EQUIPMENT FOR SCREENING

EQUIPMENT FOR SCREENING

SCREENING:

Screening is a method of separating particles according to size alone.

Under size: fines pass through the screen openings

Oversize: tails do not pass

A single screen can make but a single separation into two fractions. These are called unsized fractions, because although either the upper or lower limit of the particle sizes they contain is known, the other limit is unknown. Material passed through a series of screens of different sizes is separated into sized fractions, i.e. fractions in which both the maximum and minimum particle sizes are known.

SCREENING EQUIPMENT:

• Stationary & rotary screens
• Gyrating screens or Vibrating screens
• Centrifugal sitter.
• Sieves
• Industrial Strainer
• Sieve Shakers

Cutting diameter D_pc:

It marks the point of separation; usually D_pc is chosen to be the mesh opening of the screen.

Actual screens do not give a perfect separation about the cutting diameter. The under size can contain certain amount of material coarser than D_pc, and the oversize can contain certain amount of material that is smaller than D_pc.[1]

INDUSTRIAL STRAINERS:

Some of industrial strainers available are simplex basket strainer, duplex basket strainer, and Y strainer. Simple basket strainer is used to protect valuable or sensitive equipment in a system that is meant to be shut down temporarily. Some commonly used strainers are bell mouth strainers, foot valve strainers, basket strainers etc.

Fig. 1: Industrial Basket strainers

SIEVES:

A sieve, or sifter, is a device for separating wanted elements from unwanted material or for characterizing the particle size distribution of a sample, typically using a woven screen such as a mesh or net. The word "sift" derives from 'sieve'. In cooking, a sifter is used to separate and break up clumps in dry ingredients such as flour, as well as to aerate and combine them. A strainer is a form of sieve used to separate solids from liquid.



Fig.2: sieves or sifters
ANALYTICAL SIEVE SHAKERS:

A device used to shake a stacked column of standard sieve-test trays to cause solids to sift progressively from the top (large openings) to the bottom (small openings and a final pan), according to particle size.

Fig.3 Analytical Sieve Shaker

STATIONARY AND ROTARY SCREENS:

Rotating and stationary screens for automatic and efficient removal of coarse solids process streams and waste water effluent. Screens can be used anywhere in your plant, but are most often used as a pre-screen to further waste water treatment.

Screens Provide Great Value:

Screens provide great value in their ability to handle a wide range of flow-rates and solids loading. They work well on granular, fibrous, greasy or sticky solids. We also provide equipment for further de watering of the screened solids.

The right screen can reduce water usage, improve the efficiency of final waste water treatment, reduce maintenance, recover products, reduce hauling costs and improve processes. Contact us for assistance with your screening application.


Fig.4: Rotary Screen

Screen Applications:

• Process water
• Waste water
• Pulp and paper mill effluent
• Wood room effluent
• Stock thickening
• Broke thickening
• Rejects thickening Save all
• Poultry plant wash water
• Primary feather and offal waste water
• Peeler effluent
• Solids recovery
• Coolants

Fig. 5: Stationary Screen

GYRATORY or VIBRATORY SCREEN:

Boxlike machine with a series of horizontal screens nested in a vertical stack with downward-decreasing mesh-opening sizes; near-circular motion causes undersized material to sift down through
each screen in succession.

Or
Gyratory Screens, used in mechanical screening and sieving is based on a circular motion of the machine. Unlike other methods, gyratory screen operates in a gentler manner and is more suited to handle fragile products, enabling it to produce finer products. This method is applicable for both wet and dry screening.

A distinct difference to other techniques is that the gyratory motion applied here depends on eccentric weights instead of vibrations, which can be varied based on individual process requirement.

Gyratory equipment contains decks of screens on top of each other with the coarsest screen on top and the finest below. The feed is inserted from the top and gyratory motion triggers the penetration of particles into the next deck through screen openings.

Casings are inclined at relatively low angles (< 15°) to the horizontal plane, with gyrations occurring in the vertical plane. The eccentric masses can be varied in such as the increase of top eccentric mass leads to an increase in horizontal throw, promoting the discharge of oversize materials. Increment in bottom eccentric mass boosts the material turn over on the screen surface, maximizing the quantity of under-size-material penetration. Over size materials are discharged via tangential outlet.

Fig.5: Gyratory Screens-Schematic diagram

CENTRIFUGAL SITTERS or SCREEN SCROLL CENTRIFUGE:

Screen scroll centrifuge is a filtering or screen centrifuge which is also known as worm screen or conveyor discharge centrifuge. This centrifuge was first introduced in the midst of 19th century. After developing new technologies over the decades, it is now one of the widely used processes in many industries for the separation of crystalline, granular or fibrous materials from a solid-liquid mixture. Also, this process is considered to dry the solid material. This process has been some of the most frequently seen within, especially, coal preparation industry. Moreover, it can be found in other industries such as chemical, environmental, food and other mining fields.

Fig.6: Screen Scroll Centrifuge

References:
[1] http://lorien.ncl.ac.uk/ming/particle/cpe124p4.html[2] http://en.wikipedia.org/wiki/Sieve[3] http://www.papsco.com/Category.asp?CategoryID=15[4] http://en.wikipedia.org/wiki/Gyratory_equipment

[4] http://en.wikipedia.org/wiki/Screen_scroll_centrifuge