US3771498A

US3771498A - Furnace circuit for variable pressure once-through generator

Info

Publication number: US3771498A
Application number: US00214696A
Authority: US
Inventors: W Gorzegno
Original assignee: Foster Wheeler Inc
Current assignee: Foster Wheeler Inc
Priority date: 1972-01-03
Filing date: 1972-01-03
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13

Abstract

A vapor generator of the once-through type suitable for variable pressure operation, wherein the generator can be operated at a pressure which is proportional to load. The generator furnace enclosure is defined by at least two flow passes in series flow relationship, the first pass constituting the full periphery of the lower part of the enclosure, the second pass constituting at least part of the upper part of the enclosure. The first pass extends to an elevation above the burners and is located in the generator circuitry so that it has a subcooled fluid entering it. It is sized so that a steam and water mixture leaves the pass having a quality of approximately 30 percent at about 2,400 psi operation and about 70 percent load.

Description

United States Patent [191 Gorzegno [451 Nov. 13, 1973 [75] Inventor: Walter P. Gorzegno, Florham Park,

Assignee: Foster Wheeler Corporation, Livingston, NJ.

Filed: Jan. 3, 1972 Appl. No.: 214,696

Related U.S. Application Data Continuation of Ser. No. 144,516, May 18, 1971.

U.S. Cl. 122/406 S Int. Cl. F22b 29/06 Field of Search 122/406 R, 406 S,

122/406 ST, 448 S, 451 S References Cited UNITED STATES PATENTS 3,324,837 6/1967 Gorzegno et a1. 122/406 Primary Examiner-Kenneth W. Sprague Att0rney.lohn Maier, 111 et a1.

[57] ABSTRACT A vapor generator of the once-through type suitable forvariable pressure operation, wherein the generator can be operated at a pressure which is proportional to load. The generator furnace enclosure is defined by at least two flow passes in series flow relationship, the first pass constituting the full periphery of the lower part of the enclosure, the second pass constituting at least part of the upper part of the enclosure. The first pass extends to an elevation above the burners and is located in the generator circuitry so that it has a subcooled fluid entering it. It is sized so that a steam and water mixture leaves the pass having a quality of approximately 30 percent at about 2,400 psi operation and about 70 percent load.

9 Claims, 7 Drawing Figures TWISTED TAPE TURBULATORS INSTALLED IN TUBES IN THIS ZONE Pmmmunm m3 3.771.498

SHEET 30F 3 TWISTED TAPE TURBULATORS INSTALLED IN TUBES IN THIS ZONE.

FURNACE CIRCUIT FOR VARIABLE PRESSURE ONCE-THROUGH GENERATOR CROSS-REFERENCE TO RELATED APPLICATION The present application is a continuation of applicants copending application Ser. No. 144,516, filed on May 18, 1971, now abandoned.

DESCRIPTION OF THE INVENTION The present invention relates to a novel and improved once-through vapor generator circuit design, and more particularly to the furnace construction of a generator which is suitable for operation at variable pressure.

The present invention is particularly applicable to variable pressure start-up of a once-through vapor generator, in which the pressure is varied in direct response to load demand, and will be described with particular reference thereto, although the principles of the invention are applicable to variable pressure operation when carrying load, if desired.

Conventional ly, vapor driven turbines have been perated at a set inlet or throttle pressure, and the vapor generators therefor have been designed to operate at corresponding set pressures, load being varied by varying flow and firing rates.

Benson type vapor generators preferably have been provided with a circuit divided into a plurality of separate passes connected in series. This limits the enthalpy pick-up in any one pass, and also increases mass flow rates in the passes. As a rule of thumb, the enthalpy pick-up per pass should be limited to about 200 Btus per pound.

A turbine can be operated at variable pressure to minimize thermal stresses and increase cyclic life, the load and throttle pressure being varied in almost direct proportion to each other. This is particularly advantageous for start-up.

