US 2846185 A
Description (OCR text may contain errors)
5, 1958 H. u. WIDMER 2,846,185
FULL ADMIVSSION IMPULSE TURBINE Filed June 14, 1955 3 Sheets-Sheet 1 Fig.4 Y
- 4 INVENTOR:
H93 H. u. L/iumar i ATTuraneas Aug. 5, 1958 H. u. WIDMER FULL ADMISSION IMPULSE TURBINE 3 Sheets-Sheet 2 Filed June 14, 1955 w x. 9 w 3 2 6 E15... 2 nfi W 0O H mm,
INVEN TOR H U.. la L Cjmer Aug. 5, 1958 H. u. WIDMER 2,846,185, I FULL ADMISSION IMPULSE TURBINE Filed June 14, 1955 3 Sheets-Sheet 3 46 11 11 47 I 8 I i .J 2 3 i 7 36 I 28 79 28 5 1 "42 29 4 m 45 h 3 5 I INVENTOR: Hume/ u FULL ADMISSIQN ill [PULSE TURBINE Hans Ulrich Widmer, Zurich, Switzerland, assignor to Socit financiere dExpansion Commercials et industrielle S. A. Sfinclex, Sarnen, Switzerland Application June 14, 1955, Serial No. 515,500 Claims priority, application Switzerland February 22, 1955 11 Claims. (Cl. 253154) The present invention relates to impulse turbines and more particularly to full admission impulse turbines.
It is well known that impulse turbines must be arranged at a certain free height over the suction water level and their economical field of operation is therefore in the range of medium to higher heights of fall. Due to these relatively high fall heights the control forces are also high and in order to regulate them in the well known part-admission impulse turbines a number of means have been suggested and used. In contra-distinction to the impulse turbines having one or several nozzles which turbines were nearly always used up to now, the present invention relates to a full admission impulse turbine hav ing an axially adjustable flow control member.
The distributor, which is used in connection with the present invention is well known in the art and consists of fixed lateral limiting means, extending from the inlet spiral. Guide vanes are rigidly arranged on said limiting means and an axially adjustable flow control member is provided, which is penetrated by said guide vanes substantially free of clearance. The flow control member is mounted in the turbine so that together with stationary parts of the turbine a closed annular chamber is formed. In this type of turbine the dimensions of the distributor and therewith of the flow control member may not go below certain limits in order to achieve favorable ciliciency. In comparison to partial admission impulse turbines considerably higher forces act upon the flow control member and the problem of optimum adjustment of the flow control member by means of servo motors is still more important.
A primary object of the present invention is to provide for optimum regulating conditions in a full admission impulse turbine by sealing the annular chamber between the movable flow control member and the stationary turbine cover and by conducting the operating medium to said annular chamber, which medium is taken from the inlet upstream of the entrance to the guide vanes.
A further object of the invention is to reduce the regulating work for the axial displacement of the flow control member of a full admission impulse turbine to a fraction of the value, which would be necessary without making use of the teachings of the invention.
The inlet for the operating medium to the annular chamber must be of sufficient cross-section in order to maintain the pressure in this annular chamber at a substantially constant value, even if rapid regulating movements are necessary and in order to prevent small leakage losses from influencing the pressure in said annular chamber. The operating medium may therefore not be fed to the annular chamber along the guide vanes since the openings, which are present are not of sufficient cross-section. This is due to the fact that the clearance between the guide vanes and the flow control member must be a minimum under consideration of the possibilities. of manufacture and displacementof said flow control memaims Patented Aug. 5, 1958 2 her. Larger clearances or openings would have a disturbing effect upon the flow in the distributor, thereby reducing the efiiciency of the turbine, which would result in an uneconomical operation thereof.
A number of possibilities are present to maintain the above mentioned forces within controllable values.
1) The pressure in said annular chamber may be chosen within certain values by suitable provision of the outlet opening in the range of the inlet upstream of the operating medium.-
(2) The characteristics of the regulating forces may' be brought to an optimum by corresponding compensation of the effective surfaces of the flow control member subjected to pressure.
Further advantages of the arrangement according to the invention which are not particularly related to the regulating work are as follows: Due to compensation the load acting upon the flow control member is smaller and therefore the latter may be made of a considerably lighter construction which results not only in a direct economisation but which reduces also the masses to be displaced. Due to the feed of the operating medium under operating pressure the compensation of forces is adapted automatically to the possibly variable net fall height of the machine.
