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Silo Technology

Solving Outflow Concentration Problems Due to Cluster Segregation in Hoppers

Interruption periods during filling of a hopper can cause segregation and hence fluctuation of concentration in the outflowing powder. Observations on industrial hoppers showed, that especially for different filling velocities during the filling periods, different segregation results can occur.
Aim of the Work and Description of the Measuring Methods:
In order to investigate the above mentioned fact in more detail, filling and outflow tests on a test hopper and theoretical considerations on segregation effects were the aims of this paper. The schematic draft of the test hopper is shown in Fig. 1 (1×1 m width×depth and 2 m height, outlet area is 1×0,12 m). The test hopper is a scale model of an industrially used hopper with a wedge shaped main part.
The outlet of the hopper consists of two central sliders, which can be opened to the side. The hopper is filled through a central pipe. The front wall is made from transparent material, which allows to take photos of the infilled bulk material.
The filling material was washing powder (93.3w%, bulk density: 0.76 kg/dm3, d50,3 = 0.41 mm) mixed with black polypropylene granules (PP, bulk density 0.55 kg/dm3, d50,3 = 2.8 mm) up to 6.7 w%. Using a CCD camera, which automatically can take a sequence of single pictures in a short time period, the positions of the black particles within the white washing powder can be determined and further the horizontal velocity pattern can be calculated.
The wall material PTFE and the wall inclination of 23° should, according to Jenike shear cell tests, ensure mass flow. It has to be considered that the main part of the hopper is wedge shaped and there are only small vertical walls, what can disable proper mass flow [1,4]. The hopper was filled by 9 charges (hence 8 interruptions) of 64.3 kg mass quantities with filling velocities of 32.5 kg/min and 65 kg/min.
During the outflow procedure a partial mass flow was collected in a cylindrical collecting pipe. Thereafter, by analysing different mass sections of this collecting pipe the time dependent outflow concentration of the PP particles could be found. More details concerning the outflow concentration measurement can be found in [3].