US2776647A - Steam generating unit - Google Patents

Description

Jan. 8, 1957 c. HAWLEY I 2,775,647

STEAM GENERATING UNIT Filed April 24, 1952 INVENTOR CHARM 1 E HRH/LE) ATTORNEY United States Patent STEAM GENERATING UNIT Charles F. Hawley, Holden, Mass., assignor to Riley Stoker Corporation, Worcester, Mass, a corporation of Massachusetts This invention relates to a steam generating unit and more particularly to a means for controlling the temperature of feed-water, superheated steam, and reheated steam.

In modern power plants it is common practice to pass superheated steam from a steam generating unit into a first turbine and then return the exhaust steam to the generating unit to be reheated again for introduction into a second turbine. The steam entering the superheater tubes is at a constant temperature, irrespective of load, since it is the steam at saturation temperature; the steam leaving the superheater to enter the first turbine or first turbine stage is required to be at a constant temperature also, despite changes in load. It is the nature of the turbine to exhaust at higher temperatures at high loads, so that the steam entering the reheater tubes will be at low temperature for low loads and high temperature for high loads, the temperature varying in a generally straight-line relationship; the temperature of the steam leaving the reheater tubes and entering the second turbine is required to be at a constant temperature irrespective of load. Generally speaking, then, it is necessary to increase the heat transfer to the superheater tubes and to the reheater tubes as the load is increased, but the amount required is not the same. The increase in heat transfer to the reheater tubes with a given increase in load requirement is considerably less than the increase in heat transfer to the superheater required for the same increase in load. In the past attempts have been made to keep the exit temperatures from the superheater and the reheat tubes constant at the required level for the respective turbine irrespective of load by using various combinations of radiant and convection sections. These attempts have not been entirely successful, particularly because of the inflexibility of the system; it is practically impOssible to design the steam generating unit to give the desired characteristics. The present invention obviates the difiiculties experienced in the operation of reheater units in a novel manner.

It is therefore an outstanding object of the present invention to provide apparatus for controlling the operation of a steam generating unit making use of the reheat cycle.

A further object of this invention is the provision of an arrangement of heat transfer apparatus and associated equipment for obtaining optimum operation of a steam generating unit incorporating a reheat cycle.

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

The character of the invention, however, may be best understood by reference to certain of its structural forms, as illustrated by the accompanying drawing in which:

The single figure is a somewhat schematic, sectional view of a steam generating unit embodying the present invention, the section being longitudinally and vertically thereof.

Referring to the drawing, the steam generating unit, generally designated by the reference character 10, is shown as comprising a furnace l1 and a boiler 12. The furnace 11 is made up of a forward wall 13 and a rear wall 14 joined by two side walls 15, the four walls defining a combustion chamber 16. The Walls are covered with water tubes in a well-known manner, but these tubes are not shown in the drawings for the sake of clarity. The lower portion of the furnace 11 is provided with a single-header hopper 18, which hopper provides for the disposal of residue from the combustion process which may fall downwardly in the combustion chamber 16. In the forward wall 13 are situated a group of burners 17 which provide for the introduction of fuel into the combustion chamber for consumption therein with attendant heat release. These burners may be of the type used for burning oil, gas, or pulverized coal. Some of the waterwall tubes serve to form the walls of the hopper 18 and terminate at their lower ends in headers 19 and 20. A protuberance 32 extends from the rear wall 14 of the furnace into the combustion chamber 16 for a purpose to be explained hereinafter.

