TECHNICAL FIELD

The present invention relates to the field of cracking paraffins to olefins and more particularly to substantial fins on the external surface of the process coil(s) in the radiant section of a cracking furnace. The fins may be transverse (horizontal) or longitudinal. The fins have an array selected from the group consisting of upwardly or outwardly open grooves having a depth of less than a quarter of the maximum thickness of the fin; or protuberances having a base not exceeding 10% of the maximum thickness of the fin, and a height not exceeding 15% of the maximum thickness of the fin or both, in a regular or semi-regular pattern covering at least 10% of the surface area of at least one major surface of the fin.

BACKGROUND ART

The field of heat exchanger designs is replete with applications of fins to improve the heat transfer. Typically this is heat transfer by forced convection mechanism. Heat transfer by forced convection takes place between a solid surface and fluid in motion, which may be gas or liquid, and it comprises the combined effects of conduction and convection. This type of heat transfer occurs in most of the conventional heating systems, either hot water or electric, and industrial heat exchangers.

In the cracking of a feed comprising paraffins, typically C2-4 paraffins, such as ethane, or naphtha, or mixtures thereof, the feed typically together with diluent steam is fed into a cracker comprising a series of pipes or tubes passing through several sections of a furnace. First the feed passes through the tubes in the convection section of the furnace where exhaust gasses flowing from the

downstream radiant section of the furnace heat the external surfaces of the tubes. There, the feed is heated to a temperature at or near the level at which cracking may begin. Then the feed flows to the tubes in the radiant section of the furnace where the tubes are primarily heated by radiation from the refractory walls of the furnace and from combustion gases generated by burners typically mounted in the floor or walls of the radiant section. Some forced convection heating of the tubes is also provided by the combustion gases. Feed is heated in the furnace radiant section up to a temperature of about 800 ^Q C - 950 ^Q C. At these temperatures, the feed undergoes a number of reactions, including a free radical decomposition (cracking), reformation of a new unsaturated product and the coproduction of hydrogen. These reactions occur over a very short period of time that corresponds to the feed residence time in a coil. The residence time is typically from about 0.01 to about 10 seconds, in some cases from 0.01 to 2 seconds in some cases from 0.01 to 1 second. The reactants may be heated to temperatures from 750 ^Q C to 950 ^Q C, in some cases from 800 ^Q C to 900 ^Q C at a pressure from 200 to 500 kPa in some cases from 250 kPa to 550 kPa.

The interior of the radiant section of the furnace is lined with heat

absorbing/radiating refractory, and is heated typically by gas fired burners.

The cracked gas exits the radiant section of a furnace and then passes through a transfer line exchanger to a quencher to rapidly cool the product stream to a temperature at which the reaction stops. The resulting product stream is then separated into various components such as ethylene, propylene etc.

There is a drive to improve the efficiency of cracking furnaces as this reduces process costs and greenhouse gas emissions. There have been two main approaches to improving efficiency: the first by improving heat transfer to the furnace coils, i.e. from flame, combustion gases and refractory walls to the external surface of a process coil; and the second by improving heat transfer within the coil, i.e. from the coil internal walls into the feed flowing inside the coil.

One of the methods representing the second approach, is the addition of internal fins to the inner walls of the furnace coil, to promote the "swirling" or enhanced mixing of the feed within the coil. This improves the convective heat transfer from the coil walls to the feed as the turbulence of the feed flow is increased and the heat transferring surface of the hot inner wall of the coil is increased as well.

United States Patent 5,950,718 issued Sept. 14, 1999 to Sugitani et al. assigned to Kubota Corporation provides one example of this type of technology.

The papers "Three dimensional coupled simulation of furnaces and reactor tubes for the thermal cracking of hydrocarbons", by T. Detemmerman, G.F.

Froment,(Universiteit Gent, Krijgslaan 281 , b9000 Gent - Belgium, mars-avri, 1998); and "Three dimensional simulation of high internally finned cracking coils for olefins production severity", by Jjo de Saegher, T. Detemmerman, G.F. Froment, (Universiteit Gentl , Laboratorium voor Petrochernische Techniek, Krijgslaan 281 , b-9000 Gent, Belgium, 1998 provide a theoretical simulation of a cracking process in a coil which is internally finned with helicoidal and longitudinal fins (or rather ridges or bumps). The simulation results are verified by lab scale experiments, where hot air flows through such internally finned tubes. The papers conclude that the tube with internal helicoidal fins performs better then with internal longitudinal fins and that the results for "a tube with internal helicoidal fins are in excellent agreement with industrial observations". However, no experimental data are provided to support these conclusions. There is also no comparison made to the performance of a bare tube, with no internal ribs or fins. The authors agree that one potential disadvantage of such coils with internal fins is that carbon deposits may build up on the fins, increasing the pressure drop through the tube.

