Trommel screen

[3] Trommel screens can be used in a variety of applications such as classification of solid waste and recovery of valuable minerals from raw materials.

Trommels come in many designs such as concentric screens, series or parallel arrangement and each component has a few configurations.

These equations could be applied in the rough calculation done in initial phases of a design process.

This is done by removing inorganic materials such as moisture and ash from the air-classified light fraction segregated from shredded solid waste, thereby increasing the quality of the product fuel.

For this particular application, solids from the entering flow will settle onto the screen mesh and the drum will rotate once the liquid reaches a certain level.

The clean area of the screen is submerged into the liquid while the trapped solids fall onto a conveyor which will be further processed before removal.

[6] Trommel screens are also used for the grading of raw materials to recover valuable minerals.

The screen will segregate minuscule materials which are not in the suitable range of size to be used in the crushing stage.

It also helps to get rid of dust particles which will otherwise impair the performance of the subsequent machineries in the downstream processes.

The classification process will help to achieve the desired mass or heat transfer rate and avoid under or over-processing.

It also screens tiny food such as peas and nuts that are strong enough to resist the rotational force of the drum.

Lifter bars will not be considered in the presence of heavy objects as they may break the screen.

In addition, screw cleaning work for this design is more intensive as objects tend to get wedged in the mesh ridges.

Large chunks roll and slide to the lower end (tail discharge), while small lumps having sizes less than the openings in the bars fall through the grid into a separate collector.

[5] Based on the assumption that the particle falls perpendicularly on the screen surface, the probability of passage, P, is simply given as [18] where

impingements is simply:[18] Furthermore, the total number fraction of particles within this size range in the feed can be expressed as follows:[18] Therefore, the separation efficiency, which is defined as the ratio of the fraction of particles removed to the total fraction of particles in the feed, can be determined as follows:[18] There are a number of factors that affect the separation efficiency of the trommel, which include:[19] Two simplifying assumptions are made in the equation presented in this section for the residence time of materials in a rotating screen.

Trommel screens are used widely in industries for its efficiency in material size separation.

Larger granules will be retained inside the trommel screen until the desired aperture is met and follows the same particle behaviour.

The particles are lifted slightly from the bottom of the drum before tumbling down the free surface as shown in Figure 8.

[3] This motion generates turbulent flow of particles, resulting in a higher screening efficiency compared to slumping.

This is due to particles attaching to the wall of the rotating drum caused by centrifugal forces as shown in Figure 10.

According to Ottino and Khakhar,[21] increasing the feed flow rate of particles resulted in a decrease in screening efficiency.

Not much is known about why this occurs, however, it is suggested that this effect is influenced by the thickness of filter granules packed in the trommel body.

On the other hand, it is easier for smaller-sized particles to pass through the granules thickness in the trommel system at lower feed rates.

Increasing the area of material exposed to screening allows more particles to be filtered out.

Therefore, features that increase the surface area will result in a much higher screening efficiency and production rate.

On the other hand, decreasing the inclination angle will result in a much longer residence time of particles within the trommel system which increases the screening efficiency.

A phenomenon exist below 2° such that for a given set of operating conditions, decreasing the inclination angle will increase the bed depth resulting in a lower screening efficiency.

[3] In the wastewater treatment industry, the solids that exit the trommel will be compressed and dewatered as they travel along the conveyor.

Most often a post-washing treatment such as a jet wash will be used after the trommel screen to break down faecal and unwanted semi-solid matter.