US2228757A

US2228757A - Boiler

Info

Publication number: US2228757A
Application number: US176147A
Authority: US
Inventors: Drummond Andrew
Original assignee: Lehigh Valley Coal Sales Co
Current assignee: Lehigh Valley Coal Sales Co
Priority date: 1937-11-23
Filing date: 1937-11-23
Publication date: 1941-01-14
Anticipated expiration: 1958-01-14

Description

Jan. 14, 1941. A. DRUMMOND BOILER Filed NOV. 23, 1937 4 Sheets-Sheet l INVENT'OR.

:: ATTOIRNEY'I Jan. 14, 1941. A. DRUMMOND 2,228,757

BOILER Filed Nov. 2:5, 1937 2 4 Sheets-Sheet 2 INVEETOR ATTORNEY Jan. 14, 1941. A. DRUMMOND 2,228,757

' BOILER Filed Nov. 25, 19s? '4 Sheets-Sheet s INV NTOR ATTORN EY Patented Jan. 14, 1941 BOILER Andrew Drummond, St. George, Staten Island,

N. Y., assignor to Lehigh Valley Coal Sales Company, New York, N. Y., a corporation of New Jersey Application November as, 1937, Serial No. 176,147

8 Claims.

This invention relates to improvements in boiler construction and particularly to boilers of a type suitable for domestic or home heating with steam or hot water.

An object of the invention has been to devise a unit capable of operation at high eiiiciency with a minimum of attention and effort.

A further object has been to provide improved and simplified control or regulating means for a boiler of the character specified.

In carrying out the foregoing objects, the improved boiler is provided with a fuel storage or hopper system capable of retaining a supply of coal or the like sufficient for a considerable period of operation without unduly increasing the overall dimensions of the unit. A feature of this phase of the construction is the provision of relatively free communication between the fuel storage system and the combustion zone of the furnace and the provision of adequate means for the removal of gases resulting from the distillation and combustion of the fuel. In this connection it has been found that a combustion chamber and a hopper, each having a conical or pyramidal portion with the two portions arranged in hour-glass fashion and having the upper walls of the combustion chamber disposed at about the angle of repose of the fuel, are admirably suited for the purposes of the invention. This allows for the removal of the generated gases through ample discharge passages leading from the sloping crown of the combustion chamber and without in any way interfering with the supply of fresh fuel.

As a special feature of the unit, contributing to its high efi'iciency, there is provided a secondary combustion chamber, surrounding the throat of the hour-glass formation, into which the gases pass from the main combustion zone. Here additional air may be supplied with the result that the partially burned gases carried away from the fuel are more or less completely burned. The hot gases are then preferably led through a series of economizer tubes to reduce their temperature and utilize the continued heat thereof before they are discharged to the flue.

The water to be heated by the combustion of the fuel completely surrounds the main and secondary combustion chambers, the lower portion of the hopper and the fire tubes of the economizer section. This water is forced to circulate through a passage formed between the crown of the main combustion chamber and a parallel wall of the secondary combustion chamber by virtue of the convection currents created upon the danger of warping or burning out the grates,

the heating of the water. Heat is transferred to the water not only through the walls'of the main and secondary combustion chambers but also through the tubes or passages extending between these chambers and through the fire tubes 5 of the economizer section.

A special feature of the control system is that a single damper is adapted to regulate the rate of combustion. The arrangement is such that when maximum combustion is desired, all of the air flowing as the result of the chimney draft passes through the fuel bed whereas when a lesser rate of combustion is desired, a part of the chimney draft is diverted or by-passed so that only a desired portion of the total air drawn by the chimney passes through the fuel bed. As a further feature of this control system, the air that is by-passed and, therefore, not drawn through the fuel bed is, nevertheless, broughtinto contact with the grate bars and is, in fact, drawn across the bottom thereof so as to impart a coolingeffect to the grates. This is an important advantage inasmuch as it more or less completely obviates the formation of clinkers and minimizes which frequently happens when the heat is bottled 'up in the ash-pit as the fire is banked in a furnace of usual construction.

In the operation of the improved control system, furthermore, the draft of the chimney is never choked off. It is always permitted to have its full effect and the damper merely regulates the amount of air drawn through the fuel bed by increasing or decreasing the resistance to the flow of air past the damper from beneath the grate bars. An advantage'of this arrangement is that the combustion chamber is always left in open communication with the chimney and hence the danger of building up a back pressure in the combustion chamber, causing the leakage of coal gases from the furnace is completely avoided.

