US2819702A

US2819702A - Method of and apparatus for controlling vapor temperatures

Info

Publication number: US2819702A
Application number: US264029A
Authority: US
Inventors: Paul H Koch
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1951-12-29
Filing date: 1951-12-29
Publication date: 1958-01-14
Anticipated expiration: 1975-01-14

Description

Jan. 14, 1958 P. H. KOCH 2,819,702

METHOD OF AND APPARATUS FOR CONTROLLING VAPOR TEMPERATURES 7 Filed Dec. 29, 1951 4 Sheets-Sheet 1 INVENTOR ATTORNEY P. H. KOCH Jan. 14, 1958 METHOD OF AND APPARATUS FORCONTROLLING VAPOR TEMPERATURES Filed Dec. 29, 1951 4 Sheets-Sheet 2 INVENTOR .Pczzz/ /jfock ATTORNEY P. H. KOCH Jan. 14, 1958 METHOD OF AND APPARATUS FOR CONTROLLING VAPOR TEMPERATURES Jan. 14, 1958 P. H. KOCH METHOD OF AND- APPARATUS F017 CONTROLLING VAPOR TEMPERATURES Filed Dec. 29, 1951 4 Sheets-Sheet 4 FIG.6

lNVENTOR ATTORNEY Unite States Patent I METHOD OF AND APPARATUS FOR CON- TROLLING VAPOR TEMPERATURES Paul H. Koch, Bernardsville, N. J., assignor to The Babcock & Wilcox Company, New York, N. Y., a corporation of New Jersey Application December 29, 1951, Serial No. 264,029

11 Claims. (Cl. 122-479) This invention relates to improvements in methods of and apparatus for controlling vapor superheat temperature in vapor generating and vapor superheating units in which fuel is burned in suspension in a furnace having fluid cooled walls forming an appreciable portion of the total vapor generating surface of the unit. The vapor generated is superheated by its passage through tubular superheater elements heated mainly by convection heat transfer-from the heating gases developed in the burning of the fuel as they flow from the furnace.

The convection superheating effect is dependent upon the temperature and the mass of the heating gases flowing over the superheater tubes and these factors in turn are dependent upon the combustion conditions in the furnace and the amount of heat transferred from the gases to the fluid cooled furnace walls while the fuel is being burned and while the heating gases flow to the furnace outlet. It is known that the heat transfer to such furnace walls can be varied by shifting the proximity or level of the main combustion zone relative to the furnace gas outlet and thereby control the temperature of the heating gases flowing over the superheater tubes and correspondingly control the convection superheating effect.

In my prior application Ser. No. 167,073, filed June 9, 1950 (now Patent 2,737,931, March 13, 1956), of which this application is a continuation-in-part, I have disclosed and claimed a method of and apparatus for vapor superheat control in which the radiant transfer of heat from the combustion zone and furnace gases to the fluid cooled furnace walls is varied by the introduction of heating gases from a position wholly or partly downstream of the superheater into the furnace in such a manner that a layer or stratum of relatively low temperature gas is interposed between the main combustion zone and a substantial portion of the fluid cooled furnace wall area to vary the radiant heat transfer to the vapor generating tubes of the walls and thereby control the heat content of the heating gases leaving the furnace and flowing over the superheater.

My present invention comprises a method ofand apparatus for controlling vapor superheat temperatures in a unit of the character described which, in general, involves the concurrent use of heating gas recirculation and combustion zone level variation to obtain a range and degree of vapor superheat control which is greater than can be obtained by the use of either a corresponding amount of gas recirculation or a corresponding change in combustion zone level alone, or by the sum total of these individual effects.

In carrying out the method of the invention for the control of superheat in a steam generating unit having a vertically elongated fluid cooled furnace with a heating gas outlet from the upper portion thereof leading to a convection heated superheater, fuel and combustion air are introduced through ,a plurality of vertically spaced regulable burners or other controllable burner means in 2,819,702 Patented Jan, I 14,

a vertical furnace wall to burn a preponderance of the fuel in a main combustion zone having a mean level intermediate the height of the furnace, from which zone the heating gases flow to the furnace gas outlet and heat is radiated from the gases to the surrounding furnace wall steam generating tubes. The heat content of the gases leaving the furnace outlet is increased by introducing a controlled stream of recirculated gases into the lower portion of the furnace below the combustion zone level while concurrently varying the adjustment of the regulable fuel burners to elfect a higher mean level of the main combustion zone.

The invention effects a reduction in draft loss, as compared to a method or apparatus accomplishing the same superheat control result by recirculating gases, but lacking the concurrent variation of level of the main combustion zone. The invention also minimizes or reduces fan and duct system cost, and reduces power required to operate the recirculating fan.

Various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification, but,

for a better understanding of the invention, its operating advantages and the specific objects attained by its use,

reference should be had to the accompanying drawings and descriptive matter in which of the invention are described.

In the drawings: Fig; 1 is a vertical sectional view of a vapor generating preferred embodiments unit including the burner level control, attemperator,

and gas recirculation system as components of the superheat control system of the invention;

Fig. 2 is a partial vertical section on the line 22 of Fig. 1, showing the arrangement of rows of burners and their communications with the wind-box or secondary air chamber;

Fig. 3 is a schematic drawing of a superheat control.

system for automatic operation of the unit shown in Fig. 1;

Fig. 4 is a similar schematic view of an additional part of the automatic control system for. regulating the gas recirculation system for controlling superheat;

Fig. 5 is a chart with curves illustrating theincreased load range over which steam temperature is maintained, 7

and

Fig. 6 is a sectional elevation of a modified unit, em..

bodying the invention.

The illustrative vapor generating and superheating unit.

shown in Fig. 1 includes a vertically elongated furnace 10 having a hopper bottom 11 and a furnace gasoutlet 12 in the upper portion thereof. The-furnace boundaries such as the burner wall 14, the rear wall 16, the sidewalls (one of which is shown facing the observer), the roof 18, the arch 20, and the hopper bottom walls 22 and 24,

are defined by vapor generating tubes having their upper ends in communication with the steam and water drum to the roof as row 34 in which the tubesare more widely spaced than in the arch, providing a screen across the outlet from the furnace to the superheater chamber. The

remaining tubes 36 in the arch row are inclined 'rear-,

wardly to merge into and extendupwardly in the plane of the rear furnace wall.

