Patent Publication Number: US-6911087-B2

Title: Product discharge and cleaning assembly for an apparatus for coating tablets

Description:
This patent application is a divisional of prior U.S. patent application, Ser. No.: 09/715,855, filed Nov. 17, 2000, now U.S. Pat. No. 6,579,365 which claims the benefit of U.S. Provisional Patent Application, Ser. No.: 60/166,799, filed Nov. 22, 1999, both of which are incorporated herewith in their entirety. 

   BACKGROUND OF THE INVENTION 
   A. Field of the Invention 
   The present invention generally relates to coating machines and, in particular, to fluid-bed coating machines used for the coating of tablets. 
   B. Description of the Prior Art 
   Tablets are formed by pressing pharmaceutically active drugs, filler and binding agents together. Once formed, it may be necessary, or desirable to provide the tablet with a coating which will:
         1. prevent any portion of the drug from being released, such as in the form of dust;   2. mask any unpleasant odor or taste of the active drug, or any filler or binder used;   3. facilitate swallowing by providing a smoother and less absorbent outer layer;   4. protect the contents of the tablet from pre-mature digestion by providing a coating which is resistant to gastric fluids;   5. control the rate of absorption of the drug by the small intestine; and   6. improve the appearance of the tablet and provide a printable surface.       

   The tablets are generally coated using machines which spray a coating material, such as hydroxypropylmethylcellulose onto the surfaces of the tablets while the tablets are in motion within a product container. Two common types of machines tumble tablets within a horizontally rotatable drum during the spraying process while another type of tablet coating machines uses a vertical flow of air to circulate tablets past a vertically disposed spray nozzle. The prior art coating machines are described below. 
   1. Dragee Kettle 
   For most applications, the exact thickness of the coated layer is not critical and many different types of coating machines may be used to apply a crude, yet effective coating to the tablet. An older once popular type of coating machine is called a dragee kettle and examples of these machines are disclosed in U.S. Pat. Nos. 3,831,262 and 5,334,244. This machine includes a large drum-like vessel which is typically rotated about a horizontal axis. The vessel includes a coating chamber which is partially filled with tablets to be coated so that as the vessel rotates, the tablets roll and tumble along the inside wall of the coating chamber. During this tumbling motion, coating materials in the form of aqueous or organic suspensions of liquids are sprayed through nozzles and into contact with the rolling tablets within the coating chamber. During the coating process, a current of temperature-controlled air circulates in the coating chamber of the dragee kettle, which helps evaporate the suspension agent of the coating material so that the coating material effectively dries and adheres to the tablets. 
   One problem with the dragee kettle coating machine is that typically the tablets are not the only surfaces coated within the coating chamber. Even when a carefully controlled spraying schedule is followed (such as spraying at very short intervals while the dragee kettle rotates), much of the sprayed coating material still ends up on the inside wall of the coating chamber, as well as throughout the evaporation/venting ducting. This over-spraying creates numerous contamination and cleaning problems, and further increases the cost of the coating since much of the coating material is lost during the coating process. 
   The above-described dragee kettle type coating machine is limited to coating tablets which do not require much precision in the thickness of the coated layer because the thickness of the coating of the tablets will vary in the same batch. This process may be used to coat many different types of pharmaceuticals, vitamins, and even candy, as long as uniform coating distribution and thickness are not required. 
   2. Perforated Pan 
   The next generation of tablet coating machines after the dragee kettle is called a perforated pan tablet coating machine. This machine has improved the tablet coating process and is the most common type of tablet coating machine in use today. The perforated pan machine includes a rotatable perforated drum which rotates about a horizontal axis within a housing, and further includes a plurality of nozzles positioned within the drum. The nozzles create a spray of coating material within the drum so that any tablets located within the drum will tumble about into and out of the spray pattern and, over a period of time, will accumulate a coating on their surface. An important improvement of the perforated pan coating machine over the dragee kettle is that the perforated pan machine allows air directed through the housing (using appropriate ducting) to pass through the perorated drum and quickly reach the tablets tumbling therein. The perforations of the drum effectively expose the tumbling tablets to the current of air, resulting in more uniform distribution of drying air for each tablet. The drum further includes solid baffles which are used to enhance mixing of the tablet bed in an effort to improve the distribution of the material being sprayed onto the tablets. 
   3. Fluidized Bed Coating Machines 
   Another type of particle-coating apparatus is called a fluidized bed coating machine (also known as a Wurster machine, after inventor Dale Wurster). Several examples of the Wurster coating machine are disclosed in U.S. Pat. Nos. 3,196,827,3,110,626, 3,880,116, 4,330,502, 4,535,006 and 5,236,503. 
   The Wurster coating machine is typically used to layer, coat or encapsulate lightweight powders, particles, granules or pellets of solid materials, including pharmaceutical drugs. Often, coatings are applied to modify the release of the substrate (protective barrier, taste masking, enteric coating, delayed release or sustained release). A predetermined quantity of these coated particles are usually packaged within an edible gelatin capsule or compressed into a tablet. The distribution uniformity of the applied substance may not be critical because the capsule or tablet contains multiple units and the average coating thickness of all of the pellets within the capsule dictate the average release properties and performance of the overall dosage form. 
   As described below, the Wurster machine generates an upward stream of air or other gases such as nitrogen to circulate a substrate (particles, pellets, powders, etc.) through a vertical spray of coating liquid within a product container. As the substrate cycles through a spray, a minute amount of coating material is deposited on its surface. The number of cycles the substrate completes determines the thickness of the final coating layer. 
   The conventional Wurster machine works well when the particles are fine and lightweight (such as grains of powder). However, due to flow-related problems inherent in the design, the conventional Wurster machine fails to provide a uniform distribution of coating on heavier tablets because the heavier tablets do not uniformly cycle through the machine. The Wurster-coated tablets cannot be used for applications which require uniform, predictable and consistent distribution coatings on all tablets within a particular batch. 
   Certain types of pharmaceutical controlled-released tablets require high-precision coatings because the thickness of the coating governs the time of release and the release rate of the active ingredient of the tablet and thereby directly influences the effectiveness of the medication. The conventional Wurster machine is incapable of providing a high-precision coating on tablets, in part due to the following flow-related problems, each of which adversely effects the precision of the coating of each tablet or particle in the batch. 
   The conventional Wurster machine also creates undesirable turbulence and introduces high shear forces to the substrate as it cycles through the machine. The fine and lightweight substrates typically used with conventional Wurster machines are not adversely affected by the violent traumatic forces they must endure during each cycle. However, when a conventional Wurster machine is used to coat heavier tablets, the high shear forces generated during each coating cycle are capable of damaging the tablets and the resulting attrition rate of the tablets is unacceptable. 
   The heavier tablets are also more difficult to introduce into the high velocity airstream of the Wurster machine, usually causing some of the tablets to accelerate directly into hard structures within the machine, such as a nozzle assembly. The impact can easily shatter or otherwise damage the tablets. 
   Once a substrate is processed using the Wurster machine, the substrate must be removed from the product container. This is conventionally accomplished through a pivotal bottom door which, when opened, allows the coated substrate to simply fall by gravity into an awaiting and suitable container. Although this emptying process is effective, the process exposes both the substrate and the interior of the product container to the environment. Not only does this exposure introduce undesirable contamination to the product container, it also subjects the operators of the machine unnecessarily to potentially hazardous materials. To this end, it would be beneficial to remove the coated substrate from the coating machine using a more controlled and predictable process without undue complexity and without affecting the machine&#39;s operation. 
   Another problem with Wurster machines is that they are relatively difficult to clean. The cleaning procedure typically requires the opening of the lower end of the product container and the application of an appropriate cleaning fluid. Some coating machines have spray nozzles within the coating machine to initially wash out any residual material deposited along the interior surfaces of the machine after the coating process. The cleaning fluid from these nozzles washes the interior surfaces of the machine and typically drains through the open lower end. Sometimes, however, the material being processed within the product container comprises a drug or other material which may be hazardous if accidentally inhaled, swallowed or even touched by personnel assigned to operate and clean the coating machine. It would therefore be beneficial to ensure that a maximum amount of this potentially hazardous residue is washed from the expansion chamber and the product container while isolating the contaminated waste from the surrounding environment. (i.e., without having to opening the machine). 
   OBJECTS OF THE INVENTION 
   It is an object of the present invention to provide a fluidized-bed type coating machine which overcomes the deficiencies of the prior art. 
   It is another object of the present invention to provide a Wurster-type coating machine which encourages even and predictable flow of tablets located in the down-bed. 
   It is another object of the present invention to provide a Wurster-type particle-coating machine which encourages tablets to flow radially inwardly along a distribution plate between the down-bed and an up-bed. 
   It is another object of the invention to provide a Wurster-type particle-coating machine which includes a central nozzle assembly located at the distribution plate for discharging a spray of coating liquid and which further includes structure to redirect tablets from the down-bed to the up-bed without impacting the central nozzle assembly. 
   It is another object of the invention to provide a Wurster-type particle coating machine which is particularly suited to accurately coat heavier particles, such as tablets. 
   It is another object of the present invention to provide a Wurster-type particle-coating machine which includes a partition which is shaped to provide an atraumatic transition of the tablets moving from the down-bed into the up-bed. 
   It is another object of the present invention to provide a Wurster-type particle-coating machine which cycles the tablets within the machine between the down-bed and the up-bed in a smooth, efficient, and consistent manner so that the resulting coating distribution of each tablet is consistent and predictable and tablet-attrition is minimized. 
   It is another object of the present invention to provide a Wurster-type particle-coating machine which permits discharge of the substrate (particles or tablets) through an opening at the center of the orifice plate at the base of the insert when multiple partitions are used. 
   It is another object of the present invention to provide a Wurster-type particle-coating machine which is easy to operate during the coating process and facilitates cleaning without disassembly of the Wurster insert. 
   SUMMARY OF THE INVENTION 
   The foregoing objects of the invention are met through various improvements to a Wurster-type fluidized bed apparatus for applying a coating liquid onto the surface of particles. The coating liquid is generally comprised of substances in a solution, suspension or dispersion in water or organic solvent (in some cases a molten liquid may be used). The apparatus includes a vertically disposed cylindrical or slightly conical product container having a peripheral wall, at least one cylindrical partition defining a centrally located up bed region and a peripherally located down bed region. The product container further includes an upper end connected to an expansion chamber and a lower end including an orifice plate having a plurality of openings for passage of fluidized air. A nozzle is centrally located through the orifice plate and is adapted to generate a spray of coating liquid upwardly into the up bed. Particles located within the product container circulate upwardly through the partition and the coating liquid spray, between the up bed and the down bed. 
   A feature of the invention comprises a product discharge and cleaning assembly. The product discharge and cleaning assembly includes a discharge opening located within the orifice plate, and a conduit positioned below the orifice plate. The discharge opening and conduit plate are sized and shaped to selectively discharge material from the product container. The product discharge and cleaning assembly may further include at least one discharge jet positioned within the product container, connected to a source of fluid and being positioned so that the discharged fluid is directed towards the discharge opening to urge material towards the discharge opening. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional side view schematic (labeled PRIOR ART) of a prior art Wurster-type bottom spray particle-coating machine showing a product container, a partition, a nozzle, a down-bed, an up-bed, a nozzle sleeve, and a plurality of tablets or pills (represented by spheres) being coated as they circulate through the machine: 
       FIG. 2  is an enlarged sectional view (labeled PRIOR ART) of a portion of the prior art Wurster-type coating machine of  FIG. 1  (shown without the numerous particles, for clarity) including particle-flow arrows representing the flow of particles during their transition from the down-bed into the up-bed air-stream, and further including a dashed line representing a “dead-zone” within the product container; 
       FIG. 3  is a sectional side view schematic of an improved Wurster-type bottom spray particle-coating machine showing a product container, a partition having a peripheral skirt, a nozzle, a down-bed, an up-bed, a nozzle sleeve, a truncated cusp-shaped nozzle-ramp, and a plurality of tablets or pills (represented by spheres) being coated as they circulate through the machine, according to the present invention; 
       FIG. 4  is an enlarged sectional view of a portion of the improved Wurster-type coating machine of  FIG. 3  (shown with only selected particles, for clarity) showing details of the peripheral skirt, the nozzle-ramp, the nozzle sleeve, and the peripheral air-injection system and further including particle-flow arrows representing the flow of the selected particles during their transition from the down-bed into the up-bed air-stream, according to the invention; 
       FIG. 5  is an enlarged sectional view of a portion of the improved Wurster-type coating machine of  FIG. 3 , showing details of a dead-zone (shown in dashed line), the peripheral air-injection system, and flow arrows, according to the present invention; 
       FIG. 6  is an enlarged section view of a portion of the improved Wurster-type coating machine of  FIG. 3  showing details of the nozzle-ramp and nozzle sleeve (showing a conventional straight-edged partition), and including flow arrows, according to the present invention; 
       FIG. 7  is a plan view of the peripheral air-injection system showing an inlet conduit, a ring manifold assembly including a plurality of inwardly-directed discharge outlets, according to the invention; 
       FIG. 8  is a sectional view of the peripheral air-injection system of  FIG. 7 , taken along the lines  8 — 8  of  FIG. 7 , according to the present invention: 
       FIG. 9  is a side view of the nozzle sleeve, according to the invention: 
       FIG. 10  is a cross sectional side view of the nozzles sleeve of  FIG. 9 , according to the invention; 
       FIG. 11  is a partial-sectional side view of the nozzle-ramp, according to the invention; 
       FIG. 12  is a bottom view of the nozzle-ramp showing details of the through passages, according to the invention; 
       FIG. 13  is a sectional assembly view of the partition skirt showing upper and lower sections, taken along the lines  13 — 13  of  FIG. 14 , according to the invention; 
       FIG. 14  is a bottom view of the partition skirt, according to the invention; 
       FIG. 15  is a partial-sectional side view of the partition skirt, according to the invention; 
       FIG. 16  is a plan view of a conventional orifice plate; 
       FIG. 17  is a sectional side view of the conventional orifice plate of  FIG. 16 , taken along the line  17 — 17  of  FIG. 16 ; 
       FIG. 18  is a sectional view of a multi-partition coating machine having a central discharge assembly, according to another feature of the present invention, taken along the line  18 — 18  of  FIG. 19 ; 
       FIG. 19  is a bottom view of the multi-partition coating machine of  FIG. 18 , according to the invention, showing details of the nozzle assemblies and the central discharge; 
       FIG. 20  is an enlarged sectional side view of the central discharge assembly of  FIG. 18 , according to the invention, shown in a closed position; 
       FIG. 21  is the central discharge assembly of  FIG. 20 , according to the invention, shown in an open position; and 
       FIG. 22  is a sectional side view schematic of the improved Wurster-type bottom spray particle-coating machine similar to the one shown in  FIG. 3 , and further including a split lower plenum which divides the air or gas stream and provides independent control of the flow of air or gas in the up and down bed regions of the Wurster machine, according to the present invention. 
