Elsevier

Journal of Food Engineering

Volume 61, Issue 3, February 2004, Pages 399-405
Journal of Food Engineering

Flow property measurement of food powders and sensitivity of Jenike’s hopper design methodology to the measured values

https://doi.org/10.1016/S0260-8774(03)00147-XGet rights and content

Abstract

The flow properties and powder physical properties were measured for 13 food powders. The flow properties were measured using shear cell techniques, and the powder physical properties measured were particle size, moisture, bulk and particle densities. The flowability of the food powders, as characterised by flow index, varied from easy flow to very cohesive. Particle size and moisture content do affect flowability, however there was no strong relationship for trying to relate the flowability of the food powders based solely on these physical properties. There was no relationship between measured powder physical properties and their wall friction characteristics. As a result, surface forces between the powder particles, and between particles and the wall surface play an important role in determining the flow nature of the powders, and this is an area requiring research. Jenike’s mathematical analysis to determine the minimum hopper angle and opening size for mass flow is the engineering standard practice for designing a hopper. Applying this analysis, using values of the measured food powder flow properties, shows that this can occasionally produce some unexpected values for the hopper opening size.

Introduction

There is a large quantity and variety of food materials produced industrially in powder form, and there is a need for information about their handling and processing characteristics. Powder property measurement is important because these properties intrinsically affect powder behaviour during storage, handling and processing. Powder flow properties are important in handling and processing operations, such as flow from hoppers and silos, transportation, mixing, compression and packaging (Knowlton, Carson, Klinzing, & Yang, 1994; Peleg, 1978). One of the major industrial powder problems is obtaining reliable and consistent flow out of hoppers and feeders without excessive spillage and dust generation. These problems are usually associated with the flow pattern inside the silo. The worst-case scenario is no flow. This can occur when the powder forms a cohesive arch across the opening, which has sufficient strength within the arch to be self-supporting. Mass flow is the ideal flow pattern were all the powder is in motion and moving downwards towards the opening. Funnel flow is where powder starts moving out through a central “funnel” that forms within the material, after which the powder against the walls collapse and move through the funnel. This process continues until the silo empties or until another no-flow scenario occurs with the development of a stable rathole. Most flow problems are caused by a funnel flow pattern and can be cured by altering the pattern to mass flow (Johanson, 2002; Purutyan, Pittenger, & Carson, 1998). Measurement of powder flow properties is necessary for the design of mass flow hoppers.

Jenike pioneered the application of shear cell techniques for measuring powder flow properties. In conjunction with the measured property data, he applied two-dimensional stress analysis in developing a mathematical methodology for determining the minimum hopper angle and hopper opening size for mass flow from conical and wedge shaped hoppers (Jenike, 1964). A hopper is the lower converging section of a silo and the hopper angle is the angle between the converging section and the horizontal. The measured flow properties used in this methodology are the flowfunction, the effective angle of internal friction and the angle of wall friction. The flowfunction is a plot of the unconfined yield strength of the powder versus major consolidating stress (Fig. 1), and represents the strength developed within a powder when consolidated, which must be overcome in making the powder flow. A flowfunction lying towards the bottom of the graph represents easy flow, and more difficult flow is represented as the flowfunctions move upwards in an anticlockwise direction. The flow index (ffc) is defined as the inverse slope of the flowfunction. Jenike used the flow index to classify powder flowability with higher values representing easier flow. This was extended by Tomas and Schubert (1979) and is presented in Table 1.

The angle of wall friction represents the adhesive strength between the powder and the silo wall material, the higher the angle the more difficult it is to move the powder along the wall surface. It is the angle between the horizontal and a straight-line from the origin intersecting the measured wall yield locus (Prescott, Ploof, & Carson, 1999), as illustrated in Fig. 2. The wall yield locus often has a positive Y-intercept, thus the angle of wall friction will vary with normal stress in the hopper, where it is higher at low stresses.

Jenike’s mathematical methodology is the engineering standard practice for designing a hopper in terms of calculating the minimum hopper angle and opening size for mass flow. The method consists of the following steps:

• Values of effective angle of internal friction and angle of wall friction are used to calculate the hopper angle (θ) and the flowfactor (ff).

• The critical applied stress (CAS) is then determined from the intersection of the flowfunction and the flowfactor line, as illustrated in Fig. 3. The flowfactor line is a straight-line through the origin with a slope equal to the inverse of ff.

• The hopper opening size is then calculated using values of CAS, hopper angle and bulk density (ρB). For example, the diameter D of the opening for a conical hopper is given byD=H(θ).CASρB.gwhere H is a function of hopper angle and g is the gravitational acceleration constant. The bulk density must correspond to the consolidating stress existing at D.

• This method requires some simultaneous calculation as the value of angle of wall friction may vary with the normal stress existing in the collapsing arch at diameter D.

The objectives of this paper are

  • To present the measured flow properties of 13 food powders and to classify their flowability.

  • To investigate if physical property measurements (particle size, moisture content, bulk and particle density) can be used in comparing the flowability of the food powders.

  • To investigate the effect of the flow properties on hopper design using Jenike’s hopper design methodology for each of the powders.

Section snippets

Food powders

The powders tested were: a fine tea powder (Ceybrite variety from Lipton Soft Drinks, Ireland); powdered sugar (10 X, Kroger Co., Cincinnati, OH, USA); corn starch (Argo, Engelwood Cliffs, NJ, USA); salt (Morton Salt, Chicago, IL, USA); cellulose powder (Solka Floc, Fiber Sales and Development Corporation, Urbana, OH, USA); 10 DE maltodextrin (Maltrin, Grain Processing Corporation, Muscatine, IA, USA); cocoa powder and tomato powder (americanspice.com, Fort Wayne, IN, USA); all-purpose wheat

Flow properties

The measured flowfunctions of seven of the food powders tested are presented in Fig. 6. The flow index can be applied to the food powders tested, as the slopes of the flowfunctions increase in the anticlockwise direction from the region of easy flow to difficult flow. The flow indexes of all 13 powders are presented in Table 2, along with other flow and physical property data. Of the 13 powders, six are classified as very cohesive; four are cohesive and the remaining three are easy flow. The

Conclusions

The flow properties and powder physical properties were measured for 13 food powders. Powder flowability, as characterised by flow index, varied from easy flow to very cohesive, the effective angle of internal friction varied from 40° to 65°, and the angle of wall friction (5.9 kPa normal pressure) varied from 12° to 27°. Particle size and moisture content do affect flowability, however there is no strong relationship for trying to relate the flowability of food powders based solely on these

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