Abstract:
The present invention provides a multi stage flash long tube evaporator with flash stages arranged in a plurality of at least three tiers, wherein each tier is divided into a plurality of at least two flash stages and a plurality of at least three tube bundles are arranged in parallel and in a longitudinal direction in each tier. This configuration allows to minimize the evaporator shell volume, shell surface, foot print and weight, minimizing the cost of an evaporator and other related plant cost. This configuration is in particular suitable for large evaporator unit capacities.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     REFERENCE TO A SEQUENCE LISTING 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     A multi-stage flash evaporator is the main component of a seawater desalination plant for producing distilled water from seawater. Most evaporators for large capacity desalination plants are currently of a ‘cross tube’ type with all flash stages arranged in a single tier configuration, being build with evaporator unit capacities up to about 25 million gallons per day or about 100,000 cubic meters per day of distillate production. 
     A multi-stage flash evaporator comprises a plurality of flash stages, in existing plants typically between 15 and 30. While a heated solution, typically seawater or brine, enters the first flash stage at its highest temperature, the solution flashes down in each consecutive flash stage to a lower temperature compared to the temperature of the solution in the previous flash stage, releases some vapor which is then condensed on a tube bundle and collected as distillate. The salt concentration of the solution is increasing toward the last flash stage. A coolant enters with its lowest temperature into the tube bundle(s) at the last flash stage and its temperature increases in each flash stage relative to its temperature in the previous flash stage as vapor is condensing on the tube bundles. The coolant discharging from the tube bundle(s) of the first flash stage is further heated in a separate heat exchanger, commonly described as the heat input section or brine heater, by an external heat source to a top temperature. The coolant is than used as the solution, also described as flashing brine, fed into the first flash stage. 
     A multi-stage flash desalination system may be designed as a “once through” process in which one type of coolant is being conveyed through the tube bundles of all flash stages, starting from the last flash stage with the lowest operation temperature to the first flash stage operating at the highest temperature. 
     The most common design concept for multi stage flash desalination plants is the “brine re-circulation” system, in which the evaporator comprises a heat recovery section and a heat rejection section. The heat rejection section comprises a plurality of flash stages including the last flash stage, in which typically fresh seawater is used as a first coolant for the tube bundles. The heat rejection section is designed such, that the first coolant is capable, to remove together with the discharging distillate and the discharging concentrated solution, the majority of the heat introduced into the system through the heat input section. In the heat recovery section, which occupies typically the larger number of flash stages of an evaporator including the first flash stage, the heat released from the solution is recovered by a second coolant and used to bring the second coolant toward the desired top temperature. A mixture of a part of the concentrated solution discharging from the last flash stage of the evaporator and a part of the first coolant discharging from the heat rejection section, described mostly as re-circulating brine, is commonly used as the second coolant for the heat recovery section. The portion of the first coolant used as part of the second coolant, replaces primarily the amount of distillate and concentrated solution discharging from the system. It may be treated in order to limit scaling of the tube bundles and to limit corrosion in the evaporator. 
     Individual types of evaporators may be differentiated by the tube bundle configuration such as ‘long tube’ evaporators and ‘cross tube’ evaporators. In a long tube evaporator, the tube bundles are substantially oriented in the flow direction of the solution in the flash stages. A long tube evaporator of the prior art typically comprises a plurality of individual evaporator modules. Each module comprises typically one tube bundle with a tube sheet and a water box on each end. The individual evaporator modules are typically internally divided by stage divider walls into a plurality of flash stages. The tube bundles are also divided by the stage divider walls into a plurality of tube bundle elements, so that each flash stage comprises one tube bundle element, configured to condense the vapor released from the solution in the individual flash stages. The coolant is typically conveyed through the tube bundles of the individual modules of an evaporator unit in serial flow communication. Evaporator modules comprising two tube bundles fed with coolant in parallel have been designed and build as well. Such long tube evaporators have been preferred until about the early 1980&#39;s when the maximum evaporator capacities have been in the range of about 30% of current evaporator capacities. 
     The cross tube evaporator became for larger capacities the preferred and more economical evaporator configuration. In a cross tube evaporator, the tube bundles are oriented substantially transversally to the flow direction of the solution in the flash stages. Cross tube type evaporators typically comprise an individual single pass tube bundle in each flash stage. Evaporator configurations with double pass tube bundles or common tube bundles for a pair of flash stages are also known. The cross tube evaporators have technically only limited possibilities to increase the unit capacities beyond the maximum unit capacities of evaporators currently in operation, mainly due to limitations of available tube length for tube bundles. 
     Evaporators with large unit capacities are typically designed and build in a single tier configuration, meaning, all flash stages being arranged on the same level. Double tier configurations, with flash stages arranged in two tiers stacked on top of each other, have been designed and build as well. In some cases a common horizontal tier partition has been used between the top and bottom tier, while in other cases two individual evaporator modules, each having its own shell roof and shell bottom structure have been stacked on top of each other. 
     BRIEF SUMMARY OF THE INVENTION 
     The main concept of the present invention is a multi stage flash long tube evaporator, comprising a plurality of at least three (3) vertically stacked tiers, each tier comprising a plurality of flash stages and a plurality of at least three (3) parallel arranged tube bundles, each tube bundle being configured to extend in a longitudinal direction through all flash stages of an individual tier. The present invention is aiming to minimize the overall dimensions, foot print, volume and weight of a multi stage flash evaporator, to minimize material quantities required for the manufacturing and the space required in a plant layout. The present invention is aiming in particular to evaporators with medium to large unit capacities in the range from approximately 10 million gallons per day or about 40,000 cubic meters per day to any desired capacity of distillate production, allowing also to be expanded to evaporator unit capacities, significantly larger than the evaporator concepts of prior art. 
     The configuration of a multi stage flash long tube evaporator of the present invention allows to divide individual tiers into any number of flash stages as technically and economically feasible, without increasing the evaporator size, which is a significant advantage over a ‘cross tube’ configuration. The increase of number of flash stages leads technically to an increase of the log mean temperature difference on the tube bundles, which in turn allows to reduce the required total tube surface area for a given evaporator capacity and evaporator performance ratio (ratio of mass units of distillate generated per thermal energy unit consumed by a desalination unit, typically in the range of 7-12 lbs/1000 BTU or approximately 3-5 kg/1000 kJ). 
