Understanding Different Types of Screw Conveyors

Horizontal Screw Conveyors

Among various types of screw conveyors, horizontal screw conveyors are the most commonly utilized. They effectively transport bulk materials from one segment of a process to another. These conveyors come in diverse sizes, lengths, configurations, and materials.

Typically, horizontal screw conveyors are designed to convey bulk materials at 15, 30, or 45 percent trough loading. The loading percentage largely depends on the specific characteristics of the bulk material being moved. For example, lighter and free-flowing non-abrasive materials can handle up to 45 percent trough loading, whereas denser and more abrasive materials generally require 15 to 30 percent loading.

Feeding Mechanisms

The inlet of a screw conveyor is generally supplied either directly or with the aid of several devices including:

• Screw Conveyor
• Screw Feeder
• Belt Conveyor
• Rotary Airlock
• Volumetric or Gravimetric Feeder

For optimal performance, it is advisable to position the drive unit at the discharge end of the screw conveyor. This design effectively pulls bulk materials towards the drive unit, which not only maintains tension on the screw sections but also reduces wear and fatigue on the conveyor components.

Benefits of Screw Conveyors

• Perfect for transporting dry to semi-fluid bulk materials - from free-flowing to sluggish
• Cost-efficient in comparison with other conveying mechanisms like belt or pneumatic systems
• Facilitates efficient distribution of bulk materials to multiple locations
• Fully enclosed, making them suitable for dusty, corrosive, or hazardous environments

Inclined Screw Conveyors

Inclined screw conveyors operate at angles ranging from just above horizontal to 45 degrees. When the incline exceeds 45 degrees, the conveyor is classified as a vertical screw conveyor, requiring a design that aligns with the engineering guide for vertical systems. It's essential to note that as the incline increases, the efficiency of conveying diminishes, and the required horsepower rises due to gravity's impact and potential material backfall.

The incline’s angle and the bulk material's characteristics influence the conveying efficiency. To maximize efficiency, it is advisable to design screw conveyors with the most gradual incline possible.

Incline Design Considerations

• Inclines Up to 10 Degrees - Minimal efficiency loss, suitable for a U-trough with full pitch screw. Speed, diameter, or pitch adjustments can mitigate efficiency loss.
• Inclines Between 10 and 20 Degrees - Efficiency loss of 10 to 40 percent is typical; a U-trough with a 2/3 pitch screw works well, but extra horsepower compensates for gravitational effects.
• Inclines Between 20 and 30 Degrees - Efficiency loss ranges from 10 to 70 percent; a tubular housing with a 1/2 or 2/3 pitch screw is recommended.
• Inclines Between 30 and 45 Degrees - Significant efficiency loss of 30 to 90 percent occurs. Increasing speed and using a larger diameter can help regain efficiency.

Considering Pitch Efficiency

A pitch efficiency chart illustrates the relative efficiency levels across various incline angles and pitch configurations. As incline degrees rise, reduced pitch screws (1/2 and 2/3) tend to perform better than full pitch variants. The best efficiency achieves a combination of tubular housings with reduced pitch screws.

Horsepower Needs

The required horsepower for inclined screw conveyors rises with increasing incline degrees. The Horsepower Factor (Fi) is utilized in standard horsepower calculations to adjust for requirements stemming from gravitational pull and bulk material backslide.

TSHP (i) = Total Shaft HP for Inclined Screw Conveyor

MHP = Material HP (the horsepower necessary to transport the material)

FHP = Friction HP (the horsepower needed to operate the conveyor without any material)

If the calculated Material Horsepower is below 5HP, adjustments for overload potential should be made.

Managing Excess Conditions

Inclined screw conveyors that exceed a 10-degree incline require engineering to function effectively even under upset conditions where normal operation halts. When this happens, bulk material may slip back, filling the conveyor and demanding more horsepower to restart conveying. Consultation with engineering professionals is advisable for optimal design solutions.

Shaftless Screw Conveyors

Shaftless screw conveyors adeptly handle bulk materials coming from filter presses, centrifuges, or mixers. The shaftless design avoids clogs, simplifying the conveyance of traditionally problematic materials such as those with high moisture content.

Key Benefits

• Perfect for handling both slippery and heavy materials
• Enhanced efficiency compared to alternative conveyor types
• Offers more layout flexibility in plants due to various configurations
• Eliminates the need for internal bearings

Shaftless screw conveyors are widely used across numerous industries, including food, chemical, mineral processing, and wastewater treatment. They are designed to handle any material ranging from catalysts to dewatered biosolids.

Vertical Screw Conveyors

Vertical screw conveyors are highly efficient for elevating bulk materials at steep angles or completely vertically, categorizing any screw conveyor functioning at more than 45 degrees as a vertical model.

The compact design allows these systems to fit seamlessly into various plant layouts. Their minimal moving parts make vertical screw conveyors both economical and reliable for bulk material movement.

Advantages of Vertical Models

• Capable of managing dry to semi-fluid materials.
• Provides capacities of up to 6,000 cubic feet per hour.
• Can elevate materials up to 30 feet without requiring internal bearings.
• Encased design fulfills dust and vapor-tight specifications.

Designed for versatility, KWS supplies vertical screw conveyors that cater to numerous sectors, including chemicals, minerals, food, and wastewater treatment. An example of its application is unique shaftless vertical screw conveyors utilized in wastewater treatment facilities for efficient transport of dewatered biosolids.

For a deeper understanding of vertical screw conveyors, refer to the detailed KWS Vertical Screw Conveyor Engineering Guide available on our website.

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