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 one or several protrusions/indentations forming a multitude of stamped bumps (SB) on the envelope surface of a heat transfer tube such that the distance between adjacent stamped bumps (SB) along a line on the envelope surface parallel to the longitudinal axis of the heat transfer tube is within the range of 3 to 250 mm, said stamped bumps (SB) having a height (hp) in the range 0.3 to 5.0 mm, a width (wp) in the range 1.0-20 mm, and an inclination angle (a) versus a plane orthogonal to a longitudinal axis (CC) of the heat transfer tube in a range of 0-70 degrees so that each stamped bump (SB) is inclined and extends 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. 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 water solutions and suspensions emanating from handling of materials of biological origin, e.g. spent liquor from production of cellulose pulp, 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 sheet metal material, e.g. an iron based stainless steel material having 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 a multitude of stamped bumps (SB) formed by protrusions or indentations, wherein each stamped bump being spaced apart along the longitudinal axis (CC) of the heat transfer tube from a neighbouring stamped bump (SB) by 3-250 mm; said stamped bumps (SB) having a height (h.sub.p) or depth in the range of 0.3-5.0 mm; said stamped bumps (SB) having a width (w.sub.p) in the range 1.0-20 mm; and said stamped bumps (SB) having an inclination angle () 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 1 wherein said stamped bumps (SB) are stretching continuously from one side of the sheet metal plate to the other side of the sheet metal plate of which the tube is made.
3. A heat transfer tube (9) according to claim 1 wherein said stamped bumps (SB) are protruding on the falling film surface (20).
4. A heat transfer tube (9) according to claim 1, wherein the distance along the longitudinal axis of the heat transfer tube between adjacent stamped bumps is in the range 3-100 mm, more preferably within the range 5-50 mm and most preferably within the range 5-20 mm.
5. A heat transfer tube (9) according to claim 1, wherein the height (h.sub.p) of said stamped bumps (SB) are in the range 0.5-2.0 mm, preferably within the range 0.7-1.7 mm.
6. A heat transfer tube (9) according to claim 1, wherein at least one stamped bump (WB) is inclined in relation to said orthogonal plane, preferably with an inclination angle of 15-60 degrees.
7. A heat transfer tube (9) according to claim 1, wherein at least one stamped bump (WB) extends within a plane orthogonal to the longitudinal axis (CC) of the heat transfer tube (9).
8. A heat transfer tube (9) according to claim 1, wherein said stamped bumps (SB) are designed to have a corner roundness (Rc) of both sides of the top and the side walls of the protruding stamped bump (SB) of 0.2-2.0 mm.
9. A heat transfer tube (9) according to claim 1, wherein the side walls of said stamped bumps (SB) are designed to have an inclination angle between 26-90 degrees, preferably between 45-80 degrees, relative the base surface.
10. A heat transfer tube (9) according to claim 1, wherein said stamped bumps (SB) have the shape of an essentially symmetric trapezoid.
11. A heat transfer tube (9) according to claim 1, wherein said stamped bumps (SB) have the shape of an unsymmetrical trapezoid with a less inclined side wall on the leading side, i.e. the front side facing the film flow, compared to a more inclined side wall on the trailing side, i.e. the back side.
12. Method for manufacturing a heat transfer tube (9) for falling film evaporation of water solutions and suspensions emanating from handling of materials of biological origin, e.g. spent liquor from production of cellulose pulp, 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 stainless steel material, characterized in that the method comprises the step of forming one or several protrusions/indentations on an envelope surface of the heat transfer tube in order to form a multitude of stamped bumps (SB) on the falling film surface (20) of the heat transfer tube (9) whereby neighbouring stamped bumps along a line parallel to the longitudinal axis (CC) of the heat transfer tube are spaced apart from each other by 3-250 mm along the longitudinal extension of the heat transfer tube ; said stamped bump (SB) having a height (h.sub.p) or depth in the range 0.3-5.0 mm, said stamped bumps (SB) having a width (w.sub.p) in the range 1-20 mm; and said stamped bumps (SB) having an inclination angle () versus a plane orthogonal to a longitudinal axis (CC) of the heat transfer tube (9) in a range of 0-70 degrees.
13. Method for manufacturing a heat transfer tube (9) according to claim 12, which method comprises the step of shaping the one or several protrusions/indentations forming the stamped bumps (SB) on the falling film surface (20) onto a flat metal sheet before or while forming a flat sheet metal strip into said heat transfer tube (9).
