What is a Screw Conveyor (Auger Conveyor)?

What is a Screw Conveyor (Screw Auger) and how does it work?

 One of the most frequently asked questions is What is a screw conveyor? and Where is a screw conveyor used? The screw conveyor is one of the most reliable and cost-effective ways to material bulk handling. It is a very versatile machine that can handle a wide variety of materials, from dry, free-flowing materials like Portland cement to wet and slow materials like dehydrated biosolids.

   The purpose of this article is to help the reader understand the basics of screw conveyor design and the proper selection of the custom screw conveyor for your applications. A screw conveyor is a mechanism that uses a rotating auger flight, called a "Helix Flight", usually within a tube or channel in the shape of a "U", to handle liquid, powder and granular materials. They are used in many bulk handling industries. Screw conveyors in modern industry are often used horizontally or at a slight incline as an efficient way to move semi-solid materials, including food waste, wood chips, aggregates, cereal grains, animal feed, boiler ash, meat and bone meal, municipal solid waste, and many others. The first type of screw conveyor was the Archimedean screw, used since ancient times to pump irrigation water.

  They generally consist of a trough or tube containing a screw conveyor wound around a shaft, driven at one end and held at the other, or a shaftless screw, driven at one end and free at the other. The speed of volume transfer is proportional to the speed of rotation of the shaft. In industrial control applications, the device is often used as a variable speed feeder by varying the rotational speed of the shaft to deliver a measured speed or amount of material in a process.

 Screw conveyors can be operated with upward sloping material flow. When space permits, this is a very economical method of lifting and transporting. As the tilt angle increases, the capacity of a given unit decreases rapidly. The rotating part of the conveyor is sometimes called simply a screw complete.

Screw Complete

Screw Conveyor Inclined

Screw conveyor in operation

Screw Conveyor History

Archimedean Screw

  Archimedes designed the first screw conveyor in the 3rd century BC. C. It was used to remove water from boats and to irrigate farmland. The device consisted of a hollow cylinder with a central axis and a screw attached to the inner wall of the cylinder and the central axis. As the assembly rotated, the water was transported and rose from one place to another. The design of the screw is based on the theory of the inclined plane.

   The screw conveyor began to evolve in the late 1800s and was used as a means of increasing food and grain production to meet the needs of the rapidly growing American population. The first feed mills used screw conveyors throughout the process.

 Even today's most modern feed mills rely on screw conveyors for many of their material handling requirements. The screw conveyor has evolved up to modern times. Now it is used in almost all large industries.

   Today, thousands of processing plants in the United States and the world are using screw conveyors for bulk material handling from A to Z (adipic acid to zinc concentrate).

Where are Screw Conveyors used for?

Screw conveyors are used to handle any type of bulk material and are found in thousands of applications. The main industries that use screw conveyors are:

  • Agricultural production

  • Chemicals Food processing

  • Wood and wood products

  • Mining and Minerals

  • Processing Oil and gas extraction

  • Oil refining

  • Primary metals

  • Pulp and paper Rubber and Plastics Stone, glass and concrete

  • Environmental

Wastewater Treatment Industry

  These top industries are defined by the US Department of Labor, and each company that produces a product or service is categorized by industry. The more specific industries are within these core industries where you will find thousands of applications for screw conveyors. For example, the production of Portland cement is part of the stone, glass and concrete industry. Limestone production belongs to the same industry. Each major industry may have up to 20 more specific industries that use auger conveyors in their processes. Another example is the meat and poultry processing industries, part of the food processing industry. The screw conveyor is used in all meat and poultry processing industries to transport by-products or recycled products. The versatility and cost effectiveness of the screw conveyor make it a perfect choice for handling a wide variety of bulk materials.

