How to Design and Implement Chutes in Bulk Solids Handling Systems
Chute Design Essentials
How to Design and Implement Chutes in Bulk Solids Handling Systems
Chutes are in use in almost every bulk solids handling plant. Although everybody knows them, they are mostly overlooked, except for those cases where they cause extra-attention and -work due to malfunctioning. This article attempts to give the reader some simple rules to apply to chute design.
Adi Frittella, Abri Smit
(ed. WoMaMarcel - 20/4/2016)
The opening size must be sufficient for its purpose (see Table 1).
Where de-dusting is required, the hood should be provided with a back screen or apron seal, to limit the ingress of false air. The apron seal may be made of 3 mm (minimum) reinforced rubber cloth, attached to the chute hood entrance. The apron seal should be vertically slit to allow the material to pass. The slits are normally spaced at typically between 100 mm and 150 mm.
The hood is placed over the head or discharge pulley, which is located with respect to the equipment being fed. The location of the pulley is determined by consideration of the material trajectory over the pulley and the nature of the equipment being fed. The material trajectory is determined by the application of standard calculations. For conveyors discharging into hoppers and bins, the conveyor discharge pulley can be located to either feed the bin centrally when the bin is empty, or to allow central feed when it is full, as specified by the engineer. For cases where, for structural reasons, the bin must always be centrally loaded, the conveyor hood must be equipped with an adjustable impact plate or curved trajectory plate, in order to deflect the material stream into the desired path.
Table 1: Inspection and access openings in chutes.
4.2. Chute Body
The chute body should be designed to suit the transfer requirements, without changing the direction of the material severely. The area of the chute containing the body of the material flow must be at least 2.5 to 3.0 times the area of the material, based on the design capacity of the conveyor and the material speed at the point of consideration. The minimum area of the chute is then given by
S = Material stream speed [m/s], (which could be belt speed) D = Bulk density of the material [t/m3] Cdc = Belt design capacity [t/h]
The chute body should be designed to centralise the material onto the downstream equipment. Material flow that tends to misalign the downstream belt is to be avoided. To this end, the use of ‘Vee’ bottom chutes is often encouraged, especially for in-line transfers from one conveyor onto another. For right-angled and skewed transfers, the use of adjustable impact plates or curved trajectory plates is recommended to change the direction of the material flow. The trajectory of the material will always impart some misalignment on a right-angle transfer. For this reason, the use of deflector plates is recommended. For material that is wet or prone to plugging, the impact plate ought to be designed to be self-cleaning, which may involve some test work.
The use of drop boxes is discouraged for material having a high content of fines or moisture or both. Drop boxes are also not always acceptable on conveyors handling diamondiferous material. Drop boxes may be designed for belts of relatively high speed (belt speeds in excess of about 2.0 m/s) that carry washed and sized material of lump size greater than 30 mm. Where drop boxes are specified, they should be designed to be self-draining. The base plate of the drop box must therefore be inclined at least at 10° to the horizontal plane, see Fig. 4. The drop box is then equipped with a replaceable lip liner, located to allow the passage of water underneath it.
Fig. 4: Typical dead box design.
The chute should be designed to minimise the impact height from one conveyor onto another, as far as the plant layout allows (Fig. 5). The chute should be designed to minimise impact of the material on the sides of skirts on the conveyor being fed. The rear impact point of the material onto the conveyor is normally located 150 mm upstream of the first impact idler (Fig. 6). Loading the conveyor in the transition zone from flat to trough is to be avoided as far as possible and should only be seen as a last resort.
Fig. 5: Multiple dead boxes to reduce impact.
Fig. 6: Multiple dead boxes to reduce impact.
Where the chute sides slope, the valley angle must not be less than the minimum slope angle for the material and liner combination. The valley angle is determined from the well-known equation as
where the angles A and B (the slopes of adjoining plates) are measured from the horizontal, and angle C is the valley angle, also measured from the horizontal (Fig. 6).
The geometry of the chutes under silos and bins, feeding onto belt or apron feeders, are normally determined in conjunction with the material flow engineer.
Fig. 7: Valley angle.
4.3. Chute Exit (Spoon)
The chute exit should be dimensioned to allow the unhindered passage of the material. The exit opening minimum dimension should be at least 2.5 times the maximum lump size of the material, and must have an area at least 2.5 times the area of the material, based on the design capacity of the conveyor and the belt speed, as indicated earlier. Where the chute feeds into skirts, the chute width must not be greater than the width of the skirts. Any necking or reduction in width of the chute body must comply with the chute wall slope and valley angle requirements.
The chute exit should be designed to impart to the material some velocity in the direction of flow, where the feed is onto another conveyor, wherever possible. A common specification is for the exit velocity to be within 10% of the receiving belt speed.
For chutes exiting at right angles from screens onto conveyors, the chute should cover the full width of the screen discharge and ought to be equipped with adjustable, replaceable deflector plates, placed at approximately 70° to the horizontal, located above the conveyor belt surface. The deflector plates must be dimensioned to allow the full passage of material, without creating a cut-off area over the conveyor belt. Chute exiting screens may be provided with a cut-off or isolating mechanism, such as a clam-shell gate, or radial gate, in order to prevent flooding of the receiving conveyor under trip-out conditions, when the conveyor coasting time is less than the loaded screen run-down time. The gates may be programmed to automatically close rapidly (in less time than the conveyor coasting time), and to open smoothly and slowly, in order to deliver the screen run-off to the conveyor in a reasonably controlled manner when the system is restarted.
For chutes exiting hoppers, bins, silos and stockpiles, the chute design must accommodate the requirements of the feeding device, such as the vibrating, belt, apron or other type of feeder.