US2851015A - Method of vapor generation and vapor superheating, and apparatus therefor - Google Patents

Description

Sept. 9, 1958 P. H. KOCH 2,851,015

METHOD OF VAPOR GENERATION AND VAPOR SUPERHEATING, AND

' APPARATUS THEREFOR Filed Dec. s, 1955- 5 Sheets-Sheet 1 INVENTOR PAUL h. K00.

Sept. 9, 1958 P. H. KOCH 2,851,015

METHOD OF VAPOR GENERATION AND.VAPOR SUPERHEATING, AND APPARATUS THEREFOR Filed Dec. 3, 1953 5 Sheets-Sheet 2 INVENTOR PA M H Kocw i BY ATTORNEY Sept. 9, 1958 P. H. KOCH v2,851,015

METHOD OF VAPOR GENERATION AND VAPOR SUPERHEATING, AND

APPARATUS THEREFOR Filed Dec. :5, 1953 s Sheets-Sheet 3 & III N:

. I :3 1 I (Q'- I vC) I o O I O O O 6 1 I\\0 /9 OOOOOOKP/ Sept. 9, 1958 P. H. KOCH ,8

METHOD OF VAPOR GENERATION AND VAPOR SUPERHEATING, AND- APPARATUS THEREFOR INVENTOR P414 kocy ATTORNEY P. H. KOCH Sept. 9, 1958 2,851,015 METHOD OF VAPOR GENERATION AND VAPOR SUPERHEATING, AND

APPARATUS THEREFOR Filed Dec. 3, 1953 5 Sheets-Sheet 5 g i.m'

INVENTOR P404 H A ocH ATTORNEY United States Patent 0 METHOD OF VAPOR GENERATION AND VAPOR SUPEATING, AND APPARATUS THEREFOR Paul H. Koch, Bernardsville, N. L, assignor to The Babcock & Wilcox Company, New York, N. Y., a-corporation of New Jersey Application December 3, 1953, Serial No. 395,964

14 Claims. (Cl; 122-478) This invention relates to the art of vapor generation and vapor superheating, and the invention involves a par.-

ticular type of vapor generating and superheating unit, and a method of operation of such unit.

The invention is characterized by a vapor generating and superheating unit and a method of operation of that unit, both including the simultaneous effects of selective burner operation and correlated fiow of recirculated gas to modify furnace heat absorption and maintain a predetermined vapor superheat temperature over a Wide load range.

More specifically, as to the vapor generating and superheating unit of the invention, the pertinent unit involves a furnace fired by a plurality of burners (preferably, oil or gas burners) distributed over a substantial proportion of the area of a boundary surface of the furnace. The furnace has vapor generating Wall tubes which normally absorb heat radiantly transmitted from the burner invention involves a convection section in whichzthe gases flow horizontally from the furnace and through successive gas passes. A convection superheater is disposed in one of the gas passes and it is connected to the vapor generating wall tubes to receive generated vapor therefrom. The pertinent unit further involves agas recirculation system including a fan and duetworlc arranged to receive gases from a position beyond the super-heater and to discharge those gases in regulated quantities and at one or more selected positions in the furnace, the openings through which the recirculated gases are discharged into the furnace being disposed in a furnace wall other than the furnace wall having the burners therein. Preferably, these openings are arranged in a furnaee wall opposite the wall of the burners.

The pertinent vapor generating and superheating unit involves a furnace gas exit opposite the burner wall and at a position successively more remote from ditferent 2,851,015 1C6 Patented Sept. 9, 1958 When the pertinent vapor generating and superheating u is a e at no l fu ad a o the u bu ners a e i tende s be n QPQ Ta'F I h e rec tes- .lated gas into the furnace isv at a minimum. As the dean f generated va er ecr a e n met d f s eration of the pertinent unit involves the uniform decrease in the rates of firing of all of the burners down toa predetermined capacity or rate of vapor generation. Durit s i ar at he lo n r an o g apq ne i apa t h n mal ten en at such a un t e deficient in superheated vapor temperature is compensated or overcome by a regulated increase in the how of recirculated gases into the furnace. This increase in recirculated gas flow increasesthe heat absorbed by the superheater by reason of the increased mass flow of gases over the superheater, and deereases the heat absorption bythe vapor generating wall tubes by reason of the pertinent manner of introduction into the furnace of the lower temperature recirculated gases. Both effects so change the ratio of vapor generating absorbedheat to superheater absorbed heat that the superheat temperature is 'ntained at a predetermined valueover the pertinent part of the load range. I

