TECHNICAL FIELD OF THE INVENTION

The present invention refers generally to a heat exchanger plant for distillation. The invention refers especially to heat exchanger plants for desalination of seawater. More specifically, it is referred to a heat exchanger plant for distillation, comprising at least one proc¬ ess line with least two successive heat exchanger stages which each comprises a plate package of heat exchanger plates provided in such a way in the plate package that first plate interspaces for condensation and second plate interspaces for evaporation are formed, wherein each heat exchanger stage is adapted to perform condensation of steam and evaporation of a liquid in such a way that the first heat exchanger stage is adapted to be supplied with steam to the first plate interspaces and a liquid to the second plate interspaces, wherein the supplied steam is condensed to a liquid and the supplied liquid is evaporated and supplied to the first plate interspaces in the next heat exchanger stage for evaporation of a liquid supplied to the second plate interspaces in this next heat ex- changer stage, and wherein the plant comprises a closed casing, which encloses an inner space in which said process line with heat exchanger stages is provided.

PRIOR ART

The applicant produces since many years equipment for desalina¬ tion of seawater, where packages with heat exchanger plates form the main components in the process. The plates have no ports for steam, but instead the plate packages are positioned in containers, and the space outside the plates is used as one or several flow paths for the steam, depending on the type of process. The large plants use cylindrical pressure vessels, and the plate packages are

positioned in the longitudinal direction of the cylinder. A large plant frequently has five or even six plate packages.

The process takes place in several so-called effects at a pressure that everywhere is below the atmospheric pressure. Steam from the first effect, which has the highest pressure and temperature, passes to the second effect where it is condensed in the plate interspaces for condensation. The heat emitted provides an evaporation of salt water in intermediate interspaces for evaporation, and the steam formed passes to the next effect. The process is repeated in the other effects, and finally a condensation takes place in the con¬ denser, where the cooling medium is water. For each effect there is at least one plate package, but the plate packages should not have more than 1000-1200 plates, so that if more plates are required two parallel plate packages are included in each effect.

If one desires even larger capacities, plants having several cylindri¬ cal vessels are built. To have three parallel plate packages within one cylindrical container is not economical. The diameter has to be adapted to three plate packages beside each other, and in compari¬ son to one container for three plate packages the diameter would increase with about 50%, and that means that the thickness of the material would increase with 50% and the total amount of material would increase with more than 100%. The strongly increasing cross-sectional area is as such favourable since the steam velocity sinks, but it has a minor economical effect. There is thus no possi¬ bility to provide a decrease of the specific costs at an estimation, which one could believe.

US-A-4, 511 ,436 discloses a plant for desalination of seawater. The plant comprises a process line with several successive heat ex¬ changer stages which each comprises a plate package of heat ex¬ changer plates that are welded in pairs to each other and provided in such a way in the plate package that first interspaces for conden- sation and second interspaces for evaporation are formed. The process line of heat exchanger plates extends vertically, wherein the first stage is located at the top. In parallel to this vertical proc-

ess line, there is a vertical heat exchanger line for pre-heating the seawater to be desalinated. The two lines are provided in a closed pressure container, which is schematically disclosed in this docu¬ ment. With regard to the constructive design of the casing it is in US-A-4, 511 ,436 referred to a parallel application that has been pub¬ lished as US-A-4, 514, 260.

This document US-A-4, 514, 260 discloses a similar plant with a number of plate packages provided on each other in a vertical pile having a height which is substantially larger than the width and the length in a horizontal plane. The plate packages are enclosed in a casing. The casing has two vertical opposite plane sides and two vertical opposite outwardly curved sides.

SUMMARY OF THE INVENTION

The object of the invention is to provide a heat exchanger plant which has a very large capacity and which can be manufactured and mounted in a favourable manner with regard to the costs. A fur- ther object of the invention is to provide such a heat exchanger plant that has such a construction that the plant can be large and include a large number of heat exchanger stages.

This object is achieved by means of the heat exchanger plant ini- tially defined, which is characterized in that it comprises at least two such process lines with successive heat exchanger stages, that said process lines extend in parallel to each other in the inner space, wherein the heat exchanger stages form rows with heat ex¬ changer stages which are provided after each other and trans- versely to said process lines in the inner space within the casing, and that the casing seen in a cross-section transversely to said process lines has a rectangular shape.

