Patent Publication Number: US-6698934-B2

Title: Agitator drive

Description:
TECHNICAL FIELD 
     This application pertains to a drive for a rotary agitator. In particular, the drive provides for agitation of chemicals in high-temperature, high-pressure environments without the need for a seal around the drive shaft, and allows for the use of conventional bearings to support the shaft. 
     BACKGROUND 
     Industrial chemical processes often occur in reactor vessels and require agitation to aid chemical reactions. For example, agitation may provide for homogenous mixing, or for uniform suspension of materials having different densities or phases such as emulsions or solids suspended in a liquid. In general, agitators typically include one or more propellers or impellers inside the vessel that are attached to a rotating shaft. The shaft extends out through the wall of the vessel to a motor that rotates the shaft and, in turn, rotates the impellers or propellers. One or more bearing assemblies, generally near the vessel wall, hold the shaft in place and allow it to rotate freely and steadily under various rotational, transverse, and thrust loads. 
     It is desirable that an agitator provide consistent performance with few failures. Major industrial processing plants are extremely complex and very expensive to operate. A breakdown at one vessel can stop the operation of a major portion of a plant, and disassembly (and reassembly) of an agitator drive for repairs often takes a long time and can destroy the batch being processed in the vessel. Even worse, a breakdown in the middle of a batch may require that the vessel be carefully and laboriously cleaned before processing may resume. 
     Where the conditions inside the vessel are severe, such as where the temperature and pressure inside the vessel are both very high, a conventional agitator drive system may not provide acceptable reliability. For example, the motor for a drive system is typically located in a low-pressure area, and the drive shaft passes from the motor into the vessel so that there generally must be seals, packing, and/or bearings at the point where the shaft passes through the wall of the vessel. Seals and packing are prone to quick degradation under severe conditions where they are placed in high temperatures or across high-pressure differentials. In addition, the seals, packing, or bearings must be properly lubricated, and under severe conditions, the lubricants may degrade or may even leak into the interior of the vessel, contaminating the process. 
     Conventional solutions may not be adequate to address such problems caused by severe conditions. For example, pusher mechanical seals are often used at the vessel wall between areas of high and low pressure. These seals generally rely, however, on elastomers, which are inappropriate materials for high-temperature applications. Metal bellows (or non-pusher) seals are often used where high temperatures are expected, but they do not generally work well under high pressures. Packing materials may also be provided around a shaft where it enters a vessel. While such a solution again works well under high pressure, it can cause problems where temperatures are elevated. For example, high clamping forces around the packing material help form a tight seal that can withstand high pressure, but the forces also create friction that produces additional heat. When combined with high temperatures in the vessel, this friction can cause rapid destruction of the materials. 
     Placing the drive system—motor and all—entirely inside the vessel solves the problem of sealing across a high-pressure differential, but it is not generally acceptable. The drive motor will likely be less amenable to severe conditions than are the bearings that support the shaft because it contains bearings and other components that may not handle high temperatures or a corrosive environment well. And placing the entire drive system in the vessel simply places the bearings entirely inside the high-temperature, and potentially corrosive, conditions. In addition, access to the drive is more difficult when it is entirely inside the vessel. Moreover, the problem of potential contamination of the vessel may be worsened, particularly where the motor is hydraulically powered. 
     One solution to the problem is to break the shaft in two, placing the motor and part of the shaft outside the vessel, and the other part of the shaft inside the vessel, so that no portion of the drive passes through the vessel wall. The two parts of the shaft may be coupled through the vessel wall magnetically. The motor&#39;s shaft outside the vessel may be attached to large magnets, and the drive shaft attached to the agitator inside the vessel may be attached to matching magnets. The sets of magnets may be positioned on each side of a protruding area of the vessel wall so that rotation of the motor induces rotation of the agitator by magnetic coupling. 
     This “magnetic coupling” approach, however, is expensive and allows only limited torque to be delivered to the agitator, and still requires that the bearings supporting the shaft be located in the hostile environment of the vessel. As a result, it too may require that the bearings be made of special, expensive materials, may result in premature bearing failure, and may produce contamination of the vessel. Moreover, because the coupling force is inversely proportional to the square of the wall thickness between the magnets, there will be a practical limit to the level of coupling that can occur through a wall that is thick enough to maintain the integrity of the vessel. Furthermore, as torque requirements increase, the magnets may need to be placed further from the shafts so that the container through which the magnets operate must get larger, and its wall thickness must increase to contain the vessel pressure. As a result, practical torque and size limitations constrain the general applicability of magnetically coupled drives. 
