Belt Feeder Properly Designed

1.Introduction to Belt Feeder

Belt feeder is the means by which the rate of material from a bin or hopper is controlled. There is a close correlation between its speed of operation and the rate of discharge of the bulk material.

Belt feeders are suited for handling granular materials with lesser lump size (in comparison to apron feeder). The limitation of lump size depends upon toughness, hardness, abrasiveness and roughness of lumps.

The belt feeder is not recommended for very hard material, which have sharp cutting edges and comparatively of large lumps. Regarding basic rule for preliminary decision, the designer must take into consideration the following:

• In case of large lumpy material, the lumps can momentary blocked the belt movement.
• Lumps are pressing on belt due to pressure above and also additionally due to reshuffling reaction forces when the belt is trying to dislodge the jammed lumps by eliminating arch formation.
• Material with sharp edges of lumps can be overcome by providing thick top cover to belt, making the use of belt feeder possible.

In this article, will be discussed how to properly design belt feeders to maintain reliable flow from bins and hoppers. In addition, since the idlers can be mounted on load cells, a belt feeder can be used in either a volumetric or gravimetric application.

2. Belt Feeder function

Belt Feeder is placed bellow the hopper and it is designed to work flooded of material and take out only a required quantity of material in m3/sec. The material quantity extracted from the hopper and will be discharged at head pulley, this discharge rate Qv in mtph is given by the following formula:

Qv = skirt inside width at exit (m) x material bed height at exit (m) x belt average speed (m/s) x bulk density (t/m3) x 3600

This formula is volumetric nature and can be adjusted by the front-end opening area or by the belt speed, accordingly the belt discharge rate can be controlled as follows:

• The control gate, what allow to adjust (manually or remotely) the front opening height and thereby control the flow rate for many feeders handling granular materials with lump size not to large.
• The variable frequency drive enables to vary the feeder discharge directly proportional of belt speed.
• The hopper needs to have enough storage capacity to balance out the inflow and outflow, in an aforesaid time cycle.

3. Skirt width if function of lump size

The allowable maximum lump size is in accordance with following proportion between lump size and skirt board net width w. The values mentioned below are for skirt board of constant width. In case of skirt board of tapering width; the value at tail end side could be considered about 10% less than specified value.

• w = 3.40 x (maximum lump size): for less abrasive material, and hopper of limited height.
• w = 4.25 x (maximum lump size): for less abrasive material, and hopper of large height.
• w = 4.25 x (maximum lump size): for highly abrasive material, and hopper of limited height.
• w = 5.00 x (maximum lump size): for highly abrasive material, and hopper of large height.

4. Material bed depth height in function of skirt width

Material bed height in skirt board, mm (conveying zone), as a factor (ratio) of w, such as 0.58 w, 0.65; w, 0.71; w etc. as decided by designer.

5. Interface between the hopper outlet and belt

A critical aspect of a mass flow hopper is that the feeder must withdraw product from the full cross section of the hopper outlet. To ensure that a belt feeder increases in capacity along its length, a properly designed interface is required. A well-designed interface between the slot outlet of the hopper and the belt will progressively discharge more product onto the belt along its length. This will make the full cross section of the hopper outlet live and therefore maintain mass flow (if the hopper slope is sufficiently steep for flow along the walls). An increasing capacity is achieved by controlling the position of the shear plane between the product in the hopper moving vertically down and the product in the feeder moving horizontally.

6. Flow of the material

Basically, the vertical stresses acting through the shear plane are relatively low, what allows the belt to shear the product efficiently with a minimum amount of force. In addition, the vertical loads acting down onto the belt are relatively low and independent of the head of material in the hopper. Without a mass flow hopper and proper interface design, the loads on the belt would be high and a function of the head of material in the hopper.

Therefore, the design of the hopper, the belt feeder and the loads on the belt are very much dependent and connected.

The interface design relies on a variable speed belt to achieve a variable product flow rate.

