TECHNICAL FIELD

The disclosure pertains to a vertical-tube thermosyphon evaporator for evaporation of an evaporable fluid by means of a heating fluid, the evaporator having a vertical direction and a horizontal direction, wherein the evaporator comprises a plurality of tubes enclosed by an evaporator shell, the tubes extending in the vertical direction between an upper tube plate at an upper part of the evaporator and la lower tube plate at a lower part of the evaporator.

BACKGROUND

Large amounts of energy are used in the wood processing industry and in biorefineries to heat process liquids and/or to evaporate process liquids. The process liquids may be pure liquids or mixtures of different liquids and may contain solubilized substances as well as dispersed substances which do not evaporate.

The energy source may be steam, vapor, or hot water. Using secondary heat from process liquids as an energy source is of great interest as it reduces operating costs and improves environmental performance by lowering the use of primary heat. For example, secondary low-pressure steam is readily available from refiners and grinders in the mechanical pulping industry. Other examples are blow steam, also known as flash steam, from digesters in the chemical pulping industry or blow steam from pressurized hydrolysis reactors in biorefineries. These secondary steams and vapors are of great value, but the use of them is complicated by the fact that they are contaminated by solid, liquid, and gaseous material from the biomass processing. The usefulness of secondary steam may be limited by the presence of such contaminants since they may cause build-up on the heat transfer surfaces in a heat exchanger which leads to impaired heat transfer and an increased pressure drop, thereby lowering the efficacy of the recovery process. The contaminants may, for example, be wood fibers, lignin, spent cooking liquor, methanol, acetic acid, furfural, etc. It may be required to install a heat exchanger such that the contaminated steam is condensed, and the heat released therefrom is used to evaporate, for example, water for production of a clean steam, which may be transferred to the low-pressure steam net of a pulp mill, a biorefinery or other energy consumer. The heat dissipating fluid in an evaporator is usually a condensable vapor or a blend of condensable vapors. In the wood processing industry and in biorefineries, there is a large supply of secondary heat from pressurized vapor and vapor blends in the range from 1 to 6 bar. The secondary heat of such vapors is used as heat dissipation medium in evaporators, condensers, and other heat transfer equipment. If contaminated vapors are supplied on the outside of evaporator tubes to evaporate liquid inside the evaporator tubes, cellulose fibers and other particles may collect between the tubes and may stick to the tube surfaces. Such unwanted deposits of contaminants in the heating fluid are difficult if not impossible to remove from the evaporator by mechanical cleaning methods such as high pressure flushing, brushing, or scraping.

It has therefore been suggested to supply a contaminated heating fluid to the inside of the evaporator tubes. Such arrangement leads to a certain degree of self-cleaning of the tubes as the condensed heating fluid runs as a thin film on the inner surface of the evaporator tubes and continuously rinses the inner surface of the tubes from solid particles and other deposits. Thereby, the risk of clogging of the evaporator tubes is considerably reduced. Furthermore, the interior of the evaporator tubes is easier to clean than the exterior, such as by flushing.

However, by having the heating fluid inside the evaporator tubes, evaporation of the evaporable fluid must take place on the outside of the evaporator tubes. For efficient usage of the heat transfer surface, it is then necessary to distribute the boiling liquid evenly on the outer surfaces of the evaporator tubes and to maintain them in a uniform wet condition.

WO 81/02112 A1 discloses a method and an apparatus for vaporization of liquid in a vertical heat exchanger. The heating fluid is passed through a group of tubes in the heat exchanger, preferably from the bottom upwards. The group of tubes extends through at least one nozzle plate or nozzle basin so that nozzle openings surrounding the tubes are formed between the openings in the nozzle plates and the tubes. The liquid to be vaporized in the heat exchanger is passed to the shell side of the tubes into the upper part of the heat exchanger or into each nozzle basin of the heat exchanger separately and is guided to flow downwards on the outer surfaces of the group of tubes as a liquid film. By means of vapour flow passages formed into the group of tubes, the vapour developed from the liquid film is passed from the lower part of the heat exchanger further into heat consuming equipment or from each component unit of the heat exchanger further into heat consuming equipment. The evaporator in WO 81/02112 A1 may be referred to as a “falling film evaporator”. Such evaporators are highly efficient in that good heat transfer between the heat exchange fluids is achieved. However, the construction of such evaporators is expensive and complicated as it requires special fluid distribution equipment including pumps and piping to deliver and circulate the evaporable fluid to the one or more nozzle plates or nozzle basins at the outer surfaces of the evaporator tubes.

Another type of evaporator is the vertical, natural circulation, thermosyphon-type evaporator which may be installed e.g., at a stripper or a distillation column and be used as a so-called reboiler. The hydrostatic head in a container, such as a column containing evaporable fluid pushes evaporable fluid through the evaporator shell where it boils on the outside of the evaporator tubes. The density of the boiling liquid-vapor mixture in the thermosyphon is much lower than that of the liquid in the column. This means that the hydrostatic head of the liquid in the column is capable of circulating liquid over the evaporator and back to the column. A number of different thermosyphon designs are available on the market, including horizontally and vertically assembled units and with the boiling liquid on the tube side or on the shell side of the unit. Vertical thermosyphons with boiling on the shell side of the evaporator tubes are less common than thermosyphons where the boiling takes place in the tubes. It is more difficult to achieve good liquid/vapor distribution between the tubes on the shell side, which may have a negative impact on the natural density driven circulation over the thermosyphon. High and stable circulation over the thermosyphon is required for good heat transfer and high evaporation capacity. Furthermore, the currently available thermosyphons require external circulation piping for supplying liquid to and from the column so that the natural circulation is maintained.

The object of the present invention is to overcome or at least mitigate one or more of the problems associated with the prior art.

SUMMARY

The above and further objects may be achieved with a vertical-tube thermosyphon evaporator according to claim 1 . Variations of the disclosure are set out in the dependent claims.

The vertical-tube thermosyphon evaporator for evaporation of an evaporable fluid by means of a heating fluid as disclosed herein has a vertical direction and a horizontal direction. The evaporator comprises a plurality of tubes enclosed by an evaporator shell, the tubes extending in the vertical direction from an upper tube plate located in an upper part of the evaporator to a lower tube plate located in a lower part of the evaporator. The tubes are mounted in the upper and lower tube plates and are arranged in an annular heat-exchange space around a central fluid reservoir in the evaporator, the central fluid reservoir being arranged to contain the evaporable fluid and being in fluid communication with the heatexchange space in the lower part of the evaporator and being in fluid communication with the heat-exchange space in the upper part of the evaporator, such that the evaporable fluid can circulate from the central fluid reservoir into the heat exchange space and back into the central fluid reservoir, the evaporator comprising a first inlet for introducing the heating fluid into the tubes and a second inlet for introducing the evaporable fluid into the evaporator.

The central fluid reservoir may have a bottom wall at the lower tube plate and be delimited in the horizontal direction by a tubular fluid reservoir wall extending upward in the vertical direction from the bottom wall to the upper tube plate or terminating at a distance from the upper tube plate, the heat-exchange space being located between the evaporator shell and the tubular fluid reservoir wall. The central fluid reservoir may have a cylindrical or generally cylindrical shape.

The evaporator may further comprise an outlet for withdrawal of evaporated fluid from the evaporator and a condensate collection chamber arranged below the lower tube plate, the tubes opening into the condensate collection chamber through openings in the lower tube plate.

A vertically assembled shell and tube heat exchanger or evaporator as disclosed herein is intended for boiling of a liquid by means of a heating fluid, such as a condensable vapor or a hot fluid in the condensed phase. The disclosure concerns more particularly a thermosyphon evaporator with the heat dissipating fluid inside the tubes and with the boiling liquid on the shell side of the thermosyphon evaporator, the thermosyphon evaporator being furnished with a fluid reservoir integrated on the shell side of the evaporator.

In the thermosyphon-type vertical-tube evaporator as disclosed herein, the tubes are preferably arranged in a regular pattern in the upper and lower tube plates. In this manner uniform distribution and evaporation of the evaporable fluid between the tubes may be promoted. The evaporable fluid may be a single component liquid, such as water, or may be a blend of two or more different liquids. The evaporable fluid is a clean fluid, which is free or substantially free from solubilized components or solid contaminants.