Finally a preferred embodiment of the present invention provides a closing force in operating position of the flow control member while in the closed position of the flow control member an opening force is present, since the operating medium acting upon the flow control mem her is in the median position of equality of forces corresponding to a partial opening of the flow control member. This arrangement provides great advantages with respect to safety since upon loss of the control forces the machine will automatically take up a condition corresponding to an opening of the flow control member in the vicinity of its position of equality of forces. Racing of the turbine is prevented and the flow is not completely throttled since the pressure impulse is considerably reduced.
In order that the invention may better be understood and put into practice a number of embodiments thereof are described by way of example and with reference to the accompanying drawings, in which:
Fig. l is an axial section through a first embodiment of a turbine of circular jet according to the invention,
Fig. 2 is a detail view of Fig. l drawn to an enlarged scale,
Fig. 3 is a modification with respect to Fig. 2,
Fig. 4 is an axial section through a second embodiment of a turbine according to the invention,
Fig. 5 is a fragmentary plan view of the turbine according to Fig. 4, and
Fig. 6 is an axial section through a further embodiment of the turbine according to the invention.
Fig. 1 shows a full admission impulse turbine. From the pressure pipe l the operating medium flows into the spiral body 2 and from there into the distributor consisting of a fixed meridian surface 3 limiting the flow and having guide vanes 4 rigidly arranged thereon and consisting further of an axially displaceable meridian surface 5 which also limits the fiow and serves as the flow 'control member. The penetrations between the guide vanes 4 and the flow control member 5 are accurately stitutes a free annular flow 6 which takes up the form of a hyperboloid of rotation. The impeller 7 is arranged such that its inlet edge is situated in this annular flow 6 and is impacted by the latter on a closed annular surface. The operating medium leaving the impeller flows outwardly with a mainly axial component of velocity and may be led off in any suitable manner.
A turbine cover or limiting member 8 is seated on the spiral body 2 and carries a bearing 9 of the turbine shaft 10. Two servo motors 12 are seated on the cover 8, which motors are connected over the elongated piston push rod 13 directly with the flow control member 5 and which control the movements of the latter. Portions of this flow control member 5 the spiral body 2 and the cover 8 form together an annular chamber 11 which is sealed at the smallest effective diameter between the movable fiow control member 5 and the stationary cover 8. The clearance in the penetrations of the guide vanes is of such small order that the flow of medium in the clearance space between the channel of the distributor and the annular chamber 11 is of negligible magnitude. Operating medium is fed to this annular chamber 11 which medium is taken from a suitable point of the inlet pipe upstream of the guide vanes. In the present embodiment two infeeds are possible.
The numeral 14 indicates a bore in the spiral wall between the chamber 11 and the spiral 2, while 15 indicates a groove at the circumference of the flow control member 5 through which the operating medium may enter the chamber 11. may be devised as will be described later on in connection with the second embodiment. The two possible infeeds shown in Fig. l have in common the feature that pressure present in the chamber 11 will correspond to a pressure in the feed pipe to the distributor. In axial section the geometrical configuration of the two flow limiting surfaces of the distributor is such that in closed position of the distributor, namely, when the movable flow control member 5 is seated on the stationary limiting portion 3, there is a greater width of clearance space on larger diameters than the closing diameter between the two surfaces 3 and 5, which permits entrance of the operating medium from the spiral body below the flow control member 5. In such a construction of the turbine there are always hydraulic forces acting upon the flow limiting front face, as well as upon the rear face of the flow control member, the resultant of which is computed with respect to magnitude and direction as follows (cf. also Fig. 2)
(a) The pressure forces acting in opening direction upon the movable flow control member due to the working medium in the distributor are:
In the above equation are:
(b) In opening direction is further acting the impulse force of the water mass flow diverted at the nozzle outlet which is:
However, other possibilities of feed In the above equation are:
v the specific gravity of the operating medium g the gravity acceleration q the throughflow per time unit V,,,. the axial component of the velocity V atthe nozzle outlet.
This equation is valid in the case where the flow through the guide vanes with respect to the axially diverting portion of the distributor is directed in a vertical plane with respect to the turbine axis.
(c) In closing direction the force F is active, owing to the pressure H acting in the chamber 11 upon the flow control member 5. This force will be:
In this equation 7 and r are defined as above, in addition are:
1' the radius of the inner seal between the flow control member and the cover y the sum of the cross-sectional areas of the openings for the guide vanes H the pressure height present in the chamber 11, which in the embodiment shown will substantially correspond to the spiral pressure height.
and Hvalr will be 0.
The closure of the distributor shall take placeat the nozzle outlet so that the flow control member will be under the full water pressure at its flow limiting side as far as the closing radius r (cf. Fig. 2), and the smallest effective diameter of the annular 11 shall be smaller than the closure diameter of the distributor as indicated in the depicted embodiment. be:
whereby r indicates the closure radius of the distributor.
The impulse force F is not present since Q=O. In closed position and without movements of the flow control member also the frictional and inertia forces are equal 0. The closing force F will then be (In the closing position of the flow control member the pressure in the chamber 11 and the remaining clearance space of the distributor are equal.) The resulting force P is then P is a force acting in closing direction since F. prevails in case the indicated assumptions. It will be seen from Equation 4 that the resulting force present in the closing position depends only on the ratio between the radii r and r The value of the opening component in the operating position of the flow control member and thereby during the flow of water through the distributor is reduced according to Equation 1 by H and The force P will then-simply The influence of H and X may thereby be neglected, since the losses must be very small to achieve a good efliciency and since the member X is very small in comparison to F As a new opening force, however, the impulse force R; will now appear. The latter has a relatively small influence upon the play of forces owing to the design data which are given within certain limits by the requirements of the turbine, so that the sum of the opening forces will always be smaller during operation than in closed condition. Where r r there will result in all positions of the flow control member a closing force, so that the flow control member will automatically be moved into its closed position upon failing of the control forces.
If distributor and annular chamber 11 are so dimensioned-that a closing force will always act upon the flow control member 5, there is the possibility to control the movements of this member by servo-motors, which are adapted only to control opening movements. The maintenance of small clearances in the penetrations is a necessity for making use of an annular chamber 11 as shown in Fig. l and for the hydraulic compensation of the flow control member. If the water contains a relatively high amount of sand the narrow cross-sections are, however, endangered and may be enlarged, whereby the conditions may change to an undesirable state. As shown in Fig. 3 more suitable conditions may be provided in this respect by separating the guide vanes by covers 16 or by providing covering cups (not shown), which separate these guide vanes from the interior of the annular chamber 11. Thereby a space is formed in the flow control member which is separated from the annular chamber 11, into which space the guide vanes 4 project. This measure permits to maintain constant the control conditions of the machine, while wear, producing enlargement of the guide vane penetrations will still undesirably influence the efliciency of the turbine.
Fig. 4 shows a second embodiment of a turbine making use of the teachings according to the invention. Those members which conduct the operating medium are of the same configuration as in the first embodiment, i. e. the inlet pipe 1, the spiral body 2, the flow limiting meridional surface 3, the flow control member 5, the guide vanes 4 and the impeller 7. In this embodiment the closure diameter of the distributor is also given by the fact that the operating medium flowing from the spiral body in the closed position will impart .a pressure upon the flow limiting surface of the control member, which pressure is determined by its closure diameter. Similar to the first embodiment an annular chamber 11 is provided, to which operating medium is fed. Fig. 4 shows two possible infeeds, the one being to provide bleed bores 14, connecting the interior of the spiral body with the annular chamber 11 and the other consisting of a bleed tube 17, which extends from the spiral body into the .annular chamber 11. Such a tube permits to install throttle members therein, if desired. The cover 8 carries a tubular extension 18, which contains the sealing member 19 between the cover and the flow control member. Further sealing members 28 may be provided between the flow control member 5 and the interior wall of the spiral body. While sealing of the annular chamber 11 towards the cover 8 by means of sealing members 19 is an absolute necessity, the additional provision of sealing means for the annular chamber between the flow control member and the interior wall of the spiral body provides a substantially complete closure of the annular chamber 11 with respect to the immediately adjacent spaces. It will thus be possible to maintain a pressure in said chamber, which is differentfrorn the operating pressure present in the adjacent spaces. The annular chamber 11 has no direct control functions but serves only for a certain compensation of forces and for maintaining the end position of the flow control member 5 upon failing of the control forces. The movement of the flow control member 5, i. e. the adjustment to a desired opening for the distributor is carried out by means of an annular servomotor as shown, which is arranged in coaxial relation with respect to the turbine shaft 10. The flow control member 5 is provided with a tubular extension 20, to which the servo-piston 21 is directly connected. The servo-cylinder is formed by an annular portion 22 of the cover 8 and is closed by a supporting tube 23 connected to the portion 22 and supporting the bearing 9 of the shaft 10. The control fluid is fed to the two servo-chambers 24 and 25 by means of a control valve and over the control pipes 26 and 27. A control making use of .such an annular servo-motor contains only few movable parts, which may be strongly dimensioned and which carry out only very simple control movements. However, the type. of servo-motor used has no influence upon the functioning of the turbine according to the'invention and it is therefore possible to make use of any suitable type of servo-motor.
