Vol.4, No.5B, 90-95 (2013) Agricultural Sciences
Design parameters for a small- scale batch in-bin
maize dryer
Fashina Adepoju Bola.1, Akande Fatai Bukola.1*, Ibrahim Saula Olanrewaju2,
Sanusi Bashir Adisa2
1Agricultural Engineering Department, Ladoke Akintola University of Technology, Ogbomoso, Oyo State; *Corresponding Author:
2Agricultural Engineering Department, The Polytechnic, Ibadan, Oyo State; fbukkyakande@yahoo.com, easyprogress@yahoo.com
Received 2013
Early season maize is harvested with high
moisture content that makes it impossible to
store. The sale of early season maize in green
form is uneconomical to the farmer. Experience
had shown that farmers could hardly make the
cost of production from their sales. Also, grain
losses are high when maize is harvested green.
To minimize grain losses and thereby increase
value and the profit margin of the farmer, a grain
dryer is necessary for wet grains. Ther efore, thi s
paper presents the design and development of a
batch in-bin maize grain dryer. Some properties
of maize such as moisture content and bulk
density were determined to get information re-
quired for design of the dryer. The dimension of
drying chamber, amount of moisture to be re-
moved in a batch, quantity of air required to ef-
fect drying, volume of air required to effect dry-
ing, blower capacity, quantity of heat required to
effect drying and actual heat used to effect dry-
ing were all designed for. A maize dryer was de-
veloped with a batch size of 100 kg of threshed
wet maize. The dryer can be used in laboratory
for experimental purpose as well as on the farm
for commercial purposes. The dryer can be used
to measure drying rates of maize at different
initial moisture contents, drying air tempera-
tures, drying air velocities and grain beds. The
effects of different drying temperature, air ve-
locity, loading and agitating speed on the quality
of dried maize can be investigated with the
Keywords: Design and Development; In-bin Maize
dryer; Fresh Maize Gra in; Moisture Content; Heat
Transfer; Drying Rate
Maize is an all-important crop which provides an
avenue for making various types of foods. It also has
some medicinal values and serves as raw-materials for
many industries. Grain is the most important part of
maize crop and is put to many uses.
Maize (Zea mays L.), or corn, is the most important
cereal crop in sub-Saharan Africa and, with rice and
wheat, one of the three most important cereal crops in the
world. Maize is high yielding, easy to process, readily
digested, and relatively cheaper than other cereals. It is
also a versatile crop; growing across a range of agro
ecological zones. Every part of the maize plant has eco-
nomic value: the grain, leaves, stalk, tassel, and cob can
all be used to produce a large variety of food and
non-food products.
Maize grain could be processed into different forms,
especially in the form of maize meal, which is an impor-
tant food for large numbers of people in Africa, provid-
ing significant amounts of nutrients, in particular calories
and protein. [1] Showed that 22 of 145 developing coun-
tries had a maize consumption of more than 100 g per
person per day. In a dietary intake survey in South Africa,
[2] found that maize meal was consumed by almost, all
of the respondents, both males and females, in the rural,
farm, informal settlement and middle class urban strata.
In industrialized countries, maize is largely used as
livestock feed and as a raw material for industrial prod-
ucts, while in developing countries, it is mainly used for
human consumption. In sub-Saharan Africa, maize is a
staple food for an estimated 50% of the population. It is
an important source of carbohydrate, protein, iron, vita-
min B, and minerals. Africans consume maize as a
starchy base in a wide variety of porridges, pastes, grits,
and beer. Green maize (fresh on the cob) is eaten parched,
baked, roasted or boiled; playing an important role in
filling the hunger gap after the dry season.
The harvesting of the early season maize in Nigeria is
Copyright © 2013 SciRes. Openly accessible at http://www.scirp.org/journal/as/
F. A. Bola. et al. / Agricultural Sciences 4 (2013) 90-95 91
usually between July and October when the rain is fully
established. At this time, natural drying on the field is
difficult due to low atmospheric temperature and high
relative humidity. If the crop is left on the field to dry, it
will continue to deteriorate because of the slow rate of
drying. They are therefore harvested with the high mois-
ture content and sold cheaply to be cooked or roasted for
consumption. This decrease in farmer’s income can be
averted, if after harvesting, the maize can be dried and
Drying is one of the oldest methods of food preserva-
tion. It is the removal of moisture from an agricultural
produce/biomaterial to moisture content in equilibrium
with the surrounding air or to such moisture content that
can decrease the mould’s enzymic action and insects’
infestation. Food stuffs are usually dried to enhance
their storability, transportability, texture and retainabil-
Drying reduces the amount of water contained in the
crop after harvest to an acceptable level for marketing,
storage or processing [3]. Both grain temperature and
moisture content are critical in maintaining quality.
