Conveyors in flour mills
November 01, 1997
by Teresa Acklin
Selecting conveyors is an important part of mill management. Millers look for blending, mixing or homogenizing at various process stages for consistency, and not all conveying systems have the ability to blend. Hygiene and product purity are important to customers, and conveying efficiency and safety are important to millers.
Six main conveying methods are most common in the industry, and the capabilities and limitations of each vary.Worm/Screw Conveyors
The worm/screw conveyor, in its many forms, is perhaps the most widely used of the horizontal type of conveyor. Nearly all of this type run at 100 revolutions per minute, give or take 20 rpm.
Screw conveyors can be inclined up to 20° from horizontal, although the incline results in a capacity drop. If a greater incline is required, special adaptations can be used, such as larger horsepower or kilowatt motors, plus bolted down lids and higher speed. Before discussing the several common varieties of worm, it is well worth mentioning the auger some call it the tube screw. This is a continuous screw totally enclosed to the round shape of the rotating part, with little clearance between the moving blade and with a close-fitting static circular case.
In this mode, the auger can convey not only horizontally but also at any incline, including vertical, although vertical inclines tend to create a fair amount of comparative wear. The auger is suitable for materials, such as flour, that will not be damaged, in contrast to wheat, millfeed pellets, semolina and the like, which will be particularly harmed under normal circumstances because of attrition. Augers often run at up to 200 rpm.
Figure 1 depicts a horizontal continuously bladed screw. This is used for conveying rather than mixing and is usually surrounded by a “U” trough, as are the majority of all horizontal screw designs. Its capacity is higher than any other level worms as there is little means for stock delay. It is used for most mill stocks and products.
The ribbon blade design, also seen in Figure 1, is for gentle mixing as well as conveying. For example, some millers introduce additives, such as ascorbic acid, fungal enzyme and gluten, to flour when conveying to ensure homogeneity. Another variant shown in Figure 1 is the crescent blade in place of the continuous screw. Used widely on freshly damped wheat, it has a rolling mix action to ensure complete water coverage, as well as moving grain to its destination.
Two crescent blades bolted at short intervals through the shaft length on opposite sides cover the full 360°, 180° each. The crescent blade angle setting can be up to 45° to the direction of flow for maximum capacity. It can be adjusted to a greater angle to directional flow, say 60°, which increases the holding time and decreases capacity.
An additional common variant is the pictured paddle blade, which is the most commonly used for wheat damping. Paddle blades perform similarly to the crescent, yet provide even more action because of their placement: four blades, each at 90°, rather than two at 180°.
The paddle variety allows greater possibilities than the crescent blade. The odd blade can be angled to occasionally slow the direction of flow, forcing more rather than less mixing. The paddle is frequently used for inclined wheat damping systems.
Tapered continuous blade screws are used either singly, in quadruplicate or side by side as bin discharge horizontal conveyers. The largest diameter is near the outlet, the smallest at the opposite end. This purpose of this design is to begin to move material, such as flour or bran, on plug feed, gradually increasing in movement as it continues towards discharge. It also allows lumpy material to be accepted and broken down.
Although not often found, the double flight continuous worm has the beneficial characteristic of counteracting materials that are prone to fluidizing or flushing. This trait is advantageous on inclines.
Drives for all worm conveyors today are either direct from gear boxes or through “V” belts. Safety is always a problem if the machines are not properly limit-switch protected and guarded. More accidents have emanated from screws than perhaps any other equipment.
Hygiene and cross-contamination can be difficult because worms are not completely self emptying, although augers and continuous screws may be “almost” self-emptying. All screw conveyors except the tapered variety are capable of running in the opposite direction if necessary, which is an often used capability. Quick release casings help in case of trouble. Another characteristic is the ability to act as a dust explosion break because the design enables the central part of the worm to be always full to the brim of material.
Conveyor capacities can vary from a few grams to around 200 tonnes per hour in practice. Diameters range from the very small, say 5 millimeters, up to 600 mm.
Lengths on the larger diameters can be 30 meters or more. However, the problem with length is one of torque much power being put through a single rotating shaft. So long, high capacity screws are limited to this extent.
Screw conveyors are relatively efficient users of power. Maintenance is neither difficult nor time consuming given proper procedures and no foreign material mishaps.Chain Conveyors
Figure 2 shows an example of a chain conveyor, with arrows showing the direction of material fed into the system. The beauty of this type is the ability to convey large capacities of more than 1,000 tonnes per hour, as well as small capacities. Long distances are also not the problem they are with screw conveyors, and power consumption is proportionately low.
Chains are used commonly for wheat. They convey “en masse,” meaning in a continuous, steady non-attrition state. This makes the system singularly suitable for friable or brittle material, such as millfeed pellets. The malt and brewing industries use chain conveyors widely when moving brittle and dry barley malt, which has 4% moisture, horizontally. The method is also used to convey vertically, up inclines and in a “Z” configuration so long as it is a straight line from the drive to driven sprocket.
The machine is totally enclosed and easy to exhaust. Inlets can be at any position along the top, as can outlets along the base in certain configurations. When used as elevators, outlets can be designed at any level.
Because automation of inlets and outlets is straightforward, the chain lends itself to fail-safe underspeed devices. Most machines also are reversible, especially when inclined or horizontal. There are bar type chains set at 90° to the direction of flow. Other designs use crescent or half moon blades, but the principle remains the same.
Some applications demand a solid plate separating the outward from the return continuous chain certainly true for a vertical elevator because of friction. Chain conveyors are inherently safer from dust explosion than most other methods of transport because of their gentleness. Maintenance needed is infrequent. But should a chain break for whatever reason, repair tends to be a long and arduous process.Bucket Elevators
Another classical and time tried vertical transporter is shown in Figure 3. Always driven through the top drum due to friction, grip and weight, the bucket elevator operates continuously.
