US1986561A - High pressure boiler - Google Patents

Description

Jan; 1, 1935. H. N. DAVIS HIGH PRESSURE BOILER Filed Feb. 4, 1933 4 Sheets-Sheet l 1 l l I I llllllllllllllllll INVENTQR -B Y Z 'tTORNE'YS 4 Sheets-Sheet 3 H. N. DAVIS HIGH PRESSURE BOILER Filed Feb. 4, 1955' Jim. 1, 1935.

[NV NTOR 72 WW A TTORNE Ys n 1935. H. N. DAVIS 1,986,561

HIGH PRESSURE BOILER Filed Feb. 4, 1953 4 Sheets-Sheet 4 INVENTOR Mf m??? Patented Jan. '1, 1935 PATENT OFFICE 1 v HIGH PRESSURE BOILER Harvey N. Davis, Hoboken, N. J., assignor to 1 Claim.

This-invention relates to high pressure boilers and, while various features of the arrangement and method of operating the same are particularly adapted form in a locomotive boiler, it

5 is to be understood at the outset that the invention or at least many features thereof are.

- equally applicable to stationary or marine boiler installations.

Before proceeding with a discussion of some of the more important objects and advantages, it is first to be noted that the boiler construction of this invention is especially adapted .to the burning of fuel in suspension. The fuels which I have in mind are oil or any similar liquid fuel, gases, and finely comminuted solid fuels such as pulverized coal, any one of which is normally adapted to be ignited and burned while suspended in the combustion air.

Generally considered, the primary objects of this invention involve a method and apparatus for burning fuel in a boiler or what might be termed a boiler-furnace in such manner as to attain a high degree of efliciency with respect to steam produced from a given quantity: of fuel consumed and the attainment of such emciency by the use of apparatus which is of relatively simple construction, which is of relatively long life, which lends itself readily to inspection and repair, and which is particularly adapted for use in a locomotive.

With the foregoing in mind, this invention contemplates the use of a boiler structure of substantially elongated form, all the parts of which are well adapted to be arranged compactly with! commonly employed in locomotive boilers. It is As an additional object, I contemplate a method and means of accurate control for com- Howard L. Ingersoll, Stamford, Conn.'- Application February 4, 1933, Serial No. 655,204

bustion, so that the boiler may be operated with very high efliciency at any desired firing rate. As will readily be appreciated this is of advantage in a boiler installation of any type, although it is peculiarly advantageous in a locomotive installation, for the reason that the drain on the steam supply in a locomotive must necessarily be intermittent and irregular in view of the character of operation to which the boiler is subjected. Fine and accurate control of the rate of combustion, therefore, is of vital importance to the overall or net efficiency in the operation of a.

locomotive.

The present invention also contemplates a novel method of igniting and burning fuel in flues of the boiler, with the result that efliciency is improved, since the fuel and flame stream is in contact with evaporating surfaces at those times during which its temperature is highest.

In attaining some of the foregoing objects and advantages I employ one or more very substantially elongated primary combustion flues extending lengthwise throughout the length of the boiler proper. In the preferred embodiment a plurality of such flues are employed and-each one of them is provided with individual or separate means for feeding fuel thereto, for feeding air thereto, and for controlling the fuel and air feed. In this way, when operating at verylow rates, only one of the primary combustion flues aneed be employed, since its complete operation is entirely independent from that of the others. Moreover the fuel and air feeding means and controlling means for combustion in each one of the primary combustion flues is so designed and arranged that the rate of combustion may be varied efficiently and accurately over a wide range of firing rates. Thus, the boiler may efilciently be operated at ratings between the minimum rating of one primary flue and the combined maximum rating of the several flues provided.

