US3387769A - Multistage turbomachine - Google Patents

Description

June M, 1968 H. DAVIS 3,387,769

MULTISTAGE TURBOMACHINE Filed OCL. 3, 1966 /KA HUNT DAVIS [216.3 wif* 3,387,769 MULTXSTAGE 'EURBOMACHENE Hunt Davis, Williamsville, NX., assigner to Worthington (Iorporation, Harrison, NJ., a corporation of Delaware Filed st. 3, 1966, Ser. No. 533,743 1 Claim. (Cl. 23u-13b) ABSTRACT @F THE DISCLUSURE A centrifugal multistage rotary uid handling device which produces an overall pressure vs. flow characteristic of the normally desired negative slope, `wherein a low speed impeller of low inlet lluid energy requirements, but of head capability increasing with flow, is combined with one or more dominant successive stage elements which require this type of head capability as an inlet boost.

This invention relates generally to multistage rotary fluid handling machines, such as multistage pumps or compressors, and more particularly, to a multistage pump or compressor producing an overall decrease in head for the machine with increasing fluid ilow through all the stages of the machine, and having a low speed first stage which discharges to the inlet of a high speed second stage, said low speed stage having a low net positive suction head available while exhibiting increasing head capability with increasing flow,

ln rotary fluid handling equipment designed to impart energy to a fluid, such as rotary pumps and compressors, there have been continued efforts on the part of industry to produce more compact equipment as a means of improving proiit and competitive position. This equipment has taken the form of high speed multistage devices, and there is a particular form wherein the rst stage operates at a lower speed than some or all of the other stages. The second and succeeding stages operate at high speeds in order to take advantage of the economies of high speed design, which are well known in the art.

lt is desirable to obtain the economies of high speed design while maintaining the capability of favorable characteristics with regard to inlet net positive suction head (NPS-H) requirements for pumps or inlet relative Mach number requirements for compressors with respect to the high speed stages. The pressure of the inlet liquid in excess of its Vapor pressure which is required for a pump stage, and the acoustic velocity of the inlet gas which is required for a compressor stage both increase with increasing flow. Thus, in multistage centrifugal pumps, the second or high speed stage requires for its inlet, an NPSH value that rises as the ilow increases The production of this increasing NPSH is the primary function of the first or low speed stage, whereby the latter should accordingly have a head curve that rises with increasing flow. A similar relationship exists for the multistage compressor, wherein the second or high speed stage benets from an increase in gas temperature provided by the rst or low speed stage as flow increases.

The uoval idea presented by this invention is, in part, that the first stage design be of the type whereby the work done by the rotor per unit mass of fluid increases as the dow increases at constant speed. This can be accomplished by use of the so called low reaction forward leaning blading in centrifugal impellers. This type of blading is particularly suited for supplying the required energy to the inlet of a succeeding high speed pump or compressor stage of a much higher head capability, because of its positive head vs. capacity characteristic.

By use of this type of device as the first stage in a multistage pump, the second stage will be provided with an NPSH that rises with increasing ilow. Similarly, if.

ftates Pater used as the first stage in a multistage compressor, it will provide to the second stage, an inlet temperature which increases with flow, thereby increasing the acoustic velocity and reducing the relative inlet Mach number.

Centrifugal impellers with forward curved discharge ends of their vanes are well known in the prior art. It is a recognized favorable characteristic that they exhibit a high pressure coefficient, that is they produce more head for a given speed and diameter than impellers with backward leaning vanes. However, the positive slope of the head or pressure vs. dow capacity curve, limits their applicability. For instance, in multistage units, it is desirable to have an overall negative head vs. capacity characteristic. in the present invention this negative characteristic is supplied by having high speed pump or compressor stages that dominate the multistage machine. The high speed stages are designed with conventional impeller blading, and since these stages supply the greatest part of the overall pump head, the tendency for instability common to forward leaning blading will be overcome by the succeeding stages, so that the external circuit is provided with the usually desirable negative head vs. capacity characteristic.

ln summary, this invention provides a multistage rotary duid handling device which produces an overall pressure vs. tlow characteristic of the normally desired negative slope, wherein a low speed impeller of low inlet fluid energy requirements, but of head capability increasing with tlow, is combined with one or more dominant successive stage elements which require this type of head capability as an inlet boost.

Accordingly, it is an object of this invention to provide a more efficient mulstistage rotary iluid handling machine having stages running at different constant speeds, wherein low speed low energy booster stage is provide which exhibits a head Capability increasing with flow.

lt is another object of this invention to provide a multistage rotary fluid handling evice which exhibits an overall negative head vs. flow characteristic, said device having a high speed second stage and a low energy low speed first stage, said first stage having forward leaning blades.

