US3276383A - Pump for liquids at the boiling point - Google Patents

Description

Oct. 4, 1966 K. M. EISELE 3,276,383

PUMP FOR LIQUIDS AT THE BOILING POINT Filed May 8, 1964 5 Sheets-Sheet 1 FIG. F/G. 2

lNI/ENTOR A. M. E/SELE A TTOR/VEV Oct. 4, 1966 K. M. EISELE 3,276,383

PUMP FOR LIQUIDS AT THE BOILING POINT Filed May 8, 1964 3 Sheets-Sheet 2 F76. 4/1 Cm 5,

- I C U/ C 2 u/ C21!) 2 E (32 Oct. 4, 1966 K. M. EISELE PUMP FOR LIQUIDS AT THE BOILING POINT 5 Sheets-Sheet 3 Filed May 8, 1964 0 O \3 m EN N 1 20R EQEEQ 2st Eowfimm DEN KE wmmz 1 W I i i l l l I l l l 1 f zotzmii kzwmwmm 1 United States Patent 3,276,383 PUMP FOR LIQUIDS AT THE BOILING POINT Konrad M. Eisele, Murray Hill, N.J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed May 8, 1964, Ser. No. 366,098 2 Claims. (Cl. 10388) This invention relates to fluid pumps and more particularly to rotary pumps for pumping liquids at or near their boiling points without undue interference from cavitation.

The increased use of cryogenic liquids in laboratory and industrial processes requires convenient means for transferring them into and out of storage containers, cryostats and the like into spot coolers, or their circulation through various refrigerated devices. These liquids must frequently be transferred at or near their boiling points and, since they have low specific and latent heats of vaporization, are easily caused to boil or cavitate by the energy imparted to them in the transfer process. The resulting admixture of gas bubbles with the liquid interferes with the action of mechanical pumps, severely limiting their capacities or even halting pumping action entirely.

One priorly used technique for transferring cryogenic liquids involves pressurization of the storage container either by a supply of compressed gas or by evaporation of a part of the liquid itself by a heater within a storage container. Some types of containers, however, do not lend themselves to pressurization techniques. Cryogenic liquids heretofore could only be transferred from these by pouring or removed by evaporation.

A need has existed, for example, for a mechanical pump small enough to fit through the relatively narrow neck of the widely used twenty-five liter laboratory storage Dewar. Dewars of a currently favored design, moreover, employ plastic parts and can not be sufficiently pressurized to effect rapid and complete transfer of the liquid stored therein. In addition to small size, the me chanical pump should be capable of operating without interruption by cavitation under the low hydrostatic head conditions which prevail in containers of such relatively small capacity. It is desirable in fact, that the pump be operable under the lowest possible head in order that the container may be emptied as completely as possible, thereby reducing waste of the liquid and eliminating the need for lifting the container to pour the contents.

The stringency of the above requirements will be recognized from the following facts: The cryogenic liquid to be pumped is at its boiling point; a typical latent heat of vaporization is roughly one-tenth of that of water; and a typical heat capacity about one-half that of water. Thus, cavitation may easily result from heat conducted to the liquid through the pump itself, by heat generated in the pump bearings or by mechanical energy transferred to the liquid by the moving parts of the pump.

One of the most difficult cryogenic liquids to transfer by a mechanical pump is nitrogen. For example, it has been predicted theoretically and shown experimentally that the minimum required net positive suction head under which liquid nitrogen may be pumped without cavitation is forty-two times that required for liquid hydrogen in the same pump. Prior pumps for this purpose have been designed on the assumption that cavitation must be completely avoided. In general, they have required a hydrostatic head of several feet to suppress cavitation in liquid nitrogen. This condition is, of course, not realizable in small Dewars such as the twenty-five liter laboratory containers which are less than two feet deep.

The present invention is a small, mixed-flow rotary pump capable of pumping liquid nitrogen without interference from unwanted cavitation and with a relatively small net positive suction head.

