Conveyors and elevators are used to transport solids. An industrial plant typically contains more than one type of conveyor or elevator.
Mechanical Conveyors
Belt
Belt conveyors are the most popular type of mechanical conveyor. Mechanical conveyors use moving parts to transport material.
General Information
Belt conveyors can be used for almost any material. They come in many sizes and can be inclined. Typically they are connected in series to form a long belt conveying system.
Equipment Design
The main component of a belt conveyor is a continuous belt. It can be made of polyester coated with rubber, plain or coated canvas, woven wire, or a steel ribbon. Belt conveyors are usually flat but in bulk solids handling they are typically troughed to prevent material from falling. Belt conveyors are driven at one end by a motor, not shown, that rotates the drum to move the belt. At the other end of the conveyor, there is an idle drum.
Usage Examples
Belt conveyors are typically used for transporting, proportioning, feeding, discharging, and metering bulk solid materials. The picture below shows a series of belt conveyors that are used in an asphalt plant.
Advantages
- Can be developed to convey almost any type of solid material.
- Easy to adapt to plant layout.
Disadvantages
- Inclination angle must be less than 22°.
- Cannot transport anything at temperatures greater than 120 °C.
- Small scale experiments are needed to determine operating parameters.
Drag Chain
General Information/Equipment Design
Drag chain conveyors are a type of mechanical conveyor that can transport solids horizontally, vertically, or on an incline. Drag chain conveyors use a hopper to feed the material onto the ledges of a single or double chain. After the material is loaded onto the chain, it is conveyed through a tube until it reaches the discharge point. It is possible to have many inlet and outlet points along the chain.
Usage Examples
Drag chain conveyors are used to convey pieces of particle board in a wood processing facility. They are also used to convey wet and dry solids in the chemical and food industry.
Advantages
- Flexible in loading and unloading
- Self-loading
- Many possible discharge and loading points
Disadvantages
- Product build-up on walls
- Large amount of solid is needed for effective transport
Screw
General Information/Equipment Design
Screw conveyors are the simplest and cheapest for the transfer of materials at rates of up to 40 tons/hr over distances of up to 65 ft. Screw conveyors are flexible; depending on their diameter, they can be curved to some extent. Screw conveyors consist of a special heat-treated and tempered carbon or stainless steel spiral that rotates with ultra-high-molecular-weight polyethylene (UHMWPE) food-grade tube. The transport of material in a screw conveyor occurs by the turning of a helix screw in a trough casing. The particles should be smaller than the screw pitch. The screws can have a constant, variable, or tapered pitch. In tapered pitch, the distance between the grooves gradually decreases down the length of the conveyor.
Screws with round cross-sections are flexible and strong, and screws with flat cross-sections are more suited for high-velocity applications. The trough casing can be round, rectangular, or U-shaped. The picture below to the left shows a single helical screw conveyor, and the picture below to the right shows a dual auger design.
(Copyright PRAB, Kalamazoo, MI)
Usage Examples
Screw conveyors are used in pharmaceutical, food, and dairy production applications. The screw conveyor below to the left is used in an asphalt plant, and the one to the right is used as a feed for a metal turnings shredder.
Advantages
- A closed tube prevents product contamination
- Inexpensive to purchase and operate
- Quick installation
- Flexible conveyors can be routed around obstacles
- Product handled gently
- Product is usually very accurate and highly repeatable batches are possible
- The conveyor can be easily emptied at the end of a batch operation
Disadvantages
- Particles must be smaller than the pitch
- Abrasive materials will most likely cause wear
- Small scale experiments needed to determine operating parameters
- Need to run full of products
Vibratory
General Information/Equipment Design
Vibratory conveyors have a single trough design. This single trough vibrates on springs to produce an upward, forward flow of the material. This type of flow creates a forward hop. The capacity of a vibratory conveyor is determined by the cross-sectional area and slope of the trough. A vibratory conveyor should be used when the material has a high friction factor on steel.
(Copyright PRAB, Kalamazoo, MI)
Usage Examples
Vibratory conveyors are used in a variety of industries. In the chemical industry, they are used to transport fertilizer, detergent powders, and plastic pellets or powders. In the food industry, they are used to convey corn kernels, instant coffee, and cereal flakes. The video below shows a vibratory conveyor used to handle scrap aluminum.