Each such row of heat exchanger stages, which each comprises a plate package, form a so-called effect of the plant. Since an enclos¬ ing casing with a rectangular shape is used instead of conventional cylindrical containers, the costs may be significantly reduced. The

shape of the casing may then in a better way be adapted to the outer contour formed by the rows of heat exchanger stages in the plant seen transversally to said lines. Pressure vessels are normally made to be cylindrical since it is an optimum shape with respect to the strength and which thus gives a minimum need of material. If the pressure vessel is subjected to external over pressures, this axiom is not as self evident, since the construction could collapse due to instability. For a cylindrical vessel, a small ovality may give local bending stresses which give a deformation resulting in even larger bending stresses and finally a total collapsing. Pressure ves¬ sels subjected to external over pressures therefore have to be dimensioned to be much stronger than if the vessel merely had been subjected for a corresponding internal over pressure. In case of vacuum vessels this is particularly significant since the low pressure gives a small thickness of material if the vessel is dimensioned merely with regard to membrane stresses, and with this thickness of material the plate stiffness becomes too low for achieving a satisfactory security against buckling. In order not to have to increase the thickness of material too much, the casing is provided with strengthening rings, but in spite thereof a thickness, which is 4-5 times larger than in a cylindrical vessel subjected to an inner over pressure, is required. Since desalination plants operate at a deep vacuum the statements made above are valid. A cylindrical container does not give a particularly large saving of material in comparison with a square one, and the more parallel plate packages that are positioned in the container the smaller is the material saving. It should be remembered that the material merely constitutes a small part of the costs for a final container, and it is not at all sure that the construction having the smallest material need gives the lowest cost. There are several factors which are important for the total economy.

According to a preferred embodiment of the invention, the plant comprises at least three such parallel process lines with successive heat exchanger stages. Each row comprises thus three beside each other arranged heat exchanger stages, which together form an ef¬ fect of the plant. Furthermore, a plant may advantageously com-

prise at least four such parallel process lines with successive heat exchanger stages. The advantages of the rectangular shape in¬ crease the larger the plant is, i.e. the more parallel process lines the plant comprises.

According to a further embodiment of the invention, each heat ex¬ changer stage is designed as a module comprising a part of the casing and adapted to be connected with respect to the flow to at least one of a preceding and a successive module in the same process line. For large plants it is important that transportations can be performed in an easy manner, and that the work at the client is minimized; it is to be reduced to merely mounting work. All qualified manufacturing should take place at the supplier or his sub-supplier. With respect thereto, the rectangular casing is advantageous since it in an easy manner may be divided in two such modules which can be manufactured in a rational manner at the factory and then trans¬ ported in a relatively easy manner to the mounting site.

According to a further embodiment of the invention, each module is designed as either an inner module, which is adapted to be pro¬ vided between two adjacent modules in the same row, or an outer module, which is adapted to be provided adjacent to merely one ad¬ jacent module in the same row. From a designing point of view there are thus two modules. The outer module is present in a right design and a left design, but since these can be made completely symmetrical they only form one construction. Advantageously, each module may be adapted to be connected with respect to the flow to at least one adjacent module in the same row. Furthermore, said part of the casing of each module may be adapted to be connected mechanically to at least one adjacent module in the same row and to at least one of a preceding and successive module in the same process line.

According/to a further embodiment of the invention, each process line comprises at least three such successive heat exchanger stages, wherein at least a part of the liquid which is evaporated in the second heat exchanger stages is supplied to the first plate

interspaces of the third heat exchanger stages for evaporation of a liquid supplied to the second plate interspaces of the third heat ex¬ changer stages. Furthermore, each process line may advanta¬ geously comprise at least four such successive heat exchanger stages, wherein at least a part of the liquid which is evaporated in the third heat exchanger stages is supplied to the first plate inter¬ spaces of the fourth heat exchanger stages for evaporation of a liq¬ uid supplied to the second plate interspaces of the fourth heat ex¬ changer stages. Each process line may of course comprise further such successive heat exchanger stages, for instance five, six, seven, eight, nine or more.

According to a further embodiment of the invention, the casing is designed to permit the maintaining of a substantially lower pressure in the inner space then in the surroundings outside the casing.

According to a further embodiment of the invention, the plant is de¬ signed in such a way that the rows with the heat exchanger stages extend substantially horizontally.

According to a further embodiment of the invention, the plant is de¬ signed in such a way that the process lines extend substantially horizontally.

