WO1989010779A1

WO1989010779A1 - Apparatus and method for improving vapor quality in evaporators

Info

Publication number: WO1989010779A1
Application number: PCT/US1989/002035
Authority: WO
Inventors: Ferris C. Standiford
Original assignee: Spintech Corporation
Priority date: 1988-05-10
Filing date: 1989-05-09
Publication date: 1989-11-16
Also published as: ZA893462B; IL90224A0; CS282489A3; AU3578989A

Abstract

An apparatus and method for improving vapor quality in evaporators utilizing rotating flash devices (84). The flash devices (80) are provided with elements (87, 89, 91) to obstruct the flow of liquid in the direction of vapor exit (94). Such elements (87, 89, 91) include a slinger plate (87) affixed to the rotating flash devices (84), and various skimmer plate designs (91) which interrupt or act against liquid flow toward the vapor exit (94).

Description

APPARATUS AND METHOD FOR IMPROVING VAPOR QUALITY IN EVAPORATORS

RELATION TO OTHER PATENT DOCUMENTS

The following patent documents are hereby incorporated into this specification by reference:

U.S. Patent No. 4,329,198, issued May 11, 1982 to Standiford for APPARATUS FOR FORCED CIRCULATION EVAPORATION, and

U.S. Patent No. 4,287,019, issued Sep. 1, 1981 to Standiford for APPARATUS AND METHOD FOR ADIABATIC FLASHING OF LIQUIDS.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to an apparatus for improving the purity of vapors generated in evaporators, including multistage falling film flash evaporators. In particular, the invention relates to apparatus for improving vapor purity when liquid-vapor separation is aided by an enhanced gravitational field.

BACKGROUND OF THE INVENTION

Evaporation apparatus for concentrating solutions and slurries are employed in a wide range of industries, including seawater desalination, water treatment, food processing, chemical and pharmaceutical manufacturing, pulp and paper manufacturing, and metals processing. The evaporators may be used for the purpose of removing a solvent from a solution, to increase the concentration of a solute in a solution, or to crystallize a solute from a solution. Many types of evaporators are in use and a variety of designs are well known in the prior art. One example is a vertical tube, multistage falling film flash evaporator as taught in my U.S. Patent No. 4,287,019. Another example is a crystallizer unit as taught in my U.S.Patent No. 4,329,198.

The first referenced patent describes an improved multistage flash evaporator in which flashing is caused to occur in a series of flashers under enhanced gravitational forces, preferably provided by rotating impellers. The second referenced patent describes an improved forced circulation evaporator utilizing a rotating impeller to force liquid through a heat exchanger wherein the liquid is heated, then sent back to the flash chamber in which the impeller rotates, whereby the impeller also serves to separate liquid from the vapor evolving when the heated liquid is exposed to a reduced pressure in the flash chamber. Enhanced gravitational forces, or "G-forces" act on liquid droplets that might have become suspended in the gaseous phase, and can accelerate their removal from a two phase mixture. Phase separation efficiency should be improved in an enhanced gravity flashing device and improved separation of liquid from produced vapor should be achieved with reduced vessel volume, when compared to use of conventional velocity dependent or gravity dependent separation devices. When evaporating a liquid by spinning a gas-liquid mixture in a rotating path, such as with a bladed impeller rotating in a cylindrical or frusto-conical vessel as disclosed in the above referenced patents, high purity produced vapors are also desirable. However, in the practical application of these inventions, I have found that the anticipated savings in vessel volume for separating space required for flashing can be achieved, but that the degree of separation of liquid from vapor is not always as complete as might be desired. For instance, when applied to the multistage flash cycle for producing potable water from saline waters, the vapor may have to be separated from a brine typically containing 50,000 parts per million (ppm) of total dissolved solids. When the vapor is condensed it may be found to contain 500 ppm of dissolved solids, which may meet potability standards but may not be acceptable for other uses, such as feedwater to a steam boiler. Distillate water purity can be improved by reducing output rate but this tends to reduce the cost savings promised by use of such flashing devices. Thus, there exists a need for improved vapor purity when utilizing rotating flash devices for vapor-liquid separation in evaporator apparatus.

OBJECTS OF THE INVENTION

In order to overcome problems inherent in the known devices, there has been provided by the subject invention a new and improved method and apparatus for separation of liquid from vapor in evaporation apparatus utilizing rotating flasher devices.

