US2840055A - Method and apparatus for generating steam - Google Patents

Description

June 24, 1958 H. J. BLASKOWSKI METHOD AND APPARATUS FOR GENERATING STEAM Filed June 20, 1955 Reheuter INVENTOR Henry J. Bloskowski ATTORNEY BBE w 323. G 6E Superheater 50 5O Steam Generator Loud Steam Generator Loud .METHOD AND APPARATUS .FOR GENERATING STEAM Henry J. Blaskowski, New York, N. Y., assignorto Combustion Engineering, Inc, New York, N. Y., a corporation of Delaware Application June 20, 1955, Serial No. 516,435

11 Claims. (Cl. 122-479) This invention relates to the generation of steam and has particular reference to steam generators operating on the reheat cycle and a method of operating and controlling such generators wherein both the superheated steam and reheated steam temperatures are maintained substantially constant throughout a rather wide range of loads imposed upon the steam generator.

In accordance with the present invention there is provided a reheat steam generator or in other words a steam generator operating under the well known reheat cycle and including a furnace which is lined with steam generating tubes and into which fuel is introduced and burned therewithin with the combustion gases thus generated passing out one end of the furnace. The steam generator is provided with a superheater which is divided into high and low temperature sections and a reheater with these heat exchange surfaces being arranged so that the combustion gases generated in the furnace serially pass over the high temperature superheater section, the reheater section and then the low temperature superheater section. In order to prevent the superheated steam temperature and reheated steam temperature from falling below their desired value as the load on the steam generator decreases means are provided for recirculating combustion gases back into the furnace with the location of the introduction of these gases into the furnace being such as to reduce the amount of radiation to the furnace walls and accordingly make more heat available for the superheater and they reheater resulting in raising the superheated and reheated steam temperatures. The gases thus recirculated are withdrawn from the stream of gases flowing over the superheater and reheater at a location downstream of the reheater but upstream of the low temperature section of the superheater so that these recirculated gases bypass this low temperature superheater section. Positioned within the stream of gases bypassing this low temperature superheater section and reintroduced into the furnaceris an auxiliary reheater which is connected in series with the main reheater of the boiler. This auxiliary reheater is so constructed and arranged relative to the other steam heating heat exchange surface in the generator as to maintain a substantially constant differ-v ential between the reheated and the superheated steam temperatures over a relatively wide variation in load imposed upon the steam generator. Means are provided to regulate the amount of gas recirculated into thefurnace with this means being eifective together with the auxiliary reheater to maintain the superheated and reheated steam temperatures at their desired value throughout a wide load range with the amount of gas recirculated increasing as the load decreases. If desired additional control means may be provided which is effective tovary the temperature of the superheated and reheated steam relative to each other independently of the gas recirculation control, with tilting burners, effective to adjust the combustion zone toward and away from the furnace outlet, beinga, convenient control for this purpose.

2,840,055 Patented June 24, 1958 a It is an object of this invention to provide an improved steam generator operating on the reheat cycle and wherein the superheated and reheated steam temperatures are maintained substantially constant throughout a rather wide load range. I

-Another object of this invention is to provide an improved method whereby thesuperheated and reheated steam temperatures of a steam generator operating on the reheat cycle are maintained constant'throughout a:

rather wide load range. Other and further objects of the invention will become apparent to those skilled in the art as the description tion of the curves of the Figure 3.

proceeds. V With the aforementioned objects in view, thejinvention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawing wherein:

Figure l'is a diagrammatic representation of a reheat unit embodying the present invention.

Figure 2 is a set of curves showing the respective portions of heat absorbed by the superheater and reheater at various percentages of steam generating load.

