US2685279A - Equalization of superheated and reheated steam temperature in steam power plants - Google Patents

Description

Aug; 3, 1954 v. z. CARACRISTI 2,685,279

EQUALIZATION 0F SUPERHEATED AND REHEATED STEAM TEMPERATURE IN STEAM POWER PLANTS 10, 1952 4 Sheets-Sheet 1 Filed Jan.

yflef Reheaterlnlet I Superheuter 221 ,231 Inlet 3/ l7 Reheater 0 Generator INVENTOR r C m Y PU MW Z R .m w M W Y, g F m n e d n O C .m

g- 1954 v. z. CARACRISTI 2,685,279

EQUALIZATION OF SUPERHEATED AND REHEATED STEAM TEMPERATURE IN STEAM POWER PLANTS Filed Jan. 10, 1952 4 Sheets-Sheet 2 INVENTOR 'Virginius Z. Corocrisfi ATTORNEY Aug. 3, 1954 v. z. CARACRISTI 2,685,279

EQUALIZATION 0F SUPERHEATED AND REHEATED STEAM TEMPERATURE IN STEAM POWER PLANTS Fxled Jan 10, 1952 4 Sheets-Sheet 3 Superheater M M A m m m mu m mm vQv 3B N 4 n m a 8 9 .m M u 0 m r & e 0 m 9 4 3 4 I 4 u h 9 9 m 8 m z G 4 u a r I I I I I I l I I I I I I I I I I I I I I I 9 I M F To Condenser INVENTOR ATTORNEY Aug. 3, 1954 v. z. CARACRISTI 2,685,279

EQUALIZATION OF SUPERHEATED AND REHEATED STEAM TEMPERATURE IN STEAM POWER PLANTS Flled Jan 10, 1952 4 Sheets-Sheet 4 Fig. 4;

Rm 5Q L v c B 3. g 4 u m mu "m w. 4

Generator To Condenser ATTORNEY Patented Aug. 3, 1954 EQUALIZATION OF SUPERHEATED AND RE- HEATED STEAM TEMPERATURE IN STEAM POWER PLANTS Virginius Z. Caracristi, Bronxville, N. Y., assignor to Combustion Engineering, Inc., New York, N. Y., a corporation of Delaware Application January 10, 1952, Serial No. 265,777 14 Claims. (01. 122479) This invention relatesto vapor power plants in which superheated vapor after having given up part of its energy is reheated in a reheater. More specifically this invention-relates to steam power plants operating under the reheat cycle.

In the operation of modern high pressure, high temperature steam power plants it is essential that the temperature of-the steam entering the steam turbine be held at a constant value over a wide range of load.

Moreover, in a steam powerplantoperating under the reheat cycle it becomes necessary to control not only the primary superheated steam temperature but also the reheated steam temperature and likewise to maintain it at a constant value over a wide range of load. The operator of a steam generator of this reheat'type is accordingly confronted with controlling at the same time both the heat transfer to the superheater surface and to the reheater surface by changing either the temperature of the combustion gases flowing over the aforesaid heating surface, or the quantity thereof, or both.

* It hasbeen found in the operation of a-steam generating. power plant of the type--herein consideredthat at low loads the-superheater absorbs more heat than is required tomaintain-thedesired superheated steamtemperature if=--the reheat temperature is maintained at the-desired value and that in these cases it becomes necessary to employ auxiliary control equipment,-such as for example a desuperheater, to lower the temperature of the superheated steam to "the design value before enteringthe turbineyand utilization in the boiler furnace of a steam reheater further complicatesthe control problem by requiring supplemental means (again such as a desuperheater) for keeping the reheated-"and superheated steam temperatures at predetermined values as the turbine loading (and hence steam generator loading) varies.

Methods of obtaining equalization of the superheated steam and reheated steam temperatures as theone hereinabove described, in which a --desuperheater is utilized, are not only costly to maintain but also lower the overall plant efficiency.

, It is accordingly the main object of myinvention to provide a superior method of temperature equalization between the superheated steam temperature and the reheated steam temperature. Another object of my invention is to: provide for effective protection of the reheaterheating surfaceduring starting up. periods or during periodsavhe'n little orno reheat steam flows such as during a trip-out when the reheat intercept valve closes.

Still another object of the invention is to permit a higher heat release rate in the furnace than was heretofore possible.

A further object of my invention is to permit a wider operating range of primary and reheat steam temperature control.

