CA2384375A1 - Insert for a radiant tube - Google Patents
Insert for a radiant tube Download PDFInfo
- Publication number
- CA2384375A1 CA2384375A1 CA002384375A CA2384375A CA2384375A1 CA 2384375 A1 CA2384375 A1 CA 2384375A1 CA 002384375 A CA002384375 A CA 002384375A CA 2384375 A CA2384375 A CA 2384375A CA 2384375 A1 CA2384375 A1 CA 2384375A1
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- Prior art keywords
- ceramic insert
- integral ceramic
- insert
- integral
- diameter
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
- F28F21/00—Constructions of heat-exchange apparatus characterised by the selection of particular materials
- F28F21/08—Constructions of heat-exchange apparatus characterised by the selection of particular materials of metal
- F28F21/081—Heat exchange elements made from metals or metal alloys
- F28F21/084—Heat exchange elements made from metals or metal alloys from aluminium or aluminium alloys
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
- F28F13/00—Arrangements for modifying heat-transfer, e.g. increasing, decreasing
- F28F13/06—Arrangements for modifying heat-transfer, e.g. increasing, decreasing by affecting the pattern of flow of the heat-exchange media
- F28F13/12—Arrangements for modifying heat-transfer, e.g. increasing, decreasing by affecting the pattern of flow of the heat-exchange media by creating turbulence, e.g. by stirring, by increasing the force of circulation
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
- F28F21/00—Constructions of heat-exchange apparatus characterised by the selection of particular materials
- F28F21/04—Constructions of heat-exchange apparatus characterised by the selection of particular materials of ceramic; of concrete; of natural stone
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
- Y10T428/2913—Rod, strand, filament or fiber
- Y10T428/2922—Nonlinear [e.g., crimped, coiled, etc.]
- Y10T428/2925—Helical or coiled
Abstract
An integral ceramic insert (24) for a radiant tube (10). The insert (24) has a helical shape and contains ceramic material; and it has a thermal expansion rate of less than 6.0 x 10E-6 meters/meter/degree Celsius and a thermal conductivity of at least 0.2 calories/centimeter/second/degree Celsius. The diameter of the insert (24) is from about 2 to about 10 inches, and it has a length such that the ratio of its length to its diameter is from about 2/1 t o about 8/1. The helical shape of the insert (24) contains one or more pitches of from about 2 to about 32 inches, each which defines a helix angle of from about 15 to about 80 degrees.
Description
Description Insert for a Radiant Tube Technical Field An insert for a radiant tube which contains ceramic material with a specified rate of thermal expansion and a specified thermal conductivity; the insert is the shape of a helix with a specified number of turns per unit of length and per unit diameter.
Background Art Many industrial process furnaces require special atmospheres and, thus, cannot be directly heated by means of gas combustion. These special atmosphere furnaces are often heated by means of a system ire which gas-air combustion takes place within long metal alloy tubes which exit to the outside of the furnace wall to prevent contamination of the furnace's atmosphere. These furnaces are primarily heated by radiation coming off of the tubes; thus these tubes are called "radiant tubes."
Such "radiant tubes" are well known to those skilled in the art and are described, e.g., in applicant's United States patents 5,655,599, 5,071,685, and 4,789,506. The tubes sometimes contain "inserts" to increase heat transfer from the combustion gases to the inside surface of the radiant tube. Thus, e.g., United States patent 4,869,230 describes a "turbulator insert" in a radiant tube which is formed as a corrugated strip of metal alloy material twisted to form a helix.
Such metal alloy material inserts, while initially effective, are not very durable.
Thus, some work has been done to replace these metal alloy material inserts with ceramic inserts. Illustrative of such work are the "FLAME BUSTER" ceramic inserts sold by Ipsen Industries, Inc. of Rockford, Illinois in the 1950's. These inserts met with only limited commercial success, primarily because they would tend to break while in use and often damage the burner assemblies. Reference also may be had, e.g., to United States patent 2,861,596.
These ceramic inserts were discussed in United States patent 4,153,035of Carl-Heinz Stiasny, in which it is disclosed that "Such flame breakers may be made of ceramic material. With a design of this type, heat losses...may be relatively high....Furthermore, flame breakers of this type...involve the risk of breakage particularly where vibrations in the furnace occur..."
The Stiasny patent issued in 1979. Since that time, to the best of applicant's knowledge, no flame breaker has been described in the prior art which is made of ceramic material, which has a substantially helical shape, and which provides efficient heat transfer and consequent low energy consumption and, additionally, is durable. It is an object of this invention to provide such a flame breaker.
Disclosure of the invention In accordance with this invention, there is provided an integral ceramic insert for a radiant tube which has a helical shape, has a thermal expansion of less than 6 x 10'6 meter/meter/degree Celsius, and has a thermal conductivity of at least 0.2 calories/centimeter/second/degree Celsius.
Brief description of the drawings The claimed invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which:
Figure 1 is a sectional view of a radiant tube assembly;
Figure 2 is a side view of one preferred ceramic insert of the invention;
Figure 3 is a schematic view of several suitable ceramic insert cross-sections;
Figure 4 is a schematic representation of a novel radiant tube assembly; and Figures 5 and 6 are schematics of one preferred means for making the ceramic insert of the invention.
Best Mode for Carrying Out the Invention The insert of this invention is made from aceramic material which has resistance to thermal shock. In general, such material will have a combination of low thermal expansion rate and high thermal conductivity properties.
As used herein, the term "ceramic" includes an inorganic material (such as silicon nitride, silicon carbide), used either by itself or with an infiltrant. Thus, as used in this specification, a body consisting essentially of silicon carbide is "ceramic."
Additionally, a body which consists of a porous silicon carbide body infiltrated with infiltrant such as molten silicon,also is "ceramic." Additionally, a mixture of silicon carbide particles and either graphite and/or amorphous carbon particles may be used to prepare a "ceramic." It is preferred that at least about 40 volume percent of the final material be comprised of either silicon carbide, silicon nitride, silicon, and/or mixtures thereof.
