Patent Application
20140166640
In one embodiment a support member for a heating element coil includes at least one support beam having an opposed distal and proximal end. The distal end being arranged to be anchored in insulation of the heating element and the proximal end oriented towards a center of loops of the heating element. At least one vertical support is disposed at the proximal end of the at least one support beam. The at least one vertical support forms a barrier surface limiting inward radial movement of loops of the heating element coil and a length of the vertical member determines a pitch of the loops of the heating element coil. An interlocking feature is located on the at least one vertical support. The interlocking feature interlocks the at least one vertical support to a respective adjacent vertical support to form a contiguous, aligned vertical support column A variable surface is formed on at least a portion of a length of the at least one support beam. The variable surface provides an inward radial force to keep the loops of the heating element centered and at a minimum diameter.
In another embodiment, a method of controlling a position of a heating element coil within insulation of a heating element is provided. A plurality of support members are provided. Each support member includes at least one support beam having an opposed distal and proximal end. The distal end being arranged to be anchored in insulation of the heating element and the proximal end oriented towards a center of loops of the heating element. At least one vertical support is disposed at the proximal end of the at least one support beam. The at least one vertical support forms a barrier surface limiting inward radial movement of loops of the heating element coil and a length of the vertical member determines a pitch of the loops of the heating element coil. An interlocking feature is located on the at least one vertical support. The interlocking feature interlocks the at least one vertical support to a respective adjacent vertical support to form a contiguous, aligned vertical support column A variable surface is formed on at least a portion of a length of the at least one support beam. The variable surface provides an inward radial force to keep the loops of the heating element centered and at a minimum diameter. The plurality of vertical supports are interlocked to form the vertical support column and the loops of the heating element coil are mounted in the vertical support column, wherein the variable surface provides an inward radial force to keep the loops of the heating element centered and at a minimum diameter.
In yet another embodiment a support member for a heating element coil includes at least one support beam having an opposed distal and proximal end. The distal end is arranged to be anchored in insulation of the heating element and the proximal end oriented towards a center of loops of the heating element. At least one vertical support is connected with the proximal end of the at least one support beam. The at least one vertical support forms a barrier surface limiting inward radial movement of loops of the heating element coil and a length of the vertical support determines a pitch of the loops of the heating element coil. A variable surface is formed on a length of the at least one support beam, the variable surface providing an inward radial force to keep the loops of the heating element centered and at a minimum diameter.
These and other objects, features, aspects, and advantages will become more apparent from the following detailed description of the preferred embodiments relative to the accompanied drawings, in which:
Resistance heating element assemblies are widely used in thermal processing equipment. One of the more common configurations consists of a helically wound coil of wire surrounded by a cylinder of insulating material. These assemblies can be divided into two general groups, those having a central axis of the coil that is oriented horizontally are commonly referred to as horizontal heating element assemblies, while those where the central axis of the coil is oriented vertically are commonly referred to as vertical heating element assemblies, see
It is well known that there are two basic failure mechanisms for this type of heating element assembly. One is based on the heating element material life. This may be influenced by external contamination of the material, but is generally a function of oxidation life and driven by the material metallurgy and quality of the base alloy materials. The second is structural failures. These are influenced by process cycle times and temperatures, rates of temperature increase/decrease and heat losses. The underlying basic issues are generally the same, material strength at increased temperatures (deformation) and permanent elongation.
It is known that the FeCrAl alloy forms aluminum oxide on its outer surface at elevated temperatures. Furthermore, it is the presence of this oxide layer that protects the material from forming other oxides and nitrides that would cause the material to fail. In addition, the oxide layer, in combination with grain growth in the alloy, gives the material its creep strength and form stability. Unfortunately, the thermal expansion of the oxide and alloy are not quite the same. While the FeCrAl alloy materials exhibit thermal expansion of approximately 15×10−6° C. over the range of about 20 to about 1000° C., the protective oxide exhibits thermal expansion closer to 8×10−6° C. over the same range. Under transient thermal conditions, tensile and compressive stresses can fracture the oxide. When this occurs, additional aluminum is consumed from the alloy to form fresh oxide to “heal” the fractures. When the stresses become high enough, spallation can occur where areas of the oxide are ejected from the surface of the wire. The newly exposed material will form oxide by consuming some of the aluminum from the alloy and oxidizing it at the surface. When the material no longer has enough aluminum in the alloy matrix to properly heal this damage, then it is declared that the material is at its end of oxidation life.
This cyclical stressing and healing of the material causes a reduction in cross-section and permanent elongation in the material. Advances in material science and metallurgy have helped to minimize the permanent elongation and improved form stability by enhancing grain growth in the material, but they have not been entirely eliminated. Furthermore, any external mechanical strain placed on the material tends to exacerbate the elongation.
As the material expands during heating, the diameter of the coil increases. If the insulating materials are applied directly on the outer diameter (OD) of the coil, then the thermal expansion causes the coil to push against the insulation creating stress in the material. While this effect is much less significant in smaller diameter coils, it can be imagined that in large coils, such as those greater than about 250 mm, the expansion can be quite substantial and result in considerable stress on the resistance material. The stress created as a result of thermal expansion can result in immediate distortion of the coil as well as accelerating the permanent elongation of the material.
One approach to reducing the risk of stress due to thermal expansion is to position the insulating materials some distance from the OD of the coil. The annular space is typically chosen to accommodate the expected thermal expansion as well as some anticipated quantity of permanent elongation in the material which translates into an increase in OD. This annular space requirement is typically between about 5 mm and about 35 mm, and more typically between about 5 mm and about 15 mm. Each loop of the helical coil is supported at multiple locations around its circumference, and the insulating material is fabricated with an ID greater than the coil OD, creating the desired annular space. Unfortunately, there are other factors that can have negative impact on the heating element coil if this approach is taken.
