US1785927A - High-pressure boiler - Google Patents

Description

Dec. 23, 1930.

J. E. BELL 1,785,927

HIGH PRESSURE BOILER Filed April 16. 1924 4 Sheets-Shet 1 l INVENTQR 12 ATTORNEY Dec: 23, 1930. BELL 1,785,927

HIGH PRESSURE BOILER Filed April 16. 1924 4 Sheets-Sheet 2 INVENTQR Dec. 23, 1930. J. E. BELL 1,785,927

HIGH PRESSURE BOILER Filed April 16, 1924 4 Sheets-Sheet 3 awvoemtoz 2;, 6 sea Dec. 23, 1930. BELL 1,785,927

HIGH PRES SURE BOILER Filed April 16, 1924 4 Sheets-Sheet 4 51m: 014 f o z Patented Dec. 23, 1930 UNITED STATES PATENT OFFICE JOHN E. BELL, OE BROOKLYN, NEW YORK; LOLA B. BELL EXECUTRIX OF SAID JOHN E. BELL, DECEASED HIGH- PRESSURE BOILER Application filed April 16,

The general object of the present invention is to provide improved apparatus for generating and superheating steam. A more specific object of the invention is to provide steam generating and superheating apparatus of relatively small bulk and weight per unit of capacitywhich will be durable and efficient, and which in particular, will permit of the high temperature of combustion attainable with the use of pre-heated air for supporting combustion supplied ina relatively small excess over that theoretically required for the combustion of the fuel burned without causing unduly rapid deterioration of the furnace chamber walls. My invention comprises some features of construction and arrangement adapted, and in part especially designed, for use in generating and superheating steam at the very high pressures of 1,200 pounds per square inch or so that are now coming into use, but the general features of my invention for the most part are well adapted for use in generating and superheating steam at other pressures, and particularly at ordinary commercial pressures.

The invention is characterized by the provision of what I may call extended boiler heating surface which is adapted to absorb a relatively large amount of heat per unit of heating surface area from products of combustion coming into contact with this surface at a relatively high temperature, in conjunction with provisions for absorbing radiant heat-in the combustion chamber proper and in the passage connecting the latter to the chamher in which the extendedsurface is located, so that while gases initially coming into contact with the extended surface are still very hot their temperature nevertheless is below that at which injurious slag deposits from the heating gases will be made on the extended heating surface. The provisions made for absorbing radiant heat in the combustion chamber and passage leading away from the latter may, and ordinarily will comprise steam superheating surface as well as boiler heating surface proper, and this surface screens or is incorporated in the combustion chamber and passage walls and thereby protects the latter against overheating and 1924. Serial No. 706,851.

against the erosive action of the non-gaseous fuel residue when the fuel burned is powdered coal, for. use of which the invention is especially adaptcd.

The present invention requires a very compact disposition of the required extended boiler heating surface, which can best be formed by parallel closely spaced vgilled tubes such as the tubes encased in externally circumferentially corrugated cast iron casings employed in Foster superheaters and economizers. The effective use of these tubes requires a high gas velocity and a direction of gas fiow transverse to the length of the tubes, which should be horizontal, so that a considerable portion of the hot gases is caused to fio w through the valleys between the tube gills and furnace'dust is swept through, and prevented from depositing in the inter-tube space while at the same time necessarily high draft loss is prevented from becoming excessively large. While the invention was primarily devised for evaporating and superheating water, it has certain especial advantages for use in a power plant comprising a mercury boiler. The horizontal arrangement of the extended boiler surface tubes makes it necessary to provide external means for maintaining the desired water circulation through the tubes, particularly as with the high rate of heat absorption a relatively rapid circulation must be maintained. The external circulation producing effect might be obtained to Obtain the desired circulation by arranging the steam and water drums associated with the tubes at a level substantially above the latter with connections between the tubes and drums so disposed as to take full advantage of the difference inthe density of the water and steam mixture in the tubes and the connections leading-from the latter to the drums and the density of the water in the tube con nections leading from the drum to the tubes. Preferably the circulation producing effect thus secured is augmented by arranging the bya feed or re-circulation pump, but I prefer connections leading upward from the tubes to the drums in a position in which these connections will rapidly absorb heat and thus further decrease the density of the water and steam mixture, in this portion of the circulating system. With the preferred type of extended boiler heating surface described, it is essential to cool the gases before coming into contact with this surface below the point at whichany appreciable slag deposit will occur in the extended heating surface, as the character and disposition of the surface makes it practically impossible to remove such deposits'from any considerable port-ion of the surface. This gas cooling effect is obtained, as already explained, by the pro visions made for absorbing radiant heat in the combustion chamber in which the surface is located. As a further protection against trouble from slagging, I preferably employ tubes at the top of the bank which have the upper surfaces of their casings smooth so that it will be readily possible to remove slag therefrom.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, and the advantages possessed by it, reference should be had to the accompanying drawingsand descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings: Fig. 1-is an elevationin section on the line 11 of Fig. 2;

