US3146763A

US3146763A - Steam generating unit

Info

Publication number: US3146763A
Application number: US166008A
Authority: US
Inventors: Norman S Blodgett
Original assignee: Riley Power Inc
Current assignee: Riley Power Inc
Priority date: 1962-01-15
Filing date: 1962-01-15
Publication date: 1964-09-01
Anticipated expiration: 1981-09-01
Also published as: GB974398A

Description

Sept. 1, 1964 N. s. BLODGETT 3,146,763 STEAM GENERATING UNIT Filed Jan. 15, 1962 2 Sheets-Sheet l INVENTOR. Norman 5.BZocZ jeiT Sept. 1, 1964 N. s. BLODGETT STEAM vGENEIRATIIIG UNIT Filed Jan. 15, 1962 HIGH LOAD LOW LOAD 2 Sheets-Sheet 2 INVENTOR Norman 5. Blodqeif United States Patent 3,146,763 STEAM GENERATENG UNIT Norman S. Blodgett, Westhoro, Mass, assignor to Riley Stoker Corporation, Worcester, Mass., a corporation of Massachusetts Filed Jan. 15, 1962, Ser. No. 166,008 Claims. (Cl. 122-481) This invention relates to a steam generating unit and more particularly to apparatus arranged to produce steam for use in a steam turbine.

There are many arrangements used in connection with steam generating units to maintain the temperature of the superheated steam at a fixed value, irrespective of changes in load on the unit. For instance, the How of gases over the superheater elements has been controlled by the use of dampers; this practice has the disadvantage of being relatively insensitive and producing relatively high exit gas temperatures at certain loads so that the overall efficiency of the unit is decreased. Superheat temperature has also been controlled by de-superheaters wherein the steam generating unit produces superheated steam at the highest temperature necessary at a reduced load and the steam is cooled to produce the desired steam temperature at higher loads; this has the disadvantage of requiring extremely large and expensive superheater elements to take care of the extreme conditions. Superheat has also been controlled by recirculating a portion of the gases from the back passes of the unit to the furnace proper; this has the disadvantage of producing high dust loading of the furnace gases with resultant cutting and deterioration of the boiler tubes and the disadvantage of high power requirements. Another method of controlling superheat that has been proposed is the use of the so-called tilting burner in which situation the flame in the furnace is directed upwardly or downwardly to maintain the superheated steam at a constant value; the burners required for this practice are extremely complicated and expensive to build and maintain. Other methods and apparatus for controlling superheat have been suggested which are of lesser importance than those described above, but there is no practice which could be said to be perfect, since all previously-known methods of controlling superheat suffer from difliculties which render them less than ideal. These deficiencies in the prior art have been obviated by the present invention in a novel manner.

it is, therefore, an outstanding object of the present invention to provide an apparatus for the control of superheat in a steam generating unit, which apparatus is inexpensive to manufacture and to maintain.

Another object of the invention is the provision of an apparatus for the control of superheat whose operation is not detrimental to the overall operation of the steam generating unit.

Another object of the invention is the provision of an apparatus for the control of superheat which can be used for initial adjustment of superheat made necessary by in accuracies in design and construction and is also extremely sensitive in maintaining superheat at constant temperature, irrespective of changes in load.

A still further object of the present invention is the provision of an apparatus for the control of superheat in which the mass flow of the steam generating unit is not increased.

A still further object of the invention is the provision of a steam temperature control in which at no time does the flame impinge directly on the furnace walls and in which slagging difficulties are avoided.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

3,146,763 Patented Sept. 1, 1964 The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings in which:

FIG. 1 is a vertical sectional view of a steam generating unit embodying the principles of the present invention;

FIG. 2 is a somewhat schematic view of the unit showing the method of operation at high load; and

FIG. 3 is a schematic view of the apparatus showing the manner of operation at low load.

In the specification which follows, the expressions longitudinal, transverse, and the like refer to those directions as applied to a steam generating unit in the ordinary practice in that art and, in general, refer to the flow of gas through the unit.

