US2641206A

US2641206A - Firetube baffle insert with protected tip for heat exchangers

Info

Publication number: US2641206A
Application number: US784283A
Authority: US
Inventors: Stout Minor Woolfolk
Original assignee: Individual
Current assignee: Individual
Priority date: 1947-11-05
Filing date: 1947-11-05
Publication date: 1953-06-09
Anticipated expiration: 1970-06-09

Description

June 9, 1953 2,641,206

M. W. STOUT FIRETUBE BAFFLE INSERT WITH PROTECTED TIP FOR HEAT EXCHANGERS Filed Nov. 5, 1947 o $5 3 JNVENTOR inor' Stout Patented June 9, 1953 UNITED STAT ES N T F Fl Minor Woolfolk Stout, Webster Groves, Mo.

Application November 5, 1947," Serial N0r784,-283

7 Claims. 1.

ilTliisinvention relates'to"improvements in accessories for heat exchangers: More-particularly this invention relates to an: improved accessory that can' -be usediwith a" heat exchangerto'in crease-the efiiciency of operation of that heat exchanger" while permitting a reduction in the size of that-heat exchanger without a' decrease in' its capacity.

It is-therefore an objectof the'present invention toprovide arr'improvedaccessory that can be'used' with azheat exchanger to increase the efiiciency of operation of'that heat'exchanger while permitting a reduction in the'size of "that heat exchanger without adecrease-inits capacity.

In the operation of I heat exchangers, of" the type reierred'toas 'boilers,=itis customary to position a 'vaporizablediquid in a container or housing and to-subject that container or housing to heat; In those instances: where the containeror hous ing is heated-by the products of combustion of solid. liquid; orrga'seous fuels, it hasbeen found advantageous to: provide fire tubes that I extend through the container orhousing and conduct the products of combustion; through" that con tainer orhousingl The fire tubes greatly in-" crease the-effective'areathrough which heat can pass from: the. products oi combustion to' the vaporizable liquid, and thus they increase the eihciency of :operati'on of theheat exchangers. While the'fire tubes do act to increase the chi.- cicncy' of operation. oi: the heat exchangers, those tubesare-not as efficient as they could be because they permit theproducts of combustion to stratifv as'they passthrough'the fire tubes. Such st no cation permits the products of combustion, incontact with the-fire tubes, to lose theirheat and'thenract as a sheathor layerof relatively cool asthat acts to retard theltransferiof heat :Fr me'hotter core ofthe products "of combustionto'the surfaces oftherfire tubes. In such instances; a large proportion of the molecules in the products sot combustion do not transfer their heat tothe firea tubes, but instead carry most of their heat with them as they" pass! to the' sta'ok or chimney. 10st" since'it is dissipated in the atmosphere. Various methods have beenaproposed and devised to prevent 'stratification of the products of combus oir in firetub'es, and one such. method contcinp ted th formation of fire tubes with grooi sthat: would'cause directed flow of the products-of 1 combustion through" the fire tubes. Th is method: was exceedingly expensive because of th coast of makingsuch'tubes; and despite the hi'g-lrcost it: failed to break-up the hot core of the Theheatlof such molecules is thus products" of combustion: Anotherrmethodccona templated I the positioning 'of lengths: of: twisted wire cable inthe iire tubes to causedirected flow of the products of combustion. through those tubes; but that 'cable'imerely directed, andd-idtlnot force; themovement of thezproduc-tsof combuse tion; While these methods could be of somebem slit; in. the sense that afdirected flow of theeprode ucts iof combustion: tends. toreduce stratification; thoselrn'ethods are-far. .from' satisfactory; vsince it is necessary; to 'providerastructurethat .not only breaks upithe hot core of the .productsotcoms bustion but positively forces those-productsi=-of combustioni'against the surfaces ofrthez'iire tubes: Such a structure will cause-the heated molecules; of the .hot bore :of the products of icombustiom to move radially outward until. they; strike :th'efsuri faces 'oithe \firetubes; and it willalso' causezthose in olecules to scour those surfaceswbv-constantly rcplacing the cooled molecules with. 11621117667111.0184- cules that can transfer their heat to" the 'ifirle tubes. Theradial. movement of the molecules breakswhe stream line flow or the: productsflof combustionzandcauses turbulence; and that-tun bulen'ce will forcetheproducts of combustion'wto intermingle and admix. and assist the securing action.= Itis therefore an object ofthe present invention to nprovide .an accessory that "positively forcesthe'products of combustionin theifireltub'es of a heat exchanger to move radially'outwardand to scounthe surfaces of thefire tubes-.