The method of burning the fuel, as will be more apparent as this description proceeds, provides for admission of the fuel (comminuted, liquid or gaseous) to each primary combustion flue at a point-adjacent one end thereof so that it may pass throughout thelength of the flue and be discharged at the opposite endthereof into ,what I have termed a secondary combustion chamber. The secondary chamberis disposed at one end of the boiler proper and is arranged to reverse the direction of flow of the fuel and flame stream after which such stream passes, in reverse flow, through other boiler flues to be discharged into a smoke box located at the end of the boiler adjacent to the point of original fuel admission. After this reverse flow, the products of combustion are preferably again diverted to flow through passages surrounding or adjacent to certain auxiliaries. A discharge stack is provided beyond the said auxiliaries. It will be apparent, therefore, that in a boiler of considerably elongated shape, such as a locomotive boiler, the path of travel of the-fuel and flame stream, from the point of admission to the stack, is very long indeed, and this greatly extended path of travel has been provided, among other things, in order to permit a. rate or speed of travel of the fuel and flame stream greatly in excess of that heretofore contemplated and still ensure proper and complete combustion of all the fuel admitted. To compare the fuel and flame stream flow which I contemplate with the rate of flow more commonly adopted, it is noted that in accordance with prior practice, the pressure of the air admitted very seldom exceeds that equivalent to about five or six inches of water, while in accordance with this invention, air under a pressure equivalent to twenty-eight inches of water or even more may readily and efliciently be employed. Bearing. this in mind, it will be apparent that the rate of gas flow is very much higher than rates heretofore employed and this increased rate is highly desirable for a number of reasons. In the first place, I have discovered by tests that it makes possible the efficient burning or combustion of much more fuel per cubic foot of combustion space than was ever attained with arrangements formerly used. This, of course, is desirable in any boiler installation but it is especially useful in a locomotive in view of the fact that the total volume of the combustion space which may be provided is relatively restricted. Additionally, I have found by experiment that the use of the high rate of flow contemplated further permits operating at relatively high temperatures without the accumulation of v excessive slag, soot or the like in the primary as relatively high temperature by the time it enters the advance end of the primary combustion flue which, of course, is disposed within the boiler proper. Efllcient heat transfer, therefore, will occur immediately upon entry of the fuel and flame stream into the primary combustion flues.

Further with respect to the method of burning,

the fuel, it should be observed that when the fuel is admitted to the ignition chamber I prefer to admit therewith much less than sufllcient air to complete combustion of the total fuel admitted. For'example, the air initially introduced may be in the neighborhood of from one-quarter to onehalf the total air necessary for complete combustion. Thefuel, therefore, is ignited but only partly burned during its flow through the primary combustion flues.

In order to provide additional combustion space and air in which the remainder of the burning of the fuel may be accomplished, the secondary combustion chamber above referred to is arranged substantially as a hemispherical cap at the end of the boiler adjacent to the discharge ends ofthe primary combustion'fiues. An additional quantity of combustion air is admitted at this point and the rest of the combustion process, when the quantity of secondary air admitted is properly proportioned, takes place within this secondary chamber prior to entry of the products of combustion into the return flues.

According to this invention still further efiiciency is obtained by making provision for preheating the primary as well as the secondary combustion air admitted, such preheating eing effected by the use of novel means to b ribed more fully hereinafter.

A further advantageous feature of the invention is involved in making it possible, as a practical matter, to efliciently burn fuel in suspension,

in a locomotive boiler, thus securing more efficient utilization of the available heat energy and further eliminating grates, arches, etc., with their attendant difficulties of maintenance, removal of waste and circumscribed capacity.

.Another problem which should be considered in connection with the objects and advantages of this invention relates to the arrangement of the boiler, particularly of the fire-tube type (which, for numerous reasons, is best suited to locomotive practice) so that the very high steam pressures herein contemplated, may be employed. With this in mind, I have arranged the primary combustion flues, preferably of circular cross section and of relatively small diameter, and it is to be observed that the manner in which I contemplate burning the fuel, particularly the high rate of flow thereof, makes possible eflicient combustion in primary combustion flues of the relatively small diameters contemplated. All of the various boiler and flue parts, furthermore, with the exception of the main boiler or flue sheets or heads, are of circular or substantially circular crms section, this form being the best suited to withstand relatively-high steam pressures. The flue sheets, of course, are stayed by virtue of the extension of the fire-tubes and the primary combustion flues therebetween. If necessary, the sheets may additionally be stayed by parallelstay rods extended through the boiler from end to'end. A further object is involved in the disposition of the primary and relatively small diameter combustion flues within the boiler proper so as to be completely surrounded by the water therein and thus provide a maximum of heat transfer surfaces. A further advantage of this arrangement results from the fact that the primary combustion flues materially aid and increase circulation.

