Compressibility-What Is It?
by Joseph Marinelli

In previous articles, Mass Flow Design Considerations, (Arching parts 1 & 2), we discussed cohesive arching that can occur in a bin or hopper. In others, we discussed wall friction and its effects on hopper wall slopes required to ensure mass flow (flow along the walls). An important factor in determining the above bin design parameters is the compressibility of the material to be handled.

A material’s compressibility is its bulk density/pressure relationship. In other words, the bulk density of your product as it undergoes the range of pressures or solids loading experienced due to flow in a bin. Sometimes, bulk density is expressed as loose density or packed density. However, there are more than two values of bulk density.

Figure 1.  Compression Test Instrument

Typically, a compressibility test is run in a tester such as shown in Figure 1. This tester is composed of a 2 ½” diameter cup which is loosely filled with product. A cover, dial gauge (used to measure height deflection), and weight hanger are applied. Weights are then individually, stacked on the weight hanger and the height deflection recorded. Because the cup’s cross-sectional area and material height at each weight level are determined, along with the weight of the product in the cup, we can calculate the bulk density, lb/cu ft, as a function of pressure applied. A materials bulk density can be expressed on a plot such as that shown in Fig. 2.

Figure 2.  Bulk Density/Pressure Relationship

Compressibility is affected by your material’s moisture content, particle size, elasticity and temperature. The information gathered during the compressibility test is used to determine the following:

1. Wall friction angle-bulk density values are used in the calculation of hopper wall pressures (the pressure exerted by the material normal to the hopper wall. These pressures are subsequently used to predict the wall friction angle that develops between the material and a sample of wall material.
2. Loads on walls---bulk density directly affects the pressures acting on the walls of a hopper. Forces acting within the hopper are more accurately determined by determining the bulk density/pressure relationship.
3. Loads on feeders---knowing a material’s range of density allows you to predict the solids loads that will be applied to any feeding device or gate attached to the outlet of the hopper.

An accurate determination of your products compressibility provides important information that can affect your hopper and feeder design criteria.

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.