     The concept of using a plurality of at least three individual tube bundles arranged in parallel in each tier of an evaporator of the present invention, allows to minimize the height of the flash stages compared to an arrangement with only one or two tube bundles traditionally applied to long tube evaporators of the prior art, which leads in combination with the stacked tier configuration to a smaller volume and small foot print of the evaporator compared to a long tube or cross tube configuration of the prior art with medium to large unit capacity. 
     The width of an evaporator of the present invention is mainly determined by the space required to convey the solution through the flash stages and the area required to release the vapor from the solution, which provides in any case sufficient space to allow the parallel arrangement of a plurality of at least three tube bundles in each individual tier. 
     The relatively small cross sectional area of the individual tube bundles also allows to maintain relatively low vapor side pressure losses, which is an important factor in regard to the performance of an evaporator and the required total tube surface area. 
     An additional feature of the multi stage flash long tube evaporator of the present invention is the possibility to produce in an efficient way, beside the distillate which is substantially generated from a first vapor released from the solution, also a high purity distillate from a second vapor released from accumulated distillate. The high purity distillate may be for example used for any type of steam generation system required in conjunction with a seawater desalination plant, saving other means of water purification required to achieve the high purity water quality, typically specified for steam generators. 
     For a multi stage flash long tube evaporator of the present invention, there is technically no restriction in the possible evaporator unit capacity, since the width of the evaporator and the number of parallel installed tube bundles in individual tiers can be increased to accommodate any distillate production capacity and required solution flow, maintaining a limited allowable liquid loading for the solution (liquid loading=solution mass flow rate per ft or meter width of a flash stage). 
     The multiple tier configuration allows an easier control of the solution levels in the plurality of flash stages located in an individual tier, due to the static head available for the solution flow, when passing from one tier to the next tier below, which prevents largely any influence on the solution levels in one tier due to level changes in an other tier. 
     Applying a plurality of at least three tiers for the flash stage arrangement in an evaporator, with common horizontal tier partitions between the individual tiers, reduces the temperature differences and associated thermal stresses in evaporator parts, compared to a double tier configuration. 
     This brief summary has been provided, so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The present invention will be better understood from the following detailed description of an exemplary embodiment of the present invention, taken in conjunction with the accompanying drawings in which like reference numerals refer to like parts and in which: 
         FIG. 1  shows an example of the multi stage flash long tube evaporator comprising of 4 tiers stacked vertically and each tier comprising 6 tube bundles arranged in parallel; 
         FIG. 2  shows a right side view of the multi stage flash long tube evaporator with a plurality of at least 3 tiers stacked vertically; 
         FIG. 3  shows a front view of the multi stage flash long tube evaporator with a plurality of at least 3 tiers stacked vertically; 
         FIG. 4  shows a cross section of the multi stage flash long tube evaporator, taken along the lines  4 - 4  as indicated in  FIG. 2 , showing a plurality of flash stage elements, respectively flash stage element rows in each tier, with one tube bundle located in each of the flash stage element rows; 
         FIG. 5  shows a longitudinal section of the multi stage flash long tube evaporator, taken along lines  5 - 5  as indicated in  FIG. 3 , showing in each tier a tube bundle extending in a longitudinal direction substantially through all the flash stages of the individual tiers; 
         FIG. 6  shows a cross section of the multi stage flash long tube evaporator, taken along lines  6 - 6  as indicated in  FIG. 2 , with a first and second type of a longitudinal partition wall dividing the flash stages of individual tiers into a first and second flash stage part, configured to allow accumulated distillate to flow in parallel to the solution on top of the horizontal tier partitions or shell bottom; 
         FIG. 7  shows a cross section of the multi stage flash long tube evaporator, taken along the lines  7 - 7  as indicated in  FIG. 2 , with a third type of longitudinal partition walls dividing the flash stages in individual tiers into a first and second flash stage part, configured to allow accumulated distillate to flow in parallel to the solution on top of the horizontal tier partitions or shell bottom, and to collect a high purity distillate from a condensing second vapor in the second flash stage parts; 
         FIG. 8  shows a longitudinal section of the multi stage flash long tube evaporator, taken along the lines  8 - 8  as indicated in  FIG. 7 , showing in the top tier the solution and the first vapor released from the solution, while showing in the bottom tier and the intermediate tier located directly above, the accumulated distillate and the second vapor released from the accumulated distillate in the second flash stage parts; 
         FIG. 9  shows a cross section along the lines  9 - 9  as indicated in  FIG. 2 , showing in place of the tube bundle shrouds, longitudinal tube bundle walls, configured to create a plurality of first channels in between the tube bundles or between a tube bundle and the left side wall or right side wall, and a plurality of second channels, one of each located below each tube bundle and showing further a third type of longitudinal partition walls in the individual tiers in different positions; 
         FIG. 10  shows a longitudinal section of the multi stage flash long tube evaporator, taken along lines  10 - 10  as indicated in  FIG. 3 , with horizontal tier partitions and shell bottom having a slope in flow direction of the solution flowing on top of the individual horizontal tier partition or on top of the shell bottom; 
         FIG. 11  shows a simplified flow schematic of a desalination unit based on the ‘brine recirculation’ concept with the multi stage flash long tube evaporator; 
         FIG. 12  shows a simplified flow schematic of a desalination unit based on the ‘once through’ concept with the multi stage flash long tube evaporator; 
         FIG. 13  shows a simplified flow schematic of the bottom tier of a multi stage flash long tube evaporator in which high purity distillate generated from the second vapor released from the accumulated distillate in the second flash stage parts is collected separately from the distillate which is mainly generated from the first vapor released from the solution in the first flash stage parts; 
         FIG. 14  shows an example of the multi stage flash long tube evaporator comprising of 3 modules, wherein each module comprises at least one of the total number of tiers of the evaporator; 
         FIG. 15  shows an example of the multi stage flash long tube evaporator comprising of 2 modules, wherein each module comprises a part of the total number of flash stage element rows of each tier of the evaporator. 
