ISSN (Print) : 0974-6846
ISSN (Online) : 0974-5645
INDIAN JOURNAL OF SCIENCE AND TECHNOLOGY
February 2020, Vol 13(05), 502 – 518
DOI: 10.17485/ijst/2020/v13i05/145593,
© 2020 The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the
original author and source are credited.
Equipment Sizing and Method for the
Application of Exhaust Gas Waste Heat to Food
Crops Drying Using a Hot Air Tray Dryer
C. Ononogbo
1,
*, O.C. Nwufo
2
, C.A. Okoronkwo
2
, N.V. Ogueke
2
,
J.O. Igbokwe
2
and E.E. Anyanwu
2
1
Department of Mechanical Engineering Technology, Imo State Polytechnic,
Umuagwo, Owerri, Imo State, Nigeria
2
Department of Mechanical Engineering, Federal University of Technology, Imo State,
Nigeria
Abstract
Objectives: Equipment sizing and method of utilizing exhaust gas
waste heat for food crops drying are presented. Methods/ndings:
The sizing of the components of the equipment was achieved using
known design principles. The system uses an axial ow turbine and
a heat exchanger to harness the energy of the exhaust gas of a diesel
engine generator. A 250 kVA generator with measured exhaust gas
ow rate and temperature of 44.5 m
3
/min and 382.7 °C was selected
for this work. The purpose of the recovered energy is for the drying
of food crops. An arduino platform was used to control the operation
of the components of the dryer. The drying chamber consists of
three trays whose calculated total volume per batch is 0.0463 m
3
.
Weight losses across the trays, drying air temperature and humidity
in the chamber are monitored by sensors. The test rig of the sized
and fabricated equipment is currently undergoing extensive
experimentation. Preliminary investigation of the dryer showed
that the drying air in the chamber initially at the temperature and
humidity of 31ºC and 71.2% was heated to 88ºC and 22.3% when
the dryer operated without load for 45 min at an air speed of 2.0 m/s.
Application: After cooling and maintaining the drying chamber
averagely at 59ºC, each tray was loaded with 400 g of the grains of
freshly harvested maize. The dryer was allowed to run for 30 min
and the results obtained showed weight losses of 127.81 g, 118.36
g, and 116.91 g for the grains in trays 1, 2, and 3, respectively. The
application of this energy recovery system to the drying of food
products, would help to save a considerable amount of primary fuel
which is considered a viable means of cost saving and amelioration
of environmental degradation.
Keywords: Waste Heat Recovery, Drying, Global Warming,
Environment, Cost Saving, Food Quality.
Article Type: Article
Article Citation: Ononogbo C,
Nwufo OC, Okoronkwo CA, Ogueke NV,
Igbokwe JO, Anyanwu EE. Equipment
sizing and method for the application
of exhaust gas waste heat to food crops
drying using a hot air tray dryer. Indian
Journal of Science and Technology. 2020;
13(05), 502-518. DOI: 10.17485/ijst/2020/
v013i05/145593
Received date: June 29, 2019
Accepted date: October 18, 2019
*Author for correspondence: C.
Ononogbo
@
petermaryco@yahoo.
com Department of Mechanical
Engineering Technology, Imo State
Polytechnic, Umuagwo, Owerri, Imo
State, Nigeria
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C. Ononogbo, O.C. Nwufo, C.A. Okoronkwo, N.V. Ogueke, J.O. Igbokwe and E.E. Anyanwu
Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
1. Introduction
ere is high demand for fuel as a result of expansion in population, urbanization, and
industrialization, which has resulted to increase in fuel costs. For this reason, eorts are
being made to make energy use more ecient. Obviously, higher energy conversion or
its utilization translates to reduced cost of energy globally. One technique of ensuring
high energy eciency is the use of waste heat recovery systems. Recently, huge eorts are
directed to the reduction of the amount of energy that is wasted into the environment.
