[1]United States Patent Office 3@448,013 3,448,013 DISTILLATE COOLING MEANS FOR FLASH EVAPORATORS Robert E. Bailie, Media, Pa., assignor to Westinghouse Electric Corporation, Pittsburgb, Pa., a corporation of Pennsylvania Filed Aug. 10, 1966, Ser. No. 571,581 Int. Cf. BOld 3106,1128 U.S. Cl. 202-173 9 Claims ABSTRACT OF THE DISCLOSURE The invention comprises an evacuated flash chamber for cooling the product liqjiid condensate directed to said chamber from a multistage flash evaporator. At least a portion of the product liquid is flashed into vapor in the cooling chamber, the vapor being condensed and cooled therein by heat exchange tube structure provided in the chamber, the heat exchan,@e structure absorbin@ heat from the vapor. The present invention relates to flash evaporators and particularly to a last sta.-e cooling means for flash evaporators in which additional product cooling is obtained at reduced cost. One of the primary concerns with means and processes for convertin.- saline water to fresh (product) water is the cost. In the design of large multistage flash evaporator desaltin,a plants, it is imperative that the designer optimize the water flow cycle to achieve minimum cost of product water. Optimum design characteristics such as liquid flow velocity, number ofevaporator stages, flash range and beat transfer surface are dependent upon several cost factors which include amortization rate, power cos.ts, heating costs and cooling water costs. In many instances the cooling water temperature and the cost of installina and operating coolin- water supply facilities will dictate a last stage temperature in excess of the product water temperature specified by the customer-user. From a commercial standpoint, it is of paramount importance that the designer be allowed to optimize the design with no lirnitations involving the temperature of the last stage. In prior attempts to reduce the temperature of the last stage below the optimum design value, a considerable increase in cooling surface and cost is added to the heat reject section of the evaporator. Thus, no satisfactory means or process has heretofore been developed that allows the desi,-n engineer a flexibility in meeting specified product liquid temperature values while simliltaneously permittin.- optimum efliciency designs for multistage flash evaporator arran,@ements. Present methods of obtaining low temperature product liquid include last evaporator stage designs that maintain a low last stage temperature and are therefore highly ineffici-,nt as explained above. Another method involves the use of liquid to liquid heat exchan,@ers using incoming feed liquid on the tube-side to cool the product liquid on the shell or wall side. Either inethod adds costly heat transfer stirface with the shell and tube product cooler, often precluded in customer specifications, requiring additional piping, pumpin.- power, and maintenance. The present disclosure describes a miiltistage flash evaporator in combination with a product liquid coolin,@ means that is economical to construct, operate and maintain. The disclosure fur-ther includes an effective and efficient method of cooling the product liquid. Briefly, the present invention employs a cooling chamber in fluid communication with the last stage of a flash evaporator, and maintained at a lower pressure value than the last staae. The product liqtiid collected in the condenser space Patented June 3, 1969 2 of the last stage is permitted to flow into the cooling chamber where a portion of the product liquid flashes into vapor. The vapor is cooled by a heat exchange means, such as a plurality of tubes containing cool feed liquid, disposed in good heat exchange relationship with the vapor. The cooled vapor condenses and returns to the product stream. Preferably, the cooling chamber comprises an extension of the last stage condenser and condenser shell which 10 provide the necessary heat transfer surface. A cooling system of this type requires only an additional length of condenser heat exchange tubes and a shell or wall, a partition disposed between the last stage condenser and the thus formed cooling chamber and a small single 15 sta,@e air ejector to maintain a vacuum in the cooling charnber slightly greater than that of the last evaporator stage. The cost of such a coolin@ surface is reduced more than half that of liquid to liqui@d means and allows optimum flash evaporator design characteristics including an 20 optimum (somewhat higher) temperature parameter for the last stage. An object of the present invention, is therefore, to provide an effective and efficient means and method for cooling a product liquid produced in a flash evaporator 25 to a temperature substantially lower