WO2008036515A2

WO2008036515A2 - Radiant heat transfer system

Info

Publication number: WO2008036515A2
Application number: PCT/US2007/077951
Authority: WO
Inventors: Thomas D. Briselden
Original assignee: Storm Development Llc
Priority date: 2006-09-18
Filing date: 2007-09-08
Publication date: 2008-03-27
Also published as: JP5300725B2; JP2010503824A; EP2069692B1; WO2008036515A3; EP2069692A4; US20090277969A1; EP2069692A2

Abstract

A radiant heat transfer system has one or more radiant elements inserted in a radiant source. Radiant elements convert the combustion products from burning fuel into radiant energy. A radiant element may be formed from one or more ceramics and may be used in radiant sources, such as radiant tubes, immersion tubes, heat exchanger tubes, boiler walls, and other radiant heat applications. Each radiant element has a core section defining a longitudinal cavity enabling the insertion of a positioning mechanism. Each radiant element may have one or more normal and/or tangential wings attached to the exterior of the core section. Each radiant element may produce a more laminar or less turbulent flow of combustion products within the radiant source.

Description

Radiant Heat Transfer System

REFERENCE TO RELATED APPLICATIONS

[001] This application claims the benefit of U.S. Provisional Application No.

60/825,939 entitled "Radiant Heat Transfer System" filed September 18, 2006, which is incorporated by reference in its entirety.

BACKGROUND

[002] Radiant heating systems are used to heat steel, ceramics or other materials, water or other liquids, and the like. Many radiant heating systems have a radiant element positioned inside a radiant source. The radiant element is used to increase the heat transfer from the combustion of a fuel to the radiant source. The radiant element may prematurely or unexpectedly fail from the temperature and/or heating cycles. The radiant element also may create hot spots and other nonuniform heat transfer locations that cause the failure of the radiant source.

[003] Fuels are commonly burned (oxidized) to generate heat. Common fuels are fossil fuels, such as natural gas, oil, and coal, or renewable fuels, such as biomass, and the like. Once generated, the heat may be transferred to an oven to heat an object or to a fluid, such as a liquid or gas. The heat may be transferred by at least one of conduction, convection, and radiation. Conduction occurs in solids, where heat from one solid, or part of a solid, moves to another solid or part of the same solid. Convection occurs in fluids, such as water or air, where the heated fluid moves from one location to a second location. Radiation occurs when a heated object emits radiant energy that is absorbed by another object. Radiant heat transfer differs from conduction and convection in that contact with a solid, liquid, or gas is not needed to transfer the heat. 5ee Boyer, Howard E., Practical Heat Treating, American Society for Metals, Metals Park, Ohio, 1984, pp. 60-62. For example, the sun heats the earth by transferring radiant energy through the vacuum of space. [004] Industrial heating processes, often referred to as process heating, include the heat treating of steel or other metal parts, immersion heating where a liquid is heated to serve as a convection heat source, and steam generation for electricity production. Some industrial heating processes isolate the burning of fuel and the associated combustion products from what is being heated by containing the burning and combustion products in an enclosure, such as a tube. The fuel and oxidant are introduced at one end of the tube or enclosure and the combustion products, such as carbon dioxide, water vapor, and nitrogen, are removed from another end of the tube or enclosure. Thus, heat is indirectly transferred to what is being heated.

[005] FIG. 1 depicts a conventional radiant heat transfer system 100 for process heating that indirectly transfers the heat from a burning fuel 110 to a heating zone 120 contained by a furnace 130. The radiant heat transfer system 100 may include a diffusion flame burner 105 that includes inlets for air and fuel. A more detailed description of burners used with radiant heat transfer systems may be found in R. F. Harder, R. Viskanta and S. Ramadhyani, Gas-Fired Radiant Tubes: A Review of Literature, December 1987, Gas Research Institute, for example. While not shown in the figure, the furnace 130 may include fans or other devices to circulate a gaseous atmosphere within the furnace 130. The gaseous atmosphere my include hydrogen, nitrogen, and carbon monoxide, for example.

[006] In the conventional radiant heat transfer system 100, a flame 114 is formed from the burning fuel 110. The flame 114 generally has good radiant heat transfer properties. Combustion products 115, often referred to as products of combustion, also are formed from the burning fuel 110 and exit through outlet 142. The combustion products 115 have poor radiant heat transfer properties in relation to the flame 114. The combustion products 115 have an emissivity, or ability to radiate heat, typically less than 0.1. The combustion products 115 may include water vapor, carbon dioxide, and nitrogen when fossil fuels are burned. The temperature of the combustion products 115 may vary from about 260 degrees Celsius (⁰C) (500 degrees Fahrenheit (⁰F)) to about 1371⁰C (2500⁰F).

[007] The walls of the furnace 130 may be insulated with an insulator 135, such as firebrick and the like. The radiant heat transfer system 100 includes a radiant source 140, such as the depicted U-tube. The tube may have any inside diameter appropriate for the application, with inside diameters from about 7.6 centimeters (cm) (3 inches (in)) to about 20 cm (8 in) being common. In addition to the heating zone 120, the radiant source 140 may heat any surface in proximity to the radiant source 140, such as the furnace 130, the insulator 135, and the like. Additional details regarding the use of U-tubes as the radiant source 140 may be found in U.S. Pat. Nos. 5,655,599; 5,071 ,685; and 4,789,506. In other radiant heat transfer systems, the radiant source 140 may be a straight or other shape tube or any structure that contains the burning fuel 110, flame 114, and the combustion products 115.

[008] A first portion 144 of the radiant source 140 may radiate more heat to the heating zone 120 than a second portion 146 of the radiant source 140. The first portion 144 may radiate about 68,600 kilojoules per hour (kj/hr) [65,000 British Thermal Units per hour (BTU/hr)] and the second portion 146 may radiate about 47,500 kj/hr (45,000 BTU/hr). Thus, the second portion 146 of the radiant source 140 may radiate about 30 percent (%) to about 45% less heat than the first portion 144. The closer proximity of the first portion 144 to the burning fuel 110 and containing the flame 114 typically causes the first portion 144 to radiate more heat to the heating zone 120 than the second portion 146, which contains the combustion products 115. This uneven heat transfer from the radiant source 140 may lead to the uneven heating of objects within the furnace 130, thus increasing costs and providing a lower quality heat treated product.

[009] One reason for lower heat transfer in the second portion 146 of the radiant source 140 is the reduced ability of the combustion products 115, which are mostly gaseous, to transfer heat to the walls of the radiant source 140 in relation to the burning fuel 110. A substantial amount of heat, such as about 174,000 kj/hr (165,000 BTU/hr), may be trapped in the combustion products 115 exiting the radiant source 140 through the outlet 142. The heat lost in the combustion products may increase the operating costs of the radiant heat transfer system 100.

[0010] Conventional attempts at converting this lost heat into radiant heat at the second portion 146 of the radiant source 140 are mixed. One conventional method, as disclosed in U.S. Pat. No. 4,869,230, uses a corrugated strip of metal alloy to increase the surface area for heat radiation and increase the movement of the gaseous combustion products 115 to increase their convection within the radiant source 146. Furthermore, this increased movement of the gaseous combustion products 115, or turbulence, may increase the burn rate of any burning fuel 110 remaining in the second portion 146 of the radiant source 140. Turbulence may result in hot spots along the length of the radiant source 140 where temperatures may vary by up to about 150 ⁰C (300 ⁰F). Thus, a metal insert was used to absorb heat by convection and transfer heat through radiation.

[0011] While effective in the short term, metal inserts have the disadvantage of not being durable and have been replaced with ceramic inserts that can better withstand higher temperatures. Conventional ceramic inserts are described in U.S. Pat. Nos. 2,861 ,596; 4,153,035; and 6,484,795, for example. Some conventional inserts have wings extending in a radial manner outward from a solid longitudinal core, thus crossing at the center point of the core. However, while better able to withstand higher temperatures than metal, the ceramic inserts are inherently brittle, thus having the disadvantage of breaking or shattering due to the thermal cycling and vibrations occurring within the radiant heat transfer system 100 during use. Breakage of ceramic inserts may result in the destruction of the radiant source 140.

[0012] Accordingly, there is an ongoing need for improved radiant heating systems, especially those that may provide greater and/or more uniform heat transfer and lower costs. The systems, devices, and methods of the present invention overcome at least one of the disadvantages associated with conventional radiant heating systems.

SUMMARY

[0013] The present invention provides a radiant heat transfer system with one or more radiant elements inserted in a radiant source. Each radiant element may have one or more normal and/or tangential wings attached to a core section that defines a longitudinal cavity.

[0014] The radiant heat transfer system may include a radiant source and one or more ceramic radiant elements disposed inside the radiant source. At least one ceramic radiant element has one or more wings extending from a core section. The core section forms a longitudinal cavity.

[0015] A ceramic radiant element for insertion in a radiant source may have a core section and one or more wings. The core section forms a longitudinal cavity. Each wing extends from the core section.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views.

[0017] FIG. 1 depicts a conventional radiant heat transfer system for process heating that indirectly transfers heat from a burning fuel to a heating zone contained by a furnace. [0018] FIG. 2 depicts a radiant heat transfer system with radiant elements for process heating that indirectly transfers heat from a burning fuel to a heating zone contained by a furnace.