By supplying the potentially contaminated heating fluid to the interior of the evaporator tubes and clean evaporable fluid on the outside or shell side of the evaporator tubes, the evaporator as disclosed herein is less sensitive to the presence of solid particles and other contaminants in the heating fluid.

Furthermore, the evaporator as disclosed herein is easy to clean from deposits by flushing of the evaporator tubes. The evaporator operates under conditions reducing the risk for deposits forming on the inner tube walls as condensed fluid flows down on the inner tube walls and brings with it solid particles, fibers, etc., as well as prohibits salt crystallization on the tube walls.

The thermosyphon evaporators as disclosed herein have a simple and compact construction with a minimum of piping as the central part of the evaporator serves as an internal container for evaporable fluid, such that natural circulation of evaporable fluid takes place inside the evaporator from the the central fluid reservoir through the heat-exchange space and back into the central fluid reservoir. The simple and compact construction with a minimum of piping makes the operation of the thermo-siphon type evaporators as disclosed herein highly reliable, yet less costly and space demanding than prior art evaporators, such as the evaporators disclosed in WO 81/02112 A1 .

In an evaporator as disclosed herein, the central fluid reservoir is preferably in fluid communication with the heat-exchange space through passages arranged at the upper tube plate and at the lower tube plate. Thereby, evaporable fluid may circulate from the central fluid reservoir, upward in the heat-exchange space from the lower tube plate to the upper tube plate where non-evaporated evaporable fluid may fall back into the central fluid reservoir at the upper part of the heat-exchange space.

At the upper part of the evaporator, the central fluid reservoir may be in fluid communication with the heat-exchange space through an upper passage arranged in the tubular fluid reservoir wall below the upper tube plate as seen in the vertical direction. Alternatively, the tubular reservoir wall may terminate at a distance from the upper tube plate, leaving a gap between the end of the tubular reservoir wall and the upper tube plate which gap may serve as a fluid circulation passage between the heat exchange space and the central fluid reservoir. The upper passage preferably extends around the full circumference of the tubular fluid reservoir and may be continuous as when formed by a gap between the end of the tubular reservoir wall and the upper tube plate or may be discontinuous. In the latter case, the upper passage may be formed by a plurality of upper openings, such as two or more upper openings in the tubular reservoir wall. Such two or more upper openings are preferably arranged with an even distribution around a circumference of the tubular fluid reservoir wall such that a mixture of non-evaporated evaporable fluid and evaporated evaporable fluid may flow as freely and uniformly as possible back into the heat-exchange space.

At the lower part of the evaporator, the central fluid reservoir may be in fluid communication with the heat-exchange space through a lower passage arranged in the tubular fluid reservoir wall above the lowertube plate as seen in the vertical direction. The lower passage preferably extends around the full circumference of the tubular fluid reservoir and may be continuous or discontinuous. It may be preferred that the tubular fluid reservoir wall is connected to the bottom wall of the central fluid reservoir and that the lower passage is formed by a plurality of lower openings, such as two or more lower openings. As for the upper passage, the two or more lower openings are preferably arranged with an even distribution around the circumference ofthe tubular fluid reservoir wall such that evaporable fluid contained in the central fluid reservoir may be uniformly delivered into the heatexchange space. Alternatively, the tubular fluid reservoir wall may be attached to the upper tube plate and/or to the evaporator shell in an upper part of the fluid reservoir wall. In such embodiment the lower passage may be a gap between a lower end of the tubular fluid reservoir wall and the lower tube plate.

The tubular fluid reservoir wall may extend the full distance between the lower tube plate and the upper tube plate with the upper and the lower passages both being formed by a plurality of openings in the fluid reservoir wall, such as each being formed by two or more openings in the fluid reservoir wall.

It is preferred that both the lower passage and the upper passage extend continuously or discontinuously around the full circumference of the tubular fluid reservoir wall and that, if formed by two or more openings in the tubular fluid reservoir wall, the openings are uniformly distributed around the circumference of the tubular fluid reservoir wall, to promote uniform fluid flow through the upper and lower passages.

