Patent Publication Number: US-6655663-B2

Title: Multi-stage aerator

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of application Ser. No. 09/447,464 filed Nov. 22, 1999, now U.S. Pat. No. 6,394,423 which is a Continuation-in-part Application of application Ser. No. 08/974,086 filed Nov. 19, 1997, now U.S. Pat. No. 5,988,600. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a high efficiency impeller-type aerator for oxygenating the water supply of aquatic organisms, such as fish in a fish tank or bait in a live well. 
     DESCRIPTION OF THE RELATED ART 
     When fishing from a boat, it is a common practice to bring along bait fish in closed tanks known as live wells. Alternatively, larger boats may be equipped with a thru-hull bait well, wherein water from outside the boat is continuously pumped in, is passed through the tank, and is discharged over the side. In order to keep the bait fish alive for long periods of time, an aerator is provided to replenish the oxygen in the water as it is depleted by the bait fish. Several distinct types of aerators have been developed. 
     For example, U.S. Pat. No. 3,822,498 teaches an aerator for a live well wherein water is sucked through a pump and sprayed out a distributor manifold in the form of small jets above the surface of the water. As the jets pass through the air and then strike the surface of the water, the water picks up dissolved oxygen and entrained air bubbles. These systems are, however, disadvantageous for a number of reasons. 
     First, it is inevitable that jets of water will strike the fish and wash away the mucus outer coating which protects the fish. Second, energy consumption is high. Third, while the surface area of the live well may be aerated, the lower reaches of the bait well are not aerated, particularly when a large number of bait fish are kept in the bait well. Finally, aeration efficiency is relatively low, so that the total number of bait fish which can be kept in the well is correspondingly limited. 
     The aerator described in U.S. Pat. No. 5,582,777 (Vento et al), which utilizes a centrifugal pump, achieves a significantly improved level of oxygenation of the water in a live well while producing a gentle action which does not harm the bait fish. In fact, the unusually high level of oxygenation makes it possible to pack two to four times as many bait fish into a live well as had previously been possible. 
     The Vento et al use of the impeller cavity of a centrifugal or impeller type pump as an aerator mechanism represented a departure from conventional thinking, since it is the common experience of those in the industry that as air is introduced into the centrifugal pump it accumulates around the impeller, resulting in air-lock. That is, the accumulated air causes the impeller to spin freely, without pumping water. When water is not pumped through a live well, bait fish begin dying. Thus, conventional thinking was to take measures to prevent any air from getting into the centrifugal pump. Vento et al discovered that by regulating the amount of air introduced to the upstream (suction) side leading to a centrifugal pump, it becomes possible to induce a very thorough mincing of air and water in the pump impeller, resulting in emission of very fine mist of bubbles from the downstream (emission) side of the pump, without the problem of loss of suction. In other words, by supplying just the right proportions of air and water into the pump impeller, significant aeration can occur without the above-described problem of air lock. 
     Although the level of aeration is significantly improved with the Vento et al aerator as compared to conventional pumps using the same amperage, the inventor has noticed that there are two problems associated with this system. The first is that the Vento et al arrangement requires regulation of the input of air, either manually (via valve, clamp, etc.) or automatically (via optical turbidity sensors, etc.). The second is that the output from a centrifugal pump, once modified to introduce air according to the Vento patent, drops dramatically, for example, from 500 gallons per hour to 200 gallons per hour, thus the pump is operating at only 40% of its intended capacity. 
     In view of the foregoing, it is an object of the present invention to provide an improved centrifugal type aerator which does not require monitoring or regulating of the air input. 
     It is a further object of the present invention to provide an aerator which exhibits an improved capacity or flow rate. 
     It is a further object of the present invention to provide an aerator designed to avoid vapor lock of the centrifugal pump impeller. 
     It is yet a further object of the present invention to provide an aerator which achieves a high level of oxygenation. 
     SUMMARY OF THE INVENTION 
     The present inventor has investigated and experimented with various aerators and pumps, and produced what represents a significant improvement over the aerator invented previously by the present inventor, and which was described in U.S. Pat. No. 5,582,777 (Vento et al). 
     The present invention is built upon the Vento et al concept of introduction of air into the upstream (suction) side leading to a pump. The present inventor found he could use one of the following types of pumps: centrifugal, rotary, propeller and mixed flow. Each of these pumps can displace a liquid with a pump rotating element. The preferred pump is a centrifugal pump, such as a conventional rotary bilge pump, to cause churning and a very thorough mincing of air and water in the impeller cavity, followed by output of a mist of very fine bubbles from the downstream (emission) side discharged from the centrifugal pump. On closer examination of the Vento et al. device, the present inventor discovered and began investigating the problem of the significant inefficiency of the Vento et al. aerator. 
