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Criteria for Selecting Vibratory or Tumbler Screeners

September 30, 2013
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Material movement on a screen surface impacts the screening efficiency and effectiveness. Vibratory screeners and tumbler screeners produce two distinctly different motions. The advantages of both screening methods can be realized through the desired screening effect on material. In some cases, the suitability of both screening technologies overlaps. The different actions of both screening technologies can provide different advantages for screening materials.

Performance Action of Vibratory and Tumbler Screening
The primary action of vibratory screening can be described as having a vertical element. Vibratory screeners agitate material across the screen surface. The vertical effect elevates the material which subsequently descends through openings in the screen mesh. As material descends to the screen surface, it will either hit a screen wire to be projected upward again or it will fall through a mesh opening. Although the probability of falling through a mesh opening or bouncing off the mesh is random, the vertical action does increase particle acceleration (which can be advantageous for sticky materials or wet screening). Unbalanced motors can be utilized on a vibratory screener to provide some directional material rotation.
   The action of tumbler screening is three-dimensional. Tumbler screening relies on radial, tangential, and eccentric directional motion to rotate particles in small circles over the screen surface. The combination of the three directional forces forms an ellipse. Instead of being elevated, the material maintains contact with the screen surface with a horizontal rolling effect. The material moves over the screen surface elliptically. As the material tumbles over the screen mesh, the probability that it will fall through screen mesh is increased as a function of increased contact time with the screen. The radial, tangential, and eccentric motions can be set and tightly controlled with mechanical adjustment. The radial component controls the outward movement of the material on the screen from the center. The tangential component controls the circular motion of the material. The eccentric component controls the angular effect. These three variables can be independently adjusted to maximize screen efficiency based on the response of the material. The motion of the machine is recordable and repeatable with mechanical setting of the three controlled variables.

Advantages of Vibratory and Tumbler Screening

Vibratory Screeners
• Simple construction provides for small relatively compact profile.
• Vibratory screeners are capable of higher RPM and higher material acceleration.
• Vibratory screeners are particularly suitable for wet screening and sticky material screening.
• Cost effective for relatively simple screening operations (such as low percent scalping)
• Option for a central outlet at bottom provides compact installation for an in-line process (beneficial during pre-packaging final safety screen or a high throughput low accuracy screening)
• Ease of maintenance due to simple construction and relatively fewer working components

Tumbler Screeners
* Three-dimensional action increases material/screen mesh contact time improving screening efficiency.
* Recordable and repeatable mechanical settings for screener motion
* Ability to effectively screen irregularly shaped particles
* Ability to effectively screen fine fractions (as small as 20 micron)
* The three dimensions of tumbler screening can be adjusted (radial, elliptical, and tangential)
* The RPM can be also controlled with a frequency inverter (control of four total variables which can be adjusted for optimized screening performance).
* Tumbler screening is capable of a larger screening stroke compared to vibratory screening (up to 35 mm).
* Up to six screen decks for as many as seven separations
* Tumbler screeners can be manufactured to diameters of 114 in.
* Central drive system provides the opportunity for increased mechanical anti-blinding devices (including rotating brushes).
* Tumbler screeners are relatively quieter during operation.

Applications of Each
Vibratory screening has an advantage for applications that can utilize the vertical action. Sticky products or products prone to agglomeration can benefit from vibratory screening. Wet screening is a typical application for vibratory screeners. Another typical vibratory application is the removal of large oversize particles that represent a small percentage of material size distribution (material scalping). The simple construction of a vibratory screen can allow for a very small plant footprint. This can allow for screener installation is restricted space environments.
   Tumbler screening has advantages for fractioning fine powders due to the efficiency resulting from increased surface contact. Increased and uninterrupted contact time with the mesh surface reduces the amount of time for undersize particles to fall through the mesh. Tumbler screeners are also effective for the screening of irregular shaped particles. Particles longer than wide (often referenced as “needles”) are often technically out of specification and at risk of passing through the mesh if turned sideways. The three-dimensional rolling action of tumbler screeners reduces the risk of “needles” becoming upright and passing through the mesh. In addition, the action of the material can be closely controlled in tumbler screeners. The radial, tangential, and elliptical settings can be adjusted according to the response of the material on the screening surface. Further, the RPM of the motor can be increased or slowed to provide control of an additional variable. A total of four variables can be adjusted independently to change material movement on the screen mesh.

Special Features of Vibratory Screener
Both types of screeners provide unique advantages due to differences in design and construction. Vibratory screeners offer the potential for a central outlet at the base of the machine because the vibratory motors can be mounted to the side of the screener frame. A central position outlet is especially advantageous for use in vertical transport systems with high throughput. Central outlets are also advantageous during a pre-package or final safety scalp of the product.
   The action of the vibratory screener is complimentary to wet screening applications. Another special feature that can be uniquely added to vibratory screeners is spray nozzles. Spray nozzles can be included with the construction of a vibratory screener to facilitate a wet screening application.

Special Features of Tumbler Screener
Tumbler screeners have some unique features to prevent material blinding of the screen mesh. Both vibratory and tumbler screeners have anti-blinding features which include bouncing balls and ultrasonic rings. However, tumbler screeners have additional anti-blinding features. Tumbler screeners require a central drive system beneath to achieve the tumbling action of the material. Because of the central drive, several anti-blinding devices can be fitted via the drive shaft on tumbler screeners. Rotating brush arms can be attached to a drive shaft and rotate over the screen mesh. Conical rolling brushes can rotate beneath the screen surface and clean it without damage. Another anti-blinding option is rotating arms with air nozzles to blow out particles caught in the mesh. The air nozzles can be combined with the brushes to provide a dual solution. A passing unit with rotating arms is also a proven solution for agglomerated product. The rotating arms cause comminution (reducing solid material size) which is advantageous for agglomerated products like instant coffee.
    Craig Anderson, MBA, ALMO Process, www.almoprocess.com and Armin Heussler, MBA, ALLGAIER PROCESS TECHNOLOGY GmbH, www.allgaier.de. For more information on ALMO Process, call 513-402-2566 or visit www.almoprocess.com

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