In the present embodiment, in which the closure diameter 2r of the distributor is chosen smaller than the diameter 21' of the sealing member of the annular chamber between flow control member 5 and cover portion 18, the hydraulic forces acting upon the flow control member are as follows:
In the closed position there is again As will be seen from Equation 4 the resulting force in the closed position in the case were r r will become an opening force.
In the operating position of the flow control member and therewith upon throughflow of water through the distributor, the magnitude of the opening component will be reduced as shown by Equation 1. Therefore the influence of H and of X may'practi'cally be neglected since the losses must be very small to obtain a good efliciency and owing to the fact that the member X is very small as compared with F Due to the fact that the flow velocity V in the distributor Will increase up to nearly the velocity of the annular flow, F will be considerably smaller in the operating position of the flow control member than in its closing position. However, the impulse force FJ acting as an opening force, will also increase more and more and the closing pressure H in the annular chamber will decrease with increasing mass flow analogous to the pressure at the bleed point in the inlet. Generally however, the closing force F in the operating position of the flow control member will be higher than the sum of the opening forces F and F whereby a closing force results. A design where forces of the described magnitude will be present may be achieved by suitable choice of the sealing radius r;,. In the same manner a sealing radius r may be found with which the lowest peaks of control load for the flow control member will be present or the smallest work is necessary to carry out its control function. This arrangement, in which a closing force results in the operating position and which has a tendency to open in the closed position has further the advantage that upon failing of the control pressure for the servomotors, the flow control member will automatically carry out a closing movement, owing to the resulting closing force, until this member has reached the position of equality of forces. In this position the closing force is compensated by the opening component. This position of balance may be situated in the vicinity of the idling position by corresponding configuration of the effective parts. I
In the immediately above indicated cases of compensation, the direction of the resultant of the forces acting upon the flow control member changes and a 7 double action servo-motor must be used for the operation of this member. If the aboveindicated closing tendency is not used, the diameter of the sealing member 19 might be made so large that an opening force were always present, thus necessitating only a servo-motor acting in closing direction.
The annular chamber 11, as described in the first and second embodiment and as shown in Figs. 1 and 4 is according to the invention put under pressure by feeding operating medium thereto. While the point of feed of the pressure water into the annular chamber has practically no importance, the bleed point where the pressure medium is taken from the inlet in the operating medium is of higher importance. The pressure present in the annular chamber 11 and thus the closing force acting upon the flow control member depend on the location of this bleed point. In the choice of this bleed point and of the type of feed conduit to the annular chamber 11 there are a great number of possibilities, which may be decided independent of the design of the machine otherwise chosen. In Fig. 7 1 the already described modification consists of a bore 14 and grooves respectively, and in Fig. 4 the modification has a pipe 1 7 as shown. Fig. 5 shows a plan view of a turbine with a fragmentary view of the inlet and it will beseen that the water may be taken from a number of different bleed points and con-- ducted into the cover of the bleed tube .17 which extends from the spiral body into the cover. A pipe 31 leads from a point between the closing valve and the spiral body into the cover. 32Iindicates a pipe, which drawsofi the pressure water upstream ofthe closing valve and conducts this water to the annular chamber. In all these feed pipes closing valves (not shown) may be arranged, as desired. Bleeding-off the pressure water upstream of the closing valve 35 has the advantage that the fiow control member 5 is under the load of a closing force also if this valve is closed. This would not be possible in the other arrangements shown.
Fig. 6 shows an embodiment of a full admission impulse turbine according to the invention having additional detail modifications. The operating water flows from an inlet not shown through the spiral body 2 to the distributor, the stationary limiting surfaces 3 of which carry the rigidly arranged guide vanes 4; the axially displaceable lateral limiting surface which'is formed by the flow control member 5 is penetrated by the guided vanes substantially without clearance. The guide vanes impart a vortex How to the water which is maintained also between the nozzle members one of which is indicated by numeral 45. Due to the vortex fiow, an annular flow having the form of a rotation-hyperboloid is formed after exit from the nozzle. The inlet edges of the blades of the impeller wheel 7 are situated in the annular flow betwen the nozzle outlet and the apex of the annular flow.
