Designing a rotary dryer involves several key calculations to ensure optimal performance, efficiency, and safety. Below is a step-by-step guide to the essential design calculations:
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1. Basic Parameters
Before starting, gather the following data:
– Feed rate (W₁) – Mass flow rate of wet material (kg/h).
– Initial moisture content (X₁) – Weight % of moisture in feed.
– Final moisture content (X₂) – Desired weight % after drying.
– Inlet air temperature (T₁) – Typically 150–600°C (depends on material).
– Outlet air temperature (T₂) – Usually 10–20°C above product temperature.
– Material properties – Bulk density, specific heat, particle size, heat sensitivity.
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2. Mass and Heat Balance
# (a) Moisture Removal Rate (M)
\[
M = W_1 \left( \frac{X_1 – X_2}{100 – X_2} \right) \quad \text{(kg/h)}
\]
# (b) Heat Required for Drying (Q)
\[
Q = M \cdot \lambda + W_2 \cdot C_p \cdot (T_{out} – T_{in}) + Q_{loss}
\]
Where:
– \(λ\) = Latent heat of vaporization (~2257 kJ/kg for water).
– \(W_2\) = Dry product flow rate = \(W_1 – M\).
– \(C_p\) = Specific heat of the material (~1.26 kJ/kg·K for many solids).
– \(Q_{loss}\) = Heat loss (~10–20% of total heat).
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3. Airflow Rate Calculation
# (a) Humidity Ratio
Assume ambient air humidity (\(H_a\)) and exhaust air humidity (\(H_e\)).
The mass flow rate of dry air (\(G\)) is:
\[
G = \frac{M}{H_e – H_a} \quad \text{(kg dry air/h)}
\]
# (b) Volumetric Airflow Rate
\[
V_{air} = G \cdot v \quad (\text{m³/h})
\]
Where \(v\) = Specific volume of air at inlet temperature (~0.8–1.2 m³/kg).
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4. Rotary Dryer Sizing
# (a) Diameter (D)
Based on allowable air velocity (\(U\), typically