GB2028985A - Improvements in and relating to boilers - Google Patents

Abstract

A vertical boiler particularly suitable for fluid bed combustion coal firing, includes a furnace 10 arranged within an outer shell 12, the space between the furnace 10 and the she 12 containing water to be heated. The combustion gases from the furnace 10 flow through tubes 36 to a chamber 40 at the top of the boiler shell 12 and then make one or more passes through a second set of tubes 42 surrounding the furnace 10 before being discharged from the boiler. Thermic syphons are provided in the form of tubes 24 open to the water within the shell and extending into the furnace 10 to assist in the heat exchange. <IMAGE>

Description

SPECIFICATION Improvements in and relating to boilers The present invention relates to a vertical boiler and is particularly intended for use in fluid bed combustion coal firing.

According to the present invention, there is provided a boiler comprising a vertical furnace having an air supply at its lower end, tubes for combustion gases extending from the furnace at or near its upper end, a shell surrounding the furnace to define with the furnace a space surrounding the furnace for containing boiler fluid to be heated by the combustion in the furnace, a plurality of open ended tubes which extend into the combustion space of the furnace which are, over at least most of their lengths, at angles of at least 45% to the horizontal, and which open into the said space to act as convection driven syphons, and a plurality of tubes within the said space surrounding the fumace, the latter tubes communicating with the combustion gas tubes of the furnace so that the combustion gases make two or more passes through the fluid to be heated before being discharged from the boiler.

The plurality of open ended tubes and the latter mentioned tubes both improve heat exchange.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a vertical section through a boiler in accordance with the invention; Figure 2 is a section taken along line A-A in Figure 1; Figure 3 is a section along line B-B in Figure 1; Figures 4 and 5 are sections generally similar to Figures 1 and 2 of an alternative embodiment of the invention; Figures 6 and 7 are sections similar to Figures 1 and 2 of a further embodiment of the invention; Figure 8 is a vertical section showing a modification of the water supply to the lower inlets of the thermic syphons; Figure 9 is a detail of a further modification of the water supply to the lower inlets of the thermic syphons;; Figure 10 is a detail showing a furnace having modified thermic syphon tubes; Figure 11 is a detail of a modification of Figure 10 showing a furnace with corrugated vertical walls; Figure 12 shows the furnace of Figure 10 modified for alternative forms of fuel; Figure 13 is a vertical section of another boiler in accordance with this invention; and Figure 14 is a vertical section of a further boiler in accordance with this invention.

Figure 1 shows a vertical section through a boiler. The boiler comprises a vertical furnace 10 arranged within a boiler shell 12. The space 14 between the furnace 10 and the shell 12 is filled with water to be heated by the combustion within the furnace 10. The furnace 10 is designed for operation with a fluidised bed of coal or other solid fuel, the coal being supplied to the furnace through an inclined supply tube 1 6 and air being supplied to the fluidised bed 1 8 through a distributor plate 20 as indicated by the arrows 22.

The so-called thermic syphons 24 are provided in the furnace, the syphons being open ended tubes which lie within the furnace but whose ends open into the water space 14. The syphon action of these tubes is the result of natural convection, the water in the tube being heated by combustion and rising to the top of the tube at the same time inducting cold water from the lower part of the water space 14. Above the water level 30 for the water in the shell there is a steam space 32 and a steam outlet pipe 34.

The combustion gases escape from the furnace through a set of pipes 36. The pipes 36 are attached at their upper ends to a tube plate 38 which defines with the shell of the boiler and a tor plate 39, a chamber 40. A second set of tubes 42 distributed annularly around the furnace 10 are also attached at their upper ends to the top tube plate 38 and at their lower ends they are attached to a bottom tube plate 44 which defines in conjunction with the boiler shell 1 2 and a bottom plate 47, an annular chamber 46 surrounding the lower end of the furnace 10. A flue gas outlet 48 communicates with the lower chamber 46.

In operation, coal and air are introduced into the furnace and the coal burns as a fluidised bed.

Heat is transferred to the water in the surrounding water space 14, both through the walls of the furnace 10 and through the action of the thermic syphons which, as earlier explained, rely on natural convection. As can be seen from the section in Figure 3, a plurality of such thermic syphons are provided and circumferentially spacec in the walls of the furnace. Some of the heat generated by combustion escapes in the combustion gases and the tubes 36 which are immersed within the water to be heated therefore improve the heat exchange by allowing the combustion gases to cool down when escaping from the furnace 10. To improve the heat exchange even further, the combustion gases make a second pass through the water space, this time through the tubes 42 which, as shown in Figure 2, are arranged in a ring surrounding the furnace 1 0.On reaching the annular chamber 46 at the base of the boiler, the gases are allowed to escape through the flue-gas outlet 48.

In the embodiment of Figures 4 and 5, the chamber 40 at the top of the furnace is divided by a partition wall 50 into chambers 40a and 40b.

