Construction: Trommels are revolving screens consisting of a cylindrical frame surrounded by wire cloth or perforated plate (which acts as a screening surface). They are open at one or both ends and inclined at a slight angle to the horizontal so that the material is advanced by the rotation of the cylinder. These units revolve at relatively low speeds of 15 to 20 rpm.
A trommel is a mechanically operated screen consisting of a slowly rotating perforated cylinder with its axis at a slight angle to the horizontal.
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The perforations in the screening surface may be of the same size throughout or may be of different size in which case the small size perforations section is near the feed end. It is driven at the feed end through a gear mechanism. It has a feed point at the upper end, another-sized product discharge below the screening surface, and an oversized discharge at the opposite end.Â
Shows Fig a schematic diagram of trommels having sections of different size perforations.
Working Principle
The material to be screened is fed at the upper end and gradually moves down the screening surface towards the lower end. In doing so, the material passes over the apertures of gradually increasing size (as the single cylinder is provided with perforations ranging from the finest desired at the feed end to the coarsest at the discharge). If the single cylinder is provided with the screen having three different size perforations then we get four fractions.
The finest material (coarsest ) is withdrawn from the discharge end. Such a type of arrangement is usually adopted for smaller capacities. With this type of trommel, there is a tendency of blockage of the apertures by the large material, and the screen with the finest opening being the weakest is subjected to the heaviest wear. The operating speed of the trommel is 30 to 50% of the critical speed ( critical speed is the one at which the material is carried completely around in contact with the screening surface).
Trommels Screen ArrangementsÂ
For the separation of a given material into several size fractions, several trommels are operated in series. The first trommel of a series may have the coarsest perforations so that it produces the coarsest finished product which is delivered to the next trommel and so on. In such a case, it is most convenient to place the trommels one above the other as shown in Fig. 3.5 (a)
When the first trommel of the series has the smallest perforations, the oversize material passes to the next trommel, and so on. In such a case, it is most convenient to put the screens in line, end as shown in Fig.
If screening equipment of this type is put into service for finer separations then the cylinder may be covered with a fine wire or silk cloth instead of a perforated plate or coarse wire screen. Such an equivalent is usually called a reel.
Fig shows several concentric cylinders. The innermost is the longest and has the coarsest perforations. The outer ones are successively shorter and have finer perforations. In this arrangement, the maximum load is given to the strongest screen but the construction is complicated and expensive.
Note: Trommels are well suited for relatively coarse materials ( 1/2 in or over).Â
Advantages and Disadvantages
Advantages
1. These are revolving screens.
2. Labour requirement is low.
3. Opening in small-size feed.
Disadvantages
1. Relatively expensive construction.
2. Capacity is relatively small.
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