Chapter One: Introduction 1.1. Background Teff [Eragosties

Chapter One: Introduction

1.1. Background

Teff [Eragosties tef(Zucc.) Trotter] is indigenous cereal crop in Ethiopia with largest

share of area (22.7%, 2.4 million hectors) under cereal cultivation and third (i.e. after

maize and wheat) in terms of grain production (16.3%, 24.4 million quintals) (Central

Statistical Agency, 2007) as cited in (Geremaw, 2007). Teff is used for both human

and animal feed in Africa. As a human food source, the seeds are normally ground

into flour. The flour is fermented and used to make injera, a sour-dough type of flat

bread. It is also used for making porridge and an alcoholic drink. Because the flour is

essentially gluten free, it is gaining popularity among those who suffer from gluten

allergies (Davison, 2003).

It is well known by Ethiopians and Eritreans for its superior nutritional quality. It con-

tains 11% protein, 73% complex carbohydrate, 2.8% ash, and 2.5% fat (Geremew,

2004). It is an excellent source of essential amino acids, especially lysine, the amino

acid that is most often deficient in grain foods. Teff contains more lysine than barley,

millet, and wheat and slightly less than rice and oats. Teff is also an excellent source

of fiber and iron, and has many times the amount of calcium, potassium and other es-

sential minerals found in an equal amount of other grains. When teff is used to make

injera, a short fermentation process allows the yeast to generate more vitamins (Davi-

son, 2003).

The color of the teff grains can be ivory, light tan to deep brown or dark reddish

brown purple, depending on the variety. There are three types of teff. White teff is the

preferred type but only grows in certain regions of Ethiopia. White teff grows only in

the Highlands of Ethiopia, requires the most rigorous growing conditions, and is the

most expensive type of teff. Red teff, the least expensive form and the least preferred

type, has the highest iron content. In persons living in areas of the country where con-

sumption of red teff is most prevalent, hemoglobin levels were found to be higher

with a decreased risk of anemia related to parasitic infection. The third main type of

teff, brown teff, has moderate iron content. Ethiopia is the considered the site of origin

of teff. Teff was domesticated in Ethiopia between 4000–1000 BC

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Physical and engineering properties are important in many problems associated with

the design of machines and the analysis of the behavior of the product during agricul-

tural process operations such as handling, planting, harvesting, threshing, cleaning,

sorting and drying. Solutions to problems in these processes involve knowledge of

their physical and engineering properties (Irtawange, 2000). Principal axial dimen-

sions of teff grains are useful in selecting sieve separators and in calculating power

during the milling process. They can also be used to calculate surface area and volume

of variety seeds which are important during modeling of grain drying, aeration, heat-

ing and cooling.

Bulk density, true density, and porosity (the ratio of inter granular space to the total

space occupied by the grain) can be useful in sizing grain hoppers and storage facili-

ties; they can affect the rate of heat and mass transfer f moisture during aeration and

drying processes. Grain bed with low porosity will have greater resistance to water

vapor escape during the drying process, which may lead to higher power to drive the

aeration fans. Cereal grain kernel densities have been of interest in breakage suscepti-

bility and hardness studies (Ghasemi Varnamkhasti et al., 2007).

Differences in grain moisture content can result in a significant variation in the pro-

cessing characteristics of the grain. Hence, the objective of this study was to deter -

mine physical properties of teff seeds and pressure drop during aeration system, as a

function of moisture content in the range of 12.01 to 25.01% (wet basis) which can

help out in the design of handling for teff production.

Undesirable materials such as light grains, weed seeds, chaff, plant leaves and stalks

can be removed with air flow, when grains, fruits and vegetables are mechanically

harvested. In addition, agricultural materials are routinely conveyed using air stream

in pneumatic conveyers. If these systems are not used properly, they could cause

problems. For example, in a combine harvester, if the air speed is low, the materials

would not be separated from each other and there will be extra foreign material with

the product. If air speed is high, the product will be exhausted along with extra

material and product loss will increase. For conveying agricultural material, the range

of proper air streams should be used. With low air speed, there is stagnation in the

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system, or with high air speed, there is not only energy lost, but also grains may be

broken (Khoshtaghaza et. al, 2006)

The proper air speed can be determined from aerodynamic properties of agricultural

materials. These properties are terminal velocity and drag coefficient. If an object is

dropped from a sufficient height, the force of gravity will accelerate it until the drag

force exerted by the air, balances the gravitational force. It will then fall at a constant

velocity called the terminal velocity (Mohsenin, 1970):

Where, M is mass of the object (kg), g is gravitational acceleration (m/s2), Cd is drag

coefficient, ρ is air density (kg/m3), A is projected area (m2), and Vt is terminal velocity

(m/s). From this equation, the drag coefficient of an object can be found from its

terminal velocity:

Usually, a horizontal wind tunnel is used to measure drag coefficient of large objects.

In this method, external parameters such as size and velocity are varied and values of

drag coefficient are obtained over a wide range of Reynolds number. But for small

particles (like grain seeds), the drag force cannot be measured directly by this method.

So drag coefficient of agricultural materials are calculated from their terminal velocity

(Eq.2) which is experimentally measured.

Carman (1996) measured the terminal velocity of lentil seeds at different moisture

contents by free fall method. From the top of a dropping tube at various heights, a

seed was allowed to fall. The duration of the fall was plotted as a function of vertical

distance. The slope of the linear portion of the distance versus time curve indicated

the terminal velocity of the seed. He found that as the moisture content of the lentil

seed increased, its terminal velocity also increased linearly.

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In another experimental method, a vertical wind tunnel is used for finding the

suspension velocities of the particles in an air stream. Bilanski and Lal (1965)

measured terminal velocities of wheat kernel and straw by a vertical wind tunnel.

The drag coefficient of grains, which is a function of Reynolds number, lay within the

limits of a sphere (0.44) and of a cylinder (1.0) depending on the shape of the grain. In

this thesis the terminal velocity of teff was determined in order to find the effects of

mass and moisture content of teff seeds on terminal velocity (Khoshtaghaza et. al,

2006).

Ensuring high quality of cereal grain bulk in storage is assuming an increasingly great

importance. There are various methods of cereal grain preservation, but the most fre-

quently applied is the method of active aeration. Aeration requires a mechanical venti-

lation system that can be used to manage grain temperatures by moving air with the

desired properties through the grain mass preventing moisture movement and accu-

mulation therefore maximizing grain storage life (Foster and McKenzie, 1979) as

cited at (Maier.D et al). In the course of that process, knowledge of air flow resistance

through grain deposit is highly significant for practical purposes [Kizun, Kusińska

2004]. Airflow resistance depends on the air velocity, grain deposit thickness, and on

the properties of the grain material (i.e. kind of grain, its bulk density, porosity, con-

tent of contamination, and moisture content) [Siebenmorgen et al. 1987, Sokhansanj

et al. 1990, Jayas and Muir 1991, Dairo and Ajibola 1994, Giner and Dienisienia

1996, Waszkiewicz 1999, Nimkar et al. 2002, Ray et al. 2002, Kusińska 2005].

The value of airfl ow resistance can be also strongly affected by the pouring density of

grain that may depend on its moisture content or on the method of silo charging

[Molenda et al. 2005a, Molenda et al. 2005b]. Airfl ow resistance is not a constant

feature and depends on the duration and conditions of grain storage [Szwed 2000].