The thermal expansion rate of the ceramic material generally is less than 6.0 x 106 meter/meter/degree Celsius and, preferably, less than 4. S x 10-6 meter/meter/degree Celsius.
The thermal conductivity of the ceramic material varies with temperature. At a temperature of 25 degrees Celsius, the thermal conductivity of the ceramic material is at least about 0.2 (and preferably at least about 0.3) calories/centimeter/second/degree Celsius.
At a temperature of 1,200 degrees Celsius, the thermal conductivity of the ceramic material is at least about 0.05 (and preferably at least about 0.08) calories/centimeter/ second/degree Celsius.
The ceramic material preferably is substantially oxidation resistant in the combustion flame environment. The ceramic material preferably is also creep resistant.
When the material is heated to a temperature of at least about 1,400 degrees Celsius for at least about S years, it will not change its shape under its own weight.
In one preferred embodiment, the ceramic material is silicon carbide. In another preferred embodiment, the ceramic material is silicon nitride.
One may also use mixtures of ceramic materials which provide the required properties. Thus, e.g., one may use one or more of the materials disclosed in United States patent 5,523,133. Thus, one may use materials comprised of a silicon carbide matrix with ceramic oxide fiber reinforcements.
The ceramic insert may be comprised of a plurality of strips, each twisted longitudinally to define between its opposite end portions helical passages on opposite sides of each strip, wherein each of said strips are of a substantially uniform width; see, e.g., United States patent 5,523,133.
By way of further illustration, the ceramic insert of this invention may be a three-, four-, and/or six-leaf radiating surface tube insert, as is disclosed in United States patent 3,886,976 of Kardas et al.. In one aspect of this embodiment, the insert is the in the shape of a five-leaf cruciform; see, e.g., United States patent 2,895,508 of Drake. In another aspect of this embodiment, the insert may be in the shape of a spiral cruciform with notched edges;
see, e.g., United States patent 3,394,736 of Pearson.
In one embodiment, the ceramic insert has a cross sectional shape defined by a lateral portion inclined at right angles with respect to and on either side of a central portion. In another embodiment, the ceramic insert has a cross-sectional shape defined by lateral portions having a profile which is curved inwardly toward a plane N that is normal to the center portion. In yet another embodiment, the ceramic insert has a cross sectional shape defined by a central portion having double S-shaped curvature. These shapes are disclosed on page 104 of Topical Technical Report GRI 91-0146, published June, 1991 by the Gas Research Institute of Chicago, Illinois; and they are also disclosed in United States patent 4,700,749.
The ceramic insert may be in the shape of a swirl flow device, such as one or more of the swirl flow devices disclosed in United States patents 1,770,208, 1,916,337, 2,097,104, 2,161,887, 3,071,159, 3,407,871, 3,783,938, 3,870,081, 4,044,796, 4,090,559, 4,336,883, 4,559,998, 4,700,749, 4,823,865, and the like.
Figure 1 is a schematic representation of a radiant tube 10. Air and gas are fed into such tube through burner tip 12, creating an area of advancing flame 14. As the combustion process, continues, the mixture within tube 10 changes. Thus, at point 16, the mixture within tube 10 primarily contains gas and air. At point 18, the mixture within tube 10 contains gas, air, and combustion products. At point 20, combustion is complete, and normally all of the fuel has been consumed. As will be apparent, the temperature at pointsl6, 18, and 20 will differ.
At point 22, the mixture within tube 10 first contacts helical ceramic insert 24, and it exchanges heat with such insert. The temperature at point 22 will differ from the temperatures at points 26 and 28.
It is often desirable to have the outer surface of tube 1 exhibit the same temperature from end 28 to end 30 and, preferably, have the temperature of the exit gas at point 32 be no higher that the temperature of furnace 34. Such an ideal condition assures uniform furnace heating and maximizes the efficiency of the heat transfer.
In order to approach this ideal condition, applicant has designed a series of novel radiant tube assemblies. One of these assemblies is illustrated in Figure 2.
It will be seen that radiant tube assembly 40 is comprised of a radiant tube 42 and disposed therein a variable pitch helical ceramic insert 44. As is known, pitch refers to the distance between two adjacent turns of the helices. Thus, in variable pitch insert 44, pitch 46, pitch 48, pitch 50, and pitch 52 are not necessarily all equal to each other.
Although Figure 2 illustrates a helical insert 44 with 5 helical sections, it will be apparent that helices with fewer or more sections may be used.
The helical insert 24 will have a length such that the ratio of its length to its diameter is from about 1/1 to about 15/1, and preferably from about 2/1 to about 8/1.
The diameter of helical insert 44 is from about 2 to about 10 inches.
The pitches used in helical insert 44 range from about 2 to about 32 inches;
The pitch of the helical insert 44 divided by the diameter of the insert will determine the helix angle 45 (see Figure 2). For the preferred helical insert 44, the helix angle 45 will range from about 1 S to about 80 degrees and, preferably from about 40 to about 80 degrees, and more preferably from about 60 to about 80 degrees.
The pitches used in helical insert 44 range from about 0.5 to about 8 inches of pitch per inch of diameter of the helical insert.
The pitch at the point 22 of the insert 44 nearest the burner (not shown) is larger than the pitch at point 56 nearest the gas exit port (not shown).
Referring again to Figure 2, ceramic insert 44 is an integral assembly. In another embodiment, not shown, such assembly 44 could comprise two or more segments contiguous with each other. Thus, e.g., one could have such contiguity at point 60 between two separate segments.
