BACKGROUND

Heat exchangers in general, but mere specifically cupper, Cu, brazed plate type heat exchangers formed of a plural of surface structured stacked plates brazed together by copper, show severe problems with corrosion in some regions of the world. The present invention relate to forming a protection of the plates in order to reduce the observed corrosion problems.

Cathodic protection (CP) is a general used technique applied to structures to control the corrosion of a metal surface by making it into the cathode of an electrochemical cell. The simple idea of the protection is to connect the metal to be protected to a more easily corroded "sacrificial metal" to act as the anode. The sacrificial metal then corrodes instead of the protected metal. There are two basically two different of CP applied, one being to using passive galvanic ("sacrificial") anodes and the second impressed current systems (ICCP) using inert anodes where an external DC electrical power source is used to provide sufficient current. The last is typically applied for structures such as long pipelines, where passive galvanic cathodic protection is not adequate. When the devices to be protected are for domestic uses such as in heat exchangers for heating the water to be used in households, then hygienic becomes an issue, and for galvanic anodes magnesium anodes are an option where ICCP with advantage could make use of titanium anodes, mostly covered with mixed metal oxides (MMO).

However, the principles of CP can also be used in order to protect installations like water heating and storage tanks; these might be technical water systems, but also installations for drinking water in household applications. In the latter case, adverse effects of the CP system on the water quality must effectively be avoided.

There are systems available on the market for protecting storage heaters. These are also in use for protection of tube bundle heat exchangers. However, available systems will have to be adjusted to be used for plate heat exchangers, such as not to form an obstacle to the fluid, and at the same time leaving the exchangers with a compact design. .

SUMMARY OF THE INVENTION

The present system relate to a heat exchanger such as a plate type having an opening forming communication from fluid distributions to flow paths within the heat exchanger where an anode is inserted into said opening to provide galvanic protection to said heat exchanger.

In one embodiment the heat exchanger is connected to a power supply making the galvanic protection a impressed current cathodic protection having the advantage the material of the anode are not removed thus prolonging the life time before having to be exchanged.

The anode may be fixed to a back plate opposite to the opening in a manner where it does not interfere with the fluid connection. It optionally may be equipped with windings such that it may be fixed to the back plate by screwing, the back plate thus being equipped with the respective means to match the windings of the anode.

In an embodiment the protection operate by a Galvanic ("sacrificial") anode this having the advantage no power supply is needed. In an embodiment the anode is positioned in the area below and opening but is formed as a wire anode being spirally wound ensuring free flow of the fluids entering from the opening to the area where it is distributed to connected flow paths formed between the individual neighbouring heat exchanger plates.

Illustration of a plate type heat exchanger. Illustration of the catalytic process

Illustration of a plate type heat exchanger having an anode inserted through connection opening and having means for connection thereto,

Illustration of a plate type heat exchanger where an anode is fixed to the inner side of a back-plate of the heat exchanger.

Illustration of a plate type heat exchanger where an anode is fixed to auxiliary opening formed in a back-plate of the heat exchanger.

Illustration of a plate type heat exchanger where an anode electrically connected to an spirally formed electrical conductor reaching through an connection opening.

Fig. 7 Illustration of a plate type heat exchanger where an anode is fixed in a branch of a tap connected to the connection opening. DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 illustrate one kind of heat exchanger where to the present invention with advantage could be applied, the figure showing a plate kind heat exchanger (1 ) (PHEX) formed of a plural of structured plates (2) (formed with corrugations, dimples etc. as it is well known) arranged in a stack and brazed together in their contact areas, such as but not limited to Cu brazing. A top-plate (6), that often is unstructured and significantly thicker and more rigid than the structured heat exchanger plates (2), is positioned on top of the stack and comprise connection openings (3) forming inlets (T1 1 , T21 ) and outlets (T12, T22) communicating with a heating fluid circuit. Each heat exchanger plate (2) comprises openings (4) such that when stacked they form distribution spaces (5) (see e.g. Fig. 3) below the connection openings (3) for fluid communication to and from the flow paths formed by the plate structures between two neighbouring plates. The openings (4) are combined in pairs such that one pair form inlets and outlets for every second of the formed flow paths corresponding to the primary side with inlet T1 1 and outlet T12 to be connected e.g. to the heating fluid supply. The second pair then are connected to the other every second formed of the flow paths being at the opposing sides of the plates to the first pair of flow paths correspond to the secondary side with inlet T21 and outlet T22 to be connected e.g. to the cold fluid supply to be heated, e.g. domestic water.

