836Corrosion Control

The standard material of construction for heat exchangers is mild steel. Heat exchangers made of steel are the cheapest to buy because the material is the least expensive of all construction materials and because it is so easy to fabricate. However, when the heat transfer media are corrosive liquids and/or gases, more exotic materials may have to be used. Corrosion tables15 give the information necessary to estimate the life of the heat exchanger and life-cycle-costing studies allow

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Surface area

Figure 8.15 Cross-flow heat exchanger.

Surface area

Figure 8.15 Cross-flow heat exchanger.

valid comparisons of the costs of owning the steel heat exchanger versus one constructed of exotic materials. The problem is whether it will be cheaper to replace the steel heat exchanger at more frequent intervals or to buy a unit made of more expensive materials, but

requiring less frequent replacement. Mechanical designs which permit easy tube replacement lower the cost of rebuilding and favor the use of mild steel heat exchangers.

Corrosion-resisting coatings, such as the TFE plastics, are used to withstand extremely aggressive liquids and gases. However, the high cost of coating and the danger of damaging the coatings during assembly and during subsequent operation limit their use. One disadvantage of using coatings is that they almost invariably decrease the overall conductance of the tube walls and thus necessitate an increase in size of the heat exchanger. The decision to use coatings depends first upon the availability of alternate materials to withstand the corrosion as well as the comparative life-cycle costs, assuming that alternative materials can be found.

Among the most corrosive and widely used materials flowing in heat exchangers are the chlorides such as hydrochloric acid and saltwater. Steel and most steel alloys have extremely short lives in such service. One class of steel alloys that have shown remarkable resistance to chlorides and other corrosive chemicals is called duplex steels16 and consists of half-and-half ferrite and austenitic microstructures. Because of their high tensile strength, thinner tube walls can be used and this offsets some of the higher cost of the material.

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