However, the conventional multi-pass generator design is not suitable for variable pressure operation. A principal reason for this is the arrangement of circuits conventionally employed. It is necessary to maintain nucleate boiling in the generator at sub-critical pressures, particularly in the high temperature burner zone of the generator furnace. At reduced pressures, the quality of the flow (per cent vapor in the mixture) at any one point in the heatingcycle is higher and could, in the burner zone with conventional circuitry, be sufficiently high that the boiling becomes non-nucleate, re-

sulting in high tube metal temperatures and in some cases tube failure.

In addition, with conventional circuitry, at least some of the lower furnace passes are connected by downcomers, and it is possible that at lower pressures, with variable pressure operation, the downcomers of the circuitry couldcontain a vapor and liquid mixture. This in turn could result in phase separation at a pass inlet and resulting flow upset and tube failure resulting from high tube wall metal temperatures.

Accordingly, the multi-pass generator has as a rule been operated, at least in the furnace portion thereof where nucleate boiling is critical, at full operation pressure, even during start-up of the generator.

For operations at constant pressure, the generator has required, among other things a system downstream of the furnace circuitry for reducing the pressure of the flow, and controls therefor.

It is a principal object of the present invention to provide a multi-pass furnace design which is suitable for variable pressure operation, particularly during generator startup, without danger of flow upset and tube burn-out.

It is also an object of the present invention to provide a vapor generator which can be employed with a turbine operable at variable pressure.

It is further an object of the present invention to provide a multi-pass furnace design in which nucleate boiling is maintained in the burner zone, and wherein the pressure is varied in proportion to load, the quality of the fluid leaving the high temperature zone of the furnace being approximately30 percent at about 2,400 psi operation and about percent load.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principles of the invention may be employed.

In such annexed drawings:

FIG. 1 is a schematic section elevation view of a vapor generator furnace section in accordance with the present invention;

FIG. 2 is a section view taken along line 22, FIG.

FIG. 3 is a section view taken along line 3-3, FIG.

FIG. 4 is a graph illustrating principles of the generator of FIG. 1;

FIG. 5 is a temperature-enthalpy diagram illustrating further aspects of the present invention;

FIG. 6 is a schematic section elevation view of a vapor generator illustrating an embodiment in accordance with the invention, and;

FIG. 7 is a section view taken along line 77 of FIG. 6.

Referring to the drawings, and in particular FIGS. 1-3, the vapor generator 10 comprises a furnace section 12 defined by front and

rear walls

14 and 16 and side walls 18. The enclosure is rectangular in shape and extends vertically from a bottom hopper'20 to above roof 22. Somewhat below the roof 22, the rear wall tubes 16 are divided to provide an exit screen 24, leading to a convection area 26 of the generator (shown only in part), and a convection enclosure screen 27.

Burners 28 are positioned in the front and rear walls in the lower portion of the furnace section 12, making the lower portion the high temperature zone of the furnace.

Within the generator enclosure, extending vertically mid-way between the side walls 18 is a flat division wall 29 connected at the bottom with spaced

lower headers

30 and 32 and at the top with a longitudinally extending upper header 34. 1

The furnace circuitry as in conventional furnaces is divided into a plurality of flow passes, the use of multiple passes increasing the mass flow rate in the tubes of the furnace and decreasing the likelihood of a stagnant condition existing in any one tube.

In accordance with the present invention, the circuitry for the furnace periphery includes a first flow pass 36 which encompasses the entire periphery of the furnace in the lower portion thereof to an elevation 38 somewhat above theburners. The pass is fed by four

inlet headers

40, 41, 42, and 44 (shown in FIG. 2) which feed the front, rear and side walls of the pass, the two front and

rear wall headers

40, 41 being parallel to each other and at right angles to and embraced by the two parallel

side wall headers

42, 44; the pass terminating in a rectangular shaped upper header 46, FIGS. 1 and 3, located at the elevation 38. The inlet headers for the first pass receive their flow from a downcomer 47 which in turn receives its flow from the upper header 34 of the division wall. Notice that the division wall 29 is fed by conduits 48 which in turn are connected to the economizer of the generator and the generator inlet.

