US2465671A - Centrifugal compressor, pump, and the like - Google Patents

Description

March 29, 1949. R. D. VAN MlLLlNGEN ET AL 2,465,671

CENTRIFUGAL COMPRESSOR, PUMP AND LIKE 5 Sheets-Sheet 1 Filed Aug. 23, 1945 March 29, 1949. R. D. VAN MILLINGEN ET AL 2,465,671

CENTRIFUGAL COMPRESSOR, PUMP AND THE LIKE Filed Aug. 25, 1945 s Sheets-Sheet? R. D. VAN MlLLlNGEN ET AL March 29, 1949.

3 Sheets-Sheet 3 Filed Aug. 25, 1945 Patented Mar. 29, 1949 CENTRIFUGAL COMPRESSOR, PUIVIP, AND THE LIKE Reuel Duncan van Millingen,

Leicester, and

Geoffrey Raignallt White, Braunstone, England, assigncrs to Power Jets (Research & Development) Limited, London, England Application August 23, 1945, Serial No. 612,220

In Great Britain May 10, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires September 3, 1962 8 Claims. 1

This invention relates to impellers for'centrifugal compressors, pumps, and the like, and whilst capable of being applied to pumps for liquid is intended primarily to apply to air compressors.

The invention has for its main purpose to combine functional efliciency with practicability of manufacture, in relation to the impeller of such machines, especially when it is desired to provide machines of comparatively light weight and high rate of mass flow, which will operate with a high compression ratio per stage for the type of machine. Such attributes are especially desirable in compressors for use in aero-engine in- 1 stallations.

The invention is applicable both to compressors with unilateral and with bilateral intake, and

especially favours the latter if maximum advantage is to be gained. Impellers with which the invention is concerned are of the type in which the vanes define channels which in the region of entry are substantially axially directed and at the periphery or region of discharge are radially directed; and consequently in which the general path of the operative fluid is directed from the axial to radial (ignoring the fact of rotation).

Aerodynamic or hydrodynamic considerations show that the change of direction of the operative fluid in the channels of the impeller of a centrifugal compressor relative to the impeller, particularly in the intake region, should be as gradual as possible if losses and undesirable phenomena are to be avoided. Change of cross-sectional area of the channels, if such change is divergent in the direction of the flow, should also be gradual and substantially more gradual around a bend than would be requisite in a straight diffuser. The angle of attack of vanes at entry should be as nearly as possible zero, referred to the relative direction of the flow to the vanes; it follows that (the vane leading edges necessarily having radial length) the leading edge regions of the vanes should have a radially decreasing angle relative to the plane of rotation because of the radial velocity gradient along the leading edge, unless the tangential or axial velocity of entering air varies radially. Mechanical and manufacturing considerations however make it dificult to give the leading edge at its least radius any angle other than that of the remainder of the vane, since it is assumed that the vanes are to be made integral with the central boss or hub of the impeller and its disc part. For high rotational speeds it is moreover desirable that the vane structure should only be subject to loads which they can comfortably withstand, and cen- 2 trifugal stresses are of a high order; therefore it is mechanically desirable that the centre of mass of a vane should lie in a radius of rotation which lies within the thickness of the vane and so to design the rest of the vane that the centrifugal load due to each element of it results primarily in tensile stress, such secondary bending stress as may be inevitable being kept to the absolute minimum. The foregoing design considerations have to be compromised with ease of manufacture. Th invention seeks to provide impellers which ineach of these respects comes as near to the ideal as is practicable.

A centrifugal compressor or pump, especially for comparatively high mass flow and small dimensions, according to this invention ha an impeller of the type stated (such as will for brevity be referred to as a centrifugal impeller) in which the substantially radial vanes, boss, and disc, are made integral, and each of these vanes is made with flat surfaces (except for the regions immediately adjacent the margins of the vane) and lies in a plane which is inclined to the axis of rotation and to the plane of rotation, at an angle which corresponds substantially to the angle of attack required of the vane at its minimum radius at the boss, or is positive in incidence at this station, by an angle between zero and the critical angle for streamline flow in the design condition of operation. Preferably each vane is also so disposed that a radial line from its centre of mass to the axis of rotation, in the plane of rotation, regarding the whole vane as an element lies in the thickness of I the vane and the distribution of mass of the vane about the centre of mass is preferably such that the secondary cencent pairs of vanes, but should not be so far from corresponding to a zero angle of attack, as to 7 become critical from the streamline point of view (and it is to latitude within this compass that the word substantially is directed). It will be found that in design it is possible to arrange that this bending is such as to bring the centres of mass of relatively unsupported parts of the vane near or on radii which lie in the vane thickness,

so that centrifugal bending stresses in the already bent portion, are minimised and first-order stress is all tensile.

