TROMMELS 101 - Industrial shredders and equipment for recycling (original) (raw)

Production rate through the trommel drum is directly affected by :

• Drum diameter – for a sloping drum the larger the drum the faster material will be processed through the drum
• Drum speed – increase the speed, increase the production rate.
  • For a level drum fitted with an internal spiral screw, the production rate is determined by the pitch of the screw.
  • For a sloped drum the larger the slope the higher the production rate.

Screening Efficiency in the trommel is directly affected by :

• Retention time in the drum increases the screening efficiency. The more times the material drops to the screen bottom the more screening occurs. Every drop of material provides for a volume of material to be screened. Reducing the angle of the drum increases the number of drops of material (and therefore retention time), however, the offset to this is reduced production rate.
• Screen area – maximising the screen area used increases the area of material exposed to screening. This can be increased by:
  • Increasing the length of the drum
  • Use lifting bars to spread material
  • Increase the diameter of the drum – increases the drum surface area
  • Hole shape – a 30mm square hole has a larger area than 30mm round. The hole area is termed open hole area.
  • The Mesh Total Open Area for a screen = hole area x number of holes.
  • Drum Open Area (expressed as a percentage) of a screen mesh or plate is the % of Mesh Total Open Area compared to the total area of the drum. So the fewer gaps between holes and the less closed plate (ie no holes) through the drum the higher the open area. The higher the Drum Open Area the higher the screen efficiency for a particular product.
  • Increasing the speed of the drum (centrifugal effect)

Product being processed. The percentage of the material to be removed must be within the capability of the trommel ie: the combination of its screen area, retention time at its operational speed; to remove this amount of fines.

The longer the trommel – the higher the production rate: MYTH

There is no effect on the production rate of a trommel due to its length. Length of a trommel is all about the amount of screen area, which is about screening efficiency.

Screening efficiency is the same for all materials: MYTH

This is incorrect as the efficiency of a trommel to remove small particles is directly related to the number of small particles (of the size to be removed) in the mix. As an example, Material A may contain 25% material <10mm. The trommel may remove all this material and therefore the efficiency is 100%. Material B may contain 50% material <10mm and the trommel may only remove 50% of this material. The trommel efficiency on this material is then 50%. So we see the same trommel with two very different efficiencies on different materials. Therefore trommel efficiency is in reference to a particular material processed through it.

Material density has no effect on production rate: MYTH

Variation in input material density DOES effect the production rate. This is an easy trap to get caught with. If the density of the input product doubles, then the power required to move also doubles. If the power to run the heavier material is outside the motor capabilities, then to overcome this the trommel speed may have to be dropped to reduce the power draw to within the capabilities of the motor. So reducing the speed of the drum will decrease the production rate.

Mesh screens have more open area than perforated plate screens: MYTH

It is often considered that mesh screens have higher open areas, however, this is not correct unless it is due to the mesh having a lighter design than the plate alternative. If the designs are similar there is no reason why the open areas of both products cannot be equal, however, the holes need to be the same shape and the thickness between the holes is the same as the alternative mesh diameter. The steel plate will then provide superior life and drum rigidity.