The gas passage from the furnace gasoutlet includes a horizontally extending passage 35 and a serially con} nected downwardly extending gas pass 52. The convection superheater surface comprises a primary superheater section including a plurality of laterally spaced serially connected

multiple loop elements

50 and 48 extending transversely of the gas pass 52, and a secondary superheater including a plurality of laterally spaced serially connected multiple loop elements 37 and 38 pendentl'y' supported transversely of the passage 35. The inlet head er 54 of elements 50 is connected to the vapor space of the drum by serially connected

tube lengths

56, 58, and

60, and the outlet header 46 of elements 48 is connected by conduit 44, to the inlet header 40 of the secondary superheater, while the outlet ends of elements 38 are connected to the superheated steam outlet header 43 which in turn is connected by piping not shown to the steam user. The conduit 44 includes a section 42 which involves a spray attemperator of the type shown by Patent 2,550,683 to Fletcher et al., dated May 1, 1951.

An economizer consisting of a plurality of laterally spaced

multiple loop elements

68 and 70 is positioned in pass 52 below the convection superheater. It has an inlet header 74 and an outlet header 72, respectively, connected to a supply of feed water and to the water space of the drum 26. A gas flue 76, including dampers 78 connects pass 52 to the gas inlet of a regenerative air heater 80, from which an appropriate gas flue leads to a stack, not shown.

The furnace is provided with a plurality of vertically spaced rows of

fuel burners

62, 64 and 66, which are separately connected to a source of superatmospheric pressure preheated air for combustion and also connected to respective sources of fuel such as pulverizers C, B and A.

A regulable forced draft fan 94 is arranged to supply combustion air under pressure through duct 96 to air heater 80, and serially connected

ductwork components

102, 100, 98 conduct the heated air to the inlet 126 of windbox 104, embracing the several rows of burners. Partitions 106, 108 and 110 (Fig. 2) divide a portion of the windbox into vertically spaced

air chambers

112, 114, 116 and 118, each of which is connected to the air inlet portion 128 of the windbox by its respective separately operable set of dampers such as 130, 129, 131 and 133.

Each of the three burners in each horizontal row is connected by a separate primary air and pulverized fuel conduit to the same pulverizer.

Conduits

134, 135 and 136 connect burners 62 to pulverizer C,

conduits

146, 147 and 148 connect burners 64 to pulverizer B, while conduits 140, 141 and 142 connect burners 66 to pulverizer A. The coal pulverizers exemplify sources of a regulable supply of fluid fuel to be burned in suspension within the furnace and they are of the air swept type in which the rate of fuel delivery therefrom to the burners is regulated by changing the rate of flow of primary air flow through them.

A primary air fan 123 positioned intermediate the ends of the duct 124 is arranged to draw preheated air from chamber 118 and tempering air through dampered duct 152, and discharge into the pulverizer A under control of volume regulating damper 150. While a primary air fan has been shown and described with respect to Pl11-' verizer A only, corresponding separate primary air supply components are provided for pulverizers B and C, which receive preheated air from

connections

122 and 120 respectively.

Provision for regulable gas recirculation similar in principle to that disclosed and claimed in my copending application Ser. No. 167,073 (now Patent 2,737,931 of March 13, 1956) is made as a component of the superheat temperature control of the illustrative unit. A fan 82 has its inlet connected by duct 84 to opening 86 in the gas flue 76 connecting the pass 52 to the gas inlet of the air heater. The outlet of the fan is connected by duct 88 including a regulating damper 400 to a plurali y f nozzle or gas discharge ports which are arranged in an:

elongated row and open through inter-tube spaces be-- tween the lower ends of hopper wall tubes to the space:

below the furnace hopper throat 92.

row level aa is spaced below the furnace gas outlet 12 to such an extent that under the maximum furnace: temperature conditions to be experienced, the temperatureat the gases leaving the furnace will not be in excess of a predetermined value based primarily on consideration.

of ash fusing temperatures. The lowermost row level,. cc, is at an elevation sufliciently above the hopperbottom to permit the desired cooling of ash particles sep arating from the products of combustion and droppinginto the hopper in their passage to the ash pit below the: throat 92. The vertical spacing of the burner rows may be of a value of from 8 to 15 ft. in consideration of theabove factors as well as the desired modification in furnace wall heat absorption.

When the unit is operating at maximum load or in the upper load range, when all three rows of burners are in operation and the combustible delivery substantially uniformly the same to all burners, the main com bustion zone is in a zone opposite the burners, inasmuch as the burners are of the turbulent short flame type- The mean elevation of the main combustion zone can be. considered to be represented by the line b-b.

On the other hand when the load on the unit is at such a lower value that two rows of burners will provide sufficient fuel, and only two rows of

burners

62 and 64, are operated, being supplied, for example, by pulverizers C and B, the main combustion zone will be at a higher elevation and the mean level of the main combustion zone will be at an elevation dd intermediate lines a-a and b-b, inasmuch as burners 66 are inactive and the active main zone of combustion does not carry down into the furnace because the necessary overall gas flow is upward toward the furnace gas outlet 12.

Fig. 5 is a graph illustrating the greater extent of superheat increase which may be accomplished by the concurrent utilization of gas recirculation and modification of the elevation of the combustion zone, as compared with what is attributable to the sum total of the individual effects of gas recirculation and modification of combustion zone elevation separately utilized. This graph is representative of the results which can be effected in an installation of the types shown by Figs. 1 and 6, having means for shifting the mean elevation of the combustion zone.

The lowermost curve MM' indicates the variation of superheat temperature with operation of at least two vertically spaced rows of burners. The next curve NN', extending somewhat parallel to curve M-M', represents superheat results attainable by shifting the elevation of the combustion zone by omitting the operation of the lowermost row of burners and carrying the load by the superposed burners. Thus at a load of 40%, the curve NN' indicates a steam temperature of approximately 878 F., while curve MM at the same load is at 854 F. Shifting of the combustion zone thus gives an increase of 24 deg. at this load. If, however, the same rows of burners are operated as in the development of curve M-M', and a constant quantity of gas is recirculated to the bottom of the furnace, the superheat temperature will be in accordance with curve O-O', having a value of approximately 937 F. at 40% load. Gas recirculation will thus account for an increase in superheat from 854 F. to 937 F., or an increase of 83". However, as the unit is operated after cutting out the lowermost row of burners as per NN' but concurrently with the recirculation of the same amount of gas as in the development of curve 0-0 the superheat at 40% load will be at 982 F., or an overall rise of 982 minus 854, or 128", as indicated by curve R-P. Separately, the

' indications of boiler load.

increase due to shifting. the. elevationas .per .N-N' amounts to 24", while the effect of gas recirculation as per 0-0 is 83 so that added together 24+83=107 of temperature rise is accounted for. However, the additional superheat rise (128 minus 107-=21) is accomplished due to the concurrent gas recirculation and modification of elevation of the combustion zone,-whereby the gas recirculation becomes more effective and a higher superheat temperature rise is accomplished with no greater expenditure of recirculating gas fan power or sacrifice in essential performance characteristics of the furnace.