Fig. 2 is a section on the line 2-2 of Fig. 1; Fig. 2a is a perspective view illustrating a detail of construction employed in the apparatus shown in Figs. 1 and 2;

Fig. 2b is a section on the line 26-26 of Fig. 2a.

Fig. 3 is a view taken similarly to Fig. 1, illustrating a modified construction;

Fig. 4 is a section on the line 44 of Fig. 3; Fig. 5 is an elevation of one end of a bank of tubes shown in Fig. 3, with parts broken away and in section; I

Fig. 6 is a partial section on the l'ne 6-6 of Fig. 5; and i Fig. 7 is a partial plan on the hue 77 of Fi 1.- Figs. 1 and 2 of thedrawings illustrate a superheater boiler A primarily devised for use in generating and superheating steam at a pressure of 1,200 pounds per square inch or so, with heat derived by the combustion of powdered coal introduced into the combustion chamber B of the boiler furnace throu h fuel injectors C of known formwhich dlscharge the fuel in downwardly directed ets through openings in the roof of top wall of the combustion chamber. A small portion of the air re uired for combustion is introduced with the uel, but the major portion of the air for combustion enters the combustion chamber B through openings A in the front and side walls of the chamber from horizontal channels A which runalong the front and sides of the combustion chamber and are open at their ends to the atmosphere, and also receive atmospheric air through ports controlled by shutters A which open into the channels A through the outer walls of the later. The burning gases pass upward out of the chamber B through a neck portion or port B into a horizontal channel B from which the heating gases pass downward through a chamber B and thence into an economizer from the lower ends of which the heating gases issue through ports I. The heat absorbing apparatus of the superheater boiler as shown in Figs. 1 and 2 comprises a radiant heat superheater E, a radiant heat superheater F, radiant heat absorbing boiler heating surface G, tubular elements H swept by the heating gases passing through the chamber B and forming the major portion of the boiler heating surface proper, economizer tubes I and ash cooling or screen tubes D.

The superheater E, which in the particular construction shown is intended for use in reheating the'steam passing from one stage to another of a turbine or turbines, is composed of horizontal elements 0 in the form of flattened: tubes which may be made of cast steel and are incorporated in and form a lining for the major portion of the rear wall of the combustion chamber. The tubular elements 6 are connected at the sides of the boiler to external verticallydisposed headers E. The screen tubes D are shown as hairpin or U tubes arranged in two sets which collectively form a screen extending across the lower portion of the combustion chamber B and serve to cool the non-gaseous residue of the fuel flowing through the screen into the ash hopper A at the bottom of thecombustion chamber. The tubes D inone set are connected at their ends to inlet and outlet headers D and D in front of the combustion chamber B, while the other tubes D are connected to similar headers D and D located at the rear of the combustion chamber. The two outlet headers D are connected by corresponding pipes D and D to a correspondin steam and water drum OA preferably at a evel below but adjacent the water level in the drums. In normal operation the water level should be maintained at about the level of the line XX shown in Fig. 1. 'The lower end of each drum 0A is similarl connected by piping D to the correspon ing ends of the two headers D.

