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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:
- 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.
- 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.
- 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.
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