From the pass 1 outlet header 46, the flow is collected in risers 50 (FIGS. 1 and 3) spaced along the front side of the generator and along about half of the side walls, the flow being directed through the risers into a U-shaped inlet header 52 for a second flow pass 54 of the furnace wall enclosure. This pass is U-shaped, similar to the header therefor, and consists of the upper front wall 14 of the generator and half of each of the upper side walls 18. The second flow pass extends upwardly from the inlet header to an upper U-shaped outlet header 56 (FIG. 1) in which the second pass flow is collected. From this outlet header, the flow is transmitted via a pair of downcomers 58 on opposite sides of the generator (one of which is shown in dashed lines) leading to a U-shaped inlet header 60 embracing the rear wall of the generator. This header feeds the remaining upper portions of both of the side walls 18 of the generator and the upper rear wall 16, including the screen tubes 24 and convection enclosure tubes 27. This pass terminates in upper U-shaped header 62 (FIG. 1) and header 62a for the enclosure tubes.

From the above, it is apparent that the flow from the generator inlet and economizer is first into the division wall 29, and then into the lower first pass 36 constituting the full lower periphery of the furnace, the second pass constituting the upper front side of the furnace enclosure, and then the third pass constituting the upper rear side of the enclosure, in series.

In a particular example in accordance with the present invention, the lower furnace tubes are 1 inch tubes on 1 54; inch centers and the upper furnace tubes are 2 inch tubes on 2 9% inch centers. Normally the mix header 46 is approximately 25 feet in elevation above the burners. I

FIG. 4 illustrates operation of the generator, showing the variation in load against turbine throttle pressure in pounds per square inch, or vice versa. In the example of FIG. 4, the generator is operated to provide a full load supercritical turbine throttle pressure of about 3,500 lbs/sq. in. At start-up of the generator, the pressure is maintained at about 1,000 psi from percent load to about 25 percent load. Load is increased to 25 percent by opening the turbine stop valve bypass with turbine governor valve position at 100 percent open, as shown in FIG. 4. A control response advantage may be achieved by operating with turbine control valves 90 to 95 percent open, but discussion of this aspect is beyond the scope of this invention. At about 25 percent load, thethrottle stop valve is opened. During the loading period beyond 25 percent, the increase in load is obtained by increase in throttle pressure, in substantially proportional relationship to each other, being about 1,250 psi at about 30 percent load, and about 3,500 psi I at about 100 percent load. The increase in pressure of the steam generator, and at the turbine throttle, from 25 to 100 percent load, is accomplished by pumping and firing rate in response to a load vs. throttle pressure program. With such variable pressure operation, the quality of the fluid flow in the generator varies, having a higher vapor content at lower pressures, at a particular point in the furnace circuitry. Differently stated, as the pressure in the generator is lowered, a particular quality of fluid flow will be achieved earlier in the circuit than at higher pressures.

This is illustrated in FIG. 5 which is a temperature enthalpy diagram illustrating enthalpy pick-up in the generator for various loads. The figure shows the enthalpy pick-up for three loads, the dashed line for 30 percent load (connecting circle-points), the dot-dash line for 50 percent load (connecting square-points), and the solid line for 100 percent load (connecting crosspoints). Referring to this figure, the enthaply of the flow leaving the lower furnace at 50 percent load (1,750 psi) is about 865 Btus per pound whereas at 30 percent load (1,250 psi) it is about 900 Btus per pound. At 50 percent load, this represents a quality of about 43 percent, whereas at 30 percent load, and lower pressure, the quality is higher, about 53 percent.

With a conventional furnace circuit, the higher quality flow at lower pressures could cause a departure from nucleate boiling in the burner zone and subsequent tube overheating, depending upon the particular design of circuitry employed, and also could produce a vapor-liquid mixture flow at the inlet to a pass located in a high absorption zone.

In the circuit of the present invention, the lower furnace first pass is positioned in the furnace circuitry, and the upstream passes are sized, so that at all pressures and loads the flow entering the lower furnace pass is in a sub-cooled state. For instance referring to the 50 percent load (1,750 psi) curve, the enthalpy of the flow at the first pass inlet is about 570 Btus per pound, well within the sub-cooled range. The enthalpy in the economizer is increased from about 430 at inlet to about 480 Btus per pound at outlet, and in the division wall from about 480 at inlet to about 570 Btus per pound at outlet. Even at 30 percent load, the enthaply at the first pass inlet is only about 530 Btus per pound, or well within the sub-colled range for 1,250 psi. In the division wall at this pressure, the enthalpy is increased from about 405 Btus per pound at inlet to about 530 at outlet. Of course at full load (4,150 psi), the flow is supercritical and single phase, and there is no problem with quality of a two-phase flow.

