CA1274820A - Heat exchanger - Google Patents

Abstract

ABSTRACT

The invention relates to a heat exchanger intended to be flown through by two media and for heat exchange via heat exchanging surfaces in such a way, that the media do not directly contact each other, where the heating surfaces are located rotary symmetrically in relation to, for example, an axis, and the main flow direction of the media is in parallel with said axis. The object is to reduce the overall height and material consumption while maintaining the size of the heat exchanging surfaces. When the heat exchanger is used as an evaporator, the overall height additionally is reduced in that the expansion chamber, which normally is located above the heat exchanger unit, can be positioned centrally within the evaporator (heat exchanger).
The said improvement is achieved in that the heating sur-faces D are located in an annular area about said axis, so that a central hollow space without heat exchanging surfaces is formed about the central axis.

Description

HEAT EXCHANGER

This invention relates to a heat exchanger intended to be flown through by two media and to exchange heat via heat exchanging surfaces in such a way, that the media do not directly contact each other, and at which heat exchanger the heating surfaces are located rotary symmetrically in relation to an axis, and tlle main flow direction of the media is in parallel with said axis.

The present invention has the object to reduce the over-all height and to reduce the material consumption while maintaining the size of the heat exchanging surfaces.
When the invention is utilized as an evaporator, the oYerall height additionally is reduced in that the ex-pansion chamber, which normally is located above the hea~ exchanger unit, can be positioned centrally within the evaporator (heat exchanger)~

According to the present invention there is provided a heat exchanger intended for being flown through by two media and or heat exchange via heat exchanging surfaces in such a hay, that the media do not directly contact each other, which heating surfaces are located rotary symmetrically in relation to an axis and in an annular area about said axis, so that a central hollow space without heat exchanging surfaces is formed about the central axis, the main flow direction of the media being in parallel with said axis, characterized in that the heat exchanging surfaces consist of metal sheets assembled in pairs to plate elements and corrugated in parallel with said axis, and that the pitch of the corrugations increase with the distance from the axis and outwards, so that the corrugations of opposite elements - 30 are abutting each other and are forming supporting points for the elements.

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- 2 -Preferably the sheets are bent or curved slightly out of their plane about a line of the plane, ~hich Line is perpendicular t~ said axis.

h7ith the preferred configuration of the sheets the capacity of the structure to withstand higher internal pressure is achieved ~ithout having to increase the material thickness.
Furthermore, the invention improves the maintaining of the liquid film along th~ heat exchanging surfaces, in ~hat the running liquid during its down~ard flo~ continuously is subjected to changes in direction, whereby jets and droplets are formed which by impact on the underlying surfaces wet the same.

At a further specific embo~iment a structure is obtained, which without the arrangement of special devices, such as bello~s or the like, is capable to withstand high tensile and compressive forces, which çan arise due to high temperature differences.

Preferablyr the sheets are joined side-by-side along their two edges by welding and the joining edge is formed downwardly inclined to facilitate run-offO

This configuration eliminates or reduces a problem known in previously known structures, viz. the concentration of corroding liquid in horizontal joints.

Some embodiments of the invention are described in the following, with reference to the accompanying drawings;
in which Fig. 1 is a basic longitudinal section of a heat exchanger according to the invention~ Fig~ 2 is a basic cross-section, Fig. 3 is a longitudinal section of a plate assembled of two metal sheets, Fig._4 shows the upper end of a plate seen in the direction of the - 2a -sheet surface, Fiq. S is a view in the direction of the arrow C-C in Fig. 4, Fig. 6 is a view in the direction of ~he arrow D-D in Fig. 4, Fig. 7 is a view from above in the direction of the arrow A-A in Fig. 4, Fiq. 8 is a section along the line B-B in Fig. 4, and Fig. 9 is a longitudinal section through the invention applied as an evaporator.

Fig. 1 illustrates a heat exchanger 1~ at which both media are liquids. The apparatus is all-welded but can, of course, be provided with suitable flange connections when a simple disassembly of the apparatus for inFpection and cleaning is required.

The heating surface is annular (see Fig. 2) and consists of a plurality of radially positioned plate elements 2, each of which consists of two embossed metal sheets 3, 4 welded together along their long sides.

