Good Vibrations

April 1, 2005
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Vibratory screen cleaning devices are improving screening efficiencies in a variety of powder processing applications.



Figure 1. Examples of screen blinding from a) near-size particles and b) fine material buildup or caking.
Screen blinding is a common problem in powder processing applications. It occurs when near-size particles build up or become trapped on the wires, effectively blocking the screen openings and preventing material from passing through the screen (see Figure 1). Fortunately, a number of effective vibratory screen cleaning devices are on the market that can be used to improve screening efficiencies.

Figure 1b.

Sliders

The most common self-cleaning vibratory devices are sliders, in the form of either loose rings or clusters. Loose rings slide directly beneath the screen on a support surface, which is normally a perforated metal plate. The rings are bounced upward into the screen mesh by the vertical motion of the separator and travel around the screen (see Figure 2). They do not require a separate driving force.

Figure 2. Sliders travel across the top of a perforated plate. The motion of the vibratory separator throws the sliders into the screen as they rotate around the center of the machine.
Slider rings work in one of four different ways, depending on the properties of the material being screened:
  • With most materials, the vertical separator motion forces the slider ring to hit the bottom of the mesh, dislodging trapped near-size particles out of the screen mesh openings. This effectively cleans the screen and opens the mesh for the next particle to fall through.
  • If the material is hard or friable, the sliders shear or break large protruding particles into smaller pieces, which clears the screen and allows the particles to flow through the openings.
  • If the material is soft and pliable, the motion of the sliders can help force the material through the screen openings.
  • If the material is fibrous, the horizontal scrubbing motion of the sliders across the bottom of the screen shears trapped fibers from the screen openings, improving throughput capacity.


Slider rings work especially well on fine-mesh screens because of their low impact energy and shearing/wiping action, which cleans the screen openings. They can be used in both wet and dry screening applications. However, they are not efficient for materials that tend to ball up or agglomerate within the sliders, or for extremely hard, irregular-shaped materials, which can partially protrude through the screen opening and stop the motion of the slider. They also generate minor amounts of heat, which can fuse temperature-sensitive materials.

Figure 3. Clockwise from bottom left corner: single slider rings, slider clusters and rubber balls.
Slider clusters work on essentially the same principle as slider rings but are four to eight times larger (see Figure 3). They are normally used to facilitate maintenance on large-diameter machines. Their impact force is higher due to their larger weight and size, but fewer impacts occur due to the reduced number of clusters used. Whether individual sliders or larger slider clusters are more effective depends on the material being screened.

Figure 4. Self-cleaning kit balls bounce between a support screen and the classifying screen. The vertical motion of the separator launches the balls and generates the screen cleaning action.

Ball Trays

Rubber balls are the second most widely used vibratory screen cleaning method, but they can only be used for dry screening. The balls are generally 13/8 in. in diameter and are supported by a second coarser mesh 2 in. below the classifying screen mesh. Because the balls must be thrown against the bottom of the screen to work, a higher vertical separator motion is required to activate the ball cleaning action (see Figure 4). Like sliders, balls do not require a separate driving force.

The higher weight and more intense vertical action of the balls causes them to impart more energy with each impact compared to sliders. As a result, balls are generally used for 60-mesh and coarser screens that can withstand the higher impact energy. They are not recommended for finer screen meshes because the balls can damage the screen.

Balls are very good at cleaning near-size irregular or jagged-shaped particles wedged in mesh openings-materials that can trap sliders. Balls are also useful for shear-sensitive materials, where smearing or agglomeration can occur. They are not recommended for use with fibrous materials because they do not impart the shearing action required to dislodge fibers from the mesh.

One disadvantage of ball trays is that the balls spread radially to the periphery of the screen. This generates excellent cleaning action at the screen edge; however, blinding might continue to occur at the uncleaned screen center, limiting screening capacity.

Figure 5. (Top and close-up) A sandwich screen uses a bonded mesh on the top and bottom of the screen tension ring to enclose the cleaning devices.

Sandwich Screens

Sandwich screens are made by bonding a classifying mesh to the top of the screen tension ring and a coarser support mesh to the bottom of the ring. This creates a "sandwich," with the screen cleaning devices (typically sliders and/or balls) floating between the screens (see Figure 5).

This design improves screen cleaning because it allows the sliders or balls to more actively bounce off the bottom mesh compared to a standard perforated metal plate. The construction also locates the cleaners closer to the classifying mesh, which allows a more gentle vibratory motion that achieves better cleaning.

The screen and motor life are also increased because the improved cleaning efficiency of sandwich screens requires less vertical vibration amplitude than standard sliders and balls. Additionally, because the sandwich screens are supplied in an integrated cartridge design, they can reduce maintenance requirements and facilitate screen changes.

Figure 5b.
Sandwich screens allow the combination of smaller balls inside each slider, which can yield better results than sliders or balls used individually. The sliders provide complete coverage of the screen surface and hold the balls in the optimum position for thorough screen cleaning. The use of smaller balls compared to conventional ball trays yields the ideal impact energy for coarse or fine screen meshes.

Sandwich screens can be used for dry or wet screening and can be fabricated with metal or synthetic meshes. They are particularly useful for wet screening because liquids don't build up on the support mesh, as often occurs with perforated plates.

Another advantage of sandwich screens is that they generate less noise than screen cleaning solutions that use perforated metal plates. However, the screens are more expensive then perforated plates.

Figure 6. A top-side necklace ring slides around on top of the screen to help clean the screen mesh openings.

