US3713424A - Liquid vaporizer - Google Patents

Abstract

A liquid vaporizer or boiler for vaporizing organic working fluids without substantial decomposition thereof. A combustor of the vaporizer includes a plurality of strip burners for introducing a mixture of primary air-fuel into a baffled combustion chamber; and a strip nozzle adjacent each strip burner for introducing and turbulating secondary air with the primary air-fuel to effect combustion in the combustion chamber that is substantially free of hot-spots. A zig-zag tube bundle down stream from the baffle exchanges heat to vaporize the organic working fluid.

Description

United States Patent 1 Welz [451 Jan. 30, 1973 54] LIQUID VAPORIZER 3,368,547 2/1968 Hale .l22/367 [75] Inventor: Harold C. Welz, Littleton, Colo.

D Primary Exammer'Kenneth W. Sprague [73] Assigneez The Gates Rubber Company, ,4 R d Fink et 31,

Denver, Colo. 221 Filed: Oct. 12,1971 [571 ABSTRACT [21] APPLNO: 188,042 A liquid vaporizer or boiler for yaporizing organic workmg fluids without substantial decomposition thereof, A combustor of the vaporizer includes a plu- U.S. C, alit of tri burners for introducing a mixture of pri- [51] Int. Cl ..1F22b 21/24 mal-y aipfue] into a b ffl d combustion b a d [58] Fleld of search-"122mm, 356, 367 (31431/347 a strip nozzle adjacent each strip burner for introducing and turbulating secondary air with the primary air- [56] References C'ted fuel to effect combustion in the combustion chamber UNITED STATES PATENTS that is substantially free of hot-spots. A zig-zag tube bundle down stream from the baffle exchanges heat to 3,177,923 4/1965 Hine, Jr. et a1. ..431/347 vaporize the organic working fluid. 3,242,910 3/1966 Hale ..122/367 3,315,646 4/1967 Witten, Jr ..l22/367 7 Claims, 2 Drawing Figures PATENTEUJMI 30 ms SHEET 1 UF 2 INVENTOR PATENTEDJAN 30 ms I 3., 7 13,424

snmanrz -|NVENTOR HAROLD c. WELZ LIQUID VAPORIZER BACKGROUND OF THE INVENTION The invention relates to liquid heaters and Vaporizers, but more particularly, the invention relates to a combustor for oxidizing a gaseous fuel and heating an organic fluid contained in a zig-zag tube without substantial decomposition thereof.

Liquid heaters and Vaporizers have heretofore been constructed for adding heat to working fluids that are generally insensitive to high tube temperatures or fluctuations in temperature along the tube walls of the vaporizer. Most prior art vaporizers are directed toward vaporizing water to steam. For compactness and efficiency, the vaporizors are generally of the counterflow type where maximum heat flux is transferred from combusted gases, through the tubes, and to the water. Since water is a most accommodating working fluid which does not decompose with heat, there is no problem associated with transferring maximum possible heat from the combusted gases to the water.

Other types of devices are associated with heating, but not vaporizing, organic fluids. An example of such a heater is disclosed in US. Pat. No. 2,113,331 as issued to A. E. Nash et al. The design of the Nash et al. device is concerned with deterioration of oil as the heated working fluid. Care is taken not to vaporize the oil. However, since oil is used, a large variation in tube and combusted gas temperatures may be tolerated without seriously affecting the composition of the oil.

With the advent of high vapor pressure organic fluids, such as'halogenated hydrocarbons as the working fluid for Rankine cycle type engines, the problem of temperature fluctuations in the combusted gases and boiler tubes of a vapor generator has become more predominate. Desirable working fluids have relatively high vapor pressures at modestly low temperatures when compared to water. Consequently, the differential temperature between the combusted gas and the working fluid is much less for steam than it is for an organic working fluid. Thus, organic fluid vapor generator is very sensitive to fluctuations in temperatures because of the high heat energy represented by the larger temperature differential. Another problem is that the'organic fluid must be operated at as high a temperature as possible to obtain maximum thermal efflciencies. However, it is typical for the halogenated hydrocarbonsto have a desired temperature-pressure relationship that approaches the criticaltemperature at which a high rate of fluid decomposition occurs. For example, it may be desirable to use trichloroltrifluoroethane as the working fluid in a Rankine cycle engine and operate the fluid at 390 F. to obtain tolerable thermal efficiencies. However, such a fluid decomposes at an intolerable rate at temperatures above 450500 F. Consequently, vapor generators for such fluids must be designed with minimum temperature excursions to avoid hot-spot over-heating.