Another object of the invention is to lower the design pressure loss through the reheater.

Other objects and advantages of the invention will become apparent from the following description of illustrative embodiments thereof when taken in conjunction with the accompanying drawings wherein:

Fig. l is a diagrammatic representation of a power plant operating under the reheat cycle showing my inventive improvement including a steam generator equipped with tilting burner control.

Figs. la-lb-lc indicate in diagrammatic view how the tilting burners of the Fig. l furnace may be adjusted respectively to locate the burning fuel flame at raised, intermediate and lowered positions in the furnace combustion chamber.

Fig. 2 is a diagrammatic representation of a power plant operating under the reheat cycle incorporating my inventive improvement and including a steam generator equipped with gas recirculation temperature control.

Fig. 3 is a diagrammatic representation of a power plant operating under the reheat cycle, equipped with my inventive improvement and showing asteam generator in which steam temperature control is accomplished by means of a gas by-pass and/ or a desuperheater.

Fig. 4 constitutes a plan section taken on line 4-4 of each of Figs. 1, 2 and 3 and showing my new arrangement of superheater and reheater tubes within the furnaces upper portion.

Fig. 5 is a plan section taken on line 5-5 of Fig. 2 showing corner mounting of the furnace burners for tangential firing.

Fig. 6 is a plan section taken on line 6-5 of each of Figs. 1, 2 and 3 showing one novel feature of my invention in an enlarged scale.

Fig. '7 is an elevational section taken on line l-'l of Fig. 1 showing further details of the new superheater-reheater assemblage.

Fig. 8 is a plan section taken on line 88 of Fig. 1 showing another feature of my inventive improvement in an enlarged scale.

In the illustrative embodiment of my invention herein disclosed, a steam generator generally designatedby A comprises a furnace I the walls of which are lined with steam generating tubes (not shown) in a manner well known in the art. These tubes discharge the steam generated therein into drum 2 from which the steam leaves through tubes 3 connecting the drum with the superheater inlet header 4. There are two such headers shown in the steam generator described.

Burners 5 deliver fuel entrained in a stream of air for combustion into the furnace l. The fuel burns within the furnace: and the ash falls into the hopper bottom 8 from which it is removed by conventional means (not shown) The products of combustion rise within the furnace and pass over heating surfaces representing my inventive superheater and reheater combination, generally designated B. The gases then enter a lateral pass 7 from which they are discharged into a downward pass 8 wherein additional banks of heat absorbing tubes are located represented in this descriptive illustration by low temperature superheater section 9. Upon leaving low temperature superheater section 9 the gases may pass over other heat recovering surfaces before being discharged into the stack (not shown).

The saturated steam having entered superheater headers 4 as hereinabove described flows through the above mentioned low temperature section 5 wherein an initial degree of superheat is imparted thereto. Outlet header is and pipe ll serve to conduct this partially superheated steam to the inlet header l2 of the high tem perature superheater section Bs which forms a part of the hereinabove mentioned superheater and reheater combination B. In order to visually distinguish the superheater Bs from the later herein described reheater ET. the superheater is shown in solid lines whereas the reheater is shown in dotted linesv The steam having been superheated to a temperature corresponding to the turbine design vaue by passing through successive U-shaped tubes of superheater Bs and absorbing heat from the combustion gases, thereupon leaves by way of superheater outlet header 13 from where it is conducted through pipe it to the high pressure stage of turbine Hi. In the arrangement here shown this turbine drives an electric generator G.

The steam having passed through some of the high pressure stages of turbine l5 and having given up apart of its energy is returned to the steam generator by way of pipe line it? for reheating. For this purpose the steam enters the inlet header 5! of the reheater Br forming an integral part of my inventive superheater and reheater combination B as hereinafter described. After being reheated to the proper turbine design temperature by absorbing heat from the gases flowing over the reheater surface Br (shown in dotted lines) the steam leaves by way of reheater outlet header i3 and is returned through pipe I 9 to an intermediate pressure stage of turbine l5 wherein the remaining heat in the steam is converted into mechanical energy. The steam then leaves turbine if: to be condensed in a condenser (not shown).