A feature of the invention is a pump in which the ratio of the output to input cross-sectional areas is such that the liquid to be pumped, which is at its boiling point, is caused to undergo negative prerotation, or prerotation of liquid opposite to the direction of impeller rotation, as it enters the pump. Negative prerotation, by a mechanism not yet completely understood, produces a temperature drop in the liquid which in turn lowers the vapor pressure to a point sufficient to prevent cavitation. As a consequence, the present invention is capable of pumping cryogenic liquids at their boiling points with a net positive suction head of only two inches.

Thus, whereas the prior art designed noncryogenic pumping apparatus to suppress negative prerotation entirely in order to permit the continuous flow of liquid, the present invention is based on the fact that negative prerotation is essential to the successful operation of cryogenic pump utilizing a relatively small net positive suction head which heretofore completely precluded the use of mechanical pumps.

The objects and features of the invention will be more thoroughly understood from the following discussion taken in conjunction with the accompanying drawing in which:

FIG. 1 depicts in schematic form a cryogenic pump embodying the invention;

FIG. 2 is a detailed view of the shaft assembly of the pump shown in FIG. 1;

FIG. 3 is a detailed cross-section of the impeller end of the pump shown in FIG. 1;

FIGS. 4A and 4B are diagrams showing the relationships among various design parameters of the pump of FIG. 1;

FIG. 5 is a diagram showing the curvature and relative position of the impeller vanes of the pump shown in FIG. 1; and

FIG. 6 is a graph showing the regions of pump operation which produce negative, zero, and positive prerotation.

Referring more particularly to the drawing, there is shown in schematic form in FIG. 1 a mixed-flow rotary pump embodying the principles of the invention and adapted to fit a container such as a standard twenty-five liter laboratory Dewar which is characterized by a narrow neck and a depth of about two feet. The pump assembly comprises a motor 11 for driving; a rotary impeller 12 which is shown in the drawing with the impeller casing 13 removed in the interest of clarity. The motor 11 is connected to the impeller 12 by means of a shaft 14, which is provided with a bearing 15 at the impeller end of the assembly and a bearing 16 at the motor end.

Due to the extreme changes in temperature to which the pump may be subjected in cryogenic applications, a spring-loaded clutch 17 is provided between the shaft 14 and the motor 11 in order to accommodate considerable amount of thermal expansion and contraction of the shaft along its linear dimension. In operation, liquid is taken in at the impeller end of the pump and forced up through the tube 18 which surrounds the shaft 14. The tube 18 curves away from the shaft 14 at a point above the pump holder 19 which is adapted to seat the pump firmly in position in the mouth of the container in which it is operated.

A more detailed view of the shaft with its clutch and bearing assemblies is shown in FIG. 2. In the particular embodiment described herein the shaft is of hard-drawn stainless steel .090 inch in diameter and is coated with Teflon to bring its diameter up to .125 inch. The Teflon coating serves as a low-friction bearing surface where needed.

The clutch 17 comprises a female member 22 affixed to the shaft 29 of motor 11 and a male member 23 which is attached to shaft 14. The female member 22 is engaged by male member 23 and thereby maintains the proper alignment of the shaft 29 of motor 11 and the impeller drive shaft 14 while the latter expands and contracts. The clutch surfaces 24 are held in contact during normal operation of the pump by the action of a steel loading spring 25. The reaction thrust due to the spring 25 is borne by a pressure disc 26 aflixed to shaft 14 and mounted in a ball bearing 27. Although the use of a spring-loaded clutch exerts some extra force on the ball bearings 27 as well as on the ball bearings 28 at the impeller end of shaft 14, resulting in a somewhat higher rate of bearing wear, this step is necessitated by the fact that a shaft having a length of about two feet may contract as =much as fifty to sixty mils upon cooling from room temperature to liquid nitrogen temperature.