The enhanced superheat control efiect from concurrent gas recirculation and modification of the combustion zone position results from changes in heat transfer conditions in the furnace. Changes in average gas velocity and changes in the gas flow path length in the upper portion of the furnace alter the heat transfer exposure time period of gases flowing through the upper portion of the furnace. A modification of the combustion zone position away from a furnace wall subject to the covering eifect of the recirculated gas as, for example, by raising the mean level of the combustion zone from 11-1) to d-d, is also a factor in enhancing the superheat control effect.

The effect is a function of change in the ratio of heat absorbing furnace wall area below the level of the main combustion zone, to the furnace heat absorbing area above the main combustion zone.

The control apparatus, depicted by Figs. 3 and 4, is arranged to facilitate the operation of the unit in accord ance with the method of the invention. The left hand part of Fig. 3 of the drawings indicates a control system regulating the firing rate of the different rows of burners in accordance with indications of steam pressure and In the apparatus for this purpose, there is shown a steam pressure sensitive element 160 operatively associated with the pilot valve 162 by connections 164. This pilot valve and the other pilot valves to be described below may be constructed in accordance with the description of the patent to Johnson 2,054,464. The pilot valve 162 controls the pressure loading in the line 166 connected to the relay 168, preferably similar to the relay shown in the patent to Gorrie Re. 21,804. The modified loading from the relay 168 is transmitted through the line 170 and the boiler master selector valve 172 and line 174 to a pulverizer ratio controller 176. Regulated control pressure loadings are transmitted from this ratio controller through the lines 192-196 to the selector valves 197-199 and the outgoing lines 200-202 to the operators 203-205 of the separate primary air dampers (such as 150) of the separate pulverizers C, B, and A. The loading pressure in the line 174 is also transmitted to the averaging relay 210 by a line 178. This relay is preferably constructed in accordance with the disclosure of the patent to Dickey 2,098,913, and the outgoing loading effected by this relay in the line 212 is a result of the combined effects of the loading in the line 178 and the loading in the line 214 extending from the relay 190 to the relay 210. The line 214 transmits loadings effected by variations in boiler load as represented by steam flow and airflow variations which are respectively indicated by the steam flow meter 180 and the airflow meter 182. The indications of these meters are regulably combined through the operative connections 184 and are controllably transmitted through the pilot valve 186 through the line 188 to the relay 190 and then to averaging relay 210. The combined outgoing loading pressure from the. relay 210 is transmitted to the line 212 and through the

selector valves

216 and 218. From the selector valve 216 a control loading line 22:) transmits the ingoing loading to the calibrating relay 222 from which a modified outgoing loading is transmitted through the line 224 to the forced draft damper controller 226.

Fromthe relay 218 outgoing loading is transmitted by the line 228 to the diflferential relay 230 connected;

through a line 232 to the hydraulic tachometer 234 associated with the forced draft fan and by another line 236 transmitting loading to the relay 238. This relay is preferably constructed in accordance with the disclosure of the patent to Fitch 2,441,405. The outgoing loading from this relay is transmitted through the line 240 to the forced draft

fan valve operators

242 and 244 for the hydraulic coupling control valves 242' and 244. Thus, the airflow for the combustion air is controlled from the steam pressure and the steam flow-airflow ratio impulses to control the damper position and forced draft fan speed. The furnace draft is controlled directly from the furnace draft controller 250 to sequentially control the speed of the induced draft fan and control its damper position by the apparatus indicated at the right hand part of Fig. 3. This includes a relay 252, similar to

relays

168 and 190,

having an incoming loading pressure transmitted thereto.

through the line 254 from pilot valve 256 which has operative connections 258 with the draft controller 250. The outgoing loading from the relay 252 is transmitted through the lines 260-262 and then to the

selector valves

264 and 266. From the selector valve 264 outgoing loading pressure is transmitted through the line 268 as the incoming loading to the calibrating relay 270. The outgoing loading pressure from this relay is transmitted through the line 272 to the induced draft fan outlet damper control drive 274.

The regulated outgoing loading pressure from the selector valve 266 is transmitted through the line 276 to differential relay 278, connected by the line 280 to the hydraulic tachometer 282 associated with induced draft fan.

Also leading from the relay 278 is a line 284 transmitting loading to the relay 286. The modified and regulated outgoing loading from this relay is transmitted through the line 288 to the

operators

290 and 292 associated with

hydraulic coupling valves

294 and 296, the hydraulic coupling being utilized for speed variation of the induced .draft fan.

In conjunction with the described control system, the induced draft fan. may be considered as disposed in a conventional position in the gas flow paths of each of the Fig. 1 and Fig. 6 units, and the Fig. 6 unit may be con. sidered as having a forced draft fan and pertinent associated structures similar to those shown for the forced draft fan #94 of the Fig. 1 unit. Each fan, according to the pertinent control system, is subject to both speed control, and flow control through damper regulation.

The speed of each fan is regulated by fluid flow through hydraulic coupling valves such as 242' and 244 in the fan drive. Since the fan speed changes will not only change the volume of gas or air delivered, but also the gas or air pressure, the extent of air or gas flow regulation by fan speed alone is limited. Any increase or decrease in fan air or gas flow, beyond that obtainable by changes in fan speed, is obtained by damper regulation, as by the regulations of vanes in the fan inlet, as

positioned by the power piston or

damper operator

226 or 274, the latter becoming operative below the selected fan speed operating range.

In the above described control system, the selector valves such as 197, 216, and 266 are preferably constructed in accordance with the disclosure of the patent 1 For boiler loads for which the final steam temperature. would be lower than optimum, the temperature willbe.. raised by recirculating flue gases. Thecontrols for flue,

gas recirculation are indicated in Fig. 4. In this system the three controlling impulses are boiler load, as indicated by the boiler meter 300, final steam temperature, as indicated by the temperature controller 302, and recirculated gas flow as indicated by the flow meter 304. These components are also recorder-controllers. For flue gas recirculation, the boiler load impulse from the meter 300' calls for a decreasing amount of recirculated gases as boiler load increases. This change in flow of recirculated gas is etfected through adjustment of the damper 306 in the duct 84 leading to the recirculating gas fan 82. The flow meter 304 is efiective upon the damper through the pilot valve 308, the line 310, the relay 312, the line 314, selector valve 316 and the damper operator 318. The temperature controller 302 delivers control impulses (representative of changes in superheat, or final steam temperature) which are effective upon the recirculated gas damper 306 through the pilot valve 320, the line 322, the relay 324, line 326, line 328, the relay 330, the line 332, relay 312, and the previously described connections to the damper. Boiler load impulses as initiated by the boiler meter 300' are effective through the pilot valve 334 and the line 336 leading to the relay 330. At this relay, the effect of the boiler load impulses is modified by the effect of the steam temperature impulses arriving at the relay 330 through the line 328, the modified result being effective through the line 332, the relay 312 and its previously described connections with the damper 306.