14. Method for manufacturing a heat transfer tube (9) according to claim 13, which method comprises the feature of forming the stamped bumps (SB) on the flat sheet metal strip such that the stamped bump stretches all the way from one side of the flat sheet metal strip to another side of the flat metal sheet strip.
15. Method for manufacturing a heat transfer tube (9) according to claim 12 which method comprises the feature of shaping flat metal sheet strip to a tubular form by spiral shaping and welding the edges of the flat metal sheet strip together with a butt fusion weld.
Description
Drawings
[0037] The figures show preferred embodiments of the invention, wherein
[0038] 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;
[0039] 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;
[0040] FIG. 3a shows a first embodiment of the inventive surface modification of the heat transfer tube with an overall continuous helical shape and constant pitch of the stamped bumps on the envelope surface of the heat transfer tube, with an enlarged part shown in FIG. 3b, and FIG. 3c showing a cross section of the stamped bump formed;
[0041] FIG. 4a shows a second embodiment of the inventive surface modification of the heat transfer tube with multiple parallel and continuous stamped bumps being inclined on the envelope surface of the heat transfer tube, with an enlarged part shown in FIG. 4b; and
[0042] FIG. 5; shows a third embodiment of the inventive surface modification of the heat transfer tube with a multitude of circular stamped bumps, all arranged orthogonal to the longitudinal axis on the envelope surface of the heat transfer tube;
[0043] FIG. 6; discloses cross sectional views of alternative symmetries of stamped bumps;
[0044] FIG. 7; discloses a detailed cross-sectional view of a stamped bump having a symmetric trapezoidal shape; and
[0045] FIG. 8; discloses different patterns of and shapes of the contours of stamped bumps on the sheet metal material forming the envelope surface of the heat transfer tube.
DETAILED DESCRIPTION
[0046] 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.
[0047] 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.
[0048] 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 figure la. 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.
[0049] 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 vapours must flow through the mist separator 18.
[0050] 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 are 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 centimetres between neighbouring heat transfer tubes 9, and with a tube diameter in the range 2-10 centimetres. 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.
[0051] 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.
[0052] In FIG. 3 is a first embodiment of a heat transfer tube 9 according to the invention shown. FIG. 3a shows schematically a view of a heat transfer tube 9. FIG. 3b shows an enlarged portion of FIG. 3a, and FIG. 3c shows a cross sectional view of the stamped bump.
[0053] In the figures is: [0054] CC the longitudinal axis of the heat transfer tube 9 and D the diameter of the heat transfer tube; [0055] U corresponds to the upper end of the heat transfer tube and L corresponds to the lower end of the heat transfer tube; [0056] d the distance between adjacent stamped bumps SB perpendicular to the longitudinal extension direction of the continuous protrusion P/indentation forming the stamped bumps SB; [0057] the inclination angle of the stamped bump SB versus a plane orthogonal to the center axis CC of the heat transfer tube 9, in this embodiment close to 15 degrees; [0058] The distance D between adjacent stamped bumps along the longitudinal axis of the heat transfer tube is d divided by cos . corresponds to the angle between D and d; [0059] h.sub.p is the height of the protrusion P of the stamped bump SB measured orthogonally to and relative the falling film surface 20 and d.sub.i is the depth of the corresponding indentation I measured orthogonally to and relative the heating media surface 21; and [0060] w.sub.p is the width of the indentation I measured along the falling film surface 20 and orthogonally to the longitudinal direction of the stamped bump SB and w.sub.i is the width of said stamped bump SB measured in the same plane as the heating media surface 21 and orthogonally to the longitudinal direction of the stamped bump SB.
