ADDIS ABABA UNIVERSITY

ADDIS ABABA INSTITUTE OF TECHNOLOGY

SCHOOL OF GRADUATE STUDIES

DESIGN OF INJERA PROCESSING MACHINE

A Thesis Submitted to the Graduate School of Addis Ababa University in

Partial Fulfillment of the Requirements for the Degree of Masters of Science

In

Mechanical Engineering (Mechanical Design)

By: Yesehak Tefera

Advisor: Dr.-Ing. Tamirat T.

2019
 ADDIS ABABA UNIVERSITY

ADDIS ABABA INSTITUTE OF TECHNOLOGY

SCHOOL OF GRADUATE STUDIES

Design of Injera Processing Machine

By

Yesehak Tefera

Submitted in accordance with the requirements for the degree

MASTER OF SCIENCE (M.Sc.)

Approved By Board of Directors

Dr.-Ing. Tamirat T. ____________________ ____________________

Advisor Signature Date

____________________ ____________________ ____________________

Internal Examiner Signature Date

____________________ ____________________ ____________________

ExternalExaminer Signature Date

____________________ ___________________ ____________________

Chairman of the School Signature Date

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Declaration

I hear by declare that the research entitled “DESIGN OF INJERA PROCESSING MACHINE”
is my own independent work, and has not been previously submitted to any other university in
order to obtain a degree.

Signature_____________________

Date ______________

Advisor: Dr.-Ing. Tamirat T. Signature ______________ Date ____________

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 DESIGN OF INJERA PROCESSING MACHINE

Abstract

The present work emphasis in asserting a solution to the laborious Injera making system which
are faced by most of the communities in and around Ethiopia. The main downside of the present
existing Injera making processes is the source of power and energy consumption, tedious labor
with little or no hygiene control and the production efficiency, which is predominantly because
of the use of electric source or conventional fire wood for mitad and unskilled labors.

The proposed machines have been achieved by integrating the dough preparation and the
baking operations to work simultaneously for continuous Injera production. The dough is
prepared in a mixer container which are connected to a baking conveyor unto which the precise
quantity of dough is squeezed on to the baking chamber through the conveyor. The baking
conveyor are controlled to operate in-steps with controlled speed and appropriate delay for
injera to bake, which is achieved by using a stepper motor with a control unit. The component
design and the assembly has been done by CATIA and the real time simulation has been
performed by ANSYS.

This machine is highly recommended to save energy, minimize individual energy bill per
month and minimize operation costs of hotels and restaurants. Therefore, this injera baking
machine saves labor, energy and it is hygienic.

Keywords: Mitad, Injera, Batter, Baking, Efficiency, PLC control system

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Acknowledgment

First of all, I would like to give special thanks to God for giving me the strength during my
MSc study. I would like to thank my beloved wife and my family for their encouragement and
support.

I would like to express my deepest gratitude to my advisor, Dr.-Ing. Tamirat T. for his support
and encouragement that helped me during the time of the research work.

I would also like to thank Dr. Daniel Tilahun & the mechanical engineering department staffs.

Finally, I would like to thank my classmates and my staff friends in Debre Birhan University
for their support.

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Table of Contents

Chapter 1 Introduction .......................................................................................................... 1

1.1 Background.................................................................................................................. 1

1.2 Statement of the problem ............................................................................................. 4

1.3 Objective of the research .............................................................................................. 4

General objectives .......................................................................................................... 4

Specific objectives ......................................................................................................... 4

1.4 Limitations .................................................................................................................. 5

1.5 Organization ................................................................................................................ 5

Chapter 2 Literature Review.................................................................................................. 6

2.1 Previous works on injera processing machine .............................................................. 6

2.2 Major problems of the existing electric Injera Mitad .................................................... 9

2.3 Control System for the baking conveyor .................................................................... 11

2.4 Teflon conveyor belt for baking surface ..................................................................... 12

Chapter 3 Analytical methods, Conditions and Design ........................................................ 13

3.1. Conceptual design ..................................................................................................... 13

3.2 Embodiment design ................................................................................................... 21

3.2.1 Structural analysis of the frame ........................................................................... 21

3.2.2 Motor selection ................................................................................................... 24

3.2.3 Controller Selection for the baking system .......................................................... 29

3.2.4 Heat transfer analysis .......................................................................................... 34

3.2.5 Optimum Temperature of the resistance heater coil for the baking conveyor........ 36

3.2.6 Heat Loss Coefficients of the Injera Machine ...................................................... 38

3.2.7 Energy required for injera baking ........................................................................ 43

3.2.8 Heat loss calculation ............................................................................................ 45

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3.3 Cost Analysis ............................................................................................................. 50

Chapter 4 Result and Discussion ......................................................................................... 51

Chapter 5 Conclusion and Recommendation ....................................................................... 55

5.1 Conclusion ................................................................................................................. 55

5.2 Recommendation ....................................................................................................... 56

References .......................................................................................................................... 57

Appendix A......................................................................................................................... 59

Appendix B ......................................................................................................................... 60

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List of Tables

Table 3-1 Ranking for design concept selection of Injera Baking machine........................... 19
Table 3-2 Temperature Sensor Types …………………………………………….…………..32
Table 3-3: Common Thermocouple Sensors …………………………………………………33
Table 3-4 Material cost ……………………………………………………………………… 50

List of Figures

Figure 2-1 Double Sided Electric Injera Mitad ...................................................................... 8

Figure 2-2 Drawing of Double sided Injera Baking Mitad ..................................................... 8
Figure 3-1 Baking design concept 1 .................................................................................... 13
Figure 3-2 Baking design concept 2 .................................................................................... 13
Figure 3-3 Baking design concept 3 .................................................................................... 14
Figure 3-4 Baking design concept 4 .................................................................................... 14
Figure 3-5 Baking design concept 5 .................................................................................... 15
Figure 3-6 Baking design concept 6 .................................................................................... 16
Figure 3-7 Baking design concept 6 .................................................................................... 17
Figure 3-8 Baking design concept 6 .................................................................................... 18
Figure 3-11 equivalent stress ............................................................................................... 22
Figure 3-12 equivalent elastic strain .................................................................................... 23
Figure 3-13 Total deformation ............................................................................................. 23
Figure 3-14 Safety factor ..................................................................................................... 24
Figure 3-16 Automatic Controller Setup for Temperature Control ....................................... 33

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Nomenclatures

Q - Heat transferred in unit time

𝐾 - Thermal conductivity (W/m℃)

𝑤𝑓 - Total weight of cooked food in grams

h - convection heat transfer coefficient (W/m2 ℃)

Q̇ - heat flow (W)

dT
- Temperature gradient in plane of heat transfer in the direction of heat flow (℃/m)
dX

k - Thermal conductivity (W/m℃)

A - Area perpendicular to heat flow (m2 )

ΔT - Temperature difference between the fluid and the wall surface (℃)

σ - Stefan- Boltzmann constant

Ts - Body surface temperature (℃)

T∞ - Surrounding or ambient temperature (℃)

ε - Emissivity of the surface

Q - Heat content in (kCal)

V - Flow rate of the substance in (m3 ⁄hr)

ρ - Density of the flue gas in (kg⁄m3 )

Cp - Specific heat of the substance ( kCal⁄kg℃)

Cw - is the heat capacity of water (kJ/kg.k)

mi - Mass of the injera in (g)

hvap - Heat of vaporization of water

Eutilized - utilized energy in (kJ)

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P - Power (kW)

T1 - Hot flue gases temperature (℃)

T2 - Teflon conveyor lower surface temperature (℃)

T3 - Teflon conveyor upper surface temperature (℃)

Pr - Prattle number

Nu - Nusselt number

Ra - Raleigh number

g - Acceleration due to gravity (m/s2 )

α - Thermal diffusivity

υ - Kinematics viscosity

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 DESIGN OF INJERA PROCESSING MACHINE

Chapter 1 Introduction

1.1 Background

Injera, a soft honey-comb structured flat bread, is one of the Ethiopian staple foods, mainly
prepared from teff an indigenous cereal grain. It is a popular and nutritious food, with long
shelf-life, distinct and preferred characteristics of texture and flavor, and simplest to prepare.
Teff grain can be conserved for many years without any appreciable change or damage from
insect pests, if vermin and moisture are excluded from storage bins. Injera has broad cultural
implications for the Ethiopian society and there is always a need to hold on to it.

Injera is the most popular baked product in Ethiopia. It is fermented teff bread with a very sour
taste and is the undisputed national bread of Ethiopia. The baked product is referred to by
different names depending on the locality of production in Ethiopia. It is referred to as ‘budena’
in Oromigna, ‘taeta’ in Guragigna, and ‘solo’ in Walaytigna. The teff grains are prepared
manually or mechanically and milled to flour which is subsequently used in the preparation of
Injera.

Injera is eaten daily in virtually every household, and preparing it requires considerable time
and resources. In Eritrea and Ethiopia, the bread is cooked on a large, black, hot clay plate
called mitad in Amharic. This cooking method produces large amounts of smoke. Because of
this cooking method, much of the area's limited fuel resources are wasted.

On the basis of production procedures three types of Injera are distinguishable: (i) thin Injera
which results from mixing a portion of fermented teff paste with three parts of water and boiling
to yield a product known as ‘absit’ which is, in turn, mixed with a portion of the original
fermented flour (ii) thick Injera, which is reddish in color with a sweet taste, is a ‘teff’ paste
that has undergone only minimal fermentation for 12-24 hours; (iii) komtata-type Injera, which
is produced from over-fermented paste, and has a sour taste. The paste is baked or grilled to
give a bread-like product. Yeasts are the major microorganisms involved in the fermentation
of the sweet type of Injera.

Injera preparation usually takes two to three days; the teff is milled into powder then mixed in
water along yeast. This mix is set aside at room temperature for 2 days so it ferments and raises.

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During the second day it starts to give tangy aromas as the fermentation releases air bubbles;
this is where the Injera's slight tangy taste comes from. After the fermentation process is
finished the mix is cooked on 'Mitad'. A circular motion is used to achieve thin consistency.
When the hot pan and the fermented teff contact each other, thousands of tiny air bubbles
escape creating thousands of tiny eyes which is the familiar look of Injera.

It is no small challenge to remove the large, hot, thin Injera from the cooking surface without
either tearing it or burning one’s fingers. This challenge is further complicated when it is being
prepared in a commercial establishment, catering to numerous diners, under substantial
pressures of time and money. The repeated human hand contact, in a less than carefully run
kitchen, can also be less than completely hygienic.

