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Drying Time Calculator: Constant & Falling Rate Kinetics engineering
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Drying Time Calculator: Constant & Falling Rate Kinetics

Calculate batch tray and conveyor drying durations, constant rate surface evaporation time, internal diffusion falling rate time, and total batch cycle time.

Moisture Contents (Dry Basis X)

0.40 dry basis = 28.6% wet basis.
Onset of falling rate regime.
Bound hygroscopic moisture limit.

Drying Flux & Tray Bed Loading

Surface evaporation flux under air velocity.

Drying Time Kinetics

Total Drying Time Required
-- hours
-- minutes batch cycle
Constant Rate Period (tc)
-- hrs
Surface Evaporation
Falling Rate Period (tf)
-- hrs
Internal Diffusion Control
Falling Rate Regime Fraction: --% of total time
Average Drying Rate: -- kg/h·m²
Water Removed per m²: -- kg H2O / m²
During the constant rate period, high air velocity accelerates drying; once below Xc, internal moisture diffusion governs and air velocity has almost zero effect.

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Frequently Asked Questions

What is the physical difference between the constant rate and falling rate drying periods?

In the constant rate period, the material surface is saturated with free unbound water, and drying proceeds at the same rate as an open pool of water governed by external air velocity and humidity. In the falling rate period (below critical moisture Xc), the surface dries out and internal liquid diffusion or capillary wick action dictates drying speed.

Why does increasing air velocity have no effect on drying time in the falling rate period?

Once drying enters the falling rate regime, the limiting resistance to mass transfer shifts from the external air boundary layer to internal solid diffusion. Increasing air flow or velocity wastes fan horsepower without accelerating moisture removal; only increasing temperature or reducing bed thickness will shorten time.

What is equilibrium moisture content (Xe)?

Equilibrium moisture content is the minimum moisture a hygroscopic solid can attain when exposed to air at a specific temperature and relative humidity. No amount of drying air can dry the material below Xe unless the air humidity is decreased or air temperature is increased.