Process Equipment and Plant Design
Corrosion allowance in the design of pressure vessel/chemical equipment is not necessary, if

Material of construction is high alloy steel
Plain carbon steel and cast iron parts are used
Wall thickness is > 30 mm
Both B & C

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Process Equipment and Plant Design
In a shell and tube heat exchanger for a given heat transfer surface area, smaller diameter tubes are favoured as compared to larger diameter ones ; because the smaller diameter tubes

Are easier to clean
None of these
Are less prone to fouling
Can be fitted into a smaller shell diameter hence the cost of the heat exchanger would be less

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

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