US3297004A

US3297004A - Supercritical pressure recirculating boiler

Info

Publication number: US3297004A
Application number: US482814A
Authority: US
Inventors: Carl R Midtlyng
Original assignee: Riley Power Inc
Current assignee: Riley Power Inc
Priority date: 1965-08-26
Filing date: 1965-08-26
Publication date: 1967-01-10
Anticipated expiration: 1984-01-10

Description

Jan. 10, 1967 c. R. MIDTLYNG 3,297,004

SUPERCRITICAL PRESSURE RECIRCULATING BOILER Fil ed Aug. 25. 1965 2 Sheets-Sheet 1 FIG. 2

CARL R. M/DTLYNG INVENTOR,

Jan. 10, 1967 c. R. MlDTLYNG 3,297,004

SUPERCRITICAL PRESSURE RECIRGULATING BOILER Filed Aug. 26; 1965 v 2 Sheets-Sheet 2 CARL R. M/DTLYNG INVENTOR zimww United States Patent Gfifice 3,297,064 SUPERQIRITHCAL PRESSURE nucmcura'rnsc BQTLER Carl R. Midtlyng, Worcester, Mass., assignor to Riley Stoker Corporation, Worcester, Mass, a corporation of Massachusetts Filed Aug. 26, 1965, Ser. No. 482,814 4 Claims. ((31. 122-406) This invention relates to a supercritical pressure recirculating boiler and, more particularly, to apparatus arranged to produce steam by the burning of fossil fuel and by bringing the products of combustion into heat exchange relationship to water.

It has become common practice to generate steam at high pressure by use of the well-known once-through forced-flow boiler. Such boilers operate effectively not only at sub-critical but at supercritical pressures. However, a number of difficulties have arisen when a conventional once-through boiler is used, as compared with the use of a natural circulation boiler. As a result, a boiler which is a combination of the two and can be called a supercritical pressure recirculating boiler has been evolved. In such a boiler, a variable percentage of the flow of fluid through the main heat absorption section is recirculated back to the entrance of the section. This re-circulation permits the maintenance of adequate fluid flow through the main heat absorption section tubes at all loads. Furthermore, in starting up the boiler, it eliminates considerably the need for complicated apparatus for disposing of generated steam in that part of the start-up cycle before the steam is allowed to expand through the turbine. However, it has been found that the supercritical pressure recirculating boiler requires extremely large and expensive pumps which absorb considerable power at all loads. Furthermore, considerable difliculty has been experienced, due to the fact that the fluid temperatures coming from the absorption section vary by considerable amounts, depending on the particular part of the boiler combustion chamber inwhich the fluids originate. These and other difficulties experienced in the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a supercritical pressure boiler in which recirculation takes place with a minimum pump capacity and power absorption by the recirculating pump.

Another object of this invention is the provision of a supercritical pressure boiler in which natural forces are used to produce at least a portion of the recirculating power.

A further object of the present invention is the provision of a supercritical pressure boiler in which it is possible to select the output temperature very close to the limiting metal temperature without the danger of exceeding that temperature.

It is another object of the instant invention to provide a supercritical pressure boiler in which means is provided to assure that output temperatures do not exceed the design temperature.

It is a further object of the invention to provide a supercritical pressure boiler in which a furnace peripheral wall outlet header and piping arrangement assures that fluid to the superheater originates in wall areas having inherently higher heat absorption and fluid which is recirculated originates in wall areas having inherently lower heat absorption.

A still further object of this invention is the provision of a supercritical pressure boiler in which means is provided to limit fluid temperatures in the tubes in the high heat absorption section.

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 one of its structural forms, as illustrated by the accompanying drawings in which:

FIG. 1 is a schematic view of a boiler of a steam generating unit embodying the principles of the present invention,

FIG. 2 is a schematic view of the invention in plan view, and

FIG. 3 is a perspective view, with portions broken away of a boiler of specific details of the steam generatlng unit.

Referring first to FIG. 1, which best shows the general features of the invention, the supercritical pressure boiler, indicated generally by the reference numeral 19, is shown as consisting of a main heat absorption section 11 having a series of vertical tubes 12 joined at the upper end to a header l3 and, at the lower end, to a header 14. The upper header 13 is joined by connector tubes 15 to a header 15 connected through a superheater 17 to a turbine (not shown).

