Wall friction tester

The main difficulty facing powder process engineers and powder plant designers is the need for information on two powder phenomena firstly, the resistance to product flow generated by particles within the product and secondly, how easily a powdered product can flow over the metal, plastic or ceramic contact area or internal surfaces of plant equipment. 

When the values of shear stress are plotted against the normal applied stress the resultant line, the waU yield locus (WYL), may be a straight line or a convex curve upwards. If the WYL is linear and passes through the origin, the slope of the line is the angle of wall friction. 

If the wall shear stress line is curved, a Mohr circle may be drawn in contact with the powder yield locus of a separate shear test obtained at a pre-shear normal stress identical or similar to that of the wall friction test (Akers 1992). The intersection of the curved WYL with the superimposed Mohr circle is then extrapolated to the origin to give the angle of wall friction. 

How Wall friction (degree) Bulk density (kg/m ) Shear strength (N/m ) Hausner ratio (—) Outlet size (cm) Mass flow wall angle (degree) 

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There is a simple Wall Friction Tester available commercially, (from Ajax Equipment (Bolton) Ltd.), which uses a round plastic cell 160 mm in diameter and 25 mm in height, with a stainless... 

As the name implies, the split cell testers use a cell, usually in the form of a ring similar to that used in the Jenike shear cell or the wall friction testers, but both the cell and supporting plate are split vertically, across the diameter. The sample is compacted vertically in the usual way (by application of normal loads via a lid) and the sample is then pulled apart by moving the split halves away. There have been some research devices designed on this principle, for example by Boden33, who suspended the cell supports on air bearings to minimize friction, but such devices are too expensive for general use. 

A material s flow function is usually measured on the same tester as the wall friction angle, although the cell arrangement is somewhat different (Fig. 6). ConsoHdation values are easily controUed, and the cohesive strength of the bulk soHd is determined by measuring interparticle shear stresses while some predeterrnined normal pressure is being appHed. 

The Jenike shear cell tester is classified as a direct shear tester that is capable of providing information on a solids cohesive strength as well as its wall friction properties. The tester allows us to measure the strength of a powder blend as a function of pressure applied to it. These are two main considerations when design a bin or hopper to ensure reliable material flow. The tester consists of a base, a moveable shear ring resting on top of the base, and a top cover lid (Fig. 7.3).61 The base is fixed while the lid rotates at a constant low rate. Powder blend is placed in the ring and base and a... 

The ring or annular shear cell, was developed by Carr and Walker as early as 1968. In recent years this tester has undergone a number of modifications. Peschl has developed an annular shear cell in which the sample and shear cell consists of a full circle. This contrasts to the earlier cells that have a band of sample on the outer portion of the circle. This was done to eliminate wall friction. It is also rotated very slowly, since at low speed, velocity variability becomes more negligible in the shear measurement. In this way a full ring can be utilized and speed differences in the outside and inside of the ring become negligible. 

Comments on the Compression Tackiness Tester This is clearly a fingerprinting method of a proven practical value. The result does not represent, however, the unconfined yield stress corresponding to the compression load because the compression stress varies within the height of the briquette. This is because the load during the sample compression is partly taken up by wall friction and the stress (and the bulk density) therefore reduces in... 

The previously mentioned Janssen theory (Janssen, 1895) includes, possibly, the oldest reported attempt to calculate pressures in silos. Janssen derived an equation for the calculation of vertical and horizontal pressures and wall shear stresses. He assumed a vertical force balance at a slice element spanning the full cross section of a silo being filled with bulk solids, and determined the wall friction coefficient with a shear tester as well as the horizontal pressure ratio from pressure measurements in a model bin. He also assumed... 

POSTEC - unaxial research tester In the POSTEC - uniaxial tester, discussed by Maltby and Enstad (1993), the sample is confined in a cylindrical die and wrapped in a flexible membrane which is stretched between the outer periphery of the piston and the inner perimeter of the lower part of the die. Since the membrane is stretched and in contact with the wall and powder, the sample is compacted homogeneously thus the wall friction between the specimen and the die is reduced. Comparison of the POSTEC uniaxial tester with a biaxial and Jenike-type shear eell testers, with the standardised CRM-116 limestone powder, indicated that the fc values obtained with the POSTEC are slightly less than those obtained with Jenike-type shear cells and a biaxial tester. Since the total time for... 

Fotir granular products were employed as test materials. Their particle properties are listed in Table 1. Note that the wall friction angle listed in Table 1 was measured by using Jenike shear tester based on the similarity between the test pipeline internal condition and the shear plate (i.e. a piece of bright mild steel plate). Such similarity was determined manually, not by measuring surface roughness. Since the effect of wall friction on the slug performance is very complicated, further research is needed to address this issue. 

A free-flowing bulk solid Polypropylene (PP) lens shaped pellet was used. The bulk solid has a friction angle of wall friction angle of (pw = 15°, measured with a ring shear tester and the Jenike shear tester, respectively. The particle size of the bulk solid was about 3 mm. 

In order to develop the proper dow pattern, knowledge of a material s dow properties is essential. Standard test equipment and procedures for evaluating sohds dow properties are available (6). Direct shear tests, mn to measure a material s friction and cohesive properties, allow determination of hopper wall angles for mass dow and the opening size required to prevent arching. Other devices available to evaluate sohds dowabiUty include biaxial and rotary shear testers. 

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