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Wood Digest asked Len Beusse, CEO of Vecoplan, LLC, and Hartmut Bendfeldt, President of Weima America, to offer their opinions on the difference between chippers and grinders.

Len Beusse

1. When is a chipper appropriate for waste management in a plant? Why?

Drum chippers and disc chippers, are typically used in the primary wood processing industries, such as timber harvesting, chip mills and sawmills. Chippers produce a uniform, precision chip whose dimensions and surface area are very consistent, and very well--defined. This homogenous consistency is important if the chips are to be used in the pulp and paper industry or in the production of certain composite board products. Some large--scale energy producers require such precision for aggregate fuel mixtures.

Most mills will place a very exacting specification on outsourced chip and fiber sources, including limitations on the amount of "fines" and "pins" found in the chip supply. Generally speaking, chippers are the better option for waste management (and add the most value) if you generate the right type of wood scrap, in the right quantity and have the right local end market(s). For instance, if your process generates green softwood waste, you generate at least 5--10 tons per day of scrap and there’s a pulp mill within 50 miles, the precision chips from a chipper could be your best option. Most paper pulp, however, comes from tree farms where trees are cultivated and harvested specifically as a raw material for production of paper products. Entire trees are converted into whole tree chips and pulped into a slurry and prepared for final paper manufacturing processes. In addition to these primary sources, pulp and/or paper mills typically allow a percentage of wood chips recovered from outside sources, such as manufacturing plants. Some mills allow a small percentage of kiln dried wood, and many even allow a small percentage of lower--grade chips from grinders, shredders and hogs. Few allow chips produced from composite materials. Precision, uniform chips nearly always have more value than lower--grade chips. The value of chips from precision chippers, however, varies widely and is based on quantity, quality, supply and demand.

This is not to say that chippers should not be considered for waste management in a secondary wood products plant if you process KD hardwoods. But, the mainstream use of chipper technology is for paper and board products. There are certain other niches where a chipper could be considered over a grinder. One that comes to mind is the production of wood chips for smoker cooking. There are a few others.

Keep in mind that chippers will generally cost substantially more in parts and labor to maintain and operate. Chippers must be kept finely tuned in order to maintain chip quality. Knives must be kept sharp and they become dull rather quickly. Chipper rotors normally operate at a much higher RPM than other size reduction equipment, therefore care must be taken to insure minimal vibration, wear and tear. Chippers are not nearly as tolerant of tramp metal and other contaminants as grinders. To optimize the quality of chips from a chipper, many mills require that pins and fines are screened out prior to shipping, thus adding to the total investment cost of a system. Due to the higher maintenance costs and the somewhat sensitive nature of chippers, there should be clear reasons, in terms of increased value of the chips, to consider a chipper over a grinder.

2. When is a grinder the better option? Why?

Simply put, a grinder is a better option for any application that doesn't fall into the chipper categories described above. While low--speed, rotary wood grinders (such as the ReTech brand from VECOPLAN -- the original developers of this concept) do generally produce a lower--grade chip, they are perfect for many end markets and cost much less to maintain and operate. Chips from grinders can be used for paper and board furnish in some cases, however there are many other uses. Fuel or fuel pellets for process heat, small--scale gasification and co--generation are common uses. Animal bedding products, landscaping mulch, certain export markets, initial reduction for wood flour products, ethanol and biochemicals production are some others. Grinders are sometimes cost--justified just for volume reduction for densification of waste containers. Many of these end uses for grinder chips apply to both solid wood and composite wood waste, therefore giving options to panel processors.

Again, chippers certainly have there place and are absolutely necessary in specific precision applications. But if this level of precision is not necessary, there are inherent operational advantages with grinders. Most wood grinder manufacturers offer both vertical hopper feed models and horizontal conveyor feed models. Vertical feed models have built--in surge capacity and can be fed in a "dump & run" fashion, giving them a labor--saving aspect. These large hoppers accept large, bulky scraps and can be thrown in without prior reduction. Pallets, reject products such as furniture assemblies, batches of blocks and bundles of edge rippings can all be processed with a properly sized grinder. Grinders also operate quietly, require less maintenance, and the chip size can be altered in 5--10 minutes by changing screens. Due to the low speed operation of grinders, a tremendous amount of torque is generated allowing the processing of some very tough materials.