And the ultimate aim is to help in the conservation of exhaustible reserves of fossil fuels,
reduction of the carbon footprint and the battling of global climate change as well as
the improvement of process economics. According to Ref. [1], waste heat is heat that
is produced in a process as a result of fuel combustion or chemical reaction, and then
discarded into the environment even though it could still be harnessed for a purpose that
is useful and economic. Diminishing petroleum supplies and increasing fuel costs are
causing governments and industries to look for ways to increase the power eciency of
engines [2]. An internal combustion (IC) engine heat balance shows that the input energy is
approximately split into three equal portions such as the energy put to useful work, energy
lost to coolant, and energy lost to the environment with the exhaust gases. Hopefully, with
the emerging discoveries on exhaust heat recovery to increase the eciency of IC engines,
world energy demand for the depleting fossil fuel reserves would be reduced and hence
the impact of global warming [3–4]. It is widely believed that almost 70% of the energy
released from the fuel by an engine is lost, mostly in the form of heat. In Ref. [5], Pradip
and Hole reported that approximately 25–30% of the energy generated by engines is
dissipated in the form of energy loss through the exhaust gas. In Ref. [6–7], authors stated
that the waste heat produced from the thermal combustion process in IC engines which
is lost to the environment through an exhaust pipe could get as high as 30–40%. Basically,
the direct dumping of exhaust gases into the surroundings, not only wastes energy but
also contributes to the damage of the environment. However, several innovative cooling
and exhaust heat recovery systems have been introduced to reduce cooling loss and
regenerate the power by recovering the waste heat [4]. In recent times, large eorts have
been committed towards the recovery of waste thermal energy in vehicles and other waste
heat generating industrial machines. Interestingly, it could be surmised from the foregoing
that the energy released through the exhaust of IC engines is of the same magnitude as the
mechanical power generated by the engine.
Waste heat losses from combustion processes in IC engines are inevitable. erefore,
some facilities can be used to reduce these losses by either improving the equipment
eciency or by the installation of waste heat recovery technologies. e recovered energy
could be used in preheating combustion air, space heating, electricity generation, etc.
ree essential components are required for waste heat recovery, namely: (i) a source
of waste heat that is accessible, (ii) a technique for its recovery, and (iii) an application
for the recovered heat. e method required for the heat recovery partly depends on the
temperature of the available waste heat and the economics involved. Generally, the higher
the temperature of waste heat, the higher the value which makes the heat recovery more
cost eective. e temperatures of exhaust gases immediately leaving the engine are as
Equipment Sizing and Method for the Application of Exhaust Gas Waste Heat to Food Crops
Drying Using a Hot Air Tray Dryer
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Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
high as 450–600 °C
[2]. Table 1 shows a survey of exhaust gas temperatures from various
internal combustion engines of automotive vehicles and stationary engines.
Although technologies of energy recovery are available, there are still large potentials
for their application, which according to Ref. [8] have not yet been realized in industries.
ere are numerous technologies used for waste heat recovery such as heat exchangers,
thermoelectric devices, turbo compounds, etc. In this work, the source of waste
heat energy to be accessed is the high temperature exhaust gas from a 250 kvA diesel
generator. It involves recovery or conversion of energy of the exhaust gas to useful work.
Captured and reused waste heat is an emission-free substitute for costly purchased fuels
or electricity. By this recovery system, the impact of the pollutants to the environment is
signicantly reduced. e recovery technology to be employed is an axial ow turbine and
a heat exchanger while the energy recovered is to be used for drying agricultural products.
Large amounts of agricultural products perish yearly during post-harvest periods because
of their relatively high moisture content. Basically, Nigeria is blessed with a landmass of
98.3 million hectares of which 72% is considered suitable for agricultural production [9].
