Patent Grant
7168483
The present invention relates to a heat transfer plate for a plate heat exchanger, comprising an inlet portion, an outlet portion and a heat transfer portion which is located between the inlet portion and the outlet portion and which presents a number of ridges and troughs pressed into the plate and extending between a geometric top plane and a geometric bottom plane of the plate, said planes being essentially parallel to the geometric central plane of the plate. The invention further relates to a plate pack comprising a plurality of heat transfer plates of the type stated above, in which plate pack a fluid is intended to flow in a number of the flow areas that are formed by the interspaces between the heat transfer plates constituting the plate pack along a main flow direction extending between the inlet portion and the outlet portion. The invention also concerns a plate heat exchanger.
A plate heat exchanger comprises a plate pack consisting of a number of assembled heat transfer plates forming between them plate interspaces. In most cases, every second plate interspace communicates with a first inlet channel and a first outlet channel, each plate interspace being adapted to define a flow area and to pass a flow of a first fluid between said inlet and outlet channels. Correspondingly, the other plate interspaces communicate with a second inlet channel and a second outlet channel for a flow of a second fluid. Thus, the plates are in contact with one fluid through one of their side surfaces and with the other fluid through the other side surface, which allows a considerable heat exchange between the two fluids.
Modern plate heat exchangers have heat transfer plates, which in most cases are made of sheet bars that have been pressed and punched to obtain their final shape. Each heat transfer plate is usually provided with four or more “ports” consisting of through holes punched in the plate. The ports of the different plates define said inlet and outlet channels, which extend through the plate heat exchanger transversely of the plane of the plates. Gaskets or any other form of sealing means are alternatingly arranged around some of the ports in every second plate interspace and, in the other plate interspaces, around the other ports so as to form the two separate channels for the first fluid and the second fluid, respectively.
Since the fluid pressure levels attained in the heat exchanger during operation are considerable, the plates need to have a certain rigidity so as not to be deformed by the fluid pressure. The use of plates made of sheet bars is possible only if the plates are somehow supported. As a rule, this is solved by the heat transfer plates being designed with some kind of pattern so that the plates bear against each other in a large number of points. The plates are clamped together between two rigid end plates in a “frame” and thereby form rigid units having flow channels in each plate interspace. To obtain the desired contact between the plates, two different types of plates are manufactured, which are then alternatingly arranged in such manner that the plates in the heat exchanger are alternately of a first kind and of a second kind. Alternatively, use is made of identical plates which alternately are turned or flipped about a symmetry axis.
In most cases, the ports for the respective flow areas are located in two port portions at two opposite edges of the heat transfer plate, and said flow areas are formed by a heat transfer surface located between the port portions. In the portion of the plated located closest to the ports (the distribution surface), the plates usually have a pattern which has been specially designed to distribute the fluids over the entire width of the flow area.
In some applications, the pressure drop across the heat transfer surface represents only a small part of the pressure drop, which means that the difference in pressure drop in the transverse direction will be relatively small even if relatively large differences in fluid flow would arise across the width of the flow area. Although an uneven distribution, even if it is significant, has only a minor effect on the heat transfer in a heat exchanger with clean plates, an unevenly distributed flow is, in many cases, unacceptable since the risk of fouling increases considerably. When fouling occurs, the heat transfer capacity of the heat exchanger is drastically reduced. Besides reducing the thermal efficiency, fouling may also have a detrimental effect on the quality of the product that has passed through the heat exchanger. Furthermore, more cleaning will be required and, in serious cases, unscheduled stoppages may be necessary.
One example of processes where the pressure drop across the heat transfer surface is small is climbing film evaporation.
To obtain a sufficient distribution across the flow area also in applications characterised by low pressure drops, the pattern of the flow area must be ‘open’, i.e. a sufficient flow should be obtained even without large pressure differences. For the purpose of distribution, the pattern should thus be ‘open’ in the transverse direction, and for the purpose of main flow, the pattern should be ‘open’ in the main flow direction. An ‘open’ pattern is obtained simply by making the plates as plane as possible and providing them with only a small number of local depressions. However, with only a small number of contact points, each contact point has to bear a considerable load and the portions of the plate located between the contact points are subjected to considerable bending loads.
One problem associated with prior art is the fact that there is no structure which in a completely satisfactory manner yields the desired distribution also at small pressure drops while providing a strong plate pack formed by the individual plates.
