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

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Tapped density provides information on the susceptibility of a granular material
to compaction through vibrations. The relevant standard provides for a measurement
consisting in bringing a known mass of a granular material to the lowest volume
possible through the application of vibrations of constant amplitude and frequency
[138. The tapped density of cereal grain is higher than the bulk density by several
percent, and in some cases even by over twenty percent [159.
Chang et al. [31] showed that distributed filling of silo increases the density of
granular material by from 5.1 to 9.2% as compared to filling from centrally located
spout of conveyor. Stephens and Foster [155] observed increases in density of the
order of 3 to 5% above the bulk density values in condensed filling from spout of a
conveyor, and 7% in the case of distributed filling. Versavel and Britton [166]
showed that density depends on the falling height, the degree of contamination, and
on the filling rate. The researchers noted a considerable increase in density, of the
order of 8-10%, and in the case of high filling rates a decrease in density. Similar
relations were found by Schott and Britton [150] in laboratory studies. With
increasing grain falling height the kinetic energy of the grain increases, which
increases the packing density of the material [116]. That effect disappears above
a certain height, due to increasing aerodynamic drag during the free fall of the grain.
Table 5.1. Bulk density, porosity and specific gravity of grain [3]
Grain
Barley
Rape
Maize
Linseed
Oat
Rice
Rye
Soy
Wheat
Bulk density
(kg m -3 )
618
669
721
721
412
579
721
772
772
Moisture
(%)
9.7-10.7
6.5-6.7
9-15
5.8
9.4-10.3
11.9-12.4
9.7
6.9-7.0
9.8
Porosity
(%)
39.5-57.6
38.4-38.9
40.0-44.0
34.6
47.6-55.5
46.5-50.4
41.2
33.8-36.1
39.6-42.6
Specific gravity
(kN m -3 )
12.1-13.3
11.0-11.5
11.9-13.0
11.0
9.5-10.6
11.1-11.2
12.3
11.3-11.8
12.9-13.2
In view of the wide range of variation and considerable number of factors affecting
density, attempts are made at developing methods for the determination of „apparent
density” that would correspond to the density of a material in a silo. Therefore, it is
necessary to search for general rules applicable to the determination of the actual
density of granular media in containers that would be common for as extensive a class
of materials as possible. Basing on experimental results obtained so far it is recommended
to estimate the density of a granular material in a silo by assuming an
average density increase of 6% with relation to the density value determined from the
mass of 1 hectolitre [22]. It appears, however, that application of more accurate
methods for the prediction of density of granular material deposit is a necessity.