GB2489026A

GB2489026A - Falling film evaporator with corrugated tubes

Info

Publication number: GB2489026A
Application number: GB1104465.8A
Authority: GB
Inventors: Joesph Weinberg; Henrikh Rojanski; Amnon Levy
Original assignee: IDE Technologies Ltd
Current assignee: IDE Technologies Ltd
Priority date: 2011-03-17
Filing date: 2011-03-17
Publication date: 2012-09-19
Also published as: GB201104465D0

Abstract

An evaporator comprises a plurality of horizontal, vertically elongated tubes 110 arranged to support a vertical film 90 of saline water and to evaporate water from the film by heat transfer from condensing vapour within the tubes. The tubes are vertically and circumferentially corrugated 112 in at least a specified outer profile (120A, Figure 3F) comprising alternating outer ridges (122, Fig. 3) and grooves (124, Fig. 3) on an outer face of the tubes. The specified outer profile is selected to thin the film on the outer ridges to enhance heat transfer therethrough and evaporation therefrom. In another aspect, a method of enhancing heat transfer across evaporator tubes comprises corrugating an outer face of the tubes to thin a falling water film on at least part of the outer face, to increase heat transfer across the thinned film. Preferably, the tubes may also be corrugated in at least a specified inner profile (120B, Fig. 3F), wherein the outer and/or inner profile can be trapezoidal. The ridges may have flat or convex tops, which are angular (123, Fig. 3D) on their sides. An anti-corrosion coating (185, Fig. 4) may be provided on the outer face of the tubes.

Description

AN EVAPORATOR WITH VERTICALLY ELONGATED AND CORRUGATED

TUBES

BACKGROUND

1. TECHNICAL FIELD

[0001] The present invention relates to the field of desalination, and more particularly, to a multi effect evaporator.

2. DISCUSSION OF RELATED ART [0002] Figure 1 is a schematic illustration of a multi-effect evaporator 100 with round tubes 110, according to the prior art, as disclosed, for example in European patent document No. 1858609. Existing Multi Effect Desalination plants 100 utilize aluminum alloy horizontal tubes 110, falling-film evaporative condensers in a serial arrangement, to produce through repetitive steps of evaporation and condensation, each at a lower temperature and pressure, a multiple quantity of distillate from a given quantity of input vapor. Feed 90A entering each effect 101 is introduced as a thin falling film 90 which is supported externally by tubes 110. Vapor 85A flows internally through tubes 110. As vapor 85A condenses, feed 90A from film 90 evaporates and the vapor is introduced into tubes 110 of next effect 101. Condensate 81 is collected from tubes 110, while brine 82 is collected from film 90 after flowing over all tubes 110. Prior art tubes 110 are circular.

[0003] Any number of evaporative condensers (effects 101) may be incorporated in the plants' heat recovery sections, depending on the temperature and costs of the available low grade heat and the optimal trade-off point between investment and vapor economy.

Technically, the number of effects 101 is limited only by the temperature difference between the vapor 85A and seawater 90A inlet temperatures (defining the hot and cold ends of the unit) and the minimum temperature differential allowed on each effect 101.

[0004] The incoming seawater 90A is dc-aerated and preheated in the heat rejection condenser and then divided into two streams. One is returned to the sea as coolant discharge, and the other becomes feed for the distillation process. Feed 90A is pretreated with a scale inhibitor and introduced into the lowest temperature group. The introduction to the lowest temperature group (backward feed flow) rather than to the highest is due to an effort to maintain the thermodynamic efficiency of the plant by reducing the irreversible mixing of the colder seawater feed with the hot effects temperature. Due to the falling film 90 nature of the feed flow over tubes 110 a pump is required to move the saline water from the bottom of the effect 101 to the top of the next one 101.

[0005] Input vapor 85A is fed into tubes 110 of the hottest effect. There it condenses, giving up its latent heat to the saline water flowing over the outer surface of tubes 110, while condensation takes place on the inside of tube 110, a nearly equal amount of evaporation occurs on the outside minus the amount required to preheat the feed to the evaporation temperature. The evaporation-condensation process is repeated along the entire series of effects, each of which contributes an amount of additional distillate. The vapor from the last effect is condensed by seawater coolant in the heat rejection condenser.

BRIEF SUMMARY

[0006] Embodiments of the present invention provide an evaporator comprising a plurality of horizontal, vertically elongated tubes arranged to support a vertical film of saline water, and to evaporate water from the film by heat transfer from a condensate film of condensing vapor within the tubes. The evaporator is characterized in that the horizontal tubes are vertically and circumferentially corrugated in at least a specified outer profile comprising alternating outer ridges and grooves on an outer face of the tubes, the specified outer profile selected to thin the film on the outer ridges to enhance heat transfer therethrough and evaporation therefrom. In embodiments, the horizontal tubes are vertically and circumferentially corrugated in at least a specified inner profile comprising alternating inner ridges and grooves on an inner face of the tubes, the specified inner profile selected to thin the condensate film on the inner ridges to enhance heat transfer therethrough and condensation thereupon. The outer and inner profiles may be congruent.

