HEAT TRANSFER TUBE AND METHOD FOR MANUFACTURING A HEAT TRANSFER TUBE

Abstract

The invention relates to a heat transfer tube (9) for falling film evaporation having a heating medium surface (21) to be heated by a heating medium, a falling film surface (20) to have spent liquor passing over it, and being made from an iron based high alloy stainless steel material with an alloy content above 16.00% for Chromium and above 1% for Nickel. The falling film surface of the heat transfer tube is equipped with at least one weld ridge (WR; WR.sub.1, WR.sub.2), said weld ridge having a height (h; h.sub.2) in the range 0.3 to 5.0 mm, a width (w; w.sub.2) in the range 0.5-15 mm, and an inclination angle (; .sub.1, .sub.2) versus a plane orthogonal to a longitudinal axis (CC) of the heat transfer tube in a range of 0-70 degrees so that each weld ridge is inclined and extends helically along at least a portion of the heat transfer tube or extend within a plane orthogonal to the longitudinal axis of the heat transfer tube and forms well ridge portions on the falling film surface such that the distance along the longitudinal axis of the heat transfer tube between adjacent weld ridge portions is within the range of 0 to 250 mm. The invention also relates to a method for manufacturing said heat transfer tube.

Claims

1. A heat transfer tube (9) for falling film evaporation of spent liquor, the heat transfer tube (9) having: a heating medium surface (21) arranged to be heated by a heating medium; and a falling film surface (20) opposite and facing away from said heating medium surface (21), which falling film surface (20) is arranged to have spent liquor containing lignin and other dissolved components from cellulosic material and/or inorganics from the cellulosic material and chemicals used passing over it as a falling film while evaporating solvent from the falling film and thus increasing the dry matter content; said heat transfer tube being made from a steel sheet material, e.g. an iron based high alloy stainless steel material with an alloy content above 16.00% for Chromium and above 1% for Nickel, preferably corresponding to corrosion resistant steel qualities at least like AISI 316 or AISI 304 characterized in that the falling film surface (20) of the heat transfer tube (9) is equipped with one or several welds forming a multitude of weld ridges (WR; WR.sub.1, WR.sub.2) spaced apart along the longitudinal axis (CC) of the heat transfer tube such that the distance along the longitudinal axis of the heat transfer tube between adjacent weld ridges is within the range of 0-250 mm; said weld ridges (WR; WR.sub.1, WR.sub.2) having a height (h; h.sub.2) in the range of 0.3-5.0 mm; said weld ridges (WR; WR.sub.1, WR.sub.2) having a width (w; w.sub.2) in the range of 0.5-15 mm; and said weld ridges (WR; WR.sub.1, WR.sub.2) having an inclination angle (; .sub.1 .sub.2) versus a plane orthogonal to a longitudinal axis (CC) of the heat transfer tube (9) in a range of 0-70 degrees.

2. A heat transfer tube (9) according to claim 2 wherein the distance along the longitudinal axis of the heat transfer tube between adjacent weld ridges is in the range 2-50 mm, preferably in the range 5-20 mm.

3. A heat transfer tube (9) according to claim 1, wherein the height (h; h.sub.2) of said weld ridges (WR; WR.sub.1, WR.sub.2) are in the range 0.5-2.0 mm, more preferably within the range 0.7-1.7 mm.

4. A heat transfer tube (9) according to claim 1, wherein said weld ridges (WR; WR.sub.1, WR.sub.2) are inclined in relation to said orthogonal plane.

5. A heat transfer tube (9) according to claim 4, wherein at least two weld ridges (WR.sub.1, WR.sub.2) are inclined in relation to said orthogonal plane and arranged crossing each other.

6. A heat transfer tube (9) according to claim 1, wherein each weld ridge (WR) extends within a plane orthogonal to the longitudinal axis (CC) of the heat transfer tube (9).

7. A heat transfer tube (9) according to claim 1, wherein at least one weld ridge is applied on the heating medium surface (21) of the heat transfer tube (9).

8. A heat transfer tube (9) according to claim 1, wherein the weld ridges (WR; WR.sub.1) are formed by one or several surface welds.

