Material handling and people movement are important considerations in food plant design and operation and should dictate the plant layout. This column will briefly discuss some elements of material handling, each of which could easily merit a longer piece or even a book. First, what do we mean by material handling?
Material handling refers to steps that condition, move, control, modify, and separate raw materials and finished goods. In food processing, many raw materials are solids, and finished products are often in consumer packages. Chemical, mechanical, and food engineers receive academic training in the handling of gas and liquids, but usually have little exposure to the handling of solids. As compared with liquids and gases, solids, whether they are powders, larger pieces, or discrete packages, often have variable properties and require special equipment to execute the functions of material handling.
Conditioning and Moving
Conditioning may refer to changing the particle size of a bulk solid, removing or adding a substance, such as water, or breaking up lumps to improve flow properties. One of the important flow properties of powders or bulk solids is the angle of repose, which is the angle from the horizontal plane that a pile of the material forms when poured in a heap from a predetermined height. It is easy to measure and is important in understanding why various solids behave as they do. Why should we care about how solids behave?
Many important food ingredients are delivered in bulk trucks, large bags, or rail cars. To be used, these materials must be emptied from their delivery container and moved to storage or a point of use. This emptying and movement may be by using gravity or some form of pneumatic conveying. Pneumatic conveying involves suspending particles in a moving stream of air. The air may be under pressure from a blower or under vacuum.
When air is compressed, it is heated and becomes saturated with water due to the energy of compression and because the ambient relative humidity is increased with pressure. This means that temperature or moisture-sensitive materials may be modified unintentionally by pneumatic compression. Often, the compressed air is cooled, dried, and filtered (to remove oil from the compressor or blower) before being used to transport sensitive solids. Pressure conveying is useful for delivering solids from one point to several use points, such as flour to mixers in a bakery. Vacuum conveying is useful for collecting solids from several points to a single storage vessel. One example is dust collection or a central vacuum system for cleaning.
Pneumatic conveying may be dilute or dense phase, referring to the amount of solids in a given volume of conveying air. Dense phase is gentler and is useful for fragile solids such as sugar for which reducing fines formation is desirable.
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In gravity flow from bins, bags, or hoppers, the angle of repose governs whether the material will flow or whether it may flood. Solids with a high angle of repose have a tendency to bridge over an opening or outlet in the bottom of a bin, while solids with a low angle of repose tend to flood, or flow uncontrollably.
The angle of repose and the flow properties are affected by moisture for many solids, so conditioning may involve drying the solids to standardize the flow properties. Likewise, if the solids pick up moisture, perhaps because conveying air is not dried, the flow properties may change through the process. Sugar might be delivered to a hopper, then refuse to flow out.
A sure indicator of solids flow issues in a food plant are dented bins and hoppers, which indicate that operators have pounded with hammers to promote flow.
Metering
Measuring solids flow is challenging. Often solids flow is measured by gain or loss in weight of a container. This is achieved by mounting the bin on load cells. The precision of such measurement is related to the size, and therefore the weight, of the bin. In addition, some means of controlling flow must be provided.
Typical solids flow controllers include star wheels or rotary locks; vibratory feeders; weigh belts; slide gates; screw feeders; and “Venetian blind” gates. Vibratory and Venetian blind gates rely on the fact that the angle of repose of solids changes when the solids are agitated, as by vibration. Thus, a solid that bridges over a hole may flow if the sides of the bin are shaken. (Hence the hammer.) Controlled vibration is provided by devices that rotate an eccentric weight in a device attached to the bin wall or by introducing a periodic puff of compressed air. The Venetian blind gate has slots that are intended to cause the solids to bridge until the device is shaken.
Weigh belts and screw feeders pull solids away from a slot or slide gate at the bottom of a bin. The weigh belt passes over a load cell, which measures the flow continuously and may, in turn, control some device to change the flow rate. Normally, just changing the belt speed does not provide good solids flow control, because solids do not always stop or slow their flow when the belt slows. This is especially true for very fine solids that have a tendency to flood.
Most solids metering devices are volumetric, so weight must be measured with load cells either on the delivering vessel or the receiving vessel. Layout is influenced by the choice of device, because some require a horizontal offset from the center of the supply vessel, while others deliver on the center line. The Venetian blind device requires that a heel of solids be maintained in the vessel; otherwise, the flow of solids refilling the vessel would flow right through the gate. For start-up, the gate often has a sliding closure.
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Other Conveyors
In addition to pneumatic conveyors, which are enclosed pipes, solids are handled with belt, screw, and vibratory conveyors. Belt conveyors that are literally miles long and carry tons of materials such as coal or other minerals have been built. Belts may be polymers, cloth, or metal. They may be flat or allowed to sag in the center so as to better contain loose solids. The angle of repose influences how heavily a belt may be loaded with a given solid.
Vibratory conveyors are usually metal pans with sides and may be oscillated on springs or moved in a horizontal plane; that has been compared to the parlor trick of jerking a tablecloth from under a place setting without moving the dishes. The pan moves forward on each cycle, then quickly back, leaving the solids in their new position. Vibratory conveyors are widely used for snack foods because they are easily cleaned and are surprisingly gentle on fragile products. Glass light bulbs have been conveyed by vibration.
Screw conveyors are typically enclosed and can be mounted at an angle so as to change elevation. Screw conveyors do not completely empty because there must be some clearance between the rotating screw and the enclosure. They are also very dangerous and should have proximity switches on their covers to prevent anyone from reaching in while the screw is moving. Screw conveyors are common in meat processing because they can transport ground meat and chunks. For food applications, they are usually made of stainless steel, and care must be taken to prevent metal-to-metal contact that might cause metal contamination of the food. A variation on the screw conveyor is Spiroflow’s flexible screw conveyor in which a helical, spring-like spiral rotates inside a polymer or rubber casing (www.spiroflowsystems.com, phone 704-291-9595).
Bulk Containers
Food materials including ingredients and work in process (WIP) can be handled in large containers such as totes, Gaylords, or big bags. Totes are rigid vessels, typically about 4 feet square and about 4 feet tall with a pyramidal bottom, made of stainless steel and having hatches on the top and bottom. They can be used for solids or liquids. If necessary, they are cleaned between uses in a special chamber. Totes are fairly expensive, but last a long time if maintained properly. They may be used as returnable packages for inter-plant transfer, for WIP, and as mixers, with special equipment that tumbles them. Plants using totes must also provide space for storing empty and full units and safe paths for their movement.
Gaylords are one type of less permanent bulk containers, usually made of corrugated paperboard. They are large cardboard boxes that may contain a large polymer bag and are emptied by dumping.
Big bags are woven polymer containers that may be reused after cleaning or sent to recyclers. Special equipment is required for moving, filling, and emptying the bags. Materials that are used in quantities too small for truck delivery may be provided in these large bags. Cohesive materials, meaning those whose powders stick to one another, may clump and solidify in the bag, requiring agitation so that they can be discharged. Flexicon, which provides a full line of large-bag filling and discharge equipment, has developed a sanitary bag conditioner to solve this problem (www.flexicon.com, phone 888-353-9426).
J. Peter Clark,
Contributing Editor, Consultant to the Process Industries, Oak Park, Ill.
[email protected]