METHOD FOR MANUFACTURING A PLATE HEAT EXCHANGER HAVING PLATES CONNECTED

THROUGH MELTED CONTACT POINTS AND HEAT EXCHANGER OBTAINED USING SAID METHOD

The present invention concerns a method for manufacturing plate heat exchangers and a heat exchanger obtained using said method. As already known, a heat exchanger is a heat engine in which heat transfer between two fluids takes place, and various models of heat exchangers are available on the market and can be distinguished from one another due to their geometric configuration. Among the various models there are tube heat exchangers, concentric tube heat exchangers, plate heat exchangers: the present invention refers to plate heat exchangers.

Plate heat exchangers are used when it is necessary to have reduced dimensions and high heat exchange coefficients at the same time. For this reason, they are used in applications in which the fluids used for the heat exchange do not cause scaling (demineralised water) or when it is necessary to increase the heat exchange coefficient, which otherwise would be low due to the properties of the fluid or its low speed.

Within the category of plate heat exchangers it is possible to distinguish among different construction variants, such as those with plates that can be inspected, braze-welded heat exchangers, those with welded or semi-welded plates, etc. In particular, braze-welded plate heat exchangers are of interest in this case, which are suited to be used when the operating temperatures approximately range between -180 ⁰ C and +225°C, in applications like remote control heating, heating with solar panels, heat pumps and heat recovery units, industrial processes, refrigerating systems, hydraulic oil and engine oil cooling systems, just to mention the most important ones.

Therefore, in this specific category of plate heat exchangers there may be applications in which the fluids used for the heat exchange comprise, for example, a refrigerant, like Freon, and water or oil, water and water, water and oil, air and water.

A braze-welded plate heat exchanger comprises a set of plates, usually made of stainless steel, but available also in titanium, obtained through pressing, facing each other and aligned along a longitudinal direction to form a so-called "plate pack". The plates are adjacent to one another and joined in correspondence with

reciprocal contact points identified on each one of their side faces to define, along each one of them, in a direction substantially transversal to the above mentioned longitudinal direction, channels in which the above mentioned heat exchange fluids flow. The braze-welded plate heat exchanger also comprises a delivery duct and a return duct for each one of the heat exchange fluids.

These ducts are constituted by a plurality of through holes, made in each one of the plates, arranged coaxially along alignment directions, usually four, parallel to one another and to the longitudinal direction. For example, the through holes are obtained in pairs at the ends of each plate.

The heat exchanger also comprises a pair of holding flanges, always made of stainless steel, each one of which is positioned externally against one of the terminal plates of the pack.

The delivery and return ducts of each of the fluids communicate with the outside via four through openings, made coaxially to the through holes present in each of the alignment directions, in some construction variants on only one of the holding flanges, in other variants in pairs on each one of the holding flanges.

The delivery and return ducts contain one of the two heat exchange fluids that, furthermore, touch opposite sides, countercurrent, of the side faces of each plate, thanks to special solutions adopted for the latter in correspondence with the through holes.

In fact, the contact points between the plates are positioned in correspondence with shaped projections that alternately delimit the through holes. In practice, a shaped projection on one of the side faces of the plates corresponds to a shaped recess on the opposite side face.

In this way, one of the heat exchange fluids only is allowed to pass into one of the flow channels along one of the side faces of the plates.

Each one of the side faces of the plates is provided with shaped corrugated portions having the function to increase the turbulence, and therefore to increase the heat exchange coefficient, of the heat exchange fluids that flow countercurrent along them.

The connection of the plates is carried out through a braze-welding operation in correspondence with the already mentioned shaped protrusions and of the peripheral edge of the plates.

It is important to remember that braze-welding is a specific type of autogenous welding carried out using a weld material different from the base metal to be welded and having a lower melting point.

According to this technique, the weld material is applied between the metal edges to be welded, which are simply heated, without reaching the melting temperature.

In the case of plate heat exchangers, braze-welding consists in melting a sheet, generally in copper and less frequently in nickel, which has been previously placed between a plate and the adjacent one, in correspondence with the above mentioned contact points.

The side surfaces of each sheet of material used for braze-welding obviously have a shape that is similar to that of the side faces of the plates.

The main drawback of the heat exchangers of the type just described is directly connected to the braze-welding technique used to join the plates. In fact, in braze-welded plate heat exchangers it is necessary to place a sheet of a suitable metallic material, such as copper, between all the plates, so that they can be effectively joined.