The boiler 12 consists of a steam-and-water drum 21 located in the upper part of the unit and joined to a steam header 22. The steam-and-water drum 21 is connected in a well-known manner to downcomer tubes and steam tubes, most of which have been omitted from the drawing to clarify understanding of the invention. It is tov be understood, however, that water flows from the drum 21 into the downcomer tubes and from the lower ends of the downcomer tubes into the steam tubes, wherein a transition from water to steam takes place. The steam enters the drum 21 from the upper ends of the steam tubes and passes to the steam drum 22. The boiler 12 further comprises an economizer 23 for heating feed water and lowering the temperature of the exit gases, a superheater 24 for increasing the temperature of the saturated steam released in the drum 21, and an interstage reheater 25 for increasing the temperature of steam returned to the boiler from the turbine served by the superheater 24. The boiler 12 is enclosed within refractory and insulated wall consisting of a roof 26 extending rearwardly from the upper portion of the forward wall 13 of the furnace 11, a front wall 27 extending downwardly from and generally parallel to the upper portion of the rear wall 14 of the furnace, a back Wall 28 extending downwardly from the rear portion of the roof 26, and side walls 29 extending between the front wall 27 and the back wall 28. The front Wall 27, the back wall 28, and the side walls 29 form a back-pass chamber 30 extending downwardly and generally parallel to the combustion chamber 16 and joined at the upper portion thereto. The lower portion of the backpass chamber 30 is connected to an air heater 31. The back wall 28 of the back-pass chamber expands outwardly at a point near the upper portion thereof, thus providing a lower portion which is offset rearwardly. In line with the upper portion of the back wall is a vertical transverse baffle 33; this baffle extends from the lower portion of the back-pass chamber to a point adjacent the upper part of the back wall, but separated therefrom by a considerable distance, the separation forming an opening 34. Thus, a vertical passage 35 is formed between the back wall 28 and the bathe 33 with access thereto at the upper end through the opening 34. Another vertical transverse baffle 36 is provided in the back-pass chamber and this baffle extends from the lower portion of the chamber to a point adjacent the juncture between the front wall 2'7 of the back-pass chamber and the rear wall 14 of the combustion chamber. A passage 37 is formed between the front wall 27 and the bafile 35, this passage having an access opening 322 at the upper end. The

bafllesfilaud. 36;, thu.s,,.divide,.the backrpass chamber 30 into three passages, i. e., a front passage 37, a back.

passage. 35,. and...an.intermediate-. passage: 39..- .The. passages and 3.7..areof equal cross=sectionalarea andthey intermediatepassage. 39. is. in. the order: of twicethe sizeof each. of, the.other..two. At their; lower portions,

each, oh the. passages is provided; with. control dampers.

whi'ch may .beactuated externally, of. the boiler setting and, which; permit any. one passage. to. be. completely closed off, completely. open, or adjusted. to any intermediatev setting .ofopening. A, baffie,.41. extends; from. the 1 forward. wall 13,. of the=furnace rearwardly. over. the protuberance 32. and. closely underlies: the steam-and-water drum..21-.- Ititerminates, ata point. overlying the bafile 36. The. protuberance 32 has an; upwardly and rearwardlyinclined;surfaceand the b,afile;41 is. only slightly inclined, upwardly, and .re arwardly;; thus, a, passage 42 islformed. therebetween, which passage. narrows rearwardly. to form an.op.ening 43 into the;back-pass chamber.

Water, entersv the.unit.thr.o11gh. a conduit 44 in the lower portionof theback-passchamber.30.. The conduit 44 is connected. to anumberof coils 45 of theeconomizer 23 which reside in thelower. portion of the, front passage 37 only. These coils. are... in turn, connected in series with the main coi1's.46'of. the economizer, which main coils extend entirely across theback-pass chamber and, thus, reside, in, the back passage 35 andthe intermediate passage 39 as. well as in the frontpassage 37. The upper portion of' the main coils 4.6 of the economizer are connected to a small: header 48 and, then to the steam-andwater drum 21. The steam, space of the drum 21 is connected' to the steam header 22' and the latter is connected to the lower coils 490E the low-temperature section of the superheater 24. This low-temperature section comprises. lower coils 49 which extend across the intermediate passage 39' and. into the front passage 37 and upper coils SHWhich reside in the intermediate passage only. The upper coils 50 are connected through a small header 51 to the forward portion ofthe high-temperature section 52 of; the superheater and the rearward portion of that section. is connected to the outside of the boiler setting by means of a conduit 53'. which passesto the first-stage turbi'ne, not shown. The first-stage turbine discharges and returns steam tothe unit through a conduit 54 which leads to a. small header 55 residing externally of the furnace setting just beneath the protuberance 32'. tubes 56 extend from theheader-SS upwardly along the rear. wall 14 of the furnace over the inclined lower surface ofthe protuberance. 321 and upwardly in front of the highrtemperature section 52 of the superheater to a header 57. The header 57 is connected to the lower portion of' the low-temperature section 58'of the reheater 25; The section 58'.extends'transversely within the intermediate passage 30 only, is entirely above the top edge of the bafile'. 33; and extends for about half'its vertical height above the top edge of the baffle 36; The upper portion of the low-temperature section of the reheater is connected to the rearward coils of the high-temperature section 59 which coils depend'into the passage 42 rearwardly of" the high-temperature section 52 of, the superheater. The. forward'coils of the high-temperature section of the reheater are connecetd through a conduit 60' to the second stage turbine, not shown.