     
    
    
     For a better understanding of the present invention, the flow of liquids and vapor are shown in individual Figures in form of arrows indicating in individual positions the main flow direction. 
     DETAILED DESCRIPTION OF THE INVENTION 
     An example of a multi stage flash long tube evaporator  1  of the present invention is shown in  FIG. 1 . Details are shown in a right side view  FIG. 2 , a front view  FIG. 3 , a cross section  FIG. 4  and a longitudinal section  FIG. 5 . The evaporator  1  comprises a shell  2  with a shell bottom  2   a , a shell roof  2   b , a left side wall  2   c , a right side wall  2   d , a front wall  2   e  and an end wall  2   f . The shell  2  is internally divided by a plurality of horizontal tier partitions  6   a  into a plurality of tiers, with a top tier  3   a , at least one intermediate tier  3   b  or further intermediate tiers  3   c  . . . and a bottom tier  3   n . The horizontal tier partitions  6   a  are extending substantially in a longitudinal direction of the evaporator  1  from the front wall  2   e  to the end wall  2   f  and in a transversal direction from the left side wall  2   c  to the right side wall  2   d . The individual tiers  3   a ,  3   b  . . . ,  3   n  are divided by a plurality of flash stage partition walls  6   c  into a plurality of flash stages, with a first flash stage  4   a  located in the top tier  3   a  next to the front wall  2   e , a plurality of intermediate flash stages  4   b ,  4   c  . . . , and a last flash stage  4   n  located in the bottom tier  3   n . The flash stage partition walls  6   c  are arranged substantially vertically and in parallel to the front wall  2   e  and end wall  2   f , extending from the left side wall  2   c  to the right side wall  2   d . In the top tier  3   a  the flash stage partition walls  6   c  extend from the horizontal tier partition  6   a  which is separating the top tier  3   a  from the first intermediate tier  3   b  to the evaporator roof  2   b . In the intermediate tiers, the flash stage partition walls  6   c  extend in between the horizontal tier partitions  6   a , separating the individual tier  3   b  or  3   c  . . . from the other tiers located directly below and above, while they extend in the bottom tier  3   n  from the shell bottom  2   a  to the horizontal tier partition  6   a  located directly above the bottom tier  3   n.    
     Each of the flash stages  4   a ,  4   b ,  4   c  . . . to  4   n  are comprising in a transversal direction between the left side wall  2   c  and the right side wall  2   d  a plurality of at least three flash stage elements, with a left flash stage element  5   a  located next to the left side wall  2   c , at least one intermediate flash stage element  5   b , or further intermediate flash stage elements  5   c  . . . and a right flash stage element  5   n  located next to the right side wall  2   d , wherein all flash stages located in the same tier, comprise the same number of flash stage elements. 
     The individual flash stage elements  5   a ,  5   b ,  5   c  . . . ,  5   n  of all flash stages located in an individual tier are forming in longitudinal direction in between the front wall  2   e  and the end wall  2   f  rows of flash stage elements, with a left row of flash stage elements  5   aa , at least one intermediate row of flash stage elements  5   bb , or further intermediate rows of flash stage elements  5   cc  . . . , and a right row of flash stage elements  5   nn . Each row of flash stage elements  5   aa ,  5   bb  . . . ,  5   nn , comprises one of a plurality of tube bundles  10 , each tube bundle  10  extends in the longitudinal direction substantially from the front wall  2   e  to the end wall  2   f . Each tube bundle  10  is divided by the flash stage partition walls  6   c  into a plurality of tube bundle elements  10   f , so that each of the flash stage elements  5   a  to  5   n  in each of the flash stages  4   a  to  4   n  comprises one tube bundle element  10   f . Each tube bundle  10  comprises a plurality of straight tubes  10   a  arranged substantially horizontally and in longitudinal direction, tube sheets  10   b , one of each installed on or near to the front wall  2   e  and end wall  2   f  and, if required, a plurality of tube support plates  10   c , installed in between the tube sheets  10   b  and the flash stage partition walls  6   c  located next to the tube sheets  10   b , or in between the individual flash stage partition walls  6   c . The flash stage partition walls  6   c , the tube sheets  10   b  and the tube support plates  10   c  have tube holes arranged in the same pattern, to allow the installation of the straight tubes  10   a.    
     A pair of water boxes  11  are installed on each of the single pass tube bundles  10 , to feed a first coolant  20  or a second coolant  22  into each single pass tube bundle  10  and to collect the first coolant  20  or second coolant  22  discharging from each of the tube bundles  10 . 
     The evaporator  1  and the associated parts of the evaporator  1  are configured to convey a solution  25  from the first flash stage  4   a  through all flash stages in serial flow communication, to allow the solution  25  to flash down in each flash stage  4   a  to  4   n  to a temperature lower than the temperature of the solution  25  in the previous flash stage, to allow the solution  25  to release a first vapor  26   a  in each flash stage  4   a  to  4   n , to condense the first vapor  26   a  on the tube bundle elements  10   f , located in the individual flash stages and to collect the condensed first vapor  26   a  as distillate  24 , to convey the solution  25  in the flash stages located in the top tier  3   a  into a first flow direction from the first flash stage  4   a  toward the end wall  2   f , to convey the solution  25  either in at least one of the intermediate tiers  3   b ,  3   c  . . . or in the bottom tier  3   n  in a flow direction opposite to the flow direction of the solution  25  in the tier located directly above the at least one tier as illustrated in  FIG. 5 , or to convey the solution  25  in all of the intermediate tiers  3   b ,  3   c  . . . and in the bottom tier  3   n  in the same flow direction as the solution  25  in the top tier  3   a , to convey the first coolant  20  and the second coolant  22  in the plurality of tube bundles  10  located in an individual tier  3   a ,  3   b , . . .  3   n  in parallel and in a flow direction opposite to the flow direction of the solution  25  in the same tier, to accumulate and convey the distillate  24  through all flash stages in the same flow direction as the solution  25 , to allow the accumulated distillate  24  to flash down in the flash stages  4   b  to  4   n  to a temperature lower than the temperature of the distillate  24  in the previous flash stage, to allow the accumulated distillate  24  to release a second vapor  26   b  in each of the flash stages  4   b  to  4   n , and to condense the second vapor  26   b  on the tube bundle elements  10   f , located in the individual flash stages. 
     Mist eliminators  9  may be installed in the individual flash stages such, that the first vapor  26   a  released from the solution  25  is passing through the mist eliminators  9  before entering into and condensing on the tube bundles  10 , so that salt water droplets carried in the vapor  26   a  are largely eliminated, to achieve a desired distillate purity. 
     A plurality of solution orifices  7   a  are located in the flash stage partition walls  6   c . The solution orifices  7   a  are arranged over the width of the partition walls  6   c  between the left side wall  2   c  and the right side wall  2   d  and are sized such, that the solution  25  can pass through, from one flash stage to the next flash stage, driven by a differential pressure between two flash stages, while maintaining a level of the solution  25  above the solution orifice  7   a , so that no first vapor  26   a  would pass through the solution orifice  7   a . The differential pressure between the flash stages is maintained by the flash down of the solution  25  in each flash stage and the resulting temperatures of the first vapor  26   a  and corresponding saturation pressure in each flash stage. 
     Splash hoods  8  or similar devices may be installed downstream of the solution orifices  7   a.    
     Tube bundle shrouds  10   d  as shown in  FIGS. 4 ,  5 ,  6 ,  7 ,  8  and  10  are formed around the lower part of the tube bundle elements  10   f , to guide the first vapor  26   a  released from the solution  25  through the mist eliminators  9  before entering into the tube bundle elements  10   f . The tube bundle shrouds  10   d  serve also to accumulate the distillate  24  from the condensing first vapor  26   a  and second vapor  26   b  and may serve also to convey the accumulated distillate  24  through the flash stages. For this purpose distillate orifices  7   b  may be cut into the flash stage partition walls  6   c , allowing the distillate to pass from one flash stage to the next. 
     For the purpose to accumulate and convey the distillate  24  in the bottom tier  3   n  on top of the shell bottom  2   a  or in the other tiers on top of a horizontal tier partition  6   a  in parallel to the solution  25 , a first type of longitudinal partition wall  6   d  as shown for example in an intermediate tier in  FIG. 6  or a second type of longitudinal partition wall  6   e  as shown for example in the bottom tier  3   n  in  FIG. 6  may be installed in at least one tier. The longitudinal partition walls  6   d  and  6   e  would extend in longitudinal direction substantially from the front wall  2   e  to the end wall  2   f . The first type of longitudinal partition wall  6   d  would be arranged vertically between the shell bottom  2   a  or a horizontal tier partition  6   a  and the tube bundle shrouds  10   d  of a tube bundle  10  located directly above, while the second type of longitudinal partition wall  6   e  would be arranged on top of the shell bottom  2   a  or on top of a horizontal tier partition  6   a  and in between two tube bundles  10 . Both types of longitudinal partition walls  6   d  and  6   e  would divide each of the flash stages located in the at least one tier into a first flash stage part  12  and a second flash stage part  13 . The at least one tier together with the first type of longitudinal partition wall  6   d  or second type of longitudinal partition wall  6   e , would be configured to convey the solution  25  on top of the shell bottom  2   a  or on top of a horizontal tier partition  6   a  in the first flash stage parts  12  through the flash stages, and to accumulate and convey at least a part of the distillate  24  in the second flash stage parts  13  also on top of the shell bottom  2   a  or on top of a horizontal tier partition  6   a . The width of the first flash stage parts  12  and second flash stage parts  13  in an individual tier would be approximately proportional to the flow rates of the solution  25  and the accumulated distillate  24  conveyed through the individual flash stage parts  12  and  13  and may therefore change, in the individual tiers as the amount of accumulated distillate  24  is increasing, while the amount of solution  25  is decreasing toward the last flash stage  4   n . The second vapor  26   b  released from the accumulated distillate  24  has a higher purity compared to the first vapor  26   a  released from the solution  25  and would therefore enter into the tube bundle(s)  10  without passing through a mist eliminator  9 , as shown in  FIG. 6 . With the arrangement of the longitudinal partitions  6   d  and  6   e , the first vapor  26   a  released from the solution  25  in the first flash stage parts  12  and the second vapor  26   b  released from the accumulated distillate  24  in the second flash stage parts  13  may partly mix. 
     In place of the solution orifices  7   a , distillate orifices  7   c  would be cut in the flash stage partition walls  6   c  in the second flash stage parts  13 , allowing the accumulated distillate  24  to pass from one flash stage to the next. 
     Installing a third type of a longitudinal partition wall  6   f  as shown for example in  FIG. 7  in the lower tier  3   n  and in the intermediate tier above, in at least one tier, would also divide each of the flash stages in the at least one tier into a first flash stage part  12  and a second flash stage part  13 . The at least one tier together with the third type of longitudinal partition wall  6   f  would be also configured such, that the solution  25  would be conveyed in the first flash stage parts  12  and at least a part of the distillate  24  would be accumulated and conveyed in the second flash stage part  13 . The third type of longitudinal partition wall  6   f  would be either located in a first position in between two tube bundles  10  or in a second position, dividing each of the tube bundle elements  10   f  of one of the plurality of tube bundles  10  located in the at least one tier into a first part of a tube bundle element  10   g  and a second part of a tube bundle element  10   h , and the associated tube bundle shrouds  10   d  into a first part of a tube bundle shroud  10   j  and a second part of a tube bundle shroud  10   k . Depending on the position of the third longitudinal partition wall  6   f , the second flash stage parts  13  would comprise at least a second part of a tube bundle element  10   h  with a second part of a tube bundle shroud  10   k , or at least one tube bundle element  10   f  together with one tube bundle shroud  10   d , while the remaining tube bundle elements  10   f  and associated tube bundle shrouds  10   d  and the first parts of tube bundle elements  10   g  and first parts of a tube bundle shrouds  10   j  would be located in the first flash stage parts  12 . The first vapor  26   a  released from the solution  25  in a first flash stage part  12  as well as the second vapor  26   b  released from distillate  24  in a first flash stage part  12  would condense only on the tube bundle elements  10   f  or on a first part of a tube bundle element  10   g  located in the first flash stage part  12 , while the second vapor  26   b  released from the accumulated distillate  24  in the second flash stage parts  13 , would condense only on the tube bundle elements  10   f  or second parts of tube bundle elements  10   h  located in the second flash stage parts  13 . The distillate generated from the condensing second vapor  26   b  released from the accumulated distillate  24  in the second flash stage parts  13  would be collected as a high purity distillate  24   x  in the tube bundle shrouds  10   d  or second parts of tube bundle shrouds  10   k  located in the second flash stage parts  13 . 
     In place of the tube bundle shrouds  10   d  shown in the  FIG. 4 to 8  and  FIG. 10 , a plurality of tube bundle walls  10   e  may be installed in at least one tier as shown for example in  FIG. 9 . The tube bundle walls  10   e  are extending substantially from the front wall  2   e  to the end wall  2   f  and in the bottom tier from the shell bottom  2   a , respectively in the other tiers from a horizontal tier partition  6   a , upwards, forming in the bottom tier  3   n  together with the shell bottom  2   a  or in one of the other tiers together with one of the horizontal tier partition  6   a  a plurality of first channels  15 , one of each located in between two tube bundles  10  and in between two tube bundle walls  10   e , or between a tube bundle  10  and the left side wall  2   c  or the right side wall  2   d  limited by a tube bundle wall  10   e  and the left side wall  2   c  or the right side wall  2   d , and forming further a plurality of second channels  16 , one below each of the plurality of tube bundles  10  of the at least one tier, in between two tube bundle walls  10   e  or in between the left side wall  2   c  or right side wall  2   d  and a tube bundle wall  10   e , in case no first channel  15  is formed next to the right side wall  2   c  or left side wall  2   d . The at least one tier together with the tube bundle walls  10   e  would be configured such, that the solution  25  would be conveyed through the flash stages in the first channels  15 , while the distillate  24  would be accumulated and conveyed in the second channels  16 . 
     A third type of a longitudinal partition wall  6   f  could be installed in at least one tier in a first position in between two tube bundles  10  as shown for example in the bottom tier  3   n  of  FIG. 9 , dividing one of the first channels  15  into a first part of a first channel  15   a  and a second part of a first channel  15   b . Alternatively one of the third type of longitudinal partition walls  6   f  could be located in a second position as shown for example in  FIG. 9  in the top tier  3   a  such, that each tube bundle element  10   f  of one of the plurality of tube bundles  10  located in the at least one tier is divided into a first part of a tube bundle element  10   g  and a second part of a tube bundle element  10   h , while the second channel  16  directly below the same tube bundle  10  would be divided into a first part of a second channel  16   a  and a second part of a second channel  16   b . Furthermore one of the third type of longitudinal partition walls  6   f  could be installed in a third position as shown for example in  FIG. 9  in an intermediate tier, replacing one of the tube bundle walls  10   e . In all the three positions the flash stages of the at least one tier, would be divided by the third type of longitudinal partition wall  6   f  into a first flash stage part  12  and a second flash stage part  13 . The second channels  16  or second part of a second channel  16   b  located in the second flash stage parts  13  of the flash stages located in the at least one tier, would be configured, to accumulate and convey the high purity distillate  24   x  generated from the condensing second vapor  26   b , released from the distillate  24  in the second flash stage parts  13 , while the first channel(s)  15  or second part of a first channel  15   b  located in the second flash stage parts  13  would be configured to accumulate and convey distillate  24 , while some of the distillate  24  may be conveyed also in the second channels  16  or the first part of a second channel  16   a  located in the first flash stage parts  12 . The first channels  15  or first part of a first channel  15   a  located in the first flash stage parts  12  would be configured to convey the solution  25 . 
     Installing in at least one tier in place of the third type of longitudinal partition wall  6   f  as shown for example in the bottom tier  3   n  of  FIG. 9  in a first position, or in place of the third type of longitudinal partition walls  6   f  as shown for example in an intermediate tier in  FIG. 9  in a third position, a second type of longitudinal partition wall  6   e  as shown for example in  FIG. 6  in the bottom tier, would also divide the flash stages of the at least one tier, into a first flash stage part  12  and a second flash stage part  13  and would divide in the first position also one of the first channels  15  into a first part of a first channel  15   a  and a second part of a first channel  15   b . In this case also a part of the first vapor  26   a  released in the first flash stage parts  12  may condense on the tube bundle elements  10   f  located in the second flash stage parts  13 . Main purpose of the installation of the second type of longitudinal partition wall  6   e  would be to configure the first channel(s)  15  or second part of a first channel  15   b  located in the second flash stage parts  13  to accumulate and convey distillate  24 . 
     A configuration of the flash stages where the tube bundle walls  10   e , are arranged only on some of the plurality of tube bundles  10  in the at least one tier, while tube bundle shrouds  10   d  would be arranged at the remaining tube bundles  10 , would be possible as well. 
     The evaporator shell bottom  2   a  and horizontal tier partitions  6   a  may be arranged with a slope in the flow direction of the solution  25  conveyed on top, as shown for example in  FIG. 10 . Considering in particular that theoretically flat horizontal tier partitions  6   a  and the shell bottom  2   a  are typically warped to a certain degree, the slope would minimize the deposition of suspended solids contained in the solution and ensures a better drainage of the flash stages. Related possible corrosion problems of the horizontal tier partitions  6   a  and shell bottom  2   a  would be largely eliminated. 
     Another advantage of the sloped tier partitions  6   a  or sloped shell bottom  2   a  is, that a static head is added to the differential pressure between flash stages. This may help in particular toward the last flash stage  4   n  respectively in the lower tiers in an evaporator, where the differential pressure between the vapor  26   a  of two flash stages is significantly lower compared to the differential pressure between flash stages in the top tier  3   a , so that the sloped tier partitions  6   a  and sloped shell bottom  2   a  would also support the transport of the solution  25  during operation. 
     The shell roof  2   b  may be arranged with a slope as well, to allow the drainage of any moisture or other liquids collected on the outer surface, which cold in particular in coastal areas, where desalination plants are primarily installed, help to prevent excessive corrosion of the outer surface of the evaporator shell  2 . 
     In case the evaporator  1  is configured to convey the solution  25  in all tiers into the same flow direction the manufacturing of the evaporator  1  could be carried out with all parts in plumb position, while it could be installed in final position on site in an off plumb position, to provide a positive slope of the evaporator bottom  2   a  and the horizontal tier partitions  6   a  in the flow direction of the solution  25 . In this case also other parts like the tube bundles  10  would have a slope allowing a better drainage during a plant shut down. A uniform flow direction of the solution  25  in all tiers would however require to make provisions to transport the solution  25  discharging from one tier on the end wall side of the evaporator, to the front wall side of the evaporator to enter into the next tier below. 
     A multi stage flash long tube evaporator  1  may be used for a seawater desalination plant based on a ‘brine recirculation’ configuration as shown in a simplified schematic in  FIG. 11  or for a desalination plant based on a ‘once through’ configuration as shown in a simplified schematic in  FIG. 12 . Both schematics show in the individual tiers  3   a ,  3   b ,  3   c  . . . to  3   n  only one of the plurality of flash stage element rows  5   aa ,  5   bb , . . .  5   nn  and only one of the plurality of tube bundles  10  arranged in each of the tiers. The schematics correspond basically with the evaporator configuration illustrated in the  FIGS. 4 and 5 . 
     In the case of the brine recirculation system, the multi stage flash long tube evaporator  1  comprises a heat recovery section  1   a  and a heat rejection section  1   b , wherein the heat recovery section  1   a  comprises the top tier  3   a  and at least one intermediate tier  3   b , or further intermediate tiers  3   c  . . . , while the heat rejection section  1   b  comprises at least the bottom tier  3   n  and may comprise in addition one or more intermediate tiers located directly above the bottom tier  3   n.    
     In the brine recirculation system, the evaporator  1  and its parts are configured to receive a first coolant  20  at the tube bundle elements  10   f  located in the last flash stage  4   n , to convey the first coolant  20  through all tube bundles  10  located in the bottom tier  3   n  in parallel and in case the heat rejection section comprises more than one tier, to convey the first coolant  20  discharging from the plurality of tube bundles  10  of one tier to the plurality of tube bundles  10  of a tier of the heat rejection section located directly above, in serial flow communication. The same evaporator is further configured to receive a second coolant  22  at the plurality of tube bundles  10  located in the tier directly above the heat rejection section  1   b , to convey the second coolant  22  in parallel through the plurality of tube bundles  10  located in an individual tier of the heat recovery section  1   a , to convey the second coolant  22  discharging from the tube bundles  10  located in an individual tier to the tube bundles  10  located in a tier directly above in serial flow communication. 
     For the first coolant  20 , typically a filtered seawater is used. For the second coolant  22 , pumped by one or more brine recycle pumps  40   a  through the tube bundles  10  of the heat recovery section  1   a , typically a mixture of a concentrated solution  23  discharging from the last flash stage  4   n  and a make up water  21  branched off from the first coolant  20  discharging from the heat rejection section  1   b  is used. This mixture, used as second coolant  22 , is typically described as re-circulating brine. The make up water  21  may be passed through a deaerator  31 , which is operating under a vacuum pressure similar to the pressure of the last flash stage  4   n . The deaerator  31  has the purpose to remove the major part of dissolved gases from the make up water  21 , to minimize corrosion of the internal surface of the evaporator  1 . Furthermore, chemicals may be added into the make up water  21  to minimize scaling of the tube bundles  10 . In some cases the flow rate of make up water  21  may be equal to the flow rate of the first coolant  20 , which would mean that all of the first coolant  20  discharging from the heat rejection section  1   b , would be used as make up water  21  and no first coolant  20  would discharge from the system directly. In some cases no concentrated solution  23  may be added to the make up water  21 , so that the coolant  22  would comprise substantially of make up water  21 . 
     As the second coolant  22  flows through the tube bundles  10  of the individual tiers of the heat recovery section  1   a , its temperature is gradually increasing, as the first vapor  26   a  and second vapor  26   b  is condensing on the tubes bundles  10 , and heat is transferred into the second coolant  22 . As the second coolant  22  is discharging from the plurality of tube bundles  10  at the first flash stage  4   a , it passes then through an external heat exchanger, commonly described as heat input section or brine heater  32 , where it is heated to a top temperature by heating steam  27 , which is typically a low pressure steam supplied from a power plant. The condensate  28  collected from the condensing heating steam  27  in the brine heater  32  is normally returned to the power plant and reused as boiler feed water. 
     Similar to the arrangement of the plurality of tube bundles  10  in the individual tiers  3   a, b , . . .  3   n , also a plurality of brine heaters  32  may be installed, while the plant design with one single brine heater or any other number of parallel installed brine heaters would be possible as well. Coolant outlet headers  22   b , as indicated in  FIG. 11  may be installed to allow a fluid communication between the second coolant  22  discharging from the plurality of tube bundles  10  of the top tier  3   a  and any number of brine heaters  32 . If required for the supply of the heating steam  27  to the brine heater(s), a steam inlet header  27   a  may be installed. Similarly, a condensate outlet header  28   a  may be installed to collect the condensate  28  from a plurality of brine heaters  32 . Furthermore a coolant inlet header  22   a  may be installed for the distribution of the second coolant  22  when entering into the first flash stage  4   a.    
     As the second coolant  22  enters at its top temperature into the first flash stage  4   a  it becomes a flashing brine or in general terms a solution  25 , which flashes down, its temperature drops and some first vapor  26   a  is released. As the solution  25  is conveyed through the individual flash stages  4   a ,  4   b  . . . of the heat recovery section  1   a , the flash down of the solution  25 , the release of the first vapor  26   a , the condensation of the first vapor  26   a  and heat transfer into the second coolant  22  is repeated in each flash stage. The same procedure is continued as the solution  25  enters into the flash stages of the heat rejection section  1   b , wherein here the heat from the condensing first vapor  26   a  is transferred into the first coolant  20 . As the first vapor  26   a  is continuously released from the solution  25 , the concentration of the solution  25  is increasing. When the solution  25  has reached the highest concentration in the last flash stage  4   n , it is discharged as concentrated solution  23 , also typically described as concentrated brine. While one part of the concentrated solution  23  may be re-circulated as a part of the second coolant  22 , the remaining part of the concentrated solution  23  is typically discharged over one or more blow down pumps  40   b.    
     The process in the ‘once through’ desalination system as illustrated in the schematic  FIG. 12 , differs from the ‘brine recirculation’ system primarily in the coolant flow. Only a first coolant  20 , typically non-concentrated seawater, is used for all the tube bundles  10  of the evaporator  1 . This first coolant  20  may be also treated with chemicals to prevent scaling of the tube bundles  10 . The concentrated solution  23  is typically discharged from the last flash stage  4   n  over the blow down pump(s)  40   b.    
     In the brine recirculation process as well as in the once through process, the accumulated distillate  24  is typically discharged from the last flash stage  4   n  over the distillate pump(s)  40   c.    
     An exemplary schematic of the bottom tier in which the distillate  24  is accumulated and conveyed in the second flash stage parts  13  and the high purity distillate  24   x  is accumulated separate from the distillate  24  is shown in  FIG. 13 . This schematic corresponds with the configuration of the bottom tier  3   n  of the evaporator  1  shown in the  FIG. 7 to 9 . The schematic shows the parallel arranged flash stage element rows  5   aa ,  5   bb  . . . ,  5   nn  in the bottom tier  3   n  with the individual tube bundles  10 , fed with the first coolant  20  over the inlet header  20   a  in parallel. The solution  25  is conveyed in the first flash stage parts  12 , while distillate  24  is primarily accumulated and conveyed in the second flash stage parts  13 , which is in the example shown in  FIG. 13  identical with the row of flash stage elements  5   nn . The distillate  24  is extracted by distillate pump(s)  40   c  on the last flash stage  4   n , while the high purity distillate  24   x  is extracted by high purity distillate pump(s)  40   d . Distillate  24  accumulated from condensing first vapor  26   a  and second vapor  26   b  on the individual tube bundle elements  10   f  located in the first flash stage parts  12 , may be in each of the flash stages directly conveyed into the second flash stage parts  13  through pipe connections or ducts  24   e  as indicated in  FIG. 13 , or may be conveyed through a plurality of flash stages in the first flash stage parts  12 , before being conveyed into a second flash stage part  13 , or may be extracted from the first flash stage part  12  of the last flash stage  4   n  directly. 
     The configurations described and shown in  FIG. 1-13  provide the general concept of the present invention. The shown details should be considered as examples whereby other forms, shapes or configurations of individual parts, like for instance tube bundles  10  of circular shape may be used, solution orifices  7   a , distillate orifices  7   b  and  7   c  and splash hoods  8  may be designed in a different way than shown, shell parts like the shell bottom  2   a , shell roof  2   b , left side wall  2   c , right side wall  2   d , front wall  2   e  and end wall  2   f , horizontal tier partitions  6   a  and flash stage partition walls  6   c  may be curved or may be shaped in other forms than the flat plates shown in  FIG. 1 to 10 . 
     The transport of first coolant  20  and second coolant  22  to the tube bundles  10 , the collection of the first coolant  20  and second coolant  22  discharging from the tube bundles  10  as well as the transport of the first and second coolant  20  and  22  from the plurality of tube bundles  10  located in one tier to the plurality of tube bundles  10  located in the next tier directly above, may be realized by individual pipes connected to the water boxes  11 . Coolant inlet headers  20   a  for the first coolant, coolant inlet headers  22   a  for the second coolant, coolant outlet headers  20   b , for the first coolant and coolant outlet headers  22   b  for the second coolant as shown in the schematics  FIGS. 11 and 12  may be installed, so that the first coolant  20  or second coolant  22  fed in parallel to the plurality of tube bundles  10  or discharging in parallel from the plurality of tube bundles  10  of one tier is in fluid communication. Similarly, the transport of the solution  25  from one tier to the next tier located directly below, may be realized by a plurality of individual pipes connected to a plurality of solution inlet nozzles  25   c  and solution outlet nozzles  25   d  installed over the width of the flash stages in the front wall  2   e  and end wall  2   f  of the individual tiers  3   a ,  3   b  . . . to  3   n  as shown in  FIG. 2 to 10 . Those pipes may be also interconnected by solution inlet headers  25   a  and solution outlet headers  25   b  as shown in the schematics  FIG. 11 to 13 . Also outlet nozzles  23   a  for the concentrated solution  23  may be installed on the last flash stage  4   n  as indicated in  FIGS. 2 ,  3 ,  5  and  10 , which may be also connected with a concentrated solution outlet header  23   b . For the transport of the distillate  24 , also interconnecting distillate inlet headers  24   a  and distillate outlet headers  24   b  may be installed on the individual tiers to interconnect distillate pipes or ducts connected to the tube bundle shrouds  10  or second channels  16  as indicated in the  FIG. 11 to 13 , or distillate inlet nozzles  24   c  and distillate outlet nozzles  24   d  may be installed as indicated for example in  FIG. 8 . Instead of inlet and outlet nozzles, headers and piping connections for the transport of the first coolant  20 , second coolant  22 , solution  25  and distillate  24  as described above, also ducts located inside or outside the evaporator shell  2  or other means of transportation devices may be used for the same purpose. 
     An evaporator  1  and its parts may be configured to have the second flash stage parts  13  located in a different position than described and shown in the  FIGS. 6 ,  7 ,  9  and  13 , like for example occupying one or more than one of the intermediate rows of flash stage elements  5   bb ,  5   cc , . . . , which would eventually require additional longitudinal partition walls  6   d ,  6   e  or  6   f , but the principal purpose, allowing to accumulate the distillate  24  or accumulating the distillate  24  and producing in addition a high purity distillate  24   x  in the second flash stage parts  13 , would remain the same. 
