CA1090990A - Brazing heat exchanger tubes to headers - Google Patents

Abstract

Abstract of the Disclosure A set of metal heat exchange tubes is simultaneously brazed into a metal tube sheet more rapidly and with greater uniformity by holding tube-and-sheet assembly under radiant heater with sheet essentially horizontal and carrying brazing metal, and forcing gases from above tubes down into tubes as assembly is heated. Heater can envelope the top and sides of the sheet, or can have flat heating surface, but is preferably a ceramic fiber gas burner. Such burner, for this or other uses, can have ceramic fiber mat fitted over a combustion mixture plenum with margin of mat sealed by a high-temperature-resistant impregnant that fills its pores. Burner can be divided into two portions using a single ceramic fiber mat and separate combustion mixture supplies, with non-combusti-ble gas fed through one portion when other portion is the only one operating.

Description

1090~90 The present invention relates to the sealing of heat exchange tubes to a tube sheet, and to apparatus suitable for use in that connection.
Industry has need for relatively small all-metal heat exchangers, as for use in cooling oil that lubricates an internal combustion engine.
Such a heat exchanger can have as many as several hundred heat exchange tubes connected between two sheets in a leak-proof manner. Leak-proof connections for his purpose are generally made by a fusible metal sealant whose melting takes place at a temperature well above the naxim~m operating temperature of the heat exchanger. While tin-lead solders can be used as sealants for operating temperatures near the normal boiling point of water when no great mechanical stresses are encountered, brazing alloys including the so-called silver solders are used for higher operating temperature or higher stresses.
Such leak-proof brazing of a quantity of relatively small tubes in a tube sheet has been an awkward industrial operation that takes substantial time to assure the heating of all joints to the desired sealing temperature, and generally requires patching to seal leaks resulting from uneven heating during the original sealing.
According to the present invention more rapid and more effective sealing is accomplished by holding an assembly of the heat-exchange tubes each tube having one end in a sheet with the sheet in essentially horizontal position and carrying on its upper surface a quantity of brazing metal adequate to seal all tubes into the sheet, applying radiant heat downwardly on the sheet to substantially onl~ the top of the assembly to bring the sheet at least to the fusion point of the brazing metal and moving gases down from above the tube ends dcwn through the tube ends during the heating to cause the heating to be re uniformly applied to the tubes so that the sealant rapidly seals all tubes to the sheet.
Also according to the invention, there is provided an apparatus for simultaneously sealing a plurality of metal heat exchange tubes in a metal tube sheet having openings through which one end of each tube projects, said apparatus having support means for holding an assembly of tubes in a sheet with the sheet in essentially horizontal position and carrying on its upper surface a quantity of fusible metallic sealand adequate to seal all the tube ends in the sheet, radiant heating means mounted im~ediately over the tube-and-sheet assembly as supported by the support means, and oriented to apply radiant heat to substantially only the top of the assembly, to bring the sheet of that assembly to a te~lperature at least as high as the fusion point of the fusible metallic sealant, and gas moving ~.eans connected to at least one of the other means to propel gases from above the tube ends down through the tube ends as the assembly is heated, to effect more unifor~
heating and sealing of the tubes.
The heat for the fusion is desirably applied by a ceramic fiber burner. Such burners generally have a ceramic fiber mat made of ceramic fibers such as described in U. S. Patent 3,449,137, with the mat formation as described in U. S. Patent 3,787,194.
The most efficient heating results of the present invention are obtained when the burner that supplies the heat envolopes the top and sides of the sheet in the tube-and-sheet assembly. Such an enveloping burner is desirably divided into sections that can be operated independently or together to first direct heat principally to the outer margin of the top of the sheet-and-tube assembly, and then direct heat downwardly over the entire top. A particularly effective burner construction for this purpose uses a single porous ceramic fiber mat in the general shape of a hat with a shallow plenum divided by a wall into two parts, air or other incombustible gas being fed through one part when that part is not being operated while the other part is being operated. The porous margin of the mat can be sealed by a iU~0'~190 high-temperature-resistant impregnant like aqueous sodium silicate, and the sealed margin clamped in place. Sheets of soft material like aluminum foil can be interposed between the sealed margin and the clamping members.
The foregoing as well as additional features of the present inven-tion will be more fully understood from the following description of several of its exemplifications, reference being made to the accompanying drawings wherein:

-3a-.~;., ~(J90'390 Fig. 1 is a somewhat diagrammatic vertical sectional view of one set-up for practicing the present invention; and Fig. 2 is a similar view of a modified set-up pursuant to the present invention.
Turning now to the drawings, the apparatus of Fig. 1 includes a table 10 movable up and down as indicated by the two-headed arrow 12, and a radiant heater 50 positioned above the table. The table carries on its upper surface a block 14 having a number of vertical passageways 16 corresponding to the number of tubes 18 to be assembled into a heat exchanger, and located in a corresponding pattern. The upper ends of the passageways 16 are enlarged as at 20 to receive and position the lowest portion of each tube. The lower ends of the passageways 16 open at the bottom of block 14 over a suction opening 22 in table 10.
A blower 24 is shown as carried by table 10 and as provided with a suction tube 26 connected as by flexible duct 28 to a mounting ring 30 secured around opening 22. A butterfly valve 32 can be fitted to the suction tube 26 to enable controlling of the suction applied to the bottom of block 14 when the blower 24 is operated. Also the suction tube 26 can be spaced as by webs 27 within a wider intake mouth 29, so that when the blower operates it sucks air in around the suction tube 26 as it sucks through tube 26.
Block 14 also carries a set of supports 34 encircling the tubes 18 and holding a tube sheet 36 in position at or near ~ .

90!3~0 the tops of tubes 18. Supports 34 can be removably ,fitted in sockets 40 in block 14, and can have their lower portion cut away as at 42 to allow for the positioning of another tube sheet 37 on block 14.
Heater 50 has a porous ceramic fiber mat 52 in the general shape of a hat with a horizontal flange 54 by which it is mounted in place behind a face plate 56. -The crown section of the hat shape consists of a cylindrical portion 58 a few inches in height and a hemispherical portion 60, and a relatively shallow plenum space 62 is provided around the crown by a housing 64 to which the face plate 56 is removably secured.
The plenum space is divided by a partition 66 that extends around the inside of the housing, into a lower generally annular plenum portion 68, and an upper hemispherical shell-like portion 70. Separate inlet nipples 71, 72 are provided on the housing for separately supplying combustion mixture to the separ-ate plenum portions. In the illustrated embodiment the housing 64 is made of a lower cylindrical section 74 and an upper hemispher-ical section 76. Outwardly projecting flanges 78, 80 on these housing sections where they meet, serve as attachment structure for holding the entire hou~ing together and also holding partition 66 in place. To this end a number of threaded flange bolts 82 project through aligned sets of openings in flanges 78, 80 and in partition 66, and nuts 84 threaded on these bolts secure these members together. The bolts 82 are distributed around the housing and they also project downwardly for enough to provide securing means for the face plate 56 which is also provided with mounting openings aligned with the bolts. An extra set of nuts 86 threaded on the bolts secures the face plate in place.

1090~90' .

