US2905156A

US2905156A - Reheater steam generating unit with gas recirculation for reheat control

Info

Publication number: US2905156A
Application number: US559712A
Authority: US
Inventors: Pacault Pierre
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1955-01-25
Filing date: 1956-01-17
Publication date: 1959-09-22
Anticipated expiration: 1976-09-22

Description

P. PACAULT Filed Jan. 17, 1956 REHEATER STEAM GENERATING UNIT WITH GAS RECIRCULATION FOR REHEAT CONTROL 30 4 27 FIG] 31 7 25 Q 16 l f- '-.f 19

SPHTR 1 m Xi 'INVIENTOR. Pierre Pacaulr ATTORNEY United States Patent REHEATER STEAM GENERATING UNIT WITH GASLRECIRCULATION FOR REHEAT CON- ,TRO

Pierre Pacault, Paris, France, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey This invention relates to a method and apparatus for generating high pressure vapor, superheating the generated vapor, reheating the generated vapor, and controlling the temperature of vapor superheating and vapor reheating over a wide range or rate of vapor generation. The type of unit with which the invention is concerned is particularly adapted for supplying high temperature vapor or steam to prime movers such as plural stage steam turbines, in central power stations.

In steam generators of the pertinent type, including convection superheating and reheating units, both the superheat temperature and the reheat temperature drop from an optimum value as the load is reduced from *control'point load,- it no provisions are made for correcting this inherent action. The reheat temperature drops at a greater rate than the high pressure superheat temperature inasmuch as the superheating takes place from a substantially constant saturated temperature of the high pressure steam, while reheating is from the tempcrature of the low pressure steam, this temperature decreasing as the steam flow through the turbine is reduced.

For the highest thermo-dynamic efliciency of such a steam turbine, it is not only important that the superheated steam be supplied to the turbine at a substantially uniform temperature but it is also important to supply the reheated low pressure steam at a uniform temperature over a wide load range.

'Because of the above indicated situation, and in order to provide uniformity of steam temperatures of both the high pressure superheated steam and the low pressure reheated steam, there must be different control provisions or means having different degrees of effectiveness on the superheated steam and on the reheated steam. It is the aim of the present invention to provide apparatus and a method of control which will effect substantially uniform superheat and uniform reheat steam delivery over a wide range of generator output.

I More specifically, the invention is concerned with vapor generating and vapor heating units of the type using low temperature gas recirculation for the control of the heated vapor temperature. When the unit involves a furnace lined with vapor generating tubes constituting a radiant section, and when the furnace is supplied with high temperature heating gases by the burning of a slag forming fuel, the gas recirculation of the invention is '80 applied and controlled as to minimize slagging of the vapor heating surfaces disposed within heating gas passages leading from the furnace. With this arrangement, the most expensive parts of high capacity units of the pertinent type, i.e., radiant section of the furnace with its wall tubes can be substantially reduced in size and cost.

A specific embodiment of the invention also involves the recirculation of gases for the regulation of the final 'vapor temperature. By recirculating gases to the furnace or the radiant vapor generating section of the unit, the gases having been cooled by heat transmission there- Patented Sept. 22, 1959 from to the convection vapor heating surfaces, the radiant heat absorption may be reduced at low loads so that more heat is available for the convection section for vapor superheating and/ or vapor reheating. With a unit having different gas flow paths containing, respectively, convection vapor superheating surface and convection vapor reheating surface, the total gas flow may be proportioned between the gas flow paths by damper means in order to regulate the final temperature of vapor superheat or vapor reheat, the other of these temperatures being regulated by control of the rate of gas recirculation.

By way of example, vapor generating and vapor heating units having a gas recirculation system or systems, and methods of operating vapor generating units involving the pertinent gas recirculation for the pulpose of effecting control of vapor temperature such as reheat temperature, are disclosed in the accompanying drawings and the following description:

In the drawings:

Fig. 1 is a diagrammatic view illustrating a vapor geni erating, superheating, and reheating unit in which gas recirculation is involved in a closed circuit for controlling the reheated vapor temperature;

Fig. 2 is a diagrammatic view in the nature of a vertical section on line 2-2 of Fig. 3 illustrating a second and third modification; and

Fig. 3 is a horizontal section in the nature of a plan on the line 3-3 of Fig. 2 and looking in the direction of the associated arrows, Fig. 3 and Fig. 2 collectively, showing a third modification of the invention wherein there are two gas recirculation systems one of which controls reheat while the other controls superheat.

In each of the modifications shown in the drawings, there is a radiant section including a vertically elongated furnace chamber 1. When the unit includes a natural circulation system for vaporizable liquid, the walls and other boundary surfaces of the furnace chamber include upright vapor generating tubes such as 2 and 3, incorporated into the fluid circulation of the system by a steam and water drum 4, a lower header 5, and other appropriate circulatory connections.

