Method and apparatus for vaporizing fuel for a reformer fuel cell system

• US 20030215679A1
• Filed: 05/14/2002
• Published: 11/20/2003
• Est. Priority Date: 05/14/2002
• Status: Active Grant

• Current Assignee: NuCellSys GmbH (Mercedes-Benz Group AG), Modine Manufacturing Company
• Original Assignee: Modine Manufacturing Company and Ballard Power Systems AG
• Inventors: Thumm, Sven, Reinke, Michael J., Voss, Mark G., Wattelet, Jonathan, Granetzke, Dennis C., Motzet, Bruno, Weisser, Marc
• Granted Patent: US 6,953,009 B2
• Time in Patent Office: Days
• US Class Current: 429/13
• CPC Class Codes: F28D 2021/0043 for fuel cells
• CPC Class Codes: F28D 9/005 the plates having openings therein for both heat-exchange media
• CPC Class Codes: F28F 13/08 by varying the cross-section of the flow channels
• CPC Class Codes: F28F 3/086 having one or more openings therein forming tubular heat-exchange passages
• CPC Class Codes: H01M 8/04208 Cartridges, cryogenic media or cryogenic reservoirs
• CPC Class Codes: H01M 8/0618 Reforming processes, e.g. autothermal, partial oxidation or steam reforming
• CPC Class Codes: H01M 8/0625 in a modular combined reactor/fuel cell structure
• CPC Class Codes: H01M 8/0668 Removal of carbon monoxide or carbon dioxide
• CPC Class Codes: Y02E 60/50 Fuel cells

Abstract

A vaporizer, a fuel cell system including the vaporizer, and a method of vaporizing fuel in a fuel cell system are disclosed. The fuel cell system includes a fuel reservoir (24) for storing a liquid fuel and a fuel cell (10) for consuming a fuel and generating electricity therefrom. A fuel vaporizer (28) is interposed between the fuel reservoir (24) and the fuel cell (10) for receiving liquid fuel and vaporizing it and delivering it ultimately to the fuel cell (10). The fuel vaporizer (28) includes a heat exchanger which includes a hot fluid inlet (65), a hot fluid outlet (67) and a core (50) interconnecting the inlet (65) and the outlet (68). The core (50) has alternating fuel flow structures (68) and hot fluid structures (69) with the fuel flow structures (68,69) having an inlet (56) and an outlet (58).

Claims

1. In a fuel cell system including a fuel cell including a vaporized fuel inlet, a source of liquid fuel to be vaporized and a fuel vaporizer interconnecting the fuel cell and the source, a method of vaporizing fuel in the fuel vaporizer comprising:

(a) introducing liquid fuel to be vaporized into a fuel inlet for a fuel flow path of the fuel vaporizer;

(b) passing a heated fluid through at least one hot fluid flow path in heat exchange relation to said fuel flow path to heat and vaporize fuel therein; and

(c) controlling the pressure drop of the fuel as it passes from said inlet to an outlet whereat the fuel emerges as a vapor such that the majority of the pressure drop occurs near said inlet before virtually any of the liquid fuel is vaporized.

2. The method of claim 1 wherein said majority is at least about 70%.

3. The method of claim 1 wherein said majority is about 95%.

4. The method of claim 1 wherein there are a plurality of said fuel flow paths connected in parallel and having substantially equal fuel flow resistance.

5. The method of claim 4 wherein each said flow path is of substantially the same length and substantially the same hydraulic configuration.

6. The method of claim 1 wherein step (c) is performed by (c1) providing a highly flow resistant maze in said fuel flow path immediately adjacent said inlet and (c2) making the remainder of said fuel flow path from said maze to said outlet of low flow resistance.

7. The method of claim 6 wherein step (c2) is performed by creating said remainder of criss-crossing elongated slots whose angle to a mean direction of fuel flow through said fuel vaporizer progressively decreases from a point adjacent said maze to said outlet.

8. The method of claim 7 wherein step (c2) is further performed by causing the width of said slots to progressively increase from said point to said outlet.

9. The method of claim 7 wherein step (c2) is further performed by causing the width of at least some of said slots to increase from an upstream end of the slot nearest said inlet to a downstream end of the slot nearest said outlet.

10. The method of claim 9 wherein there are a plurality of said fuel flow paths connected in parallel and having substantially equal fuel flow resistance and wherein said majority is at least about 80%.