For the stacked heat exchanger plate (2) embodiment the heat exchanger (1 ) and the rear side relative to the side of the top-plate (6) is equipped with a back- plate (12) often thicker and more rigid than the heat exchanger plates (2), like the top-plate (6). Some of the connection openings (3) and inlet/outlets (T1 1 , T12, T21 , T22) may be formed in the back-plate (12) rather than the top-plate (6),

It has been observed at least in some areas of Europe that such stacked plates brazed together by Cu show severe problems with corrosion and the present invention relate to forming a protection of the plates in order to reduce the observed corrosion problems based on cathodic protection.

Fig. 2 illustrate the basic principles of galvanic corrosion, that occurs if metals with different potentials are connected in an electrical leading way, e. g. by direct contact, while an electrolyte covering both metallic partners is closing the circuit.

In such a case, the less noble (7) metal delivers electrons to the more noble (8) metal and thus, goes into solution in form of metals ions. The "nobility" of a metal or an alloy depends on its potential in the electrochemical series.

Thus, galvanic corrosion means that the less noble material sacrifices itself for the more noble material. This is accurately the same in the application of cathodic protection, where willingly a less noble material is brought into contact with another (more noble) material in order to protect this. For the different metals or alloys, specific protection potentials are known which have to be reached in connection with the sacrificial material. Cathodic Protection (CP) requires some basic facts being present in order to result in a proper corrosion protection of the parts to be protected. These technical preconditions are described in the following.

First of all, the protected part must provide continuous metallic conductivity of all surfaces to be protected; this is e.g. the case for copper brazed heat exchangers as illustrated in Fig. 1 . Secondly, the electrolyte being in contact with the metallic surfaces to be protected must provide continuous electrolytic connection; in most of the domestic water throughout this is not a problem as the electrical conductivity usually is sufficiently high.

Regarding the density and distribution of protective current, as a simple comparison one can imagine the dispersion of current like the dispersion of light; in this example, the anode delivering the current would be the bulb "shining" with the electrons on the surface. As with light, there will also be "shaded" areas in the dispersion of the current; however, these shaded areas are not protected as good as areas in the "light". The current density is known from published tables and e.g. for copper the protective potential is -0.20 mV; stainless steel requires a protective potential between 0 and -0.10 mV.

This current density to obtain the needed protective potential has to be provided by the chosen anode.

Cathodic Protection is often accompanied by chemical changes of the media where it is installed. In the case of drinking water, no negative influences are accepted; this applies mainly for ions being transferred into the water by dissolving anodes, but also for the formation of gases, e. g. chlorine in water with high chloride content (chlorination).

One embodiment of Cathodic Protection (CP) is to use galvanic ("sacrificial") anodes. The protective current provided by a galvanic anode is produced by the chemical reaction of the anode with the protected cathode, leading to dissolution of the anode. The dissolved material will occur in the form of sludge. The speed of dissolution depends of the surface ratio between anode and cathode. A small anode area compared to the cathode surface will lead to fast dissolution of the anode; a big anode area compared to the cathodic area will increase the lift time of the anode.

The driving force for the chemical reaction between anode and cathode is the difference in potential as mentioned above. For technical purposes, protection potentials are formulated in standards. In principal, every metal can act as an anode for a more noble metal; in practice, however, some un-noble materials are widely used for production of anodes. The anode materials being widely used are aluminium, zinc and magnesium. Looking at Table 1 it is easily to see that these metals are the least noble metals available. There is also beryllium, but this is a quite seldom metal being far too expensive for being used as anode material.

Because of physiological and toxicological reasons magnesium anodes are to be used in drinking water installations, but magnesium is also favourable from an electrochemical point of view. The values for the theoretical "current content" of magnesium is around 2185 Ah/kg, while the practical "current content" is around 1 100 Ah/kg which fits well the 1230 Ah/kg (originally "Amp-hrs per kg") for Mg mentioned in table 1 below showing properties of some materials for sacrificial anodes.

Table 1

Anodes are available in different forms, as rods, bullets, plates, wires and wire mesh. The form is not crucial and can be adjusted to the design requirements of the part to be protected.

Regarding installation of the anodes, it can be chosen whether the anodes are installed by an electrical leading mounting, e. g. a metal thread, or in an electrically isolated mode. In the latter case, the anode must be connected by a wire with the part to be protected.

Though "sacrificial" anodes offer a cheap and easy way to protect heat exchangers they also have some drawbacks. First of all they will disappear with time, the right point in time must be found to check the anode and to possibly exchange a used-up anode. In drinking water storage tanks, the usual period for checking the anodes is 2 years. Anodes must be replaced when approx. 1/3 of the initial volume is left. There is no reliable formula to calculate the lifetime of an anode; it is still suggested to check anodes in distinct intervals for their appearance.

It must be expected that the dissolution of the anode is faster than in stagnant water as corrosion products forming kind of a protective layer on the surface are most likely removed quite fast and will not accumulate on the anode surface in a plate type heat exchanger.

Furthermore, the very compact design of plate type heat exchangers makes it virtually difficult for the big anodes fit in. Thus, the size of anodes will most likely be less than in "normal" storage heaters what will additionally limit the lifetime of the anodes. Therefore an alternative and for many purposes a more suitable solution for plate heat exchanger applications would be ICCP (active cathodic protection). Inert anodes being used for impressed current CP (ICCP) do not "sacrifice" themselves but last virtually untouched for a long time. They must be installed being electrically isolated from the metallic part to be protected. The protective current is delivered by a power supply and not by chemical reaction of an anode.

Inert anodes for ICCP have to be installed electrically isolated from the part to be protected. The protective current is going through the medium between anode and metal surface of the part. Therefore, the medium must have some electrical conductivity as already mentioned above. Inert anodes for ICCP consist of noble metals, e. g. titanium; some of them are equipped with coatings of noble metals, some have mixed metal oxide (MMO) coatings. They are available in different shapes like sacrificial anodes; often relatively thin wires are used. This is possible as the anode geometry (volume) will not be changed under service.

As the current may e.g. be taken from the local power line, there must be an apparatus to reform the current to the needed type and strength; these are the so-called rectifiers. There are different types of rectifiers available on the market; ICCP can be used as constant current system, as constant voltage system and as self-controlled systems adjusting the protective current to the actual needs of the part to be protected. The rectifier must fit in its power to the type and material of the part to be protected. In the following non-limiting example a cathodic protection (CP) in copper brazed plate heat exchangers is discussed.

Major problems are caused by the specific plate heat exchanger conditions, which are their advantages compared to storage heaters: Small dimensions, big surface, compact structure. These properties make it difficult to install a proper cathodic protection.

Furthermore, most of the storage heaters available on the market are internally protected against corrosion, either by enamel layers or organic coatings. Thus, the CP system is only used to protect the "failures" of the coating which are usually below 1 %. Vice versa, a CP system in a plate type heat exchanger must protect the whole internal surface of the part.