In the case of a vertical heating element configuration, as shown in the heating furnace of
U.S. Patent Application No. 2011/0315673, assigned to the assignee of the present disclosure, discloses keeping coil loops consistent by interlocking the loops at support points while allowing the entire column of supports to move in unison. This is an effective method for controlling the creep, but is somewhat limited to higher temperature applications that have a thicker insulation profile since the assembly occupies a relatively thick profile within the insulation.
The creep issue is also discussed in U.S. Pat. No. 8,134,100, where it is suggested that a number of fixed plates can be attached to the heating element coil adjacent to some of the spacer support locations, on the side corresponding to the higher vertical position of the helical loop. This solves the problem of coil creep, but introduces issues as how to attach the fixed plates without damaging the coil, introducing thermal non-uniformity and subsequent coil deformation at the attachment points. In addition, this method is labor intensive, adding to the production costs of the assembly.
Additionally, U.S. Pat. No. 8,134,100 discloses a configuration where a series of tubular members that are declined at an angle of 50 to 200 degrees to allow the individual coil loops to move radially inward when cooled and reduce tensile stresses on the coil. While this relatively large angle of inclination can reduce tensile stresses as the coil is cooled, it can also present significant impingement forces on the OD of the coil as it expands and lead to distortion.
U.S. Patent Application No. 2009/0194521 also discloses the use of fixed plates that act as movement prevention members dispersed throughout the coil with no significant differences as to those disclosed in U.S. Pat. No. 8,134,100. These movement prevention members present the same issues with attachment and coil distortion as in the previous application.
Since none of these solutions present an optimal solution for managing creep and freely supporting the helical coil of resistance material in a mid-temperature, low mass heating element assembly, there is a need for such a solution in the industry.
The present support member allows for minimally supporting a heating coil in order to reduce stress on the element material and obtain maximum product life. Referring to
As shown in
At least some of support beams 22 have an anchoring portion 36 at the distal end to anchor the beam within insulation 42 of a heating furnace. It should be appreciated that the embedded anchoring portion can be formed into the insulation as it is molded, fit into a groove in the insulation whether it is monolithic or comprised of individual panels. In the case where the insulation is constructed from multiple panels, the embedded portion 36 may be installed at the junction of two panels (not shown). Optionally, embedded portion 36 may be cemented in place to secure it within the insulation.
In the case where the insulation is to be formed around the anchor during a molding process, it is preferential to have the surface of the retention feature incorporate an angular or radial feature 38 to insure the insulation forms fully round the anchor. In cases where the anchor will be embedded in the insulation after vacuum forming, it is alternately desirable to have the anchor at the distal end more abrupt, such as a plate perpendicular to the axis of the support beams. This configuration creates the highest resistance to extraction of the support member from the insulation.
As shown in
The profile of support beams 22 incorporate variable surface 10 formed by an inclined surface 23 having a gradation along a portion of the length of the variable surface. The profile of the variable surface may be either linear or non-linear along the length of the gradation.
As shown in
The magnitude of the gradation can be from about 0.1 mm to about 5 mm and more preferably 0.5 mm to about 2 mm. The length of the variable surface length is about 5 mm to about 35 mm. However, it should be appreciated that the disclosed embodiments need not be limited to any specific dimensions and can vary depending upon application.
Non-linear gradations can be used to supply more or less force to the outside diameter of the coil at different points in its useful life. For example, as the coil elongates over its useful life, the amount of force can be increased with additional gradation occurring toward the fiber surface. The angle created by the graded surface normal to the horizontal plane of the support member center should not exceed approximately 45°, since there is increased resistance to normal thermal expansion and risk of the coil distorting. Additionally, if the gradation of the surface is too extreme, then the coil loop can become impinged at on one side and be stuck at an angle, increasing the risk of the loop touching adjacent loops and causing an electrical short circuit or compromising the thermal uniformity of the heating element surface.
Additionally, the profile can be varied preferentially along the length of the coil to maximize this benefit. For example, the degree of gradation can be greater at the bottom of the coil than the top in order to provide a greater resistance to expansion at the bottom. Referring again to
Referring to
This force partially opposes the natural tendency of the heating element material to creep downward while increasing the diameter of the coil loops. The amount of gradation is chosen to be enough to reduce creep, while slight enough not to cause any negative effects from impingement of the OD of the coil during thermal expansion. The magnitude of gradation is selected be some multiple, (such as about 1 to about 5 times the change in vertical position of the heating element coil material from one support column to the next (loop pitch/number of columns) Loop pitch refers to the distance between two adjacent points on the coil circumference separated by 360 degrees of coil distance or in other words the distance between two axially coincident points on the coil spiral between adjacent loops. In practical applications, as described above, the magnitude of gradation will be from about 0.1 mm to about 5 mm and more preferably from about 0.5 mm to about 2.0 mm.
The support members are preferentially constructed of an aluminum-silicate ceramic to provide good mechanical performance at the typical process temperatures and electrical resistance. The material may be either fully dense (vitreous) ceramic or semi-porous material. The semi-porous material has the added advantage of reducing the thermal mass of the heating element, albeit at the cost of some mechanical strength.
The configuration of the disclosed embodiments yields preferential force vectors to return the loops of the coil to the radial center of the assembly, providing resistance to mechanical creep, while avoiding excessive impingement forces on the smaller gauge heating element resistance wire, for example, typically 2.5 mm to 5 mm diameter.
Referring to
Although the present disclosure has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred therefore, that the present disclosure be limited not by the specific disclosure herein, but only by the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| 61736956 | Dec 2012 | US |
