This application is related to U.S. application Ser. No. 10/926,900 entitled INDUCTION HEATING APPARATUS AND METHODS OF OPERATION THEREOF, filed on Aug. 25, 2004.
Field of the Invention: The present invention relates generally to induction melting apparatus for use in heating at least one material. More particularly, embodiments of the present invention relate to methods of indicating a temperature of a molten material and methods of control of induction heating apparatuses.
Induction heating apparatuses have been employed for heating a variety of materials without direct contact therewith. For instance, heat treating of metals and melting of materials may be accomplished by induction heating. Further examples of induction heating applications include, without limitation, annealing, bonding, brazing, forging, stress relief, and tempering. Additionally, powder metallurgy applications may relate to heating of a mold or other member which, in turn, heats a powder metallurgy composition to be melted. Metal or other casting applications may also utilize induction heating. Accordingly, as known in the art, induction heating may be useful in various industries and applications.
For instance, one particular application for induction heating relates to treatment and storage of such hazardous materials and is known as “vitrification.” Hazardous materials may be vitrified when they are combined with glass forming materials and heated to relatively high temperatures. During vitrification, some of the hazardous constituents, such as hazardous organic compounds, may be destroyed by the high temperatures, or may be recovered as fuels. Other hazardous constituents, which are able to withstand the high temperatures, may form a molten state, which then cools to form a stable vitrified glass. The vitrified glass may demonstrate relatively high stability against chemical and environmental attack as well as a relatively high resistance to leaching of the hazardous components contained therein.
One type of induction heating apparatus that has proven to be effective to vitrify waste materials is a cold-crucible-induction melter (CCIM). A cold-crucible-induction melter may typically comprise a water-cooled crucible disposed within an induction coil, or other inductor, usually formed along a spiral path surrounding therearound. Generally, an induction coil carries varying electric currents that generate associated varying electromagnetic fields for inducing eddy currents within electrically conductive materials encountered thereby. The varying electromagnetic fields generated by the current within an inductor may be described as the “flux” thereof.
Waste may be induction heated directly if it is sufficiently electrically conductive and thereby vitrified. However, the waste and glass forming materials used in vitrification systems may be relatively non-electrically conductive at room temperatures. Therefore, an electrically conductive material may be used to initially indirectly heat at least a portion of the waste to a molten state, at which point the waste may become more electrically conductive so that when varying current is conducted through the induction coil, conductive molten waste may be induction heated by way of eddy currents generated therein. Of course, non-electrically-conductive waste materials nearby the electrically conductive molten waste, due to the heat generated therein, may be indirectly heated and thus, melted.
As a further advantage of cold-crucible-induction melter vitrification systems, molten glass within the water-cooled crucible may form a solid layer (skull layer), which inhibits or prevents direct contact of the high temperature molten glass with the interior surface of the crucible. Furthermore, because the crucible itself is cooled with water, in combination with the insulative properties of the skull layer, high-temperature melting may be achieved without being substantially limited by the heat-resistance or melting point of the crucible.
During initial operation of the induction heating system 90 of the cold-crucible-induction melter 10 as shown in
Referring to
Also, cold cap 54, comprising granular material 55 and, possibly, condensed off-gas material, may preferably exist upon the upper surface of molten material 50 under preferred conditions. Cold cap 54 may reduce volatization of molten material 50 and may also insulate molten material 50. Impact zone 59 indicates a region of cold cap 54 that granular material 55, shown as entering the cold-crucible-induction melter 10 through feedport 14, may fall upon and accumulate. Dust, volatized material, and evolved gases 57 may exit or move upwardly away from the impact zone 59 of cold cap 54 into the plenum volume 200. Ultimately, dust, volatized material, and evolved gases 57 may subsequently condense, deposit, or settle onto cold cap 54, adhere to the inner wall of disengagement spool 40 or head assembly 20, respectively, or exit the plenum volume 200 through offgas port 12.