Further with reference to the primary combustion flues, it is observed that the construction contemplated includes the use of a relatively thin refractory lining disposed preferably only in the first or admission portion thereof and terminating, for example, somewhere in the neighborhood of a mid point between the ends. This is of advantage in ensuring progressive ignition and active increase in combustion throughout a portion of the travel of the fuel and flame throu h the combustion tubes, and I have found that only a relatively thin refractory lining need be employed in order to prevent undue cooling of the fuel and flame at the time they enter the boiler proper. By terminating the lining partway through each tube, the effective diameter of the tube is increased (beyond the point of terminaof which is indicated by the reference numeral the boiler, the" difference in effective diameter being obtained by the use of the refractory lining only in one portion thereof. Furthermore, the type of combustion provided, with high speed gas flow, is such that a very thin'layer of soot or the like will accumulate on the portion of each primary flue which is notcovered by a refractory lining. This layer, I have found, will automatically maintain itself at a thickness such as to keep 1 its inner surface hot and experimentsshow that a certain measure of protection is afforded thereby, although the heat transfer through the layer of soot is considerably greater than through any refractory lining of suflicient thickness to stand up in service. I

Further with a of the boiler in general, I contemplate employing certain tubes arranged, in a closed circuit, to be subjected over a portion of the length thereof to a heating effect or heat transfer in order to utilize heat which could not otherwise conveniently be used. The tubes of this set have the major portion of their length "disposed within the water of the boiler and thus they provide an indirect transfer to the water in the boiler of a certain portion of heat which would otherwise be lost.

Another object of especial importance in a boilerarrangement of the character herein disclosed, in which extremely high velocity for the fuel and flame stream is contemplated, is involved in the streamlining of the various passages and flues through which the gases and products of combustion pass in order to reduce aerodynamic losses to a minimum. It will be understood, of course,

that this is of much greater importance in an arrangement of the character herein disclosed,

, since aerodynamic losses would be much greater with high velocities than with the relatively low velocities heretofore commonly employed;

How the foregoing together'with various other objects and advantages which will occurto those "skilled in the art are attained will be apparent from a consideration of the following description 1 making reference to the accompanying drawings,

in which a Figures 1, 2 and 3, placed end to end in the order indicated, illustrate, in vertical section, a boiler constructed in accordance with this invention and especially adapted for 'use as a locomotive boiler;

Figure 4 is a vertical sectional view taken sub-. stantially as, indicated by the section line 4-4 on Figure 1;

Figure 5 is a view similar to Figure 4 but taken as indicated by the line 55 onFigure 1;

Figure 6 is a view illustrating certain details of I air deflecting vanes which I contemplate employing; and Figure '7 is a sectional view showingthearrangement of certain tubes and refractories incorporated in the boiler.

While I haveillustrated various features of the view to increasing the efficiency.

'7. At the rear and head ends of the barrel flue sheets 8 and 9, respectively, are provided, and

the fire-tubes or flues 10 are extended betweenthe forward and rear flue sheets.

The primary combustion flues, three of which have been employed in the present installation, are indicated by the reference numeral 11 and are shown as being extended from a point rearwardly of the back sheet 8 all the way through the boiler.

proper to terminate in the forward sheet9. The portion of each flue 11. which extends beyond the rear sheet 8 is enlarged or coned outwardly as indicated at 1 2 and then again formed cylindrically, as indicated at 13, but with a diameter substantially greater than the portion of the tube disposed within the boiler itself. The fuel admission or delivery means is arranged todischarge into the enlarged portion of the tube 11 and this means includes, as herein illustrated, an oil or liquid fuel discharge tube 14 the end of which preferably terminates in an atomizing nozzle such as shown, for example, at 15, adapted to discharge the atomized fuel generally radially outwardly. While I, have herein illustrated a fuel feeding means adapted to deliver a liquid fuel to the primary combustion tubes, it will be understood, especially in viewof the above'remarks, that this means may be replaced by means adapted to deliver a gaseous fuel or a finely divided solid fuel such as pulverized coal. The tube 14 in the preferred arrangement delivers axially of the flue 11 and in order to provide axial air admission with such an arrangement, I have disposed a fan' 16 to rotate about the supply pipe 14. ,,This fan may conveniently be mounted to be driven by the hollow shaft 17 which is driven by the motor 18.

