US2806454A - Separately fired superheaters - Google Patents

Description

Sept. '17, 1957 A. w. JACKSON 2,806,454

vSEPARATELX FIRED SUPERHEATERS Filed April 9, 1953 I 2 Sheets-Sheet l may p 7, 1957 A. w. JACKSON 2,30 ,454

SEPARATELY FIRED SUPERHEATERS Filed April 9, 1953 2 Sheets-Sheet 2 T -48 34 Y H United States Patent 2,80 6,f45 4- SEPARATELY? FIREDSUPERHEATERS Arthur W, Jackson, Elkins' Park, Pan, assignbrt'dThe Babcock' 85' Wilcox C0mpany,:New"York, N. Y., a cor p'orationofNew Jersey Application April 9, 1953, Serial Nth-347,749 6 Claims. (Cl122'4$5) This invention relates in general to vapor heaters, and more particularly to separately fir'ed' convection heated steam superheaters.

Heretofore in separately fired steam superheaters' in which steam is superheated to high temperatures by the heatingigases' generated by the-'burning of fuel in'a re fractory walled furnace chamber, difficulties-have been encountered in providing highsuperheat temperatures Without refractory wall failures, tube slaggin'g where a slagging type of fuel is used, and'tube failures due to high tube-wall'ternperatures necessary. In the high temperature ranges encountered, the maintenanceof' tubes and furnace walls has been a major problem.- Slagging of' the tubes was particularly liable" tooccur when the steam heating tubes were arranged close together, but when the-tubes were spaced far' enough" apartto avoid slag bridging,.theflow of heating gases was usually notsuflieient to 'give the desired heating effect. The existence of excessively hot gas zones in'the comb'ustion' gas p'athof such units has frequently resulted in tube or wall failures at such locations.

In'some cases these difliculties have'be'en" reducedby the recirculation of' relatively cool flue gase's intoj the furnace'chamber, or theuse' of'high percentages 'ofi excess air'in the combustionof the fuel,'to attemp'erate tl'le -heat ing: gases generated: The effectiveness of gas reeireul'a: tiorr and excess air for this purpose dependsupon their manner of introduction and degreeof mixing 'withthe burning: fuel and/ or fresh'heating gases.

The primary object of this i-nvention'is toprovidea separately fired vap'or heater with a pluralityi'of vapor heating passes in a simpleand compact construction andarianged to provide optimum heatingv gas temperature and velocity'conditions for effective heat transfer'through out-the'vapor heating passes without-danger of overheat ingthe vapor heater parts.

A further object is to provide a separately: firedvapor heater having an improved arrangement for recirculated gas-introduction andan unobstructed-gas mixingzpa'ss, to insure intimate mixingof the-relatively coolreeirculated gases with the gaseous products ofcombustionbefore' reaching the vapor heating-surface.

A still-further object is to provide a separately fired vapor heater with vapor heatingelementsdispose'd in serially-connected parallel adjoining vapor heating-passes with a tube arrangement and vapor flow therein to=provide optimum heating gas flow and heat-transfer conditionsthroughout the passes.

The various features of novelty which characterize my. invention are pointed out-with particularity in the claimsannexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained=-by-'its-use, reference should be-hadtothe accompanying drawings and descriptivematter in which I have illustrated and-dc scribed several embodiments of my invention.

Ofthe drawings;

Fig. lisa horizontal section, taken on the line 1-1 Z,8@5,4 54 Patented'sept. 17, 1957 2. of Fig.- 2, of a separately fired steam superheater constructedin accordance with my invention; Fig.2 is a sectional elevation taken on the line 2-2 of Fig. 1= through the first superheater pass;

Figs 3 is a sectional elevation taken on the line 33 of Fig 1 through the third superheater pass;

Fig. 4 is a sectional elevation taken on the line 44 of Fig-.- 1 through the second superheater pass;

Fig. 5 is a transverse section taken on the line 5-5 of-Fig. 1;

Fig.6 is a view similar to Fig. 1, taken on the line 6-6 of Fig. 7 of a second embodiment of myinvention; and

Fig.: 7 is a sectional elevation taken on the line 7-7 of Fig. 6. V

The separately fired steam superheater 1' illustrated in Figs. 1 -5 comprises a rectangular refractory walled furnace orcombustion chamber 2 defined by ,a front wall 3; rear wall 6, and side walls 7 and 8. A pair of fluid fuel burners 4,.which are enclosed by a windbox 33, are arranged to discharge through horizontally spaced burner ports421 located intermediate the heightof the front wall 3: Theinner side wa1l'8 isconstructed with a multiplicity of small-sized-reversely bent horizontal gas passages 11 arranged in vertical rows extending throughout the height of the side-wall adjacent the rear end thereof; A narrow unobstructed gas mixing pass 12 is defined by the side wallSQthe front wall 3, the rear wall 6, and a refractory wall 43" arranged parallel to and coextensive with the sidewall 8 The Wall 43 is constructed with a. multi plicityof small sized reversely bent horizontal gas passages 13 similar to those in the'side' wall 8 but adjacent the front wall. I

