Plate heat exchanger and method for constructing pressure-proof plate heat exchanger

TECHNICAL FIELD OF THE INVENTION

The object of the invention is a plate heat exchanger and a method for constructing a pressure-proof plate heat exchanger according to the preambles of the independent claims presented below.

PRIOR ART

In a typical prior art plate heat exchanger several heat exchanger plates have been fastened on top of each other the plates thus forming a stack of plates. Heat transfer media in form of liquid or gas have been alternately directed through the flow ducts formed by two heat exchanger plates. In other words, there are always different heat transfer media on the opposite sides of one heat exchange plate. It is possible to fasten two corrugated heat exchanger plates together as a plate pair. In that case, one heat transfer medium flows typically inside the plate pairs and the second heat transfer medium flows between the adjacent plate pairs. Generally, there have been arranged flow ducts passing through the stack of plates for the medium flowing inside the plate pairs, for the inflowing as well as for the outflowing heat transfer medium. A corrugated heat exchanger plate refers to a plate, which has been folded in several places so as to corrugate or the plate has otherwise been provided with corrugations, i.e. grooves and recesses.

It is known to weld several heat exchanger plates together at their outer edges or at the edges of the flow openings formed to the plates to form a stack of plates. It is also known to form a stack of plates by pressing several separate heat exchanger plates or plate pairs between supporting ends. In that case, seals are provided between the heat exchanger plates or plate pairs to prevent the leaks of the heat transfer media from the stack of plates and the mixing together of the flow media. Also known are stacks of plates, in which several heat exchanger plates or plate pairs are soldered to each other at several places and thus fastened to each other to a stack of plates.

In prior art plate heat exchangers the pressure of a stack of plates is typically received so that the stack of plates is closed between end plates, which have been firmly bonded together. If the stack of plates is placed inside a pressure- proof housing, the second heat transfer medium, e.g. the heat transfer medium flowing between the adjacent plate pairs, can be arranged to flow around the stack of plates arranged inside the housing.

The heat exchanger plates are typically designed so that when they are fastened to each other flow ducts of specific size and form are formed inside the stack of plates and between the plate pairs. The heat transfer properties of a heat exchanger can be controlled with the corrugation of heat exchanger plates, i.e. by shaping the recesses and bulges in a plate. The heat exchanger and its parts are often designed for a specific use situation. The flow rate and properties, such as temperature, density and pressure, of heat transfer media have a substantial influence on the dimensioning of a plate heat exchanger and on the choice of an optimal plate profile. Different corrugations of plates and angles between the corrugations of a plate pair need to be designed for different use conditions. In other words, different heat exchanger plates are needed for different applications.

THE AIM AND BRIEF DESCRIPTION OF THE INVENTION It is an aim of the present invention to reduce or even eliminate the above- mentioned problems appearing in the prior art.

It is a particular aim of the present invention to achieve a solution, with which a certain modifiability of the properties, especially the flow channels, of a plate heat exchanger, is achieved, without having to change the structure of the heat exchanger plates or plate pairs.

In order to realise the objects mentioned above, among other things, the plate heat exchanger and method according to the invention are characterised by what is presented in the characterising parts of the enclosed independent claims.

The embodiments and advantages mentioned in this text are in suitable parts applicable to both a plate heat exchanger and a method according to the invention, even if this is not always specifically mentioned.

A typical plate heat exchanger according to the invention comprises several corrugated heat exchanger plates. According to the invention:

- heat exchanger plates are fastened to each other in pairs as plate pairs,

- corrugation ridges of the heat exchanger plates of a plate pair are arranged in contact with each other at their contact surfaces at several places,

- at least two plate pairs are arranged successively as a stack of plates,

- between the plate pairs there are formed first flow channels for a first heat transfer medium,

- inside the plate pairs there are formed second flow channels for a second heat transfer medium,

- each heat exchanger plate has at least two openings,

- the openings in question of the heat exchanger plates are positioned facing each other so that they form an inlet and outlet channel for at least one heat transfer medium,

- adjacent plate pairs are connected to each other at the edges of said openings,

- two heat exchanger plates, which form a plate pair, are firmly fastened to each other at their contact surfaces by soldering or welding.

A typical method according to the invention for building a high-pressure resistant plate heat exchanger has at least following steps:

- corrugated heat exchanger plates, where each heat exchanger plate has at least two openings, are fastened to each other in pairs as plate pairs,

- corrugation ridges of the heat exchanger plates of a plate pair are arranged in contact with each other at their contact surfaces at several places,

- at least two plate pairs are arranged successively as a stack of plates,

- first flow channels for a first heat transfer medium are formed between the plate pairs,

- second flow channels for a second heat transfer medium are formed inside the plate pairs,

- an inlet and outlet channel for at least one heat transfer medium is formed by arranging the openings of the heat exchanger plates facing each other,

- adjacent plate pairs are connected to each other at the edges of said openings (2),

- two heat exchanger plates (1), which form a plate pair, are firmly fastened to each other at their contact surfaces by soldering or welding.

Contact surfaces refer in this context to ridges, which are in contact with each other, of the protrusions of the corrugated heat exchanger plates. The ridges of the heat exchanger plates placed towards each other are typically arranged to intersect each other, whereby contact surfaces are formed at the intersections of the ridges.

Thus, it has now been surprisingly discovered that by welding or soldering the plates of the plate pairs of a plate heat exchanger together at their contact surfaces inside the plate pairs, the plate pairs are easy to arrange to withstand even high pressure without special support structures. Thus, only the pressure of a heat exchanger medium flowing between the plate pairs is directed on ends, which possibly bind the stack of plates. If the above-mentioned pressure is low, no ends are needed or they can be dimensioned to be light-weighted.

In an embodiment of the invention two heat exchanger plates, which form a plate pair, are firmly fastened to each other by soldering or welding at at least substantially all contact surfaces. However, preferably in such an embodiment two adjacent plate pairs are, not soldered or welded together, at least not at substantially all contact surfaces.

A great advantage of the invention is that by means of it the heat exchanger can easily be made self-supportive, in other words, ends, which bind the stack of plates, can be left out. This is the fact especially, if the heat transfer medium flowing between plate pairs is not under high pressure.

A great advantage of the invention is that by means of it the production processes of various heat exchangers can be made simpler and more inexpensive. Heat exchangers often need channel systems of different size for a different heat transfer medium. Typically, for example the density of the steam to be condensed by a vacuum condenser is very low, whereby its volume in relation to the mass flow and effect of the steam is large. A problem that can occur in an ordinary plate heat exchanger is the narrowness of the plate gaps provided for steam, which narrowness limits the effect of the condenser. One solution is to manufacture plate types, which have a wider profile. That requires, however, the manufacture of expensive pressing tools. In a solution according to the invention, one and the same plate profile can be used in all heat exchangers. According to the invention the gap between the plate pairs can be adjusted so that a necessary flow cross- section is achieved on the steam side. As same kind of plates can be used for several different heat exchangers with the aid of the invention, this redounds substantial cost-benefit to the manufacturers of heat exchangers.

In an embodiment of the invention the gap between two adjacent plate pairs is left at least mainly free. This means that adjacent plate pairs are not particularly fastened to each other. This enables the heat transfer medium passing between the plate pairs to flow as free as possible. For several embodiments of the heat exchanger, a suitable distance between two adjacent plate pairs is 1-10 mm, or 1- 5 mm.

In an application of the invention each heat exchanger plate has two openings. Typically, one opening is thus arranged as an inflow channel for the heat exchanger medium passing inside the plate pairs and the second opening is arranged as an outflow channel of the same heat exchanger medium. The stack of plates is thus typically arranged inside the housing, and the second heat exchanger medium is arranged to flow in the housing, outside the plate pairs, i.e. between the plate pairs. The heat exchanger plates can also have e.g. four openings, one for the inflow and outflow channel of each heat exchanger medium.

Applications obtainable by different number of openings are not a specific aim of this invention and are thus not explained more broadly.

In an application of the invention the adjacent plate pairs are fastened to each other at the openings for the heat exchanger medium in the plate pairs. Thus, despite the free space between the plate pairs there is provided structural rigidity and strength sufficient for several applications.

In an embodiment of the invention space between two adjacent plate pairs is accomplished by fitting pieces, made of e.g. suitable metal, arranged between the plate pairs. The thickness of the fitting pieces is arranged to correspond the desired space and they are fastened or mounted between the plate pairs when assembling the heat exchanger. With the aid of the fitting pieces it is easy to arrange the spaces between the plate pairs exactly to the desired size.

In an embodiment of the invention the fitting pieces are arranged at the edges of the openings for the heat exchanger medium in the plate pairs. Thus, the plates and the fitting pieces are easy to weld together at this same point. In an embodiment of the invention the fitting piece is a ring surrounding said opening and placed between two plate pairs. This ring can thus form a sort of duct between the openings of the adjacent plate pairs. The fastening of a ring-like fitting piece to the adjacent plate pairs is easy to carry out in a leak-proof manner. The welding is easy, if the plate pairs are welded together at the inner surface of the fitting piece.

In an embodiment of the invention in a plate pair the edge of at least one heat exchanger plate is bent as a collar for a distance away from the plate pair towards the next plate pair. The next plate pair is fastened to said collar either from it's own corresponding collar or from another suitable point. The distance between the plate pairs can be determined by adjusting the length of the collar. In another embodiment of the invention said collar is formed at the edge of the opening for the flow medium formed in the heat exchanger plate. Thus, the collar can act as a sort of duct between the openings of adjacent plate pairs.

In an embodiment of the invention two adjacent plate pairs are provided with collars pointing to each other, which collars are welded together. Thus, the collars can act as a longer duct than before between the openings of adjacent plate pairs. If the collars to be placed towards each other have slightly different diameters, the collars can be fastened to each other within each other. This kind of connection is easily made firm. Thus, the distance between the plate pairs can be adjusted by changing the length, for which the collars are within each other, i.e. the overlapping length of the collars.

In an embodiment of the invention the stack of plates is placed in a housing with high-pressure resistance between ends supporting the stack. Thus, it is provided a pressure resistant channel network also for the heat exchanger medium flowing between the plate pairs.

In an embodiment of the invention the heat exchanger medium of a heat exchanger is a liquid inside the plate pairs and a gas or a steam between the plate pairs. The effective heat exchange of gas or steam often requires rather big spaces between the plates. In many heat exchanger applications the pressure of the gas or steam is not especially high, for example only a little higher than the normal air pressure, and thus it does not direct great force to the stack of plates. In a heat exchanger according to the invention the spaces between the plate pairs, i.e. the distance between the adjacent plate pairs, is easy to arrange suitable for gases or steam.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described in more detail below with reference to the enclosed schematic drawing, in which

Figure 1 shows the connection of two heat exchanger plates as a plate pair.

Figure 2 shows a partial cut-through of a stack of plates according to the invention in its radial direction. Figure 3 shows a possible arrangement according to the invention at the central opening of the stack of plates,

Figure 4 shows another possible arrangement according to the invention at the central opening of the stack of plates,

Figure 5 shows a flue gas heat recovery device according to the invention, Figure 6 shows a vacuum condenser according to the invention as a sectional view seen from the side, and Figure 7 shows a vacuum condenser of Figure 6 as seen from the end.

DETAILED DESCRIPTION OF THE EXAMPLES OF THE FIGURES

In different exemplary embodiments the same reference numerals are usually used for parts corresponding to each other.

In Figure 1 it is schematically shown, how two round heat exchanger plates 1 and 1' can be connected as a plate pair 2 according to the invention. A plate pair 2 according to the invention is formed in a manner known per se by welding or soldering two corrugated heat exchanger plates 1 , 1' together. Because of the corrugated form of the plates 1 , 1', i.e. the protrusions 22, 22', i.e. ribs, placed towards each other, and the grooves between them, a channel network for the heat exchanger medium is formed inside the stack of plates 2. In the Figures the plates have three round openings. The opening 3 formed in the middle of the plates acts as an inflow or outflow channel for the heat exchanger medium flowing outside the plate pairs. The openings 4 formed in the edges of the plates 1 act as inflow or outflow channels for the heat exchanger medium flowing inside the plate pairs. In a finished plate pair 2 the openings 3 and 4 face each other. The plates 1 , 1' and openings 3 and 4 can also have shapes other than round.

Figure 2 shows a cross-section of a stack of plates 11 formed of four plate pairs 2. The connection areas, which have been connected by welding or soldering at the protrusions 22 and 22' placed towards each other of two plates 1 and 1 ' of a plate pair, are indicated by numeral 5. According to the invention, space 6 between the plates connected together, i.e. the plate pair 2, resists even very high pressure. Typically, the plate pairs according to the invention are dimensioned to resist a pressure of at least 20 bars. Space 7 between two plate pairs is left at least mainly free, i.e. the plate pairs 2 are not fastened to each other in the area shown in

Figure 2. The stack of plates 11 can be supported at its end by an end 8 indicated by a dashed line. The stack of plates 11 can be placed inside the housing 10 indicated by a dashed line. The plates 1 and 1' connected firmly to each other according to the invention make the structure self-supportive, i.e. the stack of plates 11 does not require pressure-supporting structures, such as ends 8, when the space 7 between the plate pairs is non-pressurized or the pressure therein is low. Also the housing 10 can be supportive and high-pressure-proof. If the heat exchanger medium flowing in the space 7 between the plate pairs also has to be at high pressure, ends 8 and the housing 10 can be arranged as a pressure vessel, which can be dimensioned to resist even very high pressure, for example 100 bars. The pressure resistances of heat exchangers can of course be adjusted to be suitable for each case.

Figure 3 shows as an enlarged scale, how the size of the free space 7 between the plate pairs 2 can be arranged changeable with the fitting pieces 12 according to the invention. Herein, the ring of a suitable thickness arranged around the edge of the opening 4 formed in the stack of plates acts as a fitting piece 12. The adjacent plates 1 , 1' of two different stacks of plates are welded to the ring 12 and to each other at the ring 12 by a welding 13. The welding 13 is thus made at the edge of the opening 4.

Figure 4 shows another embodiment of the invention presenting, how a changeable space 7 between the plate pairs can be accomplished. In Figure 4 plates 1 , 1' of the plate pairs are bent at the edges of the openings 4 towards the adjacent plate pair 2. The edge of the plate 1 is bent as a collar 14 and the edge of the plate V in bent as a collar 14'. By changing the length hi of the collars 14 and 14' and the overlapping length h2 between them space 7 between the plate pairs 2 is obtained as desired. The length hi of the collar is typically between 1 — 5 mm. The overlapping length h2 is typically between 3 — 5 mm. The collars 14 and 14' are fastened to each other at the edge of the opening 4 by a welding 13. Different dimensions can of course be sized to be suitable for each case.

Figure 5 shows a flue gas heat recovery device 15. The self-supporting stack of plates 11 can be placed to the end of the flue gas channel 16, in which case the heat exchanger is formed from the stack of plates 11 and no housing structure is needed. The travel direction of flue gases is illustrated by arrows 17 and 17'. The travel directions of the liquid, e.g. water, cooling the flue gases are illustrated by arrows 18 and 18'. The flue gases are directed from the flue gas channel 16 to the central opening 3 and from there further to the spaces between the plate pairs 2, i.e. to spaces 7 shown in Figures 3 and 4. From spaces 7 the flue gases are directed out from the outer edge of the stack of plates 11 in a manner shown by arrows 17'. The water to be heated is directed according to arrow 18 into the inlet fitting 19 for water and into the stack of plates 11 via the opening 4. It is shown in Figures 3 and 4, how water is directed from the opening 4 inside the plate pairs 2. Via the channel network 6 of the plate pairs the water travels to the opening 4' and further via the outlet fitting 19', according to arrow 18', out from the stack of plates 11. Openings 3 and 4 are closed at the upper end 20 of the heat recovery device. The flue gas pressure is typically only a little higher than the normal air pressure and it does not direct great forces to the stack of plates 11. The pressure of the liquid side, i.e. the pressure prevailing inside the plate pairs 2, can be for example 5 — 20 bar, for example 10 bar. Because the plate surfaces of a plate pair are according to the invention connected to each other at several places, the stack of plate does not considerably experience deformation under the influence of the liquid pressure. The spaces between plates on the flue gas side, i.e. the size of spaces 7 remaining between the plate pairs 2, can be dimensioned suitable according to the invention. For example, the size of spaces 7 can be arranged considerably bigger than the size of the liquid channel network 6 inside the plate pairs. Thus, sooting does not block the exchanger and plate spaces can be mechanically cleaned, if necessary.

Figure 6 and 7 show a vacuum condenser 21 according to an embodiment of the invention. The steam to be condensed is directed according to arrow 17 into the inlet fitting 17 arranged to the upper edge of the condenser and further inside the vessel formed by the housing 10 and ends 8. Inside the housing 10 and ends 8 there has been arranged a stack of plates 11 according to the invention, where to

the spaces 7 between which plate pairs 2, illustrated in Figures 3 and 4, the steam to be condensed is directed. The condensed water is directed away from the stack of plates 11 and further according to arrow 17' via the outlet fitting 16' out from the vacuum condenser 21. The water, which is warming up, is directed according to arrow 18 to the inlet fitting 19 for water and further into the stack of plates 11 via the opening 4. It is shown in Figures 3 and 4, how water is directed from the opening 4 inside the plate pairs 2. Via the channel networks 6 of the plate pairs 2 water travels to the opening 4' and further via the outlet fitting 19' for water, according to arrow 18', out from the stack of plates 11. The openings 3 and 4 are closed at their other ends.

Figures show only some preferred exemplary embodiments according to the invention. Figures do not separately show matters that are irrelevant in view of the main idea of the invention, known as such or obvious as such for a man skilled in the art. It is apparent to a man skilled in the art that the invention is not limited exclusively to the examples described above, but that the invention can vary within the scope of the claims presented below. The dependent claims present some possible embodiments of the invention, and they are not to be considered to restrict the scope of protection of the invention as such.