Known compromises between the two seemingly incompatible construction requirements present too many deficiencies in terms of either distribution or strength.
An object of the invention is to provide a solution to the problems stated above or at least to achieve a compromise which does not present any appreciable deficiencies in terms of either distribution or strength.
A further object is to provide a heat transfer plate which at least offers an effective compromise concerning the problems stated above and which is easy and inexpensive to manufacture.
Another object is to provide a plate pack and a plate heat exchanger which also at least offer an effective compromise concerning the problems stated above and which are easy and inexpensive to manufacture.
These objects have been attained by means of a heat transfer plate having the features as defined in independent claim 1.
The objects have also been attained by means of a plate pack and a plate heat exchanger having the features as defined in independent claims 18 and 12, respectively.
Preferred embodiments of the invention according to its various aspects are apparent from the dependent claims.
The new pattern of the plate is a solution to the seemingly incompatible construction requirements.
The inventive concept can be summarised as a plate comprising a number of rows of elongated ridges and troughs which extend along the main flow direction and which are adapted on the one hand to support the loads arising between the plates when used in a plate pack in a plate heat exchanger and, on the other hand, to provide flow distributing flow connections, and a number of channel portions separating the rows of ridges and troughs from each other and being adapted to form main flow channels, which only cause marginal pressure drops. This results in a plate with satisfactory strength and satisfactory distribution capacity in the transverse direction also in applications where the pressure drop across the heat transfer surface has to be small. The features stated in claim 1 will be discussed in more detail below.
First, the heat transfer portion comprises a plurality of juxtaposed rows of said ridges and troughs, said rows extending along a main flow direction which extends between the inlet portion and the outlet portion. A plate of this design has a strong heat transfer surface. Strong here means, inter alia, that the plate is able to resist the pressures acting on the plate along its normal, i.e. the pressure associated with the clamping force of the rack as well as the pressure of the fluids flowing in the plate interspaces formed by the plates. The forces acting along the normal can attain considerable levels, since the plates usually have large heat transfer surfaces.
Second, the rows of ridges and troughs are separated from each other in a transverse direction which is essentially perpendicular to the main flow direction and which extends along the central plane of the plate, by essentially plane channel portions of the heat transfer portion which extend essentially parallel to the central plane of the plate. This helps make the pressing relatively uncomplicated. It also means that there will be main flow channels which extend in the main flow direction and which cause only a very small pressure drop. As mentioned above, a small pressure drop is a requirement for certain fields of application.
Third, each row presents alternating elongated ridges and elongated troughs, which extend in said main flow direction. The ridges of two juxtaposed heat transfer plates are adapted to bear against each other. Thus, the elongated ridges, which bear against an adjacent plate, will form a trough on the other side of the plate and will be located a distance from the corresponding trough on the adjacent plate on the other side. Elongated transverse connections are thereby formed between said main flow channels in the main flow direction. Thus, the flow in different main flow channels can be equalized by way of these transverse connections without causing any appreciable pressure drop. Ridge primarily means a convex side of a pressed component and trough means its concave side. Thus, a ridge on a large face of a plate forms a trough on the opposite large face of the plate. The pattern of the plate has been described as it appears on a large face of the plate.
Fourth, the transition between each ridge and an adjacent trough in the same row is formed by a continuous, essentially straight transition portion of the plate, which is inclined relative to said central plane of the plate and of which a first part forms an end wall of said ridge and a second part forms an end wall of the adjacent trough. By the portions being inclined, a pressed pattern is obtained which is relatively easy to produce. Because the inclined transition portions are essentially straight and extend directly from an ridge to a trough, a very strong structure is obtained. An upright portion of a metal sheet can support considerable loads in the plane of the metal sheet portion as compared with a metal sheet portion that is subjected to a load along its normal. By the straight sheet metal portion extending directly from a ridge to an adjacent trough, the compressive force is transmitted between two plates located on either side of an intermediate plate directly from one plate by way of the ridge contact point to the other plate by way of the trough contact point. Consequently, there are no plate portions that are subjected to any appreciable bending loads, which would lead to considerable deflections even in the case of small loads. In this connection, the angle of inclination is a question of optimisation. An orthogonal, upright portion offers a better rigidity but is more difficult to achieve without making the material too thin. Thus, the pressing properties of the material as well as its inherent rigidity, the field of application of the plate etc. need to be taken into consideration.
