AGRICULTURAL ENGINEERING

EVALUATION OF MILLING PERFORMANCE OF A 1½

BELL GRINDING MILL MANUFACTURED AT HELIDES

ENGINEERING

BY

TSOKA KUDZANAI

R049698D

SUPERVISORS

MR P RWAKATIWANA

AND

MR A CHIGARISO

An undergraduate Research Project submitted in partial fulfillment of

the requirements of the degree of Bachelor of Science Honors in

Agricultural Engineering.

UNIVERSITY OF ZIMBABWE

PO BOX MP167, HARARE

JULY 2008

i

ABSTRACT

The milling performance of a 1½ Bell grinding mill was evaluated taking

parameters such as throughput, milling efficiency, quantitative loss, power

requirement, uniformity of milled particles and major causes of breakdown

into consideration. It was hypothesized that the throughput of the grinding

mill was greater than 850kg/hr; milling efficiency would be greater than

65% whilst the quantitative losses would not exceed 10% of the initial mass

fed. Masses between 50kg and 25kg were milled to investigate the above

parameters using the 1½ Bell grinding mill for maize with the same moisture

content.

The throughput obtained was relatively higher than that expected were as the

quantitative losses and power requirement was lower than what was

hypothesized. The predicted hypotheses were proved to be true. The

particles of the milled products were relatively uniform falling within the

required standards for grinding mills. Major causes of breakdowns were

obtained from a questionnaire with most of them being due to lack of

knowledge on machine operation.

The 1½ Bell grinding mill was proved to be an effective and efficient

milling machine which can be used for different milling operations at

different industrial levels in Zimbabwe.

ii

ACKNOWLEDGEMENTS

Special thanks for the support and technical know how provided by Mr A

Chigariso from Helides Engineering.

I also articulate my sincere gratitude to my dedicated supervisor Mr P

Rwakatiwana for expert advice and encouragement during the course of the

project.

I would also like to thank the Agricultural Engineering department technical

staff for the technical assistance.

iii

DEDICATION

To the most high God for his grace is sufficient and his mercy endures

forever.

iv

TABLE OF CONTENTS

CHAPTER 1……………………………………………………………….1

1.0 INTRODUCTION…………………………………………………..1

1.1 JUSTIFICATION…………………………………………………...3

1.2.0 OBJECTIVES………………………………………………….5

1.2.1 Main objective………………………………………………….5

1.2.2 Specific objectives……………………………………………...5

1.2.3 Hypothesis……………………………………………………...5

CHAPTER 2……………………………………………………………….6

2.0 LITERATURE REVIEW……………………………………….......6

2.1 Equipment operation……… ……………………………………….11

2.2.0 Factors to consider in specifying a size reduction system….…….12

2.2.1 Feed control……………………………………………………….12

2.2.2 Milling discharge…………………………………………………12

2.2.3 Energy consumption………………………………………………12

2.2.4 Moisture content…………………………………………………..13

2.3.0 Pneumatic Conveyor System .........................................................14

2.3.1 Limitations of a conveyor System………………………………...15

2.4.0 Size reduction machine operation and performance……………..15

2.5.0 Flowchart for the milling operation………………………………16

2.6.0 Screening………………………………………………………….16

2.6.1 Factors affecting screening operation…………………………….17

CHAPTER 3………………………………………………………………19

3.0 METHODOLOGY...……………………………………………….19

3.1 Determination of throughput……………………………………….19

3.2 Determination of milling efficiency………………………………..20

3.3 Power consumption………………………………………………...20

3.4 Determination of particle uniformity…………………...…………..21

3.5 Determining losses during milling………………………………….22

3.6 Determining the major causes of breakdown.………………………22

CHAPTER 4………………………………………………………………23

4.0 Results………………………………………………………………23

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4.1 Throughput..………………………………………………………...23

4.2 Milling efficiency..………………………………………………….24

4.3 Power consumption…………………………………………………25

4.4 quantitative losses…………………………………………………..28

4.5 Uniformity of milled particles………………………………………29

4.6 Major causes of breakdown………………………………………...30

CHAPTER 5………………………………………………………………32

5.0 Discussion of Results……….………………………………………32

5.1 throughput…………………………………………………………..32

5.2 Power requirement ………………………………………………....32

5.3 Quantitative losses…………………………………………………..32

5.4 Milling efficiency…………………………………………………...33

5.5 Uniformity of milled particles………………………………………33

5.6 Major causes of breakdowns………………………………………..34

CHAPTER 6………………………………………………………………35

6.0 Conclusions…………………………………………………………35

6.1 Throughput…………………………………………………...35

6.2 Quantitative losses…………………………………………..35

6.3 Causes of losses……………………………………………..35

6.4 Milling efficiency……………………………………………36

6.5 Power requirement…………………………………………..36

6.6 Uniformity of milled particles……………………………….36

1

CHAPTER ONE

1.0 INTRODUCTION

Due to the Government initiated black empowerment program there has

been a marginal increase in the number of small to medium scale

manufacturing companies. This has also been necessitated by the exodus

of large commercial companies and the increased number of government

support programmes. For example, in 2007 fifteen billion dollars was

disbursed for small to medium enterprises revolving fund (Dr G Gono,

2007). This has also received a major boost by the launching of the third

phase Agricultural Mechanization Programme of which there is now an

increased demand for agricultural machinery. This increase in demand

for agricultural machinery has resulted in the need to perfect the

operations of small to medium scale equipment.

