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MATERIALS OF CONSTRUCTION
composition 50 per cent SiO 2 , 40 per cent Al 2 O 3 , balance CaO and Fe 2 O 3 , are used for
general furnace construction. Silica can exist in a variety of allotropic forms, and bricks
containing a high proportion of silica undergo reversible expansion when heated up to
working temperature. The higher the silica content the greater the expansion, and this
must be allowed for in furnace design and operation.
Ordinary fire bricks, fire bricks with a high porosity, and special bricks composed of
diatomaceous earths are used for insulating walls.
Full details of the refractory materials used for process and metallurgical furnaces can
be found in the books by Norton (1968) and Lyle (1947).

7.11. CARBON
Impervious carbon, impregnated with chemically resistant resins, is used for specialised
equipment; particularly heat exchangers. It has a high conductivity and a good resistance
to most chemicals, except oxidising acids, of concentrations greater than 30 per cent.
Carbon tubes can be used in conventional shell and tube exchanger arrangements; or
proprietary designs can be used, in which the fluid channels are formed in blocks of
carbon; see Hilland (1960) and Denyer (1991).

7.12. PROTECTIVE COATINGS

A wide range of paints and other organic coatings is used for the protection of mild steel
structures. Paints are used mainly for protection from atmospheric corrosion. Special
chemically resistant paints have been developed for use on chemical process equipment.
Chlorinated rubber paints and epoxy-based paints are used. In the application of paints
and other coatings, good surface preparation is essential to ensure good adhesion of the
paint film or coating.
Brief reviews of the paints used to protect chemical plant are given by Ruff (1984) and
Hullcoop (1984).

7.13. DESIGN FOR CORROSION RESISTANCE
The life of equipment subjected to corrosive environments can be increased by proper
attention to design details. Equipment should be designed to drain freely and completely.
The internal surfaces should be smooth and free from crevasses where corrosion products
and other solids can accumulate. Butt joints should be used in preference to lap joints.
The use of dissimilar metals in contact should be avoided, or care taken to ensure that
they are effectively insulated to avoid galvanic corrosion. Fluid velocities and turbulence
should be high enough to avoid the deposition of solids, but not so high as to cause
erosion-corrosion.

7.14. REFERENCES
BAINES, D. (1984) Chem. Engr., London No. 161 (July) 24. Glass reinforced plastics in the process industries.
BENDALL,K. and GUHA, P. (1990) Process Industry Journal (Mar.) 31. Balancing the cost of corrosion resis-
tance.