[0061] In FIG. 3a is disclosed a heat transfer tube 9 having a continuous protrusion P/indentation I forming a helical pattern on the envelope surface of the heat transfer tube 9. The helical pattern will thus form a multitude of stamped bumps SB crossing an imaginary line running along the envelope surface of the heat transfer tube 9 from one end to another parallel to the longitudinal axis CC. As disclosed in FIG. 3a, the continuous and helically shaped pattern from the protrusion P/indentation I running on the envelope surface of the heat transfer tube should correspond to 18 stamped bumps SB distributed along the envelope surface which intersects or passes an imaginary line on the envelope surface running from the upper end U to the lower end L of the heat transfer tube 9 parallel to the longitudinal axis CC. The pattern, or texture, on the envelope surface is preferably made such that the protrusion P is protruding on a falling film surface 20 of the heat transfer tube 9 and forming a multitude of protruding stamped bumps SB spaced apart along the longitudinal axis of the heat transfer tube. Consequently, there will be a corresponding indentation I on the opposite side, i.e. the heating media surface 21 if the protrusion P is on the falling film surface 20, and there will thus be a corresponding number of indentations or inverted stamped bumps SB on this opposite side. It should be understood that the continuous protrusion P/indentation I may be formed such that the stamped bumps SB instead are protruding on the heating media surface 21 instead of on the falling film surface. However, if it is desired to have the stamped bumps protruding on the falling film surface 20, the stamped bumps shall be oriented to be on an outer falling film surface 20 of the heat transfer tube as shown in FIGS. 1a and 1b on an inner falling film surface of the heat transfer tube as shown in FIG. 2.
[0062] The protrusion P and indentation I are formed with a trapezoidal cross section with sharp radius in corners, i.e. preferably with a radius less than 2 mm, and advantageously have essentially the same height h.sub.p of the protrusion P as the depth d, of the indentation I and essentially the same width w.sub.p of the protrusion P as the width w, of the indentation I. The height and depth is preferably in the range of 0.5-2.0 mm and the width in the range of 1.5 to 7 mm. The trapezoidal shape disclosed in the figure is symmetric, i.e. the side walls have the same inclination and length on both sides.
[0063] FIG. 4 shows a second embodiment of the heat transfer tube 9 according to the invention. FIG. 4a shows schematically the heat transfer tube 9 according to the invention, with multiple elongated protrusions/indentations forming parallel, inclined stamped bumps SB. FIG. 4b shows an enlarged part of the heat transfer tube, wherein: [0064] d is the distance between adjacent stamped bumps SB perpendicular to the longitudinal extension direction of the protrusions/indentations forming the stamped bumps SB, [0065] is the inclination angle of the stamped bumps SB versus a plane orthogonal to the center axis CC of the heat transfer tube 9, in this embodiment about 45 degrees, and [0066] D is the distance between adjacent stamped bumps in a direction parallel to the longitudinal axis CC and is calculated as d divided by cos
[0067] The shape and dimensions of the stamped bumps SB may be the same as described in FIG. 3.
[0068] FIG. 5a-b show a third embodiment of the heat transfer tube 9 according to the invention, wherein a multitude of stamped bumps SB are formed by circular protrusions P and indentations I extending along the circumference of the envelope surface of the heat transfer tube 9. The features in FIG. 5 are denoted by the same reference numbers or letters as used in FIG. 4. The protrusions P/indentations I forming the stamped bumps SB are arranged orthogonal to the longitudinal axis CC of the heat transfer tube 9. The stamped bumps SB are thus formed by the closed, ring shaped pattern textured in the sheet metal material forming the envelope surface of the heat transfer tube. In this case will the distance D between adjacent stamped bumps SB along the longitudinal axis CC of the heat transfer tube be the same as the distance d, which is the distance between the stamped bumps SB orthogonal to their longitudinal extension, since when =0 will cos be cos 0=1 why the distance D=d/cos =d/1=d.
[0069] In FIG. 6 is disclosed different shapes of the cross-sectional area of the protrusions/indentations orthogonally to the longitudinal extension of the protrusions/indentations used to form the stamped bumps SB. The stamped bumps SB have been designed such that the protruding portion is located on the surface where the film flow FF is intended to flow and the indentations are facing the side where the heating media HM is intended to flow.
[0070] In FIG. 6a is disclosed an asymmetric trapezoidal shape of the stamped bump SB. In this case is the asymmetric trapezoid stamped bump SB designed such that the side with a less inclined side wall is located on the leading side of trapezoid bump, i.e. the front side facing the film flow. Hence, by having a less inclined side facing the film flow FF will there be less risk for solid matter to get stuck and thus a reduced risk of formation of fouling and scaling due to fibres and lignin getting stuck the surface. The other side wall located on the trailing side, .i.e. the back side, is more inclined. A more inclined back side has the benefit of a sharper edge between the top portion and the back side wall which is desired in order to better induce turbulence in the flow after passing the stamped bump. Hence, an asymmetric trapezoid, designed with a less steep front sidewall side facing the film flow than the back sidewall may have the benefit of both reducing the risk of fouling as well as providing more turbulent flow when film flow is leaving the stamped bump SB on the backside wall. On the opposite side, where the heating media HM is flowing in the opposite direction, will there be a rather sharp edge when the heating media is entering the indentation I which may cause a turbulence in the flow at entering the indentations I. However, there is a rather soft curvature within the indentation and less inclined edge in the pocket at the side where the heating media HM is leaving the pocket will reduce the risk for particles or other solid matter to get stuck in the indentation.