A further advantage of the plate pattern described above is that the plates can be symmetrically designed to allow the formation of a plate pack in a plate heat exchanger using only one type of plate, every second plate in the plate pack being flipped about a symmetry line.
Advantageously, the channel portions of the plate have an extension which in the transverse direction is greater than the extension, in the transverse direction, of the respective rows of ridges and troughs. This means that there will be no appreciable pressure drop. The rows of ridges and troughs afford the plate the required strength, and the relatively wide channel portions provide channels with high flow capacity.
Preferably, the channel portions have an extension which in the transverse direction is about twice as great as the extension, in the transverse direction, of the respective rows of ridges and troughs. By designing the plate in this manner, the pressure drop will be very small and the plate will have a pattern which makes it strong.
In a preferred embodiment, each elongated ridge is narrower in a central portion thereof in such manner that the portion of the ridge coinciding with the top plane has an extension in the transverse direction which is smaller in the central portion of the ridge in relation to the extension in the end portions of the ridge. By designing the ridge in this manner, the potential heat transfer surface is effectively maintained. The part of the heat transfer surface that bears against an adjacent plate is not used to any appreciable extent for heat transfer between the two media or fluids in the plate heat exchanger. To increase the heat transfer surface while maintaining the load transmitting capacity between adjacent plates, the ridges are made narrower in their central portion, as seen in the main flow direction, than in their end portions. This can be done, for example, by making the pressed ridge narrower, but it can also be done, for example, by giving the pressed ridge a more rounded shape or by reducing the press depth, the loads during operation being allowed to act on the ridge in such manner that the required width will bear against the corresponding ridge of the adjacent plate.
According to a further preferred embodiment, each elongated trough is narrower in a central portion thereof in such manner that the portion of the trough coinciding with the bottom plane has an extension in the transverse direction which is smaller in the central portion of the trough in relation to the extension in the end portions of the trough. As described above in connection with a preferred embodiment of the ridges, this affords a high degree of utilization of the heat transfer surface and provides for a strong plate. Depending on the field of application, both the ridges and the troughs may be designed as described above, but it is also conceivable to design only the ridges or only the troughs in this way. The ridges and the troughs may, for example, be designed differently cases involving two fluids which have clearly differing characteristics in terms of the required pressure or heat transfer capacity.
In a preferred embodiment, the ridges and troughs in one and the same row have the same extension in the main flow direction. A plate which, in this respect, is symmetrical is thereby obtained. This facilitates the manufacture thereof and, in most fields of application, results in symmetrical loads on the surrounding environment.
According to a further preferred embodiment, the ridges and trough in one and the same row have different extensions in the main flow direction. By designing the plate in this way, transverse connections extending between the main flow channels can be obtained, said transverse connections compensating for the fact that the pressure of the fluids drops slightly in the main flow direction and that the fluids have already been distributed to a certain extent at a preceding stage upstream of the main flow direction. Thus, the relation between the main flow channels and the transverse connections may be optimised in terms of pressure drop and fluid distribution along the entire extension of the plate in the main flow direction.
In another preferred embodiment, the ridges and troughs located next to each other in the transverse direction have the same extension in the main flow direction. A plate which, in this respect, is symmetrical is thereby obtained, which facilitates the manufacture thereof and, in most fields of application, results in symmetrical loads on the surrounding environment.
According to yet another preferred embodiment, the ridges and troughs located next to each other in the transverse direction have different extensions in the main flow direction. By designing the plate in this way, transverse connections may be obtained which extend between the main flow channels and compensate for the fact that the flow, in most cases, is slightly lower in the outer portions of the heat transfer surface of the plate. This allows the relation between the main flow channels and the transverse connections to be optimised in terms of, for example, pressure drop and fluid distribution along the entire extension of the plate in the transverse direction.
According to a preferred embodiment, the rows of ridges and troughs are arranged in such manner that they, along a first line in the transverse direction, each present a ridge and, along a second line in the transverse direction, each present a trough. A satisfactory cross distribution of the fluids is thus obtained also in cases of small pressure drops.