The anodes (9) can be placed in the space (5) below the connection openings (3)of the plate type heat exchanger formed by openings (4), where Fig 3 illustrate an anode (9) being inserted into the space (5) having means for connecting it to the heat exchanger (1 ), where these means in the illustrated embodiment is formed as a fastener (10) adapted to be fixed to the opening (3) area, e.g. to a tap reaching out from the top-plate (6) or to the top-plate (6) itself. The parts being connected may optionally be winded to secure a fixed connection. The fastener (10) may form a conductive connection of the anode (9) and / or heat exchanger (1 ) to a power grid. The fastener (10) comprises a path forming fluid communication between the space (5) and the heating circuit connected to the connection openings (3) where the path may distribute the fluid to the external side of the anode (9) or communicate with a flow communication path of the anode (9). The anode (9) has a width smaller than that of the openings (4) of the space (5) such that fluid may be communicated along its external surface to and from the flow paths formed between the neighbouring heat exchanger plates (2), all of this formed in such a way still allowing water flow through the connection openings (3).

In an embodiment as illustrated in Fig. 4 a piece (1 1 ) is formed or fixed on the inner side of the back endplate (12) of the plate type heat exchanger reaching into a space (5) and the anode then may be fixed to this piece (1 ) such as by forming an internal winded hole in the bottom of the anode (9) adapted to match similar windings of the piece (1 1 ) such that the two parts are fixed together by screwing. The anode (9) formed such that is shorter than the height of the space (5) and thus does not interfere with the fluid communication through connection openings (3), just as it has a smaller width than the space (5).

A further problem might be the turbulent flow in the intake area. This will most likely remove corrosion products on the anodes quite fast; this makes the anode working, but on the other hand, anodes will not be protected in any kind and "corrosion", i.e. dissolution of the anode, will happen with maximum speed. It is for the time being not possible to give any figures or calculations as there is no experience in this regard. Basically, the requirements for placing an inert anode for an ICCP system are the same as for a galvanic anode. However, inert anodes can be a bit smaller than galvanic anodes as their power is not depending of the volume. Thus, places providing space for installation of galvanic anodes are suitable for inert anodes too.

For plate type heat exchangers it might be favourable to form an auxiliary opening (14) in connection to the space (5) and formed in the top- (6) or back- (12) plate not having the connection opening (3) forming fluid connection to the space (5). In the illustrated embodiment the auxiliary opening (14) is formed in the back-plate (12). In the same manner as the fastener (10) the anode (9) then could comprise a connector (13) as seen in Fig. 5 with on the rear side being fixed in or to the auxiliary opening (14) sealing it. The fixation in the usual manner may be by winding. Thus, the inert anode wouldn't interfere with the water connection and it would be easy to connect the anode electrically through the connector (13) that could form part of the electrical connection from the power grid to the anode (9) and/or heat exchanger (1 ). The auxiliary opening (14) may be formed such that the anode (9) may be positioned into the space (5) through it, this also making it easy to access the anode (9) e.g. when to be replaced.

Fig. 5 further illustrates an embodiment where the anode (9) is formed as a spiral leaving free space for the fluids to flow within the space (5). This shape could also apply to any other embodiment of fixing the anode (9) in the space (5), either as illustrated or not illustrated in the present document.

Fig. 6 illustrate an embodiment where an electric wire (15) is inserted through the connection opening (3) to contact an anode (9) within the space (5) (the anode not seen in the figure) to the power grid. The electric wire (15) may be spirally formed as in the illustration.

Another possibility is to add an L-piece (or just a bend piece) with inert anode (9) installed on the (front side) connection opening (3), see Fig. 7. The connection opening (3) comprises a tap with a branch (16) where the anode (9) may be attached by fixing means (17) e.g. similar connection (13) means as that of a previous embodiment and / or where the fixing means (17) itself form part of the anode (9). The anode (9) then further may comprise the L-piece reaching through the connection opening (3) into the opening (5), where it may be fixed at the bottom. This L-piece may be formed as a thin wire. There will be no lifetime limitations using impressed current systems with inert anodes. Titanium anodes or MMO/Mixed Metal Oxide anodes will last for decades. There might again be some influence of the turbulent water current in this area. The electrical power could be of any kind of imaginable source like the public electricity network, a solar cell, a battery, a fuel cell etc.