Statistics show that the rate of growth in the area of food production is very low, amounting
to 2.5% per annum [10]. According to Ref. [11], the poor growth is attributable to the poor
level of food preservation in the country. However, eorts are being made with a view to
achieving improvement in food production in a given number of ways such as drying
of agricultural products. is encourages farmers to embark on large scale agricultural
activities as long as ways of preserving their harvested crops are available. Many agricultural
products require long drying times ranging from 5 min to 73 h with optimum drying air
temperatures requiring large quantity of energy that results in high overhead drying cost
and high prices of dried food products [12–13]. For most food products, it is best to use
drying temperatures of 50 °C to 55 °C; and the temperature should never exceed 60 °C
[14]. is is because higher temperatures destroy nutrients contained in the food which
causes the food to lose its dietary value. When higher temperatures are used, food cooks
instead of drying, which results in case hardening where the food dries on outside but
moisture trapped inside allowing mold growth [14].
ere are many types of dryers operated by dierent energy sources like solar energy,
bioenergy, electrical energy, etc. Most of the energy sources involved in these operations
have some associated setbacks in terms of their availability and costs. is makes the end
products more expensive. Although sun drying is the cheapest and oldest method of food
TABLE 1. Temperature range for diesel engines
S/N Engine Temperature (°C)
1 Single cylinder four stroke diesel engine 456
2 Four cylinder four stroke diesel engine (Tata Indica) 448
3 Six cylinder four stroke diesel engine (Tata Truck) 336
4 Four cylinder four stroke diesel engine (Mahindra Arjun 605
DI)
310
5 Genset (Kirloskar) at power 198hp 383
6 Genset (Cummims) at power 200hp 396
Source: In Ref. [12].
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Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
preservation but it exposes the crop to contamination by dust, insect attack and infection
by microbes and yet it is not always available. ere are dierent drying methods that are
available to ensure continuous high quality food supply. In this work, a hot air tray dryer
using exhust gas waste heat as energy source is considered for the drying of food crops.
Tray dryers are widely used in agricultural drying due to their capability to dry products
irrespective of time and weather conditions [15]. Among all the drying techniques, the
tray dryer is the most extensively used because of its simple and economic design [15].
e food is spread out on trays at an acceptable thickness so that the product can be
dried uniformly. Heating may be produced by hot air stream across the trays, conduction
from heated trays, or radiation from heated surfaces. In a tray dryer, more products can
be loaded as the trays are arranged at dierent levels. Tray dryers have the capability of
drying products at high volume. e key to its successful operation is the uniform airow
distribution over the trays [16].
2. Materials and Methods
2.1. Conguration of the Waste Energy Recovery System
e conguration for the exhaust gas waste energy recovery system from a turbocharged
250 kVA Perkins diesel generator is shown in Figure 1. Here, part of the energy of the
exhaust gas is recovered by a turbine and used to drive an electrical generator (dynamo)
which in turn is used to charge a battery of 12V7.5Ah. is battery produces the electric
power needed to drive the air blower and other devices. e exhaust gas leaving the housing
Waste energy recovery
turbine
Ambient air
Heated
air to
dryer
Exhaust gas to atmosphere
Turbine
Belt pulley
arrangement
Dynamo
Heat Exchanger
Tray dryer
Exhaust gas as
source of heat to
Heat Exchanger
Intake
Compressor
Electrical
Turbocharger
Diesel Generator Engine
Blower
Moist air exiting
drying chamber to
the atmosphere
Moist air outlet fan
FIGURE 1. The system conguration of the turbine-dryer arrangement for waste energy
recovery.
Equipment Sizing and Method for the Application of Exhaust Gas Waste Heat to Food Crops
Drying Using a Hot Air Tray Dryer
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of the energy recovery turbine enters the heat exchanger to raise the temperature of the
ambient air (drying air) drawn into the heat exchanger by the air blower. e heated air is
then routed to the drying chamber of the tray dryer for the purpose of drying food crops.
e turbine rotor is directly attached to the main sha connected to the smaller pulley
(driving-pulley) as seen in the isometric view of the heat recovery system, Figure 2, while
the driven pulley of a larger diameter has a common sha with the dynamo which generates
the electrical energy required in the system. e axial turbine unit, apart from some load
bearings, has as its main part as a row of aerodynamically shaped objects (stator) which do
not move and a row of aerodynamically shaped objects (rotor) which move and provide
the torque to the sha, and the entire components are placed in a housing as seen in Figure
2. e whole unit is connected to the exhaust gas tail pipe outlet of the generator causing
the rotor blades to spin in the exhaust gas streams.
During operation, the exhaust gas travels in the axial direction and enters through the
inlet cone, where the uid slows down to the selected inlet conditions of the blades of
the stator. en the stator blades set the speed and angle at which the exhaust gas must
enter the rotor. When the exhaust gas stream strikes the turbine rotor blades by virtue of
its kinetic energy, the rotor blades spin providing torque to the sha which rotates the
driving-pulley. e exhaust gas which is still at high temperature exits the turbine housing
and enters the heat exchanger where it heats up the ambient air drawn in by the blower
(inlet fan). e heated air is then routed through the drying chamber of a tray dryer in
FIGURE 2. Isometric view of the exhaust gas heat recovery system.
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C. Ononogbo, O.C. Nwufo, C.A. Okoronkwo, N.V. Ogueke, J.O. Igbokwe and E.E. Anyanwu
Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
order to dry the product loaded in the trays. e exploded view of the heat recovery system
is shown in Figure 3.
2.2. Selection of the Axial Flow Turbine
e values of the baseline parameters such as the Exhaust gas ow rate,
V
and Inlet
temperature of the exhaust gas to the stator,
01
T
are 44.5 m
3
/min and 655.7 K, respectively.
In addition to this, the following preliminary assumptions as seen in Table 2 are used for
the selection of the axial ow heat recovery turbine.
Where
2
c
is the absolute velocity at inlet of the rotor;
3
V
is the relative velocity at exit
of the rotor;
3
c
is the absolute velocity at exit of the rotor, and
2
V
is the relative velocity
at inlet of the rotor. Based on the mass ow rate of the uid and the area upon which it
enters the turbine, the initial absolute velocity can be calculated with the ow entering
an axial turbine being in an axial direction [17]. e expressions for the annulus areas
of the turbine at the various stations,
i
A
, the mass ow rate,
m
of the exhaust gas, the
blade loading coecient,
ϕ
, the blade heights,
i
b
and the degree of reaction,
d
R
are given
below:
i
i ai
m
A
c
ρ
=
(1)
FIGURE 3. Exploded view of the exhaust gas heat recovery system.
Equipment Sizing and Method for the Application of Exhaust Gas Waste Heat to Food Crops
Drying Using a Hot Air Tray Dryer
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Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
2
mV
ρ
⋅
= ×
(2)
0
2
2
ps
m
cT
U
ϕ
∆
=
(3)
ib
i
m
AN
b
U
=
(4)
23 23
13 13
d
hh TT
R
hh TT
−−
= =
−−
(5)
Where the subscript
i
represents the dierent stations of the turbine stage, 1, 2, and 3.
However,
1
h
,
2
h
, and
3
h
are the enthalpies of the exhaust gas at inlet to the stator, at entry
to the rotor, and at exit from the rotor, respectively. Similarly, T
1
, T
2
, and T
3
are the static
temperatures at the stated positions.
2
ρ
is the density of the exhaust gas at inlet of the
rotor. e gas angles of the velocity triangles are calculated at the mean radius using the
following equations:
3
1
tan 2
22
d
R
ϕ
β
= +
∅
(6)
2
1
tan 2
22
d
R
ϕ
β
= −
∅
(7)
33
1
tan tan
αβ
= −
∅
(8)
22
1
tan tan
αβ
= +
∅
(9)
TABLE 2. Assumptions for preliminary selection
Parameter Value
1. Temperature drop,
0
∆T (K)
3
2. Mean blade speed,
m
U
(m/s)
43
3. Nozzle loss coecient,
N
ì
0.052
4. Flow coecient,
∅
0.80
5. Inlet pressure (kPa) 1.8
6. Pressure ratio,
01
03
P
P
1.52
7. Rotational speed of sha, N
b
(rpm) 5,100
8.
23
=cV
;
32
=cV
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Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
where α is the angle made by the absolute velocity with the axial direction, β is the angle
the relative velocity makes with the axial direction. Figure 4 shows the superimposed
diagram of the velocity triangles.
w
c∆ , as seen in Figure 4 is the change in the velocity of
whirl while
23
ww
c and c
are whirl velocities.