Definition and Classification of Bulk Materials

Bulk materials are defined as goods that are handled in large quantities without the benefit of individual packaging. Bulk materials are transported, stored, and processed to create the things we need to live, such as food products, plastics, construction products, paper, and thousands of finished products. The bulk materials handling industry is an engineering field focused on the design of equipment used to transport bulk materials. The Conveyor Equipment Manufacturers Association (CEMA), comprised of manufacturers of material handling equipment, creates industry standards for the design and safe operation of conveyor equipment.

Bulk Materials

CEMA and leading screw conveyor manufacturers have studied thousands of different bulk materials to determine their characteristics. CEMA classifies bulk materials by:

  • Particle size

  • Fluency Density

  • Abrasiveness

  • Other characteristics (corrosive, flammable, sticky)

   CEMA provides material classification codes for most bulk materials handled. This information is available to anyone who designs, specifies, manufactures, or uses screw conveyors and other bulk material handling equipment. Proper helical screw conveyor design depends on knowing and understanding the bulk material being conveyed and the application.

   Bulk materials are classified by particle size, flowability, and other physical properties. The Conveyor Equipment Manufacturers Association (CEMA) provides material classification codes for the most commonly handled bulk materials. This information is available in the CEMA book No. 350.

   For example, CEMA book No. 350 indicates that the material code for Portland cement is 94A10026M. The CEMA Material Classification Code Table describes material weighing 94 lbs. per cubic foot, with a very fine particle size, free flowing, moderately abrasive and can be aerated or flowable when transported. The CEMA Material Classification Code Table should be used as a general guide for the design of screw conveyors.

   Detailed information on bulk materials is available from CEMA or one of the major screw conveyor manufacturers. Most of the major screw conveyor manufacturers have in-house experts to help design screw conveyors or other bulk material handling equipment. These in-house experts can provide knowledge on many different applications and help with the right solution.

Basic Theory for Sizing a Screw Conveyor

   Screw conveyors are volumetric conveying devices. With each revolution of the screw flight, a fixed volume of material is discharged. The purpose of a screw conveyor is to transfer product from one point to the next. Screw conveyors are always controlled by being fed into the entrance by another conveyor or measuring device. Rotary valves, screw feeders or feeders, conveyors, or even other screw conveyors are typically connected to the inlet of a screw conveyor.

   Screw feeders or feeders are similar to screw conveyors, except screw feeders are always flood loaded or 100 percent full in the entry area. Screw feeders are designed to volumetrically measure material from a hopper, container, or silo at a controlled rate. Many screw feeders or feeders use adjustable speed drives to allow the material flow rate to be varied.

 Design of a Screw Conveyor

Screw Conveyor for Coconut Handling

     The flow rate or capacity of a screw conveyor is measured in cubic feet per hour. If the capacity is given in pounds per hour, tons per hour, or bushels per hour, it is converted to cubic feet per hour. Since screw conveyors are controlled feed at the entrance, the cross channel load is less than 100 percent. CEMA has developed standards for feeder loading based on material classification codes.

   Using the portland cement example above, it is described as free-flowing and moderately abrasive. CEMA recommends a minimum load of no more than 30 percent. CEMA also recommends reducing the screw conveyor speed when conveying soft to extremely abrasive materials. Reducing the channel load and speed will reduce wear on the screw conveyor.

   This information is readily available in CEMA book No. 350, from Bega Screw Conveyor or another CEMA approved vendor. The through load percentage is based on the material being handled and whether internal suspension bearings are used. Suspension bearings are located inside the screw conveyor and are used to support the screw complete.

   There are certain parameters for sizing a screw conveyor. The capacity calculation takes into account the outside diameter of the screw, the outside diameter of the pipe or central axis, the screw pitch and the load contained in the trough. The calculation determines the capacity in cubic feet per hour that will be handled with each revolution per minute of screw rotation. It is not necessary to memorize this calculation. Most CEMA approved screw conveyor manufacturers have the CEMA capacity calculation and guidelines as part of their screw conveyor design software.

   Choosing the right screw conveyor for your application requires a basic knowledge of the material being handled as discussed above, as well as some basic information such as conveyor length, degree of incline, and product temperature.

