US2481164A - Xdraft m magazine heater fob - Google Patents

Description

R. A. SHERMAN ET Al. 2,481,164

CROSSDRAFT MAGAZINE HEATER FOR BURNING SOLID FUEL Filed June 23, 1 943 4 she'tS-sneet 1 NVENTORS Pap A Sherman.

ATTNEYS pt, 6, 1949. R. A. SHERMAN ET AL 2,48Ul64 CROSS-DRAFT MAGAZINE HEATER FOR BURNING SOLID FUEL Filed June 25, 1943 4 Sheets-Sheet 2 INVENTORS 5em- 6, w49 R. A. SHERMAN ET Al. 2,48l154 CROSS-DRAFT MAGAZINE HEATER FOR BURNING SOLID FUEL vFiled June 23, 1943 4 Sheets-Sheet 3 Fig. 4

Fig. 5

INVENTORS Palph Aharmon. Barfmnd ALG dr) BYGMWIM v ATTORNEYS Sept. 6, 1949. R, A SHERMAN ET AL 2,481,164

CROSS-DRAFT MAGAZINE HEATER FOR BURNING SOLID FUEL Filed June 23, 1943 4 4 Sheets-Sheet 4 RATE OF BURNING 2056 LB. PER HR.

com. Fmen. 50o us. 6 Aven/we sTAcK Tenn 576 0&0. s

Avenne: co2. :05 Pen cfm AVERAGE SMOKE CONCENTRATION, 0.3 Fl F. FIRED n Ponen 4 RATE 0F BURNING. 2.79 La. PfR HR.

coAL man, 20.0 Le, 5 News srAcK Tenn 679 0E@4 E 5 AVERAGE co: a0 Per: CENT AVERAGE suon: rscsummou. 2.

7 regnen INVENTORS Ralph A4 Shrma n,

` Barrrand ALQndr TTORNEYS i Patented Sept. 6, 1949 FFICE CROSS-DRAFT HEATER FOB BURNING SOLID FUEL italph A. Sherman and Bertrand il. Landry,

"Columbus, Ohio Application .lune 23, iiiliii, Serial No. @1,958

(iCi. E2G- 73) It liliaima. l

@ur invention relates to an apparatus for burning solid fuel. More specifically it relates to heaters of the type in which coal, wood, or other solid fuel is tired by hand for residential or other low-duty heating.

Extensive experimental research into burning fuels smokelessly has preceded our discovery of the principles used in our invention. We have been concerned particularly with the burning of so-called smoky coals. which may be dened, roughly, as those having a volatile matter content, on a moisture-and-ash-free basis, of 23 per cent or greater.

As is well known in the art, the combustion of solid fuel in yfuel beds oi any type is a two-stage process. The iirst stage is the liberation of the volatile matter from the fuel, leaving behind a residue of coke or charcoal. The second stage is the combustionof the coke or charcoal. As the volatile matter is made up of gaseous and liquid hydrocarbons, the decomposition of these hydrocarbons or their incomplete combustion will result in the liberation of free carbon which is the smoke that is emitted to atmosphere by way of the chimney that leads from the fuel-burning equipment.

The general requirements that must be met to avoid the formation of smoke are also well known in the art. They are: (a) the supply of air at a rate required by the rate ofliberation of the volatile matter, (b) the mixture of the air and the volatile matter, and (c) the provision of a temperature within the fuel-burning equipment high enough to ignite the mixture of air and volatile matter.

Although these requirements are well known, they are not met in equipment heretofore and now available for hand-tiring of coal or wood. The usual or conventional type of heater burns fuel on the principle of overfeed burning in which the fresh charge of fuel is introduced and iiredon top of a residual bed of incandescent fuel on the grate of the heater. The contact of the fresh fuel with the hot bed, and the passage of the gases upwards through the bed, eect rapid heating of the fuel and result in a rapid rate of evolution of the volatile matter. Most of the oxygen in the air owing upwards through the grate is consumed or used up by the incandescent bed and not enough is available at the top of the bed to burn the volatile matter. Even if air is admitted above the fire bed, it is not well mixed with the volatile matter and the temperature is often below the point (1100 to 1200 degrecs Fahrenheit) required to ignite the gases.

d Consequently. the volatile gases pass off unburned, resulting in a large loss of potential heat from the fuel and in the emission to atmosphere of dense smoke.