     After extensive and careful experimentation, the present inventor found that centrifugal pumps are designed to pump a non-compressible fluid, such as water. The energy imparted to the impeller blades is normally used to move the impeller blades against water to cause flow of water through the impeller cavity, developing a negative pressure or suction on the upstream side and a positive pressure or discharge head on the downstream side. 
     However, once air is introduced into the impeller cavity, the impeller energy is diverted to first expanding air in the negative pressure side of the impeller, and then re-compressing air on the downstream side of the impeller. Further, as the volume of air is increased (due to the negative pressure) on the inlet side, this expanded air displaces water, reducing the amount of water sucked into the impeller cavity. As the air exits the impeller cavity it is compressed to reduced volume, this constant compressing having the end effect of reducing the output at the downstream side of the impeller. Thus, the conventional centrifugal pump, when used to pump a fluid containing a compressible gas, works harder to pump less fluid. 
     Following further experimentation, the present inventor was able to determine that the above problems could surprisingly be solved by placing a first stage or booster impeller before the second stage or main impeller, with air being introduced at a point downstream of the first stage impeller outlet and upstream of the second stage impeller outlet. 
     Specifically, a preferred aerator of the present invention, designed for aeration of the water supply of aquatic organisms, can comprise: a centrifugal type pump comprising a first impeller having inlet and outlet edges, a second impeller having inlet and outlet edges, and a pump casing having at least one pump water inlet, one pump air inlet, and one pump water outlet, with the first and second impellers disposed between the pump water inlet and outlet, wherein the air inlet is positioned between the first impeller outlet edge and the second impeller outlet edge and is in communication with air, and wherein the water inlet and outlet are in communication with water. 
     Alternatively, the aerator may comprise first and second water pumps, each having a water inlet and a water outlet, with the water outlet of the first pump in fluid tight communication with the water inlet of the second pump, at least the second pump being a centrifugal pump including an impeller having inlet and outlet edges, the first pump being a smaller capacity pump than the second pump, and an air inlet positioned between the first pump outlet and the second pump impeller outlet edge and in communication with air. 
     Operationally, the device of the present invention has two stages: the boost stage and the main stage. The main stage is similar in construction to the Vento et al device, but it&#39;s operation is modified by the pressure increase brought about in the boost stage. In the boost stage, water is drawn into the eye of the first impeller, or booster impeller, and is accelerated and thrown out, radially, at the impeller&#39;s outlet edge. In the main stage, air from the air inlet and pressurized water from the booster impeller are co-mingled or minced by the main impeller. Due to the increase in water pressure brought about by the booster impeller, the main impeller does not have to draw as hard on water, i.e., does not have to create a significant negative pressure gradient prior to the main impeller inlet. Since the negative pressure gradient is reduced, the air bubbles being introduced do not expand to the degree experienced in the original Vento et al device. Thus, the main impeller is not expending energy on expanding air. Further, since the air being introduced into the main stage is more compressed and less expanded, the air displaces less water, and the pumping capacity of the main impeller is significantly improved. Finally, since air is not being introduced to the booster impeller, and since the booster impeller is continuously providing water to the main impeller, i.e., is continuously priming the main impeller, it becomes impossible to “vapor lock” the main impeller. Thus, there is no need to monitor or control the aerator of the present invention to prevent vapor lock. 
     The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood and so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other aerators for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent structures do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and objects of the present invention, reference should be made by the following detailed description taken in with the accompanying drawings in which: 
     FIG. 1 shows a cross-sectional view of a preferred design of the aerator of the present invention. 
     FIG. 1 a  shows a top view of the first and second impeller blades. 
     FIG. 1 b  shows a cross-sectional view of the preferred design of the aerator of the present invention having suction cups. 
     FIG. 1 c  shows a top view of another embodiment of the first and second impeller blades. 
     FIG. 2 shows a cross-sectional view of the preferred design of the aerator of the present invention showing its operation. 
     FIG. 3 shows a cross-sectional view of the thru-hull embodiment of the aerator of the present invention. 
     FIG. 4 shows a cross-sectional view of an alternative embodiment in which two pumps are oriented in series. 
     FIG. 5 shows an enlarged cross-sectional view of the impellers in series showing direction of water flowing past the air-inlet. 
     FIG. 6 shows a sectional view of a main impeller driven by a water wheel activated by flowing water. 
     FIG. 6 a  shows an axial sectional view of a water wheel activated by flowing water and driving the main impeller of FIG.  6 . 
