TUBULAR BURNER

Abstract

A flame hole member of a tubular burner has a front plate and a rear plate both of sheet metal make. The front plate has a first flame hole in the central portion of the front plate, and a plurality of second flame holes located around a periphery of the first flame hole, each of the second flame holes being of a slit shape in a width below a quenching distance. The rear plate has a first ventilation hole in the central portion of the rear plate, and a plurality of second ventilation holes of smaller diameter than the first ventilation hole, each of the second ventilation holes being located around a periphery of the first ventilation hole. At least one of the rear plate and the front plate is provided with a cylindrical section for introducing into the first flame hole.

Description

BACKGROUND

1. Field of the Invention

The present invention relates to a tubular burner having; a mixing tube including at a rear end thereof an inlet port into which a fuel gas and primary air flow; a venturi section having a smaller diameter than the diameter of the inlet port; and a tapered tube section having a gradually larger diameter from the venturi section toward a front of the mixing tube. The tubular burner has a flame hole member with a plurality of flame holes, the flame hole member being adapted to be fitted into a front end region of the mixing tube.

2. Description of the Related Art

As this kind of burner, there is conventionally known one which is described in U.S. Pat. No. 5,186,620. In the burner described therein, a flame hole member is made of a sintered metal of larger thickness. A plurality of flame holes which penetrate in the back and forth (i.e., longitudinal) direction are formed in the flame member so that a mixture of a fuel gas and primary air (hereinafter also referred to as air-gas mixture) is ejected from these flame holes for combustion.

The flow of the air-gas mixture from the mixing tube toward the flame hole member has a directional component that is directed toward the radially outward direction under the influence of the tapered tube section. Therefore, if the flame hole member is made smaller in thickness, the flames tend to be spread radially outward. The above-mentioned conventional burner, on the other hand, has a flame hole member of larger thickness. As a result, the flow of the air-gas mixture is rectified at each of the flame holes so as to be directed forward, thereby preventing the flames from getting spread radially outward.

However, in the above-mentioned conventional burner, the flame hole member is made of a sintered metal of higher material cost, thereby bringing about a disadvantage of higher cost.

SUMMARY

Problems to be Solved by the Invention

In view of the above points, this invention has a problem of providing a tubular burner in which a flame hole member is made of a sheet metal plate to thereby reduce the cost, and in which the flames can be prevented from getting spread diametrically outward.

Means for Solving the Problems

In order to solve the above-mentioned problems, this invention provides a tubular burner comprising: a mixing tube inclusive of an inlet port, at a rear end thereof, into which a fuel gas and primary air flow, a venturi section having a smaller diameter than a diameter of the inlet port, and a tapered tube section having a gradually larger diameter from the venturi section toward a front of the mixing tube; and a flame hole member having a plurality of flame holes and being adapted to be fitted into a front end region of the mixing tube such that a mixture of the fuel gas and primary air is ejected through the flame holes for combustion. The flame hole member is made up of a front plate of sheet metal make, and a rear plate of sheet metal make located behind the front plate. The front plate has a first flame hole in the central portion of the front plate, and a plurality of second flame holes located around a periphery of the first flame hole, each of the second flame holes being of a slit shape in a width below a quenching distance. The rear plate has a first ventilation hole in the central portion of the rear plate, and a plurality of second ventilation holes of smaller diameter than the first ventilation hole, each of the second ventilation holes being located around a periphery of the first ventilation hole. At least one of the rear plate and the front plate has disposed therein a cylindrical section for introducing into the first flame hole the mixture of the fuel gas and primary air flowing into the first ventilation hole.

According to this invention, the flame hole member is made of the front plate and the rear plate, i.e., a total of two plates of sheet metal make. Therefore, as compared with the above-mentioned conventional example in which the flame hole member made of a sintered metal is used, the cost can be reduced. In addition, according to this invention, although the flame hole member is of sheet metal make, the flames can be prevented from getting spread in a radially outward direction.