At high rates of burning the required temperature is reached more quickly than at low rates, but residential heating equipment operates for a large part of the year at quite low burning rates which accentuate the loss of heat and the emission of dense smoke.

The underfeed principle of burning is used with a fair degree of success in some mechanical fuelburning equipment, such as underfeed stokers, for the efiicient combustion of coal without dense smoke. This principle has also been attempted in non-mechanical equipment. In one attempt to obtain desired results, the air is admitted above the fuel and passes downward in the same direction as the movement of the fuel toward the grate. This method presumably fulfills the requirements of supply of air and its mixture with the volatile matter and of the high temperature required since the air, passing through the fresh charge of coal, meets the volatile matter and mixes with it before passing through the incandescent bed of coke. v

If the rate of air supply is such that the rate of ignition of the raw coal is just equal to the rate of burning of the coal, which would, obviously, result in a fuel bed of constant or equilibrium thickness, the rate of evolution of the volatile matter from the raw coal would be such as to be equivalent to the rate of air supplied and smokeless combustion would result. We have found, however, by experimental research, that the rate of burning required to maintain a bed of equilibrium is of the order of 30 pounds-of coal per square foot of grate per hour, whereas in residential or domestic heaters rates of burning of the order of approximately 10 pounds per square foot of grate per hour or less, are required.

When the rate of air supply is reduced to give the rates of burning required in residentialv heaters, the rate of ignition of the raw coal is greater than the rate of burning and the active fuel bed increases in thickness until the entire charge of raw coal is devolatilized. Thus, the rate of evolution of volatile matter is greater than can be burned by the air supplied and smoky operation results. Also, we have found that with coals that cake and coke to any degree, true underfeed burning does not occur. The coal cokes and burns more rapidly around the perimeter of the bed than in the center and a monolithic mass of coke is formed. The now of air around the peaceras@ riphery is accentuated and thus inefficient and smoky combustion results.

A third principle of burning solid fuel, known as the cross-feed principle, is used in some fuelburning equipment such, for example, as in the traveling-grate stoker. The fuel is carried into the furnace in a relatively thin layer on a traveling grate and is ignited by radiation from the flame and from refractory ignition arches above the bed. vThe air flows upward through the grate across the bed, at substantially right angles to the direction of motion of the fuel. By the proper arrangement of the ignition arches and by the admission of secondary air above the bed at high velocity to obtain turbulent mixing of the air and volatile gases, essentially smokeless burning is obtained. Because of the mechanical principles involved, this principle or method of burning has not. however, been successfully applied to residential heaters.

It is, therefore, one of the important objects of our invention to apply the principle of crossfeed burning of solid fuel in a relatively simple non-mechanical type of heater so as to obtain essentially somkeless and eicient burning of a wide variety of types of solid fuels.

Another object of our invention is to apply the cross-feed principle to a residential heater that requires refueling and attention only at infrequent intervals.

A further object of our invention is to provide a. non-mechanical heater wherein, in connection with the burning of solid fuel, high thermal efflciencies are obtained by complete combustion of the volatile gases.

A still further object of our invention is to provide a non-mechanical heater which effects the essentially smokeless combustion of normally smoky coal and requires only infrequent attention.

The foregoing and other objects and advantages of the present invention will be apparent from a` consideration of the following description and appended claims when considered in connection with the accompanying drawings forming a part of this specification and wherein like reference numerals designate corresponding parts in the several views.

We have discovered that by preventing the flow of air through the bed of raw fuel, we avoid rates of liberation of volatile matter from the raw fuel which are in excess of those that can be burned by the air supplied as primary and secondary air. We have provided means for venting that volatile matter which rises to the top of the fuel chamber into the primary air stream in a manner which causes it to pass en tirely through the bed of incandescent fuel where it is efficiently burned. We have further provided means for the admission of secondary air into the fuel bed near the point of exit of gases from the bed whereby a hot zone is maintained so that ignition and burning of all volatile gases is assured. By reason of providing such means, our improved heater fullls all the known requirements of smokeless combustion of solid fuels.

As shown, by way of illustration and not of limitation, our invention is embodied in an improved heater which embodies a vertically disposed fuel chamber, the upper part of which com prises the fuel magazine, and the lower part of which, although not separated from the upper part by any particular or well defined boundary, comprises the combustion space or chamber. A

suitable grate, designed to permit removal of the ashes, forms the lower boundary of the combustion space. An ash pit is provided below the grate for the accumulation of ashes. This pit, as shown, is fitted with a door for the removal of ashes from the heater, but it is to be noted that this door has no port or opening for the admission of air and hence no air is admitted to the fuel bed through the grate. Likewise, the door through which the fuel is fed to the fuel magazine is solid and tightly fitted and thus no air is admitted therethrough and through the raw fuel.

lThe main supply of air is, as shown in the drawings, admitted to the lower part of the bed at a point above the grate andy passes across the bed substantially at right angles to the direction of the downward movement of the fuel to the outlet duct for the combustion chamber. Such outlet extends vertically, is located at one side of the combustion chamber, and has its lower end in communication with the combustion chamber. The fuel bed is of practically constant thickness a-s contrasted to an overfeed bed whose thickness changes constantly and continuously as the fuel burns. The provision of a fuel bed of constant thickness provides for uniform burning and consequently greater and uniform efficiency.

Located above, and in the immediate vicinity of, the inlet end of the outlet duct for the combustion chamber is a slot-like port which serves to introduce secondary air and is so arranged that such air flows through a portion of the fuel bed where a part or portion of the oxygen is consumed by combustion. This causes the temperature to be so increased that the proper ignition of the combustible gases passing from the outlet duct is assured. A vent is provided at the top of the fuel chamber and this leads to the inlet for the primary air. Any volatile gases that accumulate above the fuel are thus led into the primary air stream and are completely burned as they pass through the incandescent fuel bed.

One embodiment of our invention is illustrated in the accompanying drawings wherein, as heretofore pointed out, similar reference characters designate corresponding parts in the several views, and wherein:

Figure 1 is a perspective view, partly broken away, looking toward a front corner of an improved heater embodying the principles of our invention.

Figure 2 is a perspective view, partly broken away, looking toward a rear corner of the heater of Figure 1.

Figure 3 is a vertical section view through the heater embodying our invention.

Figure 4 is a horizontal section taken substantially along the line 4 4 of Figure 3, looking in the direction of the arrows.

Figure 5 is a horizontal sectional view taken substantially along the line 5-5 of Figure 3, looking in the direction of the arrows.

Figure 6 is a view taken from a recording chart showing the density of the smoke emitted by the heater when' burning high-volatile, smoky coal; and

Figure 7 is a view taken from another recording chart showing the density of the smoke emitted by a conventional or prior art heater when burning the same type of high-volatile, smoky coal as recorded in Figure 6.

With reference to the drawings, we have shown one form of our invention as embodied in a heater which includes an outer casing shown as a whole at I0. Such casing may be cast or formed from sheet metal. Furthermore it may comprise any suitable number of parts or sections and be of any desired shape. In general the casing comprises a pair of side walls,a front wall, a back wall, a top wall and a bottom wall.

As shown, the casing I comprises an ash pit or receiving portion II. The latter is located directly above the bottom Wall of the casing and separated by a grate I2 from the fuel magazine or chamber I3 and the subjacent combustion chamber I3a. Such chambers are separated from the vertical outlet duct I4 by a hollow wall structure composed, as shown, of two substantially vertical, laterally spaced, metal walls I5 and I6. These walls are located adjacent, but spaced rearwards from, the front wall of the casing and may be joined at the bottom to form an inverted horizontal arch I'I which is located a substantial distance above the grate I2 in order that the lower end of the outlet duct I4 is in communication with the combustion chamber I3a. The front and sides of the fuel and combustion chambers, and also the vertical walls I5 and I5, the lower part of the arch I'I, and certain faces of the vertical outlet duct I4 are preferably lined with suitable refractory material.

Located in, and forming a part of, the back wall of the heater casing is a rearwardly extending or offset box-like portion or extension I8 having a fixed top part I8a provided with an inlet opening I9- for the admission of primary air to the combustion chamber I3a. A hinged or otherwise adjustable valve type cover 20 is provided over the opening I9 for controlling the amount of primary air that is admitted through said opening. The adjustment of the cover 20 may be effected by a chain 2I, either manually or by connection to a suitable thermostatic control (not shown).

The vertical metal walls I5 and I6 have the ends I5a and Ilia thereof in abutment with the opposed side walls of the heater casing IIJ and provide a transverse passageway 23 which is open to the atmosphere at both of its ends by way of openings 22 in said side walls of the casing.

As clearly seen in Figs. .1 and 2, a downwardly and rearwardly inclined wall 24 is provided and this, together with the bottom wall 25a of a similarly inclined fuel chute 25, provides a closure for the top of the transverse passageway 23.

The upper portion of the wall I6 is notched out or cut away to receive and support the inner end portion of the inclined fuel chute 25, mentioned above. This chute extends rearwards from an opening in the inclined upper portion Illa of the front wall of the casing and is covered by a closely fitting hinged or otherwise movably mounted door 26. The chute 25 projects through the wall I6 and refractory lining of the fuel chamber I3 as clearly seen in Figs. 2 and 3.

The purpose of the open-ended passageway 23A is to admit outside fresh secondary air to the fuel chamber I3a. For this purpose the base portion of the inverted arch I1 is provided with a longitudinal slot 2l which is controlled by a curved or arcuate pivotally mounted valve 28. The valve has formed therein an off-center longitudinal slot 28a which may be brought into ,or out of registry with the longitudinal slot by manipulating a crank or handle 29, see Figs. 1 and 5. Thus, the passageway 23 and the combustion chamber I3a may be brought into communication with one another.

The vertical outlet duct I4 communicates at its` upper end with an upper horizontally disposed heat transfer space 30 by way of passageways 3| which are located on opposite sides of the fuel chute 25. The space 30 communicates with a rearwardly projecting hollow extension 32 upon which is mounted an upstanding smoke pipe collar 33. Mounted upon the collar 33 is a stack 34 and this acts as an outlet for the products of combustion and is provided with a rotatable butterfly damper 35 for controlling the draft in the heater. The stack 34 is connected to a chimney, not shown.

The grate I2 may be of any suitable design for efficient removal of the ash with ay minimum of unburned fuel. We have discovered, however, that a series of rocking finger type grate bars, as shown, is desirable. These grate bars may be rocked individually or may be linked together for conjoint operation by means not shown. An ash pan 36 for the collection of the ash and other material is located normally in the ash-pit II beneath the grate and may lbe removed through a tightly fitting imperforate ash door or closure 3l. The heater is shown as being supported above the floor surface by means of legs 38.

The refractory lining 40 for the back wall of the casing I0 is provided with a series of vertically extending vent channels in the form of slots in the rear face of the refractory lining. These channels extend from the top of the fuel chamber I3 to the primary air plenum chamber formed by the extension I8 and together form a vent duct for conducting the volatiles and other gases from the top of the fuel chamber I 3 to a point adjacent the primary air inlet opening I9.

The burning actions that take place in the regular operation of this heater with a bituminous coal will now be described. Assuming that the heater has been in operation, fresh fuel is supplied on top of the residual fuel bed so as to ll the fuel chamber I3. Combustion of the fuel bed in the lower portion is supported by primary air entering through the inlet opening I9, the entrance of which air inv induced by natural draft. The rate of burning is controlled by adjustment of the cover door 2U and by control of the draft by means of adjustment ofthe damper 35 in the smoke pipe 34. Part of the heat released by the combustion of the fuel is conducted or radiated above to the fresh fuel in the upper portion of the fuel chamber I3, part is transmitted through the walls to the space around the casing II), and the remainder is carried by the gases into the outlet duct.

'Ihe elTect of the heat from the bed below on the fresh fue1 above is (a) to raise the temperature of the fuel, (b) to soften and melt the tarry constituents of the fuel, (c) to cause the pieces of fuel to run and cement themselves together, provided, of course, that the fuel is of a coking type, (d) to vaporize or distill volatiles from the fuel, and (e) to form gradually a continuous layer of coke at the approximate boundary between the burning bed and the new fuel charge and somewhere above the level of the primary air inlet opening I9. The sealing action of the coke layer causes the volatiles that are released above this layer to expand upward through the fresh fuel. Because of the draft adjacent the lower or outlet end of the outlet duct I4 and the fact that the inlet opening I9 is restricted, the pressure over the grate will be less than at the top of the fuel chamber I3 and hence the released volatiles and gases escape through the vent chanthe inlet opening I9.

nels 38. The latter, as hereinafter described conduct the volatiles and gases downwardly to the stream of primary air entering the fuel bed via These volatiles and gases merge with the entering stream of primary air and pass through the incandescent bed to the outlet duct I4. The part of the volatiles which is released below the coke layer will pass directly downwardly through the incandescent bed. Heat conducted from the burning bed below the aforementioned layer of coke continues upwardly through such layer resulting in further coking of additional layers of coal in the fuel or magazine chamber. Simultaneously, volatiles continue to be released, to be vented above the bed as explained, to flow downwardly into the bed with the incoming air, and also to ow directly downwardly through the bed.

