Heat exchanger of profiled plates of metal sheet

The invention relates to a plate heat exchanger comprising one or more stacks of plate parts of metal sheet, between which two media flow that exchange heat between the plate parts. The invention further relates to a method of manufacturing plate heat exchangers and a method of manufacturing profiled plates of metal sheet, eg for use in a plate heat exchanger.

Background

The development of plate heat exchangers has contributed to improving the energy transfer process between two media, liquid as well as steam. A number of different types of heat exchangers are known, wherein media are, in different manners, guided between the plates in the heat exchanger, eg counter-current heat exchangers, rotating heat exchangers and cross-flow exchangers. Heat transmission takes place by transfer of a heat flow from a hot medium to a colder medium through the separating plates. Calculation of the heat flow relies on values of the thermal energy and the overall coefficient of heat transfer of the plates and indexes for the heat transition of the energy from primary flow through plate into secondary flow. Added to this are determination of temperature efficiency and indexes for the transmission of the heat of the energy in the plate material.

It is a common feature of all the different types of heat exchangers that the energy transfer between the plates is the foremost loss-making factor, to which leaks and leakages are to be added, meaning that the two media are not kept completely apart. Today, plate heat exchangers provide the best yielding energy transfer with temperature efficiencies in the order of 75 %. The temperature efficiency is here and in the following defined as the ratio between the difference between the input and the output temperatures of the

two circuits with a medium being heated and a medium being cooled, respectively.

The production of the plates for plate heat exchangers is limited by their particular configuration with input/output channels and sealant to avoid that the two media contaminate and hence decrease the efficiency of the heat exchanger. From a purely technical point of view, conventional plate heat exchangers are designed with gaskets mounted between each heat transfer plate. Assembled, the plates are pressed against each other with sealant there between to prevent leaks. Conventionally, the joining takes place with a prescribed pressure on the front plate and the back plate kept together by a pressure eg through a bolt fitting. It has been attempted to use various sealants and various kinds of gaskets, but the structure renders production difficult and entails increased use of material; which factors combine to increase the production costs.

If the plates are profiled, the heat transfer efficiency is increased since the surface area of the plates is hereby increased - and hence the heat transmission between the media. However, it is necessary to provide a certain minimum plate thickness in order to be able to emboss patterns with steep profiles so as to prevent cracks - and hence leakages - from occurring in the material. Alternatively, in the context of smaller plate thicknesses, tailor-made tools and production equipment are required, where the deformation is accomplished gradually in order to ensure that the necessary material is drawn along down or up in the profiling; adding, however, considerably to the cost of production. Thus, usually the manufacturer must either shape by rolling or pressing applied on relatively thick plates (plate thickness > 0.5 mm) in order to avoid to excessive deformations by the shaping with ensuing subsequent leaks, and hence accept reduced efficiency of the heat exchanger due to the plate thickness, or, alternatively, he has to

use a plate pattern with smaller deformations/angles in the profiling of the plates than would be optimal.

Certain plate heat exchangers are also made of plastics materials, wherein a smaller plate thickness can be obtained eg by moulding of the profiled plates.

However, plastics materials are inconvenient for use in the context of heat exchangers - on the one hand due to their poor properties in case of fire and, on the other hand, due to the increased risk of cracking occurring in the material at less elevated temperatures, which is critical to the performance of a heat exchanger.

As a further incentive to improve the temperature efficiencies of heat exchanger, the standards for plate heat exchangers for air-conditioning installations in residential building projects have been increased, eg as set out in Danish building regulations from Bygge & Boligstyrelsen, wherein a maximal energy efficiency corresponding to an SEL value of 1250 J/m ³ is now specified.

WO 95/30867 describes a manufacturing method for producing a plate heat exchanger, wherein a long length of metal sheet is rolled with a corrugated pattern, diagonally by 45°, following which it is folded. The two media that are to be heated and cooled, respectively, are then conveyed in and out from each their side of the folded stack of plates and then separately travel between two plates that are profiled diagonally and intersecting. However, the design of the heat exchanger is disadvantegous as the course of the profile, wherein a medium is conveyed in and out at the same side of the heat exchanger, does not yield an even flow-through transversally of the entire width of the heat exchanger, and uniform flow resistance through the exchanger and, likewise, the ratio between the surface of the heat transferring plate and the cross-sectional area of the exchanger is not optimal. The corrugated faces and the way in which they are stacked also

entails a comparatively thick heat exchanger for a given number of layers desired in the exchanger.

