US2890684A - Method of and apparatus for generating vapor - Google Patents

Description

June 16, I959 J. P. WlNKlN 2,390,684

METHOD OF AND APPARATUS FOR GENERATING VAPOR Filed Sept. 7, 1955 H71 N :0 v v. *h

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I JUSTIN P. WINKIN Q BY ATTORNEY METHOD OF AND APPARATUS FOR GENERATING VAPOR Justin P. Winkin, Fair Lawn, N.J., assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Application September 7, 1955, Serial No. 532,826

3 Claims. (Cl. 122--1) This invention relates to vapor generation and the operation of vapor generators, and more particularly to a method of and apparatus for providing high temperature low pressure vapor heating medium for process purposes.

Mercury vapors have heretofore been used as a source of heat for process heating requirements. Such uses have been attractive for many reasons, not the least of which has been the availability of a high temperature medium at low generating and distribution pressures when using mercury as the heat transfer medium. However, the prior mercury vapor process heating systems have been characterized by narrow ranges of operating loads, and by wide changes in the mercury vapor generating and distribution pressure to accommodate changes in the heat requirements of the process system. In general, mercury vapor generators are subject to circulation difiiculties when a wide range of vapor generating capacity rates are attempted. Such difficulties become apparent within the generating unit by the overheating of heat transfer surfaces, and when low capacity rates are attempted, parts of the generator are apt to fail by rupture from the localized overheating or hammering caused by poor circulation. As a general rule for safe operating conditions a mercury vapor generator is ordinarily arbitrarily operated in a capacity range of, for example 2 or 3 to 1 between the maximum and minimum vapor generating capacities. Such load ranges are entirely inadequate for process heating purposes, and for many power generating purposes where the desired range of mercury vapor generation may vary as much as 10, or even 1000, to 1.

In accordance with the present invention a mercury vapor generator is provided which is operated at a substantially uniform pressure through a permissible load range of, for example, 3 to 1, and when the heat requirements of the associated heat user is less than the minimum safe operating capacity of the generator the excess mercury vapor is passed to a heat exchanger or tubular condenser, where the mercury vapor is condensed for return to the vapor generator, with the heat abstracted by condensation of the mercury vapor transferred to water for the generation of steam. The steam produced in the condensation of the mercury vapor may be utilized for other process purposes or it may be used to generate power, and to supplement a separately fired steam generator. In the present invention the mercury vapor condenser is advantageously used as the steam and water drum for a waste heat boiler which receives its heat from the gases of combustion discharged from the mercury vapor heater or generator. In another aspect of my invention I provide a venting system for the mercury vapor system whereby any mercury vapor escaping from the system is conducted to an atmospheric discharge stack where the vapor is subjected to the cooling efiects of water spray to condense the mercury vapor and to thereby avoid the discharge of any mercury vapor to the atmosphere.

. The various features of novelty which characterize my atent O invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

In the drawing, a schematic arrangement of a mercury vapor generating system is illustrated as constructed and arranged in accordance with the invention.

The mercury vapor generator 10 shown in the drawing is disclosed and claimed in detail in a co-pending application Serial No. 532,828, filed September 7, 1955, in which I am co-inventor with George F. Friese and Ernest A. Brooks. As disclosed and claimed in the co-pending application, the mercury vapor generator 10 is fired by a fluid or gaseous fuel with the gases of combustion heating liquid mercury to produce vapor which is discharged through avalved discharge pipe 11 at a pressure, for example, of 66 pounds per square inch absolute (p.s.i.a.) and a temperature of approximately 850 F. The flue gases leaving the mercury vapor generator 10 are passed through a flue gas duct 12 to a waste heat boiler 13. The flue gases are cooled to a temperature of the order of 400 F. in the boiler 13 and are discharged through an induced draft fan 14 to a stack 15 and thence to the atmosphere.