Although there has been an increase in the number of players in the

manufacturing of agricultural equipment, there has been stunted growth

in the marketing of products due to lack of information on the operation

of these products. It has been observed that equipment operators in the

developing countries have been poorly provided with information and

guidelines to make realistic machinery evaluations. Technical

performance and data gained from machinery testing has been limited

and rarely presented with the customer in mind while other information

needed for effective evaluation of information and its use in machinery

selection is not available (FAO, 1995). In addition, according to FAO

1995 realistic methodologies for collection and evaluation of information

2

and its use in machinery selection are not yet established while

consumers have had little encouragement to participate in formulating

them.

Agricultural processing has been defined as any activity that is performed

to maintain or improve quality or to change the form or characteristics of

any agricultural product (Sahay and Singh, 1999). Processing operations

are undertaken to add value to agricultural materials after their

production of which grain milling is one of the operations. The milling

industry in Zimbabwe has seen the design and development of various

types of machines with different milling techniques. Some of these

include attrition mills, ball mills and hammer mills. Great emphasis has

been put in speed, throughput and power resulting in the need for

evaluation of other parameters such as prediction of losses during milling

process and energy efficiency especially for products produced by small

to medium manufacturing companies. This can enable upcoming

companies like Helides Engineering to compete with well established

companies since useful information will be available to the miller.

One big step in improving competition at Helides Engineering with the

well established companies like Precision Grinders, Tunroy and

AgricAfrica is through performance evaluation of some of its products.

This includes the evaluation of milling performance of the 1½ Bell

Grinding Mill manufactured by the latter. The 1½ Bell Grinding Mill is a

hammer that is relatively compact in stature hence it can be easily

transported from one place to the other. Furthermore, the rotor assembly

is evenly balanced thereby reducing noise and vibrations, which makes it

3

environmentally friendly. Moreover, it can be fitted with sieves of

different sizes depending on product requirement which makes it highly

flexible. In addition it can also be easily maintained since it is locally

made which gives it an urge over the imported grinding mills. Due to the

fact that it has not been performing as expected on the market there is a

need to evaluate it so as to make amends on the available loop holes.

1.1 JUSTIFICATION

Even though a lot has been done in the design and fabrication of grinding

mills at Helides Engineering little has been done in the evaluation of

milling machines among them the 1½ Bell grinding mill. The current

power shortages being experienced in Zimbabwe makes it imperative for

one to assess the power requirements of the 1½ Bell grinding mill during

the milling process so as to come up with energy saving measures

thereby improving its competitiveness on the market. Thus it will be able

to compete with grinding mills from well established companies like

Hippo Hammer mills, AgricAfrica Pvt Ltd and Tanroy Engineering.

Furthermore, evaluation will give the miller an insight into production

capacity for the grinding mill for example the Ndune mill in Kenya was

found to have a capacity of 70-200kg/hr for small grains and 400-

2550kg/hr for large grain (ELA Publications, 1985). The given data thus

gives the miller a prediction on the amount of grain that can be milled in

a given period of time hence it is possible to assess whether one can

meet the given targets.

4

In addition, there is a need for performance evaluation so as to assess or

to predict quantitative losses that occur during the milling process and

coming up with measures or methods that can be implemented to reduce

some of the losses. The latter is also important in the determination of

quantity of grain that is required to produce a certain tonnage of possibly

mealie meal taking losses during milling into account.

Moreover, there is a need for assessing the quality of product that is

produced with the standards set by the buyer (Krammer and Twigg,

1970, Chapter 3). Particle size analysis found that 44 to 88% by weight

of the mass exiting the mill was within the same size distribution

(Brennan, 1976). Particle size analysis is important in determining the

uniformity of the milled product, which is important in adding value to

the milling machine and also to the produced product hence maximizing

profits. Size is additionally closely related to quality and appearance of

product, and also has a direct bearing on how certain manufacturing

operations are perfumed. Colour of product can also be enriched if

particle size and shape are uniform. Hence form the foregoing statements

it will be seen that the objective of size reduction or milling is to make a

product which meet certain specifications hence marketing of grinding

mills from Helides Engineering will be enhanced if the above

specifications are determined.

There was also a need for evaluating the milling performance of the 1½

Bell grinding so that improvements on the existing designs can be made.

The study was also important to ensure availability of quantified

5

technical data on milling performance to the miller or consumer of the

product.

1.2 MAIN OBJECTIVE

To evaluate the milling performance of a 1½ Bell grinding mill

1.3 SPECIFIC OBJECTIVE

To determine throughput of the machine.

To determine losses during the milling process.

To determine the milling efficiency.

To assess the uniformity of particles after size reduction.

To assess the major causes of breakdowns.

To determining the power requirement during milling process for

different grain varieties.

1.4 HYPOTHESES

Throughput is greater than 850kg/hr.