As the steam or vapor demand decreases through a part of the load range next lower than the load range mentioned immediately above, a plurality of the burners are shutoff, these burners being disposed at positions mo remote o he a exi of the fu es? than he remaining burners, and the remaining burners, continuing in opera n bei d pssed in a o ce t PQ tion with respect to the furnace and, more particularly, with respect to the furnace boundary in which all of the humus a di ed im a e us ith hi shut n fi 9 me of the r e s, h ir late as flow is' so directed into the furnace and supplied thereto that there is an increase of the enveloping of the furnace heat emitting source by the lower temperature recirculated gases. Simultane ll l the distance from this heat emitn s ur e t e of t e at a sor n l ube nc eased, an he hea e itt n sur a o t e h t emitting source is decreased. Allof these effects deerease the heat absorbed in vapor generation. At the same time, the heat content in the gases leaving the furnace is of such increased value that the heat absorbed by the superheater is increased. These effects are so regulated ,as' to maintain a predetermined value of superheat temperature over the pertinent part of the load range. During successively lower parts of the load range all of the effects mentioned immediatelyabove are increased by successively decreasing the number of operative burners and continuing the introduction of redreulated gases, until only one burner remains in operation, with a maximum'of recirculated gas flow, a maximum decrease'in the path of travel of combustion elements from the fuel burning means to the furnace exit, a maximum centralization of burning fuel projection into the furnace, a maximum envelopment of the projected flame by the recirculated gases, and a minimum of surface of heat emitting source operative to provide heat for absorption by the vapor generating wall tubes of the furnace.

During the operative steps described above, the flow of recirculated gases may not only be controlled as to quantity and velocity, but their position or positions of an optimum effect of all ofv the combined acts. For

example, when a plurality of the burners, more remote from the furnace gas exit are shut off, the furnace gas entry may be so controlled that it is limited to a furnace position adjacent the location of the shut off burners. In other words, when the recirculated gas flow into the furnace takes place over any one of a plurality of positions successively more remote from the furnace gas outlet, the furnace gas flow may be limited under the pertinent conditions to the position most remote from the furnace gas outlet. Then, under the next successive sequence of acts, as the vapor generating load further decreases, the type of conditions obtaining during the first load decrease may be continued, and additional recirculated gases may be introduced at a position more remote from the furnace gas exit at the same time that additional burners are shut off at positions more remote from the furnace gas exit.

In conjunction with the above indicated subject matter, the invention also particularly contemplates an arrangement of oil or gas burners in an outside upright furnace wall, with the burners firing horizontally. With this arrangement, the recirculated gas system introduces partially heated gases through the rear wall of the furnace opposite the burner wall and preferably slightly below the lower row of fuel burners. The recirculated gases are directed horizontally forward through the furnace toward the burner wall in proximity to the horizontally extending furnace floor. The floor preferably includes a skeleton of floor cooling steam generating tubes connected into the circulation of the boiler and preferably covered with ceramic refractory.

The utilization of natural gas as a fuel permits the furnace to be fired with a sharp and substantially nonluminous flame. Luminosity will only occur in case there is a cracking of some of the combustible elements to develop carbon or some compound having radiant capabilities. In the burning of fuel oil the same high efliciency of combustion is not attainable in the same period of time, if at all, because the fuel oil must not only be atomized but the small atomized particles of oil must be vaporized before they can combine with the oxygen of the combustion air. These actions increase the time involved in the combustion. Also, the combustion of fuel oil involves combustion products of different'characteristics. These products are luminous and have greater radiating qualities than the products of natural gas combustion. Their radiating qualities, however, are much less than the radiating qualities of the combustion products resulting from the burning of pulverized coal.