The chamber 40a communicates with a first set of tubes 42a which communicate with the annular chamber 46, the latter also communicating with tubes 42b which lead into the space 40b which is provided with the flue-gas outlet instead of the annular chamber 46. This modification of the earlier described embodiment which involves the use of a partition wall 50 and altering the position of the fiue-gas outlet results in the combustion gases making three passes through the water space 14 as indicated by the arrows in Figure 4.

As the gases are now flowing through a smaller cross-section, and as they have to flow through a longer path, a higher pressure is required to drive the combustion gases but at the same time the efficiency of the heat exchange is improved.

The embodiment of Figures 6 and 7 has a first partition wall 54 arranged in the upper gas transfer chamber 40 and two partition walls 56 shown in dotted lines in Figure 7 arranged within the lower annular transfer chamber 46. this arrangement of the partitions results in the gases coming out of the tubes 36 making a second pass travelling vertically downwards through the water space 14 through tubes 421. On reaching the lower annular chamber, the gases are now diverted into tubes 422 and make a third pass travelling vertically upwards through the water space 14. Finally, the gases now enter the tubes 423 and make a final pass travelling downwards through the water space 14 before exiting through the flue-gas outlet 48. Clearly, by other arrangements of partitions a greater number of desired passes may be achieved.

Figure 8 shows a modification of the water supply to the lower inlets of the thermic syphons.

Here the water entering the water space 14 does so through an annular channel 60 surrounding the lower inlets of the thermic syphons 24. In this embodiment, the natural convection occurring within the thermic syphons 24 is assisted by the inlet water pressure. A further alternative for achieving the same principle is shown in Figure 9 where the inlet water circulates in an annular tube 62 having jets directed into the inlets of the thermic syphons 24 as indicated by the arrows in Figure 9.

Figure 10 shows a modification of the thermic syphons in which they are arranged as vertical tubes extending through the furnace open at their bottom and upper ends.

A further modification for improving the heat transfer directly through the walls of the furnace is shown in Figure 11 where the walls are shown to be corrugated to increase their surface area.

Whereas the previously described furnace constructions are specifically suited for use with fluidised coal beds, by substituting for the fluidised bed a conventional oil or gas burner 70 as shown in Figure 1 2 the boiler can readily be adapted for operation with other forms of fuel.

It will be appreciated that the modification of Figure 12 is also applicable to the other embodiments.

Although the embodiment of Figures 4,5 and 6 do not show the thermic syphons 24 they are provided having been omitted for simplicity.

Another embodiment is shown in Figure 1 3, in which the thermic syphons are still present each syphon having a lower portion designated at 60 which is at a slight upward angle to the horizontal, the remaining portion 62 of each thermic syphon being vertical or near vertical and extending to the top 64 of the vertical furnace 10.

It is important that the majority of the length of each thermic syphon within the fluid bed be it a significant angle to the horizontal so that if any steam is formed within the thermic syphon it rapidly rises through the thermic syphon into the space between the furnace 10 and the shell 1 2. If the whole of the thermic syphon is at a relatively small angle to the horizontal, steam formed within the syphon may be trapped and may act as an insulator which might result in overheating of the thermic syphons. In the vertical boiler shown in Figure 13, the combustion gas pipes leading away from the vertical furnace are designated at 66 and extend away from the side wall of the vertical furnace 10 near to its upper end.A plurality of pipes 66 are provided equally distributed around the vertical furnace, the pipes 66 communicating with an inner section 68 of an annular chamber 70 provided at the bottom of the boiler shell 1 2; the chamber 70 is divided into the inner section 68 and an outer section 72 by a partition wall 74. The smoke tubes 42 extend vertically upwards as a ring from the inner section 18 to an upper annular chamber 76 provided at the top of the boiler.

Further, smoke tubes 78 forming an outer ring extend downwardly from the annular chamber 76 to the outer section 72 from which the combustion gases are discharged through a chimney which cannot be seen in Figure 13. The entire boiler is supported on legs 80 shown in Figure 13. The top 64 of the furnace 10 is secured to the top of the boiler 12 by vertical rigid stays 82 providing mechanical strength.

It will be appreciated that the path of the combustion gases from the furnace 10 is through the pipes 66 to the section 68, from the section 68 to the chamber 76 through the smoke tubes 42 and from the chamber 76 to the outer section 72 through the tubes 78. Thus, the combustion gases 3 pass through the water within the boiler 1 0, the normal level of the water being indicated at 84.

The maximum level of the fluidised bed within the furnace 10 is indicated at 86 and it will be seen that thermic syphons extend above and below this level which is true of all the other vertical furnaces which have been illustrated.

Thus, the thermic syphons serve not only to cool and extract energy from the fluidised bed itself but also serve the same function in the combustion gases immediately above the fluidised bed.

An alternative foracheiving the principle of Figure 8 is shown in Figure 14, and although not limited to, is particularly applicable to the boiler arrangement shown in Figure 1 3. Figure 14 shows a number of curved pipe sections 91 which are bolted together by virtue of each having flanges 92 to forms a complete closed circular pipe assembly which is located by supports (not shown) adjacentto the top of the furnace section 10. One of the curved pipe sections 91 contains a vertical entry stub 94 which is connected to return water connection 90 by a pipe section 95.

