US3851993A

US3851993A - Washing machine pump

Info

Publication number: US3851993A
Application number: US00351630A
Authority: US
Inventors: J Foster
Original assignee: Franklin Manufacturing Co
Current assignee: Franklin Manufacturing Co
Priority date: 1973-04-16
Filing date: 1973-04-16
Publication date: 1974-12-03
Anticipated expiration: 1991-12-03

Abstract

A reversible dual function pump for a laundry appliance which recirculates detergent water in a tub at a moderate rate when driven in a first rotary direction and which removes such water from the tube at a greater rate when driven in the opposite direction. An impeller and a volute cavity of the pump are arranged to produce a positive displacement pumping action which provides sufficient reserve pumping capacity to allow exceptional dimensional latitude in the pump construction and an internal by pass circuit for regulating internal pressure in the pump. Wide dimensional latitude in the pump components allows them to be formed of plastic materials and allows the pump to be mounted on and driven directly by a motor. Pressure regulation by the internal by pass avoids the risk of pump out in an improper operational mode and the possibility of producing suds from an excessive vacuum condition.

Description

tUnitc States atent [191 Foster Dec. 3, 1974 [73] Assignee: Franklin Manufacturing Company,

St. Cloud, Minn.

[22] Filed: Apr. 16, 1973 [21] Appl. No.: 351,630

[56] References Cited UNITED STATES PATENTS 3/1947 Brady, Jr 415/53 T 3/1954 Smith 415/213 R 3,136,254 6/1964 Douglas 415/152 A 3,335,668 8/1967 Thon 415/214 3,751,179 8/1973 Wassmann 415/152 A 3,752,603 ,3/1973 Bunch 415/214 Primary ExaminerC. J. Husar Attorney, Agent, or FirmMcNenny, Farrington, Pearne & Gordon [57 ABSTRACT A reversible dual function pump for a laundry appliance which recirculates detergent water in a tub at a moderate rate when driven in a first rotary direction and which removes such water from the tube at a greater rate when driven in the opposite direction. An impeller and a volute cavity of the'pump are arranged to produce a positive displacement pumping action which provides sufficient reserve pumping capacity to allow exceptional dimensional latitude in the pump construction and an internal by pass circuit for regulating internal pressure in the pump. Wide dimensional latitude in the pump components allows them to be formed of plastic materials and allows the pump to be mounted on and driven directly by a motor. Pressure regulation by the internal by pass avoids the risk of pump out in an improper operational mode and the possibility of producing suds from an excessive vacuum condition.

15 Claims, 7 Drawing Figures 1 WASHING MACHINE PUMP BACKGROUND OF THE INVENTION PRIOR ART The present invention is particularly suited for use in the field of laundry appliances. In such appliances it is often desirable to recirculate detergent water during a wash cycle and subsequently drain the water from the washing tub after the wash cycle has been completed. In automatic clothes washers, for instance, it is generally convenient to provide a single pump for performing both of these operations and it is generally advantageous for the pump mode of operation to be selected by driving it in one or the other rotary direction.-

Prior to the present invention, bi-directional dual function pumps for this service have generally had characteristic deficiencies which limited their performance and, consequently, their acceptance. Among the common problems associated with earlier pumps has been a requirement of close dimensional tolerances in manufacture and assembly to approach satisfactory performance. These previously required dimensional tolerances have'necessitated the use of metal compo nents for dimensional stability and have limited the realization of economy associated with the use of plastic materials. An undesirable trait of certain prior pump designs is the development of excessive vacuum in the recirculation mode resulting in the production ofsuds and a consequentloss of washing efficiency. Another disadvantage of certain reversible pumps. is a tendency to drain the tub during operation in the recirculation mode causing a loss of detergent water during a wash cycle in the event the recirculation circuit becomes plugged. Typically, output volume of earlier pumps varied with the square of pump speed, as in most centrifugal pumps, such that it was difficult to reach a satisfactory design compromise between flow rates at high and' low speeds.

SUMMARY OF THE INVENTION The invention provides an improved impeller and housing arrangement for a reversible pump wherein balanced bidirectional pump performance is achieved. Positive displacement pump action. at a main pump outlet produces sufficient capacity to permit successfuluse of a bypass circuit at an auxiliary recirculation pump outlet to limit internal pressure in the pump. This self regulating effect of the bypass prevents objectionable pump out and loss of water in the event that the recirculation circuit is fouled.

