bulk-online, the Powder/Bulk Portal

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.

1. Wear on Chute Bottom

Consider the generalised case of flow through the spoon as indicated in Fig. 21.

An abrasive wear factor W_c may be determined as:

where:
  W_c = abrasive wear factor [N/ms]
  N_WR = non-dimensional abrasive wear number

N_WR is given by:

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.

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

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.

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, and
• load the belt centrally so that the load is evenly distributed in order to avoid belt mistracking and spillage.

The transfer of material onto the belt is illustrated in Fig. 22.

The following formulae have been developed as a means of estimating belt wear at a transfer point:

where:
  ρ = bulk density
  V_ey = vertical component of the exit velocity

The abrasive wear parameter W_a:

where:
  μ_b = friction coefficient between bulk solid and conveyor belt
  V_b = belt speed
The wear will be distributed over the acceleration length L_a. The wear parameter may then be expressed as

with:

where:
  θ_e = chute slope angle with respect to vertical at exit
  K_b = non-dimensional wear parameter.

In Fig. 23 K_b is plotted for a range of v_b/v_e values.

As shown, the wear is quite severe at low chute angles but reduces significantly as the angle θ_e increases. For the chute to be self-cleaning, the slope angle of the chute at exit must be greater than the angle of repose of the bulk solids on the chute surface. It is recommended that

8. Conclusion

This collection of rules and formulae has been presented as a reminder to the industry that chute design requires a lot more attention that it is currently given. While sophisticated computer software is an essential tool in modern materials handling, the basic design of particulate flow is equally essential where the software is not available. The application of the equations and principles pres­ented should provide the designer and plant engineer with the means to apply these principles in a practical manner and with sufficient accuracy to confidently predict the performance of the chutes.