Kroulík M., Hůla J., Rybka A., Honzík I. (2016): Pneumatic conveying characteristics of seeds in a vertical ascending airstream. Res. Agr. Eng., 62: 56-63.Pneumatic conveying characteristics were measured in vertical tubes for seeds of selected varieties of cereals, wide-row crops, oil crops, legumes and catch crops. The measured values were used for graphical representation of variation curves for the chosen groups of seeds. The values of the critical air velocity for seeds (velocity of seed lift) were computed. Statistical significance of differences in the mean values of critical velocities of tested seeds was determined. The critical velocity of cereal seeds ranged from 8.03 to 10.54 m/s. Among the crops grown in wide rows the highest critical velocity of seeds was found out in maize (13.29 m/s), the lowest in sunflower seeds (8.38 m/s). In oil crop seeds, the lowest critical velocity was recorded in poppy (4.67 m/s), which was the lowest value of all seed samples of the tested crops. The highest value of the critical velocity was measured in pea seeds (14.85 m/s).Keywords: sowing machines; aerodynamic properties of seeds; variation curves Aerodynamic properties of seeds represent the behaviour of seeds in airstream. They are mainly related to seed properties (weight, shape, surface), and also to the condition of the air environment -airflow evenness (Srivastava et al. 2006). The knowledge of aerodynamic properties of grains is applied to the design of grain cleaning and sorting systems as well as to the construction of the pneumatic sowing mechanism of seed drills.The seeds placed in a central hopper of the seed drill are sliding by gravity and by help of an opener to a vane turnstile that delivers them fluently to the airstream from the fan. The seeds are then conveyed by the airstream pressure usually in a vertical direction upwards to a distribution head and then horizontally or obliquely through delivery tubes to drill coulters.The vertical delivery of seeds upwards is a limiting feature for the appropriate function of the sowing mechanism; it is possible only when the volume flow rate of air (m 3 /s) and its velocity (m/s) are chosen in such a way that the airstream will convey the seeds evenly. This air velocity should be higher than so called critical velocity of seeds (velocity of seed lift) and should include a reserve for the potential increased mass flow of seeds associated with the required change in specific seeding rate. The upper limit of the air velocity is a requirement to avoid seed damage in the course of its conveyance.It follows from the experiments conducted in the past (Neubauer et al. 1986;Páltik et al. 2003) that the air velocity (v a ) in the vertical duct should be chosen in the range: 56
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