Since the first pass constitutes the full periphery of the lower part of the furnace, the only downcomer in the low enthalpy portion of the circuitry is that between the division wall and the first pass; and since the flow into the first pass is in a sub-cooled state at all loads, there is no problem with phase separation at a pass inlet of the circuitry.

A criterion for maintaining nucleate boiling when the flow is sub-critical is that the quality leaving the burner high temperature zone be no more than about 30 percent by weight at percent load and 2,400 psi. Line A drawn between the lower furnace exit points for 30 percent load, 50 percent load and full load shows that the furnace circuitry of the generator meets the above criterion, the outlet header for the first pass being at an elevationsubstantially coincident with the upper reach of the burner high temperature zone.-

For further insurance of maintaining nucleate boiling in pass 1, it is a feature of the invention that turbulators (of known design) can be installed in the tubes of this pass (in the area indicated in FIG. 1) to insure uniform flow distribution, proper mass flow rates, and maintain nucleate boiling conductance at the inside wall of the tubes.

FIG. 5 also shows that the enthalpy increase in the

upper passes

2 and 3 of the generator is limited to about 100 Btus per pound per pass, satisfying criterion for a pass receiving a steam'water mixture at inlet. In pass one, the enthalpy pick-up is higher, but is in accordance with proper design criteria in that a sub-cooled (single phase) fluid enters this pass at all loads. The upper part of the furnace is properly divided into two flow passes, since for one pass, the enthalpy pick-up would be excessive. With reference to FIG. 5, this would be about 200 Btus per pound, the upper furnace pick-up. Three passes can be employed in the upper part of the furnace, but this would be more costly, and two passes provides the optimum pick-up per pass. If three passes are employed in the upper part of the furnace, the intermediate of the three passes would constitute intermediate portions of each of the side walls of the upper part.

If desired, turbulators can also be employed at the inlet ends of the tubes of the second and third passes to insure uniform flow distribution.

As a further embodiment, the division wall can be omitted, and the flow from the economizer can be directed into pass one. In this case the economizer would be sized larger.

This embodiment is illustrated in FIGS. 6 and 7, which show a rectangular furnace enclosure 1 12 having front and

rear walls

114 and 116 and side walls 118. The flow from the economizer is directly into lower inlet headers l40144 at the bottom of the enclosure which feed a first pass 136 defining the full periphery of the lower portion of the furnace enclosure. As this portion contains burners 128, it constitutes the radiant high temperature zone of the generator. The first pass extends to above the burners and connects with header 146. The flow in header 146 is then collected in risers 150, spaced along the front side of the generator and along about half of the side walls of the generator. The flow in the risers is directed into a U-shaped inlet header 152 for a second flow pass 154 in the upper portion of the generator furnace enclosure. This pass is U- shaped similar to the embodiment of FIG. 1. This and a third flow pass 162 define the entire periphery of the upper portion of the generator furnace enclosure. The

. flow between the second and third passes is by means of a pair of downcomers 158.

In accordance with this embodiment of the present invention, the flow from the third pass 162 is then directed by means of a downcomer 170 into a partial division wall 172 installed in the upper furnace volume above the burners 128. Preferably the division wall is made up of a plurality of side-by-side panels 176 (Fig. 7) spaced laterally across the width of the enclosure. Details of the division wall are shown in US. Pat. No. 3,534,713, Walter P. Gorzegno, assigned to the assignee of the present application. As shown in the prior patent, the tubes making up the division wall are connected to inlet header 180 disposed outside the furnace in a horizontal plane to provide along their lengths substantially the same quality fluid. Tube connections between the inlet headers and division wall tubes are then bent and routed so that the total tube lengths from the inlet headers to the outlet headers of the division wall panels are kept approximately the same for all of the flow circuits of the division wall. Preferably the division wall tubes penetrate the front wall of the furnace enclosure at an elevation above the headers which connect the lower furnace pass 136 with the second upper pass 154 of the enclosure.

As. with the embodiment of FIG. 1, this embodiment assures that the flow to the inlet header for the lower first pass of the furnace enclosure will be in a subcooled state throughout the pressure and load ranges for the generator. In addition, nucleate boiling in the high temperaturee radiant zone of the enclosure is assured. Further, the use of downcomers in the high temperature radiant heat zone of the generator, located in the circuitry such that they would contain a vapor and liquid mixture, is avoided.

As with the embodiment of FIG. 1, turbulators of known design can be installed in the tubes of the lower pass to insure uniform flow distribution and proper mass flow rates, and to maintain nucleate boiling conductance at the inside walls of the tubes.

In addition, it should be clear that the principles of the invention are applicable to a sub-critical generator as well as a supercritical generator.

One advantage of the invention is that during startup, the steam generator circuits operated at about 1,000 psi pressure, eliminating the need for a pressure reducing station which is customarily employed between the steam generator and a flash tank to reduce the pressure of the flow downstream of the furnace circuitry and at the same time maintain full pressure in the furnace circuitry.

As a further advantage, the variable pressure generator of the present invention allows optimum operation of the turbine with substantially wide open control valves over the load range. This optimum operation minimizes thermal differences and resultant thermal stresses within the turbine.

Other advantages will be apparent to those skilled in the art.

What is claimed is:

1. A once-through vapor generator of the type having a circuitry including vapor generating surface and vapor superheating surface, the latter being connected to a point of use, wherein the normal flow is in series through said surfaces without recirculation, comprising means for operating said generator at variable pressure from no load to full load,

said surfaces comprising a first plurality of flow passes in series flow relationship defining a furnace enclosure;

burner means in the lower part of said furnace enclosure;.

said plurality of flow passes comprising a first upflow pass, lower inlet and upper outlet headers for said first upflow pass, said first upflow pass conprising a plurality of upflow tubes connected in parallel between the inlet and outlet headers and defining substantially the entire periphery of the enclosure in the lower part thereof to an elevation just above said burner means;

additional flow passes in series with the first upflow pass, said additional flow passes defining substantially the entire periphery of the furnace enclosure in the area above said first pass;

said additional flow passes comprising a second pass which defines the generator front wall and substantial portions of each side wall in the area above said first pass, and a third pass which defines the generator rear wall and substantial portions of each side wall also in the area above said first pass;

the first pass being located in the generator circuitry such that at normal operating pressures the fluid entering it is in a subcooled state,

the first pass being sized so that at about 70 percent load and 2,400 psi operation the quality of the flow leaving it is not in excess of about 35 percent vapor by weight whereby nucleate boiling is maintained in the pass.

2. A vapor generator according to claim 1 wherein the generator side walls in the part of said enclosure above said first flow pass are divided between said second and third passes. i

3. The generator of claim 1 including turbulators in said first pass, the enthalpy pick-up in the first pass being in excess of about 150 Btus per pound.

4. The generator of claim 3 including a division wall upstream of said first pass.

5. The generator of claim 1 including at least one additional furnace pass in the furnace enclosure above said first pass, and partial division wall means which receives the flow from said additional furnace pass.

6. The generator of claim 5 wherein said division wall means comprises a plurality of panels in side-by-side relationship spaced across the width of the furnace enclosure in the area of the enclosure above said first flow pass, tubes of the partial division wall panels penetrating the furnace enclosure in the expanse of the front wall of the enclosure above said first pass.

7. The generator of claim 3 wherein said furnace enclosure above said first pass is divided substantially equally into said second and third passes in series, the enthalpy pick-up in said second and third passes being about 100 Btus per pound for each pass over most of the normal load range of the generator.

