Separation of different types of seeds, i.e. peas and grain, is necessary when these crops are grown mixed together on the same field, and thus mixed when harvested with the combine.
The two sizes of seeds are separated using screens with perforations adapted to the size of the seed kernels. The screen allows the small kernels to fall through the perforation and holds back the large kernels. The large kernels will be cleaned from impurities and dust during the first passage through the cleaning machine and thus ready for storage. The fraction with the small kernels will be mixed up with small impurities from the screen. To clean the small kernels, a second passage through the cleaning process is necessary.