In other words, according to this invention, by providing the cylindrical section for introducing the air-gas mixture flowing into the first ventilation hole toward the first flame hole, the flow of the air-gas mixture directed toward the first flame hole is rectified by the cylindrical section. As a result, the air-gas mixture is ejected strongly out of the first flame hole. On the other hand, the flow velocity of the air-gas mixture that is ejected from the second flame holes of slit shape in the front plate through the second ventilation holes of relatively small diameter in the rear plate can be kept relatively small. Therefore, the air-gas mixture ejected from the second flame holes is attracted into the flow of the air-gas mixture that is ejected from the first flame hole at a high speed, thereby preventing the flames from getting spread radially outward.

By the way, should the first flame hole be formed into a pipe-shaped element that protrudes forward from the front plate without providing the above-mentioned cylindrical section, there may also be obtained an effect in that the flow of the air-gas mixture to be ejected from the first flame hole is rectified to thereby prevent the flames from getting spread in the radially outward direction. In this arrangement, however, that heat quantity from the flames which is inputted into the pipe-shaped element of the first flame hole will increase and, as a result, back firing is likely to occur due to overheating of the first flame hole. On the other hand, according to this invention, without the necessity of forming the first flame hole into the pipe-shaped element, there can be obtained an effect of preventing the flames from getting spread in the radially outward direction. Back firing due to overheating of the first flame hole can thus be prevented. Further, by forming the second flame holes into a slit shape in width that is smaller than the quenching distance, back firing at the second flame holes can also be prevented.

Further, according to this invention, preferably, a rear region of the cylindrical section is gradually reduced in diameter from the first ventilation hole forward, and the cylindrical section in front of the rear region is formed into a cylindrical shape of smaller diameter than the first ventilation hole. According to this arrangement, in addition to the effect of rectifying the flow of the air-gas mixture, there can also be obtained an accelerating effect in that the flow velocity of the air-gas mixture is made larger than the flow velocity thereof into the first ventilation hole. The spreading of the flames in the radially outward direction can thus be effectively prevented.

Furthermore, preferably, the length of the cylindrical section is equivalent to the longitudinal distance between the rear plate and the front plate. It is to be noted here that the term “equivalent” includes all of the following cases, i.e.,: the case in which the length of the cylindrical section is the same as the longitudinal distance between the rear plate and the front plate; and also the case in which the longitudinal length between the rear plate and the front plate is slightly shorter than the longitudinal length between the rear plate and the front plate. In the latter case, a clearance may occur between the front end of the cylindrical section that is disposed in the rear plate and the front plate, or a clearance may occur between the rear end of the cylindrical section that is disposed in the front plate and the rear plate. Even in case such a clearance may occur, the air-fuel gas that enters the cylindrical section or the air-fuel gas that has flown into the first ventilation hole will never leak through the clearance into the space outside the cylindrical section between the front plate and the rear plate. The case in which such a leak will not occur is also understood to fall under the meaning of “equivalent.” According to this arrangement, all of the air-gas mixture entering the first ventilation hole is introduced into the first flame hole, and the flow velocity of the air-gas mixture to be ejected from the first flame hole becomes larger. As a result, the flames can be effectively prevented from getting spread in the radially outward direction.

Further, in case the cylindrical section is disposed in the rear plate, the diameter of the first flame hole is preferably larger than an inner diameter at the front end of the cylindrical section. According to this arrangement, the air-gas mixture entering the cylindrical section can smoothly be ejected from the first flame hole without being hindered by the front plate. As a result, the flow velocity of the air-gas mixture that is ejected from the first flame hole becomes larger and also the pressure loss becomes smaller.