In addition, since there is no natural separation between the product moving down in the hopper and the product moving horizontally on the belt, the shear is achieved by brute force shearing the product under a head of material. In funnel flow the head of material acting on the shear plane is high.

7. Choice of outlet format

The major considerations in deciding which type of feeder to use are the properties of the bulk material being handled (e.g. cohesiveness, maximum particle size, particle friability, propensity for dust generation) and the application (e.g. geometry of hopper outlet, need for volumetric or gravimetric control, necessary throughput).

When starting to design a bin or hopper, one of the first things that should be decided is what shape the outlet will be – square, round or elongated. This decision should be based on the flow properties of the material and facility constraints.

• Hopper square outlet

Square and round outlets provide more flexibility in the choice of feeder and have fewer design constraints when compared to elongated outlets. It is possible, for example, to place a belt conveyor under a square or round outlet and turn it into a feeder without much in the way of negative consequences to either the bin’s flow pattern or conveyor’s horsepower.

There is an adjustable gate at the front of the hopper to adjust the cross section of product on the belt, see Figure 1.

 

 

Figure 1 – Hopper square outlet

 

• Hopper elongated outlet

In the case of an elongated outlet, this situation changes drastically. For example, if a belt is placed under a hopper with an elongated outlet and in terms of feeder choice, there are significant benefits of an elongated outlet with regards to flow. An elongated outlet reduces the minimum opening size required by a factor of approximately two when compared to a round outlet.

Designs with adjustable gates cannot feed material uniformly – especially from a long slot.

Elongated outlet allows for mass flow to occur in a hopper with less steep hopper walls. The side walls of a conical hopper must be at least 10° to 12° steeper than the side walls of a wedge or chisel-shaped hopper to allow mass flow with the same wall surface. Selecting and designing an appropriate feeder for an application with a specific bulk material is not a trivial matter. The following general design guidelines apply to belt feeders used under elongated outlets:

• Ensure that the interface between the belt and the hopper is large enough to prevent arching in the hopper and to ensure discharge of material over the entire cross section of the outlet.
• Make sure the hopper outlet is large enough to provide the required discharge rate. With fine materials, such as gypsum, fly ash, magnetite concentrate, and titanium dioxide, 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.
• Beware of the possibility of flooding with fine powders. This is a common problem if the interface is not designed for uniform withdrawal and the bin is not designed for mass flow.
• Provide sufficient power to operate the feeder. Sometimes the power required to shear material and operate a belt feeder is greater than the available power. This is usually a result of a poorly designed interface.
• Structurally design and reinforce the interface to withstand the pressures exerted by the bulk material against it. Otherwise, it will deform in such a way that significantly higher forces are needed to shear the material.
• These issues can be avoided with a properly designed interface such as the one shown in Figure 2.
• The minimum outlet width at the rear of the interface must be greater than or equal to the value required to prevent arching in the hopper above.
• The sloping side walls must be at least as steep as the hopper wall slope required for mass flow, and a slanted “nose” with an arch-shaped cutout should be included at the front to provide stress relief and prevent stagnation at the discharge end.
• A flexible rubber or plastic buffer should be placed at the back end to allow a typical 12mm gap for uniform material withdrawal without belt or interface damage.

Figure 2 - Hopper elongated outlet

 

8. Main features of design

 The main features to have a good design belt feeder shall be as follows:

• Apply at taper outlet format in both plan and elevation to reach a uniform material withdrawal.
• A slanted nose and/or arch-shaped lip to provide stress relief and prevent segregation at the discharge end.
• Flexible rubber or plastic buffet at the back and to allow a typically half-inch gap for uniform material withdrawal without belt or interface damage.
• Spillage skirts that expand slightly in the direction of the belt travel and that are remote from the feeder interface. This prevents the skirts from interfering with uniform material withdrawal.
• To avoid blockage, it is recommended that the bed depth height of material at the front of the hopper outlet be equal to at least 1,5 to 2 times the largest particle size.