' Figure 3 is a set of curves showing reheat and superheat steam temperatures that would prevail at various percentages of load it no special means were provided to maintain these temperatures constant. In a modern steam generator operated on the reheat cycle and wherein fuel is fired into a furnace lined with steam generating tubes and the combustion gases generated by the burning of this fuel are conveyed over a reheater and superheater to reheat and superheat respec tively the steam generated by the unit, it is essential that the reheated and superheated steam temperatures be maintained constant throughout a rather wide range of loads that may be imposed upon the steam generator. These units are generally designed so that whenthey are operat ing at their maximum load (i. e., 100% of rated load) they will produce the desired reheat and superheat steam temperature with no extraneous temperature controls being in operation. As the load decreases the firing'rate of the unit is correspondingly decreased with this firing rate being adjusted so as to maintain the pressure of the unit, i. e., the steam pressure at the throttles of the turbine, substantially constant. i

As the load on such a-reheat unit decreases from itsj maximum or 100% value there are inherent character istics within the unit itself that require the use of'supple} mental controls in order to maintain the superheated and reheated steam temperatures substantially constant. The first of these characteristics is evidenced by a considerafied as 10 and 12 indicate the temperature of the super heated and reheated steam, respectively, as the load on the unit decreases and wherein no supplemental controls are employed to maintain these temperatures substan-' tially-constant. From an observation of this Fig. 3 it is apparent that as the load decreases from the maximum or 100% value, both the superheated steam and the r'e-' heated steam temperature fall off rather sharply with that p of the reheated steam decreasing at a more rapid rate than that of the superheated steam. Because of this in-.' herent characteristic a supplemental control which may be called the primary control must be employed to offset this effect. Gas recirculation is one type of control that has been used'for this purpose with the effect of reintroducing combustion gases into the furnace at certain loca-' tions being to reduce the amount of radiation to the steam generating tubes lining the furnace walls and thereby in- These curves identicreasing the heat available in the combustion gases passing over the superheater and reheater to thereby increase the superheated and reheated steam temperatures. Thus as-the load on the unit is decreased the amountof gas thatjs'recirculated may be increasedaccordingly compensatingfor the. tendency of the superheated and reheated steamttemperaturesto fall with a decrease in load.

The other inherent characteristic in such a reheat unit and which inecessitatesstill. further supplemental controls isapparent from a consideration of Fig. 2 wherein curves 1-1 ands13; respectively, represent the percentage of the totalIheat absorbed by the superheater andreheater that is imparted to each of these steamheaters. From this Fig.,2.it;can.be seen that of the total heat absorbed, by these. twosteam heaters-the percentage absorbed by the reheater increases relative to that absorbed by the superheateramthe; load decreases with these curves representing; the percentage of heat absorption required in order: to maintain the superheated and reheated steam tempera tures-substantiallyconstant. This characteristic may also be evidenced to a degree from the curves of Fig. 3 wherein it is shown that with no control on the unit at all the reheat temperature falls off more rapidly than the superheat temperature thereby indicating that a greater percentage of heat is required to maintain the reheat temperature constant than is required to maintain the superheat constant. Because of the varying percentage of heat absorption between the superheater and reheater with varying loads a further control in addition.to the previously mentioned primary control is generally required in order that the reheat and superheat steam. temperatures may be maintained constant over a predetermined desired load, range. This additional control,,which may be termed a secondary control, has heretofore generally taken the form of a desuperheater, as shown in U. S. Patent 2,663,287 issued, in 'Dec.

22,1953 to W. H. Armacost or a bypass damper arrangement such as shown in U. S. Patent 2,649,079 issued in Aug. I8, 1953 to J. Van Brunt.

In accordance with the present invention this customarily employed secondary control may be eliminated absorbed by the reheater relative to that absorbed by the superheater must likewise be increased as the load decreases. Thus, if the stream of gas, that is being reintroduced into the furnace bypasses at least a portion of thesuperheater of the unit and is directed over an auxiliary reheater surfacethe percentage of heat imparted tothe steam to be reheated will automatically increase relative to that imparted to the steam to be superheated as ,the amount of recirculated gas is increased so that both the primary and, secondary control effects are produced merely by the recirculation of gases. The percent invention achieves this goal. Furthermore, becauseit may be impractical to design the various reheater and superheater surfaces with such accuracy as to get the exact desired. reheat and superheat temperatures throughout the entire control range and since the heat transfer characteristics of thesurfaces may well vary during the operationof the unit because of dirt deposit and the like,,an additional control may be provided to vary the superheat and reheat temperatures relative to each other with this control being independent of the control obtained bygas recirculation. With the present invention this secondary control may advantageously take the form of tiltable burners which heretofore were only applicable as a primary control and whereby the zone of' combustion in the furnace may be adjusted toward or 4- away from the outlet ofthe furnace, as shown in Patent 2,363,875 issued in Nov. 28, 1944 to H. Kriesinger et al. with this control being effective to correct any minor variation of the superheat and reheat temperatures relative to each other, although with this invention the required control eifect of this secondary control will be relatively minor and it will only act as a Vernier control to maintain the superheated and reheated steam temperature in proper relation to each other.

