1. Ph ton 140
Journal of Mechanical Engineering. Photon 123 (2013) 140-145
https://sites.google.com/site/photonfoundationorganization/home/journal-of-mechanical-engineering
Original Research Article. ISJN: 2951-8372
Journal of Mechanical Engineering Ph ton
Development of a laboratory tunnel dryer for cassava chips drying
Ajala A.S., Adeyemi A.A., Wojuade I.S., Omitoogun S.A.
Department of Food Science and Engineering, Ladoke Akintola University of Technology, PMB 4000, Nigeria
Article history:
Received: 10 December, 2012
Accepted: 13 January, 2013
Available online: 21 March, 2013
Keywords:
Tunnel dryer, cassava chips, temperature and efficiency
Corresponding Author:
Ajala A.S
Email: ajlad2000@yahoo.com
Abstract
Cassava chips are the most common form in which
dried cassava root are marketed and most
exporting countries produce them. In Nigeria,
cassava chips production is confronted with poor
quality due to conventional drying by spreading it in
the sun. In view of this, an efficient mechanized hot
air drying system is necessary which is affordable
by small scale processors. For this reason, a small
tunnel dryer was developed and tested to evaluate
its efficiency. The dryer had three trucks containing
four trays each making 12 trays. The fan and heater
were attached to one end of the tunnel. Cassava
chips were loaded in the dryer at velocity of 3.0m/s,
temperature of 60, 70 and 80 °C for drying to
evaluate the efficiency of the dryer. The results
showed that the machine had drying efficiency of
66.7%, 75% and 80% at temperatures of 60, 70 and
80 °C respectively. Also the can handle 6 kg of
chips per batch.
Citation:
Ajala A.S, Adeyemi A.A., Wojuade I.S., Omitoogun S.A.,
2013 Development of a laboratory tunnel dryer for
cassava chips drying. Journal of Mechanical Engineering.
Science Photon 123. Photon 123, 140-145.
1. Introduction
The post harvest losses of agricultural produce
can be reduced drastically by using proper
drying technique (Velic, et al., 2007). In
Nigeria and other developing countries of the
world, most of the agricultural plant produce
are harvested during rainy season which
makes preservation by drying difficult and
causing most of the harvested crop to perish.
The same preservation problem affects
cassava in Nigeria. Cassava is the fourth most
important energy staple in the tropics and the
sixth global source of calories in human diets
apart from rice, maize and wheat (FAO, 2004)
and its utilization for industrial purpose such as
starch, alcohol, adhesives and livestock feed
is yet to be maximally used in Nigeria. There is
need to develop preservation technology to
convert the product into stable form for
industrial and export purposes. Cassava is
highly perishable and undergoes post-harvest
physiological deterioration within three days of
harvesting, partly due to the high water
content as well as its rich store of
carbohydrates (Ashaye et al 2005), it is
therefore important to process it into stable
forms that can store for longer periods.
Traditionally, this problem has been overcome
by various processing methods practiced by
farmers, such as fermentation to convert it to
usable product such as garri, fufu and pupuru,
or dry product in form of chips which could
serve as raw material for value added
products such as ethanol, starch and animal
feeds. Cassava chips, a derived cassava
product, are very popular in Africa where it
forms the raw material for the bulk of cassava-
based foods (Ugwu and Ay, 1992).
1.1 Statement of the problem
As a result of growth potential of the cassava
chips, conventional sun drying cannot meet
the high demand for chips hence, some form
of artificial heat drying is required. However,
little if any work has been done in Nigeria to
improve cassava chip drying and hence there
is need to explore a better method of drying to
overcome the challenges encountered in
conventional sun drying. Therefore, tunnel
dryer could be useful to dry cassava chips
because of its adaptability to drying tubers,
fruits and vegetables. The versatility of tunnel
drying is its ability to dry large volume of
product at a time, control temperature, velocity
and volume and uniform distribution of air in
regard to heat transfer and moisture carrying
capacity to obtain the most cost effective