By the way, in case the second flame holes are of slit shape elongated in a radial direction of the front plate, preferably a plurality of inner second flame holes are formed in a portion, closer to the first flame hole, of the front plate at a circumferentially equal pitch, and a plurality of outer second flame holes are formed while being circumferentially deviated by half a pitch from the inner second flame holes so that the outer second flame holes are located between: such an intermediate portion in the front plate as is located in a diametrically inner end and a diametrically outer end of the inner second flame holes; and such a portion in the front plate as is located diametrically outward of the diametrically outer end of the inner second flame holes. According to this arrangement, the second flame holes can be disposed in the front plate in a well-balanced manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of a tubular burner according to a first embodiment of this invention.

FIG. 2 is a perspective view of the tubular burner according to the first embodiment of this invention.

FIG. 3 is a partly cut-away perspective view of the tubular burner according to the first embodiment of this invention.

FIG. 4 is a partly cut-away perspective view of a tubular burner according to a second embodiment of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 reference numeral 1 denotes a tubular burner according to an embodiment of this invention. This burner 1 is used as a heat source of a heating appliance, and is disposed so as to lie opposite to an inlet end of a heat exchange pipe P which performs heat exchanging with room air.

The burner 1 is made up of a mixing tube 2, and a flame hole member 3 which is adapted to be fitted into a front end region of the mixing tube 2. Also with reference to FIGS. 2 and 3, the mixing tube 2 has: an inlet port 21 at a rear end thereof; a venturi section 22 which is reduced in diameter relative to the inlet port 21; and a tapered tube section 23 which is gradually increased in diameter from the venturi section 22 forward. In this arrangement, a fuel gas ejected from a gas nozzle (not illustrated) which is disposed so as to face the inlet port 21, and primary air flow from the inlet port 21 into the mixing tube 2 so that a mixture of fuel gas and primary air is generated within the mixing tube 2. The mixing tube 2 is made of a thin sheet metal plate, and is formed by combining together two sheet metal plates 2a, 2a made, e.g., of press-formed stainless steel, and the like.

Although not illustrated, a plurality of the tubular burners 1 are disposed in parallel with one another. At the front end region of the two sheet metal plates 2a, 2a that constitute the mixing tube 2, there is each formed a dented portion 2b in a manner to be away from the other sheet metal plate 2a. The clearance to be generated between the two sheet metal plates 2a, 2a by means of these dented portions 2b constitutes a slit-shaped carry-over flame hole 2c which causes flames to be carried over to the adjoining burners.

The front end region of the mixing tube 2 is formed in a cylindrical shape which is elongated forward from an enlarged-diameter region 23a of a curved shape at the front end of the tapered tube section 23. The flame hole member 3 to be fitted into the front end region of the mixing tube 2 is constituted by a front plate 4 which is formed of a sheet metal plate of stainless steel make and the like, and a disk shaped rear plate 5 which is formed of a sheet metal of stainless steel make and the like and which is located rearward of the front plate 4.

The front plate 4 has a tubular member 4a which is elongated backward from a circular disk-shaped front portion so as to be fitted into the inner circumference of the front portion of the mixing tube 2. The front plate 4 is provided with a first flame hole 41 in the central portion at the front face of the front plate 4, and a plurality of second flame holes 42 which are located around the periphery of the first flame hole 41 and which are in slit shape in a width (e.g., 0.7 mm) below a quenching distance.

The second flame holes 42 are formed into slits which are elongated in the radial direction of the front plate 4. In case this kind of second flame holes 42 are formed simply at an equal pitch in the circumferential direction, the distance between the adjoining second flame holes 42, 42 becomes too large toward the radially outward portion. As a solution, in this embodiment, a plurality of inner second flame holes 42in are formed in a portion, closer to the first flame hole, of the front plate 4 at a circumferentially equal pitch. Also, a plurality of outer second flame holes 42out are formed circumferentially deviated by half a pitch from the inner flame holes 42in so as to be located between: such an intermediate portion, in the front plate 4, as is located in a diametrically inner end and a diametrically outer end of the inner second flame holes 42in; and such a portion, in the front plate 4, as is located diametrically outward of the diametrically outer end of the inner second flame holes 42in. According to this arrangement, the second flame holes 42 can be disposed in the front plate 4 in a well-balanced manner.

The second flame holes 42 may alternatively be formed in a slit shape elongated in the circumferential direction of the front plate 4. In other words, on a plurality of circles that are coaxial with the first flame hole 41, a plurality of the second flame holes may be formed.

Further, at a front end of the tubular member 4a of the front plate 4, there is formed a rounded corner portion 4b which is away from the inner peripheral surface of the front end region of the mixing tube 2. At the rounded corner portion 4b there is formed a plurality of flame retention holes 43 of a slit shape at a circumferential distance from one another.

The rear plate 5 is provided with a first ventilation hole 51 in the central portion of the rear plate 5, and a plurality of second ventilation holes 52 which are of a smaller diameter than that of the first ventilation hole 51 and which are located around the periphery of the first ventilation hole 51. The rear plate 5 has further formed therein a cylindrical section 53 which projects forward from the hole edge of the first ventilation hole 51. It is thus so arranged that the air-gas mixture flowing into the first ventilation hole 51 is introduced into the first flame hole 41 through the cylindrical section 53.

The rear region of the cylindrical section 53 is gradually reduced in diameter from the first ventilation hole 51 toward the front side. That portion of the cylindrical section 53 which lies in front of the rear region (i.e., the front portion of the cylindrical section 53) is formed into a cylindrical shape of smaller diameter than that of the first ventilation hole 51. Further, the length of the cylindrical section 53 is the same as the longitudinal distance between the rear plate 5 and the front plate 4 so that no clearance in the longitudinal direction occurs between the front end of the cylindrical section 53 and the front plate 4. Furthermore, the diameter of the first flame hole 41 is larger than the inner diameter of the front end of the cylindrical section 53. In this embodiment, the diameter of the first flame hole 41 is made substantially equal to the outer diameter of the front end of the cylindrical section 53.

According to the above-mentioned tubular burner 1 of this embodiment, the flame hole member 3 is constituted by the front and the rear, a total of two, plates 4, 5 of sheet metal make. Therefore, as compared with the conventional example in which a flame hole member made of a sintered metal is used, the cost can be reduced. Further, if the flame hole member 3 is made of a sheet metal plate, the air-gas mixture of the fuel gas and the primary air is ejected with a directional component that is directed in a radially outward direction under the influence of the tapered tube section 23 of the mixing tube 2, and the flames are likely to be spread in the radially outward direction. However, in this embodiment, the flames can be prevented from spreading radially outward, whereby the flames can surely be introduced into the heat exchange pipe P. A description will now be made of the reasons.

In this embodiment, because the cylindrical section 53 is disposed to introduce the air-gas mixture flowing into the first ventilation hole 51 toward the first flame hole 41, the flow of the air-gas mixture directed to the first flame hole 41 is rectified by the cylindrical section 53. As a result, the air-gas mixture is forcibly ejected forward from the first flame hole 41. On the other hand, the flow velocity of the air-gas mixture to be ejected from the slit-shaped second flame holes 42 in the front plate 4 via the second ventilation holes 52 of relatively smaller diameter in the rear plate 5 is kept relatively low. As a result, due to Bernoulli law, the air-gas mixture ejected from the second flame holes 42 is attracted by the flow of the air-gas mixture ejected at a high speed from the first flame hole 41. Consequently, the flames to be formed by the combustion of the air-fuel mixture ejected from the second flame holes 42 are combined into the flame to be formed by the combustion of the air-fuel mixture ejected from the first flame hole 41, whereby aggregated flames Fa elongated forward are formed and the flames can be prevented from getting spread radially outward.

