US3448013A - Distillate cooling means for flash evaporators - Google Patents

Abstract

1,120,938. Evaporators, flash type; desalina tion of sea water. WESTINGHOUSE ELECTRIC CORP. 23 June, 1967 [10 Aug., 1966], No. 29039/67. Heading BIB. In an evaporator comprising a series of flash chambers A, B, C communicating with condenser spaces 27, 26, 25 respectively, maintained at progressively lower pressures by ejector pump 50, the condensate collected in trays 33, 32, 31 is cooled by passing it into chamber 75 which is maintained at a lower pressure than chamber C by ejector pump 82 so that a portion of the condensate flashes into vapour which condenses on cooling tubes 80. The cooled condensate is withdrawn through line 88. Ejector 82 exhausts into the conduit 51 connecting chamber C to ejector 50 which discharges air from the system. The apparatus may be used for the desalination of sea water which is pumped in through tubes 80 and 45. A small portion of the sea water is then returned to the sea through conduit 56, and the major portion flows through tubes 46, 47 to heater 60. The heated feed then flows through flash chambers A, B, C. Part of the brine withdrawn from chamber C is discharged from the system, and the remainder is recirculated to tubes 46, 47 through line 72.

Description

R. E. BAILIE Filed Aug. 10, 1966 DIS-TILLATE COOLING MEANS FOR FLASH EVAPORATORS June 3, 1969 United States Patent O'Y 3,448,013 DISTILLATE COOLING MEANS FOR FLASH EVAPORATORS Robert E. Bailie, Media, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 10, 1966, Ser. No. 571,581 Int. Cl. B01d 3/06, 1/28 U.S. Cl. 202-173 9 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to dash evaporators and particularly to a last stage cooling means for iiash evaporators in which additional product cooling is obtained at reduced cost.

One of the primary concerns with means and processes for converting saline water to fresh (product) water is the cost. In the design of large multistage flash evaporator desalting 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 dow velocity, number of evaporator stages, flash range and heat transfer surface are dependent upon several cost factors which include amortization rate, power costs, heating costs and cooling water costs. In many instances the cooling water temperature and the cost of installing and operating cooling 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 limitations involving the temperature of the last stage. In prior attempts to reduce the temperature of the last stage below the optimum design value7 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 design engineer a flexibility in meeting specified product liquid temperature values while simultaneously permitting optimum etiiciency designs for multistage flash evaporator arrangements.

Present methods of obtaining low temperature product liquid include last evaporator stage designs that maintain a low last stage temperature and are therefore highly ineicient as explained above. Another method involves the use of liquid to liquid heat exchangers using incoming feed liquid on the tube-side to cool the product liquid on the shell or wall side. Either method adds costly heat transfer surface with the shell and tube product cooler, often precluded in customer specifications, requiring additional piping, pumping power, and maintenance.

The present disclosure describes a multistage flash evaporator in combination with a product liquid cooling means that is economical to construct, operate and maintain. The disclosure further 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 liash evaporator, and maintained at a lower pressure value than the last stage. The product liquid collected in the condenser space 3,448,013 Patented June 3, 1969 ICC 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 4comprises an extension of the last stage condenser and condenser shell which 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 stage air ejector to maintain a vacuum in the cooling chamber slightly greater than that of the last evaporator stage. The cost of such a cooling surface is reduced more than half that of liquid to liquid means and allows opti- -rnum liash evaporator design characteristics including an optimum (somewhat higher) temperature parameter for the last stage.

An object of the present invention, is therefore, to provide an effective and eiiicient means and method for cooling a product liquid produced in a flash evaporator to a temperature substantially lower than that of the evaporator.

Another object of the invention is to provide for optimum ash evaporator design including an optimum temperature parameter for the Hash evaporator while simultaneously 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 liuid communication with the condenser in the last stage of the flash evaporator in which the hotter than required product liquid is admitted to the cooling cham- =ber for cooling by ash evaporation and condensation therein.

These and other objects of the invention will become more apparent from the following detailed description taken in connection with the laccompanying drawing in which:

FIGURE 1 is a diagrammatic View of a multistage flash evaporator incorporating an embodiment of the invention;

FIG. 2 shows an alternative embodiment of the invention; and

FIG. 3 shows a preferred embodiment of the invention.