Other objects and advantages of the inventon will appear from the detailed description of an illustrative embodiment of the same which will now be given in conjunction with the accompanying drawings, in which:

Fig. 1 is a perspective view of the improved boiler as seen from the front and one side, an

outer shell or casing surrounding the unit being 50,

omitted but indicated in broken outline.

Fig. 2 is a central, longitudinal, vertical section through the boiler.

Fig. 3 is a transverse, vertical section through. the boiler along the line 33 of Fig. 2.

Fig. 4 is a transverse, vertical section taken along the line 44 of Fig. 2, the upper or magazine portion of the boiler being omitted.

Fig. 5 is a horizontal section taken along the line 55 of Fig. 2, and

Fig. 6 is a perspective view of one of the ash drawers forming part of the improved boiler.

Referring now to the drawings, the improved boiler may be said to consist of three principal portions, namely .a base l0 forming the ash-pit of the unit, a main boiler portion l l, and a magazine portion [2. The base is a box-like member and may be formed either of cast iron or of sheet iron or steel, suitably bent and welded or otherwise fastened to provide a sturdy supporting structure. The magazine portion I2 is adapted to retain an adequate supply of solid fuel, such as coal, to enable operation of the boiler continuously over a period of at least twenty-four hours and in milder weather, or at times when the furnace is largely banked, over considerably longer periods.

Turning first to the main boiler portion H, this includes a combustion chamber l3, in which the fuel is ignited and permitted to burn at an appropriate rate, and a fuel supply member or hopper M in open communication with the combustion chamber and arranged to deliver fuel to the latter through a restricted throat or passage l5. The combustion chamber and the hopper may be of any suitable :form, in horizontal cross-section. In the unit illustrated, these parts are square in horizontal cross-section but it will be clear that they might be of rectangular or other polygonal form or circular or oval or of any other suitable shape. The lower portion of the combustion chamber is preferably enclosed by a vertically disposed Wall or series of connected walls l6 while the upper portion of this chamber is formed by an inclined wall or series of connected walls IT. The latter are preferably inclined upwardly and inwardly at an angle of about 45, which is substantially at the angle of repose of the fuel intended to be consumed in the combustion chamber. While the improved boiler is adapted to utilize a large variety of different forms and sizes of solid fuel, it is particularly suited for the consumption of anthracite coal of the size generally designated Chestnut. By sloping-the top wall or crown ll of the combustion chamber, at substantially the angle of repose of the fuel, it is insured that the fuel will entirely fill said combustion chamber and spread out more or less uniformly over the entire area of the lower portion of the combustion chamber so that there will be substantially no air or gas space above the fuel within the combustion chamber.

The fresh fuel is constantly supplied to the combustion chamber from the hopper and associated magazine through the restricted opening [5. For this purpose the bottom wall l8 of the hopper is extended outwardly and upwardly from the throat I5 at an angle of about 45 and then preferably merges into a vertically extending wall or series of connected walls [9.

Surrounding the combustion chamber and hopper of the boiler is a water and steam-tight jacket comprised by a front wall 20, side walls 2| and 22, a bottom 23, a top 24, and a rear wall having a portion 26 set in from the back of the boiler to form a flue-gas header .27. In the operation of the unit, the space within the jacket just described is maintained filled with water to about the level indicated by the broken line 28 in Fig. 2. It will thus be seen that the water substantially completely surrounds the combustion chamber and the tubes in the economizer section and also surrounds a portion of the hopper. That portion of the space within the jacket above the water level constitutes the steam space when the boiler is employed in connection with a steam heating system. If desired, the boiler may be employed in a hot water heating system in which case the entire space within the jacket will, of course, be filled with water.

Surrounding the top portion of the combustion chamber and the lower portion of the hopper or, in other words, surrounding the restricted portion of the hour-glass construction formed by the combustion chamber and hopper, is a secondary combustion chamber 29. This chamber is formed by a top wall or series of connected Walls 33, parallel with but spaced slightly from the bottom wall l8 of the hopper, a wall or series of walls 3|, parallel with but spaced slightly from the top wall I! of the combustion chamber, and an outer wall or series of connected walls 32, vertically disposed in substantially the same planes as the walls l6 and [9 of the combustion chamber and hopper, respectively. In this way there is completely enclosed a secondary combustion zone 33 surrounding the restricted portion of the hour-glass construction.

A series of short tubes 34, passing through the wall or walls l1 and the wall or walls 3|, and having their ends secured to these walls, provides a communication between the top of the main combustion chamber l3 and the secondary combustion zone 33. A sufficient number of tubes 34 of suitable diameter should be provided to insure the ready removal from the main combustion zone of the gases resulting from combustion as well as gases distilled from the fuel. These lat: ter gases, and in fact any burna-ble constituents in the gases developed in the main combustion zone, are subjected to further combustion in the secondary combustion zone, into which the appropriate amount of air to support the further combustion is introduced through suitable openings, to be described hereinafter. The more or less fully burned gases are discharged from the secondary combustion zone through a series of tubes 35 of appropriate number and diameter to enable free passage of the gases into the header 21. From the latter the gases are discharged through a duct or flue 35 into a chimney, not shown.

In the operation of the boiler, heat developed by the combustion of the fuel will be transferred to the Water within the jacket, previously described, through the walls of the main combustion chamber and through the walls of the sets of

tubes

34 and 35. Moreover, it will be apparent that a circulation of the water will be established by convection through the passage formed between the secondary combustion chamber and the walls I! and [8. In this way the heat will be transferred more or less uniformly to all portions of the body of water. Steam generated during the operation of the unit may be withdrawn through a pipe 31 to the radiators, or other steam-consuming devices, and the condensate may be returned to the bottom of the boiler through a return pipe 38. Fresh water may be introduced into the system whenever necessary through a supply pipe 39 provided at the front end of the boiler, as shown in Fig. 1. A water gauge 40 of any conventional form may be provided to indicate the level of the water in the boiler and a steam gauge 4| of conventional form valves and the like, may be utilized.

Any suitable form of grate structure may be employed at the bottom of the main combustion chamber. For example, a series of grate bars 42, of any conventionalform, may be disposed from front to rear of the combustion chamber and these may be suitably mounted for a rocking motion in any convenient way. A connecting-bar 43, associated with a shaking member 44 attached to the base portion of the furnace, may be employed in a manner well known, to transmit a rocking motion to the grate bars when it is desired to shake down the ashes. A clinker door 45 may be provided at the front of the furnace to give access to the upper surface of the grate bars for cleaning purposes.

Immediately below the grate bars the front and side walls of the base I of the boiler are conveniently provided with a series of slots or openings 46 through which air may freely enter the ash-pit portion of the boiler adjacent to the under surface of the grate bars. This, as will be explained more fully hereinafter, provides for an adequate supply of air to support the combustion of the fuel in the main combustion chamber and to cool the grate bars when the fire is banked.

To facilitate the removal of ashes from the boiler a pair of ash drawers 4'! is preferably employed beneath the grate, thefronts of the drawers acting as ash-pit doors. These drawers, which may be of any suitable construction, such as the form best illustrated in Fig. 6, preferably have a sloping rear wall to enable a ready discharge of ashes into an ash-can or other recep tacle at the appropriate time. Each drawer is provided with a handle in the form of a bail 48, preferably pivotally attached to the drawer at points adjacent to but slightly above the center of gravity of the drawer. These bails or handles assist in carrying the drawers and in discharging their contents. If desired, the length of the bails may be such that the cross members will remain in a position out of the path of the ashes falling into the drawers as the grate is shaken. Inasmuch as a space is necessarily provided between the two drawers, a deflector 49 is preferably employed over this space and extending the full length of the grate bars. This deflector will serve to direct the ashes falling upon its upper face into one or the other of the drawers. The

deflector may be secured at its forward end to the front wall of the base H! and may be supported at its rear end by a bracket 50 depending from the bottom wall 23 of the boiler.. Any suitable guiding, spacing and supporting means may be provided for the drawers to facilitate their insertion and withdrawal.