The control system represented by Fig. 4 provides automatic starting and stopping the recirculation fan 82 by means of a ratio controller 340. This controller operates from a loading pressure for the spray attemperator control valve 342, this loading pressure being effective through the line 344 which leads from the loading line 346 leading to the operator 348 for the attemperator control valve. The other loading pressure operative upon the ratio controller 340 is efiective through the line 350 leading from the line 314 to the operator 318 of the recirculated gas damper 306. The contact points 352 and 354 of the ratio controller are set so as to close the circuit for the recirculated gas fan motor 356' as the loading pressure on the spray attemperator drops to a certain point, say 4 lbs., and when the loading pressure to the recirculation damper drive 318 rises to a certain point, say 3 lbs., to start the recirculation fan motor. As the boiler load increases, the loading pressure in the line 314 to the recirculation damper drive 318 decreases while the spray attemperation loading in the line 3 46 increases, thus providing a very positive closing point for the circuit for the motor 356. This controller may be adjusted so as to start the motor 356 at any desired relation between the two loading pressures, thus making it possible to start the fan motor while the attemperator is in operation, or to provide a gap between the period of operation of the attemperator and the start of the recirculation fan.

For higher boiler loads, where steam temperature would otherwise exceed the final steam temperature desired, the temperature of the steam is reduced by spray water into the attemperator 42 in such controlled quantity as to provide the proper final steam temperature. To control the amount of spray water required for maintaining the desired temperature for the higher loads, the attemperator control valve 342 is operated from the controlling impulses representing, respectively, boiler load from the meter 300, final steam temperature from the temperature controller 302, and attemperator outlet temperature. The impulses representing attemperator outlet temperature are effective through the temperature controller 302, the pilot valve 360, and the line 362 leading to the relay 364. The impulses representing final steam temperature are effective upon the relay 364 to modify the relay effect of the impulses from attemperator outlet temperature, through the pilot valve 320, the line 322, the relay 324 and the line 326, the relay 366 and the line 368. In the relay 366 the impulses represented by 8' boiler load are effective to modify the impulses represented by final steam temperature, through the boiler meter 300, the pilot valve 370 and the line 372.

The control system indicated in Fig. 4 will also involve an electrical interlock operating from the recirculation fan motor circuit so as to hold the recirculation damper drive 318 in the closed position unless the fan motor is running. In addition to this interlock, the system will involve an extra set of shut off dampers in the recirculation duct, in addition to the control damper 306. There will be a vent 406 connected with the duct 38, and a damper 402 in the outlet of the vent. Between the vent and the nozzles there will be a damper 400 so associated with the controlling mechanism that when the fan 82 is inoperative the damper 402 and the vent 406 will be open and the damper 400 open. At this time the damper 306 is closed. The provision of the open vent under these conditions prevents the flow of hot combustion gases from the furnace directly to the inlet 86 of the gasrecirculating system.

With the described arrangement of boiler unit, regulable fuel supply means and control apparatus, the vapor generator unit may be operated through a wide load range with the attainment of a substantially constant degree of steam temperature. The method of operation will be according to the procedure hereinafter described.

With the unit in operation at a load of or near to of capacity, fuel will be delivered to the three rows of burners from their respective pulverizers A, B and C. The regulation of the rate of fuel burning will be controlled by the automatic manipulation of the primary air supply to the individual pulverizers from vapor pressure indications as a steam demand index.

When the vapor demand falls and the vapor pressure tends to increase, the fuel supplied to the burners will be automatically decreased. As the load is progressively decreased the heat absorbed by the superheater will decrease and when the load is decreased below the control point corresponding to 1000 F. final steam temperature and about 79% load, as indicated by Fig. 5, the delivered vapor temperature would be below optimum except for the modification of operation which will be automatically initiated by the control apparatus.

When the load decreases from the control point, the steam temperature controller 302 will initiate the operation of the recirculating fan and cause the introduction of recirculated gas to bring about a modification of furnace wall heat absorption to an extent leaving sufiicient heat in the gases passing over the superheater so that the optimum temperature will be attained.

With still further reduction in steam demand or load the automatic combustion control will reduce fuel delivery and the amount of gas recirculated will be automatically increased in accordance with steam temperature requirements. At some intermediate load, the value of which may be observed by the operator from boiler load controller 300, the rate of gas recirculation will have increased to a predetermined maximum value which will be indicated by flow meter and controller 304. With these indications, the operator will modify the operation upon a further reduction of load, by manipulating selector valve 199 to cause controller 205 associated with damper in the primary air supply duct of pulverizer A, to be taken out of control. Controller 205 will then automatically close damper 150 and the fuel delivery from pulverizer A to the lower row of burners 66 will cease.

With elimination of the supply from pulverizer A, the delivery of fuel from the remaining pulverizers B and C will be corespondingly increased in order to maintain the steam pressure. However, as the mean level of the main combustion zone will be raised from a level approximating line b--b (when all burners are in operation) to one approximating line d--d as indicated in Fig. 1, the augmented superheat modifying efiect previously described would give a vapor temperature in excess of opti-.

mum if the amount of gas recirculation used with the. prior three pulverizer and three burner level operation were continued. The line d-d is intended to indicate the mean level of the main combustion zone with only the top two rows of burners in operation. The line has similar significance with only the topmost row of burners in operation, and the lines bb and cc similarly apply to their associated rows of burners.

The vapor temperature controller 302 will sense this tendency for the vapor temperature to increase and through its control connections will automatically reduce the quantity of the recirculated gas to bring the vapor temperature to the optimum value.

When the load is increased to a value where it is considered desirable to operate three pulverizers and the three coiresponding rows of burners, the operator will start the idle pulverizer such as A, and then when it is at operative speed and delivering fuel subject to manual control, make it subject to regulation by the automatic control system through manipulation of selector 199 whereby the control pressure of line 196 is transmitted to line 202.

