WO2011160992A1

WO2011160992A1 - Brazed plate heat exchanger

Info

Publication number: WO2011160992A1
Application number: PCT/EP2011/059963
Authority: WO
Inventors: Debing Zhang; Donghua Chai; Chin Shuan Ong; Tomas Dahlberg
Original assignee: Swep International Ab
Priority date: 2010-06-21
Filing date: 2011-06-15
Publication date: 2011-12-29

Abstract

A brazed plate heat exchanger (200) comprises a stack of a number of heat exchanger plates provided with a pressed pattern providing for contact points between the plates and keeping the plates on a distance from one another under formation of interplate flow channels. Port openings are provided in the plates, said port openings being in selective fluid communication with the interplate flow channels, wherein connections (100, 300) communicating with the port openings are fastened on an end plate of the heat exchanger plate stack. At least an inner surface (120) of at least one of the connections (100) is provided with a coating of copper.

Description

BRAZED PLATE HEAT EXCHANGER

FIELD OF THE INVENTION

The present invention relates to a brazed plate heat exchanger comprising a stack of a number of heat exchanger plates provided with a pressed pattern providing for contact points between the plates and keeping the plates on a distance from one another under formation of interplate flow channels, wherein port openings are provided in the plates, said port openings being in selective fluid communication with the interplate flow channels, and wherein connections in fluid communication with the port openings are fastened on an end plate of the heat exchanger plate stack.

PRIOR ART

In the art of brazed plate heat exchangers, it has hitherto been common to provide the heat exchangers with connections made from stainless steel similar or identical to the stainless steel of the heat exchangers plates from which the heat exchangers are made. Usually, the connections are fastened to a top plate of the plate pack by prior to a brazing operation brazing the plates of the heat exchanger together placing the connections concentrically with a port opening. Brazing material may be placed between the connections and the top plate.

In order to fasten a piping of an external circuitry to the connection, a common method has been soldering or brazing by silver or tin solder. An aggressive flux must be used in order to break the oxide layer covering the stainless steel. Moreover, the soldering or brazing material must contain a substantial amount of silver in order to

"wet" the stainless steel. Silver is an expensive metal, making the brazing expensive in terms of material consumption.

It is also possible to use other methods for fastening the external circuitry to the connection, but in case the external circuitry contains a coolant or the like, it is crucial that the connection is completely sealed; hence, brazing or soldering is preferred over other means of connecting the external circuitry.

In practice, it is common to apply the aggressive flux to the connection and/or the pipe and bring them together to the desired position. Thereafter the parts are heated, e.g. by a flame, to a temperature sufficient to melt a solder or brazing material, which may be applied after the connection and the pipe has reached the predetermined temperature. The brazing material may e.g. be copper alloyed with phosphor in order to decrease its melting point.

During the last decade or so, induction heating has become more and more common; by induction heating, it is possible to heat e.g. pipes with a very high power. The high power means that it is possible to have high temperature gradients in the material, meaning that it is possible to heat a portion of the pipe to a temperature high enough to melt a brazing material, whereas portions remote from the heated position remain comparatively cool.

The present invention aims to provide a heat exchanger having connections enabling brazing or soldering between the connection and the pipe without the need for aggressive flux designed to break the oxide layer of the stainless steel.

The present invention also relates to a method for performing such brazing or soldering operation, and to a heat exchanger being provided with at least one connection

SUMMARY OF THE INVENTION

According to the invention, the above and other problems are solved by providing at least an inner surface of at least one of the connections with a coating of copper.

In order to save cost, and due to its high affinity to copper, the at least one connection may be made from mild steel or carbon steel.

In order to protect the connection from rusting, the entire surface of the at least one connection may be provided with a coating of copper.

In order to provide a connection having a stainless exterior and an interior being easily covered with copper, at least one of the connections may be provided with an insert located between the connection and a pipe to be brazed to the connection.

In order to achieve a fully stainless connection while sporting a copper coating to the interior of the connection, the inner surface may be roughened prior the copper coating procedure. The surface roughness may be higher than Ra 3.2.

A method for manufacturing a heat exchanger according to the invention comprises the steps of:

providing a number of heat exchanger plates provided with a pressed pattern adapted to provide contact points between the plates and to keep the plates on a distance from one another under formation of interplate flow channels onto one another, the plates being provided with port openings allowing for selective communication with the interpolate flow channels; placing the heat exchanger plates in a stack, wherein brazing material is placed between the plates;

placing connections (100, 300) coaxially with the port openings;

Optionally, placing brazing material in the contact surfaces between the heat exchanger plate and the connections (100, 300);

brazing, i.e. heating to a temperature exceeding the melting temperature of the brazing material, the stack of heat exchanger plates, such that the brazing material melts;

Allowing the heat exchanger stack to cool down such that the brazing material solidifies and provides a metallic bond between the contact points;

Wherein the method is characterized by the further step of:

Prior to the brazing operation, coating the connections (100) with a copper- containing material or an insert having a high affinity to molten copper.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, the invention will be described with reference to the appended drawings, wherein:

Fig. 1 is a perspective view of a brazed plate heat exchanger according to the present invention;

Fig. 2 is a perspective view showing one embodiment of a connection comprised in the heat exchanger of Fig. 1;

Fig. 3 is a sectioned perspective view of another embodiment of the present invention; and

Fig. 4 is a perspective view of still another embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

With reference to figure 1 , a plate heat exchanger 200 is shown. The plate heat exchanger 200 is a brazed heat exchanger, i.e. a heat exchanger comprising a number of heat exchanger plates (not shown) provided with a pressed pattern (not shown) keeping the plates on a distance from one another by contact points between the patterns of neighboring plates under formation of flow channels, wherein the heat exchanger plates are brazed to one another in the contact points formed by the pressed pattern. The flow channels are in selective communication with port openings (not shown) formed in the heat exchanger plates.

In order connect the heat exchanger to a circuitry containing media to exchange heat,the port openings on an outer heat exchanger plate is provided with a connections 100, 300 for allowing connection to the circuitry. In the shown embodiment, the connections 100, which are in fluid communication with one another, are adapted to be connected to a circuitry containing refrigerant, whereas the connections 300, which are in fluid communication with one another, are adapted to be connected to a circuitry containing e.g. a brine solution or water.

The connections 100, 300 may be fastened to the heat exchanger in at least two ways, which ways will be described later.

Often, heat exchangers of the type concerned are used to exchange heat between a refrigerant and any other carrier of thermal energy, e.g. water or a brine solution.

As mentioned in the "prior art" section, it is crucial that the refrigerant circuit is extremely tight; even very small leakages of refrigerant will lead to malfunction of the cooling cycle in which the coolant works. Consequently, it is desired only to have brazed or soldered connections in a refrigerant circuit, since such connections have proved to have very low rates of leakage. Moreover, brazed or soldered connections are cost efficient.

When it comes to the other connections, i.e. the connections for water or brine (such connections are denoted by 300 in Fig. 1), a small leakage is not that crucial. Hence, it is possible to use mechanical fasteners for these connections.

According to one embodiment of the invention, the connections 100 are manufactured from carbon steel, the internal surface of the connection being covered with a coating of copper. Steel is very beneficial in many ways, primarily since it shows a high affinity with copper; hence, copper coated onto a connection made from mild or carbon steel will stick to the connection during a brazing operation used to manufacture the heat exchanger (this process will not be described in detail, since it is well known in the art to braze heat exchangers). Steel has the further benefit of a low cost, as compared to stainless steel.

As mentioned above, the internal surface of the connection should be covered with copper; by covering the internal surfaces with copper, it is possible to solder e.g. a copper pipe having a smaller outer diameter than an inner diameter of the connection, using standard grade tin and flux. Such standard grade tin contains about 3% silver, and has a rather low melting point. It is also possible to use a copper/phosphor brazing material and standard flux for joining e.g. copper pipes (i.e. a much less aggressive flux than necessary when soldering stainless steel).

In fig. 2, an exemplary design of the connections 100 is shown. The connection comprises a pipe-like main body 110, having an internal surface 120 and an external surface 130 and extending from an upper end 140 to a lower end 150. In order to secure that the pipe to be inserted into the connection does not extend too far into the connection, or even intrudes into the heat exchanger, the internal surface 120 may be provided with a restriction (not shown) having a smaller diameter than the outer diameter of the pipe to be inserted into the connection. Preferably, this optional restriction is placed in the vicinity of the lower end.

At the lower end, a guide means 160 may be provided. This guide means exhibits a smaller diameter than the port opening of the heat exchanger plate; thus, when the connection is placed coaxially with the port opening, the connection 100 will stay in a correct position, coaxially aligned with the port opening.

Moreover, the external surface 110 may be provided with recesses (see figs. 3 and 4) decreasing the heat transfer from the upper end 140 to the lower end 150.

In order to provide as beneficial circumstances as possible for soldering or brazing a pipe to the connection, an internal surface of the connection is coated by brazing material, e.g. copper. As mentioned, the copper coating makes it possible to use a less aggressive flux for a subsequent soldering or brazing operation for fastening e.g. a tube or pipe to the connection.

In one embodiment of the invention, the connection is manufactured from steel, since copper coated steel has proven to give excellent conditions for soldering or brazing between e.g. a copper pipe and the steel connection. Moreover, steel has a high affinity to molten copper, meaning that it will retain a thick enough copper coating during e.g. a brazing operation, wherein the temperature is elevated to a temperature exceeding the melting temperature of copper.