[0007] These, additional, and/or other aspects and/or advantages of the present invention are: set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present invention will be more readily understood from the detailed description of embodiments thereof made in conjunction with the accompanying drawings of which: Figures 2A-2C are schematic illustrations of a corrugated and vertically elongated tube, according to some embodiments of the invention; Figures 3A-3F are schematic illustrations of the corrugation form on the tubes and its production, according to some embodiments of the invention: and Figure 4 is a high level schematic flowchart illustrating a method of enhancing heat transfer across evaporator tubes, according to some embodiments of the invention.

DETAILED DESCRIPTION

0009] Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

0O10] The term "corrugate' as used herein in this application, is defined as a sequence of parallel and alternating ridges and grooves, or flutes. The ridges and grooves (or flutes) arc on both sides of the corrugated surface. The direction of grooves, or flutes 124 (see below) on tubes 110 may be vertical, or grooves 124 may he diagonal in respect to the faces of tube 110. The term corrugated tubes is not to be taken as limiting the relative angle of the ridges and grooves in respect to the tubes' faces.

[0011] Figures 2A-2C are schematic illustrations of a corrugated and vertically elongated tube 110, according to some embodiments of the invention; and Figures 3A- 3F are schematic illustrations of a corrugation form 120 on tubes 110 and its production, according to some embodiments of the invention. Figure 2A is a perspective view of tube 110 with film 90 illustrated on a part of tube 110. Film 90 falls on all or most length of tube 110, and is shown only on a part of tube 110 for clarity reasons. Figure 2B illustrates a transverse cross section of tube 110, and Figure 2C is a perspective view of a detail on the upper edge of tube 110. Figures 3A-3F illustrate a longitudinal cross section through tube 110, presenting various corrugation forms 120, and Figure 3E illustrates the cross section in an exemplary production method. Figures 3F-3H illustrate film 90 and condensing vapor 85 on the longitudinal cross section, and further illustrate the functioning of the corrugated tube wall profile.

[0012] Multi effect evaporator 100 comprises effects 101, each with a plurality of horizontal tubes 110 arranged to support a vertical film 90 of saline water, and to evaporate water from film 90 by heat transfer from condensing vapor within tubes 110.

Tubes 110 are vertically elongated to increase a contact area between tubes 110 and film 90, and to better support and control the form and thickness of film 90. The form of tubes may be oval and may have vertical parallel sides lilA connected rounded ends 111B.

[0013] Tubes 110 are vertically and circumferentially (relating to a transverse cross section) corrugated 112 in a specified profile 120. Corrugation form 120 may be selected according to various criteria, including, for example heat transfer coefficients, thickness and waviness of film 90 and of condensate film 85, downwards flow speed of film 90 and of condensate film 85 in respect to a location on profile 120. Corrugation 112 is arranged to enhance heat transfer from the vapor to film 90 and further enhances water evaporation by determining film characteristics.

[0014] Profile 120 comprises a specified outer profile 120A and a specified inner profile 120B (Figures 3A, 3F) that are selected to control the flowing characteristics, such as thickness and waviness, of film 90 and of condensate film 85, respectively, to enhance evaporation from an outer face 114 and condensation on an inner face 114 of tubes 110.

[0015] Specified outer profile 120A comprises outer ridges 122 and outer grooves 124 on outer face 114 of tubes 110, specified inner profile 120B comprises inner ridges 126 and inner grooves 128 on inner face 116. Outer grooves 124 correspond to inner ridges 126 and inner grooves 128 correspond to outer ridges 122. Outer profile 120A enhances evaporation (from outer ridges 122), while inner profile 120B enhances condensation of vapor (in inner grooves 128).

[0016] Specified outer ridge profile 120A may be congruent to specified inner ridge profile 120B, such that profile 120 is rotationally symmetric. The congruence may result from a symmetric production method of the sheets that are used to manufacture tubes 110. Corrugation 112 may be produced by two identical cogs 91, each arranged to produce a corresponding ridge profile 122, 126. Tubes 110 may be produced from planar corrugated sheets (see Figure 3E), e.g. by bending and welding them to tubes 110. Tubes may be produced in alternative ways, such as hydroforming, pressing, etc. [0017] Specified outer ridge profile 120A and specified inner ridge profile 120B may be trapezoidal, with either straight or convex sides (Figure 3B).