Object and description of the invention

It is the object of the invention to provide a plate heat exchanger with improved heat transfer and coefficient of heat transmission and with the lowest possible pressure drop, simultaneously with the above mentioned problems that occur during the production process and those of leaks are avoided.

Thus the method relates to a method of manufacturing a plate heat exchanger comprising at least one stack of plate parts at least partially from metal sheet or thin plates, between which plate parts two media flow that exchange heat through the plate parts, said method comprising configuration of a profile pattern on at least one length of plate and subsequent folding of the length of plate, to the effect that it forms a stack of continuous plate parts comprising alternatingly a plane plate part and a plate part with profile pattern.

Hereby the advantageous aspect is provided that, in a simple manner, a stack of plates is obtained, wherein, without further measures, any connection is avoided between two adjoining plate spaces to the effect that two heat-exchanging media are automatically kept apart. Problems in respect of leakages and leaks at the sides of the heat exchanger are thus efficiently obviated, whereby the temperature efficiency of the exchanger is considerably improved. Simultaneously the production time can be reduced considerably and, likewise, the production as such is simplified. Likewise, the alternating use of a plane and a profiled plate part provides an optimal distribution of the two heat-exchanging media with maximized surface area of plate and hence maximized energy output of the heat exchanger.

The present invention further relates to a method of manufacturing a plate part at least partially from metal sheet or thin plates with profile pattern on the entire or parts of the surface of the plate part for instance for a plate heat exchanger, said method comprising rolling of folds transversally of the plate part and subsequently embossing a profile pattern at least partially over the rolled folds.

According to one embodiment the subsequent embossing takes place by pressing or by rolling. By a sheet or thin plates is to be understood, here and throughout the text, a very thin metal plate with a thickness in the order of 1 mm or thinner. Metal sheets or thin plates of such thicknesses make particular demands as to which techniques can be used for processing and how these can be executed. By the above method according to the invention, it is accomplished precisely that it is rendered possible to manufacture a profiled surface on thinner plates than would otherwise be possible by conventional embossing methods, without cracks or fractures occurring. Thus, plate parts of thicknesses smaller than 0.1 mm can be profiled by the method according to the invention without involving problems. Likewise, it is possible to form profile patterns with steeper profiles and larger deformations of the material, since the material to be moved to the most extensively deformed areas is already brought closer to the area at the pre-rolled folds, and hence it is not to be fetched from so far away. Simultaneously the described method does not require cost-intensive tools or production equipment and may furthermore be carried out with short process time.

Yet an embodiment concerns a method of manufacturing a plate part according to the above, wherein the folds are rolled in the entire width of the plate part. Hereby the width of the plate can be kept approximately unchanged, whereby the procedure of having to cut the plate part to fit at the sides following embossing is eliminated and so is the waste of material

involved in this. Likewise, the design of the rollers and their manufacturing costs are simplified in that rolling is performed with simple, transverse folds and, likewise, the rollers can be used to manufacture plate parts of different widths.

Yet an embodiment relates to a method of manufacturing a plate part in accordance with the above, wherein the subsequent pressing of the profile pattern is performed at each side of the profile part. This is advantageous in the context of certain types of profile patterns, such as pattern of channels for plate heat exchangers, since the same dice can thus be used to manufacture plate parts for plate heat exchangers of different widths.

According to yet an embodiment the method of manufacturing a plate heat exchanger in accordance with the above also comprises the formation of one or more profile patterns according to one or more of the above-mentioned methods there for. Hereby the use of far thinner plates compared to previously is enabled, whereby the heat transfer can be increased considerably simultaneously with the material consumption and hence the costs of material being reduced considerably. At the same time a heat exchanger can be obtained which has a smaller thickness or height for the same number of layers - or alternatively an exchanger with far more plate layers for the same thickness (height). Likewise, hereby a slightly pebbled surface on the plate parts is accomplished which has been found to contribute to an improved transfer of heat between the media - on the one hand due to the increased surface area and, on the other hand, due to the formation of turbulence in the flow.

Moreover the invention relates to a plate heat exchanger comprising at least one stack of plate parts at least partially of metal sheet, between which plate parts media flow that exchange heat through the plate parts, which plate heat exchanger is manufactured completely or partially by one or more of the

above methods. The advantages of this are as mentioned above in the context of the manufacturing methods.

According to one embodiment at least a part of a plate part is coated with a surface coating with hydrophobic properties. Hereby the risk is minimized of condensation occurring within the heat exchanger which would otherwise constitute a problem, in particular at the exit of the hot medium. Condensation is undesirable as it entails a reduction in recovery and an increase in the risk of the exchanger icing up. Likewise, it has been found that the result of the mechanical process of rolling and embossing the metal sheet is improved for plate parts with hydrophobic surface coatings which provide a lubricating effect, meaning that the process time is reduced without, however, the coating being destroyed or damaged. Therefore the coating may also have the result that even thinner sheets can be rolled and embossed or pressed without cracking or otherwise suffering damage.

