agri process

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Agricultural and Biosystems Engineering Department 12/07/06Iowa State University Carl J. Bern

Chapter 12Mechanical Grain Conveying

INTRODUCTION

Material handling is a unit operation which changes the spacial location of material without changing its formexcept incidentally (Pinches, 1958). Material handling operations with grain involve many types of grainconveying 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 seedleft in the conveyor will be mixed with the next grain conveyed. It is of no importance in a farm design where theonly material handled is corn for feed. Product damage is important in a conveyor loading grain to be marketedsince an increase in fine material could result in a lowered value. It has a low priority for a conveyor loading agrinder.

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 aconveyor 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 toconsider as conveyors are compared. The energy expended in lifting material represents an increase in potentialenergy of the material mass and is not dependent on conveyor design. If the conveying path is horizontal, thiscomponent is zero. If the conveying path slopes down, this energy input is negative, meaning there can be an


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output of energy from the conveyor. In some cases when this output exceeds conveyor requirements, the conveyorproduces 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 compareactual 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 = mg sin (12-1)

where F = conveying force on particlem = particle massg = acceleration of gravity

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

=2

1dsFcosW (12-2)

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

==

2

1

y

y 12)ymg(ymgdyW

(12-3)

Note that Equation 12-3 represents the work necessary to convey the particle in the absence of friction. It is thusthe 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:


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t

)ymg(yP 12

=

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 thedesign of a hypothetical conveying system and compared for energy requirement to an impossible "perfect" systemwhich requires no energy to operate the conveyor. Example 12-1 will describe this system and illustrate thecomputation 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 a20-ft-diameter bin having a loading hole 27 ft above ground level. Compute power (hp) and energy per unit grainmass (hp h/ton) required assuming a "perfect" conveyor.

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

P = 2.26hplb)ft00(330(60min)(h)

hp)(min(h)lb)000(140ft)(32=

E =ton

hhp.0323

tonlb)(140,000

lb)(2000(h)hp)(2.26=

Power and energy necessary for the "perfect" conveyor, which requires only power necessary to lift the material, is2.26 hp h/ton.

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

(60)000)(33(hp)

(100)(Q)(Lh)Ec = (12-5)

whereEc = energy efficiency,hp = power from conveyor motor, hpLh = lift height, ftQ = 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,

%100(60)000)(33(2.26)

(100)000)(140(32)Ec ==

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


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Flow of grain by gravity can be utilized where slopes are adequate for reliable flow of material. Table 12-1 listsspout 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 37grain, wet 45 (minimum)pellets 45meal 60

Table 12-2 lists grain flow rates for clean, dry grain flowing through a round tube from a dead stop. This would bethe 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

Flow rate, bu/h

Wheat

6 1,686 2,023 2,5808 3,000 3,600 4,590

10 4,679 5,615 7,15912 6,741 8,089 10,31314 9,178 11,014 14,04216 11,907 14,396 18,35518 15,168 18,202 23,20720 18,632 22,356 28,50722 22,654 27,185 34,66024 26,963 32,356 41,53426 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 thecomponents 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 beltsand thus increase the allowable load cross-section (Figure 12-3a). Return idlers under the belt carry the belt flatand 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 theconveyor, and also covers the loaded conveyor. This design may be less expensive to build, but powerrequirements 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 frombeing 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, theNorth Central Railroad elevator in Baltimore was equipped with a 30-in rubber-belt grain conveyor which ran at550 ft/min. Ball or roller bearings were in common use on belt conveyors by 1920 (Hetzel and Albright, 1941).


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Although conveyor configurations remain similar to early designs, vast improvements in belts, bearings, and driveshave 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 spacedidlers 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 chutemay 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 tothe 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 bedischarged 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. Amoving tripper allows formation of a continuous pile of triangular cross section below the belt.


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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 narrowestpossible 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 thebelt, by belt slippage on the drive pulley as centrifugal force acts on the belt, and by the dangers of belt damage aslarge 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 andother free-flowing, nonabrasive material (CEMA, 1979)


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Belt width, in. Max. beltspeed, ft/min.

Approx. belt weightlb/ft #

18 500 3.524 700 4.5

30 700 636 800 942 800 1148 1000 1454 1000 1660 1000 1872 1000 2184 1000 2596 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 slidingof 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 willtend to roll down if the incline is too great. The limit for a smooth, slick material such as hulled or polished rice is8 degrees. A fibrous, interlocking material like wood chips can be conveyed at a 27-degree incline. Recommendedmaximums for grain are in the range from 8-18 degrees. Recommended maximums for specific materials are listedin 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 intothe 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 istheoretically possible (96 in. belt, maximum speed). No other material handling method can approach such acapacity. As a result, belt conveyors find wide use in applications such as grain elevators where high capacities arerequired.

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


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Conveying distance is unlimited. Belt conveyor systems can be designed like pipelines for dry material. A beltconveyor system has been proposed to carry corn 250 miles east from Storm Lake, IA to a Mississippi river bargeterminal. Although technically feasible, the conveying costs were projected to be higher than rail car rates and sothe 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 (dustcontainment, 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 beltsize, 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 showsdimensions used to compute load cross section for a troughed belt. When material is loaded on the belt it falls to itsfilling 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 , henceforth called thesurcharge angle. As the conveyor belt passes over successive carrying idlers, material on the belt is agitated and


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the cross section assumes a more flattened shape. Since lines AD and CD are perpendicular to the material topsurface, angle ADC is 2 . 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 isestimated as follows:

l = 0.371 (b) + 0.25where (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 angleof repose. See Table 12-4. Flowability is the fourth characteristic of the material code from Table A-1 inAppendix A. For example, wheat has material code 47LC25N (from Table A-2). The 2 indicates a free-flowingmaterial (Table 12-4).

The load cross section as defined here exists in a vertical plane. The effective load cross section of inclined beltsdecreases 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 thecomputation 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 ft 3).