2.3. Flow Parameters at the Outlet of the Stator
Considering the geometry of the superimposed velocity diagram above, the following
equations can be derived:
ai m
cU= ∅
(10)
cos
ai
i
i
c
c
α
=
(11)
e temperature equivalent of the exit velocity is:
2
2
02 2
2
p
c
TT
c
−=
(12)
For a single stage turbine, it is assumed that
1
0
α
=
since
1
c
is axial;
and this together
with the assumptions that
13
cc=
and
32aa
cc=
, yields,
113a
c cc= =
(13)
For choke conditions at the outlet of the stator, we have that,
2
2
22
2
I
N
p
c
TT
c
µ
−=
(14)
2
P
is calculated from the reversible adiabatic relation:
2
3
3
3
K
L
J
H
G
F
2
3
2
3
2
∆
=
2
+
3
2
FIGURE 4. Schematic of the superimposed velocity diagram of the single stage reaction
turbine.
Equipment Sizing and Method for the Application of Exhaust Gas Waste Heat to Food Crops
Drying Using a Hot Air Tray Dryer
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1
01 01
2
2
I
PT
P
T
γ
γ
−
=
(15)
Neglecting the frictional eect on the critical pressure ratio, we have that:
1
01
1
2
c
P
P
γ
γ
γ
−
+
=
(16)
Calculation of the densities at various stations of the turbine stage is obtained using
equation (17):
i
i
i
P
RT
ρ
=
(17)
e temperatures equivalent of the entry and exit kinetic energies are:
2
1
01 1
2
p
c
TT
c
−=
(18)
and
2
3
3 03
2
p
c
TT
c
= −
(19)
e reversible adiabatic relation at station 3 is given by:
1
3
3 03
03
T
PP
T
γ
γ
−
=
(20)
e radius ratio of the annulus can be found from equation (20),
2
2
m
t
m
m
b
r
r
b
r
r
+
=
−
(21)
and
2
tr
m
rr
r
+
=
(22)
where
r
r
is the root radius of the turbine,
m
r
is the mean radius, and
t
r
is the tip radius,
respectively. From [18], the power output of the turbine,
out
P
is given by:
out m w
P mU C= ∆
(23)
where
w
C∆
is the change in the velocity of whirl as seen in Figure 4.
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C. Ononogbo, O.C. Nwufo, C.A. Okoronkwo, N.V. Ogueke, J.O. Igbokwe and E.E. Anyanwu
Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
e calculated dimensions of the annulus as well as the values of the parameters at
various stations of the turbine are presented in Table 3:
2.4. Description and Sizing of the Hot Air Tray Dryer and the Heat
Exchanger
e tray dryer consists of seven major components, namely: the drying chamber, the trays,
the chimney, the heat source, the temperature and humidity sensors, the strain gauge
weight sensors, and the moist air outlet fan.
e dryer operates on the principle of batch drying. e dryer consists of a drying
chamber where the products to be dried are fed before drying commences. Heat
is supplied to the chamber by the air which is heated up by the exhaust gas passing
through the heat exchanger. e heat energy supplied to the dryer is part of the heat
recovered from the exhaust gas energy of the diesel generator. e blower powered by
the electrical energy from the dynamo is connected to the heat exchanger into which
it draws in ambient air to be heated up by the exhaust gas passing through the heat
exchanger. e air heated by the heat exchanger expands and travels upwards into the
drying chamber. e food crops to be dried are spread on wire gauze trays inside the
drying chamber. As the warm air moves and circulates across the surface of the food
crops, it picks up moisture. e air that has picked up moisture on its way through the
drying chamber leaves the chamber through the chimney provided at the top of the
dryer. e eectiveness of the evacuation of moist air through the chimney is enhanced
by the use of a moist air outlet fan placed near the top of the dryer. e cooled exhaust
gas passing through the heat exchanger is then released into the atmosphere through a
pipe whose outlet is at a reasonable height and distance away from the dryer to avoid
the contamination of the surrounding air to be used for drying. However, black carbon
(a ne particle of PM
2.5
emitted from diesel engines) and other larger airborne particles
which settle faster due to gravity [19] may pose a danger of contamination to the drying
air. erefore, the drying air to be heated is drawn into the heat exchanger by the aid of a
blower whose box is covered with a mesh air lter of 2 µm size in order to prevent these
particles from being sucked into the drying chamber. To preserve the quality of the food
crops during drying, the temperature of the drying chamber is controlled so that it does
not exceed 60 °C. e temperature of the drying chamber is monitored using sensors.
However, above this temperature, the temperature sensors give a signal to the control
system which cuts o the ow of heated air into the chamber by switching o the air
TABLE 3. Annulus dimensions and parameters of the turbine at various stations
Station A(m
2
) b(m)
/
tm
rr
/
3
p kg m
/
a
c ms
/
cms
P˙
(bar) T (K)
α(°) β(°)
1 0.0188 0.03724 1.602 0.9540 39.081 39.081 1.793 655.04 0 0
2 0.0216 0.0427 1.722 0.9464 34.400 70.510 1.775 653.53 28.33 60.8
3 0.0324 0.0641 2.323 0.6302 34.400 39.081 1.179 652.04 60.8 28.33
Equipment Sizing and Method for the Application of Exhaust Gas Waste Heat to Food Crops
Drying Using a Hot Air Tray Dryer
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blower until the chamber temperature falls to 50 °C (which as stated above is within the
range of acceptable drying temperatures) before the operation of the blower is activated
once again. e whole control processes in terms of temperature and velocity regulation
of the drying air within the drying chamber are made possible by the use of an arduino
platform. e 3-D view of the dryer is shown in Figure 2, while the schematic diagram
of the dryer is shown in Figure 5. e dryer has been fabricated using the following
materials: (i) Equal length angle steel for constructing the dryer frame; (ii) Mild steel
sheet for covering the external part of the drying chamber; and (iii) Fiber glass used as
a lagging material for the thermal insulation of the dryer. e trays of the dryer were
constructed using galvanized steel wire gauze with aluminum prole base. e aim of
using galvanized steel wire gauze is to avoid the problem of rust of the tray material and
contamination of products to be dried.
2.5. The Dimensions of the Trays and the Drying Chamber
e drying chamber is composed of three (3) trays with the total volume, of products
to be dried per batch equal to 0.04628 m
3
. e length,
t
l
, width,
t
w
and height,
t
H
of the
trays are 0.66 m,
0.468 m
, and 0.05 m, respectively. e head and bottom spaces of the
dryer from the trays,
hs
H
and
bs
H
are 0.12 m and 0.17 m while the tray thickness,
t
T
and
the space between the trays,
t
S
are
0.005
m and 0.09 m, respectively. e drying chamber
height,
C
H
, 0.71 m is obtained using the expression:
3 32
c t t t hs bs
H H T SH H=× +× +× + +
(24)
2.6. Water Removal from Products to be Dried and Heat
Requirement for Drying
Weight loss from wet to dried product is calculated from Ref. [20], where the mass of dry
product is given by:
( )
100
100
wo
d
f
m
M
−∅
=
−∅
(25)
where
w
m
is the total mass of wet product per batch,
o
∅
is the percentage maximum
moisture content of freshly harvested product, and
f
∅
is nal moisture content aer
drying. us, the mass of water to be removed,
w
M
is
ww d
MmM= −
(26)
According to Ref. [20], the quantity of heat required to remove water from the product
is given by:
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Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
( )
w c dp ic w
Q m c T T ML=× −+
(27)
where
L
is the latent heat of vaporization of water (2256 kJ/kg),
dp
T
is the temperature
of the drying product, and
ic
T
is the initial temperature of the drying chamber. e rate
of heat transfer,
Q
between the exhaust gas and the ambient air in the heat exchanger is
given by Ref. [21] to be:
( )
time s
Q
Q =
(28)
e temperature of the exhaust gas at the exit of the heat exchanger,
2g
t
can be obtained
from the heat transfer rate
( )
Q
expression:
( )
12g pg g g
Q mc t t= −
(29)
where
pg
c
is specic heat of exhaust gas at constant pressure,
g
m
is mass ow rate of
exhaust gas, and
1g
t
is the temperature of the exhaust gas at the inlet of the heat exchanger.