Less than 10% of quantitative losses occur during the milling process.

Milling efficiency should be greater than 65%.

6

CHAPTER TWO

2.0 LITERATURE REVIEW

In 1985 maize accounted for 43% of cereal production in Africa,

sorghum and millet, wheat, and rice accounting for 28%, 11% and 11%

respectively. These figures show that 93% of what is produced in crop

production consists of cereals and therefore there is much demand for the

processing of cereals and thus includes the milling operation (ELA

Publications, 1985). The aim of milling process is to achieve an

economic size reduction at minimum cost (Brennan, 1976). Thus

evaluations of technical performance should be carried out so as to give

the consumer a wide range of options in the selection of machines with

quantified values of performance.

Evaluation consists of the engineering parameters established during

testing combined with economic and ergonomic parameters, all of which

relate to the performance of the equipment, machine or tool (FAO, 1992).

Quantifying of parameters such as uniformity of milled particles, milling

efficiency, energy efficiency, throughput and maintenance requirements

gives information to the user or buyer in deciding on what milling

equipment to buy in relation to intended use. For example, the M16 from

AgricAfrica product catalogue with a 30hp, Star Delta and with a

throughput of 1500kg per hour (AgricAfrica, 2008). But there is no

adequate information on other important factors such as the milling

7

efficiency and energy efficiency, which are of great interests to the miller

during the current economic hardships being experienced in Zimbabwe.

Well established manufacturing companies have been advertising their

products with respect to how they perform during the milling process

mainly in terms of capacity and this has been a major setback on the

marketing of products produced by the small to medium scale

engineering companies venturing in design and fabrication of grinding

mills. One way in which the small to medium scale engineering

companies should advertise their product should be similar to the way

Tinytech advertise the Tinytech maize mill (www.tinytechindia.com).

This mill is referred to as:

-The simplest appropriate scientific design

-Very simple and easy to operate

-Long reliable and trouble free operation

-Pay back period against investment, less than three months

-Highly profitable for custom milling

It also includes technical details like:

-Three easily replaceable sieves are provided for desired fineness of flour

-Inner housing made of steel is easily explicable

-Design is based on hammer mill principle

In this design, there is an advantage of using three replaceable sieves,

which are responsible for improving the degree of fineness. It is also

advantageous when one of the sieves fails because there is always a

backup sieve to do the screening process. The mesh of the screen

determines the particle size; a 1mm size is suitable for most human

8

foods, whereas a 3mm hole is preferred for animal feed (IT, Publications,

1985). Though technical details on the Tinytech maize mill is provided

there is no detail when it comes to the issues involving throughput,

milling efficiency and energy efficiency which is important when

marketing products especially in Zimbabwe where there is currently a

shortage of cereal grains and also experiencing electrical shortages.

ZESA reduced their power generation by about a third from 750

megawatts to 550megawatts (The Herald, 29 October, 2007). There is a

need for using grinding mills that require minimum energy whilst getting

the desired output.

Another grinding mill that is available on the world market is the

Vibrokinetic Energy mill from MicroGrinding systems Inc. The VKE is

regarded as the most efficient, extremely fast and very energy efficient.

The VKE mill is very versatile, can be operated wet or dry, simple low

cost and practically maintenance free. Energy savings of 50% or more

over the other types of mills by use of high-speed type vibratory motion,

which increases the grinding impact rate resulting in faster grinding

(www.microgrindingsystems.com). The marketing of the VKE puts more

emphasis in energy efficiency, which is the most appropriate for the

Zimbabwean situation, but there is inadequate information provided for

other parameters such as throughput and losses incurred during the

milling process.

An example of a hammer mill manufactured in Zimbabwe is the hippo

mill from precision grinders. It is an ISO 9002 certified product. Can be

sold either as electric or diesel powered which widens its market

9

spectrum to remote locations without electricity. It is an ideal machine

for farmers, contract millers and businesses that require high performance

and reliability. It is available in three different sizes i.e. 500kg/hr to

4ton/hr. There are 7 different screens available which makes it flexible

enough to mill other cereals besides maize and even silage. There is also

a full spares backup guarantee which reduces expenses of having to

import spares in the event of a breakdown thereby reducing maintenance

costs. It is robust and can be operated for long hours which makes it

suitable for millers that operate in double shifts. It is a device that can be

easily serviced and it also comes with comprehensive operator’s manual

which is one of the things that lacks on the 1½ Bell grinding mills. The

complex rotor assembly is evenly balanced to ensure free rotation and

longer bearing life (www.pgehippo.co.zw). Figure below shows the

picture of the hippo mill

10

Assessment of how much energy in kilowatt-hours is required to achieve

a given performance rate and how many hours of labour for the mill staff

does this represent (ELA, Publications, 1985). The Swepco Vibro

Grinding mills are the most versatile type of mill available today for fast,

low cost particle size reduction to sub-micron range. Vibro Energy

grinding action achieves higher output per unit input horsepower, precise

control of size of few moving parts and replaceable linings made

abrasion resistant ceramic or elastomers. Information on performance has

11

been provided but there is no quantitative data or actual figures on the

given information. There is also missing information on issues

concerning losses or estimation of losses during milling process neither is

there any data on particle uniformity of the products.