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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.

bu/h1928)ft(1.245minft)(100h

buft)(285min)ft(35793

3

= (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/minBelt (ft2) (ft3/h)

Width Surcharge Angle Surcharge Angle(Inches) 0 5 10 15 20 25 30 0 5 10 15 20 25 30

18 .089 .108 .128 .147 .167 .188 .209 537 653 769 886 1005 1128 125424 .173 .209 .246 .283 .320 .359 .399 1041 1258 1477 1698 1924 2155 239430 .284 .343 .402 .462 .522 .585 .649 1708 2060 2414 2772 3137 3511 389736 .423 .509 .596 .684 .774 .866 .960 2538 3057 3579 4107 4645 5196 576542 .588 .708 .828 .950 1.074 1.201 1.332 3533 4250 4972 5703 6447 7210 799748 .781 .940 1.099 1.260 1.424 1.592 1.765 4691 5640 6594 7560 8544 9552 10592

54 1.002 1.204 1.407 1.613 1.822 2.037 2.258 6013 7225 8444 9678 10935 12223 1355260 1.249 1.501 1.753 2.009 2.270 2.537 2.812 7498 9006 10552 12057 13621 15223 1687672 1.826 2.192 2.560 2.933 3.312 3.701 4.102 10961 13155 15364 17599 19876 22210 2461784 2.513 3.014 3.519 4.030 4.551 5.085 5.635 15079 18089 21119 24186 27309 30511 3381396 3.308 3.967 4.631 5.302 5.986 6.687 7.411 19850 23806 27787 31816 35921 40128 44466

Power requirement

The power requirement of a belt is estimated by use of this equation:

000)(33

(V)(Te)hp =

where hp = power to drive pulley, hpTe = effective tension, lbV = belt speed, ft/min

Te is effective tension at the drive pulley which must be supplied by the drive. The torque supplied to the drivepulley shaft is the product of Te and the drive pulley radius.

Ordinarily, to compute hp, V is known and Te is estimated by summing tensions necessary to run the conveyor andlift the material. For a basic straight-line belt conveyor of the type commonly used for grain movement, Te can beestimated by use of this emperical (adaped from CEMA, 1979):

Te = L (0.00068Wm + 0.05 Wb + .58) + Wm (0.035L + H) + 225 (12-9)

where L = conveyor length, ft (use pulley-to-pulley centerline distance)Wm= weight of material, lb per ft of belt lengthWb = weight of belt, lb per ft of belt length

H = vertical distance material is raised (+) or lowered (-)

The equation can estimate effective tension for a straight-line belt conveyor with no accessories (plows, trippers forexample) operating at 32 F or above.

The drive pulley is connected to a source of shaft power (usually an electric motor) through a drive assembly.Since drive pulley speed is slower than electric motor shaft speed (usually 1725 r/min), the drive is designed to


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reduce speed by means of gears, chains or belts. The efficiency of each reduction is about 93% and drives usingtwo reductions are most common. Lost power is dissipated as heat from drive components.

An example will illustrate power computation:

Example 12-3

A 36-in belt conveyor runs at 285 ft/min and carries wheat at a rate of 245 ton/h. The conveyor is 200 ft long andlifts the wheat 40 ft. Compute necessary motor power output.

lb/ft28.66ton(60min)(285ft)h

lb)(2000hmin(245ton)Wm ==

Wb = 9 lb/ft (Table 12-3)

Substituting into Equation 12-9:

Te = 200 (0.00068 (28.66) + 0.05 (9) + 0.58) + 28.66 (0.035 (200) + 40) + 225

Te = 1781.9 lb


hp15.3800033

(285)(1781.9)hp ==

Assuming each of two speed reductions is 93% efficient, the motor must deliver:

hp17.78(0.93)(0.93)

(15.38)=

Belt conveyor application

Belt conveyors are best suited for low slope, heavily used, high capacity, stationary applications demanding highreliability. At the high end of their capacity range there may be no alternative conveying method available.

FLIGHT CONVEYORS

Flight conveyors consist of one or two endless flexible drive lines (chains, belts, cables) to which flights areattached. Flights drag along material as the drive line is pulled in a circuit. There are many variations inagriculture and industry.

As is the case with belt conveyors, half of the drive line is inactive and is continually pulled back to the loadingpoint empty. (Some circuit-type conveyors may have a drive line more than half active.)

General characteristics of flight conveyors

It is difficult to generalize about characteristics here since flight conveyors are so varied.

Speed

Flight conveyors travel at drive line speeds from 25 to 300 ft/min. Speeds in the range from 100 to 200 ft/min aremost common. Higher speeds accelerate wear and may increase product damage.


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Power

Power requirement is high (higher than belt conveyor, other things being equal) because the drive line, flights andmaterial are all dragged along a surface. This dragging also makes noise. Some designs use plastic liners onflights or on interior conveyor surfaces to reduce friction and noise.

Incline

Allowable incline depends on the flight conveyor type. Some are designed for horizontal use only. Others mayoperate at extreme slopes or even vertically.

Product damage

Some grain damage occurs in flight conveyors because of rubbing action and possible pinch points betweenconveyor components. However, damage is usually less than with screw or pneumatic systems.

Design parameters for conventional flight conveyors

Figure 12-6 is a double chain, portable flight conveyor also known as a farm elevator. Load is carried on top in theopen, with the drive line return below. Flights are rectangular. This type of flight conveyor is very versatile and,with little or no modification, can be used for grain, feed, ear corn, forage, and even bundles of shingles. This typeof conveyor is inexpensive, often noisy, and will have a long life of intermittent use since it is needed only a fewhours per year. Although driving from the bottom sprocket is not desirable (more chain and bearing stress), it isoften done because of the difficulty of transferring power to the discharge end of this type of conveyor. It will beused as an example for design computations for flight conveyors.

Figure 12-6. Double chain, portable flight conveyor (farm elevator).


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Typical design parameters

This type of conveyor is operated with chain speeds between 25 and 300 ft/min. Flight spacing is about equal toflight width and flight height is about 40% of flight width.

The theoretical volume capacity is given by Equation 12-10.

C = (V) (h) (w) (12-10)

where C = theoretical volume capacityV = drive line speedh = flight heightw = conveyor width (flight length)

Equation 12-10 neglects the volume of the flight and chain and assumes slug flow of grain. To consider flightvolume, multiply Equation 12-10 by (s-t)/s, where t is flight thickness and s is the flight spacing.

At 100% of theoretical volume capacity, the conveyor is full to the flight depth. The conveyor will operate atvarious fractions of theoretical volume capacity depending on conveyor slope and the repose characteristics of the

material conveyed. Henderson and Perry, (l976) list the percentages shown in Table 12-6. A conveyor with anenclosed conveying chamber will have less effect of slope on its capacity.

Table 12-6. Flight conveyor approximate volume capacity (Henderson and Perry, 1976).

Incline, degrees Approx. % of theoretical capacity0 115

20 7730 5540 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 lineup + force to slide material + force to lift materialup + 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 thedischarge 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 + sin ) + Wm(Fm cos + sin ) + Wc(Fc cos - sin ) + h2(0.044))

where Te = turning effort, lb (12-12)L= conveyor length, ft

Wc = weight of chain and flights, lb/fth = average depth of material in conveyor, in.

