Screw Conveyor Systems
Design, application and selection guidelines.
The screw conveyor is one of the most reliable and cost-effective ways for conveying bulk materials. It is a very versatile machine that can handle a wide variety of materials from dry, free-flowing materials such as portland cement to wet, sluggish materials such as dewatered biosolids. The purpose of this article is to help the reader understand the basics of screw conveyor design and proper selection of the right screw conveyor for their applications.
History of the screw conveyor
Archimedes designed the first screw conveyor in the third century B.C. It was used for removing water from ships and for irrigating farmland. The device consisted of a hollow cylinder with a center shaft and a spiral fixed to the inner wall of the cylinder and center shaft. As the assembly rotated, water was conveyed and lifted from one location to another. The spiral design 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 feed and grain production to serve the needs of the rapidly growing American population. The first feed mills utilized screw conveyors throughout the process. Even the most modern feed mills today depend on screw conveyors for many of their material handling requirements. The screw conveyor has evolved to modern times. It is now used in almost every major industry. Today, thousands of processing plants throughout the United States and the world are using screw conveyors to convey bulk materials from A to Z (Adipic Acid to Zinc Concentrate).
Definition of Bulk Materials
Bulk materials are classified by particle size, flowability and other physical properties. The Conveyor Equipment Manufacturers Association (CEMA) provides material classification codes for most commonly conveyed bulk materials. This information is available in CEMA Book No. 350.
For example, the CEMA Book No. 350 indicates that the material code for portland cement is 94A10026M. The CEMA Material Classification Code Chart describes the material as weighing 94 lbs. per cubic foot, with a very fine particle size, that is free flowing, moderately abrasive and can become aerated or fluid when conveyed. The CEMA Material Classification Code Chart is to be used as a general guideline for screw conveyor design.
Basic Theory and Design
Screw conveyors are volumetric conveying devices. With each revolution of the screw, 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 control fed at the inlet by another conveyor or metering device. Rotary valves, screw feeders, belt conveyors, grinders, or even other screw conveyors typically are connected to the inlet of a screw conveyor. Screw feeders are similar to screw conveyors except that screw feeders are always flood loaded or 100-percent full in the inlet area. Screw feeders are designed to volumetrically meter material from a hopper, bin or silo at a controlled rate. Many screw feeders utilize adjustable speed drives to allow for varying the material flowrate.
The flowrate or capacity of a screw conveyor is measured in cubic feet per hour. If the capacity is given in lbs. per hour, tons per hour or bushels per hour, it is converted to cubic feet per hour. Since screw conveyors are control fed at the inlet, the crosssectional trough loading is less than 100-percent. CEMA has developed standards for trough loading based on the material
classification codes. Using the previous example of portland cement, it is described as free flowing and moderately abrasive. CEMA recommends a trough loading of no more than 30-percent. CEMA also recommends reducing the speed of the screw conveyor when conveying mildly to extremely abrasive materials. Reducing the trough loading and speed will reduce the wear on the screw conveyor. This information is readily available in the CEMA No. 350 Book, from KWS Manufacturing Company or another CEMA approved supplier. The percentage of trough loading is based on the material being conveyed and if internal hanger bearings are used. Hanger bearings are located inside the conveyor and are used to support the screw.
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, the pitch of the screw and the trough loading. The calculation determines the capacity in cubic feet per hour that will be conveyed with each revolution per minute of screw rotation. It is not necessary to memorize this calculation. Most CEMA approved screw conveyor manufacturers have the capacity calculation and the CEMA guidelines as part of their screw conveyor design software.
Choosing the right screw conveyor for your application requires basic knowledge of the material being conveyed as discussed previously, as well as, some basic information such as the conveyor length, degree of incline and product temperature.
Applications and Industries
Screw conveyors are used in thousands of applications throughout most industries. The major industries that utilize screw conveyors are:
Within each of these major industries are more specific applications for screw conveyors. For example, the production of portland cement is classified as part of the stone, glass and concrete industry. The production of limestone is also found under the same industry. Each major industry can have as many as 20 more specific industries that use screw conveyors in their processes. Another example is the use of screw conveyors in the meat and poultry processing industries. These two industries are part of the major industry of food processing. Screw conveyors are used throughout the meat and poultry processing industries for conveying the byproducts or rendered products.