     FIG. 7 shows a cross-sectional view of a second alternative design of the aerator of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to an aerator for oxygenating the water supply of aquatic organisms, particularly for a live bait well or a boat&#39;s thru-hull bait well. As used herein, the terms aerator and oxygenator have the same meaning. 
     While the present invention represents a marked improvement over the aerator described in U.S. Pat. No. 5,582,777, the basic technology remains the same. Specifically, the present invention remains based upon the discovery that the introduction of air into an upstream side (suction) leading to a pump, results in the ability to optimize the air/water mixture, resulting in a very fine mincing of air and water. The pump of choice may be a centrifugal, a rotary, a propeller or a mixed flow. Preferably the pump is a centrifugal pump such as a centrifugal rotary bilge pump as well known in the art. The pump discharges an air/water mixture containing a large volume of very fine air bubbles. 
     These fine air bubbles are significantly better at oxygenating water than larger air bubbles as produced by conventional aerators since (1) the effective bubble surface area and thus air/water contact area is increased, (2) smaller bubbles take much longer to rise to the surface and thus remain in the water longer, (3) smaller bubbles are less likely to coalesce upon contact, and thus are likely to remain suspended in the form of fine bubbles, and (3) the presence of the ultra-fine bubbles during the mechanical churning during pumping has a synergistic effect resulting in enhanced oxygenation. 
     The term “centrifugal pump” as used herein is intended to mean a pump which utilizes the throwing force of a rapidly moving impeller. The liquid is pulled in at the center or eye of the impeller and is discharged at the outer rim of this impeller. By the time the liquid reaches the outer rim of the impeller, it has acquired considerable velocity. The liquid, traveling under high velocity, is then slowed down by being led through either a volute or a conical housing. The housing design is important because, either of the above designs provides an opening wide enough for the liquid passing through to slow down. The simplest method for converting dynamic pressure to static pressure is to slowly increase the volute delivery channel area (e.g., a taper of no greater than 8°). This is known as a diffuser and is often used on small pumps. As the velocity of the liquid decreases, its pressure increases so to increase suction of air for mixing. The shape of the outlet has the effect of changing the low-pressure, high velocity fluid to high pressure, low velocity fluid. That is, some of the mechanical kinetic energy is transformed into mechanical potential energy. In other words, the velocity head is partially turned into pressure head. 
     The aerator of the present invention is characterized by the employment of two rapidly rotating means, a first means to draw in water and sustain a good vacuum, and a second means to mince air and water. The first means for drawing in water may be a water wheel, a propeller, an impeller or any thing that would drive the water into the system. The simplest method of drawing water into the housing is through the force of water coming into the system by the speed of a boat having a thru-hull bait well. When water is drawing into the housing by the boat&#39;s speed, a water wheel  64 , as shown in FIG. 6, is used to move the second means for mixing the air and water. 
     The second means for mincing air and water may be a propeller, impeller, or any means operating at high rpm to beat the air and water with a spinning motion. Preferably the first and second means are impellers of a centrifugal pump—a first impeller, or booster, and a second impeller, or main impeller. The first means or booster impeller is known for priming the main impeller. 
     The precise manner in which the main impeller minces the air and water and creates the ultra fine air bubbles is not understood, but it is logical to assume that the rapid changes of direction from (1) axial at the eye to (2) radial in the impeller to (3) axial between the impeller tip and the outlet to (4) radial at the water outlet, and also the changes in speeds, pressures, shear forces, and other forces acting within the impeller have an effect on the formation of bubbles. One thing that is evident through the operation of the two rapidly rotating means is that a venturi effect occurs near the second means and the air inlet. The venturi effect is created as the high velocity fluid, created by the fluid passing through a channel formed at the flow directing boundaries, causes a decrease in pressure at the air inlet positioned adjacent the air flow boundaries. The reduced pressure causes a suction in the air conduit and the suctioned in air is minced with the flowing liquid. 
     Specifically, and with reference to the figures, a preferred aerator  1  of the present invention, designed for aeration of the water supply of aquatic organisms comprises: a centrifugal type pump comprising a first means or first impeller  12  (booster) having inlet and outlet edges  12 A and  12 B, respectively, a second means or second impeller  10  (main impeller) having inlet and outlet edges  10 A and  10 B, respectively, and a pump casing  14  having at least one pump water inlet  16 , one pump air inlet  18 , and one pump water outlet  20 , with the first and second impellers  12  and  10  disposed between the pump casing water inlet  16  and outlet  20 , wherein the air inlet  18  is positioned between the second impeller outlet edge  10 B and the first impeller outlet edge  12 B and is in communication with air, and wherein the water inlet  16  and outlet  20  are in communication with water. 