Each curved pipe section 91 has a number of small section discharge pipes 93 attached which extend vertically downwards and have at their lower extremity a closed end 97 and a row of discharge holes 98 pointing in the direction of the entry to the syphon tubes 24.

Water entering the boiler at connection 90 is therefore distributed toward the inlet of all the syphon tubes 24 thus assisting the natural circulation of the syphon tubes.

A particular advantage of the arrangement as shown in Figure 14 is that cold water entering the boiler at connection 90 is contained within a long length of pipework before mixing with water within the boiler and is thus substantially preheated before mixing with water within the boiler thereby avoiding impingement of cold water upon hot boiler plates.

The described constructions of vertical boilers offer several advantages some of which are now listed.

It may first be seen that the boilers rely on simpie design and construction so that they lend themselves to economical manufacture.

The vertical furnace and vertical tubes from the furnace provide a significant benefit in maximising the "fall-back distance" for grit and solids from the combustion process giving an automatic recycling effect leading to more complete combustion and less carryover of unburnt particles into other sections of the boiler. This process will also tend to provide a significant dust cloud aiways turbuient in the vertical smoke tubes which will aid heat transfer in the area.

The concentric arrangement of cooler outer smoke tubes together with the effect of the thermic syphons located typically as shown will promote a uniform and positive and natural water circulation within the boiler with its attendant advantages of low thermal stressing and good quality steam.

Because of this, the basic design of the boiler is eminently suitable for fully flooded hot water duty and may readily be adapted to such duty by the modifications to the water inlets earlier described.

Both the steam and hot water designs can be fired using conventional oil or gas burners as shown providing a truly multi-fuel unit.

Finally, the simplicity of design promotes ease of inspection and simplifies repairs when necessary.

Claims (14)

1. A boiler comprising a vertical furnace having an air supply at its lower end, tubes for combustion gases extending from the furnace at or near its upper end, a shell surrounding the furnace to define with the furnace a space surrounding the furnace for containing boiler fluid to be heated by the combustion in the furnace, a plurality of open ended tubes which extend into the combustion space of the furnace, which are, over at least most of their lengths, at angles of at least 450 to the horizontal, and which open into the said space to act as convection driven syphons, and a plurality of tubes within the said space surrounding the furnace, the latter tubes communicating with the combustion gas tubes of the furnace so that the combustion gases make two or more passes through the fluid to be heated before being discharged from the boiler.

2. A boiler as claimed in claim 1 , wherein both ends of each of said plurality of open ended tubes is in communication with an opening in the said wall of said furnace.

3. A boiler as claimed in claim 1, wherein a lower end and an upper end of each of said plurality of open ended tubes communicate respectively with an opening in the side wall of the furnace and an opening in the top of the furnace.

4. A boiler as claimed in claim 1, in which the plurality of open ended tubes are straight tubes passing vertically through the furnace.

5. A boiler as claimed in any of claims 1 to 4 wherein the furnace is designed forfluidised bed combustion and wherein the plurality of open ended tubes extend above and below the normal level of the bed and are over at east most of their lengths within the bed, at an angle to the horizontal.

6. A boiler as claimed in any of claims 1 to 5, in which the tubes for combustion gases communicate with a chamber above the furnace into which open the plurality of tubes arranged in the said space, the latter tubes being in communication at their lower end with an annular chamber surrounding the furnace.

7. A boiler as claimed in claim 6, in which neither the chamber at the upper end of the boiler nor the annular chamber at the lower end of the boiler is partitioned, the combustion gases being discharged from the lower chamber.

8. A boiler as claimed in claim 6, in which the chamber at the upper end of the boiler is divided into two sections by a partition, one section communicating with the tubes extending from the vertical furnace, the combustion gases being exhausted from the other section, the lower annular chamber being undivided whereby the exhaust gases make three passes through the water to be heated.

9. A boiler as claimed in claim 6, in which the chamber at the upper end of the boiler and the annular chamber at the lower end of the boiler are both partitioned and communicate with the tubes in the said space surrounding the furnace in such a manner that the exhaust gases make at least four passes through the water to be heated.

10. A boiler as claimed in any of claims 1 to 5, which has annular chambers at its upper and lower ends, between which chambers the said plurality of tubes in the space extend, the tubes for combustion gases extending from the furnace being in communication with the lower annular chamber.

11. A boiler as claimed in claim 1 0, in which the lower annular chamber is divided into two sections by a partition and the combustion gases are exhausted from one section of the lower annular chamber.

12. A boiler as claimed in any of claims 1 to 11, in which the fluid to be heated is introduced into a channel surrounding the lower ends of the open ended tubes.

13. A boiler as claimed in any of claims 1 to 11, in which the fluid to be heated is introduced into the boiler through an annular tube surrounding the lower ends of the pluralityof open ended tubes and having jets directed at the said lower ends of these tubes.

14. A boiler as claimed in any of claims 1 to 13, in which the side walls of the furnace are corrugated.

1 5. A boiler comprising a vertical furnace, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.