US3229671A

US3229671A - Marine steam generator having fluid cooled furnace

Info

Publication number: US3229671A
Application number: US448570A
Authority: US
Inventors: Leonard E Triggs
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-04-08
Filing date: 1965-04-08
Publication date: 1966-01-18
Anticipated expiration: 1983-01-18

Description

Jan. 18, 1966 E. TRlGGS 3,229,671

MARINE STREAM GENERATOR HAVING FLUID COOLED FURNACE Original Filed Nov. 26, 1963 2 Sheets-Sheet 1 IIIIIIII I I IIII III],

INVENTOR: LEONARD E. TRIGGS BYQ ATTORNEY Jan. 18, 1966 E. TRIGGS 3,229,671

MARINE STREAM GENERATOR HAVING FLUID COOLED FURNACE Original Filed Nov. 26, 1963 2 Sheets-Sheet 2 FIG. 2

INVENTOR: LEONARD E. TRIGGS ATTORNEY United States Patent 3,229,671 MARINE STEAM GENERATOR HAVING FLUID COOLED FURNACE Leonard E. Triggs, 14 Farmstead Lane, West Simsbury, Conn. Continuation of application Ser. No. 326,016, Nov. 26, 1963. This application Apr. 8, 1965, Ser. No. 448,570 6 Claims. (Cl. 122-478) This application is a continuation of application Serial No. 326,016, filed Nov. 26, 1963, now abandoned.

The present invention relates to the construction and operation of marine-type vapor generating and superheating units. More particularly, the invention relates to a novel construction of a natural circulation marine vapor generator of the two-drum type wherein the furnace chamber is completely water cooled. The invention further engenders a marine vapor generator and superheating apparatus wherein the furnace floor, superheater screen, and superheater roof are formed from a single circuit of vapor generating tubes.

In vapor generators of the marine type, it is common to employ vapor generating tubes for fluid cooling the walls and roof of the furnace chamber; however, the furnace floor in such units is normally lined with downcomer tubes connecting the liquid drum with the riser circuit, such as at the lower header of the front or side walls. This practice requires the application of a layer of insulating material upon the floor tubes so as to prevent heat absorption therein which would be sufficient to elfect the generation of vapor in these tubes and thereby impede the circulation of fluid within the system. The insulation most commonly employed consists of a layer of thermal insulating material, such as Magnesite covering the tubes, and a layer of furnace refractory applied to the top of this layer. The use of insulation and refractory material within a furnace, while being effective for the purpose intended, is undesirable since it adds materially to the cost of the unit in that, in addition to its initial cost, the effects of high temperature result in its requiring frequent repair and replacemnt throughout the life of the unit. This repeated need for maintenance gives rise to undesirable shutdown of the unit which could be dangerous to a vessel at sea, as well as increasing the operating costs of the unit due to the labor and material costs required in replacing the spent materials.

The present invention provides a vapor generating unit wherein the presence of thermal insulation and refractory material is eliminated from the furnace chamber due to the fact that, in the instant unit, the floor of the furnace chamber as well as the walls and roof thereof are completely fluid cooled. This is accomplished by lining the floor and walls of the chamber with tubes in which vapor is generated whereby the velocity of the fluid flowing through the tubes is sufficiently great such that the flowing fluid absorbs the heat generated in the furnace thereby preventing burnout of the tubes.

According to the invention the front and side walls and roof of the furnace chamber are formed of tubes extending between lower and upper headers to define the periphery of the chamber while the floor is formed by a circuit including a plurality of closely spaced, parallel tubes that connect at one end to the lower front wall header and at the other end to the vapor and liquid drum. The tubes of this circuit are so formed as to define a screen to protect the superheater tubes from slagging and from the radiant effects of the flame present in the furnace chamber. In addition they overlie the top of the superheater chamber to cool the unit in this area.

Therefore, it is an object of the present invention to provide a marine-type vapor generating and superheating unit characterized by a furnace chamber which is completely fluid cooled.

A further object of the present invention is to provide a marine-type vapor generating and superheating unit wherein the need for the application of thermal insulating and furnace refractory material to the floor of the furnace chamber is eliminated.

Another object of the present invention is to provide a marine-type vapor generating and superheating unit wherein the furnace floor, superheater screen, and superheater roof are formed of a single tubular circuit comprising a plurality of parallelly arranged vapor generating tubes.