According to one embodiment the plate parts of the plate heat exchanger are manufactured from aluminum sheet material or a sheet of an aluminum alloy. This is advantageous in that aluminum is suitable for the described production methods comprising rolling and pressing. Moreover, aluminum is suitable for plate heat exchangers as it maintains its mechanical properties at the relevant temperatures and, for instance, it does not crack in frosty weather conditions.

According to a further embodiment, the plate heat exchanger is of the cross- flow exchanger, rotating heat exchanger or cross-flow exchanger types, all three of which are efficient plate heat exchangers suitable for eg heat recovery in residential building projects.

According to yet an embodiment, the heat-exchanging media in the plate heat exchanger are in the gas phase.

According to yet an embodiment, the plate thickness in the plate heat exchanger is less than or equal to 0.1 mm, which is advantageous since the temperature efficiency of the plate heat exchanger is increased the thinner the plate material used.

Finally, the invention also comprises use of a plate heat exchanger in accordance with the above.

Brief description of drawings

In the following the invention is described with reference to the figures, wherein

Figure 1 shows a counter-flow heat exchanger with indication of the flow of the media;

Figure 2 shows a plate for a counter-flow exchanger with embossed channels;

Figure 3 is a sectional view through a stack of plates in a counter-current heat exchanger;

Figure 4 shows the principle of the folding/bending of a stack of plates for a plate heat exchanger;

Figure 5 illustrates the closure of an end by bending of a plate;

Figure 6 shows a plate heat exchanger with seals; and

Figures 7-8 show different steps in the manufacture of a plate heat exchanger according to the invention.

Description of embodiments

Figure 1 shows a sketch of a plate heat exchanger of the counter-current exchanger type 101 , illustrating the ideas behind the present invention. However, the plate exchanger according to the present invention can also be used in other types of heat exchangers, such as rotating heat exchangers and cross-flow exchangers. Plate heat exchangers are efficient for heat exchange between media in the gas or liquid states and for certain types of condensation and evaporation. The fundamental design is a stack 102 of plates 103 spaced a small distance apart. A typical plate heat exchanger consists of between 50 and 200 stacked plates, but in the figures only a few plates 103 are shown, for the sake of clarity. The heat flow (indicated by the arrow 104) at input 105 is distributed into several channels and exchanges heat with another medium 106 through plates 103 made of a material with required high heat conductivity, such as aluminum or, alternatively, plastics.

In the shown type of heat exchanger, the heat exchange takes place counter- currently. This means that the warmest primary input air 104 heat exchanges directly with already heated secondary output air 106 to maximal output temperature. After a medium has been conveyed into the upper or lower part of the heat exchanger, the medium is distributed into channels in the entire or the major part of the of the exchanger's height to obtain the largest possible heat exchange with the counter-currently flowing medium, and is then again conveyed out into the one part of the opposite end of the exchanger. Figure 1 shows the front of the open channels 107 in top/bottom for either primary input air 106 or secondary output air 104. The open channels on the opposite side of the joined plates are intended for primary output air and secondary input air, as shown by arrows 104, 106, illustrating the flow of the two media.

Figure 2 shows a profiled plate 103 with a profiled pattern of channels 201 for conveying the flow through the counter-flow exchanger. The channels for the secondary 105 and the primary 104 circuits, respectively, on the top and bottom sides, respectively, of the plate 103 are separated at each end 202 of the plate into separate channels that intersect. The system of channels is designed such that approximately the same flow resistance is obtained through the entire exchanger and, likewise, due to the pattern of the channels, all the air or the liquid is caused to flow approximately the same distance from input to output through the exchanger. The pattern of channels is continued uninterruptedly on the one side of the plate at the inlet, whereby it is ensured that the incoming medium is guided and distributed evenly throughout the entire width of the heat exchanger. Opposite, the pattern of channels is interrupted or is in sections at the other end, at the output, on both sides of the plate where the current is to reassemble, and control therefore is not as important. Hereby the same minimal flow resistance is ensured throughout the entire course of the flow-through and to both media. By thus minimising the pressure drop through the exchanger, correspondingly lower power consumption results for operation of the heat exchanger. Other channel configurations for the primary and the secondary medium flow-throughs 104, 106 than the one shown can also be used.

In order to maximise the performance of the exchanger, it is important to have as thin plates as possible and to increase the surface area as much as possible. The latter may be accomplished ia by allowing the sides of the channels to be as steep as possible, as will appear more clearly by study of Figure 3, showing a cross-section through some of the plates 103 from the central area of the heat exchanger shown in Figure 2. Here, the two heat- exchanging media 104, 106 are shown by individual grey tones. The stack of plates in the heat exchanger is formed by alternating plane plate 301 and profiled plate 302, whereby the two media are conveyed past each other in

the channel system with the largest possible contact area and hence maximised heat transfer.

In order to uphold optimal performance of the plate heat exchanger it is important that no leaks or leakages occur between the two heat exchanging media. In conventional plate heat exchangers, this is a problem in particular in connection with the seals along the sides of the plates. By the plate heat exchanger according to the present invention, this is solved by manufacturing the plate stack 102 from one or more endless lengths of plate, as illustrated in Figure 4. Following formation of the pattern of channels or the profiling 201 on a part of the plate, each individual section or plate part 204 is bent or folded 401 in accordance with the same principle as that of an accordion. Hereby the plate parts 402 remain continuous, and thereby the plate stack 102 is open only at the one side 403 inasmuch as the one medium is concerned, and correspondingly at the other end 404. As opposed to conventional plate heat exchangers, where it is necessary to seal between every plate, it is now necessary to ensure proper sealing only along the corners of the plate stack 102 and thus not between each plate part 402 of the plate stack 102. Thus, considerably reduced production time results for a plate heat exchanger and at the same time increased temperature efficiency.

According to one embodiment of plate parts, the plates are folded respectively with profiling, without profiling, with profiling, etc., whereby a plate stack with channel passageways as shown in figure 3 is obtained.

According to one embodiment of the invention, a plate heat exchanger is manufactured in that a roll or coil of plate material is, in steps or continuously, first embossed with the desired pattern of channels and with pre-embossings, where the plate length is to be folded 401 , and then folded followed by the same length of plate without profiling, which is also folded 401 , and so on

until the desired number of plate parts 402 is accomplished in the plate stack 102. This is shown in Figure 4.

Moreover, half of the front and half of the rear end between each two plates are closed off alternatingly at the top and at the bottom to separate inlet and outlet of the two heat-exchanging media as also illustrated in Figure 1. This is done in accordance with one embodiment of the invention by punching flaps 501 onto both sides of the endless length of plate, which flaps are folded upwards or downwards on top of the neighbouring plate part 402 in the plate stack between each folding of the plate length. This is outlined in Figure 5. The closure is sealed by arranging a seal compound, rubber tape or the like sealant 502 between the flap 501 and the plate part 402 on top of which the flap 501 is bent or pressed. The sealant also serves to avoid fractures or cracks caused by the folding. The shown joining method is also advantageous in also securing the two plate parts to each other and ensuring that they do not open in case of higher pressures between the plate parts, which would otherwise pose a problem where other closure means are concerned.

Following ended folding of the stack of plates 102, the length of plate is cut off the coil and the stack of plates are arranged in a U-shaped box 601 (without top and end pieces), as shown in Figure 6. Owing to the way in which the plate heat exchanger is configured, it is, as mentioned above, necessary to seal efficiently only along the edges 602 of the stacks and down along the centre of the front and rear 603 to efficiently separate input and output. As sealant for instance rubber tape, silicon compound, a hot-melt or the like can be used. Then a lid 604 is mounted on the box 601 and so is an optional, further round-going frame for further reinforcement and strength of the plate heat exchanger. Subsequently the latter is ready for being coupled to channels for supply and discharge of the heat-exchanging media.

The production costs associated with the above-described methods of manufacture are reduced considerably compared to the production of conventional heat exchangers with stacked, separate plates. Moreover, the performance of the finished exchanger is increased considerably.

Figures 7 and 8 outline a further method of manufacture of a plate heat exchanger by which ultra-thin plate thicknesses can be accomplished, in the order of 0.05 - 0.1 mm, without having to compromise on the depth and steepness of the profiling on the plate parts featuring a pattern of channels. Here the profiling of the plate sheet is performed in several steps. First, the folds 702 in a length of plate 701 is rolled 703 transversally of the entire width of the length of plate and in amount and density corresponding to the pattern of channels 201 , as it should be at the central part of a profiled plate part in the plate stack. The rolling procedure is carried out by use of rollers with corresponding pattern. The material for increasing the surface area of the plate is, during this process, taken off the plate coil, and the width of the length of plate thus remains approximately unchanged. Then a section of the length of plate is rolled without folds, corresponding to a plane plate part 704 for the stack of plates and then once again folds, etc. Between the sections of the length of plate that are to end up as plate parts in the stack of plates, folding edges 706 are pre-embossed 705 where the length of plate is subsequently to be bent.