The problem of decomposing a working fluid because of hot spotting is set forth at page 28 of the May 1 1, 1970 issue of Design News. The solution to the problem as therein presented is the introduction of a projective jacket of buffer fluid that protects organic working fluid from hot-spot decomposition.

SUMMARY OF THE INVENTION In accordance with the invention, a liquid vaporizer is provided which has a combustor that includes a plurality of strip type fuel burners positioned adjacent "secondary air control nozzles that are in the form of strips. The burners provide a primary fuel-air mixture to a combustion chamber. The arrangement of the strip burners and nozzles in relation to the combustion chamber provides means for combusting the fuel at substantially invariant temperature to heat an organic fluid contained in a zig-zag bundle.

Accordingly, it is an object of the invention to provide a liquid vaporizor suitable for vaporizing an organic working fluid without the aid of a buffer material.

Another object of the invention is to provide a combustor having fuel-air combustion means that substantially eliminates occurrence of hot-spots in the combusted gases.

Still another object of the invention is to provide a compact and economical liquid vaporizer for vaporizing sensitive organic fluids.

These and other objects or advantages of the invention will become more apparent by reviewing the drawings and description thereof wherein:

FIG. 1 is an isometric cross-sectional view of the invention.

FIG. 2 is a view taken along the line 2-2 of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to the FIGS., a liquid vaporizer 10 is provided which includes the preferred embodiments of the invention. The vaporizer includes a combustor 12 for generating heat by oxidizing a fuel; a zig-zag tube bundle 14 for exchanging heat to vaporize and heat an organic working fluid contained therein; and a flue for exiting cooled combusted gases.

Perhaps the most significant aspect of the invention is the combustor 12 where a fuel is combusted with air to effect a combusted gas of generally homogeneous temperature at the point where the gas impinges with the tube bundle. The combustor has one or more strip type burners 16 which direct a primary mixture of airfuel to a combustion chamber 18. The number of burners, of course, depends on the volume of the combustion chamber 18 and the quantity of fuel to be combusted to effect a desired heat transfer. Each burner 16 is of the venturi type where a desired quantity of gaseous fuel is metered to be mixed with air. The strip burners 16 extend generally across the width 20 of the combustion chamber 18 and the burners are positioned to occupy a major portion of the height 22 of the combustion chamber 18. Each burner 16 includes many individual nozzles or orifices 24 for emitting primary airfuel. The plurality of strip burners l6 effect distribution of a substantially uniform mixture of air-fuel across the inlet side of the combustion chamber.

The combustion chamber is a walled housing of a general box shape. The inside walls of the chamber are of a refractory material 26. One wall 28 of the chamber has a plurality of apertures in the form of slots 30 that are adapted to receive the strip burners 16. The bottom or interior wall of the chamber definesa baffle 32 and an aperture 34 which opens to the tube bundle 14. The apertured wall 28 of the combustion chamber mounts a plurality of angled strips 36 that define converging secondary air nozzles 38 that have a substantially constant rate of convergence. The secondary air nozzles 38 are juxtaposed to each strip burner 16 at a substantially constant spacing 40. The opening of each secondary air nozzle 38 is in longitudinal and parallel alignment with its adjacent burner 16, and the openings are of predetermined size to be commensurate with a designed fuel-air flow rate. Optionally, the strip burners 16 may be mounted to move to or away from the secondary air nozzles for the purpose of changing the spacing 40 which results in controlling secondary air flow rate. The burners 16 have sufficient height such that when the spacing is decreased, secondary air flow around the burners is also decreased. The burners may be permanently fixed in relation to the secondary air nozzles if the correct burner position has been predetermined with a test model in a manner which will later be explained.