One may vary the heat dissipation properties of the radiant tube by utilizing ceramic inserts with different cross sectional shapes. Refernng to Figure 3, in which dotted lines 73 indicate the center lines of the inserts, one may use the substantially tape-like shape 69, the substantially tape-like shape 70, the three-winged cross-sectional shape 72, the four winged cross sectional shape 74, the five winged cross-sectional shape 76, and the six winged cross-sectional shape 78. The inserts made from shapes 69, 70, 72, 74, 76, and/or 78 are preferably helical along their length.
It is desirable to vary the heat transfer characteristics of the radiant tube assembly comprised of the radiant tube 10 and the insert 24 so that the temperature radiated by the assembly is substantially more uniform along its length. Without the use of a ceramic insert, the temperature within the tube 10 will vary as composition of the reaction mixture within the tube, and/or its temperature, varies. The inventions described in this specification tend to minimize such variances.
One may vary the heat transfer characteristics of the insert within the tube 10, from one point to another by means such as those illustrated in Figure 2, wherein the pitch and pitch angle of the insert are varied from one end to another. Another means of doing so is illustrated in Figure 3, wherein the surface area of the insert is varied. As "wings" are added to the insert, the surface area of the insert increases, and the heat transfer characteristics increase. Referring to Figure 3, the "wing" portion is the portion denoted by a solid line extending outwardly from the centerpoint.
Figure 4 illustrates an assembly 100 comprised of a radiant tube 102 in which there disposed ceramic inserts 104, 106, 108, and 110; in the embodiment depicted, tube 102 is linear.
In the embodiment depicted, ceramic insert 104 is comprised of three wings, ceramic insert 106 is comprised of four wings, ceramic insert 108 is comprised of/
five wings, and ceramic insert 110 is comprised of eight wings. In this embodiment, inserts 104, 106, 108, and 110 are substantially contiguous with each other. In another embodiment, not shown, a ceramic insert with two wings is disposed in front of ceramic insert 104.
In one embodiment, not shown, in addition to the number of wing in adjacent insert sections, or instead of varying such wings, one may vary either the helix angle or pitch in adj acent sections.
Referring again to Figure 4, the air/gas mixture 112 is combusted in a burner (not shown) and travels in the direction of arrow 114 down tube 102. At a certain point 116 the flame caused by the combustion of mixture 112 ceases to exist. The temperature of mixture 112 decreases as it travels in the direction of arrow 114.
When the air gas mixture 112 contacts insert 104, it is relatively hot; the insert 104, because it has a relatively low surface area, has a relatively low rate of heat transfer to the inner surface of tube 102.
When the air gas mixture 112 contacts insert 106, it is cooler than when it contacted insert 104; thus insert 106 is designed with a higher surface area in order to provide a higher rate of heat transfer than that provided by insert 104; the goal is, by balancing these variables, to maintain the outer surface of tube 102 at substantially the same temperature.
Another means of varying the heat transfer characteristics of the radiant tube assembly is by utilizing discontinuous insert segments, i.e., segments which are not contiguous with each other. Such an arrangement is illustrated in Figure 1, wherein the section 120 of the tube 10 contains no ceramic insert, but the section 122 of the tube 10 does contain such an insert.
Referring again to Figure 1, radiant tube 10 is substantially linear. In another embodiment, not shown, the radiant tube 10 will be substantially arcuate, being substantially U-shaped or W-shaped. In another embodiment, not shown, the radiant tube 10 will have both linear and arcuate portions.
In one embodiment, not shown, the radiant tube 10 has two straight legs connected to an arcuate elbow. This type of radiant tube is often referred to as a U-type tube.
By way of further illustration, one may use a W-tube with four legs and three elbows, which is also comprised of linear and arcuate sections.
It is preferred to use the ceramic insert of this invention in those portions of the radiant tubes closest to the exhaust; for, in such portions, the combustion mixture will generally be at a lower temperature than it is in the portions nearer the burner.
Figure 5 is a flow diagram of one preferred process for making an ceramic insert.
The process described in this flow diagram involves the use of silicon carbide grains;
however, it will be apparent that the process is also useful with other ceramic materials.
Referring to Figure 5, a round funnel 130 is disposed within a vertical closed bottom aluminum tube 132. Thereafter, an inside flat blade forming funnel is disposed within the funnel 132. The tube 132, the funnel 130, and the funnel 134 are attached to each other by conventional means (such as screws) in order to maintain them in fixed spatial relationship vis-a-vis each other. The spatial relationship of funnels 130 and 134 is also illustrated in Figure 6.
Funnel 130 is filled with silicon carbide grains. It is preferred that at least about 99 weight percent of the silicon carbide grains have an average particle size of from about 50 to about 1000 microns and, more preferably, of from about 150 to about 250 microns. In one embodiment, substantially all of the silicon carbide grains have an average particle size of from about 160 to about 220 microns.
For the silicon carbide grains described above, the desired particle size ranges facilitate the pourability of the powder. When other powders are used for form the ceramic material, different particle size ranges may be desirable.
The silicon carbide grains 136 are preferably poured into funnel 130 until the grains reach near the top of such funnel. Thereafter, a mixture 138 comprised of such silicon carbide grains 136 and resin are poured into funnel 134.
A relatively small amount of such resin (from about 1.5 to about 5 weight percent, weight of dry powdered resin by total weight of resin and silicon carbide) is used. The resin is used as a binder which will afford structural integrity to the tape formed within funnel neck 140.
In one embodiment, the resin used is a dry powdered phenolic resin sold as "Durez 29-302" by the Occidental Chemical Corporation (Durez Division) of Niagara Falls, New York.
Once both of the funnels 130 and 134 have been filled with grains, the funnel 134/funnel 130 assembly is simultaneously rotated in the direction of arrow 142 while being pulled upwardly in the direction of arrow 144. Varying the rate of rotation for a given lift rate will vary the pitch on the helix being formed by the process.