The furnace chamber 1 is fired in its lower region by a variable rate fuel burning means 6 which may include pulverized coal burners, or combination, pulverized coal, gas and oil burners. The unit of each embodiment of the invention is so constructed as to provide a gas flow path leading from the gas outlet of the furnace chamber to atmosphere. This gas flow path includes a lateral gas pass 7 leading from the upper portion of the furnace chamber and arranged above an arch 8 including parts of the vapor generating tubes and projecting inwardly from the vertical plane of the rear furnace wall along the vapor generating tubes 3. This gas flow path further includes a downfiow gas pass 9 conducting the gases over a primary superheater and an economizer section 10 and leading thence over the surfaces of an air heater 11, and then through a stack to the atmosphere.

Extending from the upper part of the furnace chamher is a reheater gas pass 12 which may be considered as a supplementary gas passage forming a part of a closed gas circuit of a gas recirculation system and having disposed therein a convection vapor reheater 13. At the entrance of the gas passage 12, the associated wall tubes 2 of the furnace chamber are more widely spaced to form a screen 14 to permit the free entrance of gases to the reheater gas pass 12 and to provide radiant heat protection for the reheater.

In addition to the lateral reheater gas passage 12, the closed gas circuit of the Fig. 1 unit includes a gas flow path partially within furnace chamber 1, a recirculated gas fan 15, fan inlet duct-work 16 constituting the only gas outlet of the reheater gas pass 12 and leading therefrom to the inlet of the recirculated gas fan 15, and fan outlet duct-work 17 leading from the outlet of the fan to the lower part of the furnace chamber 1. The fan 15 is adapted to effect recirculation of furnace gases at a regulable rate through the reheater gas pass 12, the duct 16 and the duct-work 17 to the lower part of the furnace chamber. The duct work 17 terminates in suitable discharge means adapted to give the most favorable orientation and distribution of recirculated gases in relation to the combustion zone within the furnace chamber 1 Preferably this duct work has a plurality of regularly spaced openings provided by the screen parts 18 of the wall tubes 2 distributed throughout the width of the associated furnace chamber wall.

Steam, or generated vapor, in the operation of the unit, flows through suitable circulators from the steam and water drum -4 to the inlet header 19 of the convection primary superheater 20. From the outlet header 21 of this superheater the superheated steam flows through appropriate conduit means 22 and 23 and an interposed spray attemperator 24 to the inlet header 25 of the secondary superheater 26. From the outlet header 27 of this superheater, the superheated steam flows to the steam inlet to the high pressure stage 28 of the steam turbine. From the exhaust of this turbine stage the lower pressure steam flows through conduit means 29 to the reheater inlet header 30 and thence through the convection reheater 13. From the outlet header 31 of the reheater the steam flows through conduit means 32 to the steam inlet of the lower pressure turbine stage 33.

The walls of the superheater gas pass 7 and the reheater gas pass 12 are of appropriate construction and they may include vapor generating tubes connected into the fluid circulation of the furnace chamber in the same manner as that in which the furnace chamber wall tubes are connected.

In addition to the reheater 13, the closed gas path for the reheater may also contain a superheater section, and, in some cases, this gas flow path may be also provided with heat exchange surfaces other than vapor heating surfaces, such as the surfaces of an economizer section.

In operation, a portion of the furnace gases, instead of flowing directly from the furnace chamber to the atmosphere through the gas passages 7 and 9, is recirculated through the closed reheater circuit including the

components

12, 15, 16 and 17 and thus in heat exchange relationship with the heating surfaces of the reheater 13, and by controlling the rate of gas recirculation the final temperature of vapor reheat may be maintained at a predetermined value, over a wide range of load or rate of vapor generation, thus overcoming the inherent tendency of the convection reheater 13 to effect undesirably low reheat temperatures at low loads, and particularly those loads substantially below normal load or control point load.

The degree to which the recirculated gases are cooled depends upon the total provisions within that passage for effecting heat transfer from the gases. The gases cooled by passing through the reheater gas passage 12 and entering the furnace chamber at its lower part are subsequently mixed with the unrecirculated products of combustion from the fuel burning means 6, before they reach the entrances of the gas passages 7 and 12. The recirculated gases therefore have a tempering action which may be controlled, in the case of pulverized coal firing, to minimize slag accumulations upon the convection surfaces exemplified by the reheater 13 and the secondary superheater 26. The recirculated gases also reduce the residence time of the unrecirculated gases Within the furnace, and they reduce the area and volumetric extent of the heat emitting unrccirculated gases and thereby reduce heat radiation to the vapor generating wall tube surfaces of the furnace chamber, thus providing a higher proportion of heat in the gases available for reheating the generated vapor.