Preferably, material is fed to the upside as pictured to reduce power and complications. However, feed, such as bran or sticky stocks, can be directed to the opposite side. Capacities of 1,000 tonnes per hour or more are common, right down to a few kilograms per hour. Common belt speeds range from 45 to more than 182 meters per minute.
Bolted on to a continuous band made of variously rubberized cotton or chrome leather belting, buckets of many capacities and designs are used. Maintenance is always slow and awkward should there be a breakage, when significant jamming occurs on high capacity machines.
These machines are more prone to potential dust explosions because of their internal operation. Accordingly, an explosion vent should be placed every 6 cubic meters of elevator leg with 1 square meter of light relief panel, designed to open automatically under only slight pressure.
An additional problem is the possibility of metal sparks and elevator belts that slow down before a choke. So modern bucket elevators are fitted with safety features, including belt alignment proximity devices, underspeed rotation switches and run-back prevention devices. Inert gas injection at strategic points triggered by slight pressure sensors also is helpful all of course designed to shut down the moving plant instantly.
Elevators are driven by a variety of methods, from motors directly on to gear boxes, “V” belts and torque arm reducers. Bucket elevators no longer are widely used for mill stocks because of hygiene and maintenance demands.
Bucket elevator exhaust is necessary because of dust cloud formation. The addition of exhaust air on both legs at reasonable intervals is a must at the head (discharge) and boot (inlet) points. Automatic tensioning is common, usually by weights placed on the bottom drum bearings.Belt Conveyors
Figure 4 on page 28 depicts a belt conveyor. This type remains popular, especially operated at high capacity in grain elevators, because of its low maintenance and power consumption.
Belt conveyors are offered with either flat or concaved or curved belts. The former is of a low comparative capacity. The curved belt will run happily over very long distances, more than 100 meters at a thousand tonnes per hour or more. They don't like inclines, not usually more than 15° for wheat.
Belt systems in storage elevators are not as hygienic as chain conveyors from a dust emission standpoint, although they can be totally enclosed, at a cost. The drive drum is usually at the discharge end, whereas the driven one has a manual or automatic tension device. Designs can lead to several feed points and sometimes more than one discharge point working simultaneously.
Belt conveyors only convey, they don't mix or blend grain in what might be called the strict sense or meaning. Belt speeds of 120 to 240 meters per minute are commonplace. They are now made from a variety of man made substances as well as the classic cotton rubberized webbing.
Remote stopping of the belt (or band) in emergency presents a problem in long elevator galleries. This is overcome by having a continuous small bore light- weight cable running the length of the machine. One physical pull stops the motion.Blowline Positive Pressure Pneumatics
Used worldwide in flour mills, this type of conveyor Figure 5 tends to be reserved for wheat or finished products. The advantages include hygiene because no stock is left within the line. These conveyors are occasionally used on mill stocks in large plants, but this is relatively rare.
Capacities handled range up to 50 tonnes per hour or more over distances up to a few hundred meters. Blow lines work with pressures up to 15 psi.
Diameters range from 50 mm to 150 mm and the equipment can be “threaded,” or worked in between machinery and obstructions, which cannot be done with bucket elevators and horizontal conveyors.
Powered by Rootes type compressors, feed is introduced to the airflow via a close tolerance rotary seal. Discharge takes place, as seen in Figure 5, at a dust collector with another rotary seal at the base. The outlet can be just as easily coupled in to a properly exhausted bin.
Blow lines lend themselves to automation easily. It is possible to measure power consumed and pressure. Moreover a number of diverter valves can be added to direct flour to a number of different locations.
Power consumption is higher pro rata than bucket elevators, a criticism often heard. Maintenance is simple but there is a problem of wear, especially from wheat, on blow line bends, although wear depends on capacity.
The flexibility and cleanliness of this type of conveyor generally outweigh mechanical means, except for high capacity work, such as at a terminal elevator. Loadings range from 0.22 kilograms per 0.03 cubic meters of air to stock to 0.45 kg.Suction/Negative Pressure Pneumatics
Figure 6 depicts a system using negative pressure. Negative pressure is effective when one fan can move 50 or 100 different low capacity stocks at the same time.
The figure shows a section through a mill with three floors, a, b and c. Arrows point the direction of flow of a mixture of air and stock (e.g. break stock semolina, durst). Pick up is from rollermills (1), and separation of stock from air is carried out with cyclones and seals (2) discharging at point (3). Air is drawn into common trunking (4) and moved by a fan and dust collector attached (not shown).
Heavy loadings of 0.12 cubic meters of air per 0.45 kg of stock will operate at a negative pressure of between 25 and 60 inches water gauge, light loadings of 0.18 cubic meters of air per 0.45 kg of stock will operate at pressures between 20 and 50 inches.
All manner of sophisticated additions can be included. Hygiene, a dust free atmosphere, maintenance and machine exhaust are serious and marked advantages compared with bucket elevators, whereas power is a penalty.
Automatic diaphragm-operated air balancing valves have the capability of reducing power consumed. The more air a fan moves, the more power is used. Start-up of such large fans draws a considerable amount of initial current if the main single air valve is not shut. So this valve is closed on start-up, opening only when full speed is reached.
David Sugden, independent consultant to the grain industries, may be reached at The Coach House, Killigrews, Margaretting, Ingateston, Essex CM4 OEZ, U.K. Tel: 44-1245-352048. Fax: 44-1245-251162.