HEAT EXCHANGER AND FLUID HEATER
United States Patent 3809061
A heat exchanger is provided with a matrix of successive coils of finned tubing for the fluid to be heated which are each wound into the form of a truncated cone, the coils being closely wound and packed together so that fins of turns of each pair of coils are in at least approximate engagement with fins of two turns of the same coil and two turns of the other coil of the pair. A fluid heater incorporating such heat exchanger has a combustor with a correspondingly conical wall located coaxially within the exchanger and provided with a multiplicity of small apertures for directing flames of ignited fuel toward the matrix.
US Patent References:
WATER HEATER
Cancilla - October 1971 - 3612004
High output vapor generator for aircraft
Price - December 1940 - 2223856
VAPOR GENERATOR
Hoagland - February 1971 - 3563212
/1035992.html
Molesworth - August 1912 - 1035992
WATER HEATER
Cancilla - October 1971 - 3612004
High output vapor generator for aircraft
Price - December 1940 - 2223856
VAPOR GENERATOR
Hoagland - February 1971 - 3563212
/1035992.html
Molesworth - August 1912 - 1035992
Application Number:
05/195235
Publication Date:
05/07/1974
Filing Date:
11/03/1971
View Patent Images:
Export Citation:
Assignee:
Steam Engine Systems Corporation (Newton, MA)
Primary Class:
Other Classes:
165/163, 166/163
International Classes:
F22B27/08; F24H1/43; F28D7/02; F28F1/24; F22B27/00; F24H1/22; F28D7/00; F23D13/24; F24D17/00
Field of Search:
126/35R 122/25R 431/326,328 165/163
Primary Examiner:
Dority Jr., Carroll B.
Assistant Examiner:
Anderson, William C.
Attorney, Agent or Firm:
Kent, Edgar H.
Claims:
1. A fluid heater utilizing heated gases as a heating medium, said heater including a heat exchanger comprising:
2. A fluid heater according to claim 1 wherein said second coil precedes
3. A fluid heater according to claim 1 wherein said matrix has at least
4. A fluid heater according to claim 1 wherein said included angle is about
5. A fluid heater according to claim 1 wherein at least the outermost coil
6. A fluid heater according to claim 1 wherein each coil has fewer turns
7. A fluid heater according to claim 6 wherein each coil has two fewer
8. A fluid heater according to claim 1 wherein said means for providing hot gas within the interior of said matrix includes burner means for directing flames of ignited fuel toward the inner coil thereof substantially
9. A fluid heater according to claim 8 wherein said burner means comprises a flame holder having a generally conical wall, the included angle of the generatrix of which is substantially the same as that of the coils of the matrix, said flame holder being located substantially coaxially within the matrix and being provided with a multiplicity of small substantially uniformly spaced apertures for directing a fuel-air mixture outwardly
10. A fluid heater according to claim 9 wherein said wall is a plate and said apertures are circular and have a diameter of the order of 0.025 to
11. A fluid heater according to claim 10 wherein said apertures occupy about 10 percent to 25 percent of the area of said wall.
2. A fluid heater according to claim 1 wherein said second coil precedes
3. A fluid heater according to claim 1 wherein said matrix has at least
4. A fluid heater according to claim 1 wherein said included angle is about
5. A fluid heater according to claim 1 wherein at least the outermost coil
6. A fluid heater according to claim 1 wherein each coil has fewer turns
7. A fluid heater according to claim 6 wherein each coil has two fewer
8. A fluid heater according to claim 1 wherein said means for providing hot gas within the interior of said matrix includes burner means for directing flames of ignited fuel toward the inner coil thereof substantially
9. A fluid heater according to claim 8 wherein said burner means comprises a flame holder having a generally conical wall, the included angle of the generatrix of which is substantially the same as that of the coils of the matrix, said flame holder being located substantially coaxially within the matrix and being provided with a multiplicity of small substantially uniformly spaced apertures for directing a fuel-air mixture outwardly
10. A fluid heater according to claim 9 wherein said wall is a plate and said apertures are circular and have a diameter of the order of 0.025 to
11. A fluid heater according to claim 10 wherein said apertures occupy about 10 percent to 25 percent of the area of said wall.