In addition to the mercury vapor discharge pipe 11 leading from the vapor generator 10 to the process heat users 16, a second valved conduit 17 connects the mercury vapor generator with a relief and minimum load condenser 18. The condenser is of the tubular type and is is disclosed and claimed in co-pending application Serial No. 532,827, filed September 7, 1955. Mercury vapor passing through the conduit 17 is delivered to a distribution manifold 20 for discharge through a plurality of connecting conduits 21. Within the condenser 18 mercury vapor is delivered to a plurality of inlet headers 22 and through bundles of upright tubular elements 23 which are in contact with cooling water and steam maintained within the shell 24 of the condenser 18. The heat exchange between mercury vapor and the water within the shell condenses the mercury, with the liquid collected in a plurality of outlet headers 25 and discharged through conduits 26 for collection in a liquid drain manifold 27.

The mercury vapor which has been condensed and discharged to the drain manifold 27 is passed through the customary dirt removal and mercury seal apparatus 28 and returned to the mercury vapor generator 10 via a connecting pipe 30 which also receives mercury vapor condensate from the process heat users 16 through a pipe 31. It is also customary to provide a mercury liquid level indicator and a control system for maintaining the mercury level substantially uniform as indicated at 29, in the downcomers of the mercury vapor generator. This control system is indicated schematically at 32 in the drawing.

The water which is heated in condensing the mercury vapor is vaporized and leaves the condenser through a valved steam outlet pipe 33 for discharge to an external steam main (not shown). The condenser shell 24 is provided with the customary vent and safety valve connections 34, blowdown connections 35, and a make up line 36. Since the shell 24 also serves as the steam and water drum for the waste heat boiler 13, the boiler is connected with the condenser by downcomers 37 which withdraw water from the lower portion of the waste heat boiler 13. A series of riser tubes 38 connect the steam and water space of the waste heat boiler 13 with the vapor space of the condenser shell 24.

The flow of mercury vapor from the generator 10 to the condenser 18 through the connecting pipe 17, mani fold 20 and pipes 21 is manually or automatically regulated by a valve 39. In an actual installation of the invention disclosed, the heater or vapor generator was constructed to supply a maximum of 3,000,000 B.t.u. per hour of mercury vapor to the process heat users 16, The minimum heat load on the process heat users 16 was of the order of 1000 B.t.u. per hour, and with the generator 10 limited to a low limit of output of approximately 1,000,000 B.t.u. per hour, Le. a 3 to 1 operating range, the excess mercury vapor is diverted to the condenser 18 and the condensed mercury returned to the generator, In the installation described, the valve 39 is regulated automatically in response to the temperature of the mercury vapor in the drum of the generator 10. Thus, at low loads, with the fuel input to the generator at a minimum value corresponding to a mercury vapor output equivalent to a heat output of approximately 1,000,000 B.t.u. per hour the temperature of the vapor in the drum would rise, opening the valve 39 to divert excess vapor to the condenser.

With the arrangement and control system described, the amount of heat in the form of mercury vapor delivered can vary between extremely wide limits without encountering the circulation difliculties characteristic of mercury vapor generation. As a safety measure, a separate control system, responsive to vapor pressure in the generator, is also connected with the valve 39 to divert mercury vapor through the pipe 17 by opening the valve 39 when the pressure within the vapor system reaches a predetermined value. In the system described, the maximum mercury vapor pressure in the system serving the heat users 16 is approximately 66 p.s.i.a., and the safety pressure responsive control actuating the valve 39 is set for a value of 73 p.s.i.a. Thus, when the pressure reaches the latter value the valve 39 will be completely opened to conduct mercury vapor to the condenser 18. A separate pipe 52, containing a pressure relief valve 53 is connected with the pipe 17 both above and downstream of the valve 39. The valve 53 is set to open when the pressure in the pipe 17 exceeds a selected value above the selected opening pressure of the valve 39. This pressure could be, for example, 77 p.s.i.a., and would relieve the generator 10 when the pressure exceeded this value, even though the valve 39 was inoperative and unable to open.

The safety pressure relief valve 53, and the minimum load relief valve 39 are intended to prevent the mercury vapor pressure from rising to the set pressure of a direct relief valve 54 positioned in a pipe 55. The pipe 55 is connected with the juncture of the pipes 17 and 52 upstream from the valves 39 and 53, and is adjusted to an opening pressure of, for example, 87 p.s.i.a. The pipe 55 opens at its opposite end to a pipe 40 of larger diametcr, as hereinafter described for pressure relief of the mercury vapor system to the atmosphere. It will be noted that the valves 53 and 39 direct a flow of mercury vapor to the condenser 18, where such a flow could be considered the normal and usual mercury vapor relief flow path. On the contrary pressure relief of the mercury system through the valve 54 relieves the system to the atmosphere and would occur only under unusual pressure conditions within the system.