With oil or gas firing of the furnace, the relatively short flame burners may be located relatively close to the bottom of the burner wall, inasmuch as there is no problem of cooling ash particles as would be involved in the firing of the furnace by pulverized coal. With such a short flame burner the furnace bottom disposed in proximity to the flames developed by the lowermost burners can be formed by ceramic refractory material.

When such a furnace as that above described is operated in accordance with prior art suggestions, and without recirculated gas, the intense heat of the combustion zone developed by the lowermost burners results in radiant transfer of heat to the ceramic covering of the floor, bringing the face of the ceramic covering to a state of incandescence. In the present invention, providing for the innoduction of recirculated gas horizontally across the furnace floor and toward the burner wall, the recirculated gases sweep the floor and absorb heat from the ceramic material of the floor. This results in a decrease in temperature of the ceramic floor covering so that the heat transferred from this covering to the steam generating floor tubes is reduced, and the heat which would otherwise go into those tubes as a result of the incandescence of the ceramic floor covering is,

lated gases and carried therewith to the furnace exit. The interposed stratum of recirculated gases absorbs radiated heat but it is still of a temperature lower than the furnace gases existing at that location when there is no recirculation. The radiant heat emission from the pertinent zone is reduced as is also the reradiation from the ceramic floor covering. These are factors involved in obtaining a greater heat content in the gases leaving the furnace. They also contribute to an increase in the available convection heat in the gas pass beyond the furnace exit.

The introduction of the recirculated gases substantially horizontally across the furnace floor results in the continued flow of these gases toward the burner wall, where they are deflected upwardly. This action deflects the burner flames and products of combustion of the short flame burners upwardly and, more specifically, diagonally across the furnace toward the furnace exit. Also, the average velocity of the upwardly moving gases is greater than would be the case if there were no introduction of the recirculated gases. Thus, the residence time and the radiant transfer of heat from the gases to the furnace walls are reduced.

The invention will be specifically set forth in the claims appended hereto, but for a more complete understanding of the invention and the advantages and benefits attained by its use, reference should be had to the accompanying description which refers to the pertinent drawings disclosing a preferred embodiment of the invention.

In the drawings:

Fig. 1 is a sectional elevation on the line 1--1 of Fig. 2, indicating the differential elevations of the rows of burners, and indicating the gas recirculation system;

Fig. 2 is a plan section taken on the line 22 of Fig. 1 showing the relation of the furnace to the serially connected gas flow passes of the convection section;

Fig. 3 is a partial plan section taken on the line 3-3 of Fig. 1, showing the gas recirculation system and its gas outlets distributed over the length of the furnace;

Fig. 4 is a transverse sectional elevation taken on the line 4-4 of Fig. 2, with the positions of the burners indicated in dash lines in a portion of the drawings in which the wall tubes are not shown;

Fig. 5 is a diagrammatic view similar to Fig. 4 but with the dampered gas outlets of the gas recirculation system shown and with the vapor generating tubes removed for the purpose of clarity; and

Fig. 6 is a chart or diagram intended to indicate the coordinated steps of the illustrative method and the results produced thereby.

Figs. 1 and 2 of the drawings indicate the pertinent type of vapor generating and superheating unit. This unit is characterized by a boiler setting of rectangular outline including a front wall 10, a rear wall 12, and

the connecting side walls 14 and 15. Parts of these walls, together with the groups 17, 19 and 20 of upright vapor generating furnace wall tubes, and similar upright tubes 18 forming a vertical baffie extending from the wall 15 for a part of the width of the setting, constitute the furnace or combustion chamber 22 fired by burners A-F inclusive, arranged in two substantially vertically spaced horizontal rows, as indicated in Fig. 4. Three of these burners are indicated in Fig. 2 at C, D and F.

