Bulk material characteristics

What are material codes ?

A very fine 100 mesh material with an average density of 800 kg/m3, that has average flowability and is moderately abrasive, would have a material code 800 A100 36. If this material were very dusty and mildly corrosive the number would then be 800A10036LT.

The Material Factor is used in the horsepower formula to determine the horsepower to operate a horizontal screw conveyor. The calculation of horsepower is simply done here Calculation sheets

The Wiki Material Table lists a wide range of bulk materials that can be handled in screw conveyors. The table shows in the first column the range of density that is usually experienced in handling that material. The average density is not specifically shown but is often assumed to be at or near the minimum. The next column shows the material code number. This consists of the average density, the usual size designation, the flowability number, the abrasive number followed by those material characteristics which are termed conveyability hazards.

The indication of suitability for handling the material in a vertical screw conveyor is only a guide.

The information and data in the Material Characteristics table, has been compiled by members of CEMA and represents many years of experience in the successful design and application of screw conveyors for handling the listed materials. The indicated physical characteristics of these materials are not the result of any particular laboratory tests but were learned from the actual industrial operation of countless screw conveyors.

The Material Table includes various grains, seeds, feeds, etc. that are commonly handled in many conveyor types. The published unit weights, the component series and material factors Fm are for average conditions. For instance, wheat when dry or with a low moisture of less than 10% is very free flowing, and the Fm factor of 0.4 can be used. When higher moistures are prevalent, a material factor of 0.5 or 0.6 is suggested. This phenomena is common to all grains and some other substances.

It should also be noted that soybeans are shown as being very abrasive. Heavy conveyor construction is recommended. This is because soybeans, especially when dirty and harvested at a low moisture, are extremely abrasive. On the other hand, hard iron bearings which are commonly used with abrasive materials cannot be recommended because of spark generation and consequent dust explosions. This phenomena is also true of rough rice and to a lesser degree on other grains

THE MATERIAL TABLE IS A GUIDE ONLY. THE MATERIALS CODE AND THE MATERIAL FACTOR Fm ARE BASED ON EXPERIENCE OF SEVERAL CONVEYOR MANUFACTURERS. A SPECIFIC MATERIAL SAMPLE MAY HAVE PROPERTIES THAT VARY FROM THOSE SHOWN IN THE TABLE. THE RANGE OF DENSITIES WILL ALSO VARY DEPENDING ON MOISTURE CONTENT AS WELL AS ITS SOURCE.

Experience shows that the key to successful screw conveyor design is a thorough knowledge of the characteristics of the material to be handled. Material wiki table offers a coded classification of many bulk materials. The code is based on the physical characteristics of the listed materials and provides a basis for determining screw conveyor specifications. It is important to understand that the action of a screw conveyor is such that it tends to tumble and shear the material as it is being conveyed. Therefore, it follows that materials which tumble or shear readily are more easily conveyed than others which do not.

Materials, first of all, are classified according to particle size. It is important to have a screen analysis made of the material, if at all possible. For example if a material is said to consist of 12,7 mm and under, it may be similar to granules of plastic. Or it may have only 10% of 12,7 mm particle size, with 90% fines grading to micron sizes. Some materials may require use of cover gaskets and/or seals; others may not, depending upon material characteristics. Lumpy materials must be checked against the Lump Size. Very often larger screw conveyors must be used solely to accommodate the lumps than otherwise would be required from a standpoint of normal capacity.

Irregular, stringy, and interlocking materials that mat or cling together require special consideration. Stringy materials, particularly if long enough, may wrap around the pipe shaft of the conveyor screw or around the intermediate hanger bearings, thus effectively clogging the conveyor. Materials that mat may also be those that pack under pressure. If the material does pack under pressure, it may jam the conveyor screw and seriously damage the conveyor. All materials with these characteristics must be carefully studied in detail with respect to their actions in a screw conveyor. On particular advantage of Archimedys fligthing is their flexibility. If a clogging occurs, the flight can be bended and recover is shape after clogging removal. Materials are also classified as to their flowability. This, unfortunately, is a relative term and not easily measured. However, so far as the operation of screw conveyors is concerned, flowability is related to two factors, one the angle of slide and the other the internal friction of the material. The angle of slide may be determined by tilting a plate carrying a quantity of the material. The angle of internal friction may be evaluated from shear cell test data. Changes in moisture content, temperature, particle size distribution and chemically corrosive action of the material all affect the flowability.

It is known that some materials which are uniform in particle shape and size are quite free flowing when dry. Screened dry sand is free flowing. The addition of moisture, however, changes the flowability character. Likewise, dry granulated sugar is free flowing, but this material is hygroscopic and will pick up moisture from the air. If this happens, its flowability is changed considerably. The flowability of most materials is affected by changes in their moisture content, with consequent changes in their ability to be conveyed. The abrasiveness of materials is also a relative quantity and isn’t easily defined with accuracy. Some materials are more abrasive than others. It will be found that nonabrasive or very mildly abrasive materials may be handled with screw conveyors with standard troughs.

Very abrasive materials require heavier than standard components. Most abrasive materials in the following Material wiki table, are handled at lower cross-sectional loads than are the nonabrasive materials. This is done to attain the maximum economical life of the conveyor and its parts. The foregoing bulk materials have hazards affecting conveyability. The effect of some of these hazards as they affect screw conveyor design follows.

• K. Some bulk substances are sensitive to small changes in temperature or pressure. For example, materials containing vegetable oils or fats can become spoiled by the heat of friction in a hanger bearing.
• L. Dusty materials—especially those that are very dusty—should be carefully considered. Previous experience with similar materials is the best guide. Flange gaskets and special trough end seals may be needed.
• M. Some materials such as dry cement will aerate and develop fluid characteristics as a result of transport in a screw conveyor. The “as conveyed” apparent density is much lower than the normal apparent density. Many dusty and aerated materials can bypass an intermediate discharge spout. As the material becomes more fluid-like, the flowability increases markedly, and in some cases the aerated material will flood and run like water with the result that the cross-sectional load increases and control of the rate of flow is lost. Consult your conveyor manufacturer regarding materials which may aerate greatly.
• N. Dusts associated with certain bulk materials are flammable or even explosive when mixed with air in the proper concentration. It therefore may be necessary to contain dust laden material at all times within the conveyor enclosure. Grain dust is an example. The very nature of a screw conveyor—being an enclosed conveying device—may be used for handling materials with flammable or explosive dusts, although more sophisticated than standard enclosures may be required. The U4 material range is a conductive material created in order to avoid static accumulation on the flight and electrical spark.
• P&Q. Contaminable and degradable materials must be recognized because their salability or use may be affected by improper conveying or ill-considered conveyor specifications. Suitable non-lubricated bearings should be used. Low conveyor speeds normally will prevent excessive degradation.
• R. Materials in this category are similar to those described under L and N, except that exposure of the dust or fumes may be hazardous to personnel. Tight enclosures and spouting connections—usually gasketed—are required. Elaboration of the enclosures depends upon the severity of the hazard.
• S&T. Corrosion protection requiring the use of special metals is a common problem. Here again “corrosion” is a relative term which isn’t easily defined numerically. The choices of materials of construction, such as the types of stainless steel or other special metals, should be referred to the conveyor manufacturer. Once again, the advantage of Archimedys flightings is their high resistance to corrosion. The trough, in difficult case can also be done with a particular plastic in order to build a complete corrosion free conveyor.
• U. Certain bulk materials are hygroscopic. They absorb water from the moisture in the ambient atmosphere. The water they pick up changes their flowability, of course, and this has been taken into account for the usual behavior of such materials as listed in the wiki table.
• V&X. Bulk materials which interlock and mat usually will require screws of heavier than standard construction and flight edges that can cut their way through the material. Intermediate hanger bearings may have to be eliminated. A similar condition exists for materials which pack under pressure.
• W. Oils or chemicals that may be contained in bulk materials require special consideration. Some of these constituents may make the materials sticky and cause adherence to the working parts of the conveyor. Ribbon type conveyor screws sometimes help. It is best to consult your conveyor manufacturer when attempting to handle such materials. Archimedys module has a non sticky surface allowing the use for a wide range of materials.
• Y. Light and fluffy materials require consideration similar to those which are dusty or which tend to aerate as they are conveyed. See paragraphs L and M.
• Z. Elevated temperatures are encountered in many phases of material processing. Screw conveyors should be fabricated of heavier than standard construction and designed to withstand the inevitable expansion and contraction that takes place. Intermediate hanger bearings must be protected against heat or omitted. End bearings and drive equipment may be separated from the trough end to reduce their exposure to heat. The temperature service range for standard Archimedys flighting is -30°C to 80°C. Custom made Archimedes flighting can reach a temperature of 700°C.

Flowability and abrasiveness index and flow function are presented in the material documentation.