Drying chamber
Air blower
(inlet fan)
Wire mesh tray
Control panel
Moist air leaving the dryer
Exhaust gas to atmosphere
Heated air to drying chamber
Heat exchanger
Dryer stand
Temperature and humidity
sensor
Exhaust gas pipe
from turbine
housing
Ambient air entering
heat exchanger
Moist air outlet
fan
Chimney
Exhaust gas
entering heat
exchanger
Strain gauge load cell
FIGURE 5. Schematic diagram of the hot air tray dryer.
Equipment Sizing and Method for the Application of Exhaust Gas Waste Heat to Food Crops
Drying Using a Hot Air Tray Dryer
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Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
e logarithmic mean temperature dierence (LMTD),
m
θ
is used to determine the area
and length of the heat exchanger to be used, and is given by Ref. [22], as:
( ) ( )
( ) ( )
11 2 2
12
1
11 2 2
2
In /
In
ga g a
m
ga ga
tt tt
tt tt
θθ
θ
θ
θ
−− −
−
= =
−−
(30)
While the overall heat transfer coecient, U according to [22] is given by:
1 11
o
iio
r
Urhh
=×+
(31)
where
i
h
is inside heat transfer coecient,
o
h
outside heat transfer coecient,
i
r
and
o
r
are inside and outside radii of the tube. For a parallel ow heat exchanger, the length,
he
L
and area,
he
A
can be estimated using the expressions by [22]:
he he
π
o
A dL
(32)
and
he
π
om
Q
L
Ud
(33)
2.7. Thermal Insulation of the Drying Chamber
e drying chamber is thermally insulated to avoid excessive heat losses through its walls.
e thickness of the heat insulating (lagging) material was determined using an analogy
of heat transfer through a composite wall. e chamber wall is made of mild steel plates
at the internal and external parts while the heat lagging material (ber glass) is placed
in-between the mild steel plates of the chamber walls as shown in Figure 6.
e thermal conductivities of the three layers of the wall are as follows:
1
k
,
2
,k
and
3
k
,
while their thicknesses are as follows:
1
x
,
2
,x
and
3
x
, respectively. e temperatures of
the inner and outer surfaces of the drying chamber walls are
1
t
and
4
t
while the interface
temperatures are
2
t
and
3
t
, respectively. However, for continuity of ow to be achieved,
the rate of heat transfer
q
through the layers of the wall must be the same. In addition, the
1
4
1
2
3
2
3
1
2
3
FIGURE 6. Lagging of the drying chamber walls.
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C. Ononogbo, O.C. Nwufo, C.A. Okoronkwo, N.V. Ogueke, J.O. Igbokwe and E.E. Anyanwu
Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
internal and external parts of the chamber are of the same material and thickness. Hence,
13
kk=
and
13
xx=
. us, by Fourier’s equation of heat transfer, we have:
( ) ( ) ( )
1 12 2 23 1 34
121
kAt t kAt t kAt t
q
xx x
−−−
= = =
(34)
2.8. The Fan/Blower Selection
Blowers are sized based on the volume of air delivery and static pressure. In this work, the
blower is powered by the electricity generated by the dynamo which is driven by the waste
energy recovery turbine. e work done by the heat source per unit time,
ht
P
is equivalent
to the work done on the ambient air,
a
P
[21]. us:
ht
a a
P P cTm= = ∆
.
(35)
where
a
m
is mass ow rate of air,
c
is specic heat capacity of air, and
T∆
is the dierence
in the values of the maximum allowable temperature of the product to be dried and the
ambient temperature. From Ref. [21], discharge of the heated air,
D
is given by:
aa
Dvm=
(36)
where
a
v
is the specic volume of air. e velocity of air,
a
c
required through the drying
chamber [21], is calculated thus:
2
a
D
c
A
=
(37)
where
2
A
is the area of the drying chamber’s oor. From Ref. [21], the static pressure loss
due to sudden enlargement is given by:
2
1
se
1
2
21
U
L
A
g
A
=
−
(38)
where
1
U
is the linear velocity of air in the heat exchanger;
g
is the acceleration due to
gravity; and A
1
is the area of the heat exchanger. e horsepower of the blower,
hp
B
[21] is
obtained using the expression below:
st
hp
b
6320
Dp
B
ξ
×
=
×
(39)
where (
b
6320
ξ
×
) is called the conversion factor and
b
ξ
is the blower eciency,
D
is the
airow rate (in CFM), and
st
p
is the static pressure in inches of water.