' The passage of the vented mixture of volatiles and air along a substantially horizontal pathv through the fuel bed effectively aids complete ignition and burning of this mixture without smoke emission. Our improved heater, in this respect, provides for all known means of burning the volatiles. It provides for (a) the mixing with primary air, (b) the greatest possible length of travel in the hot fuel bed or, in other words, the greatest possible time is consumed to complete the burning reaction, (c) the maintenance of high temperatures in the gases as they converge to the outlet duct, and (d) a refractory lining adjacent the outlet duct to maintain the temperature of the gases above the ignition point of the combustible mixture.

In addition. there is provided by the slot 21, means for supplying supplementary or secondary air by natural draft. The provision of the slot 21 adjacent to, but above, the bottom of the arch Il insures that the secondary air will pass through a portion of the bed where a part of the air will act as primary air, thus maintaining a zone of high temperature to ignite and burn the mixture of volatile gases. .The remaining air will act as truly secondary air. Thus, even at low drafts and low rates of burning, a zone of high temperature is maintained in the heater adjacent the inlet end of the outlet duct, and ignition of the volatile combustible gases is assured even after firing a heavy charge of coal. Furthermore, by providing the slot 21 above or adjacent the /inlet end of the outlet duct, `a depth of devolat'ilized coal is maintained as shown in Fig. 3. When the bed below is burned and the bed above is broken by poking, undevolatllized coal cannot be brought down to the inlet end of the outlet duct.

In operation of the heater with non-coking coal, the fuel above the grate-is gradually consumed and fresh fuel automatically feeds down from above. With coking coals, owing to the suspensory properties of the coke arch which is formed and which prevents self-feeding of` the fuel above, a growing cavity is formed in the fuel bed extending under the arch toward the outlet. Eventually, in such cases, a reasonable draft will no longer maintain the required rate of burning and attention is required, such, for example, as (a) shaking of the grate bars to dispose of the accumulation of ash content, (b) opening of the firing door and insertion of a poker which is used to break the arch from above so as to let the fuel drop by gravity into the cavity which existed below, (c) simultaneously using the poker to break into smaller pieces the large coke pieces resulting from the initial fracture of the arch, in order to make the fuel bed desirably compact, (d) introducing the next fuel charge by refueling the magazine at the top and (e) closing the firing door tightly.

Our experience with the heater of the present invention is that, because of the momentary high rate of release of volatile matter on poking, a sharp spurt of smoke may pass out of the outlet duct but that, within a few minutes, the rate of devolatilization is so decreased that smoke emission ceases almost as abruptly, and smokeless operation continues until it is time to repeat the attention described above. Thus, depending upon the rate of burning and on the fuel capacity of the heater, periods from eight to more than twenty-two hours of smokeless operation have been observed, while the duration of the smoke spurt mentioned was only from two to fifteen minutes total. One such typical smoke performance produced the record illustrated in Figure 6.

In Figure 7 there is illustrated for purposes of comparison, a typical smoke record for a conventional overfeed heater. The illustration is given as evidence of the marked improvement obtained with our new and improved heater as described above. Coal from the same source was used in both tests and the rates of burning were similar. On the smoke charts shown, the outside circle represents no smoke, the innermost circle is opaque black smoke, and the intermediate scale of concentric circles represents the standard Ringelmann numbers. The comparative records illustrated were obtained by the use of a photoelectric viewer, based on the opacity effect of smoke on a constant light source.

In a small heater of the type embodied in the present invention which we have constructed and tested, the overall height of the magazine (chambers i3 and I3a), from grate to top, was 19 inches, the width from side to side inside the refractory lining was 10 inches, and the depth from front to back of the magazine was 8 inches. The top of the primary air inlet was 7 inches above the grate and the bottom of the arch was 6 inches above the grate. The dimensions of the outlet duct were 2 x 10 inches inside the refractory.

With an applied draft of 0.06 inch of water column at the stack, the following rates of burning were obtained: With Pittsburgh bed coal, a strongly-coking, high-volatile coal, 2.8 lb. per hr.; with Ohio No. 6 bed coal, a weakly-coking, high-volatile coal, 3.3 lb. per hr.; with Pocahontas No. 3 bed coal, a strongly-Coking, lowvolatile coal, 2.6 lb. per hr. When burning hard wood, with an applied draft of 0.03 in. water column, the rate of burning was found to be 9.4 lb. per hour.