Fuels for cooking Injera or other traditional bread include, fuel wood, charcoal, dung, and other
crop residues. In most developing countries like Ethiopia the household sector is the largest
energy consumer. In Ethiopia the single largest demand for energy is for subsistence, which
accounts for nearly 90% of the total energy consumption. This burden of subsistence is carried
almost entirely by women. In villages, women have to spend more times in fuel collection. This
heavy workload in the long run affects their health. This is because the energy expended is
more than the intake of food to accomplish daily task. The burden of women taking care of the
family gets worse when you include the time it takes to collect the wood using bare hand or
primitive tools and carry it over long distances.

The urban poor have even a greater problem due to the scarcity of fuel and their incomes have
not kept pace with rising cost. This group continues to rely on wood fuels, which are becoming
very scarce. Recent news has shown that urban dwellers are collecting shrubs and leaves of
trees or left over from tree cuttings making the soil open to erosion.

This is putting tremendous pressure on the family to maintain a subsistence living. Unless
something is done soon in terms of alternative energy source or a new technological
development in the near future, the country is putting itself in a dangerous situation in terms of
sustaining the availability of wood or its derivative.

As discussed earlier biomass fuels such as wood and its derivatives are used widely in
developing countries like Ethiopia, especially in rural and poor urban areas. This biomass is
composed of complex organic maters, carbohydrates that contain carbon, nitrogen, oxygen and

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other elements in trace amounts. Smoke emission from these domestic fuels is the major source
of indoor pollution, especially in rural and poor urban communities.

This smoke contains pollutants and particulates that adversely affect the health of women. It is
reported that these pollutants are the major causes of chronic bronchitis and lung diseases. This
prolonged smoke exposure associated with biomass usage also has a huge long-term effect on
eyesight of women and infants. Studies have shown that wood, charcoal and dung produced
unacceptable levels of indoor air pollution during cooking and baking.

The wide spread practice of burning dung, burning crop residues for fuel, and deforestation for
wood is undoubtedly increasing. Deforestation cannot be reduced without providing
alternatives to the current way of cooking that also addresses related health issues. It is very
important that whatever the alternatives are, they must provide better livelihood and sustained
income generation to support the family. Otherwise the people will continue this relentless
deforestation that endangers their life and the eco-system beyond repair.

Injera can be prepared from teff and other cereals like sorghum, barley, maize and wheat. The
acceptance of such a substitute is only limited to certain areas. Cereals are bought from a nearby
grain market, preparatory operations will be carried out by local women and the grain is ground
into tine powder in a local flour mill.

The steps of injera production for white teff injera are listed as follows. [1]

First Teff flour is mixed with clean water in the ratio 1:2 (w/w) and 16 % of starter (ersho) by
the weight of the flour and was kneaded. The resultant dough was allowed to ferment for 2 - 3
days at ambient temperature. After this primary fermentation, the surface water formed on the
top of the dough was discarded. After the liquid is discarded a portion of the fermented dough
is put into a boiling water by the ratio of 80 g dough to 200ml water (locally known as “absit”)
After the absit was cooled to about 450C it was added into the main part of the dough. The
batter was left covered for 2-3 hours for secondary fermentation. The main dough was thinned
by adding water and stirred well. Finally, the batter is ready for baking which takes about 2-3
minutes.

In this paper the design considers the traditional injera preparation method and replaces it with
modern industrial baking method by using the injera processing machine. Therefore, this paper
focuses on studying the injera baking system and designing a suitable machine for baking injera
for mass production.
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1.2 Statement of the problem

The Injera baking tradition in Ethiopia is done manually by electric power that wastes more
than 50% of the electric energy or by wood fuel that uses the traditional three stone stoves
which takes longer time and relatively high amount of wood. This baking method is power
wasting, laborious and it is not hygienic. [2]

Therefore, this design will introduce a mass production of injera that can greatly assist our local
or foreign community in providing an Injera processing and baking machine that will ease the
present injera baking system.

1.3 Objective of the research

General objectives
The aim of this research paper is to design Injera processing and baking machine which can be
easily installed and operated wherever electric power is available. The automation ensures that
the job is done faster, safer and skill independent for a mass production of injera. The target
groups are institutions that have high demand of injera per day such as universities, hotels,
injera exporters and supermarkets.

Specific objectives
1) To discuss on the present and recent past situations of Injera processing & baking
technologies and alternative practices in Ethiopia.
2) To identify different methods of Injera processing and baking and to select the more
suitable design based on its efficiency that bakes injera in mass production in health
and hygienic system.
3) To make design analysis for components that would be incorporated in the generalized
system of the selected Injera processing and baking machine.
4) To determine the capacity of the designed Injera processing and baking machine.
5) To select the materials that is available, suitable and cheap for the fabrication of Injera
processing and baking machine.
6) To analyze the financial feasibility of investment in manufacturing the Injera processing
and baking machine for mass production.

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1.4 Limitations
The major limitation of this paper is that the research is limited to the design and simulation
analysis so further prototyping and testing could not be done due to lack of budget and the
variation of the data on the injera possessing and baking is also the other limitation faced in
this research.

1.5 Organization
This research will study the injera processing system and design an efficient machine that will
solve the problems of Injera baking. The design includes selecting the best design concept for
the machine, selecting different material for the machine, conducting the structural analysis by
ANSYS software. The result of the research will be included in the report with the design paper,
part drawing and assembly drawing which will be drawn by using Catia software.

Review on injera baking will be carried out by

referring research papers, journals, and related
documents.

Concepts of Injera baking machine will be

generated using sketching and different ideation
tools.

Concept evaluation for selecting the final concept will be carried

out by weighted ranking method. Rendering of the selected
design models will be done using software such as Catia V6.

Material selection and analysis will be done for

each system of the machine.

Finally the result will be discussed and the part

and assembly drawings will be drawn as per the
design.

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 DESIGN OF INJERA PROCESSING MACHINE

Chapter 2 Literature Review

2.1 Previous works on injera processing machine

In this section we will review different designs of injera baking machines which have played a
significant role in improving the present power wasting &laborious injera baking method.

The first reviewed research is the thesis paper on the Experimental Investigation on
performance, characteristics and efficiency of electric injera baking pans (“Mitad”). The aim
of this thesis work is to investigate the performance characteristics and efficiency of the electric
baking pan (“mitad”) experimentally and to prove the better efficiency of the improved
“mitad”. The average power consumptions of conventional and improved electric “mitads”
were 12.86 kW/m2 and 9.08 kW/m2respectively. The improved electric “mitad” have an
advantage over conventional electric “mitad” and it recommends to use the improved baking
pans. [2]

The second reviewed research is the thesis paper on the Design and Manufacture of Laboratory
Model for Solar powered Injera baking Oven.

The thesis research uses solar powered injera baking oven, so as to avoid the problems that are
caused due to burning of fossil fuels and to assure the environmental sustainability. In the
research a laboratory model for solar powered injera baking oven system is designed and
manufactured; the laboratory model consists of the oil storage and heating tank, the piping and
pumping system, the baking pan assembly, and supporting frame and legs as its main
components. The system uses electrical heater to heat the heat transfer oil to the required
temperature, and then the heated oil is pumped to the baking pan assembly to heat the pan
surface and re-circulates in the system using an electrical driven pump. To protect heat loss;
ash insulation system for the heat transfer oil gallery, the oil storage and heating tank and fiber
glass insulation for the piping lines are used. The baking pan surface temperature in the
experiment was measured and a temperature of about 215 oC on the pan surface was registered;
so that it was possible to bake injera. But the baking cycle needs five minutes. [3]

The third reviewed research is the thesis paper on Heat transfer analysis during the process of
injera baking by finite element method. Modeling and simulation of the heat transfer during
the process of injera baking is done by mathematical models and finite element formulations.
It studied the transient heat transfer analysis during injera baking process.

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 DESIGN OF INJERA PROCESSING MACHINE

The research states that given the same thermal property (thermal conductivity, specific heat,
and density) and power input, decreasing the thickness of a baking pan results a decrease in
heat up time and idle time of the baking pan. Given the same thermal property and same pan
thickness, decreasing power input results an increase in heat up time and idle time. And also,
it shows that, heat up and idle time of a baking pan decreases as thermal conductivity increases,
so a major improvement in efficiency will be obtained if baking pan thermal conductivity is
improved. On the other hand, increase in specific heat capacity and density of baking pans
increases the heat up time and idle time. [4]

The fourth reviewed research is injera baking machine invented by Yoseph Temesgen. This
injera baking machine includes a polishing assembly, a batter application assembly, and
deposition assembly. The polishing assembly includes a polishing pad which may be positioned
over the cooking surface when the cooking surface is not in use. When engaged, the polishing
assembly applies the rotating polishing pad to the cooking surface so as to clean it prior to
applying injera batter to the cooking surface. The batter application assembly can also be
positioned over the cooking surface with a drive to rotate a batter supply line reciprocating
nozzle carrier so that the batter is poured onto the cooking surface along an inwardly spiral
path so as to generate a uniform circular pour of batter. Finally, the deposition assembly utilizes
a reciprocating conveyor equipped with a spatula and coordinated conveyor rollers so as to
remove the cooked injera wafer from the cooking surface at cook completion time for cooling,
while simultaneously discharging a previously cooled wafer generated by this fully automated,
cyclic process. [5]

The fifth reviewed research is the Injera machine built by Dr. Wudneh Admassu a professor at
University of Idaho and Chairman of Department of Chemical Engineering.

The machine can produce 500 unit per hour consistently and uniformly 24 hours a day using
available electricity. Therefore, the Injera machine can easily be installed and operated
wherever electric power is available. The Injera made using this technology has a shelf life of
at least 5 to 7 days which is another improvement over the old way of making Injera which
becomes moldy within 1 to 2 days of shelf life. [6]

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 DESIGN OF INJERA PROCESSING MACHINE

The sixth reviewed research is a Double Sided Electric Injera Mitad by MDM engineering.

In this design, after pouring the batter on one of the two Mitad plates and allowing 15-20
seconds interval the unit is rotated 1800 and the batter will be poured to the other mitad. This
design can reduce baking energy and time by 50%. [7]

Figure 2-1 Double Sided Electric Injera Mitad

Figure 2-2 Drawing of Double sided Injera Baking Mitad

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 DESIGN OF INJERA PROCESSING MACHINE

2.2 Major problems of the existing electric Injera Mitad

The existing electric Injera Mitad technology is believed to be in the market for many years.
The performance efficiency is very lower and the product has not been standardized so far. The
electric Injera Mitads are estimated to constitute about 60 % the power demand of a typical
residential household. It is customary to see the dimming of light bulbs, lessening of power
level and the high level of steam and heat generated while Injera is baked. Other electrical
devices like stoves and water heaters will not be turned on once Mitads are in operation.
Electric Injera Mitads contribute to the bulk of the electric power demand and consumption of
a typical residential household and the nation.