The accessory which is providedby thepresent invention: is preferably a flat piece 20f metal that has been'twisted intothe :form Ufa/11311.3(; This r piece-of metal can be positioned within aifiretube of a heat exchanger; and when so positionedit will positively forcethe products of combustion to change direction and. to follow a helical. path. While following this path,.the-molecules-ioflthe products of .cornbustionwill experience consider-r able-turbulence; and this 1 turbulence: will cause recurrent scouring of the surface of the firetube by theproducts of combustion. The helicalstrip of metalnot only avoids stratification andaan axial flow of. the products of combustionrbutit forces each of "the molecules of the products of combustion to contact and'scour the inner. sur-.- faces of the fire tube and transfer its heat to that tube. In this way, the helical strip ofi'metalfpre vents" the formation of a relatively cooli heat= retarding sheath of gas adjacent the-'innertsur face of the "fili-Z tube, and "thus the strip makes-it possible for the vaporizableliquid irr'the 'com tai'ner or-housing *to absorb heat from all, rather than just a few; oithe-molecules of the'products of combustion. This greatly improves the operating efficiency of the heat exchanger, because it forces the molecules in the products of combustion to transfer most of their heat to the fire tubes, thus reducing the amount of heat that goes up the stack and is lost to the atmosphere. It is therefore an object of the present invention to provide a helical strip of metal which can be positioned within the fire tube of a heat exchanger and can act to force the products of combustion to move radially outward and scour the inner surface of that tube.

To be quite efficient, the helical strip of metal should be quite thin so it does not unduly reduce the available cross section of the fire tube, but a thin strip of metal is not easily positioned within a small diameter tube. However, the helical strip of the present invention can be made quite thin, without detracting from its support, by having that strip rest directly upon the inner surface of the fire tube. The succeeding turns of the helical strip will stiffen that strip considerably; and those portions of the edges of that strip which engage the lower half of the inner surface of the tube will support that strip and hold it in position. While supported in this position, the helical strip of metal will obviate a hot core in the products of combustion; indeed it will force the molecules of those products of combustion to move radially outward and scour the inner surface of the fire tube. It is therefore an object of the present invention to provide a thin helical strip of metal which has a number of complete twists or turns and is made to rest on the inner surface of the fire tube of a heat exchanger.

For extremely high efficiency of operation of the heat exchanger, a tip of insulation should be provided for the leading edge of the strip of metal. This tip performs two important but distinct functions; it isolates the flaming products of combustion from the leading edge of the metal strip and thus keeps that strip from conducting away heat and lowering th combustion temperature, and it becomes red hot and serves to catalytically ignite any unconsumed fuel component in the products of combustion. By keeping the combustion temperature high, and by igniting all of the fuel introduced into the fire tube, the tip of insulation provides even greater emciency of operation for the heat exchanger. therefore an object of the present invention to provide a tip of insulation, for the helical strip of metal, that will keep that strip from reducing the combustion temperature and that will ignite unconsumed fuel components in the products of combustion.

