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What to Do about Lumps: Selecting Crushers and Lump Breakers

April 24, 2012
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The objective of this article is to explain crushers/lump breakers, which break lumpy and hard to handle material to improve processing, while handling challenges of many materials. I will first discuss applications and benefits, including common crusher types, factors to consider when selecting a crusher/lump breaker, how to conduct crusher tests, selection of the right machine for your application, and finally, installation considerations.

Crushers and lump breakers are used to reduce lumps in chemicals, pharmaceuticals, foods, agricultural feeds, pet food, aggregates, and other materials, to a consistent size. The offerings range from slow-speed machines for coarse or hard materials, to higher-speed machines that produce finer discharge sizes. There are crushers that excel on applications beyond hard, friable materials, and can even handle rubbery or sticky lumps.

You can use a crusher/lump breaker to solve many processing problems, including preparing your material for conveying, breaking up agglomerates, and reducing your material for further processing.

Preparing Your Material for Conveying
Reducing lumps will make your material easier to handle and insure the success of your conveying, whether in a pneumatic conveyor, belt conveyor, screw conveyor, or other conveying equipment.

Breaking up Agglomerates
Material that is stored can become compacted and/or agglomerated. For instance, bagged powders can form lumps or even be one solid lump in the lower layers, when stored on a pallet. Hygroscopic material, such as cement, can agglomerate on silo walls from exposure to humidity in the air.

Reducing Your Material for Further Processing
Crushing lumps can prepare your material for secondary size reduction, by increasing the material’s surface area to improve efficiency for drying or for better mixing.
Proper sizing can improve your material’s flow characteristics while reducing segregation.

A properly selected crusher can reduce heat buildup in your material while reducing the size and horsepower of downstream equipment.
Crushing off-spec material can eliminate waste and increase overall revenue.

Crusher Components and Operation
A typical crusher has a housing with a crushing chamber and one or two rotating shafts (rotors) with teeth or blades that are mounted to them. Typical models are shown in Figure 1. The teeth or blades are available in various shapes. A common tooth type is made of bar stock that is about 1 in. wide by ½ in. thick by 3 in. long.

There is usually a sizing screen of perforated metal in crushers to hold back larger pieces for further crushing after the material passes the rotors. For heavy-duty applications such as crushing rock, a grid of evenly spaced bars (also called a breaker grid or breaker bars) is used for sizing.

Large square or rectangular infeed and discharge flanges are used to mount the crusher to the surrounding equipment. The crusher size is typically specified by the inlet dimensions, such as 18 by 18 in., or 3 by 4 ft.

Shaft seals are used where the shaft penetrates the housing to prevent fine materials from leaking out of the housing. The bearings are usually located outboard from the housing and seals to keep material from wearing on the bearing seals. The drive can be a direct, chain, or belt drive, and is located outside the bearings. The drive may include a gear reducer after the motor.

In operation, lumps of material are fed through the inlet into the grinding chamber. In a single rotor crusher, the rotor turns and the teeth or blades impact the lumps, crushing them against the sizing screen or grid. In a dual-rotor crusher, the intermeshing shafts counter-rotate and the teeth or blades impact the lumps, crushing them between the rotors, comb teeth, and then against the sizing screen or grid. Particles that are small enough fall through the sizing screen or grid and exit the crusher. Oversize particles remain in the crushing chamber until repeated impacts reduce them to a size that passes through the screen or grid.

The crushers are typically of bolted or welded construction, available in carbon steel, many types of stainless steel, cast iron, abrasion-resistant steel, aluminum, Hastalloy, and even titanium.

A crusher for an agricultural application is typically carbon steel with bolted or welded construction. For chemical applications, the unit can be carbon or stainless steel. Stainless steel crushers using sanitary construction are used in food, dairy, and pharmaceutical applications. Aluminum and titanium crushers can be used for chemicals and resins.

Other Crusher Types
Other types of crushers are also available and can include: a drum rotor tooth style, which has a rotating drum with teeth (as shown in Figure 2); a drum compression unit or roll crusher, which has two counter-rotating drums without teeth, sometimes fitted with intermeshing ridges or rings; and a compression jaw unit or jaw crusher, which uses steel jaws or a series of jaws in multiple stages to break hard objects like rocks.

Options for Single and Dual Rotor Crushers
While standard single- and dual-crusher designs are available from various suppliers, each can be customized to suit your application and provide optimal performance. Feed hoppers, transitions, and support stands can be custom designed for your crusher. Comb-tooth assemblies can be mounted on the side plates or in the center of the rotors in place of a sizing screen or grid to handle sticky and fibrous materials. The rotor teeth or blades break the material against the comb teeth. The rotor teeth or blades, screens, grid, or comb teeth can be hard-faced or heat-treated for abrasive applications. For sticky materials, components can be coated with Teflon or other coatings.

Several types of shaft seals are available, including mechanical lip, packing gland, and air purge.

Some crushers must be cleaned frequently to meet sanitary requirements. Spray nozzles or hose connections can dispense the cleaning solution at the inlet to wash internal components. For these clean-in-place applications, make sure the crusher’s materials, shaft seals, and other components can be safely exposed to the cleaning material.

Selection Factors to Consider
Selecting a crusher can be a two-part process. First, consider your application parameters, and then conduct crusher tests on your material.
Some parameters about your application to consider are: material characteristics, your final product specification, and how you want to impact the downstream process.

Some questions about your material:
• What is the bulk density?
• How much moisture does it contain?
• What is the material’s consistency: hard, brittle, friable, fibrous, abrasive, sticky, or rubbery?
• Is it heat-sensitive or sensitive to other conditions?
• Is it flammable or explosive?

For instance, moisture content can cause material to behave differently in a single-rotor crusher than in a dual-rotor crusher. In some environments, a material with high moisture content can easily be overworked in the crusher, becoming gummy and building up on the components. Reducing the amount of energy input into the material by varying such factors as rotor tip speed, tooth design, and tooth quantity, can minimize problems with high moisture materials. Larger clearances, Teflon-coated components, screen design will help the crusher handle sticky materials.

While brittle materials are easily crushed, you can crush fibrous materials if you select the proper tooth (or blade) design, rotor tip speed, and screen or comb tooth configuration.
Heat-sensitive materials like pharmaceuticals can be crushed using a pneumatic system to draw ambient or chilled air through the crusher, by using water to cool the shaft, by adding a cooling material to the product, controlling the crusher’s operating speed, and choosing internal contact components like comb teeth rather than a screen. Using two machines can reduce the amount of work done at each machine also reducing temperatures.

Consider Your Final Product Specification:
• What particle size distribution should the crushed material have?
• Do you need to have a specific bulk density after crushing?

To achieve your desired particle size and consistency, you need to consider several process parameters, which will affect the downstream process. Answer these questions about your process:
• What is the material’s particle size distribution (specifically, the percentage of lumps versus fines or free-flowing material) as it enters the crusher?
• How is the material being fed to the crusher, such as by surging, controlling and metering, or flood feeding?
• What is the material feed rate to the crusher?
• How will the material behave in the crusher? Will its consistency change by becoming gummy or sticky during crushing?
• How is the material being taken away from the crusher?

If your material has a small percentage of lumps and large percentages of fines, it’s more efficient to separate the fines with a screener or gravity screening transition. The screening will let fines go directly to your process, while only the oversized material goes through the crusher.

Unlike a gravity discharge, a pneumatic conveyor, screw conveyor, or other conveyor can help draw material out of the crusher discharge and prevent over grinding the material by having it backup inside the crusher.

How to Conduct Crusher Tests
Having identified your selection factors will help you work with a crusher supplier while planning the testing of your material. The supplier can run your material through various test lab equipment and determine how to fine-tune the crusher’s size and configuration for your process. For instance, two different size crushers may effectively reduce your material at the speed you need, but one can require much less horsepower and less cost to operate than the other. Testing can also determine the proper crusher component clearances for your application. The tests can have a nominal charge, but this is money well-spent by selecting the best crusher and configuration for your application.

Frequently the lab staff has experience crushing your material or a similar material. By running small-volume tests with your material in their lab crushers, they can gather data on your specific application. Production-scale lab tests are also available. If it is determined that the only place to duplicate the actual processing conditions are in the plant, you can rent production-scale crushers for in-plant testing

In-plant testing should be done off-line of your production line so you can observe the crusher’s performance without impacting your normal production. Off-line testing allows you to adjust the crusher’s operation during tests, which is also helpful because most rental machines are standard units that aren’t configured for your specific application. The supplier can send a technician to help you start up the test crusher. Plan to spend a few days running the in-plant test.

To help determine which testing plan is best for you, consider these:
• Can I run a small volume of material through a small-scale lab crusher outside my plant and still obtain correct results from my material?
• Can my material’s consistency change during shipping to the supplier’s lab? Although relatively uncommon, this can happen with a high-moisture material. In such a case, also consider whether the supplier can restore your material to its as-shipped condition.
• Will the supplier’s test provide representative results for predicting material buildup or sticking inside the crusher?
• Can the test foresee screen blinding over time?
• Will a small-volume lab machine test determine the actual horsepower and machine size my crusher requires? In some cases, it is difficult to scale up crushing results from small material volumes through a lab-size crusher, so the test must be done in a production-size unit.

Installation Considerations and Maintaining the Crusher
The crushers’ location in your production line can impact its operation. In particular, the crusher’s relationship to the feeding and discharge devices is critical.

Feeding device. Establishing the proper feed rate to the crusher is an important factor in selecting the equipment. A common misconception is that the crusher can be mounted directly under a bin to serve as a feeding device for your process. This situation will create a head loading in the crusher. If you consider the typical crusher’s rotor design, you can see that such an installation would cause material to flow from the bin and down around the rotor(s), filling the housing. This put an excessive load on the rotor(s) because there is no room to move or to displace the material during rotation, as shown in Figure 3.

By metering your material to the crusher with a device such as a belt feeder or screw feeder, you can achieve more consistent and efficient crushing action.

Discharge device. The conveyor at your crusher’s discharge can be any of several types, including a pneumatic conveyor, belt conveyor, screw conveyor, or drag conveyor. The conveyor should always remove material at least as fast as the feeder meters material to the crusher, as shown in Figure 3; 1½  times feed rate is ideal. This will prevent material from backing up into the crusher at the discharge, which can cause over grinding.

Material backup at the discharge can also overload the crusher drive. Operating the crusher under such a condition is similar to trying to twist a spade that’s driven into a barrel of sand. Head loading requires tremendous torque. To handle head loads, the crusher would have to be considerably larger than a unit that is meter fed and has proper discharge conveying. There are crusher designs that can handle some loading.

In conclusion, following these simple steps where you define your product and process, perform testing and consider your installation, you will succeed in buying a crusher that will provide years of lump-free processing.

Clark Kreider is the sales manager at Machine and Process Design (Anoka, MN), and has been involved in size reduction application engineering for the past 20 years. For more information, e-mail him at clarkk@mpd-inc.com, call 612-427-9991, or visit www.mpd-inc.com.

Machine & Process Design