Patent Application
20060051715
N/A
In the processing of materials such as ceramics, thermal uniformity is often required to achieve uniform heating of the product and to minimize opportunities for distortion, bending or cracking of the product by reason of uneven heating. A system is shown in U.S. Patent Publication No. U.S. 2004-0173608 A1 (U.S. patent application Ser. No. 10/775,542), assigned to the same assignee as the present application, wherein uniform heating is provided by one or more eductors in the furnace chamber which produce high volume gas circulation in the furnace to achieve a highly uniform gas environment and temperature. The one or more eductors can also be employed for forced convection cooling of a product. The one or more eductors are preferably as shown in U.S. Pat. No. 5,795,146, which is also assigned to the assignee of the present invention. The eductors provide high volume flow necessary for improved temperature uniformity and control and can provide a thermal uniformity of about ±3.5° C. during the process cycle.
The present invention provides a thermal processing system that employs slot eductors in one or more wall or roof surfaces of the furnace chamber. For purposes of the present application the term “slot eductor” means an eductor formed by a slot of any cross sectional shape in a wall or other surface of a furnace chamber and having a nozzle at one end of the slot for directing high velocity gas along the slot. For example, the slot cross section can be of v-shape, rectangular shape or curved shape. Alternatively, the eductor slot can be provided by the corner of joined walls of the furnace chamber, or can be provided by directing a high velocity stream along a portion of the wall itself without any physical slot. As the eductors are provided in or on the furnace walls themselves, there are no constraints on furnace construction due to placement of tube type eductors. Eductors can be provided in the furnace chamber where tube eductors would not fit or would not be operationally practical. Thus, the invention eliminates the need for added tubes or other equipment in the furnace to provide the eductor structures.
The invention is particularly useful in batch type furnaces, especially those having a relatively tall furnace chamber wherein the temperature profile from the top to the bottom of the chamber can tend to be uneven. The invention can also be employed in continuous type furnaces wherein a product is conveyed between furnace sections or chambers of a furnace to perform an intended process cycle.
The invention will be more fully described in the following detailed description taken in conjunction with the drawings in which:
Referring to the drawings, there is shown in
The slot may be of any cross-sectional shape and is not limited to the rectangular configuration illustrated in the embodiment of
The nozzle or feed gas inlet can be arranged to provide an incoming gas stream of any desired configuration. For example, a conical jet can be provided. In another embodiment, the jet could be rectangular to create a sheet of gas. The cross-sectional shape of the associated slot eductor and the shape of the gas jet can be selected for compatibility. Any gas suitable for the particular application, such as air, nitrogen, argon, or hydrogen, can be used.
The slot eductors make use of the Coanda effect in which a high velocity gas stream tends to follow an adjacent surface along which it is flowing. The high velocity stream creates a low pressure area along the stream that acts to entrain gas from the chamber into the stream, thereby amplifying the volume of gas being moved. The eductor utilizes the energy of the incoming high velocity gas to move much larger amounts of resident furnace chamber gas in the desired direction for the benefit of heat transfer, causing turbulent gas contact with the product in the chamber to enhance out-gassing and to deliver required chemistry to the product. The high volume gas flow in the chamber provided by the eductors also provides uniform heat distribution within the chamber for uniform heating of the product. The slot eductor is capable of moving at least 10 times, and preferably at least 20 times, more volume of gas within the chamber than is introduced into the chamber via the eductor nozzle.
Preferably the length of the slot is at least 10 times greater than the greatest width dimension of the slot in cross-section. A slot that is shorter than this length generally will not aspirate in a sufficient volume of gas to be effective.
A further embodiment is illustrated in
The atmosphere in the furnace chamber is typically air, an inert gas such as nitrogen or argon, or a combination thereof for processing low temperature cofired ceramics (LTCC) and other ceramic products and for processing fuel cells. For processing powder metals, the atmosphere is typically a combination of hydrogen and nitrogen. For some purposes, the atmosphere may be water vapor with or without other gas. As noted above, the one or more slot eductors introduce gas into the furnace chamber and provide amplification and circulation of the gas to achieve an intended uniformity of furnace atmosphere and temperature to which the products or materials being processed are exposed. Temperature uniformity of about ±3.5° C. can be achieved by use of the invention. The one or more eductors can also provide forced convection cooling of the product.
The number of slot eductors and their positioning within a furnace chamber is determined to provide an intended gas flow pattern in the particular chamber to produce a uniform temperature and gas environment in the chamber for uniform heating of the product contained therein and for uniform exposure of the product to the gas environment. The eductors may be operated in concert or may be operated in a switched manner to provide an intended gas flow or circulation pattern. For example, the eductors on one side of a furnace chamber may be on while the eductors on the opposite side of the chamber are off, and vice versa during repeated cycles of operation.
