Loss-in-Weight Screw Feeders: Screw Trough Agitation and Screw Alternatives

Many loss-in-weight feeders incorporate a screw as the feed device due to its wide feed range capability. Each selected feed device has a range of available screws with different diameters and pitches. Each feeder manufacturer provides a range of models capable of providing feed rates over the entire range of normal applications.
    
The feed device is comprised of several components. The components discussed in this article are shown in the schematic diagram.     

Screw Trough Agitation
The screw trough is a critical component of a screw feeder. It converges dimensionally from the inlet, attached to the extension hopper, and into the screw. It transitions the mass of dry ingredient contained in the feeder from the larger dimension extension hopper into the smaller dimension screw. Most dry ingredients (powders, granules, pellets, fibers, and flakes) do not flow reliably through converging transitions. There is friction between the particles and an interlocking tendency that restricts or stops the flow. When the flow stops it is called a "bridge." All dry ingredients have a minimum bridging dimension. Flow stops and a bridge forms at this cross section dimension. The reducing cross section dimensions in the screw trough eventually reach the bridging dimension. An agitation device is incorporated in the screw trough in order to prevent the bridge from forming.

Get more information or register for the International Powder & Bulk Solids Conference & Exhibition, May 3-5, 2016

There are two common types of agitation devices: Flexible walled/external paddle and internal stirring.

Flexible Walled/External Paddle: The screw trough is constructed of a flexible material, normally polyurethane, in a wedge shape on two sides and near vertical on the other two sides. Metal paddles contact the two wedged-shaped sides and move to massage these sides, pushing the sides in and out continuously. This massaging causes an unstable support for the dry ingredient particles. A restriction movement followed by a void movement causes any bridges that are starting to form to collapse. Gravity acts on the particles to move them down and eventually into the screw.

Internal Stirring: The screw trough is constructed of metal, normally stainless steel, and is curved in a constant radius to accommodate the close movement of rotating agitating blades. The agitator blades move through the dry particles on each side of the screw, which is positioned directly below the agitator blades. Movement is down into the screw on one side of the screw trough and up on the other side. This blade movement prevents bridging in the screw trough.
    
The shape of the screw trough is critical. A loss-in-weight feeder's main purpose is to accurately feed the ingredient at the desired set point. The weight feedback control requires that each screw flight is filled with the same weight of ingredient. Repeatable weight filling of each screw flight is achieved by a properly designed agitation device (flexible walled/external paddle massaged or internal stirring). Also, there must be sufficient screw flights exposed openly in the bottom of the screw trough so that complete and repeatable filling occurs before the ingredient is enclosed in the screw tube. The best screw trough shape is a wedge as opposed to a spherical or conical shape at the bottom of the screw trough in order to expose the required length of screw to fill properly.

The flexible walled/external paddle massaged screw trough has gained preference over the internal stirring screw trough. One important reason for this is that the movement of the flexing screw trough directly contacts and massages the ingredient in the screw trough on both sides of the wedge surfaces from the top inlet to the top of the screw.

Since the stirring agitator has a circular motion, there are two dead zones (no agitation) at the top of the screw trough. The screw trough is sized to reduce the volume of the dead zone. As a result the flexible walled/external paddle massaged screw trough typically has a larger volume and inlet dimension compared to the internal stirring screw trough. Since the extension hopper is mounted directly above the screw trough, the bottom dimension of the extension hopper is typically the same dimension as the inlet to the screw trough. Loss-in-weight feeder extension hoppers are commonly sized (rule of thumb) to contain four minutes of storage at the maximum feed rate/minimum bulk density of the ingredient. Often the extension hopper can be supplied with vertical sides. This results in floor space advantages compared to conical shaped extension hoppers. Also, there is no requirement for additional stirring agitation in the extension hopper since the sides are vertical (no converging).

Generally, a screw feed device with flexible walled/external paddle massaged screw trough is a first choice of loss-in-weight feeder selection. However, there are situations where the stirring agitator is required:

A) Cohesive ingredients: Cohesive ingredients form a solid mass when exposed to compression. Some are so sensitive that the force of gravity acting on their own head mass can cause them to solidify. These ingredients are difficult, sometimes impossible, to feed reliably without excessive maintenance. Testing is highly recommended at feeder manufacturers who offer several types of feed devices. Stirring agitators may have the best chance of being successful.

B) Ingredient temperature: Polyurethane becomes too soft for ingredient temperatures above 50°C requiring a metal screw trough.
 
Screw Selection
The screw is the component of the feeder that provides the “feeding.” The agitation ensures that the screw flights fill uniformly. Screws are selected to achieve maximum and minimum feed rates. The screw diameter and pitch are sized to achieve the feed rate at an optimal screw rotational speed. At too low a screw speed the “pulsing flow” may cause an undesired variation in the final product. Too high a screw speed may cause an incomplete filling of the screw. Below are discussed three common screw types.
Spiral Screw: The single spiral screw is the most common selection. It is “open” for the ingredient to freely flow into the flights of the screw. It has a smaller surface area that helps reduce adherence of the ingredient that changes the screw volumetric geometry. Ingredients are transferred with low shear. On the negative side, aerated ingredients can easily flood past the screw flights.

Blade Screw: The single blade screw offers more resistance to aerated ingredients however, because there is more surface area, adhesive ingredients can adhere reducing the volumetric geometry. Blade screws are often preferred for heavy powders with bulk densities over 80 lb/cu ft.
 
Twin Screw: Twin concave screws can provide superior performance for poor-flowing powders. Twin concave screws are solid and the screw flights have a low volume compared to spiral and blade screws. The screws are intermeshing and as such are self-wiping. The negative effects of some adhesive powders are reduced with the self-wiping of the two screws. Twin concave screws are co rotating. The powder flows around the flights of both screws. The shear on the powder is high, particularly in the screw tube. The screw tube is typically short to reduce negative effects of high shear.

Twin concave screws are ideally selected for powders with feed rates below 5 cu ft/hr. The cross section of the two screws is wider than an equivalent single screw reducing the tendency to bridge in the transition from the screw trough into the screw. Also, screw speeds for twin concave screws can be higher than single screws for the same feed rate because of the larger inlet and low volume of screw flight. Higher screw speeds reduce the negative effects of pulsation.

Conclusion
Reliable performance of loss-in-weight feeders begins with the selection of three critical components: the screw trough, the agitation, and the screw. It is necessary to understand the different types and the advantages/disadvantages of each. Witness testing the alternatives at the feeder manufacturer's facility using a sample of the difficult ingredient to be fed is a worthwhile function in the selection process.

    Walter Folkl has a degree in business management from Westminster College in New Wilmington, PA, and has over 30 years of experience in material handling, compounding, and feeding equipment. Andy Kovats has a B.Sc. degree in chemical engineering from Queen’s University in Kingston, Ontario, and an MBA from York University in Toronto. Both are sales managers at Brabender Technologie Inc., an international company specializing in feeding equipment. For more information, visit www.brabenderti.com.

Get more information or register for the International Powder & Bulk Solids Conference & Exhibition, May 3-5, 2016

For related articles, news, and equipment reviews, visit our Feeders Equipment Zone

Click here for a List of Feeder Manufacturers