The core problem of existing electric Injera Mitad is that it is energy inefficient. The causes of
energy inefficiency are mainly attributed to the high amount of electrical energy needed to heat
up to the set to temperature of about 200 – 250 0C required to bake Injera. This energy
requirement is due to high heat load, heat losses, poor heat insulation, and the method of
production of the Mitad.

The heat loss at the bottom of the clay plate in a form of radiation constitutes the major portion
of the heat lost from the Mitad. Heat insulation is commonly made using either of Pumice,
Sandstone, Gypsum and mixture of soil. The thermal conductivity of Pumice and Sandstone,
Gypsum is relatively high. Besides, these materials add weight or heat load to the Mitad. Heat
insulants like Fiber glass have very low thermal conductivity and weigh much less.

Due to the conduction of heat from the clay plate to the support ring and then to the enclosure
and convection heat transfer, heat is lost at the side of the Mitad. The lifting cover stays closed
for over 50% of the baking cycle and gets heated up. It will be heated up to a level it can’t be
touched with bare hands. Heat is lost to the surrounding through radiation, convection and
conduction from the lifting cover.

The clay plate of electric Injera Mitad is made from sand and clay. The traditional manner in
which it is produced makes the plate require more heat energy. The clay plate is produced in
rural areas in a traditional way. Sand and clay are mixed in under surface pit using hand and
legs. There is no defined or accurate measurement as to the proportions of the sand, clay and
water either by weight or by volume. Many clay plates got broken or are rejected when brought
to the market due to the traditional way of mix proportions, preparation and firing.
Disproportional mix of clay and sand results in different level of heat requirement of the clay
plate. The Specific heat of Sand and clay are different and heat added depends on the mass of
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 DESIGN OF INJERA PROCESSING MACHINE

the sand and the clay. The mixture of sand clay and water is considered to be adequate by the
feeling on the bare leg. The mix cannot be judged accurately by the feeling on the leg. This
results in inadequate mixing thereby reduce the bonding expected from the sand and clay. The
final sizing of clay plate is not precise. Thickness of plates is not uniform across the plate
diameter. These results in the difference in the depth of grooves made while placing of the
heating element there by inducing different level of heat response from the plate. After the mix
is baked and dried it is fired on an open-air using cow dung as a fire source. When cow dung
is burnt away completely the clay plate is considered fired. Because of the prevailing wind,
the fire intensity and duration, the firing process is not perfect. There are under firing or over
firing instances. This has got an impact on how the clay plate responds to an added heat and
its mechanical strength. As a result, the mass and the strength of the clay plates produced differ
from producer to producer. The heat required for the Mitad depends on the heat load.

No temperature regulating devices are installed on the electric Mitad to control overheating.
While baking Injera, over heat and under heat conditions are managed through observation and
the effects on the Injera baked. Unnecessary wastage of energy occurs while the over and under
heat cases are managed. Different sizes of electric Injera Mitad are produced. Size ranges from
40 cm diameter to 60 cm diameter. There is different level of power requirement for the
different sizes of the clay plates. As size increases heat requirement increases. However, similar
sizes of heating elements are used for clay plate sizes starting from 56 cm to 60 cm resulting
in high or low heat responses. Two pieces of the 0.9 mm diameter electrical heating element
(resistor) locally wound are commonly used per Mitad for sizes from 56 cm to 60 cm diameter.

Resistances are mostly wound locally and the value per resistor depends on the length and
diameter winded. As electrical power equals the square of voltage divided by resistance, slight
change in the value of resistance changes the power demand. Hence, the electrical Mitads
currently produced in the country do not have equal and uniform power rating, even within the
products of the same producer. Resistor is placed in spirally made grooves at the bottom side
of the clay plate in the case of the single clay type. After the resistor is placed, the groove is
sealed with Gypsum material. There is no uniformity or accepted standard adhered to in fixing
the clay plate thickness and the size and depth of the grooves. Deep groove leads to overheating
as the resistor will be closer to the baking surface whereas shallow groove makes the clay plate
unresponsive to the heat applied.

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 DESIGN OF INJERA PROCESSING MACHINE

The energy inefficiency problems on the existing electric Injera Mitad have imposed two
effects: The sets have been excessively rated to high power capacity. This placed immense
pressure on the use of the product in residential households and electrical generating and
distribution networks, and the energy demand of the country. Frequent interruptions of power
especially during religious holidays depict the intensive and concurrent use of electric Injera
Mitad. The bill of consumers is high and energy consumption at national level has become
excessive.

2.3 Control System for the baking conveyor

PLC is an industrial control computer, which uses programmable memory to store instructions
and to execute logic, sequence, timing, counting, and calculus functions, and input and output
via digital or analog, so as to control plant machinery or production processes.

With the rapid development of the microprocessor, computer and digital communication
technologies, computer control has been widely used in all industrial fields. Modern society
requires the industry to respond quickly to market demand and produce small-volume, multi-
species, multi-standard, low-cost and high-quality products. The programmable controller is to
respond to this need. Microprocessor is a general industrial control device. Programming
controller not only can realize logic control over a variety of pre-programmed programs, it also
has advantages of being freely programmable, automatic diagnosis, versatility, small size and
high reliability.

The PLC has its origin in the motor manufacturing industries. Manufacturing processes were
partially automated by the use of rigid control circuits, electrical, hydraulic, pneumatic. It was
found that whenever change had made, the system had to be rewired or reconfigured. The use
of wiring of boards on which could connections could be changed by unplugging them and
changing them around followed. With the development of microcomputers, it was realized that
if the computer could switch things on or off and respond to a pattern of inputs, then the changes
could be made by simply reprogramming the computer and so the PLC was born. PLC is an
industrial computer control system that continuously monitors the state of input devices and
makes decisions based upon a custom program to control the state of output devices. Almost
any production line, machine function, or process can be greatly enhanced using this type of
control system. However, the biggest benefit in using a PLC is the ability to change and
replicate the operation or process while collecting and communicating vital information.

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 DESIGN OF INJERA PROCESSING MACHINE

In application design of PLC systems, PLC application system was first designed, namely,
according to requirement of controlled object’s function and process, identifying system’s
work to be done and required conditions. Then analyze functions of PLC application system,
namely through the analysis of systems function, providing the PLC control system structural
form, the type of control signal, volume, system sizing, layout. Finally, according to conclusion
based on the results of the system analysis, determine the specific configuration of model and
system of the PLC. Therefore, the PLC control system is suitable for controlling the
temperature, the conveyor speed and the other components of the injera baking machine.

2.4 Teflon conveyor belt for baking surface

Polytetrafluoroethylene (Teflon) coated conveyor belting is a small but important segment of
the lightweight belting market. Teflon belts have a service temperature of 500 F. With proper
belt design, higher temperatures are possible. Many Teflon coated belts are used to process
foods. Teflon is very inert. It will not react with or contaminate the product being conveyed.

Teflon belts are very slippery. All Teflon coated fiberglass belts are coated on both faces. This
leads to a belt that slides easily on the machine’s support structure. It also means that the
product being conveyed slides easily and is easily removed from the belt at the exit of the
process which makes it suitable to bake injera continuously on the baking machine.

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 DESIGN OF INJERA PROCESSING MACHINE

Chapter 3 Analytical methods, Conditions and Design

3.1. Conceptual design

This is a system of selecting good alternatives among many model solutions for a particular
problem. There are different types of design matrices that are used to make systematic decisions
on choosing best alternatives of given two or more alternative designs. The design matrix
method is selected so that to help reach at the best decision.

Figure 3-1 Baking design concept 1

Figure 3-2 Baking design concept 2

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-3 Baking design concept 3

Figure 3-4 Baking design concept 4

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-5 Baking design concept 5

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-6 Baking design concept 6

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-7 Baking design concept 6

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-8 Baking design concept 6

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 DESIGN OF INJERA PROCESSING MACHINE

Table 3-1 Ranking for design concept selection of Injera Baking machine
Therefore concept 6 is the selected injera baking design concept.

No Criteria Weight Concepts

1 2 3 4 5 6

% W % W % W % W % W % W

10 O. Capacity 0.182 62 11.284 75 13.65 79 14.37 77 14.014 80 14.56 82 14.924

8 O. Safety 0.164 64 10.496 65 10.66 68 11.15 70 11.48 76 12.464 80 13.12

9 Simplicity 0.127 62 7.874 65 8.255 71 9.017 73 9.271 76 9.652 79 10.033

4 Ease of O. 0.109 63 6.867 67 7.303 81 8.829 80 8.72 84 9.156 85 9.265

2 M. Cost 0.109 67 7.303 71 7.739 78 8.502 82 8.938 85 9.265 84 9.156

1 M. Cost 0.073 60 4.38 65 4.745 69 5.037 72 5.256 74 5.402 76 5.548

6 M. 0.073 66 4.818 66 4.818 68 4.964 71 5.183 65 4.745 71 5.183

7 Durability 0.073 65 4.745 75 5.475 79 5.767 80 5.84 84 6.132 81 5.913

11 Efficiency 0.054 62 3.348 69 3.726 80 4.32 82 4.428 75 4.05 78 4.212

3 Ease of A. 0.018 60 1.08 62 1.116 60 1.08 65 1.17 61 1.098 68 1.224

5 Easy of A. 0.018 59 1.062 65 1.17 63 1.134 72 1.296 61 1.098 71 1.278

Sum 63.257 68.65 74.1 75.596 77.62 79.856

7 8 2

Rank 6th 5th 4th 3𝑟𝑑 2𝑛𝑑 1st

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-9 Selected baking design concept 3D model

To bake the circular injera the batter is deposited on the conveyor belt through the rotating
cylinder while the remaining batter in the rotating cylinder will be pumped back into the
depositor.

To bake the square (rectangular) injera the batter is deposited directly on the conveyor belt and
the cutting process can be done before baking by clothing and opening the actuator while
depositing or after baking by the rotary cutting mechanism.

The baking process starts by pumping the batter into the reservoir on the top. When the
conveyor belt is heated to the baking temperature the depositor locking mechanism is opened
and the injera batter is poured on the conveyor belt continuously. For some time, the injera
passes through the oven where it is heated by the heater which is placed between the conveyor
belts.

At the end of the baking process the injera will pass through a fan and it is allowed to cool for
some time while it is on the same conveyor. After the baking process the cooling conveyor will
receive the injera and more cooling will take place.

Since the machines for mixing, fermenting, cooling, cutting, rolling and packing are available
in the market they can be selected according to the design concepts. But in this paper, we will
focus on the detail design of the injera baking machine.

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 DESIGN OF INJERA PROCESSING MACHINE

3.2 Embodiment design

3.2.1 Structural analysis of the frame
The model of the frame is designed by Catia software. This model is imported to ANSYS
software for structural analysis. The ANSYS result for the static structural analysis is shown
below.

