US3496915A

US3496915A - Vapor generators

Info

Publication number: US3496915A
Application number: US708644A
Authority: US
Inventors: Richard A Grams
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1968-02-27
Filing date: 1968-02-27
Publication date: 1970-02-24
Anticipated expiration: 1987-02-24
Also published as: FR2002673A1

Description

Feb. 24, 1970 RA. GRAMS.

VAPOR GENERATORS 2 Sheets-Sheet 1 Filed Feb. 27, 19 68 m T m m Richard A.Grams 2 ATZRNEY Feb. 24, 1970 R. A. GRAMS VAPOR GENERATORS 2 Sheets-Sheet 2 Filed Feb. 27. 1968 r||||||||l|ll II Ik United States Patent O U.S. Cl. 122-478 7 Claims ABSTRACT OF THE DISCLOSURE An improved marine vapor generator of the type which includes a reheater and the means for by-passing the combustion gases around the reheater when vapor flow through it is stopped as in the case of maneuvering and astern operation of the ship. Improvements to the generator include a first convection gas pass, located between the superheater and reheater passes, arranged with sufficient saturated surface to reduce and maintain the reheater pass gas temperatures within safe operating limits and controlling reheat steam temperature by regulating the flow of combustion gases through both the bypass portion of the reheater pass and the bypass portion of the first convection gas pass. A major portion of the economizer is exposed to the entire flow of combustion gases thereby achieving stable, efficient operation during all operating modes.

The invention relates to a ship propulsion system and more particularly to marine type vapor generators equipped with a reheater and to the means for controlling the flow of combustion gases over the said reheater.

The reheating of steam between turbine stages is well recognized as a desirable step in obtaining the high efficiencies required of todays power plant systems, however the application of the reheat principle to a ships propulsion system introduces certain factors which are not normally encountered in stationary instalations. One such factor is the practice of marine systems to exclude reheat and part of the steam cycle during astern operation, maneuvering and inport operation of the ship.

Heat exchanger surface intended for the superheating or reheating of steam is normally located in a zone where operating gas temperatures are far above recognized design temperature use limits for metals, however, such use is made possible by the metal cooling effect derived from steam passing through the tubes of these heat exchangers. A problem arises during astern operation, maneuvering and inport operation when no steam is passed through the reheater, thereby producing a condition where tube metal temperatures will exceed safe design limits unless remedial measures are taken. One manner of overcoming this problem has been to locate the reheater in one of two parallel gas passes and to provide dampers for preventing the flow of hot combustion gases over the reheater when the latter is not in use, as is shown in US. Patent No. 3,280,559.

In the present state of the art, it is costly and often impractical to provide a gas-tight damper to contain and control a hot and corrosive fluid such as boiler flue gas. An object of the present invention is to provide protection for a tubular reheater by disposing sufiicient heat exchange surface ahead of the reheater to lower the gas temperature to acceptably safe limits before it enters the reheater. This heat exchange surface may comprise saturated wall tubes lining a first convection gas pass located between the superheater and reheater gas passes. An important feature of the saturated surface arrangement is the formation of an open pass cavity. Heat transfer in the cavity is by means of radiation and is, there fore, primarily a function of gas temperature entering the cavity rather than a function of gas mass flow as is convective heat transfer. Thus for a given boiler rating the cavity heat absorption will be approximately the same regardless of whether or not the reheater is being bypassed. This in turn means that the cavity will be much more effective in reducing gas temperature during reheater by-pass operation as the gas flow to the reheater is reduced to minimum values (that corresponding to damper leakage). In this manner the reheater can be protected from overheat during by-pass operation and will also have some extra thermal potential available in the gas for normal operation.

Another object of the invention is to facilitate the control of superheated steam temperature through the use of saturated surface rather than superheater surface to modulate the gas temperature to thereby protect the reheater. It can be readily seen that if steam generating apparatus of the general type disclosed by applicant should have a portion of the superheater surface located in the reheater gas pass that portion of the superheater would remain virtually idle during astern operation, thereby effectively reducing the amount of superheater surface area exposed to the flow of combustion gases. Since superheater surface requirements are basically dependent on fuel input it would mean that, for equal firing rates, the effective superheater surface and total absorption will differ substantially for astern and forward operation of the vessel. Obviously this would impose a most difficult operating burden on the system for controlling superheated steam temperature over the units load range.

