Method for manufacturing a brazed plate heat exchanger

Abstract

A method for manufacturing a brazed plate heat exchanger, the brazed plate heat exchanger comprising a stack of heat exchanger plates, the stack of heat exchanger plates being provided with a pressed pattern adapted to provide contact points between adjacent heat exchanger plates, such that the heat exchanger plates are spaced apart from each other while forming inter-plate channels for a heat exchange medium, wherein the inter-plate channels selectively communicate with port openings for the heat exchange medium and are circumferentially sealed to prevent external leakage, the method comprising the following steps: a. calculating the positions of the contact points between adjacent plates; b. calculating the force that must be transmitted by each contact point when the heat exchanger is in use; c. calculating the amount of brazing material required for each contact point based on the above method steps; d. providing a screen for screen printing brazing material onto the heat exchanger plates, wherein the screen is provided with openings of a size, position, plate thickness, and shape adapted to provide the necessary amount of brazing material to each contact point; e. screen printing the heat exchanger plates with brazing material using the screen; f. stacking the heat exchanger plates into a stack; and g. Brazing heat exchanger plates is used to stack the plates together to form a heat exchanger.

Description

[0001] This application is a divisional application of Chinese patent application filed on January 16, 2019, with application number 201980008271.2 and entitled "Method for Manufacturing Brazed Plate Heat Exchanger". Technical Field

[0002] The present invention relates to a method for manufacturing a brazed plate heat exchanger comprising a stack of heat exchanger plates having a pressed pattern adapted to provide contact points between adjacent heat exchanger plates, such that the heat exchanger plates or portions thereof remain spaced apart from each other while forming inter-plate channels for heat exchange medium, wherein the inter-plate channels selectively communicate with port openings for heat exchange medium and are circumferentially sealed to prevent external leakage. Background Technology

[0003] SE539695 discloses a method for applying brazing material to a heat exchanger plate prior to brazing. This method, in short, involves applying the brazing material near, rather than at, the contact point between the ridges and grooves of adjacent heat exchanger plates. By this method, the brazed joint between the ridges and grooves will be significantly thinner than a brazed joint obtained by applying brazing material at the contact point between the ridges and grooves.

[0004] By using the method described in the aforementioned patent application, a high-strength brazed joint is obtained with a small amount of brazing material. This has several advantages; in addition to significantly saving brazing material, the risk of so-called "burn-through" of the heat exchanger plate is reduced when a brazing material capable of dissolving the heat exchanger plate material is used as the brazing material.

[0005] Surprisingly, it has been shown that the amount of brazing material applied near the contact point, rather than at the contact point, does not significantly affect the strength of the brazed joint. This is likely because if more brazing material is used, the metal sheet forming the heat exchanger plate will suffer more corrosion as the brazing material dissolves a portion of the plate's thickness. However, if more brazing material is used, the overall strength of the brazed joint will be higher, but the difference will be less than expected.

[0006] The purpose of this invention is to optimize the amount of brazing material applied near or at each contact point in order to obtain a brazed heat exchanger with optimal strength for a given total amount of brazing material. Summary of the Invention

[0007] This invention solves the above and other problems through a method comprising the following steps: i. Calculate the location of the contact point between adjacent plates; ii. Calculate the force that must be transmitted at each contact point when using a heat exchanger; iii. Based on steps i and ii above, calculate the amount of brazing material required for each contact point; iv. Provide a screen for screen printing brazing material onto a heat exchanger plate, wherein the screen is provided with openings of a size, location, plate thickness, and shape suitable for providing the necessary amount of brazing material as calculated in step iii to each contact point; v. Using the screen, screen print brazing material onto the heat exchanger plate; vi. Stacking the heat exchanger plates into a stack; and vii. Brazing the stack of heat exchanger plates to join the plates together to form the heat exchanger.

[0008] To achieve maximum strength with minimal use of brazing material, the wire mesh openings can provide a reduced amount of brazing material applied as the distance from the port opening increases.

[0009] To seal the internal flow channels and prevent external leakage, a circumferential seal may be provided by a circumferential skirt provided at each heat exchanger plate, wherein the skirts of adjacent plates are adapted to contact each other in an overlapping manner.

[0010] Preferably, the heat exchanger plate is typically rectangular, and the port openings are located near the corners of the heat exchanger plate. This is advantageous because the metal sheet in coil form can be utilized in a material-efficient manner.

[0011] To provide a brazing connection with minimal thickness, the brazing material can be arranged in a "double-dot" shape on either side of each contact point. The double dots can be circular, square, rectangular, crescent-shaped, or bracket-shaped. Attached Figure Description

[0012] The present invention will now be disclosed by way of examples of preferred embodiments with reference to the accompanying drawings, wherein: Figure 1 It is a planar drawing of a screen or template used for screen printing heat exchanger plates with brazing material prior to brazing; and Figure 2-5 This is a plan view of an exemplary shape of the opening in the sieve. Detailed Implementation

[0013] The present invention will now be described with reference to preferred embodiments. However, some basic properties will first be briefly explained: This invention relates to the manufacture of brazed heat exchangers. The brazed heat exchanger comprises a plurality of heat exchanger plates having a pressed pattern comprising ridges and grooves adapted to form contact points between adjacent plates when the plates are stacked. Through these contact points, the plates are kept at a distance from each other, thereby forming interplate channels for the heat exchange medium. These interplate channels are circumferentially sealed by edge seals, which are typically in the form of skirts extending along the entire periphery of the plates, wherein the skirts of adjacent plates are adapted to overlap each other, thus forming a circumferential seal that prevents leakage from the interplate channels.

[0014] The interplate channels communicate with port openings typically located near the corners of the heat exchanger plates. In most cases, four port openings are provided: an inlet and an outlet for a first fluid, and an inlet and an outlet for a second fluid. The inlet and outlet openings for the first fluid communicate with each other through a first set of interplate channels, and the inlet and outlet openings for the second fluid communicate with each other through a second set of interplate channels. Typically, the channels in each set are arranged such that every other channel is included in the first set, and the other channels are included in the second set.

[0015] Selective connectivity between port openings and flow channels is typically achieved by providing regions around the port openings at different heights, such that the regions around the port openings of adjacent plates either contact each other or do not. If these regions contact each other, there will be no connectivity between the inter-plate flow channels between the port openings and adjacent plates when they are brazed together, while if these regions do not contact each other, there will be connectivity.

[0016] During operation, the fluids exchanging heat with each other typically have pressures exceeding the surrounding atmospheric pressure. The force generated by this pressure will cause adjacent plates to move away from each other, and in order to hold the plates together, the contact points between the ridges and grooves of adjacent plates must transfer the force from plate to plate. This force will depend on the pressure and the area that each contact point must transfer the force generated by the pressure.

[0017] Understandably, since the port opening area will be exposed to fluid pressure and there are no contact points within the port opening, the brazed joint near the port opening must transmit a considerable force. Therefore, the entire force exerted by the fluid pressure must be transmitted through only a few contact points.

[0018] refer to Figure 1The diagram schematically illustrates the application pattern of brazing material to heat exchanger plates prior to brazing. It can be seen that the brazing material is applied in pairs of points in a "double-point" manner. Each point in each pair is positioned such that it forms a contact point between the ridges and grooves of adjacent heat exchanger plates. Alternatively, the brazing material can be provided as a single point, placed at the contact point between the ridges and grooves of adjacent plates. The points can have any shape, such as circular, square, or rectangular. In the case of using double points, a crescent-shaped or bracket-like symbol for each point has proven to yield good results—if either of these shapes is used, the openings of the brackets or crescents in each pair should face each other in the same way as brackets are used in text.

[0019] Preferably, the brazing material dots are applied by screen printing, i.e., a printing technique based on placing a screen or stencil on the area where the brazing material is to be selectively applied, wherein the screen or stencil is provided with openings having size, shape and position corresponding to the desired brazing material application pattern.

[0020] As is well known to those skilled in the art of brazed heat exchangers, such heat exchangers are prone to cracking near the port area if subjected to excessive pressure for the reasons mentioned above.

[0021] Therefore, the mesh opening size is larger near the port opening. A larger mesh opening will provide more brazing material, thus providing a larger brazed joint. Although a brazed joint containing a lot of brazing material will be weaker per unit area, it will be larger and therefore able to transmit greater force. Using a large amount of brazing material to braze a contact point also increases the risk of burn-through when using brazing material that can dissolve the substrate. However, it should be noted that burn-through depends not only on the amount of brazing material. Other factors, such as temperature and time, are also important. In a heat exchanger where equal amounts of brazing material are applied to all contact points, a certain percentage of brazed joints may burn through the substrate. If less brazing material is used, this percentage will be lower. It should be noted that even for the point where the maximum amount of brazing material is applied, the percentage of burn-through is very low, but if less brazing material is used for each contact point, this percentage will drop to even lower percentages.

[0022] Therefore, by varying the amount of brazing material applied near or on the contact points—applying a larger amount of brazing material near or on the contact points subjected to greater forces, and a smaller amount of brazing material near or on the contact points requiring the transmission of smaller forces—some unexpected and combined benefits can be achieved: 1. Reduced the amount of brazing material required to manufacture heat exchangers; 2. Reduced the risk of burn-through; 3. The burst strength of a heat exchanger will be equal to the degree to which all brazed joints have been subjected to brazing material and the degree to which the brazed joints have been subjected to a greater force.