   

   DETAILED DESCRIPTION OF THE FEATURES OF THE INVENTION 
   By way of background and introduction, the present invention provides improvements relating to tablet flow and handling during the coating process of an otherwise conventional Wurster-type bottom-spray particle coating machine (hereinafter referred to as “Wurster machine”). To better understand and appreciate the improvements of the present invention, a detailed description of the structure and operation of a conventional Wurster machine is first provided. 
   A. Description of a Conventional Wurster Machine 
   Referring to  FIGS. 1 and 2  (labeled PRIOR ART), a conventional Wurster machine  10  is shown, including a generally conical product container  12 , an orifice plate  14 , a lower plenum  16 , a central nozzle  18  projecting upwardly through a central opening  20  located within orifice plate  14 , a nozzle sleeve  22 , and a cylindrical partition  24 . The product container  12  may be cylindrical or conical in shape and is mounted to an upper surface  26  of orifice plate  14 , while the lower plenum  16  connects with a lower surface  28  of orifice plate  14 . The upper end of the product container  12  is connected to an expansion chamber (not shown). 
   As shown in  FIGS. 16 and 17 , orifice plate  14  includes a plurality of orifices  30  which are arranged in such a manner as to allow air to flow (represented in  FIGS. 1 and 2  by arrows  15 ) from the lower plenum  16  through the orifices  30  and into the product container  12 , to create a fluidized bed, as is described in greater detail below. Orifice plate  14  typically includes two sections of relative porosity concentrically positioned around the central opening  20 ; an up bed section  14 A (also referred to as an “up bed plate”), and a down bed section  14 B (also referred to as a “down bed plate”). Air flow from the lower plenum  16  will behave differently in the up bed plate and the down bed plate. The air flow generated by a remote blower unit (not shown) is restricted in a controlled and predictable manner so that a desired fluidity of the solid particles located within the product container is maintained. The up bed section  14 A of the orifice plate (located under the partition  24  and adjacent to the nozzle  18 ) includes a large number of orifices  30  which allow a high volume of air from the lower plenum  16  to pass through the orifice plate  14  and up through the partition  24  at a relatively high velocity. This upward flow of high velocity air is called the up bed (represented by arrows  33  in  FIGS. 1 and 2 ) and is used to pneumatically transport the substrate vertically past the spray nozzle during each cycle. 
   The ring-shaped region outside of the partition  24  within the product container  12  is referred to as the down bed (represented by arrows  37  in FIGS.  1  and  2 ). The down bed section  14 B of the orifice plate  14  has a porosity (number, diameter and distribution of orifices  30 ) which allows sufficient air flow to penetrate the substrate from the lower plenum  16  and to maintain the substrate located in the down bed in near-weightless suspension. This influx of air causes the particles  27  to behave somewhat like a fluid and travel rapidly and freely downward in the down bed  37  and subsequently be drawn horizontally through the transition gap and redirected upwardly into the up bed. The amount of air flow required to produce the near-weightless suspension of the substrate depends on the size and shape of the particular substrate. In general, tablets require significantly more air to produce this condition than pellets or fine particles and the orifice plate  14  must be designed accordingly. 
   The central nozzle  18  of the prior art Wurster machine  10  in  FIG. 1  is mounted through the central opening  20  of the orifice plate  14  so that its discharge outlet is directed upwardly into the partition  24  and product container  12 . The nozzle  18  ejects a highly atomized spray of coating liquid into the partition, creating a “coating zone”  32  which is usually shaped as a narrow volumetric ellipse. The exact shape of the coating zone may be controlled according to the size of the substrate being coated and the pattern density in the partition. The atomized spray is discharged from the nozzle at about 300 meters per second. 
   Centrally and vertically mounted within the product container is the cylindrical partition  24  whose diameter is about half the diameter of the product container  12  (as measured at the base of the product container). The cylindrical partition  24  is positioned within the product container  12  adjacent to and above the central nozzle  18  so that the spray pattern discharged from the nozzle  18  extends into the partition  24 . As is understood by those skilled in the art, the partition  24  is used to help direct particles  27  located within the product container  12  as they circulate throughout the Wurster machine, and further to protect the integrity of the spray pattern  32 . The partition  24  also helps separate particles  27  moving upwardly in a centrally located up bed  33  through the coating zone  32  from particles  27  that are falling back towards the orifice plate  14  in a peripherally oriented down bed  37  with respect to the product container  12 . 
   The partition  24  includes a lower edge or rim  25  and is positioned within the product container  12  so that the lower rim  25  is located a predetermined distance from the upper surface  26  of the office plate  14 . The space (or “transition gap”) that is created between the lower rim  25  of the partition  24  and the upper surface  26  of the orifice plate  14  forms a “transition zone” where particles  27  located in the peripheral down bed  37  are radially-inwardly drawn through a differential in pressure back into the central up-bed  33 . 
   Conventional Wurster machines do not shield the relatively hard nozzle assembly  18  and will typically result in particle attrition and breakage as the particles impact the nozzle assembly  18  as they accelerate from the down bed  37  to the up bed  33 . 
   One improvement over the conventional Wurster machine regarding the problem of particle attrition and breakage due to an exposed nozzle assembly  18  is disclosed in commonly owned U.S. Pat. No. 5,236,503. This patent discloses a protective nozzle sleeve  22  which is a tube positioned around the nozzle assembly  18 . The nozzle sleeve  22  disclosed in U.S. Pat. No. 5,236,503 (and shown in  FIG. 1  of the present patent application) prevents particles  27  from entering the spray pattern until it is more fully developed, thereby increasing productivity. Furthermore, it keeps the substrate from encountering the extremely high compressed air velocity at the tip of the spray nozzle thereby reducing particle attrition and breakage. Unfortunately, some of the particles accelerating into the up bed  33  in the prior art Wurster machine (with the improved protective nozzle sleeve  22 ) still impact the nozzle sleeve  22 , resulting in a measurable attrition rate. Such impact damage to the particles  27  will only increase as the mass of the particles increases, such as when relatively heavy tablets are cycled through the coating machine. 
   Another problem with the prior art nozzle sleeve  22 , as shown in  FIG. 1  of the present patent application and also disclosed in U.S. Pat. No. 5,236,503 is that the sleeve itself is prone to trapping particles  27  at the completion of the process, when fluidization air flow is stopped. Once trapped within the hollow nozzle sleeve  22 , the particles  27  will no longer circulate within the machine and may further interfere with the air flow around the nozzle, causing its spray pattern to be altered and resulting in undesirable agglomeration of the particles. These trapped particles must be manually removed prior to loading a subsequent batch. 
