Sizing A Drive For Your Screw Conveyor: A Comprehensive Guide

Sizing a drive for a screw conveyor involves several factors, including the desired flow rate of materials, screw diameter, and trough loading. Screw conveyors are commonly used to transport bulk materials and determining the correct size ensures efficient operation. The capacity of a screw conveyor is measured in cubic feet per hour, and manufacturers consider various factors to calculate this capacity. These factors include the desired flow rate, screw diameter, outside diameter of the center pipe, screw pitch, and trough loading percentage. Additionally, the size of the product being conveyed, as well as its density and machine capacity, play a crucial role in determining the overall screw size and trough loading.

Flow rate and screw size

The capacity of a screw conveyor is determined by the flow rate of material, which is the weight or volume of material it can convey per hour. This is affected by the size of the screw conveyor, as a larger screw inside the conveyor will be able to move material faster. The capacity of a screw conveyor is also influenced by the density of the material being conveyed, as denser materials will reduce the flow rate.

When selecting a screw conveyor, it is important to determine the desired flow rate or capacity. The capacity of a screw conveyor is measured in cubic feet per hour and is calculated based on the diameter of the screw, the outside diameter of the center pipe, the pitch of the screw, and the trough loading percentage. The actual size of the product being conveyed is also a critical factor, as the screw conveyor must be sized to accommodate both the minimum and maximum product sizes.

The recommended trough loading percentage corresponds to the screw conveyor's capacity and the type of material it processes. For abrasive, thick materials, the trough loading should not exceed 30%. When processing dense materials, it is necessary to reduce the speed of the screw conveyor.

To calculate the size and capacity of a screw conveyor, the following formula can be used:

Q = screw capacity in kg/h

D = screw diameter in m

S = screw pitch in m

N = screw speed in rpm

Α = loading ratio

Ρ = material loose density in kg/m3

C = inclination correction factor

By defining the required capacity and the characteristics of the product, the screw conveyor can be designed to meet the desired flow rate.

It is important to note that these formulas provide an estimate of the screw conveyor's size and speed and are not meant for detailed design or manufacturing. Specialized companies should be consulted for detailed design and manufacturing of screw conveyors.

Screw conveyor capacity

Q = screw capacity in kg/h

D = screw diameter in m

S = screw pitch in m

N = screw speed in rpm

Α = loading ratio

Ρ = material loose density in kg/m3

C = inclination correction factor

To determine the capacity of a screw conveyor, the following steps should be taken:

• Define the capacity requirement for the screw conveyor. The design of the screw must reach or exceed this value.
• Calculate the capacity of the screw conveyor by defining the screw pitch and estimating the loading ratio according to the characteristics of the material to be conveyed.
• If the screw conveyor is inclined, determine the correction factor.
• Adjust the screw speed so that the capacity of the screw meets or exceeds the requirement.
• Compare the calculated capacity to the maximum screw speed. If the calculated speed is greater than the maximum speed for the screw diameter, the design must be adjusted by changing a parameter, typically the diameter.

It is important to note that these formulas provide an estimate of the screw conveyor size and speed and are not meant for detailed design or manufacture. The capacity of a screw conveyor is also influenced by the type of flight used, such as cut flight, cut-and-folded flight, ribbon, and paddles. Modified flights generally reduce the conveying capacity.

When designing a screw conveyor, it is crucial to refer to capacity tables and consider factors such as the maximum lump size of the material, the characteristics of the bulk material, and the recommended trough loading percentage.

Screw conveyor diameter

The diameter of a screw conveyor is a crucial factor in determining its capacity and speed. It is influenced by various factors, including the required capacity, material characteristics, and physical properties.

To determine the screw conveyor diameter, follow these steps:

Step 1: Define the Requirement

Firstly, establish the required capacity of the screw conveyor. For instance, if you need to convey sugar, the required capacity might be 3500 kg/h.

