Multi-stage Flash Evaporator

Abstract

An improved multi-stage flash evaporator for distilling sea water, which, in transverse profile, is symmetrical about a vertical central axis. The evaporator construction includes a horizontally disposed, longitudinally extending, continuous, tubular casing, whose profile, on each side of the axis, is substantially of the shape of an ellipse which is truncated towards one end by this axis; and an inverted U-shaped shell within the casing and extending along the length thereof. The lower edges of the shell are sealingly connected to the floor of the casing, While the top of the shell sealingly contacts the upper junction connecting the elliptically shaped zones of the casing. A plurality of vertical, spaced apart, transversely extending, apertured partitions subdivide the shell into a number of serially connected chambers, for the raw sea water to flow therethrough and evaporate therein. The outer casing and the inner shall define separate, condensation chambers which are positioned laterally on each side of the serially connected evaporation chambers, the distillate vapor flowing from the evaporation to the condensation chambers through droplet-separating filters housed in the top of the shell.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multi-stage flash evaporator particularly suitable for desalting sea water. More specifically, the present invention relates to an evaporator wherein feed water to be evaporated flows continuously through a plurality of serially connected, evaporation chambers in which successively lower internal pressures exist in adjacent chambers with each evaporation chamber being connected to two separate condensation chambers.

2. The Prior Art

There have recently been disclosed a number of multi-stage flash evaporators for distilling sea water, brack water and the like.

For instance, U.S. Pat. No. 3,146,177 (to Chalmers et al.) discloses a multi-stage evaporator comprising a plurality of evaporation chambers in series and side condensation chambers. Raw sea water to be preheated, is subdivided into parallel streams flowing through heat exchangers arranged within the condensation chambers, the direction of flow of the sea water through the exchangers being opposite to that of the flow of water being evaporated through the evaporation chambers.

In accordance with Chalmers et al., each evaporation chamber is subdivided into two half chambers, with each half chamber being connected with one condensation chamber only, so that a single evaporation chamber does not communicate with both of the laterally positioned condensation chambers. Such a construction possesses several major disadvantages, since it does not uniformly distribute the vapor evolved at the evaporation zone; besides, the complexity of such construction and the presence of several sharp corners and dead zones result in an inefficient operation and, ultimately, an impure distillate.

SUMMARY OF THE INVENTION

In general terms, the multi-stage flash evaporator construction for distilling sea water and the like in accordance with the present invention, may be stated as substantially comprising an outer, continuous, generally horizontally disposed, longitudinally extending casing which is specially shaped in the manner that will be fully explained in detail below, and an inverted U-shaped shell which is positioned within and extends along the length of the casing. A plurality of vertical, spaced-apart, transverse partition walls having passages therethrough adjacent the bottom of the shell, subdivide the shell into a plurality of serially connected chambers. At the opposite ends of the shell, inlet and outlet means are provided for the continuous flow through the evaporation chambers of the pre-heated liquid (e.g. sea water) to be evaporated. Successively lower internal pressures exist in adjacent chambers. Therefore, the inner shell defines the feedwater receiving zone and the evaporation zone of the evaporator in accordance with the invention. The outer walls of the shell and the inner walls of the casing define a number of separate condensation chambers positioned laterally on each side of the central evaporation zone, the condensation chambers extending along the length of the casing. In the generally flat top surface of the shell, openings are provided for accommodating filtering means which are horizontally positioned over the evaporation chambers and which permit the flow of vapor from the evaporation chambers to the condensation chambers. Two heat exchangers, extending along the length of the casing, are positioned within the condensation chambers to cause condensation of the vapor while partially pre-heating the raw sea water, the flow of the raw sea water through the heat exchangers being in a direciton opposite that of the flow of the sea water being evaporated in the evaporation chambers.

More specifically stated, the evaporator construction in accordance with the present invention, comprises:

a continuous, elongated, generally horizontally disposed, tubular casing which in transverse profile is symmetrical about a vertical central longitudinal plane, the part of the profile on each side of the central plane being generally of the shape of an ellipse whose major plane is at right angles to such central plane and which is truncated towards one end by such central plane, the profile deviating from elliptical curvature in a zone extending on the lower side of the casing from a point located approximately below the centre of the ellipse to the central plane, the curvature in this zone being circular;

an elongated, inverted U-shaped shell, positioned within and extending along the length of the casing, the lower edges of the shell being sealingly connected to the casing, the shell thus having as its floor a central portion of the casing, this shell consisting of two vertical side walls and a top which is generally flat in its central portion and curved at the edges joining said side walls, the top meeting the casing sealingly at the junction connecting the elliptically shaped zones of the casing;

a plurality of transverse partition walls positioned vertically within the casing and spaced apart from each other, said partition walls dividing the shell into said plurality of evaporation chambers, each chamber having inlet and outlet means for the feedwater to be evaporated, such means including passages formed in the partition walls serving as the feedwater outlet means for one chamber and feedwater inlet means for the adjacent, lower pressure evaporation chamber; the inner surface of the casing and the outer surface of the shell defining two separate condensation chambers extending along said evaporation chambers;

droplet-separating filter means housed in the top of the shell, for permitting the flow of vapour from each of said evaporation chambers to both of the condensation chambers connected thereto.

a heat exchanger positioned within and extending along the length of each of the condensation chambers.