     In all cases the first and second means, whether impellers, wheels or propellers, of the aerators are operated under conditions under which no cavitation (as conventionally defined) occurs, i.e., there is no reduction in pressure to the point where the hydrodynamic pressure of the water is dropped to below its vapor pressure. Cavitation most frequently occurs in a marine environment when the vapor pressure of the water is dropped below the vapor pressure of air behind a ship&#39;s propeller blade, such that air bubbles are formed. Strictly speaking, cavitation can occur when the pressure in a container of carbonated beverage is reduced such that dissolved gasses come out of solution and form carbon dioxide “cavities”. For the purposes of the present invention, no such cavitation occurs. The second means, preferably an impeller, is operated under conditions where a smooth, continuous mincing of air and water occurs. 
     The centrifugal pumps as used in the present invention are basically similar to a wheel, with vanes or blades called impeller blades sandwiched between an upper and a lower housing. For ease of construction, one of the upper or lower impeller housings may be eliminated so long as the free upper or lower sides of the impeller blades are in close proximity to the impeller chamber housing. An impeller thus differs from a propeller mainly in that (1) an impeller operates using centrifugal force, while a propeller does not, and (2) an impeller has a upper and lower housing or case for throwing fluids out radially, while a propeller has only blades which pushes liquid in a direction axially parallel with the propeller shaft. A propeller type pump may be used, however, a propeller type pump can not achieve the ultra-fine bubbles according to the present invention. 
     An impeller may be of either the centrifugal pump type or the compressor type, with centrifugal pump type impellers being greatly preferred. Pump impellers are generally cast in one piece with a hub; compressor impellers are generally fabricated. 
     As shown in FIG. 1, the aerator  1  further comprises a water impermeable motor casing  15 . An electric motor (not shown) of any conventional design is mounted within the motor casing  15 . The electric motor may be of any suitable construction such as the type utilized in a RULE bilge pump, for example, a RULE 360 GPH bilge pump. Basically, any conventionally available centrifugal pump motor available in the fishing industry can be used for the purposes of the present invention. A major supplier of pumps containing such motors is E&amp;B Discount Marine, Inc. of 201 Meadow Road, P.O. Box 3138, Edison, N.J., as found in the E &amp; B Discount Marine, Inc. Catalog &#39;95, pages 112-115 of which are incorporated herein by reference. The motor may be powered by any suitable means such as an internal battery, an external portable battery, or via electrical connections to the main electrical supply system of a boat (in which case the electric drive motor includes insulated electrical conductors  22 ). The ends of the electrical connection means  22  may be provided with electrically conductive clamps (not shown) whereby the clamps may be clamped to the terminals of an electric battery or other source of electrical power. The portable power supply (not shown) may be provided in a casing which can be mated integral with the motor casing  15 , or may be located outside the motor housing and inside or outside the bait well, in which case external electrical connection means  22  are again required. 
     The assembly may then be placed into the live bait well or thru-hull well and anchored to the bottom thereof via suction cups  14 C as depicted in FIG.  1 B. 
     A drive shaft  2  extends through the bottom of the motor casing  15  and is connected to the second centrifugal rotary impeller, or main impeller  10 . Preferably, for ease of assembly, the main impeller  10  and booster  12  are integrally molded as one article. Therefore, the main impeller  10  and booster  12  are connected by a hollow sleeve  11 . The drive shaft  2 , therefore, extends through the hollow sleeve  11 , and is affixed to it, so that when the motor turns the drive shaft  2 , both impellers  10  and  12  are likewise turned. 
     The pump casing  14  and motor casing  15  cooperate to direct water flow as shown in FIG.  2 . This is facilitated by the preferred shape of the motor housing  15 , which comprises a generally cylindrical outer wall portion  15 A and a generally flat bottom portion  15 B. The pump casing  14  is shaped so as to encompass the impellers  10  and  12  and to define water inlet  16  and water outlet  20  areas. Further, the pump casing  14  forms flat bottom portions  14 A. In the design as shown in FIG. 1, preferably, the water inlet  16  is immediately below, and co-axial with, shaft  2 , and also immediately below the “eye” of the booster  12 . The lateral water outlet  20  is provided in the pump casing  14  above the main impeller  10  for return of aerated water to the bait well. 