     Accordingly, there is a need for an agitator drive system that can provide reliable operation to vessels that house severe conditions with little or no risk of pressure loss or of contaminating the contents of the vessel. In addition, there is a need to provide such a drive in a sealless system that can use conventional materials and parts. Furthermore, there is a need to provide a motor for such a drive that can operate reliably in a high-pressure atmosphere in which the pressure varies over time. 
     SUMMARY 
     In general, an agitator drive is disclosed for use with a chemical processing vessel. The drive may be sealed with the vessel, and may thus be under the same or similar pressure as the housing. The drive may be separated from the housing by an insulated floor, so that the temperature inside the housing is significantly lower than that inside the vessel. A pair of overlapping shields, in the form of a standpipe attached to the drive floor and a skirt attached to the shaft, may prevent fluid from the housing from entering the vessel. In addition, the bearings that hold the shaft may be immersed in one or more sumps around the shaft that are filled with lubricant. As a result, the drive shaft does not pass from a high-pressure area to a low-pressure area, and the active drive components, such as the bearings and motor, are isolated from the high temperatures in the vessel. 
     In one embodiment, a drive for an agitator assembly has a drive housing that defines an interior volume and a drive motor mounted in the interior volume. A bearing support is connected to the inside of the housing and has a bearing receptacle in which one or more bearings are mounted. A drive shaft surrounded by a sump is rotatably mounted in the bearings and is driven by the drive motor. The sump defines an inner volume for holding a lubricant in which at least a portion of the bearings are located in the inner volume so that the bearings may be immersed in the lubricant. The sump may also be connected to the drive shaft and rotate with the drive shaft. 
     The bearing support may have an upper portion, which may comprise a plurality of support arms or a solid disc, outside the sump, and a cantilevered portion, which may comprise a solid cylinder, that contains the bearing receptacle. The shaft, one or more bearings, and cantilevered portion may define a fluid galley, and the bearing support may have a fluid supply manifold. A floor may be located below the sump and may have a passage surrounded by a standpipe through which the drive shaft passes. A skirt attached to the shaft may surround and overlap with the standpipe, and may be provided with a baffle that overlaps with a baffle on the standpipe. A fluid circulation system may also receive and recirculate fluid from the motor and the bearings. 
     In another embodiment, a drive is provided having a housing with a floor, and a drive motor in the housing connected to a drive shaft that extends through the floor. A standpipe around the drive shaft may extend upward from the floor, and a shield may be attached to the drive shaft above the standpipe to prevent fluid from entering the standpipe. The shield may comprise a skirt that overhangs the standpipe, and the skirt may have a baffle that overlaps a baffle on the standpipe. A drain port may be located in the housing near the floor. A sump may be mounted around the shaft, and may have a bearing mounted in its interior volume. The sump&#39;s top edge may be above the top of the bearings, and the bearings may be cantilevered into the sump on a bearing support, which may have fluid passages for introducing lubricant near the bearings. The drive may also be provided with a media introduction line that extends through the standpipe and terminates near the shaft. Furthermore, the floor and a portion of the shaft that extends into the vessel below the floor may be insulated. 
     In yet another embodiment, a method of extending bearing life in an agitator drive is disclosed by which one or more bearings in a bearing support are provided around a drive shaft, a sump is provided around the bearings, lubricant is passed through the bearings until the sump is overflowing to lubricate and cool the bearings, and the fluid is recirculated. A divider, such as a floor, may be provided below the sump between the vessel and the drive around the drive shaft. Also, a standpipe may be provided around a hole in the divider to prevent fluid from passing into the vessel, and the fluid may be cooled outside the agitator drive. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a partial section perspective view of a drive for use with an agitator. 
     FIG. 2 is a cross-sectional view of the drive. 
     FIG. 3 shows a partial section view of a hydraulic drive motor for use in a high-pressure environment. 
     FIG. 4 shows the fluid circulation system for the drive system in schematic form. 
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     FIG. 1 is a partial-section perspective view of a drive  10  for use with an agitator. Motor  12  is coupled to shaft  14 , which extends down from motor  12  into reactor vessel  18 , and provides the power for drive  10 . Shaft  14  may be connected to an agitator (not shown) for stirring a liquid within vessel  18 . The components of drive  10  may be located inside housing  16 , which may extend outward from vessel  18 , as shown, or may be partially or wholly located inside vessel  18 . Vessel  18  may be a long-term storage vessel, a holding vessel that is intermediate to other process steps, a reactor vessel, or other pressurizable vessel useful in chemical or other process industries. 