If the funnel 134/funnel 130 assembly is lifted without being rotated, a straight extruded blade will be formed. If the funnel is lifted while being rotated in one direction and then in another direction, a tape with reversing helical portions will be formed. If a funnel 134 is used with a cross section different than the rectangular cross section depicted in Figure 5, the helical tape formed will have such different cross section (see Figure 3).
Reference may be had to applicant's United States patents 5,655,599, 5,071,685, and 4,789,506 for a discussion of other aspects of and uses for this process.
The funnel 134/funnel 130 assembly may be turned by conventional means, such as by means of a cam follower. Inasmuch as funnels 130 and 134 are attached to each other, the twisting and raising of funnel 134 also twists and raises funnel 130. The removal of the 130/134 funnel assembly leaves the formed helical tape within a bed of loose grains of silicon carbide, both of which are disposed within container 132. Thereafter, container 132 with the helical tape therein and the loose silicon carbide is transported to an oven (not shown) where it is heated to a temperature of from about 350 to about 450 degrees Fahrenheit to set the resin particles and afford structural integrity to the helical tape.
After heating, the formed helical tape is removed from the bed of silicon carbide particles. The tape as formed is then treated to transform the resin while maintaining the structural integrity of tape.
One such transformation process involves contacting the tape with molten silicon, which infiltrates and/or wicks into the body of the tape, converts the resin to elemental carbon, and thereafter converts the elemental carbon into secondary silicon carbide. It is preferred to contact the tape with the molten silicon in a vacuum chamber and/or an inert gas atmosphere to prevent oxidation of the resin (which would form carbon dioxide and remove the support from the silicon carbide grains) while subjecting the tape and the silicon to a temperature of from about 1,500 to about 1,900 degrees Celsius for a period of less than about 1 S minutes.
The infiltrated tape thus formed is allowed to cool. Thereafter it is ready to use in the structure depicted in Figure 1.
The helical tape may be treated to form other silicon infiltrated materials besides the one discussed above. Thus, e.g., one could use a graphite grain, or amorphous carbon grain, rather than the silicon carbide grain.
It is to be understood that the aforementioned description is illustrative only and that changes can be made in the insert and/or the process for making it.
Background Art Many industrial process furnaces require special atmospheres and, thus, cannot be directly heated by means of gas combustion. These special atmosphere furnaces are often heated by means of a system ire which gas-air combustion takes place within long metal alloy tubes which exit to the outside of the furnace wall to prevent contamination of the furnace's atmosphere. These furnaces are primarily heated by radiation coming off of the tubes; thus these tubes are called "radiant tubes."
Such "radiant tubes" are well known to those skilled in the art and are described, e.g., in applicant's United States patents 5,655,599, 5,071,685, and 4,789,506. The tubes sometimes contain "inserts" to increase heat transfer from the combustion gases to the inside surface of the radiant tube. Thus, e.g., United States patent 4,869,230 describes a "turbulator insert" in a radiant tube which is formed as a corrugated strip of metal alloy material twisted to form a helix.
Such metal alloy material inserts, while initially effective, are not very durable.
Thus, some work has been done to replace these metal alloy material inserts with ceramic inserts. Illustrative of such work are the "FLAME BUSTER" ceramic inserts sold by Ipsen Industries, Inc. of Rockford, Illinois in the 1950's. These inserts met with only limited commercial success, primarily because they would tend to break while in use and often damage the burner assemblies. Reference also may be had, e.g., to United States patent 2,861,596.
These ceramic inserts were discussed in United States patent 4,153,035of Carl-Heinz Stiasny, in which it is disclosed that "Such flame breakers may be made of ceramic material. With a design of this type, heat losses...may be relatively high....Furthermore, flame breakers of this type...involve the risk of breakage particularly where vibrations in the furnace occur..."
The Stiasny patent issued in 1979. Since that time, to the best of applicant's knowledge, no flame breaker has been described in the prior art which is made of ceramic material, which has a substantially helical shape, and which provides efficient heat transfer and consequent low energy consumption and, additionally, is durable. It is an object of this invention to provide such a flame breaker.
Disclosure of the invention In accordance with this invention, there is provided an integral ceramic insert for a radiant tube which has a helical shape, has a thermal expansion of less than 6 x 10'6 meter/meter/degree Celsius, and has a thermal conductivity of at least 0.2 calories/centimeter/second/degree Celsius.
Brief description of the drawings The claimed invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which:
Figure 1 is a sectional view of a radiant tube assembly;
Figure 2 is a side view of one preferred ceramic insert of the invention;
Figure 3 is a schematic view of several suitable ceramic insert cross-sections;
Figure 4 is a schematic representation of a novel radiant tube assembly; and Figures 5 and 6 are schematics of one preferred means for making the ceramic insert of the invention.
Best Mode for Carrying Out the Invention The insert of this invention is made from aceramic material which has resistance to thermal shock. In general, such material will have a combination of low thermal expansion rate and high thermal conductivity properties.
As used herein, the term "ceramic" includes an inorganic material (such as silicon nitride, silicon carbide), used either by itself or with an infiltrant. Thus, as used in this specification, a body consisting essentially of silicon carbide is "ceramic."
Additionally, a body which consists of a porous silicon carbide body infiltrated with infiltrant such as molten silicon,also is "ceramic." Additionally, a mixture of silicon carbide particles and either graphite and/or amorphous carbon particles may be used to prepare a "ceramic." It is preferred that at least about 40 volume percent of the final material be comprised of either silicon carbide, silicon nitride, silicon, and/or mixtures thereof.
The thermal expansion rate of the ceramic material generally is less than 6.0 x 106 meter/meter/degree Celsius and, preferably, less than 4. S x 10-6 meter/meter/degree Celsius.
The thermal conductivity of the ceramic material varies with temperature. At a temperature of 25 degrees Celsius, the thermal conductivity of the ceramic material is at least about 0.2 (and preferably at least about 0.3) calories/centimeter/second/degree Celsius.
At a temperature of 1,200 degrees Celsius, the thermal conductivity of the ceramic material is at least about 0.05 (and preferably at least about 0.08) calories/centimeter/ second/degree Celsius.
The ceramic material preferably is substantially oxidation resistant in the combustion flame environment. The ceramic material preferably is also creep resistant.
When the material is heated to a temperature of at least about 1,400 degrees Celsius for at least about S years, it will not change its shape under its own weight.
In one preferred embodiment, the ceramic material is silicon carbide. In another preferred embodiment, the ceramic material is silicon nitride.
One may also use mixtures of ceramic materials which provide the required properties. Thus, e.g., one may use one or more of the materials disclosed in United States patent 5,523,133. Thus, one may use materials comprised of a silicon carbide matrix with ceramic oxide fiber reinforcements.
The ceramic insert may be comprised of a plurality of strips, each twisted longitudinally to define between its opposite end portions helical passages on opposite sides of each strip, wherein each of said strips are of a substantially uniform width; see, e.g., United States patent 5,523,133.
By way of further illustration, the ceramic insert of this invention may be a three-, four-, and/or six-leaf radiating surface tube insert, as is disclosed in United States patent 3,886,976 of Kardas et al.. In one aspect of this embodiment, the insert is the in the shape of a five-leaf cruciform; see, e.g., United States patent 2,895,508 of Drake. In another aspect of this embodiment, the insert may be in the shape of a spiral cruciform with notched edges;
see, e.g., United States patent 3,394,736 of Pearson.
In one embodiment, the ceramic insert has a cross sectional shape defined by a lateral portion inclined at right angles with respect to and on either side of a central portion. In another embodiment, the ceramic insert has a cross-sectional shape defined by lateral portions having a profile which is curved inwardly toward a plane N that is normal to the center portion. In yet another embodiment, the ceramic insert has a cross sectional shape defined by a central portion having double S-shaped curvature. These shapes are disclosed on page 104 of Topical Technical Report GRI 91-0146, published June, 1991 by the Gas Research Institute of Chicago, Illinois; and they are also disclosed in United States patent 4,700,749.
The ceramic insert may be in the shape of a swirl flow device, such as one or more of the swirl flow devices disclosed in United States patents 1,770,208, 1,916,337, 2,097,104, 2,161,887, 3,071,159, 3,407,871, 3,783,938, 3,870,081, 4,044,796, 4,090,559, 4,336,883, 4,559,998, 4,700,749, 4,823,865, and the like.
Figure 1 is a schematic representation of a radiant tube 10. Air and gas are fed into such tube through burner tip 12, creating an area of advancing flame 14. As the combustion process, continues, the mixture within tube 10 changes. Thus, at point 16, the mixture within tube 10 primarily contains gas and air. At point 18, the mixture within tube 10 contains gas, air, and combustion products. At point 20, combustion is complete, and normally all of the fuel has been consumed. As will be apparent, the temperature at pointsl6, 18, and 20 will differ.
At point 22, the mixture within tube 10 first contacts helical ceramic insert 24, and it exchanges heat with such insert. The temperature at point 22 will differ from the temperatures at points 26 and 28.
It is often desirable to have the outer surface of tube 1 exhibit the same temperature from end 28 to end 30 and, preferably, have the temperature of the exit gas at point 32 be no higher that the temperature of furnace 34. Such an ideal condition assures uniform furnace heating and maximizes the efficiency of the heat transfer.
In order to approach this ideal condition, applicant has designed a series of novel radiant tube assemblies. One of these assemblies is illustrated in Figure 2.
It will be seen that radiant tube assembly 40 is comprised of a radiant tube 42 and disposed therein a variable pitch helical ceramic insert 44. As is known, pitch refers to the distance between two adjacent turns of the helices. Thus, in variable pitch insert 44, pitch 46, pitch 48, pitch 50, and pitch 52 are not necessarily all equal to each other.
Although Figure 2 illustrates a helical insert 44 with 5 helical sections, it will be apparent that helices with fewer or more sections may be used.
The helical insert 24 will have a length such that the ratio of its length to its diameter is from about 1/1 to about 15/1, and preferably from about 2/1 to about 8/1.
The diameter of helical insert 44 is from about 2 to about 10 inches.
The pitches used in helical insert 44 range from about 2 to about 32 inches;
The pitch of the helical insert 44 divided by the diameter of the insert will determine the helix angle 45 (see Figure 2). For the preferred helical insert 44, the helix angle 45 will range from about 1 S to about 80 degrees and, preferably from about 40 to about 80 degrees, and more preferably from about 60 to about 80 degrees.
The pitches used in helical insert 44 range from about 0.5 to about 8 inches of pitch per inch of diameter of the helical insert.
The pitch at the point 22 of the insert 44 nearest the burner (not shown) is larger than the pitch at point 56 nearest the gas exit port (not shown).
Referring again to Figure 2, ceramic insert 44 is an integral assembly. In another embodiment, not shown, such assembly 44 could comprise two or more segments contiguous with each other. Thus, e.g., one could have such contiguity at point 60 between two separate segments.
One may vary the heat dissipation properties of the radiant tube by utilizing ceramic inserts with different cross sectional shapes. Refernng to Figure 3, in which dotted lines 73 indicate the center lines of the inserts, one may use the substantially tape-like shape 69, the substantially tape-like shape 70, the three-winged cross-sectional shape 72, the four winged cross sectional shape 74, the five winged cross-sectional shape 76, and the six winged cross-sectional shape 78. The inserts made from shapes 69, 70, 72, 74, 76, and/or 78 are preferably helical along their length.
It is desirable to vary the heat transfer characteristics of the radiant tube assembly comprised of the radiant tube 10 and the insert 24 so that the temperature radiated by the assembly is substantially more uniform along its length. Without the use of a ceramic insert, the temperature within the tube 10 will vary as composition of the reaction mixture within the tube, and/or its temperature, varies. The inventions described in this specification tend to minimize such variances.
One may vary the heat transfer characteristics of the insert within the tube 10, from one point to another by means such as those illustrated in Figure 2, wherein the pitch and pitch angle of the insert are varied from one end to another. Another means of doing so is illustrated in Figure 3, wherein the surface area of the insert is varied. As "wings" are added to the insert, the surface area of the insert increases, and the heat transfer characteristics increase. Referring to Figure 3, the "wing" portion is the portion denoted by a solid line extending outwardly from the centerpoint.
Figure 4 illustrates an assembly 100 comprised of a radiant tube 102 in which there disposed ceramic inserts 104, 106, 108, and 110; in the embodiment depicted, tube 102 is linear.
In the embodiment depicted, ceramic insert 104 is comprised of three wings, ceramic insert 106 is comprised of four wings, ceramic insert 108 is comprised of/
five wings, and ceramic insert 110 is comprised of eight wings. In this embodiment, inserts 104, 106, 108, and 110 are substantially contiguous with each other. In another embodiment, not shown, a ceramic insert with two wings is disposed in front of ceramic insert 104.
In one embodiment, not shown, in addition to the number of wing in adjacent insert sections, or instead of varying such wings, one may vary either the helix angle or pitch in adj acent sections.
Referring again to Figure 4, the air/gas mixture 112 is combusted in a burner (not shown) and travels in the direction of arrow 114 down tube 102. At a certain point 116 the flame caused by the combustion of mixture 112 ceases to exist. The temperature of mixture 112 decreases as it travels in the direction of arrow 114.
When the air gas mixture 112 contacts insert 104, it is relatively hot; the insert 104, because it has a relatively low surface area, has a relatively low rate of heat transfer to the inner surface of tube 102.
When the air gas mixture 112 contacts insert 106, it is cooler than when it contacted insert 104; thus insert 106 is designed with a higher surface area in order to provide a higher rate of heat transfer than that provided by insert 104; the goal is, by balancing these variables, to maintain the outer surface of tube 102 at substantially the same temperature.
Another means of varying the heat transfer characteristics of the radiant tube assembly is by utilizing discontinuous insert segments, i.e., segments which are not contiguous with each other. Such an arrangement is illustrated in Figure 1, wherein the section 120 of the tube 10 contains no ceramic insert, but the section 122 of the tube 10 does contain such an insert.
Referring again to Figure 1, radiant tube 10 is substantially linear. In another embodiment, not shown, the radiant tube 10 will be substantially arcuate, being substantially U-shaped or W-shaped. In another embodiment, not shown, the radiant tube 10 will have both linear and arcuate portions.
In one embodiment, not shown, the radiant tube 10 has two straight legs connected to an arcuate elbow. This type of radiant tube is often referred to as a U-type tube.
By way of further illustration, one may use a W-tube with four legs and three elbows, which is also comprised of linear and arcuate sections.
It is preferred to use the ceramic insert of this invention in those portions of the radiant tubes closest to the exhaust; for, in such portions, the combustion mixture will generally be at a lower temperature than it is in the portions nearer the burner.
Figure 5 is a flow diagram of one preferred process for making an ceramic insert.
The process described in this flow diagram involves the use of silicon carbide grains;
however, it will be apparent that the process is also useful with other ceramic materials.
Referring to Figure 5, a round funnel 130 is disposed within a vertical closed bottom aluminum tube 132. Thereafter, an inside flat blade forming funnel is disposed within the funnel 132. The tube 132, the funnel 130, and the funnel 134 are attached to each other by conventional means (such as screws) in order to maintain them in fixed spatial relationship vis-a-vis each other. The spatial relationship of funnels 130 and 134 is also illustrated in Figure 6.
Funnel 130 is filled with silicon carbide grains. It is preferred that at least about 99 weight percent of the silicon carbide grains have an average particle size of from about 50 to about 1000 microns and, more preferably, of from about 150 to about 250 microns. In one embodiment, substantially all of the silicon carbide grains have an average particle size of from about 160 to about 220 microns.
For the silicon carbide grains described above, the desired particle size ranges facilitate the pourability of the powder. When other powders are used for form the ceramic material, different particle size ranges may be desirable.
The silicon carbide grains 136 are preferably poured into funnel 130 until the grains reach near the top of such funnel. Thereafter, a mixture 138 comprised of such silicon carbide grains 136 and resin are poured into funnel 134.
A relatively small amount of such resin (from about 1.5 to about 5 weight percent, weight of dry powdered resin by total weight of resin and silicon carbide) is used. The resin is used as a binder which will afford structural integrity to the tape formed within funnel neck 140.
In one embodiment, the resin used is a dry powdered phenolic resin sold as "Durez 29-302" by the Occidental Chemical Corporation (Durez Division) of Niagara Falls, New York.
Once both of the funnels 130 and 134 have been filled with grains, the funnel 134/funnel 130 assembly is simultaneously rotated in the direction of arrow 142 while being pulled upwardly in the direction of arrow 144. Varying the rate of rotation for a given lift rate will vary the pitch on the helix being formed by the process.
If the funnel 134/funnel 130 assembly is lifted without being rotated, a straight extruded blade will be formed. If the funnel is lifted while being rotated in one direction and then in another direction, a tape with reversing helical portions will be formed. If a funnel 134 is used with a cross section different than the rectangular cross section depicted in Figure 5, the helical tape formed will have such different cross section (see Figure 3).
Reference may be had to applicant's United States patents 5,655,599, 5,071,685, and 4,789,506 for a discussion of other aspects of and uses for this process.