Referring first to FIG. 1, wherein are best shown the general features of the invention, the steam generating unit, designated generally by the reference numeral 10, is shown in use with a furnace 12 and a boiler 13 mounted on a supporting structure 14. The furnace 12 consists of a forward Wall 1d, a rearward wall 15, and side walls 17 defining a combustion chamber 18. Underlying the combustion chamber is a slag basin 19 lined with refractory material. The forward wall 15 is provided with an abutment 21 located in its lower portion directly overlying the slag basin 1) and provided on its undersurface with a burner 22. A similar abutment 23 is provided on the rearward wall 16 and a burner 24 is mounted on the downwardly-directed surface of this abutment also. The

abutments

21 and 23 cooperate with the basin 19 to form a high-temperature cell 56 at the lower end of the combustion chamber.

The boiler 13 consists of a steam-and-water drum 25 joined by downcomer tubes 26 to lower headers 27, the steam-and-water drum 25 and the headers 27 extending transversely of the unit. Downcomer pipes 32 also extend from the steam-and-water drum 25 to headers 29 underlying the side walls 17 below the slag basin 19. Water wall tubes 28 extend upwardly along the surface of the side walls 17 from the header 29. Similarly, water wall tubes 31 extend from the headers 27 upwardly along the surfaces of the forward wall 15 and the rearward wall 16. Before reaching the side walls, however, these tubes are formed to underlie the bottom of the slag basin 19 to cool it and to form a slag tap opening 3% overlying a slag tank Zil. The water wall tubes 28 and 31 are connected at the upper part of the boiler to the steam-andwater drum 25 to release steam theretov Some of the tubes 31 on the front wall are bent rearwardly of the wall at the upper portion to form a slag screen 32, while some of the tubes 31 on the rearward wall 16 are bent forwardly in a similar manner to form a slag screen 33. The tubes forming the slag screen 32 come together into a single Vertical plane to form a baffle 34 while the tubes in the slag screen 33 are returned to a similar vertical plane to form a bafiie 35. A roof 36 extends across the upper part of the combustion chamber and considerably rearwardly thereof. A back pass 37 extends downwardly at the rear of the combustion chamber and is divided into a front portion 38 and a rear portion 39 by a generally vertical bathe 41. At the bottom of the front portion 38 and the rear portion 39 of the back pass 37 are located

dampers

42 and 43, respectively, for controlling the gas flow through these portions. The back pass is connected through an air heater 44 to a stack 4-5. A forced draft fan 46 introduces air through the air heater 44 to a duct 40, having branch ducts :7 and d8 leading to the burners 2d and 22. respectively.

As is evident in the drawing, the steam-and-water drum 25 is provided with a steam separator of the usual type; tubes 49 lead from the upper part of the steam-and-water drum to a header 51 extending longitudinally over the side walls of the back pass 37. At the bottom of these side walls is arranged another header 52 and the two are connected together by superheater tubes 53 which line the side walls of the back pass. The bottom header 52 is connected by a steam pipe 54 to a header 55 located behind the rearward wall 16 of the furnace. Located a short distance above the header 55 is a similar transverse header 56; connecting these headers are radiant superheater platens 57 arranged in longitudinal planes, each platen being of an upwardly-looped variety so that it is completely drainable into the

headers

56 and 55. The header 56 is connected by a steam pipe 58 through a desuperheater 59 to a transverse header 61 located forwardly of the forward wall 15. This header is connected to superheater platens 62 located in longitudinal vertical planes and principally made up of horizontal tubes. At their outlet ends the platens 62 are connected to a superheated steam header 63 which is connected to the high-pressure section of a steam turbine (not shown).

The output of the high-pressure section of the turbine is connected by means of a pipe 64 to a header 65 located forwardly of the forward wall 15. Immediately above the header 65 is a similar header 66 and the two are connected by radiant reheat platens 67. The reheat platens 67 are very similar in construction to the radiant superheater platens 57 in that they are composed of vertical loops which are drainable into their headers. The header 66 is connected by a steam pipe 68 to an inlet header 69 of a convection reheater '71 located in the back portion 39 of the back pass 37. At its upper end of the convection reheater 71 is connected to reheater platens 72 overlying the superheater platens 62 at the upper part of the furnace; the output of the reheater platens 72 is connected to a reheated steam header 7%) which, in turn, is connected by a suitable steam pipe 73 to the low-pressure section of the turbine.