Still another object of the invention is to achieve efiicient, stable operation of the unit throughout all operating modes by exposing the main portion of the economizer heat surface to the entire flow of combustion gases. This main portion may comprise upwards of percent of the total economizer heating surface nad may be extended surface type tubes for added heat transfer capability. The remaining portion of economizer surface may be bare tubes located in the alternate gas passage provided for bypassing the reheater, its primary function being to lower the fiue gas temperature below say 1000 F. to protect the dampers in the gas flow path. It can be readily seen that with this arrangement of economizer approximately the same feedwater temperature will be available at the boiler drum for specific firing rates regardless of the mode of operation.

A further object of the invention is the control of reheat steam temperature by regulating the gas mass flow across the reheater. This is accomplished through the selective positioning of dampers located in the alternate or by-pass gas outlet passage of the first convection gas pass. These dampers are designed to effectively control the quantity of gas flowing in the by-pass. On the other hand, it is contemplated that the dampers at the outlet of the reheater gas pass will operate either in a fully open or closed position, their sole function being to inhibit the flow of gases across the reheater during astern operation, maneuvering or inport operation. These dampers will necessarily be of the type which provide closure means rather than positive control characteristics.

In the drawings:

FIGURE 1 is a diagrammatic sectional side view along lines 1-1 of FIGURE 2 of an improved vapor generator for use in a ship propulsion system.

FIGURE 2 is a diagrammatic sectional plan view along lines 22 of FIGURE 1.

Referring to FIGURES 1 and 2 there is shown a boiler setting 10 comprising front and rear walls 11 and 12 and

sides walls

13 and 14. The boiler setting 10 is enclosed by casing 15 and insulation 16. The setting 10 is partitione'd into a furnace chamber 17, a superheater gas pass 18, a first convection gas pass 19, a reheater gas pass 20, and a second convection gas pass 21. Furnace chamber 17 is defined by side wall 13 and sections of front wall and rear walls 11 and 12 and a partition wall 22. The walls enclosing furnace chamber 17 are of tangent tube to tube construction with the exception of a portion of partition wall 22 which includes a staggered tube intermediate section forming a screened inlet 23 to the superheater gas pass 18, and the section of front wall 11 which includes the openings 24 to accommodate the fuel burners (not shown). The tubes making up the furnace floor 34, partition wall 22 and side wall 13 originate in lower header 25 and terminate in steam drum 26. The tubes in the furnace section of front wall 11 originate at lower header 27 and terminate in

upper headers

28 and 29 which are connected to steam drum 26 through riser tubes 30. The tube circuitry of the furnace section of the rear wall 12 (not shown) is identical to that of the front wall 11 with the exception that there are no burner openings.

The superheater gas pass 18 contains the entire superheater heat exchange surface composed of upright generally U-shaped tubes 35 spaced across the whole width of the gas pass and comprising a secondary superheater 36, a tertiary superheater 37 and a primary superheater 38 serially arranged in that order in the direction of gas flow. The sections of front and rear walls 11 and 12 which define the superheater gas pass are of wide spaced construction with fiat studs 39 filling-in the tube spacing. The tubes in the superheater gas pass section of front wall 11 originate at lower header 40 and terminate in steam drum 26. The tube circuitry of the superheater gas pass section of the rear wall 12 (not shown) is identical to that of the front wall 11 with the exception that there are access doors 41 to allow entry into the lanes between superheater banks. The superheater gas pass 18 has a refractory lined sloped floor 42. Floor 42 also serves as the roof of header vestibule enclosure 43. The first convection gas pass 19 is situated downstream gas flow-wise of superheater gas pass 18. Gas passes 18 and 19 are separated by partition wall 44 which includes a staggered tube intermediate section forming a screened gas inlet 45 to first convection gas pass 19. Superheater gas pass 18 includes transverse screen tube wall 46 used primarily to support the tertiary superheater 37 through brackets 47. Primary and

secondary superheaters

38 and 36 are supported by

partition walls

44 and 22 respectively through similar brackets.