From the furnace 22 the gases flow horizontally around the end of the baflle 18 and through a plurality of gas passes 2426. In all of these gas passes the gases flow horizontally over and between the spaced elements of banks of upright tubes 28-34, inclusive. The bank of tubes 28 in the first pass 24 constitutes, with appropriate inlet and outlet manifolds, a convection superheater, and the banks of tubes 31-34, in the gas passes 25 and 26, constitute vapor generating or fluid heating tubes directly connecting the upper vapor and liquid drum 40 to a lower drum 42.

The first gas pass 24 is formed between the boiler setting wall 14. anda baflle 44, including appropriate heat resistant material between rows of upright vapor generating, wall tubes 46 and 48. At the rear of the gas pass 24, the gases turn, as shown by the arrow 50, and proceed across the banks of vapor generating tubes 31-32 to the gas turning space 52 rearwardly of the baflle formed by the vapor generating tubes. 18. From this gas turning space, the gases proceed rearwardly over the banks of tubes 33-34 to the space at the rear of the gas pass 26, from which, at least a controlled proportion of the gases pass to a flue 53 or induced draft fan (not shown). The second and third gas passes 25 and 26 are separated by the baflle 54, including appropriate heat resisting material between the rows of fluid heating tubes 56 and 58.

The furnace 22 extends. to an elevation below that of the drum 42, as indicated in Fig. 1,.the furnace; having a floor 60, including vapor generating floor tubes 62, connecting the front and rear floor headers 64 and 66. The floor tubes are preferably covered with ceramic refractory material 61 forming the upper surface of the furnace floor. Fig. 1 also clearly indicates the front wall vapor generating tubes, such as 20, as connecting to the header 64 and having upper parts constituting roof tube sections 68 extending to connection with the drum 40. The rear floor header 66 is similarly connected to the drum 40. by the rear furnace wall tubes 70 some of which are included in the group 18 of Wall tubes indicated in Fig. 2. The natural circulation circuit through the tubes 20. and 70 and the floor tubes 62 is completed by tubular connections 72 affording direct communication between the lower drum 42 and the header 66,.

The furnace side Wall vapor generating tubes 17 and 19 are connected into the natural circulation by' circulators such as 74. leading from lower drum 42 to a side wall header 76 to which the tubes 17 are directly connected. The upper ends of these side wall tubes are directly connected to a header 78 which has direct connection with the drum 40. The vapor generating tubes 19 of the opposite furnace side wall are similarly connected into the natural circulation circuit through the headers 80 and 82 and similar connections to the drums 40 and 42.

The superheater 28 is a convection superheater receiving vapor generated in the unit and it is therefore subject to the inherent operative characteristic'that the vapor temperature at the outlet of the superheater decreases from an optimum value as the rate of vapor generation of the unit decreases, unless there is some compensating operative influence to prevent such drop in superheat (or superheated vapor temperature at the outlet end of the superheater). In the illustrative unit such compensating influence is provided by a superheat control which includes a plurality of coordinated factors. One of these factors is provided by the heating gas recirculation system including an inlet duct 90 particularly indicated in Fig. l as leading from an inlet 92 at the bottom of the gas space disposed rearwardly of the last gas pass 26. This duct leads to the inlet of a gas recirculating fan 94, the outlet of which is connected by duct 96 to a large manifold 98. This manifold has communicating therewith at distributed positions along its length, the three lateral ducts 100402. These ducts communicate with the bottom of the furnace 22 through openings between the tubes 70, and diflerential flow through the lateral ducts may be attained through the selective operation of louver type dampers 104106, a plurality of which is disposed in each duct. The arrangement of the recirculated gas ducts 100-102 and their outlets X, Y and Z with reference to the gas inlet to the first gas pass 24 (I to IV inclusive) is particularly indicated in Fig. 5, and it is to be noted that the gas recirculation outlets are disposed at positions of successively different degrees of remoteness, relative to the inlet of the first gas pass 24 (or the furnace gas exit), I.IV inclusive. With this arrangement the heatabsorption by approximately half load, as indicated by' LL.