In any vapor generating system utilizing two dissimilar vaporizable mediums, such as mercury and water, the operating pressures of the two vapors may differ considerably, and safety means must be provided for a possible failure of parts leading to an intermingling of the two vapors where the higher pressure vapor may enter the lower pressure vapor system. In the embodiment of the present invention, the maximum permissible mercury vapor pressure in the system is of the order of 100 p.s.i.a., while the steam generated in the waste heat boiler 13 and the condenser 18 is of a pressure of the order of 425 p.s.i.a.. Thus, a failure .of a tube .or tubes in,for example, the condenser 18 imposes the pressure of the steam circuit on the mercury system, and would necessitate an immediate pressure relief of the mercury system. Furthermore, in the mercury vapor and steam generating system illustrated, it is necessary to avoid the release of mercury vapor to the atmosphere not only because of the toxic quality of the vapor, but also due to the economic loss involved.

As shown, the pipe 40 connects the conduit 17 and the manifold 20, and the mercury relief pipe 55 with a condenser 41 which is provided with a cooling coil 56 supplied with cooling water through a valved inlet pipe 42 and a water discharge pipe 43. The condenser 41 has a relatively low capacity compared with the condenser 18, and is intended to condense only the mercury vapor vented from the vapor system through leakage of the valve 54. With a limited flow of mercury vapor to the condenser 41 a vacuum is maintained therein. Mercury vapor condensed in the drum 41 is discharged to the mercury liquid cleaning apparatus 28 through a pipe 44 for return to the mercury vapor generator 10. The condenser 41 is provided with a rupture diaphragm in the side thereof masking the opening to a pipe 45 leading to a stack 46. Depending upon the material constituting the diaphragm, it may be desirable to cool the diaphragm to avoid premature rupture from overheating rather than by the imposition of pressure thereon. For this purpose provisions are made for a controlled water cooling of the diaphragm through a valved cooling water flow pipe 3-9 supplied from the same source as the coil 56 When the flow of vapor to the condenser increases, as from the opening of the pressure relief valve 54, the condensing capacity of the condenser 41 is insufiicient and the pressure will exceed the rupture strength of the diaphragm and vapor is discharged through the pipe 45 into the stack 46. The vapor is condensed within the stack 46 by contact with cooling water spray introduced thereto through pipe 47 and spray nozzle 48. The condensed vapor, both water and liquid mercury, is withdrawn from the base of the stack through a pipe 50 and delivered to a water and mercury separating device (not shown) with the mercury returned to storage and thence to the mercury vapor generator 10, and the water discarded to .a sewer.

In the event of a rupture of a tube 21 within the con.- denser 18 the full pressure of the steam generating system would be imposed on the mercury vapor system. While such an eventuality is remote, the systems must be arranged for pressure relief so that no part of the mercury vapor system, which is designed for a pressure of p.s.i.a. for example, is subjected to the higher pressure of the steam system, which may be 425 p.s.i.a. for example. Since the mercury vapor system, including the manifold 20 is intended for operation at the lower pressure it is necessary to arrange the mercury vapor piping connections between the tube bundle 22 and the manifold 20 so that the flow of steam into the manifold will not exceed the allowable design pressure thereof. This is accomplished by sizing the pipes 21 so that the critical flow velocity therethrough will limit the pressure to the desired value.

In the illustrated embodiment of the invention the pipes 21 are selected for an internal diameter which will permit a flow of 131,000 pounds per hour of steam, with water flashing into steam into the header 20 while restricting the pressure of the steam delivered to the header 20 to a maximum of approximately 100 p.s.i.a. The dimensions of pipe 40, and the connecting portion of the pipe 17 is sufiicient to relieve the maximum steam and mercury vapor volumes present in the header 20 without exceeding an allowable pressure of for example 100 p.s.i.a., on any portion of the mercury vapor system. A valve 57, having a size consistent with the flow volumes of such a continguency, is positioned in the pipe 40 and is set to open with the imposition of a differential pressure of, for example, 40 p.s.i. therein. In a similar manner the rupture diaphragm and the pipe 45 are dimensioned to accommodate the entire flow of both the steam and mercury vapor systems so as to safely vent the systems to the stack 46.

While in accordance with the provisions of the statutes 1 have illustrated and described herein a preferred embodiment of the invention, those skilled in the art will understand that changes may be made in the method of operation and form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. Heat transfer apparatus comprising a mercury vapor generator, means for heating mercury in said generator by the gaseous products of fuel combustion to generate mercury vapor, a mercury vapor user, means for delivering a controlled quantity of mercury vapor from said generator to said user, steam generating means including tubular elements positioned in heat exchange relationship with the gaseous products of combustion leaving said mercury vapor generator, a steam and water drum spaced above and communicating with said tubular elements, mercury vapor condensing tubes disposed in the water space of said steam and water drum, means for passing mercury vapor from said generator to said condenser in response to the increase of vapor pressure avove a selected value, and an atmospheric pressure relief system for said mercury vapor generator including a stack, a water spray nozzle positioned Within said stack, pipe means connecting said mercury vapor generator, said condenser, and said stack; a diaphragm positioned in said pipe means adjacent said stack, said diaphragm rupturable at a predetermined pressure to discharge condensable vapor of said stack wherein said vapor is condensed by the water spray from said nozzle, said pipe means between said condenser and said stack being of sufilcient size to vent steam and mercury vapor in the event of rupture of a mercury vapor condensing tube without imposing excess steam pressure on said mercury vapor generator.

2. In a binary vapor system having a fuel fired mercury vapor generating zone operable in a narrow range of maximum and minimum heating rates and a mercury vapor user having a maximum heat requirement substantially equal to the maximum heating rate of said Vapor generating zone and a minimum heat requirement substantially less than the minimum heating rate of said mercury vapor generating zone, the method of operation which comprises generating mercury vapor in a mercury vapor generating zone at a substantially uniform pressure by heat exchange with hot gases of said fuel combustion, generating steam in a separate heating zone by heat exchange with the gases of combustion leaving said mercury vapor generating zone, passing a steam and Water mixture from said separate heating zone to a steam and Water separating zone, transporting said mercury vapor to said vapor user in accordance with the quantity of heat required by said vapor user, passing the mercury vapor generated in said mercury vapor generating zone in excess of the quantity required by said vapor user to a separate mercury vapor condensing zone within said steam and Water separating zone, and controlling the quantity of excess mercury vapor passed to said mercury vapor condensing zone in response to a predetermined change in a Vapor condition within said mercury vapor generating zone, and generating steam by heat exchange with said excess mercury vapor in said mercury vapor condensing zone to condense said excess mercury vapor and to return said mercury condensate to said mercury vapor generating zone.

3. In combination, means for generating vapor from a high boiling point vaporizable liquid by heat exchange with hot gaseous products of combustion over a normal range of operating loads including a relatively high minimum operating load, means forming a vapor utilizing device arranged to receive vapor directly from said vapor generating means and designed for operation over a normal range of operating loads including loads below the minimum operating load of said vapor generating means, and means permitting the operation of said vapor generating means at an operating load at or above said minimum operating load while delivering generated vapor to said vapor utilizing device in an amount below said vapor generating means minimum operating load comprising a waste heat boiler having Vapor generating surface arranged to receive hot gases from said vapor generating means and a steam and water drum positioned above said boiler and riser and downcomer tubes connecting said surface to said drum, vapor condensing means arranged within said steam and water drum and having a vapor condensing capacity at least suflicient to condense all of the vapor generated by said vapor generating means at its minimum operating load, a pipeconnecting said vapor generating means with said vapor condensing means, and a valve in said pipe operable in response to a predetermined change in a vapor condition in said vapor generating means to permit vapor to pass to said condenser.

References Cited in the file of this patent UNITED STATES PATENTS 1,692,786 Pew et a1. Nov. 20, 1928 1,765,038 Wait June 17, 1930 1,888,311 Dow Nov. 22, 1932 1,914,140 Lucke June 13, 1933

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