   B. Conventional Wurster Operation 
   In operation of the prior art Wurster machine shown in  FIGS. 1 and 2 , a supply of particles  27  that are to be coated are placed within the product container  12 . The size of the particles  27  are typically between 100 and 2000 microns in diameter. The particular operating parameters (such as the specific size, permeability, and arrangement of the orifices  30  in orifice plate  14 , and the height of the transition gap) of the Wurster machine will vary depending on the size and type of particles  27  being coated. 
   When the particles  27  are positioned within the product container  12 , a flow of filtered air is drawn from an air handling unit (not shown), the product container itself, and an expansion chamber (not shown) using an appropriate blower fan (also not shown) creating a negative pressure within the expansion chamber and the product container  12 . This negative pressure causes air (arrows  15 ) to be drawn upwardly through orifices  30  of orifice plate  14  from lower plenum  16 . As the air is drawn into the product container  12  through orifices  30 , it passes through both the up bed  33  and the down bed  37  (moving upwardly) penetrating and influencing the particles  27  located in the down bed  37 , as shown in FIG.  1 . The upward flow of air causes each particle  27  of the down bed  37  to effectively float or become suspended on a cushion of air as the air flow finds its way upward into the product container  12  to try to equilibrate the negative pressure in the expansion chamber. The “floating” particles  27  become “fluidized”, behaving more like a fluid than a mass of solid particles. Air (arrows  15 ) from plenum  16  is also drawn into the product container  12  through opening  20  and into nozzle sleeve  22  and further up into partition  24  adding to the up bed air stream  33 . 
   After the flow of air from the lower plenum  16  fluidizes the particles  27  within the product container  12 , the central nozzle  18  is activated to discharge a controlled spray pattern  32  of coating liquid upwardly into partition  24 , as shown in  FIGS. 1 and 2 . The spray liquid generally is comprised of a solution, suspension or dispersion in water or organic solvent (in some cases a molten liquid may be used) and is ejected upwardly from the nozzle  18  at a high velocity around 300 meters/second, depending on the particular type and size of particle being coated, the type of coating material used, and the desired coating characteristics. 
   As the particles  27  rise rapidly upward in the high-velocity up bed air stream  33  created by the nozzle  18 , they contact micro-atomized droplets of the coating liquid and become coated before slowing down within the expansion chamber (not shown). As the particles  27  continue to rise in the partition  24  and into the expansion chamber, excess moisture from the applied coating liquid evaporates. 
   The high-velocity air stream spouting from the upper end of the partition  24  forces the particles  27  radially outwardly in the expansion chamber (as represented in  FIG. 1  by arrows  35 ). Once away from the upstream lift provided by the up bed  33 , the particles  27  are influenced by gravity and fall within the product container  12  in the down bed  37 , eventually reaching the orifice plate  14 . 
   The high volume and velocity of the airflow into and through the partition  24 , combined with the high velocity of the air from the nozzle  18 , generates a very strong negative pressure in the transition zone lying adjacent to the nozzle  18  and the orifice plate  14  relative to the measured pressure within down bed  37 . This creates a pressure differential. The pressure differential draws the particles  27  that are located in the peripheral down bed  37  radially inwardly through the transition zone and into the up bed  33 . The up bed again accelerates the particles  27  up into the partition  24  and through the coating zone  32 . 
   The cycle is repeated for all particles  27  located within the product container  12 , until a desired coating thickness is formed on all particles of the batch. 
   C. Problems with the Conventional Wurster 
   As discussed in the Background section of this specification, the above-described Wurster machine is generally effective at coating fine particles within the product container. The longer the particles are kept circulating through the spray of the coating liquid, the greater the thickness of the coating on each particle, and the greater the consistency between coated particles of the sane batch. The Wurster machine, however, fails to provide an accurate and predictable distribution of coating on the particles as the particles increase in mass and size (such as when tablets or pills are cycled through the machine). The conventional Wurster machine includes three main particle-flow-related problems which are inherent in its design and are illustrated in FIG.  2  and described below. 
   A first flow-related problem of the Wurster machine relates to an uneven circulation flow of particles  27  located in the down bed  37  resulting in the creation of a peripheral “dead zone”  40  (shown in dashed lines in FIG.  2 ). During the coating process, the heavy tablets (or particles  27 ) within the down bed  37  exert pressure on the orifice plate  14 , particularly along the peripheral wall of the product container  12 . Owing in part to this “loading”, a region of low-flow (and in some cases, no flow) is created along the outer perimeter of the product container  12  within an outer and lower section of the down bed  37 . The dead zone  40  generally extends approximately 50 mm above the orifice plate  14  and about 50 mm out into the product container. The tablets (or particles  27 ) located within this peripheral dead zone  40  of the down bed  37  tend to slow down and even stop relative to tablets (or particles  27 ) located radially inwardly within the down bed  37 . These slower moving tablets (or particles  27 ) in the dead zone  40  fail to circulate as often as the other tablets and will therefore have an adverse effect on the consistency of coating distribution between tablets within the same batch. During the coating process, the surface properties of the tablets change, and flow behavior in most instances worsens (the flow of tablets slows down). This decrease in tablet flow tends to increase the probability that a slow or dead zone will form along the perimeter of the base of the product container. 
   Again, referring to  FIG. 2 , a second flow-related problem of the prior art Wurster machine  10  relates to a misdirected flow of particles  27 . As a pressure differential is created in the transition zone between the down bed  37  and the up bed  33  of the Wurster machine, some of the heavier particles  27  (e.g., tablets) lying close to the orifice plate  14  fail to divert upwardly into the airstream of the up bed  33  and actually impact against either nozzle  18 , or nozzle sleeve  22  (if one is used). This impact path is represented by an arrow  42  in FIG.  2  and will invariably increase attrition and breakage of the tablets. 
   A third flow-related problem of the prior art Wurster machine  10  is that a percentage of tablets  27  located within the down bed and adjacent to the partition  24  are violently and traumatically pulled into the up-bed  33  by a pressure differential, resulting in tablet attrition and breakage. This traumatic flow path is represented by an arrow  44  in FIG.  2 . 
   Another problem associated with the prior art Wurster machine becomes apparent after the coating process is complete and particles  27  must be removed from the product container  12 . The conventional process includes hinging open the lower plenum  16  (and the orifice plate  14 ) from the lower portion of the product container  12  and literally dumping the coated particles  27  from the product container  12  into an awaiting container. Not only does this crude emptying procedure expose the immediate environment (including workers) to potentially hazardous materials (such as drug residue), it also exposes the freshly coated particles and the interior surfaces of the product container and orifice plate to possible contamination. 
   Also, the conventional Wurster machine is difficult to clean, usually requiring hinging open the lower plenum  16  (as described above) and spraying a cleaning solution throughout the product container and expansion chamber, allowing the waste cleaning fluid (which is contaminated and potentially hazardous) to pour from the machine through the open lower plenum  16 . As during the above-described particle emptying procedure, this prior art cleaning process introduces potentially hazardous materials to the immediate environment which are difficult to handle and contain. 