11. The method of claim 10 wherein step (c1) is performed by creating said maze of a plurality of intersecting relatively short and narrow slots connected in hydraulic series and in fluid communication with said inlet and a relatively long manifold slot generally transverse to said mean direction of fuel flow and which in turn includes plural, orifice slots extending, at uniformly spaced intervals, to said remainder of criss-crossing elongated slots.

12. The method of claim 11 wherein step (c1) is further performed by including additional ones of said orifice slots downstream of said narrow slots and upstream of said first named orifice slots in said fluid flow path.

13. A fuel cell system including:

a fuel reservoir for storing a liquid fuel for a fuel cell;

a fuel cell for consuming a fuel and generating electricity therefrom;

a fuel reformer for receiving fuel in a vaporized state and connected to the fuel cell for providing a fuel thereto for consumption therein; and

a fuel vaporizer interposed between the fuel reservoir and the fuel reformer for receiving liquid fuel from the fuel reservoir and vaporizing the liquid fuel to the vaporized state for delivery to the fuel reformer, and including a heat exchanger having a hot fluid inlet, a hot fluid outlet and a core interconnecting said inlet and said outlet, said core having alternating hot fluid passages extending between said hot fluid inlet and said hot fluid outlet and in heat exchange relation with liquid/vaporized fuel passages, said hot fluid passages each including a fin or fins and extending the length thereof and two separator plates bonded to and sandwiching said fin(s), said heat exchanger further including a liquid fuel inlet and a vaporized fuel outlet, said liquid/vaporized fuel passages extending between said liquid fuel inlet and said vaporized fuel outlet and including two abutting plates, each having elongated slots or grooves therein and extending diagonally to the mean direction of fuel flow in said liquid/vaporized fuel passages with the slots or grooves in one plate criss-crossing the slots or grooves in the other plate to be in fluid communication therewith, said slots or grooves having a progressively decreasing angle with the mean direction of fuel flow through said fuel vaporizer from said liquid fuel inlet to said vaporized fuel outlet.

14. The fuel cell system of claim 13 wherein the cross sectional area of said slots or grooves progressively increases from said liquid fuel inlet to said vaporized fuel outlet.

15. The fuel cell system of claim 13 wherein at least some of said slots or grooves are narrower at their ends nearer said liquid fuel inlet than at their ends nearer said vaporized fuel outlet.

16. The fuel cell system of claim 15 wherein said at least some slots or grooves are tapered.

17. The fuel cell system of claim 16 wherein said slots or grooves are generally straight.

18. The fuel cell system of claim 16 wherein said slots or grooves are non-straight.

19. The fuel cell system of claim 18 wherein said slots or grooves are V-shaped.

20. The fuel cell system of claim 16 wherein said slots or grooves are in rows and include relatively long slots or grooves alternating with relatively short slots or grooves.

21. The fuel cell system of claim 13 wherein said liquid/vaporized fuel passages include a maze capable of providing a high pressure drop adjacent said inlet and a low pressure drop section extending between said maze and said vaporized fuel outlet.

22. The fuel cell system of claim 21 wherein said maze is configured to produce the majority of the pressure drop between said liquid fuel inlet and said vaporized fuel outlet.

23. The fuel cell system of claim 22 wherein said majority is at least about 70%.

24. The fuel cell system of claim 23 wherein said majority is about 95% or more.

25. The fuel cell system of claim 22 wherein said maze includes a plurality of intersecting relatively short and narrow slots or grooves connected in hydraulic series and in fluid communication with said liquid fuel inlet and a relatively long manifold generally transverse to said mean direction of fuel flow and which in turn includes plural, orifice slots or grooves extending, at uniformly spaced intervals, to said remainder of criss-crossing elongated slots or grooves.

26. The fuel cell system of claim 25 including additional ones of said orifice slots or grooves downstream of said narrow slots or grooves and upstream of said first named orifice slots or grooves in said fluid flow path.

27. The fuel cell system of claim 13 wherein each said liquid/fuel passage includes a plurality of hydraulically isolated channels extending from said liquid fuel inlet to said vaporized fuel outlet, each channel having substantially equal fuel flow resistance.

28. The fuel cell system of claim 27 wherein each said channel is of substantially the same length and substantially the same hydraulic configuration.

