What is a Screw Feeder (Doser)?

Screw feeder for dosing of bulk materials

Screw feeders are used in thousands of material handling applications and are designed to meter bulk materials at a controlled feed rate. A standard screw conveyor is normally fed by another screw conveyor and the amount of material that is discharged is directly proportional to the amount that is fed into the inlet.

In contrast, the inlet of a screw feeder is always 100 percent or flood loaded and is typically mounted directly to a storage device, such as a hopper, bin, or silo. The amount of material discharged by a screw feeder is completely controlled by design parameters.

A hopper is typically square or rectangular in shape with a sloping bottom and limited storage capacity. A container is very similar to a hopper but with greater storage capacity. A silo is cylindrical in shape with a conical or mass flow bottom and a large storage capacity.

Since the screw feeder inlet is flood loaded, the desired feed or discharge rate directly influences the screw design. The discharge rate of a screw feeder can be accurately determined by speed control and screw conveyor pitch design. The use of variable speed drives improves measurement accuracy and can provide a wide range of feed rates. Pitch control is also an important consideration in screw feeder design.

If a Screw Conveyor is designed to handle bulk materials, it is carried out in relation to the rotation speed of the screw with a controlled percentage of load in the tundish and under these conditions the power was calculated and a standard pitch was placed, this conveyor should not force yourself to handle more bulk material, because it is only designed to handle and not feed bulk material.

Consequently, it is not recommended to design screw feeders with outside diameter and standard pitch (avoid designing with outside diameter equal to the lead pitch), because bulk materials will fill the helical screw from the back of the inlet opening first. , creating holes or caverns, stagnant material and possibly a "bridging" of bulk materials above the screw feeder.

Figure 1 shows the improper design of a screw feeder with uniform outside diameter and constant pitch. To extract bulk materials evenly throughout the entire intake, each auger flight must increase the available volume as the screw conveyor moves toward the discharge. An effective and efficient system requires a variable pitch, tapered outside diameter (OD), or mass flow screw design.

Figure 2 shows a properly designed screw feeder.

Figure 1

Poorly designed screw dispenser

Figure 2

Well Designed Screw Feeder

Screw Feeder Types

There are 3 main types of Screw Feeders:

1.- Variable pitch screw feeder (Progressive Pitch)

The pitch of the screw conveyor changes from shorter to longer as the screw advances towards the discharge of the screw feeder. With variable pitch, each pitch increases in length in the inlet section creating more volume available for adding bulk materials from the hopper. With variable pitch, the flight pitch changes in increments. See Figure 3. For example, a variable pitch screw feeder may have 0.60 meter of 1/3 pitch, then 0.60 meter of 2/3 pitch in the input section.

Figure 3

Variable Pitch Progressive

2.- Conical Screw Feeder (Tappered)

The outer diameter of the screw flight tapers from the rear of the intake opening to the deck, creating more volume available for adding bulk materials from the hopper. See Figure 4.

Figure 4

Screw Feeder Tappered

3.- Mass flow Screw Feeder

The mass flow design was developed by Jenike & Johanson and is a combination of variable pitch and tapered inner shaft. The tapered shaft and variable pitch area is located in the material loading area, at the hopper or silo opening, and spans approximately to the center of the inlet opening. Short pitch auger flights are mounted on the cone creating a volume available for the addition of bulk materials from the hopper. A variable pitch is then added to the screw starting where the cone ends and continuing through discharge. See figure 5.

Figure 5

Mass Flow Screw Feeder

Screw Feeder Configuration

Multi-diameter screw feeders

They consist of a screw feeder with an extension screw conveyor which receives and only transports the bulk material already fed. A smaller diameter screw feeder sits under a hopper, bin or silo and is flood loaded. The screw feeder doses the bulk material to the larger diameter extension conveyor. When the bulk material reaches the extension conveyor, the channel load decreases and the bulk material is conveyed to discharge. Suspension bearings are allowed on the extension conveyor as long as the trough load is less than 45 percent. Figure 6 shows a typical feeder / conveyor.

Figure 6

Mass Flow

Mass Flow Screw Conveyor (Live Bottom)

Live Bottom Screw Feeders are designed for use in large silos, bins and hoppers with large discharge openings. The Live Bottom Screw Feeder uses multiple feeder screws together to create a "live bottom" to prevent bridging or cavern formation. Bulk materials are measured and drawn equally from the entire width and length of the inlet opening. Live bottom screw feeders are used in bulk materials that tend to compact or bridge easily. Figure 7 shows a four screw live bottom feeder.

Figure 7

Live Bottom (Mass Flow)

Inclined screw feeder

The inclined screw feeder measures and lifts bulk materials from hoppers, bins or silos and performs the same function as horizontal screw feeders; however, special care is required when designing inclined screw feeders. Figure 8 shows typical inclined screw feeders. Knowledge of the flow characteristics of bulk materials is extremely important to a successful inclined screw feeder design.

The angle of repose and the flowability of a bulk material will determine the screw feeder design and maximum tilt angle. Bulk material testing is required for all inclined screw feeders before a proper design can be established. At Bega we have testing facilities for bulk materials and can test your bulk material before determining the proper inclined screw feeder design.

Screw feeders are an integral part of most bulk material handling operations. Proper screw feeder design ensures successful bulk material processing.