The lower passage may be configured such that 1/3 of a total flow pressure drop from the lower passage to the upper passage takes place at the lower passage during operation of the evaporator. A stable natural circulation over the thermosyphon-type evaporator as disclosed herein and pulsation-free boiling of the evaporator may be promoted if at least 1/3 of the total flow pressure drop takes place at the inlet for the evaporable fluid b to the tubes in the heat-exchange space.

The lower passage may have a smaller extension in the vertical direction than an extension of the upper passage in the vertical direction. A larger passage at the upper tube plate allows a larger volume of the mixture of evaporated evaporable fluid and non-evaporated evaporable fluid to pass out from the heat-exchange space into the central fluid reservoir through the passage in the inner tubular wall where the evaporated fluid may be withdrawn from the top of the evaporator and the non-evaporated fluid may fall back into the central fluid reservoir.

An outlet for uncondensed heating fluid, i.e., fluid or components of a fluid which have not been condensed when reaching the lower ends of the evaporator tubes, may be arranged below the lower tube plate in the condensate collection chamber. The outlet for the uncondensed heating fluid is preferably arranged at an upper part of the condensate collection chamber and is preferably arranged directly below the lower tube plate where the lower tube ends open into the upper part of the condensate collection chamber through openings in the lower tube plate.

The first inlet, i.e., the inlet for heating fluid, may open into a reception chamber for the heating fluid. The reception chamber may be arranged above the upper tube plate with upper tube ends opening into the reception chamber through openings in the upper tube plate. The reception chamber is a space which may be open towards the central fluid reservoir or which may be closed off from the central fluid reservoir.

Alternatively, the first inlet may be arranged below the lower tube plate and may be arranged to supply heating fluid in an upwards direction through the tubes. The second inlet, i.e., the inlet for evaporable fluid, may be arranged at and above the lower tube plate and may open into the heat exchange space. The evaporable fluid which is delivered into the evaporator through the second inlet may flow into the central fluid reservoir from the heat exchange space, e.g., through passage between the tubular fluid reservoir wall and the lower tube plate, or may flow through the second inlet directly into the central fluid reservoir.

In the evaporator as disclosed herein, condensed heating fluid may be collected and removed from a bottom part of the condensate collection chamber, such as being removed through a conduit arranged at a bottom part of the condensate collection chamber.

An outlet may also be arranged in the bottom wall of the central fluid reservoir for draining off small amounts of residues, such as salts which may be present in the evaporable fluid and which may accumulate at the bottom of the central fluid reservoir over time.

The vertical-tube thermosyphon evaporators as disclosed herein have a simple and compact construction allowing for efficient and reliable operation where an evaporable fluid is evaporated by means of a heating fluid which is led through a plurality of vertically extending tubes which are enclosed by an evaporator shell and which extend between an upper and a lower tube plate in the evaporator. The tubes are concentrically mounted in the tube plates and are separated from a central fluid reservoir in the interior of the evaporator by a tubular inner fluid reservoir wall such that heat transfer may take place from the heating fluid which is lead through the tubes in a downward or upward direction to the evaporable fluid in a heat-exchange space formed between the evaporator shell and the tubular fluid reservoir wall. The central fluid reservoir is in fluid communication with the heat-exchange space at a lower part of the heat-exchange space as well as at an upper part of the heatexchange space, allowing evaporable fluid in the evaporator to circulate from the central fluid reservoir, upward along the tubes in the heat-exchange space and back into the central fluid reservoir. The natural internal circulation of evaporable fluid in the evaporator is driven by the difference in density between the fluid (liquid) in the central fluid reservoir and the less dense boiling liquid/gas mixture in the heat-exchange space. BRIEF DESCRIPTION OF THE DRAWINGS

The thermosyphon type evaporator as disclosed herein will be further explained hereinafter with reference to the appended drawings wherein:

Figure 1 shows a thermosyphon type evaporator according to prior art;

Figure 2 shows a cross-sectional view taken in the longitudinal direction through a thermosyphon type evaporator according to the invention; and Figure 3 shows a cross-sectional view taken along the line Ill-Ill in Fig. 2.

