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Introduction

Many industries use feeders to meter powders and bulk materials through various parts of their processes. Feeders are used to meter materials from bins, silos, or hoppers, or to feed mixers, reactors, extruders, dryers, and a variety of other equipment.

Feeders are an integral part of the process and can directly affect process efficiency. Selecting the wrong feeder for your application can cause a long list of flow problems, and possibly process problems upstream and downstream of the feeder. However, selecting the right feeder allows for "rash free" hoppers, less stressed operators, and a smooth process. Table Feeders can have a large inlet diameter which typically exceeds the "Critical Arching Diameter" of many materials. This simplifies the hopper design and saves considerable head room due to their low profiles.

Types of Feeders

There are numerous types of feeders on the market today. Some feeders are industry specific, however, the majority of them fall into one of the following general categories:

• Screw Feeders
• Vibratory Feeders
• Rotary Feeders
• Belt Feeders

Many articles have been written on the common feeders listed above. In addition, their dominant use in the industry proves their versatility for most materials. However, what do you do if your material does not fall in one of these categories? In the past, operators and production personnel had to live with the problem and deal with it in the "classical" fashion: beat the hopper with a mallet and hope for the best!  The use of Table Feeders may be a better solution especially for those difficult to flow materials.

Table Feeder Principles

The basic principles behind the operation of table feeders are simple and can be explained as follows:

A straight walled cylinder is filled with a fine powder and covered with a flat plate. The cylinder is then inverted and placed on a flat surface. Lift the cylinder slightly and observe the flow of powder between the partially lifted cylinder and the plate. Powder flows out from the clearance between the cylinder and the plate surface as it forms an angle of repose along the entire periphery of the cylinder.

Now insert a thin rigid wire through the powder between the cylinder and the plate, then slowly rotate the wire horizontally in a clockwise direction (see Figure 1, left). As the wire is rotated, powder flows out the cylinder around its periphery. Note that the level of powder drops evenly across the whole surface area of the cylinder allowing for mass flow conditions.

It is important to note that the powder flows from the inside to the outside of the cylinder, where no powder pressure is exerted. Additionally, the slow movement of the wire helps move the material radially to the outside.

Table Feeder Designs

Several types of table feeders, utilizing the basic principles described above, are available. The following discussion examines the design and features of some of the table feeders on the market today:

Simple Table Feeder

A simple Table Feeder consists of a table, flat scraper, discharge port, and feed hopper. The table is driven by a drive shaft from below. The table slowly turns allowing material to be radially transported out to the periphery. As the material hits the scraper, it drops through the discharge port.

Figure 2.

Features of the Simple Table Feeder:

• The clearance between the hopper and the table is adjustable for flow control
• Adjustable table rotation speed provides further rate
• Simple design

Circular Feeder with Adjustable Ring

This feeder consists of a stationary bottom plate, flow adjusting ring (weir), central rotating vanes, peripheral rotating vanes encased by a cylindrical ring, drive shaft/speed reducer/motor, and a discharge port. The drive shaft runs up through the center of the bottom plate, and is attached to the peripheral vanes which consist of four evenly distributed blades (see Figure 3 below for reference).

Figure 3

Powder pressure is exerted inside the cylindrical portion of the feeder. Due to the natural collapse of the material being fed and the slow rotation of the central vanes, the material flows out to the periphery where no powder pressure is exerted. At the periphery, the rotation of the peripheral vanes carry the material to the discharge port.

The two parameters that control the discharge rate are shaft rotation and the height of the flow adjusting ring (weir). The rotation of the central vanes is controlled by a variable speed motor and inverter.

Features of the Circular Feeder: 

• Large enough openings that can exceed the critical arch of most materials.
• The slow rotating veins move the material radially from the center to the outlet ensuring "first in - first out" mass flow
• Wide range of discharge rates are possible by variable motor speed and the height of the flow adjusting ring
• The larger diameter opening between the holding bin and the feeder allows for large bin capacity.

Rotary Plough Feeder

The Rotary Plough Feeder consists of a flat circular table, internal cone, discharge arm and drive, mounted in a silo or bin. The table has a central discharge opening, which is covered by a large internal cone, with a clearance between the cone bottom and the table (see Figure 4, left).

During the filling of the silo a natural angle of repose is formed under the internal cone sealing the discharge opening. The discharge arm rotates around the central, vertical axis, and moves the bulk material into the discharge opening. The discharge arm undercuts the silo wall eliminating any material buildup on the walls. Controlled discharge of the bulk material is accomplished by varying the speed of the drive.

Features of Rotary Plough Feeder:

• Used for silos ranging from 0.5 to 6 meter diameter
• Suitable for continuous discharge of cohesive material with high moisture content

Flat Bottom Feeder

This feeder consists of an encased stationary bottom plate which mates to a conical mass flow hopper. A drive shaft is attached to a 4 blade conditioning agitator (driven from below). The flat plate has a discharge opening, offset to one side. A screw feeder is located directly under the discharge opening in the flat plate and is separately driven (see Fig. 5).

Product in the feed chamber and the feed screw is always moving. This prevents the formation of stable arches or material bridging, ensuring "first in - first out" flow.

Features of the Flat Bottom Feeder:

• Used in applications where the discharge rate required is moderate
• Feed chamber diameters in the range of 0.3-0.8 meters are typical.

Feeder Selection

The selection of the most appropriate feeder for a given application is not often an easy task. It is advisable to work with a vendor and conduct tests on various feeders with your material.

Among the factors to be considered in the feeder selection process are:

· Material flowability · Desired Repeatability · Space Constraints · Material Characteristics (particle size, bulk density, stickiness, hygroscopicity) · Volumetric vs. Gravimetric Control · Cost

Table 1 below is a general guide that could be helpful in selecting the right feeder for your application:

About our author:

Samer Habash is a granulation specialist for LCI Corporation.   He holds a B.S. degree in Chemical Engineering from Virginia Tech.

You can contact Samer at:

Mr. Samer Habash
LCI Corporation
P O. Box 16348
Charlotte, NC 28297
Telephone:  (704) 398-7874
Email:  shabash@lcicorp.com
Web site:  http://www.lcicorp.com/

Help others by posting your comments, suggestions and experiences with bulk solids feeding or any other materials handling concerns you may have on our On-Line Help Forum.  For past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.