The positive displacement pump action further provides sufficient reserve pumping capacity to allow unusual latitude in dimensional tolerances in the components and the assembly of the pump. This flexibility permits the pump to be constructed of plastic materials with attendant savings in manufacturing costs and elimination of corrosion problems. Further, relatively large workable clearances in the improved 'pump allow the impeller to be supported directly on a motor shaft and the housing to be mounted on the motor or an adjacent bracket with minimum concern of dimensional variations in the motor construction or positioning of the pump relative to the motor.

Additionally, the pump arrangement has been found to producea recirculation flow which is approximately proportional to motor speed, a characteristic of positive displacement pumps, so that the difference in recirculation flow between high and low motor speed is at an acceptable minimum. Moreover, the present pump avoids sudsing of the wash water resulting from excessive suction at the main outlet during operation in the recirculation mode.

, cent the main outlet produces a variable volume chamher effect when a blade sweeps by the outlet. This effect provides positive pressure at the outlet 'in the pump out mode and a normally negative pressure during the washing or recirculation mode. A bypass circuit between the recirculation-outlet and the pump inlet moderates internal pump pressure to insure that pressure in the main outlet cannot become positive enough to pump out water when the recirculation outlet is blocked. The bypass circuit preferably includes an integral open faced channel forming a portion of the inlet and extending radially between the axis of the impeller and the recirculation outlet.

Recirculation flow is driven through a slot in a circumferential wall of the pump housing. Water passes through the slot .into. a recess and then to a recirculation port. In accordance with an important feature of the invention, a wall common to portions of both the recess and the main inlet may be perforated when the pump is employed in an appliance not having an external recirculation circuit. By perforation of this wall, an internal recirculation path is created such that the operational characteristics of the pump are not significantly altered. Thus, substantially the same pump and toolingmay be used whether or not it is desired to provide recirculation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded perspective view of a reversible pump constructed in accordance with the invention.

FIG. 2 is an elevational cross-sectional view of the pump assembly.

FIG. 3 is a plan view of the pump with its cover removed to reveal the impeller in a first angular position.

FIG. 7 is a fragmentary, perspective view similar to FIG. 1, illustrating a modification of the pump housing used when recirculation is not provided.

DESCRIPTION OF THE PREFERRED EMBODIMENT The drawings disclose a reversible pump 10 wherein,

water or other liquid is drawn into a central inlet port 11 and expelled from either of two

peripheral outlet ports

12 and 13 determinedby the direction of rotation of an impeller 14. A pump housing in which the impeller 14 operates comprises mating body and

cover portions

16 and 17 respectively.

Within the housing body 16 is formed an impeller cavity 21. The impeller cavity 21 is bounded radially by a generally circular or cylindrical wall 22 and a spiral or volute wall extension 23. The impeller cavity 21 is bounded axially by an end wall 26 integral with the housing body 16 and the cover or opposite end wall 17. The central inlet, generally indicated at 11, is integrally formed in the end wall 26 and includes a rounded central chamber or hollow 28, projecting axially outward of the plane of the end wall 26, and a rounded channel 29. The inlet channel 29 is preferably in open communication with the impeller cavity 21 along its full length and extends generally radially to the axis of the pump, i.e. the axis of rotation of the impeller 14. The channel 29 is slightly eccentric to the impeller axis to provide an efficient flow path to the impeller center, designated 30. A cylindrical inlet nipple 31, formed integrally with the housing body 16, joins the inlet channel 29 at a point 32 slightly within the line of the circular cavity wall 22.

The main outlet, generally indicated at 12, is formed by a cylindrical nipple 36 integrally formed with the housing body 16 in a manner similar to the inlet nipple 31. This outlet nipple or tube 36 is substantially parallel to the inlet nipple 31 extends tangentially into direct communication with a volute zone 38 of the impeller cavity 21. The volute zone 38 is formed by the spiral wall section 23 and an adjacent section 39 (shown in phantom in FIGS. 3 and 4) of an imaginary cylinder defined by an extension of the circular wall section 22.

