GB2052696A - Boiler - Google Patents

Abstract

A boiler of improved capacity and efficiency and causing less pollution and noise has a tubular casing 40, water tubes 33 forming a first wall means and water tubes 34 forming a second wall means outwardly of the first wall means. The tubes extend between end chambers 31 and 32. Annular gas ducts 39, 41 are formed between the walls and casing 40, and ducts 55 may extend from the combustion chamber 35 to the duct 39. Smoke tubes 44 within the water tubes 33 connect with the combustion chamber 35 and annular duct 39, and smoke tubes 45 in the tubes 34 connect with the duct 39 and the annular duct 41 which has an outlet 42. The water tube walls may be replaced by annular members forming annular first and second wall means. <IMAGE>

Description

SPECIFICATION Boiler The invention relates to boilers.

Heretofore, several types of boilers have been proposed and used with satisfaction to some extent.

However, in order to decrease the complaints regarding pollution, to reduce the installation cost, and to cope with the demand for saving of energy, there has constantiy been a need for improving the capacity and efficiency of a boilder while satisfying the desired conditions, such as above.

In general, there are considered two approaches to improve the capacity and efficiency of a boiler. That is: (a) increasing the passing rate of thermal energy by raising the speed of combustion gas while increasing the amount of fuel to be fed to a burner; and (b) providing as large effective heat transmitting surfaces as possible within the space of a boiler.

If the approach (a) above is relied on, the necessary power for air supplying means such as a forced draft fan may become too large resulting in an increase in total power consumption and also causing the problem of noise. This is because the air drag or resistance is proportional to the square of the flow speed and the flow rate is proportional to the amount of fuel to be burned, so the power required for the air supply is in proportion to cube of the amount of fuel to be supplied.

If the approach (b) above is taken, construction of the heat transmitting surface may require increased speed in air flow which leads to an increase in the power required, though the thermal efficiency and capacity of the boiler would be generally increased as the amount of the thermal energy transmitted per unit area is made smaller.

Among the boilers of prior art, an attempt has been made to provide a plurality of vertical water tubes between a pair of chambers making the central portion sur;ounded by the tubes as a furnace. For example, such boilers are disclosed in Japanese Patent Public Disclosure Nos.

30341/64, 1 1210/71 and 34121/71. The boilers shown in these Disclosures operates satisfactorily, however, as stated above, there has been a further need for improvement in efficiency and capacity as well as for decreasing pollutions.

Accordingly it is an object of this invention to provide a boiler of improved efficiency and capacity and causing less pollution.

It is a further object of this invention to provide a novel construction of a boiler which improves the efficiency and capacity of boiler whilst reducing environmental noise and pollution.

It is also an object of the present invention to provide a construction of a boiler usable either as a circulation type or a once-through type.

According to the present invention, the objects above are attained wherein a pair of annular chambers is connected by a tubular wall means which comprises an outer wall and an inner wall confining a space therebetween, said two chambers being in communication with each other through the space, a room defined by the both chambers and the wall means being utilized as a combustion chamber into which a burner extends through center hole in one of the annular chambers gas duct means being provided so as to direct combustion gas from the combustion chamber through said inner wall at a place adjacent one of said chambers and through said outer wall at a place adjacent the other of said chambers to outside of the boiler. The wall means may be constructed by contiguously arranging a plurality of water tubes.Within each of the wall means duct means are provided so as to direct the combustion gas to outside of the boiler.

The present invention will be further described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic illustration of a sectional view of a prior type boiler which is referenced herein for explaining the background of the present invention; Fig. 2 is a cross sectional view taken along the line 11-Il in Fig. 1; Fig. 3 shows a schematic sectional illustration of a preferred embodiment of the present invention; Fig. 4 is a cross-sectional view taken along the line IV--IV in Fig. 3; Fig. 5 is a fragmentary view taken along the line V-V in Fig. 3; Fig. 6 is a cross-sectional view similar to Fig. 3 and illustrates another embodiment of the present invention; Fig. 7 is also a sectional schematic illustration of still another embodiment according to the present invention;; and Fig. 8 is a cross-sectional view of a further embodiment of the present invention which may be regarded as corresponding to a section taken along the line VIll-VIll with an appropriate modification effected to the embodiment of Fig. 7.