29. A fuel cell system including:

a fuel reservoir for storing a liquid fuel for a fuel cell;

a fuel cell for consuming a fuel and generating electricity therefrom;

a fuel reformer for receiving fuel in a vaporized state and connected to the fuel cell for providing a fuel thereto for consumption therein; and

a fuel vaporizer interposed between the fuel reservoir and the fuel reformer for receiving liquid fuel from the fuel reservoir and vaporizing the liquid fuel to the vaporized state for delivery to the fuel reformer, and including a heat exchanger having a hot fluid inlet, a hot fluid outlet and a core interconnecting said hot fluid inlet and said outlet, said core having alternating hot fluid passages extending between said hot fluid inlet and said hot fluid outlet and in heat exchange relation with liquid/vaporized fuel passages, said hot fluid passages each including a fin or fins and extending the length thereof and two separator plates bonded to and sandwiching said fin(s), said heat exchanger further including a liquid fuel inlet and a vaporized fuel outlet, said liquid/vaporized fuel passages extending between said liquid fuel inlet and said vaporized fuel outlet and including two abutting plates, each having elongated flow passages therein and extending diagonally to the mean direction of fuel flow in said liquid/vaporized fuel passages with the flow passages in one plate criss-crossing flow passages in the other plate to be in fluid communication therewith, said flow passages having a progressively increasing width from said liquid fuel inlet to said vaporized fuel outlet.

30. The fuel cell system of claim 29 wherein said flow passages have a progressively decreasing angle with the mean direction of fuel flow through said fuel vaporizer from said liquid fuel inlet to said vaporized fuel outlet.

31. The fuel cell system of claim 29 wherein said flow passages are defined by slots or grooves.

32. The fuel cell system of claim 29 wherein said flow passages are defined by slots.

33. The fuel cell system of claim 32 wherein said at least some slots are tapered.

34. The fuel cell system of claim 32 wherein said slots are generally straight.

35. The fuel cell system of claim 32 wherein said slots are non-straight.

36. The fuel cell system of claim 35 wherein said slots are V-shaped.

37. The fuel cell system of claim 32 wherein said slots are in rows and include relatively long slots alternating with relatively short slots.

38. The fuel cell system of claim 29 wherein said liquid/vaporized fuel passages include a maze capable of providing a high pressure drop adjacent said inlet and a low pressure drop section extending between said maze and said vaporized fuel outlet.

39. The fuel cell system of claim 38 wherein said maze is configured to produce the majority of the pressure drop between said liquid fuel inlet and said vaporized fuel outlet.

40. The fuel cell system of claim 38 wherein said majority is at least about 70%.

41. The fuel cell system of claim 39 wherein said majority is about 95% or more.

42. The fuel cell system of claim 38 wherein said maze includes a plurality of intersecting relatively short and narrow elongated slots connected in hydraulic series and in fluid communication with said inlet and a relatively long manifold generally transverse to said mean direction of fuel flow and which in turn includes plural, orifice slots extending, at uniformly spaced intervals, to said remainder of criss-crossing elongated slots.

43. The fuel cell system of claim 42 including additional ones of said orifice slots downstream of said narrow slots and upstream of said first named orifice slots in said fluid flow path.

44. The fuel cell system of claim 29 wherein each said liquid/fuel passage includes a plurality of hydraulically isolated channels extending from said liquid fuel inlet to said vaporized fuel outlet, each channel having substantially equal fuel flow resistance.

45. The fuel cell system of claim 43 wherein each said channel is of substantially the same length and substantially the same hydraulic configuration.

46. A fuel cell system including:

a fuel reservoir for storing a liquid fuel for a fuel cell;

a fuel cell for consuming a fuel and generating electricity therefrom;

a fuel reformer for receiving fuel in a vaporized state and connected to the fuel cell for providing a fuel thereto for consumption therein; and

a fuel vaporizer interposed between the fuel reservoir and the fuel reformer for receiving liquid fuel from the fuel reservoir and vaporizing the liquid fuel to the vaporized state for delivery to the fuel reformer, and including a heat exchanger having a hot fluid inlet, a hot fluid outlet and a core interconnecting said inlet and said outlet, said core having alternating hot fluid passages extending between said hot fluid inlet and said hot fluid outlet and in heat exchange relation with liquid/vaporized fuel passages, said hot fluid passages each including a fin or fins and extending the length thereof and two separator plates bonded to and sandwiching said fin(s), said heat exchanger further including a liquid fuel inlet and a vaporized fuel outlet, said liquid/vaporized fuel passages extending between said liquid fuel inlet and said vaporized fuel outlet and including two abutting plates, each having elongated slots therein and extending diagonally to the mean direction of fuel flow in said liquid/vaporized fuel passages with slots in one plate criss-crossing slots in the other plate to be in fluid communication therewith, and separator plates bonded to and sandwiching said abutting plates, said slots having a progressively increasing width from said liquid fuel inlet to said vaporized fuel outlet.

47. A fuel cell system comprising:

a fuel cell;

a source of liquid fuel for said fuel cell; and

a fuel vaporizer interposed between said fuel cell and said source for vaporizing liquid fuel received from said source and providing vaporized fuel to said fuel cell, said vaporizer including a fuel passage structure in heat exchange relation with a passage for a heated heat exchange medium and defined by at least one fuel passage sheet having a plurality of fuel flow areas therein, an inlet at or near one end of said sheet connected to deliver liquid fuel to said fuel flow areas, an outlet at or near an opposite end of said sheet and including an enlarged opening in the sheet connected to said fuel flow areas and serving as a collection manifold for vaporized fuel exiting said fuel flow areas, a pair of separator plates sandwiching said at least one sheet to close said fuel flow areas and said enlarged opening, at least one of said separator sheets including a series of raised, spaced dimples in said enlarged opening and contacting and bonded to the other of said separator sheets.

48. The fuel cell system of claim 47 wherein said dimples are elongated.

49. The fuel cell system of claim 47 wherein said outlet includes a fuel outlet passage in fluid communication with said enlarged opening and said dimples are elongated and oriented within said enlarged opening to direct the flow of vaporized fuel from said flow areas to said fuel outlet passage.

50. The fuel cell system of claim 49 wherein said dimples are at different angles to a mean direction of vaporized fuel flow through said fuel flow areas.

51. The fuel cell system of claim 47 wherein said dimples are located on only one of said separator sheets.

52. The fuel cell system of claim 47 wherein there are a plurality of said fuel passage structures and a plurality of said heat exchange medium passages arranged in a stack with the fuel passages alternating with said heat exchange medium passages.

53. The fuel cell system of claim 52 wherein said dimples are elongated.

54. The fuel cell system of claim 52 wherein said outlet includes a fuel outlet passage in fluid communication with said enlarged opening and said dimples are elongated and oriented within said enlarged opening to direct the flow of vaporized fuel from said flow areas to said fuel outlet passage.

55. The fuel cell system of claim 54 wherein said dimples are at different angles to a mean direction of vaporized fuel flow through said fuel flow areas.

56. The fuel cell system of claim 55 wherein said dimples are located on only one of said separator sheets.

57. A fuel cell system comprising:

a fuel cell;

a source of liquid fuel for said fuel cell; and

a fuel vaporizer interposed between said fuel cell and said source for vaporizing liquid fuel received from said source and providing vaporized fuel to said fuel cell, said vaporizer including a stack of fuel passage structures alternating with heated medium structures defining heated medium passages, each said fuel passage structure including at least one fuel passage sheet having a fuel flow area therein sandwiched between two first separator plates, each said heated medium structure including a fin located between two second separator plates, said second separator plates extending beyond opposite ends of said fuel passage plate and having aligned enlarged openings beyond said opposite ends serving as inlet and outlet manifolds in fluid communication with said heated medium passages, said second separator plates being sealed to each other about said enlarged openings except at a location establishing said fluid communication with said heated medium passages.

58. The fuel cell system of claim 57 wherein at least some of said second separator plates also serve as said first separator plates.

59. The fuel cell system of claim 57 wherein each said fuel passage sheet includes an outlet which in turn includes an enlarged opening at one of said opposite ends and connected to said fuel flow area thereof and serving as an outlet manifold for said fuel flow area, at least one of said two first separator sheets including a series of spaced, raised dimples in said enlarged opening of said fuel flow sheet and contacting and bonded to the other of said first separator sheets.

60. The fuel cell system of claim 59 wherein said dimples are elongated.

61. The fuel cell system of claim 59 wherein said outlet includes a fuel outlet passage in fluid communication with said enlarged opening and said dimples are elongated and oriented within said enlarged opening to direct the flow of vaporized fuel from said flow areas to said fuel outlet passage.