Specifically, there is shown in FIG. 1 a multistage 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 given by way of example only. Chamber A is the first and highest pressure stage with the remaining lettered chambers forming evaporation stages decreasing in pressure in order of their alphabetical designation so that last stage C is the lowest pressure (highest vacuum) in system 1G.

As well known in the art, the flash evaporation chambers A, B and C may be formed yby metal housing structure that is of a generally parallelopiped shape comprising to top wall 12, a bottom wall 13, vertical end walls 14 and 15, as well as front and rear walls (not shown), and vertical internal partitions 17 and 1S which cooperate with the outer wall structure to form the chambers. Chambers A, B and C lare disposed in liquid communication with each other by way of interconnecting slots or orifices 21 and 22 formed in partitions 17 and 18, respectively adjacent bottom wall 13.

The housing structure further defines an equal plurality of condensing spaces 25, 26 and 27 for receiving the condensible vapors 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 ow passages 34, 35 and 36 respectively, so that vapor formed in chambers C, B and A can ow 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 condensing spaces 25, 26 and 27 are provided with suitable surface type heat exchangers or condensing tube structures 45, 46 and 47 (only diagrammatically shown in FIG. 1). The condensing space 25 along 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 A' and B provide regenerative heating of the circulating liquid by the heat extracted from condensing the vapors produced in those stages.

In order to maintain chamber stages A, B and C at successively lower pressure values, a main ejector or suction device 50 is connected to the last and lowest pressure 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 frolm the stages by the ejector device 50.

An impure liquid, such as sea or other impure water, is pressurized and fed into system 10 by a suitable (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 remaining and greater portion of the feed liquid is then directed through the tube structures in the heat recovery sections A and B'. The pressurized, impure liquid mixes 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 indicated by arrow 64, and the heated feed liquid is thence directed into the first ilash evaporation chamber A as indicated by Iline 65.

As the heated liquid for evaporation is directed into the iirst and highest 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 ow passage 36 as indicated by dashed arrows 67 into the condensing space 27. The vapor is condensed by heat transfer from the heat exchanging tube structure 47 and falls into tray 33 for collection. The unilashed liquid flows through orice 22 into the next and lower pressure stage chamber B wherein the same chain of events occur with the unashed liquid thence flowing through orice 21 into the lowest and last pressure Istage chamber C for nal evaporation.

With each event of ash evaporation, the liquid becomes more and more enriched or concentrated with its impurities which are iinally collected in the last and lowest pressure 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 '4 enriched liquid may be removed or blown down from 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.

A substantially pure product liquid is the result of the ash evaporation and condensing functions performed, respectively, in the flash chambers and condensing spaces. As mentioned earlier, the product liquid, such as pure water, is collected in trays 31, 32 and 33 as the rising vapors come in contact with heat exchange tube structures 45, 46 and 47, respectively, condense thereon and fall from the tubes as condensate into the trays. The pure liquid (condensate) ows towards the last tray 31 through 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. However, the temperature of the product liquid in this last stage is generally higher than that specified for consumption.

An effective and eiiicient means for cooling the product liquid is therefore needed, and in accordance with the present invention there is shown such a means in FIG. l wherein a cooling chamber 75 is diagrammatically 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 structure 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 exchange 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 explained hereinafter. The condensate may be simply collected in the bottom of the chamber 75, in which case, the tray 81 would be unnecessary.

Chamber 75 is maintained at a lower pressure value than that maintained in the lowest pressure flash chamber C by virtue of an auxiliary vacuum pump or ejector means 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 5'1 of the main ejector 50, as indicated by line 83.

The product liquid collected in tray 31 in the last 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 pressure maintained in chamber 75. The ashed portion (vapor) rises into the area occupied by heat exchange tubes 80 which carry a cooling uid therethrough. The cooling fluid owing 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 chamber 75 by the fluid ow 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.

The arrangement depicted in FIG. 1 is particularly adaptable for use with existing flash evaporator systems and where it is desirable that the feed liquid be fed directly into the ash evaporator as shown. With this arrangement, no extensive and costly alteration and/or modication of existing ash evaporator systems is necessary to effect cooling of the product liquid.