These and other objects and advantages of the invention will become evident from the following description with references to the accompanying drawings in which:

FGURE l is a diagrammatic sketch of a two stage rotary fluid handling machine as an embodiment of this invention;

FlGURE 2 is an elevation of the machine shown in FIGURE l in partial section to show the disposition of the impellers therein; and

FIGURE 3 is a graph illustrating the relationship between head and tlow for forward and backward leaning stage blading, respectively, and for the combined stages.

Referring to the drawings, FGURE 1 shows a multistage rotary fluid handling machine which may be either a pump or compressor. The machine as depicted consists of a low speed iirst stage l having an inlet 2 and discharge outlet 3. The discharge outlet 3 is connected by suitable conduit means 4 to the inlet 5 of a high speed second stage 6. The second stage 6 has a discharge outlet 7 which may be connected to a suitable pressure fluid system (not shown), or alternatively, there may be more than one successively connected high speed stage in the nature of second stage 6.

This equipment is designed to impart energy to a fluid and as such, it requires a prime mover or source of power as a driving means. This means may be a steam turbine, electric motor, or any other suitable means (not shown) which may be drivingly connected to a drive shaft S and conveniently operated at substantially constant speed. The drive shaft d is journaled in a gear box 9, which box for seamen purposes of illustration, comprises three gears. A driving gear It) is mounted within the gear box on the end of the shaft 8 for driving the driven gears l1 and 12. Gear i1 is of larger diameter than gear i2 and, as such, the former will of course be driven at a lower constant speed than the gear 12. The gear Il is connected to a drive shaft i3 which is suitably journaled in the gear box 9, as india cated at 14. The drive shaft i3 is coupled to a driven shaft by means of coupling 16. On the shaft 15 is secured an impeller I7 in such a manner as to be overhung in the casing 18 of the low speed first stage Trl, as more clearly shown in FIGURE 2. The casing i3 is mounted in a suitable manner to a hase or :frame I9.

In the high speed stage 6, the gear 12 is similarly connected to a drive shaft 2) which is in turn coupled by means of coupling 21 to a driven shaft 22. The shaft 22 is connected to and drives an impeller 23 in the casing 24 of the high speed second stage 6, which casing 24 is likewise suitably mounted to the base 19. The gearing as described is such as to provide substantially, constant, high and low speed rotation of the impellers 23 and 17, respectively. As used herein, the terms low speed and high speed are of course relative, with the actual speeds varying very Widely depending upon the design of the overall machine. In general, however, it may be noted that for some applications the speed of the second stage 6 may, for example, be twice that of the first stage 1.

The respective gears 11 and 12 are designed to rotate the low speed first stage impeller t7 and the high speed second stage impeller 23 in the direction indicated by the arrows on the end of the shafts 15 and 22, respectively, in FIGURE 2.

When viewed with respect to the direction of rotation the low speed rst stage impeller 17 has forward leaning blades, while the high speed second stage impeller 23 comprises blades of a conventional backward leaning design.

The forward leaning blades of impeller 17 are disposed With respect to a radius through the shaft 15 at an angle A as shown in FIGURE 2 in a manner well known in the art, and a favorable characteristic of this type of blading is that it produces more head for a given speed and diameter than impellers with backward leaning blades. The particular value of the angle A depends upon the specific design of the impeller i7, it being noted that there are no special maximum or minimum values for this angle other than those dictated by the requirement for efiicient impeller performance. This type of blading will, at low constant speeds, produce increasing pressure or head with increasing flow capacity.

As has been previously mentioned, it is desirable to have a decreasing pressure with increasing flow characteristic for the entire machine. Therefore, conventional blading or backward leaning blading 26 has been used in the high speed second stage 6 on the impeller 23. In the contemplated design, this high speed second stage with any succeeding stages will dominate the output of the machine, so that the head vs. flow characteristic of the entire machine will have the desired negative slope.

These desired relationships between head and flow for the respective first and second stages, and the combined stages or overall machine, are believed clearly illustrated by the graph of FIGURE 3, wherein curve 30 illustrates the said relationship for the first stage 1, curve 32 illustrates the said relationship for the second stage 6, and curve 34 illustrates the said relationship for the overall machine.

Curve 30 makes clear that the head provided by low speed operation of the first stage .ll increases, as flow increases, throughout the normal operating range of the overall machine; While curve 32 makes clear that the head provided by high speed operation of the second stage 6 decreases, as flow increases, throughout the same said operational range.

