US3414188A

US3414188A - Fan having hollow blades

Info

Publication number: US3414188A
Application number: US597042A
Authority: US
Inventors: Gallie Daniel Gordon
Original assignee: American Radiator and Standard Sanitary Corp
Current assignee: AMERICAN DAVIDSON Inc A CORP OF MICH
Priority date: 1966-11-25
Filing date: 1966-11-25
Publication date: 1968-12-03
Anticipated expiration: 1985-12-03

Description

Dec. 3, 1968 D. G. GALLIE 1 v 3,414,188

FAN HAVING HOLLOW BLADES Filed Nov. 25, 1966 n 1 4 sheets-sheet 1 x N y x 0 coe- /0` 4 a o a Gvo ce e .66 .e v g` r G Q Q Q 62- 72 /03 06 y ,6 Z0 o vos \/02 /00 fwn 50 I N VENTOR.

Dec. 3, 1968 D, G. GALLIE 3,414,188

FAN HAVING HOLLOW BLADES Filed Nov. 25, 196e 4 sheets-Sheet 2 Dec. 3, 1968 D. G. GALLIE 3,414,188

FAN HAVING HOLLOW BLADES Filed Nov. 25, 1966 4 Sheets-Sheet 5 j?? 5 I l INVENTOR.

Dec. 3, 1968 D. G. GALLIE FAN HAVING HOLLOW BLADES 4 Sheets-Sheet 4 Filed NOV. 25, 1966 INVENTOR.

United States Patent O 3,414,188 FAN HAVING HOLLOW BLADES Daniel Gordon Gallie, Allen Park, Mich., assignor to American Radiator & Standard Sanitary Corp.; New York, N.Y., a corporation of Delaware Filed Nov. 25, 1966, Ser. No. 597,042v 6 Claims. (Cl. 230-120) ABSTRACT OF THE DISCLOSURE One problem which occurs in connection with operation of fans is the separation of air or other fluid from the trailing faces of the fan blades as the blades rotate. This problem is particularly acute in connection with centrifugal fans having air foil blades. However, the problem also occurs in connection with other types of fans and fan blades, such as propeller fans and fans having radial blades. It will be appreciated that as a fan blade rotates, the air or other fluid upon which the fan operates tends to be compressed on the driving face of the fan blade while the layer of fluid tends to be separated from the trailing face as the blade, in essence, moves away from the trailing uid. This phenomenon is known as boundary layer separation.

There are two disadvantages associated with boundary layer separation. One is that the separated fluid assumes a direction Ihaving a large relative velocity component as the uid is deflected further rearward from the direction of wheel rotation. This condition is known as Wheelslip and as it generally lessens the magnitude of the absolute velocity vector, the tlow capacity and pressure of the fan is adversely influenced. Accordingly, the absolute velocity vector assumes a more radial direction lof ow due'to wheel slip which is detrimental to fan e'fliciency asadditional losses are incurred due to the greater tendency of the air to impinge on the 'housing surface. Accordingly, the radial dimensions of the housing may be reduced without incurring a greater loss than presently experienced in the conventional housing.

These problems are alleviated in accordance with* the present invention by providing means which reduce boundary layer separation with resultant increase in available fan capacity and improved fan eiciency. In addition, the present invention also provides eicient and simple means for controlling fan capacity from the maximum available capacity to a capacity less than what would be considered normal fan capacity. The present control of fan capacity is an impnovement over conventional fan control structures, for example, such as the provision of-variable-inlet vanes at the fan entry or variableoutlet vanes'at the fan outlet, in that the energy losses resulting from such variable vanes are considerably reduced.

It is therefore an object `of the present invention to provide means for controlling boundary (layer separation of uid from the trailing face of a fan blade.

Another object of the invention is to provide, in' one aspect, a Ihollow fan blade having perforations in t'he trailing face and a secondary impeller connected to the blade to create negative pressure within the interior of the blade which acts, throught the perforations,'to preice vent separation of uid from the trailing face ,of the blade.

A further object of the invention is to provide, in a lhollow blade, a slot at the blade trailing edge which functions to elevate-the ow and pressure capability of the fan by the diversion ofthe fluid stream on the trailing surface of the blades in a desired direction.

A still further object of fthe invention is to provide such a secondary impeller which may be adjusted to control the effect thereof on boundary layer separation andthe deilection of the air stream and thus control the fan capacity.

Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.-

In the drawings:

FIGURE 1 is a side elevational View in section of a centrifugal fau forming one embodiment of the present invention;

FIGURE 2 is a sectional view taken substantially along the line 2-2 of FIGURE l looking in the direction of the arrows;

FIGURE 3 is an enlarged View of the upper portion Vof the fan blade structure of FIGURE l illustrating the flow of lluid in one adjusted position of the secondary impeller;

FIGURE 4 is a sectional View of a fan blade illustrating the ow of fluid through the interior thereof;

FIGURE 5 is a side elevational View of the fan including a vector analysis of such a fan in a condition unmodified by the present invention and in a condition modied by the present invention;

FIGURE 6 is a front elevational view of a fan having radial-tipped blades modified in accordance with the present invention; l

FIGURE 7 is a view taken substantially along the line 7-7 of FIGURE 6 |looking in the direction of the arrows;

FIGURE 8 is a front elevational v"view of a radialbladed fan modified in accordance with the present invention;

FIGURE 9 is a sectional view of an axial flow fan embodying the principles of the present invention;

FIGURE l0 is a sectional view of a stator vane of the fan of FIGURE 9; and

FIGURE ll is a sectional viewfof ain impeller blade of the fan of FIGURE 9.

Before explaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Referring to FIGURE 1, it will be noted that the illustrated centrifugal fan 10 comprises a fan hou-sing 12 in which is mounted a primary impeller 13 having a blade structure including a plurality of air foil blades 14 mounted n spaced lapart relationship on a hub 16. The Ihub 16 is carried on a shaft 18. The shaft 18 extends exteriorly of the housing 12 and is driven by a motor 20 mounted on a suitable support structure 22. The impeller structure includes an annular rim str-ucture 24 which is carried on the edges of the blades 14. As best shown in FIGURE 5, the housing 12 is of a scroll-type and has an outlet 26. In operation, air or other iluid is drawn through the inlet 25 and is thrown centrifugally by the blades 14 toward and through the outlet 26.

Referring to FIGURES l-4, it will be noted that the bladesa 14 are hollow and comprise a driving wall mem- -ber 28, curved bottom or nose and trailing wall member 32. The ihub 16 and rim structure 24 form end closures for the blades. The trailing walls 32 have a plurality of perforations 34 which provide fluid communication between the trailing face of the blade land the interior of the blade. A slot 36 is provided on the trailing edge of the blade and extends across almost the entire width of the blade.

A secondary impeller structure is provided on the rearward side of the fan hub 16. The secondary impeller structure comprises a first annular wall member 38 having a peripheral ange 40 secured to the peripheral portion of the hub 16 fand having a web portion 42 extending outwardly therefrom to space the annular wall 38 from the hub 16. The annular wall 38 has a central opening 44 to space the inner periphery thereof from the shaft 18.

A second annular wall member 46 is secured to the rear side of the hub 16 and spaced outwardly from the first annular wall member 38. The wall member 46 has an inner peripheral flange 48 which is secured to the hub 16 and spaced radially inwardly from the inner periphery of the annular wall 38. Web portion 50 extends outwardly to space the annular wall 46 outwardly of the annular wall 38. The outer periphery of the annular wall 46 is approximately even with the web 42 of wall 48. A plurality of circumferentially spaced at blades 52 are provided in the space between the walls 38, 46. One blade 52 is provided for each fan blade 14. The number of fiat blades may vary in accordance with the application. The blades 52 extend from the outer periphery of the annular wall 46 to the inner periphery of the annular wall 38. As will be noted in FIGURE 2, the blades 52 are positioned approximately radially with respect to the center of rotation of the fan blade structure. Contrariwise, the blades 14 are positioned somewhere between a tangential and radial alignment with respect to the axis of rotation.

A plurality of openings 54, illustratively three, are provided in the hub 16 in alignment with the interior of the blades 14 to provide fluid communication between the blade interior and the annular compartment 56 defined by the hub 16 and the annular wall 38. Similarly, a plurality of openings 58, illustratively three in number, are provided inthe annular wall 46 in front of the leading surface of each of the secondary impeller blades 52. This is best illutrated in FIGURE 3. It will be noted that a portion of the wall 46 and blade 52 have been broken away in order to show the openings 58 in solid lines. An additional opening 59 is provided in the web 50 beneath each blade 52.