Still another object of the present invention is to provide a marine-type vapor generating and superheating unit characterized by simplicity of design, compactness of form and efficiency of operation.

Various other objects and advantages will appear from the following description of the embodiments of the invention, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

The invention will be understood from the following description when considered in combination with the accompanying drawings forming a part thereof wherein:

FIG. 1 is a side elevation of a marine-type steam generating and superheating unit embodying the present invention;

FIG. 2 is a section taken along line 2-2 of FIG. 1;

FIG. 3 is a partial elevation of a steam generating and superheating unit embodying a slightly modified form of the present invention;

FIG. 4 is a partial elevation of a steam generating and superheating unit embodying another modified form of the present invention; and

FIG. 5 is a sectional view taken along line 5-5 of FIG. .1.

Referring now to the drawings, there is shown a marinetype vapor generating unit 10 comprising a furnace section 12, a boiler bank section 14, and a superheating section 16. The furnace section 12 comprises a pair of spaced side walls 18, a front wall 20 and a roof 22 formed of closely spaced fluid-bearing tubular members arranged to form a chamber into which a plurality of fuel burners 24, mounted in the roof 22, is adapted to fire. The boiler bank section 14 is located in rearward alignment with the furnace section 12 and comprises an upper vapor and liquid drum 26, a lower liquid drum 28 and a plurality of substantially upright vapor generating tubes 30 connecting the two. The superheater section 16 occupies a position intermediate the furnace section 12 and the boiler bank section 14. Within this section is positioned a vertical superheater 32 formed of a plurality of vertically oriented, laterally aligned, inverted U-shaped tubes 34 arranged in a bundle that extends across substantially the full width of the unit and connected at their terminal ends by elongated headers 36 which are arranged to connect the superheater tubes in parallel.

The disclosed vapor generator embodies a generally box-like construction so as to permit unimpeded cubical expansion during operation. The base of the unit includes horizontally disposed liquid drum 28 extending transversely of the vapor generator structure. In spaced parallel relation thereto is a lower front wall header 40. Lower side wall headers 38 extend between and fluidly connect header 40 with the liquid drum 28. As shown, header 40 is located at a lower elevation than the liquid drum 28 such that lower side wall headers 38 are inclined at a slight angle with respect to the horizontal.

The periphery of the upper portion of the unit is defined by the vapor and liquid drum 26 parallelly arranged with respect to the liquid drum 28, upper side 'to expand in an unimpeded manner.

wall headers 42 and upper front wall header 44. The vapor and liquid drum 26 is supported in an elevated position above liquid drum 28 by external downcomer conduits 46 that extend between the two drums to supply the drum 28 with operating liquid and the tubes 30 which form the boiler bank. The forward ends of upper side wall headers 42 are vertically supported by upright conduits 48 which extend between and connect with the I headers 42 and lower front wall header 40. The rear ends of headers 42 do not connect with the vapor drum 28 but are closed as shown and the headers 42 are inclined in parallel with lower side wall headers 38. Upper front wall header 44 extends transversely of the unit side walls parallel to, lower front wall header 40 and is supported at its ends by headers 42. Vapor relief conduits 50 unite the vapor drum 26 and the

headers

42 and 44 in fluid. circulation. This arrangement presents a framing construction for the steam generator that is structurally sturdy and that will permit the pressure parts of the unit Augmenting this framing structure are diagonal external downco mer conduits 52 which connect the water space in vapor and liquid drum 26 with lower front wall header 40 and also s rve as diagonal bracing members tying the lower front wall header 40 to the drum 26.

The radiantly heated walls, 18 and 20, and roof 22 of the furnace section 12 are formed by substantially vertically oriented parallel vapor generating tubes. Those tubes emanating from lower front wall header 40 connect directly to upper front wall header 44 but those which emanate from the lower side wall headers 38 do so in a manner which effects formation of the roof 22 of the furnace section 12 as well as the side walls 18 thereof. As shown in FIG. 5, only alternate tubes 19 emanating from the lower side wall headers 38 connect with the upper headers 42 positioned directly thereabove. The adjacent tubes 19' extend the height of the furnace and then are normally offset and extend transversely across the top of the furnace to connect with the upper headers 42 in the opposite walls. '1=he roof 22 therefore is formed of the horizontal legs of the offset tubes 19' which emanate from the lower headers 38 of opposite side walls 18.