the vapor generating tubes of the furnace may be, regulated by differential control of the recirculated gas flow through the outletsX", Y" and Z". If, for example, the heat absorption by the furnace wall vapor generating tubes is to be decreased to the maximum extent for flow through only one outlet, the flow of recirculated gas into the furnace'may be limited to the outlet Z", with the dampers for the associated outlets Y" and X" completely closed. Under such conditions, it is to be appreciated that the total flow of recirculated gas may not only be regulated, but the time required by such gas flow to pass through the furnace is increased by reason of the fact that the outlet Z is more remote to the furnace outlet, than either of the recirculated gas outlets Y" or X". Under these conditions there would be a double effect upon heat absorption by the furnace wall generating tubes, the one effect on heat absorption resulting from the closing of the, recirculated gas outlets Y" and X to reduce or limit thegtotal flow of recirculated gases; and the second effect being simultaneously operative by reason of the increased time factor, relating to the period of time during which the furnace wall vapor generating tubes are subject to the recirculated gas flow.

The differential flow of recirculated gas through the outlets X", Y" and Z" may be also coordinated with the selective operation of the burners A-F inclusive (Fig. 5). For instance (and particularly referring to Fig. 6), the total flow through selected recirculated gas outlets may be increased as the load decreases from a control point load (or full load) MM' to a value of This load decrease is indicated by the line ML' of Fig. 6. When the load value L-L is reached, the three burners, D, E and F, may be shut off. This action increases the centralization or concentration of the ignition zone in a space immediately in front of the operating burners, relative to the surrounding heat absorbing surfaces of the furnace, and thus decreases the heat emitting surface of the heat emitting source formed by the operating burners; and increases the heat radiation distance from the emitting source to at least some of the heat receptive furnace wall areas. Both of these effects tend to reduce the heat absorbed by the vapor generating tubes. These effects in reducing the furnace wall heat absorption may be also augmented by the concentration or limitation of recirculated gas flow to the outlet Z at a position adjacent the furnace wall 15. Thus, a heat transfer retarding medium of recirculated furnace gases would be interposed'between the wall 15 and the remaining operative burners A, B and C, further limiting, for example, the radiant heat transmission from the more centralized heat emitting source formed by the remaining operative burners A, B and C.

The actions indicated immediately above might also, under some load conditions, similarly result in a double or triple effect in reducing the heat absorption by the vapor generating tubes of the floor of the furnace. This might result from the introduction of lower temperature, and therefore, greater density recirculated gases in the lower part of the .furnace between the heat emitting source and the heat absorbing surface formed by the floor tubes. This latter action might be particularly increased when the recirculated gas outlet X" is closed and both of the other outlets Y" and Z" are open.

When the three burners D, E and F are shut off as at point Y in Fig. 6, the recirculated gas fan capacity, or total flow of recirculated gas into the furnace, is reduced as indicated by the line YX. Otherwise, the superheat might increase above the point L to an amount approximately indicated by the line LP, due to the tendency of the shortened distance from the heat emitting source to the furnace gas outlet to produce higher furnace gas temperatures at the outlet of the furnace, and, in part, due to the tendency of the recirculated gas system to increase the flow of recirculated gases by reason of the reduced resistancein that part of the recirculated gas flow circuit leadingover' the convection surfaces.

7 It is contemplatedthat all six-burners will have their rateof firing reduced uniformly so that the total rate of heat input is reducedyas indicated by line MP as the load drops from M-M to LL, and as the recirculated g'as flow is increased, as indicated by the line ZY. As the load further drops from L-L' to KK, with only the three burners A, B and C in operation, the recirculated gas flow will be increased as indicated by the line XW to increase superheat absorption by an increase of gas mass flow from the furnace and to effect an accompanying reduction in furnace wall absorbed heat in the manners above indicated.