The radiant heat superheater F is incorporated in and forms a lining for the top wall of the heating gas channel B and is composed of elements similar to those employed 1n the superheater E. In the construction illustrated, the elements forming the superheater F are connected at their ends by return bends F into units each of which comprises five elements through which the steam passesin series. The'steam thus passing through each superheater unit is drawn from a corresponding steam and water drum 0 through a corresponding pipe f and steam passes from each 'unit through a corresponding outlet pipe f to the superheater outlet header F p A. characteristic feature of the invention is the formation of the-major portion ofthe heat absorbing surface of the boiler proper by a closely spaced bank of horizontally disposed elements H each of which comprises a steel tube 72. (see Figs-5 and 6) surrounded by tubular externally corrugated cast steel or iron casing sections h, or b the elements H being thus generally similar in structure to the Well known Foster superheater elements. The tubular casing sections it may be exactly similar in form to those employed in Foster superheater elements, each such casing-element being in the form of a cylinder with external circumferentially, extending corrugation grooves. Each of the elements H may be provided with casing sections 71., but, advantageously, the elements H in the two top rows of'such elements are provided with special casing sections h in lieu of casing sections it. The

casing sections it? (see Figs. 5 and 6) are formed with smooth oppositely inclined upper surface portions h into which the corrugation grooves and intervening ribs at the under sidesof the casing sections merge, as shown in- Figs. 5 and 6. This facilitates the removal of any slag which may. accumulate on the elements H at the top of the, bank of such elements. The various elements H are connected in groups for series flow through elements located at successively higher levels from the bottom to the ,top of the bank by return bend end connections J, which are advantageously, though" not necessarily similar in construction to one or another of" the forms of construction illustrated in my co-pending' application Serial No. 702,012, filed March 26, 1924,-patented April 23, 1929, Patent Number 1.710,530.

The end walls of the chamber B in which the ends of the tubes H are mounted, may

be constructed in various ways. A preferredconstruction of these'end walls and a desirable form ofend connections J for use in conjunction therewith, is illustrated in Figs. 3, 5, and 6, wherein each end connection comprisesa return bend in the form of a tube of the same diameter as the tubes k,

which is bent intoan arcwith tangential end portions which are externally bevelled ormade conical in form. In the assembled apparatus each of these conical ends J is inserted in the upwardly flared end h of the corresponding tube H. The flared tube end 72. is received in aheavy metal collar h having its bore tapered to receive the flared tube end. Each return bend is surrounded adjacent each end by a collar J formed with an internal groove into which. the return bend tube is expanded. Each collar J and the corresponding collar h are secured together by bolts J The tube sheet H, forming a part of each end wall of the chamber H is formed with openings H receiving the'collars h on the ends of the tubular elements. To facilitate the insertion and removal of the elements H, which may be withdrawn for repair or replacement through the rear wall of the chamber B", the openings H in the rear wall at least are horizontally elongated, as shown in Fig. 5, so, that each opening receives the collars h of two tubular elements H which, when inplace, may be spaced apart by removable blocks of cast iron H. This facilitates the removal and insertion of elements normally metal plates or sections,-and surrounding the end connections. This packing not only restricts the loss of heat by external radiation but also assists in minimizing air leakage through the corresponding walls of the chamber B The length of the horizontally disposed elements is reduced to a minimum by placing them directly across'the gas passage, and advantageously they are supported at their centers as by the interlaced metal bars H which extend between diagonal rows of elements in two transverse sets as shown in Figs. 5 and 6. Each bar H has an enlarged head H engaging upper portions of the surfaces of two elements H in the top row of such elements, thereby preventing the spacing bar'from slipping down out of place.

Each element H in the bottom row of those elements is connected at the rear end of the boiler by an end connector J to a bent tube K having its yoke portion located within the chamber 13 and having its outer end connected .to'the header L. The shape and ar r'angement of the tubes K tend to prevent or minimizetroubles due to thermalexpansion and'contraction. As shown, there are a plurality of these headers L arranged in an end to end series and each connected by tubes K to five'elements H in the bottom row .of those down-comer pipe M to the lower end of a Each tubular element H in the top row of the bank of those elements has its rear end connected by acorresponding end connector J to the-lower end of the inner tube 9 of al elements. Each header L is connected by a corresponding steam .and water drum 0.

corresponding element G. As shown in Fig. 2a, each element'G comprises a tube 9, preferably of the same diameter as the tubes 72., which is encased in cast iron or steel blocks g rectangular in cross section. 'Some of the blocks 9 have transversely extending supports G secured thereto by means of which the elements G are supported from a supporting metallic framework G Advantageously and as shown, the elements G are arranged to form a smooth heat absorbing and gas deflecting surface which is inclined from the vertical so that it projects over the chamber B from the rear wall of the latter, and forms the rear end of the channel B The tubes g, which have their lower ends connected through tubular elements H, end connections J and tubes K to the same header L, and steam and water drum 0, are connected to the latter at their upper ends through a corresponding horizontal header P which is connected into the steam and water drum adjacent the water. level therein.