Accordingly, it is an object of the present invention to provide improved condensate quality in evaporation apparatus utilizing rotating flasher devices.

It is a feature of the present invention that the method and device accomplishes improved condensate quality in a manner which is easily adaptable for use with prior known methods and apparatus, and which minimizes as much as possible any added complexity or cost. It is an advantage of the present invention that a rotating flasher device is provided for use in evaporation apparatus which substantially eliminates contamination of the product condensate due to carryover of droplets of the liquid being flashed. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects and in accordance with the purpose of the present invention as embodied and broadly described herein, the present invention is directed to an apparatus and method for achieving high vapor quality from a rotating flasher device.

I have discovered that by arranging an upper boundary to obstruct liquid flow inward toward the vapor outlet, the vapor quality is enhanced. Thus, the phenomenon believed to be responsible for contamination of condensate is not in the separation of liquid and vapor in the enhanced gravitational field but in the bypassing of liquid around the enhanced gravitational field. In particular, in the previously preferred version of such flash device illustrated in Figure 3, liquid appears to splash onto the skimmer plate 69 of the vessel, in the zone inward from the rotating liquid ring, where it is beyond the influence of the enhanced gravitational field of the rotating impeller. Since some clearance space must be provided between the top of the impeller and the upper plate of the flash chamber(to avoid wear) , such a film of liquid adhering to this top skimmer plate cannot be swept down into the rotating field by the impeller. A liquid film creeping along this plate toward the vapor outlet is thus aided to flow inwardly by the produced vapor. Therefore, liquid or brine may be carried to the vapor outlet where it ultimately becomes a contaminant in the condensate of the vapor.

The rotating flasher apparatus of the present invention includes means for interrupting and/or obstructing the flow of liquid to prevent liquids from flowing out of the vapor outlet of the flash devices. In the most preferred embodiment, such flow interrupting and/or obstructing means is comprised of an integral slinger plate located at the top of the rotating flash apparatus. Alternatives include means for sealing the gap between rotating flasher blades and fixed skimmer plate at the top of the flasher housing, as well as other alternatives involving various skimmer plate geometry designed so as to aid liquid removal and/or to interrupt liquid flow. Although use of the present invention is shown with a multistage flash evaporation device with multiple flashers in a vertically εtacked configuration, the device is adaptable to single or multistage evaporating apparatus.

BRIEF SUMMARY OF THE DRAWINGS

The invention may be more clearly understood by reference to the accompanying drawings thereof, wherein:

FIG. 1 is a vertical, sectional view showing a falling film multistage flash evaporator, in which the improved flash device of the present invention can be used.

FIG. 2 is a vertical, sectional view showing a forced circulation evaporator, which may also be used for service as a crystallizer, in which the present invention is utilized as an improved rotating flasher device and a pump/vapor-liquid separator.

FIG. 3 is a vertical, sectional view of an assembly showing prior art rotating flash devices with liquid-vapor separating means as taught in my U.S. Patent No. 4,287,019.

FIG. 4 is a vertical, sectional view of a stack assembly of rotating flash devices showing the preferred first embodiment of the present invention. in a multistage flash device, a stacked number of flash devices may be employed as illustrated in FIG. 1.

FIG. 5 is a bottom plan view, i.e. looking up at the bottom of a flasher device, with a fixed skimmer plate partially removed to reveal design of liquid collecting means, taken along the line 5-5 of FIG. 4.

FIG. 6 is a plan view of the stationary portion of the bottom of the flasher device, revealing liquid outlets, taken along the line 6-6 of FIG. 4. FIG. 7 is a cross section view of liquid outlet devices, taken along the line 7-7 of FIG. 6.

FIG. 8 is a bottom plan view, i.e., looking up at a rotating flasher and stationary flasher housing in the most preferred embodiment of the present invention, taken along the line 8-8 of FIG. 4.

FIG. 9 illustrates a second embodiment of my invention providing for a wiped surface to prevent liquid carryover. FIG. 10 is a vertical section view, showing the second embodiment of my present invention illustrated in FIG. 9, taken along the line 10-10 of FIG. 9.

FIG. 11 is a vertical cross section, similar to the view shown in FIG. 4, of a third embodiment of the present invention.