 

9. Belt Speed

Selecting the optimum belt speed is very important step in design of belt feeder. The belt feeder performance, life and price depend upon belt speed. The belt speed selection requires thoughtful understanding of bulk material characteristics. The belt speed is influenced by following points:

• The belt speed is less for material of higher abrasion capability. The belt speed is more for material of less abrasion capability.
• The belt speed being used is somewhat less for heavy bulk material. This is due to reason that for same column height, the material force (pressure) and belt abrasion is more in case of heavy material. The material column height is same for equal values of friction angles, equal values of external features of lumps / granules and hopper arrangement; but independent of bulk density. So, density aspect is to be incorporated in belt speed calculation.

The belt speed is comparatively less for difficult feed zone and is more for easy feed zone. This is due to consideration that the difficult feed zone results into more abrasion / wear of belt. This is based on following benchmarks:

• Belt speed up to 0.3 m/s is used in applications dealing with abrasive material and difficult feed zone.
• Belt speed of up to 0.5 m/s is used in applications dealing with nonabrasive materials and easy feed zone.
• Belt speed of up to 1,0 - 1.5 m/s there are applications handling coal product flow with flooded feeders directly onto the main belt conveyor running at this speed, flat rollers or 20 degrees troughing. A feeder requires good material handling, an accurate weighing system, an accurate belt travel detection system and an intelligent control algorithm for it to operate correctly.

 The belt speed calculation always involves certain element of subjectivity, because most suitable speed will depend upon the exact nature of material and belt feeder hopper arrangement, which are specific to application. This should be given open thought and in case of doubt, opt for somewhat lower speed.

10. Cross section features

The cross section of the belt feeders shall be according to the material and flow to be extracted from the hopper or bin as follows:

• Flat belt

The flat belt feeder does not have constrains similar to picking type or 3-roller type belt feeder, but it has only one issue of concern for “possible” spillage of leaked material. It is compact and can be constructed exceptionally strong. The short belt feeder will not have chances for spillage of leaked material.

This belt feeder with sufficient edge margin can sometimes be the only and economical choice when application of very heavy class is resulting into very stiff and thick belt and skirt board is also of expanding type. See Figure 3.

 

Figure 3 - Flat belt feeder interface

• Picking idler

The inclination of the lateral rolls has variation of 12,5o - 20o degrees, for effectiveness or for meaningful purpose of picking type idler, some portion of belt width remains on side roller, and it does not become flat belt feeder.

The unusual stiff and thick rubber belt bending in radial zone can require more value of y. The increase of y and thereby also the increase of Minimum Edge Margin will result into lesser cross section of material layer. The required y value is also influenced by troughing angle. The shallow troughing angle reduces the problem of belt bending.

As against this, more troughing angle helps to prevent spillage of leaked material. See Figure 4.

Figure 4 – Picking belt feeder interface

• 3 Rolling Troughing idler

 The inclination of the lateral rolls has variation of 20o and 35o degrees, bigger trough angle 45o degrees is favorable to prevent spillage.

More value of troughing angle is favorable to prevent spillage of leaked material. The scope for use of this belt feeder for expanding width skirt board is limited, because skirt plate cannot pass across the idler kink. So only small value of width gradient is possible. Other option is to use comparatively smaller middle roller and longer side rollers to have somewhat more gradient on width.

Even in such case, the skirt board construction would be complicated because it would be expanding on inclined side rollers. See Figure 5.

Figure 5 – 3 rolling troughing belt feeder interface

11. Shear Zone

The geometry of the shear zone of a belt feeder is quite difficult to predict precisely. According to Schulze & Schwedes, the shear zone may be divided into three regions as illustrated in Figure 6.

In their work the lengths of the regions were predicted on the basis of the 'Coulomb principle of smallest safety' which assumes that the rupture surface in a consolidated bulk solid will develop in such a way that the bearing capacity of the solid is minimized.

 

Figure 6 - Shear Zones in Belt Feeder

It is also noted that there will be a velocity gradient developed in the shear zone, as indicated in Figure 7.

The characteristic shape of this profile depends on the properties of the bulk solid, the feeder speed and the geometry of the hopper/feeder interface. In the extended skirt plate zone, the velocity distribution is more uniform.

