Compaction, Shredding and Crushing
The key waste management processes used to reduce volume and size: Compaction removes air and reshapes materials; Shredding breaks down waste into smaller pieces for further processing; and Crushing fractures brittle materials like glass. Each method optimizes space and handling, depending on the material properties.
Compaction and reduction ratio’s
Compaction is the process of reducing volume of objects*. The reduction of volume is realized by removal of air trapped in and between objects and/or changing the shape of the object by force permanently. For compaction to be effective we must have:
- sufficient power exercised on the object, indicated by press force in kgs.
- long duration of the force exercised on the object, indicated by cycle time in seconds
For vendor products evaluation these are two important factors. Keep a close eye on them.
* Reduction ratio indicated as % relate to the volume decrease from start of the compaction process or initial load up to the last possible compaction cycle. Compaction cycle are repeated until the highest possible density is achieved. For the Delitek compactor we run at least 4 to 5 full compaction cycles.
Achievable reduction ratios vary over waste fractions. High ratios are achieved for PET bottles, carton boxes, mixed household waste bags and galley packaging. Others have very little potential such as books, paper and textiles. Be on your guard if average high ratios are reported. A waste fraction capacity analysis including corresponding expected volumes is helpful for machinery selection, daily processing and storage space required during the voyages. Obviously combined with a check of power, press force and duration of the full compaction cycle.
On Shredding
This is the process of breaking down materials into smaller pieces for further processing for instance as pre-treatment for compaction. There are various shredding methods available such as: fracturing, tearing and shearing*. For typical dry waste fractions on a vessel most suitable method is shearing. Rotary Shearing or an industrial shear shredder deforms, cuts a wide variety of materials in smaller pieces via hooked cutters, knives or shearing tools which are on two or more counter-rotating shafts. It is the most common used method when processing various materials and size.
* For Marine and Offshore application and materials such as e-waste, PET bottles, Metal scrap, Plastics, Cans, Jars, 55-gallon drums and dunnage a heavy-duty shearing shredder has:
- Two or more independent counter-rotating shafts
- Rotation speeds must also differ in rpm. ideally ratio 2:1
- Slowly turning gear and shaft for high Torque in Nm
- Knives, cutters and pick-up hook at right size, angle and geometry
- High strength hardened knives, blades and scrapers
- Sufficient “surface area” for pick-up of materials
Fracturing / Crushing of Glass
Brittle materials are better to be fractured. Glass is one of those and onboard collected as segregate waste stream from mainly provision stores and galley area*. Glass requires less force to be crushed but – for consistent output in particle size – speed is important. There are various methods used amongst them single shaft shredders, rotor stator grinders and high-speed hammering devices. Glass crushers reduce the volume significantly and with that the mass density is increased, crushed glass output is heavy.
* For shipboard application it is worthwhile to check for:
- Strengthened quality steel materials hardox or special cast alloys
- High speed rotary shafts preferably above 1.400 rpm.
- Length and diameter of feeding tube, L=100 cm D=16 mm
- Proven crushing mechanism for consistent output size
- Easy collection, store and transport devices
- Glass dust prevention measures for all doors and openings
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