Agitating – With heating or cooling
Reexamination Certificate
2001-05-04
2003-04-08
Cooley, Charles E. (Department: 1723)
Agitating
With heating or cooling
C366S224000, C366S273000
Reexamination Certificate
active
06543928
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a processing vessel and a method of mixing powders with a magnetically coupled agitator mounted inside of the processing vessel. More particularly the invention is directed toward a rotating processing vessel and a method of using a magnetically coupled agitator to mix dry or moist powders inside of the rotating powder processing vessel. Although the method of the present invention has many different applications, it is described herein primarily as used for mixing dry powders inside of a tumble blender.
2. Description of the Related Art
Rotating tumble blenders are frequently used to mix dry chemical compounds and other ingredients for the pharmaceutical, food, cosmetic and other industries. Because of the nature of these highly-regulated industries, a sterile mixing environment or an environment which is free from cross-contamination is oftentimes required during the mixing process. Besides operating under ambient pressure conditions, for some applications, the blender operates under pressures greater than atmospheric. For other applications, the blender operates under less than atmospheric pressure. To satisfy all of these applications, tumble blenders must be constructed and operated to both prevent contaminants from entering the vessel, such as when the vessel is operating at less than atmospheric pressure during a mixing process, as well as prevent the vessel's contents from escaping to the environment outside the vessel walls such as when the vessel is operated under conditions greater than atmospheric pressure.
In some applications a tumble blender is outfitted with a rotating agitator designed to enhance and/or accelerate the mixing of the contents of the blender. In these instances the tumble blender typically includes a drive shaft that couples a mixing agitator (impeller) located within the interior of the vessel to a motor located outside of the blender vessel. To effect such an arrangement where the drive shaft penetrates the vessel wall, the vessel oftentimes contains mechanical seals and/or a packing arrangement located between the drive shaft and the vessel wall. These seals and packing are designed to prevent the vessel contents from migrating along the drive shaft into the bearings and ultimately out of the vessel particularly when the interior of the vessel is operating under greater than ambient pressure conditions. Additionally, the seals and packing prevent any outside contaminants from entering the vessel along the same route particularly when the interior of the vessel is operating under negative pressure conditions (less than atmospheric pressure). Such seals and packing arrangements are undesirable for the mixing applications referenced above because they are susceptible to failure, especially under pressurized or negative pressure conditions. Additionally, such seals, packing and bearing failures when the powders migrate past the seals add to the blender's maintenance costs because they are difficult to clean and replace. Cleaning and replacement is required to prevent product which becomes entrained in the seals and packing during one mixing application from cross-contaminating a second, different, product during a subsequent mixing application within the same vessel.
In a typical dry mixing application, when an agitator rotates inside of the mixing vessel, work energy is added to the dry powders situated therein, creating heat. In some instances the heat energy increases the temperature of the air inside of the vessel causing a buildup of pressure. If there is no mechanism, such as an atmospheric vent or pressure relief valve, to allow the pressure to dissipate from within the vessel to the surrounding atmosphere, internal pressure within the vessel forces the powders back along the agitator shaft inside of the vessel. This causes the bearings, mechanical seals and/or packings to fail prematurely resulting in the escape of powders from the vessel to the outside environment.
In those instances where the vessel is vented to the atmosphere to alleviate any pressure build-up, filters or filter cloths are typically placed over the vents in an attempt to prevent the powders inside of the vessel from escaping. Inevitably, however, some powders do pass through these filter arrangements thus creating an environment detrimental to worker health and safety particularly where the powders are toxic or reactive. Furthermore, after a period of use the filters often become blinded by the powder. This blinding effectively seals the vent and allows pressure to build-up within the vessel. This increased pressure causes the mechanical seals and/or packings to fail prematurely. The use of pressure relief valves in these applications presents similar difficulties as they too often become clogged by the powder.
To eliminate these problems when mixing dry or moist powders in a rotating tumble blender, the processing vessel and method of the present invention uses a magnetic coupler to couple the mixing agitator on the interior of the blender to a motor located outside of the blending vessel. The magnetic coupler comprises on the outside of the vessel walls a magnet (the “drive” magnet) attached to a shaft which is rotated by a motor and, on the interior of the vessel, another magnet (the “driven” magnet) connected to the agitator. In this manner the drive shaft does not penetrate the vessel walls and the need for seals and/or packing in the vessel walls is eliminated. The drive and driven magnets are assembled close together, although they are on opposite sides of the blender vessel wall, so that the rotation of the drive magnet rotates the driven magnet and hence the agitator. This magnetic coupling arrangement advantageously allows the mixing agitator inside the blender to be rotated by a motor outside of the blender without mechanically connecting the two members. Therefore, in the processing vessel and method of the present invention, pressurized or negative pressure conditions inside of the mixing vessel no longer present a problem for maintaining a sterile mixing environment since the drive shaft does not penetrate the vessel's walls. Since the mechanical seals and/or packing associated with a traditional agitator application are eliminated, a conduit for cross-contamination between the interior and exterior of the vessel along the drive shaft is eliminated. Additionally, the need for an atmospheric vent and any associated filter or filter cloth is also eliminated.
Magnetic couplings in general are well known in the prior art for mixing or pumping liquids. Typically, such magnetic couplings comprise a pair of axially or radially opposed magnets, or sets of magnets, formed from a magnetic material. One of the magnets is coupled to a driving member such as a shaft from a motor, and the other magnet is coupled to a driven member such as a pump impeller or agitator. The magnets are magnetically coupled to each other so that rotation of the driving member causes a corresponding rotation of the driven member to obtain the desired torque output. Couplings of this type are particularly advantageous, as described above, when it is desirable for an impermeable barrier to be interposed between the driving and driven members such as in stirred reactors, autoclaves, centrifugal pumps and the like. In such applications, the barrier assures against passage or leakage of any process fluid being mixed between the driving and driven members, and thereby prolongs the operating life of the equipment. For examples of prior art mixers, stirrers and pumps employing magnetic couplings in liquid applications see U.S. Pat. Nos. 2,495,895; 2,556,854; 2,711,306; 2,996,393; 4,207,485; 4,247,792; 4,277,707; 4,534,656; 5,292,284; 5,407,272; 5,470,152 and 5,533,803.
During operation, a magnetic coupling may generate substantial quantities of heat due to relative slippage of the magnets at excessive torque loads, induction heating effects, and the like. This is particularly true with cl
Cooley Charles E.
General Machine Company of New Jersey, Inc.
Norris & McLaughlin & Marcus
Sorkin David
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