United States Patent Application 20030015316 published Jan. 23, 2003 in the name of Burkay teaches a heat exchanger tube having internal fins and external fins. There is no teaching or suggestion in Burkay that the external fins should have additional grooves on their external surface. The patent application teaches away from the subject matter of the present application.

NOVA Chemicals United States Patent 7,128,139 issued Oct. 31 , 2006 teaches external annular fins on the cracking furnace coil to increase convection heat exchange to the coil. The patent fails to teach or suggest the fins have further grooves on the major external surface of the fins.

United States Patent 7,096,931 issued August 29, 2006 to Chang et al. assigned to ExxonMobil Research and Engineering Company teaches an externally finned heat exchanger tube in a slurry reaction (Fischer Tropsch synthesis). In the reaction, a slurry of CO and hydrogen in a hydrocarbyl diluent containing catalyst, flows over the external surface of heat exchanger tubes containing flowing cooling water. The heat exchanger tubes has ribs having an aspect ratio of less than 5. There is no teaching or suggestion in the patent that the fins have further grooves on their major external surface.

United States Patent Application 2012/0251407 published in the name of

Petela et al., assigned to NOVA Chemicals (International) S.A. teaches longitudinal fins on furnace tubes in the radiant section of a cracking furnace. The fins do not have grooves on their surface. Paragraph 54 teaches the thickness of the fin at its base. Typically the fin has a thickness at its base from 6% to 25% of the diameter of the tube, preferably from 7.5% to 15% of the diameter of the furnace tube.

United States Patent 8,790,602 issued July 29, 2014 to Petela et al., assigned to NOVA Chemicals (International) S.A. teaches furnace tubes or coils used in the radiant section of a cracking furnace having protuberances on their surface. The patent does not teach or suggest fins having protuberances on the surface of the coils used in the radiant section of the furnace.

United States Patent 7,743,821 issued June 29, 2010 to Bunker et al., assigned to General Electric Company teaches a heat exchanger tube having an annular fin which is dimpled, mechanically or molded, on at least a portion of its major surface. The heat exchanger is used to cool gas or air (i.e. air conditioners).

The heat exchanger is primarily concerned with convective heat exchange rather than radiant heat exchange. The heat exchanger is not comparable to the tubes in a cracking furnace. There is no written disclosure of the wall thickness of the heat exchanger tube, or the thickness of the fin. From the figures the dimples appear to be about a half to a third the thickness of the fin which is significantly greater than the maximum of one quarter of the thickness of the fin required in the present invention.

United States Patent 8,376,033 issued Feb. 19, 2013 to Robidou et al., assigned to GEA Batignolles Technologies Thermiques teaches a comparable fin in a convection heat exchanger except that the grooves are of diminishing depth from the base of the fin to the external edge. The patent teaches that the fin may have a thickness at its inner edge (base) from about 0.4 to 1 mm and a thickness at its outer edge from 0.15 to 0.4 mm (Col. 5 lines 25 -30). The patent also teaches that the grooves may have a depth (thickness) between 0.4 and 1 .5 mm. The grooves seem to have a thickness of about half the thickness of the fin. Again these fins are for convective heating and not for radiant heating as in a cracking furnace.

The present invention seeks to provide thick or substantial fins for furnace tubes having on at least one major surface an array selected from the group consisting of: upwardly or outwardly open grooves having a depth of less than a quarter of the thickness of the fin; or protuberances having a base with the main dimension not exceeding 10 % of the maximum thickness of the fin, and a height not exceeding 15% of the maximum thickness of the fin; or both, in a regular or semi-regular pattern covering at least 10% of the surface area of at least one major surface of said fin.