FIG. 2 shows a modification of the arrangement of FIG. 1 in which the cooling chamber 75 again forms a unit separate from system 10 and is connected thereto. In the figures, like numerals refer to like parts. In FIG.

2, instead of using a separate parallel cooling uid for flow through the heat exchange tubes 80, the cooling iluid flows in series from the distillate cooling 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. l. 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 ash chamber C as indicated by line 83a. Auxiliary ejector means 82 could, if desired, exhaust into the intake conduit of main ejector 50, as shown in FIG. 1. In either case, the air and other noucondensible gases removed from charnber 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 is cooled in chamber 75 by the cool liquid in tubes 80 absorbing 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 removed frorn chamber 75 by the ow of the cooling liquid through tubes 80 to tubes 45 in condensing space of the flash evaporator system 10.

The embodiments shown in FIGS. l and 2 provide a simple yet highly effective cooling means for existing flash evaporator systems and for new systems where it is desirable to have cooling chamber 75 separate from the ash evaporator system.

In FIG. 3 there is shown the preferred embodiment of the invention in which Hash evaporator system '10 is divided physically into two separate groups of ash evaporator chambers with chamber C forming one group and chambers A and B forming 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. k 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 wall structure forming the last condenser space 25 in flash evaporator system 10. Chamber 75 is further dened 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.

The heat exchange tube structure 46 and 47 is shown in FIG. 3 as a bundle of three long and substantially parallel rows of tubes extending horizontally 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 tube 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 ash 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 flash chamber C and the -tube structure 80 in the cooling 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 chamber 75 to opposite end walls 14a and 85 with the opposite end portions 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. q

Beneath heat exchange tubes 80 may be disposed tray 81 for receiving the product liquid and collecting condensate as explained in connection with FIG. l. 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 8'1. As explained in reference to FIGS. l and 2, chamber 75 is maintained at a lower pressure value that the last flash chamber C by an auxiliary ejector 82 which can exhaust into either the main ejector intake 51 (as shown) or into the condensing space 25 as shown in lFIG. 2.

The unheated feed liquid is directed to header 93 by the feed pump 58. Header 93 collects and equally distributes the liquid into the plurality of tubes forming heat exchange tube bundle portion 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 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 providing a convenient means for connecting 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 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 unashed portion of the liquid in the flash chamber B is conducted to the flash chamber C by way of a suitable conduit 95 shown connected between the two chambers. In a similar manner, the product liquid collected in tray 32 is conducted to tray 31 by way of a suitable conduit l96 connected between the ash chambers B and C. A suitable conduit 97 is employed to connect the condensing spaces 25 and 26, so that the main ejector 50 may remove air and other noucondensible gases serially from the ash chambers in a manner similar to -that shown in the arrangement of FIGS. 1 and 2. The remainder of the system shown in FIG. 3 functions in substantially the same manner as that described in reference to FIGS. l 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 section, comprising chamber C in FIG. 3, may consist of two or more ash chambers depending upon the size and total 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 ilash evaporator, since vapor to liquid heat transfer rates are substantially greater than say liquid to liquid rates. In using a distillate cooling system of this ltype the amount of heat transfer surface (wall and tube) is thus greatly reduced so that the cost of the surface is less than half that of liquid arrangements.

It should now be apparent from the foregoing description that a simple yet effective and eiiicient product liquid cooling arrangement and method has been provided. This is accomplished by directing the product liquid into a low pressure chamber containing a simple heat exchange structure so that a portion of the liquid flashes into vapor and then condenses on the heat exchange structure as heat is removed from the vapor by a cooling uid flowing through the structure. Furthermore, this is accomplished without adversely affecting the overall eiciency of the ash evaporator system producing the liquid since an optimum temperature may 4be 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 certain 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 cooling chamber 75 is shown disposed at one end of the housing structure forming flash evaporator 10 and in axial alignment there` with. This is the optimum arrangement since the heat exchange tubes and connecting conduits are maintained in a straight line manner for maximum ease of fluid ilow. However, it 4may 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 relationship with evaporator stage C and condenser space without departing from the spirit and scope of the invention.

