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

All of these
For carbon steel and cast iron pressure parts is 1.5 mm (except for tubes) and for severe conditions it is 3 mm
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

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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 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
Is always less than arithmetic average value
Is always less than arithmetic mean value, but more than geometric mean value

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