In accordance with the present invention there is provided a generally Z-shaped fluid circuit 54 formed of parallelly arranged vapor generating tubes 56 which connect at their lower ends to lower front wall header 40 and at their upper ends to vapor and liquid drum 26. The fluid circuit 54 is in the form of a panel which extends across the width of the furnace section 12 with the lower leg 58 forming the floor of the furnace, the upper leg 60 forming a roof over the superheater 32 and the generally vertical connecting portion 62 being positioned closely adjacent the forward legs of the superheater tubes 34 to thereby form a superheater screen. iln order to fluid cool the floor of the furnace section 12 the tubes 56 forming the lower leg 58 of the fluid circuit 54 are arranged in closely spaced, or tangent, side-by-side relation. In front of the forward tubes of superheater 32 adjacent the lower end thereof the tubes 56 are normally offset and extend substantially the full height of the furnace. 'In this portion, called the vertical connecting portion 62, alternate tubes 56 are offset, as shown best in FIG. 2, both rearwardly and laterally with respect to the adjacent tubes to thereby form spaces 63 through which gases from the furnace are permitted to pass to the superheating section 16 and boiler bank section 14. Therefore, the tubes 56 serve to provide the superheater 32 with a fluid cooled screen to protect the superheater tubes 34 from the radiant effects of the furnace.

At a level somewhat below the roof 22 but not substantially lower than the top of the superheater tubes 34 the tubes 56 are returned to tangent, or closely spaced,

' relation and are thereafter bent at an angle to the connecting portion 62 to form the upper leg 60 of the fluid circuit 54 which serves as a cover or roof for the superheater 32 to protect the generator from the adverse effects of high temperature that exist in this area. The closely spaced tube arrangement that exists in the upper portion of the connecting leg 62 also serves to form a gas baffle 64 which prevents the flow of gases into the void above the top of the superheater 32 but instead directs those gases flowing along the upper portion of the furnace into contact with the superheater tubes 34.

The operation of the disclosed steam generating and superheating unit is as follows:

With the burners 24 firing into the furnace section 12 feedwater is admitted to the vapor drum 26 where it mixes with the downcomer fluid or water separated from the water deposited in the drum. This mixture flows downwardly to the liquid drum 28 through the external downcomer conduits 46 and through downcomer tubes located within the boiler bank section 14. A portion of the liquid mixture in drum 26 flows from the drum to the lower front wall header through the diagonal downcomer conduits 52. From the liquid drum 2-8 the mixture flows into the lower side wall headers 38 which supply the tubes forming the side walls 18 and roof 22 as well as the front wall header 40. Lower front wall header 40 supplies the tubes forming the front wall 20 and those forming the fluid circuit 54 with fluid. Heat transfer occurs within the vapor generating tubes and thereby fluid circulation is established from the water drum 28 and front wall header 40 to the vapor drum 26. Due to the thermal siphonic action existing within the tubes fluid is caused to flow upwardly through the side walls 18, front wall 20 and the fluid circuit 54, the tendency to flow being enhanced by the upward inclination of the unit from front to rear. Fluid flow through these tubes serves to cool the furnace chamber 12. In addition, the fluid flowing through the tubes 56 forming the fluid circuit 54 provides the superheater 32 with both a screen to protect the tubes thereof from the radiant effects of the furnace and a roof or cover to protect the roof lining elements against overheating.

Because it is important that the velocity of the fluid flowing through the fluid circuit 54 be relatively great, it is necessary to insure an adequate liquid supply to these tubes. In the present arrangement such adequacy of supply is provided by the fact that the roof 22 is formed by tubes in the side walls 18 that are supplied by headers 38 and the tubes that form the front wall 20 emanating from the same supply header 40 as the tubes forming the fluid circuit 54 are limited in length, extending only the height of the front wall thereby limiting the amount of heating surface presented by the tubes and concomitantly the amount of liquid necessary to be supplied to the tubes. This permits the greater portion of the liquid supplied to the header 40 to be passed through the fluid circuit tubes. Were the tubes that form the front wall 20 also employed to form the roof 22 as by offsetting them over the top of the furnace chamber 12 to connect directly to the drum 26 rather than to header 42 there would be provided such a great amount of heating surface by the tubes that the amount of liquid in header 40 available to service the tubes in circuit 54 would be reduced thereby presenting the danger of an inadequate supply of operating fluid and possible burnout. By limiting the amount of heating surface presented by the tubes in wall 20 this danger is removed.