Step 2: Calculate the Capacity of the Screw Conveyor

In this step, you need to define the screw pitch based on the screw diameter and the characteristics of the material being conveyed. The loading ratio (α) should be estimated according to the flow properties of the solid. The loading ratio can vary significantly, sometimes reaching up to 95%.

For an inclined screw conveyor, determine whether it needs to be flat or inclined and calculate the corresponding inclination correction factor (C).

Adjust the screw speed so that the capacity meets or exceeds the requirement.

Step 3: Compare the Calculated Capacity to the Maximum Screw Speed

Compare the calculated speed from Step 2 to the maximum screw speed for the selected screw diameter. If the calculated speed is lower than the maximum speed, the design is suitable. If it is higher, you will need to recalculate by changing a parameter, typically the diameter.

When dealing with explosive powders, ensure that the maximum speed results in a conveyor tip speed of less than 1 m/s.

Alternative Formula by CEMA

The CEMA association provides an alternative formula to calculate the capacity of a screw conveyor:

> C = 0.7854*(Ds2-Dp2).P.K.60/1728

Where:

• C = capacity in ft3/h/rpm
• Ds = Diameter of the screw flight in inches
• Dp = Diameter of the pipe shaft supporting the screw flight in inches
• P = pitch of the screw in inches
• K = percent trough loading

This formula provides the capacity over 1 rpm. To find the capacity per hour, multiply by the rpm at which the screw is used.

In conclusion, the diameter of a screw conveyor is a critical factor that influences its capacity and speed. It is determined by various factors, including required capacity, material characteristics, and physical properties. By following the steps outlined above, you can calculate the appropriate screw conveyor diameter for your specific application.

Screw conveyor speed

The speed of a screw conveyor can be calculated using the following formula:

> Q = screw capacity in kg/h

> D = screw diameter in m

> S = screw pitch in m

> N = screw speed in rpm

> α = loading ratio

> ρ = material loose density in kg/m3

> C = inclination correction factor

By inputting the relevant values into this formula, the required speed of the screw conveyor can be determined to meet the desired capacity.

When designing a screw conveyor, it is important to consider the maximum recommended speeds for different materials. The capacity table provides information on the maximum speeds for various materials, taking into account the percentage of loading. These speeds are intended for non-abrasive, free-flowing bulk materials, and lower speeds are often recommended for industrial applications.

Additionally, the screw conveyor speed calculation should consider any special pitch or modified flight types used, as these can impact the overall capacity and speed requirements. The selection capacity, which takes into account special pitches and modified flights, is used to determine the proper screw conveyor diameter and speed from the capacity table.

In summary, screw conveyor speed is a crucial factor in the design of a screw conveyor system. It involves careful calculations and considerations to ensure the desired capacity and performance are achieved while adhering to recommended speed limits for specific materials and applications.

Trough loading and sizing

When selecting a screw conveyor, it is important to consider the capacity, which is defined as the weight or volume per hour of a bulk material that can be safely conveyed. The capacity of a screw conveyor is influenced by various factors, including the screw diameter, flight pitch, and trough loading percentage. Reducing the flight pitch, for instance, decreases the capacity of the conveyor.

The recommended trough loading percentage is directly related to the screw conveyor's capacity and the type of material being processed. For abrasive, thick materials, it is generally recommended to not load the trough more than 30%. Additionally, when dealing with dense materials, it is advisable to reduce the speed of the screw conveyor.

The Conveyor Equipment Manufacturers Association (CEMA) provides a formula to calculate the amount of product conveyed by the screw per rotation. Reputable manufacturing companies utilize capacity calculation formulas in their screw conveyor design software to ensure accurate sizing.

It is worth noting that the maximum recommended capacity for each screw diameter should not be exceeded. The exact conveyor speed can be calculated using the formula provided, taking into account the capacity in cubic feet per hour. This calculation is essential for ensuring the proper functioning of the screw conveyor within its design limits.

Frequently asked questions