Recent Development and Evolution of the Screw Conveyor

Screw Conveyor Shaftless

   As building materials such as abrasion resistant alloys and stainless steels have developed over the years, the use of screw conveyors has increased to many more industries, as discussed above. Recent improvements in production technology include the automation of specific manufacturing processes that improve quality and reduce costs. The use of automated or robotic welding in the screw manufacturing process is a good example.

   One of the most recent developments is the "shaftless" screw conveyor which uses a screw without the center tube. The screw or spiral is constructed of high-strength steel and is rigid enough to handle the torque. The spiral is usually found at the bottom of the channel in a liner made of UHMW plastic or metal. The drive shaft is directly connected to the helical without shaft.

  The shaftless screw conveyor was originally developed to convey wet and sticky materials. When handling bulk material with a shaft screw conveyor, the wet and sticky materials offer to adhere to the pipe where the span joins. Since the center tube is removed on a screw conveyor, there is no place for the material to stick. The image to the side shows a typical shaftless screw conveyor.

Differences between screw conveyor manufacturers

   Each screw conveyor manufacturer is unique and offers different products and services. Some manufacturers specialize in providing standard CEMA components, while others focus on custom designs. It is important to choose the right screw conveyor manufacturer based on your specific needs and customers in your area.

   For example, if the material to be handled is very abrasive, you will need a manufacturer that specializes in designing and building heavy duty screw conveyors. The material thickness for screw runs and channels can be as thick as one inch.

     The main screw conveyor manufacturers are active members of CEMA and can be found at https://cemanet.org/. You can contact CEMA for information if you are unfamiliar with a particular manufacturer. Also, check the individual company website or contact the company directly to discuss capabilities and quality programs.

At Bega Screw Conveyor we add value for the customer

   Screw conveyors are an important part of our customers' operations. If a screw conveyor continually fails or requires excessive maintenance, then the customer is not efficiently producing the product. Having a basic understanding of screw conveyor design along with understanding the application helps us solve the customer's problem.

For example, a customer handles bulk petroleum coke and replaces helical screws every three months. The CEMA material classification code for petroleum coke is 30D737N. The code indicates that petroleum coke weighs 30 pounds. per cubic foot, it has a large bulge size, average flow, is extremely abrasive, and can be explosive. Helical screws wear prematurely because petroleum coke is extremely abrasive. As an added value, we can offer screw conveyors constructed of abrasion resistant materials or the helical screw sections can be coated with a resistant coating material for welding.

The image to the right shows a hardfaced screw conveyor welded onto the screw and internal surfaces of the housing.

Spare parts of screw conveyor for handling abrasive powders

   Screw conveyors are often a cost-effective way to transport bulk materials, but they can be very expensive if they shut down your operations. The cost of not producing the product far exceeds the cost of a screw conveyor. This is why it is important to have screw conveyors and other bulk material handling equipment designed to meet your needs and applications.

   Many common mistakes are made when bulk material handling that can be solved with application experience. For example, you may be processing and handling an abrasive product such as dry biosolids and replacing screws every three months. The cost of ongoing maintenance and downtime far outweighs the cost of replacing screw conveyor components. As an added value, replacement screws can be made of abrasion resistant materials or guides and the screw channel can be lined with ceramic tiles (see photo on page 18). New replacement components will extend the life of the screw conveyor by up to 10 years.

   Another example could occur in a chemical processing plant. Many chemicals give off dangerous or toxic fumes when processed and handled. Containing toxic fumes is important for the health and safety of plant personnel because the fumes could ignite and cause an explosion or create health hazards. Screw conveyors can be designed to be completely steam tight and also to handle internal pressure. The screw would be completely encased in a tubular or pipe casing and special shaft seals would prevent vapors from escaping. The fully welded construction is also important to prevent leaks. Failure is not an option when handling hazardous bulk materials. The added value of the proper design and construction of screw conveyors for hazardous applications can be estimated in millions of dollars.

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