Agitating – Stirrer within stationary mixing chamber – Rotatable stirrer
Reexamination Certificate
2001-03-07
2002-10-29
Soohoo, Tony G. (Department: 1723)
Agitating
Stirrer within stationary mixing chamber
Rotatable stirrer
Reexamination Certificate
active
06471394
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to apparatus and method for industrial mixing and processing and particularly to horizontally arranged industrial mixers having horizontally disposed mixing shafts extending through the mixing chamber.
BACKGROUND OF THE INVENTION
Processing of a large variety of consumer and industrial products, such as food, plastic, pharmaceutical and chemical products, for example, usually involves one or more mixing steps for mixing the various component materials of the products. Such mixing steps are oftentimes accompanied by the simultaneous introduction or removal of heat, such as for drying the material being mixed or cooling heated products while they are mixed. Product mixing is also often accompanied by granulation or chopping of the material forming the product. Often, for numerous products, the materials are mixed in a dry, powdered or granular form, and the mixing process is referred to generally as solids mixing.
For accomplishing such solids mixing, large-capacity industrial mixers are utilized, which are able to handle very large loads of material for efficient and cost-effective mixing. One type of mixer design, which is suitable for solids mixing and is able to effectively mix large loads of material, is a horizontal mixer. Horizontal mixers have an elongated mixing chamber which is disposed generally horizontally with respect to the ground surface on which the mixer rests. More specifically, horizontal mixers generally comprise an elongated cylindrical mixing chamber, and an elongated, horizontal mixing shaft which extends through the chamber and rotates. A plurality of mixing tools depend generally perpendicularly from the horizontal shaft and rotate around the inside of the chamber when the shaft is rotated. The mixing tools are configured and dimensioned as required for the mixing process to follow the cylindrical inside walls of the chamber for proper mixing of all of the material in the chamber.
In a conventional horizontal mixer, the elongated horizontal mixing shaft extends out of the mixing chamber at both ends of the chamber through appropriate openings in the chamber end walls or head walls. At one end of the shaft, referred to as the drive end, the shaft is operably coupled to a drive motor and gearing which rotates the shaft. At the drive end, the shaft is coupled through a bearing structure located between the drive motor and the chamber. The bearing structure provides support of the shaft drive end and also ensures smooth rotation. A separate seal structure is then located further in along the length of the shaft and interfaces with the opening in the end wall through which the drive end of the mixing shaft extends.
The other end of the mixing shaft, referred to as the stub end of the shaft, is not driven, but rather rotates with the drive shaft. The stub end of the mixer also includes a seal structure to seal the stub end of the shaft and the end wall opening through which it extends. The seal structures at the ends of the shaft isolate the mixing chamber environment from the outside environment and generally prevent the passage or leakage of material into and out of the mixing chamber. The seal structures used in such horizontal mixers are therefore important to ensure the integrity and purity of the material being mixed and are also necessary for preventing leaks and protecting the health of workers in the area of the mixers.
As may be appreciated, leakage between the horizontal mixing chamber and the outside environment and atmosphere during mixing is undesirable. For example, edible products such as pharmaceuticals and foods must not be contaminated with foreign materials which may leak into the mixing chamber at the shaft end openings. Grease or oil associated with the drive motor and/or the shaft bearings must also be kept out of the mixing chamber. Furthermore, it is also equally important to contain the mixed material in the chamber and to prevent it from migrating and leaking to the outside environment through the shaft end openings of the mixer. This is particularly so if the material being mixed is a harmful chemical which cannot be directly contacted by the skin or if the mixed material produces a harmful vapor which may be released through the shaft openings.
Still further, it may be necessary to maintain a pressure differential between the horizontal mixing chamber and the outside environment to achieve proper mixing. For example, some mixing procedures require elevated pressures within the chamber which may be compromised by a leak. Furthermore, under such circumstances, a leak will tend to force mixed material through openings in the mixing chamber, such as out through the shaft end seals. Other procedures require that a vacuum be drawn in the chamber which would also be compromised by a leak. Also, a leak would draw contaminants into the chamber through the shaft end openings and seals.
While the seal structures of existing horizontal mixers operate somewhat adequately for their intended purpose, they have several drawbacks. More specifically, the seal structures at the stub end of the mixing shaft can be of particular concern, due to their location in the mixer.
First, many existing seal structures, including the stub end seal structures, are expensive. Because the rotating drive and stub ends of the mixing shaft extend through the end walls of the mixer, the seal structures must be dynamic seals which can handle both rotation and translation of the shaft, while still maintaining the seal. The seal structures, for example, may include elaborate dynamic seals with braided packing elements that surround the rotating shaft or may include expensive mechanical seals. The packing elements of certain dynamic seals are constantly worn by the rotation and linear movement of the shaft and thus are prone to wear and leakage. Therefore, constant maintenance and replacement of the dynamic seals are necessary. Some such seal structures must be coupled to an air line for preventing migration or leakage of the mixed material out of the chamber or the leakage of contaminants into the chamber. Mechanical seals, on the other hand, have highly polished faces which spin against each other under pressure. Such mechanical seals require precise, and therefore expensive, machining and polishing for proper operation and are also subject to wear and leakage. The complicated and intricate seal structures conventionally used for horizontal mixers are therefore expensive, not only to manufacture, but also to maintain and replace.
Secondly, the stub end seal structure is particularly prone to failure and leakage because of its position in the mixer. Therefore, the stub end seal and bearing structures must be maintained and replaced more frequently than the drive end seal structures. More specifically, the stub end of the shaft not only rotates during use, but also translates linearly in a longitudinal direction along the longitudinal, horizontal axis of the shaft. Since the drive end of the shaft is somewhat fixed due to the drive motor and other associated components, the longitudinal translation of the shaft caused by expansion and contraction of the shaft occurs primarily at the stub end. The shaft expands and contracts in length due to temperature changes during the mixing process. Constant exposure of the shaft to the variations in temperature caused by the heating and cooling of the mixing chamber and the heat generated by the mixing process causes some expansion and linear translation. Furthermore, the shaft itself may be actually heated or cooled such as by introducing steam, water or oil into a cavity in the shaft. Still further, the end walls or end plates of the mixing chamber will also move in and out longitudinally with respect to the shaft due to the temperature variations of the mixing chamber itself. Therefore, the seal structure and the packing elements at the stub end of the mixing shaft are exposed not only to rotational wear but also to significant translational wear, thereby making the stub end
Littleford Day, Incorporated
Soohoo Tony G.
Wood Herron & Evans LLP
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