The frame length is selected by considering the time it takes for the injera to be baked along
the moving conveyor. Whereas the height of the conveyor is as per the standards of ergonomic
heights.

Weight of batter container

Lower part volume; VA = A x h

= π(0.5m)2x 1m

= 0.7854 m3

Density of batter =1175 kg/m3 [2]

Mass of batter = density of batter∗ volume of batter

= 1175 kg/m3 ∗ 0.7854 m3

=922.84 kg

Weight = (922.84 Kg) *(9.8m/s2)

= 9043.8N

Properties of the selected material: -

Structural steel

Density 7850 kg/m3

Dimension: 40 x 40 x 4 mm

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-10 Load distribution

Figure 3-11 equivalent stress

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-12 equivalent elastic strain

Figure 3-13 Total deformation

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-14 Safety factor

As we can see from the above results, the frame structure is safe to carry the load. The
deformation shown by the ANSYS result is negligible.

3.2.2 Motor selection

Most rotary electromechanical systems have a need to accelerate, run at a constant speed, and
decelerate. To move any mechanical system with contacting elements, friction, damping, and
any external loading must be overcome by the motor. When accelerating, additional torque
must be applied to overcome inertia. While decelerating, friction and damping apply a negative
torque and assist the motor with deceleration. Motor sizing starts with a first pass estimation of
all loading and a simple move profile defining the worst-case acceleration and velocity
requirements. The most common type of general-purpose motors found in industrial motor
systems are squirrel cage induction motors. These motors are generally referred to as “general-
purpose motors.” The squirrel cage name is derived from the shape of the motor’s rotor, which
is shaped like a cylinder constructed from bars and rings, and which resembles a hamster’s
cage.

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 DESIGN OF INJERA PROCESSING MACHINE

To optimize system efficiency, it is important to select the appropriate motor to meet the needs
of the application. For more detailed information, the National Electrical Manufacturers
Association (NEMA) Motor Generator Section maintains standards for squirrel cage induction,
NEMA Standards Publication MG 1–2010.

Motor specification

The nameplate of a motor provides important information necessary for selection and
application. Below is the nameplate of a sample 15 horsepower AC motor. Specifications are
given for the load and operating conditions as well as motor protection and efficiency.

AC motors are designed to operate at standard voltages and Frequencies. This motor is
designed for use on 230 VAC 60Hz and 200VAC 50Hz. Full load current for this motor is 38
amps (60Hz) and 44 amps (50Hz).

Base speed is the nameplate speed, given in RPM, where the motor develops rated horsepower
at rated voltage and frequency. It is an Indication of how fast the output shaft will turn the
connected equipment when fully loaded with proper voltage and frequency applied.

The base speed of this motor is 1780 RPM (60Hz) and 1475 RPM (50Hz). It is known that the
synchronous speed of a 4-pole motor is 1800 RPM. When fully loaded there will be 1.1% slip.
If the connected equipment is operating at less than full load, the output speed (RPM) will be
slightly higher than name plate.

% 𝑆𝑙𝑖𝑝 = (1800 − 1780) 𝑥 100% / 1800 % 𝑆𝑙𝑖𝑝 = 1.1

A motor designed to operate at its nameplate horsepower rating has a service factor of 1.0. This
means the motor can operate at 100% of its rated horsepower. Some applications may require
a motor to exceed the rated horsepower.

In these cases, a motor with a service factor of 1.15 can be specified. The service factor is a
multiplier that may be applied to the rated power. A 1.15 service factor motor can be operated
15% higher than the motor’s nameplate horsepower. The 15 HP motor with a 1.15 service
factor, for example, can be operated at 22.5 HP. It should be noted that any motor operating
continuously at a service factor greater than 1 will have a reduced life expectancy compared to
operating it at its rated horsepower.

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 DESIGN OF INJERA PROCESSING MACHINE

In addition, performance characteristics, such as full load RPM and full load current, will be
affected.

Procedure for selection:

1. Check speed ratio of the gear head

2. Calculate the required torque
3. Select motor (ORIENTAL or SIEMENS catalogue)
4. Calculate the starting torques
5. Check moment of load inertia

Based on the above procedures,

1. Determining the gear ratio

𝑉×60 110±11×60
Speed at the gear head output shaft, 𝑁𝐺 = = = 17.5 ± 1.75 𝑟𝑒𝑣/𝑚𝑖𝑛
𝜋×𝐷 𝜋×120

Where Belt speed = 110 mm/sec

Because the rated speed for a 4-pole motor at 60 Hz is 1450 to 1550 rev/min, the gear ratio
is calculated as follows:

1450~1550 1450~1550
Gear ratio, 𝑖 = = = 82.85~88.57
𝑁𝐺 17.5±1.75

This gives as a gear ratio 𝑖 = 85

2. Calculate the required torque 𝑻𝑴

Friction coefficient of sliding surface F is calculated as follows:

Where total mass of belt and load, m1 Kg which is mass of belt + injera = 5.5 Kg

𝐹 = 𝐹𝐴 + 𝑚 · 𝑔 (𝑠𝑖𝑛 𝜃 + 𝜇 · 𝑐𝑜𝑠 𝜃)
𝐹 = 0 + 5.5 × 9.807(sin 0 + 0.8 · cos 0)
𝐹 = 43.15 𝑁

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 DESIGN OF INJERA PROCESSING MACHINE

Take η = 0.9 for between rebar and rollers

Where diameter of the roller (D) = 120 mm

𝐹𝐷 43.15 × 0.12
𝐿𝑜𝑎𝑑 𝑡𝑜𝑟𝑞𝑢𝑒, 𝑇𝐿 = =
2𝜂 2 × 0.9
= 2.876 𝑁𝑚
Taking safety factor of 2

𝑇𝐿 = 2.876 × 2 = 5.752 𝑁𝑚

Here, 5IK60GE-AW2U and 5GE50SA are tentatively selected as the motor and gearhead
respectively, by referring to the “Gear motor – of oriental motor general catalogue.

Convert this load torque to a value on the motor output shaft to obtain the required torque𝑇𝑀 ,
as follows: taking gear efficiency 𝜂𝐺 = 0.66

𝑇𝐿 5.752
𝑆𝑡𝑎𝑟𝑡𝑖𝑛𝑔 𝑡𝑜𝑟𝑞𝑢𝑒 , 𝑇𝑚 = =
𝑖 × 𝜂𝐺 85 × 0.66

= 0.1025 𝑁𝑚 [102.5 𝑚𝑁. 𝑚]

Since the starting torque of the 5IK60GE-AW2U motor is 320 mN·m, this is greater than the
required torque of 102.5mN·m

3. Checking moment of load inertia

Inertia of belt and load,

𝐷
𝐽𝑚1 = 𝑚𝑙 ( ) 2
2

= 5.5 𝐾𝑔 × (0.06)2

= 0.0198 𝐾𝑔𝑚2

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 DESIGN OF INJERA PROCESSING MACHINE

Inertia of roller,

Where mass of roller diameter is 1.5 kg

1
𝐽𝑚2 = ( ) 𝑚𝑟 𝐷2
8

1
= ( ) × 1.5 × (0.12)2
8

= 0.0027 𝐾𝑔𝑚2

𝐽 = 𝐽𝑚1 + 2𝐽𝑚2

= 0.0198 + 2 × 0.0027

= 0.0252 kg𝑚2

Form the above calculation we have:

Power rated = 2.5 KW at 50 Hz

𝑛𝑟𝑎𝑡𝑒𝑑 = 1465 𝑟𝑝𝑚

𝑇𝑟𝑎𝑡𝑒𝑑 = 5.752 𝑁𝑚

Mounting = horizontal

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 DESIGN OF INJERA PROCESSING MACHINE

3.2.3 Controller Selection for the baking system

In temperature control applications a set-point is the target value at which a controller attempts
to maintain the process variable. This can be achieved by adjusting its control output power
(the correcting variable). Controllers have a local set-point and sometimes remote or other
alternative set-points. Set-point values are limited by the instrument input range and any other
environmental or equipment limits.

These days to control the temperature of various baking systems programmable controllers are
used. A programmable temperature controller can be configured with multiple set-points that
are sequenced with preset timing criteria. In its simplest form this can be one profile made up
of multiple segments.

Heat-up, soak, and cool-down times are critical elements to consider for selecting the correct
equipment. Heat-up time is the time that the equipment takes to increase from room
temperature to the desired amount for baking or other function. Soak time refers to that time
when the product has reached the desired process temperature for the desired length of time.

A programmable controller can be programmed to remain at temperature for a specified period

of time, then cool-down to complete the cycle. For many applications, the soak cycle begins
once the time specified for the heat-up cycle has been completed. When more precise control
is necessary and it is important that the soak time does not begin until the product has reached
its temperature, a guaranteed Soak option can be utilized. In this method, the controller does
not start timing the soak cycle until a thermocouple embedded on the product, or in the
airstream, senses that the set point temperature has been reached.

Since cooling can be thought of as removal of heat, product considerations for cool-down rates
are similar to those for heat-up rates. Typically, oven cool-down is achieved by exhausting
heated air from the oven. A corresponding flow of cooler, ambient air will enter the oven to
replace the warm exhausted air. If the cool-down rate requires no control, the only need is to
size the exhaust fan large enough to remove the necessary amount of heat in the required time.

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 DESIGN OF INJERA PROCESSING MACHINE

If the cool-down rate is to be controlled, it can be accomplished in two ways:

A. Exhaust dampers are opened to the maximum position, and the heaters supply enough heat
to keep the temperature from dropping too rapidly.

B. Modulating dampers can be controlled by the programmable controller. The dampers will
modulate in order to maintain a controlled cooling rate. Consider that the initial investment
cost of this option is greater, but operating costs will be lower due to a more energy efficient
system.

Figure 3-15: Different Segments of programmable temperature controller

In a programmable temperature controller, the programs are also referred to as profiles and
recipes. There are a number of programmable temperature controllers available which can store
multiple ‘recipes’ allowing the programmable controller to be used for a variety of processes,
for example baking of edible products, heat treatment of metals and composites. With some
instruments it is possible to set the programs (profile, recipe) to be joined and/or repeated. The
complexity of the temperature control process will dictate how sophisticated the programmable
controller needs to be.

When choosing a programmable controller, the number of segments and programs that the
process and application needs will be your first concern. These ranges widely between
instruments, for example in the West Control Solutions product offering the PMA KS 40-1
includes a basic programmer function which allows up to 4 segments with 1 program however
the new Pro-EC44 dual loop temperature controller from West allows up 255 segments and 64
programs [18].

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 DESIGN OF INJERA PROCESSING MACHINE

Other options to consider when choosing a programmable temperature controller include:

Human Machine Interface and Displays: This is particularly important if you will be
expecting users to make regular profile selections via the menu interface. A text display can
make the selection process much easier and minimize errors.

