Agricultural and Biosystems Engineering 12/28/92



Agricultural and Biosystems Engineering Department 12/07/06

Iowa State University Carl J. Bern

Chapter 12

Mechanical Grain Conveying

INTRODUCTION

Material handling is a unit operation which changes the spacial location of material without changing its form except incidentally (Pinches, 1958). Material handling operations with grain involve many types of grain conveying devices. These types of mechanical grain conveying devices will be discussed in this chapter:

Belt conveyors

Flight conveyors

Bucket conveyors

Screw conveyors

These devices all find wide use in agriculture, and are very interesting from an engineering point of view.

Important design factors

Important design factors of material handling equipment are:

Capacity

Safety

Reliability

Original cost

Operating cost

Maintenance

Simplicity of design and fabrication

Product damage

Cleanability

Pollution (usually noise and dust)

Power requirements

The importance of each factor depends on the application. Cleanability is important for a seed conveyor when seed left in the conveyor will be mixed with the next grain conveyed. It is of no importance in a farm design where the only material handled is corn for feed. Product damage is important in a conveyor loading grain to be marketed since an increase in fine material could result in a lowered value. It has a low priority for a conveyor loading a grinder.

Energy considerations

As noted above, power requirement (energy) is an important consideration in the design of a conveyor. Much more attention has been given to this design aspect since the so-called energy crisis of 1973. The energy input to a conveyor is used for two things:

To operate the conveyor

To lift material

The quantity of energy expended operating a conveyor is dependent on the conveyor design and is something to consider as conveyors are compared. The energy expended in lifting material represents an increase in potential energy of the material mass and is not dependent on conveyor design. If the conveying path is horizontal, this component is zero. If the conveying path slopes down, this energy input is negative, meaning there can be an output of energy from the conveyor. In some cases when this output exceeds conveyor requirements, the conveyor produces net energy which can be used for other purposes.

The "perfect" conveyor

The hypothetical perfect conveyor is one which moves material without friction losses. In this conveyor, the energy to operate the conveyor is zero. No actual conveyor can operate without friction. However, it is useful to compare actual conveyors with a perfect conveyor doing the same job.

Power for a perfect conveyor

Figure 12-1 shows forces moving a particle during conveying from point 1 to point 2 along a frictionless surface.

Figure 12.1 Forces on particle being conveyed along a frictionless surface path.

Summing forces tangentially, we obtain:

F cos Θ = mg sin φ (12-1)

where F = conveying force on particle

m = particle mass

g = acceleration of gravity

In order to move the particle from point 1 to point 2, the work required is

[pic] (12-2)

where ds is an infinitesimal distance along the frictionless surface. Since ds sin θ = dy,

[pic] (12-3)

Note that Equation 12-3 represents the work necessary to convey the particle in the absence of friction. It is thus the energy required by a "perfect" conveyor. Note also that the work required is independent of the route taken between point 1 and point 2.

For a continuous flow of material, power required by the perfect conveyor is:

[pic]

where:

P = conveyor power

mg/t = mass flow rate

Comparing conveyors

To judge among various conveying methods as to their energy requirement, each conveyor type will be used in the design of a hypothetical conveying system and compared for energy requirement to an impossible "perfect" system which requires no energy to operate the conveyor. Example 12-1 will describe this system and illustrate the computation procedure.

Example 12-1

Corn (45 lb/ft3) is to be moved at a rate of 140 000 lb/h from the bottom of a 4-ft-deep pit to discharge 1 ft above a 20-ft-diameter bin having a loading hole 27 ft above ground level. Compute power (hp) and energy per unit grain mass (hp h/ton) required assuming a "perfect" conveyor.

The total lifting height is: 4 + 27 + 1 = 32 ft.

P = [pic]

E = [pic]

Power and energy necessary for the "perfect" conveyor, which requires only power necessary to lift the material, is 2.26 hp h/ton.

Energy efficiency of the conveyor can be defined as a ratio of the increase in potential energy of the material to energy input. In conventional units, the equation is:

[pic] (12-5)

where

Ec = energy efficiency,

hp = power from conveyor motor, hp

Lh = lift height, ft

Q = mass flow rate, lb/h

The factor 33 000 converts hp to ft lb/min; the factor 60 converts hours to minutes. For the example,

[pic]

Conveyor types, it will be seen, will fall into a high energy requirement group and a low energy requirement group. Those types which slide grain on a surface as it is conveyed will be in the high group because of friction losses. Conveyors which carry material on anti-friction bearings will be in the low group.

Gravity

Flow of grain by gravity can be utilized where slopes are adequate for reliable flow of material. Table 12-1 lists spout or flow slopes for material flow.

Table 12-1. Minimum angles for material flow (MWPS 1983).

|(Material) |Spout angle or floor slopes, degrees |

| | |

| grain, dry | 37 |

| grain, wet | 45 (minimum) |

| pellets | 45 |

| meal | 60 |

Table 12-2 lists grain flow rates for clean, dry grain flowing through a round tube from a dead stop. This would be the condition existing when the tube discharges through a gate from a bin opening.

Table 12.2 Grain flow rates through tubes (Ditzenberger, 1980.)

|Tube diameter, inches |Corn |Soybeans |Wheat |

| | |Flow rate, bu/h | |

| 6 | 1,686 | 2,023 | 2,580 |

| 8 | 3,000 | 3,600 | 4,590 |

| 10 | 4,679 | 5,615 | 7,159 |

| 12 | 6,741 | 8,089 | 10,313 |

| 14 | 9,178 | 11,014 | 14,042 |

| 16 | 11,907 | 14,396 | 18,355 |

| 18 | 15,168 | 18,202 | 23,207 |

| 20 | 18,632 | 22,356 | 28,507 |

| 22 | 22,654 | 27,185 | 34,660 |

| 24 | 26,963 | 32,356 | 41,534 |

| 26 | 31,641 | 37,969 | 48,411 |

BELT CONVEYORS

A belt conveyor consists of an endless moving belt which supports and moves material. Figure 12.2 illustrates the components of a belt conveyor. The belt is usually fabric-reinforced rubber. It is carried on idlers fitted with anti-friction bearings. On the top (load) side of grain conveyors, these idlers are usually arranged to trough the belts and thus increase the allowable load cross-section (Figure 12-3a). Return idlers under the belt carry the belt flat and can be installed at longer spacings than the load-carrying idlers (Figure 12-3b).

Some portable belt conveyor designs eliminate carrying idlers by running the loaded belt inside a metal tube. During return, the belt is carried on return idlers under the tube. The tube is the main structural component of the conveyor, and also covers the loaded conveyor. This design may be less expensive to build, but power requirements will be higher due to the sliding friction of the belt.

Most designs drive (apply power to the belt) at the head pulley since this prevents the return side of the belt from being tensioned due to load.

History

Flat belt conveyors were in use in U.S. industry by 1840 to carry clay, sawmill refuse, and stone. In 1876, the North Central Railroad elevator in Baltimore was equipped with a 30-in rubber-belt grain conveyor which ran at 550 ft/min. Ball or roller bearings were in common use on belt conveyors by 1920 (Hetzel and Albright, 1941). Although conveyor configurations remain similar to early designs, vast improvements in belts, bearings, and drives have been made through the years.

Figure 12-2. Nomenclature of components of a typical belt conveyor (CEMA, 1979).

Figure 12-3. Belt conveyor idlers (CEMA, 1979)

Loading and unloading

Belt conveyor loading is normally done through a feed chute located just ahead of the tail pulley. Closely spaced idlers in this region prevent excessive belt deflection due to dynamic loading forces (Figure 12-2).

The simplest discharge arrangement consists of discharge over the head pulley (Figure 12-4a). A discharge chute may be necessary to direct flow after it leaves the end of the belt (Figure 12-2).

Discharge along the run of a belt conveyor is difficult. A plow is one way to discharge over the side of the belt (Figure 12-4b). The plow, held solid above the belt, pushes grain off the side of the belt. The plow is attached to the conveyor frame and can be designed to be movable along the belt. It may not be usable on troughed belts.

A tripper (Figure 12-4c) is a device which lifts the belt and its contents high enough so that material can be discharged over a belt pulley and then allowed to flow down a gravity chute to a pile beneath either side of the belt. Various tripper designs allow flow on the belt past the tripper and even movement of the tripper by belt power. A moving tripper allows formation of a continuous pile of triangular cross section below the belt.

Figure 12-4. Belt discharge methods (CEMA, 1979)

General characteristics of belt conveyors

We will note here the general characteristics of belt conveyors. Some will be explained more in the design section.

