Heat Transfer
The actual temperature drop across the heating surface of an evaporator depends on the

Liquid depth over the heating surface
Pressure difference between the steam chest and the vapor space above the boiling liquid
All of these
Solution being evaporated

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Heat Transfer
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because

Total heat transfer area of all the effects is -less than that in a single effect evaporator system
Heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system
Total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect
Boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system

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