Any particulate or granular material in storage undergoes compaction under the effect

of its own weight, which results in deformation of grains and has a detrimental effect

on the grain quality features [Szwed, Kusińska 2005]. That process may cause an in-

crease in airfl ow resistance in grain bedding,

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The quality of cereal grains in storage will deteriorate to an unacceptable level if

they are not kept dry and cool. To model the drying and cooling process, an accurate

knowledge of the airflow distribution is required. Design of efficient systems for dry-

ing and aeration of grains requires proper design of electric motor and compressor or

fan selection, which can only be achieved with information on airflow resistance of

the grains. The pressure drop through a bed of grain depends on the airflow rate,

method of filling, the surface area and shape, configuration of voids, the variability of

particle size, grain bed depth and crop moisture content (Shedd , 1953).

Shedd (1953) reported that foreign materials mixed with grain increases the resistance

to airflow if the foreign material is finer than the grain. Similar results were reported

for resistance to airflow of grains, seeds, other agricultural products and perforated

sheets (ASAE, 1992a) as cited at (Jekayinfa, 2006) . Similar studies were also re-

ported by Calderwood (1973) for milled rice, Kumar and Muir (1986) for wheat and

barley, Dairo and Ajibola (1994) for sesame seed, Al-Yahya and Moghazi (1998) for

barley grain and Jekayinfa (2001) for cocoa beans. The most commonly used model is

the one proposed by Shedd (1953) where he presented curves relating airflow and

pressure drop per unit depth of grain. Because of their simplicity and ease of handling,

Shedd’s curves are widely used by many designers to estimate pressuredrops in

grains. The curves were estimated based on the formula:

1

Where,

Q = airflow rate (m3 s-1 m-2)

Δ P= Pressure drop per unit depth (Pa/m) and

a, b = constants and are related to moisture content for some grains.

Because of the limitation of Eqn. (1) for being able to predict airflow resistance over

only a narrow range of airflow rate (Q = 0.00056 to 0.203 m3 s-1 m-2), Hukill and Ives

(1955) also proposed an empirical equation which accounts for the non-linear nature

of resistance to airflow data. The equation is of the form;

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2

Where,

C, d = constants for a particular grain.

Equation 2 is applicable over a wide airflow range of 0.01 to 2.0 m3 s-1 m-2.

It is important to determine the effects of some physical properties such as moisture

content, bulk density, bed depth and airflow rate on the resistance to airflow through

the bean seeds. The knowledge of these relationships would assist in the design of

dryers and aeration systems for teff seeds.

1.2. Statement of the Problem

Due to teff seed importance many research has be conducted by different researchers. Physicochemical Characteristics of Grain and Flour in 13 teff variety (Geremew, 2007), teff demonstration planting results for 2003 (Davison, 2003), observations of commercial teff production in Nevada during 2006 (Davison, 2003), aerodynamic properties of teff grain and straw material (Zewdu, 2007), moisture dependent physi-cal properties of teff seed (Zewdu and Solomon, 2007).

Proper designing of teff storage system and handling system as well as determining the factors affect these phenomena is crucial. Since these helps maintain grain quality and reduce its loss. The unnecessary loss of grain leads the society food insecurity. The majority of the population of Ethiopia is estimated to be food insecure and the per capital food production has dropped by 13% per annual against a 3% population growth (World Bank 1994). This production drop mainly due to post-harvest losses. Post-harvest losses include the following:

Harvesting loses Drying loses Transportation loses Storage loses (loses due to spoilage the result of absence aeration system) Handling loses (loses due to lack of proper conveying)

Therefore, in order to reduce the post harvest losses above, measures should be taken to minimized by improving storage system, applying proper handling and using appropriate mechanism for transportation. The solution to handling and storing high moisture grain is to either remove the excess moisture, or cool the grain mass. This can be achieved by passing air through the grain mass, the ability to dry grain or simply control temperature. Hence, in order to reduce storage and handling loses this thesis intended to develop and determine pressure drop in bulk aeration and pneumatic conveyor for Ethiopian indigenous grain teff, so that proper storage and handling design can be done by taking the following in to consideration:

the rate of airflow passing through the grain, the distribution effectiveness of air delivery and the thermodynamic properties of the air passing through the grain.

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1.3. Thesis Objective

General objective

This study is to address the whole aeration systems and pneumatic conveying system, i.e. from the feeding point to the receiving tank, including all typical components on it. Consequently, the final aim of the study is to formulate a reliable design technique, which can be used in design of aeration system of teff seeds.

Specific objective

The main specific objective of this study is:

a. to formulate for pressure drop determination of aeration system and pipe lines of pneumatic conveying of teff seeds

b. determine the effect of moisture content, particle size and air flow rate on re-sistance characteristics of teff grain

c. determine the pressure losses of blow tank feeding with top discharge facility d. fit the data obtained to selected models to predict air flow resistance through a

bed clean bulk teff grain and pneumatic transport pressure drop across a short straight section and a standard 90o bend.

Chapter Two: Review of Literatures

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In this literature review, gas-solid flow in pipes is described with help

of some suggested mechanisms that have been proposed in

literature. The bulk aeration system and drop will be reviewed. And

also pneumatic conveying system application, type and component

parts will be presented.

2.1. Bulk Storage Aeration System and Pressure Drop

The distribution of air in a grain storage structure has a significant

effect on the ecosystem of the stored grain mass. Most grain

storage structures have a non-uniform airflow distribution due to the

variations in material properties of the grain mass, the geometry of

the storage structure, and/or the design of the aeration system.

Airflow is generally assumed to be uniform in silos with fully

perforated floors and non-uniform in silos with aeration ducts, pads

or partially perforated floors. The airflow distribution can also be

non- uniform in a silo with a hopper bottom, peaked grain from

overfilling, inverted grain from partial unloading, and high fine

material concentration in the core of the grain mass (Bartosik and

Maier, 2006) as cited at (Garg. D., Maier, D.E.).

Knowledge of the flow field of air in the grain mass and the pressure

drop is essential to designing grain aeration systems. Additionally,

most of the ecosystem models developed to predict heat and mass

transfer in grain storage structures during aeration (Maier, 1992;

Chang et al., 1993; Montross, 1999) as cited at (Garg. D., Maier,

D.E.) assume airflow rate to be uniform through the grain mass.

Solving and integrating the non-uniform airflow distribution into

existing ecosystem models will result in a more accurate estimate of

the heat and mass transfer during storage.

Stored grain moisture levels are also influenced by the

temperature of the external environment. A warm or cold

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condition outside the storage cause convection currents inside the

grain storage and this causes overall moisture migration within the

grain mass. In cool weather conditions the outer of the storage will

have a downward convection current. The middle of the storage will

remain warm and when reaching the lower part of the storage the

current will then rise with the warmth. The end result is that there

can be moisture migration to accumulate under the surface near the

peak of the storage. Alternatively if grain is much cooler while in

storage than its external environment the reverse moisture

migration will occur. In this circumstance there will be rising

currents along the outside of the storage and falling currents

in the centre with moisture accumulation at the base of the

storage. This is called moisture migration (Figure 2.1) and prevented

by aeration (CBH Group et al, 2006).

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Figure 2.1. (a) When the external temperature is lower than grain

temperature

Figure 2.1 (b) when the ambient temperature is greater than the

grain mass temperature.