     As shown, housing  16  is a hollow, pressure-tight cylinder whose inner walls support the various components of drive  10 , but a variety of forms or arrangements could be used for housing  16 . The interiors of housing  16  and vessel  18  are generally both held at a substantially similar and high pressure, such as several hundred psig. The interior of vessel  18  may be kept at several hundred degrees Fahrenheit, but the interior of housing  16  is generally kept at a substantially lower temperature. The interiors of vessel  18  and housing  16  may also be in communication, and any gases within housing  16  and vessel  18  may be allowed to intermingle. Alternatively, the pressure inside housing  16  may be kept slightly above the pressure inside vessel  18  so that gases inside vessel  18  are maintained inside vessel  18 , or their release and exit from vessel  18  via housing  16  can be controlled, for example, by lowering the pressure of housing  16  below that of vessel  18 . Likewise, gases may be introduced into housing  16  and may be moved into vessel  18 . 
     Motor  12  may be located inside housing  16 , and may be accessed via removable cover  21 . Motor  12  may be a positive displacement type, hydraulically driven motor. Alternatively, motor  12  could be any type of power source that can provide rotational motion to shaft  14 , such as electric or pneumatic motors. Cover  21  is advantageously provided so as to allow for the assembly, service, and at least partial disassembly of drive  10 . In addition, portions of drive  10  may be provided in separate housing sections to permit for pre-assembly of the major elements of drive  10 , and their subsequent attachment to the housing. 
     Mounting plate  22  may be fastened to housing  16  beneath motor  12 , and may provide a convenient anchor for motor  12 . Mounting plate  22  may be a solid plate, and could also be perforated to allow any lubricant that accumulates inside motor housing  20  to pass through to housing  16 . In addition, mounting plate  22  may be removably mounted so as to allow easy access to other components within housing  16 . Alternatively, motor  12  could be mounted by any of a number of other mounting means, such as cross-bars or mounting arms. 
     Shaft  14  can be held in place by one or more bearing assemblies  24 ,  26 . Bearing assembly  24  includes a pair of bearings  32 , a bearing support  28 , and a sump  34 . Bearing support  28  has an upper portion  28 A that extends inward from the wall of housing  16 , and a cylindrical descending cantilevered portion  28 B. Upper portion  28 A may be attached directly to the inner wall of housing  16 , or may be attached to mounting blocks (not shown) that are attached to the inner wall of housing  16 , so as to allow for easy removal of bearing support  28 . Upper portion  28 A may include a plurality of spiders, or arms, that extend from cantilevered portion  28 B to the wall of housing  16 . The spiders may be in the form of extending tabs that are formed integrally with upper portion  28 A or may be separate structures. The spiders may create a gap between the inner wall of housing  16  and upper portion  28 A so that lubricant may fall past bearing support  28 , and so that access may be had to the lower bearing assembly  26 . Alternatively, upper portion  28 A could be a solid plate or disk. 
     Cantilevered portion  28 B is cylindrically shaped and defines an inner volume appropriately sized to hold bearings  32 . Moreover, cantilevered portion  28 B may have a slight recessed portion sized to receive bearings  32  and hold them tightly in place. Cantilevered portion  28 B could also take on a variety of other appropriate forms. 
     Bearings  32  are held in position inside cantilevered portion  28 B, and form the bottom of oil galley  30 . Bearings  32  may be lubricated with oil from lubricating line  52 , either directly or by passage of lubricant into oil galley  30 . For example, oil may be pumped through lubricating line  52  and may enter oil galley  30  at one or more locations via fluid passages (shown by dashed lines). The lubricant may then flow downward through bearings  32  either from pressure from lubricating line  52  or under the force of gravity. The lubricant may also be directed toward bearings  32  by any other suitable means, such as by introducing lubricant above bearing support  28 . Preferably, the lubricant is introduced in sufficient quantities, and in a continuous or semi-continuous manner, to immerse bearings  32  and provide adequate flow over bearings  32  to afford them with fresh lubrication and cooling, and to ensure that bearings  32  do not seize or overheat during normal operation. 