Inasmuch as the lower bearing 28 generally runs submerged in a cryogenic liquid while the upper bearing 27 runs at a temperature not much above that within the cryostat, careful design of the bearings is essential to minimize the production of heat which would vaporize the liquid being pumped. While large ball bearings run more quietly and more steadily, they tend to generate more heat. It has been found, accordingly, that the bearings are advantageously carefully degreased and cleaned precision ball bearings of rather small size. In the embodiment being described the bearings used had an inner diameter of .125 inch and an outer diameter of .250 inch. While the lower bearing 28 is lubricated effectively by the cryogenic liquid itself, it is preferred that the upper bearing 27 be run with no lubricant whatsoever due to the very low temperatures it may reach when the pump is immersed in a cryogenic liquid for extended periods. It has been found that the bearing seats at room temperature are advantageously made one or two mils oversize to allow for the contraction upon cooling. Although a well-balanced and perfectly straight shaft does not require any bearings between the top and bottom if the rotational speed is not excessive, it has been found desirable to use one brass-Teflon friction bearing about midway between the ends of shaft 14 to safeguard against excessive Vibration.

A cross-sectional view of the impeller assembly is shown in FIG. 3. The impeller vanes 31 are afiixed to an impeller wheel 32 which is attached to the lower end of shaft 14. The double curved lower surface of wheel 32 forms the back shroud of the impeller 12. The front shroud, formed by the curved surface 33 which is a part of the impeller casing 13, is roughly complementary to the surface of the back shroud. The maximum size of the impeller assembly of the illustrative embodiment was determined by the diameter of the neck tube of a standard laboratory storage Dewar (.625 inch). The diameter of the impeller 12 was .490 inch. The clearance between the edges of the impeller vanes 31 and the front shroud 33 was about .010 inch.

The surface area which the impeler 32 presents to the liquid entering the impeller chamber is the input crosssectional area, denoted by A The annular area between the impeller 32 and the inner wall of the impeller casing 13 at the discharge end is the output cross-sectional area, denoted by A It was estimated that an impeller of such small dimensions should rotate at least 2500 revolutions per minute in order to impart sufficient energy to the liquid to push it to a height of two feet. It has been found, on the other hand, that a speed of 10,000 revolutions per minute causes considerable vibration of the pump mechanism, excessive wear to the degreased ball bearings, and uncontrolled cavitation due to violent tearing of the flow lines and resultant turbulence in the liquid. It was considered advantageous, therefore, to operate the pump at a spee somewhere near the midpoint of this range.

In general, it is considered desirable in mixed-flow pumps for the lines of flow to be directed at an angle between fifty degrees and eighty degrees to the pump axis. The angle between the principal surface of the front shroud 33 and the pump axis of the illustrative embodiment was about fifty-seven degrees. The impeller 12 was designed to provide a discharge angle of about 22.5 degrees.

The significant information describing the operation of the impeller may be related by means of geometrical diagrams known as entrance and discharge velocity triangles. The significant quantities are the peripheral velocity of the impeller, the relative velocity of the liquid with respect to the impeller and the absolute velocity of the liquid flow. The discharge and entrance velocity triangles, respectively, are shown in FIGS. 4A and 4B in which the peripheral velocity of the impeller at the entrance is represented by U and at the discharge by U The meridional velocity of the liquid at the entrance is designated C and at the discharge C The vane discharge angle is [3 The meridional velocities C which are the components of the absolute velocity of the liquid normal to the peripheral velocity U, may be obtained from the rotational speed of the impeller, the capacity of the pump and the available cross section of the flow at the entrance and discharge of the impeller.

The triangle drawn by means of U, C and B yields the other unknowns, i.e., the actual absolute velocity C of the liquid and the relative velocity W of the liquid with respect to the impeller surface.

