, with a total axial thickness of about 0.120 inches, resulting in a blade chord of about 0.180 inches for the first stage rotors and stators. A very important feature is that there is significant tangential space between a trailing blade edge and the leading edge of the following blade, so that much larger volume flow through the inlet end stages is able to be obtained than that taught by the prior art or that attainable in practice with prior art apparatus.

Thus, summarizing the dimensional relations preferred for a rotor or stator near the inlet end, the angle a" should be 20 to 50 and preferably about 40, s/b should be from about 1.0 to about 2.5 preferably about 2.0, and r,,/r, should be from about 0.4 to about 0.8 and preferably about 0.5 to 0.6. The aspect ratio US should be at least 1.5 and need not be greater than 4. The ratio t/w should be as small as possible consistent with strength and manufacturing feasibility.

The parameters for an intermediate stage rotor or stator should be as follows: Blade angle a" should be from 15 to 30, preferably about 20, s/h from about 0.75 to about 1.5, preferably about 1.0, and r,,/r, may be somewhat the same as in the first stage, namely about 0.5 to 0.8, preferably about 0.5 or 0.6. Trailing edge-leading edge overlap may be present, but should not be large. The ratio l/w should be as small as possible consistent with strength and manufacturing feasibility.

For the rotors and stators near the outlet end, blade angle (1" should be even smaller, from about 5 to about 20, preferably about 20, s/b may be from about 0.3 to about 1.0, preferably about 0.75 and r /r, Should be from about 0.75 to about 0.95, preferably about 0.9. 1n the outlet end stages, because of the volume flow, which is much reduced by the compression of the gas, it is not so critical that blade thickness may be very small, for instance, it may be about one half the interblade spacing. The trailing edge-leading edge overlap should be considerable, for example, about one half the blade chord.

A pump constructed as set forth herein is capable of attaining heretofore unattainable overall pressure rises as well as large pressure rises across the various individual stages. For example, when connected to a fore pump of conventional construction and capable of producing a "torr vacuum level, a pump as described herein is capable of achieving pressure rises of up to 7 factors of 10 or more therein. Thus, with an outlet pressure of 10' torr, inlet pressures of 10*" or even substantially less are readily attainable, that is, the outlet pressure is 10,000,000 times greater than the inlet pressure. With such extremely high vacuums, molecular mean free paths are sufficiently large that gas flow in the pump is of an entirely different character than gas flow in the axial flow compressor of a gas turbine or the like where the flow mechanism is entirely different. and where pressure ratios, for example, of only about 1.25 to l are usual. In contrast to this situation, pressure rises across individual pressure stages in a turbomolecular pump made as set forth herein may exceed 25 to 1.

Accordingly, it has been discovered that, in a turbo-type vacuum pump made according to the criteria set forth herein, unexpectedly excellent results have been made possible which were not heretofore believed attainable, and which in fact were not attainable with prior art turbopumps, particularly those bearing a superficial resemblance to the pumps of the present invention, but differing therefrom in the important respects referred to herein. It will thus be seen that the present invention provides an improved turbomolecular pump having a number of advantages and characteristics, including those hereinbefore pointed out, and others which are inherent in the invention.

lclaim:

1. A vacuum pump for operation in the free molecule flow pressure range, comprising, in combination, a housing, said housing having an inlet and an outlet means operatively associated therewith, and at least one rotatable impeller as sembly and a cooperating stator assembly, said stator assembly comprising a plurality of axially spaced apart stator elements disposed between said inlet and outlet means with each stator element including a row of spaced apart stator blades, said impeller assembly including a plurality of rotor elements axially spaced apart and also disposed between said inlet and outlet means, said rotor and stator elements being in axially interleaved relation to each other to define a plurality of pressure stages, at least one of said rotor elements which is located in a pressure stage generally near said inlet means including a row of radially outwardly extending inlet pressure stage rotor blades, said inlet pressure-stage rotor blades having in relation to the direction of rotation, front and rear rotor blade faces defining therebetween a given blade thickness, each of said front and rear rotor blade faces including a leading edge portion and a trailing edge portion which define therebetween the chord of each of said respective blade faces, a given portion ofa rear blade face and the corresponding portion of the front blade face of the immediately following rotor blade in the direction of rotation of said inlet pressure-stage rotor element defining a given interblade spacing, said interblade spacing being greater than the dimension of said chord, said front and rear blade faces each defining a given angle with respect to the plane of rotation of said inlet pressure-stage rotor element of from 20 to 50, and said interblade spacing being at least 1 /2 the thickness of said blade, whereby a trailing edge of a given rear blade face in said inlet pressure-stage rotor element is spaced forwardly apart in the direction of rotation of the leading edge of the front blade face of the next following blade in said inlet pressure-stage rotor element so as to permit an unobstructed line of sight through the space between said blades in a direction parallel to the axis of rotation of said inlet pressure-stage rotor element, at least in the region of said element near the radially outer portions of said blades. I

2. A vacuum pump as defined in claim 1 wherein said front and rear blade faces of said respective leading and following blades in said inlet pressure-stage rotor element are substantially parallel to each other.

3, A vacuum pump as defined in claim 1 in which said given interblade spacing is at least 4 times said thickness of said blade.

4. A vacuum pump as defined in claim 1 in which said given angle is about 40.

5. A vacuum pump as defined in claim 1 in which said interblade spaeing is up to about 2.5 times the dimension of said chord.

6. A vacuum pump as defined in claim 1 in which said interblade spacing is about 2.0 times the dimension of said chord.

7. A vacuum pump as defined in claim 1 in which each of said blades in said inlet pressure-stage rotor element has a given radial extent, and in which said radial extent is at least 1% said interblade spacing.

8. A vacuum pump as defined in claim 1 in which said inlet pressure-stage rotor element comprises a hub portion and a blade row portion, said hub portion terminating adjacent the roots of said blades, said hub portion having a given radius, and in which said tip portions of said blades define the outer radius ofsaid rotor, said hub radius being from 0.4 to 0.8 times said outer radius of said inlet pressure-stage rotor element.

9. A vacuum pump as defined in claim 1 in which said inlet pressure-stage rotor element comprises a hub portion and a blade row portion, said hub portion terminating adjacent the roots of said blades, said hub portion having a given radius, and in which said tip portions of said blades define the outer radius of said rotor, said hub radius being from 0.5 to 0.6 times said outer radius of said inlet pressure-stage rotor element.

10. A vacuum pump as defined in claim 1 in which at least one of said stator elements located in a pressure stage generally near said inlet means includes a row of radially outwardly extending blades, each blade having, in reference to the direction of gas flow therethrough, front and rear blade faces, each of said front and rear blade faces including a leading edge portion and a trailing edge portion, said front and rear blade faces defining therebetween a given blade thickness and defining between a given portion of a rear blade face and the corresponding portion of an adjacent front blade face, in said reference direction, a given interblade spacing, said faces lying substantially parallel to each other and being disposed at a given angle in relation to the plane of said stator element, said blades being disposed so that the trailing edge of a given rear blade face is spaced forwardly apart in said reference direction from the leading edge of the next following front blade face so as to permit an unobstructed line of sight through the spaces between said blades in a direction perpendicular to said plane of said stator element, at least in the region of said element near the radially outer portions of said blades.

11. A vacuum pump as defined in claim 1 in which at least one other of said rotor elements includes a row of radially outwardly extending blades, each blade of said one other of said rotor elements having in relation to the direction of rotation, front and rear blade faces which each have a leading edge and a trailing edge portion, said front and rear blade faces defining therebetween a given blade thickness and defining between a given portion ofa rear face and the corresponding portion ofa following front blade face. in the direction of rotation, a given interblade spacing, said faces being disposed at a given angle in relation to the direction of rotation thereof, and in which said given angle of said blades on said one other of said rotor elements is from to 30, measured between the plane of rotation and one of said rear faces.

12. A vacuum pump as defined in claim 11 in which said interblade spacing of said other of said rotor elements is at least W1 times said blade thickness.

13. A vacuum pump as defined in claim 11 in which said leading and trailing edges of each of said blade faces on said other of said one rotor elements define therebetween the chord of each of said blade faces, and in which said interblade spacing is from 0.75 to 1.5 times said chord.

14. A vacuum pump as defined in claim 11 in which said one other of said rotor elements comprises a hub portion having a given radius and terminating adjacent the roots of said blades, and in which the tips of said blades define the outer radius of said one other of said rotor elements, said hub radius being from 0.4 to 0.8 times the extent of said outer radius of said one other of said rotor elements.

