The interface between a hopper outlet and belt feeder plays a vital role in the feeder design. If it is not designed correctly, solids flow may be severely compromised. However, with the right know-how, these problems can be avoided.
The interface between a hopper and a belt feeder is where the interaction of the two pieces of the material handling equipment occurs. You need to consider the hopper and feeder as a whole unit when it comes to a belt feeder design. Even if you are retrofitting a feeder under an existing hopper, you need to assess the design of the hopper to prevent any flow problems down the track. Why is that? This is because that their performances can be affected by each other.
1. The flow pattern within the hopper can be influenced by the feeder’s operation.
There are two major types of flow patterns when the material is discharged from a hopper, namely funnel flow and mass flow, as demonstrated in Figure 1. In funnel flow, a first-in last-out flow sequence, an active flow channel forms above the outlet, with non-flowing material at the periphery. In mass flow, a first-in-first-out flow sequence, all material is in motion during discharge. Material from the centre and periphery moves toward the hopper outlet uniformly.
To achieve uniform draw-down from the hopper when the belt feeder is in operation, a mass flow hopper is required, which shall be selected based on the flow properties of the material being handled. Otherwise, erratic flow or ratholing may establish within the bin. However, if the interface is not designed correctly, preferential flow can still develop near the front or rear end of the hopper outlet even with a properly designed mass flow bin above as illustrated in Figure 2. The reasons and solutions to this problem will be discussed later.
2. The feeder loads, and its power consumption, can be affected by the hopper geometry.
The material in the bin and between the outlet and belt surface can exert both vertical and horizontal forces on the belt feeder. The former is the sum of the vertical force at the hopper outlet and the weight of the material out of the hopper outlet. It is used for determining belt support requirements and the required tension in the belt to overcome drag of the supports. The latter is the force required to shear the material from beneath the outlet. Knowledge of these loads is an essential component in determining feeder drive details.
Both loads are sensitive to interface geometry and material loading conditions in the hopper. An example of the relationship between the hopper geometry and the vertical force on the shear plane is shown in Figure 3.
Problems to avoid using belt feeder
The challenges in belt feeder design or the problems you may encounter when implementing belt feeders can be categorized into three groups, namely, flow issues, mechanical design issues and operational issues.
Flow issues
As mentioned previously, a mass flow hopper is required above the belt feeder to make uniform draw down in the bin possible. The outlet needs to be properly sized to avoid material arching or particle interlocking. The hopper wall angle needs to be steep enough to ensure the material will flow along the wall. If a funnel flow hopper is used, uniform material discharge over the entire cross section cannot be achieved.
The space below the hopper outlet needs to be increased along the feed direction, otherwise preferential flow can occur as shown in Figure 2. To avoid this, the hopper outlet at the interface needs to be tapered. The angles for the increase in both plan and elevation will be discussed in the design section later.
With fine powders, the discharge rate may be limited if the belt feeder is operating at a speed greater than the bulk material’s critical steady state rate of discharge. Similarly, flooding of fine powders is a common problem if the interface is not designed for uniform withdrawal or the bin is not designed for mass flow.
Mechanical design issues
One of the common concerns for feeder design is if the drive is undersized. The power required to shear material and operate a belt feeder can be greater than the available power if one of the following scenarios exists.
- an improperly designed interface that does not allow uniform withdrawal over the entire cross section of the outlet.
- the interface being not structurally designed and reinforced to withstand the pressures exerted by the bulk solid against it, it will deform in such a way that significantly higher forces are needed to shear the material.
- differential settlement between a bin and an independently supported feeder, which compacts material above the feeder.
- excessive belt sag between idlers.
Operational issues
Excessive spillage and dust generation can create housekeeping and environmental concerns. Particle falling onto the top pf the return portion of the belt may result in belt mis-tracking and premature belt and idler wear. Belt sag between idlers can allow particles to escape, increase idler loads and increase skirt wear. Sometimes, the sag can also excite the bin to vibrate at one of its resonant frequencies, which may damage the structure.