Bucket Elevators
General Information/Equipment Design
Bucket elevators are usually used when there is not enough space for a horizontal conveyor. Bucket elevators consist of buckets mounted on a single or double chain. The buckets are loaded at the bottom of the apparatus and dumped through a chute at a higher level. From the chute, the materials are usually loaded on a conveyor or stored in a silo.
Advantages
- Can operate at speeds of about 1.5 m/s.
- Can handle large capacities of material in a short time.
Disadvantages
- Buckets undergo wear and tear.
- No universally accepted design.
Non-Mechanical Conveyors
Pneumatic
Pneumatic conveying uses gas to convey solid material.
General Information
Belt conveyors can be used for almost any material. They come in many sizes and can be inclined. Typically they are connected in series to form a long belt conveying system.
Equipment Design
The two types of pneumatic conveyors are the dense phase and dilute phase. In dense phase pneumatic conveying the particle concentration is very high, as shown in the left picture. Dilute phase pneumatic conveying, shown on the right, has a small solid concentration
(Copyright MAC Equipment, Inc., Kansas City, MO)
In dense phase pneumatic conveying, the material is first placed in a hopper. The hopper feeds the material into a dense phase transmitter, which mixes the feed with a gas, usually air. The material travels by plug flow to its destination.
In the dilute phase of conveying, the material is fed into an airlock feeder. The solid material is suspended in the air at a ratio of about 3 to 5 kilograms of feed to 1 kilogram of air. The material is then conveyed as a suspension along the pipeline. Throughout the entire process, air penetrates through the feed stream. If chemical reactions with air are a concern, other gases can be used.
Usage Examples
The pictures below exemplify a pneumatic conveying system used to transport metal chips for processing. The upper picture shows the inlet receiving hopper and pick-up pneumatic piping. The lower picture shows the overhead pneumatic transfer system from the pickup lines to the main pneumatic line.
Advantages
- Can operate at speeds of about 1.5 m/s.
- Can handle large capacities of material in a short time.
Disadvantages
- Buckets undergo wear and tear.
- No universally accepted design.
Acknowledgements
- Astec, Inc., Chattanooga, TN
- Hapman Conveyors, Kalamazoo, MI
- Kaeser Compressors, Inc., Fredericksburg, VA
- MAC Equipment, Inc., Kansas City, MO; now part of Schenk Process
- PRAB, Kalamazoo, MI
References
- Degirmencioglu, A. and A.K Srivastava. “Development of Screw Conveyor Performance Models Using Dimensional Analysis.” Transactions of the ASAE Vol. 39 September-October 1996: 1757-1763.
- Gager M. and S. Tappeiner. “Additional Strain in Conveyor Belts Caused by Coarse and Transition Geometry.” Bulk Solids Handling. Vol.13 November 1993: 695-703.
- Kimbel, Kirk W. “Troublefree Pneumatic Conveying.” Chemical Engineering April 1998: 78-83.
- Kolhe, P.P. and D.J. Tidke. “Belt Design and Load Capacity Estimation for Air Supported Belt Conveyors.” Bulk Solids Handling. Vol. 14 Oct-Dec 1994: 755-758.
- Mainwaring, Nigel J. “Characterization of materials for pneumatic conveying.” American Ceramic Society Bulletin. Vol.72 August 1993: 63-71.
- Perry, Robert H. and Don W. Green. Perry’s Chemical Engineers’ Handbook. 7th ed. New York: McGraw-Hill, 1997: 21-14, 21-6 – 21-10, 21-13 – 21-14, 21-19 – 21-21.
- Podevyn, Michel. “Selecting a Conveyor” Chemical Engineering February 20089 27-29.
- Roberts, A.W. “Advances in the Design of Mechanical Conveyors.” Bulk Solids Handling Vol.14 April-June 1994: 255-281.
- Shamlou, P. A., Handling of Bulk Solids: Theory and Practice. Boston: Butterworth & Co. Ltd., 1988.
- Stewart, Gordon. Modern Steam Traps: Their Construction and Working. New York: D. Van Nostrand Co., 1907.
Developers
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