The remaining space in the unit is occupied by a convectionvap or heating section containing three steam superheating: passes 14, 17, and 20 arranged normal to the mixingpa'ss 12, this section being defined by the wall 43,

extensions'of the front and rear walls 3 and 6, and a refractory'ontersi'de wall 42. The side wall 42 terminates short ofthe rearwall'6 to form a heating gas exit from trie'pass 20'. The three passes 14, 17, and 20are'serially. connected adjoining parallel gas passes defined by parallel vertical refractory walls 39 and 4tl extending'from'the' walls 43 an'd 42 respectively parallel to' thefront and rearwall's 3an'd"6 Hollow tile 44 is'arranged along the bottom ofthe'whole superheater to cool the floor.

Spaced groups" of multi-loop'ed nested pendent superheater tubes are disposed in each of the steam superheating. passes 14, 17 and 20. Two banks of tubes 15 and- 16 are-in the first pass 14', banks 19 and 18 in the seeond pass 17, and banks-21, 22 and 23are in the third pass 20.- The supporting steel work for the unit com prises-roof beams 39, extending above a roof 41 and used to support the superheater tubes in each gas pass. Hangers 45 carried'by. supporting cross-beams 46, 47, 48 and 60 above the roof beams 39 are connected to the top intermediate loops of the superheater tubes in each pass. In the first gas pass corresponding tubes of the banks 15 and 16 are serially connectedby straight horizontal tu bular elements 61. The other ends of the tubes in the tube "banks 15 and 16 are connected to transverse inlet and intermediateheaders 36 and 37 respectively supported on the roof beams 39. The header 36is joined by a connecting pipe 36a which leads to an outlet header 36b ofth'e tube-bank 19 in the second pass 17. The other end of'thi'stube bank 19 is connected to an intermediate header 35. The tubes of the tube bank 18 in-the second pass are connected at oneend to an extension of the header 37. The other ends of the tubes of-bank 18 are connected into a-transverse steam outlet header- 38. In the third pass 20 corresponding tubes in the banks 21, 22', and 23 are serially connected by straight horizontal tubular elements 62 -and 63 ai1d extend between a steam inlet header 34 at the end of tube bank 23 and an extension of the header 35 at the end of tube bank 21. The upper looped portions of the tubes and connecting ele ments of all tube banks extend into the roof 41.

With the described construction and arrangement, the steam to be superheated or reheated is introduced through the inlet header 34-, flows successively through the tube banks 23, 22 and 21 to the intermediate header 35, then through the tube bank 19 to header 36b, through the conduit 36a to header 36, through the tube banks 15 and 16 to header 37, and then through the tube bank 18 to the superheated steam outlet header 38 and discharge conduit 38a.

With the described arrangement and connection of the tube banks, the hottest heating gases entering the first pass 14 successively flow over the tube banks 15, 16 and 18. These tube banks are arranged for steam flowin the same direction as gas flow, i. e. a parallel flow heat transfer relationship. The tubes of these banks are arranged four tubes wide on six inch centers, in contrast to the tubes of banks 19, 21, 22 and 23 which are six tubes wide on four inch centers and have a counterflow heat transfer arrangement. With this arrangement the steam mass flow in the tubes containing the higher temperature steam is greater than the others due to the smaller number of tubes. This tends to reduce the tube metal temperature and minimize the likelihood of tube failure. Although the hotter heating gases contact the tubes in the first pass and a part of the second where the steam temperature is highest, with the wider spacing of these tubes there will be a lower gas flow velocity and resulting heat transfer than where the tubes are closer spaced in the remaining part of the second and third passes. However, in the third pass and part of the second pass where the combustion gas temperature is lower, the tube banks are arranged to have the steam enter at the end of the last pass and flow in a direction opposite the gas flow for counterflow heat transfer. Here the tube spacing is closer in order to provide more gas turbulence and higher gas velocity with better heat transfer in this vapor heating section where both the steam and the heating gases are at their lower temperatures.

Combustion air is supplied to burners 4 from the windbox 33. The air is first preheated by forcing it through a rotary regenerative type heat exchanger 25 by a fan 31 and delivering the heated air through a duct 32 to the windbox 33. The gaseous products of combustion from the furnace 2 pass through the wall passages 11 into the mixing pass 12, horizontally through this pass and out through the wall passages 13. The flow restrictions provided by these small wall passages and the narrow width of the mixing pass cooperate in effecting a thorough mixing of the gases before entering the convection heating section. The gases then flow through the narrow serially connected passages 14, 17 and 20 in successive intimate contact with the superheater tube banks 15, 16, 18, 19, 21, 22 and 23, and exit through the gas outlet 24. The gases are withdrawn from this outlet through a rotary regenerative type air heater 25 by an induced draft fan 26 and discharged through a flue 27 to the stack 28. A damper 44 in the flue 27 controls the proportion of the gases discharged to the stack 28.