The illustrative unit also provides for the highest permissible gas temperature at the entry of the reheater gas pass 12, thus permitting the attainment of a predetermined high reheat temperature at low loads, and minimizing extent and cost of the reheater. Moreover, the attainment of optimum superheat temperature is facilitated since none of the heat in the gases flowing through the gas passes 7 and 9 is required for reheating. Economy in the extent and cost of the superheater is therefore attained. In addition, cooling of the furnace gases to the requisite degree, before reaching the convection heat exchange surfaces provides for the reduction in size and cost of the radiant section of the unit.

Placing the reheater within the supplementary gas passage 12 rather than within the gas flow path leading to atmosphere provides for better protection of the reheater during periods of disturbance such as during outages of the generator driven by the steam turbine, and during starting up of the unit. The arrangement facilitates protective regulation of superheat and reheat temperatures during starting up of the unit.

Fig. 2 of the drawings may be considered as illustrating two modifications of the Fig. 1 unit. Each of these additional modifications involves the primary superheater 40, the intermediate superheater 42 and the final or secondary superheater 44, with the attemperator '46 disposed in the line 48 connecting the outlet header 50 of the intermediate superheater 42 to the inlet header 52 of the final superheater 44. It will appear from the drawing that the second modification is otherwise similar to the Fig. 1 unit.

The third modification is considered as illustrated by Figs. 2 and 3 when considered together. In this third modification, the secondary superheater 44 is disposed in a gas passage 56 which is a part of one of two separate gas recirculation systems. The first of these systems includes the recirculated gas fan 58, the fan outlet duct work 60, and the fan inlet duct work 62 which is the only gas outlet from the gas exit of the gas passage 56. In the third modification, there is a second and separate gas recirculation system located in parallelism with respect to the gas recirculation system including the gas passage 56 and the secondary superheater 44. This second parallel gas passage is indicated in Fig. 3 as having therein a reheater 64 which is similar to the reheater 13 of the Fig. 1 modification. Otherwise, this second gas recirculation system is similar to that involved in either one of the Fig. 1 and Fig. 2 modifications.

In the operation of the illustrative unit of Fig. 1, the flow of recirculated gases into the furnace chamber 1 may be controlled from a plurality of operative influences or variables. For example, such flow of recirculated gases may be controlled from representations of gas tem perature at the position 70 immediately in front of the secondary superheater 26. Another variable in the control of such recirculated gas would be representations of the outlet temperature of the reheat vapor or steam at such a position as that indicated at 72, in the line conducting reheated vapor .to the turbine stage 33.

Such control may be manual or it may be automatic. In the latter case, it would be effected by known control systems such as that indicated in the co-pending application 193,832 filed in November 3, 1950 now abandoned and belonging to the same assignee.

Additional control of the Fig. 1 unit would preferably involve the control of the rate of firing of the fuel burning means 6 from representations of steam or vapor flow at such a position as that indicated at 74, in the line 76 leading from the secondary superheater outlet header 27 to the steam inlet of the high pressure turbine stage 28. Another simultaneously effective influence upon the rate of fuel firing would be the pressure of g. the vapor at such a position as that indicated Further control of the operation of the Fig. 1 unit might also involve controls such as those disclosed in the pending applications, to Moonan Ser. No. 542,927, Troutman Ser. No. 542,925, and Stallkamp Ser. No. 542,926 (filed -2655), for so controlling the operation of the recycled gas system and more particularly, the gas flow in the vicinity of the recycled gas fan, that any reverse flow of gases from the furnace into the normal outlet of the recycled gas fan is prevented at low rates of flow of recycled gas.

The second modification illustrated by Fig. 2 alone would have controls similar to those of the Fig. 1 unit but, of course, the vapor temperature control would be in the line leading from the outlet of the secondary superheater 44.

The control of the third modification illustrated by Figs. 2 and 3, when considered together, would be similar to the above indicated controls, modified by the inclusion of superheat temperature control for the superheater recycled gas system including the gas pass '56, and including reheater outlet vapor temperature control for the recycle gas flow in the parallel recycled gas system including the reheater -64.

As further illustrative of the steam temperature control of the invention, the attemperator, such as indicated at 24 or 46, may be of the type indicated in the US. patent to Fletcher et al., 2,550,683, of May 1, 1951.

Although the invention has been described with reference to a few specific modifications it is to be recognized that the invention is not limited to all of the details of any one of these modifications. It is rather to be considered as of a scope commensurate with the scope of the subjoined claims.