In this embodiment, since the front portion of the cylindrical section 53 is formed into a cylindrical shape having a diameter smaller than that of the first ventilation hole 51, there can be obtained an accelerating effect in which the velocity of the air-gas mixture is larger than the incoming velocity into the first ventilation hole 51, in addition to the rectifying effect of the air-gas mixture. Therefore, the velocity of the air-gas mixture ejected from the first flame hole 41 becomes larger, whereby the radially outward spreading of the flames can effectively be prevented.

In case the cylindrical section 53 is not provided but the first flame hole is formed into a pipe-shaped element that protrudes forward from the front plate, there can also be obtained an effect in that the flow of air-gas mixture ejected from the first flame hole is rectified and that the spreading of the flames in the radially outward direction can be prevented. In this arrangement, however, the amount of heat from the flames to be inputted into the pipe-shaped element in the first flame hole becomes too large, and backfiring is likely to occur due to overheating of the first flame hole. In this embodiment, on the other hand, there can be obtained an effect in that, without forming the first flame hole 41 into a pipe-shaped element, the radially outward spreading of the flames can be prevented. Backfiring due to overheating of the first flame hole 41 can thus be prevented. In addition, by forming the second flame holes 42 into slit shape of a width below the quenching distance, backfiring in the second flame hole 42 can also be prevented.

In place of the slit-shaped second flame holes 42, backfiring can similarly be prevented by forming, as the second flame holes, a multiplicity of circles of diameter smaller than the quenching distance. It is, however, advantageous to arrange the second flame holes 42 into slits like in this embodiment, because the fabrication becomes easier and the pressure loss can be reduced.

It is also possible to make the length of the cylindrical section 53 smaller than the longitudinal distance between the rear plate 5 and the front plate 4. In this arrangement, however, there will occur a clearance in the longitudinal direction between the front end of the cylindrical section 53 and the front plate 4. Part of the air-gas mixture flowing into the cylindrical section 53 may leak through this clearance into the space outside the cylindrical section 53 between the front plate 4 and the rear plate 5. As a result, the flow velocity of the air-gas mixture ejecting from the first flame hole 41 will be lowered. In the arrangement of this invention, on the other hand, the length of the cylindrical section 53 is made equal to the longitudinal distance between the rear plate 5 and the front plate 4 so that no clearance in the longitudinal direction occurs between the front end of the cylindrical section 53 and the front plate 4. As a result, all of the air-gas mixture flowing into the first ventilation hole 51 will be introduced into the first fame hole 41. In conjunction with the above-mentioned accelerating function, the velocity of the air-fuel mixture ejected from the first flame hole 41 becomes still faster, and the radially outward spreading of the flames can more effectively be prevented.

In this embodiment, an arrangement has been made that the length of the cylindrical section 53 is set to be the same as the longitudinal distance between the rear plate 5 and the front plate 4. However, it need not be limited to such an arrangement. In other words, take for example a case in which the length of the cylindrical section 52 is slightly smaller than the longitudinal distance between the rear plate 5 and the front plate 4 and, as a result, a clearance occurs in the longitudinal direction between the front end of the cylindrical section 53 and the front plate 4. Even in such a case, if the air-gas mixture flowing into the cylindrical section 53 does not leak into the space outside the cylindrical section 53 between the front plate 4 and the rear plate 5, there can be obtained a similar effect as the one described above.

Further, in this embodiment, the diameter of the first flame hole 41 is made larger than the inner diameter of the front end of the cylindrical section 53. Therefore, the air-gas mixture flowing into the cylindrical section 53 is smoothly ejected from the first flame hole 41 without being disturbed by the front plate 4. The velocity of the air-gas mixture ejected from the first flame hole 41 becomes larger and the pressure loss becomes smaller.

