Start with the Beginning and the End in Mind
There are several questions that need to be answered when considering a purchase of a wood grinder including: What type of material are you planning on grinding? What are the thickness and dimensions of the waste material? Will it have metal or nails in it? How contaminated might the waste be? Will you be grinding shingles or other non-wood waste? How fine does the processed chip size need to be? Are you interested in simple volume reduction or total recycling? Are you producing mulch, colored mulch, animal bedding, biofuel, fiber for engineered wood products, etc.? And ultimately, should you use low-speed, high-speed, or chipper technology?
Bandit Industries President Jerry Morey, commented, “Biomass markets require chips of very specific sizes, and they must also be extremely consistent. Some systems work best with larger chip sizes, while others require micro chips as little as ¼ inch. Markets can range from large scale power plants and pellet manufacturers, to local companies and universities. While chip size can vary, common among them all is a need for chip to be uniform. Consistency is key.”
You have to really focus on the type of material that you start with and what you hope to produce in the end because that will guide the entire process. In this article, we will focus in on a few of these questions to help shape your thinking on which technology is right for you.
Types of Grinding Solutions
First, you need to understand the three choices of grinding technology and what they are designed to process. The three major types of wood grinding solutions on the market are – high-speed grinders, low-speed grinders sometimes called shredders, and chippers; and there are some important differences between them.
High-speed grinders will run at a higher rotor RPM than a low-speed grinder, they produce higher throughput volumes, more noise, and often higher operating costs due to higher capacity.
Scott Harrington, Northeast regional sales manager for Rotochopper, said, “In the pallet industry horizontal high-speed grinders work very well. They feed pallets into the grinder much more efficiently than vertical mills.” Harrington added, “that low-speed grinders, while running at lower speeds, can handle contaminants better than high-speed grinders but offer lower production rates and reduced particle size control.”
A low-speed grinder uses high torque to shear the wood in the grinding process, and also produces a very consistent and uniform end-product by use of screens to control particle size with less noise and lower throughput volumes. Low-speed, high torque grinders are offered in horizontal-fed and vertical or hopper-fed classes. Jack Cress, CEO of Cresswood, stated, “horizontal-fed low-speed grinders are excellent for zone grinding or in-line processing/sorting, while hopper-fed machines are suited for a “dump and go” grinding application.”
According to Rotochoper, disk chippers and drum chippers are designed to very quickly process solid round wood—and they do it better than any other machines on the market. Chippers are often used to produce chips for paper, OSB board and playground cushion from green round wood. However, once you move outside a wood chipper’s design purpose, its advantages quickly disappear in favor of a wood grinder. Rotochopper further explained that if you have a steady stream of clean round wood, nothing beats the efficiency of a chipper. But they have difficulty feeding and chipping material of mixed dimensions, including tangled piles of tree trunks and branches. Many chippers lack a feed system aggressive enough to process whole trees with rigid limbs.
Morey, explained, Whole tree chippers are the machines of choice for quickly processing clean round wood into chips for biomass use; our machines have no problem processing large trees. Our small and mid-range whole-tree chippers use aggressive dual-wheel feed systems that pull in and crush large material, while our large whole tree chippers feature multi-wheel feed systems with horizontal and vertical wheels that compress wide crotches and big limbs. When shopping for a chipper, often times people find that going larger with more horsepower actually lowers operating cost because the machine can process more material in less time.”
There are two high-speed grinder style choices – tub or horizontal. According to Morbark, tub grinders are popular because they are productive, and easy to work on and maintain.
Tub grinders have a larger opening that makes it easier to feed large diameter material that are fed with large bucket loaders. Tub grinders also are known for simplicity of design relying on the force of gravity and tub rotation to regulate speed rates. This means the production capacity may vary more than with a horizontal grinder. Also, horizontal grinders tend to have a more complex feeding mechanism, which can make maintenance more challenging. The feed system of a horizontal grinder helps to maintain a consistent processing rate for a variety of raw materials, from whole pallets to wood chips to slab wood.
Horizontal grinders are viewed as safer since the grinding mechanism is not exposed. This prevents debris from escaping and makes it more difficult for people to get hurt by the machine. Proper safety controls and the use of shielding can mitigate these factors in some ways for tub grinders. When tub grinders are not fully loaded with material, the hammermill can become exposed allowing the ejection of material outside of the tub. Horizontal grinders can process longer material due to the feeding process.
Morey of Bandit said, “Tub grinders can be effective on very large, bulky chunk wood, but without a feed system the end-product isn’t very good, and they’re extremely dangerous to operate. The mill is exposed at the base of the machine, and material can be ejected up and out to several hundred feet around the grinder. This is why Bandit doesn’t offer a tub grinder.”
Speaking about horizontal grinders, Morey suggested, “Not all horizontal grinders are equal. Machines like The Beast that use a down-turn cuttermill will generally feed better because the mill is working with gravity, not against it. That will produce a better end-product, and it can also lessen the chance of material being ejected from the machine.”
One thing to consider is how much processing you will need to do and you can achieve the desired result with one pass through the system. You may need to add a secondary hammermill to a grinder to achieve very fine particle sizes, which can be added to low-speed and high-speed technologies.
“Whether a tub or horizontal grinder is best for you goes back to knowing which applications you may find your grinder working in,” declared Curt McMullen, Morbark regional sales representative. “Knowing up front, prior to purchasing is key.”
What Technology Best Suits Your Needs Based on Your Throughput Requirements?
How much material will you process or handle in an hour? The amount of material will impact the processing speed of the line. Both the volume you need to grind now, as well as the volume you will need to grind in the future, should be considered as you choose your grinder. The basic rule of thumb is that higher speeds are needed for higher volumes.
“If you want to process less than 15,000 pounds per hour, low-speed technology is the right choice for you,” said Cress.
“This ‘rule’ is not always true”, stated, McMullen. He said that shredders that run at low-mill RPM are nearly as productive as midsize grinders. He added, “You should also consider the amount of contamination and the required end product.”
Where you plan to use the grinder can also impact the decision. “A low-speed, high torque, single-shaft grinder, like Cresswood manufactures, runs anywhere from 90 – 300 rpms; it is very quiet when compared to high speed technology,” explained Cress. “You can put a low-speed grinder right into your work environment in close proximity to your work force.”
How Much Does It Cost to Grind Per Unit?
Figuring out how much it costs to grind per ton or cubic yard is a good way to choose between various options. You have three major components to evaluate – energy/power, labor, and handling and maintenance and repair costs.
“Cost to purchase is important, but owning and operating costs as they relate to cost per ton produced is most important,” said McMullen.
Diesel machines tend to be more expensive to run than electric, according to Cress, but they do have some benefits. They are usually self-contained and portable, making them a good choice for use out on location or for mobile applications. Cress also stated that one major advantage of electric-powered machines is that they can cost less to run. Another advantage of electric grinders is that they require less downtime for routine maintenance, according to Rotochopper.
“Where low-speed technology has its benefits is the cost per unit; it is typically lower than high-speed technology,” Cress said. “If a company has volumes to be processed less than 15,000 pounds per hour, and operating cost is a concern, then it should choose low-speed technology to meet its needs.”
Keep in mind that labor costs may differ from one design to another. Grinders that must be fed in batches can cost more in labor costs than one that can simply be dumped into or automatically fed by a conveyor. You want to cut down on labor where possible.
Many types of equipment offer the flexibility to be batch fed or integrated in-line with handling equipment like conveyors. In this regard, pallet recyclers can often minimize labor costs with the right planning to allow for the smoothest integration of the grinder into the facility.
Repair costs vary per machine. Ask the machinery supplier what typical wear parts are and the costs for repairs as well as the maintenance track record from other owners.
Do You Need Metal Separation?
Nails and other scrap metal need to be removed from the wood residue in order for the end product to be sellable. Knowing what percentage of separation you need will determine what kind of extraction system you need in your grinder. The amount of separation needed will most likely be determined by the specifications of the fiber buyer or based on your use for the processed particles.
There are several ways of removing metal debris, but no separator can produce 100% debris-free material. Cress stated the most effective single method in the pallet industry is the cross-belt magnetic separator; this type of separation meets most pallet customers’ needs. It is self-cleaning, meaning it removes the metal from the magnet and dumps it into a collection bin, which reduces labor costs. However, two-stage metal separation is required for some customers based on the use for their fiber (typically for bedding).
Two-stage metal separation includes a cross-belt magnetic separator and a magnetic pulley head for the highest level of metal separation, which are also both self-cleaning. According to Cress, many customers use both separators within one system in order to meet their processed fiber requirements.
“One critical aspect for optimum metal removal is ensuring the outfeed conveyor is wide enough to keep the bed depth of the discharge material low as it exits the machine,” said Mark Lyman, president of West Salem Machinery. “This is a great aid in achieving full magnetic removal of ferrous metals with the overhead self-cleaning magnet and a magnetic head pulley. This is a very important factor in producing the highest value finished fiber.”
Customized Solutions
What add-on or unique options will you need to make a grinder flow with your operation? A key consideration here is infeed and outfeed setup. It could be conveyor belt, vibrating conveyor or chain infeed system. What power feed mechanism fits your situation? What screen type might work best for you?
“The ability to get help to understand the best style and type of system is invaluable,” Lyman stated. “Horizontal or vertical feed. Low RPM or high speed. Secondary grinding for products like animal bedding. The correct screen size for your end product and ability to easily adjust sizing. All important factors in the success of the installation,” Lyman continues. “Infeed and discharge conveyors, support structures, and access platforms all need to be worked out.”
“Understanding how to integrate grinding technology into the overall process can deliver huge cost savings in labor and energy costs. This includes how to properly use screening technologies to complement the grinding machines and deliver superior finished fiber products.” Lyman explained.
Flexibility of operations can be a major consideration for some companies. Morey of Bandit, pointed out, “We have some customers that chip material with knives on The Beast cuttermill for playground chips, then switch to teeth for producing a fine finish mulch for landscaping, then switch back to knives for more chips. Customers that choose The Beast chipping drum over the standard cuttermill can swap between the two in about a day, giving them the flexibility to have both a high-production chipper and horizontal grinder.”
Mobility/Local Restrictions
Grinding wood waste can be a noisy endeavor. How much noise will the unit make and are there ways to shield the unit from the outside? Will you be moving the unit around multiple locations so it needs to be mobile? Do you need a unit that can be moved and follow road restrictions? What about the type of power that is available at the location – electric or diesel?
Worst Thing You Can Do!
Buying the wrong piece of equipment because you don’t do your research can be a nightmare and very expensive. Talk with others in the industry who have the various makes and models you are evaluating; figure out if what they are doing will work for your process flow. And the second biggest mistake is to let wood material go to waste instead of turn it into something useful. Research fiber buyers in your area, and if there are new or outside-the-box markets needing wood fiber. Consider contacting these leading suppliers listed below to learn more about what they can offer you.