The burner ~9 constructed by first assembling the housin portions 66, 74, 76, then forcing the pre-formed and prepared mat in the as~embly so that it firmly engages the inner lip of parti-tion 66, and then securing the face plate. The partition lip can be turned up as shown at 67, to make a-better seal against the mat An internally directed flange 88 at the lower end of lower housing section 74 i9 used to provide a ledge against which the mat flange 54 is held to help seal the edges of the mat agains gas leakage. A cylindrical flange 90 is also shown as integral with and projecting up from the top of the face plate, to encircle the mat edges and closely fit around the lower edge of the housing This helps hold the mat in position and strengthen the face plate.
A central hole 92 in the face plate slightly larger than the mouth of the mat 52 permits the top of the tube-and-sheet assembly to be brought into the burner a short distance above the mouth of the mat, as well as the movement of gases out from and into the work space 94 enveloped by the mat.
The burner i~ operated with gaseous combustion mixtures, and it is accordingly helpful to seal all locations through which such a mixture can leak out from the burner. Thus the joint between the housing members 64 and 66 as well as between 66 and 74 can be sealed by gasketing or as shown by painting these junctures with a liquid silicone that cures to a solid sealant. Also the margin of the mat flange 54 is shown as encircled by a sheet of aluminum ~il93cæefully folded around the upper, lower and edge faces 94, 95, 96, and sealed against ledge 88 by a sealant such as a self-curing liquid silicone rubber.
It is also helpful to fill the pores of the mat in the outer section of mat flange 54, as by impregnating that section 1(J90390' with aqueous sodium ~ilicate that dries in place or liquid silico~ e rubber that cures in place, as indicated at 97. Another desirabl feature is to water cool the outer margin of the face plate, as b~
brazing water-cooling coils 98 to its lower surface.
In operation the apparatus of Fig. 1 has its table firs fitted with the tubes and tube sheets as shown, although there wi~ 1 usually be man~ more tubes than indicated in the figure, and a quantity of powdered or granular fusible sealing material 99 sprec 1 over the upper sheet 36. The blower 24 is started and the table is raised to the position illustrated so that the upper sheet 36 has its upper surface and side edges enveloped by the burner.
Both sections of the burner are then started, followed by opening of suction control valve 32. When the tubes 18 are copper or brass with a wall thickness of about 30 mils, and the upper sheet 36 is of copper, brass or steel with a width of 8 inches and a wa] L
thickness of about 90 mils, and the burner is burning about 130,0 B.T.U. per hour of combustion mixture, a copper-phosphorus or silver-copper-flux sealing braze will in less than about 1/2 minut .
be melted and will flow into and seal each tube to the sheet with a text-book seal, regardless of how many tubes there are. Care should be used when applying the fusible sealing material so that excess material does not plug any tubes, which would impede the flow of hot gas through the tube resulting in uneven heating.
To avoid overheating, the burner is shut off as soon as the sealing is completed, although the suction can be continued.
Prolonging the suction helps cool down the heated assembly and thus further reduces surface oxidation.
If the suction is not used during the heating the heat-up of the sheet is not uniform and much more heat-up time is neede before all parts o~ the sheet are hot enough to melt the sealing material ~y that time the outer portions of the sheet are greatl i090990 .

over} ated a~d lf r.ot badly damaged ca~ a190 become sealed to the supports 34 even if the upper ends of the supports are about 3/8 inch thick steel. On the other hand when the burnt combustion gases are sucked down the tubes at a speed as low as about 1/2 linear foot per secoAd the heat-up becomes so uniform that the sealing of all the tubes is completed long before the upper ends o supports 34 get hot eno~gh to seal. The portions of the sheet 36 touched by the supports 34 will not heat-up very rapidly, with or without the foregoing gas flow, and this will also tend to maXe the immediately adjacent portions of the sheet a little slow in heating-up so that for best results it is desirable to have the tubes at least about 3/8 of an inch away from all supports. Those supports can also carry special fittings that make their upper ends more massive for even greater thermal inertia, but the 3/8 inch spacing of the tubes from their tops is still enough. Where there is considerable hardware around the margin of the tube sheet it is helpful to start the lower_section 58 of the burner 50 befor starting the upper section 60, and to start the upper section a few seconds later after the margin of the sheet has absorbed sufficient heat to be well on its way to sealing temperature. To guard against misoperation air without fuel is blown through the upper portion 60 of the mat while the lower portion is burning and the upper portion is not burning. This practically equalizes the pressures on both sides of partition 66 and thus minimizes flow of combustible mixture to undesired locations where it can be un-intentionally ignited.
Pilling the mat pores at 97 also avoids localized col-lection of stagnant combustible mixture.