When the load value KK is reached, the burner C is shut off, further centralizing the heat emitting source with reference to the roof, floor and side wall 15 of the furnace. In particular, the distance from heat emitting source to the floor of the furnace is increased to thus decrease the furnace heat absorption for vapor generation. At the point W the total flow of recirculated gas is decreased for the same reason as the decrease in such gas flow at YX. Upon further decrease in load toward the value H-H, the superheat absorbed heat is increased by an increase of gas mass flow caused by the increase of recirculated gas flow indicated by the line VU. This increase in recirculated gas flow simultaneously decreases the furnace absorbed heat by aflfording a greater amount of thermal insulating medium between the remaining operative burners A and B, and'the furnace heat absorbing surfaces, particularly the furnace floor.

When the low load value indicated by HE is reached the burner B is shut off and only burner A remains in operation thus affording the maximum in the reduction of a heat transmitting distance from a heat emitting source to a heat absorbing wall surface or surfaces. At such load value, there is a maximum flow of recirculated gases into the furnace, which may be admitted through all of the recirculated gas outlets, or be limited to the outlets Y" and/ or Z only.

Certain features of my invention are disclosed in my prior copending application, Serial No. 167,073, filed June 9, 1950, now U. S. Patent 2,737,931.

Whereas the invention has been described with reference to the details of an illustrative embodiment, it is to be appreciated that the invention is not limited to use in which all of those details are involved. The invention may rather involve the use of selected details with the omission of some of the remaining details. invention is to be considered as of a scope commensurate with the scope of the subjacent claims.

What is claimed is:

1. In a natural circulation vapor generating and superheating unit having a furnace including vapor generating tubes in its floor and walls, a ceramic refractory covering for the floor tubes arranged to form a horizontally extending closed floor, short flame burners firing the furnace from one wall at a position substantially above the elevation of the floor, a convection section including a convection superheater heated by the gases from the furnace and disposed laterally of the furnace for substantially horizontal flow of the gases over the elements of the convection section, and wide load range superheat control means including a recirculated gas system including a fan and ductwork leading from a gas flow position beyond the superheater to recirculated gas outlets disposed in a furnace wall opposite the burner wall and directed toward the burner at a position close to and above the furnace floor, said recirculated gas system directing recirculated gases into the furnace and across the floor in a direction opposite the direction of fuel firing and at an elevation below the lowermostelevation 'offiring.' r t The 2. In a natural circulation vapor generating and superheating unit having a furnace with a substantially horizontal closed floor including vapor generating tubes, means firing the furnace from one wall at a position substantially above the elevation of the floor, a convection section including a convection superheater heated by the gases from the furnace and disposed laterally of the furnace for substantially horizontal flow of the gases over the elements of the convection section, and wide load range superheat control means including a recirculated gas system including a fan and ductwork leading from a gas flow position beyond the superheater and having recirculated gas outlets in a furnace wall opposite the wall of the firing means and just above the floor, said recirculated gas system outlets directing recirculated gases across the floor into the furnace in a direction opposite the direction of fuel firing and at an elevation below the lowermost elevation of firing.

3. In a steam generating and superheating unit, a steam and water drum at the upper part of the unit, a water drum at the lower part of the unit, groups of upright steam generating tubes connecting said drums, a furnace including some of said tubes as wall tubes, short flame fuel burning means including a plurality of fuel burners disposed at positions at successively difierent degrees of remoteness from the furnace gas outlet, means forming a convection gas flow path receiving gases from the furnace, a convection steam superheater in said path, means normally conducting steam from the steam and water drum to the superheater, furnace floor tubes arranged at a level below the level of the water drum, means connecting the floor tubes to said drums, a ceramic refractory covering for the floor tubes, and wide load range superheat control means including a recirculated gas system including a fan and connected ductwork having an inlet communicating with the gas flow path beyond the superheater and having a plurality of outlets leading to the furnace at a position below the water drum, said outlets being disposed at positions of successively different degrees of remoteness from the furnace gas outlet, the fuel burning means being disposed in a wall of the furnace opposite the water drum and at a level above the level of the outlets of the gas recirculation system.