Burners 5 of the steam power plant illustrated in Fig. 1 are of the tilting type, and are preferably located adjacent the four corners of the furnace as illustarted in Fig. 5. Preferably these burners 5 direct the fuel and air into the furnace in streams tangential to an imaginary vertical cylinder within the furnace as indicated in Fig. 5. However other burner arrangements of course are usable to deliver fuel and air into the furnace i for combustion therein. The preferred tilting burner construction illustrated in Fig. 1 permits direction of the fuel and air streams upwardly towards the superheater and reheater heating surface B or downwardly away from the superheater and reheater heating surface B; such actions being shown in diagrammatic form by Figs. la, 11) and 10.

By tilting the fuel and air streams in this manner, which is well known in the art, the tem perature of the gases entering heating surfaces located above the furnace proper, such as superheater and reheater combination B, can be controlled to permit raising or lowering of the superheated and reheated steam temperature. This control is exceedingly important in the operation of modern high temperature, high pressure steam generators because it is essential for the efficient operation of the steam turbine to deliver thereto steam at a previously detenmined design temperature.

As hereinabove set forth the difficulties arising in the operation of a modern reheat power plant are aggravated by the necessity to independently regulate the steam temperature of both the reheated steam as well as the primary superheated steam. In many installations this is accomplished by the use of a desuperheater located between the high and low temperature superheater sections such as for example, in steam line H or in the steam pipe l4 leading to the turbine 15.

My invention, providing for an integrally combined superheater and reheater. eliminates the use of supplementary auxiliary control devices such as a desuperheater. This I accomplish by constructing a reheater-superheater organization in the form of panels or platens B which are relatively widely spaced such as indicated in Fig. 4. These platens comprise a plurality of tubes 28 and 21 which are arranged side by side and welded one to the other to maintain intimate metal contact. This is shown in Fig. 4 and in an enlarged scale in Figs. 6 and 8.

In my inventive combination superheated steam entering inlet header l2 flows through tube 20 to absorb heat from the combustion gases by radiation as well as by convection. The reheat steam returning from the turbine for reheating enters reheater inlet header H and flows through reheater tube 2! to also absorb heat from the combustion gases by radiation and convection.

These tubes 20 and 2| can be arranged in any convenient manner such as the here represented groups of nested U-shaped tubes forming panels B and providing within each panel parallel flow, as shown in Figs. 1, 2, and 3; each panel being serially connected to the adjacent one by connecting tubes 3!. Even though vertical runs of tubes 20 and 2| are here shown, it will be understood that these superheater and reheater tubes also can be arranged in horizontal runs.

Moreover, the tubes in and 2! can also be formed into continuous coils for pure serial flow, one coil carrying superheated steam and an adjacent coil carrying reheated steam, both maintaining intimate metal contact substantially throughout their heat exposed length. Either parallel or serial flow can be provided; however the direction of flow of the superheated steam and that of the reheated steam preferably should be the same. 7

The enlarged cross section shown in Fig. 8 indicates the intimate contact which is maintained between superheater tube 20' and reheater tube 2!. This metal-to-metal contact causes heat transfer to take place between the fluid flowmg through tube 20 and that flowing through tube 2| if a temperature difference exists between the two fluids. In most cases it is desirable to provide the turbine with superheated steam and reheated steam having equal temperatures. However a substantial difference of temperature usually exists between the saturated steam leaving drum 2 and entering superheater inlet header [2 and the reheat steam leaving an intermediate pressure stage of turbine l5 and entering reheat inlet header I1.

My invention accomplishes equalization of these reheated and superheated steam temperatures. Such equalization takes place because the superheater tubes 20 maintain intimate metal contact with reheater tubes 2| substantially throughout their entire heat exposed length; hence the temperature difference that may exist at the inlet headers 12 and His substantially if not entirely eliminated by the time the superheated and reheated steam has reached the outlet headers l3 and iii.

For example, if we consider a reheat generator operating under a steam pressure of approximately 1800 lbs. per sq. in., the temperature of the steam entering the superheater header !2 may be in the neighborhood of 600 deg. F. The temperature of the steam entering the reheater header i! on the other hand may, at low loads, be in the neighborhood of 400 deg. F., whereas at maximum load the temperature of the steam entering header I! to be reheated may reach a value as high as 700 deg. F. The final temperature leaving the superheater as well as that leaving the reheater may be in the neighborhood of 1000 deg. F. There are of course certain cases in which it is desirable for reasons of economy in steam turbine design to increase the temperature of the reheated steam over that of the primary superheated steam because the reheated steam is delivered to the turbine under a much lower pressure than that of the primary superheated steam.

