30120130406021

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 6, November - December (2013) © IAEME

AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 4, Issue 6, November - December (2013), pp. 201-213
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2013): 5.7731 (Calculated by GISI)
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IJMET
©IAEME

DESIGN AND REALISATION OF A COCOA HYBRID DRYER FOR A
RURAL ZONE
Abraham Kanmognea*,

Yves Jannotb,

Jean Nganhoua

a

b

Laboratoire d’Energétique, ENSP, BP 8390 Yaoundé, Cameroun
LEMTA, Nancy-Université, CNRS, 2, avenue de la Forêt de Haye, BP 160, 54504 Vandoeuvre
Cedex, France

ABSTRACT
We present in this article the study and the realization of a hybrid prototype of drier
solar /wood for the agro-alimentary drying of the products in general, and the cocoa in particular.
The drier is composed of four essential parts; the room of drying is of dimensions 3 m X 2 m X 1.2
m and is built out of terra cotta briquettes. The two hearths consist of stainless sheet steel of parallel
epipedic form covered with ground briquettes. Two galvanized steel tubes with diameter 135 mm
crosses the room of drying in the length’s direction and constitute the heat exchanger. The roof of the
drier is made out of plexiglass to optimize solar energy in the drying room. The study of the drier
was done while following a methodical step and by using tools of the design such as the functional
specifications, the technical functional analysis, the diagram APTE, the block functional diagram, the
life cycle and the justified choice of the technical solutions of the drier. The construction of the
prototype and the test on the prototype made it possible to dry 100 kg of fresh cocoa in 50 hours by
using energy coming from wood and 35 kg in 40 hours by using solar energy.
Key words: Hybrid Drier, Functional Analysis Technique, Cocoa, Cameroon.

1. INTRODUCTION
Cocoa is third in the world ranking of exported agricultural products after coffee and sugar.
In the majority of cases, it is produced by small farmers. 90 % of the production in rural zones of
world cocoa-plantation results from plantation of a surface area less than 5 hectares [1]. In
Cameroun, cocoa alone represents 15 % of total exports [2]. Trade tackles the problem of the quality
of cocoa bean through standard references defined in the sales contract. The French standards, a bit
different from the American or English standards stipulate that "the delivered goods must be
201


reasonably free of foreign broad beans tastes from the cocoa and of defective broad beans [3].
Currently, on the world market, the cocoa coming from Cameroon is of approximate quality because
it does not fulfill the standards of quality. This involves a low rating of the cocoa coming from
Cameroun. The approximate quality of the Cameroonian cocoa is caused by the drying system,
which caused approximately a 10% loss in post-harvest [4, 5]. These losses are caused by:
-

insufficient drying resulting to molding of the product;

-

the smell of smoke and tar in beans;

-

The presence of pebbles in dried beans.

The improvement of the activity of drying in the countries of the South passes by the taking
into account of the real needs for the users, of the characteristics of the products to be dried and the
environment in the design of the driers [6]. There is no universal mechanism in the technological
choice of a drier. Various case studies made, it is difficult to have indisputable elements of validation
and comparison between the various models of drier [7]. A study of the drying system of cocoa in
Cameroon showed that four types of dryers are used by farmers:
-

the bus dryer;

-

paved roads and flow concrete platforms;

-

Samoan ovens;

-

Soil.

Unfortunately, these dryers used are not satisfactory to farmers. The dryer wanted by
producers for drying cocoa whose harvest season lies partly in the rainy season is a dual fuel dryer
[8]. The objective of this paper is to study and build a dual fuel; solar- wood dryer for drying cocoa
in rural areas. To achieve this goal, we assumed that the design and construction of the dryer will
take into account the following functions:
-

ensure drying in any season;

-

use wood biomass and solar energy;

-

avoid the taste of smoke and tar in the product;

-

introduce and easily stir the product into the dryer;

-

protect the product against rain and insects;

-

better retain the heat in the dryer;

-

Less expensive to achieve.