At the forward end of the boiler proper arrangeda somewhat hemispherical chamber defined by the casing walls 19 and 20 which may conveniently be secured or mounted at the forward edge of the cylindrical boiler drum 7. This chamber, the secondary combustion chamber above referred to, is arranged to receive the discharge from the primarycombustion flues 11 and is smoothly streamlined or rounded to divert the flow of the fuel and flame stream upwardly and thence rearwardly after which the products of combustion enter the return flues 10. In order to provide against short circuiting of any of the fuel and flame stream, I preferably employ bent .andspaced tubes 21 which are connected with the water space in the boiler, at their lower ends, at a point just above the forward end of the primary flues 11 and,at their upper ends, at a point adjacent to the top of the shell 7. Refractories 22 are arched across or supported on the lower portion of these tubes to providea baffle around the end of which all gases must pass .before turning backto flow rearwardly through the tubes 10.

In order to efficiently utilize the heat of the gases in their passage through the secondary combustion chamber, and further toutilize the heat of the gases at this point without resorting to water'legs or sheets, I have arranged aseries of U-shaped tubes 23, lying in substantially vertical planes, and have extended such tubes rearwardly through the boiler proper near-the top and bottom of the boiler shell. At their free ends, these tubes are provided with a commonsubstantially circular header 24 and in the preferred construction the tubes 23 and the header 24 constitute a closed circuit adapted to receive water, preferably distilled, which in circulating I have forwardly and upwardly from the lower portions of the tubes and thence rearwardly through the upper portionsv thereof provides for indirect transfer of heat from the surface of the secondary combustion chamber to the water in the boiler proper.-

The tubes 23 further serve to support and position at least certain ofthe refractory lining .Before considering the admission of air for ticularly). From this header, .superheater tubes bricks 25 inside the hemispherical shell 19 and 20. The cross sectional fragmentary showing of Figure 7 illustrates the arrangement of these refractories and the tubes 23, and from Figure 7 it will also be seen that the refractories are provided with corrugationsror irregularities on the inner surface thereof as indicated at 25a, the purpose of which will be referred to morefully hereinafter.

primary as well as secondary combustion, a ntion is called to the fact that the steam produced in the boiler rises to the top thereof and passes through the openings 26 into the elongated separator chamber 2'7 and thence through the smallperforations 28 and through the valve chamber 29-to superheaterheader 30 (see Figure 5 par- 31 are extended downwardly. and thence forwardly into the rear ends of a. portion ofthe flues l0 and the superheated steam is collected in the chambers 32 and finally conducted into the live steam pipe 33.

. .The products of combustion discharged atthe rear ends -of the tubes 10 pass over the more or less exposed portions of the tubes 31 and are drawn, preferably by means of one or more 7 fans 34 driven by motors 35 rearwardly to'be discharged radially from the fan buckets or blades and thence expelled upwardly through chambers 36 and 37 (and also laterally as noted hereinafter) the latter of which communicates with a forwardly extending passage 38, preferably ing 48 which communicates with a chamber formed between the casing walls 19 and 20 and an additional casing 44, the latter casing being extended, in spaced relation to the formen'rearwardly to a point adjacent the arcuate tube sheet disposed just forwardly of the stack 39. The air in the chamber or cavities 46 is then forced into the air tubes 47 and conducted rearwardly to be discharged beyond the rear tube sheet 48 which is of arcuate shape similar to the sheet 45 (see Figures 4 and '5 particularly).

It will now be seen thatin addition to'discharge of gases upwardly and forwardly from the exhaust fan 34, the products of combustion are further dispersed laterally and forwardly through V the substantially horse-shoe shaped p the ends of which are deiiried'by the arcuate air tubesheets 45 and 48. Thus, the air taken in at the forward end of the boiler is preheated not only in around the outersurfaces of the secondary combustion chamber but also in rearwardly throughthe tubes 47 which arev subjected to a counter flow of the products of combustion in the passage of the latter forwardly to be discharged through the stack 39.

1,ose,se1

The air thus preheated is discharged into the chamber 49 provided at the back of the boiler and is delivered therefrom to the fans 16 for discharge into the primary combustion flues 11.

At the forward end of the boiler a large number of air inlet ports 50 are provided in order to supply the air necessary for the secondary I Further with a view to preventing direct flow of air from the fan 41 to the ports 50, and thus in order to prevent undue chilling'of the flame in the secondary chamber, I have provided a baiiie member 52 positioned beyond the fan and extend ed downwardly so that air in passing to the ports 50 must necessarily take a more or less circuitous path over the hot exterior surface of the secondary combustion chamber.