Induction coils 26 surrounding crucible 56 may be energized with relatively large alternating currents to induce currents within the waste material to be heated. Typically, induction coils 26 may be fabricated from a highly electrically conductive material, such as copper, and are cooled by water or another fluid flowing therein. As known in the art, waste materials, such as radioactive waste or other waste may be combined with glass forming constituents, heated, and thereby vitrified.
Conventional induction heating systems may be configured for heating in response to a temperature set-point, which may be time-varying. More particularly, conventional induction heating systems may be configured for varying the output power of the power source in relation to an error signal equal to the difference between a desired set-point in relation to a measured temperature of the material to be heated that is measured or indicated by way of thermocouple or optical pyrometer. For example, in one configuration, a desired set-point may be communicated electrically to a proportional, integral, and derivative (“PID”) type control algorithm, including user-settable or auto-setting constants, and the output of the induction heating system may be determined therewith, as known in the art.
As may be appreciated by the above discussion of the operation and configuration of a cold-crucible-induction melter 10, it may be difficult to measure or ascertain the temperature of the molten material 50 therein. Particularly, one conventional approach may include insertion of at least one thermocouple into molten material 50. However, the power source 100 of induction heating system 90 may induce heat within a thermocouple and, therefore, may potentially damage a thermocouple. Alternatively, in another conventional approach for measuring the temperature of the molten material 50, an optical pyrometer may be employed for indicating a temperature of molten material 50. An optical pyrometer, as known in the art, may indicate the temperature of a surface of a material by measuring the energy radiating from a material (for one or more wavelengths) and relating the measured energy, in consideration of the spectral emissivity of the material, to the temperature of the material. However, as best seen in
In the absence of reliable direct temperature measurements of molten material 50, conventional cold-crucible-induction melters may be controlled manually. For example, conventional cold-crucible-induction melters may be controlled by “feel” or by secondary indications such as the “frequency pulling” in relation to the applied frequency of an induction power source 100. Accordingly, it may be desired to control the output of the power source 100 of cold-crucible-induction melter 10 in relation to the temperature of the molten material 50, automatically or otherwise. Thus, there exists a need for an improved apparatus and method for indicating, controlling, or both indicating and controlling or regulating the temperature distribution within a cold-crucible-induction melter.
In view of the foregoing problems and shortcomings with conventional induction heating apparatus and methods of operation thereof, it would be advantageous to provide improved induction heating apparatus and methods of operation thereof.
The present invention relates to an induction heating apparatus and methods of operation thereof. For example, one particular type of induction heating apparatus may be a cold-crucible-induction melter. While the following discussion relates to a cold-crucible-induction melter for melting at least one material, the present invention is not so limited. Rather, the present invention relates to induction heating apparatus for use as known in the art, without limitation.
Particularly, a crucible having a wall disposed about a longitudinal axis and a bottom extending generally radially inwardly from the wall toward the longitudinal axis may be provided. Further, the walls of the crucible may be cooled and at least one material may be provided within the crucible. An inductor may be provided and disposed proximate the crucible and in operable communication with an induction heating circuit, the induction heating circuit including a power source.
Further, an electrical resistance of the at least one material may be indicated and at least one alternating current characteristic may be selected in response to the indicated electrical resistance of the at least one material. Finally, the inductor may be energized with an alternating current exhibiting the at least one alternating current characteristic. In a further aspect of the present invention, the at least one alternating current characteristic may be selected for minimizing the difference between a desired electrical resistance and the indicated electrical resistance of the at least one material. For instance, a feedback control loop configured for energizing the inductor to minimize the difference between the desired electrical resistance and the indicated electrical resistance of the at least one material may be implemented.