Belt width

Belt widths range from 18 to 96 inches. The most economical design is usually one which uses the narrowest possible belt running up to its highest allowable speed.

Belt speed

Maximum belt speeds range from 50 to 1000 ft/min. Speed is limited by the tendency of material to blow off the belt, by belt slippage on the drive pulley as centrifugal force acts on the belt, and by the dangers of belt damage as large sharp lumps are loaded.

CEMA, 1979 recommends maximum belt speeds listed in Table 12-3 for belts carrying grain or other free flowing, nonabrasive material.

Table 12-3. Recommended maximum belt speeds and belt weight for grain and

other free-flowing, nonabrasive material (CEMA, 1979)

|Belt width, in. |Max. belt speed, |Approx. belt weight |

| |ft/min. |lb/ft # |

| | | |

|18 | 500 |3.5 |

|24 | 700 |4.5 |

|30 | 700 |6 |

|36 | 800 |9 |

|42 | 800 |11 |

|48 | 1000 |14 |

|54 | 1000 |16 |

|60 | 1000 |18 |

|72 | 1000 |21 |

|84 | 1000 |25 |

|96 | 1000 |30 |

#Non-steel cable belts for material in 30 to 74 lb/ft3 range

Power requirement

Power requirement is comparatively low since the load is carried on anti-friction bearings. Since there is no sliding of material during movement, power is independent of product moisture content.

Incline

Incline is limited by the repose characteristics of the material being moved. Since the belt is smooth, material will tend to roll down if the incline is too great. The limit for a smooth, slick material such as hulled or polished rice is 8 degrees. A fibrous, interlocking material like wood chips can be conveyed at a 27-degree incline. Recommended maximums for grain are in the range from 8-18 degrees. Recommended maximums for specific materials are listed in Appendix A, Table A-2.

The limitation on incline is one factor limiting use of portable belt conveyors for grain. The belt conveyor must be quite long to discharge into grain storage structures. Some specialized designs employ rubber flights molded into the belt surface. These flights reduce the tendency for material to roll down, and allow steeper belt runs.

Capacity

A very wide range of capacities is possible with belt conveyors. A capacity of over 300 000 bu of corn per hour is theoretically possible (96 in. belt, maximum speed). No other material handling method can approach such a capacity. As a result, belt conveyors find wide use in applications such as grain elevators where high capacities are required.

Product damage

There is practically no damage to material while being conveyed on a belt conveyor since there is little relative motion between the material and the belt. There may be product damage occurring during loading and unloading.

Noise

Noise level comparatively is low since a belt conveyor has none of the usual sources of high conveyor noise (scraping of surfaces, high-speed fans, impact of particles).

Distance

Conveying distance is unlimited. Belt conveyor systems can be designed like pipelines for dry material. A belt conveyor system has been proposed to carry corn 250 miles east from Storm Lake, IA to a Mississippi river barge terminal. Although technically feasible, the conveying costs were projected to be higher than rail car rates and so the system was not built (Des Moines Tribune, 1972).

Investment cost

Belt conveyors are comparatively high in cost and designed for long life and heavy service.

Enclosure

Belt conveyors are not inherently enclosed and unless there is a reason to add the expense of enclosure (dust containment, weather protection) they are usually left open.

Combined operations

Unit operations such as weighing, sorting, or spraying can be carried out during belt conveyor transit.

Belt conveyor design

Methods will be presented here for preliminary designs of belt conveying systems. Procedures for estimating belt size, speed and power requirements will be explained.

Load cross section

The volume capacity of a belt conveyor is the product of belt speed and load cross section. Figure 12-5 shows dimensions used to compute load cross section for a troughed belt. When material is loaded on the belt it falls to its filling angle of repose with the horizontal, but then slumps to a circular profile ABC which has a center at D.

Figure 12-5. Area of belt conveyor load cross section (CEMA 1979).

At the sides of the load cross section, the top surface of the material meets the belt at angle α, henceforth called the surcharge angle. As the conveyor belt passes over successive carrying idlers, material on the belt is agitated and the cross section assumes a more flattened shape. Since lines AD and CD are perpendicular to the material top surface, angle ADC is 2α . Material is loaded to within c inches of the belt edge.

The area of load cross section is, thus, the sum of area Ab (the trapezoid) and areas As, the surcharge. Distance l is estimated as follows:

l = 0.371 (b) + 0.25

where (12-6)

l and b are as defined in Figure 12-5.

The surcharge angle is a property of the material being conveyed and is 5 to 20 degrees less than the filling angle of repose. See Table 12-4. Flowability is the fourth characteristic of the material code from Table A-1 in Appendix A. For example, wheat has material code 47LC25N (from Table A-2). The 2 indicates a free-flowing material (Table 12-4).

The load cross section as defined here exists in a vertical plane. The effective load cross section of inclined belts decreases as the cosine of the angle of conveyor slope since this cross section is measured in a plane normal to the belt. The actual loss of capacity is usually very small.

Table 12-4. Flowability - Angle of surcharge - Angle of Repose (CEMA) 1979.

For grain, a 20-degree troughed belt with three equal-length rolls is common. Load cross sections along with volume capacities for this type of belt are listed in Table 12-5.

Belt capacity

Belt capacity is the product of belt speed and load cross section. An example problem will illustrate the computation procedure:

Example 12-2

Compute the capacity (bu/h) of a 36-in. belt conveyor running at 285 ft/min and carrying wheat (1 bu = 1.245 ft3).

From Table A-2, wheat has a code of 47C25N. The fourth character (2) indicates a 10 degree surcharge angle (Table A-1, Table 1204). From Table 12-5, load cross section is 0.596 ft2 and capacity at 100 ft/min is 3579 ft3/h.

[pic] (12-7)

Table 12-5. Load cross section and capacity for 20-degree troughed belt, three equal rolls (CEMA, 1979).

| |At- Cross Section of Load |Capacity at 100 ft/min |

|Belt |(ft2) |(ft3/h) |

|Width |Surcharge Angle |Surcharge Angle |

|(Inches) |0° |

| 0 | 115 |

| 20 | 77 |

| 30 | 55 |

| 40 | 33 |

Power requirement

Power requirement of a flight conveyor can be estimated by Equation 12-8. Te is now defined as:

Te = 1.1 (force to slide drive line + force to lift drive line

up + force to slide material + force to lift material

up + force to slide drive line - force to lift drive line going down) (12-11)

The drive line consists of the chain and flights. The description assumes the conveyor slopes up toward the discharge end. In this case, gravity force on the return side of the drive line subtracts from the turning effort.

The added 10% is to account for friction in sprocket bearings.

In terms of conveyor parameters, the equation is:

Te = (1.1)L(Wc(Fc cos Θ + sin Θ) + Wm(Fm cos Θ + sin Θ) + Wc(Fc cos Θ - sin Θ) + h2(0.044))

where Te = turning effort, lb (12-12)

L= conveyor length, ft

Wc = weight of chain and flights, lb/ft

h = average depth of material in conveyor, in.

Fc = kinetic friction coefficient of chain and flights on conveyor floor (Table A-3)

Θ = conveyor slope, degrees

Wm= weight of material on conveyor, lb/ft

Fm = kinetic friction coefficient of material on conveyor floor (Table A-3)

The term 0.044 h2 is an empirical factor to account for grain friction on conveyor walls (Rexnord, 1980). It may be negligible for open, top-load conveyors.

The equation can be simplified to:

Te = (1.1)L(2Wc Fc cos Θ + Wm (Fm cos Θ + sin Θ) + h2(0.044))

If Wc is not known, it can be approximated by: (12-13)

Wc = 0.0024 (total weight of material on conveyor, lb), lb/ft (12-14)

This equation, adapted from Rexnord, 1980, assumes Wc to be a function of both conveyor length and weight of material per unit length of conveyor.

From Table A-3 it can be seen that Fm varies from one grain to another and usually increases with moisture content. Power requirement of a flight conveyor is, thus, influenced by grain moisture. An example will illustrate use of the equations.

Example 12-4

Estimate the capacity (tons/h) and motor power requirement for this flight conveyor carrying dry corn:

Flights are 12 in. long. Spacing equals length and height is 40% of length. The drive line weight is 3 lb/ft. All conveyor parts are steel. Table A-2: Bulk density = 45 lb/ft3

[pic]

Effective volume capacity is calculated using Equation 12-10 and a value from Table 12-6:

[pic]

capacity = [pic]

[pic]

[pic]

Substituting into Equation 12-13:

Friction coefficients are obtained from Table A-3 in the Appendix.