     Each impeller  10  and  12  comprise a top disk-shaped impeller plate  4  which is fixed at its center to the drive shaft  2 . The impellers  10  and  12  are provided with a plurality of impeller vanes  3 . The vanes  3  extend downwardly and are in close tolerance with the surface of the pump casing  14 . The top impeller plate  4  and the flat bottom portion  14 A of the pump casing  14  thus define the axial flow directing boundaries of the impellers  10  and  12  through which the impeller vanes  3  urge the water to facilitate low pressure that creates the suction and creates the venturi effect. The air flow boundaries define water boundaries and a channel  18 A where the water velocity is increased because of the size of the channel. The increased water flow through the channel causes pressure at the air inlet  18  to decrease. The decrease in pressure across the air inlet causes air to flow in for mincing with the water passing through the channel. 
     Specifically, the vanes  3  of the booster  12  define the booster&#39;s pump inlet edge  12 A and pump outlet edge  12 B. The vanes  3  of the main impeller  10  define the main impeller&#39;s pump inlet edge  10 A and pump outlet edge  10 B. During operation, water flows within the booster&#39;s inlet edge  12 A, into the booster&#39;s “eye”, and is expelled at the booster&#39;s outlet edge  12 B. Further, during operation, the centrifugal type pump of the present invention operates at a capacity of 500 gallons per hour but the aerator design may be sized to have greater pump capacity. Simultaneously, while the expelled water is drawn within the main impeller&#39;s inlet edge  10 A, into the main impeller&#39;s “eye”, air is pulled from the air inlet  18  and pulled within the main impeller&#39;s inlet edge  10 A, into the main impeller&#39;s “eye” by the venturi effect. The decreased pressure at the air inlet suctions in the air through the air conduit. The air and water are minced in the main impeller  10  and expelled at the main impeller&#39;s outlet edge  10 B. It is important to note that the force of the water being expelled by the main impeller increases the velocity of the water as it is urged upward by the axial flow directing boundaries. It is the increased velocity of the water as it is expelled by the main impeller that decreases the pressure at the channel. Air flow through the air conduit and into the pump housing is not directed by a secondary pumping means but solely by the venturi effect. 
     It is preferable that the vanes  3  of the booster  12  describe a curve because they are only involved in the movement of water. In contrast, it is preferable that the vanes  3  of the main impeller  10  be substantially flat to facilitate the mincing of air and water. The first means, (booster impeller) and the second means (main impeller) may be of the same size. It is preferable that the booster  12  be approximately one third the size of the main impeller  10 . No matter the size of the first means and the second means, it is important is that the first means urges water in to the axial flow directing boundaries toward the second means for the creation of the venturi effect that draws the air in for mincing with the water. 
     Referring now to the air inlet  18 , FIG. 1 shows one possible arrangement of an air conduit  24 . Air inlet  18  is shown as having an inner diameter of ⅛ inch, corresponding to the ¼ inch outer diameter of the flexible air conduit  24 , so that air conduit  24  can simply be inserted into air inlet  18  when it is desired to use the impeller pump as an aerator. Alternatively, the air conduit  24  can be disconnected from air inlet  18 , in which case the impeller pump can be used as a conventional pump, such as for a bilge pump. Suitable retaining means for retaining the aerator at the desired location, preferably at the bottom of the bait well, is provided, such as a lead weight, a snap fitting, or even a suction cup (not shown) mounted to the flat bottom of pump housing  14 , as described in U.S. Pat. No. 5,582,777. 
     As shown, the air conduit  24  is preferably a flexible transparent tube of a construction and material as readily available in pet stores for use in association with aquariums. The air conduit  24  and air inlet  18  may be of any diameter, so long as the opening of the air inlet  18  is within a critical range required for operation of the aerator. That is, if the diameter of the air conduit  24  is too large, the volume of air in the air conduit  24  will make it possible for the pump to oscillate or surge, alternatively drawing large bubbles and then no air into the impeller. Further, if the diameter of the air conduit  24  is too small, a sufficient supply of air to the main impeller  10  for optimal oxygenation is not always possible. This is not conducive to the production of fine bubbles and the smooth operation of the aerator. 
     The air conduit  24  has an opening in communication with the air, which opening is preferably above the fluid level of the bait well, but which may extend, e.g., out the side or bottom of the well. The lower outlet of the air conduit  24  supplies air to the air inlet  18  of the pump housing  14 . 
     It should further be understood that the air conduit  24  and air inlet  18  may engage via intermediate tubing (not shown), as described in U.S. Pat. No. 5,582,777 and referred to therein as “conduit”, the disclosure of which is herein incorporated by reference. The intermediate tubing may be of varying embodiments as described in Vento, et al. 