     Sump  34  surrounds a portion of cantilevered portion  28 B, and catches lubricant that passes from oil galley  30  through bearings  32 . Sump  34  may be solidly attached to shaft  14  with a liquid-tight seal and may thus rotate as shaft  14  rotates. Sump  34  may be positioned so that its top lip is above the top of bearings  32  but below upper portion  28 A. In this manner, bearings  32  will be fully immersed in lubricant, and oil galley  30  will not generally overflow onto the top of upper portion  28 A. Alternatively, sump  34  could surround shaft  14 , and not be attached to shaft  14 , and therefore could be stationary. In such an arrangement, some lubricant may be allowed to pass between the inside portion of sump  34  and shaft  14 . 
     A second bearing assembly  26  includes a single bearing  40 , along with a bearing support  36  and sump  42 . Such a second bearing assembly  26  may be particularly advantageous where shaft  14  is expected to be subjected to very high torques and/or transverse loads. Bearing assembly  26  may provide a second anchoring point that is separated from the anchoring point from bearing assembly  24 , and may thereby achieve greater leverage in holding shaft  14  in place. 
     As with bearing assembly  24 , the bearing support  36  for bearing assembly  26  has an upper portion  36 A that extends inward from the wall of housing  16 , and a cantilevered portion  36 B. Lubricant may enter via passages (shown by dashed lines) in bearing support  36  through lubricating line  54 , and may pass into oil galley  38  defined by cantilevered portion  36 B and bearing  40 . While the bearing supports  28 ,  36  are shown as having cylindrical portions for holding the bearings  32 ,  40 , the bearing supports could also take on any of a number of other appropriate forms, such as a perforated basket, multiple support arms, or a ring hung from support arms. 
     Sump  42  may be provided to catch lubricant that flows through bearing  40 . Sump  42  may be solidly attached by a liquid-tight seal to shaft  14  and may thus rotate with shaft  14 . Sump  42  may have both an upper cup  44  and a descending skirt  46 , wherein the upper cup  44  catches and holds lubricant while the skirt  46  provides a shield from the passage of lubricant. Sump  42  may be positioned so that the top lip of upper cup  44  is above the top of bearing  40  but below upper portion  36 A of bearing support  36 . In this manner, bearing  40  will be fully immersed in lubricant, and oil galley  38  will not generally overflow onto the top of upper portion  36 A of bearing support  36 . Sump  42  may also be provided separately of the descending skirt. 
     Descending skirt  46  may take a cylindrical shape and may descend downward toward, and terminate above, drive floor  48 . A divider between the interior of housing  16  and the interior of vessel  18 , in the form of drive floor  48 , may be located at or near the bottom of housing  16 , or may be located above or below the bottom of housing  16 . Standpipe  50  may in turn take a cylindrical shape, co-axial with, and inside of, skirt  46 , and may rise from drive floor  48 . Standpipe  50  may overlap vertically with skirt  46  so as to impede or prevent the passage of liquid from vessel  18  into housing  16  and vice-versa. Standpipe  50  may also take any number of other appropriate forms that prevent fluids from passing from housing  16  into vessel  18 . As a result, sump  42  may serve as a shield to prevent fluid from entering standpipe  50 . 
     Drive floor  48 , standpipe  50 , and skirt  46  conveniently separate the interior of drive housing  16  from the interior of vessel  18 . A serpentine path is created between standpipe  50  and skirt  46  so that liquid cannot easily pass between vessel  18  and housing  16 . However, it is possible to allow gases to pass between the two areas, so that the pressure differential between vessel  18  and housing  16  is minimal, e.g., one atmosphere. 
     In general, no seal is required between shaft  14  and standpipe  50 . Rather, standpipe  50  and skirt  46  block the passage of lubricating fluid out of drive housing  16  or the passage of chemical into drive housing  16 . As a result, a sealless agitator drive may be achieved. Nonetheless, the present invention is not limited to sealless drives. In particular, seals or packing (such as graphite packing material) or restriction bushings (such as labyrinth bushings) may still be provided around shaft  14 , for example, to maintain a pressure differential or to reduce heat flow from vessel  18 . It should be recognized, however, that use of such sealing techniques is not necessary to practice the present invention, and that the disclosed embodiment can encompass both sealed and sealless designs. 
     The lubricant provided to drive  10  may be any appropriate lubricating and/or cooling fluid that provides a lubricating effect to bearings  32 ,  40 , and may be a single fluid or a mixture of fluids. Examples of such fluids include hydraulic fluid, mineral oil, petroleum oil, and synthetic cooling or lubricating preparations, or water or water-based fluids. Hydraulic fluids offer the advantage of wide availability and good lubricating and cooling properties. Advantageously, the same fluid may be used for lubrication of bearings  32 ,  40  and for powering motor  12 . 