Referring now to Fig. 1 there is disclosed a steam generator Which comprises furnace 14 the inner face of the walls of which are lined with steam generating tubes 16 and into which a suitable fuel such as oil, pulverized coal or gas, together with the combustion supporting air, is introduced through burners 18. This fuel is burned within furnace 14 with the combustion gases thus generated passing upwardly through the furnace and out its outlet 20 into duct or gas pass 22 through which these gases are conveyed to a suitable stack as indicated. Steam generating tubes 16 are connected into the steam generating circuit ofthe boiler in the usual manner with a portion of the water in these tubes being converted to steam as it passes upwardly through these tubes with the heat that is utilized for this generation of steam being principally radiant heat liberated within furnace 14. The steam and watermixture leaving the upper ends of tube 16 is conveyed into drum 24 with the steam there being separated sageway 22' so that the gases leaving furnace 20 traverse,

thesersuperheatersections and impart heat to the steam being conveyed through these sections. The saturated steam in conduit 28 is first passed through the low temperature section 30 of the superheater after which it is collected in header 34 and then passed through the high temperature section 32. From this high temperature section the superheated steam is conveyed to the high pressure stage 36 of the turbine and after passing throughthis high pressure turbine it is returned through conduit 38 to the boiler to be reheated before being introduced into the low pressure stage 40 of the turbine.

The reheater through which this returned steam is conveyed and reheated to a desired temperature and pressure is divided into two sections with the main reheater section identified as 42 being disposed in passageway 22 intermediate the high temperature and low temperature superheater sections and with the supplemental or auxiliary reheater section identified as 44 and which is connected in series to section 42 being disposed in duct 46 which communicates with passageway 22 at a location between reheater section 42 and low temperature 'superheater section 30. Duct 46 is thus in by-pass rela tion with low temperature superheater section 32 and the outlet of this duet leads to an inlet of fan 48 which is driven by variable speed motor 50 with the outlet of the fan communicating with the lower end of furnace 14 through duct 52. Thus fan 48 is effective to recirculate combustion gases from passageway 22 back into furnace 14 for the purpose of controlling steam temperature in a well known manner with the gases thus recirculated bypassing low temperature superheater section 30 and being conveyed over auxiliary reheater section 44.

After the steam to be reheated passed through reheater sections 42 and. 44 it is conveyed through conduit 50 to the low pressure stage 40 of the turbine after which it passes through a condenser as is well known in the art.

In the illustrative organization the superheat and reheat steam temperatures may both be controlled and maintained at their desired values throughout a relatively wide load range by means of the gas recirculation system disclosed. As explained hereinbefore, as the load on such a unit decreases the amount of gas recirculation required progressively incre: ses, and with the organization of this invention, variable speed motor 50 (or dampers if a constant speed motor is employed) may be controlled in response to either the temperature leaving the superheater or the temperature leaving the reheater to regulate the quantity of recirculated gas as required to maintain the desired superheat and reheat steam temperatures with this control being shown in Fig. 1 as responsive to the outlet temperature of the superheater of the unit with temperature sensitive element member 52 responding to this temperature and being interconnected with control panel 54 which in turn controls the speed of motor 50.