An axially movable adjusting structure is provided on the shaft 18 for controlling passage of fluid through the openings S4, 58, and 59. rIihe adjusting structure comprises a ring element 60 which is slideably received on the shaft 18. An annular wall member having a central opening 64 is secured to the ring member 60 as by welding. The member 62 comprises a first radially extending portion 66 having opening 67 and which terminates short of the web 50 of the annular Wall 46. A portion 68 extends outwardly andl slightly upwardly from portion 66 away from the annular wall 46. A radially outwardly extending portion 70 extends from portion 68. It will be noted that the portion 70 extends slightly beyond the outermost opening 58. The ring 60 may be adjusted axially in any desired manner, either manually or by automatic means. One suitable adjusting mechanism is illustrated in FIG- URE l. The adjusting mechanism comprises a sleeve which has a rotary fit on the shaft 18. The sleeve 100 has a cylindrical recess at one end which receives the ring 60 which is keyed to the shaft 18 by means of a key 102 which is received in slot 103 provided in the shaft 18. The

sleeves

100, 60 are connected together for axial movement by `means of ball-bearing structure 104. Thus, the sleeve 100 is free t0 remain stationary while the sleeve 60 rotates with the fan impeller. A rack 108 is formed on the outer periphery of the sleeve 100. A pinion 106 engages the rack 108. As will be appreciated, rotation of the pinion will cause the

sleeves

60, 100 to move axially to thus adjust the position of the plate 62 with respect to the member 46.

For an understanding of the operation of the fan, attention is first directed to FIGURE l. It will be appreciated that as the fan wheel rotates, both the primary impeller blades 14 and the secondary impeller blades 52 effectuate what may be termed a pumping action and create a negative pressure radially inwardly of their respective

leading edges

72, 74. In the case of the fan blades 14, this results in the normal drawing of fluid through the fan inlet 24. In the case of the secondary impeller, this negative pressure results in drawing air both through the passageway defined by

wall members

46, 70 and through the passageway defined by wall member 38 and hub 16 as illustrated by the arrows. Fluid is drawn into the latter passage through the openings 54 in the hub 16 from the interior of the blades 14. Fluid enters the blade 14 through the perforations 34 and the slot 36 as illustrated in FIGURE 4.

The fluid flow path aroun-d the lblades 14 is illustrated in FIGURE 2. The arrows 74 illustrate the flow over the driving face of the blade. The arrows 76 illustrate the flow over the trailing face of the blade. It will be l'appreciated that by drawing fluid through the perforations 34, the layer of air adjacent the trailing face is inhibited from separating therefrom because of the negative .pressure and the prevention of boundary-layer growth on the trailing face. The fluid through the slot 36 causes the layer of air leaving the trailing edge of the blade to be deiiected in the `direction of blade rotation.

Referring again to FIGURE l, it will be appreciated that the less uid that flows through the passage defined by the

Walls

46, 70, the more iiuid that will be drawn from the interior of the blades 14. Thus, by moving the ring 60 closer to the web 16, the more positive will be the control of the boundary layer of air on the trailing face of the blades. Similarly, the more fluid drawn through the slot 36, the greater the deection of the fluid leaving the trailing edge of the blade.

When the ring 60 is moved away from the hub 16, as illustrated in FIGURE 3, to increase the size of the passageway defined by the

wall members

46 and 70 and to open the opening 64, little if any fluid will be drawn from the interior of the blade 14. In fact, it is even possible to obtain a back pressure so as to force air into the interior of the blade 14 through openings 67 and 59 which results in actually increasing the boundary separation of fluid from the trailing surface of the blade 14.

The effect on capacity and efficiency of the above described system may be understood by a consideration of the vector analysis illustrated in FIGURE 5. Referring first to blade 14a, operation of the fan without modification in accordance with the present invention will first be considered. Fluid leaving the trailing edge of the blade 14a is identified in terms of velocity and direction of fiow by two major vectors that are components of a resultant or absolute vector. The entire mass of the fluid is caused, as a result of blade rotation, to have a vector tangential to the dotted circle 78 defined by the axis of rotation of t-he blade wheel and identied as vector b. At the same time, as a result of the directional effect of the blade and of boundary layer separation, the uid flow relative to the trailing edge of the blade 14a is represented by a vector in the direction of the vector a. Vectors a and b have a resultant vector c. As will be appreciated, vector c is progressively reduced in length the further vector a is deected rearwardly. The length of vector c is directly related to the pressure Ypotential of the fan while the mass of iiuid fiowing through the fan is related to the length of the radial component of vector c. Thus, reduction in the length of vector c results in lower fan pressure and in most instances a reduced flow rate. The rearward deilection of vector a has the added disadvantage of causing vector c to ow in a more radial direction and impinge on the scroll surface. As will be appreciated, the resultant shock and turbulence represents an energy loss which reduces the fan eiciency.