     It can be seen that the air inlet  18  functions to supply air from air conduit  24  as close to the eye of the main impeller  10  as possible, as best illustrated in FIG.  7 . It is critical to oxygenation and optimal pump capacity that the air inlet  18  be positioned between the booster&#39;s outlet edge  12 B and the main impeller&#39;s outlet edge  10 B. The reason that the air inlet  18  does not supply air closer to the eye of the main impeller  10  is that it is necessary to maintain a wall, or water boundary  14 B in which to guide the water expelled from the booster  12  towards the eye of the main impeller  10  and to create the venturi effect to pull air from the air inlet  18 . The venturi effect occurs when the velocity of the water is increased, by passing through the channel formed between the main impeller  10  and the bottom portions of the pump housing, which in turn will cause the pressure at the air inlet to decrease, which causes the suction that pull air in through the air conduit. In FIGS. 1 and 7 the place at which the pressure drops is indicated by  18 B. 
     The approximate relationship between the essential components shall now be described. In its simplest form, air inlet  18  is approximately ¼ inch in diameter. As can be seen, the space  18 A between the impeller blades  3  and bottom flat portions of the pump housing  14 A is very small, preferably even smaller than shown by the drawings. The horizontal separation between the top of the impeller vanes  3  and the plane of the bottom portions of the pump housing  14 A is preferably within ¼ of the diameter of water inlet  16 , more preferably within ⅙ the diameter of the water inlet  16 , and most preferably within {fraction (1/10)} of the diameter of the water inlet  16 , in the case that the pump is horizontal. 
     The operation of the aerator will now be described with reference to the drawings. As seen in FIGS. 1 and 7, when the electric motor (not shown) is energized, drive shaft  2  rotates causing corresponding rotation of the main impeller  10  and booster  12  whereby water is drawn into the water inlet  16 , is accelerated by the impeller vanes  4  of the booster  12 , and is slung out at its outlet edge  12 B at which point the water has achieved maximum velocity. The activity up to this point describes the boost stage of the pump. The following describes the main stage of the pump. The water is redirected upwardly, preferably by the water boundaries  14 B of the pump housing  14  and is drawn into the eye of the main impeller  10 . While traveling axially upward, the velocity of the water is reduced and, as a consequence, the potential pressure is increased. 
     As the booster  12  begins to pump water in through the water inlet  16  and out through the water outlet  20 , a reduced pressure or suction head will form at the water inlet  16  and urge the water about the water boundaries. As similarly described in U.S. Pat. No. 5,582,777, once the absolute pressure at the water inlet  16 , below the booster, drops below the air pressure at air inlet  18 , air enters through the air inlet  18  via the air conduit  24  and enters into the pump housing  14 . In the present invention the venturi effect allows air to enter the pump housing at a point advantageously between main impeller  10  and booster  12 , as discussed above. The venturi effect which draws in air from air inlet  18  is facilitated by the water boundary  14 B. 
     Optimal oxygenation of the water can be confirmed visually. An important principle of the present invention is that optimal oxygenation does not depend upon optimal air flow through the air conduit  24 . Rather, optimal oxygenation depends upon the introduction into the bait well of very finely divided air, i.e., ultra fine air bubbles. The air bubbles should have the appearance of a fine mist or fog. The air bubbles are so small as to remain under water for a long period of time, and optimally saturate the water with oxygen. 
     To be viewed as an improvement over the single impeller aerator taught in Vento, et al., the air flow need not be controlled to achieve the maximum amount of the finest air bubbles. 
     The output from the pump is smooth and non-turbulent, so as to provide optimal habitation conditions for live bait, i.e., there is no surge, there is no high turbulence, and the flow is only so great as necessary for the re-circulation of water and for the even distribution of oxygen throughout the live bait well. 
     As an option, a strainer (not shown) of any suitable construction is mounted on the bottom of the pump casing  14 . The strainer merely serves to prevent bait fish from being drawn into the booster  12 . 
     Of course, the aerator  1  may be placed in a portable bait container such as a “minnow bucket”, placed within a bait well built into a boat, or even may be used as a temporary aerator for a fish aquarium. Further, the aerator may be used in any form of live box to aerate the water therein. Furthermore, the pump casing can have suction cup means  14 C for attaching to the floor of the bait well or other support means. 
     One characteristic of the multi-stage, two impeller aerator is an increased amount of heat generated by the motor due to the increased resistance produced by having two impellers turning the water. When the aerator of the present invention is mounted in a thru-hull mounting for a boat, as in the preferred embodiment, there is less risk of heat contaminating the water in the tank because the water is continually being replenished with water from outside the boat. However, when the aerator of the present invention is mounted in a live bait well, in which the water is being re-circulated, it is advantageous to include an air inlet and outlet in the motor casing  15 , along with an impeller which functions exclusively to circulate air around the motor, cooling it. This alternative embodiment is shown in FIG.  3 . 