What is claimed is:

1. In a fluid heat exchange unit, a single furnace having vapor generating wall tubes, fuel burning means for supplying the furnace with high temperature heating gases for the radiant transmission of heat therefrom to the fluid within the tubes, means forming a gas flow path leading from the furnace gas outlet to the atmosphere, vapor superheater subject to the heat of the gases in said gas flow path and beyond the gas outlet of the furnace, means forming with a part of the furnace a closed gas flow path leading from another part of the furnace adjacent its gas outlet and upstream of said vapor superheater to a position within the furnace remote from said gas outlet, other fluid heat exchange means disposed within said closed gas flow path and subject to the heat of the gases therein, and means for controlling the flow of recycled gases through said closed gas flow path to thereby control the temperature of the fluid heated by said other fluid heat exchange means.

2. In a fluid heat exchange unit, a single furnace having vapor generating wall tubes, fuel burning means for supplying the furnace with high temperature heating gases for the radiant transmission of heat therefrom to the fluid Within the tubes, means forming a gas flow path leading from the furnace gas outlet to the atmosphere, a vapor superheater subject to the heat of the gases in said gas flow path and extending beyond the gas outlet of the furnace, means forming with a part of the furnace a closed gas flow path leading from another part of the furnace adjacent its gas outlet and upstream of said vapor superheater to a position within the furnace remote from said gas outlet and spaced from the position of fuel burning, other vapor heating means disposed within said closed gas flow path and subject to the heat of the gases therein, and means for controlling the flow of recycled gases through said closed gas flow path to thereby control the temperature of the fluid heated by said other vapor heating means.

3. In a fluid heat exchange unit, a single furnace having vapor generating wall tubes, fuel burning means for at 74 in 6 supplying the furnace with high temperature heating gases for the radiant transmission of heat therefrom to the fluid within the tubes, means forming a gas flow path leading from the furnace gas outlet to the atmosphere,

11a. vapor superheater subject to the heat of the gases in said gas flow path and extending beyond the gas outlet of the furnace, means forming with a part of the furnace a closed gas flow path leading from another part of the furnace adjacent its gas outlet and upstream of said vapor superheater to a position within the furnace remote from said gas outlet and spaced from the position of fuel burning, a convection vapor reheater disposed within said closed gas flow path and subject to the heat of the gases therein, and means for controlling the flow of recycled gases through said closed gas flow path to thereby control the temperature of the vapor exiting from the reheater.

4. In a vapor generating, vapor superheating, and reheating unit; means forming a radiantly heated section including a single furnace chamber with vapor generat ing wall tubes; gas passage means providing a gas flow path leading from the radiant section to atmosphere; fuel burning means providing the furnace chamber with high temperature heating gases; said gas passage means providing a superheater gas passage leading from the gas outlet of the furnce chamber; a convection superheater disposed within said gas passage; means providing a reheater gas passage separate from the superheater gas passage and leading from the furnace chamber at a position adjacent its gas exit and upstream of said superheater; a gas recirculation system affording a closed gas circuit and including a recirculated gas fan and fan inlet duct work forming the gas outlet of the reheater gas passage, and fan outlet duct work communicating with the furnace chamber at a location spaced from its fuel burning means; and a vapor reheater disposed within the reheater gas passage.

5. A steam boiler having walls defining a single furnace chamber, means for burning fuel in said furnace chamber, steam generating wall tubes arranged to absorb radiant heat from said furnace chamber, means forming a gas outlet flue opening to and arranged to receive heating gases from said furnace chamber at a location remote from said fuel burning means, a convection heated steam superheater in said gas outlet flue, means forming a second gas outlet flue opening to said furnace chamber at a location spaced from said first gas outlet flue and remote from said fuel burning means, a steam reheater in said second gas outlet flue, and a gas recirculating fan arranged to withdraw heating gases from said furnace chamber through said second gas outlet flue and to return said heating gases to said furnace chamber at a location in the gas flow path through said furnace chamber upstream of said second gas outlet flue.

6. A steam boiler having walls defining a single furnace chamber, means for burning fuel in said furnace chamber, steam generating wall tubes arranged to absorb radiant heat from said furnace chamber, means forming a gas outlet flue opening to and arranged to receive heating gases from said furnace chamber at a location remote from said fuel burning means, a convection heated steam superheater in said gas outlet flue, means forming a second gas outlet flue opening to said furnace chamber at a location spaced from said first gas outlet flue and remote from said fuel burning means, a steam reheater in said second gas outlet flue, and a gas recirculating fan arranged to withdraw heating gases from said furnace chamber through said second gas outlet flue and to return substantially all of the heating gases so withdrawn to said furnace chamber at a location in the gas flow path through said furnace chamber upstream of said first and second gas outlet flues.

(Other references on following page) References Cited in the file of this patent UNITED STATES PATENTS Lacerenza Mar. 25, 1952 Kuppenheimer July 8, 1952 Blaskowski Aug. 3, 1954 Armacost et a1 1- Feb. 19, 1957 8 FOREIGN PATENTS France Aug. 4, 1954 Great Britain July 24, 1940