As a responsible supplier who cares about the success of our customers, and having interacted previously with slat floor (moving floor) style feeders, we want to share our concerns about the use of these feeder bins. It’s important you understand their limitations so everyone on your team can make an informed decision and enjoy the most efficient startup process as possible after you’ve selected your equipment.
The slat floor concept works, especially when you just want to move a lot of material in a hurry, but it’s not a precision feeding device: the discharge performance of a slat floor is based upon friction interaction between the material and the floor. When the friction changes, the output changes—and there’s a lot that can change the friction: compression, which changes the material density and bearing area; density; moisture content; particle size; and temperature. With higher flow applications, these changes are not as evident as they are in low flow. (We consider a feeder to be a low flow.)
In our experience, regulating the flow from these floors has been a challenge. These systems move the entire pile forward, unlike a screw feeder, which shears the material from the bottom of the pile. Regardless of the friction factors listed above, the slat floor output is entirely dependent on the pile height at the point where it tips over into the outlet chute. So, to predict the discharge rate, the pile height must be regulated.
We know customers that have tried using an adjustable wall to regulate the pile height at discharge, but a wall only works when you’re not trying to restrict the flow too much. (Needless to say, it did not work well for this customer.) At a certain ratio of pile size to wall opening, the material just stops moving. There isn’t sufficient friction at the floor to push the material out the opening.
You see this toward the end of the unloading process in a slat floor trailer also. To deal with this, we’ve gone so far as to design, build, and deploy a drag-back device that was added to the discharge section of a bin to achieve a constant cross-section area (pile height at discharge). Unfortunately, this was an “after the startup failed” fix, which cost more than the original floor.
Feeding the downstream conveyors with unregulated flow is the next issue. A bottom-drag conveyor cannot work in a flooded condition—that would require the return paddles to pull through the pile in the opposite direction of the material flow. If you allow the floor to puts out too much material, even in surges, you’ll damage the conveyor.
A third issue is dust containment. Operating as dust-tight as possible (per NFPA® standards) is critical. We all know the safety issues related to uncontrolled dust. Add to that the labor required for cleanup, and dust becomes two-edged sword. Watch a moving floor trailer unload and note how much dust comes through the floor.
Fourth, and last, there’s the cost involved with maintaining slat-floor feeders (trailers or other configurations). In a constant application like a feeder, you can expect to rebuild your floor annually—there are a lot of moving wear parts.
Years ago, our owner, Dane Floyd, used slat-floor trailers to transport green whole tree chips in Michigan in six-axle trailers. He ended up buying new trailers every three years because he discovered maintenance costs skyrocketed after this timeframe. Fortunately, he kept the trailers looking good, so he got a good resale value from them, but the floors were worn out.
What amazes him still is that those floors were only cycled at a maximum of ten times per week (two loads per day)! That amounted to 500 cycles per year or 1500 in three years. Considering that each cycle lasted 25 minutes, the trailer floors had a usable life of 625 hours. (In terms of cost, that equaled about $50 per hour of run time.) In a 24-hour operation, that’s only 26 days.
We acknowledge manufacturers of moving floor trailers offer higher-end packages that may last longer, but even then, issues listed above may still apply.
As mentioned, Biomass Engineering & Equipment designed a “drag back” conveyor, which can be retrofitted into a slat- or chain-feed bin to regulate the flow from these systems. We also offer SMART Floors, our stroker-based reclaim/discharge system. These floors address all the issues related to slat-floor feeders. In short, they provide:
- Consistent, metered discharge rates
- Surge control
- Dust-tight operation
- Long service life with minimal wear parts
Learn more about SMART Floors and drag-back conveyors.
Please contact us with any questions you may have.