Fc = kinetic friction coefficient of chain and flights on conveyor floor (Table A-3) = conveyor slope, degreesWm= weight of material on conveyor, lb/ftFm = kinetic friction coefficient of material on conveyor floor (Table A-3)


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

The equation can be simplified to:Te = (1.1)L(2Wc Fc cos + Wm (Fm cos + sin ) + 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 ofmaterial 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 moisturecontent. Power requirement of a flight conveyor is, thus, influenced by grain moisture. An example will illustrateuse 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. Allconveyor parts are steel. Table A-2: Bulk density = 45 lb/ft3

ft19.4630cos

40=L30=0.58

40

23.1tan ==

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

min

ft27.5=

min

ft)(1ft)(0.4ft)(125(0.55)=0.55(C)

3

capacity =h

tons37.13

hlb2000ftmin

min)(60(ton)lb)(45)(27.5ft

3

3

=

ft

lb1.1

ft

ft9.9lb46.19)0024.0(W

ft

lb9.9

ft125ftmin

(min)lb)(45)(27.5ftWm c3

3

====

in2.64ft1ft1ft125min

(12in)(min)ft(27.5h

3

==



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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

hp1.13(33000)

(125)(299.4)hp ==

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

hp.311(0.93)(0.93)

1.13requiredpowermotor ==

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 isimportant.

En masse conveyor

The en masse conveyor is a type of flight conveyor which moves grain in slug flow (en masse) rather than indiscrete 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 conveyorfloor. Grain above is carried along in a continuous stream filling the chamber up to the level of the return tracksupports. 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 orrollers on the return. During loading, grain falls through the return drive line. Discharge is under the drivesprocket, 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 usuallyin the 5- to 10-degree range


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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 asolid partition between the load and the return sections of the conveyor and grain bears against all four walls duringconveying. Some designs limit incline to 45 or 60 degrees. Others allow vertical application. Portable models inthe configuration of farm elevators are also available. These portable conveyors are driven from the discharge endthrough 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 ofcapacities 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 atdrive 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 manyhigh capacity applications.


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


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

CAPACITY CHART - UNITS PER HOUR

Conv Speed ft/minSize1 UNITS 1 50 75 100 125 150 175 200 225 250 27

12x22 CU. FT 68.4 3420 5130 6840 8550 10260 11970 13680 15390 17100 1881BU. 54.7 2730 4100 5470 6840 8200 9570 10940 12310 13680 1504

18x22 CU. FT 106.3 5310 7970 10630 13280 15940 18600 21260 23910 26570 2923BU. 85.0 4250 6370 8500 10630 12750 14880 17000 19130 21260 2338

24x22 CU. FT 144.2 7210 10810 14420 18020 21630 25230 28840 32440 36050 3965BU. 115.3 5760 8650 11530 14420 17300 20180 23070 25950 28840 3172

30x22 CU. FT 182.1 9100 13650 18210 22760 27310 31860 36420 40970 45520 5007BU. 145.6 7280 10920 14560 18210 21850 25490 29130 32770 36420 4006

18x28 CU. FT 136.9 6840 10260 13690 17110 20530 23950 27380 30800 34220 3764BU. 109.5 5470 8210 10950 13690 16420 19160 21900 24640 27380 3011

24x28 CU. FT 186.7 9330 14000 18670 23330 28000 32670 37340 42000 46670 5134BU. 149.3 7460 11200 14930 18670 22400 26130 29870 33600 37340 4107

30x28 CU. FT 236.3 11810 17720 23630 29530 35440 41350 47260 53160 59070 6498

BU. 189.0 9450 14170 18900 23630 28350 33080 37800 42530 47260 519836x28 CU. FT 286.0 14300 21450 28600 35750 42900 50050 57200 64350 71500 7865

BU. 228.8 11440 17160 22880 28600 34320 40040 45760 51480 57200 629242x28 CU. FT 335.7 16780 25170 33570 41960 50350 58740 67140 75530 83920 9231

BU. 268.5 13420 20140 26850 33570 40280 46990 53710 60420 67140 738548x28 CU. FT 385.4 19270 28900 38540 48170 57810 67440 77080 86710 96350 10598

BU. 308.3 15410 23120 30830 38540 46240 53950 61660 69370 77080 847854x28 CU. FT 435.1 21750 32630 43510 54380 65260 76140 87020 97890 108770 11965

BU. 348.0 17400 26100 34800 43510 52210 60910 69610 78310 87020 95721width x height of box cross section, in.

An example will illustrate power computation for an en masse conveyor.

Example 12-5.

An en masse conveyor is to be used to convey dry corn a distance of 200 ft along a 10 degree incline at a rate of600 000 lb/h. The conveyor is to be steel with UHMWP flights and chain wear plates. Specify the conveyor andestimate the power requirement assuming a dual reduction drive.

h

ft13333

lb45h

ftlb600000 33=

From Table 12-7, a 12 x 22 conveyor running at between 175 and 200 ft/min will handle this capacity.

19.9x

25

x

11970-13680

11970-13333==

Conveyor speed is 175 + 19.9 = 194.9 ft/min.Power is estimated by Equations 12-12 and 12-13.Weight of material on conveyor =

lb10,262(60min)ft(194.9)h

ft200hminlb000)(600=


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12-20

Figure 12-10 is a side view of a bucket conveyor type commonly used for grain. Common terminology ofconveyor parts and dimensions is also included. The figure shows a dual-leg conveyor. This means the up and thedown sides are in separate enclosures (legs). A single-leg type has the entire belt in one enclosure. The entire

bucket conveyor is sometimes called a leg. A motor drives the head pulley. Takeup adjustment is at the footpulley. It can be by bolts (as shown) or by gravity from weights hung on the shaft.

Figure 12-10. Bucket conveyor (Bloome et al. 1978).

Types of bucket conveyors

The three common belt conveyor types vary in the way material is discharged. Figure 12-11 illustrates these types.The centrifugal discharge type is discharged by centrifugal action as loaded buckets pass over the head pulley. Thehead section must be specially designed for proper discharge. This will be discussed more later. Most grainconveyors are centrifugal discharge and all discussion following this section will be about that type.

Positive discharge conveyors employ an idler below the head pulley. As the drive line (which may be chain in thiscase) runs around the idler, each bucket is inverted over a discharge spout, causing positive discharge. This typeconveyor runs at lower speeds and is used for light, fluffy, or fragile materials or those tending to stick in buckets.