The combustion chamber 18 opens to a second chamber 42 where heat is transferred from combusted gases to an organic fluid contained in the tube bundle 14. Preferably, the area of aperture 34 to the second chamber 42 is smaller than the cross-sectional area of the combustion chamber 18. The aperture defines a passageway that requires combusted gases to change their direction of flow and velocity prior to entering the secondary chamber. The change in velocity and direction turbulates combusted gases to enhance mixing thereof.

The secondary chamber 42 may be of any desired shape or at any desired location. Preferably, the secondary chamber converges or its effective flow area progressively decreases. The rate at which heat is convectively transferred to the tube bundle 14 by combusted gases is partially dependent upon gas velocity. As the combusted gases cool as heat is transferred to the tube bundle, the gas velocity is increased to compensate for the heat loss. The secondary chamber 42 is located below the combustion chamber 18 for compactness and other reasons which will later be explained. A flue 44 directs cooled combusted gases away from the secondary chamber 42. When the secondary chamber 42 is located below the combustion chamber as shown in FIG. 1, a forced draft means, not shown, maintains a differential pressure across the vaporizer for proper flow of the combusted gases through the chambers.

The design of tube bundle 14 is of special importance in the vaporizer because an organic working fluid is used which is decomposible with temperature. It is desirable in liquid vaporizer design to transfer as much heat as possible from hot combusted gases to. the vaporizer tube bundle, and thereby minimize the number of tubes for economical reasons. Consequently, it is common practice in most liquid vaporizers to counter-flow the working fluid in relation to the direction of combusted gas flow. This practice is unacceptable for organic working fluids because it results in excessive tube wall temperatures that cause fluid decomposition when minimum tube bundle size is required. Some liquid heaters, such as disclosed by Nash et al. cross flow an organic working fluid in relation to the direction of combusted gas flow. While Nash etal. discloses a device for heating an organic fluid, in-

terdependent relationships between the shape of heat transfer chamber and tubes, are not disclosed for a liquid vaporizer of minimum size and economic cost. In accordance with the invention, an interrelationship between the tube bundle l4 and secondary chamber 18 is disclosed for vaporizing an organic working fluid. As previously explained, the secondary chamber 42 crosssectional area converges to effect an increase in the rate at which heat is convectively transferred from combusted gases to the tube bundle. The rate at which heat is transferred from the tubes to a working fluid contained therein, depends on the external area of the tube in relation to the internal area of the tube. The relationship between the external and internal areas may be conveniently expressed as a ratio. When the ratio is greater than one, the external area of the tube is greatest whereas when the ratio is less than one, the intemal area of the tube is largest. By combining the effects of the converging secondary chamber and area ratio, the amount of heat transferred from the combusted gas to the working fluid in the tubes may be controlled to limit the temperature of tubes and preclude substantial decomposition of the working fluid. Differences in external and internal areas may be accomplished by providing external 46 or internal (not shown) fins on the tubes. Any external fin configuration also affects the degree of convergence of the secondary chamber because of its protrusion therein.

It has been found that a minimum size tube bundle may be used, and that heat may be properly transferred to an organic working fluid without decomposition thereof, if the combined effects of tube area and second chamber convergence are combined to limit the maximum temperature for all the individual tube walls to the same value. In other words, if a working fluid such as trichloroltrifluoroethane were used, it would be desirable to limit each tube wall temperature to a value no greater than essentially 425 F. because a substantial rate decomposition occurs at temperature approaching 425 F. Thus, the maximum wall temperature of the finned inlet tube would be no greater than the wall temperature of the bare outlet tube. Both tube walls would be limited to temperatures no greater than 450 to 500 F. Maximum vaporizer compactness with reasonable safety from working fluid decomposition is achieved when all wall temperatures are designed to reach a maximum value of 450 to 500 F.

