Hopper outlet size

A further drawback of this construction is that it is not convenient to have a taper outlet slot on a container so the practice is to slope the feeder casing from the screw diameter to a parallel hopper outlet size that matches the largest diameter of the screw. All most invariably, this results in a casing wall inclination at the smaller end of the feeder that is inadequate for product slip. In combination with the gap at the side of the screw to the casing wall that fills with static product and opposes wall, the effect is to create a narrow flow channel with non-mass flow characteristics. The economics of manufacture also detract from widespread adoption of this technique as both the screw and the casing have taper components that demand extra fabrication time. 

There are two mechanisms by which arching can occur particle interlocking and cohesive strength. The minimum outlet size required to prevent mechanical interlocking of particles is directly related to the size of the particles. The diameter of a circular outlet must be at least six to eight times the particle size, and the width of a slotted outlet must be at least three to four times the particle size. These ratios normally only govern the outlet size of mass flow hoppers if the particles are at least 0.6 cm or larger. 

B = outlet diameter or width, g = acceleration owing to gravity, m = 1 for circular opening and 0 for slotted opening, and 0 = hopper angle (measured from vertical) in degrees. A modification of this equation takes particle size into account. This modification is only important if the particle size is a significant fraction of the outlet size (8). 

When considering scale effects, the implication of the foregoing analysis is that the hopper angle required for mass flow is principally dependent on the outlet size selected for the hopper under consideration. Note that the... 

If the hopper-flow factor lies above the powder-flow function, cohesive arching is not possible with the material in that hopper. If the hopper factor lies below the powder-flow function, arching of material in the hopper is possible even for large-outlet sizes. 

Once the outlet is sized, the hopper wall sloped should be designed to be equal to or steeper than the recommended hopper angle for the given outlet size and selected wall surface. For a conical hopper, the walls should be equal to or steeper than the recommended mass flow angle for a conical hopper ( c ... 

Maximum lump size for correct hopper outlet potential abrasiveness. 

The design of a storage container for bulk materials is rarely taken in isolation, normally being influenced by various facets of the associated plant or the background of the manufacturer. There are three main steps to selection of storage hopper geometry (i) body configuration (ii) outlet size and shape and (iii) the transformation between the two. 

The outlet size and the approach to the outlet in the make-up hopper were not designed to accommodate air-retarded flow at the required discharge rate in a stable flow condition. 

Arching is caused by the outlet size being less than the critical arching size for the strength of the bulk material in the particular hopper neck construction. Air-retarded flow is the consequence of the limited rate of failure of the unconfined surface of the material. However, the solution required both a high flow rate and a bulk flow condition that is not excessively dilated. The surface area of failure demand for this latter function would require an excessively large orifice cross-section. 

The mechanistic equation for the Jenike failure locus is converted into equations of principal stress and the relationship to each other is shown graphically. The basic form of the parametric failure function equations are developed from the bulk property coefficients of the Jenike failure locus equation. It is shown how different forms of failure function are created by the type of dependency of the bulk property coefficients on the equilibrium stress and how these different forms have implications for the design of the outlet size of hoppers. 

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