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12-21

Figure 12-11. Bucket conveyor types (Thomas Conveyor Co., 1980)

Continuous conveyors have buckets placed as close as possible (continuously) on the belt. During discharge,material flows over the preceding bucket whose front and projecting ends form a chute to direct material into thedischarge spout. This conveyor type is used for heavy abrasive and lumpy materials like cement, crushed stone,and clinker.

Another design (not shown) uses hanging buckets which allow vertical, angled, and horizontal belt routing.

General features

Bucket conveyors have low power requirements since load is carried in buckets supported by antifriction bearings.Power and capacity are not affected by grain moisture content. Their noise level is relatively low. Bucketconveyors are reliable, relatively trouble free, and have a long service life. On farms they often are the commonsection of a closed-loop handling system. Horizontal conveyance in such a system is accomplished by angledgravity spouting from the bucket conveyor discharge. In grain elevators and other related industries, bucketconveyors are the preferred method of vertical grain movement. Alternatives include vertical screw conveyors and

pneumatic systems, both of which have higher power requirements and a greater potential for grain damage.During actual grain lifting, practically no damage occurs in a bucket conveyor. However, loading and unloadingoperations have a potential to break kernels. This will be discussed more later.

Bucket conveyors can be categorized by belt speed into high speed (450 to 1000 ft/min) and low speed (under 450ft/min). High speed designs are most common for the high capacity conveyors in grain elevators. Low-speed

bucket conveyors are common on farms.

Conditions for centrifugal discharge

A Centrifugal discharge conveyor must be designed with the proper combination of belt speed, head pulley radius,head section shape, and bucket shape for proper discharge. Figure 12-12 shows the forces acting on grain in aconveyor bucket as it rounds the head pulley.


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12-22

Figure 12-12. Forces on grain during centrifugal discharge.

The effective force on the grain is the resultant of the grain force, W which always acts down and C, the centrifugalforce which always acts out along a radius from the head pulley centerline. When the resultant force on a kernelpoints out through the bucket opening, the kernel will leave the bucket. Centrifugal force on a mass is given byEquation 12-15.

(3600)rg

W(Vt)C

2

= (12-15)

where C = centrifugal force, lbW = weight, lbVt = tangential velocity, ft/ming = acceleration of gravity = 32.2 ft/s2

r = effective radius of mass, ft (usually measured to a point halfway across the bucketprojection)

Hetzel and Albright, 1941 recommend that for centrifugal discharge of grain, C = W. If this condition exists theresultant on the kernel will be zero when the cup is directly above the head pulley center line. After that, theresultant force will have a direction out of the bucket and discharge will begin.

The speed for C = W will be referred to as the critical speed. If the equation C = W is combined with equation 12-15 and simplified, the result is:

gr60Vt = (12-16)

For the conveyor,

Vt = 2 r N (12-17)

where N = pulley speed, rev/minCombining 12-16 and 12-17, we obtain: (12-18)

r

54.19N =


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12-23

where N is now the critical speed. Note that the radius here is the effective mass radius and not the pulley radius.Belt speed can be computed by revising Equation 12-17:

Vb = 2 rp N (12-19)

where: Vb = belt speed, ft/min

rp = pulley radius, ft

Buckets

Figure 12-13 shows the bucket shape and size designation. Buckets are made of fabricated metal (usually steel),cast metal, or of a non-metallic material. Non-metallic buckets (polyethelene, urethane, poly vinyl chloride) reducedrive line stresses because they are much lighter than metal buckets. However, one manufacturer cautions againsttheir use in combustible environments because of their ability to retain static electrical charge and produce sparks(Rexnord, 1980).

The radius of the grain in the bucket will vary by the length B in the figure. An example will illustrate use of theequations.

Figure 12-13. Bucket size designation (Bloome et al., 1978)Example 12-6.

A bucket conveyor is designed for centrifugal discharge and the head pulley is to operate at critical speed. Thehead pulley is 18 in. in diameter, with a 0.5-in. belt thickness and a 6-in. bucket projection. Compute the correcthead pulley speed and belt speed.

ft1.04212

3)0.5(9r =

++=

rev/min53.091.041

54.19N ==

ft/min250.1812

)09.53()9(2V

b

==

Designs in use for grain conveying vary considerably from the critical speed condition for centrifugal discharge.One way to compare different designs is to compute the C/W ratio. C/W = 1 if the conveyor operates at criticalspeed. A survey of some manufacturer's specifications showed variations from C/W = 0.71 to C/W = 5.8. The lowratio design will not begin to discharge until the bucket is well past the top of the pulley. The high ratio design will

begin to discharge before it reaches the top. In each case, head section geometry must be designed to accommodateresulting grain trajectories. One design in use (not recommended for grain) uses a belt speed of 1000 ft/min andC/W = 17.1. The discharge chute extends horizontally from the top of the head section.


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12-24

If the head pulley speed is much slower or faster than the head section is designed for, grain will miss the dischargechute and fall down the down leg causing a condition known as "back legging." Back legging damages grain, cutscapacity, and wastes power.

Loading buckets

Conveyor buckets are loaded in the foot (or boot) section. A designer aims for a feed system which fills buckets toa high percent of their capacity with minimum power consumption, grain damage, and dust generation. All of theconveyors shown in Figure 12-11 are loaded into the up leg. This is the preferred loading method for farm sizeconveyors. If grain is introduced above the foot pulley shaft center, buckets are filled as they move vertically.Spillage into the foot section is minimized. If it is necessary for the system, grain can be loaded on the down side(or on both the up and down sides as shown in Figure 12-10). Grain loaded on the down side is subject tocentrifugal force as it is swept under the foot section by the buckets. The practice of making the foot pulley smallerthan the head pulley may increase centrifugal emptying forces to a point where capacity is cut and grain damageand dust generation increase. Ditzenberger, 1980, recommends that the foot pulley diameter never be smaller than66% of the head pulley diameter so that these problems are avoided.

Some high speed designs can be satisfactorily loaded only on the down side. At high belt speeds, grain must be

introduced with a velocity component in the same direction as bucket movement, as is the case with a down-angledspout into the down leg. Loading on the up side results in reduced bucket filling and increased power.

High-speed machines develop positive air pressure in the foot section as air "carried" down by buckets is displacedby grain. Ventilation pipes can be fitted to route this air to the head section which operates at negative pressure forthe same reason.