First convection gas'pass 19 is a vertically elongated pass disposed intermediate and laterally adjoining superheater gas pass 18 and reheater gas pass 20 and defined by sections of front and rear tube walls 11 and 12,

partition walls

44 and 48. Partition wall 48 includes a staggered tube lower section forming a screened inlet 50 to heater gas pass 20, two staggered tube intermediate sections forming

tube screens

51 and 52 located adjacent gas inlet 45 in partition wall 44. An economizer 49 is placed between screens 51 and -2 and comprises sinuous tubes 76 horizontally disposed across the first convection gas pass 19 with the tube ends attaching to headers 60 and -61 respectively. The tubes of

partition walls

44 and 48 originate in lower drum 53 and terminate in steam drum 26. The tubes forming the first convection gas pass section of front wall 11 originate in lower header 54 and terminate in upper header 55. The tube circuitry of the first convection gas pass section of the rear wall 12 (not shown) is identical to that of the front wall 11. The top outlet section 32 of first convection gas pass 19 situated above header 55 is defined by casing and includes a damper 56 adjacent the inlet 57 of second convection gas pass 21. Partition wall 44 has an access door 59 to header vestibule 43.

Reheater gas pass is a vertically elongated pass adjacently parallel to first convection gas pass 19 and d fin d. by part tion wa l 48, side wal 14 a d s c on of front and rear walls 11 and 12. Reheater gas pass 20 includes a reheater 62 comprising sinuous tubes 75 forming a plurality of separate banks and disposed across the width of the gas passage with their ends attaching to

headers

63 and 64 respectively. The tubes in the reheater gas pass section of front wall 11 originate at lower header 65 and terminate at upper header 55. The tube circuitry of the reheater gas pass section of the rear wall 12 (not shown) is identical to that of the front wall 11. The outlet section 33 of reheater gas pass 20 situated above header 55 is defined by casing 15 and includes a damper 66 adjacent the inlet 57 of second convection gas passage 21.

Second convection gas pass 21 is a vertically elongated gas pass located above and in communication with the outlets of both the first convection gas pass and the reheater gas pass through

dampers

56 and 66 respectively. Second convection gas pass 21 is defined by walls formed of casing 15 and includes a main economizer 67 comprising sinuous tubes 74 forming a plurality of separate banks and disposed across the width of the gas pass with the tube ends opening into

headers

68 and 69.

Downcomers 70 connect the steam drum to all of the furnace wall supply headers, for example,

headers

25 and 27 and to lower drum 53. Additionally supply tubes 71 connect lower drum '53 to

front wall headers

40, 54 and 65 and also to similarly disposed headers associated with the rear wall. Steam drum 26 includes steam-water separators 72. For the sake of clarity the connecting steam piping between drum 26 and the various superheater headers 73 is not shown.

In normal ahead or forward operation of the ship the steam-water fluid flow path of the steam generating unit is as follows: A controlled quantity of feedwater is admitted to inlet header 68 of main economizer 67 and is heated by combustion gases while passing through sinuous tubes 74 to outlet header 69. The heated feedwater is then piped (not shown) to header 60 of by-pass 49 and is further heated by combustion gases while passing through sinuous tubes 76 to outlet header 61. An alternate arrangement has the heated feedwater from main economizer 67 entering header 61 of by-pass economizer 49 and leaving through header 60. After passing through by-pass economizer 49 the heated feedwater is introduced by a conventional feed pipe (not shown) into steam drum 26 to maintain a predetermined water level. A natural circulation cycle takes place whereby water from steam drum 26 is circulated through downcomers 70 to lower drum 53 to supply

headers

25 and 27 etc. and from the lower drum through supply tubes 71 to supply

headers

40, 54 and 65 as well as similarly disposed headers associated with the rear wall. The water leaving the above mentioned lower drum and supply headers is heated becoming a steam-water mixture during its passage upwardly through the tubes of front and rear walls 11 and 12,

