Customers routinely request that we fabricate chutes before installing our equipment. For them, it’s a reasonable request, as they assume prebuilding these components will minimize their expenses during construction. However, designing and fabricating chutes prior to installing machinery almost always results in higher costs. Premade chutes virtually never fit as engineers intend, so field technicians must alter them on-site—a process that’s unplanned and which takes significantly more time than field-fabricating the component.
The reason chutes don’t line up correctly when they’re prefabricated is that models can’t take into account the real-world variables on an installation site, such as concrete that isn’t perfectly level or measurements that are off by 1/8 inch or so. When issues like these arise, premade chutes will not fit, and fabricators must cut, wrench, reform, and reweld the chutes to connect them on either end.
Trust us on this one—it’s a lesson we learned the hard way, and it’s one we’re not keen on repeating. While chutes are comparatively simple to design compared to machinery, there are still right and wrong ways to build them.
The fact that it’s better to wait to fabricate chutes isn’t to say engineers may disregard chute design entirely. On the contrary, chutes require more forethought than they often receive. But engineers can account for the things needed to make them function well without modeling them in detail. Mock chutes in layout models are more than enough to account for what’s required to make a good chute, which includes:
- A Proper Angle. A material’s coefficient of friction, angle of repose, and inherent flow characteristics determine at what angle it will flow and, by extension, how steep or shallow the chute through which it flows may be. Green woodchips, for example, require a minimum angle of about 55° to flow without issue. Other materials we’ve worked with have needed even steeper slopes. If the angle is too shallow, the material will rest in the chute and may form an obstruction.
- A Correct Shape. A chute’s shape will affect how material flows through it. Biomass and other materials with poor flow characteristics do not flow well through funnels, for example, due to their tendency to bridge over the outlet. This being the case, engineers should design chutes with as steep a taper as possible. For materials like woody biomass that don’t flow well, it’s best to make chutes with a relieving angle, with the bottom area greater than the top. Such a design helps ensure the material will not bridge over the outlet.
- A Target. When the chute discharges into a conveyor, it should spread the material over the conveyor’s width. Doing so helps ensure the conveyor’s components wear evenly and that material doesn’t pack against a sidewall. The easiest way to evenly distribute the material is to aim material at the middle of the conveyor. If the material doesn’t naturally fall in the middle of the receiving conveyor, technicians might install a plate at the chute’s outlet to direct it there.
- A Good Location. Engineers do well to place a conveyor’s discharge chute at the end of the head section (the part of the conveyor with the gearmotor opposite the receiving end). Placing the chute there avoids having material collect and pack into the resulting void on the other side of the chute. While this isn’t usually a problem with free-flowing materials or conveyors that discharge at steep angles, it can lead to obstructions or broken paddles if (and when) larger-than-normal materials collect in a horizontal conveyor or a conveyor with a shallow incline. If an engineer must locate a chute in front of the head, they should just continue the chute to the end of the head.
- Enough Width. Chutes should be as wide as possible at the discharge to prevent any ledge for a bridge to start forming, A wider chute also gives the material a better chance to spread out over the width of the receiving conveyor.
- Material Acceleration. When a chute discharges onto a belt conveyor, engineers should consider the relative velocity of the falling material relative to the belt. If the material lacks sufficient velocity in the direction of the belt’s travel, the material may bounce and tumble until it gains enough momentum to move past the inlet area. And as it bounces and tumbles, it is not moving away and eventually can accumulate and obstruct the inflow of material. Engineers can reduce the likelihood of material bounce by providing sufficient vertical distance between the discharge point and the receiving conveyor and angling the chute so that the material gains speed in the direction of the belt travel.
Sound technical knowledge is required to design chutes well, as it is possible to design and build them poorly, which results only in headaches from the time the field technicians install them onward. But you don’t have to experience the School of Hard Knocks to learn how to design chutes well. Allow the companies supplying your equipment to design and install the chutes. It’s something they (or we, at least) do regularly. Let us do what we do well and save you some frustration.
Contact Biomass Engineering & Equipment today to discuss your next bulk-handling project.