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.)

H.ρ/L
Z.H.ρ/L
L/Z.H.ρ
Z.H/L.ρ

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