The auxiliary outlet, indicated generally at 13, is provided for recirculatory flow and includes an outlet nipple 41 integrally formed in the housing body 16 adjacent the inlet 31. The auxiliary nipple 41 communicates with the impeller cavity 21 indirectly through a recess 43 peripheral to the cavity. The recirculating flow receiving recess 43 is partially formed by the circular cavity wall 22 and a wall area 44 of the inlet nipple 31. The nipple 41 communicates with the recess 43 through an aperture 45 in an associated housing wall portion 46. An elongated slot or aperture 42 is formed in the cavity wall 22 for passage of water or other liquid into the recess 43. The slot 42 is aligned lengthwise with the circumference of wall 22 and is arranged on the wall in angular alignment with the inlet channel 29 (FIG. The slot is adjacent the housing end wall formed by the cover 17 and is axially displaced from the opposite end wall 26 and the channel 29.

The housing cover 17 which closes the impeller cavity 21 is a generally planar element having integrally formed reinforcing ribs 49 and 50. A series of alternating holes 52 in the cover 17 register with corresponding holes 53 in the body 16 and receive screws 54 (one is shown in FIG. 2) to secure thecover to the body. lntervening holes 57 and aligned holes 58 are formed in the cover and

body

17 and 16 respectively for receiving screws (not shown) for securing the pump relative to a motor 80 schematically illustrated in FIG. 6. As shown in FIG. 2, a suitable gasket 59 is used to seal the body 16 and cover 17. A seal assembly of conventional design is indicated generally at 62 in FIG. 2 and includes an elastomeric element 63 in an associated recess in the cover 17 and a pair of washer-like elements 64 and 66 retained in an annular recess in the impeller 14.

A hub 67 of the impeller 14 extends through the seal assembly 62 and a clearance hole 65 in the coverl7.

The clearance hole 65 and the abutting faces of the seal elements 63 and 64 accommodate misalignment and eccentricity of the impeller relative to the

housing portions

16 and 17. The

housing body

16 and 17 cover are preferably constructed of a plastic material such as polypropylene with 20% talc fill. These

members

16 and 17, ideally, have a substantially uniform wall thickness throughout their various sections for satisfactory molding results and for minimizing the amount of material used in their construction.

The impeller 14 is, preferably, an integrally formed member which includes a central truncated conical boss 71 facing the inlet hollow 28. A plurality of substantially flat blades 72 are fixed on the circumference of a web 73 extending radially from the conical boss 71. The impeller hub 67 includes an axial bore 75 having a D-shaped cross section for mounting the impeller 14 directly on a cylindrical motor shaft having an axially extending flat on its circumference. Preferably, the impeller 14 is retained to the motor shaft by driving a cross pin through a radial hole 76 in the hub 67. The impeller 14 is, ideally, molded of the same plastic material as the housing body and cover 16 and 17.

The impeller blades 72 are preferably six in number and are of generally uniform wall thickness except for the allowance of a slight draft angle for molding purposes. As illustrated most clearly in FIGS. 3 and 4, each of the impeller blades 72 are tangentially oriented such that they each form a substantial angle with a radial line from the axis of the impeller 14 to their radially outermost portion or edge 78. This angle, preferably, is approximately 50 when six blades are used.

Study of FIGS. 3 and 4 reveals that the tangentially oriented flat blades 72 cooperate with the volute zone 38 to produce positive displacement pumping action. When a blade 72a is in a position substantially as shown in FIG. 3 where an outermost edge 78 of an adjacent blade 72 is proximate to an inner end, designated 79, of the main outlet nipple 36, the plane of the blade describes with the spiral or volute wall section 23 a volume of maximum value, represented by the cross hatched area in the subject Figure. In contrast to the position of FIG. 3, the plane of the blade 72a in FIG. 4 describes with the volute wall 23 a volume substantially smaller. As the impeller 14 turns counterclockwise, the plane of each blade 72 sweeps across the volute zone 38 diminishing the actual volume in the volute zone ahead of it to cause water or other liquid to be forced out of the outlet 36. The flat vanes 72 besides providing a positive displacement pumping action are advantageous since their uniform cross section is more readily molded and requires a minimum of material.