The production of gypsum wallboard or sheetrock requires the use of many screw conveyors. The gypsum wallboard industry is part of the stone, glass and concrete industry. The image below is a typical screw conveyor used for conveying stucco or gypsum. Construction features include heavy-duty carbon steel construction, dust-tight covers, adjustable shaft seals and pillow block bearings. The conveyor is constructed from thicker materials due to the moderately abrasive nature of gypsum. Since the gypsum is very dusty, the dust-tight covers and adjustable shaft seals keep the material contained within the screw conveyor. The pillow block bearings are mounted on pedestals away from the conveyor to avoid contamination.
As materials of construction such as abrasion-resistant alloys and stainless steels have been developed over the years, the use of screw conveyors has increased to many more industries as previously discussed. Recent production technology improvements include automating specific manufacturing processes that improve quality and reduce costs. The use of automated or robotic welding in the screw-making process is a good example.
One of the most recent developments is the shaftless screw conveyor which utilizes a screw without the center pipe. The screw or spiral is constructed from high-strength steel and is rigid enough to handle the drive torque. The spiral typically lies in the bottom of the trough on a liner made from UHMW plastic or metal. The drive shaft is connected directly to the shaftless spiral. The shaftless conveyor was originally developed to convey wet and sticky materials. When conveyed with a shafted screw conveyor, wet and sticky materials have a tendency to stick to the pipe where the flight is attached. Since the center pipe is eliminated in a shaftless conveyor, there is no place for the material to stick. The image below shows a typical shaftless screw conveyor.
Differences Between Manufacturers
Each screw conveyor manufacturer is unique and provides different products and services. Some manufacturers specialize in providing CEMA standard stock components while others focus on custom designs. It is important to choose the proper screw conveyor manufacturer based on your specific needs and the customers in your area. For example, if you have several rendering plants in your area, you will need a manufacturer that specializes in designing and building heavy-duty screw conveyors. Material thickness for screw flights and troughs can be as thick as one inch. Manufacturers that regularly build screw conveyors for the rendering industry must have special flight presses and other equipment for forming and handling one-inch thick plate. Also make sure the manufacturer that you choose has a written quality control program and is ISO 9001-2008 certified so you know you are getting quality equipment. Welders must also be certified to either AWS or ASME code standards.
The major screw conveyor manufacturers are active members of CEMA and can be located by going to www.cemanet.org. You can contact CEMA for information if you are not familiar with a certain manufacturer. Also, check out the individual company's website or contact the company directly to discuss capabilities and quality programs.
Adding Value for the Customer
Screw conveyors are an important part of your customer's operations. If a screw conveyor continually fails or requires excess maintenance, then your customer is not efficiently producing product. Having a basic knowledge of screw conveyor design along with understanding the application will help you in solving the customer's problem. For example, your customer is conveying petroleum coke and replacing screws every three months. The CEMA material classification code for petroleum coke is 30D737N. The code indicates that the petroleum coke weighs 30 lbs. per cubic foot, has a large lump size, has average flowability, is extremely abrasive and can be explosive. The screws are wearing out prematurely because petroleum coke is extremely abrasive. As an added value, you can offer screws constructed from abrasion-resistant materials or the screw flights can be hardsurfaced with a weld-on hardsurfacing material.
The right image shows a screw conveyor with weld-on hardsurfacing on the screw and internal surfaces of the housing. IDC University offers a two-day course in screw conveyor design and applications. Applications such as those mentioned in this article will be discussed in detail and you will get hands-on experience with screw conveyors. You will gain important knowledge about various industries and applications that will help you increase sales and make your company more profitable.
Bill Mecke, P.E. is Vice-President and owner of KWS Manufacturing Company, Ltd. He has over 20 years of experience in the bulk material handling industry and in the design of screw conveyors. Bill is currently the chairman of the CEMA Engineering Committee for Screw Conveyors. He received his Bachelor of Science degree in Mechanical Engineering from Texas A & M University and is a professional engineer in the state of Texas.
Source: IDC Industrial Review, July 2007.