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

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Process Equipment and Plant Design
A forced circulation long tube vertical evaporator as compared to the natural circulation evaporator

Employs high velocity, high heat transfer rate and less heating surface requirement
Employs a centrifugal pump placed between external downtake from vapor drum and inlet to tube bundle
Is economical in operation
Both B & C

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