It can well be appreciated by those skilled in the art of designing modern steam power plants, how my invention permit simplification of the control problem with which steam power plant designers are faced. This will become apparent when it is considered that my inventive improvement causes equalization of the superheated steam temperature and the reheated steam temperature regardless of whether the superheated steam temperature entering my inventive superheater-reheater B is higher than the entering reheat steam temperature, or lower. In both cases the intimate metal contact afforded between the superheater tube and the reheater tube, as shown in Figs. 6 and 8, will accomplish heat transfer between steam flowing through the superheater tube 20 and that flowing through the reheater tube 2|. If it should become desirable as hereinabove set forth to furnish the turbine with reheated steam having a temperature somewhat higher than the superheated steam, this can quite easily be accomplished by extending the heating surface of the reheater through lengthening the lower end of the reheater tubes beyond the length of the superheater tubes. My invention therefore can be applied to any temperature conditions that may be required for obtaining a maximum overall power plant eiiiciency.

Fig. 7 shows an elevational section of the upper end of two parallel superheater-reheater platens or panels, each platen formed of U-shaped tubes 20, 2 I terminating at its upper end into four headiii) ers, two superheater headers-an inlet header 22a and an outlet header 22b, and two reheater headersan inlet header 23a and an outlet header 23b. There are three tube banks 24 shown in Fig. 1 comprising spacedly arranged U-shaped platen sections B. These platens in forming the said tube banks may satisfactorily be spaced at distances approximately two feet across the width of the unit, as shown in Figs. 4 and 7. To allow for expansion between the individual U-shaped platens, guiding brackets 25 are welded to one section and extend over the width thereof to form a shoe around the adjacent platen section for guidance thereof as shown in Figs. 6 and 8. This will allow each section to expand independently of the other, yet retain the adjoining sections in alignment. To maintain the vertical sides of the platen B in alignment portions 28 oi furnace wall tubes may be bent out to form a guide channel within which the platens can expand as shown in Fig. 6. Other means well known in the art for supporting and guiding these platen sections or panels 24 of course can also be employed.

My invention as herein disclosed contemplates the use of a controlling device for regulating the temperature and/or quantity of the gases entering superheater and reheater surfaces B tomaintain constant steam temperature for various steam loads. The necessity of such a control for attaining constant superheated steam tempera ture over a given control range is widely recognized and various means are employed for this purpose, such as burner tilting as illustrated in Fig. 1 or gas recirculation as shown in Fig. 2.

Operation of the burner tilting facilities of Fig. 1 will best be understood through reference to Figs. la-lb-lc. If the temperature of the steam leaving superheater-reheater assemblage B is too low, the flame center can be raised as shown by Fig. 1a; and if the temperature is too high the flame center can be lowered as shown by Fig. 10. Such adjustments can'be effected automatically through the medium of burner tilting motor 32 as later described.

In the gas recirculation arrangement of Fig. 2 relatively cool gas from offtake 21 is forced through duct 26, as by means of fan 44, into the furnace i wherein it mingles with the combustion gases before flowing over superheater-reheater surface B; control of such recirculation being eifected by means of a damper 42 or in some other way as later explained. My invention thus can be used in combination with a gas recirculation control equally well as With tilting burner control; since with either arrangement the reheated steam temperature and the superheated steam temperature can in my inventive combination be treated as one and control of either the super-- heated steam or the reheated steam by means of gas recirculation will at the same time control the other by virtue of the'heat transfer taking place between welded tubes 29 and 2!.

In addition to the two control arrangements discussed hereinabove in connection with my invention, namely tilting burners and gas recircula tion, it is also possible to control the heat absorption of the superheater or indirectly that or" the reheater by by-passing gases around the low temperature section of the superheater as indicated in Fig. 3; an adjustable damper it then being utilized as later explained.

It is also possible in a steam generator equipped with my inventive improvement to control the temperature of the reheat steam entering the reheater inlet header H and thereby indirectly the temperature of the superheated steam, by installing a desuperheater 30 in the steam line it between the turbine 15 and the inlet header I! as shown in Fig. 3.