2. Ph ton 141
drying, with maximum product quality (Jim,
2006). This perceived versatility propel the
thrust into this study. Therefore the objective
of this work is to design, construct and
evaluate the efficiency of the tunnel dryer
using cassava chips as the raw material.
2. Material and Methods
2.1 Design Consideration for the Dryer
Development
It is adaptable to both counter current and
concurrent mode of drying. The production
rate was 6kg per batch, number of trucks were
three with four trays each. The initial moisture
content of raw cassava chips was 72% and
the desired final moisture content of the chips
was 13%. Air temperature before entering
dryer was 30 °C and the bulk density of
cassava of raw cassava was 416 kg/m
3
2.2 Description of major components and
materials of construction
2.2.1 Drying chamber (tunnel)
This is where actual drying takes places. It has
three (3) trucks which are loaded periodically
into the chamber. The inside was constructed
with iron bar which served as the skeletal
frame of the dryer which was then covered
with a galvanized plate because of its
corrosion resistance property, availability and
cheapness. The outer covering was made
from aluminium sheet because of its
malleability, ductility, lightness and cheapness.
The length of the tunnel was 1.3 m with a
breadth of 0.35 m and a height of 0.80 m.
2.2.2 Electric heater
The electric heater supplied heat to the drying
chamber for drying of the product (cassava). A
heater of 1.8 kW was used in the dryer to
supply the heat needed for the drying
operation.
2.2.3 Blower (fan)
The fan was used to circulate heated air in the
drying chamber. For effective drying system,
the use of a centrifugal fan that can deliver
3.64 inches water pressure and 2 Hp was
employed.
2.2.4 Trucks
The tunnel consists of three (3) trucks with
each truck containing four (4) trays. The truck
was made from mild steel and it contained
roller wheels for easy movement in the
chamber. It has a length and breadth of 0.30
m x0.30 m and a height of 0.80 m.
2.2.5 Trays
These are flat, square –shaped containers
which contain the product to be dried. They
are made from galvanized plate, so that the
water from the product would not corrode the
surface. The dryer consists of twelve (12) trays
in total. The tray has a length and breadth of
0.25mx0.20m and a height of 0.01 m.
2.2.6 Pipe
The pipe which is made from iron material of
diameter 50 mm allows hot wet air form the
chamber to be recycled back as dry air into
chamber.
2.2.7 Electrical components
The wires are used to connect the heater and
the blower to an electrical power source. The
thermocouple is used for censoring the
temperature rise in the dryer and the
temperature regulator is used to regulate
temperature to the actual temperature of
drying.
2.2.8 Centre exhaust
This is a small square-shaped perforated
opening for allowing the passage of wet hot air
out of the drying chamber. It is situated on the
drying chamber of the tunnel dryer.
Figure 2.1: Exploded view of the tunnel dryer
showing its different parts
2.3 Design calculations
2.3.1 Calculation for the volume of cassava
chips on the trays
Mass of cassava chips per batch= 6 kg
From the relationship
Volume= Mass/ Density
= 6/416
=0.014 m3
Each tray will carry 0.001 m
3
of raw cassava
chips

3. Ph ton 142
2.3.2 Description of the trays
Figure 2.2: Schematic diagram of the tray
0.01m
0.20m
2.3.2 Description of the trays
Length of tray=0.25 m
Breadth of the tray= 0.20 m
Height of the tray= 0.01 m
Area of the tray = LxB
= 0.25x0.20
= 0.05 m
2
Volume of the tray= LxBxH
= 0.25x0.2x0.1
= 0.005 m
3
2.3.3 Description of the truck
Figure 2.3: Sketch of the truck
0.3m
0.702m
Height of the truck = Height of the tray +
spaces between trays + height of roller
Wheel + clearance on the truck
= (0.1)4 + 3(0.1) +0.01 + 0.01
= 0.702m
Length of the truck = length of the tray +
allowance for easy movement
= (0.25+ 0.05)
= 0.30m
Breadth of the truck= 0.3m
Area of each truck = L x B
= (0.3 x 0.9) m2
= 0.09m2
2.3.4 Design for drying chamber of the tunnel
Length of the drying chamber = length of each
truck x number of trucks + spaces
between the truck
=0.3(3) m + 0.1 (4) m
=1.3 m
Breadth of the drying chamber =0.35 m
Height of the truck =0.8 m
Area of the drying chamber = LxB
= 1.3 x 0.35
= 0.455 m2
Volume of the drying chamber = LxBxH
= (1.3 x 0.35 x 0.8) m
3
=0.364 m
3
2.3.5 Design space for whole components of
the tunnel
Length of the whole tunnel =Length of drying
chamber + spaces for heater chamber+ place
for fan
= 1.3 + 0.2 + 0.5
=2.0 m
Breadth of the whole tunnel = Breadth of the
truck + allowance for clearance
= {0.35 + 0.05} m
= 0.40 m
Area of the whole tunnel = L x B
= (2.0 x 0.4) m
2
= 0.8 m
2
Volume of the whole tunnel = L x B x H
= (0.8 x 0.8) m
3
= 0.64 m
3
2.3.6 Selection of the heater
Feed (mw) = 6 kg
Intended drying time = 10 hours
Initial moisture content of the cassava chips =
72%
Desired final moisture content = 13%
Therefore,
72100 