Before explaining the present invention, discussion on the prior art boilers is presented to assist the understanding of the present invention.

In Figs. 1 and 2, a boiler 10 of prior art is illustrated which is a natural circulation boiler of water tube type. The boiler 10 comprises an upper annular water chamber 11 and a lower annular water chamber 12 between which a plurality of water tubes 13 are disposed contiguously in a spiral fashion forming a tangent tube wall as illustrated in Fig. 2 to form a combustion chamber or furnace 14 at the center of the spirally formed tangent tube wall which may be divided- into an inner portion "A" and an outer portion "B".The portions "A" and "B" form a flue 1 5 between the portions "A" and "B" extending from an intake 1 6 opening to the chamber 14 and form a flue 17 between the portion "B" and an outer wall 18 of the boiler 10, the flues 15 and 17 guiding the combustion gas from the intake 16 to an exhaust duct 19 in a direction indicated by arrows in Fig. 2. The opposite ends of each,of the water tubes 13 are narrowed and coupled to the upper and lower chambers 11 and 12 respectively so that both the chambers 11 and 12 communicate with each other through the water tubes 13.Because of the narrowed portions of the respective tubes 13, gaps 20 are formed between the tubes adjacent the bottom of the upper water chamber 11 and the top of the lower water chamber 12 and these gaps are filled with suitable packing material or insulation material to isolate the furnace space 14, flues 1 5 and 1 7 from each other. On the outer wall 18 of the boiler 10 a flexible portion 21 is provided for absorbing the deformation caused by the thermal stress. The center portion of the lower annular water chamber 12 may be filled with fire resisting packing material. At the center portion of the upper annular water chamber 11, a burner 22 is disposed in a manner to direct flame downwardly with blast air supplied from a blower or a forced draft fan not shown.

When the burner 22 is ignited for operation, inner side surfaces of the water tubes 13 in the portion "A" are exposed to heat radiation of flame and convection of combustion gas, and the surfaces of the tubes 13 in the portions "A" and "B" facing the flue 15 and the surfaces of the tubes 13 in the portion "B" facing the flue 1 7 are also exposed to the combustion gas passing through the flues 1 5 and 17 thereby effecting heat transfer to the water in the tubes 13.By such heat transfer, the inner tubes belonging to the portion "A" may become riser tubes while the tubes belonging to the portion "B" may become the down comers due to the difference in temperatures of the water induced by the heat transfer and, thus, the circulation of the water is effected, obviously the tubes in the portion "A" receive a relative larger amount of thermal energy. However, the boundary between the riser tubes and down comers may vary depending on the condition of transferring thermal energy to the water within the tubes.

It may be devised to modify the arrangement of tubes 13 in Fig. 2 in order to reduce the velocity of combustion gas passing through the flues by which increase in the power required for operating the blower may be depressed. That is to diverge the flue 1 5 from the intake 1 6 into two opposite directions and join the diverged flues at the point opposite the intake 1 6 where the combustion gas has travelled 1800 of the circumference before entering into the succeeding flue corresponding to the flue 1 7 which is again diverged into opposite directions and merged at the exhaust located around the same radial position as the intake 16.This arrangement of the flues may reduce the velocity of passing gas at the intermediate positions in the passage of combustion gas, combustion gas at the upper portion in the furnace adjacent the burner 22 such as indicated with arrows "a" involves particles of oil discharged from the burner 22 without complete combustion and these unburned relatively large particles are directed to the flue 1 5 along the arrows "a" while the gas flowing into the flue 1 5 at the lower part of the intake 16 such as indicated by an arrow "b" is completely burned. The gas containing unburned particles is cooled during the passage of the flues and, thus, such particles are discharged outside without being burned thereby increasing the amount of soot and smoke.If it is desired to reduce such soot and smoke, it is necessary to increase the mixing efficiency adjacent the burner 22 so as to assist the combustion in the furnace 1 4 to become complete. Although the amount of soot and smoke may be reduced by such mixing, such mixing leads to an increase in the capacity of an air blower or a forced draft fan thereby consuming more power. Also, the amount of NOx will be increased as the degree of mixing is made violent which causes the problem of environmental pollution and noise.