15. A vacuum pump as defined in claim 1 in which at least one other of said rotor elements includes a row of radially outwardly extending blades, each blade of said one other of said rotor elements having in relation to the direction of rotation, front and rear blade faces which each have a leading edge and a trailing edge portion, said front and rear blade faces defining therebetween a given blade thickness and defining between a given portion ofa rear face and the corresponding portion ofa following front blade face, in the direction of rotation, a given interblade spacing, said faces being disposed at a given angle in relation to the direction of rotation thereof, and in which said given angle, measured between the plane of rotation and one of said rear faces, is from 5 to 16. A vacuum pump as defined in claim 15 in which said one other of said rotor elements comprises a hub portion having a given radius and terminating adjacent the roots of said blades, and in which the tips of said blades define the outer radius of said one other of said rotor elements, said hub radius being from 0.75 to 0.95 times the extent ofsaid outer radius of said other of said rotor elements.

17. A vacuum pump as defined in claim 15 in which said leading and trailing edges of each of said blade faces on said one other of said rotor elements define therebetween the chord of each of said blade faces, and in which said interblade Lli spacing is from 0.3 to 1.0 times said chord.

18. A vacuum pump as defined in claim 11 in which said given angle of said blades on said other of said rotor elements is about 20, measured between the plane of rotation and one of said rear faces.

19. A vacuum pump as defined in claim 15 in which said given angle of said blades on said other of said rotor elements is about 10, measured between the plane of rotation and one of said rear faces.

20. A vacuum pump for operation in the free molecule flow pressure range comprising, in combination a housing, said housing having an inlet and an outlet means operatively associated therewith, and at least one rotatable impeller assembly and a cooperating fixed stator assembly, said stator assembly comprising at least three groups of axially spaced sta tor elements disposed between said inlet and outlet means with each stator element including a row of spaced-apart blades, said impeller assembly including at least three groups of rotor elements axially spaced apart and also disposed between said inlet and outlet means, all of said rotor elements in each of said groups of rotor elements including a hub portion and a row of radially outwardly extending blades, said rotor and stator elements being in axially interleaved relation to each other to define a plurality of pressure stages, each rotor element blade having, in relation to the direction of rotation, front and rear blade faces, each of said front and rear blade faces including a leading edge portion and a trailing edge portion which define therebetween the chord of each of said respective blade faces, said front and rear blade faces defining therebetween a given blade thickness, a given portion ofa rear blade face and the corresponding portion of a following front blade face defining, in the direction of rotation, a given interblade spacing, said front and rear blade faces defining a given angle in relation to the direction of rotation thereof, all of the blades in each rotor in the group nearest said inlet being disposed so that the trailing edge of a given rear blade face is spaced forwardly apart in the direction of rotation of said element from the leading edge of the next following front blade face so as to permit an unobstructed line of sight through the spaces between said blades in a direction parallel to the axis of rotation of said element, at least in the region of said element near the radially outer portions of said blade; and, for the group of rotor element blades disposed nearest said inlet, said given angle in said rotors is from 20 to 50, said interblade spacing is at least 1% times the thickness of said blade and greater than the dimension of said chord, the radial extent of each blade is at least 4 times said interblade spacing, and the radius of the hub portion of the rotor elements in said inlet group is from 0.4 to 0.8 times the radius of said rotor elements; and, for the group of blades disposed intermediate said inlet and said outlet ends, said given angle is from 15 to 30", said interblade spacing is about l /z times said blade thickness, said space between blades is about equal to the leading edge-trailing edge dimension of each blade, and the radius of the hub portion of the rotor elements in said intermediate group is from 0.5 to 0.8 times the radius of said rotor elements and, for the group of blades disposed nearest said outlet, said given angle is from 5 to 20, said interblade spacing is about equal to said blade thickness, said interblade spacing is about 0.75 times the leading edge-trailing edge dimension of said blades, and the radius of said hub portion of the rotor elements in said outlet group is from 0.75 to 0.95 times the radius of said rotor elements.

21. A vacuum pump as defined in claim 20 in which said stators are arranged in groups corresponding to the groups in which said rotors are arranged, and in which the blade dimensions and configurations are such that the stators are substantial mirror images of the rotors with which they are associated in use.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,644, 051 Dated February 22 1972 Ascher H. Shapiro It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, line 28, "2" should read V 2 Column 11, line 35, after "said" (second occurrence) insert -one Column 11, line 36, after "said delete -one Signed and sealed this 6th day of February 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM po'wso (10'69) USCOMM-DC 60376-0 69 US. GOVERNMENT PRINTING OFFICE: 1969 O3G8-334.

UNITED STATES PATENT OFFICE CERTIFICATE;@F CORRECTIQN Patent No. 051 A Dated February 22, 1972 Inventor(s) Ascher H. Shapiro It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Abstract, line 9, "on" should read --of In the Abstract, line 12, after: "thickness" insert a comma In the Abstract, line 15, after "presence" insert --or absence Col. 2, line 74, "event" should read .--extent- Col 3, line 7., "attached" should read attained-- Col 4, line 7, "in" should read -to-- Col 5, line 64, "stream" should read -upstream- Col 7, line 40', "spacings" should read spacing- Col. 8, line 70, "ration" should read -ratio Signed and sealed this 3rd day of October 1972.

(SEAL) Attest:

EDWARD MQFLE T H R JR. 7 ROBERT GOTTSCHALK attesting Officer v Commissioner of Patents -1050 (10-69) USCOMM-DC suave-P69 t i ".5. GOVERNMENT PRINTING OFFICE l9! O-366-J3|

Claims

1. A vacuum pump for operation in the free molecule flow pressure range, comprising, in combination, a housing, said housing having an inlet and an outlet means operatively associated therewith, and at least one rotatable impeller assembly and a cooperating stator assembly, said stator assembly comprising a plurality of axially spaced apart stator elements disposed between said inlet and outlet means with each stator element including a row of spaced apart stator blades, said impeller assembly including a plurality of rotor elements axially spaced apart and also disposed between said inlet and outlet means, said rotor and stator elements being in axially interleaved relation to each other to define a plurality of pressure stages, at least one of said rotor elements which is located in a pressure stage generally near said inlet means including a row of radially outwardly extending inlet pressure stage rotor blades, said inlet pressure-stage rotor blades having in relation to the direction of rotation, front and rear rotor blade faces defining therebetween a given blade thickness, each of said front and rear rotor blade faces including a leading edge portion and a trailing edge portion which define therebetween the chord of each of said respective blade faces, a given portion of a rear blade face and the corresponding portion of the front blade face of the immediately following rotor blade in the direction of rotation of said inlet pressure-stage rotor element defining a given interblade spacing, said interblade spacing being greater than the dimension of said chord, said front and rear blade faces each defining a given angle with respect to the plane of rotation of said inlet pressure-stage rotor element of from 20* to 50*, and said interblade spacing being at least 1 1/2 the thickness of said blade, whereby a trailing edge of a given rear blade face in said inlet pressure-stage rotor element is spaced forwardly apart in the direction of rotation of the leading edge of the front blade face of the next following blade in said inlet pressure-stage rotor element so as to permit an unobstructed line of sight through the space between said blades in a direction parallel to the axis of rotation of said inlet pressure-stage rotor element, at least in the region of said element near the radially outer portions of said blades.

3. A vacuum pump as defined in claim 1 in which said given interblade spacing is at least 4 times said thickness of said blade.

4. A vacuum pump as defined in claim 1 in which said given angle is about 40*.

5. A vacuum pump as defined in claim 1 in which said interblade spacing is up to about 2.5 times the Dimension of said chord.

7. A vacuum pump as defined in claim 1 in which each of said blades in said inlet pressure-stage rotor element has a given radial extent, and in which said radial extent is at least 1 1/2 said interblade spacing.

8. A vacuum pump as defined in claim 1 in which said inlet pressure-stage rotor element comprises a hub portion and a blade row portion, said hub portion terminating adjacent the roots of said blades, said hub portion having a given radius, and in which said tip portions of said blades define the outer radius of said rotor, said hub radius being from 0.4 to 0.8 times said outer radius of said inlet pressure-stage rotor element.

11. A vacuum pump as defined in claim 1 in which at least one other of said rotor elements includes a row of radially outwardly extending blades, each blade of said one other of said rotor elements having in relation to the direction of rotation, front and rear blade faces which each have a leading edge and a trailing edge portion, said front and rear blade faces defining therebetween a given blade thickness and defining between a given portion of a rear face and the corresponding portion of a following front blade face, in the direction of rotation, a given interblade spacing, said faces being disposed at a given angle in relation to the direction of rotation thereof, and in which said given angle of said blades on said one other of said rotor elements is from 15* to 30*, measured between the plane of rotation and one of said rear faces.

12. A vacuum pump as defined in claim 11 in which said interblade spacing of said other of said rotor elements is at least 1 1/2 times said blade thickness.

15. A vacuum pump as defined in claim 1 in which at least one other of said rotor elements includes a row of radially outwardly extending blades, each blade of said one other of said rotor elements having in relation to the direction of rotation, front and rear blade faces which each have a leading edge and a trailing edge portion, said front and rear blade faces defining therebetween a given blade thickness and defining between a given portion of a rear face and the corresponding portion of a following front blade face, in the direction of rotation, a given interblade spacing, said faces being disposed at a given angle in relation to the direction of rotation thereof, and in which said given angle, measured between the plane of rotation and one of said rear faces, is from 5* to 20*.

16. A vacuum pump as defined in claim 15 in which said one other of said rotor elements comprises a hub portion having a given radius and terminating adjacent the roots of said blades, and in which the tips of said blades define the outer radius of said one other of said rotor elements, said hub radius being from 0.75 to 0.95 times the extent of said outer radius of said other of said rotor elements.

17. A vacuum pump as defined in claim 15 in which said leading and trailing edges of each of said blade faces on said one other of said rotor elements define therebetween the chord of each of said blade faces, and in which said interblade spacing is from 0.3 to 1.0 times said chord.

18. A vacuum pump as defined in claim 11 in which said given angle of said blades on said other of said rotor elements is about 20*, measured between the plane of rotation and one of said rear faces.

19. A vacuum pump as defined in claim 15 in which said given angle of said blades on said other of said rotor elements is about 10*, measured between the plane of rotation and one of said rear faces.

20. A vacuum pump for operation in the free molecule flow pressure range comprising, in combination a housing, said housing having an inlet and an outlet means operatively associated therewith, and at least one rotatable impeller assembly and a cooperating fixed stator assembly, said stator assembly comprising at least three groups of axially spaced stator elements disposed between said inlet and outlet means with each stator element including a row of spaced-apart blades, said impeller assembly including at least three groups of rotor elements axially spaced apart and also disposed between said inlet and outlet means, all of said rotor elements in each of said groups of rotor elements including a hub portion and a row of radially outwardly extending blades, said rotor and stator elements being in axially interleaved relation to each other to define a plurality of pressure stages, each rotor element blade having, in relation to the direction of rotation, front and rear blade faces, each of said front and rear blade faces including a leading edge portion and a trailing edge portion which define therebetween the chord of each of said respective blade faces, said front and rear blade faces defining therebetween a given blade thickness, a given portion of a rear blade face and the corresponding portion of a following front blade face defining, in the direction of rotation, a given interblade spacing, said front and rear blade faces defining a given angle in relation to the direction of rotation thereof, all of the blades in each rotor in the group nearest said inlet being disposed so that the trailing edge of a given rear blade face is spaced forwardly apart in the direction of rotation of said element from the leading edge of the next following front blade face so as to permit an unobstructed line of sighT through the spaces between said blades in a direction parallel to the axis of rotation of said element, at least in the region of said element near the radially outer portions of said blade; and, for the group of rotor element blades disposed nearest said inlet, said given angle in said rotors is from 20* to 50*, said interblade spacing is at least 1 1/2 times the thickness of said blade and greater than the dimension of said chord, the radial extent of each blade is at least 4 times said interblade spacing, and the radius of the hub portion of the rotor elements in said inlet group is from 0.4 to 0.8 times the radius of said rotor elements; and, for the group of blades disposed intermediate said inlet and said outlet ends, said given angle is from 15* to 30*, said interblade spacing is about 1 1/2 times said blade thickness, said space between blades is about equal to the leading edge-trailing edge dimension of each blade, and the radius of the hub portion of the rotor elements in said intermediate group is from 0.5 to 0.8 times the radius of said rotor elements and, for the group of blades disposed nearest said outlet, said given angle is from 5* to 20*, said interblade spacing is about equal to said blade thickness, said interblade spacing is about 0.75 times the leading edge-trailing edge dimension of said blades, and the radius of said hub portion of the rotor elements in said outlet group is from 0.75 to 0.95 times the radius of said rotor elements.