DISCLOSURE OF INVENTION

In one embodiment there is provided a furnace tube having on its external surface one or more thick fins having a thickness at its base from 1/4 to 3/4 of the of the radius of said furnace tube and having parallel sides or sides with an upward inward taper of less than 15 ^Q relative to the major axis of said fin, said fin having on at least one major surface an array selected from the group consisting of outwardly open grooves in a regular or semi-regular pattern covering at least 10% of the surface area of said grooves having a depth of less than a quarter of the maximum thickness of the fin; and protuberances having a base not exceeding 10% of the maximum thickness of the fin, and a height not exceeding 15% of the maximum thickness of the fin; or both in a regular or semi-regular pattern covering at least 10% of the surface area of at least one major surface of said fin.

In a further embodiment there is provided a furnace tube wherein the grooves have a depth from a eighth to a tenth of the maximum thickness of the fin.

In a further embodiment there is provided a furnace tube wherein the array of grooves covers not less than one quarter of at least one major surface of the fin.

In a further embodiment there is provided a furnace tube wherein the grooves are in the form of an outwardly open V, a truncated outwardly open V, an outwardly open U, and an outwardly open parallel sided channel.

In a further embodiment there is provided a furnace tube wherein the fin forms a transverse plate in the form of a circle, ellipse, or an N-sided polygon.

In a further embodiment the base of the fins has a thickness from a third to one half of the radius of the furnace tube.

In a further embodiment there is provided a furnace tube wherein the fin is a longitudinal fin having a cross section in the form of an outwardly extending parabola, parallelogram, or Έ" shape (monolith with parallel longitudinal channels) or a blunted "V".

In a further embodiment there is provided a furnace tube wherein the array of grooves covers not less than one quarter of at least one major surface of the fin.

In a further embodiment there is provided a furnace tube wherein the grooves have a depth from a eighth to a tenth of the maximum thickness of the fin. In a further embodiment there is provided a furnace tube wherein the grooves are in the form of an outwardly open V, a truncated outwardly open V, an outwardly open U, an outwardly open parallel sided channel.

In a further embodiment there is provided a furnace tube having horizontal fins being spaced apart at least two times the external diameter of the furnace tube.

In a further embodiment there is provided a furnace tube having longitudinal fins the base of said fins covering from one third to a half of the radius of the furnace tube.

In a further embodiment there is provided furnace a tube wherein the array comprises protuberances having:

i) a maximum height from 3 to 15% of the base of the fin;

ii) a contact surface with a fin, or a base, which main dimension is 0.1 %- 10% of the fin thickness; and

iii) a geometrical shape which has a relatively large external surface containing a relatively small volume.

In a further embodiment there is provided a furnace tube wherein the protuberance has a shape selected from the group consisting of:

a tetrahedron;

a Johnson square pyramid;

a pyramid with 4 isosceles triangle sides;

a pyramid with isosceles triangle sides;

a section of a sphere;

a section of an ellipsoid;

a section of a tear drop; and

a section of a parabola.

In a further embodiment there is provided a furnace tube

wherein the furnace tube and the fin comprise the same metal composition.

In a further embodiment there is provided a furnace tube, and fin(s) comprising from about 55 to 65 weight % of Ni; from about 20 to 10 weight % of Cr; from about 20 to 10 weight % of Co; and from about 5 to 9 weight % of Fe and the balance one or more of the trace elements.

In a further embodiment there is provided a furnace tube, and fin(s) further comprising from 0.2 up to 3 weight % of Mn; from 0.3 to 2 weight % of Si; less than 5 weight % of titanium, niobium and all other trace metals; and carbon in an amount of less than 0.75 weight % the sum of the components adding up to 100 weight %.

In a further embodiment there is provided a furnace tube, and fin(s) comprising from 40 to 65 weight % of Co; from 15 to 20 weight % of Cr; from 20 to 13 weight % of Ni; less than 4 weight % of Fe and the balance of one or more trace elements and up to 20 weight % of W the sum of the components adding up to 100 weight %.

In a further embodiment there is provided a furnace tube, and fin(s) further comprising from 0.2 up to 3 weight % of Mn; from 0.3 to 2 weight % of Si; less than 5 weight % of titanium, niobium and all other trace metals; and carbon in an amount of less than 0.75 weight %.

In a further embodiment there is provided a furnace tube, and fin(s) comprising from 20 to 38 weight % of chromium from 25 to 48, weight % of Ni.

In a further embodiment there is provided a furnace tube, and fin(s) further comprising from 0.2 up to 3 weight % of Mn, from 0.3 to 2 weight % of Si; less than 5 weight % of titanium, niobium and all other trace metals; and carbon in an amount of less than 0.75 weight % and the balance substantially iron.

In a further embodiment there is provided a cracking furnace comprising a radiant section having furnace tubes as above.

In a further embodiment there is provided a method of cracking a paraffin comprising passing the paraffin in a gaseous state through the radiant section of a cracking furnace as above at a temperature from 600 ^Q C to 950 ^Q C for a time from 0.001 to 0.01 seconds, and separating the resulting olefins from the feed and co- products.

The present invention also provides any and all combinations of the foregoing embodiments.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows a furnace tube with longitudinal fins of the present invention modified with grooves on the surface.

Figure 2 shows a fin of the present invention modified with protuberances of the present invention.

Figure 3 is a graph showing the per cent increase in the surface area of the fin modified with different protuberances of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Numbers ranges

Other than in the operating examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions, etc. used in the specification and claims are to be understood as modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the properties that the present invention desires to obtain. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical values, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of to 10" is intended to include all sub-ranges between and including the recited minimum value of 1 and the recited maximum value of 10; that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. Because the disclosed numerical ranges are continuous, they include every value between the minimum and maximum values. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations.

All compositional ranges expressed herein are limited in total to and do not exceed 100 percent (volume percent or weight percent) in practice. Where multiple components can be present in a composition, the sum of the maximum amounts of each component can exceed 100 percent, with the understanding that, and as those skilled in the art readily understand, the amounts of the components actually used will conform to the maximum of 100 percent.

As used in this specification the term outwardly when referring to the grooves is outward relative to the major plane of the fin which they are on. As used in this specification fin height refers to the distance the fin extends away from the external surface of the furnace tube.

In accordance with the present invention the furnace tubes have fins which have high integrity, good stress resistance and are quite thick. Typically, the fins will have a thickness at their base of not less than about 33% of the radius of the furnace tube, typically about 40%, desirably not less than about 45%, in some embodiments up to 50% of the radius of the tube. The fins are thick or stubby. They have a height to maximum width ratio of from about 0.5 to 5, typically 1 to 3. The sides (edges) of the fin may be parallel or be lightly tapered inward toward the external edge of the fin. The angle of taper should be no more than about 15 ^Q , typically about 10 ^Q or less inward relative to the center line of the fin. The edge of the fin may be flat, pointed (at a 30 ^Q to 45 ^Q angle from each surface), or have a blunt rounded nose. The fins may have a cross section shape in the form of an outwardly extending parabola, parallelogram, of a blunt "V" shape. In some cases, preferably for longitudinal fins, the fin cross section may be Έ" shaped (monolith with parallel longitudinal extensions (having parallel grooves).

In one embodiment at least one major surface of the fin has an array of outwardly open grooves in a regular or semi-regular pattern covering at least 10% of the surface area of at least one major surface of the fin (e.g. top or bottom for horizontal fins or sides for longitudinal fins), said grooves having a depth of less than a quarter, in some instances from a eighth to a tenth of the maximum thickness of the fin. The array may cover not less than 25%, in some cases not less than 50%, preferably greater than 75%, most preferably greater than 85 % up to 100% of the of the surface area of one or more the major surfaces of the fin. The array could be in the form of parallel lines, straight or wavy, parallel to or at an angle from the major axis of the fin, crossed lines, wavy lines, squares, or rectangles. The grooves may be in the form of an outwardly open V, a truncated outwardly open V, an outwardly open U, and an outwardly open parallel sided channel.

The fins may be transverse or parallel (e.g. longitudinal) to the major axis of the furnace tube. The transverse fins could be at an angle from about 0° to 25 ^Q off perpendicular relative to the major axis of the furnace tube. However, it is more costly and difficult to make transvers fins at an angle off perpendicular to the major axis of the tube. The transverse fins may have a shape selected from a circle, an ellipse, or an N sided polygon where N is a whole number greater than or equal to 3. In some embodiments N is from 4 to 12. The major surface(s) for the transverse fins are the upper and bottom face of the fin. Transverse fins should be spaced apart at least two times in some instances from 3 to 5 times, the external diameter of the furnace tube.

The longitudinal fins may have a shape of a parallelogram, a part of an ellipse or circle and a length from about 50% of the length of the furnace tube (sometimes referred to pass) in the radiant section up to 100% of the length of the furnace tube in the radiant section and all ranges in between.