Description:
This invention relates to devices for fluid heating, particularly heat exchangers and combustors useful as vapor generators, for example, for Rankine cycle automotive systems, as water heaters, for example, for home heating systems, and for other purposes.
The object of this invention is to provide such heating devices which are more efficient than prior such devices while nevertheless lower in pollutant discharge particularly of oxides of nitrogen, and which are highly compact and relatively inexpensive to make and operate.
In attaining the foregoing object, the invention features a heat exchanger for transferring the heat from hot combustor gases to the fluid in the form of finned tubing through which the fluid flows, arranged in a matrix of a plurality of successive coils interposed in the path of hot gas flow. The coils are wound to form truncated cones about a common axis at successively greater distances therefrom, the included angle of the generatrix of the cone being at least about 50°, preferably about 60°. The turns of each coil are centered between the turns of the preceding and following coil on a line perpendicular to the generatrix and are tightly packed so that fins of turns of each pair of coils are in actual or approximate engagement with fins of two turns of the same coil and two turns of the other coil of the pair.
A heat exchanger so constructed as the outer wall of a combustion chamber is very efficient because the close packing of the finned tubes with many mutually engaged fins minimizes by-passing of flow around the fins and forces the hot gases to flow over the fins on all sides of the tubes. The frusto-conical structure permits the coils of finned tubing to be independently wound on a mandrel and then superimposed in the close packed coil arrangement specified, which is important for ease of manufacture and would not be possible, for example, with cylindrically wound coils.
The invention also features a combustor which has a flame holder with a foraminous surface, the apertures of which are very small and numerous, so that the air and the gaseous or vaporized fuel passing therethrough form a myriad of small diameter flames of very large total surface area. The flame holder surface is preferably a perforated plate with uniformly spaced apertures about 0.025 to 0.065 inches in diameter which may occupy about 10 percent to 25 percent of the area of the plate. The flame holder is most advantageously formed as a cone with a generatrix of included angle substantially matching that of the combustion chamber defined by the coil matrix, inside which it is disposed coaxially with the tube coils, so that the flame forming areas thereof are substantially equidistant from the innermost coil of the matrix.
Such a combustor with its very large ratio of flame surface area to combustion chamber volume produces very effective and rapid combustion of the fuel and air mixture, permitting high discharge rate thereof from the combustion chamber, and rapid quenching thereof by contact with the finned coils, and substantially uniform residence time of the combusted gases in the combustion chamber by reason of the parallel relation of the flame source to the coil matrix interior. Such rapid, uniform discharge and quenching rates are important not only for efficient operation and compact structure, but also in preventing the formation of excessive amounts of oxides of nitrogen, a pollutant, by reason of the short time available at high temperature for the formation thereof, and by reason of the uniformity of combustion gas residence time preventing portions of such combustion gases from occupying the combustion chamber for periods longer than the average residence time and longer than that time required for complete combustion.
In preferred embodiments, the successive coils of the tubing matrix from inside to outside are wound with fewer turns so that as the exhaust gases pass outwardly through the matrix, the flow area which they pass through as they cool and contract diminishes, which is desirable to maintain high gas velocities and high rates of heat transfer to outer turns. Such a construction also has the advantage of forming a generally cylindrical outside surface of the matrix.
The foregoing and other features, objects and advantages of the invention will be more fully understood from the following particular description of the preferred embodiment shown in the drawings, wherein:
FIG. 1 is a bottom plan view of a heater according to the invention;
FIG. 2 is an inverted cross-section view on line 2--2 of FIG. 1, in part broken away;
FIGS. 3A, 3B and 3C compare diagrammatically fin contact open spacing and gas flow obtained with coils of finned tubing wound cylindrically and assembled in two different arrangements in FIGS. 3A and 3B and conically wound and close packed according to this invention in FIG. 3C; and
FIG. 4 is a cross-section view on line 4--4 of FIG. 2, looking in the direction of the arrows.