Referring now to the burner 22, it can be seen that the air duct 48 is connected to a housing 74. Centrally of the housing is located a gun '75 for introducing a fuel such as gas. The water wall tubes 31 are bent rearwardly in the vicinity of the burner 22 to form passages for the flow of fuel and air. At the mouth of the burner below the gun 75 are located pivoted vanes 76 which are connected through a mechanical linkage to an actuating rod 77. In the upper part of the burner are located pivoted vanes 78 which are connected by a mechanical linkage to an actuating rod 79. The burner 24- is similarly provided with a fuel gun 81, lower vanes 32 whose angularity is adjustable by means of an actuating rod 83, and upper pivoted vanes 84 whose angularity is adjustable by means of an actuating rod 85.

The actuating rod 79 for the vanes 7 8 is connected to the piston rod of a hydraulic linear actuator 86, while the actuating rod 35 of the vanes 34 is connected to a similar actuator 87. The actuator 86 is connected by conduits S8 and 89 to a main control means 91, while the linear actuator 87 is connected to the controller by means of

conduits

92 and 93. The air duct 48 leading to the burner 22 is provided with a control damper 94 which is connected for pivotal action through a mechanical linkage to the piston rod of a linear actuator 95 whose piston is movable under the control of conduits 6 and 97 by which it is connected to the main control means 91. In a similar manner, the duct 47 leading to the burner 24 is provided with a pivoted control damper 93 which is connected through a linkage mechanism to the piston rod of a linear actuator 99 which is connected to the main control means 91 by means of conduits 1111 and m2. Mounted in the superheated steam header 63 is a temperatureindicating device 103 of the usual type which is connected through a line 1% to the main control means 91. A similar temperature-measuring device 105 resides in the reheated steam header 72 and is connected by a line 1% to the main control means )1. The dampers 42 associated with the front portion of the back pass are connected to a hydraulic linear actuator 167 which is opeaves superheater platens 62.

erated from the main control means 91 through lines 108 and 1&9. Similarly, the dampers 44 associated with the back portion 39 of the back pass 37 are connected to a hydraulic linear actuator 111 which is connected for operation to the main control means 91 through

lines

112 and 113. The main control means 91 is of the usual type used in temperature control applications and is provided with apparatus which is well known in the art for converting electrical signals in the

lines

104 and 106 into hydraulic flow through the

lines

38, 89, 92, $3, 95, 97, 101, and 102 leading to the hydraulic linear actuators associated with the apparatus; since the main control means is not part of the present invention in its detailed form, it is not felt that a specific description thereof is necessary adequately to describe the present invention.

The baffle 3ddefines with the front forward Wall 15 of the furnace a first pass 11 the bafile 35 defines with the rearward wall 16 of the furnace at second pass 115, and between the two baffles lies a third pass 116. The superheater platens 62 reside only in the second pass 115 and the third pass 116, while the reheat platens 72 reside only in the first pass 114- and the third pass 116. The tubes 31 lining the rearward wall 16 of the furnace extend entirely through the furnace to the roof 36 and are provided with a refractory battle 117 to a height slightly above the From the top of the refractory baffle 117 to the roof 36 these rear wall tubes are bare and provide an olftake opening 118, there being substantial spaces between the tubes in this area because of the fact that a considerable number of tubes have been removed from the backwall tubes to form the bathe 35. Between the top of the reheater platens 72 and the roof 36 is a comparatively open top pass 119. Now, each pair of tubes in the

baffles

34 and 35 are connected by a bifurcation fitting 121 to a single large tube 122. The large tube has sufficient cross-sectional area to permit Without substantial restriction the same flow of water that exists in the two small tubes with which it is connected. Each pair of tubes connected to a large tube resides on opposite sides of one of the reheater platens 72 and one of the superheater platens 62 to support these tubes and to hold them in vertical planes. These heavy tubes 122 are very well able to carry the load stresses that exists in the upper parts of the tubes. An enconornizer 123 is located at the lower part of the back pass 37 and extends not only through the front portion 38 but also the rear portion 39. Tubes from the economizer extend vertically to aid in supporting the convection reheater 71 which lies in the rear portion 3-9 of the back pass. It is of interest to note that the slag screens 32 and 33 extend across the upper ends of the radiant reheater platens 67 and the radiant superheater platens 57, respectively, to assist in supporting them and to prevent lateral sway. Furthermore, a single tube is bent outwardly from the rearward wall 16 of the furnace to extend along and protect the edge of the radiant superheater platen 57, While a similar tube 124 protects the outwardly-directed edge of each of the radiant reheater platens 67.