Assuming that the design objective is a pump capacity of three liters per minute at a speed of 6,000 revolutions per minute, it can be shown that in a pump of the dimensions of the illustrative embodiment, U equals -cm./se-c., U equals 390 cm./sec., C equals 111 cm./sec., and C equals 101 cm./sec. From the discharge triangle shown in FIG. 4A it can be determined that C the absolute "velocity of the liquid, is cm./sec. and W the relative velocity of the liquid with respect to the impeller surface, is 270 cm./sec. c is the tangental component of the absolute velocity, a quantity of little significance at the discharge end of the impeller. However, the tangental component C of the absolute velocity at the entrance of the impeller is very important as it represents the actual prerotation of the liquid in the impeller eye.

The entrance velocity triangle shown in FIG. 4B is constructed to conform with the constraint that the increase from W to W must be within certain limits to insure laminar flow in the pump. For a specific speed of 3,000 the ratio W /W =1.15 is appropriate. FIG. 4B reveals that in the illustrative embodiment W equals 310 cm./sec., C equals 127 cm./sec. and C equals 168 cm./.sec. The unusual feature of FIG. 4B is that U and C are in opposite directions. If the direction U is considered positive, then the direction of C is negative. That is, the fluid undergoes a negative prerotation or a prerotation opposite to the direction of the impeller rotation. The relatively high negative prerotation which results from the instant pump design is generally considered undesirable in pumps handling noncryogenic liquids, since it not only reduces pump efiiciency but frequently causes pumping action to stop entirely, rendering the pump completely useless. Negative prerotation is essential, however, -to the effective operation of the present pump under cryogenic conditions. Although the exact mechanism is not yet completely understood, negative prerotation, which takes place as the liquid flows through the vestibular entrance chamber formed by the lower portion of the impeller casing 13 into the impeller eye produces a .temperature drop in the cryogenic liquid. This temperature drop lowers the vapor pressure of the cryogenic liquid and thereby prevents cavitation. As a consequence, the

present invention is capable of handling cryogenic liquids at their boiling points with a net positive suction head of only two inches, and sometimes even less.

Negative prerotation of the liquid in the impeller eye is produced by designing the pump according to the following criterion. From the entrance velocity triangle shown in FIG. 4B it is apparent that (u,cu,) =Wi i (1) the condition for zero prerotation is thus the following inequalities hold for positive prerotation This relationship is plotted in FIG. 6 with A /A as the dependent variable and u A Q the independent variable. The curve ofFIG. 6, which is easily recognized as a circle having center at the origin and radius /9.03, represents those pump designs which produce Zero prerotation; that is, satisfy The circle divides the first quadrant into two regions, one in which the liquid undergoes positive prerotation and one in which it undergoes negative prerotation. If the pump is designed such that Equation is satisfied, then the liquid will undergo negative prerotation in accordance with this invention and consequently be capable of handling cryogenic liquids at their boiling points. The present invention, for instance, is designed such that A /A =1.70. Therefore, for all values of u A Q such that u A /Q 2.47, the liquid will undergo negative prerotation. One such set of values for the present invention is indicated in FIG. 6;

The aforementioned velocity triangles determine important properties of the impeller but give no information about the number of vanes necessary to obtain the desired capacity and head at maximum efficiency. Prior art practice, however, indicates that for a specific speed of 3,000 an impeller having at least six vanes should be employed. A larger number is preferred since impellers with many vanes are more apt to suppress cavitation. In the present case, on the other hand, a small amount of cavitation is desired for successful pumping of liquids at their boiling points. Moreover, in a pump of the size of the illustrative embodiment, a large number of vanes would severely restrict the available cross section of the flow especially in the impeller eye. Accordingly, it has been found that satisfactory performance is obtained with a five vane pump impeller.

In general, the vane curvature should follow the curve of an involute, but for small pumps the involute can be replaced by a section of a circle without noticeable effect on the pumping action. Front and back of such a single curvature vane are parallel. For mixed-flow impellers, in which a vane extends not purely radial to the shaft but at an angle of 30-45 degrees, it is necessary to have an entrance angle at the front of the vane that is different than the discharge angle at the back. This results in a double curvature. Furthermore, a mixed-flow vane must have a twist because the entrance portion is to transport liquid more in a radial direction whereas the discharge end of the vane is to transport liquid principally in an axial direction.

