Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1986-05-08
1989-01-24
Tung, T.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204147, 165166, 165186, C23F 1300
Patent
active
048000078
DESCRIPTION:
BRIEF SUMMARY
This invention relates to corrosion protection for a heat exchanger in which a number of metal walls delimit heat exchanging flow channels for heat emitting media and heat absorbing media, respectively, which flow channels extend between inlet channels and outlet channels for respective media, the corrosion protection at least comprising one cathode placed in the flow way for the corrosive medium within the area of the inlet channel and one cathode within the area of the outlet channel for the corrosive medium, the cathodes being isolated in relation to the metal walls which constitute an anode in a direct current circuit, a control reference electrode being arranged to influence the amperage in the mentioned direct current circuit.
Anodic corrosion protection of this kind is known since long. The basic principle means that a passive oxide film is formed on the surface of the metal by maintaining an anodic potential to the same. The metal surface is passivated by putting on the same a current of relatively high density by way of introduction. As the mentioned oxide film is built up the potential increases between the mentioned control reference electrode and the metal surface, i.e. the anode. A relatively low current density is then sufficient for maintaining the potential corresponding to a passive metal surface. If the potential should be brought to increase beyond the passive potential area, the metal surface gradually begins to corrode. Different steel qualities have somewhat different passive potential areas.
The state of things appears from the enclosed FIG. 1. Usually the system is so constructed that the necessary amperage is brought about by means of an apparatus which is influenced by a control unit with the control reference electrode as a detecting means. Thus, a value for the potential measured by the control reference electrode is preset and the apparatus provides an amperage striving to bring about the wished potential. In apparatuses of the kind mentioned by way of introduction where the control reference electrode is usually placed relatively close to a cathode, it may happen that the potential measured by the control reference electrode reaches the preset value without the metal surfaces of the whole heat exchanger being passivated. This is of course a great disadvantage and may lead to a quick corrosion of the metal when there are strongly corrosive media like concentrated sulphuric acid of high temperature, for instance 110.degree. C.
The purpose with this invention is to remove this drawback and bring about a corrosion protection of the kind mentioned by way of introduction which is reliable and secure. According to the invention such a corrosion protection is provided by the fact that the control reference electrode is arranged in a separate flow way for the corrosive medium and is spaced from the cathodes by distances such that the corrosion protection of all parts of the heat exchanger which are to be protected is secured.
Corrosion protection of the kind mentioned by way of introduction can be used in different types of heat exchangers such as heat exchangers. The separate flow way can be designed in different ways, for instance so that the corrosive medium is conducted in a partial stream to a collecting tank or returns to the actual process. In a particularly suitable embodiment of the invention the separate flow way, however, comprises a pipe line arranged like a shunt between the inlet channel and the outlet channel for the corrosive medium.
The invention shall be more closely described in the following with reference to the two accompanying figures, of which:
FIG. 1 shows an ideal, anodic passivation curve and
FIG. 2 shows, schematically, a plate heat exchanger with anodic corrosion protection according to the invention.
FIG. 1 has already been explained above. In FIG. 2 supporting plates 1 and 2 are kept together by bolts 3. Between the supporting plates 1 and 2 there are a number of heat exchanging metal plates 4 which n this case are welded together two and two such that he
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patent: 3841988 (1974-10-01), Gleason
patent: 4376753 (1983-03-01), Lucas
patent: 4588022 (1986-05-01), Sanz
"Anodic Protection of Welded Plate Heat Exchangers", Alfa-Laval, pp. 1-4, 1981.
Berthagen Leif
Karlsson Bertil
Alfa-Laval Thermal AB
Hapgood Cyrus S.
Tung T.
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