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

Need not be provided for non-pressure parts like tie rods, spacers, baffles, supports etc
All of these
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

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Process Equipment and Plant Design
When one of the fluids is highly corrosive and has fouling tendency, it should

Flow at a very slow velocity
Preferably flow inside the tube for its easier internal cleaning
Flow outside the tube, when the flow is counter-current and inside the tube when the flow is co-current
Perferably flow outside the tube

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