Wood waste processors face various considerations when choosing a screen configuration to best obtain the desired end product from their wood recycling equipment. Screen selection and grinding strategy will vary based on various factors, including the type of grinder used-horizontal and vertical-and the type of wood waste being processed, which will also vary by tree species.
“I usually tell customers about the round screens of round grinders (barrels) and the square screens of square grinders (horizontal), but there are exceptions to every rule,” said Jerry Roorda, an environmental application expert at Vermeer Corporation, a manufacturer of wood recycling equipment . “Due to the geometry of the holes, using a screen with round holes in a barrel mill will produce a more consistent end product than a square hole screen.”
Screen selection may change based on two main factors-the type of material being processed and the final product specifications.
“Each tree species is unique and will produce a different end product,” Rurda said. “Different tree species often respond differently to grinding, because the texture of the log can produce a wide variety of products, which can have a major impact on the type of screen used.”
Even the moisture content of log waste affects the final product and the type of screen used. You can grind waste wood at the same place in spring and autumn, but the final product may vary depending on the moisture content and sap amount in the waste wood.
The most commonly used screens in horizontal wood grinders have round and square holes, because these two geometric configurations tend to produce a more uniform chip size and final product in a variety of raw materials. However, there are other options, each of which provides specific functions based on the application.
This is ideal for processing wet and difficult-to-grind waste materials such as compost, palm, wet grass and leaves. The particle size of these materials may accumulate on the horizontal surface of the square hole waste wood shredder screen or between the holes of the round hole screen, causing the screen to be blocked and the waste wood recirculation, thereby reducing the overall productivity.
The diamond-shaped mesh screen is designed to guide material to the tip of the diamond, which allows the cutter to slide through the screen, helping to remove the type of material that may accumulate.
The cross bar is horizontally welded across the screen surface (as opposed to the rolled punched screen), and its function is similar to that of an auxiliary anvil. Mesh screens are often used in applications such as processing industrial wood waste (such as construction waste) or land clearing applications, where less attention is paid to the final product specifications, but more than standard wood chippers.
Since the geometric size of the rectangular hole opening is increased compared to the square hole opening configuration, this allows more wood chip material to pass through the screen. However, a potential disadvantage is that the overall consistency of the final product may be affected.
Hexagonal screens provide more geometrically consistent holes and uniform openings because the distance between the corners (diagonal) is greater on square holes than in straight hexagonal holes. In most cases, the use of a hexagonal screen can handle more materials than a round hole configuration, and a similar production value of wood chips can still be achieved compared with a square hole screen. However, it is important to note that actual productivity will always vary depending on the type of material being processed.
The cutting dynamics of barrel grinders and horizontal grinders are quite different. Therefore, horizontal wood grinders may require special screen settings in certain applications to obtain specific desired end products.
When using a horizontal wood grinder, Roorda recommends using a square mesh screen and adding baffles to help reduce the possibility of producing oversized wood chips as the final product.
The bezel is a piece of steel welded to the back of the screen-this design configuration will help prevent long scrap wood chips from passing through the hole before it is the right size.
According to Roorda, a good rule of thumb for adding baffles is that the length of the steel extension should be half the diameter of the hole. In other words, if a 10.2 cm (four inches) screen is used, the length of the steel bezel should be 5.1 cm (two inches).
Roorda also pointed out that although stepped screens can be used with barrel mills, they are generally more suitable for horizontal mills because the configuration of stepped screens helps reduce the recirculation of ground materials, which often produce The tendency of lumpy wood chips as the final product.
There are different opinions on whether using a wood grinder for one-time grinding is more cost-effective than the pre-grinding and regrinding processes. Likewise, efficiency may depend on the type of material being processed and the required final product specifications. For example, when processing a whole tree, it is difficult to obtain a consistent final product using a one-time method due to the uneven raw waste wood material being ground.
Roorda recommends using one-way and two-way processes for preliminary test runs to collect data and compare the relationship between fuel consumption rate and final product production. Most processors may be surprised to find that in most cases, the two-pass, pre-grind and regrind method may be the most economical production method.
The manufacturer recommends that the grinder engine used in the wood processing industry be maintained every 200 to 250 hours, during which time the screen and anvil should be checked for wear.
Maintaining the same distance between the knife and the anvil is essential to produce a consistent quality final product through a wood grinder. Over time, the increase in the wear of the anvil will result in an increase in the space between the anvil and the tool, which may cause the sawdust to pass through the unprocessed sawdust. This may affect operating costs, so it is important to maintain the wear surface of the grinder. Vermeer recommends replacing or repairing the anvil when there are obvious signs of wear, and checking the wear of the hammer and teeth daily.
The space between the cutter and the screen is another area that should also be checked regularly during the production process. Due to wear, the gap may increase over time, which may affect productivity. As the distance increases, it will lead to the recycling of processed materials, which will also affect the quality, productivity and increased fuel consumption of the final product wood chips.
“I encourage processors to track their operating costs and monitor productivity levels,” Roorda said. “When they start to realize changes, it is usually a good indicator that the parts most likely to wear out should be checked and replaced.
At first glance, one wood grinder screen may look similar to another. But deeper inspections may reveal data, showing that this is not always the case. Screen manufacturers—including OEMs and aftermarkets—may use different types of steel, and things that seem to be cost-effective on the surface may actually end up costing more.
“Vermeer recommends that industrial wood recycling processors choose screens made of AR400 grade steel,” Roorda said. “Compared with T-1 grade steel, AR400 grade steel has stronger wear resistance. T-1 grade steel is a raw material often used by some aftermarket screen manufacturers. The difference is not obvious during inspection, so the processor should ensure They always ask questions.”
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Post time: Sep-07-2021