Tubular Structure For Film Evaporators

Abstract

A tube-bundle evaporator through the interior of the tubes of which the liquid to be evaporated is passed. The tubes are deformed so that the flow cross-section increases longitudinally in the direction of flow of the vapor while the perimeter or peripheral extent of the tube and, preferably, the diameter of the smallest circumscribing circle remains constant over the entire length of the tube.

Description

1. FIELD OF THE INVENTION

My present invention relates to evaporators and, more particularly, to heat-exchange devices for the evaporation of liquids.

2. BACKGROUND OF THE INVENTION

A large number of evaporators, refluxers, boilers and vaporizers have been provided heretofore in accordance with fluid-fluid heat-exchange principles whereby a liquid to be evaporated is brought into contact with one surface of the heat exchanger while a heating liquid is brought into contact with another heat exchange surface in heat-exchange relationship with the first.

The device, therefore, may be an indirect heat exchanger having passages for the heating fluid and for the vaporizable liquid, separated by a thermally conductive wall through which the heat exchange is effected. Such heat exchangers are used for a wide variety of purposes and, for example, in low-temperature systems, they may be used for vaporizing a liquefied gas or a refrigerant, while in chemical technology generally, they may be used for concentration of liquids, drying of slurries, production of vapors or the like.

The present invention is concerned with tube-bundle evaporators which, while operating in accordance with the above-described principles, generally define the passages traversed by the vaporizable liquid by tubes extending parallel to one another and constituting a tube bundle. An evaporator of the latter type can comprise, therefore, a plurality of generally parallel tubes fixed at their ends in a pair of tube sheets, the latter being welded or clamped in a housing which includes a pair of end domes or closures defining manifold chambers communicating with the respective ends of the tubes at the point at which the tubes terminate in the respective tubing sheets. The casing or housing surrounding the tubes by the sheets defines a compartment traversed by the heating fluid which, therefore, passes through the interstices between the tubes. A structure of this type can have a wide variety of configurations and will be referred to hereinafter as a tube-bundle heat exchanger.

It is important at this point to note that evaporators are also classified by the orientation of the tubes and the manner in which the liquid to be evaporated is fed to the tube bundle. For example, the tubes may be oriented in either vertical of horizontal direction and, while it will be apparent hereinafter that the orientation is not critical for the purposes of the present invention, it nevertheless is preferred to apply the invention to vertical-tube evaporators. The liquid-feed direction is significant as well and, I may observe, the invention is applicable to both falling-film or climbing-film evaporators. In a falling-film evaporator, the liquid to be vaporized falls downwardly along the surface of the tube and is supplied to the tube bundle from above. Conversely, the liquid of a climbing-film evaporator moves upwardly along the tube wall from below.

In conventional evaporator tubes, i.e., the tubes of tube-bundle evaporators of the falling-film or climbing-film and vertical-tube types, the interior of the tube has a constant diameter and cross-section and, since the vapor is formed only after the liquid has been heated to the boiling point, the velocity of the vapor at the liquid-inlet is initially zero but rises to a maximum at the exit end of the tube. In tubes which are supplied with liquid at their lower ends, the increasing formation of vapor as the liquid rises produces vapor bubbles which frequently block the rise of liquid beyond a certain point. As a result, the laminar flow of liquid along the tube wall, i.e., the existance of a liquid film, is possible only in the lower portions of the tubes and, consequently, the heat-exchange characteristics between the heating fluid and the liquid vary along the tube length. The increase in heating beyond the stable liquid film gives rise to concentrated evaporation and, therefore, encrustation of the heat-exchange surfaces within the tube with solids deposited as a consequence of the non-uniform evaporation.

In falling-film evaporators, i.e., vertical tube evaporators in which the tubes receive liquid from above, it is desirable that the film of vaporizable liquid descend along the inner walls of the tubes in a uniform fashion. At the upper or inlet ends of the tubes, the vapor velocity is, of course, zero and the vapor velocity increases to a maximum at the outlet or lower ends of the tubes. However, even this system has significant disadvantages. Firstly, the formation of vapor bubbles near the inlet end of the tube tends to create vapor layer in laminar flow along the heat exchange surface, the vapor layer, deflecting the incoming liquid away from the walls of the tube and forming a strand of the liquid through the center of the tube. This coherent liquid filament, thread or strand may remain out of contact with tube wall over virtually its entire period of passage through the tube. Hence this strand of liquid is not subjected to evaporation, while residual liquid films along the surface of the tube are overheated and have a tendency to vaporize without concentration of solids within the body of the remaining liquid, thereby promoting encrustation and contamination of the tube surface. Furthermore, since the laminar gas flow is not a problem in regions at which turbulence occurs to break up this cushion, the heat exchange through the tube walls is irregular and often unpredictable. In fact, only at those tube portions at which turbulent flow occurs is the liquid to be evaporated permitted to contact the heating surface of the tube and is the formation of crusts and the like reduced.

These disadvantages have long plagued the evaporator art and numerous suggestions have been made to eliminate them. For example, it has been proposed to provide deflectors, agitators and other means for generating turbulence or providing centrifugal force to return the vaporizable liquid to a heating surface. Such means have, however, proved to be of limited suitability, not only because of their high initial and operating costs, but also because they have found to be ineffective for many types of evaporation. It has also been proposed to provide tubular inserts within the heat exchange tubes, thereby increasing the velocity of the liquid or gas traversing the tubes and promoting turbulence in accordance with the Reynold's effect. However, even this represents a cost increase and provides a large surface which is ineffective in the heat exchange so that a great deal of the liquid traversing the tube is used to wet the surface of the insert and the concentration of the remaining liquid, i.e., the liquid in contact with the heating surface is proportionally increased. This represents a greater danger of soiling and encrustation.

3. OBJECTS OF THE INVENTION

It is, therefore, the principal object of the present invention to provide an improved tube construction for a tube-bundle evaporator of the character described.

Another object of this invention is to provide a tube-bundle heat exchanger of the falling-film or climbing-film type which is free from the disadvantages of earlier heat exchangers using uniform cross-section tubes and yet can be made at low cost.

It is also an object of the invention to provide an efficient evaporator tube structure which eliminates the need for centrifugal and other turbulence-inducing devices and yet is capable of eliminating the disadvantages of climbing-film or falling-film evaporators of prior-art constructions.

4. SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter are achieved, in accordance with the present invention, with a tube construction for a tube-bundle evaporator, preferably of the falling-film or climbing-film type, in which the tube over the major part of its length spanning the tube sheets is formed unitarily (be deformation) with a plurality of troughs radiating from the center of the tube and of a flow cross-section which increases longitudinally in the direction of flow of the vapor, the peripheral extent of the tubes remaining constant over the tube's length. The term "peripheral extent" is used here to describe the perimeter of the tube in a cross-sectional plan perpendicular to the tube axis. The troughs or grooves, which should be at least three in number, collectively open into the interior of the tube at which flow remains unobstructed. As a result, the cross-section of the tube viewed from the liquid-inlet end is at a minimum, but increases progressively and preferably linearly with the distance from the liquid inlet end to the liquid outlet end at which the flow cross-section corresponds to the cross-section A = .pi.R.sup.2 where R is the radius of the interior of the cylinder tube. The perimeter of the tube, regardless of the flow cross-section is given by P = 2.pi.(R + .DELTA.R) where .DELTA.R is the thickness of the tube. At the liquid-inlet end of the tube, the flow cross-section may approach zero but is preferably a small fraction of the maximum cross-section of the tube, i.e., (A/20) to (A/3).

According to an important feature of the present invention, the walls of the troughs or grooves are parallel to one another, i.e., each trough in cross-section is defined between a pair of parallel sides lying on these walls, these sides being parallel to an axial plane passing centrally through the representative trough and lying midway between these walls. Furthermore, the walls of adjacent troughs include angles of about (360.degree./N), where N is the number of angularly equispaced troughs (preferably N = 3 or 4). I have found it to be desirable to provide free-end portions of the tubes which are of cylindrical configuration and free from the troughs to enable the tubes to be fitted or joined to the tube sheets at the liquid-inlet and outlet ends of the tubes and to deform the troughs from the walls of a cylindrical tube by spinning or drawing. As a result, it is possible to create a tube construction in which the circle of smallest diameter which can circumscribe the tube at any of its star-shaped cross-sections will correspond to the outer diameter of the original tube. The cross-section of the tube is preferably selected so that the vapor formed in the tube flows at a uniform velocity throughout its length.

If the tubes are formed by spinning with troughs having sidewalls which diverge at an angle, say, of 120.degree., it is possible theoretically to collapse the shell of the tube so that the inside surfaces of the inwardly deformed generally segmental portions contact one another and eliminate a free cross-section between them. The system of the present invention is operative even where the free cross-section of the troughs initially is zero, the fluid thus passing through the central opening in the initial stages. In all of the preferred constructions, the tube will be of star-shaped profile with the vertices of the outermost points lying on the circumference of the original tube, i.e., on the circumscribing circle or a circle centered on the tube and of a circumference equal to the perimeter of the tube. The peripheral extent or perimeter of the tube, however, remains constant over its entire length. In practice, the tube is collapsed at its inlet end so that a star-shaped free cross-section is left in the tube and the tube walls have a clearance from one another of 1 to 3 mm. The liquid to be evaporated thus can enter the tube band be immediately subjected to turbulent flow.

5. DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a vertical elevational view of a tube structure (and the associated tube sheets) of a tube-bundle evaporator embodying the invention and operating in accordance with falling-film principles;

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

FIG. 3 is a section taken along the line III -- III of FIG. 1 and showing that portion of the tube having the smallest cross-section;

FIGS. 4 - 7 are cross-sectional views taken along the lines IV -- IV to VII -- VII, respectively; and

FIG. 8 is a cross-sectional view along the line VIII -- VIII of FIG. 1 illustrating the region in which the tube is of maximum flow cross-section;

FIG. 9 is a vertical elevational view of a tube according to the invention operating under the principles of a climbing-film evaporator;

FIGS. 10 - 16 are sections taken generally along the lines X -- X to XVI -- XVI, respectively; and

FIG. 17 is a section through a tube according to another embodiment of the invention.

6. SPECIFIC DESCRIPTION

In FIG. 1, I show a falling-film evaporator in which the heating and evaporating tube 3 is anchored in the tube sheets 1 and 2 of the evaporator, the tube sheets being only fragmentarily illustrated in this drawing. Above the tube sheet 1, therefore, the usual housing (not shown) may define an inlet chamber A to which the liquid is supplied, the liquid being distributed to the upper ends of the tubes 3 of the tube bundle. Around the tubes 3, between the tube sheets 1 and 2, the housing defines a chamber B provided with the usual conduits for supplying a heating fluid to the interstices between the tubes of the tube bundle. Below the tube sheet 2, the housing defines a chamber C to which concentrated liquid is removed.

The circular top ends of the tubes 3 are rolled into the upper tube sheet 1 and the tubes are formed by spinning with three troughs 7, 8 and 9 which lie in radial planes of the tubes and angularly equispaced about the axis 10. The walls 11 and 12 defining each of the troughs 7 - 9 lie generally parallel to one another and to a radial plane P through the respective trough and midway between the sides 11 and 12 thereof. According to the principles of the present invention, the sides 12 and 13 of adjoining troughs intersect at an angle .alpha. = (360.degree./N) or 120.degree. where N = 3 as shown. The radial extend r of each trough wall is slightly less than R + .DELTA. R, where R is the internal diameter of the tube (see FIG. 8) and .DELTA.R is the wall thickness thereof. The circumferential width t of the trough initially is about 2 mm. Furthermore, at each cross-section taken perpendicular to the axis 10, several of which are illustrated in FIGS. 2 - 7, the perimeter of the tube is equal to the circumference of the cylindrical tube (FIG. 8). Hence, the lengths of the sides 12 and 13 must be equal to the arc length L between the two troughs.

In FIG. 2, I have shown a section in which the trough formations are viewed from an end while FIG. 3 represents the initial horizontal section. The troughs 7 - 9 progressively decrease in depth so that a section taken at about one-fourth of the length of the tube shows in FIG. 4 a corresponding increase of the flow cross-section to about one-fourth of the circular section of the tube as seen in FIG. 8. As the depth of the troughs is reduced, the flow cross-section increases (preferably linearly) so that midway along the tube the flow cross-section is about half of maximum (FIG. 5) about three-fourths of the way along the tube, the flow cross-section is about three-fourths of maximum (FIG. 6) and close to the end of the tube (FIG. 7 the flow cross-section is approximately equal to the maximum. The cylindrical end portions 14 and 15 of the tubes, of course, are free from the troughs and serve to anchor the tube in the sheets 1 and 2.

In FIG. 17, I have shown a four-trough structure which is otherwise similar to the embodiments previously described. Here, however, the walls 17 and 18 of adjacent troughs include an angle .beta. = (360.degree./N) where N = 4 and .beta. = 90.degree.. The walls 17 and 19 defining each of the troughs lie parallel to one another and the troughs have their apices lying along a circle 20 corresponding to the original tube circumference.