Therefore, the use of braze-welding, which has always been adopted by manufacturers for the construction of plate heat exchangers having certain performance levels and destined for specific applications, up to date has not allowed a simpler and less articulated construction technology to be designed and set up.

A further drawback is due to the fact that braze-welding is usually performed in industrial furnaces where it is necessary to maintain rather high temperatures, of the order of the melting temperature of the material used, for example approximately 1083 ⁰ C for copper, for a prolonged lapse of time, which results in considerable energy consumption for the manufacturing company.

A further drawback of braze-welded plate heat exchangers is constituted by their considerable weight in the applications where high heat exchange coefficients are required, said high coefficients being mainly obtained by providing for a high minimum number of plates inside the pack.

The present invention intends to overcome the drawbacks posed by the prior art which have just been mentioned.

In particular, the main object of the invention is to introduce in the market a plate heat exchanger that, though maintaining the same performance

characteristics and the same field of application of a braze-welded plate heat exchanger, is easier and less articulated to construct.

Practically, the invention is aimed at carrying out a heat exchanger that on one hand, as far as operation is concerned, can be compared to a braze-welded plate heat exchanger, and on the other hand is constructed with fewer components and materials.

It is a further aim of the invention to carry out a plate heat exchanger that can be obtained with less consumption of energy compared to the most similar type of plate heat exchangers, that is, braze-welded heat exchangers. Last, but not least, a further aim of the invention is to reduce the weight of the finished product compared to braze-welded plate heat exchangers, in order to facilitate material handling operations, especially when large heat exchangers are involved.

The aims mentioned above are achieved through the implementation of a method for manufacturing a heat exchanger that, according to the main claim, is of the type with plates facing each other and aligned along a longitudinal direction, joined to each other in correspondence with reciprocal contact points identified on each one of said side faces of said plates, said method being characterized in that the connection between each of said plates and the immediately adjacent one is obtained by heating almost up to the melting point the material of each of said plates in correspondence with said contact points and by compressing said plates one against the other at the same time. The subject of the present invention is also a heat exchanger that, according to the relevant main claim, comprises: - a plurality of said plates arranged facing each other and aligned along a longitudinal direction, joined to each other in correspondence with reciprocal contact points identified on each one of the side faces of said plates to define, along each one of said side faces, in a direction substantially transversal to said longitudinal direction, channels in which heat exchange fluids flow;

- a delivery and a return duct for each of said heat exchange fluids, constituted by a plurality of through holes made in each one of the plates, arranged coaxially along a series of alignment directions parallel to one another and to said longitudinal direction; - a pair of holding flanges, each positioned externally against one of said

terminal plates; and is characterized in that said plates are made of a non-ferrous metallic material and are joined to each other in correspondence with said contact points through a weld bead obtained by melting said non-ferrous metallic material of which said plates are made.

Advantageously, the invention simplifies the construction of a plate heat exchanger compared to en equivalent braze-welded heat exchanger, offering however the same performance levels and possible applications. This is possible owing to the fact that according to the method subject of the invention the plates are joined by pressure welding instead of by braze- welding.

Practically, the construction process of the heat exchanger subject of the invention eliminates the interposition of a metallic sheet, like copper, between a plate and the adjacent one prior to treatment in the apposite furnace. All the other parameters involved in the construction of the heat exchanger remaining unchanged, this also allows the cost of the final product to be reduced.

Still to advantage, in comparison with the prior art the invention ensures energy savings in relation to the operating conditions of the furnace in which pressure welding of the plates is carried out.

In fact, the production technique used according to the present invention to join the plates, together with the use of plates made of a material different from stainless steel, makes it possible to maintain temperature and pressure values lower than those required in the production of braze-welded plate heat exchangers inside the furnace.

Energy savings are also guaranteed by the fact that the correct and definitive connection of the plates at the end of the heating process inside the furnace is obtained more rapidly than for braze-welded plate heat exchangers. Still to advantage, the use of plates made of a non-ferrous metallic material, preferably aluminium, allows their weight to be reduced compared to braze- welded heat exchangers of known type, the same area of heat exchange surfaces and the same number of plates making up the pack being present. The aims and advantages described above, as well as others that will be illustrated below, will be highlighted in greater detail in the description of some preferred embodiments of the invention, given indicatively as examples without

limitation, with reference to the enclosed drawings, wherein:

- Figure 1 is an exploded axonometric view of the heat exchanger subject of the invention;

- Figure 2 shows a transversal section of Figure 1 according to cutting plane /7;

- Figure 3 is an exploded enlarged view of a detail of Figure 1 , and includes a diagram of the path of the heat exchange fluids;

- Figure 4 is a front view of a detail of Figure 2;

- Figure 5 is a rear view of the detail shown in Figure 4; - Figure 6 is a construction variant of the heat exchanger shown in Figure 1 ;

- Figure 7 is an exploded enlarged view of a detail of Figure 6, and includes a diagram of the path of the heat exchange fluids.