Te = (1.1) (46.18) (2(3) 0.57 cos 30 + (9.9) 0.27 cos 30 + sin 30) + 2.64 (0.044))

Te = 299.4 lb

[pic]

Assume the drive reduces speed in two steps, each with an efficiency of 0.93.

[pic]

Application of conventional flight conveyors

Conventional flight conveyors are inexpensive simple machines. They are best suited to intermittant use, low volume applications where power requirement is not an important factor and suitability for a variety of materials is important.

En masse conveyor

The en masse conveyor is a type of flight conveyor which moves grain in slug flow (en masse) rather than in discrete elements between flights. Figure 12-7 is a cutaway view of an en masse conveyor. Load is carried on the bottom with return on the top.

Figure 12-7. En masse flight conveyor (Huss and Schlieper, Inc. 1981).

The enclosed box design retains dust, protects grain from weather, and allows long spans without additional support. Low-height flights, which operate submerged, plus the chain move a layer of grain along the conveyor floor. Grain above is carried along in a continuous stream filling the chamber up to the level of the return track supports. Metal-to-metal sliding contact is avoided in some models by use of ultra high molecular weight polyethelene (UHMWP) wear bars (as shown) or as conveyor liners. The drive line rests on UHMWP inserts or rollers on the return. During loading, grain falls through the return drive line. Discharge is under the drive sprocket, or at any intermediate point. Ease of employing any number of intermediate discharges is an advantage. In this form the en masse conveyor is intended for no-incline or low-incline applications. Slope limits are usually in the 5- to 10-degree range

Modification of the flight design allows the en masse conveyor to be used for inclined or even vertical applications. Figure 12-8 shows en masse conveyor flight designs for various applications. Conveyors for higher inclines have a solid partition between the load and the return sections of the conveyor and grain bears against all four walls during conveying. Some designs limit incline to 45 or 60 degrees. Others allow vertical application. Portable models in the configuration of farm elevators are also available. These portable conveyors are driven from the discharge end through a shaft extending along the conveyor to a PTO or electric motor drive near the ground.

Speed, power and capacity

En masse conveyors are designed for drive line speeds of 100 to 275 ft/min. Table 12-7 illustrates the range of capacities available with en masse conveyors.

Conveyor size listed is the width x height of the conveyor box cross section in inches. Grain is assumed to flow at drive line speed in a slug the width of the conveyor and about 65% of its height.

Conveyor capacities up to nearly 100 000 bu/h and lengths to 400 ft make this conveyor type appropriate for many high capacity applications.

Figure 12-8. En masse conveyor flight configurations (Buhler-Maig (1983).

Table 12-7. Typical en masse conveyor horizontal capacity (Huss and Schieper, Inc. 1981).

CAPACITY CHART - UNITS PER HOUR

|Conv | |Speed ft/min |

|Size1 |UNITS |1 |

|500-700 |Well suited to wet and dry grain handling on |1 - Small farm needs |

| |continuous flow dryer |2 - Feed making only, with separate elevators for receiving |

| | |wet grain |

| | |3 - As wet and dry grain elevator on continuous flow dryer. |

|1000-1200 |Well matched to 6” augers. Gravity spouts: 6” |1 - Small and medium farms, feed and/or cash grain. |

| | |2 - Small batch dryers, and layer or batch-in-bin drying |

| | |methods on small to medium farms. |

|1500-2000 |Well matched to load-unload rates on many mechanized |1 - Medium to large farms, feed and cash grain. |

| |batch dryers |2 - Load-unload on batch and batch-in-bin drying systems. |

| | |3 - Primary leg in a continuous flow or batch drying setup. |

| |Maximum size for 6” gravity spouts. | |

| | | |

| |Maximum size for 8” horizontal augers in 25% corn. | |

|2500-3000 |Matched to 8” overhead augers in dry grain. |1 - Large farms, feed and cash grain. |

| | |2 - As load-unload on large batch and batch-in-bin dryers. |

| |Gravity spouts: 8” |3 - .As primary leg in two-leg installations on continuous |

| | |flow dryers. |

Figure 12-14 is a nomograph showing capacities resulting with different combinations of belt speed, bucket size, and bucket spacing. Bucket sizes are given by nominal bucket length x projection in inches (See Figure 12-12). Bucket volumes listed assume buckets are filled to line x-x on Figure 12-13 and are typical for the bucket sizes listed.

Figure 12-14. Bucket conveyor capacity nomograph (Bloome et al., 1978).

Different brands with the same nominal dimensions may vary + or - 15% from the listed volumes. Conveyor capacity assumes buckets are filled to 85% of volume. One bushel in Figure 12-14 is 1.245 ft3. In selecting a bucket size - bucket spacing combination, be sure bucket spacing exceeds bucket height (the smaller number) by at least an inch.

An example will illustrate use of Figure 12-14.

Example 12-7.

A bucket conveyor runs with a belt speed of 440 ft/min and uses 9x6 buckets. What bucket spacing is needed for a capacity of 3000 bu/h?

Line CD is drawn from 440 ft/min to 3000 bu/h. It crosses the diagonal solid line at E, which is called the turning point. Now a line is extended from F, the 9x6 volume, through E to G, a bucket center-to-center spacing of 8.3 in.

The same result can be obtained by computation:

[pic]

= 8.3 in/bucket

Power requirements

Power requirements for bucket conveyors are usually estimated by computing the necessary lifting power and then adding a component to account for friction losses. Equation 12-19 was adapted from Bloome et al., 1978.

[pic] (12-19)

where hp = power required, hp

C = conveyor capacity, ft3/h

BD = material bulk density, lb/ft3

h = lift height (distance between conveyor shaft centers), ft

Example 12-8 illustrates use of the power equation.

Example 12-8.

Estimate the motor power required for the conveyor of Example 12-7, assuming speed is reduced in two steps, the material conveyed weights 45 lb/ft3, and the lift height is 50 ft.

[pic]

[pic]

[pic]

Bucket conveyor applications

Bucket conveyors are well suited for high-rate vertical conveyance applications which find heavy use. In this type of situation, there may be no realistic alternative method. If a vertical auger or pneumatic system is an alternative, the bucket conveyor is the best choice where heavy use causes its high ownership cost and low operating costs to add to the lowest total cost.

For farms, well planned systems designed around legs are hard to match for convenience. The tall leg becomes a status symbol and landmark. However, its high investment cost is sometimes hard to justify. Because of its intermittent use pattern through the year, other more energy-intensive conveyors which cost less to buy are ultimately cheaper. The height of the leg necessitates wires for support. Wind and lightning can cause damage. Maintenance of the head section is difficult.

SCREW CONVEYORS

A screw conveyor consists of a helicoid or screw or auger which moves material as it rotates within a tube or trough. It is one of the oldest and, at first sight, simplest of the mechanical conveying devices. Archimedes is credited with using a screw conveyor to pump water from ships over 2200 years ago. For this reason, it is sometimes referred to as the Archimedean screw. It has been in continual use for countless conveying tasks since that time. Its simple appearance is deceiving. Its operating characteristics are far more complex and hard to predict than those of any of the other mechanical conveying devices.

In some references, including those of the American Society of Agricultural Engineers, a screw conveyor is called an auger. The terms will be used synonymously here.

Screw conveyor terminology

Screw conveyor terminology has been standardized by the American Society of Agricultural Engineers. Figure 12-15 shows the hand of the helicoid flighting (called "helicoid" from now on). The hand convention corresponds to that of a screw fastener.

Figure 12-15. Hand of screw conveyor helicoid (ASAE, 1983a).

Figure 12-16 shows the dimensional specifications of a screw conveyor. The illustration shows a portable or transport type screw conveyor. The terminology also applies to a fixed machine or a portable unit without a wheeled chassis.

| |SECTION 3 – DIMENTIONAL SPECIFICATIONS |

| | |

| |Auger length: The length of the tube assembly including any |

| |intake but not including any intake hopper or head drive |

| |components (dimension A). |

|[pic] |Intake length: The length of the visible flighting with the |

| |control gate (if unit is so equipped) in the full open |

| |position (dimension B). |

| |Transport angle: The angle included between the auger tube |

| |and the ground when the unit is in the lowest recommended |

| |transport position and with hitch on ground (dimension C). |

| |Maximum operating angle: The angle included between the auger|

| |tube and the ground when the unit is in the highest |

| |recommended operating position, and with the hitch on the |

| |ground (dimension D). |

| |Auger Size: The outside diameter of the auger Tub (dimension |

| |E). |

| |Reach at maximum height: The horizontal distance from the |

| |foremost part of the under carriage to the center of the |

| |discharge end when the unit is at the maximum recommended |

| |operating angle with hitch on ground (dimension F). |

| |Maximum lift height: The vertical distance form the ground to|

| |the lowest point of the discharge (excluding down spout |

| |attachments) when the unit is raised to the maximum |

| |recommended operating angle and with the hitch on the ground |

| |(dimension G). |

| |Transport height: The vertical distance from the ground to |

| |the uppermost portion with the unit in the lowest transport |

| |position and with the hitch on the ground (dimension H). |

| |Eave clearance: The vertical distance from the ground to the |

| |foremost component of the undercarriage when the unit is at |

| |the maximum raised height (dimension J) |

| |Discharge length: The total length of conveying from the |

| |outer end of the exposed flighting assembly at the intake to |

| |the centerline of the discharge (dimension K). |

Figure 12-16 Screw conveyor dimensional specifications (ASAE, 1983b).