The base of the longitudinal fin may be not less than one quarter of the radius of the furnace tube, in some instances from 1/4 to ¾ typically from about 1/3 to ¾ or in some instances1 /3 to 5/8 in other instances from 1 /3 to 1 /2 of the radius of the furnace tube. The fins are thick or stubby. They have a ratio of height to maximum width of from about 0.5 to 5, typically 1 to 3. The sides (edges) of the fin may be parallel or be lightly tapered inward toward the tip of the fin. The angle of taper should be no more than about 15 ^Q , typically about 10 ^Q or less inward relative to the center line of the fin. The tip or leading edge of the fin may be flat, tapered (at a 30 ^Q to 45 ^Q angle from the top and bottom surfaces of the fin), or have a blunt rounded nose. The leading edge of the longitudinal fin will typically be parallel to the central axis of the furnace tube. In cases where the fin extends less than 100% of the length of the furnace tube the leading edge of the fin will for the most part be parallel to the central axis of the furnace tube and then angle in to the furnace tube wall at an angle between about 60 ^Q and 30 ^Q typically 45 ^Q . In some case the fin may end in a flat surface perpendicular to the surface of the tube.

A furnace tube or pass having grooved fins will be described in accordance with figure 1 . The furnace tube 1 comprises a central channel 2 and an annular wall 3. The fins 4 and 5 in this embodiment are straight sided and do not angle or taper inwardly to the tips 6 and 7. The fins bear on their surface a series of parallel grooves-channels 10.

In a further embodiment of the invention the fins may comprise an array of protuberances.

Figure 2 shows a fin 20 of the present invention having its surface 21 covered with one or more protuberances. The protuberances may be in the shape of a square pyramid 23, an equilateral cone 24 or a hemisphere 25. The protuberances may be applied by casting or machining the fin, or by using a knurl roll so that the surface 21 of the fin has a textured surface.

The array of protuberances can cover from 10% to 100% (and all ranges in between) of the external surface of the fin. In some embodiments of the invention, the protuberances may cover from 40 to 100%, typically from 50% to 100%, generally from 70% to 100% of the external surface of the fin radiant coil. If protuberances do not cover the entire surface of the fin, they can be located at the bottom, middle or top of the fin.

A protuberance base is in contact with the external coil surface. A base of a protuberance has an area not larger than from 0.1 %-10% of the maximum thickness of the fin. Preferably, the protuberance have geometrical shapes having a relatively large external surface that contains a relatively small volume, such as for example tetrahedrons, pyramids, cubes, cones, a section through a sphere (e.g. hemispherical or less), a section through an ellipsoid, a section through a deformed ellipsoid (e.g. a tear drop), etc. Some useful shapes for a protuberance include: a tetrahedron (pyramid with a triangular base and 3 faces that are equilateral triangles);

a Johnson square pyramid (pyramid with a square base and sides which are equilateral triangles);

a pyramid with 4 isosceles triangle sides;

a pyramid with isosceles triangle sides (e.g. if it is a four faced pyramid the base may not be a square it could be a rectangle or a parallelogram);

a section of a sphere (e.g. a hemi sphere or less);

a section of an ellipsoid (e.g. a section through the shape or volume formed when an ellipse is rotated through its major or minor axis);

a section of a tear drop (e.g. a section through the shape or volume formed when a non uniformly deformed ellipsoid is rotated along the axis of deformation); and

a section of a parabola (e.g. section though the shape or volume formed when a parabola is rotated about its major axis - a deformed hemi-(or less) sphere), such as e.g. different types of delta-wings.

The selection of the shape of the protuberance is largely based on the ease of manufacturing the fin. One method for forming protuberances on the fin surface is by casting in a mold having the shape of the protuberance in the mold wall. This is effective for relative simple shapes. The protuberances may also be produced by machining the external surface of a cast fin such as by the use of knurling device for example a knurl roll.

The above protuberances are closed solids.

A protuberance may have a height (l_ _z ) above the surface of the fin from 3% to 15% of the maximum thickness of the fin, and all the ranges in between, preferably from 3% to 10% of the maximum thickness of the fin.

In some embodiments the concentration of the protuberances is uniform and essentially covers the external surface of the fin. However, the concentration may also be selected based on the radiation heat flux at the location of the coil pass (e.g. some locations may have a higher heat flux than others - corners).