The dominance of the decrease in head provided by the operation of the second stage 6, upon an increase in dow, is illustrated by curve 34 which makes clear that the head provided by operation of the combined stages, or overall machine, decreases, upon an increase in flow, in the desired manner as discussed hereinabove, despite the increasing head characteristic of the operation of the first stage.

The design point for operation of the combined stages, or overall machine, is indicated at 36 on each of curves of 30, 32 and 34, and it is believed of interest to note that, at this point, the slope of curve 30 is positive lo indicate that first stage head will increase with an increase in flow, while the respective slopes of curves 32 and 34 are negative to indicate that second stage and combined stage heads will decrease under the same conditions.

This conventional impeller blading when run at high constant speed in, for example a compressor, demands an increase in total energy of the inlet gas supplied to it, when the fiow increases, and likewise, if it were operating in a pump, it would require an increasing inlet pressure in excess of its vapor pressure with increasing fiow. Thus, the inlet requirement of the high speed second stage with conventional blading is exactly the same as the output of the low speed first stage with forward leaning blades.

In most applications of the overall machine, there is so little NPSH normally available at the inlet of the rst stage that any high speed design thereof is made impossible. Therefore, the first stage, which functions primarily to provide suflcient NPSH at the inlet of the high speed second stage as set forth hereinabove, is, of necessity, designed for low speed operation. Since the NPSH requirements of the high speed second, and succeeding stages, if any, increase as flow increases, the utilization of forward leaning blades in the nature of those of impeller 17 operates to significant advantage in meeting these requirements through the provision of increased head with increased flow, in that the NPSH available at the inlet of the second stage is equal to the NPSH available at the inlet of the first stage plus the head provided by the first stage.

Operation In operation, fluid is supplied to the inlet 2 of the low speed first stage 1 which is operating at constant speed. Energy is imparted to this fluid by the impeller 17 which itself has low NPSI-ll available. The impeller cooperates with the casing 1S and its attendant diffuser arrangement 25 in handling the fluid, so that the work done by the impeller per unit mass of fluid increases as the flow through the casing increases at constant speed. The fluid is then discharged from the casing 1S through the discharge 3 at a pressure which increases with the liow. From the discharge 3 the .fluid is passed through the conduit means 4 to the inlet 5 of the high speed stage 6. This high speed stage as previously pointed out requires for its inlet an NPSH value that rises as the dow increases if the second stage is being used as part of a multistage pump, or an inlet acoustic velocity (or temperature) which increases with flow, if the second stage is part of a multistage compressor. The production of this NPSH or inlet temperature is the primary function of the first low speed stage. The uid entering the high speed second stage 6 at its inlet 5 thus Will meet the energy requirements of this high speed stage. This fluid will, then, be acted upon by the high speed second stage having conventional blading on its impeller, which high speed second stage will impart further energy to the fluid and discharge it through the discharge 7 at a pressure or head which decreases with the increasing fiow. The combined effect of the two in series is such that the external circuit is provided with the, usually desirable negative head vs. capacity characteristic.

It will he understood that various changes in the details, materials, and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art within lthe principle and scope of the invention as expressed in the appended claim.

What is claimed is:

1. A centrifugal multistage rotary iiuid handling machine having at least two successive stages for imparting energy to the fluid handled thereby, comprising, in combination:

(a) a low speed centrifuga-l rst stage means for providing increasing head with increasing fluid flow at substantially constant speed comprising a casing with an inlet and a discharge outlet and an impeller having forward leaning blades thereon mounted within said casing and operatively disposed therewith;

(h) at least one high speed centrifugal stage means for providing decreasing head with increasing fluid flow at substantially constant speed succeeding said rst stage means, said high speed stage comprising a casing having an inlet and discharge outlet and an impeller having rearward leaning blades thereon mounted within said casing and operatively disposed therewith;

(c) means connecting the discharge outlet of said first stage means with said inlet of said second stage means; and

(d) said high speed centrifugal stage means providing an overall decrease in head for the machine with increasing fluid ilow through all the stages of the machine at substantially constant speed.

References Cited UNITED STATES PATENTS 4/1893 Rateau 103-115 l/ 1903 Lindrnark.

1/1909 Bowie 230-127 10/1913, Moss 230-127 11/1938 Harper 230-130 10/ 1952 Bowen.

8/ 1953 Wood.

8/ 1956 Carrier.

9/ 1961 Schierl.

FOREIGN PATENTS 5/ 1922 France.

8/ 1922 Switzerland.

HENRY F. RADUAZO, Primary Examiner.

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