ll i090'39~
I ., There i8 no practical upper limit to the speed wlth which the hot combustion gases are forced down the tubes. There is for example no need for gas-tight connections between the tube~
and passageways 16; indeed as shown by the open gap between suction tube 26 and suction intake 27, it is helpful to have air leaks that draw unheated air into the blower along with the hot combustion gases and thus help guard against overheating of the blower.
The tubes 18 are themselves not very wide, generally less than a half inch in inside diameter, so that it is difficult to effect extremely rapid gas movement through them. Speeds of 20 feet per second are suitable.
The seals made in a fraction of a minute pursuant to the present invention are found to have far fewer flaws than seals made in two-and-a-half minutes without the use of the gas movement down the tubes. Moreover because of the much greater uniformity of the heat-up according to the present invention the melting and flow of the sealing material is also more uniform so that less sealing material is needed. As compared to the quantities of sealing material ordinarily used in the prior art, about half as much is needed for use with the present invention. Thus for joint in which the tubes have an outside diameter about 2 mils smaller than the diameters of the holes in the sheet, only about one gram of sealing material are needed for every square inch of sheet surface in accordance with the present invention.
Fig. 2 shows a modified sealing arrangement of the present invention. Here a burner 150 having a generally flat burner face 152 i8 used. This extends the heat-up time somewhat as compared to the construction of Fig. 1, and as a result wide .

G ~ ) iO90'390 assemblies may take as much as 50% more time to seal. ~owever the sealing time i9 still far less than obtainable from the prior art.
. .
The burner 150 of Fig. 2 can be constructed such that the ceramic fiber mat has its margin merely fitted to a frame having an inert gas blow-through arrange ment in which the inert gas thus blown through the margins of the mat acts a~ to seal those margins against combustible mixture leakage. ~o other margin sealing is then needed.
In the Fig. 2 arrangement tubes 118 are sealed to a sheet 136 while the tube-and-sheet assembly is held within a tubular casing 119 which eventually forms the shell of the heat exchanger. In about a half minute such an assembly can be sealed r ~,. following which the assembly is inverted so that the opposite end is similarly sealed, and the sheets are then later brazed or welded to the shell margins. Where the shell is steel of low wall thickness it can be sealed against the sheets at the same time as the tubes are sealed, preferably using the enveloping burner arrangement of Fig. 1.
While suction provides a convenient technique for moving the hot burnt combustion gases through the tubes, they can also be forced through from above. Thus the burner of Fig. 2 can have its frame provided with a depending cylindrical extension that encircles the shell 119 and has an asbestos lining pad that closely engages the shell. Operating the burner in such an arrangement causes the hot burnt combustion gases to be discharge ¦
downwardly through tubes 118 since they have essentially no other way to escape.

. 10 11 . ' -. . - .
1090'3'3U

For the purposes of the present invention, brazing is considered a sealiny operation in which metal having a melting point at least as high as about 450F and generally a copper alloy such as an alloy of 45% silver with 55% copper by weight, is the sealant. Brazing temperatures can go as high as 1300F or even higher. Flux such as borax is frequently used with the brazing metal to protect it and the parts being joined against excessive oxidation and to promote wetting of the parts by the melted braze.
Some brazing metals are copper-phosphorus alloys or other alloys that can be used without a flux. -The heater in the sealing apparatus of the presentinvention can be operated continuously while sealing a succession of assemblies, but is preferably operated only for short intervals while the sealing metal is being melted and flows into place. Thu the heater can be completely shut off between sealing sequences, and ceramic fiber burners are particulaxly helpful in such inter-mittent operatio~sinasmuch as they heat up and cool down in only a few seconds. For such intermittent operations it is also helpful to have the burner plenum of relatively small volume, preferably not over about 1 1/2 inches deep. In this way combusti n gas can be intermittently fed to the plenum and rapidly reach the exit surface of the fiber mat where it is burned, so that the timing of the buxner action is simplified.
An igniter such as a pilot light assembly or an electric spark ignitor can be fitted near the margin of the burner to assur that it lights up each time a combustion gas feed is initiated.
A settable automatic switching sequencer can be used to time the gas feed to the different burner portions as well as the suction blower.