Figure 7. Velocity Profile in Shear Zone

12. Material discharge flow study

 The discrete element method - DEM are widely applied to simulate the dynamic behavior of granular material at the flow discharge. Here, granular material is modeled as an assembly of particles and all dynamical parameters (position, velocity, orientation, etc.) of each particle are tracked during the simulation. See Figure 8.

DEM modelling allows take into account the particulate nature of granular material, without imposing any constraints and inspires a better understanding of the fundamentals of granular flow.

 

Figure 8 – Discharge flow Profile in the transference chute

13. Main components and constructive conception

 Belt feeders need a good design, what bringing easy operation and maintenance. The main components of the belt feeder are identified in the Figure 9.

Figure 9 – Main components of the belt feeder

Regarding the steel structure conception can be used stringer to support the carry and return idlers and head and tail pulleys support as shown in the Figure 10.

Figure 10 – Belt Feeder steel structure with stringer and pulleys supports – Metso property

A monobloc structures also can be used, the belt feeder will be seated on a chassis, what can be shop assembled and dispatched to the site ready to enter into operation as shown in the Figure 11.

Figure 11 – Monobloc Belt Feeder - Metso property

In some application it is needed the belt feeder is seated on a movable chassis due to maintenance purpose, where the material flow cannot be having longer stops, and is required faster replacement thru a stand by unit ready for operation. In crushing circuits or minerals processing circuits this concept is largely applied, see Figure 12 and 13.

Figure 12 – Movable Belt Feeder 3D modeling - Metso property

Figure 13 – Lay out of the movable belt feeder

14. Special Design for Belt Feeders

In some applications of installation of belt feeder inside tunnels, above storage piles, where the available space for maintenance is very thigh, the solution is to design a belt feeder with a compact monobloc structure keeping all mechanical and structural elements inside the periphery of the endless belt. Only the external legs what support this monobloc will be out of the belt periphery and by flanged connection will be easy to remove and to connect with an auxiliary structure, allowing quick belt replacement inside the tunnel. See Figure 14.

Figure 14 – Compact modular belt feeder easy to replace the endless belt.

The endless belt, mounted an auxiliary steel support, is brought by a movable hoist by means of monorail installed in the roof of the tunnel, as shown in the Figure 15.

One side of the belt feeder carry idlers is hinged in the bottom and bolted in the top, during the belt replacement this lateral is lowered allowing the belt replacement.

Figure 15 – Endless Belt replacement device

15. Belt Feeder Effective Belt Tension and Required Power Determination

The calculation of the Effective Belt Tension and Required Power will be explained on this item, the Figure 16 shows the definition of the main dimensions, which will be used on the calculation:

Figure 16 – Main dimensions do size the belt feeder

15.1 Resistances to motion

The drag or resistance to motion can be divide into 4 categories. The sum of the 4 types of resistance to motion FM is equal to the total effective tension TTB to be transmitted to the belt:

FM     =     FP + FS + FMS + FML

where,

Primary resistance is FP

Secondary resistance is FS

Material shearing resistance is FMS

Material lifting resistance is FML

Primary Resistances

The primary resistance FP is the measure of the friction associated primarily with the rolling resistance of the idlers, indentation rolling resistance of the belt, and the flexing resistance of the belt (belt deforming due to sag). The primary resistance of carry or return can be described in the following equation:

FP = L ?? ?g? (mr + 2mb + mm)

where,

L : total length of belt feeder (m)

? : friction coefficient

g : acceleration due to gravity = 9,81 (m/s2)

mr : mass per length of belt feeder associated with rotating idler parts, at carrying and return sides (kg/m)

mb : mass per length of belt feeder associated with the conveyor belt (kg/m)

mm : mass per length of belt feeder associated with the conveyed material (kg/m)

If there are no values which have been obtained by measurement or on the basis of experience or if only an approximate dimensioning is intended, standard values for the friction coefficient ? can be selected from Table 1 for estimating the total primary resistance of the upper and lower strands on the basis of operating conditions and design features. These values are based on numerous combined upper and lower strand measurements and for the following limiting conditions:

• 3 roller fixed idler sets in the upper strand
• Carrying idlers with anti-friction bearings and labyrinth seals
• Values of relative sag less than 1%
• Filling ratio within a range of 0.7 to 1.1

Table 1 – Values for the friction coefficient ?