In Fig. 6 of the drawings the invention is illustrated as embodied in a steam generating and superheating unit having a vertically elongated furnace chamber 500 of rectangular cross section, the front and

rear walls

502 and 504 of which are delineated by upright

steam generating tubes

506 and 508 respectively. The opposite side Walls have similar Wall tubes directly connecting top and

bottom headers

510 and 512 respectively. The portions of the front and side wall tubes at the burner level are of refractory covered stud construction. All of the wall tubes are connected into the circulation system which includes an upper steam and water drum 514 and a lower water drum 516, connected by

banks

518 and 520 of steam generating tubes. The

tubes

506 and 508 discharge into the drum 514 and have

headers

522 and 524 respectively at their lower ends, connected in a well known manner to the drum. In the path of the gases flowing from the furnace are groups of bent screen tubes 526 forming continuations of the rear wall tubes 508. A convection heated pendant type superheater, formed by two banks of tubes 528 and 530, is positioned in the gas flow path between the screen tubes and the vapor generating bank 518, the screen tubes being arranged to screen the superheater tubes from furnace radiation. In this instance the superheater tubes receive heat from the gases by radiant and convection transmission from the stream of heating gases, but the convection transmission is predominant because of the small lateral spacing of the tubes and the relative temperature of the gases. If

'the superheater were located in a higher gas temperature zone and with a greater lateral spacing, the proportion of heat transmitted by radiation as compared with convection transmission would be increased. The steam flows from the steam space of the drum 514 through tubes 532 to the bank of tubes 530 and intermediate header 533 and thence through the bank of tubes 528 to the superheater outlet header 534.

The furnace chamber 500 is tired by two rows of horizontally arranged

burners

536 and 538 disposed at different levels, and directing burning fuel and air in mixing relationship through corresponding burner ports 537 and 539 respectively in the front wall 502 into the furnace chamber. The burners are shown as fired by pulverized fuel delivered from pulverizers 535 in suspension in primary combustion air through

burner supply pipes

540 and 542 respectively. The pulverizers are preferably equipped with valve mechanisms such as those shown at 6063 in Fig. 2 of the patent to Schwartz 2,275,595;

March 10, 1942, for shutting off the fuel flow in selected fuel conduits leading to the burners. Heated secondary combustion air is supplied under a positive pressure from a windbox 544 enclosing the burner ports 537, 539.

The furnace illustrated is of the water cooled hoppers bottom type with the rear furnace wall 504 and the rear.

sloping hopper wall 548 having the tubes 508 arranged in a tube-to-tube construction. Between the rear wall 548 and the sloping front wall 552 of the hopper 550, there is a throat opening 554 through which the incombustible solid residues of combustion pass into a closed ash pit 557 having

opposite walls

556 and 558.

The pulverized

fuel burners

536 and 538 are high capacity burners of the short flame turbulent type preferably having secondary control means such as the vanes 44 shown by the patent to Poole 2,380,463, July 31, 1945, for shutting off the flow of secondary air when a burner is taken out of operation. The burners are arranged to discharge pulverized fuel and combustion air in intimate mixing relationship into the furnace through the corresponding front wall burner ports. The mixtures of fuel and air are ignited immediately on entering the furnace and the burning fuel and air mixtures move toward the rear wall of the furnace at a substantial velocity, but the front to back dimension of the furnace is so designed that, even at maximum high velocity introduction of fuel and air, the combustion products will not impinge to any substantial extent upon the rear furnace wall 504.

While the burning fuel and air mixtures expand greatly on ignition, their velocity of introduction is not quickly dissipated so that the center section of the furnace, i. e. the main combustion zone, may be considered to be filled with an expanding stream of burning fuel, air and products of combustion at a substantial velocity which, under the influence of the furnace draft, turns upwardly towards the furnace gas exit 545 at the level of the water drum 516.

With the walls of the furnace defined by rows of steam generating tubes absorbing heat radiantly transmitted thereto from the burning fuel and heating gases generated, it is necessary to position the

burners

536 and 538 at elevations sufliciently below the furnace gas exit 545 so that the gases will be cooled to such a degree that slagging of the screen tubes 526 and superheater tubes 528 and 530 will not result.

The burners are also positioned a suflrcient distance above the water cooled hopper bottom 550 so that molten ash particles separating from the burning fuel stream and dropping to the bottom will pass through a lower temperature gas zone below the active combustion Zone intermediate the height of the furnace, and be chilled to a solidifying temperature for removal in a dry condition. While the main stream of high temperature gaseous products of combustion is developed above this lower furnace zone, the gas stream does not flow directly through it, the hopper space being filled with lower velocity gas eddy currents. The hopper gases however do transmit considerable heat which is absorbed by the steam generating tubes lining the hopper walls and the vertical walls below the level of the combustion zone.

The heating gas stream leaving the furnace outlet 545 flows across the screen tubes 526 and superheater tube banks and through the baflled

boiler tube banks

518 and 520. The gases leave the boiler bank 520 at the upper portion thereof and the major portion of the gases pass, as indicated by

arrows

562 and 564, to a regenerative type air heater 560 from which they pass to draft inducing apparatus, such as an induced draft fan and/ or stack (not shown), for discharge to the atmosphere. The induced draft fan connections would involve gas flow control vanes or dampers as indicated in the preceding description of the control system.

A gas recirculation system, operable in accordance with the invention to return a portion of the inert gases leaving the boiler bank 520, is provided by a vertical gas duct 563, recirculating fan 566, and serially connected

discharge duets

568, 569 and 570, the duct 570 extending along and substantially throughout one side of the hopper throat i554 and havingdischarge flow passages between waist the spaced lower ends 574 and 576 of the tubes 508 into the throat 554. Duct 568 contains an automatically operable control and shut-off damper 590. Duct 568 is also provided with a port 584 in one side thereof between the damper 590 and furnace end. A resiliently loaded damper 586 of any suitable type is positioned in this port as an automatic atmospheric communication means.

In accordance with the present invention, the tendency of the convection superheater to eifect a decrease in superheat temperatures with decreases in load is, at least, partially offset by variable operation of the inert gas recirculation provisions described. At fractional loads either row of fuel burners may be used, although for the lowest operating loads operation of the upper row of burners 536 alone is preferred from the standpoint of superheat control. When the burners of each separate row are fired by fuel from a separate pulverizer, the burners of one row may be rendered inoperative by shutting down the pertinent pulverizer.

With the lower row fuel burners 538 alone in operation and the boiler delivering heating gases to the air heater (and with the mean level of the main combustion zone at approximately the level bb'), the recirculating fan 566 may be operated to withdraw a portion of the gases from the boiler gas outlet, as indicated by the arrow 565, and discharge them through the

discharge ducts

568, 569, 570 when the damper 590 is in an open position. The recirculated inert gases are delivered by the fan to the furnace hopper throat, entering the throat with only sufficient velocity to insure uniform distribution along the length thereof. The low velocity recirculated gases move upwardly in the hopper and suppress the tendency of eddy currents of freshly developed high temperature combustion gases to circulate within the hopper zone. The continuous introduction of the cooler inert gases modifies the character of the flow path of the stream of high temperature products of combustion generated by the newly burned fuel, as the gas stream flows towards the furnace gas outlet.

The illustrated steam boiler installation has a maximum steam delivery rate of 160,000 lb. of steam per hour at a superheat temperature of the order of 900 F. and a pressure of 660 p. s. i. Tests carried out in the operation of this installation have indicated that at a steam delivery rate of 80,000 lb. of steam per hr., i. e. 50% of full load, the superheated steam temperature with only the lower row of burners 538 in service and in the customary manner without gas recirculation is only 750 F. Tests have shown that by gas recirculation in accordance with the invention, the superheat steam temperature can be raised to 900 F. by use of a recirculation rate of approximately 28% of the weight of the gases developed by the combustion of the fuel, when the lower burners alone are in service and at the same operating load.

When the three upper burners 536 only were used, for a corresponding operating load, the superheated steam temperature without gas recirculation was 810 F. It was found that a lower rate of gas recirculation than that necessary with the lower row of burners alone would readily raise the superheated steam temperature to 900 F.

The lines aa, b'-b' and c-c', indicate the approximate levels of the main combustion zone under three conditions; viz: when only the upper burners are in operation; when only the lower burners are in operation; and when all burners are in operation.

Operation of the described unit with and without gas recirculation at low operating loads has proven that the method of the invention is effective in attaining the desired superheat temperature at a very low steam delivery rate, with either row of burners in operation. Under even these low load conditions the flame is stable and flame temperature determinations by customary measurement methods show no significant change in flame temperatures due to the use of recirculated gases.

The resiliently loaded damper 586 arranged in port 584 in the side wall of the duct 568 at a position between the damper 590 and the furnace discharge openings from the discharge duct 570 is advantageous in maintaining satisfactory conditions within the gas recirculating system when it is not in use, as for example, at high load rates of operation when the desired superheat temperature is attainable without gas recirculation. Under such conditions when fan 566 is not operated and damper 590 is in a closed position for the purpose of avoiding any reverse flow of gases through

ducts

570, 569, 568, and 563, the absolute pressure on the fan side of damper 590 will correspond to that at the position where duct 563 is connected to the gas outlet from boiler bank 520. The resiliently loaded damper 586 is constructed so that when the pressure in the duct is above a predetermined pressure relative to the pressure in the throat of the furnace hopper, it will be held in a closed position. When the pressure at the damper 586 corresponds to or is below that of the predetermined pressure, atmospheric pressure will overcome the damper loading, opening the damper so that atmospheric air will be drawn through the port 584 into duct 568. This automatic introduction of relatively low temperature atmospheric air will cause a flow of cool air rather than gases from the furnace throat back through duct 568, fan 566, and duct 563 in case the damper 590 should not be gas tight in its closed position.

Variation in the rate of delivery of recirculated gases is accomplished by adjustment of the position of damper 590. This may be done manually by the operator in accordance with indications of superheated steam outlet temperature. However, a damper operating device can be adapted for connection to automatic steam temperature measurement apparatus (not shown), so that the rate of gas recirculation may automatically be regulated in accordanee with steam superheat temperature. As the superheat temperature tends to fall with decrease in load, the damper would thus be automatically adjusted to permit more gas to be recirculated, and vice versa.

While the increase of steam superheat temperature by means of gas recirculation to the furnace is of great value for superheat temperature control in connection with the normal operating load range of the boiler unit, the invention is also of particular advantage in starting up boiler units where it is desired to fire the unit and deliver a relatively low rate of steam at a predetermined minimum temperature. Inasmuch as the introduction of recirculated gases in the manner of the invention does not affect burner operation, it is possible to use the method disclosed at a very low delivery rate from the unit. A low steam delivery rate is all that is usually required to turn over and gradually bring up to speed the prime mover served by the boiler. When a non-operating prime mover, as, for example, a steam turbine, has been retained at high temperature, it is undesirable to introduce steam of low superheat to bring the turbine up to speed. The use of the invention permits the delivery of steam at a relatively high temperature even though the delivery rate may be as low as 5 to 20% of the full load steam delivery rate of the unit.

The area of the gas introduction ports as determined by their length and the width of the intertube spaces is such that the gas delivered through the ports enters the furnace at relatively low velocity in non-mixing relationship as regards relative velocities with the gases developed by direct fuel combustion. The recirculated gases so introduced will spread through the hopper, maintaining strata of relatively cool gases between the main combustion zone and the hopper walls, while the gases from the hopper zone as heated up will pass upward and through the furnace.

While the invention has been illustrated as used with 13 pulverized fuel burners, itisvalso adapted for use with other fluid fuel burners, such as oil or gas burners.

The disclosed introduction of a stream or streams of low temperature inert recirculated gas into the furnace in a manner so as to avoid mixing of the streams with either the secondary combustion air or with the mixture of fuel and air delivered through the burner ports, is particularly important from the standpoint of efiicient and stable burner performance. Dilution of the combustion air with inert recirculated gases and thereby retardation of combustion and lengthening of the flame is avoided. Retardation of combustion and lengthening of flame in a furnace normally result in a lower combustion efiiciency due to a higher unconsumed carbon loss, particularly in the fly ash.

It is within the purview of the invention that the modified unit shown in Fig. 6 is subject toa control system similar to the previously described system, including the control components shown in Figs. 1, 3, and 4.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form of my invention now known to me, those skilled in the art will understand that the changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantagewithout a corresponding use of other features.

I claim:

1. The method of maintaining a predetermined temperature of superheated steam delivered from a steam generator having a vertically elongated furnace with tubular water flow elements in the lateral walls and throughout the major extent of its height for receiving heat and generating steam, convection steam superheating elements located beyond the furnace gas outlet from the upper end of the furnace and in the path of heating gases flowing therefrom and fuel burning means arranged and constructed to deliver fluid fuel and combustion air to a combustion zone at different levels intermediate the height of the furnace, the method comprising the steps of; burning the preponderant portion of fuel in suspension in a combustion zone having a mean elevation intermediate the height of the furnace to produce high temperature heating gases, imparting heat from the gases to water as the gases flow upwardly in an elongated flow path to generate steam to meet a high load steam demand and to partially cool the gases, passing the gases in a flow path in heat transfer relationship with steam superheating elements to superheat steam and to further cool the gases, variably introducing a recirculated portion of the partially cooled gases from a position in the gas flow path downstream of the steam superheating heat transfer zone to a position within the furnace substantially below the mean elevation of said combustion zone to reduce heat transfer to the water surrounding the furnace zone subjacent the combustion zone, increasingthe rate of said recirculated gas introduction as load decreases, and as the steam demand is decreased decreasing the fuel burning rate in a combustion zone the mean elevation of which is lower than the mean elevation of the combustion zone of the higher load and less remote from the point of recirculated gas introduction, whereby the vertical extent of the furnace zone subjacent the combustion zone is increased to effect a reduction in heat transfer to the surrounding water tubes and whereby the superheat regulation effect of the gas recirculation is enhanced.

2. The method of maintaining a predetermined superheated vapor temperature in a vapor generator having a furnace with its boundaries substantially defined by tubular heat absorbing elements and having a convection superheater located beyond the furnace outlet and heated by gases therefrom, the method comprising; the burning of fuel in suspension at a first furnace position remote fromsaid outlet in a gas fiowdsense and impartingheat 1-.

the superheater; and controlling superheat under changing load by. varying the heat absorbed by said elements by concurrently controllably varying the amount of fuel burned at .the first position and variably introducing into the furnace cooled gases from a position gas flow-wise downstream of said furnace outlet, the varying of the fuel burning at said first position involving a decrease in the fuel burning rate as vapor demand is decreased and the variable introduction of cooled gases involving an increase in said introduction as the vapor demand decreases.

3. In a vapor generating and superheating installation, a vapor generating section including a furnace with vapor generating tubes generally defining the furnace combustion chamber, a superheater heated by the gases from the furnace, separately controllable first and second fuel burning means firing the furnace at different furnace positions successively more remote from the furnace gas outlet in a gas flow sense, and superheat control means including a gas recirculation system ductwork having a gas inlet communicating with furnace gas flow beyond said outlet, said ductwork having an outlet communicating with the furnace; said superheat control means including first means regulating the amountof gas recirculated by said system in accord with indications of superheat temperature and rate of vapor generation,'the first regulating means increasing the recirculated gas flow when the indications of superheat temperature and vapor generation rate decrease in value, said superheat control means also including second regulating means effective from vapor pressure indications and indications representative of rate of vapor generation to vary the rate of firing of the second fuel firing means to maintain superheated vapor temperature at a predetermined value at low rates of vapor generation, said second regulating means increasing the firing rate of the second fuel firing means when the vapor pressure decreases and when the rate of vapor generation is to be increased.

4. In a vapor generating and superheating installation, a vapor generating section including upright vapor generating tubes generally defining a furnace combustion chamber in which vapor generation results mainly from the radiant transmission of heat from combustion elements to the tubes and the vaporizable fluid therein, means forming a combustion chamber gas outlet and a convection gas pass leading therefrom, a convection section including a convection superheater in the gas pass, fuel burning means adapted to effect the mean level of the main combustion zone at different vertically spaced positions under different loads, a gas recirculation system introducing combustion gases from a position beyond the combustion gas entrance to the superheater to a position in the combustion chamber below the fuel burning means, means increasing the flow of recirculated gases when the mean level of the main combustion zone rises, and means controlling the firing means to decrease the total firing rate and elevate, I

generator having a vertically elongated furnace With tubular water. flow elements in its lateral walls and v 7 throughout the major extent of its height for receiving heat and generating steam, convection steam superheating elements located beyond the furnace gas outlet from the upper end of the furnace and in the path of heating gases flowing therefrom, and fuel burning means arranged and constructed to deliver fluid fuel and combustion air to a combustion zone at different levels intermediate the height of the furnace; the method comprising the steps of; burning the preponderant portion of fuel in suspension in a combustion zone having a mean elevation intermediate the height of the furnace to produce high temperature heating gases, imparting heat predominantly by radiation from the gases to water as the gases flow upwardly in an elongated flow path to generate steam to meet a high load steam demand and to partially cool the gases, passing the gases in a flow path in convection heat transfer relationship with steam superheating elements to superheat steam and thereby further cooling the gases, variably introducing a recirculated portion of the partially cooled gases from a position in the gas flow path downstream of the steam superheating heat transfer zone to a position within the furnace substantially below said combustion zone to reduce the radiant heat transfer to the confined water streams surrounding the furnace zone subjacent the combustion zone, increasing the rate of said recirculated gas introduction and thereby decreasing radiant heat transfer for steam generation and increasing the convection superheating effect by increased gas mass flow as load decreases, and, simultaneously, as the steam demand is decreased, decreasing the fuel burning rate in a combustion zone the mean elevation :of which is lower than the mean elevation of the combustion zone of the higher load and less remote from the point of recirculated gas introduction, whereby the vertical extent and volume of the furnace zone subjacent the combustion zone is increased to effect a reduction in radiant heat transfer to the surrounding water tubes and whereby the superheat regulation effect of the gas recirculation is enhanced.

6. The method of maintaining a predetermined superheated vapor temperature in a vapor generator having a furnace with its boundaries substantially defined by tubular heat absorbing vapor generating elements conducting confined streams of a vaporizable liquid, the vapor generator also having a convection vapor superheater located beyond the furnace outlet and convectionally heated by gases therefrom, the method comprising: the burning of fuel in suspension in a first furnace zone remote from said outlet in a gas flow sense and imparting heat predominantly by radiation from the combustion gases to portions of said confined streams located in an adjacent portion of the furnace; simultaneously burning fuel in suspension in a second furnace zone less remote from said outlet than said first zone; imparting heat from the combustion gases produced in the first zone and from the gases produced by fuel burning in the second zone to the portions of the confined streams located adjacent the second zone; convectionally superheating the generated vapor by passing the combined gases from the furnace through the superheating zone, and controlling superheat under changing load by varying the heat absorbed by said confined streams by concurrently controllably varying the amount of fuel burned in the first zone and variably introducing into the furnace cooled gases withdrawn from a position gas flow-wise downstream of the superheating zone, the varying of the fuel burned in said first zone involving a decrease in the fuel burning rate as vapor demand is decreased and the variable introduction of cooled gases involving an increase in said introduction as the vapor demand decreases.

7. The method of maintaining a predetermined superheated vapor temperature in a vapor generator having a furnace with its boundaries substantially defined by tubular heat absorbing vapor generating elements conducting confined streams of a vaporizable liquid, the vapor generator also having a convection superheater located beyond the furnace outlet and heated by gases therefrom, the method comprising: the burning of fuel in suspension in a first furnace zone remote from said outlet in a gas fiow sense and imparting heat predominantly by radiation from the combustion gases to portions of said confined streams located in an adjacent portion of the furnace; simultaneously burning fuel in suspension in a second furnace zone less remote from said outlet than said first zone; imparting heat from the gases produced in the first zone and from the gases produced by fuel burning in the second zone to the portions of the confined streams located adjacent the second zone; convectionally superheating the generated vapor by passing the combined gases from the furnace through the superheating zone; and controlling superheat under changing load by varying the heat absorbed by said confined streams by concurrently controllably reversely varying the amount of fuel burned in the first zone and variably introducing into the furnace cooled gases withdrawn from a position gas flowwise downstream of the superheating zone; the introduction of the cooled gases into the furnace being effected at a position more remote from the superheating zone than at least one of the zones of fuel burning; the varying of the fuel burned in said first zone involving a decrease in the fuel burning rate as vapor demand is decreased and the variable introduction of cooled gases involving an increase in said introduction as the vapor demand decreases.

8. The method of maintaining a predetermined superheated vapor temperature in a vapor generator having a furnace with its boundaries substantially defined by tubular heat absorbing vapor generating elements conducting confined streams of a vaporizable liquid, the vapor generator also having a convection superheater located beyond the furnace outlet and heated by gases therefrom, the method comprising: the burning of fuel in suspension in a first furnace zone remote fromsaid outlet in a gas flow sense and imparting heat predominantly by radiation from the combustion gases to portions of said confined streams located in an adjacent portion of the furnace; simultaneously burning fuel in suspension in a second furnace zone less remote from said outlet than said first zone; imparting heat from the gases produced in the first zone and from the gases produced by fuel burning in the second zone to the portions of the confined streams located adjacent the second zone; convectionally superheating the generated vapor by passing the combined gases from the furnace through the superheating zone; and controlling superheat under changing load by varying the heat absorbed by said confined streams by concurrently controllably reversely varying the amount of fuel burned in the first zone and variably introducing into the furnace cooled gases withdrawn from a position gas flow-wise downstream of the superheating zone; the introduction of the cooled gases into the furnace being effected at a position more remote from the superheating zone than the zones of fuel burning; the varying of the fuel burned in said first zone involving a decrease in the fuel burning rate as vapor demand is decreased and the variable introduction of cooled gases involving an increase in said introduction as the vapor demand decreases.

9. In a vapor generating and superheating installation, a vapor generating section including upright vapor generating tubes generally defining a furnace combustion chamber in which vapor generation results mainly from the radiant transmission of heat from combustion elements to the tubes and the vaporizable fluid therein, means forming a combustion chambergas outlet and a convection gas pass leading therefrom, a convection section including a convection superheater in the gas pass, fuel burning means adapted to effect the mean level of the main combustion zone at different positions, a gas recirculation system including a fan and ductwork introducing combustion gases from a position beyond the combustion gas entrance to the superheater to a position in the combustion chamber below the fuel burning means and more remote from the combustion chamber gas outlet, means increasing the flow of recirculated gases as the rate of vapor generation decreases and under a firing con dition involving the increasing of the mean level of the main combustion zone above its lowermost operative level, and means controlling the firing means to decrease the total firing rate and elevate the mean level of the main combustion toward its highest level as the vapor demand decreases.

10. In a vapor generating and superheating installation, a vapor generating section including upright vapor generating tubes generally defining a vertically elongated furnace combustion chamber in which vapor generation results mainly from the radiant transmission of heat from combustion elements to the tubes and the vaporizable fluid therein, means forming a combustion chamber gas outlet and a convection gas pass leading therefrom, the outlet being disposed at the upper portion of the combustion chamber, a convection section including a convection superheater in the gas pass, fuel burning means adapted to effect the mean level of the main combustion zone at difierent positions, a gas recirculation system including a fan and ductwork introducing combustion gases from a position beyond the combustion gas entrance to the superheater to a position in the combustion chamber below the fuel burning means and more remote from the combustion chamber gas outlet, means increasing the flow of recirculated gases as the rate of vapor generation decreases and under a firing condition involving the increasing of the mean level of the main combustion zone above its lowermost operative level, and means controlling the firing means to decrease the total firing rate and elevate the mean level of the main combustion toward its highest level as the vapor demand decreases.

11. In a vapor generating and superheating installation, a vapor generating section including upright vapor generating tubes generally defining a vertically elongated furnace combustion chamber in which vapor generation results mainly from the radiant transmission of heat from the combustion elements to the tubes and the vaporizable 18 fluid therein, means forming a combustion chamber gas outlet and a convection gas pass leading therefrom, the outlet being disposed at the upper portion of the combustion chamber, a convection section including a convection superheater in the gas pass, fuel burning means including upper and lower rows of burners adapted to eifect the mean level of the main combustion zone at different positions, a gas recirculation system including a fan and ductwork introducing combustion gases from a position beyond the combustion gas entrance to the superheater to a position in the combustion chamber below the fuel burning means and more remote from the combustion chamber gas outlet, superheat control means responsive to operative variables including changes in superheated vapor temperature for increasing the flow of recirculated gases as the ratc of vapor generation decreases and under a firing condition involving the increasing of the mean level of the main combustion zone above its lowermost operative level, said superheat control means regulating the firing means to decrease the total firing rate and elevate the mean level of the main combustion toward its highest level as the vapor demand decreases; and means for attemperating the superheated vapor at rates of vapor generation beyond the upper limits of superheat control by the combination of the recirculated gas introduction and the concurrent variation in level of the main combustion zone.

References Cited in the file of this patent UNITED STATES PATENTS 2,100,190 Jackson Nov. 23, 1937 2,363,875 Kreisinger et al. Nov. 28, 1944 2,575,885 Mittendorf Nov. 20, 1951 2,590,712 Lacerenza Mar. 25, 1952 FOREIGN PATENTS 504,114 Great Britain Apr. 14, 1939 516,070 Great Britain Dec. 21, 1939 525,906 Great Britain Sept. 6, 1940 OTHER REFERENCES B & W Bulletin, G67-A of 1950, page 58.