62. The fuel cell system of claim 61 wherein said dimples are at different angles to a mean direction of vaporized fuel flow through said fuel flow areas.

63. The fuel cell system of claim 59 wherein said dimples are located on only one of said separator sheets.

64. A vaporizer comprising:

a plurality of fuel flow structures, each comprising two abutting fuel flow sheets sandwiched between first separator sheets, said fuel flow sheets having elongated slots therein extending at a progressively decreasing angle to a mean direction of fuel flow through said fuel flow structures and with increasing widths from one end of the fuel flow sheet to the other, the slots in one fuel flow sheet being in criss-cross relation with the slots in the other fuel flow sheet;

a plurality of heated medium flow structures comprising a fin sandwiched between two second separator sheets, each second separator sheet extending beyond said ends of said fuel flow sheets and having first enlarged openings therein in alignment with one another at locations beyond said fuel flow sheet ends,

said fuel flow structures and said heated medium structures being located in a stack in alternating relation with the fuel flow structures in the stack being aligned with one another and the heated medium structures in the stack being aligned with one another,

a common fuel inlet to said fuel flow structures on a side of said fuel flow sheets near said one of said ends;

a common fuel outlet from said fuel flow structure on a side of said fuel flow sheets near said other of said ends;

second, aligned enlarged openings in said fuel flow sheets at said other end and intersecting some of said slots and connected to said common fuel outlet;

a highly flow resistant maze in each of said fuel flow sheets at said one end and intersecting others of said slots and connected to said common fuel inlet,

a common heated medium inlet at an end of said plurality of fuel flow structures and in fluid communication with said first enlarged opening thereat; and

a common heated medium outlet at an end of said plurality of fuel flow structures and in fluid communication with said first enlarged openings thereat, said heated medium outlet being spaced from said heated medium inlet.

65. The vaporizer of claim 64 wherein said common fuel inlet and said common fuel outlet are on the sides of said stack adjacent said one and other ends respectively.

66. The vaporizer of claim 64 wherein at least some of said second separator sheets also comprise said first separator sheets.

67. The vaporizer of claim 66 wherein at least some of said separator sheets have a series of dimples located therein which are located to extend into said second enlarged openings and into abutment with the other of said separator sheets and are bonded thereto.

68. The vaporizer of claim 67 wherein said dimples at said other end are elongated and are oriented to direct vaporized fuel toward said common fuel outlet.

69. The vaporizer of claim 64 further including flow directors in said second enlarged openings for directing vaporized fuel from said some slots to said common fuel outlet.

70. The vaporizer of claim 64 wherein some of said slots are relatively long and others of said slots are relatively short.

71. The vaporizer of claim 70 wherein said slots are in rows and said relatively long slots in each row alternate with said relatively short slots in each row.

72. The vaporizer of claim 64 wherein said fuel flow sheets each having a plurality of separated channels defined by said slots.

73. The vaporizer of claim 64 wherein said slots are straight.

74. The vaporizer of claim 64 wherein said slots are non-straight.

75. The vaporizer of claim 74 wherein said slots are generally V-shaped.

76. The vaporizer of claim 64 including a maze in said two abutting fuel flow sheets and located between said slots and said common fuel inlet.

77. The vaporizer of claim 76 wherein said maze includes a plurality of intersecting relatively short and narrow slots in said fuel flow sheets and connected in hydraulic series and in fluid communication with said common fuel inlet and a reactively long manifold slot generally transverse to said mean direction of fuel flow and which in turn includes plural orifice slots extending at uniformly spaced intervals to said others of said criss-cross elongated slots.

78. The vaporizer of claim 77 further including additional ones of said orifice downstream of said short and narrow slots and upstream of said first named orifice slots.

