There are many types of hoppers that can be used to handle your materials. Some think that conical hoppers are the only way to handle a material. We will discuss several different geometries that are available. A given bulk solids may flow in a funnel flow pattern (some material moves, most remains stagnant) or a mass flow pattern (all material moves whenever any is withdrawn). The selection of an appropriate flow pattern is largely driven by a powders’ flow properties as well as the requirements of a given application.
The hopper provides a convergence to deliver material to an outlet. The choice and style of convergence is not always intuitive. The selection process can vary for a given material; however, the following provides a brief survey of the more common hopper styles and their interaction with material flow.
By far the most common hopper style, the conical hopper converges equally in all directions to a centrally located outlet. This style of hopper can have significant cost savings as it allows for a simpler fabrication of a structurally stout hopper. For powders where funnel flow is suitable the hopper angle can be shallow allowing for reduced height.
For those powders where mass flow may be desired or those products with significant cohesion a conical hopper can be troublesome. Materials are more prone to bridge and arch in a cone and as well cones require steeper angles to achieve mass flow.
Also referred to as square hoppers, this style of hopper converges from a rectangle to a square cross section. This style of bin provides a simple construction of four flat surfaces coming together. From a flow perspective, the pyramidal bin is akin to the conical hopper as material converges in both directions. For those materials suited to funnel flow this hopper style can work quite well and allows for reduced height. The junction between side and end walls can create challenges if material clean-off is desired. The two plates form a shallower valley angle, which in additional to being less steep also serves as a pinch point for buildup.
For materials where mass flow may be desired the pyramidal hopper not only shares the challenges of conical hoppers (i.e. materials can bridge more easily over the outlet), but also require that the valleys be sloped at the conical hopper mass flow angle, and in many cases the valley angle may also need to be rounded.
The wedge style of hopper takes on a variety of configurations and is observed in traditional straight end-wall wedge hoppers, transition hoppers, and chisel hoppers. The unifying thread between these hopper styles is the material is converging almost solely in one axis to a long-slotted outlet whose length is typically three times the width.
The first challenge of this hopper style is finding a feeding device capable of feeding over the length of the slot. As such both belt feeders and screw feeders frequently find a symbiosis with wedge hoppers.
The wedge hopper offers distinct flow benefits in that cohesive solids are less prone to arch over the outlet. As well in funnel flow, a larger flow channel can often be achieved to diminish the likelihood of the formation of a stable rathole. In mass flow a wedge style hopper also allows for a less steep hopper angle to achieve flow along the hopper walls. This style of hopper typically comes at the expense of added fabrication complexity and cost.
These are the three most common styles of hoppers but are not the only available options. Other hopper styles include flat bottom bins and asymmetric bins, as well as variants on the three described here with an additional number of outlets (such as pant leg hoppers). As well, material flow in a given hopper can be altered using various inserts placed inside of an existing hopper.
This is just a brief discussion on hoppers. The choice of one hopper style over another is best decided by understanding the needs of each application. As well, these hopper styles and their design are dependent on each given material’s flow properties.
Joe Marinelli is president, Solids Handling Technologies. For more information, visit solidshandlingtech.com.