Mould and insect activities are greatly reduced below
15ºC safe moisture levels for storage. However, these
depend on grain variety, length of storage, storage struc-
ture, and geographical location.
The major input in a drying process is the heat which
raises the temperature of the inlet air which is blown
through a static grain bulk to be dried. The wet grains
can only be dried if the inlet air conditions are drier than
the wet grain. This means that the moisture contained in
the inlet air can be removed by raising its temperature,
thus, increasing its ability to remove moisture from a wet
grain. The exit air which leaves the dryer after passing
through the wet bulk of grain could accumulate and sub-
sequently condense within the dryer if there are no ade-
quate exit channels and if the airflow rate is low [4]. The
cooking of the grains and stress cracking due to the de-
velopment of high internal grains’ temperatures and
pressures can be avoided if the condition of the inlet air
introduced into the dryer systems is carefully selected.
This will ensure the good quality of out-loaded grain
bulk. Also, during drying, the conditions of grains near-
est to the inlet are always different from those nearest to
the outlet and continue along single line within the bulk.
These differences may occur as temperature of the air
voids between grains and as moisture content of the grain
when the progression of the air fronts are not uniformly
In Nigeria and other African countries, post-harvest
losses of agricultural products are very high. This is due
to the fact that each of the products has its season and it
is mostly produced in excess of what is immediately
needed. The losses are due to lack of appropriate preser-
vation and storage facilities. These losses made the
products unavailable throughout the year and where they
are available there is a sharp difference in prices at har-
vest and later after harvest. Therefore, the objective of
this study is to design and develop a batch in-bin maize
grain dryer for freshly harvested maize for on farm usage
with a view to reducing the postharvest losses faced by
farmers thereby increasing their income.
In order to develop an efficient batch in- bin dryer for
maize, the following properties and parameters were
2.1. Determination of Moisture Content
The moisture content of the maize was determined to
know the amount of moisture to be removed from the
freshly harvested maize. The sample freshly harvest
maize grain was weighed and dried in a ventilated elec-
tric oven set at 65℃ for 24 hs when constant weight was
obtained in accordance with [5]. The sample was re-
moved and allowed to air-cool. The weight of the dried
sample was determined using a digital sensitive weighing
balance of 0.01 g accuracy. The moisture content (% )
was computed using Eq.1.
2.2. Determination of Bulk Density of Maize
at Harvest
[6] Developed an empirical formula which relates bulk
density and moisture content for maize as stated in Eq. 2.
Maize harvested at maturity normally has an average
moisture content of 32% (wb) [7].
Substituting the value of Mwb = 32% (wb) into Eq.2
TWm = 0.7019 + 0.01676 (32) – 0.0011598 (32)2 +
0.00001824 (32)3
TWm = 0.6483 g/cm3
TWm = 648.3 kg/m3
2.3. Design Considerations
In designing the dryer, the following considerations
were made:
To have a uniform dried product such that the tem-
perature and moisture content are the same at every lev-
els of the grain thickness, the inlet air would be directed
as to span through the entire circular base of the dryer.
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F. A. Bola. et al. / Agricultural Sciences 4 (2013) 90-95
To avoid accumulation of vapour, the upper lid of the
dryer will be perforated so as to allow easy flow of va-
pour picked by the heated air from the grains to the at-
To reduce the static pressure which the fan must over-
come in order to supply the desired airflow, an agitator
shaft was incorporated. A cylindrical shaped dryer is
therefore considered for easy and uniform agitation.
To reduce the heat loss due to conduction and to con-
serve energy generated by the heaters, the drying cylin-
der was lagged with glass fiber because of its effective-
ness and availability.
To ensure that the grains would not be contaminated,
the inner wall, the agitation shaft and the perforated flour
of the dryer were made of stainless steel-material and
To ensure that the rate of drying of the grains is en-
hanced, appropriate size of heater was considered to raise
the temperature of the drying air.