2.1.1. Aeration Systems

When looking at the management opportunities offered by

aeration it is important to understand the physical characteristics

of grain and how it behaves while in storage. Kernels of grain are

living organisms that ‘respire’ using the same biological process as

all other living things. They take in oxygen and combined with

carbohydrates, undergo the respiration reaction to produce carbon

dioxide, water and heat.

This living process is ongoing in grain and the surrounding

environment including insect and bacteria activity. At lower

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temperatures and moisture the biological processes of the grain

and the surrounding environment are slower and more dormant.

Figure 2.2. Grain is a living mass

Limiting the level of biological activity in stored grain has two

significant benefits:

• It helps maintain the grain kernels in a dormant but living

state ready for germination or milling.

• It minimizes bacterial and insect growth so the grain kernels

are not attacked from the outside.

The following conditions support low levels of biological activity in

stored grain:

• Low moisture content

• Lower temperature

Both low moisture and temperature combine for optimal long-term

storage. A higher temperature and moisture content in stored grain

causes a higher level of biological activity (grain, bacteria and

insect). As shown in the respiration equation (Figure 2.2) this

process of biological activity produces water and heat which

then further stimulates the biological activity of the grain storage.

For this reason grain can self-heat and rapidly deteriorate in quality

if moisture and temperature are not managed.

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2.1.2. Pressure drop in bulk storage

Air flow through beds composed of seed or grain particles is

frequently used in air pollution control processes (Dairo & Ajibola,

1994) and therefore expression is needed to predict pressure drop

across beds due to the resistance caused by the presence of

particles. The drop in pressure for flow through a bed of particles

provides convenient method for obtaining a measure of the external

area of particles (Coulson et al., 1991). Air flow resistance data of

cereal grains and oilseeds have been analyzed using many different

equations (Alagusundaram and Jayas, 1990) as cited at (Nalladaru

et al, 2002). The equation is given by Shedd (1953) and Hukill and

Ives (1955) as cited at (Agullo, 2005). Shedd suggested the use of

his equation for narrow ranges of air flow rates only (.005-0.3 m3/s-

m2) (Nalladurai, et al, 2002). Shedd’s equation is:

2.1

Hukill and Inves (1955) proposed an empirical equation to represent

the air flow resistance data over an air flow range of 0.01-2.0 m3/s-

m2. The Hukill and Ives equation is:

2.2

Both Shedd and the Hukill and Ives equations were used to analyze

experimental data obtained in the current study. The empirical

equations thus developed were used to predict pressure drops at

various air flow rates. Mean relative percent: error of prediction e in

% was calculating using

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2.3

Darcy showed that the average velocity, as measured over the

whole area of the bed, is directly proportional to the driving

pressure and inversely proportional to the thickness of the bed. This

relation is known as Darcy’s law (Coulson et at., 1991) and can be

written as

2.4

Where: P is the pressure drop across the bed in N/m2; l is the

thickness of the bed in m; v average velocity of the fluid, defined as

(1/A)(dV/dt) in m/sec; A is total cross sectional area of the bed in m2;

V is the volume in m3 of the fluid flowing in time t in sec; and K is

constant depending on the physical property of the bed and fluid.

The linear relation between the rate of flow and the pressure

difference indicates that the flow is streamline. This is expected

because the Reynolds number of the flow through the pore spaces

in the granular material is low, since both the velocity of the fluid

and the widths of the channels are normally small. The resistance to

flow then arises mainly from viscous drag. Therefore, equation (2.4)

can be expressed as:

2.5

Where: is the viscosity of the fluid in kg/ms; and B is termed the

permeability coefficient for the bed, and depends only the

properties of the bed (Coulson et at., 1991) as cited at (Tabak, et al,

2004). An aeration system consists of an air fan, air plenum,

aeration ducts (or perforated floors), grain column and anemometer

shown as figure 2.3 below. The fan may be either of centrifugal or

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axial type, depending on the static pressure and airflow rate

required by the storage unit (Maier & Montross, 1997).

Figure 2.3. Scheme of the apparatus for measuring airflow resis-

tance in grain column

2.1.3. Factors affecting airflow resistance of grains in bulk

One of the primary causes of non-uniform airflow distribution is

variation in the material properties of the grain mass. Airflow

resistance is a function of particle size and porosity of the grain.

Therefore, a number of material properties like distribution of fine

material, loading method, moisture content, and compaction cause

non-uniform airflow distribution. Airflow resistance increases when

silos are filled using spreaders as the amount of fines in the grain

increases. When silos are filled using a central fill conveyor or

gravity spout, fines tend to concentrate towards the center of the

silo and chaff moves towards the silo walls. This creates a region of

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Outlet of air

Anemometer

Grain column

Perforated floor

Centrifugal fan

Pressure transducer P

Air plenum

lower resistance near the walls and higher resistance in the center

of the silo. Orientation of the grain kernels can also cause a non-

uniform airflow distribution as the grain kernels are not exactly

spherical in shape. The non-spherical shape and random orientation

of the grain kernels result in airflow resistance that is different in

every direction. Grain also undergoes compaction during storage

due to vertical pressure exerted by the grain mass in the silo, which

is influenced by the grain type, bulk density, and coefficient of

friction between the grain and the wall, moisture content, angle of

internal friction and filling method (Thompson et al., 1987) as cited

at (Garg, et al). Giner and Denisienia (1996) showed that the

experimentally determined pressure drop decreased by up to 30 %

as moisture content increased from 12.8 % to 22.3 % in clean wheat

beds. Molenda et al. (2005) concluded that the effect of grain

orientation on airflow resistance was negligible, but the fill method

significantly affected the airflow resistance.

The prediction of airflow resistance is fundamental to the design of

efficient drying and aeration systems. For long term storage of

grains, they must be kept cool and dry. Drying or cooling is done by

forcing air through the grains to remove high moisture and

temperature gradients within the bulk. An important step in

designing drying and aeration system is sizing the fans. Fans that

are misplaced or sized incorrectly lead to failure of the entire

system (Sheley, 2000) as cited at Nalladurai et al 2001. Resistance

to airflow through a bed of grains and seeds usually expressed in

terms of pressure drop. The air flow –pressure drop relationship are

useful in the mathematical modeling of air-flow pressure patterns

and air flow distribution in stored grain masses (Brooker, 1961). The

resistance to the air flow of cereal grains and oil seeds has been

studied for 70 years (Stiniman et al., 1931) as cited at (Nalladurai et

al 2001). The pressure drop depends on a number of product and

environmental factors such as: airflow rate, bed depth, bulk density,

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presence of amount, size and distribution of foreign materials, grain

moisture content and surface and shape characteristics of grain

(Dairo& Ajibola, 1994).

2.2. Pneumatic Conveying System and Pressure Drop

2.2.1. Pneumatic Conveying System and Its Application

Pneumatic conveying is a common in-plant transport system for bulk material which

has been used successfully in the chemical (soap powders, detergents), food (sugar,

flour), cosmetics (talc, face powder) or energy (coal and ash) industries. The major

advantages of pneumatic conveying systems are their enclosed nature, flexibility and

easy automation. A general pneumatic conveying system is shown in figure 2.4.

Figure 2.4: Pneumatic Conveying Plant

There are two basic types of pneumatic conveying; dilute phase (or suspension flow)

and dense phase (figure 2.5 and 2.6) where the predominant flow mechanism is a non

suspension mode of flow. While dilute phase systems are generally the most reliable

and offer the greatest flexibility in design, the relatively high conveying velocities

(generally in excess of 15 m/s) lead to significant operational problems including par-

ticle attrition and erosive wear of pipelines.