than that of the evaporator. Another object of the invention is to provide for optimum flash evaporator design includina an optimum temperature parameter for the flash evaporator while simul30 taneously providing additional product liquid cooling at reduced cost. A more specific object of the invention is to provide a multistage flash evaporator system with a cooling chamber in fluid communication with the condenser in the last 35 sta-e of the flash evaporator in which the hotter than req@uired product liquid is admitted to the coolin- chamber for cooling by flash evaporation and cond@ensation therein. These and other objects of the invention will become 40 more apparent from the foilowing detailed description taken in connection with the accompanying drawina in which: FIGURE I is a diagrammatic view of a multistage flash evaporator incorporating an embodiment of tfie in45 vention; FIG. 2 shows an altemative embodiment of the invention; and FIG. 3 shows a preferred ernbodiment of the invention. Specifically, there is shown in FIG. I a multistage 50 flash evaporation system of the recirculation and regenerative heat exchange type generally designated by numeral 10. The system employs a plurality of staged flash evaporation chambers generally designated by capital letters A, B and C, the number of chambers -iven by 55 way of example only. Chamber A is the first and' hi-hest pressure stage with the remaining lettered cham@bers formin@ evaporation stages decreasing in pressure in order of thei'r alphabetical designation so that last sta,-e C is the lowest presstire (highest vacuum) in system 10. 60 As well known in the art, the flash evaporation chambers A, B and C may be formed by metal housin@ structure that is of a generally parallelopiped shape c'omprising to top wall 12, a bottom wall 13, vertical end walls 14 and 15, as well as front and rear walls (not shown), 6,5 and vertical internal partitions 17 and 19 which cooperate with the outer wall structure to form the chambers. Chambers A, B and C are disposed in liqtiid comrnunication with each other by way of interconnecting slots ororifices 21 and 22 formed in partitions 17 and 18, 7o respectively adjacent bottom wall 13. The housing structure further defines an equal plurality of condensin- s 0 _paces 25, 26 and 27 for receivin- the con-
[2]3 densible Yapors formed in the chambers C through A, respectively. The condensing spaces are formed in the -uppermost portion of the housing structure and are further defined by generally horizontally extending trays 31, 32 and 33. The trays are provided with vertically extending vapor flow passa.-es 34, 35 and 36 respectively, so that vapor formed in chzcmbers C, B and A can flow upwardly through the flow passages in the condensing spaces 25, 26 and 27. The vertical partitions 17 and 18 are further provided with apertures 38 and 39 above the trays so that the falling condensate collected in the tray 33 is free to flow through the associated aperture 39 into tray 32 to join the condensate collected therein, and finally through aperture 38 into last tray 31 to join the condensate collected therein. From tray 31, the condensate is removed, as indicated by line 41, as the produut liquid. The condensin,@ spaces 25, 26 and 27 are provided with suitable surface type heat exchangers or condensing tube structures 45, 46 and 47 (only dia-rammatically shown in FIG. 1). The condensing space 25 alon.- with its associated tube structure 45 form heat rejection section generally designated C' while the condensing spaces 26 and 27 along with their associated tube structures 46 and 47 form respective heat recovery sections B', A'. The tube structures in the heat recovery sections X and B' provide regenerative heating of the circulating liquid by the heat extracted from condensing the vapors produced in those sta.-es. In order to maintain chamber stages A, B and C at successively lower pressure values, a main ejector or suc. tion device 50 is connected to the last and lowest @ressure stage C by a suitable conduit 51. The stages are serially connected together by way of openings 53 and 54 provided in partitions 17 and 18 respectively, so that air and other noncondensible gases can be removed from the stages by the ejector device 50. An impure liquid, such as sea or other impure water, ispressurized and fed into system 10 by a suita'ble (feed) pump 58 and directed through the tube structure in heat reject section C' where a portion of the thus heated makeup liquid is rejected from the system as indicated by line 56. The remainin- and greater portion of the feed liquid is then directed through the tube structures in the heat recovery sections A' and 13'. The pressurized, impure liquid inixes with the recirculating brine stream and is heated by the condensing vapors mentioned above. The liquid is next directed to a suitable top or brine heater 60 comprising a heat exchanging tube structure 61 disposed within a suitable vessel 62 to which steam or other heated fluid is directed as indicated by arrow 63. In the resulting heat exchange, the steam (if steam is used) is condensed and withdrawn as condensate through a drain outlet as ipdicated by arrow 64, and the heated feed liquid is thence directed into the first flash evaporation chamber A as indicated by line 65. As the heated liquid for evaporation is directed into the first and hi.-hest pressure chamber A, a portion thereof is flashed into vapor because of the reduced pressure ambient prevailing therein, and the vapor flashed therefrom is directed upwardly through flow passage 36 as indicated by dashed arrows 67 into the condensing space 27. The vapor is condensed by heat transfer froin the heat exchanging tube structure 47 and falls into tray 33 for collection. The unflashed liquid:ftows through orifice 22 into the next and lower pressure sta-,e chamber B wherein the same chain of events occur with the unflashed liquid thence:ffowin- through orifice 21 into the lowest and last pressure @,stage chamber C for final evaporation. With each event of flash evaporation, the liquid be. comes more and more enriched or concentrated with its impurities which are flnally collected in the last and lowest pressi-ire chamber stage C. From chamber C, the enriched liquid is directed through conduit 70 by a suitable pump 71. As well known in the art, a portion of the 3;448,013 4 enriched liquid may be removed or blown down fram the system 10 via a suitable conduit such as conduit 70, as shown, so that the liquid which recirculates through the system, as indicated by line 72, may not exceed a predetermined level of enrichment or concentration. 5 A substantially pure product liquid is the result of the flash evaporation and condensing functions perfor-med, respectively, in the flash chambers and condensing spaces. As mentioned earlier, the product liquid, such as pure 10 water, is collected in trays 31, 32 and 33 as the rising vapors come in contact with heat exchanae tube structures 45, 46 and 47, respectively, condense thereon and fall from the tubes as condensate into the trays. The pure liquid (condensate) flows towards the last tray 31 through I apertures 38 and 39 provided in the chamber partitions. Ordinarily, the product liquid is collected and withdrawn from the tray dividing the last evaporator stage from the last condensing space which, in the present arrangement, include chamber C and condensing space 25. 20 However, the temperature of the product liquid in this last stage is generafly higher than that specified for consumption. An effective and efficient means for cooling the product liquid is therefore needed, and in accordance with 25 the present invention there is shown such a means in FIG. I wherein a cooling charnber 75 is diagrammaticary depicted and so disposed so as to receive the hotter than required product liquid as indicated by line 41. Chamber 75 can comprise a simple shell or wall struc30 ture formed by top and bottom walls 76 and 77 respectively, vertical end walls 78 and 79 as well as front and rear walls (not shown). The walls are structurally combined to form an air-tight chamber. In the upper portion of chamber 75 is disposed a heat 35 exchan-,e means, such as a condensing tube structure 80, only diagrammatically shown. Below structure 80, adjacent bottom wall 77, may be disposed a tray 81 for collecting condensate that forms as a result of vapors condensing on tube structure 80 in a manner to be ex40 plained hereinafter. The condensate may be simply collected in the bottom of the chamber 75, in which case, the tray 81 would be unnecessary. Cha@mber 75 is maintained at a lower pressure value than that maintained in the lowest pressure flash chamber C by virtue of an auxiliary vacuun-i pump or ejector 45 eans 82. Ejector 82 removes air and other noncondensible gases from the chamber and as illustrated, may be arranged to exhaust them into the intake conduit 61 of the main ejector 50, as indicated by line 83. The product liquid collected in tray 31 in the last 50 condensing space 25 is permitted to enter tray 81 in chamber 75, where, upon entering the chamber a portion of the product liquid flashes into vapor by virtue of the reduced pressuremaintained iii chamber 75. The flashed portion (vapor) rises into the area occupied by heat ex55 change tubes 80 which carry a cooling fluid therethrough. The cooling fluid flowing through tubes 80 absorbs the heat from the product vapors with the vapors thus condensing on the tubes and falling to tray 81 as condensate. The heat absorbed by the cooling fluid is carried out of 60 chamber 75 by the fluid flow as indicated by line 84. The thus cooled condensate is collected in the tray 81, thereby mixing with and cooling the unflashed portion of the product liquid which is subsequently withdrawn for use, as indicated by line 88. 65 The arrarigenient depicted in FIG. 1 is particularly adaptable @for use with existing flash evapOTator systems and where it is desirable that the feed liquid be fed directly into the flash evaporator as shown. With this arrangement, no extensive and costly alteration and/or 70 modification of existing flash evaporator systems is necessary to effect cooling of the product liquid. FIG. 2 shows a modification of the arrangement of FIG. I in which the cooling chamber 75 again forms a unit separate from system 10 and is connected thereto. 75 In the figures, like numerals refer to like parts. In FIG.
[3]3,448,013 6 2, instead of using a separate parallel cooling fluid for flow ihrough the heat exchange tubes 80, the cooling :fluid flows in series from the distillate cooag condenser 80 to the last reject stage condenser 45. The remainder of the arrangement of FIG. 2 functions in substantially the manner described in connection with that of FIG. 1. That is, chamber 75 is maintained at a lower pressure value than that maintained in lowest pressure flash chamber C by virtue of an auxiliary vacuum pump or ejector 82 which exhausts into condenser space 25 of flash chamber C as indicated by line 83a. Auxiliary ejector means @2 could, if desired, exhaust into the intake conduit of rnain ejector 50, as shown in FIG. 1. In either case, the air and other noncondensible gases removed from chamber 75 by the auxiliary ejector and directed to either the intake 51 or condensing space 25 are removed from the system by main ejector 50. The product liquid from system 10 is cooled in chamber 75 by the cool liquid in tubes 80 absorbina heat from the vapor that is formed when the product liquid enters tray 81 and a portion thereof flashes as a result of the reduced pressure in the chamber. The heat is reinoved from chamber 75 by the flow of the cooling liquid through tubes 80 to tubes 45 in condensing space 25 of the flash evaporator system 10. The embodiments shown in FIGS. 1 and 2 provide a simple yet highly effective cooling rneans for existing :flash evaporator systems and for new systems where it is desirable to have cooling chamber 75 separate from the flash evaporator system. In FIG. 3 there is shown the preferred embodiment of the invention in which flash evaporator system 10 is divided physically into two separate groups of flash evaporator chambers with chamber C for@ming one group and cbambers A and B forining the other group. The system is divided in this way to provide an expedient for rejecting heat from the system in a manner to be more fully explained hereinafter. , In the preferred embodiment of the invention, the cooling chamber 75 is formed as an integral part of the flash evaporator 10 by a simple and economical extension of the wail structure forming the last condenser space 25 in flash evaporator system 10. Chamber 75 is further defined by an end wall 85 and a partition 86 (disposed opposite the end wall) the latter also serving to separate the condensing space 25 from the chamber 75, Ybe heat exchange tube structure 46 and 47 is shown i.n FIG. 3 as a bundle of three long and substantially parallel rows of tubes extendinghorizontally and in an unbroken manner through the condensing spaces 26 and 27 to opposite end walls 15a and 15b with the opposite end portions of the tu@be bundle opening into tube header structures 90 and 91 respectively. The number of tube rows (three) is only representative, the heat exchange tube bundles in flash evaporators being formed by a large number of relatively small size diameter tubes disposed in close proximity to each other. The tube structure 45 in the condensing space 25 of ilash chamber C and the tube structure 80 in the coolin,@ chamber 75 is shown in FIG. 3 as a bundle of three long tubes extending horizontally and in unbroken succession through the space 25, through the partition 86, and through the charnber 75 to opposite end walls 14a and 85 with the opposite end portion8 of the tube bundle opening into tube header structures 92 and 93 respectively. Again, the number of tubes is representative only. Thus, the preferred embodiment of invention employs a simple and economical extension of the heat exchange tube structure 45, in the condensing space 25, into cooling chamber 75 to form the heat exchange and cooling tube structure 80 in the chamber 75. Beneath heat exchange tubes 80 may be disposed tray 81 for receiving the product liquid and collecting condensate as explained in connection with FIG. 1. The partition 86 is further provided with an opening 87 near the lower portion thereof for admitting the product liquid collected in the tray 31 to tray Si. As explained in reference to FIGS. 1 and 2, chamber 75 is maintained at a lower pressure value that the last flash chamber C by an au@iliary ejector 82 which can exhaust into either the mam ejector intake 51 (as shown) or into the condensing space 25 as shown in FIG. 2. The unheated feed liquid is directed to header 93 by the feed pump 58. Header 93 collects and equally dis10 tributes the liquid into the plurality of tubes forming heat exchange tube bundle portion 80 employed to cool the vapors formed in the cooling chamber 75. From the tube bundle portion 80, the feed liquid is directed to heat exchange tube bundle 45, in condensing space 25, and to 15 header 92 where the liquid is collected from the tube bundle 45 for returning a portion of the thus heated liquid to its source (such as the sea) via suitable conduit 56 for the purpose of rejecting heat from the system 10, the header 92 providin- a convenient means for connecting 20 the heat rejecting conduit 56 and a second conduit 94 for directing the remaining and greater portion of the liquid to heat exchange tube bundle portions 46 and 47 by way of the header 90. From the tube bundle portion 47 the liquid feeds into the header 91 where it is collected for feeding 25 into top header 60 and thence into the flash chambers for flash evaporation as explained in connection with the arrangement shown in FIG. 1. The unflashed portion of the liquid in the flash chamber B is conducted to the flash chamber C by way of a suitable conduit 95 shown con30 nected between the two chambers. In a siinilar manner, the product liquid collected in tray 32 is conducted to tray 31 by way of a suitable conduit 196 connected between the flash chambers B and C. A suitable conduit 97 is employed to connect the condensing spaces 25 and 26, so that the 35 main ejector 50 may remove air and other noncondensible gases serially from the flash chambers in a manner similar to ;that shown in the arrangement of FIGS. I and 2. The remainder of the system shown in FlG. 3 functions in substantially the same manner as that described in refer40 ence to FIGS. I and 2. The three flash chambers shown in FIG. 3 are given by way of example only. In large multistage flash evaporator systems, the heat rejecting sec-tion, comprising chamber C in FIG. 3, may consist of two or more flash chambers depending upon the size and total 45 number of flash evaporation chamber stages in the system. The heat removing function performed in the cooling chamber 75 is highly effective in reducing the temperature of a product liquid produced in a flash evaporator, since vapor to liquid heat transfer rates are substantially greater 50 than say liquid to liquid rates. In using a distillate coolmg system of this type the amount of heat transfer surface (waH and tube) is thus -reatly reduced so that the cost of the surface is less than half that of liquid arrangements. 55 It should now be apparent from the foregoing description that a simple yet effective and efficient product liquid cooling arran,-ement and method has been provided. This is accomplished by directing the product liquid into a low pressure chamber containing a simple heat exchange struc60 ture so that a portion of the liquid flashes into vapor and then condenses on the heat exchange strlicture as heat is removed from the vapor by a coolin.- fluid flowing through the structure. FLirthermore, this is accomplished without adversely affecting the overall efficiency of the flash evap65 orator system producing the liquid since an optimum temperature may be maintained in the system that may be somewhat higher than that required by that of the product liquid. Though the invention has been described with a cer70 tain degree of particularity, it is to be understood that the disclosure has been made by way of example only. For example, in all three figures, the coolin.- chamber 75 is shown disposed at one end of the housing structure forming flash evaporator 10 and in axial alignment there75 with. This is the optimum arrangement since the heat ex-
[4]3,448,013 7 change tubes and connecting conduits are maintained in a straight line manner for maximum ease of fluid flow. However, it may not always be convenient to have chamber 75 so disposed, in which case the cooling chamber could be located to one side of or in some other relation- 5 ship with evaporator stage C and condenser space 25 without departing froni the spirit and scope of the invention. I