To facilitate the complete combustion of fuel components ignited by the heat from the tip of insulation, the present invention provides sufiicient air to support that combustion. While it isimportant to provide sufiicient amounts of air to assure complete combustion of all the fuel used to heat the material in the heat exchanger, it is also important to avoid excessive amounts of air, because excessive amounts of air would reduce the combustion temperature. The present invention provides sufiicient air for combustion but avoids reductions in combustion temperatures by limiting the amount of air used in burning the fuel. It is, therefore, an object of the present in- .vention to limit the amount of air used to support the combustion of fuel.

Other and further objects and advantages of the present invention should become apparent It is i 4 from an examination of the drawing and accompanying description.

In the drawing and accompanying description a preferred embodiment of the present invention is shown and described, but it is to be understood that the drawing and accompanying description are for the purposes of illustration only and do not limit the invention and that the invention will be defined by the appended claims.

In the drawing, Fig. 1 is a side elevational view of a helical strip of metal that is made in accordance with the principles and teachings of the present invention and is provided with a tip of insulation,

Fig, 2 is a plan view of the helical strip of Fig. 1, and

.Fig. 3 is a partially sectioned side view through a heat exchanger, of the type referred to as boilers; and it not only shows the position of the helical strips in the fire tu'bes but it also shows the spacing of the fuel and air nozzles from the ends of those fire tubes.

Referring to the drawing in detail, the numeral l0 denotes an elongated strip of metal. This strip is made from a metal which has a melting point greater than the combustion temperature of the fuel; one such metal being steel. A rustresistant steel, such as stainless steel, is preferred because of its long life; but cold rolled band steel is also usable, and in fact strips of the latter type of steel have been in use for many months. Those strips have become rusty but they still perform their intended function; moreover, the rust seems to assist heat transfer by further increasing turbulence. The strip I0 may be rolled from a bar or it may be cut from a sheet; but however formed, it is then twisted beyond its elastic limit until it assumes a helical configuration. An opening I l is provided in the trailing end of strip l3, and that opening makes it possible to hold the strip l0 extended while it is being twisted; thus making it possible to secure a very small pitch for the helix of the finished strip. Twisting the strip l0 beyond its elastic limit enables that strip to retain its helical configuration despite repeated heating and cooling cycles. The helical configuration is preferably such that the strip ID has several complete turns or twists intermediate its ends; for the greater the number of such twists, the more effective the scouring action will be. The strip I0 is made quite long; and in fact it is preferably made so its twisted length is just slightly less than the length of the fire tube of the heat exchanger. The strip in is made wide enough so it divides the space in a fire tube into two separate parts, but it is made narrow enough to facilitate its forced insertion into the fire tube. A long wire and hook are used to pull the strip into position, the hook fitting into the opening I I in the trailing end of strip l0. Where the helical strip I0 is made in this manner and is positioned within the fire tube of a heat exchanger, products of combustion introduced into that fire tube will initially and positively be rection, they experience considerable turbulence and .an appreciable reduction in velocity. The

turbulence of the masses eliminates stratification and enables most of the molecules of the products spam-woe oi-combustion to contact thesurface ofthefire tube and'transfen their: heat'to' that tube; and the reduction in the velocity' of the" products of combustion increases the eif'e'ctive -time during which the products of combustion can transfer their heat to the fire tube. The combined efict of the turbulence and the reduction in velocity to reduce the required length offire tube for a given efficiency of combustion-or a given heatexchanging capacity; thus reducing the-size and cost of the heat exchanger.