     FIG. 2 shows drive  10  in cross-section. Motor  12  and shaft  14  are mounted inside housing  16 . Hydraulic fluid for powering motor  12  is provide by supply line  70  and removed by return line  72 . Because motor  12  is a positive displacement type motor, its rotational velocity may be conveniently controlled—and may be reversed—through the provision of fluid to the motor  12 . Fluid that leaks from motor  12  may collect on plate  22 , and may pass downward through holes in plate  22 . In addition, excess pressure in housing  16  may pass up through holes in plate  22  and may be relieved via vent line  74 , which opens into the top of housing  16 . 
     Motor  12  is coupled to shaft  14  via coupling  76 . Coupling  76  may allow for easier alignment of motor  12  with shaft  14 , and may provide for easy removal of motor  12  from housing  16 . Coupling  12  may take a variety of well-known forms, and may be solid or flexible. 
     As in FIG. 1, two bearing assemblies  24 ,  26 , are shown, and each has a sump  34 ,  42 . Lubricant supply line  52  may provide lubricant to bearing support  28 , and lubricant supply line  54  may provide lubricant to bearing support  36 . Lubricant lines  52 ,  54  may communicate with fluid supply manifolds  84 ,  86  in bearing supports  28 ,  36  respectively. As shown, lubricant is supplied both above bearings  32 ,  40  into oil galleys  30 ,  38 , and below bearings  32 ,  40  into sumps  34 ,  42 . 
     Sumps  34 ,  42  are positioned relative to bearings  32 ,  40  such that the upper lip of each cup is above the corresponding bearing or bearings. In this manner, adequate lubricant may be maintained around and over the bearings  32 ,  40  in the oil galleys  30 ,  38 , so that the bearings  32 ,  40  are fully immersed. The lower sump  42  may, in one instance, be made deeper than the upper sump  34  so that the lower sump  42  holds a greater volume of lubricant, which thereby provides greater cooling to the lower, hottest portion of shaft  14 . 
     Specifically, bearings  32  are depicted as a pair of bearings, and may be a pair of ball bearings, such as angular contact ball bearings mounted front-to-front or back-to-back to be resistant to thrust forces in both directions. Alternatively, two spaced apart tapered roller bearings mounted in opposite orientations could also be used. Other types of bearings, including tapered roller bearings, needle bearings, ball thrust bearings, and sleeve or journaled bearings may also be used in appropriate circumstances. Moreover a single bearing or various combinations of bearings, including bearings such as magnetic levitation bearings, may also be used. Also, bearings  32  could be mounted to provide resistance to thrust forces in only one direction. A retainer  83  may be provided on shaft  14  to hold bearings  32  in place, and may take the form of a threaded collar, a threaded nut, a friction-fit collar, a locking pin or ring, or other appropriate structure. Likewise, bearing  40  may be any appropriate bearing or combination of bearings. 
     Drive floor  48  generally acts to collect lubricant that spills over from rotating sump  42 . Drive floor  48  is shown with insulation that slows the transfer of heat from inside vessel  18  to inside housing  16 . In addition, a drain line  56  is provided near drive floor  48  to allow for the exit of lubricant from housing  16 . Drain line  56  may be conveniently oversized so that lubricant flows easily out of housing  16 . In addition, a weir may be provided to ensure that some lubricant remains on the drive floor  48  and to provide additional insulation between vessel  18  and housing  16 . Moreover, drive floor  48  may be provided with internal channels (not shown) for the passage of cooling fluid so as to further insulate housing  16  from vessel  18 . While drive floor is shown at the interface between housing  16  and vessel  18 , it may also be located further inward of housing  16 . 
     Standpipe  50  may extend upward and downward from drive floor  48 , and may be concentric with, and in close proximity to, shaft  14 . Insulating sheath  90  may be provided around shaft  14  inside vessel  18 , and may further slow the transfer of heat from vessel  18  into shaft  14  and housing  16 . An upper lip of sheath  90  may extend upward, and may overlap with the descending portion of standpipe  50  to form a debris well, to reduce the likelihood of contamination, and to block the flow of heat upward. Furthermore, fluid baffles  92  may be provided peripherally on the outside of standpipe  50  and the inside of skirt  46  to also block the flow of lubricant out of housing  16 . Shown in the figure in plate-like form, baffles  92  may take any other appropriate form, including forms that establish a non-pressure-tight, but serpentine, passage. 