The economizer I, through which the heating gases pass downward from the chamber B comprises horizontally disposed tubes 1 which may be similar to the elements H in their construction and in the manner in which they are connected by end connectors J. I is the outlet from the economizer for the heating gases. The lowest row of economizer elements I are connected to an economizer inlet header I and the uppermost row of elements are connected to 'a header I which is connected by a tube I to a horizontal water distributing drum Q. The latter supplies water to each of the drums O and GA through a corresponding supply branch Q, which advantageously, as shown, enters the drum some distance above the lower end of the latter and comprises a downwardly extending portion within the drum which opens to the latter adjacent its lower end. The drums O and 0A, which are arranged in a row extending from one side of the boiler to the other, are each of relatively small diameter, and, in consequence, can be made of ample strength to resist the high pressure of 1,200 pounds per square inch or so for which the boiler shown in Figs. 1. and 2 was primarily devised. Each of the drums O and OA is connected to its neighbor by bent tubes 0 which open to the drums below the water level therein, and by bent tubes 0 which open to the drums well above the water level therein. These drums form effective pressure and water level equalizing connections.

In the normal intended operation of the boiler shown in Figs. 1 and 2, a very efficient combustion and correspondingly high com- '-bustion temperature is obtained by preheating the air thus supplied to an amount but little in excess of that required to support combustion Notwithstanding the high temperature of combustion, the walls of the combustion chamber are protected against overheating and are cooled to avoid injury by the erosive action of the nongaseous residue or ash content of the fuel burned, by air passing through the channels A and ports A and by the heat absorption by the elements G, the

gases entering the chamber B are cooled to a point at which they may safely be brought into contact with the elements H.

The close spacing of the elements H and the corrugated external form of the latter insures a very large amount of highly eflicient heating surface in a chamber B which is relatively small and in which the ratio of the length of the gas path to its mean hydraulic depth is great enough to insure a high coefficient of heat transfer and the desired reduction in temperature of the heating gases before they pass to the economizer. The results thus obtained would not be obtainable with smooth thin walled tubes in place of the tubular elements H. Forone thing, it is not possible to get anywhere near as much heating'surface in a' chamber B of given size with smooth tubes as with the elements H, because it is not practically possible to provide end connections for tubes spaced as closely together as the smooth tubes would have to be spaced to give the same amount of heating surface per unit of space occupied which is readily obtainable with the corrugated elements. This results from the fact that in any event, the tubes must be separated from one 7 another sutliciently far to provide space for the end connections by which the tubes are connected together and to make it possibleto mount the tubes in the tube sheets or like supports, with provisions for removing the tubes. In this connection, it should be noted that, with a tubular element H, of the form illustrated and the inner pipe it two inches in diameter, each element has about three square feet of external heat absorbing surface per lineal foot of tube length as against less than six-tenths of a square foot per foot of length of a two inch plain tube.

When, as is usual in boilers, it is attempted to increase the length of gas path andthe gas velocity through the 'inter tube space of a bank of water tubes by bafiling that space and thereby providing a plurality of passes of the heating as from the tubes, it is practically impossi 1e to maintain the baflles tight enough to avoid leakage of the gases through the baflies, which reduces the efiiciency and heat absorbing capacity of the tubes. This difiiculty is intensified by the fact that it is difficult to prevent the bafiies from burning out and that, when the gas velocity is made high enough to make it materially effective in increasing the heat transfer rate, the draft suction, must be correspondingly increased, which, of course, increases the tendency to leakage through the baifles.