Pitch and flighting terminology for some of the more common helicoid configurations are shown in Figure 12-17. The single flight, standard pitch is the most common configuration and is also the one we will be discussing at greatest length.

Figure 12-17. Pitch and flighting terminology (Thomas Conveyor Company, 1980).

Typical specifications

Typical specifications needed for power and capacity computations are listed in Table 12-9 for typical farm-type conveyors. Note that the nominal conveyor size is the outer tube diameter. For industrial horizontal conveyors (Section 12.4.5), it is usually the helicoid diameter. Industrial conveyors usually have larger shaft sizes and much lower maximum speeds.

Table 12-9. Typical farm type screw conveyor specifications

|Nominal Conveyor diameter, |Tube inside |Helicoid diameter,|Shaft diameter, |Max |

| |diameter, | | |speed, |

|in |in |in |in |rev/min |

| 4 |3.90 |3.37 |0.84 |875 |

| 6 |5.88 |5.13 |1.40 |650 |

| 8 |7.85 |7.25 |1.50 |500 |

| 10 |9.80 |9.00 |2.38 |350 |

| 12 |11.80 |11.00 |2.88 |350 |

General features

Screw conveyors are simple, compact machines. They are usable at any angle of inclination and for many bulk materials. Besides conveying, they can be used (sometimes simultaneously) for metering or feeding, heating, cooling, mixing, and even digging applications. Chapter 13 describes auger feeders for pneumatic conveyors. Chapter 6 describes application of acid preservative during conveyance in a screw conveyor. They are inherently enclosed and can be made dust tight with suitable modifications to the feed and discharge sections. There is no idle conveyor return.

Power requirements are relatively high because material is moved by sliding and is continually mixed. Grain damage can be a problem because of pinch points created between the auger tube and flighting. Pinch points (and other features) can be dangerous to operators. This is discussed later in this chapter. Some screw conveyors are noisy.

Life (in actual use time) is relatively short because of abrasion of helicoid and tube surfaces by the conveyed material. On farms, screw conveyors used occasionally will last many years. Purchase cost is relatively low because of the machine's compact and simple design. Operating cost is relatively high because of the high power requirement. Design for portability is easy because the tube can serve as a structural member.

Principle of operation

The operating principle of a horizontal screw conveyor is obvious. Material resting on the bottom of the tube is pushed along in somewhat the way a snow plow pushes snow off a road. In this case, the plow is continuous and the road slopes toward the center. Material plowed far enough to the side rolls back to the center, only to again contact the plow (helicoid) which keeps coming. The effect is conveyance along the helicoid center line and also mixing. The operation takes place regardless of the helicoid rotational speed, although as speed is increased, dynamic effects will come into play. The material will be thrown rather than pushed.

In a vertical screw conveyor, material will not move up the conveyor unless a certain critical rotational speed is exceeded. This critical speed is the speed at which material travels neither up nor down. If the helicoid is turning above critical speed, material in the conveyor is accelerated in a circular motion. Centrifugal force moves it out against the tube wall, or against other material to slide up the inclined helicoid surface as the helicoid rotates.

Material slides on both the helicoid and tube wall and moves in a spiral motion up until its discharge from the conveyor. At angles intermediate between 0 and 90 degrees, there is a transition from the horizontal mode to the vertical mode of operation.

Critical speed

The critical speed of a screw conveyor is defined as the speed at which a single particle in the conveyor will travel in a circular motion with no vertical movement up or down. The critical speed is dependent on conveyor and material parameters. We can derive an expression for the critical speed by summing the forces on a single particle.

Figure 12-18 is a view looking down on a particle within a vertical screw conveyor. The helicoid is turning at ( rad/s.

( = helicoid speed, rad/s

ro = radius of particle path

c = centrifical force

m = particle mass

Figure 12-18. A particle within a vertical screw conveyor.

The particle, at radius ro, is rotating at helicoid speed and is thus subjected to centrifugal force, C. Figure 12-19 is view AA of Figure 12-18, with the helicoid unwound to form an upward sloping surface.

α = angle of helicoid in

g = acceleration of gravity

Ft = kinetic friction coefficient between particle and tube

K = helicoid force on particle

( = angle between helicoid force and normal line to helicoid surface

Figure 12-19. Horizontal view of particle on helicoid surface.

The force C, acting normal to the tube wall, produces the friction force CFt against the tube wall. Seed weight, mg, acts down. Helicoid force K can be resolved into normal component K(cosNote: The change to pitch (12) and font (8) must be converted manually.), a normal force, and K(cos ()Fh, the friction force. Note then that:

tan .ρ = Fh (12-21)

where Fh = static friction coefficient between particle and helicoid.

At speeds above or below critical speed, Fh drops to the lower kinetic value since motion between the helicoid and particles established.

Figure 12-20. Polygon of forces at critical speed.

Figure 12-20 shows the polygon of forces on the particle. The polygon is closed at critical speed. At this condition:

[pic] (12-22)

[pic] (12-23)

where Wc = critical speed, rad/s.

[pic] (12-24)

where Nc = critical speed, rev/min

The equation indicates that critical speed will be lowered by increasing Ft and/or decreasing Fh.

Vierling and Sinha, 1960, state that with force feeding (by, for example, a horizontal feeding screw), a vertical screw can convey material when operating at critical speed. It is also important to note that this analysis assumes no interactions with other particles. Such interaction would mean different friction factors and possibly different helicoid slopes since slope increases toward the center of the helicoid.

Theoretical capacity

The theoretical capacity of a screw conveyor is the product of the free cross sectional area and the speed of advance along the conveyor. The greatest possible distance of advance is one pitch length per revoltuion. Theoretical capacity is, thus:

[pic] (12-25)

[pic]

where Ct = theoretical capacity, ft3/min.

Dh = diameter of helicoid, in.

Ds = diameter of shaft, in.

P = pitch length, in.

N = rotational speed, rev/min.

The equation neglects helicoid thickness and assumes no leakage of material around the edges of the helicoid. Note that helicoid diameter rather than tube inside diameter is used.

The ratio of actual capacity of a screw conveyor to theoretical capacity is the volumetric efficiency. It is commonly expressed as a percent. This variable will be discussed more later.

Important operating parameters

Many grain and conveyor parameters have important influences on the operation of screw conveyors. We will list all that are usually considered important, and then define some of the most important relationships.

Parameters having important influences on screw conveyor power and capacity include (not in order of importance):

material particle size

material bulk density

material flowability

material-to-tube friction

material-to-helicoid friction

conveyor intake length and geometry

conveyor length

conveyor speed of rotation

conveyor diameter

tube-to-helicoid clearance

helicoid pitch length

number of helicoids on shaft

conveyor outlet geometry

conveyor angle of inclination

The other mechanical conveyors studied do not have nearly so many parameters having large effects on power and capacity.

Many of these parameters have interacting effects. In other words, the effect on power or capacity of changing parameter A may be different for different levels of another parameter, B.

In the following discussions, parameters not mentioned are assumed to be held constant.

Intake length

The intake length is the length which the helicoid protrudes from the tube if the conveyor is loaded from a hopper or a mass of grain. It is often specified in helicoid diameters. The general effect on capacity of increasing the intake length can be predicted from intuition. Capacity must be zero with zero intake length. Capacity increases at a decreasing rate as intake length is increased. This is illustrated in Figure 12.21.

Figure 12-21. Effects of intake length on screw conveyor capacity (Rehkugler, 1967).

The expected effect is shown most clearly for 10 degrees, 300 rev/min. In this instance, there is an interaction of exposed screw speed, and inclination in their effects on capacity. Increasing the speed enhances the effect of increasing exposure length; increasing the inclination makes capacity less sensitive to intake length.