The plate heat exchanger claimed by the applicant can be used either as condenser or as evaporator, for example in conditioning or cooling systems and is shown in Figure 1 , where it is indicated as a whole by 1. It can be observed that the heat exchanger 1 comprises:

- a plurality of plates 2 arranged facing each other and aligned along a longitudinal direction Z, joined to each other in correspondence with reciprocal contact points 3, visible in Figure 2 and successive figures, identified on each one of the side faces 2a, 2b of said plates 2 to define, along each one of said side faces 2a, 2b, in a direction substantially transversal to the longitudinal direction Z, channels 4, 5 in which heat exchange fluids Fi, F ₂ flow, said fluids being for example a refrigerant and water, respectively, and being usually called primary fluid and secondary fluid;

- a delivery duct 6 and a return duct 7 for each of the heat exchange fluids Fi, F ₂ , defined by a plurality of through holes 9, made in each one of the plates 2, arranged coaxially along four alignment directions Zi, Z ₂ , Z ₃ , Z ₄ parallel to one another and to said longitudinal direction Z; - a pair of holding flanges 10, 11 , each positioned externally against one of the terminal plates 21 , 22.

According to the invention, the plates 2 are made of a non-ferrous metallic material and are joined to each other in correspondence with the contact points 3 through a weld bead 12 obtained by melting said non-ferrous metallic material of which said plates 2 are made.

In practice, the weld bead 12 is not constituted by the traditional layer of material deriving from the melting of weld material interposed between the two parts to be joined, but by the surface edges 2d, 2e of each one of the side faces 2a, 2b of adjacent plates 2, brought to melting temperature in correspondence with the contact points 3 and then hardened.

The non-ferrous metallic material is preferably, but not necessarily, aluminium, which is known to have a melting temperature of approximately 660 ⁰ C that is considerably lower than that of the materials used for braze-welding in the relevant plate heat exchanger, like copper. In other construction variants of the invention, not represented herein, the non-ferrous metallic material may be an aluminium-based alloy, for example a light alloy.

Figure 1 shows that one of the holding flanges, specifically the one indicated by 10, is provided with four through openings 13, each one of which is coaxial to the through holes 9 arranged on one of the alignment directions Z ₁ , Z ₂ , Z ₃ , Z ₄ , to obtain the configuration also known as "with single passage", in which the delivery duct 6 and the return duct 7 of each one of the heat exchange fluids F ₁ , F ₂ communicate with the outside on the same side. The holding flange 10 is provided with four tubular couplings 14 for connection to one or more external systems, each one positioned in correspondence with one of the through openings 13.

Figure 3 shows that the contact points 3, where the two adjacent plates 2 are joined, are positioned in correspondence with shaped protrusions 15, obtained on each one of the side faces 2a, 2b of the plates 2. The contact points 3 are preferably located also near the peripheral edge 2c of the side faces 2a, 2b of the plates 2.

The shaped protrusions 15 delimit the areas 16 in which there are the through holes 9 and are alternated, so that a shaped protrusion 15 on the side face 2a of the plates 2 corresponds to a shaped recess 17 on the opposite side face 2b in order to ensure, along each one of the side faces 2a, 2b, the passage of only one of the heat exchange fluids F ₁ , F ₂ in one of the flow channels 4, 5, as can be seen in the operation diagram of Figure 3.

As a consequence, a shaped recess 17 and a shaped protrusion 15 on the side face 2a of a first plate 2 respectively correspond to a shaped recess 17 and to a shaped protrusion 15 on the side face 2b of the plate 2 adjacent to

the first plate, said side face 2b being opposite to said side face 2a. Figures 4 and 5 also show that each one of the side faces 2a, 2b of the same plate 2 comprises shaped corrugated portions 18 spaced from one another, which develop along parallel directions and in opposite sense, having the function to increase the turbulence of the heat exchange fluids F ₁ , F ₂ in the flow channels 4, 5.

It must be observed that the shaped corrugated portions 18 are separated from the through holes 9 through the shaped protrusions 15. Furthermore, the shaped corrugated portions 18 protrude from the inner surface 21a, 21 b of the side faces 2a, 2b for a shorter section than that from which the shaped protrusions 15 project.

