October 1, 2013
Leverage old-fashioned geometry and physics to get more production from your VSI.
Think back — for some of us, that’s way back — to high school. Pouring over books every night trying to complete homework in biology, geometry, physics and an array of other classes, we asked our parents, “Why do I need this? When am I ever going to use physics anyway?” When you’re sitting in that classroom, studying equation after equation and theory upon theory, they all seem like useless bits of information. But take a spin around the real world, and you quickly realize nothing could be further from the truth.
Take crushing rock and mineral materials, for example. It might seem pretty basic, but the truth is it’s a science that revolves around a firm understanding of physics, metallurgy, and geology.
The technology in Vertical Shaft Impact (VSI) crushers has expanded and become more sophisticated with each passing year, along with expectations for greater productivity, more uniform product, reduced maintenance and higher profits.
And the best antidote to soaring expectations is selecting the right VSI crusher for your operation. With the ever-evolving technologies and options, choosing the right crusher is not always easy. To better understand how the crusher design will impact your productivity, it pays to break it down a little.
Begin with basics
As most operators know, VSI crushers feature a hopper to feed the unit and a rotor impact system inside that does the actual fracturing of the material. These crushers are ideal for applications where a consistent cubicle product in a high quantity is a necessity, such as the production of aggregate for roadways or chemical stripping of minerals.
But beyond producing high volumes of a uniform cubicle product, VSI crushers provide exceptional flexibility, allowing operators to make the changes needed to consistently meet evolving DOT specifications and to produce products in a tight gradation range.
VSI crushers often are added to operations that have an increasing need for the production of intermediates and fines or to more evenly balance plant production capabilities. VSI units are also a great option when expanding a plant or complementing existing equipment.
With the versatility and control offered by VSI units, there are a multitude of areas in which they can positively affect the balance and flow of an operation. But understanding the technology in today’s designs and how it affects costs and profits is a key element to finding the ideal match for an operation.
When it comes to VSI crusher designs, there are a few critical choices that must be weighed and considered. The critical components of that crusher, while invisible to the average observer’s eye, can make a ton of difference when it comes to meeting productivity and profit expectations.
Internally, VSI crushers can come in one of four configurations:
1. Closed rotor and anvil ring
2. Shoe table rotor and anvil ring
3. Closed rotor and rock shelf
4. Shoe table rotor and rock shelf
An open and closed decision
Let’s start with the easy geological decision first. Yes, those college classes pay off too! Rock and other minerals are found at different hardness and abrasion levels within Mother Nature. These abrasion levels and hardness ratings can vary from state to state, mineral to mineral or even within the same jobsite. The abrasion ratings must be considered when choosing the rotor design.
Generally speaking, the open shoe table will work best for softer materials or for processing somewhat abrasive materials in operations that require a larger feed size, normally 3 inches and larger. This boils down to a matter of dollars and sense — analyzing the bottom line.
With abrasive materials running through the crusher at high speeds, shoes may only last six to nine hours before they require replacement. With a softer material, somewhere around a 26 to 32 on the L.A. Abrasion spectrum, those shoes might last 30 hours or more depending on shoe size, which lowers costs and the amount of maintenance required. When working with the shoes, it is important to either replace them as a complete set or in pairs to maintain proper balance on the rotor.
The shape of the shoe can impact the production sizes. A straight shoe design tends to produce more fine materials because the force placed on the material is more direct. A curved shoe design produces a coarser product because some of the energy is absorbed in that curve. A manufacturer that understands the effects of the Mass x Velocity = Energy equation can work with operators to design a system that combines both curved and straight shoes to maximize production of the correct spec of material.
Some of the material operators encounter on a daily basis is extremely abrasive, registering 17 or lower on the L.A. Abrasion scale. In those instances, a closed rotor design tends to prove the most effective. Instead of using shoes, manufacturers that understand the effects of the hard minerals will design the closed rotor with a series of tungsten carbide pins that are used to trap material and build up its own internal “shoe” system. This reduces abrasion and maintenance.
Again, physics and geometry become very important when designing either the shoe or the closed rotor system. In both cases, the correct angles must be maintained to make sure the materials flow as they should and build up protection in the right places without hampering efficiency or increasing costs.
Furthermore, there is more to a closed rotor than meets the eye. When it comes to production, downtime is a critical factor. More modern designs of the closed rotor eliminate welding through a simple bolt-on approach that also makes it easy to replace all parts. All the work can be done on site minimizing downtime, yet another factor to consider when selecting rotor designs.
But the rotor decision is not necessarily an open and shut case. Beyond the initial design, the diameter of the rotor itself will impact the production performance. Increase the rotor diameter or number or ports while increasing the speed at which material exits the port and product will change. Remember, the MV=E equation tells you that this increased speed will result in greater impact energy and high fracture percentage of the material.
The heart of the matter
Perhaps the single most important component decision operators must make is whether to use a rock shelf system or an anvil ring.
Long touted as the less costly approach for VSI crushers, rock shelves are the most well known and least understood in terms of their effect on crushing. With a rock shelf system, material builds up in what is essentially an open ring, to form a shelf comprised of the rocks or minerals the unit is crushing. This shelf is designed to protect the ring from abrasion and provides a wall at which material is hurled at high speeds and with great force to break or fracture the minerals.
For a long time, the commonly held and often mistaken belief has been that the rock shelf provides the most cost-effective solution for breaking because you avoid the high costs of replacing anvils as they wear. However, that’s only half of the story, and it only factors in one cost — maintenance. As any good operator knows, a lot more plays into profit than simple maintenance costs.
A quick look at recirculation rates/efficiency and energy costs sheds a totally different light on this method.
While the theory of a rock shelf might be appealing, flaws are revealed quickly when it’s tested. First of all, the shelf is not solid and fixed. When the material thrown by the rotor impacts the shelf, it, too, moves and shifts from the force. It also absorbs energy that could otherwise be used for breaking. This significantly reduces efficiency. In essence, it eventually begins to form a drum where the material is rolled along with little force or energy. In fact, in testing, rock shelf units can result in nearly 30 percent recirculation and achieve gradation of only 74 percent in a single pass. Engineering studies have shown three to five times less efficient crushing using a rock shelf system.
Second, to get enough force to break rock, significantly more RPMs are required. Adding a larger motor and more KW can add as much as $2 to $3 per ton to the production costs.
Other technology in the crusher market uses an anvil ring that features a series of 28-percent chrome white-iron stationary anvils. The material or rock is propelled at the anvils by the rotor and virtually all of that energy goes into fracturing or crushing the material. There is no energy absorption or material rolling as found in rock shelf systems.
While it is true that the maintenance costs of the anvil system might be slightly higher, some manufacturers ensure the longest life and the best return for the dollar by using anvil systems that can be rotated to use the entire face of the anvil before it needs to be replaced. This significantly extends the wear life.
In testing, anvil style units have proven to reach 97-percent gradation in the first pass, which means recirculation is nearly eliminated. Add to that the fact that an anvil ring uses only 1.5 horsepower per ton versus 3.0 horsepower per ton on a rock shelf system, and a quick calculation shows the cost savings will more than pay for the anvil style crusher and maintenance with plenty of profit to spare.
In fact, when looking at total cost of ownership, calculating the increased initial investment and slightly higher maintenance costs as well as the increased productivity, efficiency, and energy savings, most applications will result in a 30-percent increase to the bottom line.
Making the grade
Once the geometry, physics and geology lessons are learned and the right system is in place, fine-tuning sizes for ultimate performance is the final step. It all comes down to energy, the effect of throw distance and speed on that energy and the resulting fractures. To ensure proper crushing, operators must maximize the velocity and distance relationship.
Distance can be easily altered by either adjusting the rotor diameter or the number of anvils in the anvil ring system. As a rule of thumb, the closer the throw distance, the more fines that will be produced because you are getting more energy in a short amount of space. Going from an 18-anvil ring with a typical throw distance of 10 inches to a 16-anvil ring with a throw distance of 5 inches can help operations adjust their production output and be versatile for ever-changing specs. The same applies to increasing or decreasing the diameter of the rotor.
In addition, the impact energy can also be adjusted by speed in the form of a simple change to the RPM. By speeding up the RPMs you will produce a greater percentage of fines, while slowing it down will decrease the number of fines. With today’s variable frequency drives (VFD), perhaps the single most significant advancement in VSI crusher technology, a simple computer input can adjust the speed and therefore the spec of the finished product. In addition, the VFD allows the use of smaller power sources to run a very large motor, further reducing energy costs.
The final report card
Just as in school, the true grade for the operation comes at the end of the test or, in this case, the end production results. If companies do their homework up front, understand the science of crushing and the effect of the design on production, they’re likely to have a grade A experience.
Neil Hise is the president of Cemco, Inc.