Automatic control can also be provided for my inventive combination when using any one of the four above mentioned main control arrangements. Thus in Fig. 1 a burner-tilting motor 32 is shown the action of which is controlled through suitable means 25 by the temperature actuated element 33 which responds to temperature changes of the superheated steam leaving superheater outlet header [3 (if desired element 53 can instead be placed in reheater outlet header IS). The illustrated motor 32 is arranged by means of gearing 34 and linkage 35 to raise or lower the fuel and air ejecting nozzles 36 of tilting burners 5 (preferably all of them in parallel) thereby accomplishing the raising or lowering of the mass of burning gases with in the furnace proper as diagrammatically illustrated by Figs. la-lb-lc. A change of combustion gas temperature is thereby accomplished as hereinabove set forth.

Similarly the recirculation of gases for furnace outlet temperature control as shown in Fig. 2 can be controlled by automatic means comprising a motor 38, the action of which is governed through control means 39 by temperature changes of the steam leaving either superheater outlet header 13 (wherein element 33 is here shown as being disposed) or reheater outlet header 18 (wherein element 33 may instead be disposed). The illustrated motor 33 drives a reduction gear so which in turn actuates damper 42 located in gas recirculating duct 26. It is of course also possible to provide for automatic control of gas recirculation by directly regulating the speed of fan M which in turn determines the fiow of cool gases from ofitake 21 into furnace i.

In many cases it is also desirable to provide automatic control for regulating the flow of gases over low temperature superheater section 9 by a control such as indicated in Fig. 3 wherein a temperature control device 48 responsive to the temperature registered by element 33 (disposed either in reheater outlet header i8 or in superheater outlet header [3 as shown) actuates motor 5% driving gear 52 which opens or closes damper 54. Such automatic control of damper 5:5 is effective as long as transfer switch 58 occupies the right position shown by Fig. 3.

Similarly the flow of spray water utilized in desuperheater 36 can be automatically controlled by means of motor 58 driving gearing 53 i which in turn opens or closes valve 60 located in water supply pipe 62 delivering spray water to the desuperheater from a source (not shown). Such automatic control of desuperheater 3G is effective when transfer switch. 59 is shifted to the left (from the right position shown by Fig. 3) thereby connecting temperature registering element 33 (in either superheater outlet header 13 or reheater outlet header 18) with means 63 which translate changes in the leaving steam temperature into appropriate adjusting operations by motor 56.

While illustrative embodiments of my invention have been here shown and described, it will be understood that changes in construction, combination, and arrangement of parts may be made without departing from the spirit and scope of the invention as claimed.

I claim:

1. In a steam generator of the type described adapted to operate at different loads, a boiler having a plurality of heat absorbing tubes positioned therein, a first group of said tubes comprising a superheater, a second group of said tubes comprising reheater, the relative heat absorption requirements of the superheater and reheater being different at difierent loads, the tubes of said first and second group being positioned in defined rows wherein adjacent tubes are associated with a different group, header means associated with the tubes in each of said rows effective to direct the fiow of steam serially through the tubes that comprise the superheater in each of said rows and similarly through the tubes that comprise the reheater in each of said rows, the tubes in said defined rows being positioned in direct heat exchange relation with one another whereby the temperatures of the steam within said superheater and within said reheater tend to equalize.

2. In a vapor generator of the type described operating on the reheat cycle and over a substantial load range, the combination of a boiler having a plurality of metallic heat absorbing tubes positioned therein, a first group of said tubes comprising a superheater with the tubes thereof disposed in defined spaced rows, a second group of said tubes comprising a reheater with the tubes thereof in parallel relation with and intercalated among the tubes of said superheater, the boiler, including the superheater and reheater being arranged to be heated by the same heat source, said tubes being arranged so that the vapor flows in the same direction through the tubes associated with each of said groups, means bonding the tubes in said rows together in metal to metal relationship thereby providing for good exchange of heat between said superheater and said reheater.

3. In a vapor generator of the type described adapted to operate over a wide range of loads, a boiler having a plurality of metallic heat absorbing tubes positioned therein, a first group of said tubes comprising a superheater with the tubes thereof forming defined spaced rows, the tubes in each of said rows being spaced predetermined intervals from each other, a second group of tubes comprising a reheater with the tubes thereof positioned between the tubes of said first group and substantially filling said predetermined intervals therebetween, the relative heat absorption requirements of the superheater and reheater being diiierent at difierent loads, said superheater and reheater tubes being arranged so that the flow therethrough is in the same direction, means securely bonding together in metal to metal relationship adjacent tubes in each of said rows thereby providing for a good exchange of heat between adjacent tubes.