According to the favourite embodiment of the invention described herein, the longitudinal section of each one of the shaped corrugated portions 18 has a substantially fishbone-shaped profile. As can be observed by comparing Figures 4 and 5, the shaped corrugated portions 18 present on the side face 2a are staggered in relation to the shaped corrugated portions 18 present on the opposite side face 2b of each one of the plates 2. Figure 6 shows a construction variant of the invention, in which the heat exchanger, now indicated as a whole by 100, is differentiated from the one previously described only owing to the fact that both holding flanges 104, 105 have two through openings, respectively indicated by 106 on flange 104 and by 107 on flange 105, each one being coaxial to the through holes, not visible, present on the plates 101 and arranged on one of the alignment directions defined above and not indicated for the sake of brevity.

Therefore in this case the heat exchanger 100, suitable for another use, is characterized by a type of operation with multiple passages, shown in Figure 7, which is different from the operation illustrated in the previous example, since the delivery duct 102 and the return duct 103 of the heat exchange fluid F ₃ communicate with the outside of the heat exchanger on the opposite side and not on the same side as the ducts of the other heat exchange fluid F ₄ . Consequently, each one of the holding flanges 104, 105 is provided with tubular couplings 108, 109, arranged in correspondence with the through openings 106 and 107, respectively. The process that leads to the manufacture of the heat exchanger 1 firstly

consists in placing the plates 2, obtained via a forming operation, facing each other and aligned along a longitudinal direction Z, joined to each other in correspondence with the reciprocal contact points 3 identified on each of the side faces 2a, 2b of the plates 2 and laterally closed by the holding flanges 10, 11.

According to the invention, the connection between each one of the plates 2 and the immediately adjacent one is obtained by heating almost up to the melting point the material of each one of the plates 2 in correspondence with the contact points 3 and by simultaneously compressing the plates 2 against each other, a process that is technically known as "pressure welding".

The heating operation consists in maintaining the material of the plates 2, in the case at hand aluminium, at a heating temperature Ti not exceeding the melting temperature T _f of the material itself for a given lapse of time ti to obtain the melting of the surface edge 2d, 2e of each one of the plates 2. The heating operation is carried out in controlled atmosphere into a furnace, inside which the above mentioned heating temperature Ti is preferably but not necessarily maintained at approximately 400 ⁰ C for a lapse of time ti of approximately 5 hours. The compression of the plates 2 takes place at a pressure of 1 ,5 bars. The heat exchange fluids F-i, F ₂ flow countercurrent, one along the side face 2a and the other along the side face 2b of each one of the plates 2 involved in the flow of the heat exchange fluids Fi, F ₂ according to the configuration shown in the diagram of Figure 3. The method for the manufacture of the heat exchanger 100 with plates 101 , in which the heat exchange fluids F ₃ , F ₄ flow along the side faces of the plates 101 , as indicated in Figure 7, is completely equivalent to the method just described.

The innovative concept claimed by the invention lays in that a new technique for joining the plates of a heat exchanger having specific characteristics, especially a high heat exchange coefficient, is combined with the use of plates made of a non-ferrous metallic material, preferably aluminium. In addition to the advantages described above, the use of aluminium has the advantage of reducing the final cost of the heat exchanger, though maintaining all the other conditions and parameters unchanged. This is due to the fact that, as well known, for pressing and welding operations

the mineral from which aluminium is extracted, bauxite, has a processing cost that is lower than that of stainless steel, which is the material currently used for making the plates.

According to the above, it is therefore clear that the method for manufacturing a plate heat exchanger and the heat exchanger obtained with said method achieve the aims and offer the advantages mentioned above. Upon implementation changes may be made to the heat exchanger subject of the invention, consisting in a configuration of the delivery and return ducts of the heat exchange fluids different from those previously examined. In fact, said ducts may communicate with the outside through couplings applied to the holding flanges in different positions compared to those shown in the attached drawings.

Furthermore, the heat exchange fluids may flow among the plates following paths that do not coincide with those illustrated in the attached drawings. In the cases where the technical characteristics illustrated in the claims are followed by reference marks, these have been added only with the aim to facilitate the comprehension of the claims themselves and therefore said reference marks do not have any limiting effect on the degree of protection to be granted to each element they identify only by way of example. All the variants described and mentioned, but not represented in the attached drawings, must be considered protected by the present patent, provided that they fall within the scope of the following claims.