9. A heat transfer tube (9) according to claim 1, wherein the falling film surface (20) of the heat transfer tube (9) is equipped with plastically formed protrusions (P).

10. A heat transfer tube according to claim 1, wherein the falling film surface (20) of the heat transfer tube (9) is equipped with pins (Pi).

11. Method for manufacturing a heat transfer tube (9) for falling film evaporation of spent liquor, which method comprises the step of assembling the heat transfer tube (9) having: a heating medium surface (21) arranged to be heated by a heating medium; a falling film surface (20) opposite and facing away from said heating medium surface (21), which falling film surface (20) is arranged to have spent liquor containing lignin and other dissolved components from cellulosic material and/or inorganics from the cellulosic material and chemicals used passing over it as a falling film while evaporating solvent from the falling film and thus increasing the dry matter content, said heat transfer tube (9) being made from a sheet metal material, e.g. an iron based high alloy stainless steel material with an alloy content above 16.00% for Chromium and above 1% for Nickel, preferably corresponding to corrosion resistant steel qualities at least like AISI 316 or AISI 304 characterized in that the method comprises the step of applying one or several welds forming a multitude of weld ridges (WR; WR.sub.1, WR.sub.2) to the falling film surface (20) of the heat transfer tube (9) spaced apart along the longitudinal axis (CC) of the heat transfer tube such that the distance along the longitudinal axis of the heat transfer tube between adjacent weld ridges is within the range of 0-250 mm, said weld ridges (WR; WR.sub.1, WR.sub.2) having a height (h; h.sub.2) in the range 0.3-5.0 mm, said weld ridges (WR; WR.sub.1, WR.sub.2) having a width (w; w.sub.2) in the range 0.5-15 mm, and said weld ridges (WR; WR.sub.1, WR.sub.2) having an inclination angle (; .sub.1, .sub.2) versus a plane orthogonal to a longitudinal axis (CC) of the heat transfer tube (9) in a range of 0-70 degrees.

12. Method for manufacturing a heat transfer tube (9) according to claim 11, which method comprises the step of applying the one or several welds forming the weld ridges (WR; WR.sub.1, WR.sub.2) on the falling film surface (20) of an assembled heat transfer tube (9).

13. Method for manufacturing a heat transfer tube (9) according to claim 11, which method comprises the step of applying the one or several welds forming the weld ridges (WR; WR.sub.1, WR.sub.2) on the falling film surface (20) while forming a planar steel strip into said heat transfer tube (9).

14. Method for manufacturing a heat transfer tube (9) according to claim 11, which method comprises the step of applying the one or several welds forming the weld ridges (WR; WR.sub.1, WR.sub.2) on the falling film surface (20) of a planar steel strip before form shaping the strip to a tubular form and welding the edges of the steel strip together with a butt fusion weld.

15. Method for manufacturing a heat transfer tube (9) according to claim 11, which method comprises the step of applying the one or several welds forming the weld ridges (WR; WR.sub.2) on the falling film surface (20) of a steel strip while form shaping the planar steel strip to a tubular form by spiral shaping the planar steel strip and welding the edges of the steel strip together with a butt fusion weld, wherein the weld ridges (WR; WR.sub.2) are formed integrated with said butt fusion weld.

Description

DRAWINGS

[0036] The figures show preferred embodiments of the invention, wherein

[0037] FIGS. 1a and 1b shows in orthogonal cross section views a tube evaporator where spent liquor flows as a thin film on the outer surface of the heat transfer tubes;

[0038] FIG. 2 shows schematically an alternative tube evaporator wherein spent liquor flows as a thin film on the inner surface of the heat transfer tubes;

[0039] FIG. 3a shows a first embodiment of the inventive surface modification of the heat transfer tube with a single continuous surface weld applied, with an enlarged part shown in FIG. 3b, and FIG. 3c showing a cross section of the surface weld applied;

[0040] FIG. 4a shows a second embodiment of the inventive surface modification of the heat transfer tube with multiple parallel and continuous surface welds applied, with an enlarged part shown in FIG. 4b;

