Screening Terminology

Screen Cloth

Screen cloth is a vital part of any screening machine. Virto/Elcan goes to great lengths to assure a high level of accuracy of the screen hole openings. We measure the size of the openings with the use of a digital microscopic instrument that reads in microns. This method is not influenced by the wire diameter of the cloth and is considered the only "true" and "unbiased" way to accurately compare screen cloth.

The industry also uses the term "mesh" to refer to the hole openings. This is very misleading and needs to be understood. Mesh means number of holes per linear inch in each direction of the cloth. Mesh is dependent upon the diameter of the wire used to weave the cloth. Because of this dependency, the resulting real measured hole openings can vary even when two screen cloths have the same mesh number.

Reading and Understanding Screen Cloth Charts

80 mesh market grade screen cloth has a hole opening of 180 microns. This is the correct hole opening for 80 mesh when a "market grade" wire is used.

Another type of screen cloth used in the screening industry is called TBC or Tensile Bolting Cloth. TBC cloth is weaved with a thinner wire than market grade cloth. So, a screen with 80 mesh TBC has a hole opening of 224 microns.

Some people in the industry will use the TBC cloth in place of the Market Grade, thinking that 80 mesh = 80 mesh, and end up happy with the machine's efficiency but are mad with larger particles in the product.

This is why Virto/Elcan always refers to the actual micron opening of the hole and this is why Virto/Elcan goes to great lengths to only supply cloth with accurate woven openings. Because of this need for accuracy, most of the wire cloth we use is imported from Europe not the Far East. Also, European weavers utilize a plain weave not cheaper twill weave.

Virto/Elcan maintains a large inventory of screen cloth including polyester and stainless steel ranging in size from 1000 micron to 6 micron.

Screening Machines on the Market

Vibratory: The most prevalent machine used in industry worldwide. A vibratory screening machine utilizes an imbalanced motor to shake the entire body of the machine, thus transferring this vibration to the high tensioned screen cloth (high tension is needed to help transmit the energy).

These vibratory screening machines are the industry standard because of price and easy operation. They are available with bouncing balls and sliding rings to try and keep the mesh free from blinding. The intensity of the vibration, and the direction of product movement over the screen, is done by adjusting the counter weights.

Vibratory screening machines posses a high maintenance cost due to motor failures and ripped screens due to the highly tensioned cloth. They also are not the machine of choice for fine and/or difficult to screen materials due to frequent screen blinding, inability to control material dwell time on the screen, and the inherent low energy transfer to the mesh.

Ultrasonic: This technology can be utilized as part of a complete screening machine or as a retrofittable option on a standard vibratory unit. Ultrasonic works by transmitting a high frequency signal to the screen cloth, thus exciting the screen surface. This it used mainly to keep some materials from blinding the screen. It can be easily retrofitted onto an older vibratory machine to help alleviate blinding. The control of the frequency and impulse is easy.

The electronics of the unit are sensitive and tend to fail in dirty factory environments and there is no ability to control the amplitude and it only utilizes a single frequency. Ultrasonic units are very expensive due to the manufacturing costs of the transponder needed on each individual screen which increase with multiple screen deck applications.

Gyratory: These machines impart a non-vibratory, unique gyratory - reciprocating motion to the screen surface. There is no vertical component to this motion, thus increasing material dwell time on the screen. This motion is gentle on friable materials and can utilize bouncing ball cleaning. They have a low profile which leads itself to in-line vertical plant installations. These units traditionally have high energy consumption due to their large motors and high installation costs due to the needed building structure. The screens are difficult to change compared to other screening machines.

Centrifugal Sifters: Known as "basket" sifters, these units can run high throughputs thru a single screen. They easily fit in line and have low installation costs. They require low maintenance but are prone to screen tears when foreign objects are introduced. Because of the lack of dwell time control and cleaning devises, this type of screener is utilized for scalping and not precise screening.

Tumbler Screener: They use a 3 dimensional motion and unique anti-blinding devices to achieve high efficiency separation. Their motion utilizes a low energy transfer between screen surface and the product and replicates a hand sifting motion with three adjustable force vectors. These systems work extremely well in de-dusting applications, delicate product, static bound products, high aspect ratio shaped particles, and many other difficult to screen products. Tumbler screeners utilize ball, brush, and air knife cleaning and are very specialized and only used when needed for difficult applications. They are more complicated to set-up and adjust the 3 dimensional motions.

Multi-Frequency Screener: These units are equipped with a patent protected system that amplifies the amplitude created by the unbalanced motors via adaptors that transmit this multi-frequency vibration directly to the screening surface. The multi-frequency screener utilizes untensioned screens that last much longer and the cleaning and deblinding is done with this unique vibration.

The strong impact on the screen mesh generates highly accelerated and stratified particles that allow the separation of agglomerated, fine and ultrafine dust particles. This effect allows the screening of wet products without incurring the cost of intensive drying processes.

Finger Screens - A screen used when raw material contains lots of roots and stringy type material. Finger Screens cascade on top of one another, never allowing clogging on the screen.

Definitions

Contamination: This is unwanted material in a given grade. This occurs when there is oversize or fine size material relative to the cut or grade. Another type of contamination is foreign body contamination.

• Oversize contamination- occurs when there is a hole in the screen such that the hole is larger than the mesh size of the screen. Other instances, where oversize occurs, is material overflow falling into the grade from overhead, or when there is the wrong mesh size screen in place.
• Fines contamination - is when large sections of the screen cloth is blinded over, and material flowing over the screen does not fall through. The fines are then retained in the grade outlet.
• Foreign body contamination - is unwanted material that differs from the virgin material going over and through the screen. It can be anything ranging from tree twigs, grass, metal slag to other mineral types and composition. This contamination occurs when there is a hole in the scalping screen or a foreign material's mineralogy or chemical composition differs from the virgin material.

Scalp, scalping: this is the very first cut of the incoming material with the sum of all its grades. Scalping refers to removing the largest size particles. This includes enormously large particles relative to the other particle's sizes. Scalping also cleans the incoming material from foreign body contamination such as twigs, trash, glass, or other unwanted oversize material.

Blinding - When material plugs into the open slots of the screen cloth and inhibits overflowing material from falling through.

Amplitude - This is a measurement of the screen cloth as it vertically peaks to its tallest height and troughs to its lowest point. Measured in multiples of the acceleration constant g

Frequency - Measured in hertz (Hz) or revolutions per minute (RPM). Frequency is the number of times the screen cloth sinusoidally peaks and troughs within a second. As for a gyratory screening motion it is the number of revolutions the screens or screen deck takes in a time interval, such as revolution per minute (RPM).

Gradation, grading - Also called "cut" or "cutting." Given a feed material in an initial state, the material can be defined to a have a particle size distribution. Grading is removing the maximum size material and minimum size material by way of mesh selection.

Stratification - This phenomenon occurs as vibration is passed through a bed of material. This causes coarse (larger) material to rise and finer (smaller) material to descend within the bed. The material in contact with screen cloth either falls through a slot or blinds the slot or contacts the cloth material and is thrown from the cloth to fall to the next lower level.

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