The relatively large amount of heating surface per unit of length of the elements H, and the fact that this heating surface is highly efiicient, tends, of course, to a very rapid rate of heat absorption by the individual elements with a corresponding large amount of steam generation in each element. This requires a relative rapid circulation of Water through the elements H. With the elements H horizontally disposed, as'shown, the only head for producing a water circulation in the tube bank itself is in the end connections J, in each of which there is a slight vertical rise. Steam bubbles in passing through the end connections tend to establish a Water circulation, but the tendency thus created would hardly be suflicient to insure the amount of circulation required. -Ample circulation is insured, however, by locating the steam and water drums O well above the uppermost tubes H, and by exposing the suitably elongated portions of the connections between the drums O and the elements H formed by the elements G to a high heating effect by radiant heat absorption as'well as contact with heating gases at high temperature. The circulation is augmented by the fact that the tubes '9 are nearly straight and may Well be as large in diameter as the tubes h, thus minimizing frictional resistance in this portion of the circulating system where the absence of undue frictional resistance and ample flow path cross section is especially desirable in order to obtain full advantage of the presence of steam bubbles in the water. The presence of these steam bubbles in the water materiallyreduces the weight of fluid column in the pipes G as compared with the weight of'columns of the same height and cross section in the water space of the drums O and in; the down-comer pipes M. The fact that the heating gases enter the bank of heating tubes H at the top of the bank results in the major portion of the steam formation in the upper elements H and in the elements G which are relatively close to the drums 0, thus minimizing the frictional resistance of the water circulation. Y

With the provisions made for abstracting a large portion of the heat from the gases be fore the latter enter the bank of tube elements H, little, if any trouble will ordinarily be caused either by slag formations on the elements-H or by local heating due to the combastion-of carbon-monoxide in proximity to the tube elements. With'the special construction of the upper row of elements H shown in Figs. 5 and 6, any nongaseous fuel residue which may deposit on the elements H before being cooled below the slagging temperature may readily be removed from time to time. It will be understood that any usual or suitable provisions may be made for blowing soot or other furnace dust off the surfaces of the elements H from time to'time if this is necessary or desirable, but, as the specific character of these provisions form no part of the present invention, I have not thought it necessary to illustrate or describe such provisions herein.

It will be apparent to those skilled in the art that the general'principles of my invention may be employed in apparatus quite different in form from that illustrated in Figs. 1 and 2, and that many features of the invention may well be employed in boilers in which the steam pressure is no higher than that now. commonly employed in ordinary boiler plants, and in Figs. 3 and 4 I have illustrated a boiler specially designed foruse with a moderate pressure of say 400 pounds per square inch. In theboiler shown in Figs. 3 and 4 the multiplicity of -small steam and water drums O and OA are replaced by a u single horizontal steam and water drum OB. Water passes from the drum OB down to the lowermost elements H through one or more down-comers MA and the distributing header LA. The upper ends of each of the various tubes g may be each connected directly into the drum OB adjacent the water level therein. In Figs. 3 and 4 the steam generated in the boiler furnace is superheated in the superheater E. For this purpose, the steam pipe E leading from the top of the drum OB 'is connectedto one header E of the superheater E, the superheated steam delivery pipe E leading away from the other header E. The superheater F of theconstruction first described is re laced in Figs. 3 and 4 by boiler heating sur ace composed of elements GA similar to the elements G first described, each comprising an inner tube q-encased in cast iron sectional casing blocks 9'. The upper end of the tubes of the elements GA are each directly connected to the drum OB adjacent the water level therein. The lower ends of the tubes 9 of the elements GA are connected into a common header G which is connected to the under side of the drum OB by a water circulation pipe or down-comer G at each end or side of the boiler;

In this arrangement the roof of the conduit B and the elements GA are inclined to augment the water circulation through the elements GA and make unnecessary thepump circulation of the water which might otherwise be required in this portion of the water heating aparatus. The two sets of U-shaped,

screen tubes D at the bottom of the combustion chamber B of Figs. 1 and 2 are replaced in the construction of Figs. 3 and 4, by two rows of oppositely inclined tubes DA which extend clear across the end of the combustion chamber and are connected to inlet, intermediate, and outlet headers D D and D The outlet header D is connected to the drum OB at each end adjacent the water level by a corresponding pipe D and the inlet header D is connected to branches D of the down-comer pipe G The omission in Figs. 3 and 4 of the superheater for re-heating the steam between turbine stages is not undesirable even when the steam is generated at very high pressure where provisions are made for bleeding steam from the turbine to heat the boiler feed water in stages.

It will be apparent that with either form of invention illustrated the circulation through the horizontally disposed water tubes may be augmented by the simple expedient of elevating the steam and water drum or drums and thereby increasing the gravity differential due to the greater density of the water in the down-comer connections as compared with the density of the steam and water mixture in the outlet connections from the water tubes to the steam and water drums.