Au annular chamber 11 is formed by the how control member '5, a bore in the spiral body 2, the cover 8 and its extension 18, which chamber is connected with the interior of the spiral body over a bleed pipe 17, so that the annular chamber will be filled with operating medium of operating pressure. For the feed of the operating medium into the annular chamber any other form of execution shown in the previously described embodiments may be chosen. The annular chamber 11 is sealed on its smallest effective diameter between the flow control member 5 and the extension 18 of the cover 8 by means of a sealing member 19. The diameter of this sealing member determines the effective surface of the flow control member which is under the pressure of the operating medium in the annular chamber, and consequently the closing force acting upon the flow control member. The present embodiment is designed for one of the two cases indicated in the above described embodiments Sealing members 28' may be provided between the bore of the spiral body and the flow control member. The sealing members are, however, only necessary in the case where the pressure in the annular chamber 11 is different from the pressure in the spiral body. Between the annular chamber 11 and the guide vanes passages a pressure difference will be present in the operating condition since the pressure in the guide vane passages is lower than in the spiral body by the value of the dynamic pressure. It is possible that the narrow clearances of the originally play-free penetrations will be enlarged by wear in the course of time, so that a pressure compensation would be possible through these clearances. In order to prevent this undesirable elfect sealing members 29 may be provided at the fiow control body 5 in the annular chamber 11 which ensure a tight sealing action between the flow control member and the guide vanes as shown. It would also be possible to seal the annular chamber 11 with respect to the guide vane passages by means of a cover 16 as shown in Fig. 3. This cover would then have to be provided with bores and sealing members for the penetrating bolts 36 which are shown in Fig. 6. These bolts 36 which in the present embodiment are placed under tension extend to the guide vanes 4. By their arrangement in the guide vanes 4 the bolts, which strengthen the construction, do not disturb the flow and are situated at a very suitable point with respect to strength considerations. Thus they permit a considerable reduction of the stiffening elements for the spiral body.
The shaft is journalled in the bearings 37 and 38. The bearing 37 is seated on the extension 18 of the turbine cover and takes up radial as well as axial loads. The bearing 38 is arranged below the impeller in the present embodiment and serves only for radial guidance. This latter bearing is supported at the spiral body by means of supporting arms 39. The bearing 38.could also be replaced by a collar bearing above the impeller and by overhanging arrangement of the impeller as shown in the Figs. 1 and 4.
Tests have shown that in order to obtain good How efiiciency the annular flow 6 must be ventilated on its inside and on its outside. In the present embodiment the outer. ventilation is achieved by a tapered shield 40, i. e. an annularslit 41 between the shield 40 and the spiral body 2. Feeding of the necessary amount of air for the inner ventilation takes place along the shaft and through the bore 42. The shield 40 may be formed as a portion of the outlet casing.
The nozzle efiiciency depends to a high measure on the surface condition of the nozzle flanks. Owing to the fact that during operation a certain wear of this portion must be-taken into consideration, removable nozzle inserts may be provided on the impeller side, which may be readily and rapidly replaced. Since further the configuration of'these nozzle inserts may influence the flow angle it is also possible to adjust the turbine to a changed fall height by provision of corresponding nozzle inserts.
Control of the turbine shown in Fig. 6 is eifected over three servo-motors 12 seated on the turbine cover which motors are :operated by a governor not shown. The servo-pistons 44 are connected with the flow control member over thrust rods 13. These servo-motors may be disassembled for purposes of maintenance without disassembling the turbine, which mainly faciltiates replacement of the sealing members 46 and 47. The flow control member is guided by means of its guide tube 20 in the tubular portion 18 of the cover. The length of this bearing surface is made such that upon one-sided application of load (such as owing to foreign matter entering the distributor channel) no jamming will take place. The force of the servo-motor could also be transmitted to the flow control member by means of the tube 20, omitting the sealing member 47. In this case an annular servo-motor as shown in Fig. 4 would be suitable.