FIG. 12 is a vertical cross section, similar to the view of shown in FIG. 5, of a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The rotating flash apparatus of the present invention is designed for the use with evaporators for concentrating solutions or slurries. A typical multistage falling film flash evaporator where the present invention may be employed is shown in FIG. 1. Such evaporators usually include a plurality of heat exchange tubes 1 arranged substantially vertically. The evaporator also has a shell 2 divided by horizontal partitions 3 which provide separate condensing spaces 4 which operate at various pressures. The evaporator also has an upper liquid pool reservoir or flood box 5, from which the liquid to be evaporated is fed to each tube by means such as distributors 6, or sieve tray, spray nozzles, or the like, to place a liquid film in each tube. Distributors 6 may be fitted to the top of each tube 1, from which the liquid to be evaporated flows down each tube. As the liquid flows down each tube, the liquid is heated by vapor condensing on the outside of the tubes. Vapor pressures and vapor temperatures increase from the coldest and uppermost condensing chamber 4 to the warmest and lowest condensing chamber 8. Heat required for evaporator may be provided by boiler 9 or by an external heater. If heating is done with a boiler 9, heat in the form of steam is provided to the evaporator apparatus through steam line 10, and condensate is returned to the boiler through condensate return line 11. Heated liquid which has fallen through tubes 1 is collected in the sump 12, and through return line 14 is returned to the first flasher unit 15. The flashers 15 are comprised of a series of chambers containing rotating impellers connected and driven by a common shaft 16 and driven by suitable means such as an electric motor 17. Liquid entering the bottom of flashers, as through line 14 and as through interconnecting passages 18 between flashers is accelerated by the motion of the rotating impellers and is driven toward the outside wall of the flasher. Vapor 7 is produced from the flashing liquid-gas mixture in the flasher. Produced vapor must return from the outside walls of the flasher to a vapor outlet centrally located near the common shaft 16 where it exits first vertically and then vapor 7 is secondly dispersed radially in each condensing chamber, to be condensed on tubes 1. From the top flasher 19 in a vertical stack of flashers, remaining liquid is withdrawn through line 20 to pump 21, through which it can be discharged through line 22 or at least a portion may be recycled through line 23. Fresh feed containing solvent and solutes may be introduced to the evaporator apparatus through makeup line 24. When seawater is being evaporated, the solvent is water and solutes are the various dissolved salts found therein as are well known and need not be described here, however, it can be appreciated that the evaporator apparatus and rotating flash devices of the present invention may be employed on any number of solvent-solute mixtures with thermodynamic and physical properties suitable for multistage flash evaporation. Vapor 7 produced in each stage of the apparatus is condensed on tubes 1 and forms a pool resting on partition 3 at the bottoms each compartment 4. Condensate produced from evaporating fresh feed may be moved from the highest pressure chambers at the bottom of the evaporation apparatus through lines 26 to each successive lower pressure chamber through a similar line 26. Referring again to FIG. 1, condensate may be removed f om the uppermost condensing space through line 28 and pump 29. It is the improvement in the purity of condensed vapors or condensate leaving through line 28 that is the primary object of the present invention. Vent gases may be withdrawn from the uppermost condensing space 4 through line 30 and vacuum pump 31. Where the feedwater contains temporary hardness, the teaching of my U.S. Patent No. 3,494,836 may be followed, whereby vacuum pump 31 would discharge by line 32 to return at least part of the carbon dioxide content of the vent gas into the sump where it may find its way to tubes 1 so as to contact with feedwater as it is being heated in tubes 1. Recycle of carbon dioxide as taught in my U.S.Patent No. 3,494,836 will help to minimize or eliminate the possibility of scale formations on tubes 1. FIG. 2 shows one embodiment of the present invention when utilized as combined pump and vapor-liquid separation means in an evaporator or crystallizer constructed according to the teachings of my U.S.Patent No. 4,329,198. It will be readily recognized that the present invention maybe easily adapted to other embodiments of the inventions taught in the referenced patent and the present FIG. 2 is shown by way of example only and is not in any way a limitation. FIG. 2 shows an evaporator wherein the flow of liquid through the heat transfer tubes occurs in two passes. Outer tubes 40 comprise about half of the total heating tubes in the evaporator, and operate in a downflow configuration with liquid feeding the tubes taken directly from the discharge of my rotating pump/vapor-liguid separating impeller 41. The rotating impeller is driven by means of a shaft 42 located on a central axis. Liquid exiting the outer downflow tubes 40 enter the lower water box 43, then flow upward in the inner tubes 44 of the tube bundle. The inner tubes 44 of the tube bundle then discharge to the rotating pump/vapor-liquid separating impeller 41, below diffuser plate 45. A baffle 46, defined by a surface of revolution rotating about said center axis and typically cylindrical or frusto-conical in shape, forms the outer wall of a flash chamber. The space between baffle 46 and the outer shell 47 serves as a conduit for feeding the outer downflow tubes 40. A slinger plate 48 according to the present invention is provided at the top of the rotating impeller 41 and close fitting to upper wall 49 as a means to obstruct liquid flow radially inwardly towards a vapor outlet 50, and especially as a means to prevent liquid travel inwardly along upper wall 49 toward vapor outlet 50. Other details illustrated such as inlet for heating means, construction methods used for joining vessel heads, as well as details not illustrated, such as condensate outlet or exit for heating means, are well known in the art-and there is no need to discuss them further.

