In our previous articles, we discussed factors that effect bulk solids flow, material flow properties, bin and feeder design technique and more.

Oftentimes, it is necessary to transfer solids from the outlet of a bin to processing equipment, into a truck, or another bin. Conveying material using equipment designed to mechanically convey or transfer solids works quite well; however, for short distances, this can be an expensive approach.

Chutes can be used instead of expensive conveyors to transfer solids short distances. A chute is simply a pipe or trough that is sized properly and at the correct slope angle to ensure sliding of the material to be transferred. (See Fig. 1.)

A chute must be steep and smooth enough to ensure sliding along its entire length. The impact of material on the chute is extremely important.

Whether the material is dropping in free fall from a bin or being transferred from another chute, impact pressures and the velocities necessary to keep material moving along a chute are critical. Impact pressure can be calculated as follows:

impact pressure = (bulk density)·(velocity)²·(sine of impact angle)²
                            acceleration due to gravity

Where:
The impact pressure is in pounds per square feet (psf),
the velocity before impact is in feet per second (ft/s),
the bulk density is in pounds per cubic feet (lb./ft³),
the impact angle of incoming stream is in degrees and
the acceleration due to gravity is 32 feet per second²

A test can be run in the laboratory to measure the critical chute angle resulting from impact pressure. This information can then be used to develop the minimum chute angle at the point at which it impacts the chute.

The tests involves placing a sample of solid on a surface representative of the surface to be used as the chute. A load is applied to the sample (using weights) and removed after a short time (say 20 sec.) to simulate impact pressure. The surface is then tilted until the sample begins to slide.

That angle of slide is then recorded. This test is repeated several times to ensure accuracy and under a range of loads to simulate a range of impact pressures. A safety factor of about 5° is added to the results.