The type of grinding media utilized identifies the type of grinding mill.

Each type of mill has characteristics that adopt it to certain grinding

applications. The operational characteristics of each mill type must be

evaluated in relation to the objectives of the grinding mill requirements

being studied in order to select the optimum mill

(www.miningbasics.com).

2.1 EQUIPMENT OPERATION

For proper selection and economical operation of size reduction

machinery, attention must be given to many details and procedure of

auxiliary equipment. A milling machine cannot be expected to perform

unless;

1. The feed is of suitable size and enters at a uniform rate

2. The product is removed as soon as possible after particles are of

desired size

3. Unbreakable material is kept out of the machine

4. In the event of low melting and heat sensitive products blowers

and pumps should remove the heat generated in the mill.

12

2.2 FACTORS TO CONSIDER IN SPECIFYING A SIZE

REDUCTION SYSTEM

2.2.1 Feed control

Control of the feed in the mill is the most important. The particles in the

feed must be of appropriate size. In continuous mills the feed rate must

be controlled within close limits to avoid chocking and erratic variations

in load and yet make full use of the capacity of the machine (Kukla and

Hixon 1991)

2.2.2 Mill discharge

Rate of discharge must be equal to rate of feed to avoid build up in a

continuous mill. Furthermore, the discharge rate must be such that the

working parts of the mill can operate most efficiently in the interval to be

reduced. A sweep of air removes product from the hammer mill

(McGraw Hill, Chemical Engineering Series).

2.2.3 Energy consumption

Enormous quantities of energy are consumed during the size reduction

operations. Over 99% of energy goes to the operating of the equipment,

producing undesirable heat and noise leaving less than one percent for

creating of new surface (Warren, 1973). This shows that there is a need

13

for evaluation of grinding mills so as to come up with new energy saving

designs to improve on the milling efficiency. The energy requirement for

any milling process is also affected by types and condition of products to

be produced such as grains, size of hammer screens and hence fineness of

grind and hammer mill loading conditions. These include moisture

content, bulk density, texture, size, and aerodynamic characteristic of

products. The mill condition include rate at which mill is fed with grain

and coupling efficiency of the prime move and mill (D.M.Nyang, 2003).

Certain improvements have been done in development of energy saving

technologies for the food processing industry. These include;

1. The needy to improve efficiency of each unity operation by design

improvement.

2. Development of new processes or products that consume less

energy than traditional processes for example grain conditioning

before the milling process

2.2.4 Moisture content

Materials do not flow well if they contain high moisture contents

(Coulson, 1978). Under these conditions the material tends to cake

together in the form of balls. Hence there is a need for evaluation of the

proper milling moisture content for various grains so as to improve on the

milling efficiency.

14

2.3 PNEUMATIC CONVEYOR SYSTEM

The pneumatic conveyor system plays an important role in determining

the optimum performance of a grinding mill. It is a system mainly used

for moving milled products in a closed duct system by a high velocity air

stream. It is a continuous and flexible transportation method by which the

material is carried in pipelines either by suction or blowing pressure of

air stream. The milled material because of high pressure is conveyed in a

dispersed condition and for this dispersion air velocity in the range of 15-

30m/s is necessary (Sahay and Singh, 1999).

The conveyor system needs a source of air blowing or suction, means of

feeding the product into the conveyor, ducts and a cyclone receiving

hopper for collection of products. As for the 1½ Bell a blowing or

positive pressure system is used to convey the products by using air

pressure greater than atmospheric pressure. The system used consists of a

fan or blower, an air lock feeder for introducing the product into the

system, ducts and a suitable air and product separating device. The

product is fed into the conveying system through the bottom of the

hopper. The feeder should be able to feed product at a specified rate in a

pneumatic conveyor system from the supply hopper at a certain pressure

to the other conveying pipeline at another pressure.

A fan is used to deliver the specified volume of air to transport the milled

products through the conveying pipeline (ducts) to the cyclone separator.

The supply pressure depends upon the working pressure drop required for

the length of the conveying line and properties of the conveyed products.

15

The cyclone separator is used for separation of product particles from air

or in other words it is a device which removes the bulk of the product

from the conveying air stream by centrifugal force. In some cyclone

separators, a fabric filter is attached to remove residual dust and fine

product particles from the air stream. The conveying air is first passed

through the cyclone and then goes for secondary separation of finer

particles.

2.3.1 Limitations of pneumatic conveying

1. Erosion of air ducts by abrasive product particles in a conveying air

stream

2. In case of bends and misaligned sections, the erosion of ducts is

severe.

3. There are high chances of repeated impacts between particles and

solid surfaces and this may lead to product degradation and this in

turn might diminish the market value of the products.

2.4 Size reduction machine operation and performance

To achieve good results from any size reducing machine the following

should be given due care.

Feed should be of proper size and feeding rate should be uniform.

Hard and unbreakable material should not be allowed to enter the mill.

There should be some arrangement to remove the excess heat

generated during the milling operation.

16

There should be an air lock system that separates the conveyor system

from the conveyor system for efficient transportation of products.

