No shred of compromise

Written by: Tim Fryer | Published:

Designing a waste shredding machine is something of a black art. Tim Fryer found out from Untha how clever design can make dramatic improvements in performance.

The waste hierarchy mantra is reduce, reuse and recycle, but even within that there are potential inefficiencies that could render an environmental strategy counter-productive – more damage is being done to the environment than is being saved. A strategy to reduce landfill, for example, would be undermined if it involved extra energy to burn waste, increasing emissions and not using the energy resources wisely.

Indeed, any piece of equipment involved in the waste industry is going to make the most positive contribution if it operates as efficiently as possible in its own right. Of course there are many different views of what represents good environmental performance and these will vary depending on the nature of the machine.

For a waste shredder these may include lower energy input, reduced fines, longer equipment life and quieter operation, but these may be at odds with the business drivers such as faster throughput, broader functionality and mobility. Untha aimed to meet all of these challenges when designing its latest waste shredding beasts: the XR3000 and the XR3000 mobil-e. These machines can be adapted for producing biomass, SRF and RDF (Solid Recovered Fuel and Refuse Derived Fuel, for those uninitiated in the terminology of the waste world).

The distinctions between SRF and RDF are a bit woolly, but the general rule is that RDF is made from Municipal Solid Waste (MSW), quite literally any old rubbish, while SRF - the better quality of the two - is made from Commercial and Industrial Waste (C&I), which tends to be much cleaner and better sorted. Chris Oldfield, chairman of Untha UK, summed up: “better quality in means better quality out”.

Biomass, or at least the biomass that would be processed by an equivalent to a XR3000 shredder, typically contains such things as waste wood, pallets, kitchen units and demolition timber.

SRF is largely used in power generation plants but can also be used for heating cement kilns. Biomass, as a higher quality fuel, has further uses in smaller applications and in fact, on the continent, even individual residences use biomass burners for their heating.

Machines like this shredder are aimed at the fuel and biomass producers rather than sites that produce waste and are then looking to do something useful with it.

The first design criteria when developing the new machine was to produce one platform that could be adapted to all three main output requirements, for example by changing the screens or the operating speed. For all that some internal reconfiguration is required, the XR300 achieves this although generally speaking plant of this nature will be bought to do a single thing.

A normal high speed shredder will typically produce 20% fines (dust), the low speeds of the XR3000 results in just 5% being produced.

Where this versatility becomes more relevant is with the introduction this year of the XR3000 mobil-e, which is the mobile version. A 30 tonne shredder is not naturally the most agile machine, but putting it on tracks opens it up to dealing with different waste streams around a site. Designing in reconfigurability can allow different types of fuel to be processed - or even recycling applications although that is not its primary function – but realistically it will be used in the same format.

“What this new machine does,” said Oldfield, “which is different to anything else on the market, is that we can do SRF in a single pass, whereas traditionally we've always done pre-shred and secondary shred. Secondary shredders are usually high speed and very problematical. They suffer with wear and very large motors make them very expensive to run. It also means you’ve got two machines, both depreciating and needing maintenance.”

Only one pass on the XR3000 is required even if the material has to be quite refined, e.g. down to a 30mm particle size for use in a cement kiln with all the ferrous and non-ferrous metals and ‘heavies’ (stones, brick etc) removed.

So what’s the trick? “It is a proper cutting system,” claimed Oldfield. “A lot of our competitors are not actually cutting, they use a ripping process, so it's a lot more random. Most of the people don't use this single shaft technology that we're using on the XR.They're using blades which are two shafts coming together.The answer is it's in the design of the machine, it's in the design the rotor, and the setup within the machine which has really put us out in front of everyone else technology-wise.”

Having this cutting technology, while not unique in all shredders, is not available in any other mobile shredders, according to Oldfield.

Wear is a massive challenge for any shredder, so Untha use specialised hardened steels in the main contact areas. Oldfield continued: “The way we design the machines, they are replaceable and can be swapped out easily. We've learnt from the machine tool industry where machine life is key, so we now use indexable cutters, for instance, so we get the additional life.” These indexable cutters can be turned four times to get quadruple the life.

“We've got machines out there that are 30+ years old and still in daily use,” said Oldfield. “Waste machines tend to be very heavily used, so we say to our customers you’ll get an eight- to a ten-year life. They may get 12, they may get more or less. But what we're trying to indicate there is the traditional life of the diesel hydraulic mobile machine is usually only four years, and we're saying we can double that.”

A principal reason how this longevity is achieved comes from the rotational speeds, which are typically five or six times less than a traditional hydraulic shredder. When specifying the rotation speeds the design team at Untha were looking to satisfy more than just the demands of the cutting operation.

Noise is a big issue at waste sites. HSE action points can be triggered at 80 – 85dB and many shredders can be generating 100dB and more. By using slower speeds the XR3000 is significantly quieter, below that first action point, and so can allow extended operation hours without offending the neighbours. The design team also took advantage of special steels with noise absorbent qualities.

Another benefit of slower speeds is that less dust is created – and dust, or fines, is important in this environment for two reasons. One is that it creates an explosive atmosphere, and the second is that it costs money. Typically, a normal high-speed shredder will produce 20% fines, while the low speeds of the XR3000 results in only 5% being produced. These fines cannot be used in any way, their destination is landfill which comes at a cost of £60 a tonne. With a 40 tonne an hour throughput, this works out at savings of £1.5million a year. Or to put it another way, return on capital investment is in the order of four months in terms of fines disposal alone.

It is quite an unusual design environment, as Oldfield sums up: “We're using SolidWorks and we do use some CFD modelling. But you've got to understand as well, shredders are a little bit of a black art, and our expertise lies with the individuals within the company.Some of it is scientific and some of it is expertise and experience.

“And because we we're at the premiere end of this industry we're also very conscious of quality design, and aesthetics are very important to us as well - once you see an Untha machine, it's very easily recognisable.”

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