For the dual purpose of improving the appearance of the boiler and of providing a convenient means for insulating the exposed walls of the boiler against the loss of heat by radiation, an outer shell of sheet metal, or the like, is provided at the front, sides. top and back of the unit. The location of this shell is indicated in broken lines in Fig. 1 and the shell is indicated in cross-section in other views. As best shown in Figs. 2 and 3, the walls of the shell 5| are preferably substantially in vertical alignment with the walls of the base portion H). An air space 52 is thus provided between the outer shell and the water and steam-retaining jacket previouslydescribed. This air space in itself provides some heat insulationbut this insulation may be made more effective, if desired, by the use of a lining of sheet asbestos or the like (not shown) along the inner face of the sheet 5|. -It will be apparent that any other suitable form of insulation may be employed in lieu of that described, if desired. 1

The form of the fuel magazine 12 is clearly shown in the drawings. Its particular form and arrangement may be varied but it has been found that the particular form illustratedis advantageous for the reason that it permits the storage of a relatively large quantity of fuel without unduly increasing the size of the unit. Moreover, the arrangement shown is such that the overall height of the unit may be maintained within reasonable limits and the magazine is conveniently located for charging. As shown in Fig. 2, the bottom wall 53 of the magazine is preferably inclined at a suitable angle so as to insure the automatic feeding of the coal from the magazine into the hopper. The front of the magazine is preferably inclined from the Vertical at a convenient angle so that when a charging door 54 is opened, the coal may be readilythrown into the magazine by means of a scuttle or shovel and distributed evenly over the space within the magazine. Moreover, with the arrangement shown, the door 54 will remain in either its closed or its open position byits own weight due to the location of the center of gravity with respect to the hinge axis.

At each side of the unit there is provided a door 55 which whenopened affords access to the interior of the secondary combustion chamber. A passage 55 extends from each of the door seats to the wall 32 of the secondary combustion chamber. In each of the doors 55 a rotary damper 51 is preferably provided for the admission of a desired amount of air into the secondary combustion chamber to insure substantially com plete combustion of the gases within this chamber.

As hereinbefore indicated, an extremely simple control or regulating means is provided. This is inthe form of a single damper 53 pivotally mounted on; trunnions 59 at the bottom of the flue-gas header 21. An arm 59a, formed as an extension of one of the trunnions of the damper, affords a convenient means for adjusting the damper, as desired. It will be clear that when the damper is closed, as indicated in Fig. 2, substantially the entire draft of the chimney will be effective to draw the air, admitted to the ash-pit through the openings 46, through the fuel in the main combustion chamber, and to draw the resulting gases through the tubes 34, into and through the secondary combustion chamber 29, at which point additional air may be supplied through the dampers 51, then through the tubes into the header 2'! and finally into the chimney. On the other hand, if the damper 58 is rctated into its wide-open or vertical position, the draft of the chimney will be substantially completely utilized in drawing the air directly along the line of least resistance,that is through the openings 46 directly across the under surfaces of the grate bars, past the damper 58 into the flue 36. A small portion ofthe draft will at this time be effective in, header 2'! to remove from the main combustion chamber through tubes 35 any gases generated by the slow burning of the fuel but there will be no appreciable movement of air through the fuel bed due to the fact. that.

the slight draft aforementioned will be largely, if not entirely, balanced by the suction effect downwardly through the grate produced by the movement of the air across the bottom surfaces of the grate bars. It will be clear that by adjusting the damper 58 to intermediate positions, the effect of the chimney draft in drawing air through the fuel bed may be varied or regulated to suit the particular requirements of the furnace at any given time.

Any appropriate means may be employed for regulating damper 58. The regulation may be partly or fully automatic or entirely manual. Automatic control may be effected in various Ways, as by means of a steam-pressure-controlled diaphragm or by aquastatic or thermostatic means, or other conventional control devices may be employed. If the draft of the chimney is greater than required for maximum load conditions, an adjustable stop may be provided to pre vent the complete closing of the damper.

Heretofore it has been the practice to utilize both a turn damper and a check damper in the flue connections between a boiler and a chimney and it has been customary to control the operation of the boiler largely by the adjustment of these dampers and of a damper in the ash-pit door. This has not only entailed a complicated system of connections for operating the various dampers, but has resulted in unsatisfactory control of the boiler for various reasons. In the first place, the closing of the turn damper in the flue connection tends to choke off the further discharge of gases from the furnace into the chimney and if an attempt is made to suddenly check the fire in this way after it has been burning briskly, there is danger that a slight pressure will be built up within the boiler gas pass-ages, which may cause the escape of coal gases into the surrounding room. Furthermore, the ashpit damper in a control system of this nature tends to more or less completely shut off the flow of air through or around the grate so that a great deal of heat is bottled up when the furnace is checked after a period of vigorous operation. This bottling of the heat is particularly objectionable when the fire has just been shaken down and a large quantity of red-hot cinders or coal is present in the ash-pit as well as in the combustion chamber. The high temperature created in the vicinity of the grate bars at such a time will tend to clinker the ash and will also tend to warp and burn out the grates.