Capacity

Bucket conveyor capacity depends on belt speed, bucket volume, bucket spacing, and the percent of fill attained bythe bucket. Capacity tables usually assume buckets are filled to 85% of full. In designing a line of bucketconveyors, manufacturers often select a combination of belt speed and head pulley diameter which will give propercentrifugal discharge. With this combination held constant, capacity for different models is varied by varying

bucket spacing or belt width (bucket length).

Table 12-8 shows bucket conveyor capacities for different farm applications. Capacities larger than 5000 bu/h areseldom required on farms. Grain trade applications may require capacities over 60 000 bu/h.


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Table 12-8. Bucket conveyor capacities and applications (MWPS-1978).

CAPACITYbu/h

COMMENTS APPLICATIONS

500-700 Well suited to wet and dry grain handlingon continuous flow dryer

1 - Small farm needs2 - Feed making only, with separate elevators

for receiving wet grain3 - As wet and dry grain elevator on continuousflow dryer.

1000-1200 Well matched to 6 augers. Gravityspouts: 6

1 - Small and medium farms, feed and/or cashgrain.

2 - Small batch dryers, and layer or batch-in-bin drying methods on small to mediumfarms.

1500-2000 Well matched to load-unload rates onmany mechanized batch dryers

Maximum size for 6 gravity spouts.

Maximum size for 8 horizontal augersin 25% corn.

1 - Medium to large farms, feed and cash grain.2 - Load-unload on batch and batch-in-bin

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

drying setup.

2500-3000 Matched to 8 overhead augers in drygrain.

Gravity spouts: 8

1 - Large farms, feed and cash grain.2 - As load-unload on large batch and batch-in-

bin dryers.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 sizeslisted.


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12-26

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 capacityassumes buckets are filled to 85% of volume. One bushel in Figure 12-14 is 1.245ft3. In selecting a bucket size - bucketspacing 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 neededfor 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 theturning 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:

fthbu)(3000)ft(1.245)in(1728bucketmin

in.)(12min)(60hbuft)in0.85x(200ft)(44033

33

= 8.3 in/bucket

Power requirements

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

(2490)

C

(60)000)(33

(h)(BD)(C)1.1hp += (12-19)

where hp = power required, hpC = 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, thematerial conveyed weights 45 lb/ft3, and the lift height is 50 ft.

/hft3735buh

)ft(1.245bu)(3000C 3

3

==


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12-27

hp17.62490

3735

(60)000)(33

(50)(45)(3735)(1.1)=hp =+

hp13.7(0.93)(0.93)

6.17=powermotor =

Bucket conveyor applications

Bucket conveyors are well suited for high-rate vertical conveyance applications which find heavy use. In this typeof 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 toadd to the lowest total cost.

For farms, well planned systems designed around legs are hard to match for convenience. The tall leg becomes astatus symbol and landmark. However, its high investment cost is sometimes hard to justify. Because of itsintermittent use pattern through the year, other more energy-intensive conveyors which cost less to buy areultimately 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 ortrough. It is one of the oldest and, at first sight, simplest of the mechanical conveying devices. Archimedes iscredited with using a screw conveyor to pump water from ships over 2200 years ago. For this reason, it issometimes referred to as the Archimedean screw. It has been in continual use for countless conveying tasks sincethat 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 calledan 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 tothat of a screw fastener.


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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 awheeled chassis.

SECTION 3 DIMENTIONALSPECIFICATIONS

3.1 Auger length: The length of the tube assembly includingany intake but not including any intake hopper or head drivecomponents (dimension A).

3.2 Intake length: The length of the visible flighting with thecontrol gate (if unit is so equipped) in the full open position(dimension B).

3.3 Transport angle: The angle included between the augertube and the ground when the unit is in the lowest recommended

transport position and with hitch on ground (dimension C).3.4 Maximum operating angle: The angle included betweenthe auger tube and the ground when the unit is in the highestrecommended operating position, and with the hitch on the ground(dimension D).

3.5 Auger Size: The outside diameter of the auger Tub(dimension E).

3.6 Reach at maximum height: The horizontal distance fromthe foremost part of the under carriage to the center of thedischarge end when the unit is at the maximum recommendedoperating angle with hitch on ground (dimension F).

3.7 Maximum lift height: The vertical distance form theground to the lowest point of the discharge (excluding down spoutattachments) when the unit is raised to the maximumrecommended operating angle and with the hitch on the ground(dimension G).

3.8 Transport height: The vertical distance from the ground tothe uppermost portion with the unit in the lowest transport positionand with the hitch on the ground (dimension H).

3.9 Eave clearance: The vertical distance from the ground tothe foremost component of the undercarriage when the unit is atthe maximum raised height (dimension J)

3.10 Discharge length: The total length of conveying fromthe outer end of the exposed flighting assembly at the intake to thecenterline of the discharge (dimension K).


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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 atgreatest length.


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


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Typical specifications

Typical specifications needed for power and capacity computations are listed in Table 12-9 for typical farm-typeconveyors. 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 muchlower maximum speeds.

Table 12-9. Typical farm type screw conveyor specifications

Nominal Conveyordiameter,

Tube insidediameter,

Helicoiddiameter,

Shaft diameter, Maxspeed,

in in in in rev/min4 3.90 3.37 0.84 8756 5.88 5.13 1.40 6508 7.85 7.25 1.50 500

10 9.80 9.00 2.38 35012 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 bulkmaterials. 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 inherentlyenclosed and can be made dust tight with suitable modifications to the feed and discharge sections. There is no idleconveyor return.

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

Life (in actual use time) is relatively short because of abrasion of helicoid and tube surfaces by the conveyedmaterial. On farms, screw conveyors used occasionally will last many years. Purchase cost is relatively lowbecause of the machine's compact and simple design. Operating cost is relatively high because of the high powerrequirement. 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 ispushed along in somewhat the way a snow plow pushes snow off a road. In this case, the plow is continuous andthe road slopes toward the center. Material plowed far enough to the side rolls back to the center, only to againcontact the plow (helicoid) which keeps coming. The effect is conveyance along the helicoid center line and alsomixing. 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 isexceeded. This critical speed is the speed at which material travels neither up nor down. If the helicoid is turningabove critical speed, material in the conveyor is accelerated in a circular motion. Centrifugal force moves it outagainst 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 theconveyor. At angles intermediate between 0 and 90 degrees, there is a transition from the horizontal mode to thevertical mode of operation.

Critical speed


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The critical speed of a screw conveyor is defined as the speed at which a single particle in the conveyor will travelin a circular motion with no vertical movement up or down. The critical speed is dependent on conveyor andmaterial 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/sro = radius of particle pathc = centrifical forcem = 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 isview AA of Figure 12-18, with the helicoid unwound to form an upward sloping surface.