2. MATERIALS AND METHODS
2.1.

Material

The fireplace is made of stainless steel, covered with cooked soil bricks to better retain heat
from the fuel. The frame of the dryer is made of clay briquettes due to the insulating quality and its
lower cost.
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Heat Diffusers: the two channels are made of galvanized steel pipes 125 mm diameter going through
the dryer in the longitudinal direction. They are good conductors of heat and are more resistant to
oxidation than the black iron.
Drying deck: several materials may be used for the manufacture of the grid and drying deck. We
include among other bamboo, synthetic mesh, and stainless steel grille. The synthetic mesh is too
loose to temperature. Even with the short life cycle of the bamboo, this material is chosen because it
is inexpensive and available in rural areas. The mountings mat are in black iron of profile T, coated
with antirust paint.
The chimneys are made of two filling pipes of stainless steel. They could also have been built with
briquette of cooked clay.
The thermal insulation of the base of the dryer can be done with the coconut fiber, wood sawdust,
polystyrene, glass wool, cotton or kapok. Among these insulating materials, kapok and coco fibers
are less expensive and coco fibers are available in the area of cocoa cultivation. So this material is
chosen.
Roofing: the dryer can be covered with Plexiglas, glass or special plastic film. The glass was not
retained because of its fragility and the special plastic is very rare on the local market. The roof of
the dryer will be covered with Plexiglas available on the local market and easier to maintain despite
its higher cost.

2.2.

Method

The survey of producers in the domain of drying and references of dryers permitted us to
make a list of the different types of dryers and define the functions that a particular type of dryer
should satisfy. The different types of dryers and functions of each part of the dryer are shown in a
table constructed as follows: in the first column, we present the list of different types of dryers and
on the 2nd line, the list of functions performed by a part or type of dryer. At the intersection of a type
of dryer and a function is a note ranging from 1 to 5. Note 5 means that the type of dryer in the 1st
column provides great function of the 2nd row. Note 1 means it does the function poorly. The
exploitation of the table designed permits us to choose the functions and constraints corresponding to
the life cycle of the dryer designed. An internal functional analysis of the dryer is made after the
description and afterwards implementation of the various parts of the dryer.

3. RESULTS AND DISCUSSION
3.1. Summary of existing systems
The choice of material is often delicate. They are never based solely on mechanical
considerations. Consider manufacturing methods, costs and resistance to chemical attack [9]. The
technological potential (spare parts and competence) is a major factor to work on driers adapted to
our environment [10]. We present in the following table performance of dryer system that takes into
account the function of the devices.

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Table 1: Performance of dryer system

To admit the air
for combustion
easily

easely

To stir up the product

product easily

To introduce the

goes away

Increase of moisture

from outside

To protect the product

any season

To ensure drying in

air

Good circulation of

Best conservation
of heat

To use solar energy

construction

Weak cost of

Control limited
temperature

Not to have the taste
of smoke

Functions by order of decreasing importance

Type of
drier
(Drier)

Lagdo

4

4

3

4

2

4

5

5

3

4

3

4

Tunnel

3

5

2

4

2

5

5

5

3

4

3

/

Comité
Diocésain

4

4

2

4

2

4

5

5

3

4

3

3

Solaire à
V. élect.

3

5

2

4

2

4

2

5

3

4

4

/

Autobus

5

5

4

5

2

3

2

4

2

4

4

/

ENSIAA
C1

3

5

4

5

3

3

2

5

3

4

3

/

Mvolyé

3

5

3

5

2

4

2

5

5

3

1

/

ENSIAA
C2

3

5

4

5

3

3

2

5

3

4

3

/

Basculant

3

5

4

5

3

3

2

5

3

4

4

/

Brace
Institute

3

5

3

5

4

4

2

5

3

4

4

/

Samoa

4

5

3

2

1

3

5

5

2

5

4

5

Burareiro

4

4

2

1

5

3

5

5

1

4

4

5

Cameroun

3

4

3

1

3

3

5

5

2

5

4

4

Plate
forme
CEPEC

5

5

2

1

2

4

4

3

2

5

5

5

Plate
forme

4

5

3

1

2

4

4

3

2

5

5

5

Fumoir

2

1

5

1

1

2

5

2

2

3

2

4

Wanson

4

4

2

1

2

3

4

3

2

5

3

5

204


3.2. Selection motivated devices used
Choosing a type of dryer with mixed fuel, solar- wood is guided by the advantages and
disadvantages of the various systems: the possibility of drying the product in any season, using a
reduced drying surface compared to solar drying, drying time reduced compared to solar drying. As
regards the supply in solar energy, the dryers may be classified into several categories by considering
two characteristics: the transmission mode of the heat and the nature of the airflow in the dryer. The
solar supply is direct if the solar radiation reaches the products, and indirect if the products are
shielded from sunlight. A dryer is said to be naturally ventilated if the air circulation is provided by
the thermal siphon effect, and forced ventilation is ensured by a mechanical action. A dryer is said to
be covered if the absorber is protected by a transparent cover and uncovered otherwise. Given the
cost of installation, we have eliminated the special solar systems that use photovoltaic solar energy or
solar energy by concentration. The area of use (South Cameroon) does not receive very regular
sunshine. Our choice will be limited to a covered dryer. Regarding wood combustion heating,
systems fall into two groups: direct heating dryer: the products are heated directly by the smoke
(drying / smoking) and indirect heating dryer: the smoke is not in contact with product. Table 2
summarizes the characteristics of each system.
Table 2: Characteristics of solar-wood mixed drying systems.
Solar dryers
Direct à ventilation
naturelle

Indirect à ventilation
naturelle

Indirect heating
using a natural
ventilated heat
exchanger

advantages :
Fast drying, simple
construction,
reduced cost
inconveniences:
degradation
products with
U.V, non-uniform
temperature

Advantage:
average cost, fairly
uniform temperature,
products are free
from U.V rays
inconveniences :
Heat loss , drying
slower than in direct :

Indirect heating
using forced
ventilated heat
exchangers

advantage : uniform
temperature
inconveniences:
high cost

Indirect heating
without using
forced ventilated
heat exchangers
Direct heating
without using
natural ventillated
heat exchanger

inconvenience :
Product
contaminated with
smoke
inconvenience :
Product
contaminated with
smoke

Wood dryer

Indirect à ventilation
forcée

Direct à
ventilation forcée

advantage :
Products are free from
U.V rays
inconvénients :
High cost,
Construction is
complex

advantage :
Fast drying,
average cost
inconvénient :
Degradation of
the product by the
U.V rays

advantage :
Product free from
U.V rays, uniform
temperature
inconvénients :
drying is less rapid,,
construction is
complex

advantage :
fairly uniform
temperature,
protection from rain
inconvénients :
limited temperature,
high cost, necessity of
electrical energy,
construction is complex

advantage :
fairly uniform
temperature
inconvénients :
high cost,
degradation of the
product by the
rays U.V
necessity of
electrical energy

inconvenience :
- Product
contaminated with
smoke
inconvenience :
Product contaminated
with smoke

inconvenience :
with smoke

inconvenience :
Product
contaminated
with smoke
inconvenience :
Product
contaminated
with smoke

205

inconvenience :
with smoke


Choice between the direct and indirect solar dryer
Cocoa is not very sensitive to UV rays. Traditional dried cocoa direct under sunlight (when time
permits) is of very good quality. Direct drying permitting faster drying seems to be preferable.

Choice of mode of ventilation
The dryer is often aimed at non-electrified rural areas; our choice was directed to natural ventilation.

Choice of heating mode
Product contamination by smoke is the main drawback of drying using a direct wood heating system,
hence the choice of indirect heating. The circulation of the drying air around the heating surface will
be by natural convection due to non electrification of areas for which the dryer is destined.