Equipment Sizing and Method for the Application of Exhaust Gas Waste Heat to Food Crops
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3. Discussion
Heat recovery from the high heat content exhaust gases exiting the tail pipe of diesel
powered electric generators has barely received any meaningful attention by researchers.
Yet, this enormous heat is directly dumped into the environment even though it could still
be harnessed and transferred to a productive end-use. e reason behind the unavailability
of research works done in this area could perhaps be due to the inability of researchers to
identify any meaningful and benecial application of the energy recoverable from these
systems using available techniques. However, it is pertinent to note that heat recovery from
stationary diesel generators can be used eectively to power dryers for the preservation of
agricultural products. is is achievable by an appropriate arrangement of the components
of the energy recovery turbine, the heat exchanger and the tray dryer as demonstrated in
this work. In a conventional tray dryer, the blower required for its operation is usually
attached to the dryer where it blows air across the heating source and supplies the resulting
hot air to the trays holding the materials to be dried. However, the blower used in this
work has no direct connection with the dryer. It is rather connected to the heat exchanger
into which it drives air to be heated by the exhaust gas passing through the heat exchanger,
and the heated air is then routed to the drying chamber of the dryer through a duct for
drying of products. In addition, unlike in most heat recovery systems, it is interesting that
in this work, there are two stages of energy recovery. e rst stage is the energy recovery
turbine while the second stage involves the use of a heat exchanger to further recover heat
from the exhaust gas exiting the turbine housing. e test rig of this whole equipment has
been fabricated and is currently undergoing experimental investigation.
Preliminary test of the dryer showed high drying rate of maize grains. During the test,
the tray dryer was initially allowed to run without load for a period of 45 min; and the air
temperature and humidity of 88ºC and 22.3% were recorded in the drying chamber for an
air speed of 2.0 m/s. e initial temperature and humidity of the chamber were 31ºC and
71.2%, respectively. Subsequently, the three trays of the dryer were loaded with grains of
freshly harvested maize aer the chamber had been cooled and averagely maintained at
59ºC. e trays contained 400 g each of the grains and the dryer was allowed to run for
a period of 30 min. e results obtained showed weight losses of 127.81 g, 118.36 g, and
116.91 g for the grains in trays 1, 2, and 3, respectively. Detailed results of experiments
on the drying of freshly harvested maize grains and other crops such as yam and cassava
chunks, etc. will be made available (with results analyses) in due course. Performance
evaluation of the tray dryer will be carried out; and the specic energy consumption,
eective moisture diusivity, and activation energy for the thin-layer drying of the crops
will be determined. e proximate analyses of the dried products will also be obtained in
order to determine their chemical and nutritional values.
4. Conclusion
e sizing of the equipment and method of utilizing exhaust gas waste heat for food crops
drying have been considered. e sizing of the components of the equipment was achieved
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Indian Journal of Science and Technology Vol 13(05), DOI: 10.17485/ijst/2020/v13i05/145593, February 2020
using known design principles. e concept is geared towards recovery and utilization of
waste heat of a diesel engine thereby helping to reduce the quantity of pollutants released
into the environment. e recovered energy may be put to useful work, such as drying of
products, instead of resorting to burning of fresh fuels to do the same work. It is intended
that the application of the recovered waste energy (from the exhaust gas of Internal
Combustion engines) to drying, would reduce the energy demand for the depleting
fossil fuel reserves which saves a considerable amount of primary fuel; help farmers to
minimize the amount of agricultural product losses usually incurred during postharvest,
thus improving the value and prot margin of the farmers, and hence the impact of global
warming.
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