The operation of the apparatus will now be readily understood in view of the above description. Fuel arrives at the burners through the

fuel guns

75 and 81 and is projected into the combustion chamber 18 in the general direction of the slag basin 19. The lines of action of the fuel flow intersect at an imaginary point in the central part of the high-temperature cell 50 a short distance above the basin 19. Air enters the unit through the forced draft fan 45 and, after being preheated in the air heater 44, passes through the duct 40 into the branch ducts 47 and 48 leading to the burners 22 and 24, respectively. The air enters the burner housings 74 and flows past the pivoted

vanes

76, 78, 82 and 34 into the furnace. Some of the air is controlled by the lower vanes '76 and 82 and this air blends with the fuel coming from the fuel gun to form a mass of turbulent fuel and air in the high-temperature cell 50 which underlies the

abutments

21 and 23. The ignition is maintained by a suitable igniter in the usual way. The air which passes over the

upper vanes

78 and 84 also joins the mass of turbulent fuel and air in the high-temperature cell and takes part in the combustion process. It is the nature of this particular design of furnace that combustion takes place almost completely below the horizontal line defined by the inermost points on the

abutments

21 and 23. Gases pass from this high-temperature cell through the combustion chamber 18 and leave the chamber through the first pass 114, the second pass 115, and the third pass 116. In doing so, the gases pass over the radiant superheater platens 57 and the radiant reheater platens 67. At the same time, the gases flow over the superheater platens 62 and the reheater platens 72. Gases pass upwardly into the top pass 119 and then laterally through the offtake opening 118 into the back pass 37. According to the settings of the

dampers

42 and 43 the gas is passed downwardly through the front portion 38 over part of the economizer 123, or through the rear portion 39 over the reheater 71. While the gases are passing upwardly through the combustion chamber 18 the radiant superheater platens 57 and the radiant reheater platens 67 receive heat by radiation, as well as by convection. The gases, after leaving the back pass 37, go through the air heater 42 and up the stack 45.

Feed water enters the boiler 13 through the economizer 123 and flows into the lower part of the ste-am-and-Water drum 25. Then, the water passes downwardly through the downcomer tubes 26 to the

headers

27 and 29. Water passes upwardly from the header 29 through the side water wall tubes 28 and upwardly from the headers 27 through the front and back water wall tubes 31. In these water wall tubes the water is converted to steam, which steam passes into the steam-and-water drum 25 and is purified. Purified steam passes out of the drum through the tubes 49 into the header 51. It passes downwardly along the walls of the back pass into the lower header 52 and from there through the steam pipe 54 to the header 55 of the radiant superheater platens 57. The steam passes from the platens into the header 56 and from there through the steam pipe 58 to the header 61 of the superheater platens 62. From there the steam passes into the superheated steam header 63 (where its temperature is measured by the device 103) and then to the steam turbine. Upon returning from the high-pressure section of the turbine the steam enters the boiler through the steam pipe 64 connected to the header 65 at the bottom of the radiant reheater platens 67. From there the steam passes through the platens into the header 66 which is connected by a steam pipe 68 to a header 69. The steam passes from the header 69 through thereheater 71 and from there to the reheater platen 72 at the upper part of the furnace. The steam passes from these platens to the reheater steam header 70 (where its temperature is measured by the device 105) and from there through a steam pipe 73 to the low-pressure section of the turbine.

Now, it is well-known from the patent of Miller, No. 2,947,289, that it is possible by manipulation of the

vanes

78 and 84 of the burners and the dampers 94 and 98 associated with the ducts 47 and 48 leading to the burners to cause the flame (that is to say, the line of the greatest mass flow) to occupy positions in the furnace either close to the forward wall 15, close to the rearward wall 16, or any intermediate position in the combustion chamber 18. Referring to FIGS. 2 and 3, which show the situation with the flame 11 occupying either the forward or rearward positions, respectively, it is possible to see the manner in which the manipulation of the flame in this way can be used to maintain reheat and superheat at constant predetermined values despite changes in load on the unit. At high load, as shown in FIG. 2, for instance, it is necessary to maintain the heat transfer to the reheated steam at a high value, while that heat transfer to the superheated steam should be reduced. For that reason, the flame 11 is directed toward the front wall 15, by the main control means 91 acting through the various lines to operate the hydraulic linear actuators to adjust the vanes 94 and 98 or the

