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.)
A. $$\frac{{{\text{Z}}.{\text{H}}.\rho }}{{\text{L}}}$$
B. $$\frac{{\text{L}}}{{{\text{Z}}.{\text{H}}.\rho }}$$
C. $$\frac{{{\text{H}}.\rho }}{{\text{L}}}$$
D. $$\frac{{{\text{Z}}.{\text{H}}}}{{{\text{L}}.\rho }}$$
Answer: Option A

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