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

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
Flow at a very slow velocity
Preferably flow inside the tube for its easier internal cleaning

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

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