Outputs and Event Options: Programmable temperature controllers will vary in the number
of outputs available and how these are used. In addition to main control and alarms, outputs
are often assigned as profile events and are activated for associated control whilst specific
segments are running. Again, the process will dictate your requirements here.

Remote Input: This can allow remote program selection as well as remote run, hold and reset
options.

Real Time Clock: This is not commonly available on programmable controllers but it is a very
useful if your process needs to work in real-time.

Data Logging: As one would assume, a programmable temperature controller with a built-in
data logger allows important process data to be logged and stored for later analysis. This can
be a key requirement to meet some industry regulations or where a process audit trail is
required. You may also require this data to be available on a PC, if so you will need to consider
communications option or a USB port.

Heating Element selection

Heat can be transferred to any process by natural convection, forced convection or by radiant
heat sources. Natural convection heating can be very fast, but may not be as uniform as forced
convection. There are many issues that must be considered when selecting industrial
equipment, including: the quantity of material to be processed, the uniformity, size and shape
of the products, the temperature tolerance that is permissible, whether the product or process
lends itself to be batch or continuous in-line processing.

There are many ways of heating the continuous Teflon conveyer. Many industries use:
electrical heaters or combustion heating systems that use fuels or natural gases. Most of the
time industries that produce edible products like bread and biscuits use electrical heaters
because they provide a clean and leakage free heating mechanism. Because of these reason
electrical resistors are used to heat the Teflon conveyer.

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 DESIGN OF INJERA PROCESSING MACHINE

Indirect resistance heating involves passing line alternating current through low resistance
heating elements. The resistance to the current flow generates heat in the coil; and the heat is
transferred to the baking Teflon by convection.

The whole process of heating the Teflon conveyer lies up on giving the right amount of current
to the resistance heating element. The supplied electrical power is converted to heat energy and
it is used to heat the Teflon conveyer.

𝑉
𝑃 = 𝐼2𝑅 =
𝐼

The most widely used heating element resistance is the Ni-Cr resistance. This heating resistance
type is selected because [17]: It has high melting point 14000 C − 25500 𝐶 and it doesn’t
oxidize at high temperature.

It doesn’t expand when heated and the temperature increases only by 10% between its room
temperature and its maximum temperature limit.

Temperature Sensor Selection

There are different types of temperature sensors that can be used in different industrial
environment. These temperature sensors are constructed to withstand certain amount of
temperature. The following table describes the different type of sensors and their temperature
sensing range.

Table 3-2: Temperature Sensor Types [16]

THERMOCOUPLE RTD THERMISTOR SEMICONDUCTOR

widest Range: Range: Range: Range:

–184ºCto+2300ºC –200ºCto+850ºC 0ºCto+100ºC –55ºCto+150ºC

High Accuracy Fair Linearity Poor Linearity Linearity:1ºC

and
Repeatability Accuracy:1ºC
Needs Cold Requires Requires Requires Excitation
Junction
Compensation Excitation Excitation
Low-Voltage Output Low-cost High Sensitivity 10mV/K,20mV/K,

or1µA/K Typical

Output
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 DESIGN OF INJERA PROCESSING MACHINE

Table 3-3: Common Thermocouple Sensors [16]

TYPICAL NOMINAL ANSI

JUNCTION MATERIALS USEFUL
RANGE (ºC) SENSITIVITY DESIGNATION

(µV/ºC)
Platinum (6%)/Rhodium- 38 to 1800 7.7 B
Platinum (30%)/Rhodium

Tungsten (5%)/Rhenium - 0 to 2300 16 C

Tungsten ( 26%)/Rhenium

Chromel-Constantan 0 to 982 76 E

Iron-Constantan 0 to 760 55 J

Chromel-Alumel –184 to 1260 39 K

Platinum 0 to 1593 11.7 R

(13%)/Rhodium-Platinum

Platinum 0to1538 10,4 S

(10%)/Rhodium-Platinum

Copper-Constantan –184to400 45 T
For baking injera by using Teflon continuous conveyor the Copper and Constantan alloy
thermocouple temperature sensor is selected because of its high accuracy, repeatability and the
desired temperature range that is required. The Thermocouple sensor is placed on the Teflon
conveyer and will sense the conveyer temperature. The temperature control of the Teflon
continuous time oven is described by the figure 3-16 shown below.

Figure 3-96 Automatic Controller Setup for Temperature Control

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 DESIGN OF INJERA PROCESSING MACHINE

3.2.4 Heat transfer analysis

Heat transfer, also known as heat flow, heat exchange, or transfer of thermal energy is the
movement of heat from one place to another. When an object is at a different temperature from
its surroundings, heat transfer occurs so that the body and the surroundings reach the same
temperature at thermal equilibrium. Such spontaneous heat transfer always occurs from a
region of high temperature to another region of lower temperature. There are three basic modes
of heat transfer mechanisms conduction, convection and radiation.

Conduction Heat Transfer Mechanism

Transfer of energy between objects in physical contact is the transfer of heat from a hot side to
a cooler side through a dividing medium. The hot side heats the molecules in the dividing
medium and causes them to move rapidly, heating the adjacent molecules until the cool side is
heated. The transfer of heat stops when the temperature of the hot side equals that of the cool
side

Conduction through a plane wall: The details of conduction are quite complicated but for
engineering purposes may be handled by a simple equation, usually called Fourier’s equation.
For the steady flow of across a plane wall with the surfaces at temperatures of 𝑇1 and 𝑇2 where
𝑇1 is greater than 𝑇2 ; the heat flow Q per unit area A; (the heat flux) is:

𝑄 𝑇1 − 𝑇2 Δ𝑇
= 𝐾{ }=𝐾
𝐴 𝑋1 − 𝑋2 Δ𝑋

𝑑𝑇
𝑄̇ = −𝐾𝐴
𝑑𝑋

Where: 𝑄̇ - Is the heat flow (W)

𝑑𝑇
- Temperature gradient in plane of heat transfer in the direction of heat flow (℃/𝑚)
𝑑𝑋

k- Thermal conductivity (𝑊/𝑚℃)

A – Area perpendicular to heat flow (𝑚2 )

The negative sign in the equation is provideed to account for the fact that heat is conducted
from a high temperature to a low temperature, so that (𝑑𝑇⁄𝑑𝑥) inherently negative; therefore
the double negative indicates a positive flow of heat in the direction of decreasing temperature.
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 DESIGN OF INJERA PROCESSING MACHINE

Convection Heat Transfer Mechanism

Convection is transfer of energy between an object and its environment, due to fluid motion.
Convection can be forced convection in which the flow is caused by a pump or a fan or it may
be natural convection in which the flow is caused by density differences due to differences in
temperature. It is found that the heat flux is approximately proportional to the temperature
difference between the wall and the bulk of the fluid.

𝑄
∝ (𝑇𝑓 − 𝑇𝑠 )
𝐴

This causes to define a constant proportionality called “convection heat transfer coefficient”
denoted by, h

𝑄
= ℎ(𝑇𝑓 − 𝑇𝑠 )
𝐴

Thus the rate of convection heat transfer is given by [23].

𝑄̇ = ℎ𝐴(𝑇𝑓 − 𝑇𝑠 )

Where: ℎ -convection heat transfer coefficient (𝑊/𝑚2 ℃)

𝐴 - Surface area where convection takes place (𝑚2 )

𝛥𝑇 - Temperature difference between the fluid and the wall surface (℃)

Radiation Heat Transfer Mechanism

Transfer of energy from or to a body by the emission or absorption of electromagnetic radiation.

All objects with a temperature above absolute zero radiate energy at a rate equal to their
emissivity multiplied by the rate at which energy would radiate from them if they were a
blackbody. According to Stefan–Boltzmann law, ideal radiators emit energy at a rate
proportional to the fourth power of the absolute temperature. And the net rate of exchange of
energy between two ideal radiators A and B related as; [24].

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 DESIGN OF INJERA PROCESSING MACHINE

𝑄̇ = 𝜎𝐴(𝑇𝐴 4 − 𝑇𝐵 4 )

Where: 𝜎 - Stefan- Boltzmann constant = 5.67𝑥10−12 𝑤/𝑐𝑚2 . 𝑘 4

𝑇A - Temperature of body A (℃)

𝑇B - Temperature of body B (℃)

In many physical situations, we are interested in radiation heat transfer from the surface of an
object to the surrounding uniform temperature. Thus, the net radiation from a non-black surface
to the surrounding is given by [25].

𝑄̇ = 𝜀𝜎𝐴(𝑇𝑠 4 − 𝑇∞ 4 )

Where: 𝑇𝑠 - Body surface temperature (℃)

𝑇∞ - Surrounding or ambient temperature (℃)

ε - Emissivity of the surface and has a value between 0 and 1, for a perfect reflector ε = 0 and
for a perfect emitter a so called “black body”, ε = 1.

3.2.5 Optimum Temperature of the resistance heater coil for the baking conveyor

When a medium comes in contact with another, heat exchange will occur from one point of a
medium to another whenever there is a temperature difference between the two. A
distinguishing characteristic of conduction is that it takes place within the boundary of a
medium, or across the boundary of a medium into another medium in contact with the first,
without an appreciable displacement of the matter.

Heat conduction is important in stove top cooking, where heat is conducted from the heat
source electric coils directly to the bottom side of the baking conveyor. Energy transfer from
resistance coil heater to the Teflon baking conveyor was in the way of conduction heat transfer
mechanism.

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 DESIGN OF INJERA PROCESSING MACHINE

Figure 3-17: Top view of the baking conveyor

Thus, the rate of heat transfer from the resistance heater coil to baking conveyor surface can
also be expressed as follows.

𝐾𝐴(𝑇𝑏 − 𝑇𝑢 )
𝑄̇ =
𝑡𝑐

Where: 𝑇𝑏 - baking conveyor bottom temperature (℃) (which is equivalent to the resistance
heater coil temperature)

𝑇𝑢 - baking conveyor surface temperature = 200℃

𝑡𝑐 - Thickness of baking conveyor (Teflon) = 0.003m

𝐾- Thermal conductivity of Baking conveyor (Teflon) = 0.25w/m.K

A- Area of the baking section of conveyor is calculated as follows;

𝐴 = 𝐿𝑏 × 𝑊

Where: 𝐿𝑏 - the length of baking section of conveyor = 5.4m

𝑊- The width of baking conveyor =1.2 m

𝐴 = 𝐿𝑏 × 𝑊 = 5.4 × 1.2 = 6.4 𝑚2

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 DESIGN OF INJERA PROCESSING MACHINE

Now the required resistance coil temperature can be calculated as:

𝐾𝐴 (𝑇𝑏 − 𝑇𝑢 )
𝑄̇ =
𝑡𝑐

𝑄̇ 𝑡𝑐
𝑇𝑏 = + 𝑇𝑢
𝐾𝐴

50.634 × 0.003
= + 473
0.25 × 6.4

𝑇𝑏 = 566.8 𝐾

3.2.6 Heat Loss Coefficients of the Injera Machine

Heat is lost in three directions of the bake ware; the side, the bottom and top directions of the
baking machine. All the three modes of heat transfers, (conduction, radiation and convection)
participate in the transfer of energy to the surrounding. During baking the bottom side of injera
is heated by conduction from the top bake conveyor surface temperature and heat is given off
from injera at the top surface to the surrounding by convection. So, the energy transferred to
injera is accounted as the useful energy used to cook the product and is not considered as a
loss.

