Feeder design and selection
- 30 June 2009
- Product News
For this project, Skako Comessa liaised in depth with the main contractor Whitwick Engineering (Coalville) to ensure the optimum design for each of the eight vibratory feeder positions. Many of the plant features bore a close resemblance, or identical application to a recent project at CEMEX Halkyn by the same main contractor, which incorporated 12 Skako vibratory feeders, and, on which plant, the feeder design options had been proven.
Of the 20 feeders supplied for both projects, 19 were the Skako fully modular design, whereby the tray section is fully assembled into a modular unit, which incorporates an integral flanged hopper bottom section, with skirt plates, manually adjusted bed depth regulation gate and spring suspension assembly.
However, there were then differing drive types, special design features and both open and totally enclosed dust proof designs to suit the location and specific duty. A number of these design features are highlighted in greater detail.
Feeders operating outside were fully enclosed and dust proof. Under a primary crushed material surge bin are two identical feeders, discharging into a common central discharge chute, over the takeaway conveyor. Each feeder is selected to discharge up to 625 tph of minus 350 mm primary crusher run. In order to accommodate the large feed size, whilst being totally enclosed, the feeder trough has a special increased depth and is of heavy duty design.
Each is driven by twin unbalanced motors (suitable for varying loadings on the feeder) with variable feed rate control between approximately 30-100% provided by a frequency inverter. The required plant capacity could therefore be achieved by either only one of the feeders or both at reduced feed rates. Rod gates were also supplied so either feeder could be isolated for maintenance.
When preceding the impact breakers, the feeders need to carry out a number of functions. To be able to evenly spread the material over the width of the impact rotor, to be totally enclosed to prevent safety issues, and be designed to totally integrate with the impactor feed chute arrangement. For the secondary impactor processing the minus 350 + 75 mm material generally at 450 tph, the feeder is 1800 mm wide, special increased depth and heavy duty design, incorporating a customised support frame to withstand the dynamic loads from the surge bin. A feeder of these dimensions would normally be used to deliver over 1000 tph, however, when the size is determined by the width of the impact rotor and not the capacity, it enables smaller drive motors to be utilised, thereby keeping the power consumption down to a minimum.
The same design criteria also apply to the tertiary impactor, processing minus 75mm at 250tph. The feeder is 1000mm wide to evenly spread the feed, but whereas this sized feeder would often be handling coarser material at up to 500tph, this unit only requires half that capacity, with a small maximum feed size. This feeder can therefore be of a compact ‘low height' design, with smaller drive motors and the discharge end designed to feed directly into the impactor feed chute. For both the impact breaker feeders, the variable feed rate control is between approximately 30 - 100% via frequency inverters.
Following the final screening inside the screen building, the coarse screened sizes are all handled on vibratory feeders, where product can be re-circulated to the tertiary impactor; blended with other product sizes, or diverted to load-outs. Inside the building, the feeders under the storage bins can all be open design, therefore reducing costs. Also, the simpler discharge functions mean standard sized feeders can be utilised. To give total flexibility over the feed rate and accuracy of discharge, electro-magnetic driven feeders are utilised, giving infinitely variable feed rate control between 0-100% via thyristor controllers, with instant on/off control. The 1000 mm wide feeders can handle up to 400 tph with the 800mm wide feeders handling up to 250 tph at infinitely variable feed rates.
Plant automation was paramount on these projects, with a central computerised control system. It was necessary to start and stop feeders automatically, for instance from level detectors in bins, and also to be able to select the required discharge rates in the control room. This is not a problem for the Skako feeders, because the thyristor controllers for the electro-magnetic driven units are supplied as standard, with a relay for external on/off control, and also integral terminals and selector switch for automatic regulation from an external isolated reference signal. Additionally, the frequency inverters were selected with the same control facilities for the unbalanced motor driven units.
The above examples give an insight into selecting the best suited designed feeders to suit specific applications.
Skako Comessa Ltd
Station Road Industrial Estate
T: 01937 838010
F: 01937 838011