[0019] FIGs. 3A and 3B depict axial and longitudinal cross-sectional views, respectively, of a radiant source including two radiant elements and a spacer.

[0020] FIG. 3C depicts an axial view of a retention device and positioning rod for holding a radiant element in a radiant source.

[0021] FIG. 3D depicts vertically positioned radiant elements held by a positioning rod in a radiant source.

[0022] FIGs. 4A-4D depict different views of a radiant element.

[0023] FIG. 4E depicts a perspective view of a radiant element with wings having an essentially constant pitch of about 45°.

[0024] FIG. 4F depicts a perspective view of a radiant element where the pitch each of each wing transitions from about 90° at each end to about 45° at the center.

[0025] FIG. 4G-1 and FIG. 4G-2 provide supporting calculations for Reynolds

Numbers.

[0026] FIGs. 5A-5C illustrate axial cross-sections of wings tangentially attached to a longitudinal core.

[0027] FIG. 5D illustrates axial cross-sections of wings normally attached to a core section.

[0028] FIGs. 6A-6E depict perspective views of various radiant elements with wings normal to a core section. [0029] FIG. 7 depicts a radiant heat transfer system for immersion heating that transfers the heat from a burning fuel to a fluid contained by a vessel.

[0030] FIG. 8 depicts a radiant heat transfer system for a boiler that generates steam from burning a solid fuel.

DETAILED DESCRIPTION

[0031] Radiant elements convert the combustion products from burning fuel into radiant energy. A radiant element may be formed from one or more ceramics and may be used in radiant sources, such as radiant tubes, immersion tubes, heat exchanger tubes, boiler walls, and other radiant heat applications. Each radiant element has a core section defining a longitudinal cavity. The longitudinal cavity enables the insertion of a positioning mechanism that can be used to control the location of the radiant element in a radiant source. Each radiant element may have one or more normal and/or tangential wings attached to the exterior of the core section. The wings may produce a more laminar (or less turbulent) flow of the combustion products within the radiant source. The more laminar flow may improve the heat transfer from the combustion products to the radiant element, thus improving the heat transfer from the combustion products to the radiant source. The more laminar flow decreases the turbulence that may cause failure of the radiant element and/or radiant source.

[0032] FIG. 2 depicts a radiant heat transfer system 200 for process heating that indirectly transfers heat from a burning fuel 210 and flame 214 to a heating zone 220 contained by a furnace 230. The radiant heat transfer system 200 includes a radiant source 240, having a first portion 244 and a second portion 246. Unlike the convention radiant heat transfer system 100 of FIG. 1 , the radiant heat transfer system 200 of FIG. 2 includes at least one radiant element 260 inserted in the second portion 246 of the radiant source 240. The radiant heat transfer system 200 has a burner 205 connected to the first portion 244 of the radiant source 240. The first portion 244 is where most or all of the fuel is combusted in the radiant source 240. The second portion 246 is where the combustion gases flow prior to exiting the radiant source 240. While three of the radiant elements 260 are depicted in FIG. 2, one or more radiant elements may be placed in the second portion 246 of the radiant source 240. Furthermore, the radiant element 260 may be a single element that occupies part, substantially all, or the entire longitudinal length of the second portion 246 of the radiant source 240. Preferably, the one or more radiant elements 260 occupy greater than about 50% of the longitudinal length of the second portion 246 of the radiant source 240. More preferably, the one or more radiant elements 260 occupy from about 70% to about 80% of the longitudinal length of the second portion 246 of the radiant source 240.

[0033] The radiant element 260 may be formed from any ceramic; preferably a ceramic having greater resistance to the thermal stresses within the radiant heat transfer system 200. Ceramics include true ceramics and ceramic-like materials that include additional materials, such as metals. The radiant element 260 may be formed from a powder, including silicon carbide and silicon combined with a binder that is heated at a temperature to fuse the powder into a desired ceramic structure. Thus, the fired ceramic may be a siliconized silicon carbide. Other materials may be used in forming the ceramic, such as silicon nitride, silicon- mullite, alumina, and the like. Preferably, the ceramic from which the radiant element 260 is formed has an emissivity of greater than about 0.4, preferably from about 0.4 to about 0.9. Good emissivity performance of the material from which the radiant element 260 is formed reduces fuel consumption.

[0034] As the burning fuel 210 forms combustion products and a flame 214, a nearly complete combustion zone 250 may form. In the nearly complete combustion zone 250, the burning fuel 210 is at least about 80% to about 85% converted to the combustion products 215. Preferably, at least about 90% of the burning fuel 210 may be converted to the combustion products 215 in the nearly complete combustion zone 250. Any remaining uncombusted fuel is combusted after the nearly complete combustion zone in the radiant source 240.

[0035] The radiant element 260 may be placed after the nearly complete combustion zone 250. The radiant element 260 may be placed in the combustion zone 250 where about 90% of the burning fuel 210 has been converted to the combustion products 215. If the radiant element 260 is placed too close to the burning fuel 210, the radiant element may fail. A similar failure may occur if the radiant element 260 is placed in an insulated portion of the radiant source 240. If the radiant element 260 is placed too far from the nearly complete combustion zone 250, the ability of the radiant element 260 to convert the heat trapped in the combustion products 215 to radiant energy may be reduced. Thus, appropriate positioning of the radiant element or elements 260 in the radiant source 240 is preferred.

[0036] If the radiant element 260 fills too much of the axial cross-sectional area of the radiant source 240, the turbulence and/or back pressure of the combustion products 215 may increase to the point where the radiant element 260 and/or the radiant source 240 fail. The radiant element 260 occupies less than about 20%, preferably from about 5% to about 10%, of the axial cross-sectional area of the radiant source 240. Similarly, if the radiant element 260 does not sufficiently direct the flow of the combustion products 215, the heat of the combustion products 215 may not be effectively converted to radiant energy. Thus, it is desired for the radiant element 260 to radiate heat from the combustion products 215 while not creating more turbulence in the flow of the combustion products 215 that may cause mechanical failure of the radiant element 260 or the radiant source 240.

[0037] By converting a portion of the heat within the combustion products

215 into radiant energy, the radiant element 260 may improve the uniformity of heat transfer from the first and second portions 244, 246 of the radiant source 240 and may increase the radiant heat transferred from the burning fuel 210 to the heating zone 220. For example, about 174,000 kj/hr (165,000 BTU/hr) of heat is lost through the outlet 142 of the conventional radiant heat transfer system 100 of FIG. 1. In the radiant heat transfer system 200 of FIG. 2, the radiant element 260 may recover about 15,800 kj/hr (15,000 BTU/hr) from the combustion products 215 and radiate it to the second portion 246 of the radiant source 240. Thus, the approximate 21 ,100 kj/hr (20,000 BTU/hr) difference between the first and second portions 144, 146 of the radiant source 140 of FIG. 1 may be reduced to a 5,300 kj/hr (5,000 BTU/hr) difference in FIG. 2 with the radiant element 260.

[0038] FIG. 3A depicts an axial cross-section of a radiant source 340. FIG. 3 B depicts a longitudinal cross-section of two radiant elements 360 positioned within the radiant source 340. Each radiant element 360 includes a central longitudinal core section 370. The interior of the core section 370 defines a longitudinal cavity 375. The exterior of the core section 370 defines an exterior 372 that attaches at least one wing 390. Terminal surfaces 392 are farthest from the longitudinal core section 370 in an axial direction and may or may not contact the interior wall of the radiant source 340. The shape of the terminal surfaces 392 may provide for better positioning accuracy of the radiant element 360 including when contacting the inner wall of the radiant source 340 or in relation to additional radiant elements. Due to the increased surface area of the radiant element 360 in relation to a tube of equivalent axial diameter, the radiant element 360 may provide a greater heat emissivity than a circular tube of the same outside diameter and length. The radiant element 360 has an element surface area, which is the surface area of all the radiant elements in the radiant source 340. The radiant source 340 has a source surface area, which is the surface area of the interior wall of the radiant source facing the radiant element 360 or corresponding to the length the radiant element 360. The ratio of the element surface area to the source surface area is greater than about 1.1 :1. The ratio of the element surface area to the source surface area may be from about 1.1 :1 to about 3:1. The ratio of the element surface area to the source surface area may be from about 1.2:1 to about 1.5:1. Other ratios of the surface areas may be used. In this manner, the radiant element 360 may increase energy adsorption and radiation, thus increasing heat transfer to the radiant source 340.

[0039] The cavity 375 may be accessible from each longitudinal end of the radiant element 360. While depicted as an essentially circular tube in FIG. 3B, the cavity 375 may be any shape, such as spherical, triangular, polygonal, rectangular, elliptical, combinations of these or other shapes, and the like. The cavity 375 may vary in size and shape along the longitudinal length of the radiant element 360. Thus, the axial cross-section of the cavity 375 may be symmetrical or asymmetrical along the longitudinal axis of the radiant element 360. The cavity 375 has a diameter of at least about 0.635 cm (0.25 in), preferably from about 1.27 cm (0.5 in) to about 1.91 cm (0.75 in). In another aspect, the thickness of the core section 370 between the cavity 375 and the exterior 372 is at least about 0.31 7 cm (0.125 in), preferably from 0.635 cm (0.25 in) to about 1.27 cm (0.5 in). Other cavity diameters and core thicknesses may be used.

[0040] A positioning mechanism may be used to control the location of the radiant element 360 in the radiant source 340. The positioning mechanism includes a position rod 380, a stop device 386, and a retention device 387. The positioning rod 380 is disposed in the longitudinal cavity 375 of one or more radiant elements 360. By passing the rod 380 through the cavity 375, the radiant elements 360 may be held. The rod 380 may be made of steel, ceramic, intermetallic, a combination thereof, or like material. The rod 380 may enter a first end, extend the length of, and exit through a second end of the cavity 375. When more than one radiant element 360 occupies the rod 380, a spacer 382 of sufficient outside diameter to prevent the core sections 370 of the radiant elements 360 from contacting may be placed over the rod 380. The spacer may be from about 2.5 cm (1 in) to 32 cm (12.5 in) in length. The spacer length may be selected in response to the inside diameter of the radiant source 340. Other spacer lengths may be used. In addition to containing a portion of the rod 380, the cavity 375 may provide for the injection of a fluid, such as a gas other than a fuel gas, into the radiant element 360.

[0041] At a first end 384, the rod 380 may be provided with a stop device 386 sufficient to prevent the core section 370 from sliding past the first end 384 of the rod 380. The first end 384 of the rod 380 may be threaded. A washer and bolt or a washer and a nut may be placed on the rod 380 to prevent the radiant element 360 from sliding past the first end 384 of the rod 380. The stop device 386 may be provided by bending the first end 384 of the rod 380 to prevent the radiant element 360 from sliding. Other stop devices may be used to prevent the radiant element 360 from sliding off of the rod 380.

[0042] In FIG. 3C, the rod 380 may have a retention device 387 at a second end 388. The retention device 387 may be one or more cross pieces, a cap, a metal bar, or the like that fixes or connects the rod 380 to the radiant source 340. The second end 388 of the rod 380 may be bent or equipped with a washer and/or nut 389 to hold the rod 380 in the retention device 387. The retention device 387 may include any apparatus that fixes the rod 380 in relation to the radiant source 340. Thus, the rod 380 may locate the radiant element 360 at a particular place or with a particular orientation within the radiant source 340.

[0043] If the radiant elements 360 are positioned horizontally in the radiant source 340, the rod 380 may include sufficient radiant elements and/or spacers to place a compressive force on the radiant elements 360. For example, by tightening the bolts at the first and second ends 384, 388 of the rod 380, the radiant element or elements 360 may be held in compression. This horizontal compressive force applied by tightening the bolts may overcome the tension force being vertically applied to the radiant elements 360 by gravity.

[0044] In FIG. 3D, the radiant elements 360 are positioned vertically in the radiant source 340. In a vertical position, the radiant elements 360 may be placed under compressive force without filling the rod 380 with spacers and radiant elements. In this aspect, by holding the rod 380 at the top of the radiant source 340 with the retention device 387 and by holding the radiant element or elements 360 onto the rod 380 with the stop device 386, gravity maintains a compressive force on the radiant elements 360.

[0045] The radiant element or elements 360 in FIG. 3D are held in compression as opposed to being under tension. The ceramic, from which the radiant element 360 is formed, has excellent mechanical strength when held under compression, but have poor mechanical strength when placed under tension. As previously described, conventional ceramic inserts often fail due to vibration and thermal shock. Thus, by holding the radiant element 360 in compression, whether it resides vertically or horizontally within the radiant source 340, the need for a ceramic material that resists thermal and/or mechanical shock may be reduced. By holding the radiant element 360 in compression, the failure rate of the element may be reduced.

[0046] FIGs. 4A-4D depict different views of a radiant element 460. FIG. 4E depicts a perspective view of a radiant element with wings having an essentially constant pitch of about 45°. FIG. 4F depicts perspective views of a radiant element where the pitch of each wing transitions from about 90° at each end to about 45° at the center. Combustion products may pass across the radiant element 460 in a laminar or turbulent manner. The radiant element 460 may have a surface area geometry that directs combustion products in a more laminar or less turbulent flow over the surface while radiating heat absorbed from the combustion products. Preferably, the flow of the combustion products over the radiant element 460 is a laminar or nearly laminar flow. The lower turbulence levels provided by the radiant element 460 in relation to conventional ceramic inserts may allow for increased heat radiation while avoiding the hot spots and other disadvantages of turbulent flow that may lead to failure. [0047] A Reynolds Number (Re) describes whether a flow is laminar, turbulent, transitional, or a mixed. For example, in tubes, a Re below 2300 is considered laminar while a Re above 4500 is considered turbulent. A Re between 2300 and 4500 is considered transitional or mixed. Thus, a lower Reynolds Number indicates a more laminar flow. As a further example, combustion products moving through a radiant tube with an inside diameter of 10.16 cm (4 in) have a Re of 3742, thus being transitional or more turbulent than laminar. In comparison, combustion products flowing past a radiant element with three wings in a radiant tube with an inside diameter of about 10.16 cm (4 in) have a Re of 1914, which is laminar flow. The supporting calculations for these Reynolds Numbers are shown in FIG. 4G-1 and FIG. 4G-2. These calculations are for a radiant element with three wings and show the Reynolds Number calculated for flow between two of the wings. Tubes with other Reynolds Numbers indicating laminar, turbulent, or mixed flow may be used. Thus, the radiant elements of the present invention may significantly increase the laminar flow of combustion products through a radiant source. The radiant elements may provide a Re below 2300, more preferably from 1500 to 2300 for combustion products flowing through a radiant source. The radiant elements may provide flows of the combustion products with other Reynolds Numbers.

[0048] As depicted in the FIG. 4A perspective and the FIG. 4D axial cross- section, the radiant element 460 includes a central longitudinal core section 470 defining a longitudinal cavity 475 and an exterior 472 attaching to three wings 490. The wings 490 may be attached to the exterior 472 in a normal, tangential, a combination these, or another geometry in relation to the exterior 472 or outside surface of the core section 470. Preferably, the wings 490 are attached in a normal, tangential, or in a combination of these geometries. More preferably, the wings 490 are attached in a tangential geometry. [0049] The wings 490 may increase the surface area of the radiant element

460. While the radiant element 460 is depicted with three wings, one or more wings may be used. If the radiant element 460 includes greater than four wings, the resulting decrease in the open cross-sectional area of the radiant source may result in an undesirable drop in the flow velocity of the combustion products. The core section 470 may have portions with and without the wings 490.

[0050] FIG. 4B depicts wings 490 having a helical shape with a pitch angle of about 45°. The pitch angle of the wings 490 is the orientation of the wings in relation to the center axis of the radiant element 460 or in relation to the axis of the core section 470. Pitch angles from about 20° to about 90° are preferred, with angles from about 30° to about 60° being more preferred. Other pitch angles may be used. The pitch angles of the wings 490 may remain constant or may vary along the longitudinal length of the radiant element 460. For example, FIG. 4E depicts a perspective view of the radiant element 460 with wings having an essentially constant pitch of about 45°. In contrast, FIG. 4F depicts a perspective view of a radiant element where the wing pitch transitions from about 90° at each end to about 45° at the center or middle of the radiant element. The smooth transition between the about 45° pitch angle at the center and the about 90° pitch angle at the ends may provide for reduced turbulence in relation to designs having stepped transitions between angles.

[0051] FIG. 4C is a longitudinal cross-section of the radiant element 460.

FIG. 4D illustrates that the wings 490 may be thicker where attached to the exterior 472 than at a terminus 492. Thus, the wings 490 may have a non-uniform thickness and taper from the exterior 472 to the terminus 492. For one or more of the wings, the ratio of the height of the wing (the distance from the exterior 472 to the terminus 492) to the diameter of the core section 470 may be greater than about 4:1. The ratio of the height of the wing to the diameter of the core section may be from about 4:1 to about 50:1. The ratio of the height of the wing to the diameter of the core section may be from about 5:1 to about 1 1 :1. Other ratios of the height of the wing to the diameter of the core section may be used.

[0052] FIGs. 5A-5C illustrate axial cross-sections of wings 590 tangential to a central longitudinal core section 570. In FIG. 5A the wings 590 are straight and are tangential with a circle 593 representing the exterior of the central longitudinal core section 570. In this illustration, the angle of the wings 590 to the circle 593 is about 0°. In FIG. 5B, the wings 590 also are tangential to the circle 593, but are curvilinear in shape. Unlike the straight wings of FIG. 5A, the curvilinear wings of FIG. 5B would not lay flat on a table if removed from the core section 570. Thus, the terminus of a curvilinear wing does not align with the portion of the wing attached to the central longitudinal core section 570. In FIG. 5C the central longitudinal core section 570 is triangular in shape, thus allowing the curvilinear wings to be tangential to the central point 593 of the core section 570. In contrast to FIGs. 5A-5C, the wings 590 of FIG. 5D are not tangential, but normal (nearly 90°) to the longitudinal core section 570.

[0053] FIGs. 6A-6E depict perspective views of various radiant elements with wings 690 normal to the exterior 694 or outside surface of a longitudinal core section 670. In FIG. 6A the approximately 90° normal attachment of the wings 690 is seen at the top of the radiant element. FIG. 6B depicts curvilinear wings 690 also having an approximately 90° normal attachment to the longitudinal core section 670. FIG. 6C depicts wings 690 normal to a longitudinal core section 670, but where the wings 690 transition from an about 90° pitch at either end to an about 45° pitch in the central region. FIG. 6D depicts wings 692 that extend farther from the longitudinal core section 670 than wings 691, thus establishing that a radiant element may include wings of different heights. FIG. 6E depicts a radiant element having normal to the center point 694 wing attachment, but where the wings 690 have a complex curvilinear shape. [0054] FIG. 7 depicts a radiant heat transfer system 700 for indirect immersion heating that transfers heat from a burning fuel 710 to a fluid 720 contained by a vessel 730. The fluid 720 may be a liquid, such as water, oil, salt solution, or the like. The radiant heat transfer system 700 includes a radiant source 740, having an exhaust portion 746. The exhaust portion 746 includes at least one radiant element 760. While multiple of the radiant elements 760 are depicted in FIG. 7, one or more may be placed in the exhaust portion 746 of the radiant source 740. Furthermore, the radiant element 760 may be a single element that occupies part, substantially all, or the entire longitudinal length of the exhaust portion. Preferably, the one or more radiant elements 760 occupy from about half to all of the longitudinal length of the exhaust portion 746 of the radiant source 740.

[0055] FIG. 8 depicts a radiant heat transfer system 800 for a biomass or other boiler that generates steam in tubes 820 from a bed of solid burning fuel 810, such as coal or biomass. Air may be introduced from below the burning fuel 810, for example. Radiant element 860 is held on positioning rod 880 above the burning fuel 810. Preferably, the multiple radiant elements 860 are held in compression above the burning fuel 810. Combustion products 815 flow from the burning fuel 810 over the radiant elements 860. The radiant elements 860 adsorb heat from the combustion products 815 and radiate the energy to the steam tubes 820.

[0056] The following definitions are included to provide a clear and consistent understanding of the specification and claims.

[0057] The term "emissivity" or "emissivities" is defined as the relative power of a surface to emit heat by radiation, which may be expressed as the ratio of the radiant energy emitted by a surface to the radiant energy emitted by a blackbody (an ideal surface that absorbs all radiant energy without reflection) at the same temperature. Thus, the ability of a first material to transfer heat to a second material. 5ee Webster's New Collegiate Dictionary, G. & C. Merriam Company, Springfield, Massachusetts, 1977, pp. 1 15, 372. [0058] The term "compressive" or "compression" refers to the act or action of squeezing together.

[0059] The term "tension" refers to the act or action of stretching.

[0060] The term "laminar" in relation to the flow of combustion products refers to the flow condition when the individual particles move in a regular or steady motion resulting in a smooth flow line or path. The particles passing through a given point follow the same path. 5ee Pritchard, R. et al., Handbook of Industrial Gas Utilization, Van Nostrand Reinhold Company, New York, 1977, pp. 30-31.

[0061] The term "turbulent" in relation to the flow of combustion products refers to the flow condition when the individual particles move in an irregular or unsteady motion resulting in an uneven flow line or path. The transition from laminar flow to turbulent flow occurs when the fluid velocity or speed exceeds a critical value. 5ee Pritchard, R. et al., Handbook of Industrial Gas Utilization, Van Nostrand Reinhold Company, New York, 1977, pp. 30-31.

[0062] The term "radial" refers to a wing extending outward along a radius from a centerpoint of a radiant element.

[0063] The term "normal" refers to a wing extending at a perpendicular or nearly 90° angle from a surface, such as the exterior of the longitudinal core of a radiant element. A "normal" wing also may be "radial" if the wing extends outward along a radius from a centerpoint of a radiant element.

[0064] The term "tangential" refers to a wing extending at an angle other than

90° from a surface, such as the exterior of the longitudinal core of a radiant element.

[0065] While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that other embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.

Claims

What is claimed is:

1. A radiant heat transfer system, comprising: a radiant source; and at least one ceramic radiant element disposed inside the radiant source, where the at least one ceramic radiant element has at least one wing extending from a core section, and where the core section forms a longitudinal cavity.

2. The radiant heat transfer system of claim 1 , comprising a positioning mechanism disposed in the longitudinal cavity, where the positioning mechanism is connected to the radiant source.

3. The radiant heat transfer system of claim 2, where the positioning mechanism comprises: a positioning rod disposed in the longitudinal cavity; a stop device connected to one end of the positioning rod; and a retention device connected to the other end of the positioning rod.

4. The radiant heat transfer system of claim 1 , comprising a first ceramic element and a second ceramic radiant element with a spacer therebetween.

5. The radiant heat transfer system of claim 1 , where the at least one ceramic radiant element occupies less than about 20% of an axial cross-sectional area of the radiant source.

6. The radiant heat transfer system of claim 1 , where the at least one ceramic radiant element occupies from about 5% to about 10% of an axial cross-sectional area of the radiant source.

7. The radiant heat transfer system of claim 1 , where the at least one ceramic radiant element has an element surface area, where the radiant source has a source surface area, and where a ratio of the element surface area to the source surface area is greater than about 1.1 :1.

8. The radiant heat transfer system of claim 7, where the ratio of the element surface area to the source surface area is about 1.1 :1 to about 3:1.

9. The radiant heat transfer system of claim 7, where the ratio of the element surface area to the source surface area is about 1.2:1 to about 1.5:1.

10. The radiant heat transfer system of claim 1 , where the at least one ceramic radiant element has an emissivity of about 0.4 to about 0.9.

1 1. The radiant heat transfer system of claim 1 , where the flow of combustion products in the radiant source has a Reynolds Number less than about 4500.

12. The radiant heat transfer system of claim 1 , where the flow of combustion products in the radiant source has a Reynolds Number less than about 2300.

13. The radiant heat transfer system of claim 1 , where the flow of combustion products in the radiant source has a Reynolds Number of about 1500 to about 2300.

14. The radiant heat transfer system of claim 1 , where the radiant source is a radiant tube.

15. The radiant heat transfer system of claim 14, comprising: a radiant tube having a first portion and a second portion; a burner connected to the first portion; and the at least one radiant element disposed in the second portion.

16. The radiant heat transfer system of claim 15, comprising a first radiant element and a second radiant element with a spacer therebetween.

1 7. The radiant heat transfer system of claim 15, where the at least one radiant element occupies greater than about 50% of the second portion.

18. The radiant heat transfer system of claim 15, where the at least one radiant element occupies from about 70% to about 80% of the second portion.

19. The radiant heat transfer system of claim 1 , where the ceramic radiant element has at least one wing normal to the core section.

20. The radiant heat transfer system of claim 1 , where the ceramic radiant element has at least one wing tangential to the core section.

21. The radiant heat transfer system of claim 1 , where the at least one wing has a substantially helical shape defining a helix angle, and where the helix angle is from about 20° to about 90°.

22. The radiant heat transfer system of claim 1 , where the at least one wing has a substantially curvilinear shape.

23. A ceramic radiant element for insertion in a radiant source, comprising: a core section forming a longitudinal cavity; and at least one wing extending from the core section.

24. The ceramic radiant element of claim 23, where the at least one wing is normal to the core section.

25. The ceramic radiant element of claim 23, where the at least one wing is tangential to the core section.

26. The ceramic radiant element of claim 23, where the at least one wing has a substantially helical shape defining a helix angle.

27. The ceramic radiant element of claim 26, where the helix angle is essentially constant.

28. The ceramic radiant element of claim 26, where the helix angle is from about 20° to about 90°.

29. The ceramic radiant element of claim 26, where the helix angle is from about 30° to about 60°.

30. The ceramic radiant element of claim 26, where the helix angle is about 45°.

31. The ceramic radiant element of claim 26, where the helix angle is about 90°.

32. The ceramic radiant element of claim 26, where the helical shape transitions from one helical angle to another helical angle.

33. The ceramic radiant element of claim 32, where the helix angle transitions from about 90° to about 45°.

34. The ceramic radiant element of claim 32, where the helix angle transitions from about 90° at one end of the radiant element to about 45°in a middle portion of the radiant element, and where the helix angle transitions from about 90° at the other end of the radiant element to about 45° in the middle portion of the radiant element.

35. The ceramic radiant element of claim 23, where the at least one wing has a substantially curvilinear shape.

36. The ceramic radiant element of claim 23, where the at least one wing has a substantially constant height.

37. The ceramic radiant element of claim 23, where a ratio of a height of the wing to a diameter of the core section is greater than about 4:1.

38. The ceramic radiant element of claim 37, where the ratio of the height of the wing to the diameter of the core section is from about 4:1 to about 50:1.

39. The ceramic radiant element of claim 37, where the ratio of the height of the wing to the diameter of the core section is from about 5:1 to about 1 1 :1.

40. The ceramic radiant element of claim 23 having an emissivity greater than about 0.4.

41. The ceramic radiant element of claim 40 having an emissivity from about 0.4 to about 0.9.