How do you calculate or design the diameter and pitch of a horizontal screw conveyor?

Designing the diameter and pitch of a horizontal screw conveyor involves considering several critical parameters to ensure efficient operation. Here's a step-by-step guide to help you calculate these factors:

1. Define Operational Requirements:

• Conveying capacity (Q): The amount of material to be conveyed, usually in cubic feet per hour (ft³/hr) or cubic meters per hour (m³/hr).
• Material characteristics: Include bulk density, particle size, abrasiveness, and flowability.
• Conveyor length (L): The distance the material needs to be transported horizontally.

2. Choose Preliminary Diameter:

Refer to standard screw conveyor diameters and select a preliminary size based on conveying capacity. Common diameters are 6, 9, 12, 14, 16, 18, 20, and 24 inches.

3. Calculate Screw Capacity:

Use the following formula to estimate screw capacity: \[ Q = (D^2 \times P \times N \times C) / 60 \] Where:

• ( Q ) = capacity in ft³/hr
• ( D ) = screw diameter in inches
• ( P ) = pitch length in inches
• ( N ) = speed in RPM (revolutions per minute)
• ( C ) = capacity factor (depends on the material, usually found in capacity tables)

4. Adjust for Material Factors:

Consider the bulk density of the material: \[ Capacity{actual} = Q{screw} \times Bulk Density \]

5. Determine Screw Pitch:

The pitch of the screw is typically equal to the diameter for standard materials, but it can vary.

• For free-flowing materials, the pitch can be increased.
• For sluggish, semi-fluid materials, the pitch may need to be equal to or less than the diameter.

6. Calculate Horsepower:

Use horsepower calculations to ensure the motor can handle the load. This involves friction and material handling considerations.

7. Consider Design Adjustments:

Check for any surges, material buildup, and desired feed rates. Adjust the conveyor diameter and pitch if necessary.

Example Calculation:

Let’s say we want to design a screw conveyor for a capacity of 500 ft³/hr, using a material with a bulk density of 50 lb/ft³, and we preliminarily select a 12-inch diameter screw operating at 60 RPM with a pitch equal to the diameter:

1. Preliminary Capacity Calculation: \[ Q = (D^2 \times P \times N \times C) / 60 \] \[ Q = (12^2 \times 12 \times 60 \times 0.5) / 60 \] \[ Q = (144 \times 12 \times 60 \times 0.5) / 60 \] \[ Q = 864 ft³/hr \] Here, 0.5 is an example factor for illustration purposes.

2. Considering Bulk Density Adjustments: \[ Capacity{actual} = Q \times Bulk Density \] \[ Capacity{actual} = 864 \times 50 \] \[ Capacity_{actual} = 43200 lb/hr \]

This example simplifies some complexities and assumes general material behavior. Always consult design guides and standards specific to the industry and material handled for precise calculations.

For more accurate and tailored screw conveyor designs, consider specific factors like material abrasiveness, operating conditions, feeder types, and professional software or consultations.