Referring now to blade 14b, it will be noted that three sets of vectors are represented. First, t-he vector c which is resultant of vectors a and b is illustrated. The vector d represents the deflection of uid rearward from the blade when the blade is Aprovided only with the perforations 34 and the secondary impeller. It will be noted that vector d is not deected rearwardly as much as vector a. Consequently, the resultant vector of vectors d and b, namely vector f, is longer than vector c. Vector f thus represents a considerable increase in fan pressure potential and flow capacity due to the increased radial component. Tlhe angular displacement of vector f is slightly to the rear of vectors c but the 'slight reduction in efficiency which this represents is many times compensated for by the increase in performance. Vector e represents the forward deflection of uid from the trailing edge of the blade when the blade is provided with both the perforations 34 and slot 36. It will be noted that vector e is deflected rearwardly much less than vector d. The resultant vector g is considerably longer than either vectors f and c. Additionally, vector g is less radially directed t-hus representing an increase in efficiency.

The improvement in fan capacity of the three systems is represented by the intervals X, Y and Z which are derived from the above blade variations in the respective order c, f and g. It will be noted that interval X is the smallest, with interval Y being the next large-st and interval Z being the largest, This is indicative of the increased capacity of the fan by -use of the present invention.

It will be understood from the discussion of the structure and operation of the fan, the increased fan capacity capable of being delivered by the present construction is not always desired. That is, it is frequently desired to have the fan output be considerably less than the maximum potential of the fan. Whenever it is desired to reduce the capacity, it is only necessary to move the ring member 16 away from the annular wall 46.

Another desirable feature in connection With the use of the secondary impeller is that the fluid pumped by the secondary impeller does not represent an overall energy loss. The uid from the secondary impeller is discharged into the fan housing and forms part of the uid pumped by the fan. This is dissimilar to a system, for example, in which a fluid re-circulating or siphoning structure is utilized with the result that the work done in circulating or siphoning represents an entire energy loss.

The present invention may be applied to types of fan structures other than the air foil centrifugal fan illustrated in FIGURES l-5. For example, FIGURES 6 and 7 illustrate the present invention as applied to a radiall bladed fan structure in which the blades 80 have a curved or air foil leading edge portion 82 which blends into radially directed portion 84. Normally, such blades are provided as single thickness, that is, they are not hollow as illustrated in FIGURE 6. However, in accordance with the present invention, the blades are constructed as hollow members in order to permit application of the invention.

It Will be noted that four openings 86 are provided for communication with the secondary impeller structure 88 as shown by the dotted arrows in FIGURE 7. The trailing edge of each blade 80 is provided with a slot 90 and the trailing face has perforations 91 for the purposes heretofore described in connection with the embodiment of FIGURES 1-5.

The invention may also be applied to a fan blade construction such as illustrated in FIGURE 8 wherein the blades 92 are straightradial blades. Such fans are used, for example, in handling owable materials other than gases, such as particulated solids. Radial blades are also usually constructed of a single thickness. However, for the purpose of the present invention, the radial blades 92 are hollow and have openings 94 for communication with a secondary impeller and each blade has a slot 96 in the trailing edge and perforations 97 on the trailing face for operation in accordance with the principles set forth in FIGURES 1-5.

FIGURES 9, l0 and 11 illustrate the application of the present invention for boundary layer control in connection with an axial flow fan 98. When applying the concept to a axial ow fan, lit is necessary that both the impeller blades 100 and stator blades 102 have an aerodynamic prole of hollow construction. The trailing surface 104 of the impeller blades is provided with a series of perforations 106 extending over the entire surface thereof and a slot 108 located along the trailing edge of the blade. As illustrated by the arrows, air is removed through the perforations as a means of boundary layer control. The boundary layer represents that portion of the air stream adjacent to the xed surface where a velocity gradient exists. The viscosity of the air causes a shearing action to exist across the boundary layer with the air velocity increasing from 0 at the boundary to the stream velocity some distance from the surface. The boundary layer continues to grow in thickness as the fluid moves .along the surface. A crucial point is reached Where the air separates from the surface for two reasons. The inertia of the air may not allow it to follow the blade profile where extreme curvature is involved. The air then moves along in a path representing a directional change per unit of time that is less severe than the blade surface with separation being the end result. This phenomonen is more common to axial flow type fans than centrifugal fans but does also occur in centrifugal fans having an extreme lade curvature. The second reason for separation occurs where the pressure increases as the air stream flows along the length of the blade. The fluid that has been incorporated into the boundary layer, however, has insufficient velocity for conversion to higher pressure. Thus a reverse ow is experienced with the boundary layer separating at this point.