The choice of whether to design for dilute phase conditions or for dense phase can be

a difficult choice for the designer. In general, dilute phase conveying has a greater tol-

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erance and can be safer with regard to reliability and the sensitivity of the system to

changes in material properties. However, for materials that is erosive or abrasive and

for materials that are fragile, dilute phase systems are generally not suitable. Lowering

conveying velocities can have a very significant effect in reducing the unwanted side

effects of product degradation (or attrition) and erosive wear of the system. In these

situations, there is a strong justification for using dense phase conveying.

Figure 2.5. Schematic diagram of typical dilute phase conveying system

Figure 2.6. Schematic diagram of typical dense phase conveying system

2.2.2. Advantages and Limitations of Pneumatic Conveying

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In recent years, pneumatic transport systems are being used much more often,

acquiring market sectors, in which other types of transport were typically used,

especially in the fields of bulk solids handling and processing.

A well designed pneumatic conveying system is often more practical and economical

method of transporting materials from one point to another than alternative

mechanical system (belt conveyor, screw conveyors, vibrating conveyors, drag

conveyors and other methodologies) because of key reasons:

By pneumatic conveying system, one can reduce the maintenance and

manpower cost.

It is possible to move material vertically by simply installing a vertical section

of pipe section pipe with sufficiently high velocity of the gas to transport solid.

Pneumatic systems are totally enclosed and if can operate entirely without

moving parts coming into contact with conveyed material. Being enclosed

these are relatively clean, more environmentally acceptable and simple to

maintain

They are flexible in terms of routing and expansion. A pneumatic system can

convey a product any place a pipe line can run.

On the other hand, high power consumption, wear and abrasion of materials and

equipment and the limited conveying distance (1km maximum due to the economical

purpose) are the disadvantages of pneumatic conveying.

2.2.3. Major Components of Typical Pneumatic Conveying

There are a number of components in a pneumatic conveying plant, which are

required to achieve the particular duty condition. Usually, a typical conveying system

comprises different zones where distinct operations are carried out. In each of these

zones, some specialized equipments are required for successful operation of the plant.

Any pneumatic conveying system usually consists of four major components

(Chandana, 2005):

1. Conveying gas supply :– to provide the necessary energy to the conveying gas,

various types of compressors, fans, blowers and vacuum pumps are used as a

primer mover

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2. Feeding mechanism: – to feed the solid to the conveying line, a feeding

mechanism such as rotary valve, screw feeder, etc, is used.

3. Conveying line: - this consists of all straight pipe lines of horizontal and/or

vertical sections, bends and other auxiliary components such as valve.

4. Separation equipment:- at the end of the conveying line, solid has to be

separated from the gas stream in which it has been transported. For this

purpose, cyclones, bag, filters, electrostatic precipitators are usually are

usually used in the separation zone.

2.2.4. Pneumatic Conveying System Pressure Drop

Certain minimum conveying velocity must be maintained to keep the material in sus-

pension and flowing. A velocity that is too small will impede the material conveying

capability of the system and unnecessary high velocities will increase pressure drop

and therefore, additional energy will be required to overcome the resistance. The con-

veying velocity and hence air flow rate is greatly influenced by material characteris-

tics. Particle sizes, size distribution, mean particle size and particle density; all have

an effect on minimum conveying velocity, pressure drop, air flow etc. Properties such

as moisture content, cohesiveness and adhesiveness may cause flow problem through

the vessel and valves. It is not just different materials! Different grades of

exactly the same material can exhibit totally different performances.

Thus a conveying system designed for one material may be totally

unsuitable for another. For practical purpose, a conservative design

approach is to keep the ratio of material to air below 1:2 proportions

by mass (Bhetia. A).

2.2.5. Pressure Drop Determination

The basic step in design of pneumatic conveying system is the

correct estimation of total pressure drop in the conveying line and is

estimated by either summing the individual of contributions of air

and solid by using empirical development equations. The first

method considers contribution associated with wall friction, particle

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friction, particle acceleration and support of particles. Cramp and

Priesty (1924) proposed this approach and it has been followed by

Vogt and White (1948), Hariu and Molstad (1949), Pinkas and Troy

(1952) and may others either in horizontal or vertical pneumatic

conveying as cited at (Raheman, et al, 2001). In general, the

pressure drop in horizontal pneumatic conveying line is represented

by the following equations:

2.3

The performance of a pneumatic conveying system, in terms of

achieving a given material flow rate, depends essentially on the

system resistance. Higher the system resistance, higher will be the

pressure drop in the system or higher will be the static pressure of

fan (Bhetia. A). The usual assumption of pressure drop

determination in gas-solid two-phase flow is to consider the total

pressure drop as being comprised of two hypothetical pressure drop

components, i.e., due to the flowing gas alone and the additional

pressure drop attributable to the solid particles. In this classic

approach, the pressure loss of air remains constant with respect to

different loadings and qualities of the conveyed materials. Under

steady-state operation, the acceleration losses drop out and

equation (2.3) reduces to

2.4

2.2.5.1. ‘Air-Only’ Pressure Drop

The procedure involved in the determination of the air only pressure

drop component, is quite straightforward, since single-phase flow is

well established with reliable mathematical models such as Darcy-

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Weisbach’s. The system resistance (pipe wall friction per unit area)

can be estimated using following equation:

2.5

If we carry energy balance over differential length, L1 and L2, of a

straight horizontal pipe diameter D, the total force required to

overcome friction drag must be supplied by a pressure force giving

rise to a pressure drop ∆P along the length L1 and L2. The pressure drop

force is

∆P*Area of the pipe

The friction force (force/area)*wall area of pipe

So from equation (2.5)

Therefore equating pressure equation and friction force

From above expression pressure drop per unit length due to air will be as follows:

2.6

Thus, we can see that the pressure drop is

Directly proportional to velocity squared

Directly proportional to the conveying distance i.e. length of

the pipe and

Inversely proportional to diameter of the pipe (Bhatia. A)

21 | P a g e

2.2.5.2. Pressure Drop Due to Solid Particles

Although the possibility of the existence of a unique mathematical model to determine

the pressure drop component due to the presence of dispersed solid particles is very

low, because of the complex nature of two-phase gas solid flow in pipes, many

correlating equations have been proposed by various authors in different publications.

Some of these methods, which show comparatively better agreements with

experimental consequences. Pressure drop per unit length due to solid friction is given

by:

2.7

Chapter Three: Mathematical Modeling

3.1. Bulk Aeration Mathematical Modeling

The relationship between the pressure gradient and velocity of the air through the

grain mass must be known in order to estimate the airflow distribution. The air veloc-

ity used in equations describing this relationship is the superficial velocity, which is

calculated as the volume flow rate divided by the cross-sectional area of the flow.

Shedd (1953) plotted data for numerous grains with a wide range of airflow rates and

proposed a relationship that has been widely used by engineers for the design of aera-

tion systems and the sizing and selection of fans. Unfortunately, Shedd’s expression is

empirical in nature and contains no information about the properties of the product be-

ing aerated, or the fluid flowing through the product. Several researchers developed

expressions for pressure drop through packed beds that have some physical basis.