According to a further preferred embodiment, the rows of ridges and troughs are arranged in such manner that, along a line in the transverse direction, every second row presents a ridge and every second row presents a trough. The transverse connections between the main flow channels will essentially follow a number of diagonal lines across the heat transfer surface of the plate, which results in a satisfactory distribution of the fluids over the width of the plate, since a flow through a transverse connection can easily pass the next transverse connection (to yet another main flow channel) without its direction of flow being altered to any appreciable extent.
Preferably, each channel portion is stepwise divided into a number of essentially plane step portions which are arranged one after the other in the main flow direction and displaced in relation to each other along a normal to the central plane of the plate. This design makes the plate considerably more rigid and strong than before, on the one hand because the portions interconnecting the step portions will extend at least partially along the normal to the plate and, thus, support some of the load and, on the other hand, because the relatively displaced portions will considerably increase the moment of inertia of the plate in bending and, thus, the section modulus. This means that the deflection caused by a certain load will be drastically reduced since, for most plate designs, the relation between the deflection and the length of the portion subjected to the force is more than linear. By designing the channel portions in this way, an additional advantage is obtained, namely that the steps formed in the main flow channels will effectively prevent the formation of a film of fluid which may otherwise occur across the heat transfer surface of the plate. The formation of a film has a detrimental effect on the heat exchange, i.e. the heat exchange is reduced, and also increases the risk of fouling.
Advantageously, every second step portion is located in a first step plane, which is essentially parallel to the central plane of the plate, and the other step portions are located in a second step plane, which is essentially parallel to the central plane of the plate. From the point of view of manufacture, this is a preferred embodiment, which also affords a symmetric distribution of forces.
Preferably, each step portion has an extension in the main flow direction which is about half of the extension of the ridges and troughs in the main flow direction. This affords a particularly favourable distribution of forces between the juxtaposed rows of ridges and troughs while affording the channel portion surfaces a suitable film-preventing capacity.
According to a preferred embodiment, the position of each step portion along a normal to the central plane of the plate is constant in the main flow direction, the step portions being arranged to form, together with the corresponding step portions of another plate, a channel which has a corrugated extension and a channel width along said normal which is constant in the main flow direction. Every second step portion is tangent to a first plane and the other step portions are tangent to a second plane, the first plane and the second plane being essentially parallel to the central plane of the plate. From the point of view of manufacture, this is a preferred embodiment which, at the same time, affords the channel portion surfaces a suitable film-preventing capacity. Furthermore, the step portions of adjacent plates will interact to further increase the film-preventing capacity.
In a further preferred embodiment, the position of each step portion along a normal to the central plane of the plate varies along the main flow direction, the step portions being arranged to form, together with the corresponding step portions of another plate, a channel which has a channel width along said normal which varies in the main flow direction. According to a variant thereof, every second step portion is tangent to a first plane and the other step portions are tangent to a second plane, the first and second planes being essentially parallel to the central plane of the plate. The variation in the width of the channel in the main flow direction affords an excellent film-preventing capacity. Alternatively, it is possible to have a certain degree of inclination of the planes to which the step portions are tangent in order to obtain a more or less continuous increase or decrease of the width of the channel in the main flow direction. This design allows pressure drops or any changes in phase (and associated changes in volume) of the fluids to be taken into account.
According to a preferred embodiment, the position of each step portion along a normal to the central plane of the plate varies in the transverse direction, the step portions being arranged to form, together with the corresponding step portions of another plate, a number of channels which have channel widths along said normal which vary along the transverse direction. Owing to this design, any unsymmetrical positioning of ports or inlet and outlet portions, which will result in flow paths of varying length across the plate, can be taken into account. By varying the position of the step planes in the transverse direction, the desired pressure drop for different portions of the plate in the transverse direction can be chosen, which allows a uniform heat exchange to be obtained even if the ports are unsymmetrically positioned or if, for other reasons, there is any other dissymmetry.
The plate pack of the invention comprises a plurality of heat transfer plates according to the invention. The problems solved and the solutions obtained by means of the preferred embodiments of the heat transfer plates are, in most cases, associated with the use of the plates in a plate pack and a plate heat exchanger, respectively, and will not be reiterated. However, some of the problems solved and advantages obtained will be described in more detail, since they can be understood more clearly in relation to the use of the plates in a plate pack or a plate heat exchanger.
