US2618539A

US2618539A - Flashback-proof gas mixer

Info

Publication number: US2618539A
Application number: US82314A
Authority: US
Inventors: Lewis D Conta; Teti John
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1949-03-19
Filing date: 1949-03-19
Publication date: 1952-11-18
Anticipated expiration: 1969-11-18

Description

NOV 18, L, D, CQNTA ET AL 2,618,539

FLASHBACK-PROOF GAS MIXER Filed March 19, 1949 v 3 Sheets-Sheet 1 FIG. I

NVENTORS LEWIS 0. OONTA JOHN TETI Nov. 18, 1952 L. D. CONTA ET AL 2,618,539

' FLASHBACK-PROOF GAS MIXER Filed March 19, 1949 5 Sheets-Sheet 2 JOHN T;

BY g g T ozmagYsaw Nov. 18, 1952 L. D. CONTA ETAL 9 FLASHBACK-PROOF GAS MIXER Filed March 19, 1949 3 Sheets-Sheet 3 FIG. 7

FIG. 6

INVENTO RS LEWS D.CONTA JOHN TETI ATTORNEYS Patented Nov. 18, 1952 FLASHBACK-PROOF GAS MIXER Lewis D. Conta, Rochester, N. Y., and J ohn Teti, Plainfield, N. J., assignors to Air Reduction Company, Incorporated, a corporation of New York Application March 19, 1949, Serial No. 82,314.

6 Claims.

This invention relates to gas mixers, especially mixer for oxyacetylene gas-burning apparatus, such as cutting and welding torches, flame hardening apparatus, etc.

Heretofore it has been necessary to develop a special mixer for each type of apparatus, with considerable care being given in the design to the problem of flame stability. If a piece of apparatus produces an unstable flame, frequent backfiring is likely to occur, and while backfires in themselves are not so serious, they are likely to result in more dangerous and destructive flashbacks, i. e., a continued burning of the gas mixture at some point within the apparatus upstream of the discharge orifice where the mixture is supposed to burn. It has therefore been necessary, in designing the gas mixer for each type of apparatus, to take into consideration the conditions peculiar to that type of apparatus and design the gas mixer accordingly with a view to eliminating, so far as possible, flashbacks in that type of apparatus.

It is the principal object of this invention to provide a universal type of gas mixer that is suitable for use with a wide variety of gas-burning apparatus and at the same time will make each piece of apparatus flashback-proof, although not necessarily backfire-proof.

According to the invention, the improved gas mixer has a body portion provided with two separate inlets for the gases to be mixed, and also provided with an outlet through which the mixed gases are discharged. Within the body portion there is a mixing region. for the gases and there is provided at least one passage in the form of a narrow slot interposed between each gas inlet and the mixing region. The mixer can take various forms, as will hereinafter appear, but the principle on which all of them work is as follows: When the mixer is incorporated in a piece of gas-burning apparatus such as an oxyacetylene torch, with the separate gas inlets of the mixer properly connected to oxygen and acetylene supply conduits, and th outlet of the mixer properly connected to the torch tip so that the gas mixture is fed to the flame or flames of the torch, and a backfire occurs, all of the mixed gases in the system burn with explosive rapidity, thus creating a rapid increase in pressure within the mixer, forcing the burned mixed gases upstream through the above-mentioned narrow slot passages for the two gases and perhaps back into the oxygen and acetylene conduits leading to the mixer. As soon as the pressure in the mixer subsides, these burned gases come back down through the narrow slot passages. By passing through them the burned gases are cooled and deactivated by contact with the walls of the slot passages. The burned gases, thereby cooled, then purge the downstream passages in the apparatus of the hot active products of combustion. Fresh oxygen and acetylene follow the cooled burned gases down through the mixer to the mixing region, but since no active or hot gas is present to ignite the fresh mixed gases, no flashback occurs. By active products of combustion is meant dissociation products such as free atoms and free radicals that release energy on recombining and which are capable of reigniting the fresh gases if not properly deactivated. One way of deactivating these so-called chain carriers is by promoting their collision with adjacent walls and with each other.

Several forms of an improved gas mixer embodying the invention are illustrated in the accompanying drawings, in which:

Figure 1 is a longitudinal section through one form of the mixer;

Fig. 2 is an exploded perspective view of the stack of laminations forming part of the mixer of Fig. 1, fewer laminations being shown, however, than are in the stack illustrated in Fig. 1;

Fig. 3 is a view similar to Fig. 2 illustrating a modified form of lamination stack that may be used in the mixer of Fig. 1;

Fig. 4 is a longitudinal section through a modified form of the improved mixer;

Fig. 5 is an exploded perspective view of a portion of the stack of laminations forming part of the mixer of Fig. 4;

Fig. 6 is a longitudinal section through a further modified form of the improved mixer; and

Fig. 7 is a transverse section taken along the line l? of Fig. 6.

Referring first to Figs. 1 and 2, the gas mixer illustrated therein comprises a body portion lfl having inlets l I and I2 for the gases to be mixed, such as oxygen and acetylene. The body portion als has an outlet I3 through which the mixture of the two gases is discharged. The inlets II and I2 communicate by means of passages II' and I 2, respectively, with a cavity I4 in one section of the body portion Ill. Within this cavity is placed a stack of laminations I5 that will be described in detail later. The stack of laminations is held in place by a plug I5, which constitutes another section of the body portion, and which is screwed into the firstnamed section of the body portion, a gas-tight seal being eiiected between the two sections of 3 the body portion of an O-ring IT, or in any other suitable way. The plug I6 is provided with an outlet passage l3 which leads from the cavity 14 in the body portion to the outlet I3.

Fig. 2 is an exploded perspective View of the laminated stack 15 (with fewer laminations in the stack) and best illustrates the construction of the Various members that make up the stack. The parts designated [8 are shims or plates, preferably in the shape of discs, and the parts designated [9 are spacers which are preferably given an annular shape when the plates or shims is are in the shape of circular discs. Each of the shims I8, except the uppermost one I8a, has a central opening 23 which constitutes a mixed gas port. Each of them, except the lowermost one 181), also has openings 2| and 22 at opposite sides of the opening which constitute ports for the gases to be mixed. The spacers have an outside diameter equal to the diameter of the shims and an inside diameter at least large enough to encompass the projections of the openings 2! and 22 provided in the shims. When the shims and spacers are properly stacked in alternating arrangement and positioned in the body portion as shown in Fig, 1, a passage is formed through the stack of laminations by the openings 2| in the shims in line with inlet I! and inlet passage II for distribution of one of the gases through the stack. Similarly, the openings 22 form a passage through the stack in line with inlet I2 and inlet port 12' for distribution of the other gas through the stack. The openings 20 in the shims likewise form a central passage through the lamination-stack for the mixed gases. In the space between each two adjacent shims created by the interposed spacer, the two gases are free to flow inwardly toward the center axis of the stack where they meet and flow through the center passage formed by the aligned openings 29. This center passage constitutes the mixing region heretofore referred to, and the gas mixture formed therein flows through the outlet passage IE to the outlet 13.

The flow of the two gases through the lamina- 7 tion stack is indicated by the arrows in Fig. 2.

The spacers [9 are formed of thin material so that the space between each two adjacent shims forms a narrow slot passage of the kind hereinbefore referred to for each of the two gases on its way to the center opening 20 where it meets the other gas. The exact thickness of the spacers is determined by the nature of the gases to be mixed. In the case of oxygen and acetylene the spacers should have a thickness of the order of .012". The shims 58 are preferably considerably thicker than the spacers so that the narrow slot passages for the gases will be formed between masses of metal that are sufliciently thick to cool the burned mixed gases in their travel through the lamination stack when a backfire occurs as hereinafter described.

Tests have shown that when the spacers are thin enough to make the narrow slot passages between the shims not more than .004" in width the mixer is completely flashback-proof under all conditions. Spacers which give the slot passages a width of .012" make the mixer flashbackproof for all practical conditions, and for special flow and orifice conditions, spacers having a thickness of .030" have even made the mixer work successfully. The thickness of the spacers is determined solely by the necessity for sufficient quenching and deactivation of the burned gases as they pass through the narrow slot passages and contact with the closely positioned adjacent walls of the shims.