x
m
mm
w
dw
Where mw is the mass of wet cassava chips
md is the mass of dry cassava chips
72100
6
6


x
md
md = 1.68 kg
Mass of water to be removed = mass of wet
cassava- mass of dry cassava
= (6- 1.68) kg
=4.32 kg
Quantity of heat required to remove the water
= quantity of heat on the cassava
chips + latent heat of evaporation of
water inside the chips
Specific heat of fresh cassava chips = 3.41
kJ/kg °C (Grace, 1971)
0.25m

4. Ph ton 143
Latent heat = 4.186 x10
3
{(597- 0.56(Tpr)}
(Youcef- Ali et al., 2001)
Where Tpr is the product temperature
Q = mass of cassava chips x specific heat of
the chips x temperature difference +
Mass of water x 4.186 x10
3
{(597- 0.56(Tpr)}
= 6 x 3.41 x (80-30) + 4.32 x 4.186 {(597- 0.56
(60)}
= (1023 + 9985.72) kJ
= 11008.72kJ
Power of heater to be used = Quantity of heat
/Time
= 11008.72/ (2.5 x
3600)
= 1.22 kW
From the above calculation, a heater of about
1.8kW should be used for better efficiency.
2.3.7 Selection of fan
Length of the drying chamber (previously
calculated) = 1.3 m
Breadth of the drying chamber (previously
calculated) = 0.35m
Height at which chips fill each tray
= 0.05 m
Total depth of chips for 12 trays
= 12 x 0.05
=0.6 m
Volume of the material in the tunnel (m
3
) = 1.3
x 0.35 x 0.6
= 0.234 m
3
Range of air velocity necessary for drying food
products as recommended is 0.5-3.7 m/s
(Bullent et al., 2009; Ndukwu 2009)
In this design, a maximum velocity of 3 m/s
was used
Air flow rate = air velocity x area of drying
=3x1.3x0.35=1.365 m
3
/s
It is necessary to convert the value of the
volumetric flow rate to cubic per minute (cfm)
for standard fan selection
1 cfm = 4.91747 x 10
-4
m
3
/sec (Adzimah and
Seckley, 2009).
Therefore, 1.365 m
3
/s = 2775.8 cfm
Static pressure of cassava has moisture
content close to that of potato, so a static
pressure of 1.2 inches per foot depth is taken
(Tavernetfi and Henderson, 1959).
From previous calculation,
Total depth of chips = 0.6 m = 1.97 ft
Static pressure loss equation = total depth of
chip x static pressure per foot
= 1.97 x 1.2
= 2.364 inch of water
If there are foreign materials in the chips, the
static pressure is multiplied by 1.5 (Adzimah
and Seckley, 2009).
Therefore, the static pressure due to
resistance of air flow by chips = 1.5 x 2.364
=3.64 inches of water
efficiencyfanx6320
pressurestaticx totalratesflowairvolume=(P)powerhorseFan
Most industrial fan have efficiency between
70- 85% (Adzimah and Seckley, 2009)
Hence,
85.06320
64.32775.8
x
x
P 
= 1.88 Hp
A centrifugal fan with 2.0 Hp and 3.64 inches
water pressure was used. A centrifugal flow
fan is used to ensure proper distribution of air
to the drying chamber and for effective heat
distribution (Holman, 1998)
Assuming a loss of 2% of the quantity of heat
produced,
Quantity of heat per second= 1.8 kW (from
previous calculation)
2.3.8 Design for insulation
Figure 2.4: Thickness of the materials for construction
xg xs xa
T1 = 30
°
c
T2 = 80
0
c
Aluminium
Fiber glass
Galvanized plate

