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International Journal of Advanced Research in Engineering RESEARCH IN ENGINEERING
INTERNATIONAL JOURNAL OF ADVANCED and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online)TECHNOLOGY (IJARET)
AND Volume 4, Issue 7, November – December (2013), © IAEME

IJARET

ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 4, Issue 7, November - December 2013, pp. 228-235
© IAEME: www.iaeme.com/ijaret.asp
Journal Impact Factor (2013): 5.8376 (Calculated by GISI)
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©IAEME

AMELIORATING THE ENERGY PERFORMANCE OF ELECTRIC DRYER
FOR AGRO-FOOD PRODUCT. CASE STUDY: THE DRYER OF THE
TRANSFORMATION UNIT FOR CASSAVA AT POUMA IN CAMEROON
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

SUMMARY
In this article, we-have carried out the power control of a dryer, all which uses forced air
circulation, imported from Ghana and installed in Pouma for drying cassava in Cameroon. The
performance of the dryer does not meet up with the expected needs. This power control has made us
detect the causes of a high power consumption rate of the dryer and therefore carry out by changes to
Obtain Optimal Functioning year; we-have added a scythe of ceiling, blown out Mittal heating
resistors, changed air ventilation turbines, Multiplied and Uniformly Redistributed sections for the
heated air passages in the dryer. We have identified the different dead zones in the dryer. The
changes made on the dryer-have led to a reduction in power control of about 20%.
Keywords: Power Control, Dryer, Energy Savings, Cassava, Cameroon.
1. INTRODUCTION
Agriculture occupies a prominent place in the economy of African countries, particularly in
the tropics south of the Sahara. It contributes nearly 80% to exports and employs about 70% of the
population [1]. Agricultural production are often characterized by the temporal surplus crops, the
development of appropriate techniques can allow conservation to reduce post-harvest losses. Drying,
being the oldest conservation methods one of the most appropriate techniques. However, drying
problems are numerous and do not seem to be resolved despite the transfer of efficient
technologies. The mastery and analysis of the operation of the drying technology can lead to reduced
operating costs of exploitation by reducing its energy consumption. A study was conducted on an
electric dryer belonging to a Cameroonian NGO, the AID-Cameroon in its processing cassava plant.
Pouma, geographic coordinates latitude 3° 51 'north and longitude 10 ° 31 ' east [2], a locality in the
Littoral region on the Yaoundé-Douala main road 150 km from Yaoundé. The objective in this work
228

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME

is to modify the electric dryer to address the various causes of malfunctioning registered so as to
improve energy efficiency and reduce the drying time. To attain our goal, we have assumed that the
mode of fresh and warm air circulation in a dryer, hot and fresh air flow in the drying channel all
influence the energy consumption of a dryer, and even on the reduction of the drying cost. An energy
audit of the dryer was conducted to determine the causes of excessive consumption and measures
prescribed have been implemented to increase the energy performance of the dryer and reduce drying
time.
2. MATERIALS AND METHODS
2.1. Technical equipment
2.1.1 Dryer
The dryer at the cassava processing unit (UTM) Pouma is the electric type with internal
dimensions 3.7 m long; 1.6 m wide and 1.85 m in height [3] (cf. Figure 1).

Figure 1: Drawings of the electric dryer of the Cassava Processing unit (UTM) Pouma
229