The apparatus disclosed is characterized by the relatively small amount of liquid space required, particularly in the case of the form illustrated in Figs. 1 and 2, and the invention is therefore well adapted for use in a mercury boiler. For such use, the required amount of mercury, which is expensive, may be minimized by inserting cores in the heating elements. The relatively low height of the furnace and boiler characteristic of my invention, particularly in the form shown in Figs. 1 and 2, is also advantageous for use in a mercury boiler over which a mercury turbine may be placed.

While in accordance with the provisions of the statutes, I have illustrated and described the best forms of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the forms of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. Steam generating apparatus comprising a chamber, a bank of horizontally disposed water tubes located in said chamber, a steam and water space, means connectmgsaid tubes and space in a circulating system through which water passes downward from said space to tubes at the bottom of said bank, and thence upward through tubes located atsuccessively higher levels, and thence back to said space through vertically disposed tubular elements located in the upper portion of said chamber, and means for passing heating gases into the upper end of said chamber and thence downwardly over the tubes.

2. The apparatus of claim 1 characterized by a plurality of vertical steam and water drums, each enclosing a portion of the boiler steam and water space and eachconnected in a separate circulating system with a portion only of said tubes.

3. A boiler furnace comprising a steam and water space, a combustion chamber, heating I surface screening the walls thereof and receiving heat by radiation from said chamber, a second chamber alongside the combustion chamber, a heating gas conduit connecting said chambers at their up er ends, water tubes in said second cham er, circulating connections between the tubes and space through which water passes from said space to said tubes, and other circulating connec- .tions through which water passes from said tubes back to said space, the last mentioned connections including Vertically disposed portions heated by the gases passing through said conduit.

4. A boiler furnace comprising a steam and water space, a combustion chamber, heating surface screening the walls thereof and receiving heat by radiation from said chamber, a second chamber alongside the combustion chamber, a heating gas conduit connecting said chambers at their upper ends, water tubes in said second chamber, circulating connections between the tubes and space through which water passes from said space to said tubes, and other circulating connections through which water passes from said tubes back to said space, the last mentioned connections including vertically disposed portions forming a screen for a portion of the wall of said conduit.

5. A boiler furnace comprising a steam and water space, a combustion chamber, heating surface screening the walls thereof and receiving heat by radiation from said chamber, a second chamber alongside the combustion chamber, a heating gas conduit connecting said chambers at their upper ends, water tubes in said second chamber, circulating connections between the tubes and space through which water passes from said space to said tubes, and other circulating connections through which water passes from said tubes back to said space, the last mentioned connections including tubes surrounded by metal casing elements and extending upward along a portion of the wall of said conduit.

6. A bank of horizontally disposed tubes with the-tubes at the top of the bank formed each with a smooth upper surface and a corrugated under surface, and with the remainder of the tubes formed with. external circumfer'entially extending corrugations.

. 7. A high pressure boiler comprising a bank of horizontally disposed water tubes, a

series of vertical steam and water drums, and,

means connecting each drum and a group of said tubes ina circulation circuit comprising vertically disposed tubular heated portions through which the flow is from the tubes into the drum.

8. n a superheater boiler furnace, a combustion'chamber, a second chamber alongside the combustion chamber, a conduit of inverted U shape passingheating gases from the top of the combustion chamber into the top of said second chamber, and steam generating and superheating surface screening the walls of the combustion chamber and the top wall of the conduit and'the end wall thereof remote from the combustion chamber and disposed in said second chamber-to be heated by contact with the heating gases passing through the latter.

9. In a boiler, a chamber traversed by heating gases, a bank of horizontally disposed water tubes therein, a plurality of vertical steam and water drums, and connections be tween said tubes and drums providing a plurality of water circulating systems, each including a group of tubes and one of said drums and comprising a down-comer from said drum, aheader through which said down-comer is connected to a plurality of tubes at the bottom of the bank, a header connected to the drum and to each of a plurality of tubes at the top of the bank and 'connections between the tubes of the group providinga plurality .of flow paths between said headers each including tubes at different levels.