Perhaps, the features and advantages of the invention may best be brought forth by way of example where the liquid vaporizer is in use or operation. An organic working fluid is effectively heated without decomposition or the aid of a buffer fluid. When the liquid vaporizer is in use, a gaseous fuel such as natural gas, is supplied to the inlet of the burners 16 where it is mixed with primary air. The primary air-fuel mixture is substantially uniformly distributed through the apertured wall 28 of the combustion chamber 18 by the plurality of nozzles 24 of the strip burners 16. As the primary air-fuel enters the combustion chamber 18, it is ignited by a means not shown. Secondary air is mixed with the primary air to achieve stoichiometric combustion. Secondary air enters the combustion chamber through the converging secondary air nozzles 38. The amount of secondary air which enters the combustion chamber depends on the position of the strip burners in relation to the secondary nozzles. When the strip burner is positioned close to the secondary nozzles, the effective opening to the secondary nozzles 38 is blocked off by the edge portions of the strip burners 16. In a similar manner, secondary air-flow is increased as the strip burners are moved away from the secondary nozzles. The precise position for the strip burners may be determined by selectively positioning them and analyzing the products of combustion for carbon monoxide content. The strip burners may optionally be adjustable or fixed for position or spacing 40 once a proper secondary air-flow is established.

Here, the features and interrelationships between the secondary air nozzles and strip burners should be noted with emphasis and particularity. The strip burners l6 distribute primary air-fuel in somewhat uniform fashion through the apertured wall 28 of the combustion chamber. The large area distribution of fuel-air shortens the time required for combustion in comparison to a few individual nozzles having the same rated flow capacity. The converging secondary nozzles direct air to impinge and mix with the primary air-fuel mixture. The impingement turbulates the combusting gases to effect a substantially homogeneous mixture. The void areas behind the secondary nozzles create eddies which aid in turbulation and mixing. Turbulent mixing avoids occurrence of fuel rich mixtures which could result in gaseous hot-spots or a heterogeneous mixture. A salient feature of the burner and secondary air nozzle arrangement is that precise control is maintained over secondary air flow throughout the length of the burners. One adjustment or position of the burners assures the same amount of secondary air is emitted by the strip burners for all the primary air-fuel. Thus, the problem of having too much or little secondary air is avoided and homogenity of the combusting gas mixture is enhanced. In comparison, if several independent nozzles were used, it would be difficult to assure homogenity of the gases because each nozzle would have to be individually balanced.

The length of the combustion chamber 18 is chosen I so complete combustion occurs by the time the gases reach the apertured wall. This assures that no flame will directly contact the tube bundle and create hot-spots. The change in direction and increase in combusted gas velocity at the aperture 34 further turbulates and mixes the gases to further avoid heterogeneous temperatures in the combusted gas.

The combusted gases proceeded to the secondary chamber to heat and vaporize an organic fluid contained in the tube bundle 14 in a manner as previously explained. The secondary chamber 42 is located below the combustion chamber 18 which yields two advantages. First, the total length of the vapor generator is held to a minimum.

Secondary, the tube bundle is located at the lowest possible location in the liquid vaporizer. The tube bundle is also located below the inlet 48 and outlet thereto 50 Thus, when the liquid vaporizer is used in conjunction with other equipment such as a Rankine cycle engine, the vaporizer may easily be located at a point where the tube bundle is the lowest point where a liquid working fluid may accumulate when the liquid vaporizer is not being operated. Consequently, when the liquid vaporizer is operated, it is assured that liquid working fluid is contained within the tube bundle during the starting transient. This further assures the unlikelihood of burning through a boiler tube.

The foregoing detailed description is made for pur-' pose of illustration only and is not intended to limit the scope of the invention which is to be determined from the appended claims.