FIG. 3 shows details of a prior art flasher, as disclosed in U.S.Patent No. 4,287,019. The flasher consists of a stationary, substantially cylindrical vessel 60, closed at the bottom except for one or more inlet lines 61, and a bearing or close fitting seal 62 for a common rotating shaft 63. Secured to the shaft and rotating with it is a rotating impeller 64 comprised of a deflector 65 and blades 66 , The rotating impeller 64 imparts a rotating motion on the flashing liquid, and the centrifugal force created thereby forces the flashing liquid to the walls of vessel 60, and a vapor-liquid interface 67 is formed. Vapor escapement is through opening 68, from which it can freely flow throughout the condensing spaces 4 of FIG. 1. A fixed skimmer plate 69, suitably dimensional given the desired throughput provides the means to allow escape of essentially only liquid through the gap between the outer circumference of the skimmer plate 69 and the inside diameter of vessel 60. Liquid remaining after flashing is collected in spaces 70 and withdrawn by one or more lines 71 for transport to the next flasher or for removal from the system. FIG. 4 shows a cross sectional view of a plurality of flashers vertically stacked and configured according to the most preferred embodiment of present invention. The flasher consists of a stationary, substantially cylindrical vessel 80, closed at the bottom, except for one or more liquid discharge ports 81 and a shaft seal or bearing 82 fitting on a common rotating shaft 83. Secured to the shaft and rotating therewith is a rotating impeller 84, comprised of a deflector 85, blades 86, and a slinger plate 87 secured to the top of impeller blades 86. As the rotating impeller 84 imparts a rotating motion on the flashing liquid, the centrifugal force created thereby forces the flashing liquid to the inside walls of the vessel 80. Liquid remaining after flashing is forced outwardly and upwardly to a liquid exit port 88. Liquid exiting is directed in an inward swirling pattern between diffuser vanes 89 and is carried to one or more liquid discharge ports 81 below the next flasher stage, said liquid discharge outlets 81 being more easily recognized in FIG. 5. A fixed skimmer plate 91 is located below diffuεer vanes 89, and serves to keep liquid exiting the flasher from returning by forming a conduit for the liquid in conjunction with said diffuser vanes. A seal between flasher stages is achieved by use of an O-ring 92. The connection between flasher stage is secured by means of molded elastomeric ring seal 93, which also serves to seal the gap between flasher vessels 80 and the horizontal partitions 3 between condensing sections 4 as described in FIG. 1.

Vapor flashed must first travel outwardly below the deflector 85, then may disengage from the liquid near the inside walls of the vessel 80, and then must flow back toward the center to vertically exit the axially positioned vapor outlet 94. From there, the vapor is redirected substantially horizontally to flow beyond the flashers throughout the condensing space. It is in FIG. 4 that my invention can be readily understood. The rotating impeller 84 has been modified from the prior art device shown in FIG. 3 by adding a slinger plate 87 which is really an annular shaped cap or top attached to the impeller 84. This cap has in the center a void defined by an inner diameter _ substantially the same size as the vapor outlet opening 94 from the flash chamber 80, and an outer diameter extending approximately to the outer edge of impeller blades on the rotating impellers. This outer diameter more particularly must define a circumferential liquid outlet adjacent to the upper circumferential end portion of a flash chamber εidewall, so that the sizing of the gap between the flash chamber sidewall and the slinger plate 87 is such that only liquid remaining after flashing is permitted to exit, and vapor exiting the flasher is not permitted to escape through the liquid outlet 88.

To illustrate how the new slinger plate improves vapor-liquid separation of the flasher device, the following example is provided.

