The trend in new process plant design is moving towards the multi-screened large capacity modular design. As a result, multiplicity of process equipment will be minimized by the use of fewer, but larger units of higher capacity, or units specially designed for such circuits.
One of the essential pieces of equipment in any process plant is the vibrating screen. Single deck screens with screening areas of 450 square feet and widths are of 18′ are presently successfully used. It is possible to provide vibrating screens of such dimensions with a maximum of operational reliability.
As screens are subject to vibrations, they follow the laws of oscillation and execute forced oscillations in the horizontal plan. The forced oscillations are produced by an external excitor which determines the screen frequency.
For instance, a large screen with a mass of 20,000 kilogram requires a 100,000 kilogram force to achieve a machine acceleration of 5g. If the screen has a frequency of 1,000 per minute, the direction of the acceleration force changes 1,000 times from positive to negative and produces alternating stresses in the screens structural components.
For the assessment of the fatigue strengths, the alternating stress strengths is decisive.
The material selection for the screen construction is quite important. Every material is in principle non homogeneous. It has internal notches. The result is that in the structure of each material, notch effects occur and stress peaks are formed which may considerably exceed the nominal stress.
The design of a screen unit is based on the excitor force. The excitor force also affects the dimensioning of all the individual components of the screen.
In the design of large screens with screening areas of 30 to 50 square meters (320 to 540 sq.ft.) additional phenomena arise which are relevant to the process building itself. The screen with the above mentioned screening areas will produce air vibrations which can be detrimental, especially if the building is relatively small.