Heat lost coefficients are the heat transfer coefficients at the three sides of the stove and the
heat transfer to the product were not considered as a loss since this is the heat used to cook the
product. The heat source to the bake ware (conveyor) is the current flow through Ni–Cr coil
(or wire) that is placed at the bottom side of the bake ware. The bake ware gets heated due to
the resistance to the flow of current through the heating element; what is termed as ohmic
heating or joules heating.

1. Surface heat transfer coefficient

The surface heat transfer coefficient is influenced by the composition of the fluid, the nature
and geometry of the surface, and on the hydrodynamics of the fluid moving past the surface.
From the boundary conditions the heat transfer coefficient over the bake ware is the combined
effect of the heat transfer coefficient due to convection and radiation.

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 DESIGN OF INJERA PROCESSING MACHINE

ℎ = ℎ𝑐 + ℎ𝑟

Where ℎ𝑐 convective heat transfer coefficient in (𝑊/𝑚2 . 𝐾), ℎ𝑟 radiation heat transfer
coefficient (𝑊/𝑚2 . 𝐾) and h is surface heat transfer coefficients in (𝑊/𝑚2 . 𝐾). The convective
heat transfer coefficient over the bake ware is given by:

𝐾
ℎ𝑐 = 𝑁𝑢
𝐿

Where: - Nu is the Nusselt Number, K is the thermal conductivity of the fluid (w/m. k), L is the
characteristic length (m).

1.1 Convective Heat Transfer Coefficient

In the determination of convective heat transfer coefficient, a dimensionless functional

relationship derived using dimensional analysis between hc and the relevant physical properties
of the flow situation are used. The functional of this relationship is given by:

ℎ𝑐 = 𝑓(𝜌, 𝑉, 𝐿, 𝜇, 𝐶𝑝 , Δ𝑇, 𝑘, 𝛽, 𝑔)

Where ρ is Density of fluid (𝑘𝑔/𝑚3 ), V is Average fluid velocity (m/s), L is Characteristic

length (m), μ is Dynamic viscosity (𝑁. 𝑠/𝑚2 ), Cp is Specific heat of fluid (J/kg. k), β is
volumetric expansion coefficient (1/k), g is Gravitational acceleration (m/s2). By performing
a dimensional analysis on the above equation, the following functional relation can be derived:

ℎ𝑐 𝐿 𝜌𝑉𝐿 𝐶𝑝 𝜇 𝜌2 𝑔𝛽𝐿3 Δ𝑇 𝑉 2
= 𝑓( , , , )
𝑘 𝜇 𝑘 𝜇2 𝐶𝑝 Δ𝑇

𝜌𝑉𝐿 𝐶𝑝 𝜇
Where: 𝑅𝑎 = is the Reynolds number, 𝑃𝑟 = is the Prandtle number,
𝜇 𝑘

𝜌 2 𝑔𝛽𝐿3 Δ𝑇 𝑉2
Gr = is the Grashof number and 𝐶 is the Eckert number.
𝜇2 𝑝 Δ𝑇

In natural convection, many correlations contain the product of the Grashof number and the
Prandtle number; the product of the two is defined as the Rayleigh number:

𝑅𝑎 = 𝐺𝑟 × 𝑃𝑟

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 DESIGN OF INJERA PROCESSING MACHINE

As the motion of fluid in free convection is caused by buoyancy force, for horizontal plates
either from the plate or toward the plate, two situations should be distinguished.

1. Heat transfer occurs in the direction of gravitational force (lower surface heated upper
surface cooled), and
2. Heat transfer against the direction of gravitational force (Upper surface heated lower
surface cooled).
A general correlation between the Nusselt number and Rayleigh number is developed based on
the above two cases as:

𝑁𝑢 = 𝐶 (𝐺𝑟 × 𝑃𝑟)𝑚

Where, m and C are constants. The value of the constants depends on geometry. Since heat
transfer during injera baking is against the direction of gravitational force, only the second case
is used for this study.

Fuji and imura (1972) proposed the following correlation for heat transfer to or from the
horizontal plate with constant heat flux [19, 20, 21, and 22]:

1
𝑁𝑢 = 0.13(𝐺𝑟 × 𝑃𝑟)3 , 𝑓𝑜𝑟 𝐺𝑟 × 𝑃𝑟 < 2 × 108

1
𝑁𝑢 = 0.16(𝐺𝑟 × 𝑃𝑟)3 , 𝑓𝑜𝑟 2 × 108 < 𝐺𝑟 × 𝑃𝑟 < 2 × 1011

In these equations 𝛽 is evaluated at 𝑇∞ + 0.25[𝑇𝑠 − 𝑇∞ ], and all other properties at

𝑇𝑠 − 0.25 [𝑇𝑠 − 𝑇∞ ]. 𝑇𝑠 is the average wall temperature.

Correlations are also presented for flat surfaces without constant heat flux as [19, 20, 21, & 22]

1
𝑁𝑢 = 0.54(𝐺𝑟 × 𝑃𝑟)4 , 𝑓𝑜𝑟 104 < 𝐺𝑟 × 𝑃𝑟 < 107

1
𝑁𝑢 = 0.15(𝐺𝑟 × 𝑃𝑟)3 , 𝑓𝑜𝑟 107 < 𝐺𝑟 × 𝑃𝑟 < 1011

To determine the convictive heat transfer coefficients of the bake ware air properties are first
evaluated at the average film temperature. This is the average temperature between the bake
conveyor top surface and the temperature of the environment.

𝑇𝑠 + 𝑇∞ 473 + 293
𝑇𝑆 = = = 383 𝐾
2 2

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 DESIGN OF INJERA PROCESSING MACHINE

The film temperature to evaluate the properties of air is

𝑇𝑊 = 𝑇𝑆 + 0.25 [𝑇𝑠 − 𝑇∞ ] = 383 + 0.25 [473 − 293) = 360.5 𝐾]

Properties at the average film temperature:

𝑘𝑔⁄
𝜌 = 0.996 𝑚3 , 𝐶𝑝 = 1010 𝐽⁄𝑘𝑔. 𝐾 𝑘 = 30.5275 × 10−3 𝑊⁄𝑚. 𝐾

2 𝐾𝑔⁄
𝛽 = 2.773 × 10−3 𝐾, 𝑣 = 2.216 × 10−5 𝑚 ⁄𝑠, 𝜇 = 2.1193 × 10−5 𝑚. 𝑠

2
𝑔 = 9.81 𝑚⁄𝑠 2 , 𝛼 = 3.131 × 10−5 𝑚 ⁄𝑠 𝑃𝑟 = 0.71

𝐴𝑆
The characteristic length for horizontal surface is calculated as 𝐿 = where, 𝐴𝑆 is the surface
𝑃

area and p is the perimeter of the baking conveyor [19, 21]. The dimensionless Grashof and
Rayleigh numbers become:

𝑔𝛽𝐿3 Δ𝑇 9.81 × 2.773 × 10−3 × (0.5122)3 × (383 − 293)

𝐺𝑟 = =
𝑣2 (2.216 × 10−5 )2

= 6.70 × 108

𝑅𝑎 = 𝐺𝑟 × 𝑃𝑟 = 6.70 × 108 × 0.71 = 4.76 × 108

Thus,

1 1
𝑁𝑢 = 0.16(𝐺𝑟 × 𝑃𝑟)3 = 0.16(6.70 × 108 × 4.76 × 108 )3 = 124.91

The convective heat transfer coefficient becomes:

𝑁𝑢 𝐾 124.91 × 30.5275 × 10−3

ℎ𝑐 = = = 7.44 𝑊/𝑚2 . 𝐾
𝐿 0.5122

The radiative heat transfer coefficient at the surface of the baking pan is given by:
𝜎𝜀(𝑇𝑠4 − 𝑇∞2 )
hr =
(𝑇𝑠 − 𝑇∞ )

Where; 𝜎 is the Stefan Boltzmann constant (5.67 ∗ 10−8 𝑊⁄ 2 4 ),

𝑚 𝐾

and 𝜀 is the emissivity of the surface conveyor (0.91).

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 DESIGN OF INJERA PROCESSING MACHINE

Therefore: hr = 0.91 × 5.67 ∗ 10−8 (4732 + 2932 )(473 + 293)

hr = 12.47 𝑊⁄𝑚2 . 𝐾

The surface heat transfer coefficient on the top surface of the baking pan will be:

ℎ = ℎ𝑐 + ℎ𝑟 = 7.44 + 12.47 = 19.91 𝑊⁄𝑚2 . 𝐾

1.2 Bottom Heat Transfer Coefficient

Heat is lost from the plate to the ambient, by conduction through the insulation to the plate
casing and subsequently by convection and radiation from the bottom casing surface to the
ambient:

−1
𝑡𝑖𝑛 𝑡𝑝 1
𝑈𝑏 = [ + + ]
𝑘𝑖𝑛 𝑘𝑝 ℎ𝑏

ℎ𝑏 - The total heat transfer coefficient on the bottom plate surface to the ambient air

𝑡𝑖𝑛 - Insulation thickness = 40 mm

𝑘𝑖𝑛 - Thermal conductivity of gypsum = 0.15w/m.K

𝑡𝑝 - Plate thickness = 3 mm

𝑘𝑝 - Thermal conductivity of plate = 40 w/m.k

And the total heat transfer coefficients from the bottom casing surface to the ambient is given
as:

ℎ𝑏 = ℎ𝑐,𝑏 + ℎ𝑟,𝑏

Where; ℎ𝑟,𝑏 is the bottom radiative heat transfer coefficient, It can be determined as by
considering the bottom surface temperature is 55 ℃

𝜎𝜀(𝑇𝑠,𝑏 4 − 𝑇∞4 )
ℎ𝑟,𝑏 = = 𝜀𝑏 𝜎(𝑇𝑠,𝑏 2 + 𝑇∞ 2 )(𝑇𝑠,𝑏 + 𝑇∞ )
(𝑇𝑠,𝑏 − 𝑇∞ )

= 0.10 × 5.67 ∗ 10−8 ((318)2 + (293)2 )(318 + 293)

ℎ𝑟,𝑏 = 0.648 𝑊⁄𝑚2 . 𝐾

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 DESIGN OF INJERA PROCESSING MACHINE

And, ℎ𝑐,𝑏 is the bottom convective heat transfer coefficient = 11.8 𝑊⁄𝑚2 . 𝐾

ℎ𝑏 = ℎ𝑐,𝑏 + ℎ𝑟,𝑏 = 11.8 + 0.648 = 12.44 𝑊⁄𝑚2 . 𝐾

Then the back-loss coefficient formula and its analytical value can be determined as:

−1
𝑡𝑖𝑛 𝑡𝑝 1 0.04 0.003 1 −1
𝑈𝑏 = [ + + ] =[ + + ]
𝑘𝑖𝑛 𝑘𝑝 ℎ𝑏 0.15 40 12.44

𝑈𝑏 = 2.89 𝑊⁄𝑚2 . 𝐾

1.3 Lateral or Edge Heat Transfer Coefficient

Energy lost from the lateral side of the baking pan casing may be taken to have similar value
with the bottom side of the baking pan if the thickness and area of the edge insulation has the
same to that of back (or bottom) insulation. But they have different thickness and area.

−1
𝑡𝑖𝑛 𝑡𝑝 1
𝑈𝑒 = [ + 2( ) + ]
𝑘𝑖𝑛 𝑘𝑝 ℎ𝑠

0.04 0.003 1 −1
=[ + 2( )+ ]
0.15 40 12.44

𝑈𝑒 = 2.880 𝑊⁄𝑚2 . 𝐾

3.2.7 Energy required for injera baking

Baking injera requires intensive energy and this energy can be defined as the energy necessary
to raise the batter to a particular temperature, and evaporate the amount of water that is to be
lost during the baking process. To measure the energy utilized in cooking injera, the initial
mass of batter, and the total amount of injera produced from this batter were measured.

Thus, the mass of water vapor can be obtained by reducing the mass of the injera produced
from the initial mass of batter. It is assumed that the energy utilized in cooking the injera is the
energy required in raising the temperature of the batter from room temperature to the boiling
point of water which is called sensible heat, plus the energy required to evaporate water which
is called latent heat.

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 DESIGN OF INJERA PROCESSING MACHINE

It is also assumed that the heat capacity of injera batter is the same as that of water in order to
calculate the energy required to raise the batter temperature to boiling point [27]. Therefore,
the utilized energy is:

𝐸𝑢𝑡𝑖𝑙𝑖𝑧𝑒𝑑 = 𝑚𝑏𝑎 𝐶𝑤 (𝑇𝑏𝑜 − 𝑇𝑟 ) + (𝑚𝑏𝑎 − 𝑚𝑖 )ℎ𝑣𝑎𝑝

Where:

𝑚𝑏𝑎 -is the mass of the batter expected for one injera= 400𝑔

𝑇𝑏𝑜 - is the boiling temperature of water in Addis Ababa= 92℃

𝑇𝑟 - is the room temperature in the baking pan test room = 20℃

𝐶𝑤 - is the heat capacity of water = 4.187 kJ/kg.k

𝑚𝑖 - is the mass of the injera produced = 320g

ℎ𝑣𝑎𝑝 - is the heat of vaporization of water = 2260 kJ/kg

𝐸𝑢𝑡𝑖𝑙𝑖𝑧𝑒𝑑 = 0.4𝑘𝑔 × 4.187𝑘𝐽 /𝑘𝑔. 𝑘 × (92 − 20) 𝑘 + (0.4

− 0.32) 𝑘𝑔 × 2260 𝑘𝐽/𝑘𝑔

𝐸𝑢𝑡𝑖𝑙𝑖𝑧𝑒𝑑 = 120.6 𝐾𝐽 + 180.8𝑘𝐽

𝐸𝑢𝑡𝑖𝑙𝑖𝑧𝑒𝑑 = 301.4 𝑘𝐽

During injera baking there are losses in the sides, bottom side of electric heater support, and
conveyor, so considering the losses and assuming a safety factor of 1.4 the total energy required
will be 301.4 kJ x 1.4 = 421.96 kJ.

The time taken for cooking of one injera is about 2 to 3 minutes taking 2.5 minutes; the power
required for injera baking can be calculated as:

𝐸𝑢𝑡𝑖𝑙𝑖𝑧𝑒𝑑
𝑃=
Δ𝑡

Where: P- the power required for baking one injera

Δ𝑡 = 2.5𝑚𝑖 × 60𝑠𝑒𝑐 = 150 𝑠𝑒𝑐

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 DESIGN OF INJERA PROCESSING MACHINE

421.96𝑘𝐽
Then; 𝑃 = = 2.813 𝑘𝑊
150 𝑠𝑒𝑐

Since the overall length of baking (𝐿𝑏 ) section of the Teflon conveyor was designed to bake N
injera in lengthwise, and also it can be used to bake 2 injera along its width, Therefore, the total
number of injera and the total amount of power required can be determined as:

𝐿𝑏 = 𝑁 × 𝑙𝑓𝑜𝑟 𝑜𝑛𝑒 𝑖𝑛𝑗𝑒𝑟𝑎

Thus,

𝐿𝑏 5.4 𝑚
𝑁= =
𝑙𝑓𝑜𝑟 𝑜𝑛𝑒 𝑖𝑛𝑗𝑒𝑟𝑎 0.6

𝑁=9

Since the injera is baked in two rows on the Teflon conveyor belt the total number of injera
that can be baked in 2.5 minute is 18 injera.

Then, 𝑃𝑡𝑜𝑡𝑎𝑙 = 2𝑁 × 𝑃

= 2 × 9 × 2.813 𝑘𝑊

𝑃𝑡𝑜𝑡𝑎𝑙 = 50.634 𝑘𝑊

3.2.8 Heat loss calculation

Heat loss from the cover oven of the Teflon baking conveyor

L – length of Cover = 5 m

t – thickness of cover = 0.001 m

w – width of the cover = 1.186 m

𝑇𝑜 - outer cover temperature = 52 ℃

𝑇𝑖 - internal cover temperature = 100 ℃

𝐾- Thermal conductivity of Aluminum = 205 w/m.K

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 DESIGN OF INJERA PROCESSING MACHINE

The heat transfer through the wall is considered to be at steady state.

The heat transfer through the top cover is can be determined as

KA(T1 − T2 )
Q=
L

= (205 w/m.K) (5 m x 1.186 m) ( 100 ℃ − 52 ℃ )/5m

= 11.67 kW

Figure 3-18: Baking conveyor thermal analysis by ANSYS

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 DESIGN OF INJERA PROCESSING MACHINE

The heat loss from the side walls of the baking conveyor

L - length of the side walls = 5 m

h – height of the side walls = 0.154 m

𝑡𝑖 – thickness of inner sheet metal = 0.001 m

𝑡𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛 – thickness of insulation foam = 0.01 m

𝑡𝑜 – thickness of outer sheet metal = 0.001 m

𝑇𝑖 - temperature of inner side of interior sheet metal = 70 ℃

𝑇𝑜 - temperature of outer surface of outer sheet metal = 36 ℃

𝐴𝑡𝑜𝑡𝑎𝑙 = 2[𝐿 × ℎ]
= 2 [ 5m x 0.154 m] = 1.54 m2

The thermal resistance network for the heat transfer is through a three-layer plane wall of the
internal wall and the external wall.

Thermal resistance of each layers can be determined as

𝑡1
𝑅1 =
𝐾𝐴𝑙 𝐴𝑡𝑜𝑡𝑎𝑙

0.001 m
= w
(205 m . K) (1.54 𝑚2 )

= 3.16 x 10-6 K/W

𝑡𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛
𝑅2 =
𝐾𝑖𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝐴𝑡𝑜𝑡𝑎𝑙

0.01 m
= w
(0.028 . K) (1.54 𝑚2 )
m

= 0.23 K/W

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 DESIGN OF INJERA PROCESSING MACHINE

𝑡3
𝑅3 =
𝐾𝐴𝑙 𝐴𝑡𝑜𝑡𝑎𝑙

0.001 m
= w
(205 m . K) (1.54 𝑚2 )

= 3.16 x 10-6 K/W

𝑅𝑡𝑜𝑡𝑎𝑙 = 𝑅1 + 𝑅2 + 𝑅3

= (3.16 x 10-6 K/W) + (0.23 K/W) +(3.16 x 10-6 K/W)

= 0.23 K/W

The heat transfer for the side walls can be determined as

(Tin − Tout )
Q=
R total

= ( 70 ℃ − 36 ℃ ) / (0.23 k/w)

= 147.8 W

The heat loss for the bottom part of the baking oven

L - length of the bottom walls = 5 m

𝑡𝑛𝑠𝑢𝑙𝑎𝑡𝑖𝑜𝑛 – thickness of insulation foam = 0.01 m

𝑇𝑖 - temperature of inner side of the bottom cover = 150 ℃

𝑇𝑜 - temperature of outer side of the bottom cover = 41 ℃

(Tin − Tout )
Q=
R total

= ( 150 ℃ − 41 ℃ ) / (0.23 k/w)

= 473 W

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 DESIGN OF INJERA PROCESSING MACHINE

Q total loss = Q top loss + Q side loss+ Q bottom loss

= 11.67 kW + 147 W + 473 W

= 12.29 kW

For 12 heaters in the machine of diameter 50 cm

Q input = Nheaters X Pfor each heater

= 12 X 2.813 kW

= 24. 813 kW

Q utilized = Q input − Q loass

= 24.813 kW − 12.29 kW

= 12.523 kW

𝑄𝑢𝑡𝑖𝑙𝑖𝑧𝑒𝑑
=
𝑄𝑖𝑛𝑝𝑢𝑡

= 12.523 kW / 24. 813 kW

= 50.46 %

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 DESIGN OF INJERA PROCESSING MACHINE

3.3 Cost Analysis

Table 3-4 Material cost

No. Part Name Qty Unit cost Total cost

(birr) (birr)
1 Angle iron (40*40*6000) 8 500 4000

2 Sheet metal (1200*1200*4) 1 2000 2000

3 Sheet metal (Stainless steel) 3 2500 7500

(2000*1000*1)
4 Sheet metal (Stainless steel) 4 3000 12,000
(2000*1000*2)
5 ¾'' pipe (L = 2000) 1 1000 1000

6 ¾'' Gate valve 1 150 150

7 Oven Heaters 1 6000 6000

8 Foam insulation 1 3000 3000

9 Industrial oven control panels 1 4,000 4,000

10 Arduino Uno is a microcontroller board 1 800 800

11 Relay switch 8 300 2400

12 Teflon Conveyor belt 1 10,000 10,000

13 Conveyor rollers 2 400 800

14 Conveyor Gear Drive Motor 1 3500 3500

15 Bearing 6 90 540

16 Fan 6 400 2400

17 Fabrication 10,000

18 Other extra process charges 3000

Total 73,090 Birr

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 DESIGN OF INJERA PROCESSING MACHINE

Chapter 4 Result and Discussion

To decrease the heat loss from the baking oven insulation is used on the baking oven covers.
The foam insulation is selected in this design which is the common type of insulation in the
baking industries. For the baking conveyor Teflon belt is selected for its resistance in high
temperature. The Teflon coated belt is suitable to bake the injera because it will not react with
or contaminate the product being conveyed.