Secondary Resistances

Secondary resistances FS include frictional resistances and inertia resistances. These resistances are calculated from several individual resistances and include frictional losses due to skirting drag on the belt, material drag on skirting, cleaners and plows as described in the following equation:

FS = FSM + FCP

where,

 FSM : resistance due to skirtboard drag on the belting as well as material resistance against the skirtboard.

 FSM = h ? ρ ? [(1 - senΦ)/ (1 + senΦ)] ?μs ? [(2,4 w – h)?Le + h?Ls)] + 9?L

where,

h, w, L, Le, Ls  : as defined on the Figure 16

ρ   : bulk density (Kg/m3)

Φ   : material repose angle (degrees), see Table 2

μs  : friction coefficient of bulk materials with wall steel, see Table 2

9    : added 4,5 Kg/m for each skirtboard, to overcome friction of the rubber edge

Table 2 – Material repose angle Φ and friction coefficient μs

Case-1: This is applicable to plain and fairly even surface of steel, aluminum, average plastic; without protruding edges. Flush and welded steel surface would be of this type.

Case-2: This is applicable to surface of medium smoothness / evenness, such as welded steel flush surface (below average), smooth concrete, wood planks, protruding edges, bolt head rivets etc.

FCP         : resistance due to belt cleaners and plows. 

FCP = (nc ?μc + np ?μp)? B

Where:

nc   : quantity of cleaners

μc   : friction of the cleaners = (112 Kg/m)

np   : quantity of plows

μp   : friction of the plows = (41 Kg/m)

B    : belt width (m)

Material shearing Resistance

Material Shearing Resistance is the resistance within the moving material and stopped material inside the hopper. This resistance is given by the following equation:

FMS = (1,2 w - h) ?ρ?Ls?fm? w

where,

w   : skirt board net width (m)

h    : height of the material layer (m)

The height of material layer can be considered a proportion of the skirt board width as follow h = 0.58 w, 0.65 w, 0.71w. This proportion is adopted in function of the designer criteria. 

ρ    : bulk density (Kgf/m3)

Ls   : shear length (m)

fm   : material friction = tang Φ (repose angle of material) 

Material Lifting Resistance

Material Lifting resistance is simply the resistance to elevate the material lift (or change in height). This material lifting resistance can be calculated for any flight with the following equation:

FML = w ? h? ρ ?H 

where,

FML : material lifting resistance (Kgf)

w  : skirt board net width (m)

h : height of the material layer (m)

ρ : bulk density (Kgf/m3)

H : belt feeder lift or change in height (m)

15.2     Power calculation

The required power of the belt feeder is given by the following formula:

PBF =  1 / (75?η ) ? TTB ? v   

where,

TTB : effective tension TTB = FM to be transmitted to the belt (Kgf)

v : speed of the belt feeder (m/s)

η    : drive efficiency

References:

• Jenike & Johanson – How to design efficient and reliable feeders for bulk solids, by John W. Carson, Ph.D. and Greg Petro, P.E.
• Some extracts from the book – Belt Feeder Design and Hopper, Bin and Silo.
• Feeder or conveyor: what’s the difference and why does it matter? By Carrie E. Hartford, Andrés D. Orlando and John W. Carson, Jenike & Johanson.
• Feeding of bulk solids – Feeder interface, loads and power focusing on belt and apron feeders  - Alan W. Roberts, Emeritus Professor – Center for Bulk Solids and Particulate Technologies – University of Newcastle, NSM, Australia.
• Schwedes + Schulze Schüttguttechnik GmbH - Consultants + powder testing laboratory, partner for silo design, trouble-shooting and powder testing.