The operation of the apparatus will now be, evident from the above description. Fuel enters the furnace 11 through the burners 1'7, and combustion takes place in the combustion chamber, 16 with residue falling into the hopper"l'8"for-disposal.' The hot products of combustion pass'upwa'rdly overtheprotuberance 32and'into the passage" 42; from which theyenterthe back-pass chamber 30through the opening43. The'gases then pass down- A number of wardly through the back-pass chamber, the amount of flow in the various passages which make up the chamber being determined by the settings of the dampers 40. Gases passing through the passage 37 enter through the opening 38, while gases passing through the passage 35 enter through the opening 34. After passing through the back-pass chamber, the gases enter the air-heater where they serve to heat'theincoming' air and then discharge into the stack, not shown. Feedwater enters the unit through the conduit 44; passes into the coils '45 and passesupwardly-therethrough to the'main coils 46'of the economizer. The water passes upwardly through the main coils ofr the. economizer in, counterflow. to, the: flow of gases and eXits at the upper portion thereof into the header 48 and goes'fromthereinto-thewater of the steamand-water drum 21. Water from the drum 21 enters the downcomer tubes, passes downwardly, enters the steam tubes, makes the transition from water to steam, and the steam is discharged, into,the. steam portion of the drum 21. Steam from the drum 21 passes into the steam. header 22 and from there enters the low-temperature;

section of, the, superheater. Thesteam, flows upwardly therein. ll'lzCOUl'ltCIfiOW to the flow of. gases and passes at the. upperportion. thereof into the lower portion of the upper. coils. 50. The steam. flows upwardly through the upper coils 5ll.and. leaves. the upper. portion thereof and passes to. the, forward. portion. of. the high-temperature section. 5.2. of. the superheater. The steam flows through this section in the same directiona gas flow and leaves at the rearward portion thereof to exit. through the conduit, 53.. to the;first-stage turbine. vThe exhaustv from.the first-stage turbine enters the unit through the conduit. 54 and. enterstheheader 55. from which it flows upwardly along the back. wall 14 of the furnaceandover the pro.- tuberance,32.in the, tubes 56, being subject to radiantheat from, the, furnace, in. those tubes. tubes 56 and enterstheheader 57' from which it passes into the lower portion of, the low-temperature section 58 of the reheater., The, steam, flowsupwardly through the section 58' and passes from the upper portion thereof. into the rearward, portion of the highrtemperature section 59 of the reheater. The. steam flows forwardlyin. counterfiow to the flow of gases inthe passage 42 andleaves the forwardportion of the section 59 to exit throughthe conduit60 to.the second-stage, turbine.

It can be. seen,.then,. that the economizer is subject to heat transfer. from the flow. of gases in any of. the passages,

of the, backfpass chamber, and that it has an additional number of coilsresiding in the front passage 37,; this is because gases passing through that passage pass over only a small portion of the superheater tubes which reside in that. passage and would leave the passage at an unduly high, temperature if a large number of economizer coils were not introduced to lower the exit temperature of the gases. Any gases which pass through the intermediate passage 39 pass over all the coils in the low temperature section of the reheater, all of the upper coils in the lowtemperature section of' the superheater, most of the lower coils in the low-temperature section of the superheater, and a large number of coils in the economizer. Gases which pass through the passage 35 pass over most ofthe coils inthe' low-temperature section of the reheater and the small portion of= the main coils in the economizer which extend into-thatpassage. Itcanalsobe seen that steam entering the reheater is first subjected to radiant.

heatinthetubes 56, then toheat transfer from any gases which subsequently pass-into-the back. passage 35 and the intermediate. passage 39, andfinally to heat transfer from the flow of gases through the, passage 42. Steam enter ing the superheater is subject to heat transfer from gases fi0wingthr ough.the.front passage 37 becauseof the lower. coils extending into, that passage; superheater steam is also subject to heat transfer from the gases passing through the, intermediate passage 39, this transfer taking place through the-walls of the portion of the lower coils' The steam leaves thev which lie in that passage and through the walls of all the upper coils, since the latter coils lie entirely within that passage. superheater steam is further heated by convection in the high-temperature section due to flow of gases through the passage 42.

