Materials Handled on a Screw Conveyor
Characteristics of the bulk materials that determine in the Design of a Screw Conveyor
Abrasiveness, corrosivity and fluidity
Bulk abrasive materials
Bulk abrasive materials can cause excessive wear on screw conveyor components. Screw conveyors must be made of abrasion resistant alloys. Typical materials of construction are AR235, AR400, AR500, or chrome carbide coating. These materials are harder and stronger than A36 carbon steel to resist abrasion.
The screw handling surface may be coated with a welded hardfacing material. It is also very important to reduce the channel load and the speed of the screw conveyor when handling abrasive materials in bulk.
Highly corrosive materials
Highly corrosive bulk materials require the use of corrosion resistant alloys. Screw conveyors must be manufactured from alloys that are not affected by the corrosive product. Typical materials of construction for highly corrosive products are 304, 310, 316, 410 and 430 stainless steels. In addition, high nickel alloys such as Inconel, Monel and Hastalloy can be used.
Slightly corrosive material
Slightly corrosive bulk materials, or bulk materials that tend to become corrosive under certain conditions, may require the use of corrosion resistant alloys. Corrosion is a relative term that is defined by each particular application. Screw conveyors must be manufactured from alloys that are not affected by the corrosive product.
Typical materials of construction for mildly corrosive products are 304 and 316 stainless steels.
Material that builds up and hardens
Certain bulk materials build up and harden in the "U" trough or tubular jacket of the screw conveyor. These bulk materials also tend to pack or compress under pressure and absorb moisture. Screw conveyors with clearance between screw and trough are required to avoid accumulation in the trough. Additionally, high torque drive units are required to cut through hardened bulk material and carbide tipped teeth can also be used.
Material that generates static electricity
Static electricity can be generated as the rotation of the screw conveyor causes bulk material to fall during handling. Plastic pellets will generate static electricity when handled by a screw conveyor. Grounding the screw conveyor trough in several places will help eliminate static electricity. Low channel loading and reduced operating speeds will also help.
Materials that decompose or deteriorate in storage
Some bulk materials will decompose and deteriorate if stored for long periods of time. Bulk materials stored in hoppers, containers, or silos must be unloaded and processed before being allowed to decompose or deteriorate. As part of the entire process, the design of the screw feeder and conveyor must take into account the characteristics of specific bulk materials that tend to decompose or deteriorate and use food grade stainless steels to prevent rusting, such as type 304 and type stainless steel. 316.
Flammable bulk materials will burn when oxygen and an ignition source are present. Screw conveyors can be designed with non-sparking materials to eliminate a potential source of ignition. Pressure tight housings with mechanical shaft seals will prevent oxygen from entering the screw conveyor. The inside of the screw conveyor channel or tundish can also be purged with an inert gas such as nitrogen.
Materials that turn plastic or tend to soften
Certain bulk materials are sensitive to changes in temperature or pressure. Rotation of the screw conveyor can cause friction and heat and soften the bulk material. Many food products must be transported gently. Lower loads on troughs and slower screw conveyor speeds are recommended for bulk materials that tend to become plastic or soften.
Very dusty materials
Some bulk materials can become very dusty and run off screw conveyors. These dusty bulk materials can become a hazard or even be explosive. Screw conveyors must be completely sealed with special shaft seals to contain dusty bulk material. The design of channels or troughs for screw conveyors depends on the severity of the hazard.
Materials that aerate and become fluid
Fluidizing bulk materials will aerate and tend to flow like liquids when handled. The "as handled" bulk density is much lower than the static or packed bulk density. In some cases, fluidized bulk material will flood and "run like water" with loss of control of feed rate or capacity. The trough load will increase and cause the equipment to flood at the opposite end of the load. Lower trough load and slower screw conveyor speeds are recommended to reduce the possibility of aeration of the bulk material.
Explosive bulk materials will ignite when oxygen and an ignition source are present. Screw conveyors can be designed with non-sparking materials to eliminate a potential source of ignition. Pressure tight troughs or housings with mechanical shaft seals will prevent oxygen from entering the screw conveyor. The interior of the screw conveyor trough or housing can be purged with an inert gas such as nitrogen. Explosion ventilation is also an option to relieve pressure due to an explosion.
Materials with high adhesiveness - Adhesion
Viscous or sticky bulk materials contain a high percentage of moisture or oil and are difficult to handle. Sticky bulk materials tend to adhere to the center tube of a screw conveyor where the run is welded to the center tube. The Ribbon type screw conveyor is an excellent choice for handling sticky bulk materials. Shaftless screw conveyors provide even better performance than pole-type batten screw conveyors.
