Screens that are rapidly vibrated with small amplitude, keep the material moving and prevent binding as far as possible. Vibrating screens are commonly used in industries where large capacity and high efficiency are desired.
The capacity of the vibrating screens, especially in the finer sizes, is so high as compared to the other screens that they have practically replaced all other types where the efficiency of the other screen is of prime importance. These screens are classified as mechanically vibrated screens and electrically Vibrating screens.
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The vibrations can be produced either mechanically or Electrically with the frequency of 1800 - 3600 or even more per minute. Mechanical vibrations are usually transmitted from high speeds eccentric to the casing and from them to the screens so that the whole assembly is vibrated.
Electrical vibrations are usually transmitted from heavy-duty solenoids directly to the screens so that only screens are vibrated. Vibrating screens may be mounted in a multideck fashion with the coarsest screen at the top, either horizontally or inclined up to 45°.
A directly vibrated screen. Electrically Vibrated screens are widely used in the chemical industry. The Vibrating screens have an accuracy of sizing, increased capacity per square meter, and low maintenance cost per ton of material handled.
Principle and Operation
 In the case of a vibrating screen, the vibrations are given to the screen to effect the separation of solid particles into two size fractions. Vibrations given keep the solid particles moving. Proper selected frequency and amplitude keep the capacity constant.
In the case of a vibrating screen, the material to be separated is fed to the top screen and simultaneously the screens are vibrated either electrically or mechanically at a frequency of 1000 to 3500 per minute. Due to vibrations, the particles on the screen are kept moving and due to the inclination given to the screens, the oversize material travels along the screen and is collected separately. The undersized material passes through the screens and is collected. Four fractions are obtained with a three-deck screen.
Variables in Screening OperationsÂ
There are many variables in a screening operation that can be very easily changed. The best combination of these results in the excellent performance of screening equipment.
Method of FeedingÂ
To obtain maximum capacity and efficiency, the screening equipment must be fed properly. The material should be spared evenly over the full width of the screening surface and should approach the screening surface in a direction parallel to the longitudinal axis of the screen. It must be fed at as low a particle velocity as is possible.
Screening SurfacesÂ
Use of single-deck screens in series results in the most efficient operation as in the case of multiple-deck screens lower decks are not fed so that their entire area is not used and each separation requires a different combination of angle, speed, and amplitude of vibration for the best performance.
Screen SlopeÂ
As the screen slope increases, the rate at which the materials travel over the screening surface increases, and at the same time, it reduces the bed thickness. The increase in the rate of travel means an increase in tonnages passing over the screen per unit of time.Â
Reduction in the bed thickness allows the fines to work their way to the screen surface and pass through the openings. However, the slope cannot be increased beyond a certain value because beyond that value the material will travel down the screen much faster without getting screened and the screening efficiency reduces drastically.
Vibration Amplitude and FrequencyÂ
One has to select the proper amplitude of the vibrations to prevent binding of the cloth and for long bearings life. The frequency of vibration affects the capacity of the screening equipment by regulating the number of impacts between the material and the Screening surface. While selecting both these parameters, one has to see that the feed is properly stratified for the most efficient operation.
Moisture in FeedÂ
The moisture associated with the feed material adversely affects the screening operation and should be removed.
Advantages and Disadvantages
Advantages
1. To a strong vibration of the screens box, so that the vibration screen is high efficiency and productive.
2. Dry material screening to the demand, and lightweight, more layer.
3. Easy to use and replace the screen surface.
4. Less consumption of electricity per ton of material.
Disadvantages
1. Not suitable for high moisture.
2. The Vibration Screen is used for large scale and the model has relatively high energy consumption.
3. Work noise and dust are heavier.Â
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Take these Notes is, Orginal Sources:Â Unit Operations-II, KA Gavhane