FIG. 5 shows the geometric construction of the pump impeller of the illustrative embodiment. R is the radius of curvature of the vanes, with all centers located on a circle with radius R about the center zero of the impeller wheel.

It has been found that the open impeller design, i.e., with the front shroud stationary, a vapor bubble forming in the impeller eye being in a volume of lower pressure, will collapse when it comes into the high pressure zone near the stationary front shroud. Thus, the bubble formed by the cavitation cools the liquid in the impeller eye slightly below its boiling point but tends to collapse as it 'moves out of the eye. Thus the pump operates without the undesirable and often disabling interference with smooth liquid flow which would result if the bubble expanded as it passed through the impeller. In addition, the clearance between the impeller vanes and the front shroud permits a small amount of back leakage which supplies sufficient liquid to displace any gas bubbles which might stop the flow.

The pump described herein as the illustrative embodiment of the invention has been built and operated. Contrary to prior art belief that liquid nitrogen at its boiling point could not be pumped successfully because of cavitation in the absence of a hydrostatic head of several feet, the pump described operated with net positive suction heads under two inches. At 5,600 revolutions per minute, for example, it pumped this fluid at a rate of two liters per minute with a two inch head. With an eight inch head and a speed of 6,000 revolutions per minute, capacity was about three liters per minute.

Although the invention has been described with particular reference to a specific illustrative embodiment, many modifications and variations are possible and may be made by those skilled in the art without departing from its scope and spirit.

What is claimed is:

1. A mixed flow rotary pump for pumping cryogenic liquids at their boiling points, comprising means forming an impeller chamber having intake and discharge ends,

the interior surface of said chamber being a surface of revolution flaring in a compound curve from. a. smaller diameter at the intake end to a larger diameter at the discharge end,

an axially rotatable impeller wheel having a front surface of revolution with a compound curvature substantially complementary to the interior surface of said chamber,

said impeller wheel having a surface area being the input cross-sectional area,

said interior surface of said chamber and said impeller wheel forming an annular area at said discharge end through which said liquid flows,

said annular area being the output cross-sectional area,

a plurality of curved fluid-engaging impeller vanes extending from said front surface of the impeller wheel, the curvature of the vanes being substantially that of an involute about the axis of said wheel,

said vanes extending outwardly from a circular impeller eye about the center of said wheel with a diameter approximating that of the intake end of said chamher,

said impeller being rotatably mounted in said chamber with the free edges of said vanes spaced from and substantially complementary to the curvature of the interior surface portion thereof,

the interior surface of said chamber and the surface of said impeller wheel forming front and back shrouds, respectively, of said vanes,

means forming a vestibular entrance chamber adjacent said impeller chamber at the entrance thereof,

said entrance chamber having an interior surface which is a surface of revolution flaring in a smooth curve from a large diameter at the open end to a diameter matching that of the intake end of the impeller chamber at the end adjacent thereto,

and means for rotating said impeller to cause a liquid at its boiling point to enter said impeller chamber,

said impeller chamber constructed such that the ratio of said output and input cross-sectional areas is such as to cause liquid entering said impeller chamber to undergo negative prerotation, and to flow through said impeller chamber and out said discharge end thereof.

2. A mixed flow rotary pump for pumping cryogenic liquids at their boiling points, comprising means forming an impeller chamber having intake and discharge ends,

the interior surface of said chamber being a surface of revolution flaring in a compound curve from a smaller diameter at the intake end to a larger diameter at the discharge end,

an axially rotatable impeller wheel having a front surface of revolution with a compound curvature substantially complementary to the interior surface of said chamber,

said impeller wheel having a surface area being the input cross-sectional area,

said interior surface of said chamber and said impeller wheel forming an annular area at said discharge end through which said liquid flows,

said annular area being the output cross-sectional area,

a plurality of curved fluid-engaging impeller vanes eX- tending from said front surface of the impeller wheel,