The improved control system, on the other hand, obviates all of these difficulties. In the first place, the combustion chamber is never closed off from direct communication with the chimney so that no back pressure can possibly be created within the combustion chamber. In the second place, when the fire is checked by the opening of the damper 58, cool air is drawn across the bottom surfaces of the grate bars, thus preventing clinker formation and injury to the grate bars. It will be apparent that this relatively cool air passing to the chimney will also check the draft to substantially the same extent as air admitted through a check damper in the side of the fiue pipe of a conventional installation.

In view of the possibility that a small amount of fine ash may be carried along with the air that is drawn across the under surfaces of the grate bars when the damper 58 is opened, provision ismade for cleaning out the back portion of the ash-pit from time to time. For this purpose a door 60 is provided either at one side of the ash-pit or at the back. It will be found that substantially all of the fine ash carried by the air under these conditions will be dropped out at the rear of the ash-pit as the air is forced to sharply change its direction in passing into the header 2! and fiue pipe 36.

In line with the tubes 35, a clean-out door 6| may be provided in the rear of the boiler so that access may be had to the tubes for cleaning the latter from time to time.

The operation of the boiler will be apparent from the foregoing detailed description and no rsum is believed to be necessary at this point. Suffice it to say that the only attention which the improved boiler requires, particularly if it is equipped with automatic regulating means for the control of the damper 58, is the occasional charging of the magazine l2 with a fresh supply of coal, an occasional shaking of the grates, and the occasional removal and emptying of the ashdrawers 4'1. During colder weather the maga- 'zine may be supplied with coal every day or every second day. During warmer weather, when the consumption of the furnace is relatively low, the magazine need be filled only once or twice a Week. The grates, as in usual practice, should be shaken at least once a day, preferably just prior to the checking of the fire for night operation. The ash-drawers may be removed daily, or at less frequent periods, depending upon the rate of consumption of the fuel. On the average it should not be necessary to remove the ash-drawers more than about twice a Week.

The improved boiler lends itself readily to construction in units of various sizes. A typical unit of the form illustrated may comprise, by way of example, a main combustion chamber having a grate surface of nineteen inches by nineteen inches. A series of twenty-eight tubes 3d of 2 /8" outside diameter will be found adequate for conveying the gases from the main combustion chamber into the secondary combustion chamber. A series of ighteen tubes 35 of the same diameter as the tubes 34 will be found adequate to convey the flue gases to the header 2'1. A unit of the type mentioned may have an overall height of about forty-six inches, a width of about twentyseven inches, and a depth of about forty-six inches. Boilers of larger capacity may readily be formed with a larger grate area, a larger number of

tubes

34 and 35 or larger diameter tubes and somewhat larger overall dimensions without unduly increasing the overall height of the unit.

While an illustrative form of the invention has been described in considerable detail, it will be understood that numerous changes may be made in the construction and arrangement of the various portions of the boiler without departing from the general principles and scope of the invention. Moreover, while the invention has been disclosed in relation to a boiler for the generation of steam or hot water, certain of the features are applicable also to a hot air furnace or heating system or the like. The terms and expressions employed herein should be understood to be terms of description and not of limitation.

What I claim is:

1. In a boiler of the class described an outer wall structure, inner walls within said outer wall structure forming a combustion chamber and superposed fuel storage hopper, certain of said walls having reentrant portions at substantially the same elevation intermediate the top and bottom thereof to provide a restricted throat between said chamber and hopper, and additional wall means between said outer wall structure and said inner walls within the space provided by said reentrant portions, said wall means enclosing secondary combustion space, and conduit means connecting said secondary combustion space with said chamber for the passage of products of combustion, said outer wall structure being adapted to retain water around at least portions of said combustion chamber, wall means and conduit means.