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12-33

= angle of helicoid ing = acceleration of gravityFt = kinetic friction coefficient between particle and tubeK = 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(cos), a normal force, and K(cos )Fh, thefriction force. Note then that:

tan = 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 andparticles established.


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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 thiscondition:

FtC

mg)(90tan = (12-22)

Ftr

)(tangWc

o

+= (12-23)

where Wc = critical speed, rad/s.

Ftr

)(tang30Nc

o

+= (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 verticalscrew can convey material when operating at critical speed. It is also important to note that this analysis assumesno interactions with other particles. Such interaction would mean different friction factors and possibly differenthelicoid 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 advancealong the conveyor. The greatest possible distance of advance is one pitch length per revoltuion. Theoreticalcapacity is, thus:

3

3222

in1728minrev4

ftrev)(N(Pin)in)Ds(DhCt

=

(12-25)

2200

PN)Ds(DhCt

22 =

where Ct = theoretical capacity, ft3/min.Dh = diameter of helicoid, in.Ds = diameter of shaft, in.P = pitch length, in.


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12-35

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 listall 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 ofimportance):

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 changingparameter 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 ora mass of grain. It is often specified in helicoid diameters. The general effect on capacity of increasing the intakelength 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.


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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 ofexposed screw speed, and inclination in their effects on capacity. Increasing the speed enhances the effect ofincreasing 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 conveyorcapacity. 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 forcefeeding of grain to the intake through a horizontal screw conveyor. Vertical screw conveyors for unloading shipshave helicoid flighting welded to the outside of the tube above the intake region. This tube is rotated in a directionwhich 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).


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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. Theonly 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 isvery 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. Manyscrew 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 goesdown almost linearly with slope and, in a vertical position, is usually 30 to 40% of the horizontal value. Powergoes 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 regionapparently 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).


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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 oftheoretical capacity carried by the conveyor. The figure shows experimental results for a 1.5-in. standard pitch conveyor with anintake 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


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12-39

Unlike the previous three conveyor types, screw conveyor power and capacity are significantly influenced byproduct 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. Speedand 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 seldompresents any problems. Radial discharge through an opening and chute can result in compaction of material andreduction of capacity if the opening is too small or configured incorrectly. Precise requirements for dischargedimensions 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 90rev/min percent bu/min hp/10a bu/min hp/10a bu/min hp/10a bu/min hp/10a bu/min hp/10a

200 14 9.9 .28 9.2 .41 8.3 .44 6.7 .44 4.6 .3225 6.2 1.37 5.3 1.40 4.7 1.31 3.4 .97 2.6 .32

400 14 18.1 .56 16.8 .82 14.2 .88 11.5 .83 8.6 .7025 11.6 1.84 10.3 1.89 8.5 1.78 6.7 1.45 5.0 .70

600 14 25.2 .84 23.4 1.22 19.4 1.28 15.1 1.16 12.4 1.0525 15.8 2.32 13.7 2.34 11.3 2.27 8.6 1.92 6.8 1.09

800 14 29.4 1.07 27.6 1.54 22.8 1.62 18.0 1.46 14.8 1.3225 18.3 2.80 15.8 2.85 12.9 2.75 9.7 2.44 7.9 1.55

aHorsepower is that required at auger drive shaft. Horsepower loss in drive train must be added to determine thetotal horsepower requirement of the conveyor.

Design of industrial horizontal screw conveyors

Horizontal screw conveyors in sizes from 6 to 24 in. diameter are used in applications similar to those for beltconveyors and en masse conveyors. Capacities attainable are not as high as for these other conveyors. A 24-inhorizontal screw conveyor turning at its maximum speed of 100 rev/min can move grain at a rate of about 13 000

bu/h when loaded to 45% of theoretical capacity.

Procedures for sizing conveyors and for estimating power requirements are presented here for standard flightingconveyors carrying grain.

Sizing horizontal screw conveyors

In the design procedure presented in CEMA, 1980, horizontal screw conveyor maximum speed and degree ofloading are governed by material characteristics. This is illustrated in Table 12-11. An example will illustrate the

procedure.


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Example 12-9.

Specify the size and speed of a horizontal screw conveyor to move corn at a rate of 500 000 lb/h.

From Table A-2 the material code for shelled corn is 45C25.

The conveyor, then, must move:

h

ft11111

45

000500 3=


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Table 12-11. Horizontal screw conveytor capacity (CEMA,1980).


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From Table 12-11, a material class of C-25 allows a 45% fill (first group). The percent of fill must be governedby the auger loading procedure. It is not self regulating by the auger. The capacity will require use of a 24-in.screw conveyor. The required speed is computed as follows:

min

rev67.75

min)ft(164h

rev1hft111113

3

=

The 24-in screw conveyor turning at 67.75 rev/min will move 500 000 lb of corn per hour.

Power requirement of horizontal screw conveyors

To estimate total power requirement, the power to overcome conveyor friction is added to the power to transportmaterial (CEMA, 1980):

500000

FdLNhpf= (12-26)

0000001

(Fm)(BD)CLhpm = (12-27)

hp = (hpf + hpm) Fo (12-28)

where: hpf = power to overcome conveyor friction, hpL = conveyor total length, ft.

N = conveyor rotational speed, rev/min.Fd = empirical diameter factor (Table 12-12)hpm = power to transport material, hpC = capacity, ft3/hBD = bulk density of material as conveyed, lb/ft3

Fm = empirical material factor (Table A-2)hp = power required at conveyor shaft, hp

Fo = empirical small motor overload factor (Figure 12-26)

Total length L is limited by the torque which can be transmitted through shafts and couplings. Computationprocedures for this are not presented here. Factor Fd, Fm, and Fo are all empirically derived. Fd (Table 12-12) isproportional to the conveyor weight per foot. Fm has been formulated from experience and has no measurablerelation to any material physical property. Fo causes larger motors to be applied to small (under 5.2 hp)installations. The increased motor size here has proven effective in avoiding stalling due to minor overloads orchoke conditions.

Table 12-12. Conveyor diameter factor, Fd (CEMA, 1980)

Screw Diamter Factor, Fd

ScrewDiameter

inches Fd

ScrewDiameter

inches Fd4 12.0 14 78.06 18.0 16 106.09 31.0 18 135.0

10 37.0 20 165.012 55.0 24 235.0


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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:

hp1.59000500

(235.0)(67.75)(50)hpf ==

hp10.00000001

(0.4)(45)(50)111)(11hpm ==

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


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hp = 1.59 + 10.0 = 11.59 hp

Assuming a double reduction, motor power required is:

hp13.40(0.93)(0.93)

11.59powermotor ==

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

The design procedure shown here tends to be quite conservative and most applicable to grain elevator and industrialprocessing 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 sizerun 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 systemof 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 operationwith dry corn at 90% of theoretical capacity.


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

AUGERdiameter, in

PULLEYdiameters, in

rev/min CAPACITY PER100 rev/min AT 90%

LOAD

NET CAPACITYbu/n

4 2.5 - 5 875 60 5252.5 - 8 547 60 3282.5 - 10 437 60 2622.5 - 12 365 60 219

5 3 - 7 750 90 6753 - 8 656 90 590

6 3 - 12 438 240 10513.5 - 12 510 240 12243.5 - 15 429 240 10295 - 12* 263 240 631PTO 625 240 1500

8 3 - 12 438 480 21023.4 - 15 397 480 19055 - 12* 263 480 1262PTO 540 480 2592

10 3 - 15 350 1200 42005 - 12* 263 1200 3156PTO 320 1200 3840

12 3 - 15 350 2000 70005 - 12* 263 2000 5260PTO 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 showthe effect of angle of elevation, speed, diameter, exposure length, and grain type on capacity and powerrequirement. These are the most often quoted tables for screw conveyor characteristics.


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

Length ofAuger exposed helix Angle of elevationspeed 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 .1012 150 .12 120 .15 60 .11

200 18 150 .13 120 .17 70 .1224 150 .14 120 .18 80 .13

6 270 .23 180 .25 90 .1912 290 .29 220 .29 130 .24

400 18 290 .33 240 .32 150 .2624 300 .38 240 .36 160 .27

6 410 .33 280 .40 160 .2912 470 .43 350 .52 220 .41

700 18 480 .51 380 .64 250 .4724 480 .60 380 .76 270 .49

6 490 .41 320 .61 200 .4612 650 .63 460 .81 310 .67

1,180 18 740 .85 530 1.01 360 .7924 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 totalpower required for the conveyor.


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

Angle of elevation of screw

Auger Intakespeed exposure 0 22.5 45 67.5 90

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

6 210 .15 180 .21 150 .22 100 .17 80 .1712 215 .16 190 .22 160 .24 140 .23 110 .19

300 18 220 .21 190 .25 160 .26 150 .25 120 .2124 220 .21 190 .26 170 .27 150 .26 130 .24

6 330 .23 280 .31 230 .34 170 .32 130 .2712 340 .27 300 .39 260 .43 200 .40 160 .33

500 18 350 .35 310 .45 270 .47 230 .43 180 .3524 360 .38 310 .48 280 .49 250 .46 220 .41

6 420 .28 360 .41 290 .45 210 .41 170 .3712 450 .37 400 .54 350 .60 270 .57 210 .47700 18 470 .49 410 .63 380 .66 290 .60 240 .49

24 500 .53 435 .66 380 .71 330 .65 290 .58

6 465 .33 400 .49 330 .55 240 .51 200 .4512 520 .47 470 .67 410 .74 310 .71 250 .60

900 18 570 .62 520 .81 460 .85 350 .77 300 .6324 640 .69 550 .87 470 .93 400 .84 350 .73

6 490 .38 420 .56 340 .64 265 .60 220 .5512 600 .55 530 .78 460 .86 320 .82 280 .71

1,100 18 690 .77 610 1.00 530 1.03 390 .92 340 .8124 780 .84 650 1.06 540 1.14 450 1.01 400 .92

aHorsepower is that required at auger drive shaft. Horesepower loss in drive train must be added to determine thetotal horsepower requirement of the conveyor.

Table 12-16. Performance data for a 6-inch nominal diameter screw conveyor handling shelled corn (bushelweight -54 to 56 pounds); moisture content 14.5 percent wet basis (White et al., 1962).


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Auger Intakespeed exposure 0 22.5 45 67.5 90RPM inches bu/hr hp/10ft

a

bu/hr hp/10fta

bu/hr hp/10fta bu/hr hp/10fta

bu/hr hp/10fta

6 590 .20 520 .30 370 .33 280 .31 220 .2512 590 .28 550 .41 500 .44 400 .44 280 .32

200 18 620 .32 570 .43 510 .47 430 .45 310 .3624 630 .44 590 .50 550 .55 470 .54 350 .40

6 970 .35 850 .52 650 .60 480 .57 380 .4612 1090 .56 1010 .82 850 .88 690 .83 520 .70

400 18 1170 .74 1070 .92 940 1.02 720 .92 560 .8024 1190 .97 1110 1.13 1010 1.18 830 1.07 660 .92

6 1210 .49 1050 .72 820 .82 590 .77 490 .6412 1510 .84 1400 1.22 1160 1.28 910 1.16 740 1.05

600 18 1650 1.17 1500 1.42 1270 1.52 1010 1.42 800 1.2324 1700 1.47 1570 1.74 1440 1.80 1140 1.60 920 1.40

6 1320 .58 1100 .86 890 .95 640 .92 540 .7712 1760 1.07 1660 1.54 1370 1.62 1080 1.46 890 1.32

800 18 1990 1.57 1790 1.96 1510 2.08 1220 1.94 1000 1.6424 2140 1.95 1910 2.32 1740 2.39 1360 2.12 1100 1.89

aHorsepower is that required at auger drive shaft. Horsepower loss in drive train must be added to determine thetotal horsepower requirement of the conveyor.


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Table 12-17. Performance data for a 6-inch nominal diameter screw conveyer handling soybeans (bushelweight - 54 to 56 pounds); moisture content 11 to 12 percent wet basis (White et al., 1962).


Auger Intakespeed exposure 0 22.5 45 67.5 90

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

6 490 .30 410 .41 320 .41 240 .38 180 .3412 500 .40 430 .53 360 .57 290 .50 220 .40

200 18 520 .50 500 .60 440 .66 360 .60 240 .4524 540 .60 520 .67 470 .68 390 .64 290 .52

6 880 .52 710 .71 570 .77 400 .70 310 .6012 990 .84 830 1.14 690 1.20 540 1.04 390 .79

400 18 1110 .98 1030 1.18 880 1.29 740 1.23 460 .9524 1180 1.36 1040 1.62 900 1.63 800 1.54 560 1.14

6 1080 .68 890 .96 700 1.07 510 1.00 390 .87

12 1350 1.20 1130 1.61 930 1.71 710 1.48 500 1.10600 18 1620 1.45 1510 1.74 1280 1.94 1050 1.88 660 1.4724 1690 2.13 1520 2.52 1320 2.51 1100 2.32 790 1.76

6 1180 .78 960 1.12 740 1.28 550 1.22 420 1.1012 1610 1.51 1310 1.98 1080 2.10 820 1.84 640 1.50

800 18 1980 1.93 1840 2.29 1530 2.54 1230 2.44 810 1.9824 2020 2.93 1850 3.43 1640 3.48 1320 3.24 1000 2.56

aHosepower is that required at auger drive shaft. Horsepower loss in drive train must be added to determine thetotal horsepower requirement of the conveyor.

