Furnace coil modified fins

Abstract

The present disclosure provides for thick fins on the surface of coils or tubes in a steam cracking furnace. The fins have a thickness at their base from ¼ to ¾ of the radius of the furnace tube. The fins have grooves or protuberances on not less than about 10% of a major surface. The fins help increase the radiant heat taken up by the tube from the walls and combustion gases in the furnace.

1 . A furnace tube having on its external surface one or more thick fins having a thickness at its base from ¼ to ¾ of the of the radius of said furnace tube and having parallel sides or sides with an upward inward taper of less than 15° relative to the major axis of said fin, said fin having on at least one major surface an array selected from: outwardly open grooves in a regular or semi-regular pattern covering at least 10% of the surface area, said grooves having a depth of less than a quarter of the maximum thickness of the fin; protuberances having a base 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. 
     
     
         2 . The furnace tube according to  claim 1 , wherein the array covers not less than one quarter of at least one major surface of the fin. 
     
     
         3 . The furnace tube according to  claim 2 , wherein the fin has a thickness at it base from ⅓ to the radius of the furnace tube. 
     
     
         4 . The furnace tube according to  claim 3 , wherein the fin has a cross section in the form of an outwardly extending parabola, parallelogram, an “E” shape, or a blunted “V”. 
     
     
         5 . The furnace tube according to  claim 4 , wherein the array comprises grooves having a depth from a eighth to a tenth of the maximum thickness of the fin. 
     
     
         6 . The furnace tube according to  claim 5 , wherein the grooves are in a form selected from an outwardly open V, a truncated outwardly open V, an outwardly open U, and an outwardly open parallel sided channel. 
     
     
         7 . The furnace according to  claim 3 , 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 ;   iii) a geometrical shape which has a relatively large external surface containing a relatively small volume.   
     
     
         8 . The furnace tube according to  claim 7 , wherein the protuberance has a shape selected from:
 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; and.   a section of a tear drop;   a section of a parabola.   
     
     
         9 . The furnace tube according to  claim 5 , wherein the fin forms a transverse plate in the form of a circle, ellipse, or an N sided polygon. 
     
     
         10 . The furnace tube according to  claim 7 , wherein the fin forms a transverse plate in the form of a circle, ellipse, or an N sided polygon. 
     
     
         11 . The furnace tube according to  claim 5 , wherein the fin is a longitudinal fin having a cross section in the form of an outwardly extending parabola, parallelogram, or an “E” shape. 
     
     
         12 . The furnace tube according to  claim 7 , wherein the fin is a longitudinal fin having a cross section in the form of an outwardly extending parabola, parallelogram, or an “E” shape. 
     
     
         13 . The furnace tube according to  claim 1 , wherein the furnace tube and the fin comprise the same metal composition. 
     
     
         14 . The furnace tube according to  claim 13 , 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. 
     
     
         15 . The furnace tube according to  claim 14 , 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 %. 
     
     
         16 . The furnace tube according to  claim 13 , 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 %. 
     
     
         17 . The furnace tube according to  claim 16 , 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 %. 
     
     
         18 . The furnace tube according to  claim 13 , comprising from 20 to 38 weight % of chromium from 25 to 48, weight % of Ni. 
     
     
         19 . The furnace tube according to  claim 18 , 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. 
     
     
         20 . A cracking furnace comprising a radiant section having furnace tubes according to  claim 1 . 
     
     
         21 . A method of cracking a paraffin comprising passing the paraffin in a gaseous state through the radiant section of a cracking furnace according to  claim 20 , at a temperature from 600° C. to 1000° C. for a time from 0.001 to 0.01 seconds.