4. In a steam generating and superheating unit, a steam and water drum at the upper part of the unit, a water drum at the lower part of the unit, groups of upright steam generating: tubes connecting said drums, a furnace disposed generally at one side of a plane including the drum centerlines and including some of said tubes as wall tubes, fuel burning means including a plurality of fuel burners disposed at positions at successively different degrees of remoteness from the furnace gas outlet, means including a plurality of seriallyconnected horizontal gas passes forming a convection gas flow path receiving gases laterally from the furnace, a convection steam superheater in the first of said gas passes, means normally conducting steam from the steam and water drum to the superheater, furnace floor tubes arranged at a level below the level of the water drum, the floor tubes being connected to said drums, and wide load range superheat control means including a recirculated gas system including a fan and connected ductwork having an inlet communicating with the last of said gas passes beyond the superheater and having a plurality of outlets leading to the furnace at a position below the water drum, said outlets being disposed at positions of successively different degrees of remoteness from the furnace gas outlet, the fuel burning means being disposed in a wall of the furnace opposite the water drum and at a level above the level of the outlets of the gas recirculation system.

5. In a natural circulation steam generating and superheating unit, a steam and water drum at the upper part of the unit, a water drum at the lower part of the unit, upright steam generating tubes in communication with said drums,. a.furnace.including some of saidtubes as wall tubes, fuel burning means, means forming a convection gas flow path receiving gases from the furnace, a convection steam superheater in said path, means normally conducting steam from the steam and water drum to the superheater, furnace floor tubes arranged along a flat closed floor for the furnace at a level below the level of the water drum, the floor tubes being connected to said drums and to some of the furnace wall tubes, and wide load range superheat control means including a recirculated gas system including a fan and connected ductwork having an inlet communicating with the gas flow path beyond the superheater and having an outlet leading to the furnace through a wall at a position below the water drum, the fuel burning means being disposed in a wall of the furnace opposite the water drum and at a level above the level of the outlet of the gas recirculation system.

6. The combination of claim 5 further characterized by the disposition of the recirculated gas system outlet leading through the furnace wall immediately adjacent and parallel to the water drum.

7. The combination of claim 6 further characterized by a plurality of separately operable fuel burners arranged in succession from a wall of the furnace near one end of the water toward the opposite furnace wall, and a plurality of separately dampered recirculated gas flow openings arranged in succession along the furnace wall immediately adjacent the water drum and constituting the gas outlet of the gas recirculation system.

8. In a method of operating a natural circulation vapor generating and superheating unit having a plurality of fuel burners distributed over a substantial part or area of a boundary surface of a furnace having vapor generating tubes along its walls, said burners being disposed at positions successively more remote from the furnace gas exit, said unit also having a convection vapor superheater receiving the vapor generated in said tubes and heated by the gases from the furnace, the method of controlling steam superheat temperatures over a relatively wide load range which comprises successively shutting off burners in sequence as the demand for generated vapor decreases, simultaneously returning to the furnace an increasing amount of gases which have been partially cooled by passage over the superheater, said returning of partially cooled gases also involving the entry of the partially cooled gases at differing degrees of remoteness from the furnace gas exit, said successive shutting off of burners also involving first shutting off a burner most remote from the furnace gas exit and then shutting off other burners in sequence moving toward the burner nearest said exit, and reversing the direction or sequence of said acts as the demand for generated vapor increases toward a predetermined value.

