A drying-load calculation first fixes whether inlet and outlet moisture are wet basis or dry basis. Conserving dry-solids flow gives product rate and water removal per unit time; sensible heat, evaporation heat, gas duty and real-system losses are then evaluated together.
What are wet-basis and dry-basis moisture?
| Basis | Definition | Equation (decimal moisture ratio) |
|---|---|---|
| Wet basis, X_wb | Water mass / total wet mass | X_wb = m_w / (m_w + m_ds) |
| Dry basis, X_db | Water mass / dry-solids mass | X_db = m_w / m_ds |
FAO references also distinguish these bases explicitly. Convert percentages to decimal ratios by dividing by 100 before using the equations.
How are the moisture bases converted?
The two values for one sample are different. Conversion equations are X_db = X_wb / (1 − X_wb) and X_wb = X_db / (1 + X_db). Use one basis consistently and state it beside every reported moisture value.
The common error
Subtracting a wet-basis inlet value directly from a dry-basis outlet value does not produce a physical water balance. Convert both to one basis first.
Dry-solids balance
For a basic steady-state balance with no reaction or solids loss, dry solids are conserved through the dryer. With wet-basis inlet moisture, ṁ_ds = ṁ_feed × (1 − X_wb,in). Product flow at the outlet target is ṁ_product = ṁ_ds / (1 − X_wb,out).
Dust loss, reaction, volatile solids or product decomposition must be added as separate streams when relevant.
Water evaporation duty
Water removal can be written from total streams as ṁ_evap = ṁ_feed − ṁ_product. Using dry-basis moisture, the equivalent expression is ṁ_evap = ṁ_ds × (X_db,in − X_db,out).
Unit check
If feed is in kg/h, the result is kg water/h. Keep every mass term in the same unit; enter moisture as a decimal ratio, not as a percentage number.
How is preliminary heat duty structured?
Sensible heat is written Q_sens = ṁ × Cp × ΔT and phase-change duty Q_evap = ṁ_evap × h_fg. A real balance includes heating dry solids, inlet water and process gas, water phase change, product and exhaust outlets, and environmental losses.
Why is no single value published?
Cp, h_fg, gas humidity and losses vary with temperature and operating conditions. Without material tests and design conditions, a specific-energy, burner-rating or efficiency figure would not be reliable.
Material properties that affect drying duty
- Free and bound moisture distribution
- Particle size, shape and surface area
- Porosity and internal moisture diffusion
- Stickiness, agglomeration and crust formation
- Thermal sensitivity and possible phase changes
- Bulk density and flow behaviour
- Abrasiveness and dust generation
Why does water load not directly give equipment size?
Water load is fundamental, but required drum volume and contact area depend on drying kinetics, residence-time distribution, internals, gas velocity, pressure loss, fill and temperature limits.
| Calculation result | Next verification |
|---|---|
| Dry-solids flow | Transport, fill and product collection |
| Water evaporation rate | Drying test and gas humidity capacity |
| Theoretical heat terms | Real losses, heat source and control range |
| Target outlet moisture | Sampling, measurement method and product stability |
Engineering checklist
- State basis and method for every moisture value
- Close the dry-solids balance
- Cross-check evaporation with both equivalent forms
- Separate normal and design flow rates
- Measure drying kinetics on representative samples
- Define product and exhaust temperature limits
- Add gas, fan, filter and duct balances
- Document site losses and design margins
This guide is a preliminary engineering framework; it does not create an equipment guarantee or quotation value by itself.