2.3.1. Design of the Dryer
The design of the maize dryer (Figure 1) was based on
the following: amount of moisture to be removed, quan-
tity of air required to effect drying, volume of air to ef-
fect drying, blower design and capacity, quantity of heat
required, heat transfer, actual heat used to effect drying,
rate of mass transfer, thermal efficiency, and the drying
rate. The design was based on ambient temperature (T1)
of 32℃; this is applicable for steady flow systems as
stated by [8]. The initial humidity ratio (Hr1) is deter-
mined to be 0.01 kg/kg dry air using the psychometric
chart under normal temperature and 101.325 kPa baro-
metric pressure. The safe drying temperatures (T2) re-
quired for drying maize is 43 as stated by [9]
2.3.2. Design of the Drying Chambers
The dimension of the drying chamber was determined
with the assumptions that, the configuration is cylindrical
and mass of maize grain per batch is 100 kg.
The bulk density of the maize grain depict that 6 482
kg of freshly harvested maize occupies 1 m3 by volume,
1 kg of freshly harvested maize occupies 1/648.2
= 0.001542 m3
100 kg will occupy 0.001542 × 100 m3
= 0.1542 m3
Since the dryer is cylindrical, assuming a diameter of
600 mm
Volume = base area x height.
0.1542 = × 0.32 × height π
height = 0.1542/ × 0.32 π
= 0.5453 m.
The dimension of the drying chamber was therefore
determined to be 600 mm diameter and 546 mm height.
2.4. Amount of Moisture to be Removed
Amount of moisture to be removed in kg (MR) is given
in Eq.3 as:
Where, M is dryer capacity per batch (kg), Q1 = initial
moisture content of the maize to be dried 35%, Q2 =
maximum desired final moisture content, which is 13%
based on experimental results [10]. MR is therefore de-
termined to be 25.29 kg.
2.5. Quantity of air Required to Effect Drying
Quantity of air required to effect drying in kg (Qa).
This can be calculated from Eq. 4 by [11]
where Hr1 and Hr2 are initial and final humidity ratios
in kg/kg dry air respectively; and MR is as determined in
Eq.3. The average ambient temperature and relative hu-
midity are 31 for dry bulb temperature, 28 for wet
bulb temperature and 35% for relative humidity. The
initial humidity ratio Hr1 is determined to be 0.01 kg/kg
dry air using the psychometric chart under normal tem-
perature and 101.325 kPa barometric pressure. After the
heat has been supplied, the temperature of the product
rises to 50 giving the final humidity ratio (Hr2) as
0.028 kg/kg dry air. Substituting these values of Hr1 and
Hr2, and Q1 and Q2, into Eq.4 gives the quantity of air
required to effect drying (Qa) as 1,405 kg.
2.6. Volume of Air to Effect Drying
Volume of air to effect drying in m3 (Va ) can be deter-
mined using Eq.5 by [11]:
Figure 1. Exploded view of the dryer.
Copyright © 2013 SciRes. Openly accessible at http://www.scirp.org/journal/as/
F. A. Bola. et al. / Agricultural Sciences 4 (2013) 90-95 93
where, δa is the density of air in kg/m3 which is deter-
mined at 0℃ to be 1.115 kg/m3 based on properties of
common fluids presented by [12]. The volume of air to
effect drying was therefore calculated to be 1,260.10 m3
2.7. Blower Design and Capacity
The blower serves the purpose of transferring heated
air from the heat exchanger to the dryer cabinet. The
selection was based on the characteristics of centrifugal
fan performance curve based on the Eqs.6-8:
where N is the speed (rpm) of the electric motor, H is the
static pressure (Pa ), q is the volumetric flow rate of air
(m3/min), D is the diameter of the blower (m) and hp is
the motor horse power. Based on the selection from the
chart presented by [13] on the performance curve of a
backward-curved centrifugal fan showing system char-
acteristics, N1 is 1000 rpm, D1 is 0.46 m, H1 is 1.41, H2 is
1.09, q1 is 226.4 m3/min, q2 is 198.1 m3/min and hp1 is
2.28. Based on Eq.6, N2 is taken to be in 1000 rpm since
an electric motor of 1000 rpm is selected. The value of
D2, (m) is calculated based on Eq. 7 while hp2 is calcu-
lated from Eq. 8 for which a 2hp electric motor is se-
The Blower Capacity (BC) is calculated from Eq.9
where Qa = Qm + Re + Zk and Qm = δa × q2 = 1.115
kg/m3× 198.1 m3/min = 220.88 kg/min; Re = 25% of Qm
which is 55.22 kg/min; Zk = 1-2% of Qm which is 4.42
kg/min at 2%; and n= percentage safety factor that en-
sures an adequate supply of air in all operating condi-
tions at 15% but usually 10% - 20%. Substituting, BC is
therefore calculated to be 322.6 kg/min.