[0071] In FIG. 6b is a somewhat different shape disclosed wherein the leading sidewall of the protrusion is designed to be rounded having a concave shape towards the film surface and being essentially circular. The backside wall is shaped essentially the same way as disclosed in FIG. 6a. In this case may there be further advantages in that there is a very steep angle between the concave leading sidewall and the top side of the stamped bump which most probably will induce turbulence in the film flow FF when passing this edge. Hence, it may be advantageous to have a longer top side since the film flow flowing passing over the top side surface will probably be turbulent after changing direction at this edge. In addition is the backside designed in the same way as described in FIG. 6a and the angle at the edge between the top side and the trailing side wall will thus be the same as in FIG. 6a. In addition the rounded design of the leading side wall facing the flow will reduce the risk of fouling due to the smooth transition between the planar falling film surface and the leading side wall. On the opposite surface, facing the heating media HM, will the flow entering the indentation pass over a rather sharp edge when entering into the indentation and likewise meet a rather sharp angled corner at the bottom of the indentation where the side wall and the bottom surface meet. On the other side of the indentation, where the flow leaves the indentation, is there a circular shaped side wall.
[0072] In FIG. 6c is an alternative design of a symmetric shape of the stamped bump SB disclosed which essentially is v shaped and thus not have any, or at least very short, top side and essentially identical leading sidewall facing the flow and trailing sidewall on the back side of the stamped bump. On the other side is a corresponding V-shaped indentation where the heating media flow HM passes.
[0073] In FIG. 6d is still an alternative design of the stamped bump SB disclosed in which the leading sidewall of the protrusion on the film flow FF surface is designed to be rounded, as in the design disclosed in FIG. 6b, but in this case is the leading sidewall convex, and somewhat elliptical, towards the film flow surface. The trailing edge is essentially shaped the same way as in FIGS. 6a and 6b. The indentation on the heating media HM surface corresponds to and fits onto the contour of the bump as has been the case also in FIGS. 6a-6c. Also in this case, as in FIGS. 6a and 6b, is there a more abrupt change of the flow for film flow FF at the trailing side wall, at the backside, than at the leading sidewall facing the flow.
[0074] The above disclosed shapes only exemplify a few further examples in addition to the symmetric trapezoidal shape disclosed in FIG. 3c. The desired shape and dimensions is dependent on the flow properties, e.g. flow velocity and viscosity. In addition, there may also be restrictions in what shapes and dimensions that is possible to achieve when modifying the sheet metal material and producing the stamped bumps.
[0075] To use stamped bumps SB in the heat transfer tubes will provide for an efficient heat transfer surface manufacturing process with no or minimal waste of materials and high automation potential. The use of a surface modification and formation of a specific texture on both sides of the heat transfer tube envelope surface by stamped bumps will enable an enhanced heat transfer while also having a low fouling propensity for water solutions containing fibres, particles and dissolved organic substance and salts. By designing the protrusions P/Indentations I properly and arranging the stamped bumps SB in specific patterns may also reduction or even elimination of rip currents and dry out surface areas be possible by enhanced falling film flow equalization design and it may be possible to considerably enhance the efficiency and the heat transfer capability by enhancing turbulence in both the evaporation side and the condensation side falling films.
