Heat Transfer
In which mode of heat transfer, the Biot number is important?

Radiation
Forced convection
Transient heat conduction
Natural convection

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

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

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Heat Transfer
Extended heat transfer surface like fins are used to increase the heat transfer rate. Fin efficiency is defined as the ratio of heat transferred across the fin surface to the theoretical heat transfer across an equal area held at the

Average temperature of the fin
Temperature of the fin end
Surrounding temperature
Constant temperature equal to that of the base

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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

Heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system
Boiling point elevation in a single effect system is much higher 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
Total heat transfer area of all the effects is -less than that in a single effect evaporator system

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