2.8. Quantity of Heat Required for Effective
Drying (Hr) in kJ
The quantity of heat required for effective drying is as
presented in Eq.10
where M = dryer capacity per batch (kg) = 100 kg; Hk =
CT (T2-T1), whereas CT is specific heat of maize = 1.8
kJ/kg℃; and T2-T1 = 50 - 32℃, HL = latent heat of va-
porization =1248.1 kJ/kg; and MR = amount of moisture
was removed (kg) substituting these values into Eq.10,
Hr is calculated to be 31,888.50 kJ.
2.9. Heat Transfer Rate
The heat transfer rate (Qht) can be determined from
Eq.11 by [12] as:
where h = heat transfer coefficient = NuK/d and with Nu
(Nusselt) = 121.3 = 0.13a
0.33 with Ra = 109; K as thermal
conductivity = 0.0305 kW/mK and d as diameter of the
heat exchanger = 0.56 m, the value of h is 6.607
kW/m2oC; Ae = surface area of the heat exchanger =
0.7389 m2; and TB = temperature of hot air in the blower,
. The value of heat transfer rate (Qht) is therefore de-
termined to be kJ.
The quantity of heat that can be lost through the
blower in the process is calculated from Eq.12
where qL = quantity of heat lost (kJ); K = thermal con-
ductivity of mild steel = 58
W/m.K; Ab = surface area of the blower = 0.88 m2; TBE
= temperature difference between the hot air in the
blower and the environment = T2-32℃; and δk = distance
= 1. The value of qL is therefore calculated in kJ. The net
heat transfer rate (Qhtr) that will reach the cabinet is (Qht -
qL) kJ.
2.10. Actual Heat used to Effect Drying (HD)
The quantity of heat used in effecting drying HD can
be determined from Eq. 13
where Ca = specific heat capacity of air = 1.005 kJ/kg℃;
MR = amount of moisture to be removed kg; and Tc =
temperature difference in the dryer cabinet = T2-32℃
=50-32℃. The quantity of heat is therefore calculated to
be 457.50 kJ.
The component parts of the dryer are; frame, drying
cylinder, agitating shaft, heat exchanger, air blower and
electrical control panel. The components were fabricated
and assembled according to design. The exploded view
of the dryer is show in Figure 1.
The dryer was fabricated using the following materi-
(i) Stainless steel sheet: was used for lining the drying
chamber of the dryer;
(ii) Angle bar: was used for constructing the frame;
(iii) Bearings: was used for holding the agitating shaft at
both ends;
(iv) Stainless steel rod: was used as the agitating shaft;
(v) Mild steel sheet: was used as external cover for the
drying chamber and the lid;
Copyright © 2013 SciRes. Openly accessible at http://www.scirp.org/journal/as/
F. A. Bola. et al. / Agricultural Sciences 4 (2013) 90-95
(vi) Fibre glass: was used as a lagging material;
(vii) Electrodes (both stainless and mild steel): was used
for welding the various parts.
Description of the Dryer
The dryer is an in-bin type with agitator. It has a cy-
lindrical bin as the drying chamber, with slanting and
perforated floor. A shaft placed at the centre of the cyl-
inder with spikes at alternate sides to each other serves as
agitator. The agitating shaft is driven by a gear type elec-
tric motor. The upper lid of the cylinder is perforated and
an opening cut-out to serve as inlet (grain hopper) for the
grain to be dried. Heated air is forced into the cylinder by
a centrifuged fan blowing directly on a set of heater ele-
ments. The heated air will pick moisture from the grains
as it comes in contact with the grains in the chamber and
releases same to the atmosphere through the perforated
lid of the bin. The agitating shaft ensures even distribu-
tion of the heated air by reducing the resistance to air
flow. The isometric view of the dryer is shown in Figure
2 while Figures 3-5 show the orthographic view and
Figure 6 shows the sectional view of the in-bin dryer.