The funnel 134/funnel 130 assembly may be turned by conventional means, such as by means of a cam follower. Inasmuch as funnels 130 and 134 are attached to each other, the twisting and raising of funnel 134 also twists and raises funnel 130. The removal of the 130/134 funnel assembly leaves the formed helical tape within a bed of loose grains of silicon carbide, both of which are disposed within container 132. Thereafter, container 132 with the helical tape therein and the loose silicon carbide is transported to an oven (not shown) where it is heated to a temperature of from about 350 to about 450 degrees Fahrenheit to set the resin particles and afford structural integrity to the helical tape.
After heating, the formed helical tape is removed from the bed of silicon carbide particles. The tape as formed is then treated to transform the resin while maintaining the structural integrity of tape.
One such transformation process involves contacting the tape with molten silicon, which infiltrates and/or wicks into the body of the tape, converts the resin to elemental carbon, and thereafter converts the elemental carbon into secondary silicon carbide. It is preferred to contact the tape with the molten silicon in a vacuum chamber and/or an inert gas atmosphere to prevent oxidation of the resin (which would form carbon dioxide and remove the support from the silicon carbide grains) while subjecting the tape and the silicon to a temperature of from about 1,500 to about 1,900 degrees Celsius for a period of less than about 1 S minutes.
The infiltrated tape thus formed is allowed to cool. Thereafter it is ready to use in the structure depicted in Figure 1.
The helical tape may be treated to form other silicon infiltrated materials besides the one discussed above. Thus, e.g., one could use a graphite grain, or amorphous carbon grain, rather than the silicon carbide grain.
It is to be understood that the aforementioned description is illustrative only and that changes can be made in the insert and/or the process for making it.
Claims (9)
1. An integral ceramic insert for a radiant tube, wherein said integral ceramic insert has a substantially helical shape, and wherein:
(a) said integral ceramic insert is comprised of at least 40 volume percent of a material selected from the group consisting of silicon, silicon carbide, silicon nitride, and mixtures thereof, (b) said integral ceramic insert has a thermal expansion rate of less than 6.0 x 10-6 meters/meter/degree Celsius, (c) said integral ceramic insert has a thermal conductivity, at a temperature of 25 degrees Celsius, of at least 0.2 calories/centimeter/second/degree Celsius, (d) said integral ceramic insert has a diameter of from about 2 to about 10 inches and a length such that the ratio of said length to said diameter of said integral ceramic insert is from about 2/1 to about 8/1, and (e) said helical shape is comprised of one or more pitches of from about 2 to about 32 inches, each which defines a helix angle of from about 15 to about 80 degrees, provided that there is from about 0.5 to about 8 inches of pitch per inch of diameter of the integral ceramic insert.
(a) said integral ceramic insert is comprised of at least 40 volume percent of a material selected from the group consisting of silicon, silicon carbide, silicon nitride, and mixtures thereof, (b) said integral ceramic insert has a thermal expansion rate of less than 6.0 x 10-6 meters/meter/degree Celsius, (c) said integral ceramic insert has a thermal conductivity, at a temperature of 25 degrees Celsius, of at least 0.2 calories/centimeter/second/degree Celsius, (d) said integral ceramic insert has a diameter of from about 2 to about 10 inches and a length such that the ratio of said length to said diameter of said integral ceramic insert is from about 2/1 to about 8/1, and (e) said helical shape is comprised of one or more pitches of from about 2 to about 32 inches, each which defines a helix angle of from about 15 to about 80 degrees, provided that there is from about 0.5 to about 8 inches of pitch per inch of diameter of the integral ceramic insert.
2. The integral ceramic insert as recited in claim 1, wherein said ceramic insert is comprised of at least 40 volume percent of silicon carbide.
3. The integral ceramic insert as recited in claim 1, wherein said helix angle is from about 60 to about 80 degrees.
4. The integral ceramic insert as recited in claim 3, wherein said ceramic insert has a cross-sectional shape defined by a centerpoint and extending radially outwardly therefrom at least five wings.
5. A radiant tube comprised of a first integral ceramic insert and a second integral ceramic insert disposed inside of tube, wherein each of said first integral ceramic insert and said second insert ceramic insert has a substantially helical shape, and wherein:
(a) each of said first integral ceramic insert and said second integral ceramic insert is comprised of at least 40 volume percent of a material selected from the group consisting of silicon, silicon carbide, silicon nitride, and mixtures thereof, (b) each of said first integral ceramic insert and said second integral ceramic insert has a thermal expansion rate of less than 6.0 x 10-6 meters/meter/degree Celsius, (c) each of said first integral ceramic insert and said second integral ceramic insert has a thermal conductivity, at a temperature of 25 degrees Celsius, of at least 0.2 calories/centimeter/second/degree Celsius, (d) each of said first integral ceramic insert and said second integral ceramic insert has a diameter of from about 2 to about 10 inches and a length such that the ratio of said length to said diameter of said integral ceramic insert is from about 2/1 to about 8/1, and (e) each of said first integral ceramic insert and said second integral ceramic insert has a helical shape which comprised of one or more pitches of from about 2 to about 32 inches, each which pitches defines a helix angle of from about 15 to about 80 degrees, provided that there is from about 0.5 to about 8 inches of pitch per inch of diameter of said each of said integral ceramic inserts.
(a) each of said first integral ceramic insert and said second integral ceramic insert is comprised of at least 40 volume percent of a material selected from the group consisting of silicon, silicon carbide, silicon nitride, and mixtures thereof, (b) each of said first integral ceramic insert and said second integral ceramic insert has a thermal expansion rate of less than 6.0 x 10-6 meters/meter/degree Celsius, (c) each of said first integral ceramic insert and said second integral ceramic insert has a thermal conductivity, at a temperature of 25 degrees Celsius, of at least 0.2 calories/centimeter/second/degree Celsius, (d) each of said first integral ceramic insert and said second integral ceramic insert has a diameter of from about 2 to about 10 inches and a length such that the ratio of said length to said diameter of said integral ceramic insert is from about 2/1 to about 8/1, and (e) each of said first integral ceramic insert and said second integral ceramic insert has a helical shape which comprised of one or more pitches of from about 2 to about 32 inches, each which pitches defines a helix angle of from about 15 to about 80 degrees, provided that there is from about 0.5 to about 8 inches of pitch per inch of diameter of said each of said integral ceramic inserts.