The helicalstrip of metal can be positioned in the fire tube of a boiler; and when so positioned it will provide the desired radial movement and-scouring action or" the productsofcombustion, and it will reduce the velocity of those products of combustion. Where desired, how'- ever; that strip of metal'can have its leading end provided with a tip of insulation. This tip may he lormedfrom two sheets ii of insulation which have their trailing ends out, or otherwise formed, to provide a'recess in which the leading'endof the helical strip Hi can be positioned. The sheets it of insulationmust be capable'of withstanding temperatures above two thousand (2000) degrees Fahrenheit, and they must have suflicient structural strength to withstand handling during shipment and installation; One material that has been found to be ver effective is a mixture of asbestos fibers and Portland cement which is prepared and. sold by the Ruberoid Company under the designation Stone Wall. That material has. a composition of fifteen percent (15%) asbestos fibers and eighty five percent (85%) Portlandcemenu. and it is q ite sturdy; The trailing ends of the sheets i 2 have openingstherein, and those openings are in register with each other and in register with an opening in the leading end of the helical strip it. These-openings receive a rivet Hi'which secures-the sheets ii to the strip Ill; and whiiethe rivet Hi may be made of several metals, copper is preferred. The expansion of the copper rivet Hi, when heated, is sufficient to compensate for any expansion of the sheets 12 and strip iii; and thus the sheets i2 will. not be unduly compressed and crushed during heating cooling. The leading ends of the sheets if) of insulation also have openings, and those openings receive the rivet it. This rivet also is preferably ofcopper; and its holds the abutting surfaces of' the sheets 52 so close togetherthat little if anyof theprod nets of combustion introduced into the fire tube can pass to and engage the leading end'of the helical strip Iii. Consequently, the metal strip ill cannot conduct. away heat from-the flaming products of combustion enteringthe fire-tube and cannot reduce combustion temperatures; instead the heat of the flaming products of'combustion will be held by the tip'of insulation and will serve to ignite any unconsumed fuel inthose products of combustion; By preventing reductions in combustion temperaturesand by catalytically igniting unconsurned fuel components; the tip of insulation further increases the efficiency of operation of the heat'exchanger;

Sufficient air must be supplied to support both the initial and secondary combustion of thefuel; In the construction shown in the drawing; part of the air is admixed with fuel and is expelled through nozzles, while the rest of the air isclrawn into the fire tubes the jet action of the nam ing products of combustion. By regulating'the diameter of thefire tubes, thepressure-on tlie mixtureof air and fuel issuing from the nozzles,

and. the spacing betweenthe ---n-ozz1es-' and: the fire tubes; it is 5 possibleto attain enoughzrsecondary air to support' the complete combustion" of fuel ignited by the=tip of insulation', while avoiding an excess of air which would lower thecombustion temperature. In some instances where it is desirable'to avoid reliance on secondary'air; as where that -air'is dirty or has unde sirable components, the'nozzles can be placed shown, and that heat 'exchanger'has aplurality' of fire tubes'lfl'which are disposed in parallel relation to each other. supported at eachendbytubesheets 2t, only'one of which is shown; and the connection between .i the-tube sheets "'and "the fire tubes iii is made ister' withthe pipes 22.

water' tight by rolling or 'weldingthe tubes it into place. A=series ofpipes 22 disposed in registerwith the endsof the fire tubes l8; and each of' the-pipes 22'. is provided with a nozzle 2d. The nozzles '23 have restricted orifices, and they may bemadebydidlling openings of the desired diameters in rods and then pressing the drilled rodsinto the exit ends of the-pipes 22. The exit ends or the nozzles '24 are preferably spaced inwardly about. one-half inch from the exit ends of the pipes-22; A number of tubes 26 extend transversely across the end of t e heat exchanger immediately below the aligned pipes 22; andthose tubeshave openings in reg- By connecting pipes 253 to a' source of inflammable" gas or vapor, it is possible tomake tubes '25 serve as automatic pilot lightsfor the -n1ain nozzles 2 of the heat exchanger. The mixture of fuel and air issuing from nozzles '24 will beignited by the pilot lights and will-pass intothe ends of fire tubes i8. The strips-Ill of metal'in those tubes will be spaced inwardl fromthe-ends of those'tubes; and the tips-of the flames issuing from the nozzles 2 should-impinge-upon and strike the tips of in sulation'on the'sti'ips lti of metal, overlapping the leadingend's of those tips about one-half inch;