Referring to the drawings, the fluid heater shown has a combustion chamber 19 formed within a generally cylindrical sidewall 10 and flat bottom wall 12 lined with insulation 14, and top wall of insulation 16. A combustor designated generally 18 has a cylindrical base 20 secured in an opening in bottom wall 12, and a conical wall 22 exposed within the combustion chamber and generally coaxial therewith, provided with numerous substantially uniformly distributed circular apertures 24. A duct 26 extends from the base 20 of combustor 18 to the pressure side of a blower 28. A nozzle 30, opening into duct 26 between the blower and the combustor, is connected to a source (not shown) of liquid or gaseous fuel. (In the case of liquid fuel, the liquid is vaporized by the air stream heated by means not shown). A sparkplug 32 in the base of the chamber adjacent the base of wall 22 provides initial ignition of the fuel streams escaping through apertures 24 of the combustor.
Surrounding the combustion chamber 19 and wall 22 of the combustor is a heat exchanger designated generally 34 formed of a matrix of coils of tubing 36 provided with fins 38, the coils being wound in truncated conical form about the axis of the chamber and combustor 18. Four such coils are shown and are designated from inner to outer, respectively, 40, 42, 44 and 46. The conical coils and conical wall 22 of the combustor have substantially the same generatrix which has an included angle of at least about 50° and preferably about 60° as shown. The turns of coils 42, 44 and 46 are substantially centered between the adjacent two turns of the preceding coil on a line perpendicular to the generatrix which is the direction of gas flow. The coils are closely packed so that the cylindrical envelopes circumscribing the fins on each pair of coils are in actual or approximate contact with such envelopes of the fins of two turns of the same coil and two turns of the other coil of the pair. Thus, in FIG. 2, fins on interior turns of coils 42 and 44 are in actual or approximate engagement with fins on each of the six turns of coils 40, 42 and 44 and 42, 44 and 46, respectively, which immediately surround it.
Preferably, as shown, each of coils 42, 44, and 46 has fewer turns than the coil preceding it in the direction of flow. This arrangement not only provides desirable reduction in flow area as the hot exhaust gases flow outwardly and contract, but the reduction of two turns per coil as shown also enables the matrix to be formed with a generally cylindrical exterior for a substantial part of its length and with a substantially flat top which is advantageous from the standpoint of closely encasing the matrix in a cylindrical chamber. The inclined exterior portion of the matrix is surrounded by an exhaust manifold 48 connected at its outer end to a suitable exhaust duct (not shown).
The fluid to be heated, usually water, is supplied to the lowermost turn of coil 46 through inlet pipe 50, flows through coil 46 to the top turn thereof; then, through a 180° bend fitting 52 to the top turn of coil 44; then through that coil to 180° bend fitting 54 connecting its bottom turn to the bottom turn of coil 40; then through coil 40 and a 180° bend fitting 56 connecting its top turn with that of coil 42; then through coil 42 and exits as fluid, vapor or gas through outlet pipe 58. I have found this arrangement, in which the feed fluid exits from the next innermost coil to the hot gas source, preferable to having it exit from the innermost coil as would be conventional.
The compact frusto-conical matrix herein provided may be simply supported out of contact with the surrounding walls. Thus, as shown, the bottom turn of the innermost coil may be seated on notched supports 60 on the base of the chamber and held fast on the supports by clamps 62 which have an inclined upper end with a hook which engages over the topmost turn and a vertical shank with a threaded end which extends through an aperture in insulation 14 and base 12 and is fastened by a nut 64. Due to the conical shape of the matrix and the nesting of all adjacent coils, the force exerted by these simple clamps serves to hold the coils of the matrix tightly together and on supports 60.
