Grain cleaners are agricultural machines that remove unwanted material from harvested grain before storage, milling, or sale. A combine harvester brings in wheat kernels mixed with chaff, straw pieces, weed seeds, dust, stones, and broken kernels. Cleaning matters because impurities reduce grain quality, increase spoilage risk, and can damage processing equipment.
A well adjusted grain cleaner improves food safety, market value, and storage life.
Key Facts
- Cleaning efficiency = mass of impurities removed / initial mass of impurities x 100%
- Clean grain recovery = mass of clean grain collected / initial mass of usable grain x 100%
- Air separation works because light chaff has a lower terminal velocity than dense grain kernels.
- Screen separation uses size differences: small particles pass through holes, while larger kernels or debris are retained.
- Mass balance: input grain mixture = clean grain output + screenings + dust and chaff losses
- Moist grain is harder to clean because kernels and impurities can stick together, reducing separation accuracy.
Vocabulary
- Hopper
- A funnel shaped container that feeds the grain mixture into the cleaner at a controlled rate.
- Scalper screen
- A coarse screen that removes large debris such as straw, pods, stones, or clumps before finer cleaning begins.
- Aspiration
- The use of moving air to lift away light material such as dust, chaff, and empty kernels from heavier grain.
- Screenings
- The separated waste material from a grain cleaner, including undersized kernels, weed seeds, broken pieces, and debris.
- Clean grain recovery
- The percentage of usable grain that exits the machine as cleaned product rather than being lost with waste.
Common Mistakes to Avoid
- Using only one screen size for every crop is wrong because wheat, corn, rice, and barley have different kernel sizes and shapes, so the screen must match the crop and impurity type.
- Increasing fan speed until all dust disappears is wrong because too much airflow can blow good kernels into the waste stream and reduce clean grain recovery.
- Ignoring feed rate is wrong because an overloaded cleaner forms thick grain layers that block air and prevent particles from reaching screen openings.
- Judging cleaning quality only by appearance is wrong because small weed seeds, cracked kernels, or hidden stones may remain and should be checked by weighing and sampling.
Practice Questions
- 1 A 1000 kg batch of harvested wheat contains 8% impurities by mass. After cleaning, 70 kg of impurities are removed. What is the cleaning efficiency?
- 2 A grain cleaner receives 1200 kg of mixture. It produces 1110 kg of clean grain, 60 kg of screenings, and the rest as dust and chaff. How many kilograms leave as dust and chaff, and what percent of the input is that?
- 3 A farmer notices many good wheat kernels in the chaff outlet after increasing the fan speed. Explain what adjustment should be made and why.