By gradually extracting the boundary layer formation through the perforations, an air Velocity is maintained in the region where the boundary layer normally existed. This results in an equal pressure rise in the oW channel between the blades with back ow and separation being eliminated in the entire blade configuration becoming effective. 'This occurs4 in centrifugal fans. In axial fans, air velocity upon the blade surface represents lower pressures within that region in accordance with the general energy equations where energy may be present in various forms with the sum remaining constant. Due to the lower pressure along the blade surface, the ow path of the air is less likely to separate because the inertia of the air is offset by the attracting effect of the low pressure. As the air reaches the trailing edge of the blade without separating, air is withdrawn through the suction slot 108 to further enhance the fans performance. It has been proven by -tests that air removed through a slot in this manner diverts the `main air stream in the direction of the slot as long as the air remains attached to the blade surface upstream of the slot position. Consequently, the tangential component of the main air stream velocity is increased with the fan being able to perform at higher pressure levels, or the fan size may be reduced to offer a performance level equivalent to the standard fan.

Air is removed from the hollow impeller blade 100 through openings 110 located in the Wall of the hub 112. An auxiliary centrifugal fan structure 114 equipped with radial blades is located within the hub to lcreate a suction level that will provide the necessary outow from the blade. The auxiliary fan discharges into the inner casing behind the impeller that supports the fan stator vanes 102. The inner casing is sealed forward of the pulley 116 to prevent air leakage through an opening in the inner casing through which belts pass in driving the impeller from a motor (not shown) located outside of the casing. Thus, the casing is pressurized and air is forced into the hollow stator vanes 102 through openings 122 in the casing surface.

A series of narrow slots 120 are located in the trailing surface of the stator vanes 102. The slots 120 are inclined rearwardly. Air is discharged through the slots 120 in the direction of the main air stream illustrated by the arrows. This air moves along the blades exterior surface. As in the method of boundary layer control previously explained, the purpose of the air jets released through the slots is to prevent air separation. The blowing technique of boundary layer control associated with this method requires that motion be maintained in a boundary layer formation by the action of the jets. The boundary layer otherwise moves at a reduced velocity and maintains a higher static pressure in compliance with the general energy equation.

When the boundary layer is forced to flow at a higher velocity, a portion of the static pressure is converted to velocity pressure with a lower pressure potential existing along the blade surface. In turn, the main air stream more closely adheres to the blade surface due to the attraction offered by the low pressure zone with separation being delayed. The objective is to eliminate separation along the length of the vane in applying this technique. The advantages are full control of the air stream along the vane. This adds to fan efficiency. Turbulence and random flow occur with separation and have no purpose other than to detract from an available energy level that could otherwise -be applied in doing useful work. Useful work is reflected in higher pressure and flow rates which is the principal advantage of the slots 120. Similarly, the vane length may be reduced as the vanes are presently constructed with a gradual curvature leading to an extended length when flow separation is to be avoided. The positive control afforded by boundary layer control aids in guiding the main air stream at an increased rate of deflection and thereby moving through a given angular change in reduced time. This shortens the vane length. Vanes of reduced length offer less resistance to air flow because the surface area is reduced with the frictional losses being decreased. This factor enhances fan performance.

Having thus described my invention, I claim:

1. A fan comprising a casing having a fluid inlet and a fluid outlet, an impeller structure rotatably mounted in said casing, said impeller structure including a primary impeller and a second impeller coupled to said primary impeller for rotation therewith, said primary impeller including a plurality of like hollow blades, means defining a plurality of openings in the trailing face of said blades communicating with the hollow interior of said blades, means defining an elongate slot extending along the trailing edge of said blades and communicating with the hollow interior thereof, said secondary impeller having a plurality of inlets respectively communicating with the hollow interior of said blades and being operable to create a negative pressure in the interior of said hollow blades to inhibit boundary layer separation of fluid from said trailing faces of said blades and to divert the fiow of fluid from the trailing edge of the blades from paths tangent to the blades surface to paths inclined towards the direction of fan rotation.