Figure 2. The Isometric view of the dryer.
Figure 3. Plan view of the dryer.
Figure 4. Front view of the dryer.
Figure 5. Side view of the dryer.
Figure 6. Sectional view of the dryer.
Copyright © 2013 SciRes. Openly accessible at http://www.scirp.org/journal/as/
F. A. Bola. et al. / Agricultural Sciences 4 (2013) 90-95
Copyright © 2013 SciRes. http://www.scirp.org/journal/as/
4. DATA ACQUISITION PROCE DURE [4] James, T.K., Henry-Luka, A.R. and Mazza, G.( 2006)
Flavour retention during rehydration of onion, Food
Process Engineering, 1, 399- 406.
A thermostat is incorporated with the dryer to vary the
temperature. It also has a blower control unit to vary the
air velocity. Once the drying chamber is loaded, the
blower knob is set to the required air velocity and the
thermostat is also set to the required temperature, drying
is then carried out for a specified time.
[5] Ratti, C. (2001) Hot air and freeze-drying of high-value
foods; a review. Journal of Food Engineering, 49,
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[6] Bakker Arkema, F.W., Debaerddomacker, P., Amirante,
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Debaerddomacker, P., Amirante, M., Ruiz Altisent and
Sltudman, C. (1999) CIGR. Handbook of Agriculture En-
gineering. Reinhold Publishing Corporation, 143-145.
The dryer can be used to measure rate of drying
freshly harvested maize grain at different initial moisture
contents, drying air temperatures, drying air velocities
and grain beds. The effects of different drying tempera-
ture, air velocity, loading and agitating speed on the
quality of dried maize can be investigated with the dryer.
[7] Owolade, E.O., Akinjide, T.O and Afolabi, A.R. ( 2005)
Effects of drying of different varieties of maize. Journal
of Food Technology, 11, 159-182.
[8] Otoniel, C., Nelson, B., Alberto, V., & Angel, P. (2009)
Determination of suitable thin layer model for air drying
of coroba slices (Attalea Maripa) at different air tem-
peratures and velocities. Journal of Food Processing and
Preservation, 34, 587-598.
A batch type in-bin maize grain dryer has been devel-
oped which is capable of drying fresh maize grain at
varying drying air temperature , air velocity and batch
size, depending on the intended end use of the maize.
The locally fabricated dryer is affordable with a total cost
of sixty thousand naira (N 60,000 = 375 USD). The dryer
can be used in laboratory for experimental purpose as
well as on the farm for commercial scale.
[9] Akanbi, C.T., Adeyemi, R.S. and Ojo, A. (2006) Drying
characteristics and sorption isotherm of tomato slices.
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Mx - dryer capacity per batch, kg
NOTATION Mmoisture content, wet basis
wb -
N - speed of electric motor, rpm
Ab - surface area of the blower, mm2 Q1 - initial moisture content of sample, %
A surface area of the heat exchanger, mm2
e -
BC - Blower capacity, kg/min Q2 - final moisture content of sample, %
Qa - quantity of air required for drying, kg
Ca- specific heat capacity of air, kJ/kg℃ Qht - heat transfer rate, kJ
CT - specific heat of maize, kJ/kg Qnet heat transfer rate, kJ
D - diameter of blower, m htr -
q - volumetric flow rate,m3/min
d - diameter of heat exchanger, m q quantity of heat lost, kJ
H - static pressure, kPa L -
TB - temp. of hot air in the blower,
Hactual heat to effect drying
D -
Hr - quantity of heat reqd for effective drying, kJ TBE - temp. difference between blower air& environ,
Tc - temp. difference in dryer cabinet,
Hr1 - initial humidity ratio Tm bulk density at a given moisture content, g/cm3
Hr final humidity ratio w -
Va - volume of air, m3
2 -
h - heat transfer rate, coefficient, NuK/d w1 - initial weight of sample, kg
H latent heat of vapourization, kJ/kg
L -
K - thermal conductivity of mild steel, W/m.K w2 - final weight of sample, kg
δa - density of air, kg/m3
mc - moisture content, %
δk - distance = 1 m
MR - amount of moisture remove, kg
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