6. The radiant tube as recited in claim 5, wherein the thermal expansion rate of said first integral ceramic insert differs from the thermal expansion rate of said second integral ceramic insert.
7. The radiant tube as recited in claim 5, wherein the thermal conductivity of said first integral ceramic insert differs from the thermal conductivity of said second integral ceramic insert.
8. The radiant tube as recited in claim 5, wherein said ratio of said length to said diameter of said first integral ceramic insert differs from said ratio of said length to said diameter of said second integral ceramic insert.
9. The radiant tube as recited in claim 5, wherein said ratio of said pitch per inch of said diameter of said first integral ceramic insert differs from said ratio of said pitch per inch of said diameter of said second integral ceramic insert.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US15330699P | 1999-09-10 | 1999-09-10 | |
| US60/153,306 | 1999-09-10 | ||
| PCT/US2000/024755 WO2001018476A1 (en) | 1999-09-10 | 2000-09-08 | Insert for a radiant tube |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2384375A1 true CA2384375A1 (en) | 2001-03-15 |
Family
ID=22546632
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002384375A Abandoned CA2384375A1 (en) | 1999-09-10 | 2000-09-08 | Insert for a radiant tube |
Country Status (3)
| Country | Link |
|---|---|
| US (1) | US6484795B1 (en) |
| CA (1) | CA2384375A1 (en) |
| WO (1) | WO2001018476A1 (en) |
Families Citing this family (22)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE03726967T1 (en) | 2002-01-03 | 2005-05-04 | Pax Scient Inc | EDDY RING GENERATOR |
| AUPR982502A0 (en) * | 2002-01-03 | 2002-01-31 | Pax Fluid Systems Inc. | A heat exchanger |
| AUPR982302A0 (en) | 2002-01-03 | 2002-01-31 | Pax Fluid Systems Inc. | A fluid flow controller |
| AU2003903386A0 (en) | 2003-07-02 | 2003-07-17 | Pax Scientific, Inc | Fluid flow control device |
| KR101168098B1 (en) | 2003-11-04 | 2012-07-24 | 팍스 싸이언티픽 인코퍼레이션 | Fluid Circulation System |
| ITMI20040040A1 (en) * | 2004-01-15 | 2004-04-15 | Maurizio Spoto | INCREASED HEAT EXCHANGER ELEMENT |
| WO2005073560A1 (en) * | 2004-01-30 | 2005-08-11 | Pax Scientific, Inc | A vortical flow rotor |
| GB2424695A (en) * | 2005-03-24 | 2006-10-04 | Modine Mfg Co | A heat exchanger tube of a gas fired heater |
| DE102005029321A1 (en) * | 2005-06-24 | 2006-12-28 | Behr Gmbh & Co. Kg | Heat exchanger for exhaust gas cooling has structural elements arranged so that duct has internal variable heat transfer increasing in direction of flow |
| US8162040B2 (en) | 2006-03-10 | 2012-04-24 | Spinworks, LLC | Heat exchanging insert and method for fabricating same |
| WO2008036515A2 (en) * | 2006-09-18 | 2008-03-27 | Storm Development Llc | Radiant heat transfer system |
| US8328522B2 (en) * | 2006-09-29 | 2012-12-11 | Pax Scientific, Inc. | Axial flow fan |
| CA2681281C (en) * | 2007-03-28 | 2016-02-09 | China Petroleum And Chemical Corporation | A tubular cracking furnace |
| US20100279007A1 (en) * | 2007-08-14 | 2010-11-04 | The Penn State Research Foundation | 3-D Printing of near net shape products |
| US20090301699A1 (en) * | 2008-06-05 | 2009-12-10 | Lummus Novolent Gmbh/Lummus Technology Inc. | Vertical combined feed/effluent heat exchanger with variable baffle angle |
| WO2015062619A1 (en) | 2013-10-28 | 2015-05-07 | Erbicol Sa | Inserts for burners and radiant tube heating systems |
| DE102013224038A1 (en) * | 2013-11-25 | 2015-05-28 | MAHLE Behr GmbH & Co. KG | Exhaust gas heat exchanger for exhaust gas cooling of an internal combustion engine, preferably for a motor vehicle |
| US20150300745A1 (en) * | 2014-04-16 | 2015-10-22 | Enterex America LLC | Counterflow helical heat exchanger |
| US20190346216A1 (en) * | 2018-05-08 | 2019-11-14 | United Technologies Corporation | Swirling feed tube for heat exchanger |
| USD910829S1 (en) | 2019-04-12 | 2021-02-16 | Saint-Gobain Ceramics & Plastics, Inc. | Flame diffuser insert for immersion tube furnace |
| USD910830S1 (en) | 2019-04-12 | 2021-02-16 | Saint-Gobain Ceramics & Plastics, Inc. | Flame diffuser insert for immersion tube furnace |
| US20230071567A1 (en) * | 2021-09-03 | 2023-03-09 | Saint-Gobain Ceramics & Plastics, Inc. | Bodies configured for use in radiant tubes |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3071159A (en) * | 1958-04-19 | 1963-01-01 | Coraggioso Corrado Bono | Heat exchanger tube |
-
2000
- 2000-09-08 CA CA002384375A patent/CA2384375A1/en not_active Abandoned
- 2000-09-08 US US09/658,143 patent/US6484795B1/en not_active Expired - Fee Related
- 2000-09-08 WO PCT/US2000/024755 patent/WO2001018476A1/en active Application Filing
Also Published As
| Publication number | Publication date |
|---|---|
| WO2001018476A1 (en) | 2001-03-15 |
| US6484795B1 (en) | 2002-11-26 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |