Radiant gas burner

Nov. 3, 1964

T. N. STACK

RADIANT GAS BURNER

3,155,142

Filed Feb. 13, 1961

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United States Patent Office

3,155,142

Patented Nov. 3, 1964

3,155,142

RADIANT GAS BURNER

Thomas N. Stack, St. Paul, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Pan!, Mina., a corporation of Delaware

Filed Feb. 13, 1961, Ser. No. 88,845

Claims. (CI. 158--99)

This invention relates to structures so designed as to be useful as gas burners. More particularly, the invention relates to composite gas burner structures useful to create intense radiant energy.

Transfer of heat energy by radiation is known to be one of the most efficient ways in which to accomplish energy transfer. It is useful under a wide variety of temperature conditions; and radiation greatly increases as source temperature increases, since the rate of transfer is proportional to the fourth power of the absolute temperature of the radiator. Thus, the advantages of radiant heat transfer become particularly pronounced at higher and higher source temperatures.

In order to increase the source temperature of a radiant gas burner, however, one must increase the rate of combustion for reactant gases passing therethrough. Preferably, this should be done to a point where the temperature gained as a result of rapid and confined combustion approaches as closely as possible the limit set by the dissociation temperature of the products of combustion. From the standpoint of creating the necessary radiant source for practical use applications, it is important that as much as possible of the heat generated by the combustibles during reaction is absorbed by a radiation emitting member of the burner. In this manner waste is minimized.

Many different types of radiant burners are, of course, known to the art. One such known device employs a radiant burner block of refractory material having parallel passages therein. However, attempts to increase the flow of reactant gases through such passages tend to cause the flame front to blow off the exhaust surface of the block. The flame must rest next to (or in the end of passages just beneath) the exhaust surface of a block in order for the exhaust surface to reach incandescence. But flashback of flame must be guarded against, and is usually done by employing passages of small cross sectional area in the block. The maximum radiant source temperature attainable when using a block as described is quite low, generally not exceeding about 1800° F. as measured by an infrared pyrometer. Further, a blast of air (such as encountered in a normal exposed outside environment) across the face of a block-type radiant burner is apt to cause flame blow off or blow out.

Other known radiant burners have been designed so as to cause hot burning gases to flow or wipe over the surface of a solid ceramic piece of varied shape. But the thermal inertia (i.e., bulk mass) of the solid ceramic piece works against that piece rapidly reaching higher temperatures for improved radiation transfer; and much of the heat from the hot burning gases of combustion is lost due to lack of contact between them and the massive ceramic material. Generally, the maximum temperature attained using devices of this sort in the open will not exceed about 2300° F.

Radiant burners of refractory block material having tortuous passages therethrough have also been used. Gas 5 and air mixtures usually must be forced through these blocks under several pounds of pressure in order to feed the flame, which preferably is confined within the tortuous passages on the exhaust side of the blocks. In operation these blocks may also attain rather high radiation tem10 peratures (e.g. 2100° F.), but a source of vexing trouble is the problem of non-uniform porosity which sometimes creates hot and cold spots on the exhaust side of the block.

Still other design possibilities for radiant burners have 15 been suggested, including an assembly having a relatively bulky or massive top or radiation plate separated from mixing plates, as taught in German Patents Nos. 464,692 and 466,586. The teaching of these patents emphasizes the mixing function of the plates below the radiation 20 plate, and stresses that combustion takes place within the radiation plate or at the base of the holes thereof. Experiments with the burners of these patents have revealed that their design is such that rapid combustion of the order attainable using burners of this invention cannot be 25 realized when using the burners of these patents, simply because the flame is blown from the face of the burner (or blown out) when attempts are made to increase gasair flow much beyond the slow rate required for ignition. Also ignition of the burners of these patents is difficult; 30 and the flame tends to remain at ports of the top plate instead of flashing back beneath that plate.

The burner assembly of this invention is so designed as to permit operation at a variety of inlet gas velocities and pressures. It is useful as a radiant energy source at lower 35 temperatures, e.g., 1500° F„ or even lower, as well as higher temperatures, e.g., 2500° F., and even higher. A burner constructed according to the preferred embodiment of the invention illustrated in the drawings has been operated in the open successfully to create source temper⁴⁰ atures on the order of 3000° F. as measured by an optical pyrometer. With proper materials of construction and use of proper reactant gases, as well as the mixing and introduction thereof, even higher radiation temperatures may be possible.

⁴⁵ The tendency for flame blow off or blow out is greatly minimized by the design of the burner of this invention. Combustion is largely confined within a combustion zone located between a radiation plate which promotes flash₅₀ back of flame into the combustion zone, and a separate gas feed plate Avhich inhibits flashback of flame therethrough and out of the combustion zone. Once the burner is ignited at low inlet gas pressures, retrogression of the flame into the combustion zone readily takes place, 55 and the rate of introduction of reactant gases can thereafter be increased greatly (at least four or five times) without flame blow off.

The burner of this invention, particularly when designed so as to reach the higher radiant energy emissions 60 is useful in processing applications such as annealing of materials (e.g., glass objects, metals, etc.), uniformly

3,1' ³ . .

rapid heating of liquids flowing through pipes, sealing of thermoplastic members of cartons rapidly moving along a conveyor, space heating, cooking, etc. Radiant burners hereof may also be employed to elevate the temperature of objects already raised to high temperatures. For example, objects elevated to approximately 2300° F. may be further elevated in temperature using a radiant burner hereof in a final heating process step. A particularly unique use of radiant burners of the invention is that of firing or maturing enamel coatings on metal. The technique using a burner hereof obviates the need for the bulky large enclosed furnaces of the prior art.

By taking advantage of the design principles herein disclosed, burners other than those relied upon primarily for radiant heating may be constructed and used as incandescent light-emitters in applications such as markers, various attention-getting devices, signs and the like.

Also, the design principles disclosed herein may be advantageously employed to make burners primarily relied upon as hot gas producers for convection heating, if such is desired.

For convenience in describing the invention hereof, reference wiil be made to a preferred embodiment thereof as illustrated in the drawing, wherein:

FIGURE 1 is a cross sectional view of the burner assembly;

FIGURE 2 is an exploded and enlarged cross sectional view of the burner assembly to facilitate illustration of its component parts;

FIGURE 3 is a top elevation of a preferred configuration for plates of the burner assembly; and

FIGURE 4 is a top elevation of a gas diffuser.

As illustrated in the drawing, a preferred and simplified burner assembly of the invention comprises a radiation plate 10, a combustion plate 11, and a gas feed plate 12, each spaced from the other and mounted in a refractory housing 13, such as, for example, fire brick. A pipe 14 is suitably provided for introduction of combustible gas-air mixtures to the feed plate 12. In order to prevent stratification of gases so introduced, a diffuser means or plate 15, suitably a stainless steel strip in propeller shape as illustrated in FIGURE 4, may be interposed, if desired, in the gas stream to the feed plate 12. In the specific illustrated embodiment, inlet pipe 14 is provided with a flange 16, preferably square in area. The flange is recessed in a mating square indentation of the section of the housing holding inlet pipe 14 so as to prevent rotational movement when a further pipe connection is made on the threaded portion of pipe 14. The several sections of the housing 13 of the burner may be fastened together securely along abutting planes by any suitable means such as refractory cement, external clamp members, and the like. It will be evident that the joints between the flange 16 of inlet conduit 14 and the housing sections 13α and 13&amp; should be gas tight; and for this purpose, a refractory cement filler has been found satisfactory.

Because of the tremendous temperature variations to which radiation plate 1Θ and combustion plate 11 are subjected during operation of the burner in cycles, they are each preferably loosely mounted within recessed portions along the sides of the internal gas flow zone 17 of the housing, whereas feed plate 12 is preferably cemented in its section of the housing. By loosely mounting plates 1Θ and 11 in the housing, sufficient provision is made for their differential expansion during on-off cycling of the burner without excessive cracking. Slight cracking of these members, however, has not been found to seriously affect the efficiency of burner operation.

In preferred structures, as illustrated in the drawing, the cross section area of the internal gas flow zone 17 or combustion chamber, transverse to the flow of gases therethrough, is gradually increased from the feed plate 12, through combustion chamber 17, onto the outer surface 18 of the radiation plate 10 so as to approximately )5,142 provide for the increase in volume exhibited by the gases as they undergo chemical reactions generating heat and exhaust fumes during passage through the burner.

Each plate of the burner may be made from a webbed 5 structure such as illustrated in FIGURE 3. The formation of this webbed assembly is described in a copending application of the assignee of this case, Johnson U.S. Serial No. 26,372; and the disclosure of that application is here incorporated by reference. As disclosed in that 10 application, this webbed structure may be formed as follows: A plasticized raw material mix containing finely divided sinterable particles, plasticizing ingredients such as for example organic polymeric resins, and volatile liquid viscosity adjusting media, is formed into a thin film or 15 sheet material by any suitable process such as knife coating, spraying, casting, calendering, etc. Generally, these films will be at least about 2 or 3 mils thick up to about Vs inch thick, and contain at least about 80% sinterable particles.

Part of the plasticized green ceramic film so formed is then corrugated. A laminated structure is formed by placing strips of flat sheet 30 between strips of corrugated material 31 in repetitive fashion as illustrated in FIGURE 3. In order to temporarily bond ridges of corru25 gations to contacting portions of flat sheet members of the structure, it is suitable to paint the surfaces of the fiat sheet members with a slip of the plasticized raw material mix used in making the sheet members. The slip is formed by adding volatile liquids to the raw material mix.

The coating of slip tends to solvate contacting surface portions of the corrugated sheet and fiat sheets so that a strong temporary bond between these contacting portions is formed on evaporation of the volatile liquid of the slip. Then usually the structure is sawed to shape, and fired 35 under appropriate sintering conditions to form a refractory plate.

The passages formed in the valleys between ridges of corrugations and contacting portions of flat sheet members (or mated ridges of corrugated sheets, should one prefer -° to employ corrugated members solely in making the plate so as to end up with a honeycomb-type structure) serve as ports through which gases are entrained in the operation of the burner assembly hereof. Of course, the selection of specific materials from which to form plates of 45 the burner structure may vary depending on the temperature conditions and thermal shock conditions to which the burner will be subjected in any specific practical use. Refractory materials such as zircon, alumina and the like preferably are used to fabricate the radiation plate 10 and 50 combustion plate 11. Since the thermal shock conditions to which feed plate 12 is subjected in use are relatively less severe than those to which plates 10 and 11 are subjected, some greater liberality in selection of ths material from which to fabricate feed plate 12 is permissible; and 55 metals may possibly be suitably employed in fabricating this plate where the burner is designed for low radiant temperature operation. It is, however, preferable, even in the case of feed plate 12, to employ refractory material, since the flame front may recess and essentially lie very 60 close to the surface of feed plate 12 at low pressure operation of the burner. Also refractory materials of the inorganic oxide type are usually preferably employed in fabricating plate 12 since its front face is exposed to the radiation from the combustion zone 17.

⁶⁰ I will now proceed to a description of detailed features of various parts of the burner.

Radiation plate 10, as has been indicated, is especially designed to cause flashback of flame therethrough into ₇₀ the primary zone or chamber 17 of combustion located beneath that plate. This is an essential property of the radiation plate. Characteristically, the ports through the radiation plate must have their closest diametrically opposite wails spaced at least about 0.03 inch away from 75 each other in order to promote flashback. Stated another

3,155,143 way the minor dimension of the passages or ports of the radiation plate should be at least about 0.03 inch. In the webbed corrugated design for the plate as illustrated in FIGURE 3, it is also useful to be governed by the square inch area of the port openings in the plate; and in such cases, the port openings will generally have an area at least about 0.001 square inch (preferably at least 0.0025 square inch). However, where slotted port openings are employed, the minor dimension becomes a more useful criterion. Smaller port openings tend to inhibit flashback and cause an increase in the pressure drop across the radiation plate, together with a decrease in the temperature attainable in the radiation plate.

The radiation plate preferably should present a large surface area to hot exhaust gases passing therethrough. This means, first of all, that the superficial surface area of the plate (i.e., total internal surface presented by the walls of its passages) should be large. Superficial surface area is generally measured as the total square inch area presented by the internal surfaces of passages in a cubic inch volume of the plate. For the radiation plate, superficial surface area should range between about 30 and 100 square inches per cubic inch volume of this plate. Secondly, the thickness of the radiation plate should be such that it presents, in combination with the superficial surface area measurement, a sufficient total surface area to exhaust gases passing therethrough in any square inch area transverse to the exhaust flow so that adequate absorption of the heat from the exhaust gases is accomplished to elevate the plate in temperature during the dwell time of the gases passing therethrough, as is desired in order to gain the higher radiation emission properties. In terms of thickness, while a radiation plate as thick as 1½ inches, or possibly thicker, may be employed, it is generally preferred to employ a plate as thin as possible consistent with the heat absorption and radiation emission properties desired. Extremely thick plates may absorb most of the heat of combustion in their lower portion and therefore exhibit somewhat lower surface radiation emitting properties than otherwise possible. Also, the thinner the plate consistent with adequate heat absorption for radiation emissions, the less the thermal inertia of the plate and the more rapid radiation emission temperatures are reached after burner ignition. Thin plates reduce the total bulk volume occupied by the burner, as well as reduce the pressure drop across the radiation plate, as is desired. Plates on the order of Vs inch are quite suitable to use; and those as thin as approximately %e inch or so may be employed (especially where compactness of the burner is desired even at the expense of some loss of potential radiation emission). While thickness of the plate is a significant factor, it indeed is a variable one depending upon the superficial surface area of the radiation plate. A convenient expression, which succinctly takes into account the thickness as well as the superficial surface area concept is deemed to be “superficial surface value.” This value is arrived at by multiplying the plate thicknesses (in inches) times the superficial surface area (in square inches per cubic inch volume). For the radiation plate, the superficial surface value usually should lie in the range between about 6 and 75, preferably between 10 and 60.

