Circulating water systems, cathodic

The use of sacrificial anodes in circulating water systems is limited to the application of cathodic protection to stop gates, coarse screens and other plant that are readily accessible so that the anodes can be replaced when they are consumed. Such anodes are not normally used in condensers, pumps and auxiliary coolers for the following reasons ... 

When coolers or condensers are shut-down but remain full of water, the amount of current required to maintain satisfactory cathodic protection is considerably reduced. If the current is not reduced over-protection occurs and excessive amounts of chlorine can be generated which would tend to accumulate in the upper section of the water boxes causing considerable corrosion, not only to the water boxes, but also possibly to the tubes. To ensure against this a stand-by condition should be included on the control panel which effectively reduces the current required under shut-down conditions. This control is effected by a limit switch fitted to the outlet valve of the condenser or cooler concerned. It is impossible to determine exact requirements for the protection of circulating water systems in advance and it is normal to adjust the current to provide protection during commissioning. 

Some ions are inhibitors and will reduce either the anodic or cathodic reactions that occur during the corrosion of aluminum. Examples are chromates (an anodic inhibitor) and phosphates (a cathodic inhibitor). Inhibition of circulating water systems is complex and professional consultation is recommended for the design of water treatment systems. 

Manually Controlled System A manually controlled system comprises one or more transformer-rectifiers each with its associated control panels which supply the d.c. to the various anodes installed in the water box spaces. Each transformer-rectifier is provided with its own control panel where each anode is provided with a fuse, shunt and variable resistor. These enable the current to each anode to be adjusted as required. Reference cells should be provided in order to monitor the cathodic protection system. In the case of a major power station, one transformer-rectifier and associated control panel should be provided for separate protection of screens, circulating water pumps and for each main condenser and associated equipment. 

Most of the common forms of corrosion of metals and alloys in a cooling system involve oxygen. The water is normally saturated with O2, and it is the rate of diffusion to a metal surface that is an important factor in the subsequent rate of corrosion reactions. The rate of diffusion is, in turn, affected by the circulating water velocity, the metal surface, temperature and microbiological foulants, etc. Oxygen is capable of accepting electrons and acts as a cathode in the electrochemical cell. At the cathode area, several reactions take place (as described in Section 4.1). 

The most efficient system devised by Monsanto uses electrodes fabricated from carbon steel plate, electro-coated on one face with cadmium. These are stacked in parallel so that the electrolyte can be pumped through the gap between successive plates. Overall tire system forms a series of electrochemical cells with a cadmium cathode and a carbon steel anode. Each plate of metal forms the cathode of one cell and the anode of the next in the stack. Electric current is passed across the stack. The electrolyte contains phosphate and borate salts as corrosion inhibitors, EDTA to chelate any cadmium and iron ions generated by corrosion together with hex-amethylenebis(ethyldibutylammonium) phosphate to provide the necessary telraal-kylammonium ions. This electrolyte circulates through the cell from a reservoir and there is provision for the introduction of acjylonitrile and water as feedstock. The overall cell reaction is ... 

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

Cathode-ray polarography has been very useful in a number of analyses of water used in reactor circulating systems. To determine the extent of contamination from some aluminum and steel containers, aluminum and iron were determined on 25 ml. samples of water after evaporation and ignition to determine the weight of the residue. The residues, weighing about 3 mg. each, were qualitatively identified as chlorides. The residues were dissolved in 5M HCIO4 and sodium acetate to obtain a pH of... 

Water continues to flow across the membranes when the current to the cells is shut off. This transfer dilutes the circulating caustic. A certain amount of dilution is useful, and the concentration can be allowed to fall into the range used for cell startup (27-32% NaOH is desirable). From that point, addition of concentrated caustic is necessary to maintain some minimum concentration. The most convenient design would include a system for return of product caustic to the circuit from a suitably lined tank. The caustic added must have a low iron content in order not to contaminate the circulating material and adversely affect performance of the cathode coating. 

For small systems, partial recirculation of the moist cathode exhaust air to the cathode inlet can be used for humidification (Fig. 4.25). In such a configuration, air is circulated at a high flow rate through the cathode compartment. Sufficient oxygen supply and water removal is achieved by opening the cathode exhaust valve, dependent upon the relative humidity in the cathode loop. 

Hence, unlike mobile electrolyte AFC, the electrolyte does not need to be pumped out. Moreover, there is no problem of an internal short-circuit, which makes it more favourable for space applications. The KOH electrolyte in an asbestos matrix possesses excellent porosity, corrosion resistance and strength. Due to the stationary nature of the electrolyte, it becomes mandatory that pure oxygen should be supplied at the cathode. In case CO2 is present in the air, carbonate formation takes place and the complete fuel cell needs to be rebuilt. Hydrogen is pumped at the anode and circulated to remove water. The cooling system comprises fluorinated hydrocarbon dielectric liquid. The product water is used for humidification of the cabin, cooking and drinking in space orbiter systems. As the water is used at the cathode, its level must be kept sufficiently high at the anode side. 

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