Trommels 101

Just a big drum with holes that spins around ?

Not quite.

There is a lot more to trommel screen design than you might expect. Learn about it here.

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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)
<10mm 25% 2,500
10-50mm 20% 2,000
>50mm 55% 5,500
Total 100% 10,000

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:


Drum With Internal Screw

Drum With Internal Screw - Used where the drum angle is either level or a slight slope ie <5 deg. Level drums require an internal spiral to move the material through the drum. Sloped drums rely on the material moving through the drum by being rolled or lifted (by lifter bars) further down the drum each movement (or drop)

 

Drum With Lifter Bars

Internal Lifter Bars - these aid the agitation of the material which helps separate the materials. Lifter bars also provide the lift and drop action which moves the material through the drum. With out lifter bars material rolls through the drum at a much slower rate. Typically heavy material such as C&D waste will not include lifter bars as the dropping of the material can damage the screen.

 

Perforated Plate Screen

Perforated Plate Screen - Plate when rolled and welded to form the drum is significantly stronger than the typical steel skeleton with mesh attached. Rolled steel plate drums have the holes cut out of the plate and are less prone to damage from foreign objects like steel bars. Steel plate is easier to clean due to the lack of ridges (which are found  in mesh designs).  The lack of ridges also reduces the wear in plate screens compared to mesh.

Plate thickness of the drum is determined by the impact loadings which will occur in the drum from heavy objects. To a lesser extent thicker drums will be used to increase the life from wear.
 

 

Mesh Screens

Mesh Screen Designs - this type of design is weaker than a perforated plate design and is more prone to damage and wear.  For this reason, many mesh designs utilise replaceable screen sections.  Cleaning is more difficult with mesh designs due to entrapment of material on mesh ridges and in broken/damaged sections of the mesh.

 

Plate Thickness -  The thickness of the plate used to manufacture the drum is determined by the impact loadings which will occur in the drum from heavy objects. To a lesser extent thicker drums will be used to increase the life from wear.

Hole Sizes and Shape - Round or square is a decision based on : 

  • The size of product needed as an undersize from the screen
  • The open area requirement  - square holes are larger in area than round ones.
  • Product agitation - this is is higher with square holes due to the flat edge "grabbing" more material (somewhat similar to what steel mesh screens can achieve) and potentially retaining it, which leads to:
  • Drum Cleaning -  Cleaning requirements with square holes will be more than for round holes due to the extra "grab" of the hole on the materail in the trommel. 

Wear Protection Bars are used to protect the drum from damage from large abrasive material ie concrete.


Covers for Noise and Dust Suppression

Noise and Dust Suppression - The trommel can be fitted with covers to allow for dust extraction and minimisation and for noise suppression.

 

Speed Variation - provides the ability to change the drum speed, which will increase production rate, and can also expose more screen area due to centrifugal force on the material making it "climb higher" in the drum, thus using more screen area around the drum circumference. This increases screen efficiency.

Hydraulic or Electric Drive is usually dependent on the power source available. Mobile trommels are often driven hydraulically for this reason. Electric trommels have the advantage of higher efficiencies in the drive than hydraulic drives.

Electrical Starter Selection for Electric Drives - Direct On Line (DOL) or Variable Speed Drive (VSD). Variable speed drives provide the advantage of a ramped start, which gently increases the speed from zero to the desired operation speed. This requires less current draw than DOL starters, reduces the tendency to skid wheels in friction drives, reduces the starting forces on all drive components and provides variation of the operation speed. Speed variation is generally limited to 50% of the 50Hz motor speed by the ability of the motor to cool and maintain motor efficiency. Maximum speed is limited by the power available and the rigidity of the trommel screening machine.
 


Friction Drive via supporting wheels

Drive Systems - Several drive methods are available including friction drive from the supporting wheels, chain drive using a chain wrapped and welded around the drum,  or by gear drive using a gear tooth profile on the drum. This is a decision based on the amount of friction available through the support wheels for the job being done. The friction is a function of the number of drive wheels, size of drive wheels, friction coefficient of the wheels to the driven surface of the drum (in the operating condition - which may be outside in the rain). If the available friction is not achievable, then chain and gear drives must be utilised, however there are additional costs involved in these alternatives.

 

Walkways for maintenance or inspection.

Water spray for dust suppression.
 


Bag Breakers

Bag Breakers -  for opening plastic bags - typically used to release materials retained in bags such as household waste which has not been shredded before screening.

 

Cleaning Brush

Cleaning brushes for keeping holes clean are efficient as long as the product does not contain long stringy material as this can easily clog brushes. Brushes are rarely used for very dry product as these materials do not clog holes.

 

Trommel Design Rules : Things That Really Matter

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 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 percentage) of a screen mesh or plate is the % of Mesh Total Open Area compared to the total area of the drum. So the less 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 it's screen area, retention time at it's operational speed; to remove this amount of fines.

Trommel Myths : And Other Often Confused Facts About Trommels

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

There is no effect on the production rate of a trommel due to it's 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 amount 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 effects 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.