Chutes & Feeders: A Frequent Source of Blockages

When problems arise with a feeding device finding the culprit can become confusing. From a distance there is the possibility that the feeding device has malfunctioned when material does not arrive at a process or a final delivery point. However, in a linear system a failure at any point means a complete failure.

This fragility leads to a feeder’s design being just as important as the storage bin supplying material. For a feeder to function properly the bin must be working with the material. Simultaneously, for a bin to be capable of reliable discharge, the feeder must be functioning correctly. Past the feeder often sits another area of concern, the discharge chute.

When material exits a feeding device it can go any number of places including a conveyor, a process vessel, or a final package or container. The route between the feeder and its’ next destination is typically a transfer chute. These chutes often serve at least two roles.

Depending on the equipment and space constraints a transfer chute will often need to move product horizontally. For example, a screw feeder’s outlet may be located 3 ft away from the material’s next destination. This means to get material there, a sloped transfer chute is used to allow material to slide that 3-ft distance.

Next there is typically a difference from the cross section of the feeder outlet and the inlet to the next piece of equipment. This difference could be as simple as a 6-in. circle to a 6-in. square. To mate with the next piece of equipment a unique transition must be utilized to connect the two equipment pieces.

When both of the above tasks are put together it frequently leads to a chute which has a bizarre shape with sharp corners and shallow angles. For the material trying to leave the feeder this can lead to problems. Typically, this manifests as plugging problems from the feeder, buildup on the surfaces, and downtime for the equipment.

To prevent these problems requires careful design and fabrication. Several key choices early in the design often help. Three of these to incorporate include.

1. Do not leave chutes as an afterthought. If the need for a transfer chute at the feeder’s discharge is forgotten or ignored, then a mechanical contractor may be forced to become both fabricator and designer. In such cases the design is not usually based on what is necessary to reliably handle the bulk solid. Rather the design looks to see how to connect cross section A to cross section B in the simplest fashion using whatever height and space remains.

2. Understand that for a chute to work correctly the slope and surface must be sufficiently smooth and steep to overcome the friction between the bulk solid and surface. This will require actual physical properties from testing at representative conditions. As well, considerations for sharp corners when present and the ability of certain products to build up over time on surfaces may require additional testing. Testing takes time, work, and real samples to complete.

3. When initial layouts may precede the testing, additional height and space should be made available between equipment. As well, some systems may over their lifetime be required to handle additional products that may be more frictional and more prone to chute plugging. Having the space and height to retrofit as needed may allow for greater flexibility.

There are other considerations for chutes at the discharge of a feeding device including sizing based on the rate and bulk density, considerations for particle interlocking based on lump and particle size, the wear and the velocity and state of product loading the next piece of equipment. At a minimum when selecting and designing a feeding device we must not limit our focus to the feeder and the bin. Failure to consider the discharge chute can and often has often led to problems.

Scott Miller has been providing consulting services for Solids Handling Technologies Inc. since 2016. For more information, visit www.solidshandlingtech.com.