[0076] In FIG. 7 is disclosed a detailed view of the cross-section perpendicular to the longitudinal extension of the symmetric trapezoidal stamped bump SB in FIG. 3c. As earlier disclosed, the stamped bump is preferably protruding on the surface side utilized as the evaporation surface side and the surface side with indentation is facing the heating media. The height of the protrusion h.sub.p should preferably be in the range of both the falling film thickness and the thickness of the heat transfer surface t for best performance. The width of the protrusion W.sub.p is defined as the distance perpendicular to the longitudinal extension of the protrusion P at the base of the protrusion P. In an alternate way, the width may be measured at half the height the protrusion (h.sub.p/2), still perpendicular to the extension direction of the stamped bump SB, between the side wall facing the falling film flow, i.e. the leading wall LW, and the side wall on the backside, i.e. the trailing wall TW. This method may be easier to define since it may sometimes be hard to decide where the base starts to incline. However, in this context has the width at the base been used and in case there should be any doubt about if where to measure the base point is defined as the location where each of the leading side wall angle L and the trailing side wall angle T is above 5 degrees, i.e. where the inclination angle of the leading wall LW respectively of the trailing wall TW relative the surface is more than 5 degrees. The opposite side of the surface, which normally is the heating media HM surface heated by condensing steam, will be formed as an indentation shaped as an inverted image of the protrusion on the falling film FF surface side. The indentation may also function as a turbulence inducer for enhancing the condensation heat transfer process. In addition to the height of the protrusion h.sub.p, the inclination angles of the of the leading side wall angle L and the trailing side wall angle T of the protrusion, the roundness of both sides of the top of the protrusions (Rc) and the roundness of the inside corners fillet radius (Rf) are all of utmost importance for the creation of high heat transfer intensification, low fibre and particles fouling propensity and low manufacturing cost. The inclination angle (S) of the leading side wall angle L and the trailing side wall angle T of the protrusion are defined as the angle of inclination from the base surface measured at the half height of the protrusion (e/2). the leading side wall angle L and the trailing side wall angle T may differ, as is readily understood from some of the examples in FIG. 6, but they should all be within the specified range of the chosen design side wall parameters.
[0077] The height of the ridges or protrusions (e) should, for best performance, be in the range of both the falling film thickness and the thickness of the heat transfer surface t which gives a preferred range of the height of the protrusion h.sub.p of 0.3 to 5.0 mm. Also important for best performance is the inclination angle of the two side walls of the protrusion, which are L for the leading side wall facing the falling film flow, and T for the trailing side wall. The side wall angles should preferably be 26 to 90, more preferably 45 to 80. In general, the angle of the leading side wall L and the trailing side wall are equal or the trailing side wall is more inclined. In order to assure a desired turbulence in the falling film flow is it important that the trailing side wall is sufficiently inclined why the trailing side wall angle T could be more acute than the leading side wall angle L. The side wall angles should be designed while taking into account heat transfer enhancement due to flow properties and inducing turbulence in the falling film, low fouling behaviour and ease of manufacturing. The corner radius (roundness) (Rc) of both sides of the top of the protrusions should be 0.2 to 2.0 mm preferably 0.3 to 1.2 mm, more preferably or 0.4 to 0.8 mm and a suitable value during normal circumstances is around 0.5 mm) for low fouling behaviour and ease of manufacturing. The fillet radius (Rf) and both corner radius and fillet radius for the pocket side are allowed to be 25% higher than Rc because of less sensitivity of the enhanced heat transfer behaviour for these positions. Hence, the stamped bumps SB should be designed to keep fibre and particle fouling during the evaporation process within acceptable limits which thus is limiting both high inclinations and low corner radius on top of the protrusions. On the other hand, heat transfer enhancement and fluid flow equalization over the width of the heat transfer surface are improved by high inclinations and low corner radius on top and the sides of the protrusions why there is need to find a design inducing a desired turbulence and keeping fouling low. For the indentation on the heating media side of the heat transfer surface is essentially the same height and radius preferred as for the falling film side but smaller deviations (+25%) can be tolerated
[0078] Further parameter which affects the efficiency of the heat transfer is the width of the protrusion W.sub.p which is 1-20 mm, more preferably 1.0 to 15 mm, even more preferably 1.5 to 10 mm and most preferably 2.0-8 mm). The length of the protrusion may vary widely and is generally not considered to be of the same relevance as the parameters listed above. However, the length may be of interest in finding desired patterns of the stamped bumps adapted to provide a desired flow and avoid dry zones or ensuring the falling film will not find its way down bypassing the stamped bumps SB. The stamped bumps SB may be formed by protrusions P/Indentations I stretching several turns around the envelope surface of the heat transfer tube. However, the length of the protrusions P/Indentations I if not extending more than one turn shall generally be at least 10 mm. From the point of view of manufacturing, the length of the protrusions P/Indentations I may be decided by the size of the sheet material used for manufacturing the heat transfer tube since it may be an advantage to form protrusions and indentations which stretches from one side to another of a sheet metal material in order to reduce stresses induced in the material while forming the protrusions P/Indentations I. For any individual (not continuous) protrusion in horizontal arrangement, the outer left and right corners, by viewing in the direction of the falling film flow, should have the same corner radiuses (Rp) in the plane of the metal surface as defined for the corner radius Rc for both sides of the top of the protrusion. This will facilitate an even flow field across the surface and reduce the tendency of the surface tension to create rip currents and dry out surface areas
[0079] Protrusions are preferably utilized on the falling film surface and its corresponding indentations subsequently used on the heating condensation side but it is obvious that the opposite arrangement also could be used as well as having both protrusions and indentations on the same side.
