No shredder can do it all. Every design has its advantages and disadvantages and sometimes only a shredding test will reveal the best possible solution.
By Andres Schwarz
There are pros and cons for each type of shredder and sometimes a compromise has to be made. But it is imperative to realize that there isn’t a single machine that can do every job perfectly. This article will point out differences between the various shredding technologies, so you can make a more educated decision when it comes time to make that important purchase.
Stationary Shredders: Single-Shaft
Stationary shredders have a very wide range of configurations and are typically used in a facility as part of a system that processes a waste stream. The fundamental difference between these shredders and others are in the shredding shaft design. The most popular are single-shaft, dual- and quad-shaft designs.
The single-shaft shredder uses exchangeable cutting knives mounted on one single rotor. These knives, typically made of hardened tool steel, cut against replaceable counter knives and then pass by a screen that determines the output size. Material goes by this screen until small enough to fall through. A shredder like that can shred material into a small and pre-determined maximum size. The most common use of this type of shredder is in the plastic recycling industry, which needs to reduce post-consumer or production waste so it can be processed back into a plastic product. Plastic recycling shredders tend to produce a 2″ minus product and range from a few hundred pounds per hour output to about four tons per hour.
Larger, stationary, single-shaft shredders with an output of up to 50 tons per hour are used in the processing of MSW, commercial, C&D and other waste streams of caloric value. In this application, the waste is shred into 12″ minus fractions and then cleaned of organics, metal and inerts. The remaining material goes to a high-speed secondary shredder that will further reduce the caloric fraction to 2″ or 1″ minus. This material is then sold as an alternative fuel to cement kilns or other high thermal energy consuming industries.
All of these single-shaft shredders have in common is that they have one single rotor and typically a ram that pushes the material into the rotor. However, there are vast differences in tooling design and rotor speed. It is important to point out that one particular configuration can work very well with one material stream and be a total failure with another. “One size fits all” does not exist, thus, it is extremely important to specify what type of material needs to be processed (in as much detail as possible).
Material that contains metal or other inerts are typically processed with slow speed (50 to 80 RPM) and large knives to reduce the risk of breaking knives when metal or stones are encountered. The slower speed results in less output; thus, these shredders tend to have very large diameter rotors that are up to 11 feet long. This allows more knives being installed, hence, increasing the output again.
Clean material streams can be processed with shredders that rotate between 100 and 350 RPM. Higher RPM increases output, but it also increases tool wear. Not all materials are suitable for high-speed shredding. To make the proper recommendation, it is important to consult with an expert that knows your industry, material and particular processing challenges.
Lindner America’s stationary shredder, the Komet.
Lindner America’s mobile shredder, the Urraco.
Photos courtesy of Lindner America.
Stationary Shredders: Multi-Shaft
Multi-shaft shredders have either two or four rotors and work in either a sheering or ripping fashion. The sheering type uses disks with replaceable knives or replaceable disks. Shredders with replaceable discs require total disassembly to replace worn or broken disks.
Ripping type shredders are typically two shaft shredders. They do not have replaceable wear knives; instead, they have to be welded to replace worn parts of the shafts. After a certain amount of wear, the entire shaft assembly has to be removed for a total overhaul.
Multi-shaft shredders without a screen cannot control the output size very precisely and are typically used for rough sizing or in applications where small fractions are not desired. They usually rotated at a rate below 40 RPM and are most of the time powered by hydraulic motors that are generating a lot of heat, thus, drive efficiency is not very high. Up to 25 percent of power can be lost to heat.
Mobile shredders, as the name gives it away, are mobile in nature. They come in many different configurations. Self-powered, chain track driven, hook lift type that requires a truck to move them or trailer based. Most of them are powered by a diesel engine that powers a hydraulic unit. This hydraulic unit drives hydraulic motors that turn the shafts. Most low speed, high torque shredders are two shaft type, but some use only one shaft. Because they use very high torque to rip material apart, they typically do not have replaceable knives. Instead the shafts have to be re-welded frequently to replace worn off material.
Mobile shredders are mostly used outdoors when it is necessary, or advantageous, to move the shredder to the material. For that reason they need their own power source. Diesel engines in conjunction with hydraulic drives make very powerful shredders, but efficiency is much lower than an equally powerful shredder that is electrically driven. It can be as much as three times as expensive to operate a mobile shredder versus a stationary electric driven shredder. Thus, a diesel powered shredder should not be used in a stationary installation. Noise, heat and exhaust fumes, as well as the much higher operating expense (cost of diesel and motor maintenance) do not make it a good long-term investment for stationary applications.
Mobile shredders are designed to size reduce large material into medium size fractions, typically smaller 20″ at very high rates. Mobile shredders can achieve rates of more than 100 tons per hour with C&D and landfill material. Because the primary purpose of a mobile shredder is volume reduction for landfills, or so more material fits into a truck for transport, the shafts are typically designed for a course output. This allows for thicker and stronger cutting arms that allow the shredder to process most material. Large metal inerts go through the shredder most of the time without any issue. With the right shaft configuration, entire cars and engines can be processed. On the other hand, ropes and material that wraps and is very strong do not process through these shredders very well.
The Best Solution for You
As mentioned earlier in this article, there is no shredder that can do it all. Every design has its advantages and disadvantages and sometimes only a shredding test will reveal the best possible solution. The main lesson here is, “Don’t misapply a shredder or assume it will work if you have never done it.” Without proper testing, or seeing a shredder operating in the exact same environment as what the client is planning on doing with the shredder can lead to a very expensive and frustrating experience. | WA
Andres Schwarz is President for Lindner America (Raleigh, NC), a subsidiary of Lindner Recyclingtech, focusing on providing size reduction solutions to the American and Canadian markets. Since 1948, Lindner has engineered and manufactured size reduction equipment. The core competence of Lindner is shredding technology—everything from development, construction, production and distribution, to comprehensive customer service—is from one source. Lindner has become the specialist for shredders and two step shredding systems for alternative fuel from municipal, industrial and commercial waste. Andres can be reached at (919) 610-2835.