The present invention, as stated earlier, obviates such problems as explained in the foregoing in connection with the prior art boiler, for example, as illustrated in Figs. 1 and 2.

Referring to Figs. 3 and 4, there is shown a boiler 30 as a preferred embodiment according to the present invention. The boiler 30 is illustrated as a water tube boiler of a natural circulation type and comprises an upper annular water chamber 31, a lower annular water chamber 32, a group of vertical water tubes 33 contiguously arranged to form a tangent tube wall of a cylindrical shape and another group of vertical water tubes 34 also contiguously arranged to form another tangent tube wall of a second cylindrical shape coaxially with the one first mentioned and outwardly disposed with an annular space between the two as shown in Fig. 4.The opposite ends of each of the tubes 33 and 34 are reduced in diameter and coupled with the bottom plate of the upper water chamber 31 and the top plate of the lower water chamber 32 so that both the chambers communicate with each other through the tubes 33 and 34. The inner side space surrounded by the group of tubes 33 is adapted to become a combustion chamber or furnace 35 into which a burner 36 is disposed and directed downwardly through a center aperture of the upper water chamber 31. The adjacent tubes 33 and 34 are, as explained above, contiguously arranged to form a tangent tube walls 37 and 38 and the adjacent tubes may be welded to each other to make sealing complete therebetween if necessary.

The space between the tangent tube walls 37 and 38 is utilized as a flue 39 and the space between an outer wall 40 of the boiler and the tangent tube wall 38 is also utilized as a flue 41 which is lead to an exhaust 42 for the combustion gas of the boiler.

As illustrated in Fig. 4 a cross section of the inner cylindrical tangent tube wall consisting of tubes 33 and the outer cylindrical tangent tube wall 38 consisting of tubes 34 is a complete circle, respectively and, thus, there is no intake similar to the intake 1 6 shown in Fig. 2 for directing the gas from the chamber 35 to the intermediate flue 39. However, as explained above, the opposite ends of respective tubes 33 and 34 are narrowed so that the portions of the tangent tube walls adjacent the bottom and top plates of the upper and lower water chambers, respectively are given gap passages permitting air to flow therethrough. For example, such passages 55 are shown in Fig. 5 for the lower part of the inner tangent tube wall. These gap passages 55 permit the combustion gas to pass from the combustion chamber 35 to the intermediate flue 39.As will be made clear, the passages formed at the opposite ends of the tubes 33 and 36 except for the passages 42 illustrated in Fig. 3 are filled with a suitable fire resisting packing material 43 to close the passages.

In order to complete the passage for the combustion gas from the furnace chamber 35 to the exhaust 42 through the flues 39 and 31, gas guiding means is provided which extends through the tangent tube walls 37 and 38. The gas guiding means are gas ducts 44 extending from the furnace chamber 35 to the intermediate flue 39 through inside some or all of the water tubes 34 and other gas ducts 45 extending through from the flue 39 to the outer flue 41. Each of the ducts 44 and 45 is comprised of a pipe.The pipe for the duct 44 opens to the furnace chamber 35 at an intake opening 46 which is located adjacent the chamber 32 at the lower position of the tube 33 and extend upwardly inside of the tube 33 and opens to the flue 39 at a discharge opening 47 diametrically opposite the intake 46, the discharge opening 47 being positioned at the upper portion of the tube 33 adjacent the bottom plate of the water chamber 31. Similarly, the pipe for the gas duct 45 opens to the flue 39 at an intake opening 48 located around the same height as that of the exhaust opening 47 and extends downwardly inside the tube 34 and opens to the flue 41 at a discharge opening 49 disposed in the lower portion of the tube 34 and diametrically opposite the intake opening 48.By the arrangement of the ducts explained above, the passages for the combustion gas from the furnace chamber 35 to the exhaust 42 are completed. The center space in the lower water chamber 32 may be filled with packing material similar to the material 43 to complete the bottom of the furnace chamber. At an appropriate portion of the boiler wall 40, a flexible portion 50 is provided to absorb the thermal stress induced in the wall.

When the burner 36 is ignited, due to the heat radiation of the flame directed downwardly by an air flow supplied from the blower etc. and the convection of the combustion gas, the inside surface of the tangent tube wall 37 is heated. The combustion-gas is directed into the intake openings 46 and gap passages 55 and is passed through the gas ducts 44 and the intermediate flue 39 and from the discharge openings 47 and the flue 39 to the ducts 45 through the intake openings 48 and discharge openings 49 in the tubes 34 and finally to the exhaust 42 through the flue 41. During the passage of the combustion gas explained above, water in the tubes receives the transfer of thermal energy from the gas whereby the tubes 33 become riser tubes and the tubes 34 become down comers to effect natural circulation of the water in the boiler 30.

In the embodiment explained above, the combustion gas flow directed to pass the inner tangent tube wall 37 is regarded as divided into the two courses, namely one entering into the gap passages 55 and the other entering into the intake openings 46. By this division of the gas flow, it is possible to maintain relatively low velocity of the combustion gas flow passing through the gap passages 55 and the intake openings 46 and, thus, the power consumption for the forced draft fan need not be increased.

Further, due to the increase of the heat transferring surface by the arrangement of the gas ducts 44 and 45 in this embodiment, the capacity and efficiency are remarkably improved and the generation of soot, smoke and NOx is restrained because intake openings 45 and gap passages 55 are located at the position remote from the burner 36 so that only completely burned gas is substantially directed to the intakes 46 and gap passages 55 without need of especially violent mixing.

The following might be the repetition of the foregoing in a somewhat analytical way. In the embodiment above, flow of the burned gas under the high temperature is divided into the two categorized routings, one entering into the intakes 36 and the other entering to the intermediate flue 39 through gap passages 55.

Resistance of a gas duct against the gas flowing therethrough may be generally expressed by the following equation: AParV2xy where: AP: resistance V: velocity of gas flow y: density of gas Therefore, the velocitu is the most predominant factor regarding the resistance or drag. Thus, diverging the gas passage into the two or three passages at a portion or portions of the whole duct may contribute to reduction of AP by reducing the velocity in the diverged ducts.

Further, as seen from the equation above, the density or volume is also related to the resistance and the volume (V) of gas is proportional to its absolute temperature as expressed by the equation 273+t(OC) 273 Accordingly, the resistance of duct against the gas flow may be effectively reduced if the gas flow is diverged when its temperature is relatively high.

In the embodiment illustrated in Figs. 3 and 4, the combustion or burned gas under high temperature is diverged into the two routings of the gas ducts 44 and the intermediate flue 39 and the diverged combustion gases merge again at the portion adjacent the discharge openings 47 after the diverged gases are cooled by heat transfer and thus their volumes are reduced.

Therefore, the merged gas flow may not make the resistance in the gas ducts 45 higher. Further, as seen from the illustration in Fig. 4, the number of gas ducts 45 may be increased compared to that of the gas ducts 44 and the volume of the gas has been reduced by heat transfer, the total resistance in the ducts 45 may be kept low even though the merged gas is directed to the undiverged ducts 45. However, it is possible to provide diverging route after the point of merging, i.e. around the intakes 43. To such end, the packed material 43 at the respective upper portions of the water tubes 34 may be removed to provide plural gap passages similar to the gap passages 55. Such provision of additional gaps will also reduce the velocity of gas flow.

In Fig. 6, a further embodiment according to the present invention is illustrated as a boiler 30'.

