Trommels 101 : Understanding Trommel Screen Design
An understanding of the design factors for Trommel Screens is essential in the development of the correct specification for a particular application. The following details are designed to assist in this process and minimise the consequences of poor selection. these consequences could include excessive fines in the oversize fraction, insufficient production rate, inability to adapt to different materials or production rates, and screen damage.
In selecting or designing a trommel screen, the following factors need to be considered and understood:
Input Material Specification : What Goes In
Bulk density : How many Kg’s per cubic meter is the material as it enters the screen ? Trommels essentially move a volume of material, so the space a known weight of material takes up is critical to performance and the power required to do so.
Granulometry : What is the material size split of each material size that needs to be processed ? This is best expressed in weight (ie kgs) as a split by volume does not necessarily provide accurate information because the sum of the volumes of each size after separation rarely equals the input volume. This occurs because voids in the input materials can filled by smaller product without changing the total volume.
So, if the trommel is required to split a material running at 10 tonne/hr into 3 sizes say <10mm, 10-50mm and plus 50mm, the designer therefore needs to know:
|Output Stream Size||% Split||Weight Split (kg)|
Moisture Content of the Material : This provides an understanding as to how “sticky” the material may be. A material laden with excessive moisture will increase the tendency to stick to the trommel drum or inside the screen holes and then possibly blocking the screening holes. Moisture content is expressed as the percentage of water in the total weight of material (including the water).
Maximum and Average Granule Weight : This is the weight of any one item entering the screen. This is required to correctly size the screen thickness. A drum processing concrete blocks will require significantly thicker drum thickness than one processing compost or bark
Process Specifications : What Needs To Be Done
Production Rate : This is the rate of infeed material needed to be processed in any given period. Peak rates which could be experienced, become the limiting factor of the design and therefore should be understood. A processing plant may process 30t/hr as an average over an 8 hour day, however if in 15mins the material inflow could be 10 tonne, which equals 40t/hr. In this case the design production rate must be the peak rate of 40t/hr.
Trommel Specifications : What Features Can I Have
Trommel Screens can be built with a combination of the following specifications: