Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

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is recommended in the area with the least airflow, typically in the centre of the silo,
about 30 cm under the grain surface [23]. The goal is to assure safe storage
environment everywhere in the silo rather than just average conditions for the silo as
a whole. According to Navarro and Noyes [121], the airflow resistance calculated
from recommended equations or read from figures is meant for loose-filled, clean, dry
grain with airflow in vertical direction and in general gives a conservative estimate.
The authors recommend that magnitudes of increase or decrease in such obtained
airflow resistance must be determined experimentally. They also point out that the
performance efficiency of an aeration system depends primarily on the uniformity of
the airflow distribution in different regions of grain bed.
In experimental investigations, density (interrelated with porosity) was recognized
first as a factor determining resistance to airflow through a layer of grain. Then
the influence of the content of fine material was examined, because it decreased the
amount of void space among grains increasing airflow resistance. Calderwood [28]
examined resistance to airflow of different types and forms of rice and found that
medium-grain rice offered more resistance to airflow than did long-grain rice. This
author also stated that the resistance to airflow of brown and milled long-grain rice
was nearly the same, but the resistance of brown medium-grain rice was significantly
greater than that of medium-grain milled rice. Stephens and Foster [155] in their
experiments with corn in commercial silo found increased resistance to airflow of up
to 300% when using grain spreaders, as compared to that when no spreader was used.
The same authors [156] conducted a similar test program with wheat, corn and
sorghum. They observed that spreaders decreased the uniformity of fine material in
sorghum, while there was little difference in wheat. Filling the bin with the grain
spreader produced airflow resistances 110 % greater in sorghum and 101 % greater in
wheat than those produced by filling from the central spout. Probably the crucial
factor in these experiments was fine material content which was from 2.6 to 5.5 % in
the case of corn, from 1.5 to 2.0 % in the case of sorghum, and 0,2 % in the case of
wheat. The authors suggested that a possible reason for the observed increase in bulk
density and resistance to airflow could be, in part, compaction due to the action of the
spreader, while in grains with higher amounts of fine material, part of the increase
arose from fine material occupying spaces between whole kernels.
The direction of airflow also appeared to influence resistance of the bedding to
airflow. Kumar and Muir [87] found that at an air velocity of 0.077 m s -1 the
resistance to vertical airflow compared with horizontal airflow was up to 60% higher
for wheat and 115% for barley. Based on air velocity of 0.077 m s -1 , airflow
resistance for layer filling was higher than for end filling by 25 to 35% for vertical
airflow and 50 to 75% for horizontal airflow. Hood and Thorpe [63] found that for the
velocity range up to 0.2 m s -1 and for ten grains the resistance to airflow in the vertical
direction was about double that in the horizontal. These authors indicated that