vanes

78 and 84 of the burners. For instance, the vanes 84 may be raised and the vanes 7 8 lowered to cause the flame to move toward the forward wall 15. At that time, the greatest mass flow of the products of combustion pass over the radiant reheat platens 67 transferring heat to them not only by radiation but also by convection. The gases then flow to a predominant extent through the first and forward pass 114 upwardly passing over a considerable portion of the reheater platens 72 before passing into the top pass 119. From there the gases move under the roof 36 of the unit, moving rearwardly into the offitake opening 118 into the back pass. At high load, the

dampers

42 and 43 will be set so that a great deal of the flow is downwardly through the rear portion 39 of the back pass so that the reheater 71 receives considerable heat. Since the gas is considerably cooled by the reheater 71, it is not necessary to have a great deal of surface of the economizer 123 exist in the rear portion 39. Naturally, some of the gases will pass up the rear part of the furnace "but the emphasis will be on heat transfer to the reheater portions of the boiler and reduction of transfer to the superheater portions to maintain these temperatures constant despite changes in mass flow due to change of load.

Now, referring to FIG. 3, which shows the low load situation, we find that the reheat portion of the boiler may be neglected but it is necessary to increase the heat exchange into the superheated steam to maintain it at its pre-determined value. For that reason, the control means 91 manipulates the

vanes

78 and 84 or the dampers 94 and 98 in such a way as to cause the flame 11 to move up the rear wall 16. This maybe done, for instance, by causing the vanes 78 to tilt upwardly and the vanes 84 to be directed downwardly. The predominant portion of the mass flow, therefore, moves upwardly along the rearward wall 16 of the furnace over the radiant superheater platens 57. The gas then passes through the second pass 115 adjacent the rearward wall 16 and over the superheater platens 62 which reside in that pass. The gasses then pass through the oiftake opening 118 into the back pass; now, the

dampers

42 and 43 are set to promote fiow of gas down the front portion 38 of the back pass, since the heat transfer to the reheater portions of the boiler must be diminished. The gases, therefore, pass downwardly through the front portion 38 and considerable surface of the economizer 123 must exist in this pass to cool the gases and maintain furnace efliciency.

Naturally, at intermediate loads, the flame may pass up the center of the furnace in which case a considerable portion of the mass flow will pass through the third or center pass 116. In that case, the convection heat transfer to the superheater platen 62 and the reheater platens 72 takes place more or less equally. At the same time, the heat radiation to the superheater platens 57 and the reheater platens 67 is more less equal. Furthermore, the

dampers

42 and 43 will be set to promote exactly the right amount of convection heat transfer to the reheater 71.

While these operations are taking place, the water tubes 123 along the edges of the radiant superheater platens 57 and the water tubes 124 along the edges of the radiant reheater platens 67 protect the outer steam carrying tubes of these platens against destruction because of extreme metal temperatures.

Screens

32 and 33 not only serve to support and prevent the radiant superheaters and reheaters from swaying but also prevent slag from passing into the upper passes of the boiler. It will be understood that the tubes in the

baflles

34 and 35 are not on tangential spacing, so there will be a certain amount of gas mixing from one pass to the other in this area, but the effect is to substantially limit the gas flow to the first pass 114, the second pass 115 and the third pass 116. Because of the bifurcated fittings 121 and the peculiar arrangement of connections of small tubes to large tubes 122, the longitudinal movement of gas through the top pass 119 of the furnace will not be inhibited; the spaces between the large tubes 122 will be considerable and will not restrict gas flow. It should be noted that the upper edge of the back pass bafiie 41 is considerably higher than the upper edge of the refractory baffie 117 at the top of the rearward wall 16 of the furnace, so that when the

dampers

42 and 43 are in their open position the tendency will be for the gas fiow to be divided more or less evenly between the front portion 38 and the rear portion 39. Also, it will be understood that the main control means 91 will operate through the line at to regulate the degree of desuperheating taking place in the desuperheater 59. The main control means 91 will also operate through conduits to regulate the settings of the