     As shown in FIG. 3, in the thru-hull embodiment, the motor casing  15  is external of the pump casing  14 , and connected to the pump casing  14 . It is for this reason that the motor casing  15  should again be water-tight. It is important to understand that an empty motor casing  17  still remains within the pump casing  14  to direct water flow toward the water outlet  20  and provide a surface in close proximity to the impeller plate  4  of the main impeller  10  in order to facilitate suction. To facilitate mass production of the thru-hull embodiment, the motor casing  15  merely engages with the live well embodiment, the live well embodiment having no motor within its motor casing  17 . Further, preferably disposed upon the same drive shaft  2  that the main impeller  10  and booster  12  are disposed on, is disposed an air impeller  30 . The air impeller  30  is identical in structural components to the main impeller  10  and booster  12 . Because the impellers  30 ,  10 , and  12 , share the same drive shaft  2 , when the motor  40  (which is shown only in FIG. 3) is operated, the drive shaft  2  rotates, thereby rotating the air impeller  30 , the main impeller  10 , and booster  12 , simultaneously. As seen in FIG. 3, the drive shaft  2  extends from the motor  40 , through the empty motor casing  17 . The motor casing  15  defines an air inlet  26  and an air outlet  28 . The air inlet  26  and air outlet  28 , of course, are in communication with air via similar air conduit  24  to that used to engage pump casing&#39;s air inlet  18 . It is preferable that the ends (not shown) of the air conduit  24 , which are in contact with air, be sufficiently separated from each other to facilitate the drawing in of fresh, un-circulated (unheated) air. As shown in FIG. 3, in this embodiment the motor casing  15  further defines a taper  15 C which functions to efficiently direct air from the air impeller&#39;s outlet edge  30 B to the motor  40 , so that the motor can efficiently be cooled. Preferably, the motor  40  defines aluminum cooling fins  42 , which function to increase the surface area of the motor  40 , and facilitate liberation of heat to the inside of the motor casing  15 . It should be understood that the extra work required of the motor  40  to turn the air impeller  30  is negligible compared to the benefit received in the cooling effect produced by the air impeller  30 . 
     Referring now to FIG. 4, as an alternative embodiment, the aerator may comprise first and second water pumps  52  and  54 , respectively, each having a water inlet  16  and a water outlet  20 , with the water outlet  20  of the first pump  52  in fluid tight communication with the water inlet  16  of the second pump, at least the second pump  54  being a centrifugal pump including an impeller (shown generally in FIG. 4) having inlet and outlet edges  10 A and  10 B, the first pump  52  being a smaller capacity pump than the second pump  54 , and an air inlet  18  positioned between the first pump outlet  20  and the second pump impeller outlet edge  10 B and in communication with air. 
     In the alternative embodiment of FIG. 4, the impeller of the first pump  52  acts as the booster  12 , while the impeller of the second pump  54  acts as the main impeller  10 . Each impeller  10  and  12  is merely driven by two independent motors and housed within two pumps in fluid communication. Specifically, the water outlet  20  of the first pump  52  is in communication with the water inlet  16  of the second pump  54  via a water conduit  50 . The water conduit  50  is preferably a plastic sufficiently durable to carry accelerated water from the first pump  52  to the second pump  54 . Further, as shown in FIG. 4, it is preferable that the second pump  54  be positioned lower than the first pump  52  and that the water inlet  16  of the second pump  54  be defined laterally, so that the water pumped by the first pump  52  does not need to be pumped through the water conduit  50  in an upward direction. Both pumps  52  and  54  define flat lower surfaces  14 A to facilitate suction produced by the impellers  10  and  12 . The flat lower surface of pump  54  defines axial blow boundaries  14 B, which defines channel  18 A. Further, the second pump  54  defines a deliver chamber  56 . Defined laterally within the delivery chamber  56  are the water inlet  16 , opposing air inlet  18 , and a delivery aperture  58 . Preferably, the air conduit  24  runs through the air inlet  18 , terminating at the delivery aperture  58 . The delivery aperture  58 , which is located beneath the eye of the main impeller  10 , is small in comparison to the second pump&#39;s water inlet  16 , to facilitate suction by the main impeller  10  by the venturi effect caused by an increase in water velocity through the channel. 
     It should be understood that, in the alternative embodiment of FIG. 4, the boost phase of the aeration process occurs within the first pump  52  and the main phase of the aeration process occurs within the second pump  54 . As discussed, the second pump  54  has a larger capacity than that of the first pump  52 , the water flow differential facilitating suction. For example, if the first pump  52  has a capacity of 500 gallons per hour, it is preferable that the second pump have a capacity of 700 gallons per hour. 