In the operation of the heat exchanger, that is'partially shown in Fig; 3, a mixture of air and inflammable'gas or'vapor isintroduced into the pipes- 22 under pressure'and is caused to issue from the-nozzles 2!} in streams. The streams will beignited by the pilot lights from tubes 25, and they will form flames that extend into the fire tubes ifi'and'impinge upon the insulation tips "on the ends of the-helical strips it. As prc viously indicated, the flames andthe ducts ofcombustion ineaoh tube it will then be r vided into twomasses; one passing-to one side of the helical strip it and the other passing to the other side of that strip. Because their inertia, the flames and'products of combustion will tend to move axially of thetubes in streams,

' but as they move in" that direction they will strike-the helical strips ill which positively prevent uninterrupted linear axial movement of the names and productsof combustion in streams. The engagement between the strips i3 and the products ofcombustion positively forces the products-of combustion to move radially outward and to scourthe inner surfaces of theme tubes, and that engagement also-causes a" considerable reductionirr the velocity of flow of those products of combustion.

The tubes" 18 are Operation of heat exchangers of thetype shown in Fig. 3 proves that the use of helical strips 10 in the fire tubes !8 causes decreases in the velocity of the products of combustion of as much as forty percent (40%). Such decreases are of tremendous value since they increase the time during which any particular molecule in the products of combustion remains within a fire tube, thus increasing the total amount of heat which that molecule transfers to that tube. The decrease in velocity of the products of combustion cooperates with the ignition of unconsumed fuel, when it strikes the red hot tip of insulation, to make possible reductions of as much as fifty percent (50%) in the length of fire tube travel for a given combustion efficiency or given heat exchanging capacity. Such reduction greatly reduce the dimensions and costs of the heat exchangers.

An impressive illustration of the effectiveness of the helical strips It can be made by operating a heat exchanger with and without the helical strips HI. Where the strips H] are removed from the fire tubes, the stack temperatures will be in the range of from seven hundred (700) to eight hundred (800) degrees Fahrenheit, but where the strips in are positioned in the fire tubes the stack temperatures will be as low as four hundred (400) to five hundred (500) degrees Fahrenheit. The strips 10 of metal thus cause the products of combustion to yield an additional three hundred (300) degrees Fahrenheit of their heat to the fire tubes, thus showing conclusively the improved heat transferring ability of the strips ID. Moreover, the operation of boilers equipped with helical strips It has shown that the strips make it possible to increase the unit overall heat transfer rate of the fire tubes as much as seventy-five (75) to eighty (80) percent. I

In one instance, a one pass heat exchanger was built in the manner indicated in Fig. 3, and it was equipped with helical strips as shown in Fig. 3. The combustion temperature of the gas used with that heat exchanger was approximately two thousand (2000) degrees Fahrenheit, the temperature of the water in the heat exchanger was two hundred and thirty-five (235) degreesFahrenheit,and the stack temperature was only three hundredand five (305) degrees Fahrenheit; and when a sample of stack gas was analyzed it was found to contain nine percent (9%) carbon dioxide (CO2), three percent (3%) oxygen (02), and no carbon monoxide (CO). This is an amazing record since the stack temperature is about seventeen hundred (1700) degrees below the combustion temperature of the gas used, and it is only seventy (70) degrees above the temperature of the water being heated. Moreover, the high percentage of carbon dioxide (CO2), the low percentage of oxygen (02), and the complete absence of carbon monoxide (CO) in the stack gases shows that all of the fuel was consumed without the use of excessive amounts of air that could reduce combustion temperatures. Reference to the proper Nomograph of the American Gas Association shows that this heat exchanger has an American Gas Association efficiency of eighty-four percent (84%) an efliciency that one pass heat exchangers dont ordinarily have, and that two pass and three pass heat exchangers rarely attain despite their added cost. In all respects, the helical strips l I greatly increase the efficiency and value of the heat exchangers in which they are used; and in comparison with the results which they make possible, their cost is trivial.

