Piping
The invention relates to piping for use as a pyrolysis tube in a cracking furnace. The tube is formed such that it has at least one section whose centreline curves in three dimensions, to induce swirl flow in the tube. Preferably, the tube is formed as a helix, more preferably a low-amplitude helix.
1. A cracking furnace with at least one pyrolysis tube passing through the furnace, the pyrolysis tube defining a flow passage with a cross-section that is substantially circular, wherein the flow passage of substantially circular cross-section has at least one pyrolysis tube portion having an internal diameter, the pyrolysis tube portion having a helical centerline that has an amplitude and curves in three dimensions, and wherein the amplitude of the helical centerline is less than or equal to the internal diameter of the pyrolysis tube portion.
2. A cracking furnace as claimed in claim 1 , wherein the curvature of the centerline varies along the length of the pyrolysis tube portion.
3. A cracking furnace as claimed in claim 1 , having a part with a helical curvature providing certain flow conditions where the tube enters the furnace, and a part with a helical curvature providing different flow conditions where the tube exits the furnace.
4. A cracking furnace as claimed in claim 1 , wherein an inside surface of the pyrolysis tube portion is substantially smooth, with no surface features.
5. A cracking furnace as claimed in claim 1 , wherein a cross-sectional area of the pyrolysis tube portion varies along the length thereof.
6. A cracking furnace as claimed in claim 1 , wherein the pyrolysis tube has a portion with a straight centerline and a portion with a centerline that is helical.
7. A cracking furnace as claimed in claim 6 , wherein the pyrolysis tube is “U” shaped, wherein the portion with a straight centerline is a straight leg of the pyrolysis tube, and wherein the portion with a helical centerline is another leg of the “U” shaped pyrolysis tube.
8. A cracking furnace as claimed in claim 7 , wherein the legs of the pyrolysis tube are joined by a two-dimensional bent portion.
9. A cracking furnace as claimed in claim 1 , wherein the amplitude of the helical centerline is less than or equal to one half of the internal diameter of the pyrolysis tube portion, so as to provide a line of sight along the flow passage defined by the pyrolysis tube portion.
10. A cracking furnace as claimed in claim 1 , wherein the amplitude of the helical centerline is greater than or equal to one half of the internal diameter of the pyrolysis tube portion.
11. A cracking furnace as claimed in claim 10 , wherein the helical centerline has a helix angle which is less than or equal to 20°.
12. A cracking furnace as claimed in claim 1 , wherein the pyrolysis tube is supported where it enters the furnace and where it exits the furnace and is otherwise unsupported.
13. A cracking furnace with at least one pyrolysis tube passing through the furnace, the pyrolysis tube defining a flow passage with a cross-section that is substantially circular, wherein the flow passage of substantially circular cross-section has at least one pyrolysis tube portion having an internal diameter, the pyrolysis tube portion having a helical centerline that has an amplitude and curves in three dimensions, and wherein the amplitude of the helical centerline is less than or equal to the internal diameter of the pyrolysis tube portion, and wherein the helical centerline has a helix angle that is less than or equal to 45°.