1. A full admission impulse turbine, comprising a spiral casing for the operating medium and having a fixed meridian surface, guide vanes rigidly mounted upon said surface for imparting a vortex flow to said operating medium, a movable flow control member penetrated by said guide vanes substantially without clearance, said meridian surface, said guide vanes and said flow control member constituting a distributor channel of adjustable cross section for the passage of the operating medium, a turbine shaft, said flow control member enclosing said turbine shaft, an impeller carried by said shaft and having inlet edges impacted by the operating medium leaving said distributor channel, a cover connected with said casing, said flow control member and said casing and said cover enclosing an annular chamber of variable volume, inner sealing means between said flow control member and said cover, servo-motor means controlling the movement of said flow control member, a bleed opening in said casing upstream of said guide vanes, and means operatively connecting said bleed opening with said annular chamber to provide pressure in said annular chamber channel corresponding to pressure in front of said distributor channel.
2. A turbine in accordance with claim 1, wherein the closing radius r at the exit of said distributor channel is greater than the radius 2' of the inner sealing means between said fiow control member and said cover to provide a closing force in all positions of the flow control member.
3. A full admission impulse turbine as claimed in claim 1, wherein said annular chamber between the movable flow control member and said stationary limiting member is formed by portions of said spiral casing, said stationary limiting member and said movable flow control member and wherein additional sealing means are provided between said flow control member and the central bore of said spiral casing in order to maintain a higher medium pressure in the annular chamber than in the spiral casing.
4. A full admission impulse turbine as claimed in claim 1, wherein a chamber is provided in said flow control member sealed with respect to said annular chamber and serving to receive said guide vanes penetrating into said flow control member in order to maintain constant the pressure in the annular chamber acting upon said fiow control member despite an increase of clearance in the penetrations due to wear.
5. A full admission impulse turbine as claimed in claim 1, wherein the operating medium fed to said annular chamber is bled of from said inlet upstream of a closing valve intermediate said inlet and said spiral casing in order to maintain a pressure in the annular chamber also in the closed position of the closing valve.
6. A full admission impulse turbine as claimed in claim 1, wherein said operating medium is fed to said annular chamber from the spiral casing by a groove between said flow control member and the central bore of said spiral casing.
7. A full admission impulse turbine as claimed in claim 1, wherein said operating medium is fed to the annular. chamber from said spiral casing by a bore in the wall of said spiral casing.
8. A full admission impulse turbine, comprising a spiral casing for the operating medium and having a fixed meridian surface, guide vanes rigidly mounted upon said surface for imparting a vortex flow to said operating medium, a movable flow control member penetrated by said guide vanes substantially without clearance, opposed nozzle members adjacent to said meridian surface and said flow control member, respectively, said meridian surface, said guide vanes, said flow control member and said nozzle members constituting a distributor channel of adjustable cross section for the passage of the operating medium, a turbine shaft, said flow control member enclosing said turbine shaft, an impeller carried by said shaft and having inlet edges impacted by the operating medium leaving said distributor channel, a cover connected with said casing, said flow control member and said casing and said cover enclosing an annular chamber of variable volume, said cover having an inner extension enclosing a portion of said flow control member, a sealing member between said portion of the flow control member and said inner extension of the cover, at least one servo-motor having a casing mounted upon said cover, a piston within said casing and a thrust rod connected with said piston and engaging said flow control member, said casing having a bleed opening formed therein and located upstream of said guide vanes, and a bleed pipe connecting said bleed opening with said chamber to provide pressure in said annular chamber corresponding to pressure in front of said distributor channel.
9. A turbine in accordance with claim 8, comprising sealing means between said spiral casing and said flow control member, and sealing means between said flow control member and said guide vanes.
10. A turbine in accordance with claim 8, comprising penetrating bolts extending through said annular cham her and at least some of said guide vanes and connected to said casing and said cover.
11. A turbine in accordance with claim 8, comprising a tapered shield having an outer edge connected to said casing and an inner edge extending close to the exit of said distributor channel and to the inlet edges of said impeller, an annular ventilating slit being formed between said inner edge and said casing.
References Cited in the file of this patent UNITED STATES PATENTS 1,603,973 Moody Oct. 19, 1926 1,623,446 Taylor Apr. 5, 1927 1,656,006 Lieber Jan. 10, 1928 1,656,012 Nagler Jan. 10, 1928 1,703,081 Moody Feb. 19, 1929 1,901,771 Pfau Mar. 14, 1933 2,336,450 Voorhess et a1. Dec. 7, 1943 2,683,419 Schneider July 13, 1954 FOREIGN PATENTS 286,965 France May 5, 1899