The batter container has a volume of 0.7854 m3 and it can contain 785.4 liters of injera batter.
And the injera batter can be added in a given interval to the container. The total cost of the
machine is 73,090 Birr. After considering the heat energy losses the efficiency achieved in this
design is 50.46 %.

For the designed injera machine the PLC control system is selected with programmable
memory to store instructions and to execute logic, sequence, timing, so as to control the baking
conveyor speed and to control the temperature of the heaters.

The heat will be transferred to the injera by using natural convection heating mechanism
through a continuous oven. The baking system controls the temperature of the Teflon conveyer
to give the optimum amount of heat that is required by the injera to be baked. The Pro-EC44-
Dual-Loop-Controller-300px is selected to control the temperature of the Teflon conveyer.

Figure 4-19: The complete temperature controller setup

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 DESIGN OF INJERA PROCESSING MACHINE

In the baking system the heater, the fans, the actuator valves and the conveyor are controlled
by the PLC control system as shown in the two diagrams below.

Temperature Relay Heaters

Sensor

Timer Relay Fans

Microcontroller

Start Conveyor
Relay

Stop Relay Actuator

Valves

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DESIGN OF INJERA PROCESSING MACHINE

53
 DESIGN OF INJERA PROCESSING MACHINE

The existing injera Mitad design has many drawbacks. For example, the high resistance,
inadequately sized electric wiring, and incorrectly adjusted combustion element; use of poor
construction materials; poor insulation: dissipation of energy during the baking session is said
to roughly range from 40 to 50 percent; lack of temperature control device such as a thermostat,
encouraging loss of heat; and overall, sub-optimal and inefficient design and workmanship.

The result is loss of energy. Compared to a continuous baking process, one-at-a-time baking
process is basically less efficient.

An automation of Injera baking process to produce reasonably priced Injera would help relieve
the labor of daily cooking, while minimizing harmful air pollutants from use of biomass, wood
or other similar fuels, frequently used as alternative fuels since power is unreliable, and
undependable in the current electric situations.

In spite of high daily demand, currently, there are very few wholesale bakers and sellers of
Injera. The few bakers supply primarily to restaurants, leading hotels, some supermarkets,
universities, and other institutions.

Currently, the peak load demand is growing rapidly. Two major contributing factors are:
industrial expansion and lifestyle changes such as more appliances in the home. The increased
power demand overloads Ethiopian electric power corporation’s existing power infrastructure,
causing increased blackouts, low voltage, rotating load cracking. The lack of reliable power
leads to increased utilization of alternatives, such as diesel power generation. Reduction of
peak load demand will also reduce the high fuel costs of serving these peak demands.

At present, regular power shortages, in particular with the country’s industrial sector suffers
significant economic losses on a daily basis. It is therefore crucial that sound planning,
supported by effective implementation and monitoring, are put in place on a priority basis. Use
of well planned, proven measures such as demand-side management, power conservation and
implementing energy efficiency programs, is recommended, must be included in power
moderation strategies.

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 DESIGN OF INJERA PROCESSING MACHINE

Chapter 5 Conclusion and Recommendation

5.1 Conclusion
The number of injera baked on the conveyor and the energy consumed in a specific time is the
crucial factor for comparing the result with the present electric injera mitad. The injera baking
machine can bake 18 injera in 2.5 minutes which means 432 injera in 1 hour with 50.634 kW.
The saved labor force is also a big achievement in this injera baking technology. Therefore, the
injera baking machine can make injera supplier companies very profitable by allowing them to
provide injera to the community by reducing the labor and energy costs. The demand
observation was conducted in the Debre Berehan University Students cafeteria kitchen. From
the survey result the average number of injera baked in Debre Berhan University per day is up
to 36,000 injera. This means 12,960,000 Injera per year. This number will increase as the
number of students increase. Covering such large injera demand with the present fuel wood
baking system is very expensive and its quality is very low. Due to the low energy efficiency
of the existing electric Injera Mitad, there is a huge electrical energy consumption and power
demand in the country. This indicates that majority of the conventional Mitads have low
efficiency.

However, the demand for the product is growing at a high rate due to the rapid economic
growth, the shortage of fire wood and biomass, and the huge electrification programs underway
in the country. There is a huge power demand and energy consumption imposed on the electric
generation and distribution infrastructure. There are many injera bakers and suppliers and few
exporters. Most hotels and restaurants get supply of injera from individual bakers and recently
established small scale enterprises. The interest of Injera bakers, suppliers and exporters is
reduction on the cost of energy and better efficiency of newer products. They prefer efficient
mass producing injera machines.

Injera baking is considered the most energy intensive activity in Ethiopia. The Injera electric
baking mitad are highly energy inefficient. Also, the daily baking’s power load becomes
coincident with peak load requirements, thereby overloading the distribution system.
Notwithstanding the above, the manufacturing of competitively-priced low-cost energy
efficient Injera electric baking stoves is of paramount importance to cost-effectively manage
peak load demand and reduce daily blackouts. Daily use of Mitad, with the most grid-connected
households, in view of serious daily power supply shortfalls, adversely impacts the daily peak
load demand.
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The impact is especially severe on week days, given the timing of Mitad use mid-morning and
mid-noon and the high-power needs. Compounding the situation is Mitad’s design has not
evolved in terms of energy efficiency. Cost reflective pricing is needed to lower power demand,
eliminate wasteful energy use, and encourage use of energy efficiency measures.

Injera baking is the major component of household energy use in Ethiopia. It is estimated that
over 50 percent of the overall households’ energy consumption is for ‘Injera’ baking. For the
urban poor households, and many rural Ethiopians, fuelwood is the major source of energy
consumed. Many middle-income urban households also use biomass, given lack of reliability
of power supply. However, there is no credible quantification of the extent of biomass used as
fuel. Nevertheless, continued fuelwood supplies are unsustainable and wood fuel use has
adverse health impacts, in addition to being environmentally degrading.

The design of the injera baking machine is used for an efficient injera baking mechanism. It is
well known that the present injera mitad consumes high amount of energy and the baking
system is a burden to the women. Since there is high degree of repeated human hand contact,
in a less than carefully run kitchen, it is not hygienic. The removal of the baking injera is also
one challenge due to the high temperature of the mitad and this will lead to the tearing of the
injera and burning one’s fingers.

Therefore, this design is dedicated to improve the injera baking technology by providing the
injera baking system that will reduce the energy consumption and the labor force to increase
the production rate. This machine is used mainly by injera providing companies to produce a
large number of injera continuously. By this method many injera mitads can be replaced by
one machine that has high production and energy efficiency.

5.2 Recommendation

This injera baking machine is specially recommended for the provision of injera to high
community institutions such as military, university, hotels and etc. If we can use the injera
baking machine we can minimize the energy bill, thereby it would minimize individual energy
bill per month, improves individual economy and minimize operation costs of hotels and
restaurants.

This machine is highly recommended to replace the wood fuels which are used to bake injera
so that we can reduce deforestation and high waste of electricity.
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References

1. Improvement of Injera shelf life through the use of chemical preservatives. Z., Ashagrie.
2012, African Journal of food, agriculture, nutrition and development.

2. Tekle, Awash. Experimental Investigation on performance, characteristics and efficiency of

electric injera baking pans (“Mitad”). s.l. : AAU , 2011.

3. Mesele, Mekonnen. Design and Manufacture of Laboratory Model for Solar powered Injera
baking Oven. s.l. : AAU, 2011.

4. Getenet, Gashaw. Heat transfer analysis during the process of injera baking by finite element
method. s.l. : AAU , 2011.

5. Temesgen, Yosef. Inhera Baking Machine. US 7,421,943 B1 US, Sep 9, 2008.

6. Admassu, Wudneh. INJERA MANUFACTURING SYSTEM . US 7,063,008 B2 US, Jun. 20,

2006.

7. Injera Electric Baking: energy use impacts in Addis Ababa Ethiopia. s.l. : A World Bank-
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8. Tareke, Assefa Ayalew. Heat transfer analysis of injera baking pan by finite element method.
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9. Preparation of Injera from pre-fermented flour. Tadesse, Hiwot. s.l. : International Journal
of Science Innovations and Discoveries, 2013.

10. Design of biogas stove for Injera baking application. Kebede, Dejene. Issue 1, s.l. :
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11. Improving energy consumption and durability of the clay bakeware (MITAD). Gebresas,
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13. [Online] http://www.orientalmotor.com .

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14. Aman, Abdulkadir. Design Manufacture and Experimental investigation of Solar Powered
Injera Baking Oven for indoor cooking. s.l. : AAU , 2014.

15. Holman, J.P. Heat Transfer. New York : s.n.

16. Janssens, Prof. Luc. Advanced Sensors. 2017.

17. A guide to selecting the right oven for your processing application. 2015 .

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19. Van Buskirk, R.Haile,T. and Ezana,N. The Effect of Clay and Iron Cooking Plates on
Mogogo Efficiency and Energy Use.

20. Yener, Sadik Kakac and Yaman. Convective Heat Transfer. Tokyo : s.n., 1995.

21. Rahman, Shaifur. Food Properties Handbook. Washington, D.C : s.n., 1995.

22. Yunus, A. Cengal. Heat Transfer: A practical Approach. USA : McGraw-Hill series in
Mechanical Engineering, 1998.

23. Wolverine tube heat transfer data book

24. DeWitt, Incropera F. P. and D. P. Fundamentals of Heat and Mass Transfer. New York :
Wiley, 2002.

25. Bejan Adrian, Kraus Allan D. Heat Transfer Handbook. New Jersey : s.n., 2003.

26. Bailis, Rob,. Controlled Cooking Test. s.l. : University of California-Berkley, 2004.

27. Negusse, Ezana. Electric Injera Cooker (mogogo) efficiency. Asmara, Eritrea : s.n., 1996.

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Appendix A

Traditional Injera baking currently taking place at Debre Berehan university baking 36,000
injera per day.

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Appendix B
Part and Assembly drawing for both circular and square injera baking machine

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