How to Design and Implement Chutes in Bulk Solids Handling Systems
How to Design and Implement Chutes in Bulk Solids Handling Systems
In the generalised case of a belt conveyor transfer point the material leaves the discharge pulley with some inertia in the horizontal direction and hence the material stream has to be channelled into a cohesive stream and controlled through the vertical section and onto the spoon chute. This is illustrated in Fig. 18.
Fig. 18: Flow through an inline transfer chute.
Hence the material flow in the upper hood portion is represented by the free body diagram in Fig. 19.
Fig. 19: Flow model in hood portion.
The formulae developed for the spoon section may be developed for the hood section as
(27)
For a constant radius and assuming μ_{E} is constant at an average value for the stream, the solution for the velocity Eq. (17) is
(28)
For v = v_{0} at θ = θ_{0} then
(29)
The above principles may also be applied to the case of a convex curve in the chute as indicated below (Fig. 20).
(30)
This is applicable for sin θ ≥ v^{2}/Rg.
Fig. 20: Flow in a convex section.
It is noted that Fig. 19 also applies in this case with the vertical axis now representing the maximum value of the velocity for chute contact.
For a constant radius and assuming μ_{E} is constant at an average value for the stream, the solution for the velocity Eq. (28) is
(31)
For v = v_{0} at θ = θ_{0} then
(32)
7.4. Design Principle 4 – Minimise Abrasive Wear of Chute Surface
A critical aspect in the efficient design of transfer chutes is the wear that is imposed on the chute surface by the abrasive nature of material flowing on the chute surface.
7.4.1. Wear on Chute Bottom
Consider the generalised case of flow through the spoon as indicated in Fig. 21.
Fig. 21: Flow through spoon.
An abrasive wear factor W_{c} may be determined as:
(33)
where W_{c} has units of N/ms N_{WR} is the nondimensional abrasive wear number and is given by
(34)
The various parameters are:
∅ = chute friction angle [°]
B = chute width [m]
K_{c} = ratio v_{s}/v
V_{s} = velocity of sliding against chute surface
Q_{m} = throughput [kg/s]
R = radius of curvature of the chute {m]
V = average velocity at section considered [m/s]
θ = chute slope angle measured from the vertical [°]
The factor K_{c} < 1. For fast or accelerated thin stream flow, K_{c} = 0.6. As the stream thickness increases, K_{c} will reduce. Two particular chute geometries are of practical interest: straight inclined chutes and constant radius curved chutes.
7.4.2. Wear on Chute Side Walls
Assuming the side wall pressure increases linearly from zero at the surface of the stream to a maximum value at the bottom, then the average wear may be estimated from
(35)
K_{v} and K_{c} are as previously defined. If, for example, K_{v} = 0.8 and K_{v} = 0.4, then the average side wall wear is 25% of the chute bottom surface wear.
7.4.3. Impact Wear
Impact wear in transfer chutes may occur at points of entry or at points of sudden changes in direction.
For ductile materials the greatest wear occurs when impingement angles are low, say 15° – 30°. For hard, brittle materials the greatest impact damage occurs at steep impingement angles of the order of 90°.
7.5. Design Principle 5 – Minimise the Wear of the Belt
A critically important aspect in the design of transfer chutes is to reduce the effects of the material stream on belt wear and damage. The primary objectives are to:
 match the horizontal component of the exit velocity as closely as possible to the belt speed
 reduce the vertical component of the exit velocity so as to reduce abrasive wear due to impact
 load the belt centrally so that the load is evenly distributed in order to avoid belt mistracking and spillage.
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