[0023] Refer again Figure 1 This illustrates an example of how to apply patterns using different brazing materials. Figure 1 In the process, the brazing material used for contact points near the opening is applied in a pattern according to... Figure 2 The forming shown involves applying brazing material in the form of kidney-shaped dots, where, for each pair of dots, the open ends of each kidney-shaped dot face each other. The total area of ​​these two dots is relatively large, meaning that a relatively large amount of brazing material will be applied to the contact points near the port openings. Figure 2 The pattern is applied in region G.

[0024] Region F adjacent to region G is based on Figure 3 The area where the brazing material is applied is defined as two circular surfaces. In the example shown, the circular surfaces have a radius of 0.77 mm. In adjacent region E, according to Figure 4 Brazing material is applied, i.e., applied as a circular surface with a radius of 0.67 mm, while in region D adjacent to region E, brazing material is applied as a circular surface with a diameter of 0.58 mm.

[0025] In regions D, E, and F, the area ratio between the largest circular point (0.77 mm in diameter) and the smallest circular area (0.58 mm) is 1.76. However, without departing from the scope of the invention, the area ratio between the largest and smallest points can vary within a wide range. For example, depending on the force requirement of each brazed joint, the area ratio can range from 1.1 to 10.

[0026] It should be noted that the amount of brazing material applied in the DG area decreases with increasing distance from the port opening. This is because the average surface density of the contact points increases with increasing distance from the port opening, meaning that each contact point experiences less force.

[0027] exist Figure 5 The image shows another possible shape for the pattern of the brazing material being applied. According to... Figure 5 The brazing material is applied in two rectangular areas. This shape is advantageous because it allows for the application of a considerable amount of brazing material without deviating from the central portion of the ridge where the brazing material is applied.

[0028] As an alternative or supplement, in order to accommodate the shape of the opening in the template to obtain various amounts of brazing material application, a wire mesh with varying thickness can also be used. In areas where the wire mesh is thicker, a certain application pattern will provide a large amount of brazing material application, while if the wire mesh is thinner, the amount of brazing material will be smaller.

[0029] By combining wire meshes with openings of varying thicknesses and shapes, the amount of brazing material applied can be varied over a wide range. For a given wire mesh thickness, there exists a minimum possible opening size because there is a "release limit" for each given wire mesh thickness. Simply put, the release limit is the limit to which more brazing material will adhere to the opening walls than it is applied to the surface. If the release limit is exceeded, no brazing material will be applied. The release limit will vary depending on many factors, but generally, thinner wire meshes will allow for smaller openings than thicker wire meshes. Therefore, by varying the opening size and wire mesh thickness, a wider range of different amounts of brazing material can be achieved.

Claims

1. A method for manufacturing a brazed plate heat exchanger, the brazed plate heat exchanger comprising a stack of heat exchanger plates, the stack of heat exchanger plates being provided with a pressed pattern adapted to provide contact points between adjacent heat exchanger plates, such that the heat exchanger plates are spaced apart from each other while forming inter-plate channels for a heat exchange medium, wherein the inter-plate channels selectively communicate with port openings for the heat exchange medium and are circumferentially sealed to prevent external leakage, characterized in that... The following are the steps: a. Calculate the location of the contact point between adjacent plates; b. Calculate the force that must be transmitted at each contact point when using a heat exchanger; c. Based on the above steps, calculate the amount of brazing material required for each contact point; d. Provide a screen for screen printing brazing material onto a heat exchanger plate, wherein the screen is provided with openings of a size, location, plate thickness, and shape suitable for supplying the necessary amount of brazing material to each contact point; e. Screen printing the heat exchanger plate using the aforementioned screen and soldering material; f. Stacking the heat exchanger plates into a stack; and g. Brazing the stack of heat exchanger plates to join the plates together to form a heat exchanger.

2. The method according to claim 2, wherein, The wire mesh opening provides a reduced amount of applied brazing material as the distance from the port opening increases.

3. The method according to claim 1, wherein, The circumferential seal is provided by a circumferential skirt disposed at each heat exchanger plate, wherein the skirts of adjacent plates are adapted to contact each other in an overlapping manner.

4. The method according to any one of claims 1 to 3, wherein the heat exchanger plate is generally rectangular, and the port opening is located near a corner of the heat exchanger plate.

5. The method according to any one of the preceding claims, wherein the brazing material is placed on either side of each contact point in a "double-dot" shape.

6. The method of claim 5, wherein the two dots have a circular circumference, a square shape, an elliptical shape, a crescent-shaped shape, or a bracket symbol shape.

7. The method according to any one of the preceding claims, wherein the ratio between the amount of brazing material applied at or near the contact point where a large force transmission is required and the amount of brazing material applied at or near the contact point where a small force transmission is required is in the range of 1.1 to 20, preferably in the range of 1.1 to 10.

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