What is claimed is:

1. In a liquid vaporizer of the type operable with a mixture of air-fuel, and having a walled housing that defines a combustion chamber and a secondary heat exchange chamber communicable with the combustion chamber, a tube bundle disposed within the secondary chamber to effect means for transferring heat to a working fluid contained therein, and a flue for directing combusted air-fuel away from the chamber, the improvement comprising:

an apertured wall of the combustion chamber having at least one slot extending substantially across the width thereof;

a converging secondary air nozzle mounted to the interior of the apertured wall adjacent to and extending substantially the length of each slot, said secondary air nozzle extending into the combustion chamber with substantially uniform convergence throughout its length;

a strip burner mounted at the exterior of the apertured wall juxtaposed at a substantially constant spacing from each slot, said strip burner extending substantially the length of each slot, the space between the burner and apertured wall defining a secondary air-flow area and path to the secondary air nozzle, said strip burner having sufficient height such that when the spacing of the strip burner from the apertured wall is decreased, the secondary air-flow area is decreased, said strip burner having a plurality of nozzles for emitting and directing a primary air-fuel mixture through each slot and secondary air nozzle and into the combustion chamber.

2. A liquid vaporizer as set forth in claim 1 and further including an interior wall defining a baffle and opening between the combustion and secondary chambers, the opening defining a cross-sectional flow area that is less than the cross-sectional area of the combustion chamber.

3. A liquid vaporizer as set forth in claim 1 wherein the secondary chamber is disposed below the combustion chamber and separated by at least one wall between the chambers, said wall defining an opening having a cross-sectional area which is less than the cross-sectional area of the combustion chamber.

4. A liquid vaporizer as set forth in claim 3 wherein the tube bundle includes a plurality of tubes arranged in zig-zag fashion across the width of the combustion chamber.

5. A liquid vaporizer as set forth in claim 4 wherein the walls of the secondary chamber in combination with the tube bundle defines a converging secondary chamber.

6. A liquid vaporizer as set forth in claim 5 wherein the tubes are adapted to transfer heat from the secondary chamber to the working fluid at a generally constant tube wall temperature.

7. A liquid vaporizer as set forth in claim 1 wherein said strip burner is mounted to be adjustably moved to or away from the slit to effect adjustment of the secondary flow area.

Claims (6)

1. In a liquid vaporizer of the type operable with a mixture of air-fuel, and having a walled housing that defines a combustion chamber and a secondary heat exchange chamber communicable with the combustion chamber, a tube bundle disposed within the secondary chamber to effect means for transferring heat to a working fluid contained therein, and a flue for directing combusted air-fuel away from the chamber, the improvement comprising: an apertured wall of the combustion chamber having at least one slot extending substantially across the width thereof; a converging secondary air nozzle mounted to the interior of the apertured wall adjacent to and extending substantially the length of each slot, said secondary air nozzle extending into the combustion chamber with substantially uniform convergence throughout its length; a strip burner mounted at the exterior of the apertured wall juxtaposed at a substantially constant spacing from each slot, said strip burner extending substantially the length of each slot, the space between the burner and apertured wall defining a secondary air-flow area and path to the secondary air nozzle, said strip burner having sufficient height such that when the spacing of the strip burner from the apertured wall is decreased, the secondary air-flow area is decreased, said strip burner having a plurality of nozzles for emitting and directing a primary air-fuel mixture through each slot and secondary air nozzle and into the combustion chamber.

2. A liquid vaporizer as set forth in claim 1 and further including an interior wall defining a baffle and opening between the combustion and secondary chambers, the opening defining a cross-sectional flow area that is less than the cross-sectional area of the combustion chamber.

3. A liquid vaporizer as set forth in claim 1 wherein the Secondary chamber is disposed below the combustion chamber and separated by at least one wall between the chambers, said wall defining an opening having a cross-sectional area which is less than the cross-sectional area of the combustion chamber.

4. A liquid vaporizer as set forth in claim 3 wherein the tube bundle includes a plurality of tubes arranged in zig-zag fashion across the width of the combustion chamber.

5. A liquid vaporizer as set forth in claim 4 wherein the walls of the secondary chamber in combination with the tube bundle defines a converging secondary chamber.

6. A liquid vaporizer as set forth in claim 5 wherein the tubes are adapted to transfer heat from the secondary chamber to the working fluid at a generally constant tube wall temperature.

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