The evaporator in accordance with the invention appears more suitable than prior evaporators to obtain high output capacities, such as 500 m.sup.3 /hour of distillate product or even more. A remarkable decrease in the evaporator weight and overall dimensions is achieved. Furthermore, the special arrangement of the present construction results in the lessening of the corrosion phenomens, since the formation of stagnation zones of products which cannot be condensed is hindered and no sharp corners are present. The highly symmetrical arrangement also allows a better strain distribution, with attendant stress decrease.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective, partly cut-away view of a multi-stage flash evaporator in accordance with the present invention:

FIG. 2 is a transverse sectional view taken along line II--II of FIG. 1;

FIG. 3 is an enlarged, somewhat schematic, view of a portion of FIG. 2;

FIG. 4 is a fragmentary longitudinal sectional view, taken along line IV--IV of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Construction

As shown in the drawings, the flash evaporator construction of the present invention is, in transverse profile, symmetrical about a vertical central axis A. The evaporator includes a continuous, generally horizontal, longitudinally extending, tubular casing 1, which is supported on a plane surface by conventional supporting means (not shown). As shown in detail in FIG.3, the profile of the casing 1 on each side of the axis A is generally of the shape of an ellipse which is truncated towards one end by the central axis A. The ratio between the major axis M of the ellipse and its minor axis m preferably ranges from 1.4/1 to 1.6/1. The profile of the casing 1 deviates from elliptical curvature in a zone B extending on the underside of casing 1, from a point C located below the centre of the ellipse to a point D on the central axis A. Zone B is generally circular and, preferably, the angle .alpha. subtending the arc CD ranges from 13.degree. to 18.degree..

As shown in FIGS. 1 and 2, within the casing 1 and extending along its length is, an inverted U-shaped shell 2, consisting of side walls 3 and 4 and a flat top 5, which is curved at the edges 6, 7 joining side walls 3 and 4, respectively. The bottom edges 8 and 9 of the side walls 3 and 4, are sealingly connected to the central portion of the floor 10 of the casing 1, this portion forming the floor of the shell 2. The top 5 of the shell 2 meets the casing 1 sealingly at the joint J connecting the elliptically shaped upper zones 11 and 11' of the casing 1. Two vertical partitions 12 and 13, having passages 12a, 12b and 13a, 13b, respectively, and end walls 17 and 18, subdivide the interior of the shell 2 into three evaporator chambers 14, 15 and 16, each chamber forming a feed water receiving stage and also an evaporation stage. Connected to the shell 2 is a sea water heater, not shown. Connections between this heater and a raw sea water inlet 19 on the end wall 17 are also not shown. The end wall 18 has an outlet 20 for discharging the raw sea water which has not evaporated while flowing through the evaporation chambers 14, 15 and 16.

In each of the evaporation chambers 14, 15 and 16, a dam 21 is positioned vertically and extends transversely of shell 2. As shown in FIG.4, the upper edges 21' of the dams 21 lie above the upper edges of the passages 12a, 12b, 13a and 13b, to maintain the sea water in each chamber at least at a minimum level sufficient to prevent vapor from being blown between adjacent chambers through said passages, while still permitting flow of the sea water to be evaporated.

In order to regulate the flow of the sea water to be evaporated through the evaporation chambers, as well the level of the sea water in said chambers, the passages 12a, 12b, 13a and 13b could be equipped with selectively adjustable, constriction means (such as a sluice-gate valve).

The top 5 of the shell 2 has longitudinally extending openings 22 accommodating horizontal droplet-separating filters 23 which allow the flow of vapor, while the droplets entrained by the vapors are captured by the filters and drip back in the evaporation chamber. The outer wall surfaces of the shell 2 and the inner wall surfaces of the casing 1 define six side condensation chambers 24, 24a, 24b and 25, 25a and 25b.

Two heat exchangers 26 and 27 comprising a plurality of condensing tubes 29 are positioned within and extend along the length of the condensation chambers. Conventional distillate drain means (not shown in the drawings) are connected to the condensation chambers, for discharging distillate water therefrom.