5. Ph ton 144
2% of 1.8kW =36w
The maximum temperature in the drying
chamber should not exceed 120 °C
][
a
a
s
s
g
g
k
x
k
x
k
x
A
q
T






T = T1-T2 is the change in temperature °C
T1 = Outside temperature = 32 °C
T2 = Temperature in the drying chamber =100
°C
q = Quantity of heat loss from the chamber =
31.65W
A = Area of the drying chamber = 0.28 m
2
xg = Thickness of the galvanized plate = 0.24
mn = 2.4 x 10
-4
m
xs = Thickness of the fibre glass = ?
xa = Thickness of the aluminum sheet = 0.26
mm =2.6x 10
-4
m
Kg = Thermal conductivity of the galvanized
plate = 56w/m °C
Kf = Thermal conductivity of the fibre glass =
0.048 W/m °C
Ka = Thermal conductivity of the aluminium
sheet = 204 w/m °C
]
204
106.2
048.056
104.2
[
28.0
36
30120
33 



xxx s
90 =128.57{4.29x10
-5
+ 048.0
sx
+1.27 x10
-5
]
sx = 0.033m=33mm
Hence fibre glass of 35 mm thick should be
used for safety reason.
2.4 Experimental set up in the dryer
2.4.1 Objective of research
The research is aimed at providing solution to
the poor quality of cassava chips produced in
Nigeria. Previous study has shown that the
major challenge facing chips production in
Nigeria is the drying system. This is the
problem the research wanted to address by
developing a better drying system that can
produce a better cassava chips for both
domestic and foreign uses
2.4.2 The drying process
Freshly harvested cassava roots from Ladoke
Akintola University of Technology Teaching
and Research Farm, Oyo state, Nigeria was
used for the experiment. The experiment was
carried out in 2012. The cassava was sorted to
select the appropriate roots, washed with
clean, potable water and allowed to drain. The
cassava roots was peeled and diced with
knives into dimensions: 5 cm x1 cmx 1 cm.
Drying of the chips was carried out in the
tunnel dryer developed. The dryer was run for
1 hour to stabilize the environmental condition
by adjusting it to the desired temperature and
air velocity before introducing fresh cassava
chips on the trays in the drying chamber and
be subjected to drying to safe moisture level of
13 % (Ashaye et al., 2005). The mass of
cassava chips in the truck used was 1.2 kg.
Once the moisture content of the chips
reached 13 %, the chips were packaged into
polythene bags. Three temperatures namely
60, 70 and 80 °C were used in the drying
experiments to determine the efficiency of the
machine. Also the effect of temperatures on
moisture loss was studied.
2.4.3 Determination of the dryer efficiency
The energy efficiency for convective dryers
can be calculated using equation below based
on the temperature of the drying medium at
the inlet (Tin), outlet, (Tout) and the ambient air
temperature (Tamb) according to Mujumdar
1995.
Efficiency of the convective dryer ( con) =
1
%100



ambin
outin
TT
TT
Temperature of drying air at inlet were, 60, 70
and 80 °C, temperature ambient was 30 °C
while outlet air temperature from the dryer was
40 °C.
3 Results and discussion
3.1 Result of the efficiency of the dryer
The result of the efficiency of the dryer is as
shown in Table 3.1. There was increase in
dryer efficiency as temperature of drying
increased from 60 °C to 80 °C. This is true
because at higher dry bulb temperatures, air
delivers higher enthalpy to remove moisture
from food products thereby reducing drying
time. Therefore researchers such as Shawn et
al. (2010) recommended higher temperature
and air velocity for drying. Although Tin and Tout
tended to fluctuate as fresh products were
introduced into the dryer, higher temperature
drying would quickly compensate for the
fluctuations and stabilize the system.
3.2 Effect of temperature on the drying pattern
of the chips
Figure 3.1 shows the effect of temperatures on
moisture loss of cassava chips during drying. It
took 10 hours to dry the product at 80 °C, 12
hours at 70 °C and 13 hours at 60 °C. The
reason was that higher temperatures induced
faster drying in food products than lower