side walls

13 and 14 and

partition walls

22, 44 and 48 and screen wall 46. The steam-water mixture is returned to steam drum 26 directly from the partition walls and sidewalls and by way of

discharge headers

28, 29 and 55 via riser tubes 30. The steam-water mixture upon entering the steam drum 26 passes through separators 72. The water fraction leaving separators 72 is returned to the water space of drum 26 from whence it enters downcomers 70 to repeat the circulating flow cycle. The saturated steam leaving separators 72 is passed through primary scrubbers 77 and secondary scrubbers 78 and after leaving steam drum 26 is conveyed through appropriate piping (not'shown) to be passed serially through the primary,

secondary and

tertiary superheaters

38, 36 and 37. These superheater sections are arranged for series flow and may include an attemperator (not shown) between the outlet of the primary superheater 38 and the inlet to the secondary superheater 36 for steam temperature control. The superheated steam upon leaving tertiary superheater 37 is introduced into the high pressure turbine (not 5 shown). Steam leaving the high pressure turbine is conveyed through appropriate piping (not shown) to reheater inlet header 63 to be reheated during passage through tubes 75 and is discharged from reheater outlet header 64 to be conveyed to the low pressure reheat turbine through appropriate piping (not shown).

The combustion gas path is as follows: Fuel and combustion air are introduced through burner openings 24, and after transferring a portion of their heat content to the water cooling tubes lining furnace enclosure 17 the gases are discharged through opening 23 into superheater gas pass 18 where heat from the gases is transferred to the superheater and saturated wall enclosure surfaces. From passage 18 the gases are discharged through opening 45 into first convection gas pass 19 where the flow of gases may be divided into two flow paths, one being directed downwardly toward opening 50 with heat being transferred to a portion of the water cooled enclosure walls of passage 19. The other gas flow path is directed up wardly across

screen tubes

51, 52 and 31, and economizer 49 toward outlet 32 and damper 56 with heat being transferred to these heat absorbing surfaces and to adjacent portions of the saturated Walls of passage 19. The gases discharging into reheater gas pass 20 are directed upwardly across reheater 62 and screen tubes 31 through outlet 33 toward damper 66 with heat being transferred to reheater 62 and the saturated water wall enclosure surfaces. Whenever steam is being passed through reheater 20, damper 66 will be in wide open position. Reheater outlet steam temperature is controlled by modulating damper 56 thereby varying the gas mass flow being directed to gas pass 20 and across reheater 63. A closing of damper 56 will force more gases through inlet opening 50 and into gas passage 20 resulting in a rise in reheat steam temperature for equivalent steam flows, and the converse effects will result when the quantity of gas flowing is reduced. The gases emerging from

gas passage

32 and 33 form a single flow path through second convection gas pass 21 wherein heat is transferred to the feedwater flowing through economizer 67.

During normal astern operation of the ship the steamwater cycle differs from normal ahead operation in that the steam upon leaving tertiary superheater 37 is directed to a high pressure condensing type turbine, (not shown) normally referred to as an astern turbine, and whose direction of rotation is opposite from that of the turbine used for ahead operation. Since for this service there customarily is no fiow through the reheater, the steam leaving the astern turbine is condensed and returned to the condensate system (not shown) for use as feedwater for the boiler.

The combustion gas path for astern operation differs from that of ahead operation in that substantially all of the gases leaving superheater gas pass 18 are directed upwardly through first convection gas pass 19 to damper 56 and on through second convection gas pass 21. With this mode of operation, i.e. no steam flow through the re heater 62, damper 66 will be closed and damper 56 wide open, thereby substantially stopping the flow of combustion gases into and through reheater gas pass 20.

Since the medium to be controlled is a hot corrosive gas of up to 1000 F., it is economically unfeasible to construct a commercially serviceable gas tight shut-off damper. Accordingly as a practical consideration damper 66 is designed for an expected leakage of upwards of 10 percent of the total gas flow when in the closed position. Similar considerations respecting damper 56 resulted in the first convection gas pass 19 being arranged with sufficient heat absorbing saturated Wall surface to reduce the gas temperature entering gas passage 20 to below 1000 F. assuming leakage of upwards of 20 percent of the total gas flow.