FIG. 9 shows a climbing-film evaporator, where a heating and evaporating tube 3 is disposed between partly shown tube plates 1 and 2. The tube 3 is heated by steam, which is supplied to the chamber defined by the tube plates 1, 2 and a shell, which is not shown. The circular lower end of the tube is rolled into the lower tube plate 2. By spinning, the tube has been formed with three troughs so that the cross-section of the tube behind the rolled-in portion is reduced to a fraction of the original cross-section of the tube while gaps having a width of about 2 millimeters are left. FIG. 16 illustrates a view taken into the tube. The periphery of the three-pronged figure has the same peripheral extent as the original tube. FIG. 15 is a horizontal transverse sectional view taken through the tube 3 along line XV -- XV. FIG. 14 is a sectional view taken through the tube along line XIV -- XIV at about one-fourth of the length of the tube, where the cross-section of the tube is about one-fourth of the original cross-section of the tube. The cross-section increases continuously as the troughs decrease in depth. FIG. 13 is a sectional view taken through the tube along line XIII -- XIII at one-half of the length of the tube. The cross-section of the tube is one-half of the cross-section of the original circular tube. FIG. 12 is a transverse sectional view taken through the tube along line XII -- XII at three-fourths of the length of the tube, where the cross-section of the tube is about three-fourths of the original cross-section of the tube. FIG. 11 is a sectional view taken along line XI -- XI shortly before the termination of the troughs. FIG. 10 is a sectional view taken on line X -- X through that portion of the tube which is rolled in the tube plate 1 and has again the original circular cross-section.

7. SPECIFIC EXAMPLES

Example 1

The falling-film evaporator shown in FIG. 1 was used, which was provided with heating and evaporating tubes having a length of 4 meters. The tubes had an inside diameter of 32 millimeters before they were formed with the troughs and at various points of their length, measured from top to bottom, had approximately the following cross-sections in square millimeters:

Tube cross-section at one-fourth of tube length 200 Tube cross-section at one-half of tube length 400 Tube cross-section at three-fourths of tube length 600 Tube cross-section at the exit end of the tube 800

Liquid containing 20 percent solids was fed into the tube 3 from above at a rate of 36.7 kilograms per hour. The evaporating temperature was 60.degree.C and the heating steam had a temperature of 75.degree.C. After a single pass through the tubes of the evaporator, the liquid had been evaporated to a solids content of 50 percent.

The heat transfer rate was about 2,000 kilocalories per square meter/hour/.degree.C.

When water was evaporated at a total rate of 22 kilograms per hour, the vapor velocity was uniform in all parts of the tube and amounted to 58 meters per second.

Example 2

The climbing-film evaporator shown in FIG. 9 was used, which was provided with heating and evaporating tubes 3 meters long. Before the tubes were formed with the troughs, they had a diameter of 32 millimeters. At various points of their length, measured from bottom to top, the tubes had the following cross-sections in square millimeters:

At one-fourth of tube length 200 At one-half of tube length 400 At three-fourths of tube length 600 At the exit end of the tube 800

Liquid having a solids content of 30 percent was fed into the tubes at the boiling temperature at a rate of 40 kilograms per hour. The tube was heated on the outside with steam. Water at a rate of 20 kilograms per hour was evaporated in the tube when the evaporating temperature inside the tube was 60.degree.C and the heating steam had a temperature of 75.degree.C on the outside surface of the tube. The vapor produced in the tube forced the liquid to be evaporated upwardly through the tube and had a virtually constant velocity of 67 meters per second.

A single pass through the heating and evaporating tube produced liquid at a rate of 20 kilograms per hour and water vapor at a rate of 20 kilograms per hour. The resulting liquid had a solids content of 60 percent.

Claims

I claim:

1. A tube structure for an evaporator, comprising at least one tube traversed internally by a vaporizable liquid and heated by a fluid in contact with the exterior of said tube, said tube being formed internally having equal-diameter cylindrical end portions and between said end portions with a plurality of rectilinear, angularly equispaced troughs deformed from the tube wall extending along the tube and defining a flow cross-section of the tube increasing substantially linearly over the distance between said end portions and defining a direction of flow of fluid from the small to the large cross-section whereby the flow velocity through the tube is maintained substantially constant, said troughs having arcuate floors lying on a circle of the same diameter as said end portions whereby said tube is circumscribed by a circular cylinder of substantially constant diameter over its entire length, said troughs flooring from said small cross-section to said large cross-section.

2. The tube structure defined in claim 1 wherein at least three angularly equispaced troughs are provided about said axis.

3. The tube structure defined in claim 1 wherein said troughs in cross-section each have a pair of parallel sides and the sides of neighboring troughs include angles of substantially (360.degree./N), where N equals the number of troughs.

4. The tube structure defined in claim 1, further comprising a pair of tube sheets at opposite ends of said tube and secured to said end portions.

5. The tube structure defined in claim 1 wherein four such troughs are provided at the vertices of a star centered on the axis of the tube.

6. The tube structure defined in claim 1 wherein three such troughs are provided at the vertices of a star centered on the axis of said tube.