Generally, the ports extending through the radiation plate have integral internal wall surfaces (i.e., relatively smooth internal wall surfaces). The ports are essentially uniformly distributed over the area of the radiation plate so that hot spots and cold spots are essentially obviated. Usually between about 50 and 90% of any cross section transverse to the flow of gases through the radiation plate is open area.

Opposite in function to the radiation plate, insofar as flashback properties are concerned, is the gas feed plate or member 12. In any specific burner, the minor dimension (i.e., maximum distance between the closest spaced diametrically opposite wall portions) of the ports or pas6 sages of the feed plate should be less than the minor dimension of the ports employed in the radiation plate of the burner. Flashback control, in general, is influenced by the width of port openings, in combination with the length of the ports or passages, the velocity of gases through the ports, the thermal conduction characteristics of the material of the plate, the specific gases employed in combustion, etc., as is generally well understood in the art. Thus, while the dimensions or cross-sectional area of ports play a significant part, that feature need not alone be relied upon. However, the ports in the gas feed plate, in addition to generally having a minor dimension relationship to the ports of the radiation plate as aforenoted, generally will have a minor dimension no greater than 0.05 inch where the higher pressure gas sources are employed thus maintaining a high flow rate in the passages of the feed plate (e.g., about 600 or more feet per minute). However, at lower flow rates usually required for ignition of the burner (e.g., about 100 to 200 feet per minute), the minor dimension will be no greater than 0.03 inch, or preferably 0.02 inch.

In addition to the flashback control, feed plate 12 also is designed so as to introduce mixtures of reactant gases into the combustion zone 17 of the burner at an essentially uniform rate from all portions of the exhaust surface of the feed plate. Accordingly, the plate should have an essentially uniform pattern of passages or ports extending therethrough from the rear to the front or exhaust surface. These ports have integral internal wall surfaces so as to facilitate smooth uniform introduction of reactant gases into the combustion zone. While aligned ports or passages are preferred, it will be evident that parallel aligned passages are not absolutely essential so long as an essentially uniform escape rate of gases from all portions of the feed plate into the combustion zone is maintained. From the standpoint of non-turbulent flow, it is of course, preferable to employ essentially parallel aligned passages from the back to the front face of the feed plate. For high combustion rates, it is necessary that at least approximately 65% (possibly as little as 50%) of the total area of any cross section of the feed plate transverse to the flow of gases therethrough should be open; at lower combustion rates, the percent of open area may be as low as 40% or so. The open area may reach a value as high as approximately 80%, or possibly slightly higher.

Since contact between the surface of passages in the feed plate and gas passing therethrough is not primarily accomplished in order to gain surface catalyzed combustion or heat transfer (aside from the cooling effect the gases have on the feed plate as they pass therethrough), it is not so important that the feed plate itself exhibit any specified superficial surface value as discussed in connection with the radiation plate. However, the thickness of the feed plate should be at least approximately % inch inasmuch as the flashback arresting action of longer thin passages is quite pronounced whereas extremely short thin passages serve less effectively in this function. Usually the feed plate passages will not exceed about 2 inches in length, although even longer ones may be used (with increased pressure drop across the plate), if desired.

Combustion within the relatively short distance between the feed plate and the radiation plate may significantly be accelerated by using a combustion plate 11. In the absence of a catalytic surface-combustion promoter such as combustion plate 11, the combustion chamber between the radiation plate and feed plate would of necessity be approximately four or five times the shorter length possible to use where the combustion plate is included. This is so because space combustion proceeds at a slower rate than surface-catalyzed combustion. Thus, the combustion plate becomes essential where compactness of burner design is critical. Gases wiping the surfaces of a combustion plate tend to increase in their rate of combustion. Thus, the larger the surface area presented to the flow of gases through the combustion

3,155,142 before exhaust gases enter the ports of the radiation plate. Because heat conduction away from the radiation plate is preferably kept at a minimum, this spacing serves to maintain the radiation plate in non-con5 ductive relation to the combustion plate. (Joining the plates together would, in effect, reduce the tendency for optimum attainment of the primary functions of each plate, as described.) Of course, the radiation plate may indeed function as a scavenger to surface-catalyze the IO combustion of any gases incompletely reacted in the combustion zone; but it does not primarily serve as a burning promoter. Advantageously, it serves to protect the combustion zone against external interference (e.g., flame blow out) such as a gust of wind; and this 15 is particularly significant in connection with various sign uses of the burner.

The distance separating the combustion plate from the feed plate will generally be greater than the distance between the radiation plate and the combustion plate so 20 as to achieve some space combustion prior to entrance of the fuel gases into the ports of the combustion plate where rapid surface-catalyzed combustion is to be realized. Some space combustion immediately prior to entering the ports of the combustion plate tends to cause 2a the combustion plate to reach incandescence rapidly, and therefore function more effectively as a surface catalyzer, as is desired. Additionally, a space between the feed plate and combustion plate allows for redistribution of gases emerging from ports of the feed plate, and serves 30 to isolate the combustion plate from the feed plate. In this manner heat conductive losses back through the feed plate are essentially obviated. In practice, gases emerging from the feed plate tend to lose some velocity at the point of emergence, and this further contributes to gain35 ing space combustion prior to entry of the gases into the combustion plate.

Illustratively, the separation space between the radiation plate and the combustion plate is preferably maintained at least between about Vs inch and preferably 40 no greater than 1½ inches or even 2 inches, whereas the separation between the feed plate and combustion plate is preferably at least Vi inch and no greater than about 5 inches.

Where a burner of this invention is designed for a 45 use application involving prolonged on-off operation, especially where the off periods are sufficiently long to allow the burner to cool, the radiation plate should exhibit sufficient thermal shock resistance so as to avoid excessive cracking. A radiation plate (preferably con50 sisting of corrugated members along with ridges of corrugations between each corrugated sheet unified together) formed by firing a refractory composition of the following type has been found to possess satisfactory thermal shock resistance: 67.54% zirconium silicate, 27.06% 55 aluminum silicate, 4.05% mullite fibers, and 1.35% glass fibers. Generally, a burner employing a radiation plate of this composition should be limited in its operation to maximum radiation temperatures around 2400° F. or possibly 2500° F. as measured by an optical pyrometer, go For higher temperature operation, radiation plates of alumina are quite suitable. However, a burner having an alumina radiation plate subjected to on-off cycling as well as operation at the higher radiation emission temperatures may suffer crackage of its radiation plate. 65 But so long as the crackage is not sufficient to cause the plate to collapse, no significant impairment of the radiation emission has been noticed. Of course, various refractory materials such as thoria, zirconia, bonded silicon carbide, cordierite, magnesia, etc., with or without 70 fillers, insulating ingredients and the like blended therewith, may be employed as the material from which to fabricate plates.

Of course, those skilled in the art will readily appreciate that materials employed in fabricating the radia75 tion plate and combustion plate, as well as to a lesser zone, the greater the catalysis of surface combustion effects. (In this sense, the combustion plate may be called a flame-holder.) However, for effective burner operation with higher radiation emission, there is a practical limit to the surface area thus to be presented by the combustion plate inasmuch as a superficial surface value (determined as described in connection with the radiation plate) greatly in excess of the value required to accomplish essentially complete surface-catalyzed combustion of gases will tend to detract from the heat remaining in gases passing from the exhaust side of the combustion plate on their way to the radiation plate. While great liberality is permissible in the superficial surface value employed in the design of the combustion plate, it usually should not exceed about 75, and preferably lies in a range of about 5 to 60 for most burner designs of the invention where compactness of burner design is desired in combination with an attempt to gain the higher energy emissions from the radiation plate of the burner. Additionally, the thickness of the combustion plate usually is at least Vs inch and should not exceed about two inches, or possibly three inches where extraordinary high velocity for the reactants is employed.

As in the case of the radiation plate, the combustion plate must promote flashback of flame for the surface combustion principle described to be realized in this plate. Indeed, since the efficiency of surface combustion greatly increases as temperature increases, and becomes particularly significant at temperatures in excess of about 1600° F., it generally is preferred to employ ports in the combustion plate larger than those in the radiation plate. At least, the minor dimension for the ports of this plate will always exceed 0.03 inch. Also, for best results with desired low pressure drop across the combustion plate, the percent of open area transverse to the flow of gases through the feed plate should be at least about 60%, preferably 65 to 90%.

The spacing distance between the plates of the burner is also significant where higher rates of combustion with radiant energy emission is desired. Between the radiation plate and the combustion plate, the spacing should be less than the spacing between the combustion and feed plate. Maintaining the zone of the greatest combustion intensity as close as possible to the radiation plate, without conductive contact, permits achieving a temperature in the radiation plate as high as the burner is capable of reaching under any particular set of conditions. Generally, as the superficial surface value of the combustion plate is increased, the separation, between the combustion plate and the radiation plate is decreased in order to maintain the desired close relation between the radiation plate and the zone of greatest intensity of combustion. However, the space between these two plates will always remain at least about ^J/s inch. Where the highest possible flow rates of gases through the burner are desired, the separation between the radiation plate and combustion plate will be increased; and the spacing will also be increased as the thickness of the combustion plate is decreased. Such relationships will readily be understood when it is realized that the surface catalyzed combustion effects are reduced with a reduction in exposed surface area of the combustion plate, as well as to some extent with an increase in the velocity of gases passing therethrough (i.e., reduced dwell time for the gases). It must be realized, however, that where higher flow rates for the gases are contemplated, the separation between the radiation plate and the combustion plate is not increased proportionally to the increased flow rate, since the higher temperatures created by higher velocity flows tend also to increase combustion intensity or rate in the combustion zone.

The space between the radiation plate and the combustion plate also allows for flow redistribution of the gas streams before they enter the ports of the radiation plate. It permits some space combustion to take place

3,155,142 extent the feed plate, serve as a limiting factor on the radiation emission temperature one may reach during burner operation. Thus, materials which melt at a temperature below 2800° F. are quite unsuitable to employ as radiation or combustion members in a burner designed to operate at 2800° F. Nevertheless, the materials from which parts of the burner may be fabricated vary widely since useful results based on the teaching hereof may be gained without necessarily reaching the highest radiation temperatures contemplated for preferred structures.

The following is offered as a specific illustration of a burner constructed and operated according to the invention. The housing members were formed of light weight high refractory insulating bricks good to 3000° F. or more (suitably consisting of 71% A1₂O₃, 27% SiO₂1% TiO₂, and 1% iron oxide and other impurities). Housing section 13α was equipped with a pipe 14 having an internal diameter of % inch and a 2 inch square terminal flange. The flange was lodged and sealed in housing section 13α using a commercially available refractory cement consisting largely of powdered alumina with calcium aluminate binder and clay (e.g., 80% alumina, 15-20% calcium aluminate and up to 5% or so kaolin clay). Housing section 13Ζ» had a 1¾ inch square centrally located opeing. Its dimension i&gt;i was 2 inches and dimension b₂ about 1 inch. The slope of the expanding portion of its centrally located opening was such that it terminated in a square of about 2¼ inches on the outer face of section 13Z&gt;.

The feed plate for housing 13.6 was formed of alumina using a corrugated webbed pattern as illustrated in FIGURE 3. Both the corrugated sheet and the flat sheet members were only about 3 mils thick and the frequency of corrugations in the corrugated sheet were about 20 per inch. The average port area in this plate was about 0.0002 square inch and about 70 to 80% of the plate was open area. The thickness of the plate was about Vz inch, and its area dimensions just under 1¼ x 1¼ inches. It was fittted into the housing· section 135 as illustrated in the drawing and glued in place by using the aforementioned refractory cement.

The combustion plate for this structure was also formed of alumina using honeycomb pattern formed by bonding corrugated members together along meeting ridges. Its dimensions were about 2¼ x 2¼ inches. The sheet members of the honeycomb were about 20 mils thick; and the frequency of corrugations was about 3 per inch. The plate had an average hole area of 0.013 square inch, an average superficial surface are of 25 square inches per cubic inch volume of the plate, a thickness of about Vz inch, and a superficial surface value of about 13.