8. A variable pressure once-through generator compIlSlng furnace circuit means defining the generator furnace enclosure;

a flow circuitry including a preheating circuit, the furnacecircuit and superheating circuits wherein the normal flow is in series through said circuits without recirculation, the superheating circuit being connected to a point of use;

means for varying the pressure in said furnace circuit inresponse to load change;

burner means in the lower portion of said furnace enclosure defining a high temperature radiant heat zone of the enclosure;

said furnace circuit means comprising a first upflow pass, lower inlet and upper outlet headers for said first upflow pass, said first upflow pass comprising a plurality of upflow tubes connected in parallel between the inlet and outlet headers and defining at least a substantial portion of the periphery of the furnace enclosure lower portion to an elevation just above the burner means;

said second pass and said third pass each comprising a plurality of tubes which are connected in parallel for upflow therein,

said first flow pass being located in the flow circuitry to receive a subcooled fluid throughout the normal pressure range of the generator, and being sized so that at about percent load and 2,400 psi operation the quality of the flow leaving it is not in excess of about 35 percent vapor by weight whereby nucleate boiling is maintained in the pass.

9. The vapor generator of claim 8 including turbulator means disposed within the tubes of said flow pass.

Claims

1. A once-through vapor generator of the type having a circuitry including vapor generating surface and vapor superheating surface, the latter being connected to a point of use, wherein the normal flow is in series through said surfaces without recirculation, comprising means for operating said generator at variable pressure from no load to full load, said surfaces comprising a first plurality of flow passes in series flow relationship defining a furnace enclosure; burner means in the lower part of said furnace enclosure; said plurality of flow passes comprising a first upflow pass, lower inlet and upper outlet headers for said first upflow pass, said first upflow pass conprising a plurality of upflow tubes connected in parallel between the inlet and outlet headers and defining substantially the entire periphery of the enclosure in the lower part thereof to an elevation just above said burner means; additional flow passes in series with the first upflow pass, said additional flow passes defining substantially the entire periphery of the furnace enclosure in the area above said first pass; said additional flow passes comprising a second pass which defines the generator front wall and substantial portions of each side wall in the area above said first pass, and a third pass which defines the generator rear wall and substantial portions of each side wall also in the area above said first pass; the first pass being located in the generator circuitry such that at normal operating pressures the fluid entering it is in a sub-cooled state, the first pass being sized so that at about 70 percent load and 2,400 psi operation the quality of the flow leaving it is not in excess of about 35 percent vapor by weight whereby nucleate boiling is maintained in the pass.

2. A vapor generator according to claim 1 wherein the generator side walls in the part of said enclosure above said first flow pass are divided between said second and third passes.

3. The generator of claim 1 including turbulators in said first pass, the enthalpy pick-up in the first pass being in excess of about 150 Btu''s per pound.

7. The generator of claim 3 wherein said furnace enclosure above said firsT pass is divided substantially equally into said second and third passes in series, the enthalpy pick-up in said second and third passes being about 100 Btu''s per pound for each pass over most of the normal load range of the generator.

8. A variable pressure once-through generator comprising furnace circuit means defining the generator furnace enclosure; a flow circuitry including a preheating circuit, the furnace circuit and superheating circuits wherein the normal flow is in series through said circuits without recirculation, the superheating circuit being connected to a point of use; means for varying the pressure in said furnace circuit in response to load change; burner means in the lower portion of said furnace enclosure defining a high temperature radiant heat zone of the enclosure; said furnace circuit means comprising a first upflow pass, lower inlet and upper outlet headers for said first upflow pass, said first upflow pass comprising a plurality of upflow tubes connected in parallel between the inlet and outlet headers and defining at least a substantial portion of the periphery of the furnace enclosure lower portion to an elevation just above the burner means; additional flow passes in series with the first upflow pass, said additional flow passes defining substantially the entire periphery of the furnace enclosure in the area above said first pass; said additional flow passes comprising a second pass which defines the generator front wall and substantial portions of each side wall in the area above said first pass, and a third pass which defines the generator rear wall and substantial portions of each side wall also in the area above said first pass; said second pass and said third pass each comprising a plurality of tubes which are connected in parallel for upflow therein, said first flow pass being located in the flow circuitry to receive a subcooled fluid throughout the normal pressure range of the generator, and being sized so that at about 70 percent load and 2,400 psi operation the quality of the flow leaving it is not in excess of about 35 percent vapor by weight whereby nucleate boiling is maintained in the pass.