HEAT EXCHANGER

Abstract: 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 rotation symmetrical 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 surfaces 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 rotation symmetrically in relation, for example, to the axis, and the 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 overall height additionally is reduced in that the ex¬ pansion chamber, which normally is located above the heat exchanger unit, can be positioned centrally within the evaporator (heat exchanger). The characterizing features of the invention as defined in the attached claim 1 render it possible to achieve the aforesaid object and advantage merely by a very modest increase in the diameter (for example 10-20$). At a specific embodiment of the invention according to the attached claim 2, the capacity of the structure to withstand higher internal pressure is achieved without having to increase the material thickness. Furthermore, the invention improves the maintaining of the liquid film along the heat exchanging surfaces, in that the running liquid during its downward flow continuously is subject¬ ed to changes in direction, whereby jets and droplets are formed which by impact on the underlying surfaces wet the same. At a further specific embodiment according to the attached claim 3, a structure is obtained, which without the arr¬ angement of special devices, such as bellows or the like, is capable to withstand high tensile and compressive forces, which can arise due to high temperature differ¬ ences. A problem known at previously known structures, viz. the concentration of corroding liquid in horizontal joints, is eliminatedby a further embodiment of the invention which is defined in the attached claim 4. 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. 5 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 to the sheet surface, Fig. 5 is a view in the direction of the arrow C-C in Fig. 4, Fig. 6 is a view in the direction of the arrow D-D in Fig. 4, Fig. 7 is a view from above in the direction of the arrow A-A in Fig. 4, Fig. 8 is a sect¬ ion along the line B-B in Fig. 4, and Fig. 9 is a long¬ itudinal section through the invention applied as an evaporator. Fig. 1 illustrates a heat exchanger, at which both media are liquids. The apparatus is all-welded but, of course, can be provided with suitable flange connections -when a simple disassembly of the apparatus for inspection and cleaning is required. The heating surface is annular and consists of a plur¬ ality of radially positioned plate elements, each of which consists of two embossed metal sheets welded tog¬ ether along their long sides. In order to reduce, or entirely eliminate, 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" as illustrated by a longitudinal section in Fig. 3. Pre- -stressed heat exchanger tubes have been commercially available since several years ago. The plates can be embossed in a press in optional widths and lengths. Figs. 8 and 9 show one type of plate emboss¬ ing and the forming of the end connecting rings. Fig. 1 shows, that the plates are bevelled at the ends, where¬ by so-called dead corners are avoided due to an improved flow pattern. Furthermore, the risk of crevice corrosion is reduced, due to the better run-off and exchange of flowing liquid. The heating surface, of course, also can consist of tubes which, for example, are pre-stressed, or of con¬ ventional spot-welded plate elements. A further variant is the use of plane lamellar sheets with spacer members of some kind. The design of the heating surface according to Figs. 7 and 8 and the embossing offer the great advantage, that the thickness of the sheet material can be reduced, com¬ pared with 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 boxes are so connected to the heating surface, that the medium flows internally through the elements. 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 in view of the corrosion attacks to be expected. As regards the mechanical strength, the norm¬ al calculation standards apply. The inner shell "E" 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, whereafter the medium continues to flow upward countercurrent to the medium flowing downward internally in the element. At the upper end of the heating surface the medium is dir¬ ected inward to the centre and then flows out of the apparatus through the upper central connecting piece. The length of the internal shell is so adjusted as to provide for the necessary inflow and outflow openings. The inner shell can be manufactured of a very thin mat¬ erial, because the shell is subjected only to internal overpressure corresponding to the maximum pressure drop for the medium flow through the heat exchanger. The outer shell "C" in Fig. 1 is seal welded to the external connecting rings "B". From a strength aspect, the outer shell shall be dimensioned for an internal overpressure corresponding to the pressure and temper¬ ature 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. OMPI One interesting advantage of the annular, radially posit¬ ioned plate heating surface is that it also serves as a part of the vacuum reinforcement. The extent, to which the plates contribute to the reinforcement, depends on the width and length of the sheets on the shell to be vacuum reinforced. Vacuum reinforcing rings can be a relatively rather expensive part of an apparatus and, therefore, the possibility of, entirely or partially, taking the plates into account can be a reasonable value. 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. 4-6 has proved very efficient and withstands;ι 2 bursting pressures of more than 300 kp/cm . It is also known, that the heat load at welding is of great import¬ ance for the service life of the apparatus, due to the stresses induced at welding into the structure. As reg¬ ards heat exchangers, the longitudinal welds of the plate elements as well as the connecting welds to the external and internal connecting rings ("A" and "B" 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 adjacent the welds. When, besides, the softest possible design is chosen, for example by pre-stressing the plate elements, a structure substantially vree of stresses can be obtain¬ ed. By embossing/pressing the sheets of the plate element at 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-bear¬ ing and self-supporting, irrespective of the pressure con¬ ditions internally and externally of the plate elements. Spot welding or seam welding internally on the sheets for holding together the elements at internal overpressure 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 requir¬ ed also imply, that the stresses in the sheets are rel¬ atively insignificant, and the risks of crack formation in-the depressing points are eliminated. Since no weld contact surfaces are required - disregarding the long sides - the tool is simple to manufacture, and the service life is long, owing to low pressing stresses. Spot welding for holding together the plate elements re¬ quires 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 apparat¬ uses it is further necessary to use sheets with a thick¬ ness rendering the spot welds sufficiently strong to with¬ stand the loads arising in operation. The plate surface according to Figs. 7 and 8 can be manufactured 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 crevice corrosion is eliminated, compared to the spot weld surf¬ aces according to above. The risk that deposits of solid particles may accumulate at the supporting points also is reduced considerably. The pre-stressing of the plates yields a structure substantially free of stresses, which is an essential advantage. OMP comprised in conventional evaporators. In the lower port¬ ion, an inner cylinder is located, which is seal welded to the lower inner connecting ring and also is seal welded to the apparatus bottom, 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 at the bottom of the apparatus to the distribution chamber about the upper portion of the heat¬ ing surface. The annular gap for ingoing circulated solut¬ ion is dimensioned for a certain moderate pressure drop, thereby ensuring uniform distribution. In the first phase, the entire free cross-section outside the plate elements 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 boil¬ ing/steam emission starts substantially immediately. The free space, thus, is filled with heavily oversaturated/ soaked steam, which contains water droplets of all sizes from mist upward. Due to the design of the heating surf¬ ace, including grooves, ridges and supporting points, turbulence rapidly is caused in the steam, whereby moist and droplets are thrown outward against the walls. The turbulence, of course, grows in violence as the steam amount increases on the downward flow. The design of the heating surface, thus, ensures that all parts of the heating surface are thoroughly wetted, which 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 render 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 a gap is loc¬ ated 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 Ijiquid flows past a series of baffle plates where a first coarse sep¬ aration takes place. The main part of the liquid is thrown outward against the cylinder, which is an extens¬ ion of the lower inner connecting ring of the heat ex¬ changer, and thereafter flows down into the buffer space for circulating solution. The steam performs a U-turn, whereby liquid droplets additionally are separated, and thereafter flows upward through the main separation chamber. O ?I

Claims: Claims 1. 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 rotation symmetrically in relation to an axis, and the main flow direction of the media is in parallel with said axis, c h a r a c t e r i z e d i n that the heating surfaces 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. 2. A heat exchanger as defined in claim 1, c h a r ¬ a c t e r i z e d i n that the heat exchanging surf¬ aces consist of metal sheets assembled in pairs to plat¬ es and folded in parallel with said axis, and that the pitch of the folds increases with the distance from the axis and outward. 3. A heat exchanger as defined in claim 2, c h a r ¬ a c t e r i z e d i n that the sheets are bent or curved slightly out of their plane about a line laid in the plane, which line is perpendicular to said axis. 4. A heat exchanger as defined in claim 2 where the sheets are joined side to side at their two end edges by welding, c h a r a c t e r i z e d i n that the joining edge is formed downward inclined to facilitate 'run-off. 5. A heat exchanger as defined in claim 1, c h a r ¬ a c t e r i z e d i n that the heat exchanging surf¬ aces consist of tubes in parallel with said axis. OMPI

Inventors: BENGTSSON FOLKE, BENGTSSON, FOLKE
Applicants: FOLBEX AB
Application year: 1984
Main classification: F28D1
IPC codes: F28D7, F28D9, F28F3