Process Equipment and Plant Design
In a rotary drier, the average retention time of solid is(where Z = length of the drier, metres ρ = apparent solid density, kg/m³ L = flow rate of dry solids, kg/see. m² drier cross-section H = hold up of solid.)

Z.H.ρ/L
H.ρ/L
L/Z.H.ρ
Z.H/L.ρ

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Process Equipment and Plant Design
Number of evaporators which can be used in service in a multiple effect evaporation system is limited by the

Both A & B
Practical limit reached, due to low heat transfer rate because of inadequate temperature difference
Neither A nor B
Economic limit reached, when the plant cost exceeds the saving of steam

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