22 | P a g e

Darcy showed that the velocity of the fluid flowing through a porous medium is di-

rectly proportional to the pressure drop (Darcy’s Law). According to Reynolds

(1900), the total energy loss for a fluid flow is the sum of the viscous and kinetic en-

ergy losses. At low airflow rates (laminar flow), the resistance offered by friction to

the motion of the fluid is directly proportional to the viscosity and velocity of the

fluid. Darcy’s law holds for low flow rates where viscous forces predominate and in-

ertial forces can be neglected. Ergun’s, equation can be derived by starting with the

relationship between pressure drop due to friction, ∆Pf and the fanning friction, for the

laminar and turbulent flow.

3.1

By setting the values for the superficial velocity, v’ and hydraulic radius, rH equation

for pressure drop can be develop.

3.2

3.3

3.4

Experimental data concluded that 3fp =1.75. The relationship between pressure drop

and fluid velocity was interest of many of experimenters who tried to find relation-

ships relating the factors. Blake-Kozeny, Scientists, developed an expression that cor-

related the pressure drop to low (laminar) fluid flow rates (Ergun, 1952).

3.5

Where de, L, , ∆P, k1, gc, u, are particle equivalent diameter, the height of the bed,

fluid viscosity, porosity, the pressure drop, the coefficient viscosity energy, the accel-

eration of gravity at sea level, the fluid velocity.

23 | P a g e

At high airflow rates (turbulent flow), pressure loss is proportional to the product of

the air density and the square of the fluid velocity as viscous forces then become rela-

tively negligible. Ergun (1952) presented an equation for resistance to fluid flow

based on the Reynolds Theory. According to this equation, the total energy loss in a

packed bed should be treated as the sum of the viscous and kinetic energy losses. He

examined the equation from the point of view of its dependence upon flow rate, prop-

erties of fluids (µ - viscosity and – density), and porosity (), orientation, particle

diameter (de), shape, and surface of the granular solids:

3.6

In the above equation, k2 is the kinetic energy and is the density of the fluid in the

column. Prior to 1952, Sebri Ergun and A.A Orning tried to focus their research on

the idea that the total energy lost in a packed bed could be treated as the sum of vis-

cous and kinetic energy losses. It was believed that the transition for viscous to kinetic

dominance was smooth (Ergun, 1952, pg.89). Therefore, a relationship was developed

that related the pressure drop in packed bed over the entire range of velocities when

the flow rate the properties of the fluid, the fractional void volume, shape, size and the

surface of the granular solids were known.

3.7

Finally, Ergun completed the expression by finding values for k1 and k2 and combin-

ing all previous equations.

3.8

The sphericity of the packing material () can usually be ignored when assuming per-

fect symmetry of packing. It is therefore presumed to be value of 1. When packing in

the bed are all spherical, the mixture can be modeled as crystalline, which involves an

ordinarily structured arrangement of the particles.

24 | P a g e

3.1.2. Pneumatic Conveying System Mathematical Modeling

In most pneumatic conveying, both solids flux and gas phase Reynolds number are

high and flow is usually turbulent. The effects of particle collisions and particle phase

turbulence must be considered in any mathematical model for simulating gas particle

flows. The shape and size of individual particles of material being transported may

differ and parameters characterizing shape and size must therefore be determined. The

characteristic relationships for pneumatic transport are generally measured in model

installations. The results are applicable to equipment of other dimensions of if similar-

ity criteria are known and the measurement results are plotted as the function of a sim-

ilarity characteristic. The equivalent diameter of a particle (grain

) may be calculated from volume using the equation:

Where a, b, and c are major diameter, intermediate diameter and minor diameter re-

spectively.

3.9

The shape coefficient expresses as the actual surface area as compared to the surface

area calculated using equivalent diameter

25 | P a g e

3.10

In the case of plug transport, when the tube is filled completely by material, the hy-

draulic diameter of bulk material, calculated as the ratio of the pore volume between

the grains to their surface area, is the characteristics parameter introducing the concept

of bulk porosity,

3.11

The hydraulic diameter may expressed as

3.12

Geometrical similarity is ensured by the condition

3.13

In addition, the shapes of the tube and material transported must also be similar. Simi-

larity of operation condition is ensured by adapting the specific load, i.e.,

3.14

Stationary flow for the transport medium is characterized by the Reynolds number

(ratio of the force of friction and inertia):

3.15

The rato of inertia and gravitational forces gives the Froud number:

26 | P a g e

3.16

Chapter Four: Materials and methods

4.1. Sample Preparation

Samples that their moisture content should be raised were moistened with a calculated

quantity of distilled water by using the following Eq. (1) and conditioned to raise their

moisture content to the desired two different levels (Coskun et al., 2005):

1

Where Q is the mass of added water (kg),

Wi is the initial mass of the sample (kg),

Mi is the initial moisture content of the sample (%, w.b.) and

Mf is the final moisture content of the sample (%, w.b.).

After making four levels of moisture contents, for selected teff seeds, the samples

were poured in polyethylene bags and the bags sealed tightly. The samples were kept

at 5 ºC in a refrigerator for a week to enable the moisture to distribute uniformly

throughout the sample. Before starting of each test, the required quantities of the sam-

27 | P a g e

ples were taken out of the refrigerator and allowed to warm up to the room tempera-

ture for about 2 h. The rewetting technique to attain the desired moisture content in

kernel and grain has frequently been used (Coskun et al., 2005; Garnayak et al., 2008;

Pradhan et al., 2008). All the physical properties and pressure drop of the grains were

determined at moisture levels of 12.01, 16.08, 20.71 and 25.01 % (w.b.).

4.2. Determination of engineering property tef

The seeds were cleaned manually to remove all foreign matter such as dust, dirt,

stones and chaff. To determine the size of the seeds, samples seeds were randomly se-

lected and their linear dimensions namely major dimension (length, L) and the minor

dimension (width, W) (Zewdu and Solomon, 2007) were measured using a digital

vernier caliper to an accuracy of 0.02 mm. The mass of seeds also measured by an

electronic digital balance to an accuracy of 0.0001 g. To evaluate 1000 grain seed

mass, randomly selected seeds from the bulk were weighed five times and averaged.

The moisture content of seed 1000 samples were determined using oven drying

method at temperature of 1052 for 24 hour (ASAE, 1994). Geometric mean diame-

ter (Dg), sphericity (), arithmetic mean diameter (Da) values were found using the

following formulae (Mohsenin, 1970; Sitkei, 1986).

4.1

4.2

4.3

The bulk density (ρb) was determined with a weight per hectoliter tester which would

calibrate in kg per hectoliter. The grains would not compacted during the test (Jain &

28 | P a g e

Bal, 1997 et al). The true density (ρt) and volume (V) were determined by using the

toluene displacement method (Mohsenin, 1970; Sitkei, 1986). The porosity was calcu-

lated from the following equation (Mohsenin, 1970):

4.4

The terminal velocities of the seeds are measured using a wind column. A roots

blower (rotary positive displacement blower) was used to develop air velocities. Air

flow is regulated by adjusting the blower speed by a motor with frequency converter.

For each test, a sample was dropped into the air stream from the top of the air column.

Air was blown upward to suspend the seed in the air stream. The air velocity near the

location of the seed suspension was measured by digital anemometer. The terminal

velocity for each of the seed was measured ten times and the average terminal veloc-

ity for each seed was determined. The dimensionless drag coefficient characterizes the

interaction between seed and air flow and is expressed by the formula (Mohsenin,

1970; Sitkei, 1986).