It will now be seen that when the mixer is installed in an oxyacetylene torch, or in a similar piece of gas-burning apparatus, and a backfire occurs, the burning gas mixture will be forced backwards through the narrow slot passages between the shims l8 and perhaps into the oxygen and acetylene passages leading to the stack of laminations. As soon as the pressure caused by the backfire subsides, the burned gases are forced downstream again by the incoming oxygen and acetylene, and as they pass back through the narrow slot passages they are cooled and deactivated by contact with the shims between which they pass. The burned gases, cooled in this manner, purge the downstream passages in the apparatus of the hot active products of combustion. The fresh oxygen and acetylene which follow the cooled burned gases down through the mixer form, of course, a new combustible gas mixture in the mixing region, but there is no active or hot gas present to ignite it, so no flashback occurs. Even if the backfire forces the burning gases back into the oxygen and acetylene passages leading to the lamination stack, no harm is done since the incoming oxygen and acetylene will force the burned gases back down through the mixer as soon as pressure of the backfire subsides as previously explained, and moreover there cannot be a continued burning in the oxygen and acetylene passages leading to the lamination stack since a combustible gas mixture is not formed until the gases reach the lamination stack and pass through the narrow slot passages to the central mixing region of the mixer. The capacity of the mixer may, of course, be varied by changing the number of members in the lamination stack.

A modified form of lamination stack that may be used in the mixer of Fig. 1 is illustrated in Fig. 3. The members designated 23 are the shims and those designated 24 are the spacers. In this case the spacers are not given the shape of annular rings as in Fig. 2, but each spacer has a relatively large opening 24a and a relatively small opening 2%. When the lamination stack is assembled alternate spacers are reversed in position so that the larger openings 24a of successive spacers lie at opposite sides of the central axis of the lamination stack and so that the same is true of the smaller openings 242). Thus, one set of alternate spacers has the larger openings 24a. in alignment and the other set of alternate spacers has the smaller openings 24b in alignment. Each larger opening 24a has a diameter sufilciently great to encompass the projections of the central opening 20 of a shim and one of the side openings 2| or 22. The smaller openings 241) have the same size as the side openings 2| and 22 in the shims.

It will therefore be seen that the oxygen, flowing downwardly at one side of the lamination stack through the successive openings in the shims and spacers, flows inwardly toward the axis of the lamination stack through the narrow slots formed between every other pair of shims by the larger openings 2% in one set of alternate spacers. and thence downwardly through the mixed gas passage at the axis of the lamination stack. Similarly, the acetylene, flowing downwardly at the other side of the lamination stack through the successive openings in the shims and spacers at that side of the stack, flows inwardly toward the axis of the stack through the narrow slot passages formed. between every other pair of shims by the larger openings 24a in. the. other set of alternate spacers, and thence down through the central passage of the mixer where it mixes with the: oxygen. The arrows in. Fig. 3 indicate the flow of the two gases through the lamination stack in the manner just described..

The lamination stack may be made in various other ways to provide a. number of narrow slot passages interposedbetween each of the gas inlets of the mixer and the mixing region.

Another form of mixer operating on. the same principle is illustrated in Fig. 4. The body portion of this mixer is generally similar to the body portion of the mixer of Fig. 1 except that. the: inlet passages H and. 12 leading from the gas inlets ll and 12 respectively, communicate with the cavity [4 within the body portion through the side walls of the cavity instead of through the bottom wall as in Fig. 1. Moreover, the cavity is divided into two compartments by a partition member 25 which is rendered gas-tight around its periphery by an O-ring. 2B or in any other suitable way. The inlet passage ll communicates with the cavity l4 above this partition and the inlet passage l2 communicates with the cavity below it. This type of mixer also has a stack of laminations, or more strictly speaking, two portions [5a. and l5b of a lamination stack, the portion 1541 being positioned above the partition 25 and the portion l5b being positioned below the partition. The shims and spacers that make up each portion of the lamination stack are constructed somewhat-differently from the shims and spacers previously described, as shown in Fig. 5, which is an exploded view of some of the shims and spacers of one of the stack portions. As shown in this view, both the shims and the spacers have a central opening 21 through which a bolt 2% (Fig, 4)v passes to clamp the laminations of each portion. of the lamination stack together and to clamp the two portions of the lamination stack to the. partition 25. A plug [8, which may be similar to the corresponding plug of Fig. 1, screws into the. section of the body portion that contains the lamination stack until it. bears against the lower end of the stack, and is sealed by the O-ring I! as previously described in connection with Fig. l. The plug, as in the case of the mixer shown in Fig. 1, has an outlet passage 3' for the mixed gases leading to the mixed gas outlet l3.

Referring again to Fig. 5, it will be. seen that each shim 29 of the lamination stack. has a plurality of gas ports 30 arranged in a circular series. Each of the spacers 3! is in the general form of a hub with a plurality of radiating spokes. The. spokes on. each spacer are spaced apart circumferentially of the spacer by a distance equal to to circumferential length of the gas ports 33 in the shims. The spacers have the above-described critical thickness for the particular gas mixture employed; When. the shims and spacers are assembled to produce the lamination stack, the radiating. spokes of the spacers are positioned in alignment with the spokes separating the gas ports in the shims. Thus, the lamination stack has a circular series of longitudinal gas passages extending the full height of the stack, and there is formed between each two adjacent shims by the interposed spacer a circular series of spaces each of which is in effect a portion. of one of the gas passages through the lamination stack but which is in open communication at the periphery of the stack With the cavity l4 within the body portion of the mixer. It will therefore be seen that the gas which enters the cavity [4 above the partition flows laterally into the lamination stack through the spaces between the spokes of the spacers and then through the longitudinal gas passages that extend through the stack. The partition 25 has a circular seriesv of openings corresponding to the circular series of gas ports in the shims so that the gas which enters the longitudinal passages in the upper portion of the lamination stack can flow through the longitudinal passages in the lower portion of the stack as well. The other gas entering the portion of the cavity l4 below the partition 25 enters the lower portion of the lamination stack in the same manner and mixes with the first-mentioned gas while the two are passing through the portions of the longitudinal gas passages formed in the lower portion of the lamination stack. The mixed gases then flow through the outlet passage l3" and are discharged through the outlet 13. It should be noted that before any portion of each gas can reach one of the longitudinal gas passages in the lamination stack it must pass through a nar-' row slot passage formed between closely positioned edge portions of two adjacent shims. Thus, as in the case of the mixers previously described, there are a plurality of narrow slot passages interposed between each gas inlet and the mixing region within the body portion, the mixing region being formed by the longitudinal gas passages in the lower portion l5?) of the lamination stack.

Still another form of the mixer is illustrated in Figs. 6 and 7. In this form a single. narrow slot passage is interposed between each gas inlet and. the mixing region rather than a plurality of slot passages formed by a lamination stack as previously described. In order that the single slot passage may accommodate the total flow of each gas the slot passage is preferably in the form of a circular fiat and narrow space with an annular discharge passage leading from such space intermediate its center and its outer edge. One of the gases is delivered to this slot passage through an inlet passage which meets the slot passage radially inward of the circumference along which the annular discharge passage joins the slot passage, and the other gas is delivered to the slot passage through an inlet passage which meets it radially outward of such circumference, whereby the two separate gases upon. reaching the circular slot passage flow radially inward and radially outward respectively until they meet at the entrance to the annular discharge passage. Such a circular flat space forming anarrow slot passage for each of the gases before they mix is shown at 32 in Fig. 6 and may be formed as follows: A circular shallow recess having a depth of the order of the width of the slot passages previously described is provided in the lower face of one section 33 of the body portion [0 of the mixer. The open lower side of this recess is closed by the. upper end of another section 3 5 of the body portion and by the base of a cone-shaped member 35 which is positioned in the upper end of a central passage 36 in the section 34 of the body portion. The two

sections

33 and 34 of the body portion may be clamped together by a nut 31 constituting a third section of the body portion. The upper portion of the passage 38 that receives the cone-shaped member 35 converges downwardly so that the wall of this convergent portion of the passage is almost parallel to the exterior surface of the cone-shaped member 35. Longitudinal spacing ribs 38 are provided on the cone-shaped. member to form an annular space 38" through which the gas mixture discharged from the slot passage 32 can flow to the outlet passage 36 and thence to the outlet 39 of the mixer. The upper face of the section 33 of the bod portion is provided with an annular recess 40 and a central recess 4|. These recesses are closed by a cover plate 42 which may be secured to the section 33 of the body portion in any suitable way. One of the gases, such as oxygen, is delivered to the body portion of the mixer through a conduit 43 and passes through an opening 44 in the cover plate 42 and thence to the annular recess 45 This recess communicates with the circular slot passage 32 by means of a circular series of passages 45 (see also Fig. '7). The other gas, such as acetylene, is delivered to the body portion of the mixer through another conduit 46 and passes through an opening 41 in the cover plate 42 into the center recess 4| in the section 33 of the body portion. A series of passages A8 in the section 33 of the body portion deliver the acetylene to the slot passage 32. The passages 45 communicate with the circular slot passage 32 radially outward of the circumference along which the annular discharge passage 38' joins the slot passage 32, and the passages 48 communicate with the circular slot passage radially inward of such circumference. It will thus be seen that when the oxygen reaches the circular slot passage 32 near its periphery it flows radially inward to the entrance of the annular discharge passage 38'. When the acetylene reaches the circular slot passage 32 it flows radially outward until it reaches the entrance to the annular discharge passage.

Here the two gases meet and flow downwardly through the annular discharge passage around the cone 35 and through the outlet passage 36 to the outlet 39. The annular discharge passage, and particularly its entrance, therefore constitutes the mixing region of the mixer.

It will thus be seen that, as in the case of the mixers previously described, there is a narrow slot passage interposed between each gas inlet and the mixing region of the mixer. Although there is only one slot passage through which every portion of each of the gases must flow to reach the mixing region, the passage has sufficient capacity because of the circular shape given to the narrow space 32 and because of the fact that each slot passage has an elongated exit formed along the circumference of a circle, 1. e., the circle formed by the entrance to the annular discharge passage for the mixed gases at the base of the cone 35.

From the foregoing it will be evident that a flashback-proof mixer has been provided that can take many forms, all of which operate on the same principle and in each of which there is a narrow slot or slots upstream of any mixed gases extending through walls of relatively great mass and heat-absorbing capacity. The quenching surfaces between which the slot passages are formed must be adequate to cool and deactivate the burned gases after a backfire, and the mixed gas volume must be sufficient to cause a substantial backflow through the mixer. In the case of a backfire, all of the mixed gases burn, expanding up into the narrow slot or slots and up into the oxygen and acetylene passages. When the gases are completely burned and normal flow returns, the burned gases then pass down again through the narrow slot or slots and are cooled, thus forming an insulating barrier between the hot products of combustion and the fresh incoming gases, thereby preventing ignition of the fresh gases in the mixer.

The terms of orientation and direction used in the foregoing description apply to the various forms of mixer when used in the vertical position, i. e., with the gas mixture outlet directed down wardly, but it should be understood they may be used in any position.

In our co-pending application Serial No. 82,313, filed March 19, 1949, we have claimed a flashbackproof gas burner embodying certain features of the gas mixer claimed herein.

We claim:

1. An anti-flashback gas mixer for a burner system in which a fuel gas and a combustionsupporting gas are mixed and the mixture passes to a place of combustion at which the mixture is burned, comprising a body portion having separate inlet passages for the fuel gas and the combustion-supporting gas and also having an outlet for connection to the place of combustion, means forming Within the body portion a mixing chamber for the gases associated with said outlet passage, said chamber being separated from said inlet passages by walls of relatively great mass, at least one passageway through one of said Walls communicating at opposite sides with the inlet passage for fuel gas and the mixing chamber, respectively, and at least one passageway through another of said walls communicating at opposite sides with the inlet passage for the combustion-supporting gas and the mixing chamber, respectively, said passageways having a width in a direction normal to the direction of the flow of the gases therethrough of from about .004 inch to .030 inch, the walls defining said passageways having a thickness in said direction greater than said width of the passageways and also having sufiicient heat-absorbing capacity to cool burned mixed gases passing therethrough on a backfire sufiicient to prevent flashback.

2. An anti-flashback gas mixer as defined in claim 1 in which a, plurality of the passageways of said dimensions are formed in the respective walls for the passage of both the fuel gas and the combustion-supporting gas to the mixing chamber.

3. An anti-flashback gas mixer as defined in claim 1 in which said passageways are annular flat spaces, the mixing chamber includes an annular gas-mixture passage communicating with said passageways, one of said passageways extends radially inwardly of the circumference along which the annular gas-mixture passage joins said passageways to communicate with one of said gas inlet passages and the other of said passageways extends radially outwardly of said circumference to communicate with the other of said gas-inlet passages.

4. An anti-flashback gas mixer for a burner system in which a fuel gas and a combustionsupporting gas are mixed and the mixture passes to a place of combustion at which the mixture is burned, comprising a body portion having separate inlet passages for the fuel gas and the combustion-supporting gas and also having an outlet passage for connection to the place of combustion, at least one stack of laminations within the body portion comprising a plurality of parallel plates separated by spacers, said plates having registering openings forming Within the body portion a mixing chamber for the gases associated with said outlet passage, portions of said plates forming walls of relatively great mass between said mixing chamber and the respective inlet passages, said spacers forming passageways in said wall forming portions of the plates for the flow of gases thercthrough from said respective inlets to said mixing chamber, said passageways having a width of from about .004 inch to .030 inch in a direction normal to the direction of the flow of gases therethrough, the portions of the plates of said stack which form said walls having a thickness greater than said width of the slots and also having sufficient heat-absorbing capacity to cool mixed gases passing therethrough on a backfire sufficiently to prevent flashback.