In another method of controlling an induction heating process according to the present invention, a temperature of at least one material may be indicated via measuring the electrical resistance of the at least one material and at least one alternating current characteristic in response to an indicated temperature of the at least one material may be selected. The inductor may be energized with an alternating current exhibiting the selected at least one alternating current characteristic. In a further aspect of the present invention, the at least one alternating current characteristic may be selected for minimizing the difference between a desired temperature and the indicated temperature of the at least one material. For instance, a feedback control loop configured for energizing the inductor to minimize the difference between the desired temperature and the indicated temperature of the at least one material may be implemented.
The present invention also relates to a method of determining a temperature of at least one material within a cold-crucible-induction melter. In further detail, a crucible having a wall disposed about a longitudinal axis and a bottom extending generally radially inwardly therefrom may be provided. Further, the walls of the crucible may be cooled and at least one material may be provided within the crucible. An inductor may be provided and disposed proximate the crucible and in operable communication with an induction heating circuit, the induction heating circuit including a power source.
The electrical resistance of at least one region of the at least one material within the crucible may be measured and an average temperature of the at least one region of the at least one material may be determined by correlating the measured electrical resistance of the at least one region of the at least one material to an average temperature thereof. Extrapolating further, an average temperature of each of more than one region may be determined by measuring an electrical resistance of each of more than one region and correlating the measured electrical resistance of each of the more than one region of the at least one material to an average temperature thereof, respectively.
The present invention also relates to an induction heating apparatus. More specifically, an induction heating apparatus of the present invention may include a crucible and a cooling structure disposed about the crucible for cooling thereof. In addition, an inductor may be disposed proximate the crucible and an induction heating circuit including a power supply having an electrical output may be operably coupled to the inductor and configured for delivering an alternating current therethrough. Further, the induction heating apparatus may comprise a measurement device configured for indicating an electrical resistance of an anticipated at least one material positioned within the crucible for inductive heating via energizing the inductor. Additionally, the induction heating apparatus may include a controller configured for selecting at least one characteristic of the alternating current for energizing the inductor in response to the indicated electrical resistance of the at least one material.
While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which:
The present invention relates to control of an induction heating process. More particularly, the methods of the present invention may pertain to controlling or regulating induction heating processes employed in a cold-crucible-induction melter 10 as shown in
In one aspect of the present invention, the resistance of the molten material 50 may be measured, estimated, or indicated. Generally, an induction heating circuit model pertaining to the induction power source 100, induction coil 26, molten material 50, and various other electrical properties that affect the electrical behavior of the induction coil 26 may be produced, and a solution for the resistance of the molten material 50 may be obtained.
For instance, the induction heating system 90 and molten material 50 may be modeled, approximated, or simulated as shown by the induction heating circuit model 300 shown in
Further, by Ohm's law,
Also,
Setting Equation 1 equal to Equation 2 and then solving for both the imaginary component and the real component gives respective solutions for RM. For instance, in the case of heating a material that is initially nonconductive, at least one measurement relating to the heating circuit may be performed when the resistance of RM is infinite (i.e., nonconductive). Such at least one measurement may provide respective values for the variables other then RM in Equation 2. Then, RM may be solved for responsive to the material becoming electrically conductive, since RM would be the sole unknown.
Thus, RM may be determined by appropriate analysis of Equation 2. However, it should be noted that the above analysis pertaining to a mathematical solution for RM may be substantially varied, depending upon the underlying induction heating circuit model 300 that is employed. The present invention also contemplates that modifications, additions, simplifications, or other variations of the induction heating circuit model 300 shown in
Thus, in one method of control or regulation of an induction heating system 90 of the present invention, a desired melt resistance set point may be selected and a difference between the desired resistance of molten material 50 and an indicated resistance of molten material 50 may be used to determine the output from the induction power source 100. Put another way, the heating of the molten material 50 via induction heating system 90 may be controlled, via selecting at least one characteristic of an alternating current for energizing the induction coil 26 to minimize the difference between a desired electrical resistance of molten material 50 and an indicated electrical resistance of the molten material 50. For instance, at least one of the amplitude and frequency of the alternating current communicated through the induction coil 26 may be selected.