The inherent characteristic of the superheater and reheater to vary their required percentage of heat absorption with varying load, i. e., of the total heat absorbed by these two steam heaters the percentage absorbed by the reheater increases relative to that absorbed by the superheater as the load decreases, is automatically compensated for through the positioning of auxiliary reheater section 44 in gas recirculation duct 46 which is in turn inparallel relation with the portion of passageway 22 in which low temperature superheater section 30 is positioned. Thus the stream of gas that is recirculated does not pass over low temperature superheater section 30 but does pass over auxiliary reheater section 44 thereby effectively increasing the relative percentage of total heat available to the reheater relative to that available to the superheater with this effect being increased as the amount of gas recirculated is increased whereby the aforemendesired a diagrammatic control organization is shown connected to control panel 58 which in turn controls tioned characteristic of the superheater and reheater is automatically compensated for. I 7 r V The various superheater and reheater surfaces may be proportioned in such a manner that both the superheated and reheated steam temperatures may be maintained at their desired value solely by the gas recirculation control. However, since it may be difiicult if not impractical to precisely calculate and design these surfaces for this re-' sult and since the heat exchange characteristic of the various surfaces may vary during operation because of deposits and the like forming on the surfaces it maybe desirable to provide a control in addition to the gas re-, circulation control disclosed which will act as a vernier to adjust the reheat and superheat steam temperatures relative to each other and independently of the gas recirculation control. This may advantageously be accomplished in the organization of this invention by making burners 18 of the tiltable type so that the zone of combustion in furnace 14 may be moved longitudinally of the furnace toward and away from furnace outlet 20. With this supplemental control, if the reheat temperature rises above its desired value while the superheat temperature remains at its desired value, or in other words, the predetermined differential between the superheated and reheated steam temperature changes because of an increase in the reheated steam temperature, this may be corrected for by tilting the burners upwardly so the zone of combustion is moved toward outlet 20 while if the opposite effect occurs, i. e., the reheated steam temperature falls below its desired value while the superheat steam temperature remains at its desired value, this may be corrected by tilting the burners downward to move the zone of combustion away from outlet 20. With this supplemental control of tilting burners a more flexible organization is of course produced with regard to the control of steam temperature with the amount of this supplemental effect thatshould be needed being relatively small. Of course if burners 18 are tilted to bring the superheat and reheat steam temperature back to their proper relation with respect to each other this will change somewhat the control effect of the gas recirculation which will be automatically readjusted to give the desired superheated and reheatedsteam temperatures. While the tiltway downstream of this economizer.

which includes differential temperature sensitive device 56 which responds to the difierential temperature of the superheated and reheated steam with this device being motor 66 that regulates the tiltable burner 18,-

While the tiltable burner control is preferred as a secondary control for use with the inventive organization other secondary controls such as desuperheaters at the outlet of the superheater and/or reheater mayof course be used although they are somewhat less desirable from an economic standpoint than the preferred secondary control of tilting burners.

In order that the temperature of the gases leaving passageway 22 and entering the stack. may be sufficiently reduced for economic operation of the unit economizer 62, of conventional construction, is positioned in passageway 22 downstream of low temperature superheater section 30 and air heater 64 is also disposed in this passage- A portion of economi'zer 62 is positioned within gas recirculation duct 46 so that the temperature of the gases to which fan 48 is subjected is reduced 'sufi'iciently so that they can be handled by this fan.

I With the organization of the present invention the gases that are recirculated do not pass over the low temperature superheater section 30 and the economizer 62 so that the gas temperature leaving the economizer section is lower than that obtained with known organizations utilizing gas recirculation and the draft loss across the low temperature superheater section and the economizer is likewise lower than that obtained with such previously known organizations wherefore the eificiency of the steam generator is improved.

While I have illustrated and described a preferred embodiment of my novel organization it is to be understood that 'such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the inven tion. I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fallwithin the purview of my invention.

What I claim is:

1. In a steam generator operating on the reheat cycle and wherein fuel is fired into a furnace lined with steam generating surface with the streams of combustion gases produced from the burning fuel being serially conveyed over reheater and superheater surfaces for reheating and superheati ng, respectively, the steam generated and with the reheating and superheating being effected from the heat evolved by burning fuel in said furnace, the method of controlling the reheat and superheat steam temperatures over a substantial load range comprising withdrawing combustion gases from the stream of gases issuing from the furnace at a location where they have passed over the reheater but before they have passed over a portion of the superheater, introducing the gases so withdrawn into the furnace at a location to cause the reheat and superheat steam temperatures to rise relative to those obtained without this recirculation of combustion gases,

