US2555619A - Pump - Google Patents

Description

June 5, 1951 v c 2,555,619

PUMP

Filed May 11, 1946 3 Sheets-Sheet l I IEl.

' INVENTOR. A a ward 7.' l l'ncenf MM. 5L

HTTOANEXF June 5, 1951 E T, V NCENT 2,555,619

PUMP Filed May 11, 1946 s Sheets-Sheet 2 JIYVENTOR. fawara 7'. l/lncenf Patented June 5, 1951 PUMP Edward T- Vincent, Ann Arbor, Mich., assignor to Continental Aviation & Engineering Corporation, Detroit, Mich., a corporation of Virginia Application May 11, 1946, Serial No. 669,010

2 Claims.

This invention relates to centrifugal pumps, especially to a centrifugal pump which is satisfactory as a circulating pump for the coolant liquid for aircraft at high altitudes.

A primary consideration in the design of aircraft is weight. Some items of equipment perform less efficiently at high altitudes due to the decreased pressure, and in order to obtain the desired high altitude performance, it has been the practise to use larger and heavier equipment than would be necessary at ground-level operation.

One such item of equipment is the coolant pump for aircraft engines. It has been found that, in the conventional type of centrifugal pump heretofore used for the purpose, the output volume has fallen off rapidly at increasing altitudes from sea level due to the drop in inlet pump pressure. In order to compensate for this drop in output, it has been the practise to use pumps having a far greater ground-level output. so as to get the desired high-altitude output, or to totally enclose the cooling system. Both expedients have disadvantages. Using a larger pump adds weight; using an enclosed system subjects elements of the system to undesirably high pressures, and designing the equipment to withstand these pressures adds to the expense andv to the weight of the equipment.

It is the object of this invention to provide a coolant pump for aircraft engines which maintains a substantially constant delivery flow up to altitudes in the vicinity of 30,000 feet. This object is accomplished by a centrifugal pump design which is a radical departure from conventional designs. In the pump of this invention, the impeller is concentric with the casing. The vanes of the impeller are backwardly curved, and the exit area of the impeller is larger than the inlet area. An auxiliary inlet impeller is provided immediately ahead of the main pump impeller.

In the drawings:

Fig. 1 is a longitudinal section through the pump.

Fig. 2 is a view of the pump with the cover and the auxiliary impeller removed showing the main impeller in the casing.

Fig. 3 is an end elevation of the auxiliary impeller.

Fig. 4 is a top view of the auxiliary impeller.

Fig. 5 is a curve in which the output of gallons per minute in conventional pumps is plotted against altitude. v

Fig. 6 is a curve in which the output of forward-curved vane, impeller pumps is compared with backward-curved vane impeller pumps, where the two types have about the same groundlevel performance.

In Fig. 7 are shown curves comparing the output of centrifugal pumps having volute casings against those having concentric casings.

Fig. 8 compares the output characteristics according to the ratio of exit area of the impeller toinlet area.

Fig. 9 is a curve showing relative efficiencies of pumps with and without auxiliary impellers; and

Fig 10 is a curve showing the output characteristics of a pump embodying allthe features of this invention.

Inthe use of centrifugal pumps for the circulation of the coolant fluid of aircraft engines, it was foundthat a conventional pump having the desired ground-level characteristics could not maintain the desired output with any appreciable decrease in the inlet pressure. Inasmuch as the inlet pressure in an open cooling system falls 01f as the altitude increases, the output in gallons per minute can be plotted against pump inlet pressure or altitude. In the curves shown in the drawings, output is plotted against altitude, except in Fig. 10, wherein both pressure and altitude are indicated on the horizontal scale.

As can be seen from Fig. 5, in which the performance curvesfor two conventional centrifugal pumps are set forth, in order to obtain the desired output Y1 at altitude Xi with a conventional pump, a pump has to be installed which. has a much greater output Y2 at ground level than is necessary.

It has been commonly assumed that the vanes of centrifugal pump impellers should be curved forwardly in the direction of rotation for maximum delivery pressure. The theory behind this reasoning was that it was necessary to curve the planes forward to permit a greater conversion from velocity to pressure energy, thus giving higher pump pressures resulting in greater flows for a given engine and radiator resistance. Actual tests indicate that, for high altitude operation, this theory does not seem to apply.

The. comparative curves shown in Fig. 6 indicate that the advantage is decidedly with the centrifugal pump having backward-curved vane impellers. As can be seen from this figure, the output of the forward-curved vane impeller falls off rapidly with increased altitude; the output of a pump having backward curved impellers also falls off but much more slowly.

Conventional centrifugal pumps, as heretofore designed, have usually been built with volute casingsi. e. casings in which the clearance between the periphery of the impeller and the outside diameter of the casing increases from a minimum to a maximum at the point at which fluid is withdrawn from the casing.

Fig. '7 compares the characteristics of pumps having volute casings with those having concentric casings. These curves are for pumps having substantially the same impellers, and having casings of approximately the same maximum diameter. It will be seen that actual tests indicate considerable advantage in a concentric casing. It should be added here that it is desirable to have that shown in Fig. 2 has been found to be satisfactory. With the clearance there shown, the 7 ratio of easing diameter to impeller diameter is approximately three to two.

Conventional centrifugal pump design has held that the exit area in the pump impeller should be less than the inlet area because of the increase in velocity. Fig. 8 shows that a pump impeller having exit areas of the order of magnitude of 72% greater than the inlet areas has far better performance characteristics at high altitudes than a pump in which the inlet and exit areas conform to normal practice.

j In view of the fact that decrease in inlet pressure accounts in large part for a decrease in output, .it was decided to experiment with an amiliary impeller immediately up-stream from the main impeller. The curves in Fig. 9 show the efficiencies of pumps with and without auxiliary impellers.

In Fig. 1 is shown a pump casing 2 in which is rotatably mounted a drive shaft 4. On one end of shaft 4 is disposed a gear 6 adapted to be driven by any suitable power source, not shown. At the other end of shaft 4 is mounted a pump impeller 8, held on the end of shaft 4 by means of a nut Ill.

As can be seen more readily in Fig. 2, casing 2 and impeller 8 are concentric. Fig. 2 also shows the backward curvature of the vanes 12. As seen in Fig. 2, impeller 8 is intended to be rotated counterclockwise. Fluid enters the impeller by way of inlet l4 and leaves the casing by way of outlet 16.

In the preferred embodiment of the invention, the impeller is shrouded, as shown at 18 in Figs. 1 and 2. Shrouding was found to improve high altitude operation sufficiently to warrant the additional expense involved.

The auxiliarly impeller is shown in Fig. 1, and

retically, vanes 20 should have helical surfaces.

It has been found however that plane vanes, as shown in the drawings, give satisfactory practical performance, and are actually much cheaper to manufacture. It will of course be understood that the auxiliary impeller can be separate from nut 10.

As shown in Fig; 4, the vanes form an angle of with the axis of rotation of the impeller. Experiments conducted to determine the optimum angle for the pump used indicated that best performance could be expected when the auxiliary vanes formed an angle with the axis of retation not less than 30 and not greater than 45. speed of rotation, the inlet velocity, and the diameter of the impeller.

Referring again to the exit area of the impeller as compared with the inlet area discussed above in connection with Fig. 8, experiments made on pumps with impellers having backwardcurved vanes indicated that improved performance at high altitudes could be expected with impellers in which the exit area was as much as 72% greater than the inlet area. Tests were not This optimum angle will vary with the conducted on impellers on which differences were greater than 72 Although there was no reason to believe that the improvement in performance characteristics should not continue, it should be noted that the rate of improvement had begun to decrease at that percentage of difference. In the experiments conducted, the exit area was increased by increasing the dimension to in Fig. 1. However, it may be noted here that there probably are some design limitations on the differences between exit and inlet areas.

Referring now to Fig. 10, the performance curve shown represents the performance in output at different altitudes of a pumpembodying the features of this invention. In this curve, pump output as represented by Y3 is maintained substantially constant to an altitude X1; at altitude X2, pump output Y2 is approximately 90% p I of the ground'level output. Thereafter the output falls oif rapidly to a value of Y1 which is approximately of ground level flow, at which altitude X3 the pump inlet pressure Pi is within a half or quarter inch of mercury of the vapor pressure P2 of the coolant used.

I claim:

1. A casing having a substantially circular cross-section, an impeller rotatably mounted in the casing and concentric therewith, the impeller having a centrally located inlet and a plurality of outlets, backwardly-curved vanes forming passages through the impeller of which the total outlet area is greater than the inlet area, and an auxiliary impeller rotatably mounted in the easing immediately upstream from the first-named impeller and operable to deliver fluid direct to the inlet side of said impeller, said pump being thereby operable to deliver a substantially constant output up to altitude; in the neighborhood of about 30,000 feet.

2. A casing having a substantially circular cross-section, an impeller rotatably mounted in the casing and concentric'therewith, the impeller having a centrally located inlet and a plurality of outlets,backwardly-curved vanes forming passages through the impeller of which the total outlet area is greater than the inlet area, and an auxiliary impeller rotatably mounted in the casing immediately upstream from the first-named impeller and operable to deliver fluid direct to the inlet side of saidimpeller, said pump being thereby operable to deliver a substantially constant output up to altitudes in the neighborhood of about 30,000 feet, said total outlet area being at least 72 greater than the inlet area.

EDWARD T. VINCENT.

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

UNITED STATES PATENTS Number Name Date 111,026 Williams Jan.'17, 1871 451,827 Lister et a1; May 5, 1891 1,496,633 Hertzler June 3, 1924 2,013,079 Slocum Sept. 3, 1935 2,164,869 Brady July 4, 1939 2,319,230 Harrington May 18, 1943 FOREIGN PATENTS Number Country Date 33,041 France June 11, 1928 376,936 Germany June 7, 1923

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