When using a hopper with a slotted outlet configuration, you are pretty much limited to using a screw feeder or belt feeder to withdraw your product. Screws and belts can be designed as volumetric devices, since they discharge a particular volume of material per unit time. This is done by control of belt speed or screw rpm.

A screw is preferable to a belt when handling certain bulk solids and you would typically use a screw feeder when:

• Require an enclosed device
• Headroom is restricted
• Handling dusty or toxic materials

Also, consider that if your product is friable, a screw may cause particle breakage.

Assuming that your hopper is designed to give you reliable mass flow, the screw feeder must be capable of maintaining the mass flow pattern and withdraw material uniformly over the entire outlet cross-sectional area. The key to this approach is to ensure that the feeder increases in capacity in the discharge direction. This is an extremely important statement that will be made several times during our discussion.

In this week’s article we will discuss the wrong way to design screw feeders. Take, for example, the constant pitch, constant diameter screw shown in Figure 1. Notice that the material is withdrawn preferentially from the back of the screw. The constant pitch flights do not allow any increase in capacity in the direction of feed. Therefore, the last flight fills with material and there is no more capacity to take material over the entire outlet length.

Any modifications that you made to your hopper to ensure mass flow are now rendered useless. The improperly designed screw feeder creates a preferential flow channel that enforces a funnel flow pattern with its resulting problems of ratholing, erratic flow, flooding, segregation, etc.

Remember the statement above, ensure that the feeder increases in capacity in the discharge direction. One way to provide this increase is to provide an increasing diameter screw flight, as shown in Figure 2. Notice that the screw flights increase in diameter from back to front.

You will also notice that the capacity of each flight increases in the discharge direction. We therefore have satisfied our increasing capacity requirement. Our screw should promote mass flow---WRONG! Consider that our solid may require a 12" wide slotted outlet to prevent arching. The small diameter section of a 12" screw is likely about 4" in diameter. The active flow channel through which material flows over a screw is approximately the screw diameter. It is highly likely that your material will arch over most of the feed section, because of its narrow diameter.

Another important consideration is that an increasing diameter screw and trough will be difficult to interface with your hopper outlet. The taper may impede flow along the hopper walls destroying the mass flow pattern required for your material.

There are ways to design a screw feeder to provide uniform flow along an outlet by increasing flight capacity over the entire feed section. We will discuss these approaches in our next article.

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.