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PRESENTED BY:
OWUSU-ADUOMIH IBRAHIM
SUPERVISOR: DR. ANTHONY SIMONS
Problem statement
Objectives
Methods used
Facilities employed
Scope of work
Main work
Conclusions
Recommendation
The gratis firewood dryer is a freeconvection dryer used for drying agricultureproduce.
The fuel used is wood which leads todeforestation
The amount of heat it generates cannot bewell controlled.
The produce get burnt when the systemoverheats.
To redesign the heat supply system to allow the
use of liquefied petroleum gas (LPG)
To use forced convection to supply hot air
To include an automatic fuel supply regulator
in the design to regulate supply of fuel to the
burner.
A careful study of how the existing design
operates
Interaction with operators of the dryer
Review of relevant literature on hot air drying
The Gratis firewood dryer.
The Internet
The UMaT library.
This project work is limited to improving the
design of the firewood dryer to use LPG.
According to the Ghana Statistical Services, about
sixty percent of the country’s population are
involved in agriculture.
In ideal terms, this will mean a lot of competition on
the food market and hence cheaper food prices.
But the situation on the ground is quite different
due to crude food production and preservation
methods.
Increasing the use of mechanised dryers will go a
long way to limit the perennial food price inflation.
Dryers can be classified according to thefollowing.
Source of heat
Mode of heat transfer
Drying procedure employed
Equipment used
In hot air drying, food is exposed to a stream ofpreheated air which absorbs moisture from the food.
For effective drying, air should be hot, dry and flowing.The dryness of air is termed humidity (the lower thehumidity, the drier the air).
Increase in Temperature reduces relative humidity andallows air to carry more water vapour.
Initial moisture content of the air will reduce the amountit can remove.
Temperature of the air is kept at a degree that preventschanges to the microstructure of the food but it is highenough to remove moisture from food.
FOOD CATEGORY SAFE DRYING TEMPERATURE
Grains 45oC
Meat 68oC
Fish 60oC
Vegetables 54oC
Appromedia.com, 2010
The drying of most hygroscopic materials follows a
characteristic pattern.
The characteristic drying curve is divided into three
basic parts:
the constant rate period,
the uniform falling rate period,
and the varying falling rate period.
One or more of the components may be missing
under certain conditions.
(Anon, 1956)
Anon(1956)Fig. 1.1 Drying curve
heating coil
Fig. 1.2 Existing design
pro
Fig. 1.3 Proposed design
Fig. 1.4 Exploded view
Front view Side view
Top view Fig 1.5 Projected views
Type 1 Type 2
Type 3 Type 4
Fig. 1.6 Drying trays
Fig. 1.7 Fuel supply system
Fig. 1.8 Air flow pattern
A flow velocity of 0.1m/s is chosen for the design.
Volume flow rate = flow velocity × Area perpendicular to the
direction of flow
Mass flow rate
Where ρ is the air density and is the volume flow rate.
Density of air at 27 oC (average ambient temperature) = 1.176
kg/m3
Let us consider a case where an average air temperature of
45 oC is required.
Density of air at 45o C = 1.109 kg/m3
Air flow velocity
Conventional cross flow dryers 0.3 - 0.7 m/s
Conventional platform dryers 0.1 m/s
Anon(2001)
V
Mass flow rate of heated air,
Temperature difference (ΔT) = Tc – Ta
Where Tc is temperature inside cabinet and Ta is ambient
temperature.
Specific heat capacity of air (cP) from 15 to 70oC = 1007 J/kg.k
Required rate of heat addition,
Expected volume flow rate from fan,
Cengel et al., 20..
The rate of heat transfer from the hot plate to the air stream is
given by
At 1 atm. pressure, air temperature of 27oC will give a Prandtl
number (Pr) of 0.72876 (Cengel et al., 2008)
For flat plates under forced convection Pr > 0.5 means the flow is
turbulent.
The Nusselt number (Nu) is given by
l -The characteristic linear factor, = 0.3m
λ - (Thermal conductivity of the air) at 27o C =0.025732 J/kg.K
(Processasociate.com)
Also,
Velocity of air inside the heating section,
Assuming a steady frictionless incompressible flow, A1 C1 =A2 C2,
Where A is flow area and C is flow velocity.
Fig. 1.9
Section A(square m) C(m/s)
1-1 0.350 1.616
2-2 0.045 12.570
4-4 0.084 6.774
5-5 1.011 0.559
6-6 6.000 0.094
Vapour pressure (Pv) = RH(Psat.@T), Where Pv is the vapour pressure, and
Psat@T is the saturated vapour pressure at temperature T.
In the worst case scenario where the air at ambient temperature is
already saturated, RHo=100% and Pv = 3.602 kPa. After heating air in the
absence of water to 45 ºC. Psat@45 ºC = 9.87kPa
365.087.9602.3 RH
(Cengel and Boles)
Less attention will be required from the
operator.
The design has better ergonomics.
The quality of the end product will also be
better.
I recommend that Gratis Foundation should
adopt this design and build a prototype fortesting and evaluation.