Referring to FIG. 6, there is shown a typical arrangement of the pump 10 with a laundry tub 81 such as provided in a domestic automatic clothes washer The pump 10 is conveniently mounted on a motor 80 located under the tub 81. The tub 81 may be a splash tub in which a slightly smaller spin tub 82 is rotatably mounted. The inlet 11 of the pump is connected to a drain flange (not shown) at the bottom of the splash tub 81 by a suitable hose 83. A hose 84 extends from the pump recirculation outlet 13 to a point over the splash tub 81. The main outlet 12 of the pump 10 is connected to a discharge hose 86 which extends vertically slightly higher than the expected water level in the tub 81. An end 87 of the hose 86 may be fitted with an additional line to direct discharged water to a sewer or a holding tub. As illustrated, the hose circuitry associated with the pump is valveless. I

A suitable control system in accordance with principles familiar to those skilled in the art, may be provided to direct detergent water into the tubs 81 and 82 (the inner tub 82 may be perforated). An agitator (not shown) may be driven by the motor 80 through a conventional transmission (not shown) at an appropriate time in a control cycle. The transmission may be conventionally arranged to drive the agitator when driven by the motor 10in one direction and the spin tub 82 when driven by the motor in the other direction.

In operation, the pump 10 is adapted to remove used detergent water from the tub 81 when the impeller 14 is driven counterclockwise as viewed in FIGS. 1, 3 and 4. This mode of pump operation may coincide witha spin or centrifugal cycle of the inner tub 82. During such operation water enters'the inlet 11 from the tub 81 and is forced from the central inlet hollow 28 radially outwardly through the main outlet 12 and associated hose 86 by the impeller blades 72; The disclosed pump 10 is capable of draining a tub of conventional size in approximately two minutes. The pump, moreover, has sufficient reserve pumping capacity to allow an eccentricity as high as .062 inches of the true impeller axis of rotation from the ideal axis defined by the cavity 21 where the outside diameter of the impeller 14 is approximately 3.75 inches.

When theimpeller l4 is driven in a clockwise direction, such aswhen the motor 80 is driving an agitator, the pump 10 recirculates water through the

tubs

81 and 82 to permit such water to be filtered by a lint filter (not shown). In this recirculation mode, liquid is drawn from the central inlet 11 and driven with centrifugal force radially outwardly by'the blades 72 through the slot 42 into the recess 43. From the recess 43 liquid travels outward through the auxiliary or recirculation nipple 41 and the hose 84. The geometry of the slot 42 and its relationship to the inlet channel 29 in combination with the size of the recirculation aperture 45 is chosen to provide an adequate recirculation flow ranging, for example, between 2 and 4 gallons per minute. Recirculation flow of the pump has been found to be approximately directly related or linear with pump speed. This characteristic is highly desirable where the pump is driven by a multispeed motor since it reduces flow variation from an exponential variation usually produced in a centrifugal pump. I

Under normal conditions in the recirculation mode, the pump is arranged to develop a vacuum at the outlet 12. The vacuum is created by the blades 72 as they sweep in a clockwise direction again in a positive displacement action' which may be visualized as blade' movement from the position of FIG. 4 to that of FIG. 3. The internal pressure of the pump is balanced or regulated to maintain this vacuum under normal conditions by a bypass circuit between the recirculation outlet 13 and the inlet 11. The bypass circuit comprises a zone between the impeller 14 and a cavity wall area 91. This wall area 91 extends circumferentially between imaginary lines 92' extending from the slot 42 illustrated in FIG. 5 and axially from the slot to the channel 29.

The pressure modulation produced by the bypass circuit normally maintains the internal pump pressure at a point where the vacuum at the pump outlet 12 is substantial, e.g., 10 inches of water, but not enough to completely drain the associated hose 86 so as to introduce air into the pump and thereby create suds. The bypass circuit by permitting this normally negative or vacuum biasing at the main outlet l2, further, limits internal pressure in the pump in the event that the recirculation circuit, including the nipple 41 or hose 84, becomes obstructed by foreign material. Pressure at the main outlet 12 is thus limited during operation in the recirculation mode such that it cannot become positive enough relative to the inlet 11 to pump out laundry water from the tub 81 during the recirculation mode of the pump in the event of blockage of the recirculation circuit. I

The pump 10 may be used in systems where a recirculation circuit is not necessary or desirable as when recirculation is only provided as an extra option on a machine. It has been found that the. pump maintains substantially all of its desirable characteristics when the recirculation outlet or port 13 is blocked by not providing the hole 45 and where the adjacent wall 44 of the inlet nipple 31 is perforated or removed in the area of the recess 43 (FIG. 7). This internal alteration of the pump structure in effect changes the recirculation flow path from an external flow to an internal flow but does not substantially change the characteristics of the pump either in the main pumping mode or in the recirculation mode.

