Process Equipment and Plant Design
In a double pipe concentric heat exchanger, the equivalent diameter of annulus for heat transfer would be (where, D₁ and D₂ are I.D. of inner and outer pipes)

D₂ - D₁
(D₂² - D₁²)/D₂²
None of these
(D₂² - D₁²)/D₁²

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Process Equipment and Plant Design
Ponchon Savarit method is based on the use of enthalpy concentration diagram, which contains the bubble point curve (saturated liquid curve), dew point curve (saturated vapour curve) and equilibrium tie lines. As compared to McCabe-Thiele's method, this method

Accounts for the enthalpy changes in the process
All of these
Facilitates direct calculation of heat load on reboiler & condenser from, the diagram used in this method
Is more accurate in finding the number of equilibrium stages

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