Some of the advantages of the matrix according to the invention will be appreciated from a comparison of FIGS. 3A and 3B with FIG. 3C. FIGS. 3A and 3B illustrate three successive coils of finned tubing cylindrically wound about the same axis, separately wound and then assembled in two different possible relationships. It will be appreciated that in order to assemble such coils, each successive coil from inside to outside must be wound to an inside diameter (fin diameter) which exceeds the inside fin diameter to which the preceding coil was wound by at least a full tube plus fin diameter;-- otherwise the coils could not be assembled one over the other.
In FIG. 3A, the coils are assembled with the centers c of corresponding turns located on the same line 1 perpendicular to the generatrix. With this arrangement the fins of each coil can at most be in contact with the fins of the two adjacent turns of the same coil and in approximate contact with the fins of one turn of a coil at either side of it; it being necessary to provide some clearance between fins of successive coils to permit assembly. In FIG. 3B the coils are assembled with corresponding turns staggered so that the centers of corresponding turns lie on different lines 1 perpendicular to the generatrix which are between succeeding turns of the adjacent coil or coils. With this arrangement, the fins of each coil can at most contact the fins on the two adjacent turns of the same coil.
In FIG. 3C, which illustrates a conical close packed array of three coils according to this invention, the coils are staggered so that the centers of corresponding turns lie on different lines 1 perpendicular to the generatrix which are between succeeding turns of the adjacent coil or coils, as in FIG. 3B. However, due to the conical shape, successive coils from inside to outside can be separately wound to an inside fin diameter which exceeds the inside fin diameter to which the preceding coil was wound by less than a full tube plus fin diameter. The coils may be prewound and assembled with turns of each coil nested between and in fin contact with a pair of turns of the adjacent coil or coils, such assembly being possible because each lower turn of a coil is at a greater distance from the cone axis than the next upper turn. Thus, in FIG. 3C, the interior turns of the intermediate coil are in fin contact with all six immediately surrounding turns of the three coils.
The gas flow pattern is shown by arrows in the three Figures. In FIG. 3A the flow is essentially straight through between the fins of each aligned pair of turns. There is little flow around the front or back side of the tubes and fins. In FIG. 3B, while there is flow around the back sides, it is primarily through the gaps between fins of successive coils. In FIG. 3C the extensive engagement between fins of turns of successive coils forces the gas to flow much more closely about the tubes between the fins on the back side of the turns, with consequently superior heating action.
The conical concentrically located flame holder provides roughly uniform gas residence time as flow path length is approximately uniform between it and the inner coil of the heat exchanger. Its conical shape also provides good gas distribution within it since the gas velocity remains more uniform as it flows axially in the cone than it would in a cylindrical burner, in which the velocity decreases along the burner. It has been found that perforations between 0.025 inches and 0.065 inches diameter of a density such that from about 10 percent to about 25 percent of the plate area is open results in a flame holder which has desirable insensitivity to flash-back and blow-off and is operable over a wide range of fuel velocities.
The spacing of the flame holder from the inner coil of the heat exchanger has a bearing on performance, particularly as respects the amount and type of pollutants given off, and should be related to the capacity (heat release rate) thereof. I have found the following formula a useful guide in determining such spacing for the preferred 60° cone heat exchanger and flame holder:
L = C 1 [Q/(0.866 D/L -1) 2 + 1/3] 1/3
wherein L is the distance in feet between the combustor cone and the conical surface inscribed within the inner coil of the heat exchanger, C 1 is equal to or between 0.0036 and 0.0072, D is the base diameter in feet of the inscribed conical surface, and Q is the heat release rate of the burner in British Thermal Units per hour. The value chosen for C 1 in the given range affects the amount and type of pollutants given off. The low values favor reduced oxides of nitrogen which form as a function of hot gas dwell before quenching while the high values favor reduction of carbon monoxide and unburnt fuel which burn off as a function of such dwell.
While the ribbon type helically wound fin forming material used in the preferred embodiment is effective and desirable from the standpoint of relative ease of manufacture, other types may be employed such as serrated or segmented fins arranged to lie in planes generally perpendicular, parallel or oblique to the axes of the tubes, or simply pins which may be arranged at equal spacings circumferentially and axially along the tubes. It will be understood that the metal used for the finned tubing and the combustor cone is such as to be durable under the heat conditions involved and of good heat transfer properties in the case of the tubing and may be stainless steel in both cases.