The pump arrangement disclosed above is the result of extensive imperical investigation and development to provide a valveless pump circuit in which adequate pumping capacity is achieved independently of wide dimensional variations in the pump components and in which adequate recirculation flow rates are achieved without risking pump out of detergent water in the event the recirculation circuit is obstructed. The interrelationship of the various pump elements, therefore, is not fully understood and the above description is based on observation and present understanding.

Although a preferred embodiment of the invention is illustrated, it is to be understood that various modifications and rearrangements of parts may be resorted to without departing from the scope of the invention disclosed and claimed herein.

What is claimed is:

1. A reversible pump for a laundry appliance having an internal impeller cavity, an inlet and a main outlet communicating separately with said impeller cavity, an impeller rotatable within said cavitylsaid impeller including means to pump said water from said inlet to said outlet with positive displacement action when driven in a first direction during a pump out mode, a secondary outlet communicating with said impeller cavity, said impeller including means to pump water from said inlet through said secondary outlet port when driven in an opposite direction in a recirculation mode, and bypass means between said inlet means and said auxiliary outlet means to control pressure in said main outlet when said impeller is rotating in said recirculation mode. v

2. A pump as set forth in claim 1 wherein said positive displacement means includes means to normally produce a substantial vacuum in said main outlet when said impeller is driven in said recirculation mode.

3. A pump as set forth in claim 2 wherein said impeller includes a plurality of circumferentially spaced blades, said blades each having a substantially flat outer face, said faces having a tangential orientation.

4. A pump as set forth in claim 3 wherein each of said blade faces are oriented at an angle of approximately 50 to a radial line extending from the axis of the impeller to a radially outermost portion of each respective blade.

5. A reversible pump for a laundry appliance comprising a housing having an internal impeller cavity, an inlet and a main outlet communicating separately with said impeller cavity, an auxiliary outlet communicating with said cavity, an impeller rotatable within said cavity, said impeller including means to pump water from said inlet to said main outlet when driven in a pump out mode and to pump water from said inlet through said auxiliary outlet when driven in an opposite recirculation mode, said housing including an end wall, said auxiliary outlet being arranged on said housing adjacent the circumference of said impeller, said inlet being integrally formed in said end wall and extending radially across said end wall in a direction substantially towards said auxiliary outlet, and bypass means between said inlet and said auxiliary outlet to control pressure in said main outlet during operation in said recirculation mode.

6. A pump as set forth in claim 5 wherein said inlet is in open communication with said cavity substantially from the center of said impeller to the circumference of said impeller.

7. A pump as set forth in claim 6 wherein said bypass means includes an aperture in a wall of said housing adjacent a housing end wall opposite said first mentioned end wall.

8. A pump as set forth in claim 7 wherein said aperture comprises an elongated slot extending lengthwise along the circumference of said impeller.

9. A pump as set forth in claim 7 wherein said inlet includes a wall portion radially outward of said aperture, said wall portion forming a section of a recirculating flow receiving recess external of said cavity, said wall portion being adapted to be perforated to provide an alternate flow path within the housing when an external recirculation circuit is not provided.

10. A reversible pump for a laundry appliance comprising a housing having an impeller cavity, said cavity being defined by a circumferential curved housing wall and opposed housing end walls, a rotatable impeller in said cavity, an inlet communicating with the impeller cavity through a plane defined by one of said end walls, a main outlet communicating with said cavity tangentially to said curved wall, and a recirculation outlet communicating with said cavity through an aperture in said curved wall, said aperture being axially displaced from said one end wall and being adjacent the opposite wall.

11. A pump as set forth in claim 10 wherein said inlet comprises an integral channel in said one end wall, said channel extending radially from the rotational axis of said impeller in angular alignment with said auxiliary outlet, a wall portion within said housing forming both a portion of said inlet channel and a zone in direct communication with said recirculation outlet whereby said wall portion may be perforated where external recirculation is not desired.

12. A bi-directional pump for a laundry appliance comprising a housing having an internal impeller cavity, an inlet and an outlet communicating separately with said impeller cavity, a rotatable impeller in said cavity, said impeller including means for pumping water from said inlet to said outlet when driven in one direction, a recess external of said cavity for receiving recirculation flow when said impeller is driven in a direction opposite to said first direction, said recess communicating with said cavity through an aperture in a wall forming said cavity adjacent the periphery of said impeller.