DETAILED DESCRIPTION

Different aspects of the present disclosure will be described more fully hereinafter with reference to the enclosed drawings. The thermosyphon evaporator disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the thermosyphon evaporator shown in the figures.

Fig. 1 shows an example of a prior art vertical-tube evaporator 30 of the thermosyphon type. The evaporator 30 is installed at a stripper and is used as a reboiler. The hydrostatic head of evaporable fluid b in a column 31 pushes evaporable fluid b through the evaporator jacket 33 by means of an inlet pipe 34. In the evaporator 30, the evaporable fluid b boils on the outside of the vertically arranged evaporator tubes 35. The density of the boiling liquidvapor mixture in the evaporator 30 is much lower than that of the liquid b in the column 31. This means that the hydrostatic head of the liquid b in the column 31 can circulate liquid over the evaporator 30 and back to the column 31 by means of an outlet pipe 36. The thermosyphon shown in Fig. 1 is further equipped with a level control system 37 for the evaporable fluid b in the column 31.

A thermosyphon-type vertical-tube evaporator 20 according to the present invention is shown in Fig. 2. The evaporator 20 is intended for boiling of an evaporable fluid b by means of a heating fluid a. The evaporator 20 has a vertical direction V and a horizontal direction H perpendicular to the vertical direction V. The evaporator 20 comprises a plurality of tubes 4 arranged inside an evaporator shell 3. An upper tube plate 1 is attached to the evaporator shell 3 at an upper part 20’ of the evaporator 20 and a lower tube plate 2 is attached to the evaporator shell 3 at a lower part 20” of the evaporator 20. The tubes 4 extend in the vertical direction V from the upper tube plate 1 located at the upper part 20’ of the evaporator 20 to the lower tube plate 2 located at the lower part 20” of the evaporator, such that the upper tube plate 1 is arranged above the second tube 2 plate in the vertical direction V.

The tubes 4 are attached to each tube plate 1 , 2 allowing the heat-emitting heating fluid to pass through the tubes 4 from the upper part 20' of the evaporator 20 to the lower part 20" of the evaporator 20.

As seen in Fig. 3, the tubes 4 are annularly mounted in the upper and lower tube plates 1 ,2 in a regular pattern with even distribution of the tubes 4 around a central fluid reservoir 10 in the evaporator 20. With reference to Fig. 2, the central fluid reservoir 10 has a bottom wall 17 at a level with the lower tube plate 2 and is delimited in the horizontal direction H by a tubular fluid reservoir wall 11 which extends upward in the vertical direction V from the lower tube plate 2 to the evaporator shell 3 at the top of the evaporator 20. The tubes 4 are arranged in a heat-exchange space 18 formed between the evaporator shell 3 and the tubular fluid reservoir wall 11 . As set out herein, it is not a necessary feature of the evaporator as disclosed herein that the tubular fluid reservoir wall extends all the way to the top of the evaporator 20, the only requirement on the tubular fluid reservoir wall being that it has a sufficient extension in the vertical direction to allow an annular heat-exchange space 18 to be formed between the evaporator shell 3 and the tubular fluid reservoir wall 11 and between the upper and the lower tube plates 1 , 2.

The central fluid reservoir 10 is in fluid communication with the heat-exchange space 18 through an upper passage 13 arranged in the tubular fluid reservoir wall 11 below the upper tube plate 1 as seen in the vertical direction V.

The central fluid reservoir 10 is also in fluid communication with the heat-exchange space 18 through a lower passage 12 arranged in the tubular fluid reservoir wall 11 above the upper tube plate 1 as seen in the vertical direction V.

The lower passage 12 is shown in Fig. 2 to have a smaller extension in the vertical direction V than the extension of the upper passage 13 in the vertical direction V. The evaporator 20 comprises a first inlet 16 for introducing the heating fluid a, usually in the form of vapor, into the tubes 4 and a second inlet 5 for introducing the evaporable fluid b into the central fluid reservoir 10.