[0080] FIG. 8 discloses a multitude of different possible arrangements of the stamped bumps SB and serves as further examples in addition to the ones disclosed in FIGS. 3-5. In FIG. 8a is disclosed a multitude of individual protrusions P/Indentations I located along the circumference of the envelope surface of a heat transfer surface. The protrusions P/Indentations I are forming a multitude of discontinues lines in several different planes orthogonal to the longitudinal axis of the heat transfer tube and the longitudinal extension of the protrusions P/Indentations I are also aligned with those planes. The individual protrusions P/Indentations I are staggered relative each other as seen in a direction parallel to the longitudinal axis of the heat transfer tube.
[0081] In the next pattern disclosed in FIG. 8b are the same kind of protrusions P/Indentations I used as in FIG. 8a and the protrusions P/Indentations I are also located in several different planes orthogonal to the longitudinal axis of the heat transfer tube as in FIG. 8a. However, in this case is each of the individual protrusions P/Indentations I inclined relative a plane orthogonal to the longitudinal axis of the heat transfer tube. The formation is expected to cause the falling film to be slowly screwed on its way from the top to the bottom while slowly flowing on the heat transfer tube envelope surface.
[0082] FIG. 8c is merely disclosed in order to show that there may be a multitude of alternative shapes of stamped bumps SB which may be used. There is a considerable improvement in the heat transfer efficiency regardless of the shape of the stamped bumps as long as there is an induced turbulence in the falling film flow. Turbulence will be induced regardless of the shapes of the stamped bumps.
[0083] In FIG. 8d are there a multitude of the same kind of protrusions P/Indentations I used as in FIGS. 8a and 8b. The protrusions will shape a pattern as disclosed in FIG. 4 of parallel, inclined stamped bumps SB made up of individual rectangular protrusions P/Indentations I being aligned in rows wherein the longitudinal axis of each individual protrusion P/Indentation I is inclined and aligned with the longitudinal axis of the discontinuous inclined rows of protrusions P/Indentations I. Hence, this design corresponds to the suggested design in FIG. 4 except for that this design is made up of shorter individual protrusions instead of longer continuous protrusions. If the inclination angle is decreased it may be possible to provide a helically inclined pattern as in FIG. 3 but with the same difference as when compared to FIG. 3, i.e. that this design is made up of shorter individual protrusions instead of longer, possibly only one in the design in FIG. 3.
[0084] In FIG. 8e is disclosed an arrangement in which continuous protrusions P/Indentations I are provided on the surface of the heat transfer tube envelope surface. In this case it is obvious that these protrusions P/Indentations I may not continue too long since they will coincide with each other since the different protrusions P/Indentations I not are parallel and it will not be possible to manufacture such crossing patterns by using ordinary stamping methods. It may for example be possible to produce such a pattern for certain restricted distances.
[0085] In FIG. 8f is designed a pattern of the protrusions P/Indentations I forming the stamped bumps using both continuous protrusions P/Indentations I, in every second ring line of stamped bumps in separate planes orthogonal to the longitudinal axis of the heat transfer tube, and individual smaller segments of protrusions P/Indentations I in every other ring line of stamped bumps, also in separate planes orthogonal to the longitudinal axis of the heat transfer tube so as to form a discontinuous ring of inclined protrusions P/Indentations I in between the continuous protrusions P/Indentations I. Another way of describing this arrangement is that it is a combination of the embodiments disclosed in FIG. 5 and FIG. 8b and every second ring shaped on the envelope surface of the heat transfer tube comes from the design in FIGS. 5 and 8b.
[0086] 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 stamped bumps, in the figures may be discontinuous and they may be arranged on any of the inner and outer surfaces of the heat transfer tubes.