2. A fan comprising a casing having a fluid inlet and a fluid outlet, an impeller structure rotatably mounted in lthe casing, said impeller structure including a primary impeller and a secondary impeller which rotate together, hollow blades on said primary impeller operative to draw fluid through said casing inlet and expel said fluid through said casing outlet, said secondary impeller having an inlet and an outlet, the interior of said hollow blades being in fluid communication with the inlet of the secondary irnpeller, said hollow blades having opening means in the trailing portions thereof whereby fluid is drawn from the space adjacent said trailing portions into the hollow blades and thence into the inlet of the secondary impeller to thereby inhibit separation of the fluid layer adjacent said trailing portions and to provide a deflection of the fluid stream in the direction of blade rotation as the fluid stream is discharged from the blade during fan operation, a stator structure including plurality of stationary hollow blades downstream from the impeller structure, `the interior of said stator blades being in fluid communication with the outlet of said secondary impeller, said stator blades having opening means in the trailing portions thereof whereby fluid flows through the stator blades and out of the opening means thereof to inhibit separation of the fluid layer adjacent said trailing portions.

3. A fan as claimed in claim 2 and further characterized in that said opening means comprises a plurality of spaced apart apertures over substantially the entire trailing face.

4. A fan comprising a casing having a fluid inlet and a fluid outlet, an impeller structure rotatably mounted in said casing, said impeller structure including a primary impeller and a secondary impeller which rotate together, said primary impeller including a hub plate having a plurality of circumferentially spaced hollow blades mounted on one face thereof, said secondary impeller including an annular wall member spaced from the opposite face to the hub and having a plurality of circumferentially spaced blades mounted between the annular wall and the opposite face of the hub, the hollow blades on said primary impeller being operative to draw fluid through said casing inlet and expel such fluid through said casing outlet, said secondary impeller having an inlet and an outlet, said hub having opening means therein providing fluid communication between the interior of said hollow blades and the inlet of the secondary impeller, said hollow blades having opening means in the trailing portions thereof whereby fluid is drawn from the space adjacent said trailing portions into the hollow blades and thence into the inlet of the secondary impeller to thereby inhibit separation of the fluid layer adjacent said trailing portions during operation of the fan and to cause the main air stream to be deflected in the direction of blade rotation as the air moves beyond the trailing edge of the blade.

5. A fan as claimed in claim 4 and further characterized in the provision of a second annular wall member between the blades of the secondary impeller and said opposite face of the hub to define a passageway leading from the hollow blades to the inlet of the secondary impeller, a third annular movable wall forming a passageway leading from the fan casing interior to the inlet of the secondary irnpeller, said third annular wall being adjustable to different positions to vary the size of the passageway defined thereby to thereby modify the amount of fluid drawn from the interior of the hollow blades into the secondary impeller.

6. A fan as claimed in claim 5 and further characterized in that the secondary hub includes a first portion radially inwardly of the secondary impeller blades and extending generally axially, a second portion extending radially outwardly from said first portion, said first and second portions each having opening means communicating with the secondary impeller inlet, said second wall having a first portion with opening means therein radially inwardly of the first portion of the secondary impeller hub and a second portion extending radially outwardly from said first portion, said second wall being movable to a position where the first portion thereof block the opening means of the first portion of the secondary hub and the second portion thereof reduces the amount of fluid which can pass through the opening means in the second portion of the secondary hub and movable to a position where the finst portion thereof permits fluid flow through the opening means in the first portion of the hub and increases the amount of fluid which can pass through the openings in the second portion of the hub to `thereby reduce the amount of fluid 9 drawn from the interior of the hollow blades of the primary impeller, 875,984

References Cited UNITED STATES PATENTS 5 89,617

2,935,245 5/1960 McDonald 23o- 127 92479 3,346,235 10/1967 Papst 230-122 10 FOREIGN PATENTS 5/ 1953 Germany.

5 1955 Great Britian.

8/ 1956 Great Britian. 10/ 1955 Netherlands.

8/ 1959 Netherlands.

HENRY F. RADUAZO, Primary Examiner.