     Referring now to FIG. 7, as a second alternative embodiment of the present invention. The aerator of the second alternative embodiment designed for aeration of the water supply of aquatic organisms comprises: a centrifugal type pump comprising a first means or first impeller  12  (booster) having inlet and outlet edges  12 A and  12 B, respectively, a second means or second impeller  10  (main impeller) having inlet and outlet edges  10 A and  10 B, respectively, and a pump casing  14  having at least one pump water inlet  16 , one pump air inlet  18 , and one pump water outlet  20 , with the first and second impellers  12  and  10  disposed between the pump casing water inlet  16  and outlet  20 , wherein the air inlet  18  is positioned between the second impeller outlet edge  10 B and the first impeller outlet edge  12 B and is in communication with air, and wherein the water inlet  16  and outlet  20  are in communication with water. 
     The pump casing  14  and motor casing  15  cooperate to direct water flow as shown in FIG.  2 . This is facilitated by the preferred shape of the motor housing  15 , which comprises a generally cylindrical outer wall portion  15 A and a generally flat bottom portion  15 B. The pump casing  14  is shaped so as to encompass the impellers  10  and  12  and to define water inlet  16  and water outlet  20  areas. Further, the pump casing  14  forms a delivery chamber  56  defined by flat bottom portions  14 A and constricted portion  62 . The constricted portion has flat top portions  62 A and flat bottom portions  62 B. In the design as shown in FIG. 7, preferably, the water inlet  16  is immediately below, and co-axial with, shaft  2 , and also immediately below the “eye” of the booster  12 . The lateral water outlet  20  is provided in the pump casing  14  above the main impeller  10  for return of aerated water to the bait well. 
     Each impeller  10  and  12  comprise a top disk-shaped impeller plate  4  which is fixed at its center to the drive shaft  2 . The impellers  10  and  12  are provided with a plurality of impeller vanes  3 . The vanes  3  extend downwardly and are in close tolerance with the surface of the pump casing  14 . The top impeller plate  4 , of the first impeller and the flat bottom portions  14 A of the pump casing  14 , and the top impeller plate  4  of the second impeller and the flat top portions  62 A, define the axial flow directing boundaries of the impellers  10  and  12  through which the impeller vanes  3  urge the water to facilitate low pressure that creates the suction and creates the venturi effect. Specifically, water boundaries  14 B are formed at the flat bottom portions  62 B. The water boundaries define a channel  18 A where the water velocity is increased because of the size of the channel. The increased water flow through the channel causes the pressure the air inlet  18  to decrease. The decrease in pressure at the air inlet causes air to flow in for mincing with the water passing through the channel. 
     It should be understood that, in the alternative embodiment of FIG. 7, the operation of the aerator is like the operation of the aerator in FIG.  1 . 
     Referring again to the aerator in general, the mixture of water and air which enters the impeller is violently agitated and leaves the outlet  20  of the impeller pump in the form of water with very fine air bubbles giving the appearance of fogging the water. In some cases the air bubbles may be so fine that it will be difficult to tell whether the pump is aerating or not. In that case, placement of a hand in front of the outlet  20  will either cause a rapid buildup of bubbles on the skin, showing that the aerator is working, or will result in no bubbles forming on the skin, in which case no aeration is occurring. 
     While a RULE  360  works well for large bait tanks as found on fishing boats, the amount of aeration would be too large for smaller bait tanks such as “guppy buckets”. In that case, a correspondingly smaller capacity pump, such as a 40 gph pump, may be used. 
     Various structures and connections may be resorted to. All that is important is that air and water are intimately and violently minced within a second impeller, the second impeller being primed by a first impeller. The pump may be operated right-side-up (with the water inlet opening downwardly), up-side-down (with the water inlet opening upwardly) or sideways. 
     The invention is applicable to bait wells for fresh water fish as well as for salt water fish, though best results have been observed with salt water. The invention is not limited to bait wells, and is applicable to aeration of aquariums, lobster holding tanks, etc. 
     The aerator of the present invention is rather powerful and need not be run full time. The aerator may be energized cyclically in a pattern set by a timer. Alternatively, the aerator may be energized responsive to sensor input, such as oxygen saturation sensors, as discussed in, e.g., U.S. Pat. No. 5,320,068, which teaches a system for the automatic control of oxygenation for agriculture. 
     Although the aerator was first designed as an aerator for bait fish in a bait well, it will be readily apparent that the device is capable of use in a number of other applications, such as in mincing various liquids and gasses. Although this invention has been described in its preferred form with a certain degree of particularity with respect to an aerator, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of structures and the composition of the combination may be resorted to without departing from the spirit and scope of the invention. 
     Now that the invention has been described,