[0018] Outer ridges 122 and inner ridges 126 may have flat tops which are angular 123, 127 (respectively) on their sides. Alternatively, outer ridges 122 and inner ridges 126 may have convex tops which are angular 123, 127 (respectively) on their sides. Angled outer ridges 123 are shaped to control film characteristics. For example, angle 123 may he selected to promote evaporation from film 90 by thinning or breaking film 90 and enhancing film instability, as illustrated in Figure 3K [0019] The form of tubes 110 influences film characteristics and may stretch and thin film 90 under operation of gravity, surface tension and flow forces (Figures 3F-3H).

Outer ridges 122 may enhance the wavy character of falling film 90 on outer face 114 of tubes 110 and thereby enhance evaporation. Inner ridges 126 and inner grooves 128 may enhance the wavy character of falling condensate on inner face 116 of tubes 110 and thereby enhance condensation.

[0020] Corrugation 112 of both inner and outer faces 114, 116 allows optimizing surface characteristics that maximize evaporation and condensation, and thus maximize the process efficiency. In particular, generating stronger waviness, internal turbulence vortices inside the films 90 and condensate film, and shear forces on film 90.

[0021] The inventors have discovered, that corrugation 112 changes flow characteristics and improve heat transfer in some embodiments in the following manner (Figures 3G, 3H). Downwards flow of film 90 (on outer face 114) and/or condensate film 80 (on inner face 116) has a larger volume and a lower speed in grooves 124, 128 (flows 124A, 128A) than on ridges 122, 126 (flows 122A, 126A), all designation respective to outer face 114 and inner face 116. Due to the different flow speeds, the intermediate parts of the film flow with a horizontal component 124B, 128B that compensates the eater masses and generates waviness in films 90, 85, which enhances evaporation. As a result of surface tension forces, film 90, 85 on ridges 122, 126, denoted in Figure 3H by 90A and 80A, are thinner and flow faster than without corrugation 112, and their thinness improves heat transfer (denoted 80B in Figure 3H by 90B and respectively) from tube 110 across film 122A, 126A, and hence a stronger evaporation therefrom. Indeed in grooves 124, 128 heat transfer becomes somewhat worse, but overall, due to the larger area of the areas with a thinner film, heat transfer improves. These effects of the corrugation are much more significant on outer face 114 as the amount of water in film 90 are much larger than in film 80 (as film 90 is feed water, while film 80 is condensate).

110022] Alternatively, profile 120 may comprise only an outer corrugation (Figure 3B) a wavy profile (Figure 3C), which may also provide some of the presented benefits.

[0023] In embodiments, the inventors have discovered the following profile characteristics to be most effective in some cases. in profile 120, a horizontal distance between sequential grooves 131 is 3.2 times (±10%) a tube wall thickness 132, and a depth of the grooves 133 is a fifth (±10%) of the horizontal distance between sequential grooves 131. Tube wall thickness 132 may be between 0.7 and 1.6 mm. In embodiments, tube wall thickness 132 may be between 1 and 1.25 mm. Tubes 110 may be made of aluminum to enhance heat transfer properties.

[0024] Parts or all of tubes 110 may be coated by an anti-corrosion coating such as a ceramic coating. Inner face 116 may also be coated by an anti-corrosion coating 113. The coating may be deposited on tubes 110 before or after their production from the sheets, in the latter case to protect strained areas of tubes 110.

[0025] The inventors have found, that overall in some embodiments, corrugated tubes have a total heat transfer coefficient (evaporation and condensation) that is higher by a factor of 2.5 to 3.5 in respect to oval smooth tubes in the same hydraulic and thermodynamic conditions. a

[0026] Evaporator 100 may further comprise a surfactant unit aiTanged to add a surface active agent to the saline water to control film 90 thickness on tubes 110. The surface active agent may enhance the waviness of film 90 and further enhance evaporation.

[0027] Figure 4 is a high level schematic flowchart illustrating a method 150 of enhancing heat transfer across evaporator tubes, according to some embodiments of the invention. Method 150 comprises the following stages: corrugating (i.e. forming ridges and grooves) an outer face of the tubes (stage 155) to thin a falling water film on at least part of the outer face (stage 156), to increase heat transfer across the thinned film (stage 157), and optionally corrugating an inner face of the tubes (stage 160) to thin a falling condensate film on at least part of the inner face (stage 161), to increase heat transfer across the thinned condensate film (stage 162).

[0028] Method 150 may further comprise flattening the corrugation ridges (on either inner or outer faces, or both) to thin the corresponding film supported thereupon (stage 165). The corrugated ridges may be fully or partly flattened (to become either flat or convex) to create angled ridge edges.

[0029] Corrugating of the outer face and of the inner face (stage 155 and 160 respectively) may be carried out alternately (stage 170), to yield a correspondence between ridges on the outer face and grooves on the inner face, and between ridges of the inner face and grooves on the outer face.