Thus for all normal operating conditions the gas temperature at dampers 56 and '66 will remain within safe allowable operating limits for damper metal temperature.

A particular example of a marine reheat vapor generator incorporating the present invention could operate at the conditions set forth in Table I.

TABLE I Normal Rate:

superheated steam flow, lb./l1r 166, 200 210,000

Reheated steam flow, lb./hr 162, 800 Steam temperature:

superheater outlet temp, F 1, 000 1,000 Reheater outlet temp, F 950 Gas weights:

Total gas. lb./hr 238, 300 255, 820 Gas across superheate 238, 300 255, 820 Gas across reheater. 190, 700 51, 200 Gas by-passing reheater 47, 600 204, 620 Gas temperature:

Leaving furnace, F 2, 350 2, 380 Entering bypass economizer 1,070 1,135 Leaving by-pass economizer.-. 695 880 Entering by-pass dampers" 685 850 Leaving reheater 710 970 Entering reheater dampers 705 950 Entering main economizer 700 860 Leaving main economizer 320 335 Feedwater temperzture:

Entering economizer, F 280 28) Leaving main economizer 420 475 Leaving lay-pass economizer 450 535 What is claimed is:

1. In a ship propulsion system wherein superheated and reheated steam is selectively supplied to a forward drive turbine and only superheated steam to an astern drive turbine, the combination with the propulsion system of a steam generating superheating and reheating unit comprising walls including steam generating tubes forming a setting, partition wall means including steam generating tubes dividing the setting into a furnace, a superheater gas pass laterally adjoining the furnace and opening at its inflow end to the furnace across the full width and along the major portion of the height thereof, a reheater gas pass, and a vertically elongated first convection gas pass disposed intermediate and laterally adjoining the superheater and reheater gas passes and arranged to receive all the superheater gas pass outflow, said first gas pass having one gas outflow portion opening to the lower end of the reheater gas pass and another gas outflow portion for by-passing gases around the reheater gas pass,

means for firing the furnace,

a bank of vertically arranged return bend superheater tubes in the supeheater gas pass,

an economizer in said other portion of the first convection gas pass,

a bank of reheater tubes in the reheater gas pass and constituting the entire reheater heating surface,

a second convection gas pass arranged to receive gas outflow directly from the reheater gas pass and from said other portion of the first convection gas pass,

another economizer in the second convection gas pass connected for flow of fluid with the first named economizer and constituting a major portion of the total economizer heating surface, and

damper means for regulating gas outflow from said other portion of the first convection gas pass and from the reheater gas pass.

2. A ship propulsion system according to claim 1 wherein the damper means includes a first set of dampers at the outlet of the reheater gas pass and a second set of dampers at the outlet of said upper portion of the first convection gas pass.

3. A ship propulsion system according to claim 2 wherein the first set of dampers is fully open and the second set of dampers is used to proportion the gas flow between the reheater gas pass and the first convection gas pass when reheated steam is supplied to said forward drive turbine.

4. A ship propulsion system according to claim 2 wherein the first set of dampers is fully closed and the second 7 set of dampers is fully open where only superheated steam is supplied to said astern drive turbine.

5. A ship propulsion system according to claim 1 wherein the other economizer is connected for series flow of fluid to the first named economizer 6. A ship propulsion system according to claim 5 wherein fluid flow through the first named economizer is in indirect parallel flow heat absorbing relation with the gases.

7. A ship propulsion system according to claim 5 wherein fluid flow through the first named economizer is in direct parallel flow heat absorbing relation with the gases.

References Cited UNITED STATES PATENTS 5/1947 Boland 122478 12/1958 Hutchings et a1. 122480 10/1961 Hamilton et al. 122478 12/1967 Signell 122-480 KENNETH W. SPRAGUE, Primary Examiner US. Cl. X.R.