regulating this introduction of combustion gases into the furnace so as to maintain the superheated steam temperature substantially constant throughout said substantial load range with the regulation increasing and decreasing the recirculation with decreasing and increasing load respectively, and after withdrawing said combustion gases from said stream imparting heat from this stream of gases that is to be reintroduced into the furnace to the steam to be reheated prior to reintroduction of the stream of gases into the furnace and in sufficient quantity so that the tendency of the reheat steam temperature to vary relative to the superheat steam temperature with varying load is substantially offset so as to maintain the reheat steam temperature substantially constantthroughout said substantial load range; g v p 2; The method rof"'.'clai m 1' including correcting any minor discrepancies between the desired superheat and reheat temperatures obtained byinitiating'a control action independently ofthe control obtained by gas recirculation and effective to vary these temperatures relative to each other in a manner to maintain themin a predetermined relative relation by bringing these temperatures closer together or further apart as required to maintain said predetermined relative relation. r V

3. In a steam generator operating on the reheat cycle andwherein fuel in'introduced into an elongated furnace having heat exchange surface including steam generating surface onits walls with the fuel being burned therewithin and with the stream of combustion gases thus produced passing from .said furnace at one end thereof and serially passing over reheaterand superheater surface for reheating and susperheating, respectively,. the steam generated, the method ofcontrolling the reheat an'd'superheat steam temperatures over. a substantial load range comprising withdrawing combustion gases from said stream at a location where they have passed over the reheater but before they have passed over a portion of the superheater, introducing the gases so withdrawn into the furnace at a location to cause the reheat andsuperheat steam temperatures to rise relative to those obtained without this recirculation of combustion gases, regulating this introduction of combustion gases into the furnace so as to maintain the superheated steam temperature substantially constant throughout said substantial loadrange with the regulation increasing and decreasing the recirculation with decreasing and increasingload respectively, imparting heat from the stream of gases to be reintroducedv into the furnace to the steam to bereheatedafter. said? withdrawal of said gases but prior to reintroduction of the same into the furnace to maintain the reheat steam temperature substantially constant over said substanti'al load range, correcting any material variations in temperature differential between the reheat and superheat temperatures thus obtained by adjusting the zone of combustion longitudinally within the. furnace. I "T 4. The method of operating a steam generator on the reheat cycle and controlling the reheat and superheat steam temperatureover a wide range of load comprising injecting and burning fuel in a furnace at least a portion of the walls, of which are lined with steam generating tubes, imparting a portion of the radiant heat thus evolved in the furnace to water in these tubes to convert at least a portion of this water to steam, passing the stream of combustion gases generated by this burning fuel in heat exchange relation with the steam generated in said tubes at at least two locations one of which is downstream of the other relative to this combustion gas flmvthereby superheating said steam, reheating this steam after a portion of its energy has been utilized by passing it in heat heat exchange relation with said stream of gases at a location intermediate said two locations, withdrawing a portion of the gases from said stream at a location downstream of the reheat location and upstream of the downstream one of said two locations, reintroducing these gases into the furnace so thatthe radiation to the furnace walls is decreased, imparting heat from these withdrawn gases after the same have been withdrawn but prior to introduction thereofinto the furnace to the steam being returned for reheating thereby aiding in reheating. the same, regulating the quantity of gas recirculated in a mannerto maintain the superheated and reheated steam temperatures substantially constant with varying. load with the regulation increasing and decreasing the recirculation with decreasing and increasing load respectively, and correcting any material variations of temperature differential between the reheat and superheat; temperatures that may be evidenced with varying load by initiating a control action independent of the control obtained by the, recirculation of combustion gases andeffective to varyth'ese temperatures relative to each other in a manner'to; maintain-them in a predeterminedrclative relation By-bringing these temperatures closertogether or further apart as requiredtomaintain said predetermined relative relation:

5. In-areheattype steam gencratorthe combination of a furnace lined with steam generating tubes and having a combustion gas'outlet at one end, means for superheating and reheating-the steam generated by the generator including a convection reheat heat exchanger and a convection superheat heat exchanger, means for conveying the combustion gases issuingfrom the furnace serially over these heat exchangers, means adapted to by-pass regulated quantities of combustion gases aroundsaid convection superheat heat exchanger and introduce,

changer over which the stream of bypassing gases are c011- veyed and which is connected in series with-said reheater, said last named heat exchanger being effective as a supplement to said reheater and being constructed and arranged to retain a substantially constant temperature differential between the reheat and superheat temperature throughout the control range obtained by the recirculation of combustion gases, means for regulating the gas flow through the bypass and into the furnace. I

6; The organization of claim 5 including means effective to vary the superheat and reheat steam temperatures relative to each other and independently of the gas recirculation control.

7. The organization of claim 5 including means to adjust the zone of firing toward and away from the furnace outlet.

8. A steam generator operating on the reheat cycle and including an elongated furnace lined with steam generating tubes and into which fuel is introduced and burned with the combustion gases passing through an outlet adjacent one end thereofrhigh temperature superh eater-means,1reheater means and low temperature, superheater means, means for conveying the combustion gases generated in the furnaceserially there over, said two superheater means being'interconnected andet'fectivc to superheat the steam generated by the generator and said reheater being effective to reheat this steam after a portion of its energy has; been utilized; means for withdrawing combustion gases from a location intermediate the reheater and low temperature superheater, by-passing said low temperature superheater therewith and introducing the same into the furnace at a'location where they are effective to decrease the radiation to the furnace walls and thereby raise the superheat and reheat steam temperatures over those obtainable without this recirculation of combustion gases; means for regulating the quantity of combustion gases so withdrawn and reintroduced into the furnace so as'to maintain one of the steam temperatures substantially constant over a predetermined load range; and additional reheater means in series steam flow with the first mentioned reheater means anddispo sed so that the stream of gases being reintroduced into the furnace are directed thereover prior to'being introduced into the furnace but after being formed into a separate stream, said additional reheater means being so constructed and arranged to maintain-a substantially constant temperature differential between the reheat and superheat steam temperature throughoutsaid predetermined load range.

9. In avapor generator operating on the reheat cycle and wherein fuel is introduced into a furnace having heat exchange surface, including steam generating surface on its walls with the fuel being burned therewithin and with the stream of combustion gases-thus produced passing from said furnace and over reheater and superheater surface for reheating and superheating respectively the vapor generated with the reheating and superheating being effected from the heat evolved by burning fuel in said furnace, the method of controlling the reheat and superheat vapor temperatures over a substantial load range comprising withdrawing combustion gases from said stream at a location where they have passed over the reheater and a portion of the superheater thereby forming a separate gasiform stream, introducing this gasiform stream into the furnace at a location to cause the reheat and superheat temperatures to rise relative to those obtained without this recirculation of combustion gases, regulating the quantity of gases so introduced so as to maintain the superheat temperature substantially constant with varying load with the regulation increasing and decreasing the recirculation with decreasing and increasing load respectively, and imparting heat from this separate gasiform stream prior to its introduction into the furnace to the vapor returned to the generator for reheating and in sufficient quantity to tend to maintain said reheat temperature also at a substantially constant value.

10. In a reheat type vapor generator the combination of a furnace lined with vapor generating tubes and having a combustion gas outlet at one end, means for superheating and reheating the vapor generated by the generator including a convection reheat heat exchanger and a convection superheat heat exchanger, means for conveying the combustion gases issuing from the furnace serially thereover, a bypass means adapted to bypass regulated quantities of combustion gases around the superheater and introduce them into the furnace at a location where they are eifective to raise the superheat and reheat vapor temperatures over those obtainable without this recirculation of combustion gases to thereby provide a control for the superheat and reheat temperatures over a substantial load range, a heat exchanger disposed within this bypass means so that the gases flowing therethrough are conveyed thereover, said heat exchange means being connected in series with said reheater, and means for regulating the gas flow through the bypass means.

11. A vapor generator operating on the reheat cycle and including a furnace lined with vapor generating tubes and into which fuel is introduced and burned, a superheater and reheater, means for conveying combustion gases generated in the furnace thereover, said superheater and reheater being respectively effective to superheat the vapor generated by the generator and reheat this vapor after a portion of its energy has been utilized, said superheater and reheater being disposed in a passageway through which said combustion gases pass, means for recirculating back to the furnace regulated portions of the gases traversing said passageway to tend to maintain the superheat and reheat vapor temperatures substantially constant as the load decreases said means including a duct leading from said passageway at a location downstream of the reheater to said furnace and a heat exchanger disposed in said duct and connected in series with the reheater.

References Cited in the file of this patent UNITED STATES PATENTS

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