Any feature which changes the flow pattern or grain pressure in the conveyor intake region will change conveyor capacity. Hopper geometry and fill level are important. Placement of intake guards can also have large effects. An intake guard meeting ASAE Tentative Standard ASAE S361.1T (ASAE, 1983c) reduces capacity about 17%, compared to the unguarded condition (Sevart et al., 1984). Vertical conveyor capacity can be increased by force feeding of grain to the intake through a horizontal screw conveyor. Vertical screw conveyors for unloading ships have helicoid flighting welded to the outside of the tube above the intake region. This tube is rotated in a direction which causes the flighting to force grain down to the intake and thereby increase capacity.

White et al., 1962 compared six different conveyor inlet configurations (Figure 12-22).

Figure 12-22. Performance of 6-in. screw conveyor under different inlet conditions (White et al., 1962).

Most modifications resulted in lower capacity than the usual 2-diameter exposure of standard-pitch helicoid. The only arrangement to give a higher capacity was a 2-diameter exposure of double helicoid auger.

Power requirement per unit length of conveyor increases with increasing exposure length. The rate of increase is very rapid at first since more helix is being turned and more grain is being moved. Beyond two diameters, the power increase is less and is due mainly to powering the helicoid against the friction of the grain mass. Many screw conveyors are designed with a 2-diameter exposure length.

Slope and rotational speed

Figures 12-23 and 12-24 show the effects of slope and speed on capacity and power. At any speed, capacity goes down almost linearly with slope and, in a vertical position, is usually 30 to 40% of the horizontal value. Power goes up with rotational speed at any slope.

Power is at a maximum at slopes in the 40- to 60-degree range. It is lower at greater and lesser slopes. Several effects cause this relationship. Capacity is changing with slope, as is the vertical distance of conveyance.

Capacity increases with rotational speed up to a point where centrifugal force on the grain in the intake region apparently prevents further increases and may cause a decrease in capacity.

Figure 12-23. Capacity, slope, speed relationships for a 4-in. screw conveyor carrying 56.5 lb/bu wheat (Millier, 1958).

Figure 12-24. Power, slope, speed relationships for a 4-in. screw conveyor carrying 55.5 lb/bu wheat (Millier, 1958).

Conveyor power per unit length and conveyor capacity are not influenced by conveyor length.

Figure 12-25 shows the effect of incline and rotational speed on volumetric efficiency. The volumetric efficiency is the fraction of theoretical capacity carried by the conveyor. The figure shows experimental results for a 1.5-in. standard pitch conveyor with an intake length of 2 diameters. The conveyor carried dry millet.

Figure 12-25. Volumetric efficiency versus speed for various angles of inclination (Roberts and Willis, 1962)

Moisture content

Unlike the previous three conveyor types, screw conveyor power and capacity are significantly influenced by product moisture content. Other things equal, power goes up and capacity goes down as moisture is increased. Most tables and equations assume dry grain, meaning not over 15% moisture. An extension engineer's rule-of-thumb says conveyor capacity will be halved and power doubled when grain is wet (over 20% moisture).

Table 12-10 shows capacity and power for a 6-in. screw conveyor carrying wet (25% and dry (14%) corn. Speed and slope are seen to interact with moisture content in their effects on power and capacity.

Discharge

Discharge geometry can have large effects on power and capacity. Axial discharge out the conveyor end seldom presents any problems. Radial discharge through an opening and chute can result in compaction of material and reduction of capacity if the opening is too small or configured incorrectly. Precise requirements for discharge dimensions were not found in the literature.

Table 12-10. Effect of corn moisture on conveyor performance (White et al., 1962).

Comparison of performance data for a 6-inch screw conveyor handling 14 and 25 percent moisture shelled corn (wet basis); 12 inches exposed helix at the screw inlet.

|Auger |Corn |Angle of elevation of screw conveyor |

|speed |moisture |0° |22.5° |45° |67.5° |90° |

|rev/min |percent |bu/min |hp/10’a |

| 4 |12.0 |14 |78.0 |

| 6 |18.0 |16 |106.0 |

| 9 |31.0 |18 |135.0 |

|10 |37.0 |20 |165.0 |

|12 |55.0 |24 |235.0 |

Figure 12-26. Small-motor overload factor (CEMA< 1980)

An example will illustrate use of the equation

Example 12-10.

The conveyor of Example 12-9 is 50 ft long. What is its power requirement?

Substituting into equations 12-26, -27, -28:

[pic]

[pic]

From Figure 12-26, Fo = 1 since hp is greater than 5.

hp = 1.59 + 10.0 = 11.59 hp

Assuming a double reduction, motor power required is:

[pic]

Among the three conveyor types used for mechanical conveying (belt, flight, screw), the screw conveyor is often chosen for relatively short runs (less than 50 ft) and/or where processing is done during conveyance (heat transfer or mixing, for example). Its initial costs would probably be lowest and its operating cost would probably be highest.

The design procedure shown here tends to be quite conservative and most applicable to grain elevator and industrial processing applications. An indication of this can be seen in the recommended maximum speed for the 6-in. conveyor. Table 12-11 lists it at 60 to 165 rev/min, depending on the material class code. Farm augers of this size run at speeds form 263 to 625 rev/min.

Power and capacity of screw conveyors

Because of the number of important variables affecting power and capacity of inclined screw conveyors, no system of easy-to-use prediction equations is available. Reliance on tables of empirical information is a common design procedure.

Performance tables

Table 12-13 lists capacities and speeds for a line of screw conveyors. Table values assume horizontal operation with dry corn at 90% of theoretical capacity.

Table 12-13. Approximate screw conveyor capacities in bu/h for horizontal operation with dry grain (Hutchinson, 1983).

|AUGER diameter, |PULLEY diameters, |rev/min |CAPACITY PER 100 rev/min AT |NET CAPACITY bu/n |

|in |in | |90% LOAD | |

|4 |2.5 - 5 |875 | 60 | 525 |

| |2.5 - 8 |547 | 60 | 328 |

| |2.5 - 10 |437 | 60 | 262 |

| |2.5 - 12 |365 | 60 | 219 |

| | | | | |

|5 |3 - 7 |750 | 90 | 675 |

| |3 - 8 |656 | 90 | 590 |

| | | | | |

|6 |3 - 12 |438 | 240 | 1051 |

| |3.5 - 12 |510 | 240 | 1224 |

| |3.5 - 15 |429 | 240 | 1029 |

| |5 - 12* |263 | 240 | 631 |

| |PTO |625 | 240 | 1500 |

| | | | | |

|8 |3 - 12 |438 | 480 | 2102 |

| |3.4 - 15 |397 | 480 | 1905 |

| |5 - 12* |263 | 480 | 1262 |

| |PTO |540 | 480 | 2592 |

| | | | | |

|10 |3 - 15 |350 | 1200 | 4200 |

| |5 - 12* |263 | 1200 | 3156 |

| |PTO |320 | 1200 | 3840 |

| | | | | |

|12 |3 - 15 |350 | 2000 | 7000 |

| |5 - 12* |263 | 2000 | 5260 |

| |PTO |320 | 2000 | 6400 |

| | | | | |

*Reducer Drive

Capacity decrease for angle of operation: 20% at 45° (unless pressure fed)

50% at 90° (unless pressure fed)

Capacity decrease for 25% moisture grain: 40%

Tables 12-14, 12-15, 12-16, and 12-17 are the results of the classic experimients of White et al., 1962. They show the effect of angle of elevation, speed, diameter, exposure length, and grain type on capacity and power requirement. These are the most often quoted tables for screw conveyor characteristics.