79. A fuel cell system including:

a fuel reservoir for storing a liquid fuel for a fuel cell;

a fuel cell for consuming a fuel and generating electricity therefrom;

a fuel reformer for receiving fuel in a vaporized state and connected to the fuel cell for providing a fuel thereto for consumption therein; and

a fuel vaporizer interposed between the fuel reservoir and the fuel reformer for receiving liquid fuel from the fuel reservoir and vaporizing the liquid fuel to the vaporized state for delivery to the fuel reformer, and including a heat exchanger having a hot fluid inlet, a hot fluid outlet and a core interconnecting said inlet and said outlet, said core having alternating hot fluid passages extending between said hot fluid inlet and said hot fluid outlet and in heat exchange relation with liquid/vaporized fuel passages, said hot fluid passages each including a fin or fins and extending the length thereof and two separator plates bonded to and sandwiching said fin(s), said heat exchanger further including a liquid fuel inlet and a vaporized fuel outlet, said liquid/vaporized fuel passages extending between said liquid fuel inlet and said vaporized fuel outlet and including two abutting plates, each having elongated slots therein and extending diagonally to the mean direction of fuel flow in said liquid/vaporized fuel passages with slots in one plate criss-crossing slots in the other plate to be in fluid communication therewith, and separator plates bonded to and sandwiching said abutting plates, said slots having a progressively decreasing angle with respect to the mean direction of fuel flow through said fuel vaporizer from said liquid fuel inlet to said vaporized fuel outlet.

80. A vaporizer comprising:

a plurality of fluid flow structures, each comprising two abutting fluid flow sheets sandwiched between first separator sheets, said fluid flow sheets having elongated slots therein extending diagonally to a mean direction of fluid flow through said fluid flow structures and with increasing widths from one end of the fluid flow sheet to the other, the slots in one fluid flow sheet being in criss-cross relation with the slots in the other fluid flow sheet;

a plurality of heated medium flow structures comprising a fin sandwiched between two second separator sheets, each second separator sheet extending beyond said ends of said fluid flow sheets and having first enlarged openings therein in alignment with one another at locations beyond said fluid flow sheet ends,

said fluid flow structures and said heated medium structures being located in a stack in alternating relation with the fluid flow structures in the stack being aligned with one another and the heated medium structures in the stack being aligned with one another,

a fluid inlet common to said fluid flow structures on a side of said fluid flow sheets near said one of said ends;

a common fluid outlet from said fluid flow structure on a side of said fluid flow sheets near said other of said ends;

second, aligned enlarged openings in said fluid flow sheets at said other end and intersecting some of said slots and connected to said common fluid outlet;

a highly flow resistant maze in each of said fluid flow sheets at said one end and intersecting others of said slots and connected to said common fluid inlet,

a common heated medium inlet at an end of said plurality of fluid flow structures and in fluid communication with said first enlarged opening thereat; and

a common heated medium outlet at an end of said plurality of fuel flow structures and in fluid communication with said first enlarged openings thereat, said heated medium outlet being spaced from said heated medium inlet.

81. The vaporizer of claim 80 wherein said common fluid inlet and said common fluid outlet are on the sides of said stack adjacent said one and other ends respectively.

82. The vaporizer of claim 80 wherein at least some of said second separator sheets also comprise said first separator sheets.

83. The vaporizer of claim 82 wherein at least some of said separator sheets have a series of dimples located therein which are located to extend into said second enlarged openings and into abutment with the other of said separator sheets and are bonded thereto.

84. The vaporizer of claim 83 wherein said dimples at said other end are elongated and are oriented to direct vaporized fluid toward said common fluid outlet.

85. A vaporizer comprising:

at least one fluid flow structure having an inlet, an outlet, a plurality of fluid flow passages of relatively low flow resistance connected in hydraulic parallel and located between said inlet and said outlet;

a plurality of highly flow resistant mazes, one for each of said fluid flow passages and located upstream thereof;

a common manifold connecting said mazes to said inlet;

said outlet being connected to said flow passages at a downstream end thereof; and

at least one heated medium flow structure in heat transfer relation with said fluid flow structure and the mazes and flow passages therein.

86. The vaporizer of claim 85 wherein said flow passages are each defined by criss-crossing slots or grooves.

87. The vaporizer of claim 85 wherein said mazes are each defined by criss-crossing slots or grooves.

88. The vaporizer of claim 87 wherein said flow passages are also each defined by criss-crossing slots or grooves.

89. The vaporizer of claim 88 wherein said fluid flow structure(s) includes two abutting plates having an interface and said criss-crossing slots or grooves are at said interface.

90. The vaporizer of claim 89 wherein said common manifold is defined by two aligned slots or grooves, one in each of said plates at said interface.

91. The vaporizer of claim 90 wherein there are a plurality of said fluid flow structures arranged in a stack in alternating fashion with a plurality of said heated medium flow structures, said inlet and said outlet being common to each of the fluid flow structures in the stack.