6. Ph ton 145
temperatures thereby reduce drying time. In
other words, increase in air temperature
improved drying by affecting both the external
and internal mechanisms of moisture flow.
This observation has been reported severally
by other researchers such as Ajala et al 2012,
Paul et al. (2009)
Table 3.1: Efficiency of the dryer at different
temperatures
Temperatures (°C)
Dryer efficiency ( con) %
60
70
80
66.7
75
80
Figure 3.1: Moisture content against drying time
Moisture Content against Time
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
0 5 10 15
Time (hr)
Moisturecontent(kgwater/kg
solid)
(60°C)
(70°C)
(80°C)
Conclusion
The dryer could dry 6 kg per batch operation
and has galvanized plate to prevent corrosion
which can consequently contaminate the food
products. It is hoped that it will pave way for
success in cassava chip production for
medium scale farmers which will eventually
attract large scale investors and consequently
improve the economy through proper material
handling and production.
Further study could be optimization of the
dryer and more important is the automation of
the tunnel dryer system.
References
Adzimah K.S., Seckley E., 2009. Improvement on
the design of a cabinet grain dryer. American J. of
Engineering and Applied Sciences 2 (1), 217-228.
Ajala, A.S., Babarinde G.O., Olatunde S.J. 2012.
Effect of temperatures, air velocity and flow rate on
quality attributes of dried cassava chips. Asian
Journal of Agriculture and Rural Development.
Volume 2(4), 528
Ashaye O.A., Adegbulugbe T.A., Dawodu O.J.,
2005. Evaluation of the processing technologies of
cassava chips and flour in Oyo and Ogun States of
Nigeria. World Journal of Agricultural Sciences 1(1),
56-58
Bulent K., Murat T., Ibrahim H., Hassan V., 2007.
Solar drying of red peppers: Effect of air velocity
and product size. Journal of Applied Sciences,
7(11), 1490-1496
FAO. 2004. On line statistical database Rome, Italy.
Food and Agriculture Organization of the United
Nations (www.fao.org)
Fellows J.P., 2000. Food processing technology,
principle and practice, 2nd edition, Woodhead
publishing limited, p46
Grace M., 1971 Cassava processing: In Food and
Agricultural Organization (FAO) of the united
nations. Agricultural Service Bull. 8. Rome.
Holman J.P., 1998. Heat transfer. 9th Edn.,
McGraw Hill, New York.
Jim V., 2006. Fruit Drying Tunnels Design &
Operation. USAID/ADP report, pp1-13
Mujumdar A.S., (Ed.), 1995, Handbook of industrial
drying, 2nd edition, Marcel Dekker, New York. Pp 5-
8
Ndukwu 2009. Effect of drying temperature and
drying air velocity on the drying rate and drying
constant of cocoa bean. Agricultural Engineering
International, Vol. XI
Paul M., Carine M., Claude D., Thaddée M,,
François B., 2009. Influence of drying temperature
on functional properties of wet-milled starch
granules. Carbohydrate Polymers 75, 299-306
Shawn S., Mark H., Dana P., 2010. Managing high-
temperature grain dryers for energy efficiency, Iowa
State University extension bulletin, pp1-2
Tavernetfi J.R., Henderson S.M., 1959. New Potato
Dryer. California Agricultural Journal, 2, 14-15
Ugwu B.U., Ay P., 1992. Seasonality of cassava
processing in Africa and tests of hypothesis.
COSCA Working paper No. 6, IITA, Ibadan, Nigeria,
pp.23.
Velić D., Bilić M., Tomas S., Planinić M., Bucić-Kojić
A., Aladić K., 2007. Study of the drying kinetics of
Granny Smith apple in tray dryer. Agric. conspec.
sci. 72 (4), 326
Youcef-Ali S., Messaoudi H., desmons J. Y., Abene
A., Le Ray M., 2001. Determination of the average
coefficient of internal moisture transfer during the
drying of a thin bed of potato slices. Journal of food
engineering, 48(2), 95-101.