Conclusion on the choice of dryer
The dryer that caught our attention was the mixed dryer that uses solar energy with natural
ventilation and indirect heating with wood using natural ventilated heat exchanger because despite
some drawbacks, it remained the most suitable in the production area [11].

3.3.

Functional specifications

The functional analysis is divided into two parts: the outer functional analysis that lead to
Functional Specification (FS) and internal functional analysis permits us to have a Table of
Functional Analysis (TAF).
The functional specification is a legal agreement between the giver of finance and the
subcontractor. The purpose and implications of the FS are many knowing that it helps to clarify and
formalize the responsibilities of the applicant, the design and realization of the director. In the case of
our study, the FS presents the system (dryer) abstraction of solutions. This is a model that other
researchers can take to develop alternative dryers. The FS is also a way of comparing solutions
proposed to the drier with existing alternatives or future ones. There are several methods of
functional analysis (FA) for the design in general: FIT, FAST, SADT etc. In mechanics the following
functional analysis methods are used: the Intuitive method that identifies functions thus come to
mind, systematic method (FIT), which consists of writing the expression of specific functions and the
SADT (Structural Analysis Design Technic) method, adapted to the processing of data is useful for
maintenance [12]. FAST (Functional Analysis and System Technic) method that prioritizes technical
functions is particularly suited to the redesign. Intuitive method involves a significant risk of
omission when the functions are enumerated. The SADT and FAST methods are cumbersome and
ill-suited to the first design [13]. We used the systematic method given its precision to the functions
used in the design of the dryer.
The functional specification relates to a dryer for small cocoa producer. The dryer can be
used to dry other agricultural products (groundnuts, maize, cassava, coffee, etc.). The principle of
operation of the dryer is as follows: air heated by natural convection in contact with two channels
circulates around the products placed in a thick layer on drying deck. Products also heat up by
absorbing some of the heat flux radiated by the sun, the atmosphere and the transparent cover. This
dryer can be implemented in many regions in Cameroon and some countries in Central and West
Africa. Its adaptation requires a careful choice of materials used in manufacturing. The life cycle of a
product is the set of actions that the product undergoes during the period of construction and use. The
choice of materials of construction is very important. The life cycle of the dryer includes two
phases. The normal usage phase which corresponds to the period of production of cocoa (OctoberJanuary) and the intermediate phase during which the dryer can be used to dry other products such as
mango (March-September), corn (June-August) or peanuts.
206


Technical functions and constraints
The functions of a system are of two types: the service functions which are the answers to the
need of the owner of the system and the technical functions that are internal to the system and the
result from the designer's choice.
A function is characterized by its assessment criteria, its level (tolerance compared to another
criteria), its flexibility is the degree of importance of the assessment criteria in the system (essential
(0) important (1), negotiable (2), for guidance (3)) and its control that can be measured on the
prototype, testing or simulation.
Table 3 summarizes the functions corresponding to the life cycle of the mixed dryer and allows
better monitoring during construction of the dryer ensuring that all functions are applied.
Table 3: Functions corresponding to the life cycle of the dryer
Function

AssessmentCriteria

Characterization

Flexibility

Control

F1 :Insert the product
into the dryer

amount of product
Load time
Know how
hardship

m < 200 kg
< 30 min
Worker’s level
achievable by a woman

1
3
3
3

Prototype
testing

temperature
temperature variation

< 60°C
room temperature< 30°C

0
2

cycle time
no mechanical or chemical
reaction with the
support

< 90 heures

0

Intake and exit air flow

Opening of air extraction
traps

F2 : Heat air

F3 : Keep the product
in the dryer

F4 : Circulate the air

F5 : The user controls
the quality of the
product

F6 : Extract the
product

F7 : Protect product
in the dryer outside
F8: Conserve heat
F9 : Esthetic
F10 : Clean after
several cycles

color, touch

1

Opening of air renewal
traps
90% product good

measurement
(providing a
control)

prototype
testing

1
1
3

water content
weight at the end of drying
quantity of product
discharge time
know how
hardship