Before considering the operation of the arrangement shown, some few additional structural features should be noted. In the first place I prefer to employ, for reasons which will appear more'fully hereinafter, a positive fuel ignition means. This means is preferably arranged within the enlarged ignition chamber 12 of the primary combustion flues and in this instance takes the form of a device 53 adapted to provide an electrical ignition spark. The device 53, furthermore, preferably disposed, for example, as shown in Figure 1, oifset slightly from the fuel nomle 15, so as not to be exposed. to thefuel discharged therefrom.

Asa precautionary measure the boiler may be provided with a safety valve or valves 7a.

A relatively thin refractory lining 54 is also preferably used internally of the primary combustion flues 11. Such a lining, as shown in Figures 1, 2 and 3, is extended in the enlarged ignition chamber 12 and forwardly through each flue 11 preferably to approximately the mid point thereof as shown, for example, at 55.

A feed water inlet 56 is desirably located as shown, in Figure 1, toward the bottom of the boiler shell 7 and adjacent to the lower and rear portions of the indirect heating tubes 23. With thisarrangement the relatively cool feed water will be subjected to the heating action of thetubes 23 immediately upon entrance into the boiler.

At the point where the preheated airin chamber 49 is delivered to the fans 16, I prefer to employ curved baiile members 57 dedgned to change the direction of flow of the air. into the fans and theme the primary lines, with a minimum of aerodynamic losses. Additionally, a set of vanes 58 (see Figures 1 and 6) is employed just beyond each fan 16 with respect to the direction of air flow, the vanesbeing designed to further reduce aerodynamic losses and to provide for discharge oftheairwithaminimumof residual spinor swirl. These vanes 58, furthermore, are of greater width toward the circumference of the mixing and ignition chamber than toward the center thereof so that their forward edges substantially define a cone and. in accordance with the preferred arrangement, the fuel discharge nozzle 15 is disposed Just forwardly or beyond the vanes 58 at the center thereof and is further arranged to discharge the fuel in a conicalstream of a configuration substantially conforming to that of the forward edges of the vanes.

While I have presented in the foregoing discussion as well as in the drawings, an installation particularly suitable for use in locomotive practice, it will be apparent that numerous of the novel features may just as readily be applied to or adopted in stationary practice, or on ships, especially where the total space available is materially restricted, since, as will be brought out still more fully hereinafter, according to this invention, a maximum of fuel may be burned with high efliciency in a minimum of total space. It will also be apparent that the steam delivered from the boiler may be utilized in any manner desired, for example, todirectly drive cylinders or any suitable prime mover or to indirectly produce driving power as by operating a turbo-generator. Suitable valves and the like for the steam supply and feed water, as well as manual controls for the rate of operation of the fan motors 14, 35 and 40, may be provided and, indeed, I contemplate rather accurate control of various of these devices in order to attain a degree of efficiency even at widely varying rates of firing which is not contemplated or possible with any prior arrangement even in stationary boiler practice and which has heretofore been even more remote in locomotive practice.

In considering the operation, it is assumed that the fuel being employed is one of the liquid type, for example, a heavy grade of fuel oil, although it is to be understood that as noted above any fuel adapted to burn in suspension is suitable for use in accordance with this invention.

The fuel, of course, is continuously introduced through the supply pipe 14 and the nozzle 15, and the motor 18 is continuously operated to drive the fan 16 and create a blast of air of very high velocity. I prefer to employ a motor and fan arrangement adapted to deliver air at very high pressures as compared to those heretofore employed, for example, between 0.5 and 2 pounds per square inch, or that equal to about 12 inches to 50 inches of water. This results in the high velocity flow which I desire, and upon discharge of the fuel adjacent to the forward edges of the vanes 58, the air, since it has very little swirl or spin at this point, more or less gradually deflects the fuel particles forwardly and in the preferred embodiment the force with which the fuel is discharged from the nozzle 15 and the air pressure are so relatively proportioned as to prevent direct impingement of the fuel particles on the inner surface of the refractory lining 54. While it is true that turbulence at this point would aid in mixing, I have found that adequate mixture may be obtained without it and further, that such turbulence, particularly spin, is likely to cause excessive and premature impingement of the fuel particles against the inner surface of the refractory lining. It is undesirable to have such impingement at this point although impingement of the fuel particles during their course oftravel through the fines 11 has no detrimental effect. Thus, at the time of ignition, I have provided against impingement or striking of the fuel against the refractory lining, and the ignition is therefore facilitated, not only since the action of the air, at the time of fuel delivery, aids in atomizing the fuel and thoroughly mixing it with the incoming air, but also since the fuel particles are retained in suspension in the primary air.