I claim as my invention:

1. A multistage ash evaporator having a plurality of evaporating stages maintained at progressively lower pressure values and adapted to ash evaporate a heated irnpure feed liquid at progressively lower temperatures,

a plurality of condensers physically and respectively associated with each of said stages for condensing vapors created therein, said condensing vapors forming a substantially pure product liquid,

means for directing the impure feed liquid from the last and lowest pressure stage of the evaporator at a first temperature value,

apparatus yfor cooling only the product liquid after the impure feed liquid has undergone ashing in the last and lowest pressure stage of the flash evaporator and the product liquid has issued therefrom as the nal product liquid, said apparatus comprising means defining a flash chamber separate from the last and lowest pressure stage for receiving only the nal product liquid,

heat exchange tube structure provided in said separate chamber, and disposed above the maximum level of product liquid attained in said separate chamber,

means for directing the iinal product liquid from the condenser associated with said last stage to said separate chamber,

means for reducing the pressure in said separate chamber to a value lower than that attained in said last stage so that at least a portion of said product liquid flashes into vapor in said chamber, said vapor contacting said heat exchange structure,

said heat exchange structure being effective to cool and condense said vapor to form a relatively cool product liquid which recombines with the unilashed product liquid, and

means for withdrawing the cool product liquid from said separate chamber at a temperature value below that of said first temperature value.

2. The ash evaporator of claim 1 wherein the means for maintaining the ash chamber at the lower pressure includes a main and an auxiliary ejector means,

said main and auxiliary ejector means having intake and outlet portions,

the intake of the main ejector means being directly connected to the vapor space of the condenser associated with the lowest pressure ash evaporating stage, and

the intake and the outlet of said auxiliary ejector means being respectively connected to the separate ash chamber and the vapor space of said last mentioned condenser.

3. The flash evaporater of claim 1 wherein the means for maintaining the separate ash chamber at the lower pressure includes a main and an auxiliary ejector means,

said main and auxiliary ejector means having intake and outlet portions,

the intake of the main ejector being directly connected 8 i to the vapor space of the condenser associated with the lowest pressure flash evaporating stage,

means for connecting the intake of said auxiliary ejector means to the vapor space of the separate flash chamber, and

means for connecting the outlet of said auxiliary ejector means to the intake of said main ejector means.

4. The ash evaporator of claim 1 in which the condensers and heat exchange structure include tube structures that are serially connected for transporting a coolant iluid therethrough.

5. The ash evaporator of claim 1 in which the condenser associated with the lowest pressure flash evaporating stage is provided with wall structure, said wall structure further defining in part the ash chamber in conjunction with a partition disposed between the condenser and the cooling chamber.

6. The ilash evaporator of claim V1 in which the condensers include a tube structure, the tube structure associated with the lowest pressure ilash evaporating stage having a portion thereof extending into the ash chamber, means for admitting a coolant fluid into the tube structure, the tube structure portion comprising the heat exchange structure for cooling and condensing the ashed vapor of the product liquid.

7. The flash evaporator of claim 1 in which the apparatus for cooling the product liquid is a unitary structure physically separate from the plurality of ash evaporating stages and condensers, with the means for conducting the product liquid to the cooling apparatus comprising a conduit serially connected between the cooling apparatus and the condenser associated with the lowest pressure value stage.

8. The ash evaporator of claim 1 in which the heat exchange structure comprises a tube structure for carrying a coolant fluid therethrough for cooling the ashed vapor in the ash chamber.

9. The flash evaporator of claim 8 in which the coolant iluid is the feed liquid.

References Cited UNITED STATES PATENTS 2,759,882 8/ 1956 Worthen et al. 202-174 3,119,752 l/ 1964 Checkovich 202-1-73 2,908,618 10/ 1959 Bethon 202--174 3,259,552 7/ 1966 Goeldner 203-11 X FOREIGN PATENTS 855,550 1960 Great Britain.

958,522 1964 Great Britain.

965,750 1964 Great Britain.

OTHER REFERENCES Paper presented November 1965, Desalination Symposium, H. T. Holton and vLionel S. Galstaun, page 1 and FIG. 2.

NORMAN YUDKOFF, Primary Examiner.

I. SOFER, Assistant Examiner.

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