Further, the air-gas mixture ejected from each of the flame retention holes 43 gets collided with the inner circumference at the front end of the mixing tube 2, and is then diffused in the circumferential direction in the annular clearance that is generated between the rounded corner portion 4b and the inner circumference at the front end of the mixing tube 2. The air-gas mixture is thereafter ejected forward from this clearance. Since the velocity of ejection of the air-gas mixture from this clearance is lowered due to the collision of the air-gas mixture with, and diffusion thereof into, the inner circumference of the front end of the mixing tube 2, there can be formed flames Fb that are hard to be lifted off, thereby securing the flame retention property.

Now, a description will be made of a second embodiment of this invention as shown in FIG. 4. The basic construction of the second embodiment is not particularly different from that of the first embodiment. The same reference numerals as those in the first embodiment are therefore assigned to the similar members and the parts. The difference of the second embodiment from the first embodiment is that the diameter of the first flame hole 41 is made larger than the outer diameter of the front end of the cylindrical section 53.

Also in the second embodiment, the air-gas mixture entering the cylindrical section 53 is smoothly ejected from the first flame hole 41 without being disturbed by the front plate 4. Therefore, in the same manner as in the first embodiment, the flow velocity of the air-gas mixture ejected from the first flame hole 41 is accelerated and also the pressure loss becomes smaller.

Descriptions have so far been made of embodiments of this invention with reference to the accompanying drawings. This invention is however not limited to the above embodiments. Although the cylindrical section 53 is formed in the rear plate 5 in the above embodiments, the following arrangement may also be made. For example, the front plate 4 is provided with a cylindrical section that protrudes backward from the hole edge of the first flame hole 41 toward the first flame hole 51 so that the air-gas mixture flowing into the first flame hole 51 is introduced into the first flame hole 41 through the cylindrical section. In this arrangement, preferably, the length of the cylindrical section is made equal to the longitudinal distance between the rear plate 5 and the front plate 4 so that, between the rear end of the cylindrical section and the rear plate 5, there occurs no clearance which allows air-gas mixture flowing into the first ventilation hole 51 to leak into the space outside the cylindrical section between the rear plate 5 and the front plate 4. It is also possible to make the length of the cylindrical section larger than the longitudinal distance between the rear plate 5 and the front plate 4 so that the rear end region of the cylindrical section is fitted into the first ventilation hole 51. Further, in case the cylindrical section is disposed in the front plate 4, it is preferable to gradually reduce the diameter of the rear region of the cylindrical section from the first ventilation hole 51 toward the front side so that the front portion of the cylindrical section is formed into a cylindrical shape having a smaller diameter than the first ventilation hole 51 but the same diameter as the first flame hole 41. However, in case the cylindrical section is formed in the front plate 4, the fabrication of the cylindrical section becomes troublesome. Therefore, it is advantageous to form the cylindrical section 53 in the rear plate 5 as in the above-mentioned embodiments.

In addition, the following arrangement may also be employed. For example, the rear plate 5 is provided with a cylindrical section that protrudes forward from the edge of the first ventilation hole 51. The front plate 4 is provided with a cylindrical section that protrudes backward from the edge of the first flame hole 41. Then, both the cylindrical sections are fitted between the rear plate 5 and the front plate 4 so that the air-gas mixture flowing into the first ventilation hole 51 is introduced into the first flame hole 41 through both the cylindrical sections. It is possible to provide at least one of the front plate 4 and the rear plate 5 with an independent cylindrical section other than these plates 4, 5. Although the mixing tube 2 of sheet metal make is used in the above-mentioned embodiments, it is also possible to use a mixing tube made of a cast steel. Further, the flame retention holes 43 in the above-mentioned embodiments may be omitted. In addition, in the above-mentioned embodiments, this invention was applied to a tubular burner for heating appliances. This invention can, however, be applied to tubular burners which are used in a combustion equipment other than a heating appliance.