[0030] For example, the alternate corrugation (stage 170) may be carried out by two opposing cogs to form planar corrugated sheets (stage 175), and method 150 may further comprise folding the sheets to generate the tubes, to yield elongated tubes with parallel planar faces (stage 180). The tubes may be formed by any other production method, such as hydroforming, pressing, etc. [0031] The inventors have found out, that heat transfer efficiency was maximized in one case, by corrugating the tubes (stages 155, 160, 170) to yield a horizontal distance between sequential grooves that is 3.2 times (±10%) a tube wall thickness, and a depth of the grooves is a fifth (±10%) of the horizontal distance between sequential grooves.

[0032] Method 150 may further comprise coating the outer face of the tubes by an anti corrosive coating (stage 185).

[0033] In the above description, an embodiment is an example or implementation of the invention. The various appearances of "one embodiment", "an embodiment" or "some embodiments" do not necessarily all refer to the same embodiments.

[0034] Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment.

[0035] Furthermore, it is to he understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above.

[0036] The invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described.

[0037] Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined.

[0038] While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention.

Claims

CLAIMSWhat is claimed is: 1. An evaporator comprising a plurality of horizontal, vertically elongated tubes arranged to support a vertical film of saline water, and to evaporate water from the film by heat transfer from a condensate film of condensing vapor within the tubes, characterized in that: the horizontal tubes are vertically and circumferentially corrugated in at least a specified outer profile comprising alternating outer ridges and grooves on an outer face of the tubes, the specified outer profile selected to thin the film on the outer ridges to enhance heat transfer therethrough and evaporation therefrom.
2. The evaporator according to claim 1, wherein the horizontal tubes are vertically and circumferentially corrugated in at least a specified inner profile comprising alternating inner ridges and grooves on an inner face of the tubes, the specified inner profile selected to thin the condensate film on the inner ridges to enhance heat transfer tberethrough and condensation thereupon.
3. The evaporator according to claims 1 or 2, wherein at least one of: the outer profile and the inner profile, is trapezoidal.
4. The evaporator according to claim 3, wherein at least one of: the outer ridges and the inner ridges, is trapezoidal with convex sides.
5. The evaporator according to claim 3, wherein at least one of: the outer ridges and the inner ridges, has flat or convex tops which are angular on their sides, wherein the angles ridges are shaped to control film characteristics.
6. The evaporator according to claim 2, wherein the specified outer profile is congruent to the specified inner profile.
7. The evaporator according to claim 6, wherein for both specified inner and outer profiles: a horizontal distance between sequential grooves is 3.2 times (±10%) a tube wall thickness, and a depth of the grooves is a fifth (±10%) of the horizontal distance between sequential grooves.
8. The evaporator according to claim 7, wherein the tube wall thickness is between 0.7 and 1.6 mm.
9. The evaporator according to claim 2, the tubes are produced from planar corrugated sheets.
10. The evaporator according to any of claims 1 to 9, wherein the tubes are oval.
11. The evaporator according to any of claims 1 to 9, wherein the tubes have vertical parallel sides and rounded ends.
12. The evaporator according to any of claims 1 to 11, wherein the tubes are coated with an outer anti-corrosion coating.
13. The evaporator according to claim 12, wherein the outer anti-corrosion coating is ceramic.
14. The evaporator according to any of claims 1 to 13, further comprising a surfactant unit arranged to add a surface active agent to the saline water to control the film thickness on the tubes.
15. Corrugated tubes usable in the evaporator of any of claims 1 to 14.
16. A method of enhancing heat transfer across evaporator tubes comprising corrugating an outer face of the tubes to thin a falling water film on at least part of the outer face, to increase heat transfer across the thinned film.
17. The method according to claim 16, further comprising corrugating an inner face of the tubes to thin a falling condensate film on at least part of the inner face, to increase heat transfer across the thinned condensate film.
18. The method according to claim 16 or 17, further comprising flattening corrugation ridges to thin the corresponding film supported thereupon.
19. The method according to claim 17, wherein the corrugating of the outer face and of the inner face are carried out alternately, to yield a correspondence between ridges on the outer face and grooves on the inner face, and between ridges of the inner face and grooves on the outer face.
20. The method according to claim 19, wherein the corrugating is carried out by two opposing cogs to form planar corrugated sheets, and further comprising folding the sheets to generate the tubes, to yield elongated tubes with parallel planar faces.
21. The method according to claim 19, wherein the corrugations are selected to yield a horizontal distance between sequential grooves that is 3.2 times (±10%) a tube wall thickness, and a depth of the grooves is a fifth (±10%) of the horizontal distance between sequential grooves.
22. The method according to any of claims 16 to 21, further comprising coating the outer face of the tubes by an anti corrosive coating.