< 7%
m < 107 kg
m < 107 kg
< 30 min
worker level
achievable by a woman

0
0
0
3
3
3

insect, , flies, smoke
dust
Return of moisture

ban

prototype
testing
expert during
prototype
testing

0
1
1

heat loss
form
ease of cleaning cycles
number before cleaning

minimum energy loss
Rectangular
20 min/grid
5 cycles

207

1
2
2

prototype
testing

prototype
testing
modeling
on prototype
on prototype


3.4 Description of the elements of the dryer
The base of the dryer
The dryer is based on a concrete of 3.5 m x 2.5 m at least 5 cm thick. A polyethylene film of
the same surface area as the base layer is incorporated into the concrete thereby preventing rising of
moisture. Above the layer of concrete is found a 5 cm layer of coconut fiber which is for thermal
insulation (fig. 1)

Polyethene film

Coconutfiber

concrete

Figure 1: Cut of the base of the dryer

Heat exchanger
The heat exchanger consists of two tubes of galvanized 134 mm in outer diameter and 125
mm in inner diameter each steel. Each tube measure 3 m in length. One end of the tube opens into
the fireplace and the other end closed, allows the pipe smoke in the drawing chimney.

Fireplace
It consists of a parallelepiped block of steel 50 cm wide, 75 cm high and 50 cm deep. The
block is welded to the end of the exchanger. A circular opening 134 mm in diameter on one side of
the fireplace provides the passage of smoke to the heat exchanger (fig. 2). In the lower part of the
furnace a metal screen placed 15 cm from the floor serves to support pieces of firewood, and also
delimits the height of the ashtray. The lower part of the face opposite to the heat exchanger is small
with perforated holes of 6 mm in diameter across the width allowing the supply of air for combustion
in the fireplace.
A door with metallic flap is fitted on the upper part of the opposite junction so that the
fireplace can be supplied with wood. Clay briquette walls provide thermal insulation around the
fireplace.

Chimneys for drawing off smoke
260 cm long, the drawing chimneys are welded each 5 cm from the sealed end of the heat
exchanger tubes. They are made of stainless steel and have an outer diameter of 82 mm and an inner
diameter of 78 mm. At the upper end of each pipe is welded a cone with a diameter slightly greater
than the external diameter of the chimney. At the other end of the chimney (welded junction between
the tube and the chimney exchanger), a circular hole of diameter equal to the inner diameter of the
chimney is arranged on the heat exchanger tube for the evacuation of smoke from the exchanger
to chimney (fig. 2).

208


Figure 2: Diagram showing layout of the fireplace, heat exchanger, chimney

The frame of the dryer
The frame of the dryer of dimension 3m x 2m x 1.2m is made up of briquette from cooked
soil 22cm x 11cm x 6.5cm. Three rectangular holes of 12.5cm x 30cm are arranged 12.5 cm below
the heat exchanger wall on each side to permit the renewal of air in the dryer. The intake airflow rate
into the dryer can be regulated depending on the opening or closing of these orifices.

Platform drying
The drying platform has dimension 3m x 2m consists of iron with T profile painted with rust
proofing material going through the drying chamber in the direction of the width. The ends of the
chains are embedded in the two sidewalls. The spacing between the irons is 25 cm. A grid of 3 m x 2
m in woven bamboo mat is placed on the irons. The drying platform is 37.5 cm above the heat
exchangers.

Roofing
It is made of plexiglas of 4 mm thickness divided into four modules of 1.5 m x 1.5 m. It is
removable allowing access on the drying platform. Eight circular holes 80 mm in diameter are
arranged on the roof and are used for extracting humid air from the dryer. The exhaust air flow may
be controlled by closing or opening of the orifices through the shutters designed for this purpose.

3.5.