5. An anti-flashback gas mixer for a burner system in which a fuel gas and a combustion-supporting gas are mixed and the mixture passes to a place of combustion at which the mixture is burned, said mixer having separate inlet passages for the fuel gas and the combustion-supporting gas, means forming an outlet passage for connection to the place of combustion, and a mixing chamber communicating with said outlet passage and with each of said inlet passages, the communication between each of said inlet passages and said mixing chamber having at least a portion thereof in the form of a plurality of narrow slot quench passageways which are of rel-- atively great length in the direction of flow of the gas therethrough and which have a relatively short width of about .012 inch in a direction normal to the direction of flow of the gas therethrough, the walls defining said passageways having a thickness in said direction greater than said Width of the passageways and also having sufi'icient mass and heat-absorbing capacity to cool burned mixed gases passing therethrough on a backfire sufficient to prevent flashback.

6. Means for preventing flashback in a burner system having a flame-producing burner provided with a burner orifice supplied with mixed fuel gas and combustion-supporting gas from a mix- 10 ing chamber upstream of said burner orifice, comprising a mixing chamber, means for supplying said fuel gas to said mixing chamber through one or more quench passages having at least one dimension normal to the direction of flow of the fuel gas therethrough not exceeding .030 inch, means for supplying combustion-supporting gas to said mixing chamber through one or more quench passages having at least one dimension normal to the direction of flow of the combustion-supporting gas therethrough not exceeding .030 inch, said mixing chamber downstream of said quench passages being adapted to mix said fuel gas and said combustion-supporting gas prior to delivery of said gases to said burner orifice, said quench passages being so positioned relative to said mixing chamber and having such a flow capacity that they are effective to provide for the backfiow of products of combustion resulting from a backfire in said mixing chamber upstream into said quench passages, and said quench passages having a quench surface area eifective to deactivate said products of combustion therein, so that said quenched products of combustion will form an isolating barrier of deactivated gas separating said active products of combustion and the combustible mixture of fresh gas upon the restoration of normal gas flow.

LEWIS D. CONTA. JOHN TETI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,032,071 Le Chantelier July 9, 1912 1,517,313 Pease Dec. 2, 1924 1,551,634 Bennett Sept. 1, 1925 2,229,441 Carlson Jan. 21, 1944

Claims

1. AN ANTI-FLASHBACK GAS MIIXER FOR A BURNER SYSTEM IN WHICH A FUEL GAS AND A COMBUSTIONSUPPORTING GAS ARE MIXED AND THE MIXTURE PASSES TO A PLACE OF COMBUSTION AT WHICH THE MIXTURE IS BURNED, COMPRISING A BODY PORTION HAVING SEPARATE INLET PASSAGES FOR THE FUEL GAS AND THE CONBUSTION-SUPPORTING GAS AND ALSO HAVING AN OUTLET FOR CONNECTION TO THE PLACE OF COMBUSTION, MEANS FORMING WITHIN THE BODY PORTION A MIXING CHAMBER FOR THE GASES ASSOCIATED WITH SAID OUTLET PASSAGE, SAID CHAMBER BEING SEPARATED FROM SAID INLET PASSAGES BY WALLS OF RELATIVELY GREAT MASS, AT LEAST ONE PASSAGEWAY THROUGH ONE OF SAID WALLS COMMUNICATING AT OPPOSITE SIDES WITH THE INLET PASSAGE FOR FUEL GAS AND THE MIXING CHAMBER, RESPECTIVELY, AND AT LEAST ONE PASSAGEWAY THROUGH ANOTHER OF SAID WALLS COMMUNICATING AT OPPOSITE SIDES WITH THE INLET PASSAGE FOR THE COMBUSTION-SUPPORTING GAS AND THE MIXING CHAMBER, RESPECTIVELY, SAID PASSAGEWAY HAVING A WIDTH IN A DIRECTION NORMAL TO THE DIRECTION OF THE FLOW OF THE GASES THERETHROUGH OF FROM ABOUT .004 INCH TO .030 INCH, THE WALLS DEFINING SAID PASSAGEWAYS HAVING A THICKNESS OF SAID DIRECTION GREATER THAN SAID WHICH OF THE PASSAGEWAYS AND ALSO HAVING SUFFICIENT HEAT-ABSORBING CAPACITY TO COOL BURNED MIIXED GASES PASSING THERETHROUGH ON A BACKFIRE SUFFICIENT TO PREVENT FLASHBACK.