10. In a boiler, a chamber traversed by heating gases, abank of horizontally disposed water tubes therein, a plurality of vertical steam and water drums, and connections between said tubes and drums providing a plurality of watercirculating systems, each including a group of tubes and one of sa d drums and comprising a down-comer from said drum, a header through which said down-comer is connected to a plurality of tubes at the bottom of the bank, a header connected to the drum, separate connections from the last mentioned header to each of a plurality of tubes at the top of the bank, the last mentioned connections extending upward from the bank through the upper portion of the chamber, and connections between the tubes of the group providing a pluralit of flow paths betweensaid headers, each of w ich includes thetubes" of the grou at different levels, and means for passing eating gases downward through said chamber.

11. Apparatus for generating and superheating steam at high pressure comprising a combustion chamber and means for supplying powdered fuel and air for its combustion to said chamber, a second chamber, a conduit for passing heating gases from the top of the combustion chamber into the top of said second chamber, a plurality of vertically disposed steam and water drums, steam generating and superheating surface comprising superheating surface screening the side walls of said combustion chamber and the roof of said conduit, screen tubes extending across the lower portion of said combustion chamber, conduits connecting said screen tubes in a circulating system with one of said drums, a bank of horizontally disposed tu'beslocated in said second chamber and swept by theheat ing gases passing downward therethrough, and conduits connecting the last mentioned tubes in circulating systems with the drums not connected to said screen tubes.

12. Apparatus for generating and superheating steam at high pressure comprising a combustion chamber and means for supplying powdered fuel and air for its combustion to said chamber, a second chamber, a conduit for passing heated gases from the top of the combustion chamber into the top of said sec-. ond chamber, a plurality of vertically disposed steam and water drums, and steam generating and heating surface comprising a portion screening the side walls of said combustion chamber and the roof of said conduit, screen tubes extending across the lower portion of said combustion chamber, conduits connecting said screen tubes in a circulating system with one of said drums, and a bank of horizontally disposed tubes located in said second chamber and swept by the heating gases passing downward therethrough, and

connections between the last mentioned tubes and the drums not connected to said screen tubes forming water circulating systems including said last mentioned tubes and the drums connected thereto.

13. Apparatus for generating and super- 'heating steam at high pressure comprising a combustion chamber and means for supplying powdered fuel and air for its combustion to said chamber, a second chamber, a conduit for passing heated gases from the top of the combustion chamber into the top of said second chamber, a plurality of vertically disposed steam and water drums, steam generating and heating surface comprising superheating surface screening the side walls of said combustion chamberand the roof of said conduit, screen tubes extending across the lower portion of said combustion chamber and conduits connecting saidscreen tubes in a circulating system with one of said drums, a bank of horizontally dis osedtubes located in said second chamber an swept by the heating gases passing downward therethrough, and

ing powdered fuel and air for its combustion to said chamber, a second chamber, a conduit for passing heated gases from the top of the combustion chamber into the top of said second chamber, and steam generating and superheating-surface comprising a portion screening the'side walls of said combustion chamber, a portion screening the roof of said conduit, a portion screening the end of said COIldUlll remote from the combustion chamber, and a portion distributed in said second chamber to be heated by contact with the gases passing through the latter.

15. Apparatus for generating and superheating steam at high pressure comprising a bank of horizontally disposed water tubes, a row of steam and water drums adjacent one end of the bank of tubes, a row of radiant heat superheating elements, all so arranged that the vertical axes of said drums lie in a plane perpendicular to said tubes and superheating elements, means connecting each of said drums in a circulating system with an adjacent group of the tubes in said bank, and means for passing steam from each drum in series through the superheating elements in an adjacent group of such elements.

16. Apparatus for generating steam at high pressure comprising a bank of horizontally disposed water tubes, a plurality of vertical steam and water drums, and means for connecting each steam and Water drum in a circulating system with a plurality of said tubes, said means comprising upper and lower headers and a connection from each of the drums, and means connecting the tubes of each group between the headers to provide a plurality of flow paths between the latter each comprising a plurality of tubes at different levels through which a series'fiow occurs.

17. In a boiler comprising a bank of horizontally disposed water tubes anda steam and Water space to which said tubes are connected by means comprising a row of downcomer pipes extending in front of the bank at one end thereof, removable return bends connecting the ends of adjacent tubes and a tube supporting sheet at the end of the bank adjacent the down-comer pipes formed with horizontally elongated slots each receiving the ends of a plurality of said tubes Signed at New York city, in the county of New York and State of New York, this 14th day of April, A. D. 1924.

JOHN E. BELL.

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