The lower header 14 is connected by tubes '18 to a header 1& which, in turn, is connected by a downcomer 21 to a mixing chamber 22. The upper end of the mixing chamber is connected to a pipe 23 leading to the main feed pumps (not shown).

The header in is connected to the mixing chamber 22 by a pipe 24 containing a flow-regulating valve 25. The header in is also connected through a shutoff valve 26 to the suction side of a recirculating pump 27, the output of which is connected through a shutoff valve 28 and a checkvalve 2-9 in series to the mixing chamber 22. The checkvalve 29 is arranged to allow flow from the pump 27 to the mixing chamber 22 but not in the opposite direction.

FIG. 2 shows in somewhat greater detail the interconnections at the top of the boiler. The steam generating unit is provided, of course, with an upper side wall header 13 and, at the opposite side, to an upper side wall header 31. Extending between these headers transversely of the steam generating unit is the header 16 to which are connected the tops of the tubes on the front and rear wall of the apparatus. The connecting pipes 15 join the ends of the upper side wall header 13 to the adjacent end of the header 16. Similar connecting pipes 32 connect the ends of the upper side wall header 31 to the adjacent end of the header 1%. The central portion of the header 13 is connected by a tube 33 to the tube 34 leading from the center of the header in to the superheater 17. A similar tube 35 connects the center of the upper side Wall header 31 to the tube 34.

Tubes

36 and 37 connect the central portion of the header 16 to the

tubes

33 and 35, respectively, adjacent their connection to the tube 34. Tubes 38 and 39 connect the ends of the header 16 to the valve 26 leading to the pump 27.

Referring now to FIG. 3, which shows the details of construction of the steam generating unit, it can be seen that the steam generating unit it is provided with a front wall 41, a rear wall 42, and side walls 4-3 and 44 forming a main combustion chamber. At the upper end of the side wall 43 is the header 13 and, at the lower end, the

header 14. At the upper end of the side wall 44 is the header 31 and, at the lower end, a header 45, these headers being joined by vertical side wall tubes in the usual way. The tubes making up the front wall 41 and the slde wall 42 are joined at their upper ends to the collector header 16 and at their lower ends approach one another to form a lower hopper 46. The other end of the tubes making up the front wall 41 terminate in a transverse header 47, while the tubes making up the rear wall 42 terminate in a transverse header 43. This perspective view shows the manner in which the pipes join the ends of the header 13 to the adjacent end of the header 16. Similarly, they show the way in which the pipes 32 join the ends of the header 31 to the adjacent end of the header 116. The pipe 33 joins the central portion of the header 13 to the pipe 34, while in a similar way the pipe 35 joins the center of the header 31 to the pipe 34. Pipes 36 and 37 join the central portion of the header 16 to the

pipes

33 and 35, respectively, adjacent their connection to the pipe 34. The ends of the header 16 are joined by the

pipes

38 and 39 to the pump apparatus 27.

The pump apparatus 27 consists of a network including pumps 51 and 52. The pumps are connected through shutoff valves 28 and checkvalves 29 to the mixing chamber 22 and are connected to the

pipes

33 and 39 through shutoff valves 26.

The

tubes

38 and 39 are also connected to the end of the tube 24 leading to the valve 25 and, eventually, to the mixing chamber 22. The upper end of the mixing chamber is connected by the tube 23 to main feed pumps, while the bottom end is connected by the downcomer Ell to the drum 19 which, for the purposes of illustration, is shown as a spherical drum. This drum serves to feed the various lower headers. For instance,

feeder tubes

54 and 55 lead from the drum 19 to the header 48 at the bottom of the hopper. Similarly,

tubes

53 and 56 lead from the drum 19 to the header 47. A feed tube 58 lead from the drum to the header 45, while a similar tube 57 leads to the header 14.

The operation of the apparatus will now be readily understood in view of the above description. Feedwater under pressure enters the system through the tube 23 and passes into the mixing chamber 22 down the downcomer 21 into the drum 1%. From there it flows through the feeder tube system 18 consisting of the

tubes

53, 54, 55, and 56 to the header 14 as well as the

headers

45, 47, and The water passes upwardly through the main heat absorption section 11, through the tubes 12, consisting of the tubes in the front wall 41, the rear wall 42, the side wall 43, and the side wall 44. When the section 11 is operated at supercritical pressure and temperature, there will be no sharp line of conversion from liquid into steam. Most of the fluid in the system can be treated as phase fluid. The fluid which has been heated passes from the walls into the

headers

13, 31, and 16. Because of the fact that the combustion chamber is of a rectangular form, the amount of radiant heat received by the tubes will be less for those tubes at the corners of the enclosure than it will those in the central portions of the sides. Therefore, the tube 33 leaving the center of the header 13, the tube 35 leaving the center of the upper header 31, and the

tubes

36 and 37 leaving the center of the header 16 receive effluent of higher than average temperature and feed it into the tube 34 going into the tubrine. Tubes 15 lead fluid from the ends of the header 13 to the ends of the header 16, while tubes 32 carry cool fluid from the ends of the header 31 to the other end of the header 16. Then, this cooler than average fluid originating in the ends of

headers

13 and 31 as well as the ends of the header 16 itself are carried through

tubes

33 and 39 to the recirculating system passing from the upper ends of the

tubes

38 and 39 into the mixing chamber 22. At low loads, the

valves

26 and 28 are open, so that the pumps 51 and 52 cause fluid to pass from the

tubes

38 and 39 to the mixing chamber 22 where fluid joins the fluid coming from the feed pumps through the feed line 23. At higher loads, there is less need for the pumps 51 and 52 because the same circulation of the boiler which serves to operate a natural circulation boiler also causes fluid to flow through the bypass 24 through the open valve 25, into the mixing chamber 22, and down the downcomer 21 to the lower part of the boiler. When auto-circulation takes place through the bypass valve 25, the checkvalves 29 prevent back flow through the pumps.

It can be seen that with the present construction the fluid for recirculation is taken from the least heat-absorbing portion of the combustion chamber. In addition, the arrangement requires that the fluid from the superheater for use by the turbine is taken from the highest heat-absorption portion of the furnace. The circulation in the absorption section 11 is maintained at a value sufficient to assure that the tube metal temperature does not exceed a safe value. That circulation is normally in the range from 3.5 to 6.0 feet/ second. Auto-recirculation is due to the gravity head of the downflow pipe 21 which contains relatively cool, dense fluid, this head equaling the opposing gravity head in the upflow portion plus all the flow resistances in this closed circuit. Pumps 51 and 52 are connected in parallel with the pipe 24 and these pumps are of a head capacity characteristic which is typical of centrifugal pumps having a steeply-falling headcapacity curve. Each pump and its constant speed motor are sized for the cold start-up condition of the boiler with the water initially at, say, F. It is a fortunate result of an optimum combination of pump and closed-circuit characteristics that, when the boiler is in the hot running condition, almost all of the total fluid recirculated passes through the pumps, but over the entire load range the pumps operate at or near the maximum capacity zerohead point. Each pump is virtually a floating pump for, with the motor energized, the pump is neither fluid driven nor does it develop any significant head in excess of flow losses. Over a lower portion of the load range a small fraction of the total recirculated fluid passes through the tube 24 and the valve 25. This bypass is provided so that, when the pumps are shut down and isolated, natural circulation flow through the pipe 24 will provide at least a minimum safe fluid velocity at the tube entrances of the absorption section 11.

It can be seen, then, that there are certain advantages which may be derived from this specific boiler arrangement. First of all, it is not necessary, as is ordinarily the case, to provide a pump motor which is sized considerably in excess of the hot running requirements in order to meet the power requirements when the same pump is used for cold startup. Furthermore, the hot running pump power requirement is negligible at all boiler loads. The flow resistance in the closed circuit is minimized, so that the natural recirculation tendency is maximized. Each pump is logically a vertical shaft canned pump with water-lubricated bearings. It is desirable to avoid occasions for starting and stopping the pump, since the bearing wear occurs almost entirely when stopping or starting. The present arrangement permits having one pump in use continually at all times, from cold start-up all the way up through the highest loads. Although the pump requires negligible power while in the hot running boiler load condition, recirculation is augmented over that which would occur naturally, if the pump were deenergized and isolated. However, with this pump normally energized and passing most or all of the recirculated fluid, any reduction in normal mixed fluid volume flow rate would encounter an opposing and compensating pumping action by the uncontrolled free running pump.

