Process Equipment and Plant Design
The necessary wall thickness for a metallic storage vessel is a function of the

Diameter of the vessel
Ultimate tensile strength (or yield point) of the material and operating temperature
All of these
Operating pressure and welding/joint efficiency

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

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