For completeness, it should be recognized that the method of control of induction heating system 90 via resistance of the molten material 50 may be employed in combination with other methods of controlling induction system 90. Particularly, as described above, since the electrical resistivity of granular material 55 may be substantially infinite (i.e., non-conductive) for temperatures under about 800° Celsius, other modes of control may be employed until at least a portion of granular material 55 becomes molten.
One approach for melting at least a portion of granular material 55 may be to select a substantially constant (frequency and amplitude) electrical output from the power source 100 for energizing the induction coil 26 for a selected amount of time. The specific characteristics of the electrical output of the power source 100 for energizing the induction coil 26 may be selected based on one or more of the following: the amount of granular material 55 within the crucible 56, the melting temperature of the granular material 55, the relative amount of electrical power generated within the susceptor 120 via the induction coil 26, the material comprising the susceptor 120, the size of the susceptor 120, and the ambient conditions (the temperature, humidity, etc.) influencing the induction heating system 90, or the granular material 55. Of course, simulations or modeling may be used to predict the heating response to energizing induction coil 26. For instance, heating of susceptor 120, the granular material 55 therewith, or both may be simulated or modeled.
Alternatively or additionally, there may be other methods for determining whether at least a portion of the granular material 55 has been melted. For instance, if the susceptor 120 is visually or otherwise observable, such observation may indicate that a portion of granular material 55 has been melted. For instance, if the susceptor 120 is initially in contact with granular material 55, melting of the granular material 55 in proximity to susceptor 120 may cause the susceptor 120 to become visually observable. Alternatively, if the susceptor 120 changes position (i.e., floats or sinks within molten material 50), such a change in position may be detected and may indicate the presence of molten material 50.
Upon at least a portion of granular material 55 becoming molten and, therefore, electrically conductive, the molten material 50 may be heated directly via the electromagnetic flux of induction coil 26. Upon at least a portion of the granular material 55 forming molten material 50, control or regulation of an alternating current for energizing the induction coil 26 to minimize or reduce the difference between a selected electrical resistance set point and an electrical resistance of the molten material 50 may be employed.
The electrical resistivity of molten material 50 may be determined according to the approach described above, automatically or as otherwise known in the art. For instance, a measurement device, such as a computer including, optionally, a data acquisition system, may be employed to indicate the electrical resistivity of at least one material to be inductively heated. Additionally, a measurement device may be configured to measure at least one electrical characteristic of portions of the induction system 90 for calculating RM.
Extrapolating further, the ability to calculate or measure RM may provide a feedback signal for controlling the output from the induction power source 100 for energizing the induction coil 26. As shown in
Controller 306 may implement a so-called proportional, integral, and derivative type (“PID”) control algorithm for regulation of RM of molten material 50. Of course, controller 306 may comprise a controller as known in the art, without regard to the design of the algorithm implemented therewith. Furthermore, controller 306 may implement logic, timers, limits, alarms, or other controlling functions as known in the art or as otherwise desired. Thus, the control signal 308 may be developed in consideration of a number of inputs, measurements, or indications, without limitation.
For instance, in recognition that the amount of molten material 50 may be relatively small initially in comparison to the amount of granular material 55, it may be desirable to limit the amount of power that is applied or generated therein, to avoid overheating. Thus, an upper limit may be imposed on the electrical power communicated through the induction coil 26 for a selected amount of time.
Indicated resistance feedback 303 may be calculated by measurement of one or more electrical properties or operational conditions related to induction system 90. At least one sensor 302 may measure voltage, resistance, inductance, capacitance, or, more generally, at least one property of an induction heating circuit for use in calculating, estimating, or otherwise determining RM.
Such a configuration may be termed an estimator 310, because control or regulation of the induction power source 100 is performed via an indirect measurement of the resistance of the molten material 50. Put another way, the indicated resistance feedback 303 is determined by indirect indication, prediction, or estimation of the resistance of molten material 50.
In another method of the present invention, a temperature set point, which is obtained via a resistance measurement or indication thereof of the molten material 50, may be used for controlling the output from the induction power source 100. Explaining further, the electrical resistance of the molten material 50, RM, may be determined and the temperature of the molten material 50 may be also determined therewith. The temperature of the molten material 50 may be indicated by the electrical resistance thereof, since the electrical resistance of molten material 50 may vary with temperature, as shown in greater detail hereinbelow.
Generally, the electrical resistance of a material may vary by either increasing or decreasing with increases or decreases in temperature. For example,
Of course, once a mass of molten material 50 has been established, as shown in
In a second method of operation of an induction system 90 of the present invention, generally, a selected or desired temperature set point may be selected and control of the induction heating process may proceed with reference thereto. Particularly, heating of at least one material via induction heating system 90 may be controlled, via selecting at least one characteristic of alternating current 312 for energizing the induction coil 26 so as to reduce the difference between the desired temperature of the at least one material being heated and a temperature thereof which is estimated or indicated by determining the electrical resistance of the at least one material and correlating the electrical resistivity of the at least one material to the temperature thereof.
As shown in
As explained hereinabove, indicated temperature feedback 403 may be calculated by measurement of one or more electrical properties or operational conditions related to induction heating system 90. Sensor(s) 402 may measure voltage, resistance, inductance, capacitance, or other parameters that are useful in calculating, estimating, or otherwise determining a resistance and, ultimately, a temperature of at least one material heated by the inductor. For instance, with reference to molten material 50, RM may be measured and then may be correlated to a temperature of molten material 50, as described hereinabove in relation to
In a further aspect of the present invention, it should be noted that the electrical resistance RM that may be indicated pertains to the region of the molten material 50 under the influence of the flux of the induction coil 26. Thus, the electrical resistance RM may indicate an average temperature of a portion or region of the molten material 50 influenced by the electromagnetic flux of the induction coil 26. Such a configuration may be advantageous, since conventional temperature sensors may indicate the temperature at a particular position (e.g., a thermocouple) or of a particular surface area (e.g., an optical pyrometer).
Generally, the skin depth of the electromagnetic flux may be defined as the depth to which eddy-currents are induced within a material heated by electromagnetic flux. The theoretical depth of penetration or skin depth (d0) within a material to which an electromagnetic wave travels to is defined to be the depth at which the electromagnetic field or flux is reduced to 1/e or approximately 37 percent of its value at the surface. In the case of induction heating, the theoretical skin depth of the varying electromagnetic fields and the resulting eddy currents may be computed by the following equation:
As may be appreciated by inspection of Equation 3, a relatively low frequency of oscillation of the electromagnetic wave may, according to Equation 3, increase the skin depth of the electromagnetic flux. Correspondingly, a relatively high frequency of oscillation of the electromagnetic wave may, according to Equation 3, decrease the magnitude of the skin depth d0 of the electromagnetic flux of the induction coil 26. Also, as mentioned hereinabove, electrical resistivity of molten material 50 may vary widely in relation to their temperature. Therefore, one factor that influences the skin depth d0 may relate to the temperature of the molten material 50.
Accordingly, in another aspect of the present invention, it may be desirable to select the region of influence of the electromagnetic flux of the induction coil so as to indicate the temperature of the region of interest. Put another way, the electrical parameters of the power source 100 may be adjusted so as to generate a flux having an anticipated penetration depth (inwardly from the exterior of the molten material 50 and not including the skull layer 52) or skin depth d0, which corresponds to a selected region of the molten material 50 for which the average temperature is of interest.