Operation

Raw sea water enters the condensing tube inlet 30 of evaporation chamber 16, which is maintained at the lowest pressure and temperature. This sea water is pre-heated by forming the condensing medium flowing through the condensing tubes 29, passing progressively through each of the evaporation chambers 16, 15 and 14, to leave finally through the condensing tube outlet 31 of the chamber 14, which has the highest temperature and pressure. The partially pre-heated sea water then passes in the heater (not shown), where the sea water is heated to the desired temperature. This fully pre-heated sea water is then fed into the feed water receiving section of the chamber 14, to begin the desalinization process. In the evaporation chamber 14, a certain amount of water flashes off into vapor, which rises and passes through the droplet-separating filters 23 into condensation chambers 24 and 25, as indicated by the arrows in FIG.2. The filters 23 capture droplets of unvaporized sea water which drip back into the evaporation chamber 14, thus preventing any contamination of the condensed vapor 33 contained in the condensing chambers 24 and 25. The elliptically shaped zones 11 an 11' at the top of the casing 1 prevent deposition of the distillate vapor on the upper part of the casing 1. As distillate vapor contacts the heat exchangers 26 and 27 within the condensation chambers 24 and 25, it gives up a certain amount of heat to the raw sea water in the condensing tubes 29 and the distilled vapor then condenses and collects at the lower portion of condensation chambers 24 and 25. The raw sea water which has not undergone distillation within the first evaporation chamber 14, flows continuously through the passages 12a and 12b into the next adjacent evaporation chamber 15, thus flowing in the opposite direction to the raw sea water being pre-heated within the condensing tubes 29. In the evaporation chamber 15, the same process as in the evaporation chamber 14 takes place. The sea water entering evaporation chamber 15 is, however, at a lower temperature than sea water in chamber 14 and the pressure in chamber 15 is also correspondingly lower. The sea water, then, flows continuously through the evaporation chambers 14, 15 and 16 at progressively decreasing temperature and pressure, finally leaving from the chamber at the lowest temperature and pressure. The distillate 33 is drained from the condensing chambers 24 and 25 through a known distillate drain, not shown in the drawings.

Claims

Having thus described the invention, what is desired to be secured by Letters Patent and hereby claimed is:

1. A flash evaporator for distilling sea water and the like in a plurality of multi-stage, generally horizontally disposed, serially connected, adjacent evaporation chambers, wherein feed water to be evaporated flows continuously through successive stages in which progressively lower pressure exists in adjacent stages, the evaporator comprising:

a continuous, elongated, generally horizontally disposed tubular casing which in transverse profile is symmetrical about a vertical central axis, the part of the profile on each side of the central axis being generally of the shape of an ellipse whose major axis is at right angles to such central axis and which is truncated towards one end by such central axis, the profile deviating from elliptical curvature in a zone extending on the lower side of the casing from a point located approximately below the centre of the ellipse to the central axis, the curvature in this zone being circular;

an elongated, inverted U-shaped shell, positioned within an extending along the length of the casing, the lower edges of the shell being sealingly connected to the casing, the shell thus having as its floor a central portion of the casing, this shell consisting of two vertical side walls and a top which is generally flat in its central portion and curved at the edges joining said side walls, the top meeting the casing sealingly at the junction connecting the elliptically shaped zones of the casing;

a plurality of transverse partition walls positioned vertically within the casing and spaced apart from each other, said partition walls dividing the shell into said plurality of evaporation chambers, each chamber having inlet and outlet means for the feedwater to be evaporated, such means including passageways formed in the bottoms of the partition walls serving as the feedwater outlet means for one chamber and feed water inlet means for the adjacent, lower pressure evaporation chamber; the inner surface of the casing and the outer surface of the shell defining a plurality of separate condensation chambers extending along said evaporation chambers;

droplet-separating filter means housed in the top of the shell, for permitting the flow of vapour from each of said evaporation chambers to both of the condensation chambers laterally connected thereto;

a pair of heat exchangers, one each positioned within and extending along the length of each condensation chambers.

2. A flash evaporator as claimed in claim 1, wherein the ratio between the major and the minor axis of each ellipse ranges from 1.4/1 to 1.6/1.

3. A flash evaporator as claimed in claim 1, wherein the angle subtending said circular curvature ranges from 13.degree. to 18.degree..

4. A flash evaporator as claimed in claim 1, further comprising a plurality of dams positioned vertically within said shell on the floor thereof, each of said dams extending transversally of said shell and each positioned within an evaporation chamber approximately midway between said transverse partition walls, the upper edge of said dams being positioned at a height greater than the height of the upper edge of the passageways in said transverse partition walls to maintain the liquid level above the upper edge of said passageways.