After the rolling procedure, a pressing procedure 801 follows with respect to sections of the length of plate. Here, two (or more) dice 802 emboss the desired input and output patterns 803 onto a profiled plate part at each side and over the rolled folds 702, as shown in figure 8. In order to ensure that the length of plate is unable to bulge, it can be secured during the pressing procedure around each die. By pressing the profile pattern over the folds 702, it is accomplished that even patterns or profiles with steep recesses and elevations can be formed on even very thin metal sheets without cracks or

fractures occurring even in the narrow profile peaks on the plate. This is possible because the additional material required for forming the profile peaks and the increased surface area are already present in the pre-rolled folds 702 and is not to be drawn further inwards from further away, which is otherwise not possible without specially made pressing tools - and, even in that event, only with difficulty. The shown embodiment is further advantageous in that the same pressing dice 802 can be used on different plate widths, and thus the production can, without further ado, be switched to manufacture of plate heat exchangers of other widths. Following ended pressing of a plate part, end flaps 501 are punched out at both sides of the length of plate and are folded perpendicular to the length of plate, a profiled plate part 402 is bent and folded on top of or into the stack of plates 102, and the end flaps 501 are bent around the neighbouring plate part 402 for closure thereof. Then follows folding of a plane plate part 402 upwards into the plate stack 102, closure with folding of the end flaps 501 and so forth. In Figure 8, the folding of the length of plate is outlined to take place by the stack of plates being folded around the next plate part in the stack. The folding can likewise be accomplished by the plate stack being held more or less securely in a position (eg vertically), following which the two next plate parts in the length of plate are tilted and folded upwards into (inverse) V-shape from the remainder of the length of plate and are thus folded upwards into the stack. Other folding techniques can also be used. Optimised, all the described process phases can take place in sequence and continuously with an ensuing elevated production rate. The surface on a ready-embossed plate part can, in response to the pressing pressure and the plate material, appear slightly pebbled. However, this has been found to be advantageous for use in a plate heat exchanger where it further enhances efficiency, on the one hand due to the surface area being further increased compared to a smooth surface and, on the other hand, since the coarse surface creates turbulence in the flow, whereby the transmission of energy is increased.

The sequence of the individual process elements may, of course, to a certain extent take place in another sequence; the essential aspect according to the invention being that the pressing procedure taking place completely or partially over the rolled folds. Moreover the entire pattern of channels - and not only the inlet and outlet portions thereof - can be pressed into the pre- rolled plate. In that case the pre-rolled folds may assume random, different shapes and directions relative to the length of plate. Likewise it is avoided that the pressing procedure is to be adapted, with respect to location, to the rolled folds, which, however, has turned out in practice not to be problematic.

According to a further embodiment the final pattern of channels is embossed by rolling over an already profiled plate instead of by pressing, as described above.

According to a further embodiment, metal sheet is used with a coating on the one or both surfaces of a material which has water-repellent or hydrophobic properties. Such coating has been found to entail that the rolling and pressing of the metal sheet takes place more easily, more quickly and with lower friction with an ensuing improved result. Moreover, the coating is still intact following the processing.

Of course, the described method can also be used to advantage for embossing plate sheets for other purposes than plate heat exchangers, eg as intermediate material in sandwich panels, for filters or as an alternative to conventional deep-drawing methods. It also applies that the embossing is not limited to being configured on a continuous length of plate either; rather it may just as well be accomplished in two more or less separate processes on sections of plate. Further, the embossing by pressing or rolling may also be performed from just the one side against a fixed and optionally plane support, depending on which profiling is desired for the faces of the plate.

As mentioned in the introductory part, the Danish Bygge & Boligstyrelsen has laid down more rigorous standards for plate heat exchangers for use in air conditioning installations in addition to the official building regulations to the effect that an energy efficiency of 1250 SEL is not exceeded. This has been successfully tested on the plate heat exchanger according to the invention. The efficiency is determined on the basis of a test on a heat exchanger under defined design coefficients and required accuracies of the measurements. Moreover the heat exchanger is tested to have a temperature efficiency of between 85 and 90 %. This shows a marked improvement of actually a 20 % increase in the efficiency of the heat transmission in a plate heat exchanger compared to other models.

It will be understood that the invention as taught in the present specification and figures can be modified or changed while continuing to be comprised by the protective scope of the following patent claims.