In FIG. 3 is shown a somewhat modified embodiment of the invention. In this embodiment the front wall header 40' is displaced rearwardly from the position it occupied in the FIG. 1 arrangement and the tubes forming the front wall 20 are offset at an angle adjacent the lower end of the wall to form the forward portion of the furnace chamber floor before connecting with the header 40'. The tubes 56 which form the fluid circuit 54 also con nect with the header 40' and extend in the opposite direction from the header to form the remainder of the furnace floor, indicated as 58', before being normally offset to form the upright connecting leg 62 adjacent the forward end of the superheater 32.

In this embodiment fluid circulation through the tubes is the same as in the preferred arrangement shown in FIG. 1. Side wall tubes 18 are fed with downcomer liquid by the lower side wall headers 38. The front wall tubes 20 and the tubes 56 of fluid circuit 54 are served by the header 40' which receives downcomer liquid from the diagonal external conduits 52.

FIG. 4 discloses a slightly modified formation of the gas baflle located on the upper portion of the upright connecting leg 62 of the fluid circuit 54. In this arrangement the fluid circuit tubes 56 remain in spaced relation through the connecting leg portion 62 of the circuit and through the upper leg portion 60. The gas baflle 64' is formed by applying refractory material to the tubes 56 that form the top portion of connecting leg 62 from the roof 22 to a level not substantially below the top of the superheater tubes 32. With this arrangement the gases which flow along the surface of the roof 22 are similarly directed into contact with the superheater tubes in a manner not unlike that of the FIG. 1 arrangement.

By means of the present invention there is provided a marine-type steam generating and superheating unit having a completely fluid-cooled furnace section thereby eliminating the need for insulation and refractory material in this section of the unit. Marine-type boilers constructed in accordance with the present invention are simple in design yet eflicient in operation and, because of the fact that the furnace section is completely fluid cooled, the units are substantially less expensive to build and to maintain.

It will be apparent to those skilled in the art that many alterations or modifications of the herein disclosed structure can be made without departing from the spirit of the invention. For example, the walls and roof of the unit as well as the floor-forming and roof-forming portions of the Z-shaped fluid circuit could be formed of welded tubular panels or the like. For this reason the invention is to be limited only by the scope of the appended claims.

What is claimed is:

1. A vapor generating and superheating unit comprising rectangularly disposed tubular front, rear and opposed side walls defining a gas enclosure including a furnace chamber, a superheating chamber and a boiler bank, said chambers and boiler bank extending across the width of said unit and being disposed in side-by-side relation; said superheating chamber being positioned intermediate said furnace chamber and said boiler bank in open gaseous communication with each; means forming a flue opening in said rear wall; said boiler bank including a horizontally extending upper drum, a lower drum underlying said upper drum in parallel relation therewith and a bank of upwardly extending tubes connecting said drums; said front wall being forwardly spaced from said boiler bank and including vertically spaced upper and lower headers and a plurality of closely spaced vapor generating tubes connecting said headers; said side walls including opposed lower headers connecting said lower drum and said lower front wall header, upper side wall headers overlying said lower side wall headers in parallel relation therewith, and a plurality of closely spaced, upwardly extending vapor generating tubes connecting said upper and lower side wall headers; a portion of said side wall tubes being laterally offset to overlie the top of said furnace chamber thereby forming the roof thereof; vapor relief means connecting said upper headers to said upper drum; burner means operatively associated with said furnace chamber; superheater means disposed in said superheater chamber; a fluid circuit including a plurality of vapor generating tubes disposed in parallel relation across the width of said unit and extending between said lower front wall header and said upper drum, said fluid circuit tubes emanating from said lower front wall header and extending rearwardly therefrom along the bottom of said furnace chamber in closely spaced relation to a point forwardly adjacent the lower end of said superheater chamber thereby forming the floor of said furnace chamber, said fluid circuit tubes further being upwardly offset from said point and extending to the roof of said unit in screening relation to said superheater means and connecting with said upper drum, a number of said tubes in said upwardly offset portion being longitudinally and laterally offset with respect to the plane of said portion to form openings in said portion.

2. The organization of claim 1 including means external of said gas enclosure for supplying operating liquid to said lower front wall header.

3. The organization of claim 1 including downcomer conduit means external of said gas enclosure connecting said upper drum and said lower front wall header for supplying operating liquid to said header.

4. A vapor generating and superheating unit comprising rectangularly disposed tubular front, rear and opposed side walls defining a gas enclosure including a furnace chamber, a superheating chamber and a boiler bank, said chambers and boiler bank extending across the width of said unit and being disposed in side-by-side relation; said superheating chamber being positioned intermediate said furnace chamber and said boiler bank in open gaseous communication with each; means forming a flue opening in said rear wall; said boiler bank including a horizontally extending upper drum, a lower drum underlying said upper drum in parallel relation therewith and a bank of upwardly extending tubes connecting said drums; said front wall being forwardly spaced from said boiler bank and including vertically spaced upper and lower headers in longitudinal alignment with, but lower than, the respective drums, and a plurality of closely spaced vapor generating tubes connecting said headers; said side Walls including opposed lower headers inclined with respect to the horizontal connecting said lower drum and said lower front wall header, upper side wall headers overlying said lower side wall headers in spaced, parallel relation therewith, and a plurality of closely spaced, upwardly extending vapor generating tubes connecting said upper and lower side wall headers; a portion of said side wall tubes being laterally offset to overlie the top of said furnace chamber thereby forming the roof thereof; vapor relief means connecting said upper headers to said upper drum; burner means operatively associated with said furnace chamber; superheater means disposed in said superheater chamber; a fluid circuit including a plurality of vapor generating tubes disposed in parallel relation across the width of said unit and extending between said lower front wall header and said upper drum, said fluid circuit tubes emanating from said lower front wall header and extending inclinedly rearwardly therefrom with respect to the horizontal along the bottom of said furnace chamber in closely spaced relation to a point forwardly adjacent the lower end of said superheater chamber thereby forming the floor of said furnace chamber, said fluid circuit tubes further being substantially normally offset from said point and extending to the roof of said unit in screening relation to said superheater means and connecting with said upper drum, a number of said tubes in said upwardly offset portion being longitudinally and laterally offset with respect to the plane of said portion to form openings in said portion; and downcomer conduit means external of said gas enclosure connecting said lower front wall header to a source of operating liquid.

5. The organization of claim 4 wherein said downcomer conduit means connects said lower front wall header to said upper drum.

6. A vapor generating and superheating unit comprising rectangularly disposed tubular front, rear and opposed side Walls defining a gas enclosure including a furnace chamber, a superheating chamber and a boiler bank, said chambers and boiler bank extending across the width of said unit and being disposed in side-by-side open gaseous communication with each; means forming a flue opening in said rear wall; said boiler bank including a horizontally extending upper drum, a lower drum underlying said upper drum in parallel relation therewith and a bank of upwardly extending tubes connecting said drums; said front wall being forwardly spaced from said boiler bank and including vertically spaced upper and lower headers in longitudinal alignment with, but lower than, the respective drums, and a plurality of closely spaced vapor generating tubes connecting said headers; said side walls including opposed lower headers inclined with respect to the horizontal connecting said lower drum and said lower front wall header, upper side wall headers overlying said lower side wall headers in spaced, parallel relation therewith, and a plurality of closely spaced, upwardly extending vapor generating tubes connecting said upper and lower side wall headers; a portion of said side wall tubes being laterally offset to overlie the top of said furnace chamber thereby forming the roof thereof; vapor relief tubes external of said gas enclosure connecting said upper headers to said upper drum; burner means cperatively associated with said furnace chamber; superheater means disposed in said superheating chamber including a plurality of vertically arranged U-tubes and header means connecting said U-tubes for parallel flow; a fluid circuit including a plurality of vapor generating tubes disposed in parallel relation across the Width of said unit and extending between said lower front wall header and said upper drum, said fluid circuit tubes emanating from said'lower front wall header in coplanar relation and extending inclinedly rearwardly therefrom with respect to the horizontal along the bottom of said furnace chamber in closely spaced relation to a point forwardly adjacent the lower end of said superheater chamber thereby forming the floor of said furnace chamber, said fluid circuit tubes further being substantially normally offset from said point and extending parallel to said superheater tubes to the roof of said unit in screening relation to said superheater tubes, alternate tubes in said normally offset portion being longitudinally and laterally offset with respect to the adjacent tubes to form openings therebetween, said fluid circuit tubes being offset to overlie the top of said superheater chamber in closely spaced relation and connecting with said upper drum; and downcomer conduit means external of said gas enclosure conmeeting said upper drum to said lower front wall header for supplying operating liquid thereto.

References Cited by the Examiner UNITED STATES PATENTS 1,815,086 7/1931 Wood 122336 2,112,202 3/1938 Lucke 122--336 2,633,831 4/1953 Langrand 122478 2,834,325 5/ 1958 Banker 122-478 FOREIGN PATENTS 846,340 6/ 1939 France.

474,087 10/ 1937 Great Britain.

794,046 4/ 195 8 Great Britain.

FREDERICK L. MATTESON, 111., Primary Examiner.

Claims

1. A VAPOR GENERATING AND SUPERHEATING UNIT COMPRISING RECTANGULARLY DISPOSED TUBULAR FRONT, REAR AND OPPOSED SIDE WALLS DEFINING A GAS ENCLOSURE INCLUDING A FURNACE CHAMBER, A SUPERHEATING CHAMBER AND A BOILER BANK, SAID CHAMBERS AND BOILER BANK EXTENDING ACROSS THE WIDTH OF SAID UNIT AND BEING DISPOSED IN SIDE-BY-SIDE RELATION; SAID SUPERHEATING CHAMBER BEING POSITIONED INTERMEDIATE SAID FURNACE CHAMBER AND SAID BOILER BANK IN OPEN GASEOUS COMMUNICATION WITH EACH; MEANS FORMING A FLUE OPENING IN SAID REAR WALL; SAID BOILER BANK INCLUDING A HORIZONTALLY EXTENDING UPPER DRUM, A LOWER DRUM UNDERLYING SAID UPPER DRUM IN PARALLEL RELATION THEREWITH AND A BANK OF UPWARDLY EXTENDING TUBES CONNECTING SAID DRUMS; SAID FRONT WALL BEING FORWARDLY SPACED FROM SAID BOILER BANK AND INCLUDING VERTICALLY SPACED UPPER AND LOWER HEADERS AND A PLURALITY OF CLOSELY SPACED VAPOR GENERATING TUBES CONNECTING SAID HEADERS; SAID SIDE WALLS INCLUDING OPPOSED LOWER HEADERS CONNECTING SAID LOWER DRUM AND SAID LOWER FRONT WALL HEADER, UPPER SIDE WALL HEADERS OVERLYING SAID LOWER SIDE WALL HEADERS IN PARALLEL RELATION THEREWITH, AND A PLURALITY OF CLOSELY SPACED, UPWARDLY EXTENDING VAPOR GENERATING TUBES CONNECTING SAID UPPER AND LOWER SIDE WALL HEADERS; A PORTION OF SAID SIDE WALL TUBES BEING LATERALLY OFFSET TO OVERLIE THE TOP OF SAID FURNACE CHAMBER THEREBY FORMING THE ROOF THEREOF; VAPOR RELIEF MEANS CONNECTING SAID UPPER HEADERS TO SAID UPPER DRUM; BURNER MEANS OPERATIVELY ASSOCIATED WITH SAID FURNACE CHAMBER; SUPERHEATER MEANS DISPOSED IN SAID SUPERHEATER CHAMBER; A FLUID CIRCUIT INCLUDING A PLURALITY OF VAPOR GENERATING TUBES DISPOSED IN PARALLEL RELATION ACROSS THE WIDTH OF SAID UNIT AND EXTENDING BETWEEN SAID LOWER FRONT WALL HEADER AND SAID UPPER DRUM, SAID FLUID CIRCUIT TUBES EMANATING FROM SAID LOWER FRONT WALL HEADER AND EXTENDING REARWARDLY THEREFROM ALONG THE BOTTOM OF SAID FURNACE CHAMBER IN CLOSELY SPACED RELATION TO A POINT FORWARDLY ADJACENT THE LOWER END OF SAID SUPERHEATER CHAMBER THEREBY FORMING THE FLOOR OF SAID FURNACE CHAMBER, SAID FLUID CIRCUIT TUBES FURTHER BEING UPWARDLY OFFSET FROM SAID POINT AND EXTENDING TO THE ROOF OF SAID UNIT IN SCREENING RELATION TO SAID SUPERHEATER MEANS AND CONNECTING WITH SAID UPPER DRUM, A NUMBER OF SAID TUBES IN SAID UPWARDLY OFFSET WITH RESPECT TO THE PLANE OF SAID PORTION TO FORM OPENINGS IN SAID PORTION.