The invention can be embodied in a variety of batch type or continuous type furnace systems to suit particular products or materials to be processed and to suit other manufacturing requirements. A furnace system is shown in diagrammatic form in
The heating source can employ convection heaters, radiation heaters, or microwave heaters or combinations thereof. Microwave heating can suitably be employed for debinding and sintering applications. If microwave heating and non-microwave heating are employed, the non-microwave source must be compatible to avoid unwanted microwave reflections or absorption in the furnace chamber. Typically microwave heating can be employed with convection heating in the same chamber. If radiation heating is to be employed, the radiant heaters must usually be in a furnace chamber separate from the chamber heated by microwave energy since the usual radiant heaters are made of silicon carbide or other material, which is microwave absorptive.
The heating source 32 is controlled by controller 38 during the heating cycle to provide an intended thermal profile and to provide uniform volumetric heating of the materials or products throughout the heating cycle. The heating source is controlled during the process cycle in accordance with the particular material or product being processed including the composition of the material and its shape or mass.
A batch furnace chamber is shown in cross section in
Slot eductors 56 are disposed in each sidewall of the furnace. (Only one side is illustrated in
One or more openings 47 extend through the hearth 46 from one side of the chamber to the other side. An eductor 60 is disposed in the side wall adjacent each end of the opening 47. The eductor 60 is preferably a tube type eductor as shown in U.S. Pat. No. 5,795,146, assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference. The one or more openings 47 provided through the hearth provide a circulation path through the hearth from one side of the furnace chamber to the other side of the chamber. The eductors 60 operate in concert with the slot eductors 56 to provide recirculation of the gas within the chamber to achieve intended uniformity of temperature and gas exposure to the product.
As noted above, the high velocity gas flow from the eductors causes entrainment of gas in the furnace chamber into the gas stream and amplification and circulation of the gas. Ratios of the volume of entrained gas with respect to the volume of injected gas of up to 50:1 can be achieved. Preferably, a ratio of at least 10:1 and more preferably at least 20:1 is achieved for good furnace operation.
In one embodiment, the eductors can be operated in complementary manner such that for one time interval the eductors on one side of the furnace chamber are on, while the eductors on the opposite side of the chamber are off. For the next time interval, the operation of the eductors is reversed such that the formally off eductors are on, while the formerly on eductors are off. The alternating operation of the eductors provides further uniformity of the atmosphere within the chamber by reason of the alternating circulatory flow paths. In the off mode, the nozzles to the slot eductors are not completely shut off, but provide a small amount of gas flow, typically about 5% of full flow, to cool the gas nozzle to avoid damage to the gas nozzle at the high operating temperatures of the furnace and to avoid air or other contaminants entering the furnace chamber through the nozzle assembly.
The eductors can also be employed to provide forced convection cooling of the product such as during the cool down portion of a thermal cycle. The gas flow from the eductors is controlled in conjunction with control of the heat sources to achieve an intended rate of cooling of the product.
The number and arrangement of eductors in the furnace chamber is determined to provide an intended gas flow pattern within the chamber to achieve the requisite temperature uniformity and uniform gas environment. The arrangement illustrated in
The materials or products to be processed are retained in a suitable support assembly. One typical form of support is a tray having multiple compartments for respective items to be processed, the trays being stackable, one on top of the other, such that a relatively large quantity of items can be processed at a single time within the furnace chamber. The support assembly can be of other types such as a suitably configured rack for holding particular products or items to be processed. For some purposes it is useful to sandwich the product between upper and lower plates or other supports to prevent distortion of the product during the heating cycle. The product holders are made of a refractory material capable of withstanding the operating temperatures of the furnace.
The invention can also be embodied in a continuous thermal process and system. In such a system a product is conveyed along a furnace chamber which typically can have multiple zones to provide an intended heating and cool down cycle which is suitable for the particular product or material being processed. The slot eductors can be configured within the furnace chamber in similar manner as described above to provide uniform heating and amplified gas volume. One or more eductors can also be arranged along the length of the furnace chamber to propel gas resident within the chamber along the furnace length such as to ensure that dirty furnace atmosphere is being pushed to the exhaust area near the front end of a furnace.
The invention is not to be limited by what has been particularly shown and described. The invention can be embodied in batch and continuous type furnaces of various constructions and in single and multi zone furnaces. The invention can also be utilized with a variety of conveyer mechanisms to move products into and out of the furnace or to convey products between furnace sections or zones. It is therefore intended that the invention should comprehend the full spirit and scope of the appended claims.
This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 60/607,681, filed Sep. 7, 2004, the disclosure of which is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 60607681 | Sep 2004 | US |