According to a further embodiment of the invention, the plant is de¬ signed in such a way that the process lines extend substantially vertically. For very large plants, the costs for the casing may be fur¬ ther reduced if the plate packages are positioned in several rows in several planes. With such a design, the outer surface and the re- quired ground area may be minimized.

According to a further embodiment of the invention, the first plate interspaces and the second plate interspaces in the plate packages are sealed by means of gaskets. In such a way the plate packages may be opened for cleaning and repair.

According to a further embodiment of the invention, a liquid separa¬ tor is provided in each process, line in connection to substantially each heat exchanger stage.

According to a further embodiment of the invention, the plant com¬ prises a thermo compressor which is adapted to be operated through the supply of external steam at a high pressure and which is adapted to receive at least a part of the steam produced in at least the last heat exchanger stages for mixing of this part and the external steam, wherein the mixture forms said steam supplied to the first heat exchanger stages.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now to be explained more closely through a de¬ scription of various embodiments and with reference to the draw¬ ings attached hereto.

Fig. 1 discloses a sectional view from the side through a heat exchanger plant according to a first embodiment of the invention.

Fig. 2 discloses a sectional view from above through the plant in Fig. 1.

Fig. 3 discloses a side view of a heat exchanger stage of the plant in Fig. 1.

Fig. 4 discloses a sectional view through the plant in Fig. 1 along the line IV-IV.

Fig. 5 discloses a view of a first outer module of the heat ex¬ changer plant in Fig. 1. F Fiigg.. 6 6 discloses a view of an inner module of the heat ex¬ changer plant in Fig. 1.

Fig. 7 discloses a view of a second outer module of the heat exchanger plant in Fig. 1.

Fig. 8 discloses a side view of the plant in Fig. 1. F Fiigg.. 99 discloses a first sectional view from the side through a heat exchanger plant according to a second embodiment of the invention.

Fig. 10 discloses a second sectional view through the heat ex¬ changer plant in Fig. 9 along the line X-X.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

Figs. 1-4 disclose a heat exchanger plant for distillation, especially for desalination of seawater. The heat exchanger plant disclosed comprises a four process lines 1. Each process line 1 , which ex- tends in a longitudinal direction of the plant, comprises five succes¬ sive heat exchanger stages 2a-2e, which each comprises a plate package 3 of heat exchanger plates 4 provided in such a way in the plate package 3 that first plate interspaces 5 and second plate interspaces 6 are formed. The four process lines 1 extend in paral- IeI to each other in such a way that the heat exchanger stages 2a- 2e form rows 8 of heat exchanger stages 2a-2e. Each row 8 with heat exchanger stages 2a, 2b, 2c and 2d, respectively, forms a so- called effect.

These rows 8 lies after each other and extend in a transversal di¬ rection of the plant, i.e. transversally to the process lines 1. The rows 8 extend in the embodiment disclosed substantially perpen¬ dicularly to the parallel longitudinal process lines 1. It is to be noted that the plant according alternative designs may comprise another number of rows 8 and process lines 1 then the ones disclosed.

The plant comprises a closed casing 10, which encloses an inner space 11 in which the four process lines 1 with heat exchanger stages 2a-2e are provided. The casing 10 is designed as a pressure container permitting the maintaining of a substantially lower pres¬ sure in the inner space 11 than in the surrounding atmosphere di¬ rectly outside the casing 10. A separation wall 12 extends substan¬ tially horizontally in the casing 10 and divides the inner space 11 into substantially two longitudinal halves. Furthermore, the different heat exchanger stages 2a-2d are separated from each other through vertical walls 18, 18'. The walls 12, 18 and 18' thus create a number of upper spaces 13 and a number of lower spaces 14, see

Fig. 3. Each plate package 3 is provided in such a way that it ex¬ tends through the separation wall 12, wherein an upper part of the plate package 3 is located in such an upper space 13, and a lower part of the plate package 3 in such a lower space 14.

One of the plate packages 3, which each may comprise a large number of heat exchanger plates 4, for instance 500-1500, is dis¬ closed more closely in Fig. 3. Each plate package 3 may be kept together by for instance four tie bolts (not disclosed), which extend through a frame plate and a pressure plate (not disclosed) of each plate package 3. In each plate interspace 5 and 6 in the plate pack¬ age 3, gaskets 15 and 15', respectively, are provided for sealing the plate interspaces 5 and 6. More specifically, gaskets 15 are pro¬ vided in such a way that the first plate interspaces 5 for condensa- tion are sealed against the respective lower space 14, and gaskets 15' are provided in such a way that the second plate interspaces 6 for evaporation are sealed against the respective upper space 13, see Fig. 3.

