Oxygen Boiler

Abstract

An oxygen reboiler is provided with a brazed plate fin-type heat exchanger at least partially submersed in a body of liquid oxygen. The flat vertical oxygen passages thereof are open at the top and bottom and have no fins or other separate extended heat transfer surface therein which may tend to cause accumulations of hazardous acetylene. These passages are closed at their vertical edges to promote percolator action of the liquid oxygen therein when heated by nitrogen passing through adjacent passages. The conventional closing bar or other elongated member normally brazed between plate surfaces to close these vertical edges has been substantially eliminated from the oxygen passages to thereby reduce the diffusion of molten braze metal onto the plate surfaces thereof. The vertical edges of these passages have been closed with a vertical panel spaced from the plate surfaces.

Description

SUMMARY OF THE INVENTION

In processes involving the cryogenic separation of air it is now common practice to use aluminum plate fin heat exchangers for reboiling oxygen by heat exchange with warmer nitrogen gas. It is also common practice to construct the oxygen passages free of obstacles such as extended heat transfer surface in the form of fin packing which is considered responsible for retention of acetylene contaminants likely to cause dangerous explosive reactions with the oxygen.

Plate fin heat exchangers are normally constructed by assembling the various plate, fin and bar elements and subjecting the assembly to a brazing furnace or bath wherein brazing metal placed at the desired areas melts and flows within the joints of the assembly. After the assembly is cooled, the brazing metal sealingly bonds the various elements together.

The finless oxygen passages may be formed in a manner described in my copending U.S. Pat. application Ser. No. 467,623 filed June 28, 1965 now U.S. Pat. No. 3,359,616 granted Dec. 26, 1967. This application involves a method of constructing a heat exchanger with fin packing to lend support to the assembly during the brazing step. The fin is later pulled out to form the finless passage. In such cases it is desirable that the faces of the oxygen passages be kept free of braze metal, lest the pullout fin be permanently bonded in place. However, molten braze metal in the presence of a braze flux readily diffuses along surfaces and may flow from the joint of the closing bar (commonly employed to form the edges of passages) onto the heat transfer plate surfaces thereof. As aforementioned, this is undesirable as it may prevent removal of the pullout fin. The present invention teaches how to prevent this braze metal diffusion onto these plate surfaces for whatever purpose desired.

The instant invention contemplates an oxygen boiler plate-type heat exchanger wherein the oxygen passages are free of fins and wherein the vertical edges of the oxygen passages are closed in such a manner as to minimize the diffusion of molten braze metal onto the oxygen side of the heat exchanger separator plates. This is accomplished in the instant invention by closing the edges of a plurality of oxygen passages with a panlike panel member which is arranged in spaced relationship with the edges of the heat transfer plates. Since no attempt is made to provide a brazed seal in contiguous relationship with the edges of the oxygen passages, the problem of molten braze metal diffusion is substantially eliminated.

It is thus a prime object of this invention to provide an oxygen boiler heat exchanger with oxygen passages free of extended heat transfer surface that protrudes substantially into the passage which may cause accumulation of acetylene.

It is a further object to provide an oxygen boiler heat exchanger which may be constructed by bulk brazing, i.e., furnace or bath brazing, without substantial diffusion of brazing metal on the heat transfer surfaces of the oxygen passages.

Specifically, this invention involves an oxygen boiler comprising in combination: a vessel containing a body of liquid oxygen; first means within said body of liquid oxygen defining a first plurality of generally planar first passages for passage of a first heat exchange fluid and each provided with extended heat transfer fins therein; second means within said body of liquid oxygen defining a second plurality of generally planar substantially finless second passages for passage of liquid oxygen and interleaved with said first passages; a metallic heat conductive separator plate disposed between each pair of first and second passages disposed in generally parallel relation to a generally vertical axis for maintaining separation of the fluids within said first and second passages and conducting heat therebetween; said plates being of generally similar configuration, spaced one from the other, and each having first and second faces exposed respectively to said first and second passages; first closing means for closing said first passages at the edges thereof including a brazed seal extending between adjacent faces of each of said first passages along the margins thereof; a brazed bond between said extended heat transfer fins and said first faces; first inlet and outlet means for passing said first heat exchange fluid through said first passages; second inlet and outlet means for passing liquid oxygen in said body of liquid oxygen through a major portion of each of said second passages in a direction generally parallel to said axis; and a second closing means for closing said second passages at the edges thereof including a closing panel generally parallel to said axis traversing several of said first and second passages in overlying relationship to said first closing means and in spaced relationship to said second faces whereby the edges of said second passages are closed for establishing a percolator action of the liquid oxygen within said second passages with a minimum number of brazed joints at said second faces and a minimum braze metal diffusion on said second faces.

Other objects and advantages will become apparent as this specification proceeds to describe the invention illustrated in the accompanying drawings wherein like elements have been designated by like numerals throughout and in which:

FIG. 1 is a perspective of the oxygen reboiler portion of an air-fractionating column and having portions shown in section for viewing features of the invention;

FIG. 2 is a side elevation of one of the oxygen boiler heat exchangers employed in the oxygen reboiler of FIG. 1;

FIG. 3 is a plan view of the heat exchanger shown in FIG. 2;

FIG. 4 is a horizontal section taken at line 4-4 in FIG. 2;

FIG. 5 is a vertical section taken at line 5-5 in FIG. 4 through one of many similar nitrogen passages; and

FIG. 6 is a vertical section taken at line 6-6 in FIG. 4 through one of many similar oxygen passages.

Now with reference to FIG. 1, there is shown a fractionating column 10 having a high-pressure section 12 and a low-pressure section 14. The lower portion of low-pressure section 14 includes an oxygen reboiler section 16 having several plate-type oxygen boiler heat exchangers 18 circumferentially spaced and supported by lugs 20 welded or torch brazed to the vertical side plates 21 thereof and supported on the inner side of annular wall 22 of the vessel forming the low-pressure section. In certain of the passages of each heat exchanger 18, oxygen is vaporized by heat from warmer nitrogen passing through adjacent passages. An oxygen inlet conduit 24 connects the interior of the reboiler section 16 with a source (not shown) of liquid oxygen such as oxygen rich liquid air. The flow of oxygen rich liquid through conduit 24 may be controlled to maintain the desired liquid level in the reboiler section with respect to heat exchanger 18.

Nitrogen gas for vaporizing the oxygen enters the reboiler section 16 from the high-pressure section 12 via supply conduit 27 and annular header 29 and is conducted to the heat exchangers 18 via nitrogen inlet conduits 26. Nitrogen leaves each of heat exchangers 18 through a nitrogen outlet conduit 28 which may be connected to return condensed nitrogen to the high-pressure section 12. The specific structure of heat exchanger 18 is more clearly seen in FIGS. 2--6.

Each heat exchanger 18 has a plurality of imperforate aluminum separator plates 30 of rectangular configuration spaced in superposed parallel relationship forming platelike or flat passages therebetween.

Certain of these passages 32 (see FIGS. 3, 4 and 5) for nitrogen are filled with extended heat transfer surface such as fin packing 34 for conducting heat from the nitrogen fluid to the plates 30. Fin packing 34 may take various forms such as shown in FIGS. 9--10 of U.S. Pat. No. 3,282,334 and is oriented to provide good distribution of the nitrogen fluid across the passage 32 from nitrogen inlet 36 to nitrogen outlet 38. The top and bottom edge of each passage 32 are sealingly closed by bars 40. The rear edge of each passage 32 is sealingly closed by a bar 42 while a bar 44, somewhat shorter than bar 42 sealingly closes the front edge of each passage 32 and terminates short of bars 40 to form nitrogen inlet 36 and nitrogen outlet 38. Semicylindrical headers 46 and 48 which may be welded or torch brazed to the heat exchanger core assembly communicate the inlets 36 with inlet conduits 26 and the outlets 38 with outlet conduits 28 respectively. Fin packing 34 and bars 40, 42 and 44 are bulk brazed, i.e., furnace or bath braze bonded, to the first faces 30a of plates 30 which form the sidewalls of passages 32.

Certain other of the passages 50 (see FIGS. 3, 4 and 6) for conducting oxygen enriched liquid are interposed between the nitrogen passages 32. Passages 50 are constructed free of any separate extended heat transfer surface such as the fins used in passages 32, since fins are considered to be responsible for acetylene accumulation which increases the risk of explosive reactions with oxygen. The bottom and top edges of each passage 50 are completely open for circulation of oxygen.