Claims

1. A tubular burner comprising:

a mixing tube inclusive of an inlet port, at a rear end thereof, into which a fuel gas and primary air flow, a venturi section having a smaller diameter than a diameter of the inlet port, and a tapered tube section having a gradually larger diameter from the venturi section toward a front of the mixing tube; and

a flame hole member having a plurality of flame holes and being adapted to be fitted into a front end region of the mixing tube such that a mixture of the fuel gas and primary air is ejected through the flame holes for combustion,

the flame hole member being made up of a front plate of sheet metal make, and a rear plate of sheet metal make located behind the front plate,

the front plate having a first flame hole in a central portion of the front plate, and a plurality of second flame holes located around a periphery of the first flame hole, each of the second flame holes being of a slit shape in a width below a quenching distance,

the rear plate having a first ventilation hole in a central portion of the rear plate, and a plurality of second ventilation holes of smaller diameter than the first ventilation hole, each of the second ventilation holes being located around a periphery of the first ventilation hole,

at least one of the rear plate and the front plate having disposed therein a cylindrical section for introducing into the first flame hole the mixture of the fuel gas and primary air flowing into the first ventilation hole.

2. The tubular burner according to claim 1, wherein a rear region of the cylindrical section is gradually reduced in diameter from the first ventilation hole forward, and wherein the cylindrical section in front of the rear region is formed into a cylindrical shape of smaller diameter than the first ventilation hole.

3. The tubular burner according to claim 1, wherein a length of the cylindrical section is equivalent to a longitudinal distance between the rear plate and the front plate.

4. The tubular burner according to claim 1, wherein the cylindrical section is disposed in the rear plate, and wherein a diameter of the first flame hole is larger than an inner diameter at the front end of the cylindrical section.

5. The tubular burner according to claim 1,

wherein the second flame holes are of slit shape elongated in a radial direction of the front plate,

wherein a plurality of inner second flame holes are formed in a portion, closer to the first flame hole, of the front plate at a circumferentially equal pitch, and

wherein a plurality of outer second flame holes are formed while being circumferentially deviated by half a pitch from the inner second flame holes so that the outer second flame holes are located between: such an intermediate portion in the front plate as is located in a diametrically inner end and a diametrically outer end of the inner second flame holes; and such a portion in the front plate as is located diametrically outward of the diametrically outer end of the inner second flame holes.

6. The tubular burner according to claim 2,

wherein the second flame holes are of slit shape elongated in a radial direction of the front plate,

wherein a plurality of inner second flame holes are formed in a portion, closer to the first flame hole, of the front plate at a circumferentially equal pitch, and

wherein a plurality of outer second flame holes are formed while being circumferentially deviated by half a pitch from the inner second flame holes so that the outer second flame holes are located between: such an intermediate portion in the front plate as is located in a diametrically inner end and a diametrically outer end of the inner second flame holes; and such a portion in the front plate as is located diametrically outward of the diametrically outer end of the inner second flame holes.

7. The tubular burner according to claim 3,

wherein the second flame holes are of slit shape elongated in a radial direction of the front plate,

wherein a plurality of inner second flame holes are formed in a portion, closer to the first flame hole, of the front plate at a circumferentially equal pitch, and

wherein a plurality of outer second flame holes are formed while being circumferentially deviated by half a pitch from the inner second flame holes so that the outer second flame holes are located between: such an intermediate portion in the front plate as is located in a diametrically inner end and a diametrically outer end of the inner second flame holes; and such a portion in the front plate as is located diametrically outward of the diametrically outer end of the inner second flame holes.

8. The tubular burner according to claim 4,

wherein the second flame holes are of slit shape elongated in a radial direction of the front plate,

wherein a plurality of inner second flame holes are formed in a portion, closer to the first flame hole, of the front plate at a circumferentially equal pitch, and

wherein a plurality of outer second flame holes are formed while being circumferentially deviated by half a pitch from the inner second flame holes so that the outer second flame holes are located between: such an intermediate portion in the front plate as is located in a diametrically inner end and a diametrically outer end of the inner second flame holes; and such a portion in the front plate as is located diametrically outward of the diametrically outer end of the inner second flame holes.