General design aspects
Belt feeders are often an excellent choice when feeding material from an elongated hopper outlet but can also be used with square or round outlets. Based on the issues we have discussed previously, more attention needs to be given on the following aspects, especially when an elongated hopper outlet is used.
1. A mass-flow bin
It is important to choose a properly designed mass-flow bin to ensure reliable material flow and try to use the feeder to control the flow rate instead of a gate at the outlet. The minimum outlet width at the rear of the interface must be greater than or equal to the value required to prevent a stable arch from forming. The sloping side walls must be at least as steep as the hopper wall slope required for mass flow. Both the minimum outlet width and required hopper angle can be calculated from measured flow properties. The slot length should be at least three times the width to realise the benefits of a rectangular outlet. Often it is advantageous to use a much longer slot with large silos or with wedge or chisel-shaped hoppers containing vertical end walls. The latter are useful geometries if the bulk solid being handled is very frictional and/or cohesive.
2. A tapered outlet
A tapered outlet can provide increasing capacity along the length of the bin outlet by providing expansion in both plan and elevation. A flexible rubber or plastic buffer at the back end to allow a typical 12 mm gap for uniform material withdrawal without belt or interface damage. Once the gap at the rear end of the interface is determined, divergence angle as shown in Figure 4 can be selected to define the taper of the outlet.
Model tests or experience are often necessary in this case as no verified guidelines are published. Roberts, in his paper an overview of feeder design focusing on belt and apron feeders (2001), stated the relationship between an optimum divergence angle and the ratio of the outlet length to width for an assumed volumetric efficiency and velocity profile as shown in Figure 5.
Generally, taper of the outlet slot in elevation must be sufficient for particles to freely form an angle of surcharge on the belt. The gap between the interface and the belt at the discharge end must be at least three times the normal maximum particle size, and this gap must be greater than the maximum possible particle size.
3. Estimation of the feeder loads
Using an approach based on the work of Jenike and assuming the bin and belt feeder interface provide mass flow, the vertical load can be expressed as
A x F +B
Where:
A = integrated vertical material force at the shear plane
F = dimensionless multiplier used to correct for different loading conditions
B = weight of material between shear plane and belt
The shear load can be approximated as
C x F
Where:
C = Value A multiplied by the effective coefficient of internal friction, usually taken as ( ).
A lot of methods are available to estimate the vertical force on the shear plane. According to the research done by Holmes (thesis 2011), some of the methods may be applicable for only certain types of materials. However, the theories developed by Jenike (1964) and Arnold, Mclean and Roberts (1980) generally can provide the most accurate estimations for the tested materials in his thesis.
There is considerable latitude in choosing the remaining variables but there are trade-offs to be considered. As example, in both capital and operating costs between a wide belt operating at low speed compared to a smaller belt operating at higher speed. Each component of the wider belt is likely to initially cost more; however, since abrasive wear of a belt is a function of both the belt’s speed and the stress exerted on it by the material, the effect of varying belt width is difficult to generalize. A wider belt will have higher solids stresses acting on it but may experience less wear because of its lower speed.
Another consideration is the power consumption of different belt sizes. Wider belts require more belt tension but may require less power if the speed is reduced sufficiently. Low speed belts require high ratio gear reducers, which have more drive loss than standard units. For most situations, a practical approach is to start with standard 20° picking idlers with the centre roll wider than the interface at the front; this sets an initial belt width. Using a conservative (i.e. shallow) angle of surcharge, one can quickly determine whether a wider belt is required to prevent spillage or allow a slower belt speed.
Conclusion
By following well-proven design rules, belt feeders can control the flow rate of the material reliably and volumetric efficiency can be maximized, with minimum power consumption. In the meanwhile, the service life of the belt feeder can also be prolonged.
Do you have a bulk solids handling question? Jenike & Johanson has developed the science of bulk solids flow and specialises in applying it to solving the most challenging bulk solids handling problems. So why not put them to the test with your question?
Note: The advice here is of a general nature. Specific solutions are very sensitive to their circumstances; therefore, you should consult with a specialist in the area before proceeding.