The object of this invention is to provide such heating devices which are more efficient than prior such devices while nevertheless lower in pollutant discharge particularly of oxides of nitrogen, and which are highly compact and relatively inexpensive to make and operate.
In attaining the foregoing object, the invention features a heat exchanger for transferring the heat from hot combustor gases to the fluid in the form of finned tubing through which the fluid flows, arranged in a matrix of a plurality of successive coils interposed in the path of hot gas flow. The coils are wound to form truncated cones about a common axis at successively greater distances therefrom, the included angle of the generatrix of the cone being at least about 50°, preferably about 60°. The turns of each coil are centered between the turns of the preceding and following coil on a line perpendicular to the generatrix and are tightly packed so that fins of turns of each pair of coils are in actual or approximate engagement with fins of two turns of the same coil and two turns of the other coil of the pair.
A heat exchanger so constructed as the outer wall of a combustion chamber is very efficient because the close packing of the finned tubes with many mutually engaged fins minimizes by-passing of flow around the fins and forces the hot gases to flow over the fins on all sides of the tubes. The frusto-conical structure permits the coils of finned tubing to be independently wound on a mandrel and then superimposed in the close packed coil arrangement specified, which is important for ease of manufacture and would not be possible, for example, with cylindrically wound coils.
The invention also features a combustor which has a flame holder with a foraminous surface, the apertures of which are very small and numerous, so that the air and the gaseous or vaporized fuel passing therethrough form a myriad of small diameter flames of very large total surface area. The flame holder surface is preferably a perforated plate with uniformly spaced apertures about 0.025 to 0.065 inches in diameter which may occupy about 10 percent to 25 percent of the area of the plate. The flame holder is most advantageously formed as a cone with a generatrix of included angle substantially matching that of the combustion chamber defined by the coil matrix, inside which it is disposed coaxially with the tube coils, so that the flame forming areas thereof are substantially equidistant from the innermost coil of the matrix.
Such a combustor with its very large ratio of flame surface area to combustion chamber volume produces very effective and rapid combustion of the fuel and air mixture, permitting high discharge rate thereof from the combustion chamber, and rapid quenching thereof by contact with the finned coils, and substantially uniform residence time of the combusted gases in the combustion chamber by reason of the parallel relation of the flame source to the coil matrix interior. Such rapid, uniform discharge and quenching rates are important not only for efficient operation and compact structure, but also in preventing the formation of excessive amounts of oxides of nitrogen, a pollutant, by reason of the short time available at high temperature for the formation thereof, and by reason of the uniformity of combustion gas residence time preventing portions of such combustion gases from occupying the combustion chamber for periods longer than the average residence time and longer than that time required for complete combustion.
In preferred embodiments, the successive coils of the tubing matrix from inside to outside are wound with fewer turns so that as the exhaust gases pass outwardly through the matrix, the flow area which they pass through as they cool and contract diminishes, which is desirable to maintain high gas velocities and high rates of heat transfer to outer turns. Such a construction also has the advantage of forming a generally cylindrical outside surface of the matrix.
The foregoing and other features, objects and advantages of the invention will be more fully understood from the following particular description of the preferred embodiment shown in the drawings, wherein:
FIG. 1 is a bottom plan view of a heater according to the invention;
FIG. 2 is an inverted cross-section view on line 2--2 of FIG. 1, in part broken away;
FIGS. 3A, 3B and 3C compare diagrammatically fin contact open spacing and gas flow obtained with coils of finned tubing wound cylindrically and assembled in two different arrangements in FIGS. 3A and 3B and conically wound and close packed according to this invention in FIG. 3C; and
FIG. 4 is a cross-section view on line 4--4 of FIG. 2, looking in the direction of the arrows.