The unit will be designed, as is the usual practice, so that at a given load, say 75%, the operation will be such that, with the dampers in the passages 35 and 37 closed and with all gas flowing through the intermediate passage 39, the superheater exit steam and the reheater exit steam will be .at their desired temperatures, say 1000 degrees F. In most cases, the required steam temperature going to the turbine will be the same, whether it be first-stage or second-stage. If the load is changed, the combustion rate will be greater, the steam produced will. be greater, and the flow of hot products of combustion will be greater. However, a greater differential exists between the temperature of the steam entering the superheater and the temperature needed at its exit than exists between the temperature of the incoming steam to the reheater and that needed at its exit. This means that the superheater must be subjected to proportionately more heat transfer than took place at design load. Opening the dampers in the front passage 37 will change the proportion of gas flowing over the superheater to gas flowing over the reheater; as a matter of fact it will increase that proportion. If it is necessary to raise the proportion of heat transfer to the reheater with respect to that to the superheater, the dampers in the back passage 35 may be opened along with those in the intermediate passage 39, and the dampers in the front passage 37. In that condition, the reheater coils will be subjected to heat transfer from gases which never reach the low-temperature section of the superheater tubes. As a general rule the passage 35 will be used only to temper the results obtained by the opening ofpassage 37. It is to be appreciated that the gas flow to the front and back passages can be increased by closing the dampers in the intermediate passage and, at the same time, the proportion of gases going to the front and back passages can be determined by the relative settings of their respective dampers. It is thus possible at any load to obtain the desired superheat and reheat temperatures.

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

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

1. A steam generating unit, comprising: a furnace having a vertically elongated combustion chamber, a boiler having a vertically elongated back-pass chamber through which hot gases from the furnace pass downwardly, the combustion chamber and the back-pass chamber being joined at their uppermost portions by a convection pass, two baifles extending vertically throughout the length of the back-pass chamber and dividing it into three passages, the passages consisting of a large intermediate passage and smaller front and back passages, dampers at the lower portions of the three passages for controlling gas flow therethrough, a superheater having a low-temperature section residing in the intermediate passage and a high-temperature section residing in the said convection pass, a reheater having a low-temperature section residing in the intermediate passage and a high temperature section residing in the convection pass, and an economizer extending across the back-pass chamber and having a portion in each of the said three passages, the superheater having a portion residing in the front passage, the reheater being subject to heat transfer from the gas flowing into the back passage.

2. A steam generating unit as recited in claim 1, wherein: the said economizer has a sect-ion which resides in the front passage only and which is in series with the remainder of the economizer.

3. A steam generating unit, comprising a furnace having a combustion chamber, a boiler having a back-pass chamber through which hot gases from the furnace pass downwardly, the combustion chamber and the back-pass chamber being joined at their uppermost portions by a convection pass, means dividing the back-pass chamber into three passages, the passages consisting of an intermediate passage and front and back passages, dampers at the lower portions of the three passages for controlling gas flow therethrough, a superheater having a low-temperature section residing in the intermediate pass and a high-temperature section residing in the said convection pass, a reheater having a low-temperature section residing in the intermediate passage and a high-temperature section residing in the convection pass, and an economizer extending across the back-pass chamber and having a portion in each of the said three passages, the superheater having a portion residing in the front passage, the reheater being subject to heat transfer from the gas flowing into the back passage.

4. A steam generating unit, comprising a furnace having a vertically-elongated combustion chamber, a boiler having a vertically-elongated back-pass chamber through which hot gases from the furnace pass downwardly, the combustion chamber and the back-pass chamber being joined at their uppermost portions by a convection pass, tube baffles extending vertically throughout the length of the back-pass chamber and dividing it into three passages, the passages consisting of a large intermediate passage and smaller front and back passage-s, dampers at the lower portions of the three passages for controlling gas flow therethrough, a superheater having a low-temperature section residing in the intermediate pas-s and a high-temperature section residing in the said convection pass, a reheater having a low-temperature section residing in the intermediate passage and a high-temperature section residing in the convection pass, and an economizer extending across the back-pass chamber, the superheater having a portion residing in the front passage, the reheater being subject to heat transfer from the gas flowing into the back passage.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain Oct. 19,

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