Furthermore, the plant comprises a passage through the separation wall 12 between each heat exchanger stage 2a-2b, 2b-2c, 2c-2d and 2d-2e. In substantially each such passage, a liquid separator 16a-16d is provided.

The plant also comprises a thermo compressor 20 which is adapted to be operated through the supply of external steam at a high pres¬ sure in a manner known per se. The external steam is supplied to the thermo compressor 20 via a supply conduit 21. The thermo compressor 20 supplies steam at a pressure and a temperature to the first heat exchanger stages 2a via an inlet conduit 22. This pressure and this temperature correspond to the pressure and tem¬ perature in the first heat exchanger stages 2a, but is lower than the surrounding atmospheric pressure and the surrounding tempera¬ ture, respectively. The pressure and the temperature decrease then successively in the successive heat exchanger stages 2b-2e. A part of the steam which is discharged from one or several of the last heat exchanger stages, in this example the penultimate heat ex-

changer stages 2d, is returned to the thermo compressor 20 via a conduit 23. The thermo compressor 20 comprises a nozzle for the recirculation of the returned steam to the inlet conduit 22 by means of the external steam.

The liquid to be distilled, in this example a salt-bearing liquid, so called brine, is supplied via a schematically disclosed feed conduit 30. The feed conduit 30, which may be more complex than dis¬ closed in Fig. 1 , is arranged to provide a salt-bearing liquid at a temperature that is adapted to the temperature in each stage 2a-2d. The salt-bearing liquid is supplied to the second plate interspaces 6 in each plate package 3 in the four first heat exchanger stages 2a- 2d via the feed conduit 30 and a port channel 31 in each plate package 3, see Fig. 4. The liquid supplied will be heated and at least partly evaporated by the steam in the adjacent first plate inter¬ spaces 5. The steam in the first plate interspaces 5 will then be condensed and discharged as liquid via two port channels 34 in each plate package, see Fig. 4, and a discharge conduit 35, see further below. It is to be noted here that in the embodiment dis- closed in Figs. 1-4, the last heat exchanger stages 2e are pure condensation stages for the condensation of the steam from the preceding heat exchanger stages 2d. The condensation may be provided through circulation of an external cooling agent by means of a circulation conduit 32 and suitable port channels in each plate package 3 in the last heat exchanger stages 2d. At least a part of the external cooling agent may via the feed conduit 30 be supplied to the different heat exchanger stages 2a-2d. The heat exchanger stage 2e is then used for preheating of a salt-bearing liquid, see Fig. 1.

Each heat exchanger stage 2a-2e is thus adapted to perform con¬ densation of steam in the first plate interspaces 5. Furthermore, each heat exchanger stage 2a-2d, except the last heat exchanger stages 2e, are adapted to perform evaporation of a liquid in the second plate interspaces 6. More specifically, the first heat ex¬ changer stages 2a are supplied with steam to the first plate inter¬ spaces 5 via the inlet conduit 22 and the upper space 13. A salt-

bearing liquid is supplied to the second plate interspaces 6 of the first heat exchanger stages 2a via the feed conduit 30. The supplied steam is condensed to a liquid that is discharged from the first heat exchanger stages 2a via the port channels 34 and the discharge conduit 35. All liquid, which is discharged via the discharge conduit from all heat exchanger stages 2a-2e has a high purity with a very low salt content. The supplied liquid is partly evaporated and dis¬ charged in the lower space 14. From the lower space 14, the steam may pass to the upper space 13 via the first liquid separator 16a. Liquid droplets of the salt-bearing liquid, which has not been evapo¬ rated and which in the embodiment disclosed contains salt, will then be catched and conveyed back as excess liquid to a bottom space 37 in a lower part of lower space 14. This bottom space 37 is thus in the embodiment disclosed adapted to contain a salt-bearing ex- cess liquid, so called brine. It is to be noted that in order to ensure wetting of the heat exchanger surfaces in the first plate interspaces 5 several times more salt-bearing liquid is supplied then is evapo¬ rated.

