Factor Influencing Screw Conveyor Design

Highly Corrosive

Highly corrosive bulk materials require the use of corrosion resistant alloys. Screw conveyors must be fabricated from alloys not affected by the corrosive product. Typical materials of construction for highly corrosive products are 304, 310, 316, 410 and 430 stainless steels. Also, high nickel alloys such as Inconel, Monel and Hastalloy may be used.

Builds Up and Hardens (A)

Certain bulk materials build up and harden in the screw conveyor trough. These bulk materials also tend to pack under pressure and absorb moisture. Screw conveyors with close clearance between the screw and trough are required to prevent build up in the trough. Also, high-torque drive units are required to shear through the hardened bulk material and carbide tip teeth can be used as well.

Decomposes – Deteriorates in Storage (C)

Some bulk materials will decompose and deteriorate if stored for long periods of time. Bulk materials stored in hoppers, bins or silos must be discharged and processed before being allowed to decompose or deteriorate. As part of the complete process, screw feeder and conveyor design must take into account the characteristics of specific bulk materials that tend to decompose or deteriorate.

Becomes Plastic or Tends to Soften (E)

Certain bulk materials are sensitive to changes in temperature or pressure. The rotation of the screw may cause friction and heat and cause the bulk material to soften. Many food products must be gently conveyed. Lower trough loading and slower screw  speeds are recommended for bulk materials that tend to become plastic or soften.

Aerates and Becomes Fluid (G)

Fluidizing bulk materials will aerate and tend to flow like liquids when conveyed. The ―as conveyed‖ bulk density is much lower than the static or packed bulk density. In some cases the fluidized bulk material will flood and ―run like water‖ with the loss of control of capacity or feed rate. Trough loading will increase causing the flooding of downstream equipment. Lower trough loading and slower screw speeds are recommended to reduce the chance of aeration of the bulk material 

Contaminable, Affecting Use (J)

Contaminable bulk materials lose their value if contaminated by foreign materials. Screw conveyors must be completely sealed with special shaft seals and utilize hanger bearings that will not contaminate the bulk material 

Degradable, Affecting Use (K)

Degradable bulk materials are easily broken down to smaller particles when conveyed and could lose their value. Screw conveyors must be designed for very slow speeds and lower trough loading. Larger screw conveyor sizes are recommended 

Gives Off Harmful or Toxic Gas or Fumes (L)

Toxic bulk materials give off vapors or dusts that may be hazardous to personnel. Screw conveyors must be completely sealed with special shaft seals to contain the toxic bulk material. Design of screw conveyor troughs or housings depends on the severity of the hazard. Screw conveyors can be designed to contain internal pressure and even an explosion.

Hygroscopic (M)

Hygroscopic bulk materials readily absorb moisture and can become contaminated. Screw conveyors with vapor-tight covers are required. It also may be necessary to insulate or provide a jacket for the trough to maintain a consistent product temperature. Purging the troughs with dry gas or air may also be necessary.

Interlocks, Mats or Agglomerates (N)

nterlocking bulk materials have a tendency to mat together and become resistant to being conveyed. Heavier construction with high torque drives for screw conveyors are required. Intermediate hanger bearings can create an obstruction to flow so may need to be eliminated. Shaft less screw conveyors are a good option for interlocking bulk materials.

Oils Present (O)

Oils contained in the bulk material may cause the bulk material to become sticky and adhere to the flights of the screw as well as the trough. Sticky bulk materials tend to stick to the center pipe of a screw conveyor where the flight is welded to the center pipe. Ribbon fighting is an excellent choice for handling sticky bulk materials.

Shaft less screw conveyors provide even better performance than shafted conveyors with ribbon fighting.

Elevated Temperature (R)

Hot bulk materials are encountered in many phases of bulk material processing. Screw conveyors should be designed and manufactured with heavy-duty construction and may require 304, 316 or even high- temperature stainless steel alloys. Hot materials can be cooled with Thermal Processors that utilize jacketed troughs and hollow- flight designs.