It will be understood that the heater of our invention can be and has been constructed both in smaller and larger sizes. It will be understood further that the ratios of the dimensions given, although found desirable, may be varied without departing from the scope and spirit of our present invention.

While we have discussed above our heater primarily from the viewpoint of its use with bituminous coals, it will be apparent that theheaters may be used to burn all types of solid fuels, such as bituminous coal, anthracite coal, coke, charcoal, lignite, and wood.

It will be apparent from the above that we have provided a heater of the crossfeed type having many advantages. Many of the advantages have been mentioned and others will be apparent.

The principles of our crossfeed heater may be applied to a variety of useful heating devices o! 9 which the space heater shown is but one example. It is to be expressly understood that the invention is not to be restricted to the details set forth since these may be modif-led within the scope of the appended claims without departing from the spirit and scope of the invention.

Having thus described our invention, what we claim is:

1. A heater for the combustion of solid fuel, including a vertically disposed casing having therein a chamber consisting of an upper fuel space and a lower combustion space;` a substantially horizontal perforated grate constituting the bottom of said combustion space; said casing also including an ash-receiving chamber normally closed against admission of outside air to it; a transversely disposed vertical wall spaced above said grate to provide an outlet passage for the gases emanating from the combustion space; said casing having an outwardly offset portion at that side of the chamber which is opposite said wall to provide an air space free of fuel, said olset portion having a damper-controlled air inlet located above the grate and adapted to direct a primary air stream so that it iiows horizontally over and across the grate in the direction of said outlet passage, a duct within said casing extending from the upper portion of said fuel space to said air space for conducting volatile matter released from the fuel in the top of the fuel space to said air space to join with the primary air' stream, said vertical wall being hollow and in communication with the atmosphere and having a latitudinally continuous rearwardly directed slot extending the entire width of the Vertical wall for passing secondary air into the combustionv space, and a means for varying at will the area of said slot.

2. A heater adapted to burn solid fuel without appreciable smoke and comprising a box-like casing formed of a first pair of opposed vertical walls, a second pair of opposed vertical walls between and at right angles to the rst pair, a top wall, and a horizontal bottom wall, a horizontal perforate grate disposed in the casing directly above the bottom wall and dening with said bottom wall and the lower portions of the vertical walls a normally closed and air-tight ashpit, a vertical hollow wall structure positioned in the casing in closely spaced and substantially parallel relation with one of the vertical walls of the first pair, extending between and connected to the second pair of vertical walls, defining between it and said one vertical wall of the rst pair an upwardly extending outlet duct for the products of combustion and between it and the other vertical Wall of the rst pair a vertical chamber consisting of an upper fuel space and a lower combustion space directly over the grate, and having the lower portion thereof terminating above the grate in order that the combustion space is in communication with the lower end of the outlet duct, means in the upper portion of the casing for sealing the upper end of the fuel space from the upper end of the outlet duct, normally sealed means for introducing successive charges of solid fuel into said fuel space, horizontally elongated primary air supply means disposed wholly above the grate, associated with, and extending completely and transversely across, said other vertical wall of the iirst pair, connected to receive outside air, and having a single, horizontal, full length opening arranged so that it causes primary air to flow substantially horizontally over the grate, through the fuel in the lower portion of the combustion space and tosi) wards the lower end of the outlet duct, and means within the casing forming a vertical duct leading downwards from the top of the fuel space to a charge slot facing inwards and downwards and serving to discharge the secondary air so that it ows part way through said fuel in the lower portion of the combustion space.

3. A smokeless heater adapted to burn solid fuel and comprising a box-like casing formed of a rst pair of opposed vertical walls, a second pair of opposed vertical walls between and at right angles to the first pair, a horizontal top wall, and a horizontal' bottom wall, a horizontal pcrforate grate disposed in the casing directly above the bottom wall and defining with said bottom wall and the lower portions of the vertical walls a normally closed and air-tight ash-pit, a vertical hollow wall structure positioned in the casing in closely spaced and substantially parallel relation with one of the vertical Walls of the first pair, extending between and connected to the second pair of vertical walls, defining between it and said one vertical wall of the first pair an upwardly extending outlet duct for the products of combustion and between it and the other vertical wall of the first pair a vertical chamber consisting of an upper fuel space and a. lower combustion space directly over the grate, and having the lower portion thereof terminating above the grate in order that the combustion space is in communication with the lower end of the outlet duct, a substantially horizontal imperforate plate extending across and serving permanently to seal the upper end of the fuel space, spaced beneath the top wall of the casing, and defining with said top wall a horin zontal outlet duct extension communicating with, and leading from, thel upper end of the outlet duct and extending across said upper end of the fuel space, normally sealed means adapted to introduce successive charges of solid fuel into said fuel space and embodying an imperforate door, horizontally elongated primary air supply means disposed above the grate, associated with, and extending transversely across, said other vertical wall of the first pair, connected to receive outside air, and having a single, horizontal, full length opening arranged so that it causes primary air to `iiow substantially horizontally over the grate,

through the fuel in the lower portion of the cornbustion space and towards the lower end of the outlet duct, and means within the casing and adjacent said other vertical wall of the first pair forming a substantially vertical duct with the upper end thereof in communication with the upper end of the fuel space and its lower end terminating immediately adjacent said primary air supply means, the hollow wall structure being connected to receive outside air as secondary air and having in the lower portion thereof a single, horizontal, full length discharge slot facing inwards and downwards and serving to discharge the secondary air so that it flows part way through said fuel in the lower portion of the combustion space.

4. A smokeless heater adapted to burn solid fuel and comprising a box-like casing formed of a first pair of opposed vertical walls, a second pair of 1l opposed vertical walls between and at right angles to the first pair, a horizontal top wall, and a horizontal bottom wall, a horizontal perforate grate disposed in the casing directly above the bottom wall and defining with said bottom wall an ash-.pit normally closed against admission of outside air to it, a vertical hollow wall structure positioned in the casing in closely spaced and substantially parallel relation with one of the vertical walls of the rst pair, extending between and connected to the second pair of verg tical walls, defining between it and said one vertical wall of the' first pair an upwardly extending outlet duct for the products of combustion and between it and the other vertical wall of the first pair a vertical chamber consisting of an upper fuel space and a lower combustion space directly over the grate, and having the lower portion thereof spaced above the grate in order that the combustion space is in communication with .the lower end of the outlet duct, a substantially horizontal imperforate plate extending across' and servingto seal the fuel space, spaced beneath the top wall of the casing, and defining with said top wall a horizontal outlet duct extension communicatlng with, and leading from, the upper end' of the outlet duct and extending across said upper end of the fuel space, normally sealed means adapted to introduce successive charges of solid fuel into said fuel space and embodying an imperforate door, elongated hollow means connected to, and in'ofl'set relation with, the other vertical wall of the first pair, disposed wholly above the grate, defining an airspace free of fuel and for outside primary air, and having a single, horizontal, full length opening arranged so that it causes the primary air to flow substantially horizontally over the grate, through the fuel in the lower portion of the combustion space and towards the lower end of the outlet duct, and

means within the casing and adjacent said other vertical wail of the nrst pair forming a substantially vertical duct with the upper end thereof in communication with the upper end of the fuel space and'its lower end terminating adjacent said hollow means, the hollow wall structure being connected to receive outside air as secondary air and having in the lower portion thereof a single. horizontal. vfull length discharge slot facing inwards and downwards and serving to discharge the secondary air so that it flows part way through said fuel in the lower portion of the combustion space.

RALPH A. SHERMAN.

BERTRAND A. LANDRY.

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

UNITED STATES PATENTS Number Namo Date Re. 3,424 Woodson May 4, 1869 220,529 Greene Oct. 14, 1879 223,041 Greene Dec. 30, 1879 529,072 I-Iinstin Nov. 13, 1894 552,911 Knauss Jan. 14, 1896 554,499 Schnigge Feb. 11, 1896 567,398 Lanner Sept. 8, 1896 "659,161 Washborn Oct. 2, 1900 690,431 Hugentobler I Jan. 7, 1902 1,501,847 Howell July 15, 1924 1,544,582 Howell July 7, 1925 1,568,866 Barault .L Jan. 5, 1926 2,403,829 Sanford July 9,1946

FQREIGNSPATENTS Number Country Date 5,856 Germany June 27, 1879 52,313 Denmark Sept. 30, 1936 131,699 Germany June 26, 1902 479,039 Germany July 9, 1929

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