EXAMPLE Utilizing the teachings of prior art as set forth in my U.S. Patent No.4,287,019, a plurality of rotating flasher devices was constructed substantially as shown in FIG. 3. A vertically nested stack of 24 flashers was constructed, installed, and operated in a multistage flash apparatus, with a rated capacity of 5,000_U.S. gallons per day, according to the teachings of the referenced patent. After startup of the unit and upon stabilization of unit operation, the apparatus consistently produced condensate with a purity of approximately 750 parts per million(ppm) . Various methods were evaluated to decease the presence of dissolved solids in the condensate, and thus to increase the purity thereof. One method involved decreasing the rated capacity of the evaporator system from 5,000 U.S. gallons per day to 4,000 U.S. gallons per day. This resulted in a decrease in contamination of in the condensate from 750 ppm to 300 ppm. rsince it is economically undesirable to lower^"the rated capacity of an evaporator apparatus, another solution was sought.

Rotating flashers constructed according to the- most preferred embodiment of the present invention were constructed and installed in the very same evaporation apparatus used above. These improved rotating flashers were tested under operating conditions substantially similar to operating conditions utilized during previous tests. With the new rotating flashers in place, condensate quality of approximately 80 ppm was observed while continuing operation at an evaporation capacity of 5,000 U.S. gallons per day. FIG. 5 is a sectional view from FIG. 4, and shows a bottom plan view looking up at the flasher device. Rotating shaft 83 and bushing or bearing 82 are shown at the center. Vapor exit space 94 is formed by an annular area at the center of the flasher device. Stationary skimmer plate 91 extends radially from the edge of the vapor exit 94 to the edge of the liquid exit 88. The gap between skimmer plate 91 and the flasher housing 80 must be sized such that only liquid is permitted to escape upwardly through liquid exit 88. Once liquid has escaped through the exit 88, liquid is directed in a swirling pattern by diffuser vanes 89 toward liquid exit passageways 81 which connect to the bottom of the successive flasher stage. It can be seen from.,FIG. 5 that it is necessary for efficient vapor-liquid separation to direct liquids outward towards the vessel wall 80 while simultaneously directing clean vapors inwardly toward the vapor exit 94.

FIG. 6 is a section taken through FIG. 4, and shows a plan view of the stationary portion of the bottom of a flasher device, revealing liquid outlets 81. Referring back to FIG. 5, it can be seen that liquid is directed by diffusers vanes 89 up through liquid outlets 81 to enter the bottom of the flasher device. Referring again to FIG. 6, also shown is the central rotating shaft 83 and bushing 82.

FIG. 7 is a sectional view taken through line 7-7 of FIG. 6. FIG. 7 shows the contruction of liquid outlets 81 and illustrates how side diffuser vanes 89 are formed to redirect liquid upward through liquid outlets 81.

FIG. 8 is a bottom plan view taken as a section through FIG. 4, and shows a rotating flasher 84 in the most preferred embodiment of the present invention. Here can be seen the flasher housing 80, rotating shaft 83, and bushing 82. Vapor must exit inwardly between impeller blades 86 and towards vapor exit 94. The slinger device 87 of the present invention is mounted above impeller blades 86 and rotate therewith. To achieve the objects of the present invention, there must be no gap between the upper portion of impeller blades 86 and the rotating slinger plate 87. It can readily be seen here that as liquid tries to exit with vapor 94, it will be caught by rotating impeller blades 86 or it may impact against slinger plate 87. In either case, such liquid droplets will tend to be accelerated outwardly toward the flasher wall 80. Thus, use of the slinger plate 87 prevents buildup of liquid at the top of the flasher device and forces liquid attempting to escape to impinge on rotating impeller blades or slinger plate and be accelerated toward the liquid outlet 88. Also shown in hidden lines are grooves on the upper side of the slinger plate which are useful to redirect any liquids attempting to escape between slinger plate 87 and -skimmer plate 91 of FIG. 4, so that such liquid returns toward liquid exit 88. Ridges on the slinger plate may be equally useful.

FIG. 9 illustrates a second embodiment of my present invention. FIG. 9 is a plan view looking down on a rotating impeller assembly. Impeller blades 86 have wiper blades 130 attached thereto. As the rotating impeller 84 turns on shaft 83, wiper blades 130 seal the gap between blades 86 and the fixed skimmer plate 91 above. These flexible wiper blades would provide a serviceable seal. However, this means may be less preferable than the most preferred embodiment as this method involves solid-to-solid contact and hence wear. Thus, a more limited selection of materials for fabrication of the wiper blades 130 and the skimmer plate 91 must be made.

FIG. 10 is a vertical section of the apparatus earlier illustrated in FIG. 9, taken along line 10-10 of FIG. 9. Here, fixed skimmer plate 91 is shown being wiped by blades 130, which are secured to impeller blades 86 by securing means 131.

FIG. 11 is a vertical cross section of a stack of flasher devices, similar to FIG. 4, and illustrates a third embodiment of the present invention. Here, the fixed skimmer plate 100 has been constructed at an angle which inclines upwardly toward the center vapor exit 94, in generally frusto-conical shape. Also it can be seen that the outer edge of vapor exit 94 extends down below the inner terminus of the inclined annulus formed by skimmer plate 100. Thus, water carried inward toward vapor exit 94 will be hindered from flowing further by the effect of gravity which will tend to pull any liquids on the skimmer plate 100 back down towards the liquid exit 88. Also, if liquid reaches the obstruction at the inner terminus of skimmer plate 100, it will tend to drop downward towards the impellers 84 and will be accelerated towards the inside wall of flasher vessel 80. FIG. 12 is yet another embodiment of the present invention, similar to the view shown in FIG. 4 above. Here, it can be seen that liquid tending to move towards the center of the rotating flasher in an attempt to escape through vapor exit 94 will, when traveling along skimmer plate 110, reach a point where it will have no further surface to adhere to as it travels. At the corner 111, liquid will tend to fall vertically downward towards the impeller blades 84 and will thereby be sent back towards the inside wall 112 of the flash vessel 80.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

CLAIMSWhat is claimed is:

1. A multistage flash evaporator comprising: a. means providing a compartment; b. a vertical stack of flashers in said compartment; and c. means for inputting a liquid to be evaporated to the lowermost flasher in said stack, for transferring unevaporated liquid from said lowermost to each succeeding flasher in said stack, and for outputting evaporated liquid from the compartment; d. each of said flashers comprising: (i) a cylindrical vessel,

(ii) a rotatable impeller in said vessel for so accelerating heated liquid in said vessel toward the periphery of the vessel as to promote the conversion of said liquid to vapor,

(iii) means in the upper reaches of said vessel providing a centrally located vapor outlet from said vessel for the evaporated liquid generated in the vessel, and

(iv) means above and immediately adjacent said impeller which keeps unevaporated liquid in said vessel from migrating toward said centrally located outlet; and e. said evaporator further comprising means for rotating the impellers of said several flashers.

2. A multistage flash evaporator as defined in claim 1 wherein the means for inhibiting the migration of liquid toward the vapor outlet of the vessel in which the impeller is located comprises an annular slinger fixed to the top of said impeller for rotation therewith.

3. A multistage flash evaporator as defined in claim 2 which includes means providing a vapor-tight seal therebetween said impeller and said slinger.

4. A multistage flash evaporator as defined in claim 2 wherein the flasher vessel has a liquid outlet and wherein said slinger has an upper side and grooves on said upper side for redirecting liquid migrating toward said vapor outlet toward said liquid outlet.

5. A multistage flash evaporator as defined in claim 2 or in claim 4 wherein the flasher vessel has a liquid outlet and wherein said slinger has an upper side and ridges on said upper side for redirecting liquid migrating toward said vapor outlet toward said liquid outlet.

6. A multistage flash evaporator as defined in claim 1 wherein the means for inhibiting the migration of liquid toward the vapor outlet of the vessel in which the impeller is located comprises wiper means above and rotatable with the impeller.

7. A multistage flash evaporator as defined in claim 1 in which each flasher vessel has a liquid outlet and in which the means for inhibiting the migration of liquid toward the vapor outlet of the vessel in which the impeller is located comprises a frustoconical skimmer located above the impeller in the flasher vessel and so configured that liquid adhering to said skimmer-will flow downwardly and outwardly toward said liquid outlet.

8. A multistage flash evaporator as defined in claim 1 wherein the means for inhibiting the migration of liquid toward the vapor outlet of the vessel in which the impeller is located comprises a εkimmer having a first, generally horizontal component and a second, generally vertical component extending toward and so abutting said first component that unevaporated liquid can migrate across εaid first component and then downwardly along εaid εecond component.