9. In a method of operating a steam generating and superheating unit including a bent tube natural circulation steam generating system including an upper steam and water drum and a lower water drum, said unit including a plurality of serially connected horizontal gas flow passes arranged in succession lengthwise of and between the levels of the drums, a furnace having steam generating tubes along its walls and having a furnace gas outlet leading to the first of said gas passes, a convection superheater in one of said gas passes, fuel burning means at distributed positions in a wall of the furnace opposite said furnace gas outlet, said distributed positions having different degrees of remoteness from said outlet, and wide load range superheat control means including a gas recirculation system including a fan and ductwork having successive gas outlets leading into the furnace at a furnace wall other than the wall having the fuel burning means therein and having a gas inlet communicating with gas flow beyond the superheater, said increasing of the flow of recirculated gases as steam demand decreases also involving the variation of recirculated gas flow through said successive outlets at different degrees of remoteness from the furnace gas exit, said method comprising the steps su ccessively-shutting ofl burners in sequence circumferentially of the area over which they are distributed, as the demand for steam decreases, simultaneously increasing the flow of recirculated gases, said successive shutting off of burners also involving first shutting off a burner most remote from the gas exit of the furnace and then shutting off other burners in sequence moving toward the burner nearest said exit, and reversingsaid sequence or direction of said recirculated gas flow and fuel burning operative acts as the steam demand increases.

10. In the natural circulation generation and superheating of a high-pressure elastic fluid; effecting the combustion of fuel to provide high temperature gases; transmitting heat from the gases to confined streams of liquid to generate high-pressure elastic fluid; superheating the generated elastic fluid by the transmission of heat from the gases after loss of heat therefrom in the generation; and controlling the temperature of the superheated elastic fluid over a wide range of the rate of generation by withdrawing a percentage of the gases after loss of heat therefrom in superheating, introducing the withdrawn gases into the combustion zone selectively at positions of different degrees of remoteness relative to the gas exit of the combustion zone, and simultaneously selectively varying the remoteness of the main combustion zone relative to the gas exit of the combustion zone.

11. In the natural circulation generation and superheating of a high-pressure elastic fluid; effecting the combustion of fuel by the projection of fuel and air into a combustion zone to provide high temperature gases; transmitting heat from the gases to confined streams of liquid to generate high-pressure elastic fluid; superheating the generated elastic fluid by the transmission of heat from the gases after loss of heat therefrom in the generation; and controlling the temperature of the superheated elastic fluid over a wide range of the rate of generation by withdrawing a percentage of the gases after loss of heat therefrom in superheating, introducing the withdrawn gases into the combustion zone selectively at positions of different degrees of remoteness relative to the gas exit of the combustion zone, and simultaneously selectively varying the degree of remoteness of the main combustion zone relative to the gas exit of the combustion zone, the introduction of withdrawn gases being separate from the projection of fuel and air into the combustion zone.

12. A steam generating and superheating unit comprising vertical walls, a horizontally arranged closed floor and a roof defining a setting of rectangular horizontal crosssection, means forming a division wall arranged to divide said setting into a furnace chamber and a laterally adjoining convection heating section communicating at one end thereof, a natural circulation steam generating system comprising an upper horizontal steam and water drum and a lower horizontal water drum extending parallel thereto, a vertically arranged bank of steam generating tubes in said convection section connected to said upper and lower drums, and steam generating tubes arranged to cool vertical walls and the floor of said furnace chamber, a convection heated steam superheater arranged in said convection section adjacent the gas inlet end thereof, fuel burning means in one of said furnace chamber vertical walls above the level of said floor, and means for controlling the final steam superheat temperature over a relatively wide load range including a gas recirculating fan having its gas inlet connected to the gas flow path in said convection section downstream of said superheater, and means for discharging recirculated gases substantially horizontally across said furnace chamber floor at a level below the lowermost fuel burning means.

13. The combination of claim 12 further characterized by the furnace floor being disposed at a level substantially downwardly spaced from the level of the lower drum, the disposition of the recirculated gas system as a plurality of separately dampered openings through the furnace wall References Cited in the file of this patent UNITED STATES PATENTS Lucke May 31, Blizzard July 12, De Baufre Ian. 28, Kennedy July 29, Behr July 24,

FOREIGN PATENTS Belgium June 30, Belgium Oct. 31,

Images