In accordance with my invention, relatively cool gases are withdrawn from the flue 27 and introduced into the furnace chamber 2 in a manner providing effective protection of the refractory Walls and permitting a thorough mixing of the recirculated gases with the fresh gaseous products of combustion before the combined gases contact the steam superheating tubes. The quantity of gases recirculated is controlled by the relative adjustment of the damper 44 and a damper 29 in a recirculated gas duct 36 connected into the flue 27 at the induced draft fan side of the damper 44. The duct 30 delivers recirculated gas to an inverted V-shaped manifold surrounding the top and sides of the burner windbox 33. The manifold 10 has a multiplicity of small ducts 9 of square cross-section extending from the manifold 10 through the combustion chamber wall 3 for the admission of recirculated gases. With the described flue arrangement the recirculated gases enter along the roof 41 and the side walls 7 and 8 of the furnace chamber so that the walls and roof are protected from the high temperature gaseous products of combustion generated by the burning fuel by a moving stream or layer of relatively cool recirculated gas which acts to absorb a substantial portion of the heat otherwise radiated to the furnace chamber walls. A thorough mixing of all of the gases is effected by the described mixing pass and the gas inlet and outlet openings thereto.

In the modification of my invention shown in Figs. 6 and 7, burners 50 are arranged to discharge through the burner ports 50a in the outer side wall 7, intermediate the height thereof and in a direction toward the opposite side Wall 8. With this burner arrangement, the recirculated gas manifold 10 is arranged as previously described relative to the burner ports and the recirculated gases are introduced along the furnace chamber front wall 3, rear wall 6, and roof 41, through a multiplicity of ducts 57. Additional wall protection is provided by the use of a vertical manifold 58 connected to the recirculated gas duct and manifold discharge ducts 54 adjacent the chamber side wall 8 to provide streams of cool flue gases along the wall 8.

In both embodiments gas recirculation is used to control the final temperature of the steam. By varying the quantity of gas recirculated, the quantity and temperature of the heating gases is varied with a resultant change in heat transferred to the steam.

In operation the gaseous products of combustion generated in the combustion chamber 2 are in most cases at temperatures too high for safe contact with the steam superheating tubes. The radiant heat generated from the combustion of the fuel in the combustion chamber is also too great to permit the direct exposure of the steam superheating tubes to this radiant heat. To eliminate the possibility of tube metal failure the steam superheating tubes are shielded from the furnace chamber radiation by the refractory walls 8 and 43. The recirculated flue gases effectively maintain the refractory furnace chamber walls at a safe temperature and the mixture of these CO'Oll gases with the high temperature gaseous products of combustion by their passage through the mixing pass insures a gas temperature at the superheater which will provide the desired superheat tempenature, without overheating the tubes.

With the plurality of serially connected side-by-side steam heating passes of my invention, the refractory walls are less likely to be exposed to destructive temperature extremes, than would be so with a single pass arrangement which would involve long refractory walls with great variations of temperature from one end to the other. The interior transverse walls 39 and forming the heating passes have very little temperature variation from one wall side to the other, since both sides are exposed to the heating gases. The provision of three s-ide-by-side heating passes provides an effective method of obtaining the desired superheat temperatures with a minimum of connections between the superheater tube banks.

' While in accordance with the provisions of the statutes I have illustrated and described herein the best forms of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by the claims.

What is claimed is:

1. A separately fired high temperature vapor superheater comprising refractory walls defining a combustion chamber, means for burning fuel in suspension in said chamber, recirculated gas inlet means arranged to discharge gases along a vertical wall of said combustion chamber, an unobstructed gas pass of relatively narrow cross section arranged along one side of said combustion chamber having restricted gas inlet and outlet openings at opposite ends thereof for the flow of heating gases from said combustion chamber, refractory walls defining a vapor heating chamber having a plurality of serially connected adjoining parallel gas passes arranged for the horizontal flow of gases in a direction normal to said .unobstructed gas pass and in communication with said unobstructed gas pass outlet, a bank of vapor heating tubes disposed in each of said vapor heating chamber gas passes, said vapor heating tube banks being interconnected for vapor flow parallel to the direction of gas fiow through an initial portion of the gas flow path through said vapor heating passes and for counterflow during the remaining portion of the gas flow path through said vapor heating passes, and means for recirculating a portion of the gases leaving said vapor heating chamber to said recirculated gas inlet means.