The evaporator 20 further comprises an outlet 15 for withdrawal of evaporated evaporable fluid e from the evaporator 20 at the top of the central fluid reservoir 10 and a condensate collection chamber 7 arranged below the lower tube plate 2 and the bottom wall 17 of the central fluid reservoir 10. The tubes 4 open into the condensate collection chamber 7 through openings in the lower tube plate 2.

An outlet 8 for uncondensed heating fluid, i.e., vapor c is arranged below the lower tube plate 2 in an upper part 7’ of the condensate collection chamber 7. The uncondensed heating fluid is a part of the heating fluid a which is introduced into the evaporator through the first inlet 16 and has passed downward through the tubes 4 in the heat-exchange space 18 without condensing.

The outlet 8 for uncondensed heating fluid c is arranged directly below the lower tube plate 2 where lowertube ends 4” open into the upper part 7’ of the condensate collection chamber 7 through openings in the lower tube plate 2.

The condensed heating fluid d is collected in the bottom part 7” of the condensate collection chamber 7, the condensed fluid d being collected together with impurities such as particles and fibres and removed from the bottom part 7” of the condensate collection chamber through an outlet pipe 9.

Accordingly, as shown in Fig. 2, condensate d and uncondensed vapor c are taken out through the outlets 9 and 8. Alternatively, the first inlet 16 for introducing the heating fluida into the tubes 4 may change position with the outlet 8 for the uncondensed heating fluid c, so that uncondensed vapor meets condensate in countercurrent in the tubes 4. The condensate flowing downward in the tubes 4 has a washing effect on the heating fluid a, so that solid material is separated from the vapor.

A further outlet may be arranged in the bottom wall 17 of the central fluid reservoir 10 for draining off small amounts of residues f, such as salts which may be present in the evaporable fluid b and which may accumulate at the bottom of the central fluid reservoir 10 over time.

A countercurrent process may also be advantageous in condensing multicomponent mixtures when there is a desire to enrich a component with a low boiling point in the uncondensed fraction, e.g., methanol which can be enriched and removed through the outlet 8 when a water vapor/methanol mixture is condensed.

The first inlet 16 for the heating fluid a opens into a reception chamber 6 for the heating fluid a, the reception chamber 6 being arranged above the upper tube plate 1. In the example shown in Fig. 2, the reception chamber 6 is delimited in the horizontal direction H by the evaporator shell 3 and a continuation of the tubular fluid reservoir wall 11 and is delimited in the vertical direction V by the upper tube plate 1 and the evaporator shell 3, with upper tube ends 4’ opening into the reception chamber 6 through openings in the upper tube plate 1 . However, as set out herein, the reception chamber 6 needs not be walled-off from the central fluid reservoir.

The second inlet 5 for introducing evaporable fluid b into the evaporator 20 is arranged at and above the lower tube plate 2. In the example shown in Fig. 2, the evaporable fluid b is introduced into the heat-exchange space 18 and passes through the heat-exchange space 18 upward in the vertical direction V and inward in the horizontal direction R to the central fluid reservoir 10.

The heat-absorbing evaporable fluid b is introduced into the evaporator 20 via the second inlet 5. The heating fluid a, which enters via the first inlet 16 into the reception chamber 6 for the heating fluid a, is distributed therefrom into the tubes 4 and emits heat through the tube wall to the cooling evaporable fluid b on the outside of the tubes 4. As set out herein, the heating fluid a may be a condensable vapor or a mixture of condensable vapors. The fluid a, if it is vapor or a vapor mixture, condenses if the surface temperature of the tube wall is below the dew point at the total vapor pressure.

As shown in Fig. 3, the tubes 4 are arranged annularly so that the central fluid reservoir 10 is formed around the central axis of the evaporator 20. The cylindrical partition wall, i.e., the tubular fluid reservoir wall 11 , separates the central fluid reservoir 10 from the tubes 4 in the heat-exchange space 18. The lower passage 12 at the bottom of the central fluid reservoir 10, allows the evaporable fluid b to pass from the central fluid reservoir 10 to the tubes 4 in the heat-exchange space 18. At the upper passage 13 in the tubular fluid reservoir wall 11 , the evaporable fluid b can pass from the heat-exchange space 18 with the tubes 4 back to the central fluid reservoir 10 of the evaporator 20, thereby allowing a natural internal circulation of evaporable fluid b in the evaporator 20.

