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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.
(ed. WoMaMarcel - 20/4/2016)
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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 in-line 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 μE is constant at an average value for the stream, the solution for the velocity Eq. (17) is

                              (28)

For v = v0 at θ = θ0 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 ≥ v2/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 μE is constant at an average value for the stream, the solution for the velocity Eq. (28) is

                              (31)

For v = v0 at θ = θ0 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 Wc may be determined as:

                              (33)

where Wc has units of N/ms NWR is the non-dimensional abrasive wear number and is given by

                              (34)

The various parameters are:

∅ = chute friction angle [°]
B = chute width [m]
Kc = ratio vs/v
Vs = velocity of sliding against chute surface
Qm = 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 Kc < 1. For fast or accelerated thin stream flow, Kc = 0.6. As the stream thickness increases, Kc 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)

Kv and Kc are as previously defined. If, for example, Kv = 0.8 and Kv = 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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