In designing the protuberances care must be taken so that they adsorb more radiant energy than they may radiate. This may be restated as the transfer of heat through the base of the protuberance into the fin surface must exceed that transferred to the equivalent surface on a bare smooth fin at the same operational conditions. If the concentrations of the protuberances become excessive and if their geometry is not selected properly, they may start to reduce heat transfer, due to thermal effects of excessive conductive resistance, which defeats the purpose of the invention. The properly designed and manufactured protuberances will increase net radiative and convective heat transferred to a fin, and subsequently to a coil from surrounding flowing combustion gasses, flame and furnace refractory. The positive impact of protuberances on radiative heat transfer is not only because more heat can be absorbed through the increased fin external surface so the contact area between combustion gases and fin is increased, but also because the relative heat loss through the radiating fin surface is reduced, as the fin surface is not smooth any more. Accordingly, as a protuberance radiates energy to its surroundings, part of this energy is delivered to and captured by other

protuberances, thus it is re-directed back to the fin surface. The protuberances will also increase the convective heat transfer to a fin, due to increase in fin external surface that is in contact with flowing combustion gas, and also by increasing turbulence along the fin surface, thus reducing the thickness of a gaseous boundary layer adjacent to the fin surface.

Figure 3 is a plot of the percent increase in the area of the surface 21 of the fin 20 when the protuberances are an equilateral pyramid 26, a square pyramid 23, an equilateral cone 24 and a hemisphere 25, having a main dimension 'a' (side length of a pyramid or diameter for a cone or hemisphere) in mm.

The size of the protuberance must be carefully selected. Generally, the smaller the size, the higher is the surface to volume ratio of a protuberance, but it may be more difficult to cast or machine such a texture. In addition, in the case of excessively small protuberances, the benefit of their presence may become gradually reduced with time due to settlement of different impurities on the fin surface. However, the protuberances need not be ideally symmetrical. For example an elliptical base could be deformed to a tear drop shape, and if so shaped preferably the "tail" may point down, in line with the overall direction of flue gas flow, when the coil is positioned in the furnace.

Another important advantage of the fins with grooves or protuberances is that although the fin has the increased contact surface, its weight might be reduced.

The fins and the furnace tube may comprise the same material. In some embodiments the fins are easiest to cast as part of the furnace tube. In other embodiments the fins may be cast separately and welded in place.

The tube and the fin(s) may comprise from about 55 to 65 weight % of Ni; from about 20 to 10 weight % of Cr; from about 20 to 10 weight % of Co; and from about 5 to 9 weight % of Fe and the balance one or more of the trace elements. The alloy from which the tube and fins are made may further comprising from 0.2 up to 3 weight % of Mn; from 0.3 to 2 weight % of Si; less than 5 weight % of titanium, niobium and all other trace metals; and carbon in an amount of less than 0.75 weight % the sum of the components adding up to 100 weight %.

The furnace tube and fins may comprise from 40 to 65 weight % of Co; from

15 to 20 weight % of Cr; from 20 to 13 weight % of Ni; less than 4 weight % of Fe and the balance of one or more trace elements and up to 20 weight % of W the sum of the components adding up to 100 weight %. The alloy from which the furnace tube and fins are made may further comprise from 0.2 up to 3 weight % of Mn; from 0.3 to 2 weight % of Si; less than 5 weight % of titanium, niobium and all other trace metals; and carbon in an amount of less than 0.75 weight % the sum of the components adding up to 100 weight %.

The furnace tube and fins may comprise from 20 to 38 weight % of chromium from 25 to 48, weight % of Ni. The alloy from which the furnace tube and fins may be made may further comprise from 0.2 up to 3 weight % of Mn, from 0.3 to 2 weight % of Si; less than 5 weight % of titanium, niobium and all other trace metals; and carbon in an amount of less than 0.75 weight % and the balance substantially iron, the sum of the components adding up to 100 weight %.

The grooves or protuberances could be machined on the surface of the cast fin. In some embodiments it is preferred to cold roll (at a temperature below the recrystallization temperature of the steel) the fin to produce the grooves

/protuberances without removing any material. This may be particularly useful where the fins are substantially flat.

The grooves or protuberances could be in a geometric pattern such as longitudinal or transverse parallel lines, diagonal lines, a cross hatch pattern, squares, rectangles, circles, ellipses, etc. The pattern could be regular or semi- regular.

INDUSTRIAL APPLICABILITY

The invention provides furnace tubes for cracking paraffin to olefins having an improved efficiency.