The evaporable fluid b, which is supplied in liquid form and which is to evaporate completely or partially, is introduced into the evaporator 20 via the second inlet 5 and fills the space between the tube plates 1 ,2 so that a liquid level 14 is obtained in the central fluid reservoir 10. The heat-exchange space 18 with the tubes 4, outside the tubular fluid reservoir wall 11 communicates with the central fluid reservoir 10 so that the tubes are partially immersed in the evaporable fluid b. The fluid b boils due to heat being transferred from the heating fluid a through the walls of the tubes 4. Thereby, the tubes 4 are enveloped by a two-phase mixture consisting of liquid with vapor bubbles. The density of the two-phase mixture is significantly lower than that of the liquid in the central fluid reservoir 10. This means that the liquid level in the heat-exchange space 18 outside the tubular fluid reservoir wall 11 due to the hydrostatic head formed by the liquid in the central fluid reservoir 10 is higher than the level 14 of the evaporable fluid b in the central fluid reservoir 10 and rises until unevaporated liquid b overflows the edge ofthe tubular fluid reservoir wall 11 and flows back to the central fluid reservoir 10. The vapor in the two-phase mixture formed in the heat-exchange space 18 separates from unevaporated liquid in the upper part ofthe heat-exchange space 18, as well as in the space of the central fluid reservoir 10 above the liquid level 14. Steam e is diverted from the evaporator 20 to heat consuming processes through the connection 15. Fig. 2 shows the steam e being diverted through a space being formed by a continuation of the tubular fluid reservoir wall. However, it is to be understood that the steam may be drawn off through a conduit having different dimensions than the tubular fluid reservoir wall, e.g., through a conduit having a smaller diameter than the tubular fluid reservoir wall. Furthermore, as set out herein, it is not a necessary requirement that the space above the central fluid reservoir 10 be walled-off from the reception chamber 6 as shown in Fig. 2.

As set out herein, the heating fluid a is usually condensing vapor or vapor mixtures but can alternatively be a liquid that emits sensitive heat to boiling.

The evaporator as disclosed herein has several advantages over current vertical constructions in which a fluid is boiled on the outside of the heat-exchange tubes. The central fluid reservoir 10, which is separated from the tubes 4 by the tubular fluid reservoir wall 11 , functions as a circulation container for the liquid to be evaporated, thus eliminating the need for an external pressurized container with associated piping to and from the evaporator.

The annular arrangement of the tubes 4 allows the evaporable fluid or liquid b to be distributed very evenly through the lower passage 12 to the tubes 4 in the heat-exchange space 18, since the lower passage 12 is arranged around the entire periphery (360°) of the tubular fluid reservoir wall 11 . The even distribution of evaporable fluid b means that boiling of the evaporable fluid b, takes place uniformly on all tubes 4 in the heat-exchange space 18.

Also, the upper passage 13 for vapor and circulating liquid is arranged around the entire periphery (360°) of the tubular fluid reservoir wall 11. Thus, outflow of evaporated and unevaporated evaporable fluid b through the passage 13 is evenly distributed around the circumference of the central fluid reservoir 10. This ensures a uniform flow pressure drop axially along all the tubes 4 of the evaporator, in all directions of the periphery. Liquid and vapor flow in the radial or tangential direction between the tubes 4 is eliminated or at least minimized, which ensures an even heat load over all tubes 4, i.e., boiling takes place uniformly over all tubes 4.

A stable natural circulation over a thermosyphon and pulsation-free boiling of the thermosyphon may be promoted if at least 1/3 of the total flow pressure drop takes place at the inlet for the evaporable fluid b to the tubes in the heat-exchange space 18. The size of the lower passage 12 in tubular fluid reservoir wall 11 may therefore be adapted so that 1/3 of the total flow pressure drop from the lower passage 12 to the upper passage 13 takes place at the lower passage 12.