   Finally, the prior art nozzle sleeve  22 , described in commonly owned U.S. Pat. No. 5,236,503 fails to adequately prevent particles impacting its surface, and further is prone to accumulating and trapping particles at the completion of the coating process. U.S. Pat. No. 5,236,503 is hereby incorporated by reference into this specification. 
   D. Description of the Present Invention 
   Referring now to  FIG. 3 , a Wurster machine is disclosed including improved features according to the present invention. The improved features solve the above-discussed problems of the conventional Wurster machine so that the improved Wurster machine may be used to accurately and efficiently coat heavier particles  27 , such as tablets and pills (hereinafter referred to as “tablets”  27 ). 
   1. Air-Injection Manifold 
   As discussed above in the background section of the specification, one problem inherent in the design of the conventional Wurster machine is the existence of dead zones, wherein tablets  27  become stagnant and cycle fewer times than other tablets in the same batch. According to a first feature of the invention, as shown in  FIGS. 3 ,  4 ,  5 ,  7  and  8 , an air-injection manifold  50  is provided around the lower end of the product container  12  adjacent to the orifice plate  14  and which overcomes the problems associated with the creation of dead zones in prior art coating machines. 
   As shown in  FIGS. 5 ,  7  and  8 , ring manifold  50  is circular and includes an inner surface  52  having a lower edge  54 , an upper surface  56  having a circumferential channel  58  located immediately adjacent to the inner surface  52 , a bottom surface  59 , and a plurality of openings  60  evenly spaced along the inner surface  52  immediately adjacent to the lower edge  54 . Openings  60  are directed radially inwardly towards the center of the circular manifold  50 . Each of the openings is in fluid communication with an internal circumferential conduit  62  which is shown in section in  FIGS. 3 ,  4 ,  5 , and  8 . An inlet conduit  64  ( FIG. 7 ) connects with the internal conduit  62  so that air supplied under pressure to inlet conduit  64  flows within conduit  62  and discharges evenly throughout the plurality of openings  60  around the inner surface  52 . This discharge of air flow from openings  60  results in a radially directed flow of air (airflow from selected openings  60  is represented by arrows  65  in FIG.  7 ). 
   Since manifold  50  is intended to be used in a clean environment, it is preferably made as an assembly of parts which may be selectively disassembled to access and clean all surfaces. To this end, internal conduit  62  is preferably formed by fitting a conduit ring  66  and a bottom sealing ring  68  with an outer main ring  70 , as shown in  FIGS. 3 ,  4 , and  8 . 
   According to the invention, manifold  50  is positioned between a side wall  72  of product container  12  and orifice plate  14 , as shown in  FIGS. 3 ,  4 , and  5 , and is sized and shaped so that a lower end  74  of side wall  72  snugly fits within the circumferential channel  58  and forms a smooth transition between an inner surface  75  of side wall  72  and inner surface  52  of manifold  50 . Inner surface  52  of ring manifold  50  preferably angularly aligns with the conical product container  12 . Bottom surface  59  of manifold  50  is mounted to the upper surface  26  of orifice plate  14  so that the openings  60  of manifold  50  lie immediately adjacent upper surface  26  of orifice plate  14 . With this arrangement, according to the invention, air (or any fluid, including cleaning liquids or rinse water) that is introduced under pressure into inlet conduit  64  will discharge through openings  60  in a radially inward direction across the upper surface  26  of orifice plate  14 . As illustrated in  FIG. 5 , this inwardly directed blast of air  65  from manifold  50  effectively prevents the dead zone  40  from forming by forcing all tablets  27  located in this region to move horizontally towards the transition zone and nozzle  18  (as represented by arrows  61  in FIG.  5 ). Tablets  27  are not shown in dead zone  40  of  FIG. 5  for clarity so that air flow arrow  65  can be seen and understood. According to the invention, manifold  50  keeps all of the heavy tablets moving evenly from down bed  37  to up bed  33  so that an otherwise conventional Wurster machine may be used to coat heavy tablets  27  without creating a peripheral dead zone  40  within the product container  12 . Additionally, as discussed in greater detail below, the volume and velocity of the air from openings  60  nay be adjusted to compensate for changing tablet surface flow properties during the application of coating material to the tablets. This manifold air adjustability is independent of the process air flow through the orifice plate in the up bed  33  and down bed  37  regions. 
   The effectiveness of introducing a radially directed blast of air from the periphery of the product container  12  along the upper surface  26  of the orifice plate  14  can be appreciated through the illustrations of  FIGS. 4 and 5 . Arrow  65  of  FIG. 5  represents the force of the radially inwardly directed blast of air while arrow  78  represents the “loading” or force exerted by the tablets  27  located in the down bed  37  on orifice plate  14 . The horizontal force  65  generated by the discharged air from openings  60  of manifold  50  move the lower tablets  27  of the down bed  37  radially inwardly as shown by arrow  61 , to effectively make room for other tablets  27  of down bed  37  and to keep all of the tablets  27  moving in a smooth and consistent flow from down bed  37  to up bed  33 . 
   Referring to  FIG. 4 , a representative tablet  27  moves on a radially inward path (arrow  65 ) in response to the radially inwardly directed flow of air discharged by openings  60  of manifold  50 . The horizontal air injection flow created by manifold  50  cooperates with the conventional flow of air passing through openings  30  of orifice plate  14  to maintain fluidization of tablets  27 . 
   The air pressure used to feed manifold  50  will vary depending on the size and shape of the tablets  27  being coated, their changing flow properties during a coating process, the size and shape of the particular product container  12 , the particular configuration of the orifice plate  14 , and other operational and structural parameters of the machine. The air pressure measured in one operational example was about  20  pounds per square inch (p.s.i.). 
   According to another aspect of the invention, the supplied air pressure may be controlled so that the velocity of the air discharged from openings  60  will vary at predetermined time periods during a coating procedure. Tablets  27  are typically provided with a lubricant on their surface which allows them to flow easily throughout the coating machine during the first few minutes of the coating process. As the coating is applied, however, the surface of each tablet  27  tends to become a bit tacky, resulting in a slower descent rate in the down bed  37 . By controlling the radially directed air injection (independent of the flow of air through the orifice plate) over time, the flow resistance caused by the “tackiness” of tablets  27  in the down bed  37  can be accounted for and minimized, resulting in a consistent down bed (and up bed) behavior. For most instances, during the coating process, the air flow through outlets  60  of manifold  50  may be controlled to gradually increase in velocity and volume. 
   2. Nozzle Sleeve 
   Referring to  FIGS. 3 ,  4 ,  9 , and  10 , a nozzle sleeve  90  is shown according to a second feature of the present invention. Nozzle sleeve  90  is hollow and includes a generally cylindrical base portion  92 , having a circular bottom edge  94 , a truncated conical upper portion  96  having a circular upper edge  98  (defining an upper opening  99 ), and a central hollow passage  100 . Nozzle sleeve  90  is sized and shaped to fit around nozzle  18  (see FIGS.  3  and  6 ). Passage  100  and the diameter of upper opening  99  may be sized with respect to the diameter of nozzle  18  so that air may flow freely up through passage  100  and through upper opening  99 , adjacent to nozzle  18  during the operation of the machine, as described below. By directing air through the passage  100  of nozzle sleeve  90  in this manner, the discharge of air through upper opening  99  adjacent to nozzle  18  may assist in shaping and controlling the shape and characteristics of the spray pattern generated by the nozzle. Alternatively, the upper opening  99  may be sized to tightly receive nozzle  18  so that no air (or minimal air) will pass through nozzle sleeve  90  during the operation of the machine. Nozzle sleeve  90  is secured in position with bottom edge  94  abutting against orifice plate  14 . 
   Once in position around nozzle  18  within product container  12 , and during the coating operation, nozzle sleeve  90  serves three functions. First, nozzle sleeve  90  protects tablets  27  from directly impacting the harder surfaces of nozzle  18  during operation, as described below. Second, conical upper portion  96  is shaped to accommodate the natural flow of tablets  27  as they are drawn into the up bed  33  from the down bed  37 , as shown in FIG.  3 . Third, the hollow passage  100  and the conical upper portion  96  direct air from the lower plenum  16  to assist in shaping the up bed  33  and the coating zone  32 . 
   Nozzle sleeve  90  is preferably made from a strong, somewhat resilient plastic, such as a PTFE or Delrin, or an appropriate rubber, such as silicone, and is preferably adapted to be easily installed within a coating machine and quickly and easily replaced to minimize setup time. The particular dimensions and shape of nozzle sleeve  90  may vary according to particular parameters of the coating machine, as is understood by those skilled in the art. The nozzle sleeve may be separate or integrated into the nozzle ramp, and may be solid or perforated to permit air flow in proximity to the spray nozzle. 
   3. Nozzle Ramp 
   As mentioned above, a problem with the conventional Wurster coating machine is that heavier tablets  27  are traumatized during their transition from the down-bed  37  of the product container  12  to the central up-bed  33  through the partition  24 . The heavier tablets used in a conventional Wurster machine may also be damaged by impacting the nozzle assembly during bed transition. The up bed  33  moves much faster than the peripheral down bed  37  within the product container and a strong negative pressure is developed around the center of the orifice plate and within part of the partition. As discussed above, this negative pressure rapidly draws tablets  27  from the peripheral down bed  37  radially inwardly along a horizontal path into the airstream of the up bed  33 . Owing to the mass of the tablets  27 , the horizontal component of the inertia imparted to the tablets  27  by the negative pressure is often too great for the upwardly moving airstream of the up bed  33  to completely vertically redirect the horizontally moving tablets  27  before some of the tablets  27  impact the centrally located nozzle assembly  18 , nozzle sleeve  90  (if one is used) and/or other tablets  27  entering from opposing directions along the orifice plate  14 . 
   Referring to  FIGS. 4 ,  6 ,  11 , and  12 , a nozzle ramp  102  is shown, according to a second feature of the present invention, which overcomes the above-mentioned problem of tablets  27  impacting nozzle  18  as they enter into the up bed  33 . Nozzle ramp  102  includes a circular base  104  having a perimeter  106 , a hollow cylindrical center  108  having a side wall  110  and a top edge  112 , and an arcuate ramp surface  114  (having a shape that is similar to a cusp) positioned between perimeter  106  of base  104  and top edge  112 . As shown in  FIG. 3 , nozzle ramp  102  is centrally positioned within product container  12  with its base  104  mounted flush against upper surface  26  of orifice plate  14  (using bolts, for example). Nozzle ramp  102  may be used within the product container with or without a nozzle sleeve. Should a nozzle sleeve be used, hollow center  108  is preferably sized and shaped to accommodate both nozzle  18  and nozzle sleeve  90 , described above, or alternatively, a conventionally shaped nozzle sleeve  22 , such as the one shown and described in U.S. Pat. No. 5,236,503. Further, nozzle ramp  102  may be formed with an integral nozzle sleeve (not shown), however, it is preferred that nozzle sleeve remain as a separate and attachable part so that tablet and air flow characteristics can be better controlled. 
   Nozzle ramp  102  further includes a plurality of vertically disposed passages  116  which are preferably arranged in concentric rings passing between the arcuate ramp surface  114  and circular base  104 , as shown in  FIG. 11  (in section). These vertical passages  116  are sized and shaped to exactly align with corresponding openings  30  of a conventional orifice plate  14 , which is shown in  FIGS. 18 and 19 , so that air passing through openings  30  from lower plenum  16  freely passes through the aligned vertical passages  116  of nozzle ramp  102  and becomes discharged at an upper end of each respective passage  116  along arcuate ramp surface  114 . Passages  116  are preferably either equal to or larger than the corresponding openings  30  of orifice plate  14 . Orifice openings  30  are sized to control the flow of air within passages  116 . 
   The purpose of the nozzle ramp  102  is to direct air from the lower plenum  16  along a curved circular ramp surface so that the fast moving, horizontally driven tablets  27  can be atraumatically coerced to follow a vertical trajectory using a cushion of air. As discussed below, the use of nozzle ramp  102  will minimize undesirable impacting of tablets  27  against nozzle  18  or nozzle sleeve  90 . The conical upper portion  96  of nozzle sleeve  90 , according to the above described feature of the present invention, preferably generally aligns with arcuate ramp surface  114  of nozzle ramp  102 , as shown in  FIGS. 3 and 4  so that tablets  27  being diverted to the up bed by the nozzle ramp  102  may follow a less severe arcing path (as shown as arrow  118  in  FIG. 4 ) and still avoid impacting any portion of nozzle sleeve  90 . 
   4. Partition Skirt 
   The tablets  27  located in the down bed  37  ( FIG. 1 ) of a Wurster machine generally move downwardly at a rate of one meter in approximately 10 to 30 seconds. When these tablets  27  are drawn into the up bed  33 , where they accelerate to approximately 5 to 10 meters per second, they encounter an atomizing air velocity of about 300 meters per second. This violent change in velocity causes great transitional trauma and high shear to the relatively fragile tablets  27  and will likely increase the attrition rate of the tablets of the batch early in the coating process. The relatively sharp lower edge  25  of the conventional partition  24  which separates the down bed  37  and the up bed  33  only exacerbates the transitional trauma to the tablets  27  as they are drawn into the fast moving upward current of the up bed. 
   Referring to  FIGS. 3 ,  4 ,  13 ,  14 , and  15 , a partition skirt  120  according to another feature of the present invention is provided at the lower edge  25  of partition  24  which overcomes the problems relating to transitional-trauma of the tablets  27  entering the high velocity up bed  33 . Skirt  120  is a ring-shaped cone having a angled outer surface  122  at angle A (FIG.  13 ), a cylindrical inner bore  124 , and a lower surface  126 . Bore  124  is sized and shaped to snugly receive partition  24  so that skirt  120  may be secured to the lower end of partition  24  (preferably in a manner that allows skirt  120  to be quickly and easily removed from partition  24  if necessary). The diameter of bore  124  is preferably equal to or slightly larger than the outside diameter of partition  24 . 
   Lower surface  126  of skirt  120  is preferably beveled at a prescribed angle B, as shown in  FIG. 13 , forming an inwardly directed funnel shape which extends between angled surface  122  and bore  124 . Lower surface  126  functions as an “on-ramp” allowing tablets  27  adjacent to skirt  120  to “get up to speed” before entering the high velocity up bed  33 . The exact degree of angle B of the lower surface  126  will vary depending on the size, shape and weight of the tablets  27 , the dimensions of the product container  12 , coating characteristics, as well as other structural and operational factors and parameters, but angle B will generally be in the range of 15 and 30 degrees. Lower surface  126  preferably includes a rounded outer edge  129 , as shown in FIG.  15 . 
   Tablets  27  located in down bed  37 , in particular adjacent to the partition  24 , will be directed away from partition  24  as they descend down bed  37  by angled surface  122  until they reach lower surface  126  at which point tablets  27  will gradually pick up speed due to the pressure differential created by the up bed  33  and move inwardly along lower surface  126 . As tablets  27  move inwardly along lower surface  126 , they will gradually accelerate before “falling upward” into the high velocity up bed  33 . Lower surface  126  allows tablets  27  to gain speed and thereby reduces the effects of shear and other mechanical stresses created by the gradient between down bed  37  and up bed  33 . This less traumatic introductory path into up bed  33  is represented by the arrow  127  in FIG.  4 . 
   Skirt  120  is preferably made from a strong resilient plastic, such as PTFE or Delrin, but may be made from any appropriate material including other plastics, rubber, and metal, such as stainless steel. To simplify the manufacturing of skirt  120  and to introduce versatility, skirt  120  may be made from two pieces, as shown in  FIG. 13  including a lower ramp ring  128  and an upper conical sleeve  130 . Ramp ring  128  and conical sleeve  130  may be formed separately (milled or molded) and thereafter secured along a mating surface  132  to each other using any appropriate bonding, fastening or welding technique, as understood by those skilled in the art. In one embodiment, the two pieces making up skirt  120  are attached in an easily removable manner so that one of many conical sleeves  130  having a particular angle A may be fitted with one of many ramp rings  128  having a particular angle B. Although it is preferred that skirt  120  be provided as a part to be attached to the lower portion of partition  24 , it is also contemplated that skirt  120  and partition  24  be made integrally as a single piece. 
   The purpose of skirt  120  is to provide a somewhat horizontal surface (lower surface  126 ) on which particles  27  may gradually and atraumatically accelerate in the up bed  33 . The lower surface  126  is formed between the wall of partition  24  and the angled surface  122 , as shown in FIG.  4 . Some prior art Wurster machines use a partition that includes an outwardly flared lower end. This flared lower end does not define or otherwise establish an angled surface  122  (or any horizontal surface between the down bed and the up bed). The purpose of the prior art flared lower end is to allow particles to be drawn into the up bed without “crowding” the nozzle and disrupting the spray pattern. Although the flared lower end of the prior art partition forces particles located in the down bed outwardly towards the peripheral wall of the product container, the particles are still traumatically drawn into the up bed because there is no angled (or generally horizontal) surface  122 , as in the present invention. 
   As described above and according to the invention, skirt  120  diverts tablets  27  located in the down bed  37  away from lower edge  25  of partition  24  and provides an inclined ramp (angled lower surface  126 ) so the adjacent tablets  27  are not harshly and traumatically drawn into the up bed  33 . Skirt  120  also helps channel the “loading” of down bed on orifice plate  14  outwardly near the periphery of the end product container  12 . By doing this, a larger transition zone is created. Tablets  27  located under skirt  120  are more easily suspended by the air flowing through orifice plate  14  from lower plenum  16  because there is less or no downward force exerted on them by tablets  27  located higher in the down bed  37 . The result is that tablets  27  move more easily and less traumatically from the down bed  37 , through the transition zone, and into the up bed. To maximize this effect, it is preferred that the distance between the outermost point of skirt  120  and the wall of the product container  12  (represented by arrow C in  FIG. 4 ) be approximately equal to the distance between the lowermost point of skirt  120  and the upper surface  26  of orifice plate  14  (represented by arrow D in FIG.  4 ). 
   The particular dimensions and shapes of all the above-described components of the improved machine have a mathematical relationship wherein the particular parameters of one component are related and determined by the particular parameters of another component, and the characteristics of the particles being coated, and the desired coating results. Finite analysis techniques may be used to establish the relationship between the components. 
   5. Central Discharge 
   A rate limiting factor in the productivity of the Wurster process is the relatively narrow coating zone which, in turn, restricts the diameter of the partition  24  to about nine inches. To increase the efficiency and the productivity of a Wurster machine, the size of the product container may be increased if multiple partitions and nozzles are used. For instance, to operate efficiently, a Wurster machine having an eighteen inch diameter product container uses a nine inch diameter partition. However, a thirty two inch Wurster may require three, nine inch diameter partitions spaced evenly within the product container, and a forty six inch Wurster may include six or seven, nine inch diameter partitions. 
   Referring to  FIGS. 18 ,  19 ,  20  and  21 , an improved product discharge assembly  140  of the present invention is shown, suitable for efficiently removing tablets  27  (or particles, powders, granules, pellets, or grains) in a sealed and controlled manner from a product container  12  of the type having multiple partitions  24  and nozzles  18 . An exemplary coating machine  142  having three nozzles  18  and three partitions  24  is shown and described herein to explain the structure and operation of discharge assembly  140 , according to the invention. The discharge assembly  140 , according to the invention, may be used with any multi-partition/nozzle Wurster machine or with product containers in conventional fluidized bed drying or spray granulating equipment. The above-described features of the invention including the nozzle ramp., the lower skirt assembly, and the air-discharge manifold are not shown in  FIGS. 18 and 19  for clarity. Any and all of the features described in this specification may be used in any combination in a coating or drying machine. 
   Referring to  FIG. 18 , partition  24  may include an outwardly flared upper end  145 , as shown, having a shape which allows particles  27  to quickly exit up bed  33  (of partition  24 ) and enter down bed  37  without substantially impacting the side wall of partition  24 . The specific shape of the flare is preferably cusp shaped, but flared upper end  145  may alternatively be conical in shape. Flared upper end  145  allows particles  27  to disperse from partition  24  without particle attrition or breakage and encouraging a smooth transition of the particles from the up bed  33  to the down bed  37 . Flared upper end  145  may be used in single partition machines, described above, or multi-partition machines, (only one of the partitions  27  of  FIGS. 18 and 19  is shown with a flared upper end  145  to illustrate the flared feature). Also, the upper end of partition  24  may include a resilient or impact absorbent material, such as a rubber or suitable plastic to help minimize particle or tablet attrition. The absorbent material (not shown in the figures) may be in the form of a coated layer or an attachable sleeve or layer. 
   For these larger coating machines, once a coating process is complete for a particular batch of tablets  27 , the tablets are typically removed by opening a pivotal lower end of the machine and literally dumping the contents of the product container  12  into an awaiting container (not shown). As described above, this prior art process for removing coated tablets  27  may easily introduce contamination to both the tablets and the interior portions of the machine  10 , as well as expose workers to potentially hazardous materials. 
   The improved machine  142 , shown in  FIGS. 18 and 19  includes a discharge conduit  144  which extends from an orifice plate  146  to an accessible location remote from the machine  142 . Orifice plate  146  is similar to the above-described orifice plate  14 , except that it is designed for three nozzles  18  positioned 120° apart from each other and therefore includes three large openings (not shown). According to this feature of the invention and referring to  FIGS. 18-21 , orifice plate  146  further includes a central discharge opening  150  which is sized and shaped to accommodate discharge conduit  144 , as shown in  FIGS. 20 and 21 . A conduit cover  152  is movably fitted above orifice plate  146  in alignment with central discharge opening  150 . The conical cover  152  is oriented with its apex directed upward and is movable between two positions, a sealed position (shown in  FIG. 20 ) and an open position (shown in FIG.  21 ). A linear actuator  154  is connected to conical cover  152  by one or more armatures  155  (only one armature  155  is shown in the figures) so that when activated, actuator  154  linearly displaces armature  155  which, in turn, displaces conical cover  152  between the sealed position ( FIG. 20 ) and the open position (FIG.  21 ). Actuator  154  may be any appropriate type, such as an electromagnetic actuator (e.g., a solenoid), a pneumatically driven cylinder, an hydraulically driven ram device, or a mechanically operated device such as a system of cables and/or levers (not shown). The purpose of actuator  154  is to open or close conduit cover  152 , as desired, and as further discussed below. 
   When conduit cover  152  is closed, as shown in  FIG. 20 , tablets  27  remain sealed within product container  12  and, if coating machine  142  is operating, tablets  27  will circulate in a manner similar to the earlier described single partition coating machine  10  (represented by arrows  153  of FIG.  20 ), without being obstructed or otherwise affected by conduit cover  152 . Conical cover  152  will function as a divider, evenly directing the tablets  27  of a central common down bed to each of the three nozzles  18 . 
   When the coating process is complete (or it is otherwise desired to remove tablets  27  from product container  12 ), actuator  154  is activated causing conduit cover  152  to be vertically displaced above orifice plate  146 , as shown in  FIG. 21 , forming a gap  156  between the perimeter of discharge opening  150  and the perimeter of conduit cover  152 . The opening of conical cover  152  exposes an open end of discharge conduit  144  which causes tablets  27  to be drawn into discharge conduit  144 , as represented by arrows  158  of  FIG. 21 , as they continue to circulate within the coating machine  142  between the down bed and the up bed, as described above in connection with earlier features of the invention. Tablets  27  are preferably drawn into discharge conduit  144  using a pressure differential between the product container  12  and the discharge conduit (i.e., by creating a vacuum within discharge conduit  144 ). The tablets  27  drawn into discharge conduit  144  (represented by arrow  160  of  FIG. 21 ) exit product container  12  and may be collected in an awaiting container (not shown), while remaining in a sealed and controllable environment. 
   The central discharge assembly described above and shown in  FIGS. 18-21 , is intended to be used only for larger coating machines which require three or more nozzles  18  and partitions  24 . The central discharge assembly is preferably located in the center of the product container  12 , but could be located elsewhere along orifice plate  146 . The central discharge feature of the invention may be used alone or in combination with any of the other features of this invention and further with any conventional coating machine, or other type of tablet-processing/handling machine including fluidized bed granulating and/or drying machines. The central discharge assembly is preferably used in combination with the above-described compressed air manifold which would assist in forcing product located along the periphery of the product container  12  inwardly towards the central discharge assembly. This assistance to the tablets is particularly useful near the end of the discharge process when few tablets remain in the product container and the fluidization air has been reduced to a minimum or stopped completely. The central discharge system is not limited to use with tablets, but is equally effective with smaller substrates such as pellets, granules, crystals or powders. 
   6. Cleaning 
   After a predetermined period of cycle time, the interior surfaces of the multi-partition coating machine  142  described above and shown in  FIGS. 18-21  must be cleaned using a cleaning fluid. As described above, it is known to position spray nozzles within the expansion chamber and product container and apply a cleaning fluid along most of the interior wall surfaces, rinsing drug residue (or other materials) and other contaminants down towards the orifice plate  14 . However, much of the drug residue includes relatively large particles which are too large to pass through the openings  30  of orifice plate  14  or through a fine screen (not shown), if one is used in combination with the orifice plate  30 . These large particles wash down the wall surfaces of the product container and become deposited onto upper surface  26  of the orifice plate  14  or fine screen, typically in the dead zone  40  (see  FIG. 2  PRIOR ART), while the waste cleaning fluid passes through the fine screen and the lower plenum  16  as it drains. 
   In accordance with another feature of the present invention, referring to  FIGS. 3 ,  7 ,  8  and  18 - 21 , during the cleaning process, cleaning fluid may be injected under high pressure through inlet conduit  64  and inner conduit  62  so that the fluid discharges through openings  60  and across orifice plate  14  (or across a fine screen, not shown, which may be placed on top of orifice plate  14 ). The radially discharged cleaning fluid will force any of the large particles deposited on orifice plate  14  to also move radially inwardly towards the central discharge assembly. According to the invention, conical cover  152  is moved to its open position (as shown in  FIG. 21 ) during the cleaning process so that the now exposed central discharge conduit  144  may serve as a central drain for any of the larger particles unable to pass through the openings  30  of the orifice plate. 
   While the use of the compressed air manifold in combination with the central discharge is described for use with a Wurster tablet coating machine, these components may also be installed and used in the same manner for removing powders, granules, and/or coated particles from conventional fluidized bed drying and/or spraying granulating equipment. Also, each of the above-described improvements of this invention may be used alone or in any combination with each other in any type of vertical-spray fluidized bed granulating machines or drying granulating machines, if appropriate. 
   7. Split Plenum Arrangement 
   According to another embodiment of the invention, referring to  FIG. 22 , a Wuster machine  10  is shown similar to the one shown in FIG.  3  and described above, however the lower plenum  16  now includes a slit-plenum arrangement. The split plenum arrangement includes a central conduit  170  and a peripheral conduit  172 . The central conduit  170  extends from the lower surface of orifice plate  14  and is sized and shaped to generally direct air or gas upwardly through nozzle sleeve  90 , nozzle ramp  102  (through passages  116 ) to define the up-bed flow of air in the product container  12 . The central conduit  170  is connected to a source of air flow (pressurized gas or appropriate fan) not shown, and further includes a metering system  174  for measuring the speed, pressure, volume, humidity, and/or temperature of the passing central up-bed air flow. The metering system  174  is connected to the source of air flow (not shown) so that an air flow having desired flow characteristics can be achieved and maintained using conventional feedback controlling subsystems, for example. 
   The peripheral conduit  172  is sized and shaped to supply air flow through the remaining exposed portion (everything around the central conduit) of the orifice plate  14 , thereby controlling the fluidized bed characteristics of the down-bed and the transition bed of the product container. Similar to the central conduit  170 , the peripheral conduit  172  is connected to a dedicated source of air flow (not shown, and similarly includes a metering system  176  for measuring the speed, pressure, volume, humidity, and/or temperature of the passing peripheral air flow. The metering system  176  may be similar to the central air-flow metering system  174  and may similarly be used as a feedback controlling system to maintain air-flow having desired preset characteristics. 
   By separating the flow through the lower plenum  16  into central and peripheral regions, up-bed, down-bed, and transitional-bed flow and fluidization characteristics may be more accurately and independently controlled.