in the nature of coke, may accumulate on the" walls of the ignition-chamber and this accumu lation, especially at the higher rates of firing, will materially aid ignition and may even be suflicient by itself under some conditions to provide proper ignition, at the same'time, I prefer not to rely entirely on ignition by virtue of the coking but also provide means in the nature of a sparking device 53 for ensuring thorough fuel ignition.

The ignition chamber, it will be noted, gradually tapers to the reduced cross sectional area'of the fiues 11 and this gradual taper is highly desirable in accordance with this invention since the velocities contemplated are extremely high and must be provided for aerodynamically in order to reduce the friction losses to a minimum. Since the cross sectional areaof the flue 11 is in the neighborhood of one-half that of the ignition chamber, the velocity of the fuel and air upon entering the flue is very materially increased. The increase is still further augmented by virtue of the formation of gases resulting from combustion and in operation I contemplate very high velocities in the primary combustion flues, for example, as high as say from 3000 feet to 20,000 feet per minute. 'Before proceedingwith the action of the fuel in passing through the fines 11, it should be observed that it is of vital importance to this invention that the air originally admitted with the fuel constitute only a fraction, at maximum firing rate, say approximately half of the total air necessary to complete combustion. Thus, in effeet, I have provided an arrangement in which the fan means are adapted to deliver or drive as much air through the flues 11 as is efficiently possible when friction losses and the like are considered, and then I admit a quantity of fuel which may be as much as twice or even more than twice that which would provide an efficient'combustion mixture. In this way, I provide for rather unusually high temperatures in the primary combustion flues although, as will readily be understood, the air and fuel may travel a small distance into the fiues before the highest temperatures are attained.

In considering the travel of the fuel and air through the forward portion of the flues 11, that is, through the portion of the flues which are lined with the refractory 54, it is to be observed that I have found by experiment that only a relatively thin refractory lining need be employed in order sufficiently to protect the fuel and flame from the chilling action of the boiler. Another desirable factor which I have determined by experiment resides in the fact that while a certain amount of soot, coke-and the like may accumulate from time to time on the inner surface of the refractory, at the same time, under the operatingconditions which I contemplate, such accumulation never exceeds one which would impair proper and efficient operation, it being noted particularly that the high velocity of flow at least in part prevents excessive accumulatiomv This also applies to the latterportions of the flues 11, i. e., those portions which are not protected by refractories, but in these portions I have found that a certain accumulation of soot or the like is desirable in order to prevent the otherwise completely uncovered inner surface of the flues from unduly chilling the flame. In normal operation the accumulation in the latter half of the fines 11 is automatically maintained as a thin layer. The inner surface of this layer, of course, becomes quite hot and thus prevents undue chilling of the flame at this point. I therefore contemplate operation in such manner as to produce a thin layer of soot in order to afford protection for the surfaces, such thin layer, in effect, acting as a refractory but being of substantially greater efficiency from the standpoint of heat transfer than any layer of refractory which could be inserted as a practical matter and which would stand up under normal service conditions.

The termination of the refractory lining 54, for example, at point 55 is of advantage as it provides a slightly increased cross sectional area for the fuel and flame stream in the latter portion of its travelthrough the tube 11 and further because it provides for more direct heat transfer to the water in the boiler. By applying a refractory lining to the admission end of each tube 11.

I amenabled to provide for an increase in the cross sectional area of the tube, for example, just beyond the mid point, without resorting to the use of a flue of irregular section, this being a structural feature of considerable advantage. It should be borne in mind in this connection that V by the time the fuel 'and flame stream passes through the latter portion of each tube 11 its velocity is'even greatly in excess of that in the entrance end of the tube, this increase being the result of the gases liberated in combustion, and the high temperatures caused by the progressive combustion.

