Heat Transfer
In sub-cooled boiling,

Very large vapour space is necessary
Bubbles from heating surface are absorbed by the mass of the liquid
Temperature of the heating surface is more than the boiling point of the liquid
Temperature of the heating surface is less than the boiling point of the liquid

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Heat Transfer
Vacuum is generally maintained in the vapour space of an evaporator mainly to

Concentrate heat sensitive materials
Achieve very high concentration of the final product
Facilitate forward feeding in multiple effect evaporation
Get economical temperature difference by using moderate pressure steam

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