     As noted, a number of features help to block heat flow from vessel  18  into housing  16 . Insulated sheath  90  prevents direct flow into the body of shaft, and the sheath&#39;s upwardly projecting portion helps block the convective flow of heat. Drive floor  48  may be insulated and further block the flow of heat upward from vessel  18 . In addition, lubricant on drive floor  48  may further absorb heat just before the fluid exits via port  56 . Standpipe  50  may also block convective heat flow, both by its close proximity to shaft  14  and its overlapping with skirt  46  (and via their overlapping baffles  92 ). Moreover, lubricant in rotating sumps  34 ,  42  and in oil galleys  30 ,  38  may be in contact with shaft  14  and may thereby remove additional heat that is able to propagate upward through shaft  14 . 
     In operation, motor  12  provides a rotary force to shaft  14 . Lubricant is provided via lines  52 ,  54  and flows into oil galleys  30 ,  38  and rotating sumps  34 ,  42 . Lubricant may alternatively be provided by other suitable means, including entry through the walls of housing  16  and across the top of bearing supports  28 ,  36 . Lubricant from oil galleys  30 ,  38  flows downward under the force of the supply pump or gravity through bearings  32 ,  40  and into sumps  34 ,  42 . When lubricant in oil galleys  30 ,  38  rises above the level of the upper lips of sumps  34 ,  42 , respectively, lubricant is forced to overflow sumps  34 ,  42 . Where bearings  32 ,  40  are lower, respectively, than the upper lips of sumps  34 ,  42 , bearings  32 ,  40 , will be immersed in lubricant. In this manner, the height of the upper lips can be used to control the depth of the lubricant in sumps  34 ,  42 . The upper portion  36 A of bearing support  36  may be comprised of a plurality of arms that extend inward from the wall of housing  20  toward cantilevered portion  36 B, so that lubricant that overflows sump  34  may fall through the arms and onto drive floor  48 . In the case of rotating sump  42 , overflowing lubricant falls directly to drive floor  48 . Fluid on drive floor  48  may exit through drain port  56 . The general flow of lubricant is indicated in the figure via small arrows marked ‘a.’ 
     Media, such as gases, liquids, or powders, may be introduced into the system via media introduction line  88 . As shown, media introduction line  88  enters housing  16  through the center of drain port  56 , and opens in the wall of standpipe  50  near shaft  14 . In this manner, introduced gases may migrate down along the shaft  14  into vessel  18  and up along shaft  14  into housing  16 . Such gases may be provided, for example, where the process vessel  18  requires a particular environment, or where the introduced gas reduces the entry of deleterious gases from the process into housing  16 . The gases may include any gas used in the process vessel, or gases introduced to maintain a particular pressure differential between housing  16  and vessel  18 , including air. 
     Advantageously, the pictured arrangement separates the bearings from the high temperature inside vessel  18 . As a result, the arrangement can make use of conventional bearings, thereby lowering the costs and improving the reliability of the system. Also, the arrangement does not place the bearings or other components in between two areas of greatly differing pressure. Therefore, high-pressure seals or other precautions are generally not needed for the drive to operate. The pictured arrangement allows the bearings to be isolated from the high temperature inside vessel  18  without requiring a pressure-tight seal around shaft  14  that separates the inside of vessel  18  from the ambient atmosphere. Rather, the interior of housing  16  is generally kept at the same or a similar pressure to that inside vessel  18 . By eliminating such a seal, the risk of contamination, early seal failure, and escaping gases or other materials can be greatly minimized. 
     FIG. 3 shows a partial section view of a hydraulic drive motor  130  for use in a high-pressure environment. In particular, standard hydraulic motors are filled with hydraulic fluid that is under pressure. In normal use, the internal pressure in the motor generally pushes hydraulic fluid from inside the motor through bearings at the edge of the motor. In this manner, the motor bearings, which are usually located near the ends of a motor, may receive a continuous supply of fluid and may be lubricated by the fluid. The fluid may be caught at the edge of the motor and recycled, or it may be allowed to drip out of the motor into the ambient atmosphere. However, when a standard hydraulic motor is placed in a high-pressure atmosphere, where the ambient pressure is higher than the fluid pressure inside the motor, the hydraulic fluid is not able to flow out of the center of the motor and lubricate the bearings. As a result, the bearings may be starved of lubricant, may be exposed to corrosive gases, and may wear out quickly. 