4. A steam generator operating on the reheat cycle and over a wide load range, said generator comprising a generally vertical boiler having burner means disposed at one end thereof and provided with a plurality of vertically disposed horizontally spaced rows of tubes adjacent the other end, adjacent tubes in each of said rows being physically united throughout at least a major portion oi their length, alternate tubes in each of said rows forming the superheater for said steam generator and the remaining tubes in each of said rows forming the reheater for said steam generator, header means for said tubes constructed and arranged so as to provide for serial flow through said superheater and said reheater tubes in each row and further to provide for the flow through said superheater and said reheater tubes in the same direction.

5. A steam generator operating on the reheat cycle and over a substantial load range, said generator including a gas pass through which the hot combustion gases flow, a plurality of vertically disposed horizontally spaced rows of metallic tubes positioned in said gas pass to absorb heat from the hot gases of combustion flowing therethrough, said rows being parallel with the gas flow and comprising numerous U-shaped tubes nested within one another and united in metal to metal relation to form a rigid substantially flat panel, alternate tubes in each of said rows being interconnected in such a manner as to form the superheater for said steam generator with the remaining tubes in each of said rows being interconnected in such a manner as to form the reheater for said steam generating means, with the direction of flow through adjacent tubes being the same.

6. In a system of the type described, a steam generator operating on the reheat cycle and at substantially different loads, said generator having a steam superheater means and a, steam reheater means disposed therein, the generator including the superheater means and reheater means being arranged to be heated by the same source of heat, control means responsive to the temperature of the steam leaving one of the aforementioned means and operable to maintain said temperature substantially constant, said superheater means comprising a plurality of spaced metallic tubes communicating with superheater inlet and outlet headers, said reheater means comprising a plurality of metallic tubes intercalated among and parallel with said superheater tubes, said reheater tubes being bonded in metal to metal relation with adjacent superheater tubes and communicating with suitable inlet and outlet headers whereby said superheater means and said reheater means are in heat exchange relationship with one another resulting in automatically re ulating within limits the temperature of the steam at the outlet of both by a single control means.

'7. In a steam generator operating on the reheat cycle and over a substantial load range, said generator includin a furnace, means for supplying fuel to and burning it in said furnace and steam generating surface absorbing heat from the burning fuel and from the gases of combustion; steam heating elements disposed in the path of the gases discharged from the furnace comprising a panel of heat absorbing elements made up of metallic tubes disposed in side by side relation in rows spaced transversely of the gas path; means for regulating the heat exchange relationship between the steam flowing through said tubes and the stream of gases contacting the latter; means including headers connected so as to direct a stream of steam from said generator through certain tubes in said panel and for conducting the superheated steam to a steam user; and means including header means for directing steam from said user through other tubes in said panel adjacent to said certain tubes and in the same direction with respect to the flow of gases thereover as the steam flowing through said certain elements, the tubes of said panel being in direct heat exchange relationship with each other to equalize the temperature of steam heated therein by contact with the mass of gases flowing thereover.

8. In a steam generator operating on the reheat cycle and over a substantial load range, said generator having a furnace, means for supplying fuel to and burning it in said furnace, and steam generating surface extending generally throughout the length of the furnace for absorbing heat from the burning fuel and from the gases of combustion; steam heating elements disposed in the path of the gases discharged from the furnace comprising a plurality of parallel panels of heat absorbing elements each made up of metallic tubes disposed in side by side relation in rows spaced transversely of said gas path; means for regulating the differential between the temperature of steam flowing through said tubular elements and the temperature of the mass of gases contacting said elements consisting of means for moving the zone of combustion of fuel in the furnace toward. or away from its outlet to alter the amount of heat absorbed by said steam generating tubes therein; means including headers connected so as to direct a stream of steam from said generator in series relation through alternate tubes in said panel and for conducting the superheated steam to a steam user; and means including header means for directing steam from said user through the intermediate tubes in said panel in series relation and in the same direction with respect to the flow of gases thereover as the steam flowing through said alternate elements, said alternate and intermediate tubes of each panel being in direct heat exchange relationship with each other throughout a major portion of their length to substantially equalize the temperature of steam heated therein.

