Process Equipment and Plant Design
In the design of a shell and tube heat exchanger, the corrosion allowance

All of these
For internal cover and tube sheet is provided on both the sides
Need not be provided for non-pressure parts like tie rods, spacers, baffles, supports etc
For carbon steel and cast iron pressure parts is 1.5 mm (except for tubes) and for severe conditions it is 3 mm

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Process Equipment and Plant Design
In a shell and tube heat exchanger, triangular pitch arrangement as compared to square pitch arrangement

Results in higher shell side pressure drop
Can accomodate less number of tubes for a given shell diameter
Facilitates easier shell side cleaning; hence is more suitable for handling high dirt factor shell side fluid
Creates relatively lower turbulence on the shell side resulting in lower shell side heat transfer co-efficient

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