Chip it up - Shred it down
July 2015 at the Rieger company waste wood processing plant in Austria: A Crambo 6200 is running at full speed, fed by an orange-peel grab. The shredder drums chew their way through the material coming in, while a never-ending stream of shredded wood goes out to a Multistar L3. Under each star screen discharge conveyor is a container which is carefully weighed after each test run. Engine data and screening analyses of the material streams complete the benchmark test of a high-speed shredder and two low-speed shredders.
Recycled wood as a valuable material
The three machines went through a scenario that is a daily routine at the site. Waste wood arriving at the site was separated into different categories, shredded, and cleared of magnetic contraries as far as possible. The resulting shred is a useful raw material, for example for making particle board. More and more, recycled wood is taking the place of logged wood in this application. In this test, the ability of the machines to deliver a homogeneous grain from 20 to 130 mm was compared. A Multistar L3 was set up to separate this fraction out as the medium fraction.
First shred, then screen
Preliminary testing showed that two-stage processing has advantages when shredding large quantities over long periods. It’s better to shred a little coarser, screen out the overs and run them back through the shredder, than to restrict the machine’s throughput with a very fine screen basket. But which technology, which screen baskets and which teeth are the most efficient? At the end of the day, what counts is the overall balance of useful fraction versus energy used. And - is the process practical in extended use?
High-speed shredder: Yes but...
Shredding waste wood that has not been pre-broken up is a challenge for any high speed shredder. The less pre-sorted it is, the higher the danger of massive metal contraries. The magnetic junk pulled out by the overbelt magnet included nails, bolts and a car wheel. The Axtor features newly developed free-swinging teeth, and stands up relatively well to these contraries. But long-term, these items are going to mean more wear and higher risk of damage from a large contrary. So in continued operation a procedure needs to be in place that makes sure the input material contains no large ferrous items. On the positive side, the Axtor delivers high throughput with large screen baskets, and a low percentage of overs. With the 150 mm screen basket it made over 30 tonnes. This means, that over 70 percent was usable fraction and only 1-2 percent was overs. Energy efficiency was also good, with almost half a tonne of useful fraction per litre diesel fuel used - in terms of volume, that’s about two and a half cubic metres.
Terminator: A though do-it-all machine
The Terminator was set up for the test with the XXF shredding unit. XXF stands for “extra extra fine,” and sure enough, it didn’t turn out much in the way of overs. The Terminator also created less dust than the high speed shredder, and less fines. Another advantage is its insensitivity to metal contraries, so it can shred very contaminated input. But in terms of specific fuel consumption the Terminator couldn’t quite keep up - high speed shredders like the Axtor, and the low-speed Crambo dual-shaft shredder, do better in this area. That’s really no surprise, since the Terminator was designed as an all-purpose shredder. It can do waste wood, but its real strength is in mixed use.
Waste wood - where the Crambo shines
The Crambo direct was the last machine tested. It’s no secret that it is suitable for shredding all kinds of waste wood. And the mechanical direct drive is known to save fuel. But what screen basket gives the highest efficiency?
The low-speed Crambo will always tend to produce less fines than the Axtor. But the screen basket used makes a big difference - smaller screen baskets mean less overs and more fines, and vice-versa for large baskets. Throughput is much higher with large screen baskets, and the largest size tested turned out to be the most efficient, as compared with the other screen baskets and also the other machines. Result: over 700 kg usable fraction with a litre of diesel fuel. Even if you count the effort to re-shred the overs that go back into the input, the Crambo with 250 screen basket still comes out ahead.
Heavy Duty Outfit
Even with high amounts of ferrous metal, up to 2.5 percent by weight in the processed material, the Crambo can still stand up to extended usage. The Heavy Duty version has armoured tooth mountings, wear plates on the drum, and screen baskets that are twice as strong. Another option is a special tooth design for waste wood that gives up to 1000 operating hours.
Naturally this comparison test cannot answer every question. The material requirements, available space and logistic situation differ from user to user, and other processes often have their place. But we consider the fact that many customers have set up similar processes at their sites to be a confirmation that our results are sound.