4.5

4.3. Determination of pressure drop

4.3.1. Experiment Apparatus

Figure 3.1 shows a diagram of equipment. It consists of fan, control valve, air duct, air

plenum chamber, a test column, a pressure drop measuring system and air flow mea-

surement system. The centrifugal fan was used to create air pressure and is preceded

by manually operated control valve (not seen on the picture). The manometer was in-

stalled next to control valve to measure pressure created by centrifugal fan. To avoid

propagating vibrations, a flexible rubber tube was used to connect the air plenum. The

flow was conducted to the inlet of air plenum to create a uniform velocity profile. To

29 | P a g e

assist this effect, the air was passed through a perforated sheet metal or mesh with di-

ameter 0.40mm of holes between the air plenum and test column.

The test column consists of a cylinder of 1.0m long and 0.1m of internal diameter.

The pressure measuring taps was installed at four points with interval of 0.15m above

the air plenum to compare pressure drops. After the column, the air was flow towards

an outlet with diameter of 0.05m. This outlet air tube was used to measure air veloci -

ties. To measure air velocity digital-hot anemometer was used in order to minimize

errors.

Outlet of air

Anemometer

Grain column

Perforated floor

Centrifugal fan

Pressure transducer P

Air plenum

Fig.3.1. Aeration system experimental apparatus

4.3.2. Determination of Pressure drops

To determine pressure drops the test column will be filled with teff seeds to the depth

of 0.60m at a given level of moisture content. Two method of filling was used: dense

and loose. Pressure drops of dense and loose fill method was compared. The first

0.15m of test column above air plenum chamber was used for straightening the air

(Agulo et al, 2005). Pressure drop measurement was started from the air tap (T1) at a

height of 0.15m above air plenum. This pressure was taken as a reference tap and

30 | P a g e

pressure drops for subsequent taps was the differences in static pressure between this

tap and the subsequent above the air plenum.

Pressure in a Column of Liquid or Gas

Invention of the U-tube manometer allowed the early investigators into fluid mechanics to

confirm that pressure was directly related to the sum total of the forces acting on a

surface. If you were to stand on the seashore, the pressure on you would be the weight of

the air column directly above you. That pressure has been given the name of ‘one

atmosphere’. If you were to dive to a depth of 10 meters (about 32 feet) there would now

be an added pressure on you of the weight of water above plus the weight of the air

column. By international agreement (a convention) ‘absolute’ pressure includes the

pressure of the column of air whereas ‘gauge’ pressure does not. Gauge pressure is the

pressure showing on a pressure indicator dial and is one atmosphere less than absolute

pressure. The pressure of 1-m depth of water is found from the formula –

Pressure = Density x Gravity x Height of liquid column P = ρ gh

The difference of the level of the liquid on both sides of the U tube, the unknown

pressure P for gas fluid C can be determine with fluid static formula as below,

31 | P a g e

(1)

The gauge pressure of P can be determinate with

(2)

The unit is a Pascal. Gravity has the value of 9.8 m/sec2 at sea level. For simplicity of

multiplication the value 10 m/sec2 will be used. The density for water is 1,000 kg per

cubic meter at 20 oC. Putting all the know values back into the pressure equation gives –

P = ρ g h = 1000 kg/m3 x 10 m/sec2 x 1 m = 10,000 Pa = 10 kPa

The calculation shows that 1-meter of water is equal to about 10 kPa, which means 30

meters of water produces a pressure of nearly 300 kPa. One atmosphere of air pressure at

sea level is 101 kPa. This means the pressure at 30 meters depth below sea level is 300

kPa gauge pressure or about 400 kPa absolute pressures. The same formula can be used to

calculate negative, or vacuum, pressures.

32 | P a g e

The pressure difference in a inclined u-tube can be expressed as

ΔP = ghsin( ) B ρ θ Eq (3)

where

θ = angle of column relative the horizontal plane

How to Use a U-Tube Manometer

Figure No. 1 shows three manometers open to atmosphere. The left one has the same

pressure in both legs and the liquid levels are the same on both sides. The U-tube in the

center shows a pressure applied to the left leg of 100 kPa. The water level in the left leg

has gone down and the level in the right leg has gone up. The difference in the height of

water between the two legs is 10 meters. Since the liquid is water, each meter height

represents 10 kPa and a 10 meter high water column represents 100 kPa gauge pressure.

The remaining U-tube shows a pressure of 100 kPa as well but this time mercury is used

in the tube. The height of mercury is now 750 mm. The density of mercury is 13.6 times

that of water. Because mercury is so much heavier than water the same pressure raises a

correspondingly lower column of liquid.

33 | P a g e

Figure No. 1 U-tube Manometers with Water and Mercury

Figure No. 1 U-tube Manometers with Water and Mercury If a manometer were used to

measure a vacuum the column of liquid would be drawn up toward the vacuum and the

difference in the height of liquid between the two legs would be a measure of the vacuum

pressure below atmospheric pressure.

Making a U-Tube Manometer

To make a U-tube manometer requires a clear plastic tube mounted in the shape of a ‘U’

onto a board marked with a graduated scale. The pressure to be measured determines the

selection of the liquid used in the tube. The U-tube liquid’s density and the pressure being

measured determine the height of the liquid column and the corresponding height of the

backing-board.

4.4. Determination of power requirement and capacity

To determine the conveying power requirement, the voltage, the current, and the

power factor drawn by the electric motor for the roots blower was measured by using

voltmeter, ampere meter, and power factor meter, respectively. Conveying capacity

34 | P a g e

depends on mass of seed, air moved, and the physical characteristics of the seeds

(Hellevang, 1985).

Chapter Five: Results and discussion

5.1. Determination of engineering properties of teff seeds

The teff seeds were cleaned and all foreign materials such as dust, dirt, stones and

chaff were removed. Engineering properties of the teff seeds were determined using

different level of moisture contents. Different moisture contents of samples were

determined using oven drying method at temperature of 105 2 for 24 hours (ASE,

1994).

The size of seeds at different moisture contents were determined by randomly selected

samples and their linear dimensions major dimension (length) and minor dimension

(width) were measured using a digital vernier caliper to an accuracy of 0.02 mm. The

35 | P a g e

thousand grain of teff seeds were measure by electronic digital balance to accuracy of

0.001 gm the teff seeds selected from the bulk were weighed five times.

The level zero and the moisture content average result was found on wet basis was

12.01% and the engineering properties at this moisture content level were found as

follows.

Major dimension (length) 1.05 mm and minor dimension (width) 0.62 mm, thousand

grains mass was 0.322 gm.

Geometric mean diameter (Dg), sphericity () and mean diametric (Da) values were

found using the following formula (Mohsenin, 1970, Sitikei, 1986).

The equivalent mean diameter (de) at the level zero moisture content was determined

using the procedure adapted by the Kaleemullah and Gunnasker (2002) and Socilik et

al. (2003), by considering its effective diameter of the thousand grain mass and true

density as:

The bulk density (b) was determined with weight per hectoliter tester which would

calibrate in kg per hectoliter. The grain was not compacted during the test (Jain & Bal,

et al. 1997). The result at the zero level moisture content of bulk density was 795.53

Kg/m3.

36 | P a g e

The true density (t) and volume were determined by using the toluene displacement

method (Mohsenin, 1970, Sitikie, 1986). The result at the zero level moisture content

was 1320.5 Kg/m3. The porosity was calculated from the following equation

(Mohsenin, 1970):

The same procedure was followed for the other remains moisture content and the

following results were found.

The level one moisture content average result found on wet basis was 16.08 and the

engineering properties at this moisture content level were found as follows. The two

dimension major dimension (length) and minor dimension (width) were 1.07 mm and

0.63 mm respectively and average thousand grains mass was 0.328 gm.



The equivalent mean diameter (de) at the level one moisture content was determined



density as:

37 | P a g e



et al. 1997). The result at the level one moisture content of bulk density was 791.41

Kg/m3.


method (Mohsenin, 1970, Sitikie, 1986). The result at the level one moisture content


(Mohsenin, 1970):

The level one moisture content average result found on wet basis was 20.71% and

the engineering properties at this moisture content level were found as follows. The

two dimension major dimension (length) and minor dimension (width) were 1.09 mm

and 0.65 mm respectively and average thousand grains mass was 0.330 gm.



38 | P a g e

The equivalent mean diameter (de) at the level two moisture content was determined



density as:



et al. 1997). The result at the level two moisture content of bulk density was 740.88

Kg/m3.


method (Mohsenin, 1970, Sitikie, 1986). The result at the level two moisture content


(Mohsenin, 1970):

The level one moisture content average result found on wet basis was 25.01% and

the engineering properties at this moisture content level were found as follows. The

two dimension major dimension (length) and minor dimension (width) were 1.13 mm

and 0.68 mm respectively and average thousand grains mass was 0.351 gm.



39 | P a g e

The equivalent mean diameter (de) at the level three moisture content was determined



density as:



et al. 1997). The result at the zero level moisture content of bulk density was 740.88

Kg/m3.


method (Mohsenin, 1970, Sitikie, 1986). The result at the level three moisture content


(Mohsenin, 1970):

Table 1. Effect of moisture content on engineering properties of teff seeds

Moisture Engineering property of the teff seeds

Content

(%)

Length

(mm)

Width

(mm)

Geo

dia

(mm)

Equi

dia.

(mm)

M1000

(gm)

Sphericity

(%)

True

density

(Kg/m3)

Bulk

density

(Kg/m3)

Porosity

(%)

12.01 1.05 0.62 0.74 0.775 0.322 70.38 1324.03 795.53 39.92

40 | P a g e

16.08 1.07 0.63 0.75 0.78 0.328 70.24 1319.23 791.41 39.47

20.71 1.09 0.65 0.77 0.79 0.33 70.84 1272.54 741.88 40.06

25.01 1.13 0.68 0.78 0.85 0.351 71.27 1094.2 630.63 41.13

5.1.1. Effect of moisture content on teff seed linear dimensions

Mean values of the size dimensions of teff seeds at different moisture contents are

presented in Table 1. As also seen in Table 1, all the dimensions increased with in-

crease of moisture content within the moisture range of 12.01-25.01% (w.b.). The re-

lationships between the axial dimensions (L and W) and moisture content of grain (M)

can be represented by the regression equations:

With values for R2= 0.9661

With values for R2= 0.9569

By means of which the regression relationship was determined. This results show that

there is an important and positive relationship between moisture content of grain and

axial dimensions of grain.

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

10 12 14 16 18 20 22 24 26

Moisture content, %, w.b.

Lin

ear

dim

en

sio

ns,

mm

Length

Width

Linear(Length)Linear(Width)

41 | P a g e

Fig.5.1. Effect of moisture content on teff seed linear dimensions

5.1.2. Effect of moisture content on teff seeds diameters

The variation of geometric mean diameter of teff seeds, with moisture content is pre-

sented in (fig ). This show that the geometric mean diameter of teff seeds increased

from 0.74 to 0.78 mm in the moisture content range of 12.01 to 25.01% (w.b). A lin-

ear relationship between geometric mean diameter and moisture content was obtained

and can be expressed using equation:

With the coefficient of determination R2 of 0.985, Where Dg was geometer diameter

of teff seeds (mm) and M was moisture content at wet basis (w.b).

The relationship between equivalent diameter and moisture content of grain is shown

in Fig. The equivalent diameter of the teff seeds increased from 0.775 to 0.85 mm de-

pending on the increase of moisture content. The relationship between diameter and

moisture content can be represented by the following equation (R2=0.767):

42 | P a g e

0.72

0.74

0.76

0.78

0.8

0.82

0.84

0.86

10 15 20 25

Moisture content, %, w.b

Dia

mete

rs,

mm

Equi dia

Geometric M dia

Fig.5.2. Effect of moisture content on teff seed diameters

5.1.3. Effect of moisture content on teff seed thousand grains mass

The variation of one thousand teff seeds grains mass weight, M1000, with moisture con-

tent is presented in (fig ). This show that the one thousand teff seeds weight increased

from 0.322 to 0.351 gm in the moisture content range of 12.01 to 25.01% (w.b). A lin-

ear relationship between M1000 and moisture content was obtained and can be ex-

pressed using equation:

With the coefficient of determination R2 of 0.8254, Where M1000 was one thousand

teff seeds weight (gm) and M was moisture content at wet basis (w.b).

43 | P a g e

0.315

0.32

0.325

0.33

0.335

0.34

0.345

0.35

0.355

10 15 20 25Moisture content , % w.b.

M10

00,

gm

Fig.5.3. Effect of moisture content on teff seed thousand grain mass

5.1.4. Effect of moisture content on teff seed Sphericity

The relationship between sphericity and moisture content of grain is shown in Fig.

The sphericity of the teff seeds first decreased from 70.38 to 70.24% then increased

from 70.24 to 71.27% depending on the increase of moisture content. The relationship

between sphericity and moisture content can be represented by the following equation

(R2=0.8298):

70

70.2

70.4

70.6

70.8

71

71.2

71.4

10 15 20 25

Moisture content, % w.b

Sp

her

icit

y, %

Fig.5.4. Effect of moisture content on teff seed sphericity

44 | P a g e

5.1.5. Effect of moisture content on teff seed Densities

The true density of teff seeds at 12.01, 16.08, 20.71 and 25.01% (w.b) moisture con-

tents the values varied from 1324.23 to 1094.20 Kg/m3 (fig ). The relationship be-

tween true density of grain of with moisture content was as follows:

With values for R2 of 0.779

Where M is moisture content at wet basis (w.b).

The values of bulk density of teff seeds for different moisture contents level varied

from 795.53 to 630.63 Kg/m3 (fig ). The bulk density of grains was found to be linear

the following relationship with moisture content:

With values for R2 of 0.8443

The regression equations indicate that the increase of moisture content caused a de-

cline both in bulk density and in true density. It was also observed that the increase of

moisture content of grain depending on structure of fiber in grainy products affected

bulk density and true density in studies made by Gupta and Das (1997), Baryeh

(2001), Sahoo and Srivastava (2002), Aviara et al. (2005), Altuntaþ et al. (2005),

Mwithiga and Sifuna (2005) and Yalçýn (2006) as cited at Kiber et al. (2008).

45 | P a g e

1000

1080

1160

1240

1320

1400

10 15 20 25

Moisture Content, % w.b.

Tru

e D

ensi

ty,

Kg

/m3

600

700

800

900

Bu

lk D

ensi

ty,

Kg

/m3

True density

Bulk density

Fig.5.5. Effect of moisture content on teff seed densities

5.1.6. Effect of moisture content on porosity of teff seed

The change of porosity with moisture content is shown in Fig. 2d. The porosity of teff

seeds increased from 39.92 to 42.38% depending on the increase of moisture content.

