In our last article, we discussed a means to measure solids flow properties.

By using the Jenike Shear Tester, we can simulate storage of solids in bins and hoppers. This test procedure allows us to simulate several conditions which effect material flowability, such as pressure, moisture content, particle size, etc. We can adjust the sample's moisture content and particle size while the direct shear tester allows us to simulate the effects of temperature and time of storage at rest.

Temperature

Some materials are sensitive to the temperature at which they are handled or stored. A solid's temperature environment can affect its cohesiveness. For example, many chemicals and plastic powders become more difficult to handle as their temperature rises. These type materials soften or form stronger cohesive bonds as their temperature increases. Usually increases in temperature are troublesome; however, freezing temperatures can cause the individual particles of materials like coal, sand, etc. to freeze together forming very strong bonds.

Some materials exhibit more strength at constant temperature, while others gain cohesive strength as the temperature changes during heating or cooling. An example of this phenomenon is soybean meal. If soybean meal is stored at temperatures of 90 °. Or less, it is usually not difficult flowing material.

If, however, the soybean meal is allowed to heat to 100 ° or greater (such as during summertime conditions), it behaves completely different. At 100 ° or greater, soybean meal becomes extremely cohesive and capable of bridging or arching over very large openings.

Time

Storage at rest is responsible for many of our industries flow problems.

Many materials are free flowing if handled in a continuous fashion. In other words, as the material is placed in a storage vessel, it is immediately discharged and not allowed to remain stagnant for extended periods of time. Unfortunately, it is not practical to design storage vessels for only continuous flow conditions.

Most solids are required to be stored at rest for some period of time.

Large silos are expected to store quantities of material at rest for some time, to be supplied to trucks, processes, etc. Whether they are stored just overnight or for a weekend, most solids are sensitive to time of storage at rest. Cohesive bonds become stronger as the materials remain stagnant. In time, some solids can gain tremendous strength which leads to bridging or ratholing over even very large outlets.

In our next article, we will discuss ways to minimize the effects of temperature and time of storage at rest on material flow properties. We will also present another article written by our guest author Lyn Bates.