Since the construction and effect of the elements in the boiler are almost the same except for a few points, the elements corresponding to those in the boiler 30 (Figs. 3, 4 and 5) are given the same numerical references with a prime added to each thereof, respectively. In the boiler 30', a combustion chamber or furnace 35' is arranged to be a return flow burning type and, thus, a burner 36' of a long flame type is employed and installed in the furnace chamber so that the tip thereof is somewhat low compared to the position of the burner in the boiler 30 The combustion gas is arranged to once reach the bottom of the furnace where it is reversed to rise along the circumference of the combustion chamber 35'.The fuel is expected to be completely burned by the return flow burning and, therefore, reduction in the amount of NOx may also be expected. In this embodiment, the respective positions of gap passages 55', intake openings 46', discharge openings 47', intake openings 38' and discharge openings 49' are reversed to those corresponding thereto shown in Fig. 3 with respect to the vertical positions, respectively and, accordingly, the direction of gas flow in from the combustion chamber to the last floe 41' is also opposite to that in Fig. 3. Other elements not specifically rererred to are the same as those without prime in Fig. 3 with respect to their construction and effect.

In Fig. 7, a further embodiment according to the present invention is illustrated as a boiler 130.

In this boiler too, almost all the elements are similar to those in Fig. 3 and, thus, the corresponding elements are given the same numerical references as in Fig. 3 but with "100" added thereto, respectively. Thus, the respective functions and constructions thereof may be referred to those regarding Fig. 3 by deleting "100", respectively therefrom. As similar to Fig.

3, two tangent tube walls 137 and 138 are coaxially arranged within a boiler 1 30 to form a combustion chamber 135 and a first flue 139 and a second flue 1 41 as shown. The tangent tube walls 137 and 138 comprise a plurality of water tubes 133 and 134, respectively which are contiguously arranged.In order to complete the passage for the combustion gas from the combustion chamber 135 to an exhaust opening 142 through the first flue 139 and the second flue 131, gas ducts 144 and 145 are provided within the water tubes 1 33 and 134, respectively the gas duct 144 communicating with the furnace chamber 135 at an intake opening 146 disposed at a lower portion of the tube 133 and the opposite end of the duct 144 communicating with the first flue 139 at a discharge opening 147 disposed at an upper position of the tube 133 diametrically opposite the intake opening 146.

The duct 134 also opens to the first flue 139 at an intake opening 148 disposed at a lower position of the tube 134 around the same vertical position as the intake opening 146 and the opposite end of the duct 145 opens to the second flue 141 at a discharge opening 149 disposed diametrically opposite the intake opening 148 at an upper portion of the tube 134. By the arrangement of the ducts 144 and 141 in combination with the flues 139 and 141 and the exhaust opening 142, the combustion gas is guided to outside of the furnace from the furnace 135 in a direction indicated by the arrows.In the boiler 130, the gap passages similar to the gap passages 55 (Figs. 3 and 5) are not provided in view of the direction of flow of the combustion gas especially the flow in the first flue 139 and the corresponding portions are filled with a suitable fire registering material 143. However, the increased total surface for effecting heat transfer improves the efficiency of the boiler.

The arrangement of the intake openings 146 and 148 and the discharge openings 147 and 149 may be reversed with respect to their vertical positions in the same way as is done in the embodiment of Fig. 6 so that the boiler may be converted to a return flow burning type by properly adjusting the height of the tip of the burner of a long flame type.

In the foregoing explanation, it has been touched upon that it is not mandatory to provide a gas duct in every water tube and some of the water tubes may be arranged without such ducts.

On such occasion, the number of ducts and distribution thereof in each of the tangent tube walls may be appropriately determined in combination with the dimensions of the tubes, ducts and openings so as to facilitate the smooth operation of the boilder with high capacity and efficiency as well as to restrain the causes for environmental pollution. Similar consideration may also be applied to the provision of gap passages such as shown in Figs. 3 and 5. The consideration above might be also advantageous in the boiler 130 (Fig. 7) since there is no diverging in combustion gas flow. For example, the number of the ducts in the inner tangent tube wall may by made more than that of the ducts in the outer tangent tube wall by which the resistance in the ducts against the flow of combustion gas may be equalized throughout the entire passage of the combustion gas.Also, absence of the ducts in some of the water tubes may facilitate circulation of the water in the boiler.