Upon discharge of the flame stream into the secondary combustion chamber at the forward end of the boiler I have found that substantially the only remaining combustiblespresent are hydrogen and carbon monoxide, and upon the mixture of the stream with a substantial quantity of air entering through the ports 50, the combustion of all the fuel is completed in the chamber as the gases bend or turn upwardly and then rearwardbaflle or arch formed by the refractories 22, as

noted above, prevents short circuiting of the fuel and flame and thus premature entrance into the return flues. The bent tubes 21 on which the refractories 22 are supported are also of substantial advantage since they are connected with the water space in the boiler at a point substantiallybelowthesurfaceofthe watersoastomaterlally augment the circulation'therein.

In the preferable'arrangement and as shown Figure3theU-shapedtubes23arecoveredat least partiall by the refractories 25 so as to prevent undue chilling of the combustible gases by contact with these tubes. -It is advantageous to form these refractories with corrugations, ribs or other projections (as suggested in Figure 'l) to increase the surface presented to the impinging flame, as, by this means, a certain degree of surface combustionis effected, the surface combustion being desirable because of its extreme rapidity. This factor together with the relatively large volume, especially of the upper portion of the secondary chamber, serves to ensure complete combustion before the gases are chilled on entering the return tubes. The arrangement of the refractories 25 and the tubes 23 also provides a substantial saving since the heat transferred to the water in the tubes 23 (directly from the fuel and flame stream as well as from contact with the refractories) is indirectly carried to the water in the boiler.

At the delivery ends of the return tubes 10 the heat remaining in the products of combustion is further utilized by contact with the superheater elements 31 as well as by contact with the separating chamber 27 which is disposed centrally above the boiler substantially between the rear tube sheet 8 and the stack 39.

While it may not be essential to employ a fan such as illustrated at 34, I prefer its use since it serves to further increase discharge 'of gases andthus the velocity of flow through the various tubes and flues in the boiler.

Turning now to the air preheating system, it is observed that still further economies are effected by virtue of the flow of a large quantity of the products of combustion at each side of the boiler around the air tubes 4'7. Thus, when the air is drawn in by the forward fan 41, it is passed over the secondary combustion chamber into the forward portion of the boiler proper and thenceinto the tubes 47 for still further heating, and a further desirable feature to be pointed out lies in the fact that the tubes 47 are arranged in two arcuate nests adjacent to the outer surface of the boiler shell 7. Heat losses from the boiler are, therefore, brought to a very low value.

In conclusion, reference is again made to the novel character of combustion and heat transfer herein contemplated and especially to the fact that-by the use of a plurality of relatively small diameter primary combustion flues, any one or all of which may be used at any time, very great efliciency may be, obtained over widely varying rates of firing. The independent control which I contemplate for the several fans, furthermore, and the manner in which the combustion is effected, are such as to permit the use of only one primary firing tube at widely varying rates. That is, even a single tube may efliciently be used to burn only a very small quantity of fuel, or such tube may be operated at its maximum rating. With the foregoing variable factors in mind it will clearly be seen that efllciency may be obtained over operating ranges never before contemplated.

It is also a feature of considerable advantage to arrange the primary air intake and its draft fan at the forward end of the boiler, since, in a locomotive, the entrance or admission of air will be facilitated by virtue of the disposition of the nlet or intake at this point.

Since the boiler proper and all the various tubes therein are all constructed of circular cross section, the operating pressures may be relatively high even in stationary and marine practice, and in locomotivepractice such pressures may be carried far beyond those which were practical with fire tube boilers in any prior construction. This, of course, is of considerable advantage since the higher pressures permit greater economies and efiiciency in net production of power from a given unit of fuel consumed.

What I claim is:-

A boiler-furnace including a cylindrical and substantially elongated boiler shell, a primary combustion tube or passage of relatively small diameter extended through the boiler, fuel and air delivery means arranged toward one end of the boiler to discharge fuel and air into said tube, a secondary combustion chamber at the other end of the boiler, means for supplying air for the primary combustion, the last mentioned means including an air intake adjacent said secondary chamber and air passages extended therefrom rearwardly toward the fuel and air delivery end of the boiler to communicate with the air delivery means, and means for directing the products of combustion from said secondary chamber adjacent to the air passages whereby to effect transfer of heat thereto, said last mentioned means including return fire-tubes or flues extended from said secondary chamber through the boiler to the fuel delivery end thereof and means for reversing the flow of gases discharged from the return tubes to provide for passage thereof forwardly, and means for effecting heat transfer from said forwardly extending gas passages to said rearwardly extending air passages. 15

HARVEY N. DAVIS.

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