     Hydraulic motors may be provided with drain holes that open into internal cavities near the motor bearings. Therefore, in a high-pressure environment, it is possible to pipe lubricant through the drain holes at sufficient pressure so that the lubricant flows out and over the bearings. To ensure adequate and substantially continuous lubrication, however, it may be necessary to vary the pressure at which the fluid is introduced to compensate for changes in the ambient pressure around the motor. Such compensation may require the use of a closed-system control loop having pressure transducers and control valves-an expensive and complicated solution. 
     Motor  130  is adapted to operate without complex structures within a high-pressure environment in which the pressure may change over time. Motor  130  includes a hydraulic fluid inlet  132  and a hydraulic fluid outlet  134 . Hydraulic fluid enters inlet  132  under pressure and leaves through outlet  134 . The hydraulic fluid passing through motor  130  may provide a force to cause motor shaft  136  to rotate. Motor shaft  136  may be connected to an agitator, such as that described above, or to other equipment. 
     Motor shaft  136  may be held in place by upper bearing  138  and lower bearing  140 , which may be ball bearings or any other appropriate type of bearing. Bearings  138 ,  140  may be removable, and may be held in place by upper bearing retainer  142  and lower bearing retainer  144 , respectively. A lip seal  146  may be provided to catch fluid that has passed through lower bearing  140 . Lip seal  146  may be provided with a drain so that fluid that collects there may be passed to another location and recycled. 
     Lower case drain  148  can provide access to a lower cavity  152  behind lower bearing  140  and may thereby allow ingress and egress of fluid from lower cavity  152 . Likewise, upper case drain  156  may allow access to an upper cavity (not shown) behind upper bearing  138 . Fluid conduits  150 ,  158  may connect, respectively, to lower case drain  148  and upper case drain  156 , so that fluid in the conduits may flow into motor  130 . Conduits  150 ,  158  may terminate, respectively, at fluid receptacles  154 ,  160 . Fluid supply conduits  164 ,  162  can extend through a wall  166  of a pressurized housing and terminate above fluid receptacles  154 ,  160 , separated from fluid receptacles  154 ,  160 , by air gaps  170 ,  168 . Fluid receptacles  154 ,  160  could also be combined into a single receptacle, and fluid supply conduits  164 ,  162  could also be combined with each other. 
     In operation, lubricant may be introduced through fluid supply conduits  162 ,  164  and may pass into fluid receptacles  154 ,  160 . Fluid receptacles  154 ,  160  may be cup-shaped and sized so as to catch a substantial portion of the lubricant from fluid supply conduits  164 ,  162 . The lubricant held in fluid receptacles  154 ,  160  may supply a slight head of pressure that pushes lubricant down through conduits  150 ,  158  and into motor  130  so that the lubricant may pass over bearings  140 ,  138 . Advantageously, the pressure of the fluid in fluid receptacles  154 ,  160  is always referenced to the ambient pressure around motor  130 , so that if the ambient pressure changes, the lubricant will still flow into motor  130 . 
     Lubricant may be introduced through fluid supply conduits  162 ,  164  at a relatively steady rate or a varying rate. Fluid may be supplied at a relatively steady rate, for example, where the fluid is driven by a constant velocity positive displacement pump. To the extent the lubricant flow exceeds the demands of the motor, the lubricant may be allowed to overflow fluid receptacles  154 ,  160 , and may be collected, filtered, and recycled. Where it is not possible to have the fluid overflow fluid receptacles  154 ,  160 , the flow rate through fluid supply conduits  162 ,  164  may be varied or stopped. For example, the amount of fluid supplied may be decreased as the hydraulic motor speed is decreased. Alternatively, fluid level sensors (not shown), such as floats, may be provided in receptacles  154 ,  160  so as to turn off the supply of fluid before the fluid overflows. 
     Alternatively, an air gap may be provided at one or both of the case drains  148 ,  156 . Lubricant may, in that instance, be sprayed or misted across the gap and into the motor, so that case drains  148 ,  156 , act as fluid receptacles. 
     Advantageously, the disclosed embodiment provides for a motor that can be operated in a high-pressure environment without the need for specialized pressure compensation mechanisms. The reservoir of fluid is constantly referenced to the pressure around motor  130 , so that the pressure of the fluid behind bearings  138 ,  140  is slightly higher than the pressure in front of bearings  138 ,  140 . In this manner, bearings  138 ,  140  may be provided with an appropriate level of lubricant throughout the operation of motor  130 . 
     FIG. 4 illustrates, in schematic form, a fluid circulation system  100  for drive  10 . The circulation system  100  may be a generally closed system, and may provide both power for operating drive  10 , and lubrication and cooling for the bearings and other components inside drive  10 . Advantageously, both functions may be performed with the same fluid. In addition, system  100  may provide for the capture, filtering, and reuse of fluid from drive  10 , and may control any gases inside drive  10 . 