The steam that passes through the first liquid separator 16a is sup¬ plied to the upper space 13 and the first plate interspaces 5 in the second heat exchanger stage 2b for evaporation of the liquid sup¬ plied to the second plate interspaces 6 in the second heat ex¬ changer stages 2b via the feed conduit 30. The steam that is con- densed in the first plate interspaces 5 in the second heat exchanger stages 2b is discharged via the port channels 34 in the plate pack¬ ages 3 and via the discharge conduit 35. The supplied liquid is evaporated and discharged in the lower space 14. From the lower space 14, the steam may pass to the upper space 13 of the third heat exchanger stages 2c via the second liquid separator 16b. Liq¬ uid, which in the embodiment disclosed contains salt, will then be catched and conveyed back as excess liquid to the bottom space 37.

The steam that passes through the second liquid separator 16b is supplied to the upper space 13 and the first plate interspaces 5 in the third heat exchanger stages 2c for evaporation of the liquid

supplied to the second plate interspaces 6 in the third heat ex¬ changer stages 2c via the feed conduit 30. The steam that is con¬ densed in the first plate interspaces 5 in the second heat exchanger stages 2c is discharged via the port channels 34 in the plate pack- ages 3 and via the discharge conduit 35. The supplied liquid is evaporated and discharged in the lower space 14. From the lower space 14, the steam may pass through the upper space 13 of the fourth heat exchanger stages 2d via the third liquid separator 16c. Liquid, which in the embodiment disclosed contains salt, will then be catched and conveyed back as excess liquid to the bottom space 37.

The steam that passes through the third liquid separator 16c is supplied to the upper space 13 and the first plate interspaces 5 in the fourth heat exchanger stages 2d for evaporation of the liquid supplied to the second plate interspaces 6 in the fourth heat ex¬ changer stages 2e via the feed conduit 30. The steam, which is condensed in the first plate interspaces 5 in the fourth heat ex¬ changer stages 2d, is discharged via the port channels 34 in the plate packages 3 and via the discharge conduit 35. The supplied liquid is evaporated and discharged in the lower space 14. From the lower space 14, the steam may pass to the upper space 13 of the fifth heat exchanger stages 2e via the fourth liquid separator 16d. Liquid, which in the embodiment disclosed contains salt, will then be catched and conveyed back as excess liquid to the bottom space 37.

The steam that passes through the fourth liquid separator 16d is supplied to the upper space 13 of the fifth heat exchanger stages 2e. From this upper space a part of the steam is sucked to the thermo compressor 20 via the conduit 23 whereas the rest of the steam is supplied to the first plate interspaces 5 in the fifth heat ex¬ changer stages 2e. The steam, which is condensed in the first plate interspaces 5 in the fifth heat exchanger stages 2e, is discharged via the discharge conduit 35. It is to be noted that the fifth heat ex¬ changer stages 2e, which are adapted to perform the final condens¬ ing, may comprise heat exchanger stages of another kind than the

preceding stages 2a-2d, for instance plate packages with plates of another type or completely different types of heat exchangers, for instance tube condensers.

One or several of the heat exchanger stages 2a-2d may also com¬ prise a preheater 40 for preheating the salt-bearing liquid which is to be supplied to the first plate interspaces 5 via the feed conduit 30. Such a preheater 40 is schematically disclosed in Fig. 1 for pre¬ heating of the salt-bearing liquid by means of the steam supplied to the heat exchanger stages 2c.

It is also to be noted that it is possible to let at least a part of the excess liquid from the heat exchanger stages 2a, 2b, 2c pass di¬ rectly from the lower space 14 to the lower space 14 of the next row 8 with heat exchanger stages 2b, 2c, 2d via a flash chamber 42, see Fig. 3. Excess liquid from one row 8 with heat exchanger stages 2a, 2b, 2c is conveyed via one or several conduits 41 into the flash chamber 42, where the same pressure as in the next row with heat exchanger stages 2b, 2c, 2d prevails. Due to the pressure decrease the excess liquid will thus be evaporated. The liquid thus formed is conveyed via one or several relatively large openings 43 in the lower space in the next row 8 with heat exchanger stages 2b, 2c, 2d.

