Powder flow is, indeed, a science. The field of bulk solids handling was developed mainly through the work of Dr. Andrew W. Jenike, who pioneered bulk solids flow theory. In the 1950s, Jenike developed a scientific approach to the storage and flow of bulk solids that is still relevant today. In fact, the Jenike shear test is now the American Society for Testing and Materials (ASTM) standard in the United States and Europe for determining a powder's cohesive and wall friction properties.1
Typical flow patterns
To avoid flow problems, you first must understand the flow patterns that can develop in a bin or silo. Second, to identify a flow pattern, you must have knowledge of material flow properties and be able to apply that knowledge.
Funnel flowoccurs when some material in a bin moves while the rest remains stationary. The walls of the hopper section are not steep enough or smooth enough to force the material to flow along them. The friction that develops between the bin hopper walls and the material inhibits sliding, resulting in the formation of a narrow flow channel, usually directly over the outlet.
The first material entering the bin usually is the last material out (first-in/last-out flow sequence). If the material has sufficient cohesive strength, it can bridge over the outlet or, if the narrow flow channel empties out, a stable rathole could form. This stable rathole can:
Decrease the bin's live or usable capacity.
Result in stagnant material that can cake or spoil.
Heighten segregation problems.
Cause structural failure.
Two common funnel flow bin designs are shown in Fig.1.
Figure 1. Funnel Flow Bins
Funnel flow bins are suitable for coarse free-flowing materials that do not degrade.
Funnel flow bins are suitable for coarse free-flowing materials that do not degrade. They also can be used when segregation is not important. For example, plastic pellets typically are suited to funnel flow applications.
The major benefits of a funnel flow bin are reduced headroom requirements and lower bin fabrication costs. Shallow cones (60 degrees or less), pyramidal hoppers and flat-bottomed bins exhibit funnel flow.
Most funnel flow bins are designed to save height and/or cost. However, flow problems for materials such as fine powders can far outweigh the size and cost benefits.
Mass flowoccurs when all the material in a bin is in motion during withdrawal. Material slides along the hopper walls, which are steep and smooth enough to overcome the friction that develops between the wall surface and bulk solid.
The hopper outlet must be large enough to prevent arching (mass flow alone will not prevent arching). Stable ratholes cannot form in a mass flow bin. Consequently, mass flow bins are suitable for cohesive solids, fine powders, materials that degrade or spoil and solids that segregate.
Two typical mass flow bin designs are shown in Fig. 2.
Figure 2. Mass Flow Bins
The hopper outlet must be large enough to prevent arching.
The flow sequence is one of first-in/ first-out. Therefore:
Solids that degrade with time can be stored.
Powders cannot flood if the material's residence time is sufficient for deaeration.
Particle segregation is minimized as the fines and coarse particles are reunited at the outlet.
Typically, steep cones and wedge-shaped hoppers (chisels, transition hoppers, etc.) are used to ensure mass flow.
Material flow properties
Solids flow property tests will identify, prior to equipment fabrication, just how a solid will flow in a particular geometry. You can confirm the validity of a proposed geometry by determining a material's cohesive properties and wall friction properties.
Flow property tests also can be used to identify a flow problem in an existing bin. These tests measure a material's cohesive strength, wall friction properties, compressibility and permeability.2
Cohesive strength measurements determine the opening size required to prevent arching and ratholing. Cohesive strength is the measure of a physical, chemical or electrical attraction holding a material together ," and potentially causing a flow obstruction to occur. When poured from a container, many bulk solids will flow like a liquid. Under these conditions, the material has no cohesive strength. However, when squeezed, a solid can gain enough strength through compaction to retain the shape of your hand.
Cohesive strengthis measured using a bench-scale laboratory testing device such as a direct shear tester. The material's cohesive strength is measured as a function of applied consolidation pressure. In a laboratory, a sample of the material is placed into a shear cell on the direct shear tester, and both compressive and shear loads are applied to simulate bin flow conditions.
Once material in the shear cell is consolidated, you can measure its strength by shearing the material to failure. By repeating this procedure under different conditions, you can develop the resulting value of strength vs. consolidating pressure. The process is fairly straightforward; however, it requires time to simulate a range of pressures acting in a bin. Typically, three pressure levels are used to simulate the range of pressures that represent the pressures the material will experience in storage.
This test procedure simulates several other conditions that affect material flowability. The sample's moisture content and particle size can be controlled, and the direct shear tester simulates the effects of temperature and time of storage at rest on the sample.