Patent Application
20120051387
The present invention relates in general to heating systems and methods and in particular to the use of heating elements that are resilient in the face of very high temperatures and the possibility of flux or other debris being deposited near or on the heating elements.
On occasion, laboratory personnel using such devices will also add a halogen chemical compound, to facilitate the removal of the end-product. Upon heating the crucible, the lithium borate melts and dissolves the sample. This dissolution reaction is enhanced by the agitation of the crucible. After complete reaction, the resulting hot solution is poured into a plate-shaped mold and cooled, to produce a glassy disk that is then placed in an elemental analyzer.
The temperature of the crucible-heating process can reach 1200 degrees Celsius, which poses tremendous challenges to the durability of the materials and parts used. At such high temperatures, most materials will likely burn, melt, rapidly oxidize in the presence of oxygen in the air and possibly chemically interact with the halogen gases released by the heating process.
Thus, there is a need in the art for an apparatus that would durably support the crucibles and the molds and reliably heat the furnace chamber, to minimize repair operations by laboratory personnel in remote areas, while maintaining a clean environment in the furnace, and while avoiding contaminating the samples being processed.
In existing systems, crucibles are typically held either using metallic clips (as in the case of gas fluxers from Corporation Scientifique Claisse, Canada), or placed in a shallow tube (as practiced in the electrical furnace by ModuTemp, Australia), or on horizontally running parallel ceramic rods (as offered by Katanax and Corporation Scientifique Claisse on their respective electric fluxers). In the last of the above-listed configurations, crucibles are held apart by small ceramic spacers (Katanax) or by a single scalloped metallic part (Corporation Scientifique Claisse).
Crucible holders are typically designed to enable repair or replacement to be as easy and as fast as possible. The ease and speed of replacement are needed because flux tends to spill onto the holder, and cleaning must be performed quickly in a production setting to avoid costly downtime.
Molds can be secured with metallic clips (as offered on gas fluxers by Corporation Scientific Claisse, Canada), or the mold may rest on a metal plate with circular openings. Both the mold and the crucible holder may increase thermal expansion of their respective lengths in the range of 3 to 8 mm (millimeters). If compliance is not built into the surrounding parts to accept this expansion, the heat-exposed parts will be longitudinally compressed between their fastening points (located outside the furnace), and are likely to sag and fail.
The only heating elements that are known to reach temperatures suitable for resistive heating fusion machines are: SiC—Silicon carbide. One drawback of SiC is ageing. Due to heat exposure, each heating element's electrical characteristics will change over time, making it nearly impossible to replace a single element in a multi-element configuration. Doing so would strongly reduce the life expectancy of the heating elements that are not being replaced.
MoSi2—molybdenum disilicide is another element commonly used for heating elements. The main challenge encountered when designing with MoSi2 is that it sags heavily at high temperature, and becomes very brittle upon cooling. The heating element is typically installed in a U-shaped free-hanging configuration. Molybdenum disilicide is known to potentially react with halogens, and degrade prematurely.
The effects incurred at high temperatures cause the drawback of this alloy. At high temperatures, FeCrAl becomes soft, and coil-shaped elements will deform to the point that the turns of the helically wound resistive heating coil tend to approach one another, leading to element failure by localized overheating. This type of element can also experience surface reaction with halogen gases or flux spills, and then fail rapidly.
Accordingly, there is a need in the art for improved configurations of heating elements and associated parts of resistive-heating furnaces.
According to one aspect, the invention is direct to a heating apparatus that may include a ceramic rod having at least one circumferential groove extending substantially circumferentially about a perimeter of the rod; and a coil located about the perimeter of the rod and having turns of the coil embedded with the grooves of the rod.
Other aspects, features, advantages, etc. will become apparent to one skilled in the art when the description of the preferred embodiments of the invention herein is taken in conjunction with the accompanying drawings.
For the purposes of illustrating the various aspects of the invention, there are shown in the drawings forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
In the following description, for purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one having ordinary skill in the art that the invention may be practiced without these specific details. In some instances, well-known features may be omitted or simplified so as not to obscure the present invention. Furthermore, reference in the specification to phrases such as “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of phrases such as “in one embodiment” or “in an embodiment” in various places in the specification do not necessarily all refer to the same embodiment.
In the embodiment of
Consistent with the goal of making mold holder 200 resilient to heat, very little metal is present in mold holder 200, thereby helping to avoid contamination, and to make for a more sag-proof assembly. End supports 230 are ceramic, and the holder central portion is freely suspended, thereby eliminating the concern for thermal expansion issues. In a preferred embodiment, brackets 240 are the only metal portion of mold holder 200. Brackets 240 may be made of a Nickel/Chrome (80/20) alloy, that is 80% nickel and 20% chrome. However, other alloys, metals, and/or alloy compositions may be employed.
Heating element 300 may include ceramic rod 310, coil 320, and/or shield 330. However, in some embodiments, shield 330 may not include shield 330. Rod 310 preferably includes grooves around the perimeter thereof, which are preferably in a spiral pattern. Coil 320 is preferably configured as a single helical coil having turns that match the geometry of the spiral groove in rod 310. However, the present invention is not limited the specific geometry of the spiral coil 320 and the grooves in rod 310 shown in
When coil 320 is assembled onto rod 310, the turns of coil 320 become embedded within respective grooves around the exterior of rod 310. Thus, the ridges in between the grooves of the exterior of rod 310 end up being located in between adjacent turns of the coil 320. With this arrangement, the ridges restrain any possible movement of the turns of coil 320, and thus keep adjacent turns of coil 320 properly spaced apart from one another during high temperature conditions within the furnace. Thus, even when high temperatures tend to create expansion forces within coil 320, the ridges between the grooves on the exterior of rod 310 prevent the turns of coil 320 from approaching one another. Thus, the prior-art problem of excess localized heating arising from turns of coil 320 moving toward one another under high temperature conditions is prevented by the presence of grooves and ridges on rod 310.
After coil 320 has been assembled onto rod 310, shield 330 may be slid over the combination of coil 320 and rod 310 to form heating element assembly 300, as shown in
In the embodiment of
As shown in
The disclosed embodiment overcomes problems in the prior art that arose when users needed to extract heating elements 300 through restricted openings in housing 510 under conditions providing limited access, poor visibility, and the possibility of damaging heating element assemblies 300 upon removing same from the housing 510.
Attention is now directed to benefits observed due to some of the inventive embodiments disclosed herein. Employing the removable module, the crucible holders can be removed and disassembled in under a minute, without the use of tools. Individual parts can then be replaced or cleaned easily. The materials used for heat shield 330 operate to minimize the risk of sample contamination, and reduce the likelihood of thermal expansion and sagging. Durability is greatly improved with the inventive embodiments. As for the heating elements, life expectancy was dramatically increased, in order to minimize costly downtime in laboratories. Flux spills and halogen vapors are much less of a problem with the inventive embodiments.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