the curvature of the vanes being substantially that of an involute about the axis of said wheel,

said vanes extending outwardly from a circular impeller eye about the center of said wheel with a diameter approximating that of the intake end of said chamber,

said impeller being rotatably mounted in said chamber with the free edges of said vanes spaced from and substantially complementary to the curvature of the interior surface portion thereof,

the interior surface of said chamber and the surface of said impeller wheel forming front and back shrouds, respectively, of said vanes,

means forming a vestibular entrance chamber adjacent said impeller chamber at the entrance thereof,

said entrance chamber having an interior surface which is a surface of revolution flaring in a smooth curve from a large diameter at the open end to a diameter matching that of the intake end of the impeller chamber at the end adjacent thereto, and means for rotating said impeller to cause a liquid at its boiling point to enter said impeller chamber, said impeller chamber constructed such that the ratio of said output and input cross-sectional areas is such as to cause liquid entering said impeller chamber to undergo negative prerotation, said negative prerotation causing the temperature and subsequently the vapor pressure of said liquid to decrease thereby to prevent cavitation and allow said liquid to flow through said impeller chamber and out said discharge end thereof.

References Cited by the Examiner UNITED STATES PATENTS 86,264 1/ 1896 White et al 10'3--88 1,090,066 3/1914 Johnston 10390 1,795,588 3/1931 Wilson 103-89 1,981,991 11/1934 Cline et al. 103-90 2,865,295 12/ 1958 Laing 103-89 2,868,132 1/1959 Rittershafer 10389 3,028,140 4/ 1962 Lage 230-134.45 3,168,048 2/1965 Toyokura et a1. 103-89 FOREIGN PATENTS 1,316,212 2/1962 France.

MARK NEWMAN, Primary Examiner.

HENRY F. RADUAZO, Examiner.

Claims (1)

1. A MIXED FLOW ROTARY PUMP FOR PUMPING CRYOGENIC LIQUIDS AT THEIR BOILING POINTS, COMPRISING MEANS FORMING AN IMPELLER CHAMBER HAVING INTAKE AND DISCHARGE ENDS, THE INTERIOR SURFACE OF SAID CHAMBER BEING A SURFACE OF REVOLUTION FLARING IN A COMPOUND CURVE FROM A SMALLER DIAMETER AT THE INTAKE END TO A LARGER DIAMETER AT THE DISCHARGE END, AN AXIALLY ROTATABLE IMPELLER WHEEL HAVING A FRONT SURFACE OF REVOLUTION WITH A COMPOUND CURVATURE SUBSTANTIALLY COMPLEMENTARY TO THE INTERIOR SURFACE OF SAID CHAMBER, SAID IMPELLER WHEEL HAVING A STRUCTURE AREA BEING THE INPUT CROSS-SECTIONAL AREA, SAID INTERIOR SURFACE OF SAID CHAMBER AND SAID IMPELLER WHEEL FORMING AN ANNULAR AREA AT SAID DISCHARGE END THROUGH WHICH SAID LIQUID FLOWS, SAID ANNULAR AREA BEING THE OUTPUT CROSS-SECTIONAL AREA, A PLURALITY OF CURVED FLUID-ENGAGING IMPELLER VANES EXTENDING FROM SAID FRONT SURFACE OF THE IMPELLER WHEEL, THE CURVATURE OF THE VANES BEING SUBSTANTIALLY THAT OF AN INVOLUTE ABOUT THE AXIS OF SAID WHEEL, SAID VANES EXTENDING OUTWARDLY FROM A CIRCULAR IMPELLER EYE ABOUT THE CENTER OF SAID WHEEL WITH A DIAMETER APPROXIMATING THAT OF THE INTAKE END OF SAID CHAMBER, SAID IMPELLER BEING ROTATABLY MOUNTED IN SAID CHAMBER WITH THE FREE EDGES OF SAID VANES SPACED FROM AND

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