How to Calculate Dryer Evaporation Load from Moisture Content
Dryer capacity is often quoted as tonnes per hour, but wet feed, dry solids, finished product and water evaporation are different quantities. A water-removal calculation helps establish the process duty before a belt dryer or another dryer is selected.
Use a consistent moisture basis
Wet-basis moisture is the mass of water divided by the total wet material mass. Dry-basis moisture is the mass of water divided by the dry solids mass. They are not interchangeable. For the equations below, express both inlet and outlet moisture as wet-basis fractions: 60% becomes 0.60.
Calculate dry solids, product output and water removal
Let F be wet feed in kg/hour, x be inlet wet-basis moisture and y be final wet-basis moisture. Assuming steady operation, conserved dry solids and water as the only component removed:
- Dry solids, S = F × (1 − x).
- Finished product, P = S ÷ (1 − y).
- Water evaporated, E = F − P.
These equations give a mass balance. They do not determine drying time, belt area, fuel consumption or installed motor power.
Worked example: 1,000 kg/hour from 60% to 10% moisture
This is an illustrative calculation, not a GEM equipment rating. With F = 1,000 kg/hour, x = 0.60 and y = 0.10:
- Dry solids = 1,000 × 0.40 = 400 kg/hour.
- Finished product = 400 ÷ 0.90 = 444.4 kg/hour.
- Water evaporation = 1,000 − 444.4 = 555.6 kg/hour.
The incoming water is 600 kg/hour. Approximately 44.4 kg/hour remains in the product and 555.6 kg/hour is removed. The finished product is therefore not 400 kg/hour: it still contains water.
Convert dry-basis measurements before using the equations
If dry-basis moisture is reported as a fraction d, the corresponding wet-basis fraction is d ÷ (1 + d). For example, 50% dry-basis moisture means d = 0.50 and wet-basis moisture is 0.50 ÷ 1.50 = 33.3%. Ask the laboratory to identify its reporting basis.
Why evaporation load is only the starting point
Two materials with the same water-removal duty can require different drying areas and residence times. Particle size, bed permeability, sticking, allowable product temperature and the final-moisture target influence selection. A heat balance must also account for heating the feed, exhaust conditions, losses and the proposed heat source.
If solids are lost, additives are introduced, reactions occur or liquids other than water evaporate, extend the balance accordingly. For variable feed, calculate representative and peak conditions rather than relying only on an average.
Information to include in a dryer enquiry
Send the material identity, wet feed rate, operating hours, inlet and target moisture with their basis, particle size, bulk density, temperature limits and available utilities. Mark estimated values clearly.
Use the equipment and operating cost checklist when comparing proposals. Share your drying duty with GEM Drytech and discuss pilot trials when material behaviour or product quality needs to be established.