[0041] FIG. 5a shows a third embodiment of the inventive surface modification of the heat transfer tube with both a continuous surface weld and a crossing butt weld, with an enlarged part shown in FIG. 5b and FIG. 5c showing a cross section of the butt weld applied;

[0042] FIG. 6a shows a fourth embodiment of the inventive surface modification of the heat transfer tube with only a single continuous butt weld applied, with an enlarged part shown in FIG. 6b and FIG. 6c showing a cross section of the butt weld applied;

[0043] FIG. 7a shows a fifth embodiment of the inventive surface modification of the heat transfer tube with a butt weld applied and the surrounding surface equipped with ridges as shown in cross section in FIG. 7b; and FIG. 7c showing supplemental pins arranged on the falling film surface of the heat transfer tube; and

[0044] FIG. 8 a-c show a sixth embodiment of the inventive surface modification of the heat transfer tube with a multitude of closed circular surface welds applied, all arranged orthogonal to the longitudinal axis of the tube.

DETAILED DESCRIPTION

[0045] Throughout this description, a heating medium surface is a surface arranged to be heated by a heating medium, whereas a falling film surface is a surface arranged to have spent liquor passing over it as a falling film.

[0046] FIGS. 1a and 1b illustrate schematically a tube evaporator for evaporating spent liquor. The evaporator comprises a shell 1 containing a set 2 with multiple heat transfer tubes 9 arranged vertically in the shell 1.

[0047] FIG. 1 is seen in a cross-sectional view through the shell 1, with the heat transfer tubes 9 exposed. FIG. 1b is also seen in a cross-sectional view through the shell 1, but seen from the left-hand side of FIG. 1a. A liquid to be concentrated, in this case spent liquor, is fed through an inlet connection 3 into the shell 1, to the bottom thereof forming a volume of liquid with the surface level below the tubes 9. Liquor is discharged from the bottom of the evaporator through an outlet connection 4 and part of it is pumped by means of a schematically shown pump 5 through a circulating tube 6 into a distributing basin 7 above the set 2, from which basin it flows substantially evenly on steam distribution chambers 8 of the evaporating elements and from there further along outer falling film surfaces of separate heat transfer tubes 9 downwards. At the lower end of the heat transfer tubes 9, the concentrated spent liquor flows along the outer surface of steam collecting chambers 10 and falls subsequently into the liquor in the lower part of the shell 1 and mixes therewith.

[0048] To provide evaporation, vapor is led through the heat transfer tubes 9, and it is at first fed in through an inlet channel 11 in the upper part of the set 2 to steam distribution chambers 8 connected to upper parts of the heat transfer tubes 9. From there the vapor enters firstly a connecting chamber 12, which is connected to the upper collecting chambers 8 of the evaporating elements, so that the vapor is distributed through these evenly to all heat transfer tubes 9. Correspondingly, the remaining part of the vapor and condensate are collected, after having passed downwards along inner heating medium surfaces of the heat transfer tubes 9, in the steam collecting chambers 10 of the lower end of the evaporating elements, these collecting chambers being connected to a lower connecting chamber 13. From the lower part of the lower connecting chamber 13 starts an outlet channel 14 for condensate, through which channel the condensate is discharged, and respectively, from the upper part of the connecting chamber 13 starts an outlet channel 15 for vapor, through which channel the remaining heating vapor is exhausted. The water evaporated from the spent liquor under the influence of heating is exhausted as vapor through an outlet connection 16 at the upper end of the shell 1, and respectively, the concentrated liquor is bled off from the recirculation through a pipe 17. Inside the evaporator, in front of the outlet connection 16, there is further a mist separator 18 in such a way that water or liquor drops possibly contained in the exhaust vapor is caught on the mist separator and led back downwards. The mist separator is mounted to be enclosed by a closed housing 19 on each side so that all exhaust vapor must flow through the mist separator 18.

[0049] FIG. 2 illustrate schematically an alternative tube evaporator for evaporating spent liquor, with the difference that the spent liquor is flowing as a thin film on an inner falling film surface of the heat transfer tube. Details with same function as those shown in FIGS. 1 and 2 is given the same reference number. FIG. 2 is seen in a cross-sectional view through the shell 1, with only one of the heat transfer tubes 9 exposed. In a real evaporator are several tubes arranged in parallel, with a distance of about 1-4 centimeter between neighboring heat transfer tubes 9, and with a tube diameter in the range 2-10 centimeter. A spent liquor to be concentrated is fed through the shell 1, to the bottom thereof forming a volume of spent liquor with the surface level below the heat transfer tubes 9. Spent liquor is discharged from the bottom of the evaporator through an outlet connection 4 and part of it is pumped by means of a schematically shown pump 5 through a circulating tube 6 into a distributing basin 7. From the upper surface level of the basin is spent liquor flowing over the upper edge of the tube and onto the inner falling film surface of the heat transfer tube 9 as a thin film and further downwards. At the lower end of the heat transfer tube 9, the concentrated spent liquor falls into the volume of liquid. While flowing as a thin film over the inner falling film surface of the heat transfer tube 9 is the tube heated by a heating medium at the outer heating medium surface of the heat transfer tube 9, and the film is thus exposed to evaporation during passage. Heating media is supplied via inlet channel 11, and in the lower end is residual steam extracted via outlet channel 15 and clean steam condensate is drained off via outlet channel 14. The dirty steam evaporated from the spent liquor may be bled off via upper outlet connection 16a and lower outlet connection 16b, and preferably are condensate deflectors/mists separator 18 used. The concentrated liquor is bled off from the recirculation through a pipe 17. It should be noted that the heating media may also be steam evaporated from other evaporation stages, and in such cases would the condensate collected in outlet channel 14 not be classified as clean water, instead dirty condensate containing turpentine or other liquids that has a condensation temperature close to that established in the heating media chamber.

[0050] The invention may be used on both types of tube evaporators, i.e. where the spent liquor flows as a thin film on an outer falling film surface of the heat transfer tube, as shown in FIGS. 1a and 1b, and where the spent liquor flows as a thin film on an inner falling film surface of the heat transfer tube, as shown in FIG. 2.

[0051] In FIG. 3a is a first embodiment of a heat transfer tube 9 according to the invention shown. In this case is a continuous surface weld applied on an outer falling film surface 20 of the heat transfer tube 9 and forming a multitude of weld ridges spaced apart along the longitudinal axis of the heat transfer tube. It should be understood that the surface weld may be applied instead on the inner surface of a heat transfer tube, if the thin film of spent cooking liquor flows on said inner surface. FIG. 3b shows an enlarged portion of FIG. 3a, and FIG. 3c a detailed cross section of the surface weld.

[0052] In the figures is: [0053] CC the longitudinal axis of the heat transfer tube 9; [0054] d the distance between adjacent weld ridges WR perpendicular to the longitudinal extension of the weld forming the weld ridges WR; [0055] the inclination angle of the weld ridges WR versus a plane orthogonal to the center axis CC of the heat transfer tube 9, in this embodiment close to 15 degrees; [0056] The distance between adjacent weld ridges along the longitudinal axis is d divided by cos [0057] h the height of said weld ridges WR, measured orthogonally to the falling film surface 20, preferably in the range 0.3-5.0 mm; and [0058] w the width of said weld ridges WR, measured in the same plane as the falling film surface 20 and orthogonally to the longitudinal direction of the weld ridges WR, preferably in the range 0.5-15 mm.

[0059] FIG. 4a shows a second embodiment of the heat transfer tube 9 according to the invention, with multiple surface welds applied to form parallel and continuous weld ridges WR. FIG. 4b shows an enlarged part of the heat transfer tube, wherein: [0060] d is the distance between adjacent weld ridges WR perpendicular to the longitudinal extension direction of the welds forming the weld ridges WR, [0061] is the inclination angle of the weld ridges WR versus a plane orthogonal to the center axis CC of the heat transfer tube 9, in this embodiment about 45 degrees. [0062] The distance between adjacent weld ridges in a direction parallel to the longitudinal axis CC is calculated as d divided by cos

[0063] FIG. 5a shows a third embodiment of the heat transfer tube 9 according to the invention. A surface weld forms a multitude of first kind of weld ridges WR.sub.1, identical to the weld ridges WR shown in FIG. 3a, and a crossing butt weld forms a multitude of protruding second weld ridges WR.sub.2. FIG. 5b shows an enlarged part of the heat transfer tube 9, and FIG. 5c shows a cross section through one of the second kind of weld ridges WR.sub.2. The height h.sub.2 and width w.sub.2 of the second kind of weld ridges WR.sub.2 are of the same magnitude as for the first kind of weld ridge WR.sub.1, whereas the inclination angle .sub.2 is somewhat steeper for the second kind of weld ridges WR.sub.2 and the distance d.sub.2 between adjacent second kind of weld ridges WR.sub.2 perpendicular to the longitudinal extension direction of the welds forming the weld ridges WR.sub.2 is longer. The distance between adjacent weld ridges along the longitudinal axis is d.sub.2 divided by cos .sub.2.

[0064] FIG. 6a shows a fourth embodiment of the inventive surface modification of the heat transfer tube 9 with only a single continuous butt weld forming a multitude of weld ridges WR with an enlarged part shown in FIG. 6b disclosing the inclination angle and FIG. 6c showing a cross section of the butt weld. A spirally running metal strip has been butt welded to form said heat transfer tube 9. Consequently, the distance d between adjacent weld ridges WR is equal to the width of the metal strip. However, the distance between adjacent weld ridges along the longitudinal axis of the heat transfer tube is calculated as d divided by cos . The weld ridges WR are formed on an inner falling film surface of the heat transfer tube 9.

[0065] FIG. 7a shows a fifth embodiment of the heat transfer tube 9 according to the invention with a butt weld forming weld ridges WR at a falling film surface 20. The falling film surface 20 is also equipped with protrusions P and an opposite heating medium surface 21 is provided with corresponding grooves G. The protrusions P and grooves G extend orthogonally to said weld ridge WR on opposite sides of the heat transfer tube 9, as shown in cross section in FIG. 7b. The grooves and ridges are preferably formed with a trapezoidal cross section with sharp radius in corners, i.e. preferably with a radius less than 2 mm, and advantageously have the same height h.sub.p (ridges) and depth d.sub.g (grooves) and width w.sub.p, w.sub.g as the height h and width w of the weld ridge WR.

[0066] FIG. 7c shows a sixth embodiment of the invention, wherein supplemental pins Pi are arranged on the falling film surface 20 of the heat transfer tube 9. The pins may be butt fusion welded to said falling film surface 20. Pins may additionally or alternatively be arranged on the heating medium surface 21 to increase the surface area exposed to the heating medium. The pins Pi may be arranged in a net by means of thin fixation wires FW, fixing the distance between pins at equidistant distance between neighboring pins, before attaching the net on the surface by butt fusion welding. The pins Pi may be built from spirally wound wire.

[0067] FIG. 8a-c show a seventh embodiment of the heat transfer tube 9 according to the invention, wherein a multitude of closed circular surface welds forming weld ridges WR are applied to the outer falling film surface 20 of the heat transfer tube. All weld ridges WR are arranged orthogonal to the longitudinal axis CC of the heat transfer tube 9. The weld ridges are thus formed by welds forming closed circular rings, with a multitude of welded rings applied over the surface; and with [0068] a distance d between adjacent weld ridge portions WR perpendicular to the longitudinal extension direction of the welds forming the weld ridges WR, [0069] a height h, preferably in the range 0.3 to 5.0 mm; and [0070] a width w, preferably in the range 0.5-15 mm.

[0071] In this case will thus the distance between adjacent weld ridges along the longitudinal axis of the heat transfer tube be th same as d since the distance is calculated as d divided by cos , =0 and cos =1.

[0072] The scope of protection is not limited to the above described embodiments. The skilled person understands that the embodiments can be modified and combined in many different ways without parting from the scope of the invention. For example, the weld ridges, protrusions, grooves and pins in the figures may be discontinuous and they may be arranged on any of the inner and outer surfaces of the heat transfer tubes.