9. In a steam generator operating on the reheat cycle and over a substantial load range, said generator including a furnace, means for supplying fuel to and burning it in said furnace and steam generating surface absorbing heat from the burning fuel and from the gases of combustion; steam heating elements disposed in the path of the ases discharged from the furnace and comprising a plurality of parallel panels of heat absorbing elements made up of metallic tubes disposed in side by side relation in rows spaced transversely of said gas path; means for regulating the heat exchange relationship between the steam flowing through said tubular elements and the stream of gases contacting said elements comprising means for controllably returning to the furnace a volume of gases after they have passed in contact with said steam generating and steam heating means; means including headers connected so as to direct a stream of steam from said generator through certain tubes in said panel and for conducting the superheated steam to a steam user; and means including header means for directing steam from said user through tubes in said panel adjacent said certain tubes and in the same direction with respect to the now of gases thereover as the steam flowing through said certain tubes, said certain and other tubes of each panel being in direct heat exchange relationship with each other to substantially equalize the temperature of steam heated therein.

10. Inv a steam generator operating on the reheat cycle and over a substantial load range, said generator including a furnace, means for supplying fuel to and burning it in said furnace and steam generating surface absorbing heat from the burning fuel and from the gases of combustion; steam heating elements disposed in the path of the gases discharged from the furnace and comprising a plurality of parallel panels of heat absorbing elements each made up of metallic tubes disposed in side by side relation in rows spaced transversely of said gas path;

means for regulating the heat exchange relationship between the steam flowing through said tubular elements and the stream of gases contacting said elements comprising a low temperature superheating section disposed beyond said steam heating means in the path of gas flow from the furnace and a damper control by-pass for apportioning the fiow of gases over said primary superheater and through said by-pass to effect said temperature regulation; means including headers connected so as to direct a stream of steam from said generator through certain tubes in said panel and for conducting the superheated steam to a steam user; and means including header means for directing steam from said user through tubes in said panel adjacent said certain tubes and in the same direction with respect to the fiow of gases thereover as the steam flowing through said certain tubes, said certain and other tubes of each panel being in direct heat exchange relationship with each other to substantially equalize the temperature of steam heated therein.

11. In a steam generator operating on the reheat cycle and over a substantial load range, said generator including a furnace, means for supplying fuel to and burning it in said furnace and steam generating surface absorbing heat from the burning fuel and from the gases of combustion; steam heating elements disposed in the path of the gases discharged from the furnace and comprising a plurality of panels of heat absorbing elements each made up of metallic tubes disposed in side by side relation in rows in said gas path; means including headers connected so as to direct a stream of steam from said generator through certain tubes in said panel and for conducting the superheated steam to a steam user; means including header means for directing steam from said user through other tubes in said panel adjacent to said certain tubes and in the same direction with respect to the fiow of gases thereover as the steam flowing through said certain elements, said tubes of each panel being in direct heat exchange relationship with each other to equalize the temperature of steam heated therein by contact with the mass of gases flowing thereover; and desuperheating means interposed between the outlet of said superheater and the inlet of said reheater.

12. A steam generator operating over a wide load range and comprising a boiler provided with steam generating means, means for superheating the steam produced by said steam generating means, said superheating means including a first heating conduit for how therethrough of the steam to be superheated, a steam reheating means including a second conduit for flow therethrough of the steam to be reheated, the relative heat absorption requirements of the superheater and reheater being different at different loads, said first and second conduits being physically united throughout at least a substantial portion of their lengths with these imited conduit porions arranged so the steam flow therethrough is in the same direction.

13. A vapor generator adapted to operate at substantially different loads and having a plurality of heat absorbing tubes positioned therein, a first group of said tubes comprising a superheater, a second group of said tubes comprising a reheater, the relative heat absorbing requirements of said superheater and reheater being different at difierent loads, at least a major portion of the tubes of said first and second group being arranged so that the direction of vapor fiow therethrough is the same, with these portions of the tubes of each group being in direct heat exchange relation with each other.

14. A steam generator operating on a reheat cycle and over a substantial load range, comprising a boiler provided with steam generating means, means for superheating the steam produced by said steam generating means, a steam reheating means, said generator means, superheating means and reheating means being heated by the same source of heat, a substantial portion oi said superheating means and said reheating means being arranged in direct heat exchange relation with one another with these portions having steam flow in the same direction so that the temperature at the outlet of said superheater means and said reheater means will tend to be equal.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 258,933 Litchfield et a1 June 6, 1882 1,952,542 Ehlinger Mar. 27, 1934 2,602,433 Kuppenheimer July 8, 1952

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