In another embodiment of the invention, the connections are made from stainless steel. An internal surface of the connection is roughened, e.g. by sandblasting, sanding, checkering or the like. The rough surface may have an Ra number of more than 3.2, e.g. 6.3. Prior to the brazing operation connecting the heat exchanger plates to form the brazed heat exchanger, the rough surface of the connection is covered with copper, e.g. in form of a paste or a thin plate. Another option is to use connections made from ferritic stainless steel, since such steel has proven to have excellent retaining properties when it comes to molten copper.

In still another embodiment, shown in Fig. 3, the connection 100 may be provided with an insert 400 made from a material showing a high affinity to molten copper, while the remainder of the connection is made from stainless steel. The insert 400 is placed between the pipe to be brazed to the connection 100 and the connection 100 itself. Preferably, the insert is brazed to the connection in the same manner the connection is brazed to the heat exchanger (see below). This embodiment will make it possible to gain an excellent fastening between the insert and the connection, while maintaining the stainless properties of the connection as a whole; even if the insert is made from steel, it will be protected from moist, and hence rust, by the pipe to be brazed into the insert and the remainder of the connection.

Fig. 4 shows an embodiment of the present invention, wherein the connection 100 is provided with an annular recess 410 in the outer surface 110 of the connection. This recess has the double purpose of serving as a holding means for a testing adapter, and to decrease the heat required for brazing the pipe to the connector. The recess 410 of the embodiment of Fig. 4 can be used for all other embodiments of the connector according to the present invention.

The port openings 100, 300 are preferably fastened to the port openings of the brazed heat exchanger in the ordinary manner, i.e. during the brazing operation normally used to manufacture the brazed heat exchanger. An example of such a brazing procedure will be described below:

In a first manufacturing step of a heat exchanger according to the present invention, a number of heat exchanger plates are provided with a pressed pattern adapted to provide contact points between the plates and to keep the plates on a distance from one another under formation of interplate flow channels, the plates being provided with port openings allowing for selective communication with the interplate flow channels.

In a second manufacturing step, the heat exchanger plates are placed in a stack, wherein brazing material (e.g. copper) is placed between the plates.

In a third manufacturing step, the connections 100, 300 are placed coaxially with the port openings. Brazing material may be placed in the contact surfaces between the heat exchanger plate and the connections 100, 300. If inserts 400 are to be used in the connections, the inserts are preferably placed in the connections during or prior to this manufacturing step. Brazing material is preferably provided between the insert and the connection.

In a fourth manufacturing step, the stack of heat exchanger plates, including the connections, are brazed, i.e. heated to a temperature exceeding the melting temperature of the brazing material, such that the brazing material melts. The heating may be performed in a brazing furnace under vacuum or in a protective atmosphere, and the temperature should be sufficient to melt the brazing material. After the brazing operation, the heat exchanger is allowed to cool, such that the brazing material solidifies and provides a metallic bond between the contact points.

Optionally, the copper coating of the connections 100 may be carried out during the brazing operation in the fourth manufacturing step; for example, it is possible to manually coat the connections with a copper-containing paste containing e.g. copper powder. During the brazing operation, which preferably is performed in a vacuum environment, the copper in the copper paste will melt, hence coating the connection with copper.

Claims

1. A brazed plate heat exchanger (200) comprising a stack of a number of heat exchanger plates provided with a pressed pattern providing for contact points between the plates and keeping the plates on a distance from one another under formation of interplate flow channels, wherein port openings are provided in the plates, said port openings being in selective fluid communication with the interplate flow channels, and wherein connections (100, 300) communicating with the port openings are fastened on an end plate of the heat exchanger plate stack, characterized in that at least an inner surface (120) of at least one of the connections (100) is provided with a coating of copper or an insert (410) being covered with copper containing material.

2. The heat exchanger of claim 1, wherein the at least one connection is made from mild steel or carbon steel.

3. The heat exchanger according to any of the preceding claims, wherein the entire surface of the at least one connection (100) is provided with a coating of copper.

4. The heat exchanger according to any of the preceding claims, wherein at leas one of the connections (100) is provided with an insert (400) located between the connection (100) and a pipe to be brazed to the connection.

5. The heat exchanger of any of the preceding claims, wherein the inner surface (120) is roughened prior the copper coating procedure.

6. The heat exchanger of claim 5, wherein the inner surface (120) has a surface roughness of more than Ra 3.2.

7. Method for manufacturing a heat exchanger according to any of the preceding claims, comprising the steps of: providing a number of heat exchanger plates provided with a pressed pattern adapted to provide contact points between the plates and to keep the plates on a distance from one another under formation of interplate flow channels onto one another, the plates being provided with port openings allowing for selective communication with the interpolate flow channels; placing the heat exchanger plates in a stack, wherein brazing material is placed between the plates; placing connections (100, 300) coaxially with the port openings;

Optionally, placing brazing material in the contact surfaces between the heat exchanger plate and the connections (100, 300); brazing, i.e. heating to a temperature exceeding the melting temperature of the brazing material, the stack of heat exchanger plates, such that the brazing material melts;

Wherein the method is characterized by the further step of: