60/262,011, filed January 17,2001.

Field of the Invention [0002] The present invention generally relates to the heating of granules and more particularly to heating granules as they travel through the interior of a rotating furnace by transfer of magnetically induced heat in the material of the furnace or a separate susceptor.

Background of the Invention [0003] Granules encompass a broad range of particulate materials that include powders of varying grades, and may exhibit magnetic or non-magnetic properties. Ferromagnetic powders, such as a steel powder, can be used to form an article in any shape by the application of high temperature and pressure to the powder in a mold. The metallurgical properties of the powder will effect how well the article is formed. In general, a"soft" textured powder is desirable for forming the article.

[0004] A conventional method of forming a metal powder is by passing a chilled air or water stream across a flow of molten metal. The chilled fluid freezes the metal into granules of powdered metal. The process produces a metallurgically hard powder that would be very difficult to compress into a finished form. To soften the powder, as illustrated in FIG. 1, the powder 100 is transferred by feeder 112 to a fine-mesh steel conveyor belt 114 that transports the metal powder through a tunnel furnace 116. Gas or electric radiant tube heaters 118 radiate heat in the furnace to heat the powder being conveyed through the furnace. The furnace heats the powder and produces a soft, annealed metal powder. The conventional furnace has a tendency to unevenly heat the granules of metal powder and the gas or electric radiant heat source is a relatively inefficient method of heating the powder.

[0005] Therefore, there exists the need for apparatus and a method that will efficiently and more evenly heat granules of a material.

Brief Summary of the Invention [0006] In one aspect the present invention is an apparatus and a method for heating granules by application of induction power to a rotating furnace body that uses internal conveying means to move the granules through the furnace chamber. These and other aspects of the invention are set forth in the specification and claims.

Brief Description of the Drawings [0007] For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

[0008] FIG. 1 is a cross sectional view of a conventional prior art tunnel furnace that is used to heat granules.

[0009] FIG. 2 is a cross sectional view of one example of the furnace of the present invention for heating granules.

[0010] FIG. 3 is a cross sectional view of another example of the furnace of the present invention for heating granules.

[0011] FIG. 4 is a cross sectional view of another example of the furnace of the present invention for heating granules.

Detailed Description of the Invention [0012] There is shown in FIG. 2 a first example of the induction furnace 10 of the present invention for heating granules. Granules include a range of particles, including what are known as powders used in powder metallurgical processes. In these processes, metal powders, or granules, after heating by the apparatus and method of the present invention, are compacted in a die to a desired shape, and then sintered or heated in a furnace to produce an article. Furnace body 12 is formed substantially from a suitable electrically conductive material. For some applications, a ferromagnetic material, such

as steel, can be used. As shown in this example of the invention, the furnace body is generally cylindrical in shape and forms a tubular structure. Variants of this configuration are suitable for use of the invention, provided that the configuration allows advancing of the granules through the furnace by conveying element 14 as the furnace rotates. The interior passage of the tubular structure forms the heating chamber.

Conveying element 14 is inserted inside of the interior passage. The function of the conveying element is to advance the granules 24 through the furnace in the direction of the arrow as the furnace is rotated. Rotation is generally about the longitudinal axis of the furnace chamber, but off-axis, for example, ellipsoidal rather than circular rotation is contemplated within the scope of the invention. Conveying element 14 can be a continuous and screened helical structure that runs the length of the furnace chamber, but other structures are contemplated as being within the scope of the invention, so long as the conveying element functions to advance the granules through the furnace. For example, conveying element 14 may consist of a series of discrete elements rising from the interior wall of the chamber arranged in a manner to advance the granules through the furnace. Conveying element 14 is formed from a suitable high temperature material, such as steel, and may be inserted and fastened to the wall of the interior passage, or fabricated as an integral feature of the furnace chamber. In this example of the invention, suitable thermal insulating material, or refractory material 16, such as a silicon composition, surrounds furnace body 12 and serves to retain heat within the furnace body and to electrically isolate the furnace body from induction coil 18.

Induction coil 18 surrounds the outer wall of the furnace body and the refractory material. The coil is suitably connected to ac power source 20. Current supplied from the power source flows through the coil and produces a magnetic field that induces eddy current heating in the electrically conductive material that comprises the furnace body.

[0013] In operation, feeding element 22 delivers granules 24 into the furnace chamber.

Furnace body 12 is rotated by conventional rotational drive means (not shown in the figures) at a relatively slow rate that is determined by the length of the furnace chamber and desired degree of heat"soaking"of the granules at the temperature inside of the chamber. As the furnace body rotates, granules are advanced through the furnace by helical conveying element 14. Preferably, the height of granules along the length of the furnace is approximately equal to the height of the screened conveying element so that

the granules are sifted and remain loosely packed as they travel through the furnace to achieve a uniform heating effect. Inductive heating is controlled by a conventional temperature feedback circuit in the chamber to maintain a chamber temperature that, in some application, can be limited up to the Curie point of the granules.

[0014] Further, the output of the power supply and the thickness of the furnace body are selected to maximize the depth of current penetration into the furnace body. The depth of current penetration into the furnace body, Am, is defined (in meters) by the following equation: [0015] [0016] where pm = resistivity of the molten metal (in ohms/m); [0017] Uo * Um == the product of absolute and relative permeability, [0018] with 4Tc x10-7 H/m, and Um, the relative permeability of the furnace body, is in H/m ; and [0019] f = the frequency of the induction coil current (in Hertz), which is controlled by the output of the power supply.

[0020] Maximizing the depth of current penetration into the furnace body assures high resistance of the furnace body and, therefore, higher electrical efficiency of the heating process.

[0021] A second example of the invention is shown in FIG. 3. This embodiment is of particular value in heating granules to temperatures that are higher than the Curie point of the granules. An electrically conductor material 26, or susceptor, is placed around a furnace body 13 composed of a high temperature material, such as stainless steel. The susceptor may be attached to or separated from the exterior of the furnace body. If separated from the furnace body, the susceptor may or may not rotate with the furnace body. Suitable, but not limiting, susceptor materials are graphite and silicon carbide. In this embodiment, the magnetic field created by current flowing in induction coil 18

inductively heats the susceptor to temperatures above the Curie point of the granules 24.

Heat generated in the susceptor by the induced eddy current conducts through furnace body 13 and is transferred to the granules 24 traveling through the interior of the furnace chamber in a similar fashion as that in the first example of the invention. Similar to the first example of the invention, thermal insulating material 17 can be placed around the exterior of the susceptor to assist in furnace retention of the inductively generated heat.

[0022] Another example of the invention is illustrated in FIG. 4. In this embodiment, three induction coils 31,32, and 33 (any combination of two or more induction coils can be used) are selectively connected to ac power source 20 by switching means 35, which can be any suitable power switching elements, such as solid state switching devices. In this example, different temperatures can be maintained throughout the length of the furnace chamber by applying varying amounts of inductive power to each of the three induction coils to heat the granules to different temperatures as they travel through the furnace chamber in a similar fashion as that in the first example of the invention.

Suitable alternative induction coil winding and switching schemes are disclosed in U. S.

Patent No. 6,121,592, entitled Induction Heating Device and Process for the Controlled Heating of a Non-electrically Conductive Material, which is incorporated herein in its entirety. The selective switching of multiple coils in this example of the invention can also be applied to the above second example of the invention wherein the single induction coil 18 in FIG. 3 is replaced by two or more induction coils connected to suitable switching means.

[0023] The foregoing embodiments do not limit the scope of the disclosed invention.

The scope of the disclosed invention is further set forth in the appended claims.