Design, Fabrication And Performance Testing Of Trommel Screen For Solid Waste Processing

Project and Seminar Material for Civil and Environmental Engineering

Design, Fabrication And Performance Testing Of Trommel Screen For Solid Waste Processing


Abstract


Theoretical and practical considerations influencing the design and use of trommel screens as devices for segregating various components from municipal solid waste have been identified and studied. Reported are design considerations derived from the results of a comprehensive analytical modeling effort of the dynamics and screening phenomena occurring during trommel screening and from the results of experiments carried out on unprocessed and processed municipal solid waste under various operating conditions and using various screen configurations.


Chapter One


1.1 Introduction

A trommel screen, also known as a rotary screen, is a mechanical screening machine used to separate materials, mainly in the mineral and solid-waste processing industries. It consists of a perforated cylindrical drum that is normally elevated at an angle at the feed end. Physical size separation is achieved as the feed material spirals down the rotating drum, where the undersized material smaller than the screen apertures passes through the screen, while the oversized material exits at the other end of the drum.

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. However depending on the application required, trommels have several advantages and limitations over other screening processes such as vibrating screens, grizzly screens, roller screens, curved screens and gyratory screen separators.

Some of the main governing equations for a trommel screen include the screening rate, screening efficiency and residence time of particles in the screen. These equations could be applied in the rough calculation done in initial phases of a design process. However, design is largely based on heuristics. Therefore, design rules are often used in place of the governing equations in the design of a trommel screen. When designing a trommel screen, the main factors affecting the screening efficiency and production rate are the rotational velocity of the drum, mass flow rate of feed particles, size of the drum and inclination of trommel screen. Depending on desired application of trommel screen, a balance has to be made between the screening efficiency and production rate.


1.2 Objective Of The Project

The objective of this work is to design a Trommel screen. Trommel screen is consist of five parts as drum, frame, hopper, reducer and motor. After the broken stone into the drum, on the one hand, materials being screened along with the rotation of drum, on the other hand, the bigger materials flow forward along the direction of slope and be separated by the screen with different number of meshes. The different materials fall into different hopper and then be sent to different finished product stacks by belt conveyor.


1.3 Advantages Of The Project

Trommel screens are cheaper to produce than vibrating screens. They are vibration free which causes less noise than vibrating screens. Trommel screens are more mechanically robust than vibrating screens allowing it to last longer under mechanical stress.


1.4 Limitation Of The Project

Lesser material can be screened at once for a trommel screen compared to other type of screen. This is because only one part of the screen area of the trommel screen is utilised during the screening process whilst the entire screen is used for a vibrating screen. Trommel screens are also more susceptible to plugging and blinding, especially when different sized screen apertures are in series. Plugging is when material larger than the aperture may become stuck or wedged into the apertures and then may be forced through which is undesirable. Blinding is when wet material clump up and stick to the surface of the screen.


1.5 Scope Of The Project

One of the available designs of trommel screens is concentric screens with the coarsest screen located at the innermost section. It can also be designed in parallel in which objects exit one stream and enter the following. A trommel in series is a single drum whereby each section has different apertures size arranged from the finest to the coarsest.

The trommel screen has many different configurations. For the drum component, an internal screw is fitted when the placement of the drum is flat or elevated at an angle less than 5°. The internal screw facilitates the movement of objects through the drum by forcing them to spiral.

For an inclined drum, objects are being lifted and then dropped with the help of lifter bars to move it further down the drum which the objects will otherwise roll down slower. Furthermore, the lifter bars shake the objects to segregate them. Lifter bars will not be considered in the presence of heavy objects as they may break the screen.
As for the screens, perforated plate screens or mesh screens are usually used. Perforated plate screen are rolled and welded for strength. This design contains fewer ridges which makes it easier for the cleaning process. On the other hand, mesh screen are replaceable as it is susceptible to wear and tear compared to perforated screen. In addition, screw cleaning work for this design is more intensive as objects tend to get wedged in the mesh ridges.


1.6 Application Of The Project

Municipal and industrial waste

Trommel screens are used by the municipal waste industry in the screening process to classify sizes of solid waste. Besides that, it can also be used to improve the recovery of fuel-derived solid waste. 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. In addition, trommel screens are used for the treatment of wastewater. 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.

Mineral processing

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.

Other applications

Other applications of trommel screens can be seen in the screening process of composts as an enhancement technique. It selects composts of variable size fractions to get rid of contaminants and incomplete composted residues, forming end products with a variety of uses. Besides this, the food industries use trommel screens to sort dry food of different sizes and shapes. 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.


Design, Fabrication And Performance Testing Of Trommel Screen For Solid Waste Processing


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