2.5 FLOW CHART FOR THE MILLING OPERATION

Feed

Product

2.6.0 SCREENING

Screening is one of the most important parameters that affect the milling

performance of a hammer mill. By definition screening is the unit

operation in which a mixture of various sizes of solid particles is

separated into two or more fractions by passing over a sieve

(J.G.Brennan at al, 1969). A screen contains a number of equally sized

apertures. On a grinding mill these are selected in relation to the required

particle size of the product. The smaller the apertures on the screens the

higher the energy required for size reduction since the feed will have take

longer in the action zone before they could pass through the sieves and

Grinding

chamber

Screens Pneumatic

conveyor

Cyclone separator

17

the higher will be the cost of production (Leninger ,1911). The former

statement is closely related to throughput since throughput decreases with

increasing fineness.

2.6.1 Factors affecting screening operation

1. Rate of feeding

If the rate of feeding is too high, insufficient residence time on the

screening surface will result. This causes overloading on the screen

surface and as a result material capable of passing through the sieve

will leave with the oversize and thereby increasing the milling time.

2. Moisture

The presence of high moisture content in the feed can cause adhesion

between small and large particles and this will result in blinding or

clogging of the sieves. Aspect of moisture content is of paramount

importance in determining milling performance hence there is a great

need for identifying the optimum moisture content to prevent

clogging of screens.

3. Worn or damaged sieves

Oversize particles will pass through the damaged area and the

efficiency of the separation will be impaired. This has got a huge

effect on the uniformity of milled particles and It tends to reduce the

18

price of the product produced hence damaged screens should be

repaired immediately

19

CHAPTER THREE

3.0 METHODOLOGY

Determination of milling efficiency, throughput and energy consumption

measurements was carried out at Helides Engineering and Betta milling

company located at 128 Willovale road just opposite High Glen shopping

center. Whilst particle size analysis were investigated at the University of

Zimbabwe laboratories

3.1 Determination of throughput

Masses of 50kg and 25kg were measured using a mass balance and these

were milled respectively with time taken to feed each sample recorded

using a stop watch. Four replicas for each mass were milled using the

above procedure so that we can get a better estimate of the throughput.

The throughput was then calculated using the formula below.

Throughput = mm Tm

Where; mm is mass of milled grain

Tm time taken to mill the grain

20

Figure 3.1

Figure 3.1: The setup that was used for throughput measurements at Betta milling

company

3.2 Determination of milling efficiency

Masses of grain between 25 and 50kg masses were measured using a

mass balance. The milling efficiency of the 1½ Bell grinding mill was

obtained by the percentage ratio of mass of milled product to mass of

feed as shown by the equation below

Milling efficiency= mp*100

mf

Where; mp is the mass of product and mf is the mass of feed

3.3 Power consumption during milling process

To measure power requirement an ammeter was connected between

the electric motor of the grinding mill and the electrical supplies.

21

The current taken up by the machine when there is no input of grain

i.e. the idle current before commencement of milling process was

measured using an ammeter.

The feed was then emptied into the milling machine at a constant feed

rate. Meter readings were taken after every 5seconds intervals until all

the grain was milled. This was shown when the meter reading goes

back to the idle power.

Voltage readings were taken using a voltmeter across the power

supplies.

Power was then obtained by using P=VI where P is the power

required during the milling process, V is the average voltage obtained

and I is the current difference between the maximum current and the

idle current

3.4 Determination of particle uniformity

Tyler meshes were used to determine particle size. In the analysis of

milled products a set of standard sieves was arranged sequentially

according to size with the pan at the bottom and the smallest sieve at the

top. The samples were placed on the top most sieve and the set was

placed on a sieve shacking machine for 5 minutes.

The fineness modulus which indicates the uniformity of ground products

was determined by adding the weight fractions retained above each sieve

divided by 100 (Sahay and Singh, 1999).

22

3.5 Determining losses during milling

Quantitative losses were determined by subtracting mass of milled

product from mass of feed. Mass of feed and product was measured using

a mass balance. This was then obtained by the following formula;

L=mp-mf

Where; mp is the mass of product in kg

mf is the mass of feed in kg

3.6 Assessing frequency of breakdown.

Frequency of breakdown was assessed by the use of a questionnaire

which was disbursed to individual people who operate the grinding mill

or companies who have purchased the product. Data was analysed using

SPSS (Statistical Package for Social Studies). The basic structure of

questions is shown below;

23

CHAPTER 4

Results From obtained data of throughput from different trials graphs of

throughput and number of trials was plotted. This is important so that

a better estimate of the through is obtained.

Figure 4.1: shows the general variation of average throughput

with the number trials.

From the graph in figure 4.1 it is seen that the throughput was almost

constant through out the different trials carried out.

24

The average throughput for the 1½ bell grinding mill was found by

first calculating the average mass milled and also average time taken

to mill the masses fed using Microsoft excel. The following equation

was the used with Microsoft excel.

Throughput = mm Tm

Where: mm is the average mass of the milled grain and Tm is the

average time taken to mill the grain fed into the grinding mill.