     The accumulation of the distillate  24 , release of the second vapor  26   b  from the accumulated distillate  24 , condensation and accumulation of high purity distillate  24   x  outside the evaporator  1 , in a separate attached apparatus, would be possible as well. 
     Details like the venting of non-condensable gases from the tube bundles  10  are not shown, since those are commonly known details for tube bundles or evaporators. 
     The ring space between tubes  10   a  and tube holes in the flash stage partition walls  6   c  may be minimized by installation of suitable sleeves (details are not shown) to minimize the vapor passage from one flash stage into the next flash stage through the ring spaces, if considered necessary. 
     For structural reasons, an evaporator  1  may be designed such, that at least a part of the flash stage partition walls  6   c  located in a plurality of tiers are lined up vertically, however, flash stage partition walls  6   c  may be located in individual tiers in any position, regardless of the location of partition walls  6   c  in other tiers. Also each individual tier may be designed with a number of flash stages as considered as required or most suitable. 
     Also an evaporator may be configured having in individual tiers different numbers of flash stage element rows  5   aa - 5   nn , respectively different numbers of tube bundles  10 . 
     With the configuration of a plurality of flash stage element rows  5   aa ,  5   bb  . . . ,  5   nn  in the individual tiers, each comprising an individual tube bundle  10 , there is basically no limitation in unit capacity of an evaporator  1 , since the width of the evaporator  1  between the left side wall  2   c  and the right side wall  2   d  and the number of flash stage element rows  5   aa  to  5   nn  with individual parallel fed tube bundles  10  can be adjusted as required for a desired evaporator unit capacity. 
     The multi stage flash long tube evaporator  1  of the present invention may be divided horizontally into a plurality of at least two individual evaporator modules, wherein each evaporator module would comprise at least one of the plurality of tiers  3   a  to  3   n.    
     A typical example is shown in  FIG. 14  with a first evaporator module  100 , a second evaporator module  200  and a third evaporator module  300 . In the example shown in  FIG. 14 , the first evaporator module  100  comprises the top tier  3   a , the second evaporator module  200  comprises the intermediate tiers  3   b  and  3   c  and the third evaporator module  300  comprises the intermediate tiers  3   d ,  3   e  and the bottom tier  3   n . Such separation of an evaporator into individual evaporator modules may be applied if considered more feasible or more economical. For example, the first evaporator module  100  may be designed for a higher operation pressure than the second and third evaporator module  200  and  300 , reducing eventually the overall cost, compared to an evaporator  1  not divided into individual modules and designed completely for the higher operation pressure. 
     Similarly an evaporator  1  may be divided vertically and in longitudinal direction into a plurality of at least two evaporator modules. In this case each of the evaporator modules would comprise a part of the plurality of rows of flash stage elements  5   aa  to  5   nn  of each of the plurality of tiers  3   a  to  3   n  of the evaporator  1 . As an example,  FIG. 15  shows a first evaporator module  100  comprising the rows of flash stage elements  5   aa  to  5   dd  of each of the tiers  3   a  to  3   n , while the second evaporator module  200  comprises the rows of flash stage elements  5   ee ,  5   ff ,  5   gg  and  5   nn  of each of the tiers  3   a  to  3   n.    
     In both described options of evaporator configurations, each of the evaporator modules comprises an individual evaporator module shell, like indicated for example in  FIG. 14 , the evaporator module  100  comprising the evaporator module shell  102 , the evaporator module  200  the comprises evaporator module shell  202  and the evaporator module  300  comprises the evaporator module shell  302 . Each evaporator module shell comprises an individual shell bottom, shell roof, left side wall, right side wall, front wall and end wall, like indicated as examples in  FIG. 14-15  for the evaporator module  100 , a shell bottom  102   a , a shell roof  102   b , a left side wall  102   c , a right side wall  102   d , a front wall  102   e  and a end wall  102   f , or as indicated as example in  FIG. 15  for the evaporator module  200  a shell bottom  202   a , a shell roof  202   b , a left side wall  202   c , a right side wall  202   d , a front wall  202   e  and a end wall  202   f  or as indicated as example in  FIG. 14  for the evaporator module  300  a shell bottom  302   a , a shell roof  302   b , a left side wall  302   c , a right side wall  302   d , a front wall  302   e  and a end wall  302   f.    
     The wall thickness of evaporator shell parts and sizes of individual parts shown in the  FIGS. 1-10  and  14 - 15 , are partly not shown in true proportion to the evaporator size, but are shown significantly larger for clarity purpose of the design concept. 
     The basic concept of the evaporator  1  of the present invention, which is a multi stage flash long tube evaporator with a multiple tier configuration, comprising a plurality of at least 3 tiers and comprising a plurality of at least 3 parallel arranged tube bundles  10  in each tier  3   a  to  3   n , may be also used for desalination systems other than the described ‘recirculation system’ or ‘once through’ system. Also coolants of different types than the described seawater and re-circulating brine may be used and different type of coolant or make up water treatment may be applied. 
     The expression ‘longitudinal’ used in the description of the evaporator  1  is the direction of the flow of the solution  25 , while the expressions ‘transversal’ relate to the orientation perpendicular to the flow direction of the solution  25 . The expression ‘front’ relates to the evaporator side where the first flash stage  4   a  is located in the top tier  3   a , while the expression ‘end’ relate to the opposite side of the evaporator  1 . The expression ‘left’ and ‘right’ are orientations related to a view direction from the ‘front’ toward the ‘end’. 
     Expressions like ‘front’, ‘end’, ‘left’, ‘right’, ‘top’, ‘bottom’, ‘longitudinal’, ‘transversal’ in conjunction with the description of the evaporator parts, parts configuration, flow direction etc. are used for the purpose to provide a clear understanding of the design concept of the present invention. However, an evaporator may be designed and build for example in mirror image configuration, or other orientation related expressions may be used. 
     Although an exemplary embodiment of the invention has been described above by way of example only, it will be understood by those skilled in the field that modifications may be made to the disclosed embodiment without departing from the scope of the invention, which is defined by the appended claims.