Calculate drying rate as moisture content lost per hour or minute from initial and final moisture and drying time, or solve for the drying time.
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Drying Rate Formula
The calculator uses a constant drying rate based on the change in moisture content over time. Moisture content is entered as a percentage, so the drying rate is expressed in percentage points per unit of time.
- DR = drying rate
- Mi = initial moisture content, in percent
- Mf = final moisture content, in percent
- t = drying time
Select the value to solve for. Rate mode divides initial minus final moisture content by elapsed time. Initial mode adds rate × time to the final content; final mode subtracts it from the initial content. Time mode divides the moisture change by the signed nonzero rate and requires a positive elapsed time. Use one consistent moisture basis: wet basis is 0–100%; dry basis may exceed 100%.
Drying Rate per Exposed Area
Select Drying rate per area to convert a measured dry-basis moisture change into an average water-removal flux. Enter the dry-solid mass, exposed area, initial and final dry-basis moisture percentages, and elapsed time. The result is in kg water/(m²·h), with total water removed and kg/h shown separately.
Here Ms is dry-solid mass in kg, A is exposed drying area in m², Δt is elapsed time in hours, and X is kg water per kg dry solid. The calculator divides the entered dry-basis percentages by 100 to obtain X. Dry-solid mass and exposed area are assumed constant. If moisture is reported on a wet basis, first convert its fraction w to X = w / (1 − w); wet-basis and dry-basis percentages are not interchangeable.
Example: Average water removal per area
For 10 kg of dry solids, 2 m² of exposed area, and a drop from 50% to 10% dry-basis moisture over 4 hours, water removed is 10 × (0.50 − 0.10) = 4 kg. The average drying flux is 4 / (2 × 4) = 0.5 kg water/(m²·h), and the water-removal rate is 1 kg/h.
Does this calculate a drying curve?
Two moisture readings determine an interval average. They do not establish the instantaneous drying rate, a critical moisture content, or a material-specific drying curve. A measured curve needs readings at several times under known drying conditions. Plot moisture content against time, then estimate interval rates using R = −(Ms/A) × ΔX/Δt. Plot those rates against moisture content to examine any constant-rate and falling-rate periods. The calculator does not assume either period from two endpoints.
Drying Rate Units and Result Checks
The calculator converts time and drying rate units internally, so matching units is not required. The table below shows equivalent rates.
| Drying rate | Equivalent in %/min | Equivalent in %/hr |
|---|---|---|
| 0.001 %/s | 0.06 %/min | 3.6 %/hr |
| 0.01 %/s | 0.6 %/min | 36 %/hr |
| 0.1 %/s | 6 %/min | 360 %/hr |
Use this table to check whether the result makes physical sense.
| Result | Meaning |
|---|---|
| Positive drying rate | Final moisture content is lower than initial moisture content. |
| Zero drying rate | Moisture content did not change during the time period. |
| Negative drying rate | Final moisture content is higher than initial moisture content, so the material gained moisture. |
Example Problems
Example 1: Calculate drying rate
A material starts at 40% moisture content and ends at 25% after 5 hours.
The drying rate is 3 %/hr.
Example 2: Calculate final moisture content
A product starts at 18% moisture content and dries at 0.2 %/min for 30 minutes.
The final moisture content is 12%.
FAQ
Is drying rate the same as percent moisture removed?
No. A moisture-content change is measured in percentage points, while a relative percentage change uses the initial content as its denominator. Dropping from 30% to 20% changes moisture content by 10 percentage points. Over 2 hours, the average rate is 5 percentage points/hour. This does not directly give the mass of water removed.
Why does the calculator use percentage points?
The calculation subtracts one moisture percentage from another. A drop from 40% to 30% is a 10 percentage point drop, not a 10% relative decrease. The drying rate is therefore percentage points per second, minute, or hour.
Can the drying rate be negative?
Yes. A negative result means the final moisture content is greater than the initial moisture content. That usually indicates moisture gain, reversed input values, or a process that is not actually drying during the time interval entered.