The side and top walls are made of two steel plates separated by the glass wool used for
thermal insulation. The floor of the drier consists of a steel sheet resting on the rafters. The
circulation of air in the dryer is provided by two centrifugal fans. The motors are attached to the
outside on the roof of the dryer. The turbines of the motor-fans are coupled to the engines and are
positioned inside the dryer at about 15 cm from the ceiling. Turbines take in air through the drying
chamber and propel the air into a 25 cm wide space between one side of the wall and a plate arranged
parallel to it. In this space are arranged 30 regularly spaced electrical resistors in the direction of the
length of unit power 1.7 kilowatts. The electrical supply to these resistors is controlled in all or
nothing law by an adjustable thermostat connected to a temperature probe placed inside the
dryer. The thermostat cuts off the power supply of the resistors when the temperature inside the dryer
has reached the required temperature set-point. Four vertical slits each of 2 cm width formed in the
sheet allow the entry of hot air heated by the resistors inside the dryer. Two holes of diameter 9 cm
on the roof of the dryer ensure the intake of the new air into the dryer. Two other openings 15 cm in
diameter located just above the air outlet of the centrifugal fan permit the extraction of moist air
from the dryer. Be it at the intake or extraction port, air flow is manually adjustable through a
butterfly valve mounted in the holes. When opened, part of the exterior air enters the dryer (Case of
the intake valve) through the inlet opening or a portion of the interior moist air leaves the dryer (case
of the extraction valve) through the extraction port. The degree of valve opening is adjustable
manually by means of a command stem.
2.1.2. Trolleys and Grid
In the dryer, cassava is arranged on grids carried by trolleys. The dryer can hold 10 trolleys
with 3 of 14 grids and 7 of 11 grids. The average dimensions of the grids are 0.6 m wide and 0.75 m
long, giving a total area of the drying grid of 53.5 m 2 for 119 grids. The grids are made up of thick
mesh (expanded metal) and their frames are made of angles 3 cm tile-gutters. On the thick mesh is a
plastic mosquito netting on which is placed the cassava to be dried. The frame of the mosquito
netting is made of white wooden that can slide within the frame bracket allowing loading and
unloading of the material to be dried. Trolleys are made of tile-gutter of 4 cm. The medium size of
each carriage is of width 0.7 m , 0.8 m in length and 1.5 m in height. On a carriage, the grids are
placed one above the other at 14 or 11 per trolley.
2.2. Measuring equipment
The measuring equipment consists of :
−
A thermo anemometer of the Testo brand for measuring the velocity of the air through the slits
2 cm in the dryer ;
−
A digital thermo hygrometer for measuring the air humidity and the temperature in the dryer ;
−
An electronic scale for measuring the weight of the product ;
−
A ventilated oven [4] to determine the water content of the lumped cassava ;
−
A tape measure for different measurements relating to the dryer [5];
−
An electronic timer to measure the dryer’s operating time;
−
An energy meter for measuring the electric power consumption of the dryer ;
−
A digital multi-meter.
2.3. Method
Before changing the dryer, vacuum and load tests of the dryer has been made. Careful
inspection and a number of measurements were carried out on the dryer :

230


Dryer off
The 30 resistors heaters were dismantled and the values of their electrical resistance were
verified using the digital multi-meter. The different compartments of the dryer (control table, drying
chamber, trolley and grid) were inspected.
Dryer running and empty
The thermostat is set to a fixed temperature above the ambient temperature. Both fan motors
rotate, using the thermo anemometer, we measured the temperature at different points within the
dryer and the hot airflow over the four slots at 3 mid-planes; 0.35 m, 0.9 m and 1.50 m from the
ceiling of the dryer. The thermostat was then set to a temperature below room temperature. Both fan
engines were functional and we noted the indicator of the time and energy counter of the fans, using
an electronic timer.
Dryer running and loaded with cassava
The exhaust air valves are set at a given position. Lumped Cassava is loaded on the grid with
the cup. The weight of the lumped cassava is measured in advance by using the electronic balance.
The racks are always loaded on carriages which are introduced into the dryer. Using a hygrometer,
the humidity of the air at the outlet of the extraction orifice is measured at predetermined time
intervals. The ambient temperature is measured using the thermo-anemometer. A sample mass of
cassava is weighed at regular time intervals. At the end of the cycle, the dry mass of the sample is
determined using the balance and the ventilated oven. This mass is used to determine the water
content of the product. The maximum temperature for drying of the product should be 80°C [6].
Product is dry if the water content on a dry basis is between 0.12 and 0.15.
After modifications on the dryer, we repeated the same tests as above when the dryer was running
and empty. When the dryer was running and loaded, the only difference was that measurements of
the velocity of the air will be made on 12 slots instead of 04 as previously.
3. RESULTS AND DISCUSSIONS
3.1. Energy audit
The tests and measurements on the dryer before modification yielded the following results. Of
the 30 electrical resistors that provide heat to the air in the dryer we have:
− 2 missing ;
− 1 is defective ;
− 7 are disconnected.
It has been proposed to connect the disconnected resistors, replace blown resistors and
complete missing resistors. The speeds of the air blown into the dryer were also measured and the
average air flow blown into the dryer was evaluated at 2000 m3/h. The performances of the dryer had
been limited from the designing by the fact that the heated air was admitted into the dryer by 4 slots
of width 2cm disposed over a length of 3.7 m. So there were dead zones between these slots (spaces
of 90 cm) where the air velocity was very low. It was proposed to develop other slots to increase the
hot air flow to standardize the drying chamber. The spacing of the slots should not exceed 20 cm.
The recovery of air blown by the turbines is done at the center of the dryer which means that part of
the hot air blown is sucked by the turbine without passing through the product. It has been proposed
to find a system that allows us to force the hot air to pass through the entire product before being
sucked through the turbines. The solution to fix a false ceiling for channeling hot air on the cassava
before being drawn to the side opposite the resistor has been advocated. Following the rupture of the
turbine during the startup of the dryer, they were replaced. One of the 2 turbines is smaller than the
turbine initially put in place, which provokes falling airflow pulsed through the heaters. It has been
231


proposed to replace the two existing turbines by 2 turbines of larger diameters to increase the air
velocity in the dryer and overcome losses. The dryer can hold 10 trolleys; only 7 were manufactured.
It is essential to make three additional trolleys to occupy the entire dryer and on allowing constant
drying, to reduce the thickness of the layer of cassava on each grid. Stocking density of cassava must
be between 10 and 11.2 kg/m2, which correspond to a specific surface area of the product between
0.09 and 0.10 m2/kg for good drying results [7]. The optimization of drying of cassava at UTM
Pouma consists also to determine the drying channel which ensures minimal energy consumption. To
do this, we conducted a series of tests in which we varied the cassava loading density on the grids
and the airflow of extracted air in the dryer. Previously, we conducted a series of load tests to
determine certain characteristics of the dryer. All these were done after the implementation of the
measures recommended by the energy audit.
3. 2. Performance of the modified dryer
The measured speeds are speeds of the air which is blown into the chamber through the 14
slots after passing over the heating resistors. We have measured the speed at three heights and
varying the positions of the air extraction and intake valves.
Figure 2 shows the distribution of an air flow per unit mass in the dryer.
6

Air velocity (m/s)

5
4
3
2
Extraction open
1

Extraction close

0
1

2

3

4

5

6

7
8
9
Slots number

10

11

12

13

14

Figure 2: Distribution of air flow per unit mass in the modified dryer at UTM Pouma
The observation of speed profiles shows that in the top of the dryer speeds are very low (1.3
m/s – 1.6m/s) relative to the speed of the medium (2.2 m/s- 3.1 m/s) and those of the bottom of the
dryer (2.6 m/s-3.6 m/s) This reveals the existence of a dead zone vis-à-vis these fans that is areas
where air velocities are very low. This zone goes from 1.5 m from floor to the ceiling, which means
that the load of the product will be limited to 1.5 m from the floor of the drier.
Kinetics of drying
The drying operation is to extract part of water from a product to obtain at the end of the
treatment a solid product of given residual moisture [8]. The drying kinetics are either the variation
of water content as a function of time (x = f (t)), or the variation of drying rate as a function of time
or of the water content (dX / dt = f (t) or (dX / dt = f (X)). The water contents of the product at the
instant t i on dry basis, is determined by relationship : Xi = (m i - Md)/md [9] where mi is the mass
232


of the product at time t i and md the mass of the anhydrous product. Figure 3 represents the variation
of moisture of cassava for an initial mass of 781 kg and a final weight of 430 kg.

Water content(kgeau/kgms)

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0

5

10

15

Drying time (h)

20

25

30

Figure 3: Variation of the water content during drying 781 kg of cassava with broken clods
Heat balance of the dryer
The total energy consumed by the dryer is determined by the equation : ET =EV+ER where
EV is the energy consumed by fans and ER is the energy consumed by the heating elements.
Neglecting the heat losses through the walls of the dryer and assuming that all the energy consumed
by the heaters is transformed into heat energy, the transformation efficiency of electrical energy ER
to thermal energy is equal to 1. ER is the useful energy for drying. Tests in the modified dryer yielded
the results in table 1.
Table 1: Energy Performance of modified drying
Mass of
cassava clods
(kg)

Mass of dry
cassava (kg)

Drying time
(hours)

Energy
required
(kWh)

Loss (energy
consumed by
fans) kWh

781

430

21

398

78

572

305

18

337

67

425

235

16

306

59

Energy consumption of the modified dryer
To assess the thermal consumption of dryers, we define a number of criteria which include
the energy consumption per unit mass (ECM) of a dryer which is the amount of heat needed to dry 1
kg of water. For a dryer 3300 kJ/kg of water, ECM ≤ 6500 kJ/kg of water [10]. The ECM of the
electric dryer for cassava in Pouma is: 4637 kg/kg of water. This is consistent with the theory for this
type of dryer.

233


Energetic gain of the dryer
The useful energy for drying is proportional to the drying time. Figure 4 shows the tonnage of dried
cassava with time used to dry for two configurations of the dryer: The original and the modified
dryer.
700

Mass of dry cassava (kg)

600
500
400

After Modification
Before Modification

300
200
100
0
0

5

10

15

20

Drying time (h)

25

30

35

40

Figure 4: Influence of changes in the dryer on the drying time
It appears from the curves of the figure that for the same quantity of cassava with broken
clods, the drying time is reduced by about 20% for the modified as compared to the drying time for
the initial drying machine. This induces an energy consumption of about 20% less in the modified
dryer.
4. CONCLUSION
An energy audit of the UTM Pouma dryer was performed to determine deficiencies in the
construction and operation of the dryer. The correction of these deficiencies has increased the flow
rate per unit mass of drying air in the dryer, ensured uniform distribution of hot air in the dryer,
rationalize the use of hot air in the drying chamber and define a dead zone in the dryer area in which
we must not place the product to dry. These factors have enabled after comparing the energy
performance of the original and the modified dryer to release an energy saving of around 20%.
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