Radiation plate 10, formed of the thermal shock resistant zirconium aluminum silicate formula above described, was about 2¾ x 2¾ square inches and about '-A inch thick. It consisted of a structure such as illustrated in FIGURE 3. The corrugated and flat sheet members were about 10 mils thick. The frequency of corrugation was about 7 per inch. The average area of the ports of this plate was 0.0026 square inch. Its average superficial surface area was about 60 square inches per cubic inch, and its superficial surface value about 15.

Both the combustion plate and radiation plate were loosely mounted (i.e. mechanically seated under the lip portions of the firebrick) in the structure so as to allow for about 5 to 8% free expansion before contact with the internal recessed portions under the lip flanges of fire brick members 13c and 13d. About %e inch clearance on all sides is suitable. The spacing between the radiation plate and the combustion plate was about % inch and between the combustion plate and feed plate about 1 inch. The average slop of the walls of the combustion zone below the combustion plate was approximately 20° from a line perpendicular to the plate as illustrated in the drawing. Above the combustion plate the slope of the walls of the combustion zone was about 30°. The resulting burner, with its fire brick housing sections cemented together using the refractory cement above 5 noted, was confined with a housing about 4% inches high and 5 inches square.

This burner was connected to a line from a mixing unit for proportioning 1 part of natural gas (a gas of about 1000 B.t.u. per cubic foot) per 9.7 parts of air. The 10 burner was ignited at a gas-air introduction flow rate of 50 c.f.h. (cubic foot per hour). Shortly after ignition, the flame passed through the radiation plate into the combustion zone between that plate and the feed plate, enveloping the combustion plate. About 20 seconds fol15 lowing flashback, the rate of introduction of the gas-air mixture was turned up to about 210 c.f.h. over a period of about 20 seconds. Combustion remained in the combustion zone of the burner. The combustion plate, as well as the radiation plate, rapidly became incandescent. 20 It attained cherry red. incandescence immediately following flashback to the combustion zone. The temperature of the radiation plate after turn up of gas introduction rate was measured to be at 2450° F. using an optical pyrometer. (By using an alumina radiation plate instead 25 of the zircon mullite one here employed, and a gas-air input rate to about 400 c.f.h., a temperature of about 3000° F. for such a radiation plate may be attained.)

A significant advantage of this burner is that the pressure drop across the radiation plate as well as the com30 bustion plate is extremely low (in the realm of only a few hundredths of an inch of water at higher gas-air input rates) as compared to the pressure drop across the feed plate. And the pressure drop across the feed plate also is very low (under about 1 inch at higher gas-air 35 flow rates). Furthermore, the burner has a turn-down ratio of about 10 to 1, which means tht the burner may successfully be operated at as low a burning rate as 35 c.f.h., as well as at rapid combustion rates (greatly increased gas-air flow) as high as 400 c.f.h. Its versatility 40 in this respect contributes to its usefulness inasmuch as small variations of gas introduction rate are not accompanied by flame blow out or blow off or backflash. At combustion rates approaching the maximum for a zircon mullite radiation plate (e.g., 210 c.f.h. of a mixture of 1 45 part natural gas and 9.7 parts air), the radiant heat emission of this burner was approximately 1600 B.t.u. square inches/hour at its radiation plate temperature of 2450° F. as measured by an optical pyrometer.

Those skilled in the are will readily appreciate that 50 physical configurations of the burner of this invention may vary greatly without upsetting essential features and characteristics described. For example, it is visualized that more than one feed plate for reactant gases may be employed in the design of a burner. Feed plates may be 55 positioned opposite each other on the side walls of a combustion zone, with refractory material otherwise outlining the confines of the zone except for the radiation plate opening. Surface combustion may be induced in a combustion zone by employing several combustion plates 60 in series. The port openings in each may vary in size; but each preferably should have essentially uniform port openings distributed essentially uniformly over the area thereof. These plates may be separate or sinter-welded together to form a complex combustion plate. The ra65 diation plate also may be formed of two or more plates in series, each sinter-welded to the other to form a complex structure. It should here be noted that the ports through a complex structure may indeed be of different geometry at various levels throughout the plate; but the 70 geometry at any one level is essentially uniform for all of the ports in the plate. Thus, the ports may properly be described as having integral internal wall surfaces of essentially uniform geometry and essentially uniform distribution throughout any one cross sectional portion of 75 a plate transverse to the flow of gases therethrough.

3,165,142 tributed over the area thereof such that between 50 and 90% of any cross section of said plate transverse to the flow of gases therethrough is open, said gas burner assembly further having a combustion chamber within 5 said refractory housing beneath said radiation plate, with a flashback-arresting gas feed plate mounted transverse to the flow of gases into said combustion chamber, said gas feed plate having multiple port openings therethrough distributed over the area thereof such that any cross 10 section of said feed plate transverse to the flow of gases therethrough presents a total open area less than the lowest total open area of any equivalent-sized cross section of said radiation plate transverse to the flow of gases therethrough, the mounting of said plates in said 15 housing being such that substantially all gaseous material flowing through the housing must flow through each of said plates.

4. A gas burner assembly operable under a variety of inlet gas velocities and pressures, comprising a corrugated^ 20 type multiport aligned-passaged refractory ceramic radiation plate which promotes flashback of flame therebeneath, a separate multiport refractory ceramic feed plate which arrests flashback of flame therethrough, and a combustion chamber between said radiation plate and 25 feed plate, said radiation plate having a superficial surface area between 30 and 100 square inches per cubic inch volume thereof, a superficial surface value of between 6 and 75, and ports essentially uniformly distributed over the area thereof such that between 50% and 30 90% of any cross section of said radiation plate transverse to the flow of gases therethrough is open, said ports further being at least 0.03 inch in minor dimension and also wider in that dimension than the ports of said feed plate, the minor dimension of the ports of said feed 35 plate being no greater than 0.05 inch, the arrangement of said plates within said gas burner assembly being such that substantially all gaseous material flowing through the gas burner assembly must flow through each of said plates.

5. A gas burner assembly operable under a variety of inlet gas velocities and pressures, comprising a refractory housing in which is mounted: (1) an exposed multiport flashback-promoting refractory ceramic radiation plate having a superficial surface area between 30 and 100 45 square inches per cubic inch volume thereof, a superficial surface value of between 6 and 75, and ports distributed over the area thereof such that between 50 and 90% of any cross section of said plate transverse to the flow of gases therethrough is open, the minor dimension of 50 said ports being at least 0.03 inch; (2) a separate multiport refractory ceramic combustion plate adapted to surface-catalyze the combustion of combustible fuel mixtures passing therethrough, having a superficial surface value up to 75, with ports essentially uniformly distrib55 uted over the area thereof such that at least 60% of any cross section of said plate transverse to the flow of gases therethrough is open, the minor dimension, of said ports being at least 0.03 inch; and (3) a separate multiport refractory ceramic flashback-arresting gas feed plate ₈0 having ports of integral internal wall surfaces essentially uniformly distributed over the area thereof such that between 50 and 80% of any cross section of said plate transverse to the flow of gases therethrough is open, the minor dimension of said ports being less than the minor ₆5 dimension of the ports selected for said radiation plate as ’«veil as less than the minor dimension selected for the ports of said combustion plate, and no greater than 0.05 inch; the mounting of said plates being such as to dispose them transverse to the flow of combustible gases through said housing and being so arranged that substantially all of the material flowing through the housing must flow through each of said plates.

6. A gas burner assembly operable under a variety of inlet gas velocities and pressures, comprising a multiport 75 refractory ceramic radiation plate which promotes flash11

By choosing an assembly of plates having minimum pressure drop, it is possible to operate the burner on inspired air. Another modification of design readily possible using the principles hereof is that of employing several composite burner members as described, all facing a central area equipped with ducts to carry away hot gases. Such arrangement for several burner members hereof would generally be used where hot exhaust gases are desired for some processing step. It is conceivable that an annular or cylindrical burner may be constructed with a radiation plate, combustion plate and feed plate concentrically disposed between a sandwich of refractory brick material. In such structures, the cylindrical radiation plate could be disposed innermost (where creation of hot exhaust gases is desired) or disposed outermost (where an annular emission of radiation is desired).

While great flexibility of design is possible using the principles hereof, it generally has been found that burners most closely approximating the design features specifically illustrated are superior from the standpoint of attaining desired high rates of combustion as well as radiation emission properties in combination with safety of operation. In these burners, the ratio of the area of the feed plate to the combustion plate to the radiation plate preferably lies within the limits of 1:3/2:2 to 1:5:8. General expansion of the combustion zone of the burner from the inlet to exhaust openings is desired so as to allow space for the gases as they expand during combustion, and thereby reduce back pressure effects.

The foregoing is to be construed as illustrative and nonlimitative of my invention, as it is further pointed out in the following claims.