Estimating capacity and power of inclined screw conveyors

Most inclined screw conveyor designs are based on tests of power and capacity since these parameters are verydifficult to estimate. A rational computational procedure will be presented here. This procedure can be used forinitial estimation of power and capacity. Because of the complexity of the problem, it is much less reliable than

procedures for any of the other conveyor types.

Assumptions

The procedure assumes a steel standard-pitch single helicoid conveyor loaded from a hopper or grain mass by twodiameters of exposed helicoid.

Procedure

1. Compute conveyor size and speed using specifications from Table 12-9, along with Equation 12-25.

Estimate volumetric efficiency using Figure 12-25. This will require a trial and error procedure. If grainmoisture is over 17%, use half of the volumeteic efficiency predicted from Figure 12-25.

2. Compute hpf and hpm using Equations 12-26 and 12-27. If grain moisture is over 17%, double Fm.

3. Compute the lifting power, hpl, using Equation 12-29:


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(60)000)(33

(Fl)(h)(BD)Chpl = (12-29)

where hpl = power to lift material, hpC = capacity, ft3/hBD = bulk density as conveyed, lb/ft3

h = lift height = L sin , ftFl = approximate lift factor = 4 for moisture < or = 17%, or = 8 for moisture > 17%

4. Add power components:

hp = hpf + hpm + hpl (12-30)and apply drive efficiency factor.

Example 12-11.

Specify nominal size, speed, and power for a 30-ft-long screw conveyor to move 25% moisture corn at a 45-degreeincline at a rate of 500 bu/h.

1. Try a 6-in conveyor. Substituting into Equation 12-25:

0.0568N2200

(5.13)N)(1.4)((5.13)Ct

22

=

=

min

ft10.373

min)(60buh

h)ft(1.245bu)(500 33=

Assume volumetric efficiency at 0.5(0.62) = 0.31

0.0568N0.31

10.375=

N = 589.2 rev/min.

This is below the 650 rev/min maximum speed, so it is acceptable.

2. hp0.636000)(500

(18)(589.2)(30)hpf ==

hp0.672000)000(1

(0.8)(45)(30)(1.245)(500)hpm ==

3. hp2.40000)(33

(8)45)(sin(30)(45)(1.245)(500)

hpl ==

4. hp = (0.636 + 0.672) 1.85 + 2.40 = 5.54 hp

A 6-in. conveyor turning at 589 rev/min and a power input of 5.54 hp is required. Table 12-10 estimates a 6-8inconveyor turning at 389 rev/min and requiring 5.26 hp is required.

SAFETY CONSIDERATIONS


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As was noted in Section 12.1.1, safety is an important consideration in grain conveyor designs. Since there issome overlapping among conveyor types, they will be discussed in a separate section.

On-farm conveyors

Table 12-18 shows some statistics for farm machinery accidents. The statistics show that elevators are the mostdangerous machines on the farm, in terms of accidents per million hours of exposure. Also, not-fatal elevatoraccidents are more severe than accidents with other farm machines, in terms of days lost per accident.

The statistics do not differentiate among types of conveyors, so screw conveyors, flight conveyors, and possiblybucket conveyors are included. In the period from 1978 through 1982 farm screw conveyors outsold farm flightconveyors by over 8 to 1 (Implement and Tractor, 1983). It can then be assumed that most elevators are screwconveyors.

There are a number of ways to be injured by a portable screw conveyor on wheels (a transport auger):1. The intake region presents a pinch point between the tube and the turning helicoid and a rotating shaft

for possible entanglement.2. The auger tube can fall upon failure of the hydraulic or cable-actuated lift mechanism.

3. The entire machine can tip sideways.4. The tube assembly can contact overhead electrical lines during transport.5. Many screw conveyors are power take off (PTO) driven. Entanglement in the PTO shaft is, thus,

possible.

Table 12-18 Farm machinery accident statistics.

Accident frequency per million manhours use

Average days lost pernon-fatal accident

Michigan OhioTractor 8.4 7.4 58Corn picker 48.6 62.3 22Wagon 71.9 51.0 76

Baler 106.4 ---- 5Combine 112.0 90.1 209Elevator 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 ASAES361.1T (Safety for agricultural auger conveying equipment) is Appendix B. The purpose of the Standard is toestablish safety recommendations which will minimize the possibility of injury during normal operation of augerconveying equipment used to convey agricultural materials on farms. The standard specifies intake guarddimensions (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.


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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 feedingconveyors, 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).


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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 mechanicalconveyors will be discussed here.

The importance of grain breakage during conveying differs with circumstances. Gentleness to grain is not a veryvaluable characteristic for a conveyor carrying grain to a grinder. The breakage of any grain destined for livestockfeed (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 determinebreakage 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 grainvelocities after discharge from 8- and 12-in. orifices (Figure 12-29). Velocities of dry corn, wheat, and soybeansdiffered 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 thestream do not react with the air like individual kernels, the stream attains velocities higher than the terminalvelocity of individual kernels at about a 50-ft drop height. Kernel velocity exceeded zero at zero drop height due tomotion within the grain bin.


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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 forsoybeans. Wheat breakage was much lower and was not reported. The breakage relationships are shown in Figure12-30. Breakage is seen to be an exponential function of velocity.


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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 ismuch worse for corn than for soybeans, that breakage is higher for lower moisture, and for lower graintemperatures.

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- to19-% 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 forcedair up the tube against grain flow.


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

Breakage increaseFlow retarder per handling, % % of controlRetro-air 3.64 107

No retarder (control) 3.41 100

Spout retarder 3.22 94Cushion box 2.83 83Spout 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 grainand 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 reducedcorn breakage to some extent.

Two other findings from this study are worth noting. Drying treatment had a greater effect on breakage than didflow retarder action. Breakage for all tests averaged 5.87% per handling for corn dried in a batch dryer with 90 to100 C air, 2.66% per handling for corn dried in a bin with 50 to 60 C air, and 0.

agri process

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