     The pressure inside housing  16  may be maintained in part via trap  104  attached to vent line  74  and drain line  122 . Drain line  122  may be adequately sized so that it is not entirely filled with lubricant leaving housing  16 , and may thereby allow gas to escape. Drain line  122  may also be sized to be completely filled with lubricant. 
     Trap  104  may act as a liquid/gas separator, and hydraulic fluid storage tank  102  may release any remaining gases through vent  110 . For example, vent  110  may bleed off undesired gases, or such gases may be released to atmosphere, and may be ignited from a flare on vent  110 . Such gases could also be reclaimed and stored for later use, collection, or disposal, or for processing, as appropriate. To capture gas and fluids, vent line  74  connects to trap  104 , which in turn empties into storage tank  102 . Trap  104  may receive lubricant that has passed through drive  10  from drain line  122 , and may likewise pass the lubricant to storage tank  102 . Lubricant that has been used to power the drive  10  also passes from return line  72  to storage tank  102 . Catch filters  106 ,  108  may be provided in the lines leading to storage tank  102  to remove debris that enters the lubricant in drive  10 . 
     Lubricant in storage tank  102  may be withdrawn for powering the motor in drive  10  and for lubricating and cooling components in drive  10 , such as bearings. As shown, a shell-and-tube heat exchanger  112  is provided to remove heat that has accumulated in the lubricant from drive  10 . The cooling fluid for heat exchanger  112  may be, for example, water from a cooling tower or another source of available relatively cool fluid. Other types of heat exchangers may also be used. In addition, heat exchanger  112  may be provided in the returns that are upstream from storage tank  102 . 
     Positive displacement pumps  114 ,  118  may be provided for supplying lubricant, respectively, for powering the motor in drive  10 , and for lubricating and cooling bearings in drive  10 . Pump  114  is a positive displacement gear-type pump, and is powered by drive  116 . Pump  114  draws fluid from storage tank  102  and provides the fluid via supply line  70  to the motor in drive  10 . Drive  116  may be a variable speed drive. Because the motor in drive  10  is a positive displacement motor, and pump  114  is a positive displacement pump, the rotational speed of the motor may be controlled by the speed of drive  116 . Other types of pumps, including non-positive displacement pumps, may also be used. 
     Pump  118  is also a positive displacement gear-type pump, and may be coupled to drive  120 . Pump  118  may draw fluid from storage tank  102  and provide the fluid under pressure as lubricant for bearings and other components in drive  10 . The lubricant may be supplied to the bearings via lines  52 ,  54 . Lubricant may also be supplied via lines  162 ,  164  to lubricate motor bearings in drive  10 . In addition, media introduction line  88  may provide a medium, such as a gas stream, to the interior of housing  16 , as discussed above. 
     Advantageously, the disclosed system separates the drive bearings and other drive components from the heat of the vessel and also collects lubricant from the bearings so that low-cost conventional bearings may be used. For example, open-type bearings, i.e., those that are not sealed, allow for the flow of lubricating fluid throughout the various parts of the bearings, such as the rollers, balls, needles, or other intermediate members that may be positioned between the bearing races. In addition, because there are no substantial pressure differences across the bearings, the bearings need not be selected to maintain a pressure, and may have longer service lives. 
     In addition, the same fluid may provide power, lubrication, and cooling. As a result, fewer parts are needed for the drive system, and the system can thereby be built and operated more reliable and less expensively. Of course, other fluid supply and circulation systems having different arrangements of components other than that shown in the Figures can also be used and provide a similar function. 
     Other arrangements of the disclosed embodiment are also within the scope of the invention. For example, various numbers, types, and arrangements of bearings may be provided to hold the shaft, and additional bearings would provide for a more even distribution of the load from the shaft and improved cooling of each bearing. In addition, a “tandem” system, in which two (or more) shafts are provided inside a single housing, can be provided, in which each shaft would be provided with one or more bearings and associated rotating sumps, and could be driven by a single motor or multiple motors. Such an arrangement would be particularly advantageous for a process requiring two or more agitators in one vessel, particularly where the agitators are counter-rotating. 
     It should be understood that various modifications could be made without departing from the spirit and scope of the invention. In particular, the invention is intended to be operable in any of a number of environments, and using any of a number of arrangements of elements. Accordingly, other implementations are within the scope and coverage of the following claims.