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

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

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Heat Transfer
Heat transfer by conduction results due to the transfer of free electrons, kinetic energy & vibrational energy from one molecule to another. Conduction heat transfer can not take place

Both B & C
Between two bodies not in physical contact with each other
From one part of a body to the another part of the same body
Between two bodies in physical contact with each other

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Heat Transfer
A 10 cm dia steam pipe, carrying steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m.°C). It losses heat to the surroundings at 30°C. Assume a heat transfer co-efficient of 0.8 W/m².°C for heat transfer from surface to the surroundings. Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cms, the rate of heat loss from this insulated pipe will be

Greater than that for uninsulated steam pipe
Equal to that of the uninsulated steam pipe
Less than the steam pipe with 5 cms insulation
Less than that of the uninsulated steam pipe

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