The impeller is found to be such that the vanes can be machined by straightforward milling methods, no complicated profiling machinery being required. It may also lend itself to casting, moulding, or extrusion methods of manufacture, without the complexity of awkward re-entrant shapes in recesses of deep and narrow character;

The vanes may be formed with some extent of overhang at their leading edges; that is to say the overall axial length over the vanes is greater than that of the boss portion. In such cases the angle of the plane of a vane to the axis may be reduced to a degree according to the extent to which the overhang can be bent to take up further angle; or, the overhang may be flat and merely constitute an unbent or uncambered leading edge, in effect decreasing the pitch/chord ratio of the vanes.

The invention is preferably applied to compressors with bilateral intakes; in such cases the impeller is'double-sided and symmetrical about its plane of rotation in the disc. As viewed on Fig. 1 is a perspective view of a complete impeller.

Fig. 2 is a (fragmentary)-view looking radially inwardly at the periphery.

Fig. 3 is a (fragmentary) view in axial elevation.

Fig. 4 is a half-section through the axis and is related to the sectional views 4A to 4E, taken on the lines A to E of Fig. 4.

Fig. 5 is an axial view of a blank forging, showing the attack of a milling cutter to form the vanes.

It will be seen from a general view of the drawings that the impeller is of bilateral type, that is to say it is for a compressor having double inlet eyes. The vanes, on each side of a central web or disc, are so disposed that they present a herringbone pattern at the periphery. The vanes. the disc, and the main boss or hub of the impeller, are integral, and though it is not possible to define precisely what is disc and what is boss because these parts merge together, they are nevertheless convenient terms to use, since they are adequately descriptive in the context in which they are used.

Fig. 4 gives a good indication of the parts. The vanes l spring from the boss 2 and disc 3. The boss is axially bored at 2A.

Each vane l is initially formed (except at its margins which will be described) by flat surfaces, such as result from cutting with an edge cutter (represented at 5, Fig. 5) with a straight feed. By setting up the work (comprising an appropriately shapedblank) and the cutter 5 so that the axis of the cutter is inclined or skewed relative to the axis of the impeller, to the plane of rotation of the impeller, and to the radius of the impeller where the cut is made (as illustrated in Fig. 5), the features of the invention result. Fig. 5 shows first cuts being made; these are followed by a second series of cuts which define the second surface of each vane.

nels of the impeller, and further operations to 7 remove irregularities, burrs etc. as may be necessary, also to trim up the margins and "radii. The machining operations can be performed by straightforward tools, where desired, and thus no elaborate profiling needinghighly specialised machine tools etc. is called for.

In the course of cutting the two-surfaces of a vane, two or more cuts may be made with slight changes of the cutter angle, in order to taper the section of the vane axially or radially or both, and some blending between one so-formedfiat and another, may be provided. It can be discerned from Fig. 2 for example, that the vanes are tapered towards the leading edge, in the region of a bend afterwards mentioned; as cut and.before bending, the vane is made with flat surfaces.

The inner margins of the vanes, at IA, are blended into the disc and hub by radiusing, the cutter being appropriately shaped to this end. The free'margin IB is preferably left square and' can be formed simply by turning on a lathe. The tip |C- may be likewise left square or may be faired off in streamline manner. The leading edge ID is trimmed carefully to the aerodynamically required profile.

The vanes are formed, with an overhanging leading edge, by undercutting as at 6; this permits the whole length of the leading edge to be bent to required angles of incidence, whereas the angle of the vanes at the entry end of the inner margin IA (at 4F of Fig. 4) is of course fixed in manufacture by the skewing of the cutter. This angle which is that of the vane as a whole is selected to correspond (in the design condition of operation) with zero incidence or positive incidence within the critical angle for streamline flow.

The vanes, when formed in the manner described, can be so located with reference to the axis of the. impeller, that the centre of mass (as nearly as it can be ascertained) of each vane lies in a radius from the axis of rotation which is included in the thickness of the vane. Thus if the centre of mass is the point M (in Figs. 3, 4, 4C), a radius R. drawn from the axis to the point M, would be wholely within the metal thickness. This obviates any major bending loads being set up by centrifugal force. Moreover, owing to the skewed position of the vanes, the centres, of mass of particles of each vane, are not very far from likewise being on contained radii, though they cannot all be exactly so, so that suchbending stresses in vanes as do arise due to centrifugal force, are kept reasonably low, and all major loads are transmitted practically in tension.