Materials that should not be Contaminated when being handled
Contaminating bulk materials lose their value if contaminated by foreign materials. Screw conveyors must be completely sealed with special shaft seals and use suspension bearings that will not contaminate the bulk material, and be made of materials such as Type 304 or Type 316 stainless steel.
Degradable materials that can be damaged or broken when handled
Degradable bulk materials easily break down into smaller particles when handled and could lose their value. Screw conveyors must be designed for very slow speeds and lower loads. Larger screw conveyors are recommended.
Materials that Emit noxious or toxic gases
Toxic bulk materials emit vapors or dusts that can be hazardous to personnel. Screw conveyors must be completely sealed with special shaft seals to contain toxic bulk material. The through design of screw conveyors depends on the severity of the hazard. Screw conveyors can be designed to contain internal pressure and even an explosion.
Hygroscopic bulk materials easily absorb moisture and can become contaminated. Screw conveyors with vapor tight covers are required. It may also be necessary to insulate or provide a cover for the channel to maintain a constant product temperature. It may also be necessary to purge the channels with dry gas or air.
Materials that are interlocked or agglomerated by interlocking
Bulk materials that interlock tend to stick together and become resistant to being transported. Heavier construction is required with high torque units for screw conveyors. Intermediate suspension bearings can create an obstruction to flow, so they may need to be removed. Shaftless screw conveyors are a good choice for conveying interlocking or clumping materials.
Bark of wood
Materials with Oils
The oils contained in the bulk material can make the material sticky and stick to the screw as well as the trough. Sticky bulk materials tend to stick to the center tube of a screw conveyor where it is attached to the center tube. The ribbon helix is an excellent choice for handling sticky bulk materials. Shaftless screw conveyors provide even better performance than screw conveyors with shaft.
Materials that are compressed and packaged
Compressible materials tend to pack under pressure. Bulk materials will clump or accumulate as moisture content increases and pressure is applied. Special screw conveyors with "lump breakers" or "breaker bars" can help break up lumps and reduce packing. Additional horsepower and torque are required to break up lumps and transport bulk material.
Very light and fluffy materials
Bulk materials weighing less than 240 kilograms per cubic meter tend to be dusty and airy when transported. A larger diameter screw conveyor with a lower trough load and slower screw speed is recommended to reduce the possibility of aeration of the bulk material.
High temperature materials
Hot bulk materials are found in many phases of bulk materials processing. Screw conveyors must be designed and manufactured with strong construction and may require 304, 316, or even high-temperature stainless steel alloys. Hot materials can be cooled with Bega Thermal Processors using jacketed channels (Jacketed Screw Conveyors) and hollow screw designs.
Carga permisible en una artesa de Transportador Helicoidal
The allowable load in the trough is the volume of a bulk material contained in the trough of a screw conveyor during its operation and is measured in percentage, taking into account that 100% means the full trough. The recommended minimum loads of 15, 30 and 45 percent were developed based on the characteristics of the bulk materials. The minimum recommended load for a screw conveyor is a function of the density, abrasiveness, and flowability of a bulk material. For a given capacity, the size and speed of the screw conveyor are determined by the minimum load percentage. This is based on screw conveyors with "U" trough working in horizontal configuration, for inclined applications with tubular jacket, consult your Bega consultant.
15% loaded Trough
Bulk materials with a density range of 800 to 2000 kg / m 3 that are extremely abrasive and slow, such as alumina and glass are difficult to handle and do not flow easily through a screw conveyor. The trough load must be kept well below the center tube to reduce undue wear on conveyor components such as bolts, suspension bearings, coupling shafts and trough. The minimum recommended load for bulk materials with similar characteristics is 15 percent.
30% loaded Trough
Bulk materials with a density range of 200 to 1000 kg / m 3 that are slightly abrasive and free flowing, such as black carbon, fish meal or barley grain, will flow through a screw conveyor. The trough load can be raised to a level below the center tube without causing undue wear on conveyor components such as screws, suspension bearings, coupling shafts and troughs. The minimum recommended load for bulk materials with similar characteristics is 30 percent.
45% loaded Trough
Bulk materials with a density range of 80 to 650 kg / m 3 that are non-abrasive and free flowing, such as alfalfa, baking soda, or paddy rice, will easily flow through a screw conveyor. The trough load can be raised to the level of the center tube without causing undue wear on conveyor components such as bolts, suspension bearings, coupling shafts and channels. The minimum recommended load for bulk materials with similar characteristics is 45 percent.
What materials can a Vertical Screw Conveyor handle?
Many bulk materials can be easily transported on a vertical screw conveyor. Non-abrasive, free-flowing, medium-density bulk materials are ideal for vertical transport. The use of bulk materials that tend to aerate and flow, interlock, pack under pressure, are very light and fluffy, or degradable on a vertical screw conveyor is not recommended. Consult our with your Bega advisor for vertical screw conveyor applications.
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