Pinned furnace tubes

Abstract

In an embodiment of the invention, furnace tubes for cracking hydrocarbons having a longitudinal array of pins having i) a maximum height from 0.5-1.3 cm; ii) a contact surface with the tube, having an area from 0.1%-10% of the tube external surface area iii) a uniform cross section along the length of the pin. (i.e. they are not tapered); and iv) a length to diameter ratio from 1.5:1 to 0.5:1 have an improved heat transfer over bare fins and reduced stress relative to a fined tube.

Claims

1. A tube for use in the radiant section of a furnace for cracking hydrocarbons to produce olefins having on its exterior surface a series of pins in one or more linear arrays parallel to the longitudinal axis of the tube, said pins having: i) a maximum height from 0.8 to 1 cm; ii) a contact surface with the tube, having an area from 0.1% to 10% of the tube external surface area; iii) a uniform cross section along the length of the pin; iv) length to diameter ratio from 1.5:1 to 0.5:1; v) a distance between consecutive pins within a given linear array is from 0.1 to 5 times the diameter of the pin; vi) from 2 to 6 linear arrays of pins radially spaced from 180 to 20 apart; and vii) the aggregate weight of the pins comprises from 5 wt. % to 25 wt. % of the weight of the tube.

2. The tube according to claim 1, wherein the pins have a maximum length from 3% to 15% of the tube outer diameter.

3. The tube according to claim 2 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.

4. The tube according to claim 3 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 %.

5. The tube according to claim 2, 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 %.

6. The tube according to claim 5, 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 %.

7. The tube according to claim 2, comprising from 20 to 38 weight % of chromium from 25 to 48, weight % of Ni.

8. The tube according to claim 7, 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.

9. The tube according to claim 3, wherein the cross section of the pin is round.

10. The tube according to claim 5, wherein the cross section of the pin is round.

11. The tube according to claim 7, wherein the cross section of the pin is round.

12. The tube according to claim 4, wherein the cross section of the pin is round.

13. The tube according to claim 3, wherein the cross section of the pin is quadrilateral.

14. The tube according to claim 5, wherein the cross section of the pin is quadrilateral.

15. The tube according to claim 7, wherein the cross section of the pin is quadrilateral.

16. The tube according to claim 4, wherein the cross section of the pin is quadrilateral.

17. Tube according to claim 2, wherein the pins in a linear array are of uniform height.

18. The tube according to claim 2, where in the spacing between pins in a linear array is from 0.5 to 5 times the diameter of the pin.

19. The tube according to claim 2, wherein the pins in a linear array are of different heights to provide a profile to the array.

20. The tube according to claim 19, wherein at least part of the profile is a taper.

21. The tube according to claim 2, where in the central axis of the pin is at an angle from 90 to 60 relative to the external surface of the tube.

22. A method for making a tube according to claim 1, electrically welding a strip of any stud shaped material to the surface of the tube and then cutting the strip at the desired length.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing of an ethylene cracker.

(2) FIG. 2 shows the geometry of a single longitudinal vertical fin with rectangular cross section.

(3) FIG. 3 shows an axial fin temperature distribution with increasing height of the fin.

(4) FIGS. 4A, 4B and 4C show a half cross section of bare tube (FIG. 4A), axial finned tube (FIG. 4B), and pinned tube (FIG. 4C).

(5) FIGS. 5A, 5B and 5C show the outside wall stress distribution for bare tube (FIG. 5A), axial finned tube (FIG. 5B), and pinned tube (FIG. 5C).

(6) FIGS. 6A, 6B and 6C show the inside wall stress distribution for bare tube (FIG. 6A), axial finned tube (FIG. 6B), and pinned tube (FIG. 6C).

EXAMPLE

(7) The present invention will now be illustrated by the following non limiting example.

Example 1

(8) A finite element model of the ethylene 1 furnace tubes was performed in ANSYS Mechanical 14.0. This is a commercial finite element analysis (FEA) software used to create numerical models for stress/strain and heat transfer analysis.

(9) Prior to performing a FEA analysis a heat transfer model of a rectangular fin (FIG. 2) was created for a one-dimensional heat distribution. The net heat conducted through the fin is equal to heat transferred to the fin external surface from surroundings,

(10) $Q_{x+\mathrm{dx}}-Q_x=Q_{}\text{:}$$\frac{d}{\mathrm{dx}}\left(A\frac{d}{\mathrm{dx}}\right)=\frac{O}{}$

(11) Where =t.sub.gt.sub.xthe temperature difference between combustion gases, t.sub.g, and local temperature in the fin, t.sub.x, at location x (0xLz)

(12) i) O=2(L.sub.s+L.sub.h)the perimeter of the cross section of the rectangular fin,

(13) thermal conductivity of the fin material,

(14) total heat transfer coefficient (=.sub.rad+conv)

(15) from the above equation.

(16) $\frac{d^2}{{\mathrm{dx}}^2}=B^2$$B\sqrt{\frac{O}{A}}$

(17) The general solution of this equation takes the form:
.sub.x=C.sub.1e.sup.Bx+C.sub.2e.sup.Bx
where the constants C.sub.1 and C.sub.2 are determined from two boundary conditions:

(18) $\mathrm{for}x=0={}_p=C_1+C_2$$\mathrm{for}x=L_z={}_k=C_1{e}^{\mathrm{BL}}+C_2{e}^{-\mathrm{BL}}$$\mathrm{and},Q_{}=Q_{}=-A{\left(\frac{d}{\mathrm{dx}}\right)}_{x=\mathrm{Lz}}=A{}_k$

(19) So, after calculating C.sub.1 and C.sub.2, the temperature distribution in the fin takes the form:

(20) ${}_x={}_p=\frac{\cosh \left[B\left(L_z-x\right)\right]+\frac{}{B}\sinh \left[B\left(L_z-x\right)\right]}{\cosh {\mathrm{BL}}_z+\frac{}{B}\sinh {\mathrm{BL}}_z}$

(21) This temperature distribution is shown in FIG. 3 for a base temperature of 900 C. which was used for generating temperature loads on the axial finned tube.

(22) A static structural FEA was performed on three different furnace tubes; a bare tube, an axial finned tube, and a pinned tube. Half models were created with symmetric boundary conditions. A cross section of each of the tubes is shown in FIGS. 4A, 4B and 4C. The temperature distribution described above was applied to the external surface of the finned and pinned tube. Since the above heat transfer analysis was not performed for a pinned tube, the external surfaces of the pinned tube were assumed to follow the same distribution. An average process temperature of approximately 750 C. and an average convective heat transfer coefficient of 998 W/m.sup.2K were used to define the thermal boundary condition on the inner surface of the tube. Both gravity and an internal tube pressure of 0.336 MPa were also applied to the furnace tube model. The temperature distribution described above was determined for an axial finned tube and the assumption was made that the distribution would be similar in a pinned tube.

(23) External and internal stress distributions are shown in FIGS. 5A, 5B and 5C and FIGS. 6A, 6B and 6C. As seen in these figures, the finned furnace tube is in a much higher state of stress than the bare furnace tube. The difference in thermal expansion of the tip and base of the axial fin causes the base tube to be put in a high state of tension.

(24) The advantage of the pinned tube is that it is not constrained in any direction and is free to expand. There is a slight stress concentration at the base of the pin; however the overall state of stress is much lower than that of the axial finned tube. The overall state of stress in the furnace tube is comparable to that of a bare tube. However, there is an increase in heat transfer in the pinned tube over the bare tube.