., . 11. - ' ,`` ll .-) . . -, , , ` 1090~90 Instead of, or in add~tion to, ving the table up and down to bring the work to the burner, the burner can be moved toward and away from the table. In the construction of Fig. 2 no vertical movement is needed by the table or the burner.
An auxiliary heater can also be provided around and above the lower tube sheet 37 in the construction of Fig. 1, and operated to seal the lower tube ends into that sheet while the tube-and-sheet assembly is held in the illustrated position. Thus a layer of sealing mixture can be applied to the upper surface of the lower sheet and the auxiliary heater started even before the burner 50 is lit inasmuch as the heat-up of the lower sheet takes longer than that of the upper sheet.
Where the margin of a ceramic fiber mat has its pores well sealed, as by the silicone or sodium silicate or other alkali metal silicate impregnant, the mat margin can be clamped in place without wrapping the aluminum foil 93 around those edges. The aluminum foil or other gasketing can still be inserted between the mat margin and the plenum margin, or the silicone or alkali metal silicate can also be used to seal the mat edge to the plenum.

` ` 10~0~90 .
Obviously many modifications and variations of the pre-sent invention are possible in the light of the above teachings.
It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than a~
specifically described.
.', ' .
. ~

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for simultaneously sealing a plurality of metal heat exchange tubes in a metal tube sheet having openings through which one end of each tube projects, said apparatus having support means for holding an assem-bly of tubes in a sheet with the sheet in essentially horizontal position and carrying on its upper surface a quantity of fusible metallic sealant adequate to seal all the tube ends in the sheet, radiant heating means mounted immedi-ately over the tube-and-sheet assembly as supported by the support means, and oriented to apply radiant heat to substantially only the top of the assembly, to bring the sheet of that assembly to a temperature at least as high as the fusion point of the fusible metallic sealant, and gas moving means connected to at least one of the other means to propel gases from above the tube ends down through the tube ends as the assembly is heated, to effect more uniform heating and sealing of the tubes.

2. The apparatus of claim 1 in which the radiant heating means envelopes only the top of the tube-and-sheet assembly.

3. The apparatus of claim 1 in which the radiant heating means is a ceramic fiber burner.

4. A process for simultaneously brazing a plurality of metal heat ex-change tubes in a metal tube sheet, which process comprises, holding an assembly of such tubes each having one end in a sheet with the sheet in essen-tially horizontal position and carrying on its upper surface a quantity of brazing metal adequate to seal all tubes into the sheet, applying radiant heat downwardly on the sheet to substantially only the top of the assembly to bring the sheet at least to the fusion point of the brazing metal, and moving gases from above the tube ends down through the tube ends during the heating to cause the heating to be more uniformly applied to the tubes so that the sealant rapidly seals all tubes to the sheet.

5. The process of claim 4 in which radiant heat is applied from a radiant heater that envelopes only the top of the tube-and-sheet assembly.

6. The process of claim 4 in which the radiant heat is applied from a ceramic fiber burner.

7. The process of claim 4 in which the heating to at least the fusion point of the braze is effected in about a minute or less.

8. The apparatus of claim 1 in which the radiant heating means has a heat output that brings the sheet to the sealant fusing temperature in about half a minute or less.

9. The apparatus of claim 1 in which the radiant heating means has two independently operable heat radiating zones, one zone being arranged to principally direct heat down on the top of the tube-and-sheet assembly, and the other zone being arranged to principally direct heat to the periphery of the assembly top.