The side edges of the strips [0 rest upon the surfaces of the fire tubes l8, and thus those side edges enable the strips to intercept all of the products of combustion and force them to scour the inner surfaces of the fire tubes l8. The leading ends of the strips H] are preferably spaced from the nozzles 24 a distance suflicient to permit the flames to impinge upon the tips of insulation. This keeps those tips red hot, thus enabling them to ignite unconsumed fuel components in the products of combustion. The air to support the secondary combustion of the fuel, as where part of the gas has been converted to carbon monoxide (CO) and must be converted to carbon dioxide (CO2) may be provided in the form of secondary air, as shown in Fig. 3, or it may be provided in the form of excess primary air. In either case, however, it will be found that the amount of air required, where the strips I!) are used, can be regulated more closely than when those strips are not used. This is partially due to the reduced velocity of the products of combustion, it is partially due to the scouring action of the products of combustion, and it is also partially due to the completeness of combustion effected by the tips of insulation; and that close regulation of air avoids the use of such excesses of air as would lower the combustion temperature.

Whereas a preferred embodiment of the present invention has been shown and described in the drawing and accompanying description it should be obvious to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.

What I claim is:

1. In a heat exchanger which comprises a fire tube, a tube sheet to support an end of said fire tube, a burner nozzle in register with and directed into said end of said fire tube, and a pipe to conduct fluid fuel to said nozzle for burning in said fire tube, the improvement that comprises a strip of metal extending longitudinally within said fire tube, said strip of metal having a tip of low heat conductive refractory material thereon in register with and opposite to said burner nozzle, said strip being substantially as wide as the inner diameter of said fire tube and extending rearwardly from said tip and away from said burner nozzle, said strip and said tip thereon being arranged diametrically of said fire tube and being in the path of products of combustion in said tube and coacting with said tube to divide the products of combustion into a plurality of masses, said tip igniting unconsumed fuel components in the products of combustion from the fuel directed into said fire tube by said burner nozzle, said tip comprising fiat, thin sheets of low heat conductive refractory material that are held in coplanar, abutting relation with, and that overlie, the leading end of said strip of metal to protect said one end of said strip of metal.

2. In a heat exchanger which comprises a fire tube, a tube sheet to support an end of said fire tube, a burner nozzle in register with and directed into said end of said fire tube, and a pipe to conduct fluid fuel to said nozzle for burning in said fire tube, the improvement that comprises a strip of metal extending longitudinally within said fire tube and being substantially as wide as the inner diameter of said fire tube, said strip of metal having a tip of low heat conductive refractory material thereon in register with and opposite to said burner nozzle to ignite unconsumed fuel components in the products of combustion from the fuel directed into said fire tube by said burner nozzle, said tip comprising flat, thin sheets of low heat conductive refractory material that are held in coplanar, abutting relation with, and that overlie, one end of said strip of metal to protect said one end of said strip of metal, said tip and strip being arranged in substantially bisecting relationship of said fire tube to force the products of combustion within said fire tube to divide into two masses and said strip being twisted along its length to force those masses to scour the inner surface of said fire tube.

3. In a heat exchanger which comprises a fire tube, a tube sheet to support an end of said fire tube, a burner nozzle in register with and directed into said end of said fire tube, and a pipe to conduct fluid to said nozzle for burning in said fire tube, the improvement that comprises a strip of metal extending longitudinally within said fire tube, said strip of metal having a tip of low heat conductive refractory material thereon in register with and opposite to said burner nozzle to ignite unconsumed fuel components in the products of combustion from the fuel directed into said fire tube by said burner nozzle, said tip comprising elongated flat, thin sheets of low heat conductive refractory material, said sheets of refractory material having the confronting surfaces at the rear ends thereof cut away, said sheets of refractory material being disposable in abutting co-planar relation so the cut-away confronting surfaces at the ends thereof form a recess, said sheets of refractory material being dimensioned so the recess formed by the cutaway confronting surfaces thereof receives and telescopes over the said one end of said strip of 1 metal, the forward ends of said sheets of refractory material projecting ahead of said one end of said strip of metal to protect the edge of said one end of said strip of metal, said strip being substantiall as wide as the inner diameter of said fire tube and extending diametrically of said fire tube.

4. In a heat exchanger which comprises a fire tube, a tube sheet to support an end of said fire tube, a burner nozzle in register with and directed into said end of said fire tube, and pipe to conduct fluid fuel to said nozzle for burning in said fire tube, the improvement that comprises a strip of metal extending longitudinally within said fire tube, said strip of metal having a tipof low heat conductive refractory material thereon in register with and opposite to said burner nozzle to catalytically ignite unconsumed fuel components in the products of combustion from the fuel directed into said fire tube by said burner nozzle, said tip comprising elongated, fiat, thin sheets of low heat conductive refractory material, said sheets of refractory material having the confronting surfaces at the rear ends thereof cut-away, said sheets of refractory material being disposable in abutting co-planar relation so the cut-away confronting surfaces at the ends thereof form a recess, said sheets of refractory material being dimensioned so the recess formed by the cut-away confronting surfaces thereof receives and telescopes over the said one end of said strip of metal, the forward ends of said sheets of refractory material projecting ahead of, and overlying, said one end of said strip of metal to protect the edge of said one end of said strip of metal, said strip having a plurality of turns or twists intermediate its ends.

5. An axially extended helicoidal strip of metal that is adapted to be positioned within a fire tube of a heat exchanger, said strip having curved, axially extended, helicoidal surfaces, said strip being adapted to be arranged diametrically of said fire tube to force the products of combustion within said tube to change direction from normal axial flow and to experience turbulence, and a tip of low heat conductive refractory material on one end of said strip of metal, said tip comprising flat, thin sheets of low heat conductive refractory material that are held in coplanar, abutting relation with, and that overlie, one end of said strip of metal to protect said one end of said strip of metal, said tip being adapted to be heated to incandescence to catalytically ignite unconsumed fuel components in the products of combustion adjacent said tip.

6. An axially extended helicoidal strip of metal that is adapted to be positioned within the fire tube of a heat exchanger, said strip being adapted to bisect said fire tube and having curved, axially extended, helicoidal surfaces adapted to force the products of combustion within said tube to change direction from normal axial flow and to experience turbulence, and a tip of low heat conductive refractory material that comprises thin sheets of low heat conductive refractory material which are attached to and supported on one end of said strip of metal, said tip enclosing the end of said strip of metal and extending ahead of said end of said strip of metal.

7. A formed strip of metal that is adapted to be positioned within the fire tube of a heat exchanger, said strip being adapted to bisect said fire tube and having twisted, axially extended curved surfaces adapted to force the products of combustion within said tube to change direction from normal axial flow and to experience turbulence, and a tip of low heat conductive refractory material on one end of said strip of metal, said tip comprising elongated, fiat, thin sheets of low heat conductive refractory material, said sheets of refractory material having the confronting surfaces at the rear ends thereof cut-away, said sheets of refractory material being disposable in abutting co-planar relation so the cut-away confronting surfaces at the ends thereof form a recess, said sheets of refractory material being dimensioned so the recess formed by the cut-away confronting surfaces thereof re ceives and telescopes over the said one end of said strip of metal, the forward ends of said sheets of refractory material projecting ahead of and overlying said one end of said strip of metal to protect the edge of said one end of said strip of metal, said tip of refractory material being adapted to keep said strip of metal from conduct ing away the heat of said products of combustion.

MINOR WOOLFOLK STOUT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,388,238 Chappelle Aug. 23, 1921 1,540,535 Burke June 2, 1925 1,875,856 Craver Sept. 6, 1932 1,918,935 Sellers July 18, 1933 2,035,970 Maclldowie Mar. 31, 1936 2,059,523 Hepburn et al. Nov. 3, 1936 2,067,133 Wales Jan. 5, 1937 2,348,901 I-Iandley May 16, 1944 2,379,017 McCollum June 26, 1945