13. A pump as set forth in claim 12 wherein said recess is formed by at least two walls, one of said walls forming a portion of said inlet, the other of said walls forming a portion of a recirculation outlet, said one wall being adapted to be perforated to establish internal recirculation flow in said housing when external recirculation is not provided.

14. A reversible pump for a laundry appliance comprising a plastic housing of generally uniform wall thickness, said housing including an internal volute impeller cavity, an inlet communicating with the center of the cavity, a main outlet and an auxiliary outlet each communicating with the cavity at its periphery, an integrally molded impeller, said impeller including a plurality of flat tangentially oriented blades, an impeller hub including means for mounting the impeller directly on a motor shaft, said impeller including means to pump water from said inlet to said main outlet when driven in one direction and from said inlet to said auxiliary outlet when driven in the opposite direction.

' 15. A reversible pump for a laundry appliance, comprising a housing having an internal impeller cavity, said impeller cavity being defined by a pair of axially spaced end walls and a circumferential wall, said circumferential wall having a generally circular major portion and a volute minor portion, a main outlet in said volute wall portion and a recirculation outlet adjacent the circular major portion and angularly spaced from said main outlet, an inlet channel integrally formed in one of said end walls, said inlet channel extending in open communication from the center of said impeller cavity along a radial direction in line with said recirculation outlet, said recirculation outlet communicating with said impeller cavity through a recess external of said cavity, an aperture in said circumferential wall adjacent the other of said end walls providing communication between said cavity and said recess, said inlet channel and said recess having a common'wall adapted to be perforated to provide a recirculating flow path internal of the housing when external recirculation flow from said recirculation outlet is not provided,

an impeller rotatable in said cavity in a first direction cumferential wall providing a bypass circuit between said aperture and said inlet to regulate the pressure at said main outlet when said impeller is driven in said recirculation direction.

Claims

1. A reversible pump for a laundry appliance having an internal impeller cavity, an inlet and a main outlet communicating separately with said impeller cavity, an impeller rotatable within said cavity, said impeller including means to pump said water from said inlet to said outlet with positive displacement action when driven in a first direction during a pump out mode, a secondary outlet communicating with said impeller cavity, said impeller including means to pump water from said inlet through said secondary outlet port when driven in an opposite direction in a recirculation mode, and bypass means between said inlet means and said auxiliary outlet means to control pressure in said main outlet when said impeller is rotating in said recirculation mode.

4. A pump as set forth in claim 3 wherein each of said blade faces are oriented at an angle of approximately 50* to a radial line extending from the axis of the impeller to a radially outermost portion of each respective blade.

14. A reversible pump for a laundry appliance comprising a plastic housing of generally uniform wall thickness, said housing including an internal volute impeller cavity, an inlet communicating with the center of the cavity, a main outlet and an auxiliary outlet each communicating with the cavity at its periphery, an integrally molded impeller, said impeller including a plurality of flat tengentially oriented blades, an impeller hub including means for mounting the impeller directly on a motor shaft, said impeller including means to pump water from said inlet to said main outlet when driven in one direction and from said inlet to said auxiliary outlet when driven in the opposite direction.

15. A reversible pump for a laundry appliance, comprising a housing having an internal impeller cavity, said impeller cavity being defined by a pair of axially spaced end walls and a circumferential wall, said circumferential wall having a generally circular major portion and a volute minor portion, a main outlet in said volute wall portion and a recirculation outlet adjacent the circular major portion and angularly spaced from said main outlet, an inlet channel integrally formed in one of said end walls, said inlet channel extending in open communication from the center of said impeller cavity along a radial direction in line with said recirculation outlet, said recirculation outlet communicating with said impeller cavity through a recess external of said cavity, an aperture in said circumferential wall adjacent the other of said end walls providing communication between said cavity and said recess, said inlet channel and said recess having a common wall adapted to be perforated to provide a recirculating flow path internal of the housing when external recirculation flow from said recirculation outlet is not provided, an impeller rotatable in said cavity in a first direction to pump fluid from said inlet through said main outlet, and in an opposite direction to pump Fluid from said inlet through said aperture, said impeller including a plurality of substantially flat, tangentially oriented blades, a clearance between said impeller and said circumferential wall providing a bypass circuit between said aperture and said inlet to regulate the pressure at said main outlet when said impeller is driven in said recirculation direction.