Top-Side Screen Cleaning Methods

As their name indicates, top-side cleaning devices operate on top of the classifying screen mesh. These can be divided into three types: brushes, dams and wipers.

Top-side rotary brushes are exceptional for clearing fibrous materials that tend to mat on top of the screen and block the mesh openings. The brushes are propelled around the screen diameter by the vibratory motion of the separator; no additional driving motors are required. The brushes ball up the fibrous mat above the screen to expose the screen openings, and the fiber balls are then discharged from the top of the screen. However, one disadvantage to top-side brushes is that the bristles can fall off and be lost in the product stream.

Top-side necklace rings create a radial edge dam to keep materials on the active screen longer and thereby increase the capacity throughput (see Figure 6). In some cases, this can increase the yield of smaller machines to match the capacity of larger machines.

Top-side wiper rings provide the same radial edge dam as a necklace, but also have a wiper that provides an additional shearing action to improve capacity. This shearing action can help wipe agglomerated materials through the screen mesh, or break up friable clumps to improve capacity or reduce loss. The disadvantage to top-side wipers is that they can slow or impede oversize material discharge from the machine. They should not be used with materials that tend to ball.

Vibro-Rim Screens

Vibro-rim screens are composed of metal ball bearings inside a hollow screen tension ring. The vibratory motion of the separator moves the ball bearings, creating a secondary vibrational impact against the inside screen tension ring that is transmitted to the screen mesh. This excitation helps clean the radial edge of the screen and is useful for synthetic meshes, where unacceptable wear and damage can occur from sliders or balls. However, because the additional vibration occurs only at the screen rim, only the outer 2 in. of the screen tends to be excited and cleaned with this method.

Water Sprays

Water sprays clear screen mesh openings, help eject solids, and keep slurries from drying out and building up inside separators. They also eliminate the risk of contamination that occurs with slider, ball and brush materials. Stationary spray nozzles are used to clean mesh openings and wash fine particles from oversized solids, while rotary spray nozzles are used to deblind screens and eject fibrous or lint-type materials. However, water sprays tend to cost more than other screen cleaning methods and also dilute the material being screened. As a result, this method is typically only used for unique wet screen blinding issues.

Figure 7. a) An ultrasonic screen cleaning system installed in a round vibratory separator. The externally mounted electrical control box powers the transducer affixed to the screen tension ring. The screen is bonded to the exciter ring, which vibrates the mesh, as illustrated in b).

Ultrasonic Cleaning

With ultrasonic screen cleaning, vibrational energy is generated by an ultrasonic frequency transducer attached to the metal screen mesh. The ultrasonic energy breaks down electrostatic charges and surface tension that would otherwise agglomerate particles and prevent efficient screening (see Figure 7, p. 20).

Figure 7b.
The ultrasonic vibrational amplitude is typically 0.000005 in. and occurs at a frequency of 35,000 times per second. The lower amplitude and increased frequency means that the particle bounces on the screen openings approximately 1000 times more often than with a standard vibratory separator, thereby increasing the statistical chance that the particle will go through the mesh opening. Ultrasonic-assisted screening is useful for high-accuracy applications where the particle size of the material is similar to the mesh opening of the screen.

However, ultrasonic screen cleaning is the most expensive screen cleaning process. Ultrasonic equipment generally costs 10 to 20 times more than sliders, ball trays, sandwich screens or vibro rings, and the consumable screen costs are up to five times higher because the screen tension ring includes an ultrasonically tuned resonator ring weldment. For this reason, ultrasonic screen cleaning is typically used only with high-value materials and difficult screening operations that cannot be accomplished efficiently by any other method.

Figure 8. A pneumatic screen energizer attached below the screen surface.

Energizers

The energizer is a pneumatic screen vibration generator that operates at frequencies and amplitudes that are lower than ultrasonic screen cleaners but higher than standard round separators. It can be used on both synthetic and metal screen meshes (see Figure 8).

The multiple transducers generate a more uniform vibration across the entire screen surface on large-diameter screens compared to ultrasonic screen cleaning. Additionally, the pneumatic generators do not use electrical energy, and simple mechanical maintenance is required.

The energizer adds the effectiveness of high frequency screen excitation at a lower cost-energizer systems typically cost one-third that of ultrasonic equipment, and the disposable screens cost one-quarter that of an ultrasonic screen. However, as with ultrasonic screening, the energizer screen cleaning method should only be used for higher-value materials where sliders, ball trays and other screen cleaning methods are inefficient.

Efficient Screening

A variety of vibratory screen cleaning devices are available to help prevent blinding. Slider rings and clusters are the most common screen deblinding method for the majority of wet and dry vibratory separator processes. Ball trays are used for coarse mesh dry screen cleaning, while sandwich screens generate more efficient screen cleaning action then either ball trays or sliders on perforator plates.

Screen cleaning devices on the top side of the screen are less common and are used for more specialized cleaning applications, while liquid spray cleaning methods are used for unique wet screen blinding issues. Ultrasonic screen cleaning is very effective but also expensive. Pneumatic screen energizer systems can yield the benefits of high frequency excitation at a lower cost.

Each material and application has unique variables that affect screening efficiency. By working closely with screening equipment suppliers, ceramic manufacturers can ensure that they choose the right screen cleaning method to optimize throughput in their facility.

For more information about screen cleaning, contact Sweco, 8029 U.S. Highway 25, Florence, KY 41022; (800) 849-3259; fax (859) 727-5123; e-mail eric.Johnson@sweco.com or greg.brock@sweco.com; or visit http://www.sweco.com.

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