Referring to the drawings, the fluid heater shown has a combustion chamber 19 formed within a generally cylindrical sidewall 10 and flat bottom wall 12 lined with insulation 14, and top wall of insulation 16. A combustor designated generally 18 has a cylindrical base 20 secured in an opening in bottom wall 12, and a conical wall 22 exposed within the combustion chamber and generally coaxial therewith, provided with numerous substantially uniformly distributed circular apertures 24. A duct 26 extends from the base 20 of combustor 18 to the pressure side of a blower 28. A nozzle 30, opening into duct 26 between the blower and the combustor, is connected to a source (not shown) of liquid or gaseous fuel. (In the case of liquid fuel, the liquid is vaporized by the air stream heated by means not shown). A sparkplug 32 in the base of the chamber adjacent the base of wall 22 provides initial ignition of the fuel streams escaping through apertures 24 of the combustor.
Surrounding the combustion chamber 19 and wall 22 of the combustor is a heat exchanger designated generally 34 formed of a matrix of coils of tubing 36 provided with fins 38, the coils being wound in truncated conical form about the axis of the chamber and combustor 18. Four such coils are shown and are designated from inner to outer, respectively, 40, 42, 44 and 46. The conical coils and conical wall 22 of the combustor have substantially the same generatrix which has an included angle of at least about 50° and preferably about 60° as shown. The turns of coils 42, 44 and 46 are substantially centered between the adjacent two turns of the preceding coil on a line perpendicular to the generatrix which is the direction of gas flow. The coils are closely packed so that the cylindrical envelopes circumscribing the fins on each pair of coils are in actual or approximate contact with such envelopes of the fins of two turns of the same coil and two turns of the other coil of the pair. Thus, in FIG. 2, fins on interior turns of coils 42 and 44 are in actual or approximate engagement with fins on each of the six turns of coils 40, 42 and 44 and 42, 44 and 46, respectively, which immediately surround it.
Preferably, as shown, each of coils 42, 44, and 46 has fewer turns than the coil preceding it in the direction of flow. This arrangement not only provides desirable reduction in flow area as the hot exhaust gases flow outwardly and contract, but the reduction of two turns per coil as shown also enables the matrix to be formed with a generally cylindrical exterior for a substantial part of its length and with a substantially flat top which is advantageous from the standpoint of closely encasing the matrix in a cylindrical chamber. The inclined exterior portion of the matrix is surrounded by an exhaust manifold 48 connected at its outer end to a suitable exhaust duct (not shown).
The fluid to be heated, usually water, is supplied to the lowermost turn of coil 46 through inlet pipe 50, flows through coil 46 to the top turn thereof; then, through a 180° bend fitting 52 to the top turn of coil 44; then through that coil to 180° bend fitting 54 connecting its bottom turn to the bottom turn of coil 40; then through coil 40 and a 180° bend fitting 56 connecting its top turn with that of coil 42; then through coil 42 and exits as fluid, vapor or gas through outlet pipe 58. I have found this arrangement, in which the feed fluid exits from the next innermost coil to the hot gas source, preferable to having it exit from the innermost coil as would be conventional.
The compact frusto-conical matrix herein provided may be simply supported out of contact with the surrounding walls. Thus, as shown, the bottom turn of the innermost coil may be seated on notched supports 60 on the base of the chamber and held fast on the supports by clamps 62 which have an inclined upper end with a hook which engages over the topmost turn and a vertical shank with a threaded end which extends through an aperture in insulation 14 and base 12 and is fastened by a nut 64. Due to the conical shape of the matrix and the nesting of all adjacent coils, the force exerted by these simple clamps serves to hold the coils of the matrix tightly together and on supports 60.
Some of the advantages of the matrix according to the invention will be appreciated from a comparison of FIGS. 3A and 3B with FIG. 3C. FIGS. 3A and 3B illustrate three successive coils of finned tubing cylindrically wound about the same axis, separately wound and then assembled in two different possible relationships. It will be appreciated that in order to assemble such coils, each successive coil from inside to outside must be wound to an inside diameter (fin diameter) which exceeds the inside fin diameter to which the preceding coil was wound by at least a full tube plus fin diameter;-- otherwise the coils could not be assembled one over the other.
In FIG. 3A, the coils are assembled with the centers c of corresponding turns located on the same line 1 perpendicular to the generatrix. With this arrangement the fins of each coil can at most be in contact with the fins of the two adjacent turns of the same coil and in approximate contact with the fins of one turn of a coil at either side of it; it being necessary to provide some clearance between fins of successive coils to permit assembly. In FIG. 3B the coils are assembled with corresponding turns staggered so that the centers of corresponding turns lie on different lines 1 perpendicular to the generatrix which are between succeeding turns of the adjacent coil or coils. With this arrangement, the fins of each coil can at most contact the fins on the two adjacent turns of the same coil.
In FIG. 3C, which illustrates a conical close packed array of three coils according to this invention, the coils are staggered so that the centers of corresponding turns lie on different lines 1 perpendicular to the generatrix which are between succeeding turns of the adjacent coil or coils, as in FIG. 3B. However, due to the conical shape, successive coils from inside to outside can be separately wound to an inside fin diameter which exceeds the inside fin diameter to which the preceding coil was wound by less than a full tube plus fin diameter. The coils may be prewound and assembled with turns of each coil nested between and in fin contact with a pair of turns of the adjacent coil or coils, such assembly being possible because each lower turn of a coil is at a greater distance from the cone axis than the next upper turn. Thus, in FIG. 3C, the interior turns of the intermediate coil are in fin contact with all six immediately surrounding turns of the three coils.
The gas flow pattern is shown by arrows in the three Figures. In FIG. 3A the flow is essentially straight through between the fins of each aligned pair of turns. There is little flow around the front or back side of the tubes and fins. In FIG. 3B, while there is flow around the back sides, it is primarily through the gaps between fins of successive coils. In FIG. 3C the extensive engagement between fins of turns of successive coils forces the gas to flow much more closely about the tubes between the fins on the back side of the turns, with consequently superior heating action.
The conical concentrically located flame holder provides roughly uniform gas residence time as flow path length is approximately uniform between it and the inner coil of the heat exchanger. Its conical shape also provides good gas distribution within it since the gas velocity remains more uniform as it flows axially in the cone than it would in a cylindrical burner, in which the velocity decreases along the burner. It has been found that perforations between 0.025 inches and 0.065 inches diameter of a density such that from about 10 percent to about 25 percent of the plate area is open results in a flame holder which has desirable insensitivity to flash-back and blow-off and is operable over a wide range of fuel velocities.
The spacing of the flame holder from the inner coil of the heat exchanger has a bearing on performance, particularly as respects the amount and type of pollutants given off, and should be related to the capacity (heat release rate) thereof. I have found the following formula a useful guide in determining such spacing for the preferred 60° cone heat exchanger and flame holder:
L = C 1 [Q/(0.866 D/L -1) 2 + 1/3] 1/3
wherein L is the distance in feet between the combustor cone and the conical surface inscribed within the inner coil of the heat exchanger, C 1 is equal to or between 0.0036 and 0.0072, D is the base diameter in feet of the inscribed conical surface, and Q is the heat release rate of the burner in British Thermal Units per hour. The value chosen for C 1 in the given range affects the amount and type of pollutants given off. The low values favor reduced oxides of nitrogen which form as a function of hot gas dwell before quenching while the high values favor reduction of carbon monoxide and unburnt fuel which burn off as a function of such dwell.
While the ribbon type helically wound fin forming material used in the preferred embodiment is effective and desirable from the standpoint of relative ease of manufacture, other types may be employed such as serrated or segmented fins arranged to lie in planes generally perpendicular, parallel or oblique to the axes of the tubes, or simply pins which may be arranged at equal spacings circumferentially and axially along the tubes. It will be understood that the metal used for the finned tubing and the combustor cone is such as to be durable under the heat conditions involved and of good heat transfer properties in the case of the tubing and may be stainless steel in both cases.