Process Equipment and Plant Design
In a double pipe (concentric) heat exchanger, the hydraulic radius for heat transfer (for a fluid flowing through the annulus) would be

Same as that for fluid flow
D₂ - D₁ (D₁ and D₂ are I.D. of inner and outer pipes respectively)
More than that for fluid flow
Less than that for fluid flow

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Process Equipment and Plant Design
Pick out the wrong statement pertaining to the design of a horizontal tube evaporator.

Liquor flows inside the tube, while the steam is outside submerging the tube
Its usual dimensions are: tube dia = 2-3 cms; evaporator body dia = 1-4 metres and evaporator height = 2.5-4 metres
It is suitable for process, in which the final product is a liquor instead of solid
It is unsuitable for concentrating those liquids, which form a scale or deposit salt

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