2. In a boiler of the class described an outer wall structure, inner walls within said outer wall structure forming a combustion chamber and superposed fuel storage hopper, certain of said walls having reentrant portions substantially midway between the tops and bottoms thereof to provide a restricted throat between said chamber and hopper, and additional wall means between said outer wall structure and said inner walls within the space provided by said reentrant portions, said wall means enclosing secondary combustion space, and conduit means connecting said secondary combustion space with said chamber for the passage of products of combustion, said outer wall structure being adapted to retain water around at least portions of said combustion chamber, wall means and conduit means.

3. In a boiler of the class described an outer wall structure, inner walls within said outer wall structure forming a combustion chamber and superposed fuel storage hopper, certain of said walls having reentrant portions at substantially the same elevation intermediate the top and bottom thereof to provide a restricted throat between said chamber and hopper, said reentrant portions providing also a top for said combustion chamber inclined at substantially the angle of repose of the fuel, and additional wall means between said outer wall structure and said inner walls within the space provided by said reentrant portions, said wall means enclosing secondary combustion space, and conduit means connecting said secondary combustion space with said chamber for the passage of products of combustion, said outer wall structure being adapted to retain water around at least portions of said combustion chamber, wall means and conduit means.

4. In a boiler of the class described an outer wall structure, inner walls within said outer wall structure forming a combustion chamber and superposed fuel storage hopper, certain of said walls having reentrant portions at substantially the same elevation intermediate the top and bottom thereof to provide a restricted throat between said chamber and hopper, said reentrant portions providing also a top for said combustion chamber inclined at substantially the angle of repose of the fuel, and additional wall means between said outer wall structure and said inner walls within the space provided by said reentrant portions, said wall means enclosing secondary combustion space and being in part parallel with the contour of said reentrant portions and spaced only a short distance therefrom, and conduit means connecting said secondary combustion space with said chamber for the passage of products of combustion, said outer wall structure being adapted to retain water around at least portions of said combustion chamber, wall means and conduit means.

5. In a boiler of the class described an outer wall structure, side and end walls within said outer wall structure forming a combustion chamber and superposed fuel storage hopper, said side and end walls having reentrant-portions at substantially the same elevation intermediate the top and bottom thereof to provide an hour-glass effeet with a restricted throat between said chamber and hopper, and additional wall means between said outer wall structure and said inner walls within the space provided by said reentrant portions, said wall means enclosing secondary combustion space, and conduit means connecting said secondary combustion space with said chamber for the passage of products of combustion,

said outer wall structure being adapted to retain water around at least portions of said combustion chamber, wall means and conduit means.

6. In a boiler of the class described an outer wall structure, inner walls within said outer wall structure forming a combustion chamber and superposed fuel storage hopper, certain of said wallshaving reentrant portions at substantially the same elevation intermediate the top and bottom thereof to provide a short neck with a restricted throat of an area less than one-quarter the cross-sectional area of said chamber between said chamber and hopper, and additional wall means between said outer wall structure and said inner walls within the space provided by said reentrant portions, said wall means enclosing secondary combustion space, and conduit means connecting said secondary combustion space with said chamber for the passage of products of combustion, said outer wall structure being adapted to retain water around at least portions of said combustion chamber, wall means and conduit means.

7. In a boiler of the class described an outer wall structure, inner walls within said outer wall structure forming a combustion chamber and superposed fuel storage hopper, said walls being constructed and arranged to provide an inwardly and downwardly slopping bottom for said hopper and an inwardly and upwardly sloping top for said combustion chamber, said hopper and chamber communicating through a short neck having a restricted throat, additional wall means between said outer wall structure and said inner walls forming enclosed secondary combustion space a portion of the enclosing walls of which are spaced only a short distance from portions of said neck, and means forming short gas conveying passages between said combustion chamber and said enclosed space, said outer wall structure being adapted to retain water around at least portions of said combustion chamber, wall means, and means forming gas conveying passages, the construction being such as to induce circulation of the water between said chamber and wall means.

8. In a boiler of the class described an outer wallstructure, inner wall means within said wall structure forming a combustion chamber and superposed fuel storage hopper, said wall means having a reentrant portion intermediate the top and bottom thereof to provide an hourglass effect with a restricted throat between said chamber and hopper, and additional wall means between said outer wall structure and said, inner wall means within the space provided by said reentrant portion, said additional wall means enclosing secondary combustion space, and conduit means connecting said secondary combustion space with said chamber for the passage of products of combustion, said outer wall structure being adapted to retain water around at least portions of said combustion chamber, additional wall means and conduit means.

ANDREW DRUMMOND.