Explaining further, for example, as shown in
It should also be noted that while the electromagnetic flux envelope 130 may be described and may be mathematically treated as being substantially symmetric, substantially cylindrical, or being both substantially symmetric and substantially cylindrical, the distribution of electrical heating within molten material 50 by way of an induction coil 26 may be uneven in nature, depending on the geometry and properties of the molten material 50, the proximity of the induction coil 26 to the molten material 50, the geometry of the induction coil 26, or other environmental conditions that may influence the electromagnetic flux of the induction coil 26 in relation to the molten material 50. The present invention contemplates that such unevenness may be modeled, predicted, or otherwise compensated for so as to increase the efficiency of the induction heating process.
Thus, such an electromagnetic flux may indicate, in combination with measurements of at least one electrical property of the induction heating system 90 and by using Equations 1 and 2, the electrical resistance of a selected region 60 of molten material 50 influenced by the electromagnetic flux. Then, an average temperature may be estimated or determined by determining the electrical resistance of the region of molten material 50 influenced by the electromagnetic flux and correlating the electrical resistance with a temperature, by way of, for instance, the relationship depicted in
By way of extension, one or more indications of the temperature related to one or more regions of the molten material 50, respectively, may be indicated by selecting the operational parameters of the power source 100 so as to generate an electromagnetic flux having differing anticipated skin depths. Accordingly, a respective measurement or indication of a temperature associated with each of a plurality of differing regions of molten material 50 may be obtained. For instance,
The average temperature of region 60 may be obtained by energizing the induction coil 26 with an alternating current that produces an anticipated electromagnetic flux envelope 130 as follows. First, the electrical resistance of region 60 may be measured or indicated by employing the above-described circuit analysis and solving for RM. Then, the average electrical resistance of region 60 may be correlated to the temperature of region 60 by way of a relationship therebetween (e.g., as shown in
Similarly, average temperature of regions 60 and 61 may be obtained by energizing the induction coil 26 with an alternating current that produces an anticipated electromagnetic flux envelope 131 as follows. First, the electrical resistance of regions 60 and 61 may be measured or indicated by employing the above-described circuit analysis and solving for RM. Then, the average electrical resistance of regions 60 and 61 may be correlated to the temperature of regions 60 and 61 by way of a relationship therebetween (e.g., as shown in
However, by knowing the volume of each of regions 60 and 61, the average temperature of region 61 may be calculated by knowing both the average temperature of region 60 as well as the average temperature of both of the combination of regions 60 and 61.
Moreover, average temperature of regions 60, 61 and 62 may be obtained by energizing the induction coil 26 with an alternating current that produces an anticipated electromagnetic flux envelope 132 as follows. First, the electrical resistance of regions 60, 61 and 62 may be measured or indicated by employing the above-described circuit analysis and solving for RM. Then, the average electrical resistance of regions 60, 61 and 62 may be correlated to the temperature of regions 60, 61, and 62 by way of a relationship therebetween (e.g., as shown in
However, by knowing the volume of each of regions 60, 61, and 62, the average temperature of region 62 may be calculated by knowing both the average temperatures of region 60, region 61, and the average temperature of all of regions 60, 61, and 62.
Alternatively or additionally, a value for RM, in combination with other induction heating circuit measurements such as inductor voltage, current, and phase may be useful in determining a so-called melt temperature profile, which may be used for approximating or predicting the general behavior of an induction heating system during operation thereof. Determining a melt temperature profile according to a plurality of different regions (i.e., varying the frequency so that the size and shape of the electromagnetic flux changes) of a material that is induction heated, as described hereinabove with respect to
While the present invention has been described herein with respect to certain preferred embodiments, those of ordinary skill in the art will recognize and appreciate that it is not so limited. Rather, many additions, deletions and modifications to the preferred embodiments may be made without departing from the scope of the invention as hereinafter claimed. In addition, features from one embodiment may be combined with features of another embodiment while still being encompassed within the scope of the invention as contemplated by the inventors. Therefore, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.
The United States Government has rights in the following invention pursuant to Contract No. DE-AC07-99ID13727 between the U.S. Department of Energy and Bechtel BWXT Idaho, LLC.
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