From Microsoft excel the average throughput was found to be

964.07kg/hr

Figure 4.2: Variation of milling efficiency with time.

varation of milling efficiency with time

97

97.2

97.4

97.6

97.8

2 months 5 months

time/months

% efficien

cy

25

Figure 4.2 shows that milling efficiency was decreasing with an

increase with time of operation. But on average it was found to be

97.50%.

Figure 4.3: Relationship between current and time for a hammer

mill with 5 month old beaters

The graph shows the relationship between current and time of which the

current increased from an idle current of 18A to a maximum of 40A and

then started top decrease though there were some fluctuations throughout the

Graph of current against time

0

5

10

15

20

25

30

35

40

45

0 50 100 150 200 250

Time (sec)

Current

26

whole graph The current then starts to drop to its idle current of 18A which

shows that the grinding process is over and there is no longer anything in the

grinding chamber.

The above data is used to calculate the energy requirement during the

grinding process for the 1½ bell grinding mill. The idle or minimum current

before any grain is added into the grinding mill is obtained from graph so as

the maximum current during the milling process. Calculations are carried out

as follows.

Current required =Maximum current-Idle current

Hence I=40-18

=22A.

The current obtained above is the current required for the milling process

only i.e. the actual size reduction of the grain without taking current required

for running the electric motor without any grain fed into the grinding mill.

From voltage measurements the value obtained was a constant throughout

the whole experiment, of which it was found to be 330V

The power required during milling process is obtained by the formula:

P=IV

Where P=power required during milling process

V=voltage required by the electric motor

I=current drawn during milling

Therefore; power required during milling process = 22*330

= 7260W

27

= 7.26Kw

Figure 4.4: Relationship between current and time for a hammer mill

with 2 month old beaters.

G ragh o f curren t against tim e

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

tim e/sec

cu

rren

t/A

current

Scale 1unit represents 5seconds on the x-axis.

Basically the variation between current and time is the same as in fig 4.3

despite the age difference. The idle current is 15 amperes and the maximum

28

current obtained from the graph is 30 amperes. The voltage across the motor

remained the same i.e. 330V

Using P=IV

Therefore power = 30*330

= 4950W

= 4.95Kw

The power requirement for the 1½ Bell Grinding mill increases with time of

use as shown by the above calculation since the one with 2 month old

beaters was 4.95Kw whilst the one with 5 month old beaters was 7.26Kw.

Figure 4.5: Comparing average quantitative losses between open and

closed air inlet ducts.

Period of

operation

Mass of mealie

meal lost when

aperture is open/kg

Mass of mealie

meal lost when

aperture is

closed/kg

Throughput of

machine when

aperture is

open/kg/hr

Throughput of

machine when

aperture is

closed/kg/hr

2 months 1.000 0.500 962.07 961.53

5 months 0.750 0.625 514.90 513.60

Figure 4.5 shows the variation of quantitative losses with time of which the

losses tend to increase with time whilst the throughput is decreases. The

quantitative losses are decreasing for a mill with open apertures (air ducts)

from 1.000kg for a hammer mill with 2 month old beaters to 0.750 whilst

that with a closed aperture is increasing from 0.500kg for mill with 2 month

old beaters to 0.625kg for mill with 5 month old beaters. Generally much

29

losses are incurred when the air duct is open than when it is closed but as

shown by the above table the throughput tends to be reduced when the duct

is closed than when it is open. This is shown by a reduction from

962.07kg/hr to 961.53kg/hr. From the above figure it is also shown that

throughput decreases with an increase in operational time.

Plate 4.1: showing some of the quantitative losses that occur during the

milling process.

The mealie mill that is found on the ground is the one that is generally lost

through the air and as shown the quantities of products lost could greatly

increase with time.

30

-10

0

10

20

30

40

50

100 70 50 40 30 20 15 pan

sieve num ber

% w

eig

ht

reta

ined

S eries1

Figure 4.6: variation of average %mass retained with sieve number.

The figure above shows that there was an increase ion the % mass retained

as the sieve number or aperture size decreased up top a maximum the a drop

in the last to sieves. A maximum of 46.28% was observed in sieve number

20 which has an aperture size of 0.894mm (see Appendix 2).

31

Results obtained from questionnaire

Major causes of breakdown

-Presents of foreign matter.

-Seizure of bearings

-Wear of beaters.

-Failure of electric motors

-damaged sieve.

-fan belt failure

-over-loading causing the grinding mill to stop during milling process.

-Motor should not exceed the optimum required speed of between

4000- 4200rpm of which if exceeded this will result in increased rate of wear

of bearings.

Summary of results

Figure 4.6

Age of

beaters

throughput

(kg/hr)

%

Quantitative

losses

Milling

efficiency

Power

requirement

(Kw)

2months 961.80 2.25% 97.75% 4.95

5months 513.70 2.75% 97.25% 7.26

Figure 4.6 is summarizing the relationship between the time of operation and

parameters that determine the performance the 1½ Bell Grinding mill.

32

CHAPTER 5

5.0 Discussion of results

5.1 Throughput

The first hypothesis was proved to be true since an average experimental

throughput of 964.07kg/hr was obtained as compared to expected throughput

of 850kg/hr from the operator’s manual. The throughput was found to be

greater than the expected and this is observed to increase with the increase in

time of use.

5.2 Power requirement

The power requirement for the 1½ bell grinding mill was found to increase

with time from 4.95Kw for a grinding mill with 2month old beaters to

7.26Kw for a grinding mill with 5month old beaters this is due to an increase

in resident time inside the mill as the beaters wore out. Current required

during the milling process was obtained by taking readings on the ammeter

for which the current was determined by subtracting the minimum current

from the maximum.

5.3 Quantitative losses

The average quantitative losses increased with time from about

0.750kg/50kg which is approximately 1.5% to 1.00kg/50kg which

approximately 2% for 2month old hammer mill to 5month old hammer mill

respectively the latter results are for open air ducts whereas for closed ducts

the range was from 0.500kg/50kg (1%) to 0.625kg/50kg(1.25%) for

hammer mill with 2month old beaters and 5 month old beaters respectively.

33

From the above data it is observed that the greatest losses occurred when the

air duct was open. This is so due to a reduction in the pressure required for

the conveyance system. And from the figures obtained the second hypothesis

was proved true since less than 10% quantitative losses occurred for the 1½

Bell Grinding mill.

5.4 Milling efficiency

The milling efficiency decreased from 97.75% for a hammer mill with

2month old beaters to 97.25% for a hammer mill with 5month old beaters.

This is mainly due to wear of beaters and an increase in the quantitative

losses of which as a recap the milling efficiency is the ratio between mass of

product and the mass of feed hence if there is a marginal decrease in the

mass of milled product the efficiency is assumed to be low. The obtained

experimental means for both 2 month old beaters and 5 month old beaters is

greater than the expected efficiency of 65% from literature hence the third

hypothesis was proved to be true..

5.5 Uniformity of milled particles

The particles obtained from the milled mealie mill from the 1½ Bell grinding

mill were proved to be relatively uniform since the majority of the particles

fell within sieve number 30 and 20 with % masses retains of 27.28% and

46.28%. These are proved to be uniform since from literature 44-88% by

weight of particles exiting a milling machine should fall within the same size

distribution (Brennan, 1976) of which 46.28% from sieve number 20 falls

with in the required range.

34

5.6 Major causes of breakdowns

From the questionnaire it was observed that the causes of breakdowns due to

lack of the technical know how on the use of grinding mills. This is shown

by breakdowns caused by overloading and damaged sieves due to presents

of foreign matter of which these can be avoided if there is proper inspection

of the feed by the operator of the machine. Also such cases of overloading

and presents of foreign matter led to other causes such as seizer of bearings

and increased wear rate of beaters which are responsible for the actual size

reduction process. Causes due to electric motor failure where not much of an

issue due to overloading or presents of foreign matter due to the presents of

a fail safe mechanism in the form of fan belt linking the motor and the

rotating shaft with beaters.

35

CHAPTER 6

6.0 Conclusions and recommendations

6.1 Throughput

The first hypothesis was proved to be true since an average experimental

throughput of 964.07kg/hr was obtained as compared to expected throughput

of 850kg/hr from the operator’s manual. This figure shows that the 1½ Bell

Grinding mill has a throughput that is relatively large for use in small to

medium scale milling processes. The mill can safely compete with other

mills on the local market for example it exceeds the 500kg/hr hippo mill

from precision grinders (www.pgehippo.co.zw) hence I can strongly

recommend it for use by companies or individuals who want to meet given

targets.

6.2 Quantitative losses

The second hypothesis was proved to be true for which the expected %

quantitative losses was expected to be less than 10% whilst the obtained

experimental means were found to be between 1% and 2% despite the

different conditions discussed in chapter 5. The major causes of the losses

are highlighted below.

6.2.1 Causes of losses

The major causes of loses was found through the upper covers and the air

ducts mainly due to over loading and air getting through the feeding hopper.

36

Overloading caused the grinding chamber to discharge more than what is

required by the pneumatic conveying system and this tended to increase the

pressure inside the conveyor system and thereby causing the mealie meal to

escape through the lower air inlet/duct. As a result to reduce these losses a

rotary hopper can be used since it meters the supply of products into the

conveying system and it affects the air lock which is necessary for the

efficient operation of the pneumatic conveyor system and thereby reducing

quantitative losses in the process. In addition losses also occurred through

the fan housing and for this I recommend the use of gaskets to produce an air

tight seal which prevents ground products from escaping outwards from the

conveyor system.

6.4 Milling efficiency

From the conducted research the 1½ bell grinding has proved to have a high

average efficiency in terms of the ratio of milled product to the mass of feed

hence it can be safely used by companies that require high efficiency

grinding mill or individuals who do service milling.

6.5 Power requirement

The energy required during the milling process was observed to be

increasing marginally with time of use though initially the energy required is

relatively low. This could be due to an increase in wear rate of beaters which

increase the residents’ time of feed within the grinding chamber.

6.6 Uniformity of milled particles

The 1½ Bell grinding mill proved to produce milled particles with a

uniformity that fell within the required ranges from literature hence it has

37

proved to be a standard size reduction machine. But there is still need for

research on ways of improving the uniformity from 46.28% to higher values

especially when it comes to baking and colour enriching products which

require particles with a higher degree of uniformity.

38

REFERENCES

A.Chakraverty, 1998. “Post Harvesting Technology of Cereals,

Pulses and Oilseeds”. Oxford and IBH Publishers. New Dehli,

Bombay, Culculta

Brennan.J.G, Butters.J.R, Cowell N.D, Lilly A.E. (1976). Food

Engineering Operations. Applied Science Publishers, Essex,

England.

J.M Coulson, J.F.Richardson. J.R Buckhurst and J.H Harker.

(1978).Chemical Engineering Unit Operations, Vol 2,3rd

ed.

Leniger H.A and Beverlov W. (1993). Food Process

Engineering. Riede Publishers.Pordretch,

Holland/Boston, USA.

Warren L, Mcobe, Julian.C.Smith, Peter harriot.”Unit

Operations in Chemical Engineering, 5th ed”.

Sahay and Singh. (1999). Unit 0perations in Agriculture.

www.tinytechindia.com

www.microgrindingsystems.com

www.rbz.co.zw, 2007

The Herald 29 October 2007

FAO (1992) Machine Performance Evaluation.

39

Appendices

Appendix 1

QUESTIONARE FOR MILLING PERFORMANCE OF A 1½ BELL GRINDING

MILL

NAME OF COMPANY: ……………………………………….................................

TYPE OF MILLING MACHINE: ………………………………………………………..

DATE OF PURCHARSE: …………………………………………………………………

SIZE OF MOTOR USED: …………………………………………………………………

HOW MUCH MAIZE DO YOU USE PER DAY…………………………………………

DOES IT MILL OTHER PRODUCTS BESIDES MAIZE (If yes specify): ………….......

………………………………………………………………………………………………

WHICH GRAINS DOES IT PERFORM BEST…………………………………………...

HOW OFTEN DOES THE GRINDING MILL BREAK DOWN…………………………

………………………………………………………………………………………………

WHAT ARE THE MAJOR CAUSES……………………………………………………...

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

ARE YOU SATISFIED WITH THE MAINTANANCE COSTS…………………………

IS IT USER FRIENDLY…………IF NOT WHAT DIFFICULTIES ARE YOU

HAVING WHEN OPERATING THE HAMMER MILL………………………………....

40

………………………………………………………………………………………………

………………………………………………………………………………………………

DOES THE GRINDING MILL IMPOSE ANY DANGERS TO THE

OPERATOR……….

IF YES WHERE EXACTLY………………………………………………………………

………………………………………………………………………………………………

WHAT WEAKNESSES HAVE YOU OBSERVED ON THE MACHINE……………....

………………………………………………………………………………………………

………………………………………………………………………………………………

ARE CUSTOMERS SATISFIED WITH THE PRODUCTS PRODUCED……………….

………………………………………………………………………………………………

WHAT AREAS DO YOU FEEL NEED TO BE IMPROVED AND WHAT ARE

YOUR SUGGESTIONS……………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

41

Appendix 2

Test sieves and their respective sizes.

BSS ASTM ISS Width of

Opening/mm

Width of

Opening/inches

5

6

7

8

10

12

14

16

18

20

22

25

30

36

40

44

48

50

52

60

72

80

85

100

120

150

170

200

240

300

325

350

400

4

6

7

8

10

12

14

16

18

20

-

25

30

35

40

-

45

-

-

50

60

70

-

80

100

120

140

170

200

220

270

-

325

-

480

340

280

240

200

170

140

120

100

85

-

70

60

50

40

-

35

-

-

30

25

20

-

18

15

12

10

9

8

7

6

5

-

4

4.750

3.250

2.818

2.399

2.032

1.676

1.405

1.201

1.000

0.954

0.894

0.708

0.592

0.500

0.420

0.401

0.351

0.336

0.295

0.296

0.251

0.211

0.186

0.177

0.157

0.124

0.104

0.089

0.075

0.064

0.053

0.044

0.044

0.038

0.1870

0.1252

0.1109

0.0945

0.0800

0.0659

0.0553

0.0473

0.0394

0.0332

0.0322

0.0279

0.0233

0.0197

0.0165

0.0158

0.0133

0.0132

0.0116

0.0117

0.0099

0.0083

0.0073

0.0070

0.0060

0.0049

0.0041

0.0035

0.0030

0.0025

0.0021

0.0017

0.0015

0.0015

42

Appendix 3

Expected

mean 850

Exp mean 964.07

Median 954.04

stdev 28.10813

sample var 790.0672

n 5

conf level 95,0%

43

Appendix 4

time current

5 18

10 24

15 26

20 30

25 34

30 38

35 33

40 35

45 34

50 36

55 38

60 36

65 38

70 36

75 40

80 38

85 38

90 40

95 36

100 38

105 34

110 36

115 32

120 33

125 30

130 28

135 30

140 28

145 30

150 36

155 34

160 32

165 35

170 32

175 30

180 26

185 24

190 18

195 18

200 18

44

Appendix 5

sieve no

% weight of mealie meal retained

100 0

70 0.48

50 5.44

40 13.8

30 27.28

20 46.28

15 2.88

pan 3.84