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.