Physical properties of agricultural materials

Physical properties of  agricultural materials

Properties of agricultural materials

Physical

     – Size and Shape – useful in handling, separation and storage

Mechanical

    – Hardness, friction coefficient – useful in size reduction and conveying operations

Thermal

    - Thermal Conductivity and diffusivity – useful in heating and cooling operations

Electrical

   - Conductivity and resistivity - separation and determination of moisture content

Physical Properties

·        Size and Shape – useful in handling,

·        Separation

·        Storage

Geometric characteristic used in describing different shapes

Round - approaches circular shape

Long - longitudinal length greater than the lateral.

Oblate - flattened stem end

Oblong - vertical diameter diameter greater than horizontal diameter

Conic - tapered towards the apex

Ovate - Egg shaped and broad at tail end

Regular - Horizontal section approaches a circle

Size and Sphericity

v The three linear dimensions of  seed namely major, intermediate and minor diameters are measured with a micro meter screw gauge

v The equivalent diameter and sphericity of seed are determined using the following equation proposed by Mohsenin (1986)

v Equivalent Diameter,

DΕ= (L X B X T) ^1/3

v Sphericity,

ψ = (LXBXT)^ 1/3/L

where:

v  L = Longest intercept, (Length) in mm;

v  B = Longest intercept normal to ‘L’ (Breadth) in mm;

v  T= Longest intercept normal to ‘L’ and ‘B’(Thickness) in mm.

Bulk Density

v The bulk density of seed at different moisture content is determined by filling a container of known self-weight and volume to the brim with seeds and weighing to determine the net weight of the seeds.

v The bulk density is calculated as

Bulk Density (g/cm3) = Weight of sample (g)/Volume   occupied                                                                  (cm3)

Porosity

v The porosity of an unconsolidated agricultural material can either be determined experimentally using the porosity tank method

v  Or theoretically from bulk and true densities of the material.

Porosity = (1 – (Bulk Density/ True Density) )/100

Thousand Kernel Weight

v For small seeds, 1000 kernels are weighed

v And a parameter known as the thousand-kernel weight (TKW) is determined.

v An electronic weighing balance having a sensitivity of 0.10g is used.

Fineness Modulus

v Fineness modulus is the sum of the weight fractions retained in each of the seven sieves divided by 100.

v It indicates the average distribution of fines and coarse in a feed.

Uniformity Index

v Uniformity index is a measure of the relative uniformity of the different sizes of particles in a ground feed sample.

v  It is expressed as a ratio of 3 figures which indicate the proportions of coarse, medium and fine particles in the feed.

Aerodynamics properties

v The properties include particle diameter, frontal area, terminal velocity and drag coefficients.

v The major characteristics used in separation are size, shape, density, surface texture, terminal velocity, electrical conductivity, colour and  resilience (Koya and Adekoya, 1994; Lucas and Olayanju, 2003).

v These determine what methods of cleaning can be used and their level of efficiency.

v Most cleaning operations used physical and aerodynamics properties of grain either singly or in some combination.

Terminal velocity

v Terminal velocity of seed, the velocity at which the seed remains in suspension, is measured by using a vertical air tunnel

v The value of CD is then used in an equation proposed by Kashayap and Pandya, 1986 for calculation of terminal velocity as:

Vt = √2Mg/ Ap Sf CD

where:

v M = Weight of particle (kg)

v AP = Projected area of seed, LW (m2)

v CD = Drag Coefficient

v δf = Density of fluid (air), (kg/m3) = 1.150

v N.B - Density and Viscosity of air were assumed constant at the temp and pressure when

v the experiment was carried out

v g = Acceleration due to gravity, m/s2 = 9.81

Isometric View of the Terminal Velocity Test Equipment

A – Manometer;                         D – Pitot Tube;

B – Manometer Box;            E – Wind Duct;

C – Rubber Hose;                 F – Electric Motor;

G – Blower                                                                                

Test Equipment

v Terminal velocity of seed, the velocity at which the seed remains in suspension, is  measured by using a vertical air tunnel.

v  It consists of the following components:

a frame, wind tunnel, plenum chamber, flow straightener, centrifugal blower, electric motor, pitot tubes and inclined manometer filled with coloured water.

Measurements of Terminal Velocity

v The test equipment is initially run without any seed while response of the measuring instrument:

v Pitot – static tube and manometer are observed.

v The seed sample is placed on a mosquito wire netting within the duct and is blown upwards using a centrifugal blower whose speed is controlled by a variable speed motor.