In some conditions (for example where there is controlled pre-whirl or radially varying axial flow velocities of entering fluid) an impeller as so far described, 1. e. with entirely flat vanes, may be practical and eflicient as it stands. It is however preferred to cater for straight constant velocity axial fluid entry without pre-whirl, and this necessitates a variation of angle of incidence at the entry, at different radial stations. Such variation is afforded by bending the vanes. The nature of such bending is illustrated by the drawings and in particular by Fig. 4 and its related sectional figures which show that at lesser radius (4E) the leading edge of a vane is slightly bent forwards in the direction of rotation, and the degree of bending increases radially outwards to that shown at 4A at the tip of the leading edge. By this means the change in peripheral velocity due to increased radius is related to the fluid path so that the incidence at entry is zero, or is positive within the critical angle. At the ,location 41 however, the vane cannot be bent because its margin is not free, hence the above-mentioned fact that the angle of the whole vane is deflned by the incidence required at its minimum radius.

The bending of the vanes .is preferably by manipulation whilst hot, use being made of blocks or formers appropriately contoured, and temporarily held in the channels between one vane and the next, in known manner. Such bending as that shown has the incidental advantage that it brings thecentre of mass of the least well supported part of the vane, i. e. the region of the outer part of the leading edge, nearer the condition-where it lies on a contained radius.

In putting the invention into practice, it may be noted that turning operations to finish the free margin, or on the leading edges, should be performed with the vanes well damped against vibration; this can be accomplished by casting the whole impeller solid in wax prior to turning, and turning off both metal and wax, subsequently melting oil? the remaining wax.

It will be seen that the double sided impeller has its vanes so arranged that they are raked forwards in the intended direction of rotation,

shown by the arrow in Fig. 1, that is to say so that the free margins IB and ID lead the margin IA.

We claim:

1. A centrifugal fluid impeller comprising in-* tegral boss, disc, and vanes, each of said vanes comprising a radially disposed main portion and an entry portion forming a continuation of the main portion, the vanes defining curved channels substantially axially directed at the fluid entry region and substantially radially' at the fluid discharge region and each of said vanes having flat surfaces except at the margin of the entry portion and lying in a plane which is inclined to the plane of rotation of the impeller at an angle which corresponds substantially to the angle of attack required of the entry portion of the vane at its minimum radius in the design condition of operation.

2. An impeller according to claim 1 in which the leading edge region of each vane at the eye of the impeller is bent towards the direction of rotation so as to present an angle of attack at each radial station corresponding substantially to the relative direction of fluid flow at entry in the design condition of operation having regard to the velocity of the leading edge at each such station.

3. A centrifugal fluid impeller comprising integral boss, disc, and vanes, each of said vanes comprising a radially disposed main portion and at an angle which corresponds toan angle of attack of the entry portion of the vane .at its minimum radius which is positive but less than the critical angle for streamline flow in the design condition of operation.

4. An impeller according to claim 3 in which the leading edge region of each vane at the eye of the impeller is bent towards the direction of rotation so as .to present an angle of attack at each radial station corresponding substantially to the relative direction of fluid flow at entry in the design condition of operation having regard to the'velocity of the leading edge at each such station.

5. A centrifugal fluid impeller comprising integral boss, disc, and vanes, each of said vanes comprising a radially disposed main portion and an entry portion forming a continuation of the main portion, the vanes defining curved channels substantially axially directed at the fluid entry region and substantially radially at the fluid discharge region and each of said vanes having flat surfaces except at the margin of the entry portion and lying in a plane which is inclined to the plane of rotation of the impeller at an angle which corresponds to an angle of attack of the entry portion of the vane at its minimum radius which is within the range from zero to the critical angle for streamline flow in the design condition of operation, and the approximate center of mass of each vane lying in a radius which is included in the thickness of the vane when struck from the axis of rotation.

6. An impeller according to claim 5 in which the leading edge region of each vane at the eye of the impeller is bent towards the direction of rotation so as to present an angle of attack at each radial station corresponding substantially to the relative direction of fluid flow at entry in the design condition of operation having regard to the velocity of the leading edge at each such station.

REUEL DUNCAN VAN MILLINGEN. GEOFFREY RAIGNALUI' WHITE.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Campbell et al. May 7, 1946 Patent No. 2,465,671

Certificate of Correction I March 29, 1949. REUEL DUNCAN VAN MILLIN GEN ET AL. I

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

In the heading to the printed specification, line 9, foreign filing date, for May 10, 1944 read September 3, 1942;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and seged this 16th daybf August, A. D. 1949.

THOMAS F. MURPHY, Assistant G'ommissioner of Patents.

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