It will be understood that, by selecting the fluid for recirculation from portions of the main heat absorption section that produce the lowest temperature of eflluent, it is possible to reduce the average fluid temperature in the section. This permits safer operation of the boiler. In other words, selective recirculation of cooler-than-average furnace wall efliuent results in a significant reduction in temperature of eflluent from all tubes and a corresponding reduction in tube metal temperature.

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 supercritical pressure boiler having a combustion chamber, comprising (a) a main heat absorption section having an inlet header at its lower end and an outlet header at its upper end, the section being located in the combustion chamber and means to heat said section whereby one part receives less heat than another part and delivers fluid of lower than average temperature to a portion of the outlet header,

(b) a recirculating circuit including a mixing device joining the said inlet and outlet headers,

(c) a feed circuit including a main feed pump connected to the mixing device,

(d) a recirculating system connected in the recirculating circuit,

(e) means connecting said portion of the said outlet header receiving fluid of lower than average temperature to the mixing device, and

(f) a superheater conduit connected to a portion of the outlet header receiving fluid of higher than average temperature.

2. A super-critical pressure boiler as recited in claim 1, wherein the recirculating system includes two circuits arranged in parallel, each circuit containing a pump and a shut-01f valve following the pump, a further shutofl? valve being provided in each circuit ahead of the pump, and a check valve being provided in each circuit.

3. A supercritical pressure boiler as recited in claim 1, wherein the mixing chamber consists of an elongated vertical cylindrical housing, wherein the main feed pump is connected coaxially to the upper end of the housing, wherein the downcomer extends downwardly from the lower end of the housing to the lower end of the evaporator section, and wherein the recirculating circuit is connected tangentially to the central portion of the mixing chamber.

4. A supercritical pressure boiler, comprising (a) a main heat absorption section forming at least part of a combustion chamber Wall, means to heat said section so that the section has a first part which receives relatively less transferred heat and a second part which receives relatively more transferred heat, so that the fluid reaching the outlet end of the first part is at relatively low temperature and the fluid reaching the outlet end of the second part is at relatively high temperature,

(b) a recirculating circuit joining the inlet and outlet of the said section,

(c) a feed circuit including a main feed pump connected to the inlet of the section,

(d) means connecting the outlet of the said first part of the main heat absorption section to the recirculating circuit, and

(e) a superheater conduit connected to the outlet of the said second part of the main heat absorption section.

References Cited by the Examiner UNITED STATES PATENTS 3,135,251 6/1964 Kane 122-406 3,185,136 5/1965 Cozza 122-406 3,213,835 10/1965 Egglestone 122406 KENNETH W. SPRAGUE, Primary Examiner.

Claims

1. A SUPERCRITICAL PRESSURE BOILER HAVING A COMBUSTION CHAMBER, COMPRISING (A) A MAIN HEAT ABSORPTION SECTION HAVING AN INLET HEADER AT ITS LOWER END AND AN OUTLET HEADER AT ITS UPPER END, THE SECTION BEING LOCATED IN THE COMBUSTION CHAMBER AND MEANS TO HEAT SAID SECTION WHEREBY ONE PART RECEIVES LESS HEAT THAN ANOTHER PART AND DELIVERS FLUID OF LOWER THAN AVERAGE TEMPERATURE TO A PORTION OF THE OUTLET HEADER, (B) A RECIRCULATING CIRCUIT INCLUDING A MIXING DEVICE JOINING THE SAID INLET AND OUTLET HEADERS, (C) A FEED CIRCUIT INCLUDING A MAIN FEED PUMP CONNECTED TO THE MIXING DEVICE, (D) A RECIRCULATING SYSTEM CONNECTED IN THE RECIRCULATING CIRCUIT, (E) MEANS CONNECTING SAID PORTION OF THE SAID OUTLET HEADER RECEIVING FLUID OF LOWER THAN AVERAGE TEMPERATURE TO THE MIXING DEVICE, AND (F) A SUPERHEATER CONDUIT CONNECTED TO A PORTION OF THE OUTLET HEADER RECEIVING FLUID OF HIGHER THAN AVERAGE TEMPERATURE.