dampers

42 and 43 at the lower end of the back pass 37.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope of the claims.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent 1. A steam generating unit, comprising vertical walls defining a vertically-elongated combustion chamber, a fuel burning apparatus located in the lower part of the combustion chamber, flame positioning means associated with the fuel burning apparatus selectively to cause the main stream of the products of combustion to flow adjacent a first wall or adjacent a second wall which lies in opposition to the first wall, two vertical baflies lying in the upper part of the chamber parallel to and between the said first and second walls and serving to divide the upper part of the chamber into a first pass lying adjacent the first wall, a second pass lying adjacent the second wall, and a third pass lying between the other two passes, a reheater having a substantial portion lying in the first pass and the third pass, a superheater having a substantial portion lying in the second pass and the third pass, and a main control operating the said flame positioning means to move the main stream to positions from adjacent the ([1: first wall to adjacent the second wall with change in load to assist in maintaining the temperatures of reheat and superheat at predetermined values.

2. A steam generating unit as set forth in claim 1, wherein the lower part of the chamber is provided with opposed abutments defining a high temperature cell, the fuel burning apparatus consisting of directional flame burners mounted on downwardly-directed surfaces of the abutments.

3. A steam generating unit as set forth in claim 1, wherein the said superheater and reheater are predominantly convective in nature, radiant superheater platens are provided adjacent the second wall underlying the second pass, and radiant reheater platens are provided adjacent the first wall and underlying the first pass.

4. A steam generating unit as set forth in claim 1, wherein a back pass extends parallel to the chamber and is connected at its upper end to receive gas flow from the three passes, a bafile extending generally vertically through the back pass parallel to the said two bafiies and dividing the back pass into a front and a rear portion, a convection superheater lying in the rear portion, an economizer lying in both portions, and dampers underlying the back pass and controlling the division of flow of gas through the said portions.

5. A steam generating unit as set forth in claim 1, wherein the superheater and the reheater are in the form of a series of vertical platens and each of the baflies consists of a number of large tubes arranged in a row and extending downwardly at the top of the furnace, each of the larger tubes being joined at its lower end to a pair of small tubes which lie on opposite sides of one of the said vertical platens, the small tubes extending downwardly to a level below the passes where they extend laterally to form slag screens underlying the passes.

References Cited in the file of this patent UNITED STATES PATENTS 2,947,289 Miller Aug. 2, 1960 2,985,153 Dickey May 23, 1961 3,033,177 Koch et al May 8, 1962 FOREIGN PATENTS 758,230 Great Britain Oct. 3, 1956

Claims

1. A STEAM GENERATING UNIT, COMPRISING VERTICAL WALLS DEFINING A VERTICALLY-ELONGATED COMBUSTION CHAMBER, A FUEL BURNING APPARATUS LOCATED IN THE LOWER PART OF THE COMBUSTION CHAMBER, FLAME POSITIONING MEANS ASSOCIATED WITH THE FUEL BURNING APPARATUS SELECTIVELY TO CAUSE THE MAIN STREAM OF THE PRODUCTS OF COMBUSTION TO FLOW ADJACENT A FIRST WALL OR ADJACENT A SECOND WALL WHICH LIES IN OPPOSITION TO THE FIRST WALL, TWO VERTICAL BAFFLES LYING IN THE UPPER PART OF THE CHAMBER PARALLEL TO AND BETWEEN THE SAID FIRST AND SECOND WALLS AND SERVING TO DIVIDE THE UPPER PART OF THE CHAMBER INTO A FIRST PASS LYING ADJACENT THE FIRST WALL, A SECOND PASS LYING ADJACENT THE SECOND WALL, AND A THIRD PASS LYING BETWEEN THE OTHER TWO PASSES, A REHEATER HAVING A SUBSTANTIAL PORTION LYING IN THE FIRST PASS AND THE THIRD PASS, A SUPERHEATER HAVING A SUBSTANTIAL PORTION LYING IN THE SECOND PASS AND THE THIRD PASS, AND A MAIN CONTROL OPERATING THE SAID FLAME POSITIONING MEANS TO MOVE THE MAIN STREAM TO POSITIONS FROM ADJACENT THE FIRST WALL TO ADJACENT THE SECOND WALL WITH CHANGE IN LOAD TO ASSIST IN MAINTAINING THE TEMPERATURES OF REHEAT AND SUPERHEAT AT PREDETERMINED VALUES.