2. A separately fired high temperature vapor superheater comprising refractory walls defining a combustion chamber having a gas outlet in one vertical wall thereof, means for burning fuel in suspension in said chamber, refractory exterior Walls and at least one vertical interior wall defining a vapor heating chamber having a plurality of serially connected adjoining parallel horizontally extending gas passes arranged for the horizontal flow of gases in a direction normal to the gas outlet wall of said combustion chamber and in communication with said combustion chamber outlet, and one or more banks of vapor heating tubes disposed in each of said vapor heating chamber gas passes, said vapor heating tube banks being interconnected for serial flow of vapor parallel to the direction of gas flow through an initial portion of the gas flow path through said vapor heating passes and for counterfiow during the remaining portion of the gas flow path through said vapor heating passes.

3. A separately fired high temperature vapor superheater comprising refractory Walls defining a combustion chamber having a gas outlet in one vertical wall thereof, means for burning fuel in suspension in said chamber, recirculated gas inlet means arranged to discharge gases horizontally along a vertical wall of said combustion chamber, refractory exterior walls and at least one vertical interior wall defining a vapor heating chamber having a plurality of serially connected adjoining parallel horizontally extending gas passes arranged for the horizontal flow of gases in a direction normal to the gas outlet wall of said combustion chamber and in communication with said combustion chamber outlet, a bank of vapor heating tubes disposed in each of said vapor heating chamber gas passes, said vapor heating tube banks being interconnected for serial flow of vapor parallel to the direction of gas flow through an initial portion of the gas flow path through said vapor heating passes and for counterfiow during the remaining portion of the gas flow path through said vapor heating passes, and means for recirculating a portion of the gases leaving said vapor heating chamber to said recirculated gas inlet means.

4. A separately fired high temperature vapor superheater comprising refractory walls defining a combustion chamber, means for burning fuel in suspension in said chamber, an unobstructed gas pass of relatively narrow cross-section arranged along one side of said combustion chamber having restricted gas inlet and outlet openings at opposite ends thereof for the flow of heating gases from said combustion chamber, refractory exterior walls and at least one vertical interior wall defining a vapor heating chamber having a plurality of serial connected adjoining parallel horizontally extending gas passes arranged for the horizontal flow of gases in a direction normal to said unobstructed gas pass and in communication with said unobstructed gas pass outlet, and a bank of vapor heating tubes disposed in each of said vapor heating chamber gas passes, said vapor heating tube banks being interconnected for serial flow of vapor parallel to the direction of gas flow through an initial portion of the gas flow path through said vapor heating passes and for counterflow during the remaining portion of the gas flow path through said vapor heating passes.

5. A separately fired high temperature vapor superheater comprising refractory walls defining a combustion chamber having a gas outlet in one vertical wall thereof, means for burning fuel in suspension in said chamber, refractory exterior walls and at least one vertical interior wall defining a vapor heating chamber having a plurality of serially connected adjoining parallel horizontally extending gas passes arranged for the horizontal flow of gases in a direction normal to the gas outlet wall of said combustion chamber and in communication with said combustion chamber outlet, and a bank of vapor heating tubes disposed in each of said vapor heating chamber gas passes, said vapor heating tube banks being interconnected for serial flow of vapor parallel to the direction of gas flow through an initial portion of the gas flow path through said vapor heating passes and for counterflow during the remaining portion of the gas flow path through said vapor heating passes, said tubes being positioned further apart in the initial portion of the gas flow path through said vapor heating passes than in the latter portion.

6. A separately fired high temperature vapor superheater comprising refractory walls defining a combustion chamber, means for burning fuel in suspension in said chamber, a series of recirculated gas inlets arranged along one or more vertical walls of said combustion chamber, refractory walls defining an unobstructed gas pass arranged along one side and in communication with said combustion chamber having curved restricted gas inlet and outlet openings at opposite ends thereof, refractory exterior walls and at least one interior vertical wall defining a vapor heating chamber having a plurality of serially connected adjoining parallel horizontally extending gas passes arranged for the horizontal flow of gases in a direction normal to and in communication with said unobstructed gas pass, one or more banks of pendant-type nested multi-loop vapor heating tubes in each of said vapor heating chamber gas passes, said vapor heating tubes being interconnected for serial flow of vapor parallel to the direction of gas flow and also spaced further apart during an initial portion of the gas flow path through said vapor heating passes and for counterflow and with closer tube spacing during the remaining portion of the gas flow path through said vapor heating passes, and means for recirculating a portion of the gases leaving said vapor heating chamber to said recirculated gas inlet means.

References Cited in the file of this patent UNITED STATES PATENTS 1,064,250 Primrose et al June 10, 1913 1,102,361 Stevens July 7, 1914 1,691,698 Broido Nov. 13, 1928 1,891,879 Folliet et al. Dec. 20, 1932 1,929,890 Huet Oct. 10, 1933 FOREIGN PATENTS 970,916 France June 28, 1950

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