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The heat exchanger is rotary symmetrically formed and consists of an outer shell 5, an inner shell 6 covered by a top end-wall, plate elements 2 placed between the inner shell 6 and outer shell 5, an inlet pipe 7 going S to thecenterof the outer shell 5 atthe bottom end,an outlet p~e 8 going from the center of the outer shell 5 at the top end, an inlet piece 9 going to t~e space be~ the ~ shell 6 and the outer shell 5 at'~e u~pper end thereof'and an cutlet piece10 going from said space at the lower end thereof. The inlet piece 9 is connected to a distributing box 13, which passes one of the media to the interior of the plate elements ~ and a collecting box 15 collects the said one medium coming out of the plate elements 2 and passes it to the outlet piece 10O Whole arrows show the passage of one of the media (e g the heat emitting one) through the heat exchanger and the dash~d arrows show the passage of the second meclium. The sha~e~of the plate elements are more closely shown in figures 7 and 8. Each one of the plate elements consists of two parallelly corrugated sheets 3 and 4, which are welded together along the sides 11, 12 being parallel with the corrugations. A passage parallel with said sides (channel) is thus formed between the two sheets. As can be seen from fig. 8 the pitch h1, h2, h3 of the corrugations are increasing from the center to the outside of the heat exchanger so that opposite plate ele-ments 2 are abutting each other at the top of the corru-gations whereby the plate elements support each other.

Herebyare reduced, or entirely eliminated, stresses in the structure which may arise in apparatuses, in which great differences in temperature between the media exist, or in which strong transient variations in temperature and flow occur. The elements can be "pre-stressed".
Pre-stressed heat exchanger tubes have been commercially available since several years.

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The plates can be embossed in a press in optional widths and lengths. E~igs. 7 and 8 show one type of plate embos~
sing and the forming of the end connecting rings. Fig. 1 shows, that the plates are cut obliquely at the ends at 18 and 19, whereby so-called dead corners are avoided due to an improved flow pattern. Furthermore, the risk of crevice corrosion is reduced, because of the better run-off and exchange of flowing liquid.

The design of the heating surface according to Fiys. 7 and 8 and the embossing offer the great advantage, that the thickness of the sheet material can be reduced, com-pared to spot-welded structures, without thereby caus-ing any loss of mechanical strength. This is of special importance when the structure is manufactured of titan-ium or a corresponding very expensive material.

The external collecting box 15 is so connected to theheating surface, that the medium flows internally through the elements 2. The material for the collecting boxes, externally as well as internally, and also for the heat-ing surfaces with inner and outer connecting rings, thus,must be selected considering the corrosion attacks to be expected. As regards the mechanical strength, the nor-mal calculation standards apply.

The inner shell 6 shown in Fig. 1 forces the medium flowing in from below in the centre to flow on the heat-ing surface externally of the elements 2, whereafter the medium continues to flow upward countercurrent to the medium flowing downward internally in the elements 2. At the upper end of the heating surface the medium is directedinwards to the centre and then flows out of the - apparatus through the upper central outlet channel 8.
The length of the internal shell 6 is so adjusted as to provide for the necessary inflow and outflow openings.

The inner shell 6 can be manufactured of a very thin mat-8~

erial, as the shel] is subjected only to internal overpressure corresponding to the maximum pressure drop for the medium flow through the heat exchanger.

The outer shell 5 in Fig. 1 is tight~welded to the external connecting ring 20. From a strength aspect, the outer shell shall be dimensioned for an internal overpressure corresponding to the pressure and tempera-ture prevailing in the medium flowing externally of the elements. The outer shell normally also should be dimensioned for full internal vacuum. In such cases the shell mostly is reinforced by so-called vacuum rein-forcing rings suitably spaced along the length of the shell.

One interesting advantage of the annular, radially posi-tioned plate heating surface is that it also serves as a part which increases the vacuum. The extent, to which the plates 2 contribute to the increase, depends on the width of the sheets 3, 4 and length of the shell to be vacuum increased. Rings increasing the vacuum can be a relatively expensive part of an apparatus and, therefore, the possibility of, entirely or partially, taking the plates into account can be of a reasonable valueO

The mechanically weakest point in a plate heat exchanger is the connection to the collecting boxes where the strength of the weld connection between the sheets and the connecting portion is entirely decisive for the pressure and temperature, at which the apparatus can be permitted to operate. The connecting method shown in Figs. ~-6 has proved very efficient and withstands bursting pressures of more than 300 kp/cm2. It is also known, that the heat load at welding is of great impor-tance for the service life of the apparatus, due to the stresses induced at welding into the structure. As re-gards heat exchangers, the longitudinal welds of the plate elements as well as the connecting welds to the qt~

external and intexnal connecting rings (20 and 21 in Fig. 1) are carried out by fusion welding without filler metal. This implies welding with moderate heat load and with substantially reduced risk of crack formation adja-cent the welds. When, besides, the softest possible de-sign is chosen, for example by pre-stressing the plate elements, a structure suhstantially free of stresses can be obtained.

By e~bossing/pressing the sheets of the plate element ~t a certain angle and with supporting points according to Figs. 7 and 8, an annular heating surface with circular cross-section can be produced which is entirely self-bea-ring or self-supporting, irrespective of the pressure conditions internally and externally of the plate elements.
Spot welding or seam welding internally on the sheets for holding together the elements at internal overpressures in the passageways is superfluous. This implies, that the necessary pressing forces for the forming of passage-ways with transverse grooves and supporting points are substantially (probably 70-90%) lower than those required for the production of perfect contact surfaces for spot or seam welding. The low pressing forces required also imply, that the stresses in the sheets are relatively in-significant, and the risks of crack formation in the de-pressing points are eliminatedO Since no weld contactsurfaces are required - disregarding the long sides -the tool is simple to manufacture, and the service life is long, owing to low pressing stressesO Spot welding for holding together the plate elements requires relatively large plane depressed surfaces for each point (about 10 mm diameter), which involves an increased risk of both stress and crevice corrosion.
-At the designing of the structure of such plate appara-tuses it is further necessary to use sheets with a thick-ness rendering the spot welds sufficiently strong to 7~

withstand the loads arising in operation. The plate sur-face according to Figs. 7 and 8 can be ~nanufactured of very thin sheet metal by using a suitable embossing pattern with supporting points. As the supporting points have a small area and are well rounded, the risk of cre-vice corrosion is elin~inated, compared to the spot weld surfaces according to above. The risk that deposits of solid particles may accumulate at the supporting points is also reduced considerab~y. The pre-stressing of the plates yields a struc~ure substantially free of stresses, which is an essential advantage.

In Fig. 1 and Fig. 9 the pla~ce elements are cut obliquely both upwardly and downwardly whereby the flow cbnditions for inflowing and outflowing media ex~ernally of the elements are improved.

The evaporator according to Fig. 9 operates with the driving steam internally of the elements 2. The steam is received in a distribution chamber at the top of the apparatus, from where it flows into the upper portion of the elements. The distribution chamber is somew~at enlarged so that it can be entered for inspections. The driving steam, together withthe formedcondensate, flows downward through the elements. Due to the heating sur-face being formed with grooves, ridges and supporting points, the condensate film on the wall is continuously broken and thinned, thereby improving the heat transfer to the wall (5-10%). The condensate flows down into a condensate space 23 beneath the heating surface t from where it is discharged. Non-condensable gases are re-moved from said space via a separate connectingpiece 24, which is provided with a "fender" 25, which also may have annular configuration.

The outer and inner shells 5, 6 join substantially sea-lingly the annular heating surface along substantially the entire length. In an upwardly located distribution chamber 26 the outer shell 5 has been cut down a dis-tance so as to provide an annular gap 27 around the upper portion of the heating surface for the intake and uniform distribution of solution circulating outside the plate elements. In the lower portion an opening gap 28 is pro-vided for discharge, in that the inner shell has been given a smaller diameter. Centrally in the apparatus, the inner shell forms by means of its top end wall 29 a large separation chamber 30, which corresponds to -the expansion vessel or steam separator, which normally is comprised in con-~entional evaporators. In the lower portion, an inner cylinder 31 is located, which is tight-welded to the lower inner connecting ring 22 and is also tight-welded to the apparatus hottom at 33, which can be a head as shown in the sketch, or a curved end wall.
When required, a droplet separator can be built-in in the central separation chamber.

The solution to be evaporated is circulated by a pump from the buffer space 34 at the bottom of the apparatus to the distribution chamber 26 about the upper portion of the heating surface. The annular gap 27 for ingoing circulated solution is dimensioned for a certain mode-rate pressure drop, thereby ensuring uniform distribu-tion. In the first phase, the entire free cross-section ~5 outside the plate elements 2 is filled completely with inflowing circulation solution, which thereafter flows downward in the form of a film on the heating surfaces.
The temperature of the circulated solution is very close to the boiling point, so that boiling/steam emission starts practically immediately The free space, thus, is filled with heavily oversaturated/soaked steam, which contains water droplets of all si2es from mist and upward.
Due to the design of the heating surface, including grooves, ridges and supporting points, turbulence rapidly is caused in the steam, whereby moist and droplets are thrown outward towards the walls. I'he turbulence, of ~ 7~

course, grows in violence as the steam amount increases on the downward flow.