The porosity () and the moisture content of teff seed can be correlated as:

Coefficient of determination R2= 0.912

39.5

40

40.5

41

41.5

42

42.5

43

10 15 20 25Moisture content, %, w.b

Po

rosi

ty,%

Fig.5.6. Effect of moisture content on teff seed porosity

46 | P a g e

5.2. Aerodynamic Property of teff

The proper air speed can be determined from aerodynamic properties of agricultural

materials. These properties are terminal velocity and drag coefficient. If an object is

dropped from a sufficient height, the force of gravity will accelerate it until the drag

force exerted by the air, balances the gravitational force. It will then fall at a constant

velocity called the terminal velocity (Mohsenin, 1970):

Where, M is mass of the object (kg), g is gravitational acceleration (m/s2), Cd is drag

coefficient, ρ is air density (kg/m3), A is projected area (m2), and Vt is terminal velocity

(m/s). From this equation, the drag coefficient of an object can be found from its

terminal velocity:

Usually, a horizontal wind tunnel is used to measure drag coefficient of large objects.

In this method, external parameters such as size and velocity are varied and values of

drag coefficient are obtained over a wide range of Reynolds number. But for small

particles (like grain seeds), the drag force cannot be measured directly by this method.

So drag coefficient of agricultural materials are calculated from their terminal velocity

(Eq.2) which is experimentally measured.

Carman (1996) measured the terminal velocity of lentil seeds at different moisture

contents by free fall method. From the top of a dropping tube at various heights, a

seed was allowed to fall. The duration of the fall was plotted as a function of vertical

distance. The slope of the linear portion of the distance versus time curve indicated

the terminal velocity of the seed. He found that as the moisture content of the lentil

seed increased, its terminal velocity also increased linearly.

47 | P a g e

In another experimental method, a vertical wind tunnel is used for finding the

suspension velocities of the particles in an air stream. Bilanski and Lal (1965)

measured terminal velocities of wheat kernel and straw by a vertical wind tunnel.

The drag coefficient of grains, which is a function of Reynolds number, lay within the

limits of a sphere (0.44) and of a cylinder (1.0) depending on the shape of the grain. In

this thesis the terminal velocity of teff was determined in order to find the effects of

mass and moisture content of teff seeds on terminal velocity.

5.3. Determination of Pressure drops

To determine pressure drops the test column was filled with teff seeds to the depth of

0.60m at a given level of moisture content. Two method of filling was used: dense and

loose. Pressure drops of dense and loose fill method was compared. The first 0.15m of

test column above air plenum chamber was used for straightening the air (Agulo et al,

2005). Pressure drop measurement was started from the air tap (T1) at a height of

0.15m above air plenum. This pressure was taken as a reference tap and pressure

drops for subsequent taps was the differences in static pressure between this tap and

the subsequent above the air plenum.

Four moisture levels 12.01, 16.08, 20.71 and 25.01% were used to determined pres-

sure drop through the grain column, the result indicate that when the moisture content

increases the pressure drop or static pressure decreases. With the above moisture con-

tents in the loose method the following pressure drops were observed 518.52, 503.70,

488.889 and 481.482 Pa per meter respectively. For the loose fill the calculated pres-

sure drops were 315.501, 309.809, 245.889 and 197.414 Pascal per meter respec-

tively.

Incase dense fill method with moisture contents 12.01, 16.08, 20.71 and 25.01% the

following pressure were observed 525.93, 522.963, 500.00 and 496.296 Pascal per

meter and calculated pressure drops for the dense fill were 333.454, 328.01, 295.16

and 225.57 Pascal per meter respectively.

5.4. Determination of power requirement and capacity of Blower Motor

48 | P a g e

To determine the conveying power requirement, the voltage, the current, and the

power factor drawn by the electric motor for the roots blower was measured by using

voltmeter, ampere meter, and power factor meter, respectively. Conveying capacity

depends on mass of seed, air moved, and the physical characteristics of the seeds

(Hellevang, 1985).

5.5. Mathematical Modeling

5.5.1. Bulk Aeration Mathematical Modeling

The relationship between the pressure gradient and velocity of the air through the

grain mass must be known in order to estimate the airflow distribution. The air veloc-

ity used in equations describing this relationship is the superficial velocity, which is

calculated as the volume flow rate divided by the cross-sectional area of the flow.

Shedd (1953) plotted data for numerous grains with a wide range of airflow rates and

proposed a relationship that has been widely used by engineers for the design of aera-

tion systems and the sizing and selection of fans. Unfortunately, Shedd’s expression is

empirical in nature and contains no information about the properties of the product be-

ing aerated, or the fluid flowing through the product. Several researchers developed

expressions for pressure drop through packed beds that have some physical basis.

Darcy showed that the velocity of the fluid flowing through a porous medium is di-

rectly proportional to the pressure drop (Darcy’s Law). According to Reynolds

(1900), the total energy loss for a fluid flow is the sum of the viscous and kinetic en-

ergy losses. At low airflow rates (laminar flow), the resistance offered by friction to

the motion of the fluid is directly proportional to the viscosity and velocity of the

fluid. Darcy’s law holds for low flow rates where viscous forces predominate and in-

ertial forces can be neglected. Ergun’s, equation can be derived by starting with the

relationship between pressure drop due to friction, ∆Pf and the fanning friction, for the

laminar and turbulent flow.

3.1

49 | P a g e

By setting the values for the superficial velocity, v’ and hydraulic radius, rH equation

for pressure drop can be develop.

Experimental data concluded that 3fp =1.75. The relationship between pressure drop

and fluid velocity was interest of many of experimenters who tried to find relation-

ships relating the factors. Blake-Kozeny, Scientists, developed an expression that cor-

related the pressure drop to low (laminar) fluid flow rates (Ergun, 1952).

Where de, L, , ∆P, k1, gc, u, are particle equivalent diameter, the height of the bed,

fluid viscosity, porosity, the pressure drop, the coefficient viscosity energy, the accel-

eration of gravity at sea level, the fluid velocity.

At high airflow rates (turbulent flow), pressure loss is proportional to the product of

the air density and the square of the fluid velocity as viscous forces then become rela-

tively negligible. Ergun (1952) presented an equation for resistance to fluid flow

based on the Reynolds Theory. According to this equation, the total energy loss in a

packed bed should be treated as the sum of the viscous and kinetic energy losses. He

examined the equation from the point of view of its dependence upon flow rate, prop-

erties of fluids (µ - viscosity and – density), and porosity (), orientation, particle

diameter (de), shape, and surface of the granular solids:

50 | P a g e

In the above equation, k2 is the kinetic energy and is the density of the fluid in the

column. Prior to 1952, Sebri Ergun and A.A Orning tried to focus their research on

the idea that the total energy lost in a packed bed could be treated as the sum of vis-

cous and kinetic energy losses. It was believed that the transition for viscous to kinetic

dominance was smooth (Ergun, 1952, pg.89). Therefore, a relationship was developed

that related the pressure drop in packed bed over the entire range of velocities when

the flow rate the properties of the fluid, the fractional void volume, shape, size and the

surface of the granular solids were known.

Finally, Ergun completed the expression by finding values for k1 and k2 and combin-

ing all previous equations.

The sphericity of the packing material () can usually be ignored when assuming per-

fect symmetry of packing. It is therefore presumed to be value of 1. When packing in

the bed are all spherical, the mixture can be modeled as crystalline, which involves an

ordinarily structured arrangement of the particles.

51 | P a g e

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Documents

Chapter One: Introduction 1.1. Background Teff [Eragosties