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

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

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Heat Transfer
In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred

Through a metallic wall
By direct contact of hot flue gas with air
None of these
By heating an intermediate material (like chequor bricks) and then heating the air from this hot material

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