The plate pack is characterised in that the heat transfer portion has a plurality of juxtaposed rows of said ridges and troughs, said rows extending along the main flow direction, that the rows of ridges and troughs are separated from each other in a transverse direction, which is essentially perpendicular to the main flow direction and extends along the central plane of the plate, by essentially plane channel portions of the heat transfer portion, which extend essentially parallel to the central plane of the plate, that each row presents alternating elongated ridges and elongated troughs which extend in said main flow direction, that the transition between each ridge and an adjacent trough in the same row is formed by a continuous, essentially straight transition portion of the plate, which is inclined relative to said central plane of the plate and of which a first part forms an end wall of said ridge and a second part forms an end wall of the adjacent trough, that a main part of the fluid stream flows in the main flow direction in main flow channels which extend along the main flow direction and which are formed by the essentially plane channel portions of two adjacent heat transfer plates, and that a small part of the fluid stream flows in the transverse direction in the portions where the troughs of two adjacent heat transfer plates form open transverse connections between the main flow channels.
This design is a satisfactory compromise between the seemingly incompatible construction requirements according to which the plate pack is to be strong without causing any appreciable pressure drop. The ridges of the rows bear against each other, and since the material extends directly between the ridges and the troughs (which form an ridge relative to the adjoining plate on the other side) a strong plate is obtained. Owing to the essentially plane channel portions the fluid is conducted through the plate pack without any appreciable pressure drop. Furthermore, the transverse connections allow the fluids to be distributed over the width of the plate without the need for any appreciable pressure to achieve the distribution.
According to a preferred embodiment, the plates constituting the plate pack are identical. Every second plate in the plate pack is usually flipped or rotated about some kind of symmetry line in order for the different interspaces to communicate with different ports of the heat exchanger. Using identical plates in the plate pack, as opposed to using several different plates, allows the number of pressing tools to be reduced.
According to another preferred embodiment, the plates constituting the plate pack are of two different types, so that every second plate is of a first type and every second plate is of a second type. This construction makes it easier to optimise the plate design in terms of fluid flow and transmission of forces between the different plates.
The invention will be described in more detail below with reference to the accompanying schematic drawings, which by way of example illustrate currently preferred embodiments of the invention.
As shown in
The plate 1 is intended to be mounted together with a plurality of similar plates in a plate heat exchanger 100, as shown in
As shown in
The gaskets 112 are used to respectively seal off and allow a fluid flow by the connections 111a–c and the ports 11a–c communicating with every second plate interspace 111d and the connections 110a–d and the ports 10a–e communicating with the other plate interspaces 110e. Thus, a first fluid will flow in a flow area in every second plate interspace 111d and a second fluid will flow in a flow area in the other plate interspaces 110e. There is no direct contact between the two fluids. Instead, heat is exchanged by the intermediary of the heat transfer surfaces C of the plates 1.
As shown in
In a transverse direction G, which is perpendicular to the main flow direction F, the rows 200 of ridges (210) and troughs (220) are separated or delimited by channel portions 240 extending in the main flow direction F.
A straight or plane transition or connecting portion 230 extends between each of the elongated ridges 210 and troughs 220 of the rows 200, said portion 230 being inclined relative to the central plane P1 of the plate 1. The connecting portions 230 are continuous and present a straight unbroken flank, which means that they transmit the compressing forces between the ridges 210 and troughs 220 in a very advantageous manner.
The ridges 210 are narrower in their central portion 211 than in the end portions 212. Thus, the central portion 211 is tangent to the top plane P2 along a width H1 which is smaller than the width H2 along which the end portions 212 are tangent to the top plane P2 (see
The channel portions 240 are divided into a number of step portions 241, 242 which are arranged one after the other in the main flow direction F. Each step portion 241, 242 extends over the width of the entire channel portion 240 between two rows 200. Every second step portion 241 is arranged in a first step plane P4 and every second step portion 242 is displaced along the normal N in the direction of the central plane PI of the plate 1 and lies in a second step plane P5 (see
In the Figures, the same reference numerals are used to designate the ridges 210, the troughs 220, the channel portions 240 etc. for the different embodiments in
In the embodiment shown in
In the embodiment shown in
The step portions 241, 242 are configured so that, along a line which is parallel to the transverse direction G, all channel portions 240 present step portions which are tangent to the same step plane. Along a line which is parallel to the transverse direction G, all channel portions 240 present the step portion designated 241 and, along another line which is parallel to the transverse direction G, all channel portions 240 present the step portion designated 242.