Internal Functional Analysis dryer

The objectives of the internal functional analysis are: the improvement of the dryer with
respect to the needs and justification of the cost of the dryer. It is to define the purpose of the dryer,
namely services rendered, by identifying the relationship it has with its environment [12] .The block
diagram below reflects these relationships

Function Block Diagram (FBD) of the dryer
The functional block diagram permits us locate the dryer in its operating environment,
identifies the relationships established by the dryer and the surrounding environment and expresses
the purpose of each of these relationships. Two types of relationships are expressed: the relationship
through the product or feature usage and relationships that bind the product directly to an exterior
element or adaptation function. Figure 3 shows the different relationships between the dryer and its
surroundings.
209


Figure 3: APTE diagram of the dryer
The diagram of the dryer with the surrounding environment does not show the various
internal components of the dryer, or the connections between its elements. Relations between the
components can be internal or external contacts to the dryer. The contact between the two
components is materialized in a FBD by a route connecting these components (fig. 4) shows the
functional block diagram of the dryer.

Figure 4: Function block diagram of the dryer

Array of functional analysis (AFA) dryer
The functional analysis array is a complementary tool of the functional block diagram. This is
a representation of table describing: the list of components of the dryer; the elementary functions
associated with each component and the cost of each component of the dryer. Two types of functions
are useful for AFA: the design function FK and the service function JN. The design function is an
internal function of the dryer not directly involved in the service to be rendered. It corresponds to the
technical functions that the designer adds just necessary to perform the service. It is represented by
the material, labor force and energy, which are not directly useful to the user. The service function is
210


that expected for a product to meet the needs of a user. AFA can analyze the distribution of costs
before deducing design performance (RC) which is an indicator for positioning the actual product
compared to its optimum corresponding to that exactly necessary. The design performance in % is
defined by the expression:

RC =

100 C J
C J +C F
N

N

[14]
K

CJN is necessary cost and CPK is just the cost of the design function. The performance of the
joint design of the dryer is 90% and the cost is 2 000 000 FCFA.

3.6.

Implementation and operation of the dryer

The Joint dryer measures 3 m x 2 m x 1.2 m. It offers the possibility of drying various
products (cocoa, fruits, vegetables, cassava etc. ...) throughout the year. The frame of the dryer is
built of clay bricks. The roof consists of a transparent cover, which is removable to allow access to
the product to be dried. The drying platform 5.8 m² of useful surface is made of woven bamboo mat
plated on T-shaped metallic tile-gutters that traverse the drying chamber in the direction of the width
and the ends of which are sealed in the walls briquette earth. The heat transfer in the drying chamber
takes place by natural ventilation around two steel pipes placed below the drying platform and
through the drying chamber in the direction of the length. The hot flue gases from the fireplace pass
through the steel pipes, which serves as heat exchanger between the hot flue gases and the
surrounding air, before being discharged to the outside through two chimneys mounted to the
opposite side of the fireplace. The admission of air into the drying chamber is equipped with
openings on both walls 3 m below the smoke pipes. The extraction of humid air takes place through
eight small chimneys arranged on the roof of the dryer (fig.5).

7
6
4

5

1

Air
2
3

8

1: Fire place (2), 2: Heat exchanger (2), 3: air admission orifices (3), 4: decanting chimney (2),
5: drying platform, 6: roof made of plexiglas, 7: air extraction orifices (3), 8: base of dryer
Figure 5: diagrammatic representation of the dryer
211


3.7.

Test of the dryer

Having designed, studied and built the dryer, we proceeded to dry the cocoa. The test on the
prototype made it possible to dry 100 kg of fresh cocoa in 50 hours by using energy coming from
wood and 35 kg in 40 hours by using solar energy.

4. CONCLUSION
We have designed and built a mixed solar / wood dryer after first defining the terms of
reference (working document) which was prepared to fulfill the primary function which is to dry.
The functional specifications fixed all the main features of the dryer and operating conditions to be
satisfied. Then, a block diagram of the dryer is designed with different functions to be performed by
each system clearly stated. The overall design is decomposed into subsystems which are all subject
to the study of variants in order to choose the best technological solutions. This phase was completed
by a detailed description and drawings of the entire dryer. The dryer was built and field test of the
dryer presented a satisfactory result.

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