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

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

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Process Equipment and Plant Design
In the case of a shell and tube heat exchanger, the logarithmic mean temperature difference

Is always less than arithmetic mean value, but more than geometric mean value
Is always less than arithmetic average value
Is always more than arithmetic mean value and the geometric mean value
May be either more or less than geometric mean and arithmetic mean value depending upon whether the flow of stream is co-current or counter-current

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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

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
Results in higher shell side pressure drop
Creates relatively lower turbulence on the shell side resulting in lower shell side heat transfer co-efficient

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