The purpose of the step portions 241, 242 being relatively displaced is to provide a plate 1 which is significantly stronger than what was previously possible. Furthermore, owing to the flank 243, which interconnects the step portions 241, 242, film formation in the channels can be prevented, which is an advantage.
As described above, the plates 1 are adapted for use in a plate pack 101 in a plate heat exchanger 100. For this purpose, every second plate is flipped about a symmetry axis S which is parallel to the main flow direction F. The ridges 210 of one plate 1 will bear against the corresponding ridges 210 of an adjacent plate 1. In the same way, the troughs 220 of said plate 1 will form ridges 210 on the other side, which will bear against the ridges 210 of another adjacent plate. This is clearly illustrated in
The embodiment described above leads to a construction in which the main part of the fluid stream over the heat transfer surfaces C between the port portions A, B will flow in the main flow channels F′ without any appreciable pressure drop. Furthermore, the embodiment described allows the fluid flow to be distributed between the different main flow channels F′ so that a uniform flow is obtained over the entire heat transfer surface C. Owing to this design, the required transverse flows will occur without the need for any appreciable pressure. Thus, the major part of the fluid stream will flow in the main flow channels F′ and only a minor part of the stream will flow between the main flow channels F′ via each individual transverse connection G′.
In
As shown, particularly in
As shown in
It will be appreciated that a number of modifications of the embodiments of the invention described herein are possible within the scope of the invention, which is defined by the appended claims.
For example, the ridges and troughs of one and the same row may have different extensions in the main flow direction. The extension of the ridges may be greater or smaller than that of the troughs. According to another alternative, the extension of the ridges and/or troughs may vary in the main flow direction. In a further alternative, the extension of the ridges and troughs relative to each other may change in the main flow direction, whereby a solution compensating for pressure drops and/or any changes in state of one or both fluids. The relative extension of the ridges and troughs may be varied in a large number of ways depending on the field of application. Furthermore, the extension of the ridges and troughs and the relation between them may, for example, be varied along the transverse direction, to compensate, for example, for the fact that, in most cases, the fluid flow will initially be slightly unevenly distributed.
According to an alternative embodiment, the step portions may be arranged in such manner that the channel width of the main flow channels along the plate normal is constant and the sidewalls of the channel (i.e. the step planes) are moved in the same direction in the same position in the main flow direction. This may be achieved, for example, by alternating the different step portion planes along a line in the transverse direction.
According to a further alternative embodiment, the step planes are inclined so that the channel width will change continuously in the main flow direction. The channel width may also be changed by arranging the step portions in a number of different planes whose relative distance varies in the main flow direction, and not only in two planes. The relative position and height of the step portions, both in the main flow direction and in the transverse direction, can be varied in a large number of ways.
It is also conceivable to have various embodiments, in which two or more different types of plates are used to form the plate pack in the plate heat exchanger. A common solution is to use two different plates which are alternatingly arranged in the plate pack in the plate heat exchanger. Another common variant is to use identical plates (the pressed sheet-metal plate) and two different types of gaskets, so that two different heat transfer plates can be obtained by means of only one pressing tool. However, the advantage of the plate pattern described above is that is allows one type of plate to be designed, which can be flipped and used to form all the plates of the plate pack.
The gaskets 112 may be replaced by other types of gaskets, such as ridges bearing against the adjacent plates and being welded onto these plates.
The above description refers to a plate heat exchanger with only one plate pack. However, it is conceivable to use several plate packs in one and the same plate heat exchanger. In that case, the different plate packs may be completely separated from each other or they may communicate in terms of flow.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0100028 | Jan 2001 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE02/00009 | 1/4/2002 | WO | 00 | 11/12/2003 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO02/053998 | 7/11/2002 | WO | A |
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| 4635714 | Almqvist et al. | Jan 1987 | A |
| 4781248 | Pfeiffer | Nov 1988 | A |
| 4915165 | Dahlgren et al. | Apr 1990 | A |
| 5398751 | Blomgren | Mar 1995 | A |
| Number | Date | Country |
|---|---|---|
| 863816 | Jan 1953 | DE |
| 2 348 460 | Nov 1977 | FR |
| 1 201 151 | Aug 1970 | GB |
| WO 8701795 | Mar 1987 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20040069473 A1 | Apr 2004 | US |