Table 12-14. Performance data for a 4-inch nominal diameter screw conveyor handling shelled corn (bushel weight: 56 pounds); moisture content 13.2 to 14.2 percent wet basis (White et al., 1962).

| |Length of | |

|Auger |exposed helix |Angle of elevation |

|speed |at intake |0° |45° |90° |

|rev/min |inches |bu/hr |hp/10 fta |bu/hr |hp/10 fta |bu/hr |hp/10 fta |

| | 6 |140 | .11 |110 | .13 | 40 |.10 |

| | 12 |150 | .12 |120 | .15 | 60 |.11 |

| 200 | 18 |150 | .13 |120 | .17 | 70 |.12 |

| | 24 |150 | .14 |120 | .18 | 80 |.13 |

| | | | | | | | |

| | 6 |270 | .23 |180 | .25 | 90 |.19 |

| | 12 |290 | .29 |220 | .29 | 130 |.24 |

| 400 | 18 |290 | .33 |240 | .32 | 150 |.26 |

| | 24 |300 | .38 |240 | .36 | 160 |.27 |

| | | | | | | | |

| | 6 |410 | .33 |280 | .40 | 160 |.29 |

| | 12 |470 | .43 |350 | .52 | 220 |.41 |

| 700 | 18 |480 | .51 |380 | .64 | 250 |.47 |

| | 24 |480 | .60 |380 | .76 | 270 |.49 |

| | | | | | | | |

| | 6 |490 | .41 |320 | .61 | 200 |.46 |

| | 12 |650 | .63 |460 | .81 | 310 |.67 |

| 1,180 | 18 |740 | .85 |530 | 1.01 | 360 |.79 |

| | 24 |770 | 1.08 |560 | 1.21 | 380 |.88 |

| | | | | | | | |

aHorsepower is that required at auger drive shaft. Power loss in drive train must be added to determine the total power required for the conveyor.

Table 12-15. Performance data for a 4-inch nominal diameter screw conveyor handling soybeans (bushel weight - 54.5 to 56.0 pounds); moisture content 11.0 to 11.2 percent wet basis (White et al., 1962).

| | | |

|Auger |Intake | | | | | |

|rev/min |inches |bu/hr |

|Auger |Intake | | | | | |

|RPM |inches |bu/hr |

|Auger |Intake | | | | | |

|rev/min |inches |bu/hr |

| |Michigan |Ohio | |

|Tractor |8.4 |7.4 | 58 |

|Corn picker |48.6 |62.3 | 22 |

|Wagon |71.9 |51.0 | 76 |

|Baler |106.4 |---- | 5 |

|Combine |112.0 |90.1 | 209 |

|Elevator |573.6 |981.5 | 340 |

References: Doss and Pfister, 1972 and National Safety Council, 1974.

The ASAE has established a tentative standard for auger conveying equipment. ASAE Tentative Standard ASAE S361.1T (Safety for agricultural auger conveying equipment) is Appendix B. The purpose of the Standard is to establish safety recommendations which will minimize the possibility of injury during normal operation of auger conveying equipment used to convey agricultural materials on farms. The standard specifies intake guard dimensions (hazard 1 above), winch and cable requirements (hazard 2), lateral stability requirements (hazard 3), and PTO guarding (hazard 5).

Bucket conveyor safety considerations

In grain elevators, bucket conveyors are the most common known location of primary explosions (see Figure 12-17). Friction in bucket conveyors ranks next to "cutting and welding" and "unknown" as an ignition source of primary explosions in grain elevators (see Figure 12-28).

Johnston, 1979 describes this likely scenario of the start of a fire or explosion:

1. Material stops leaving the conveyor and the belt and buckets plug and jam.

2. The drive motor increases its torque output and belt slippage begins.

3. At the slippage point, the belt rapidly heats up, begins to melt, and lubricates further slippage.

4. The belt begins to burn and spreads burning embers within conveyor.

5. Since grain elevator bucket conveyors routinely contain dust concentration exceeding the minimum explosive concentration, explosion and/or fire can result.

The scenario can be avoided by a control system which can detect blockage conditions and shut down feeding conveyors, and can detect belt slippage and shut down the conveyor when a certain level of slippage occurs.

Figure 12-27. Locations of primary explosions in grain elevators (Johnston, 1979).

Figure 12-28. Ignition sources of primary explosions in grain elevators (Johnston, 1979).

GRAIN BREAKAGE IN CONVEYORS

The general topic of grain breakage is discussed at greater length elsewhere. Breakage in specific mechanical conveyors will be discussed here.

The importance of grain breakage during conveying differs with circumstances. Gentleness to grain is not a very valuable characteristic for a conveyor carrying grain to a grinder. The breakage of any grain destined for livestock feed (about 80% of Iowa corn) does not decrease its value directly. Lowered storability and handling the fines may be problems with livestock feed.

Breakage of grain may result in lowered market value and lowered value as a feedstock for milling.

Grain breakage in various handling operations

Fiscus et al. 1971a and b carried out series of experiments with corn, soybeans, and wheat (all dry) to determine breakage resulting from various operations.

Grain breakage in free-fall drop tests

Gravity conveyance can damage grain because of impact at the end of a fall. Fiscus et al. 1971a measured grain velocities after discharge from 8- and 12-in. orifices (Figure 12-29). Velocities of dry corn, wheat, and soybeans differed little and all data was pooled to compute the regression lines shown on the graph. The free fall line is the velocity attained by a particle accelerated by gravity but not subject to air resistance. Since kernels within the stream do not react with the air like individual kernels, the stream attains velocities higher than the terminal velocity of individual kernels at about a 50-ft drop height. Kernel velocity exceeded zero at zero drop height due to motion within the grain bin.

Figure 12-29. Grain velocity versus drop height (Fiscus et al., 1971a).

Breakage was measured after grain impacted upon grain in a bin. Breakage was the percent weight of particles passing through 0.159 x 0.159-in. screen openings (corn) and through 0.158 x 0.5-in. screen openings for soybeans. Wheat breakage was much lower and was not reported. The breakage relationships are shown in Figure 12-30. Breakage is seen to be an exponential function of velocity.

Figure 12-30. Grain breakage versus velocity (Fiscus et al. 1971a)

Other tests showed that grain falling on grain is damaged less than grain falling on concrete. Note that breakage is much worse for corn than for soybeans, that breakage is higher for lower moisture, and for lower grain temperatures.

Many different devices and methods have been tried in effects to avoid high velocity impact after gravity conveyance. Stephens and Foster, 1977 tried various flow retarders on a 50-m inclined grain spout carrying 11- to 19-% moisture corn at temperature of 4 to 11 C. Damage was the weight percent of fines passing through a 4.76-mm (12/64-in) round hole screen. Table 12-19 shows results. The retro-air employed a 2.2-kW fan which forced air up the tube against grain flow.

Table 12-19. Corn breakage per handling (Stephens and Foster, 1977)

| |Breakage increase |

|Flow retarder |per handling, % |% of control |

|Retro-air |3.64 |107 |

|No retarder (control) |3.41 |100 |

|Spout retarder |3.22 | 94 |

|Cushion box |2.83 | 83 |

|Spout retarder and cushion box |2.65 | 78 |

The spout retarder is a cone-shaped device installed near the end of the spout. Inside, a 5-L bucket fills with grain and then continually spills over as grain continues to impact on its opening. The cushion box employs the same principle, but grain makes a 45 degree direction change through the device. All devices except the retro air reduced corn breakage to some extent.

Two other findings from this study are worth noting. Drying treatment had a greater effect on breakage than did flow retarder action. Breakage for all tests averaged 5.87% per handling for corn dried in a batch dryer with 90 to 100 C air, 2.66% per handling for corn dried in a bin with 50 to 60 C air, and 0.92% per handling for corn dried in the field. Also, breakage is approximately cumulative. About the same breakage will occur each time corn is dropped.

Bucket conveyor breakage

In another series of tests, Fiscus 1971b measured breakage in a bucket conveyor. Breakage for wheat was defined as the weight percent passing through 0.065 x 0.25-in screen openings. Corn and soybean breakage were measured as described earlier. Table 12-20 shows the results.

Several points can be noted from the results:

Corn had by far the highest breakage, followed by soybeans, and then wheat.

Breakage went up with decreasing grain moisture.

Breakage went up with decreasing grain temperature.

There was no difference in breakage between the two bucket styles used.

Half full buckets increased breakage 0.2 points for corn, compared to full buckets. There was no difference for other grains.

Feeding corn on the up leg side increased breakage 0.3 points compared to feeding on the down leg side. There was no difference for other grains.

Table 12-20. Bucket elevator percent breakage (Fiscus et al. 1971b)

| |

Grain | |

Corn | |

Soybeans | |

Spring wheat | |Winter Wheat | | | |Moist, % | |13.3 |12.7 |15.1 |14.8 | |10.8 |12.6 | |10.9 |12.9 | |11.5 | | | |Temp, °F | |43 |85 |28 |84 | |58 |43 | |28 |36 | |48 | | | |Test wt, lb/bu | |54.3 |54.4 |54.2 |53.6 | |57.8 |57.9 | |61.1 |61.1 | |63.5 | |TEST CONDITION | | | | | | | | | | | | | | | |Belt speed, fpm |Boot feeding method |Bucket loading |Bucket style | | | | |

Mean percent breakage

| | | |650 |Front |½ Full |Nu-Hy | |3.18 |1.03 |1.17 |0.30 | |0.42 |0.43 | |0.11 |0.11 | |0.13 | |650 |Front |Full |Nu-Hy | |2.74 |0.68 |0.92 |0.21 | |0.22 |0.25 | |0.11 |0.16 | |0.13 | |940 |Front |½ Full |Nu-Hy | |2.89 |1.06 |1.30 |0.33 | |0.43 |0.34 | |0.17 |0.15 | |0.12 | |940 |Front |Full |Nu-Hy | |2.68 |0.95 |0.80 |0.26 | |0.53 |0.37 | |0.15 |0.16 | |0.13 | |650 |Back |½ Full |Nu-Hy | |2.21 |1.03 |0.78 |0.20 | |0.32 |0.27 | |0.11 |0.17 | |0.11 | |650 |Back |Full |Nu-Hy | |1.64 |0.81 |0.28 |0.19 | |0.22 |0.24 | |0.11 |0.14 | |0.12 | |940 |Back |½ Full |Nu-Hy | |2.01 |0.90 |0.41 |0.24 | |0.42 |0.27 | |0.12 |0.12 | |0.10 | |940 |Back |Full |Nu-Hy | |2.67 |0.82 |0.29 |0.29 | |0.51 |0.28 | |0.33 |0.11 | |0.20 | |650 |Front |½ Full |Link | |2.95 |1.06 |1.00 |0.21 | |0.37 |0.36 | |0.15 |0.15 | |0.11 | |650 |Front |Full |Link | |2.81 |0.79 |0.35 |0.18 | |0.37 |0.32 | |0.17 |0.13 | |0.14 | |940 |Front |½ Full |Link | |3.03 |0.96 |0.39 |0.32 | |0.46 |0.40 | |0.18 |0.13 | |0.10 | |940 |Front |Full |Link | |2.36 |0.89 |0.82 |0.35 | |0.40 |0.33 | |0.22 |0.12 | |0.08 | |650 |Back |½ Full |Link | |2.48 |0.67 |0.24 |0.38 | |0.29 |0.24 | |0.15 |0.12 | |0.13 | |650 |Back |Full |Link | |2.26 |0.65 |0.20 |0.22 | |0.30 |0.24 | |0.16 |0.12 | |0.10 | |940 |Back |½ Full |Link | |2.67 |1.38 |0.79 |0.28 | |0.68 |0.33 | |0.18 |0.12 | |0.18 | |940 |Back |Full |Link | |1.98 |0.92 |0.83 |0.31 | |0.51 |0.26 | |0.19 |0.12 | |0.20 | | | | |Average | |2.54 |0.91 |0.66 |0.27 | |0.40 |0.31 | |0.16 |0.13 | |0.13 | |

Grain breakage in screw conveyors

The screw conveyor has long had a reputation of being a major cause of grain damage. Grain damage can occur by crushing between the helicoid and the tube wall, and by abrasion against the tube and helicoid surfaces.

Hall and Sands, 1970, conducted tests using multiple passes through a 12-ft-long nominal 6-in screw conveyor. Inside tube diameter was 5.875 in. Helicoid diameter was 5 in. Corn was at 13% moisture. Fines were defined as material which would pass through a 16/64-in square hole. Figure 12-31 shows results. Fines production is much higher for partial auger loading and for higher drying temperatures, and is much greater at higher auger speeds.

Figure 12-31. Cumulative fines production in a screw conveyor (Hall and Sands, 1970).

PROBLEMS

12-1. What percent increase in volume capacity could be expected if a 24-in., 225 ft/min. belt conveyor designed for hulled rice is fully loaded with flaxseed?

#12.2. Compute the load cross section of a 48-in., 20-degree troughed belt carrying a material with a surcharge angle of 17 degrees.

12-3. Specify belt width and speed needed to carry dry navy beans at 10 000 lb/min. (Use the smallest belt possible).

12-4. Design a 20-degree troughed belt conveyor to do the conveying job outlined in Example 12-1. Assume pulley centers must be 1 ft below the loading point and 1 ft above the discharge point. Thus the top pulley center is 39 ft above the bottom pulley center. Use the narrowest belt which will carry the volume. Drive is double reduction. Specify conveyor length, width, belt speed, hp, hp x h/ton and energy efficiency.

12.5. A 48-in, 20-degree troughed belt conveyor is carrying corn down a 10-degree decline to a loading dock fully loaded and at maximum speed. At what conveyor length will the required motor output be theoretically zero?

12.6. Assuming a motor efficiency of 75%, compute the power dissipated from the motor and from the drive in Example 12-3. Express in kW.

#12-7 Trajectories of material discharged from a belt conveyor must be known for proper design of discharge chutes. Write a computer program which will print coordinates of (or plot) the trajectory of top and bottom particles discharged from the end of a horizontal belt conveyor. Load thickness is 100 mm. Material leaves the belt at the initial point of tangency of the belt with the pulley (directly above pulley center). Assume this point to have coordinates (0.0). Belt speed is 3 m/s. Neglect air resistance. Follow the trajectory for 1 s in 0.2-s increments.

12-8. A belt conveyor is to be designed for loading corn on ships. In the loading operation, any one of the 3 grains is to be conveyed at a rate not less than 3.5 x 106 lb/h up a slope. The belt should have the volume capacity to carry the lightest grain at the specified rate, and the power necessary to carry the heaviest grain at the belt's volume capacity. The loading point is 80 ft below the discharge point. Specify belt slope, width, speed, and power required at motor output shaft.

#12-9 When material having no velocity component in the direction of conveyance is loaded on a belt conveyor, power is required to accelerate the material up to belt speed. This power is not usually recovered. Compute the power necessary to accelerate the load in problem 12-3.

12-10 Shelled corn is to be conveyed 1000 ft horizontally at a rate of 10,000 bu/h, by a 20-degree troughed belt conveyor. (Assume the corn weighs 45 lb/ft3).

(a) Compute minimum belt size and speed necessary.

(b) Compute power required by conveyor.

(c) What power is required from the drive motor, assuming two speed reductions?

12-11. A double chain all steel flight conveyor is needed to carry dry barley (45 lb/ft3) at a rate of 300 000 lbs per hour up along a 30-degree slope for a distance of 82 ft. The conveyor is to operate at maximum allowable speed. Flight dimensions follow normal proportions. Assume chain and flights weight 10 lb/ft and the drive reduces power in two steps. Compute flight dimensions and power requirement

12-12

A standard dimension (w = s, h = 0.4w) all steel flight conveyor carries a grain having a zero angle of repose and a bulk density of 720 kg/m3. Drive line speed is 60 m/min. Friction coefficient for grain on metal is 0.3. Drive efficiency is 90%. Write a computer program which will print specific conveyor energy per unit mass of grain per unit lift height (kJ/(kg x m)). What angle should the conveyor be operated at for minimum specific conveyor energy?

12-13. Design a conventional flight conveyor to do the conveying job outlined in Example 12-1. Assume sprocket centers must be 1 ft below the loading point and 1 ft above the discharge point. Assume conveyor is standard design, all steel, and operates at 200 ft/min. and 40 degree slope. Specify conveyor width, flight height, hp, hp x h/ton, and energy efficiency.

12.14. An en-masse conveyor is needed to convey soybeans at a rate of 800,000 pounds/hour 100 ft horizontally. The conveyor is to be steel with UHMWP flights and chain wear plates. Select the smallest conveyor size which will do the job. The conveyor is to be powered by an electric motor through a double speed reduction. Specify conveyor size, conveyor speed, and motor hp required.

12-15. A bucket conveyor having a 0.5-in-thick belt and a 5-in bucket projection is to run at critical speed with a head pulley speed of 45 rev/min. Compute head pulley diameter and belt speed.

#12-16. A bucket conveyor is to operate at critical speed with a belt speed of 500 ft/min. Bucket projection is 6 in and belt thickness is 0.5 in. Compute head pulley diameter and speed.

#12-17. Compute centrifugal force, as a percent of kernel weight, for kernels at the inner wall, center, and outer wall of the bucket of problem 12-15.

12-18. A bucket conveyor is equipped with 10 x 5 buckets at a spacing of 10 in, and is lifting corn (45 lb/ft3) to a height of 60 ft.

(a) What should be the belt speed for a capacity of 2500 bu/h?

1. Answer using Figure 12-14.

2. Calculate answer.

(b) Calculate motor power required, assuming two speed reductions. Use calculated speed.

12-19. Design a bucket conveyor to do the job outlined in Example 12-1. Specify bucket size, belt speed, bucket spacing, head pulley diameter, motor HP.

#12-20. Compute the tangent of the slope angle and length in units of diameters per revolution for the outer edge of a standard-pitch helicoid when its axis is vertical.

12-21 Design a bucket conveyor to move dry corn a height of 40 feet at a rate of 1500 bushels/hour. Specify bucket size, belt speed and bucket spacing. Compute motor hp requirement, assuming two speed reducers. Compute energy per unit grain mass (hp•hour/ton), and energy efficiency (%).

12-22. Dry soybeans are being conveyed in a 6-in, 60-ft-long, standard-pitch farm-type screw conveyor operating at 650 r/min and set at a 45-degree angle. Intake exposure is 18 in. (Use tables 12-9 and 12-17.)

Compute:

(a) Power required.

(b) Capacity, bu/h.

(c) Percent of theoretical capacity.

12-23. Using Figures 12-23 and 12-24, define a relationship between energy per unit mass per unit vertical distance versus slope for 500 rev/min operation. Use W x h/kg x m as the energy unit. At what slope is the conveyor most efficient?

12-24. A horizontal (industrial) screw conveyor is needed to carry cottonseed flakes a distance of 22 ft at a rate of 85,000 lb/h. Specify conveyor diameter, speed, and power required at conveyor input shaft.

12.25. Estimate maximum possible capacity (bu/h) and power required for a 20-ft, 12-in standard-pitch vertical farm-type screw conveyor handling dry corn.

12-26. Design a screw conveyor to do the conveying job outlined in Example 12-1. Assume the conveyor extends from 1 ft below the loading point to 1 ft above the discharge point at an angle of 45 degrees. The drive is double reduction. Specify conveyor length, diameter, speed, power required, hp x h/ton, and energy efficiency.

12-27. Locate a portable farm screw conveyor (on wheels) on a farm or dealer's lot. Assume that you have been asked, as part of a product liability case, to ascertain if this auger is in compliance with Sections 4.1, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5.1, 5.3 of ASAE S361.2 (Safety for Agricultural Auger Conveying Equipment). See Appendix B.

- Identify auger completely (brand, size, model, serial number, age, location, etc.)

- State date of examination.

- Explain your procedure for determining compliance for each Section

- State clearly your conclusions.

For lateral stability, do a static analysis assuming the auger is in transport position and is horizontal. For the crank force test, stand on a bathroom scale while cranking and record force changes as you crank slowly.

12-28. Write an equation for corn breakage as a function of drop height for 13% moisture corn at 40 F falling from an 8-in. orifice. Compute the expected breakage for 60 ft drop.

12-29. The grain ladder concept has been proposed as a means of reducing corn breakage. With the grain ladder, grain drops in 23-ft steps rather than dropping the total bin depth. Compare damage due to one 92-ft drop to damage in 4 23-ft drops.

12-30. One million bushels per year of corn at 13% moisture is to be dropped 100 ft from an 8-inch orifice onto pile (drop height stays constant). A grain ladder is to be built to allow corn to make multiple short drops instead of one big drop. Each "rung" of the ladder has an annual ownership cost of $500. Each point of breakage decreases corn value by 0.2c/bu. How many rungs should the ladder have to minimize total cost? (0 rungs = 1 drop, 1 rung = 2 drops, etc)

12-31 Design a screw conveyor to move dry corn a height of 40 ft at a rate of 1500 bushels/hour. Use Table 12-16. Assume an intake exposure of 18 inches. Specify angle of elevation and rotational speed. Compute motor hp requirement, assuming one speed reducer. Compute energy per unit grain mass (hp•hour/ton) and energy efficiency (%).

12.32 An 8-inch auger is set up to convey corn at a 45° angle of elevation at 600 rev. per minute. Compute its % of theoretical capacity with corn at 14% moisture and with corn at 25% moisture. Assume a 1.5-inch diameter shaft size.

12-33 What standard electric motor horsepower size is needed to power a 6-in auger 25 feet long, handling dry corn at a 45 degree angle of elevation? Repeat for wet corn.

#Relatively difficult.

REFERENCES

ASAE 1983a. Auger flighting design considerations. ASAE Engineering Practice: EP3899. American Society of Agricultural Engineers, St. Joseph, MI.

ASAE 1983b. Terminology and specification definitions for agricultural aguer conveying equipment. ASAE Standard ASAE S374. American Society of Agricultural Engineers, St. Joseph, MI.

ASAE 1983c. Safety for agricultural auger conveying equipment. ASAE Tentative Standard ASAE S361.1T. American Society of Agricultural Engineers, St. Joseph, MI.

Bloome, P., S. Harp, J. Garton. 1978. Bucket elevators. OSU Extension Facts No. 1106. Oklahoma State University, Stillwater, OK.

Buhler-Maig. 1983. Chain conveyor design and applications manual. Buhler-Maig, Minneapolis, MN.

CEMA 1979. Belt conveyors for bulk materials. Second edition. CBI Publishing Company, Inc., Boston, MA.

CEMA, 1980. Screw conveyors. CEMA Book No. 350. Conveyor Equipment Manufacturer's Association, Washington, DC.

Des Moines Tribune 1972. Belt conveyor across northern Iowa. December 17, 1972.

Ditzenberg, D. 1980. Elevator design, spouting and distribution for small grains. Presented at Greater Iowa Grain Elevator and Processing Society meeting, March 11, Boone, IA.

Doss, H.J. and R.G. Pfister. ca 1972. Farm machinery use study. Agricultural Engineering Dept., Michigan State University, East Lansing, MI.

Fiscus, D.E., G.H. Foster, H.H. Kaufamann. 1971a. Grain stream velocity measurements. Trans. of ASAE 14(1):162-166.

Fiscus, D.E., G.H. Foster, H.H. Kaufmann. 1971b. Physical damage of grain caused by various handling techniques. Trans. of ASAE, 14(3):480-485, 491.

Hall, G.E., L.D. Sands. 1970. Operating a screw conveyor with minimum damage to corn. Illinois Research 12(2):14-15.

Henderson, S.M., R.L. Perry. 1976. Agricultural process engineering. Third edition. AVI Publishing Co., Westport, CT.

Hudson, W.G. 1954. Conveyors and related equipment. Third edition.

John Wiley and Sons, Inc., New York.

Huss and Schlieper, Inc. 1981. Kleen Flo drag conveyors. Huss and Schlieper Inc., Decatur, IL.

Hutchinson Division, 1983. Hutchinson 1983 catalog. Hutchinson Division, Lear Siegler, Inc., Clay Center, KS.

Implement and Tractor. 1983. Shipments of farm machines and equipment. Implement and Tractor. 98(13):50.

Johnson, J.A. 1979. Grain elevator monitoring systems. In R.C. Gordon(ed.) A practical guide to elevator design. National Grain and Feed Association, Washington, DC.

MWPS 1983. Structures and environment handbook. MWPS-1, Fifth edition, Midwest Plan Service, Ames, IA.

National Safety Council 1974. Accident facts. Nation Safety Council, Chicago, IL.

Pinches, H.E. 1958. Materials handling: farm production integrator. Agricultural Engineering 39(9):517.

Rexnord Inc. 1982. Rexnord catalog R80. Rexnord Inc. Milwaukee, WI.

Rboerts, A.W. and A.H. Willis. 1962. Performance of grain augers. Proceedings of the Institution of Mechanical Engineers. 176(8):165-187

Sevart, J.B., B.L. Klausmeyer, J.E. Wray. 1984. Designing a safer grain auger inlet guard. ASAE Paper MCR-84-196. American Society of Agricultural Engineers, St. Joseph, MI.

Stephens, L.E., G.H. Foster. 1977. Reducing damage to corn handled through gravity spouts. Trans. of ASAE. 20(2):367.

Thomas Conveyor Company. 1980. Bucket elevators catalog BE-980. Thomas Conveyor Company, Fort Worth, TX.

Thomas Conveyor Company. 1981. Thomas screw conveyor engineering guide SC-581. Thomas Conveyor Company, Fort Worth, TX.

Vierling, A., G.L. Sinha. 1960. Investigations into the process of conveying by vertical screw conveyor. Fordern V. Heben (10(8):587-592 (in German). NIAE Translation No. 95, Journal of Agr. Eng. Res. 5(4):445-451.

White, G.M., L.A. Shaper, I.J. Ross, G.W. Isaacs. 1962. Performance characteristics of enclosed screw conveyors handling shelled corn and soybeans, Research bulletin No. 740. Purdue University, Lafayette, IN.

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