In-store grain drying: high-temperature and near-ambient air approaches
Learn about the two basic methods of grain drying – high-temperature and near-ambient air drying – including the pros and cons associated with each approach. You can also access our grain drying review from these pages.
High-temperature grain drying
- Air heated to at least 40°C
- Heat generated from oil or gas
- Uses batch or continuous dryers
- Generally, grain is moved to limit overdrying and heat damage
Capital costs: High
Speed: Fast – grain is in a shallow layer with high airflow
Management skills: Follow manufacturer’s instructions
Weather: Independent of weather conditions
Temperatures: Max of 65°C at 20% MC, reducing by 1°C for every 1% increase in initial MC
For feed grain: Max of 120°C for 1 hour or 100°C for 3 hours
Spoilage risks: Low risk of slow drying, risk of overheated grain and some risk of overdrying
- Maltsters, millers and seed producers require that grain temperatures do not exceed 50°C
- Do not leave grain in the dryer for too long
- Ensure adequate airflow
- After drying, cool grain to prevent breeding of insects and mites
- Rapeseed becomes brittle at low MC, so overdrying can be a problem
- Free-fatty-acid content increases rapidly in broken seed and may cause oil degradation after crushing
If a direct drying system uses an oil-fired energy source, manage the hydrocarbon contamination risks. Ensure that the fuel meets commercial ISDN/ISO standards and that there is efficient combustion (by setting air:fuel ratios to manufacturer’s recommendations). Provide adequate ventilation to prevent the recirculation of burner exhaust gases into the intake cowling to reduce taint risk. An additional risk is the development of polycyclic aromatic hydrocarbons (PAHs), if combustion is incomplete.
Near-ambient air grain drying
- Used for bulk grain stored in bins or on floor
- Air, up to 5°C warmer than the grain, is blown through the bulk
Capital costs: Low
Speed: Slow – typically, 10 days with recommended airflows of 180 m3/hr/tonne or 6,357 ft3/hr/tonne
Management skills: Need to respond to moisture content and weather conditions
Weather: Wet weather slows drying
Moisture content: If initial moisture content is high, drying capacity is reduced
Spoilage risks: Higher risk of slow drying, low risk of overheated grain and some risk of overdrying
- Drying occurs in a layer that develops at the air intake, before moving through the bulk – grain ahead of the drying zone remains wet and may also be warm, increasing the risk of spoilage
- Individual seeds present different resistances to airflow, so bed depth is a critical factor – check airflow is adequate by taking measurements at several points using an anemometer
In most years, on-floor drying can be effective and economical. However, in some seasons, it will incur considerable costs and may not achieve good enough results. Grain stirrers can be used to mix the grain vertically, resulting in a mix of dry and undried layers. An AHDB-funded project demonstrated that stirring reduces the length of time that the upper layers of the grain spend at the initial moisture content – providing there are sufficient stirring augers to be effective for the whole bed.
Spoilage risk increases as grain depth exceeds a fan’s design maximum – as airflow reduces, it slows the advance of the drying zone. If grain is normally stored at 2.8 m deep, this depth should be reduced by 0.5 m for each percentage point increase in initial grain moisture above 20%.
Grain moisture calculator
Use the AHDB grain moisture calculator to estimate the weight loss associated with drying 1 tonne of cereals. Simply enter the ‘initial moisture’ and ‘final moisture’ values from representative samples of grain. Alternatively, use the look-up table for a range of specific values.
Grain drying and energy efficiency review 2021
In cereal production, most energy consumption for fixed equipment is that needed for grain drying, cooling, and storage, with heat being the largest energy use. Produced as part of the AHDB GrowSave programme, a review of grain drying technologies was completed in 2021. The review looks at the technology/approaches and the opportunity to reduce energy use and the carbon footprint of the sector.