To avoid the diffusion of braze metal on faces 30b the vertical edges of the oxygen passages 50 are closed at the front by a shallow generally planar panlike vertical front panel 52 disposed between headers 46 and 48 and overlying the front edges of plates 30 and bars 44 in spaced relationship to the front edges of faces 30b of passages 50. Closing panel 52 is bonded in place along its upper edge to header 46 and along its lower edge to header 48. The vertical sides of panel 52 are similarly bonded to side plates 21. The vertical edges of the oxygen passages 50 are closed at the rear by a similar shallow panlike vertical rear panel 54 positioned to overly the rear edges of plates 30 and bars 42 in spaced relationship to the rear edges of faces 30b of passages 50. Closing panel 54 is bonded in place at intervals 55 (FIG. 3) along its upper and lower edges to bars 42 and the edge of plates 30 in the vicinity of the ends of bars 42 thereby supporting plates 30 in spaced relationship at their upper rear corners. The vertical sides of panel 54 are similarly bonded to side plates 21. The heat exchanger side of headers 46 and 48 in the areas coextensive with passages 50 are suitably closed by a short bar member 56 which may be similar in cross section to bars 42 and 44. These bar members 56 also support plates 30 in spaced relationship at the top and bottom front corners. If desired, the side plates 21 may be constructed with substantial thickness to provide the assembly with sufficient strength.

Thus it will be seen from FIGS. 3, 4 and 6 that the front and back edges of passages 50 are closed to promote percolator action of the liquid fluid within passages 50. However, close clearances and wide laps which may collect dangerous acetylene have been avoided.

During operation of the oxygen boiler, nitrogen gas which is warmer than the liquid oxygen in reboiler section 16 is passed from high-pressure section 12 through supply conduit 27, header 29, through inlet conduits 26 to inlet headers 46. In each of the passages 32 nitrogen gas enters the inlet 36 from a header 46 and is distributed uniformly across the width of the passage whereupon it moves downward giving up heat to bar 42, bar 44, and plates 30 partially via fin packing 34. All or part of the nitrogen may be condensed. The nitrogen leaves passage 32 via outlet 38 which discharges into lower header 48. Outlet conduits 28 conduct the discharged nitrogen from headers 48 to return the same to high-pressure section 12. The heated bars 42, 44 and plates 30 induce a percolator of the oxygen liquid within the confines of front and rear panels 52 and 54 causing the liquid oxygen to be swept rapidly over the bar and plate surfaces for efficient heat transfer to the oxygen liquid. A portion of the heated liquid oxygen is thus vaporized and may be withdrawn from the reboiler through a conduit not shown.

Having now described in detail the preferred embodiment of my invention, I contemplate that many changes may be made without departing from the scope or spirit of my invention and I accordingly desired to be limited only by the claims.

Claims

I claim:

1. An oxygen boiler comprising in combination: a vessel containing a body of liquid oxygen; first means within said body of liquid oxygen defining a first plurality of generally planar first passages for passage of a first heat exchange fluid and each provided with extended heat transfer fins therein; second means within said body of liquid oxygen defining a second plurality of generally planar substantially finless second passages for passage of liquid oxygen and interleaved with said first passages; a metallic heat conductive separator plate disposed between each pair of first and second passages disposed in generally parallel relation to a generally vertical axis for maintaining separation of the fluids within said first and second passages and conducting heat therebetween; said plates being of generally similar configuration, spaced one from the other, and each having first and second faces exposed respectively to said first and second passages; first closing means for closing said first passages at the edges thereof including a brazed seal extending between adjacent faces of each of said first passages along the margins thereof; a brazed bond between said extended heat transfer fins and said first faces; first inlet and outlet means for passing said first heat exchange fluid through said first passages; second inlet and outlet means for passing liquid oxygen in said body of liquid oxygen through a major portion of each of said second passages in a direction generally parallel to said axis; and a second closing means for closing said second passages at the edges thereof including a closing panel generally parallel to said axis traversing several of said first and second passages in overlying relationship to said first closing means in spaced relationship to said second faces whereby the edges of said second passages are closed for establishing a percolator action of the liquid oxygen within said second passages.

2. The apparatus as defined by claim 1 including a pair of side plates each disposed at one of the opposite sides of the group of interleaved first and second passages in generally parallel relation to said separator plates; and means bonding vertical edges of said closing panel to said pair of side plates.

3. The apparatus as defined by claim 1 including means bonding an upper and a lower edge of said panel to a point on each of said separator plates.

4. The apparatus as defined by claim 2 wherein said first inlet and outlet means includes an inlet header and an outlet header connected in supporting relation to said separator plates; and means bonding upper and lower edges of said closing panel between said inlet and outlet headers.

5. The apparatus as defined by claim 4 including means for closing the heat exchanger side of said inlet and outlet headers in the areas coextensive with said second passages.

6. The apparatus as defined by claim 1 where said second closing means includes a second closing panel generally parallel to said axis traversing several of said first and second passages in overlying relationship to said first closing means in spaced relationship to said second faces and disposed in remote relation to said first mentioned closing panel.