In the explanation of the foregoing embodiments, the water tubes have been explained as being reduced in diameter at their opposite ends. However, straight tubes may also be employed except for the portions requiring the gap passages such as the gap passages 55 shown in Figs. 3 and 5. The reduction of the tube diameter may be accomplished by any suitable means such as swaging the end of the tube or welding a small diameter tube at the end of the water tube.

In case where tubes having axial fins are employed, the adjacent fins are connected to form a combustion chamber or flue and such tubes are generally uniform in diameter throughout their length and, thus, some portions of the fins may be cut out to provide necessary gap for passages.

This connection of the wall is also referred to as a tangent tube wall in the description of this specification and claims.

Referring to Fig. 8, there is shown a crosssection of a still further modified form boiler 60 according to the present invention. While in the embodiments hereinbefore explained, a plurality of water tubes has been employed to construct tangent tube walls. In the boiler 60, such tangent tube wails are replaced with double-walled members. An inner double-walled member 61 comprises an inner cylindrical wall 62 and an outer cylindrical wall 63 between which water may be filled or passed through. Similarly outer double-walled member 64 comprises an inner cylindrical wall 65 and an outer cylindrical wall 66 the space between which is used for water passage.Inside space of the member 61 is utilized as a combustion chamber 67, the space between the member 61 and 64 is utilized as a first flue 68 and the space between the member 64 and a casing or outer wall 69 of the boiler is utilized as a second flue 70 communicating with an exhaust opening 71. The illustration shown in Fig. 8 may be regarded as equivalent to the crosssectional view taken along the line VIll-Vill in Fig. 7 and, thus, only intake openings 72 for ducts 73 communicating the combustion chamber 67 with the first flue 68 and only intake openings 74 for ducts 75 are shown in Fig. 8. Of course, the ducts 73 and 75 open to the first and second flues 68 and 70, respectively so that a gas passage from the combustion chamber 67 to the exhaust opening 71 is completed.The arrangement illustrated in Fig. 8 may provide freedom in determining the distributing the gas ducts. The gap passages similar to those shown in Figs. 3 and 5 as 37 may be also provided in the boiler 60 by disposing tubes extending through the member 61 at appropriate portions thereof.

By the provision of gas ducts in the water tubes or the double-walled member, the surface concerning the heat transfer may be increased without scaling up all the dimensions of the boiler whereby the installation cost for the boiler of this invention may be held down with the improvement in the capacity and efficiency as well as the reduction in the problems relating to environmental pollution and noise.

In the accompanying drawings, the upper and lower water chambers have been illustrated as circularly annular shape, respectively. However, the shape of the water chambers is not iimited to circular and it may take any annular shape provided that there is a central hole. For example, the shape may be an oval, square or rectangular provided that there is a center hole through which the burner extends in case the chamber is an upper one. Accordingly, the arrangement of the tubes, double-walled members and the casing may not necessary be circular in cross-sectional view. Therefore, the term "annular" used in the specification and claims is to be regarded as not being limited to circular.

In the foregoing, the boiler of this invention has been explained as a water tube type effecting circulation of the water through the upper water chamber, down comers (or double walled member), the lower water chamber and the riser tubes (or double-walled member). However, the boiler according to the present invention may be used as a once-through type wherein the water level is considered as existing in the intermediate portion of the tangent tube walls or the doublewalled member.

Further, though the embodiments have been explained as the vertical type, the construction of the boiler according to the present invention may be also applicable to a boiler of horizontal type with similar effects and advantages wherein the boiler is preferably canted relative to the horizontal direction.

While the present invention has been explained in detail referring to the specific embodiments, it will be apparent to those skilled in the art that modification and changes are available within the spirit and scope of the present invention which will be defined in the claims annexed.

Claims (14)

Claims

1. A boiler comprising: a tubular casing; a pair of annular chambers disposed at opposite ends of said tubular casing and joined thereto, respectively; a first wall means disposed within said casing to form a tubular body spaced from said casing and having an outer wall and an inner wall defining a spaced therebetween which is coupled with said both chambers at opposite ends thereof so as to allow communication between said chambers through said space;; second wall means forming a tubular body within said casing and outwardly surrounding said first wall means with a first gap between said first wall means and said second wall means and a second gap between said casing and said second wall means, said second wall means having an outer wall and an inner wall confining a space therebetween which is coupled with said both chambers at opposite ends of said tubular body so as to allow communication between said chambers through said space; a combustion chamber defined by said first wall means and said pair of annular chambers the center hole of the annular shape of one of the chambers being closed; a burner extending through the center hole of the other annular chamber and directed toward said combustion chamber; an exhaust opening located on the casing to communicate with a second gap;; first gas duct means extending through said first wall means from said combustion chamber to said first gap; said first duct means having a first intake opening means opening to said combustion chamber adjacent one of said chambers and a first discharge opening means opening to said first gap adjacent the other of said chambers; and second gas duct means extending through said second wall means from said first gap to said second gap, said second duct means having a second intake opening means opening to said first gap adjacent one of said chambers and a second discharge opening means opening to said second gap adjacent the other of said chambers.

2. A boiler as claimed in Claim 1 wherein additional gas passage means is provided so as to extend through said first wall means and opening at both walls thereof adjacent said one chamber.

3. A boiler as claimed in Claim 2 wherein other additional gas passage means are provided so as to extend through said second wall means and opening at both walls thereof adjacent the other of said chambers.

4. A boiler as claimed in Claim 1 wherein said first and second wall means are comprised of plural water tubes contiguously arranged to form a first and a second tangent tube walls, respectively, and said first and second duct means are a plurality of tubes which are inserted within some or all of said water tubes.

5. A boiler as claimed in Claim 2 or 3 wherein said first and second wall means are comprised of plural water tubes contiguously arranged to form a first and a second tangent tube walls, respectively, and said first and second duct means are a plurality of tubes which are inserted within some or all of said water tubes at least one ends of some or all of said tubes being reduced in diameter to form said additional gas passage means in said first wall means or in said first and second wall means.

6. A boiler as claimed in Claim 4 wherein said water tubes are finned type water tubes and adjacent water tubes are joined with each other at said fins to form first and second tangent tube walls, said first and second gas duct means are a plurality of tubes which are inserted within some or all of said water tubes.

7. A boiler as claimed in Claim 6 wherein the fins are cut out in said first wall means adjacent one of said chambers to provide additional gas passage means extending from said combustion chamber to said first gap.

8. A boiler as claimed in Claim 7 wherein the fins in the second wall means are cut out adjacent the other of said annular chambers to provide another additional gas passage means extending from said first gap to said second gap.

9. A boiler as claimed in any one of Claims 1 through 8 wherein said tubular body is arranged to be vertical and one of said annular chambers is disposed at the upper end of said tubular body and the other of said chambers is disposed at the lower end of said tubular body, the lower chamber being used as a water chamber so that said boiler is a once-through type.

10. A boiler as claimed in Claim 9 wherein said upper chamber is utilized as a water chamber so that said boiler is a circulation type.

11. A boiler'as claimed in Claims 9 or 10 wherein each of said tubular casing and said tubular body is cylindrical and each of said annular chambers is circular.

12. A boiler as claimed in Claim 11 wherein said intake and discharge opening means are a plurality of intake openings and discharge openings, said intake openings in the first tangent tube wall and said discharge openings in said second tangent tube wall are approximately in the same level adjacent one of said chambers and said discharge openings in said first tangent tube wall and said intake openings in said second tangent tube wall are approximately in the other same level adjacent the other one of said chambers.

13. A boiler as claimed in Claim 11 wherein said intake openings and discharge opening means are a plurality of intake openings and discharge openings, said intake openings in the first tangent tube wall and said intake openings in said second tangent tube wall are approximately on the same level adjacent one of said chambers and said discharge openings in said first tangent tube wall and said discharge openings in said second tangent tube wall are approximately on the other same level adjacent the other one of said chambers, said additional gas passages being plugged.

14. A boiler constructed substantially as herein described with reference to and as illustrated in Figs. 3 to 8 of the drawings.