The desiyn of the heating surface, thus, ensures that all parts of the heating surface are thoroughly wetted, which S is a prerequisite for satisfactory operation and one hundred percent utilization of the heating surface in-stalled. The conditions on the falling film side, as described above, also renders it possible to operate the apparatus with a minimum of excess liquid, which implies lower power consumption in the circulation pump.

In the lower portion of the heating surface the gap 28 is located between the heating surface and the inner shell, so that steam formed and excess circulation liquid can leave the heating package. The mixture of steam and liquid flows past a series of baffle plates 35 where a first coarse separation takes place. The main part of the liquid is thrown outwards towards the cylinder, which is an extension of the lower inner connecting ring of the heat exchanger, and ther~after flows down into the buffer space 34 for circulating solution. The steam performs a U-turn, whereby liquid droplets additionally are separa-ted~ and thereafter flows upward through the main separa-tion chamber 30~

Claims (6)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
   PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
   
   A heat exchanger intended for being flown through by two media and for heat exchange via heat exchanging surfaces in such a way, that the media do not directly contact each other, which heating surfaces are located rotary symmetrically in relation to an axis and in an annular area about said axis, so that a central hollow space without heat exchanging surfaces is formed about the central axis, the main flow direction of the media being in parallel with said axis, characterized in that the heat exchanging surfaces consist of metal sheets assembled in pairs to plate elements and corrugated in parallel with said axis, and that the pitch of the corrugations increase with the distance from the axis and outwards, so that the corrugations of opposite elements are abutting each other and are forming supporting points for the elements.
2. 2. A heat exchanger as defined in claim 1, characterized in that the sheets are bent or curved slightly out of their plane about a line of the plane, which line is perpendicular to said axis.
3. 3. A heat exchanger as defined in claim 1, where the sheets are joined side to side along their two end edges by welding, characterized in that the joining edge is formed downward inclined to facilitate run-off.
4. 4. A heat exchanger for the exchange of heat between two separate fluids, comprising;
   a fluid body having an interior, a first fluid inlet, a second fluid inlet, a first fluid outlet, and a second fluid outlet, said fluid body having a sidewall which is generally cylindrically disposed about a central axis;
   a second body disposed generally coaxially within said sidewall of said first body, an annular gap being defined between said body and said second body;
   a plurality of heat exchange elements generally radially disposed in said annular gap, each of said plurality of heat exchange elements having walls including a first plate and a second plate in facing engagement together forming a plurality of first channels therebetween adapted to receive said first fluid therein, a plurality second channels being defined between adjacent ones of said plurality of heat exchange elements, said second channels being adapted to receive said second fluid therein;
   wherein said first fluid and second fluid each flow through said first body in a direction substantially parallel to said central axis without mixing, such that heat is transferred between said first fluid and said second fluid across said walls of said heat exchange elements, said first and second channels being formed by similar but opposite parallel corrugations formed in said first and second plates of each said heat exchange element, adjacent ones of said plurality of first channels formed in each said heat exchange element being separated by longitudinally-directed corrugations of said first and second plates, the longitudinal direction of said longitudinally-directed corrugations being parallel to said central axis, adjacent ones of said plurality of second channels being separated by outwardly-projecting corrugations of said first and second plates adjacent said longitudinally-directed corrugations thereof, said outwardly-projecting corrugations being pitched, the pitch of said outwardly-projecting corrugations increasing with the distance radially outwardly from said central axis, the respective top parts of said outwardly-projecting corrugations of adjacent ones of said heat exchange elements abutting each other.
5. 5. A heat exchanger as claimed in claim 4, wherein said first and second plates of said heat exchange elements are each curved slightly about a line generally perpendicular to said central axis.
6. 6. A heat exchanger as claimed in claim 4, wherein said first and second plates of each said heat exchange element are joined along respective side edges thereof by welding, upper and lower ends of each said heat exchange element being included obliquely upwardly and downwardly, respectively, to facilitate runoff of fluid.