The discharge conduit 35 may also be connected to a flash tank 39 downstream at least some of the heat exchanger stages, in the em¬ bodiment disclosed downstream the heat exchanger stages 2b, 2c and 2d. The condensate from the respective plate package 3 is conveyed via the discharge conduit 35 to the flash tank 39 where a lower pressure than in the respective plate package 3 prevails. Due to the pressure decrease, a part of the condensate will be evapo¬ rated through flashing. The steam formed is returned to the process in the next row 8 with heat exchanger stages via suitable conduits (not disclosed). The remaining condensate is discharged from the tanks 39 via the conduit 40.

The casing 10 has, seen in the cross-section shown in Fig. 4, a rec¬ tangular shape. The opposite upper and lower walls 51 and 52 are plane, substantially horizontal and substantially parallel. The oppo¬ site side walls 53 and 54 are plane, substantially vertical and sub- stantially parallel. The plant is also constructed of a number of modules 61-63 for an easy premanufacturing at a factory and an easy mounting at the site where the plant is to be mounted. Each module 61-63 comprises one of the plate packages 3 and a part of the casing 10. Each module 61-63 is adapted to be connected with respect to the flow to at least one of a preceding and a successive module in the same process line 1. Furthermore, each module 61- 63 is adapted to be connected with respect to the flow to at least one adjacent module 61-63 in the same row 8. In the embodiment disclosed, the steam flow may pass from one heat exchanger stage to the next. There is however no partition between adjacent plate package as in each row 8, which means that the steam flow in one process line 1 may be spread over to adjacent process lines 1 in the successive row 8.

Each module 61-63 may be designed as either an inner module 61 , which is adapted to be provided between two adjacent modules in the same row 8, or as an outer module 62-63, which is adapted to be provided adjacent to merely one adjacent module 61 , 63 and 61 , 62, respectively, in the same row 8. An inner module 61 is disclosed in Fig. 6. Each outer module 62, 63 may be designed as a left mod¬ ule 62 or a right module 63. A left module 62 is disclosed in Fig. 5 and a right module 63 is disclosed in Fig. 7.

The above-mentioned part of the casing 10 of each module 61-63 is adapted to be connected mechanically to at least one adjacent module 61-63 in the same row 8 and to at least one of a preceding and a successive module 61-63 in the same process line 8. Accord¬ ing to one embodiment, the mechanical connection may be achieved by connecting the modules 61-63 to each other by means of weld joints, i.e. the casing 10 of each module 61-63 is welded to the casing 10 of an adjacent module 61-63.

According to another embodiment, each inner module 61 may com¬ prise vertical longitudinal flanges 70 adapted to abut corresponding vertical longitudinal flanges 70 of an adjacent module 61-63. The modules 61-63 may then be connected to each other through suit- able connections, for instance screw connections. The outer mod¬ ules 62-63 differ from the inner modules 61 since they merely com¬ prise flanges 70 on one side. Furthermore, each module 61-63 may comprise vertical transversal flanges 71 adapted to abut corre¬ sponding vertical transversal flanges 71 of an adjacent module 61- 63 in the same process line 1. These flanges 71 are indicated in Fig. 8. The first and last module 61-63 in each process line may be closed by means of a cover 73 of a suitable design. In the joints be¬ tween different modules 61-63 in the longitudinal direction and the transversal direction, gaskets 74 may be provided, see Figs. 5 and 7.

The flash tanks 39 have in the embodiment disclosed been located outside the casing 10, but it is also possible to arrange them inside the casing 10.

Figs. 9 and 10 disclose schematically a heat exchanger plant ac¬ cording to a second embodiment. Elements having substantially the same function have been given the same reference signs in the two embodiments. According to the second embodiment, the process lines 1 with successive heat exchanger stages 2a-2g extend not in a longitudinal horizontal direction but in a longitudinal vertical direc¬ tion. The rows 8 with plate packages 3 extend as in the first em¬ bodiment horizontally and transversally to the longitudinal process lines 1. The width of the last row 8 with the last heat exchanger stages 2g, which are adapted for the final condensation, is in this embodiment larger than the width of the preceding rows 8 with re¬ spect to the heat exchanger stages 2a-2f. The heat exchanger stages 2g that are disclosed in Figs. 9 and 10 have been realised by means of a tube condenser. The second embodiment is suitable for very large plants and comprises as appears three thermo com¬ pressors 20 with three feed conduits 22. The casing 10 is in this embodiment approximately cubic, which means that the outer sur-

face area of the casing 10 is minimized. The compact construction also results in very short distances for piping. The required ground area is very small in comparison with the ground area required for a plant with horizontally lying process lines 1.

The invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims.