solar powered chalk dryer

8/18/2019 Solar Powered Chalk Dryer

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1.0 INTRODUCTION

1.1 Background Information

The power from the sun intercepted by the earth is approximately 1.8 x l0ll MW (Wikipedia

2009). This makes it one of the most promising unconventional energy sources. Solar energy is

available in abundance in most parts of our country throughout the year and, so, the drying of

various agricultural products in open sunlight is an age-old practice. The sun has produced

energy for billions of years and this energy is in form of the sun’s rays (solar radiation) that reach

the earth. Solar energy can be converted into other forms of energy such as heat and electricity.

The angle of insolation (Incoming Solar Radiation) of the sun is different at different locations

on earth because the earth is split into three main regions: polar, temperate and tropical. Since the

earth is a sphere, the middle is more direct to the sun, unlike the other regions. So, the sun would

"give" the heat to the tropics, closer to the equator, faster and later spread to the other regions,

where it takes some time to heat them up. Drying of fruit and vegetables is one of the energy

intensive processes in the food processing industry and a promising method of reducing post

harvest losses. Escalating prices and shortages of conventional fuels have lead to an increased

emphasis on using solar energy as an alternative energy source, especially in developing

countries [1]. The legislation on pollution and sustainable and ecofriendly technologies has

created greater demand for energy efficient drying processes in the food processing industries.

Drying practices for chalk, particularly in Nigeria, are largely traditional, which. In traditional

drying, the produce is spread on an open floor under sun, which has limitations like dust

contamination, spoilage due to rains, insect infestation etc. Moreover, as there is no control over

the drying rate, the chalk may be over dried, resulting in discoloration, loss of writing qualities


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and sometimes even complete damage. The moisture content and the temperature at which chalk

should be dried is always fixed, which is possible only in controlled drying.

1.2 A Brief History of Drying

Large-scale continuous drying of foods began in the 1940’s and in 1945, the first conveyor dryer

for foods was manufactured. By 1956, the first continuous dryer for breakfast cereal in the

United States had been manufactured (Aeroglide, 2006) and by the 1950’s (Nyrop, 2007), the

development and supply of complete spray dryers, for industrial drying had begun. Alternative

air dispensing and atomization techniques were incorporated in the drying systems in the 60s, a

period which also witnessed the development of flash and fluid bed dryers.

According to Pier (2007), the 70’s saw the development of the Regenerating desiccant type air

dryers which were designed largely due to the vulnerability of railway pneumatic systems to

water condensation (which was as a result of the cooling compressed air). Allgemeiner (1955)

stated that, as at 1869, chalk slurries were still being dried using the power of the sun and wind.

However, by 1906, chalk plants construction had begun and had incorporated dryer drums,

screening machines and several other machines which made the drying process faster.

Development of various solar devices for thermal applications such as water heating, space

heating, drying, cooking and power generation, however, began during the last century.

Chalk is a soft white, porous sedimentary rock, a form of limestone composed of the mineral

calcite. It is formed under relatively deep marine conditions from the gradual accumulation of

minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores (Wikipedia

2009). Relatively, chalk is resistant to erosion and also because it is porous it can hold a large

volume of underground water. Chalk has been quarried since pre-history, serving as a building


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material and marl for fields. For instance, in Southeast England, den holes are a notable example

of ancient chalk pits. The chalk group is a European stratigraphic unit deposited during the late

cretaceous period; it forms the famous white cliffs of Dover in Kent, England. The champagne

region of France is mostly underlain by chalk deposits, which contain artificial caves used for

wine storage (Wikipedia, 2009).

Chalk board, which necessitates the use of chalk as its writing material, could be dated back to

1801 when teachers and schools had no means of presenting information to roomful students all

at once, no means of presentation of concepts and historical overviews for the entire class to

view, grasp and discuss. Supplies of pencils and paper were often in short supply or unaffordable

for making mass copies; hand outs too were a rarity, since the teachers had to hand a set for each

of their students. The expense of material and the individual attention required by such

presentation methods caused small class enrolment and slowed down instructions.

The history of solar energy stretched back into the dim recesses of pre-history, perhaps as far as

the clay tablet era in Mesopotamia when the temple priestesses used polished golden vessels to

ignite the altar fibres. In 1615, Salomon de Caux published a description of a working solar

“motor”. He used a number of glass lenses mounted in a frame that concentrated the Sun’s rays

on an air tight metal chamber partially filled with water. The sunlight heated the air, which

expanded and forced the water out as a small fountain (Aden B. Meinel, 1976)

Solar ovens appear in the literature, as described by the English astronomer John Herschel, son

of the famous astronomer Sir William Herschel. John Herschel constructed a simple device for

practical use on his expedition to the Cape of Good Hope in 1837. His oven was simply a black

box that was buried in sand for insulation and had a double layer glass cover that allowed


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sunlight to enter and prevented heat from escaping. The oven was used by Herschel’s staff to

cook meat and vegetables for the dinner table of the expedition. (Aden B. Meniel, 1976).

In 1875, Mouchot made a notable advance in the solar collector design by making one in the

form of a truncated cone reflector. The spherical or parabolic mirror arrays of his predecessors

had focused all the light at one small sport in space where the absorber was placed.

The earliest attempts to convert solar energy into other forms revolved around the generation of

low-pressure steam to operate steam engines. According to Wikipedia (2009), solar energy is the

radiant light and heat from the sun that has been harnessed by humans since ancient times using

a range of even-evolving technologies. Solar energy (solar power) technologies can provide

electrical generation by heat engine or photovoltaic means. A partial list of application would

include space heating and cooling through solar architecture, portable water through distillation

and disinfection, daylighting, hot water, thermal energy for cooking and high temperature

process heat for industrial purposes. Solar concentrating technologies such as the parabolic dish,

trough and schettler reflectors can provide processing heat for commercial and industrial

applications. The first commercial system was the solar total energy project (STEP) in

Shenandoah, Georgia, USA, where a field of 114 parabolic dishes provided 50% of the process

heating, air conditioning and electrical requirements for a clothing factory.

Photovoltaics (PV) has mainly been used to power small and medium sized applications, from

the calculator powered by a single solar cell to off-grid homes powered by a photovoltaic array.

On large-scale generation, concentrating solar power (CSP) plants like SEGS have been the

norm, but recently multi-mega watt PV plants are becoming common. Completed in 2007, the

14MW power station in Clark County, Nevada and the 20MW site in Benixama, Spain are

characteristics of the trend toward larger photovoltaic power stations in the US and Europe.


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The first solar cell was constructed by Charles Fritts in the 1880’s. Although the prototype

Selenium cells converted less than 1% of incident light into electricity, both Erust Werner Von

Siemens and James Clerk Maxwell recognized the importance of this discovery. Following the

work of Russel Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin

created the Silicon solar cell in 1954. These early solar cells cost 286USD/Watt and reached

efficiencies of 4.5-6%

Concentrated sunlight has been used to perform useful tasks since the time of ancient China. A

legend claims that Archimedes used polished shields to concentrate sunlight on the invading

Roman fleet and repel them from Syracuse. Auguste Mouchout used a parabolic trough to

produce steam for the first solar steam engine in 1866 and subsequent developments led to the

use of concentrating solar-powered devices for irrigation, refrigeration and locomotion.

Concentrating solar power (CSP) systems use lenses or mirrors and tracking systems to focus a

large area of sunlight into a small beam. The concentrated light is then used as a heat source for a

conventional power plant. In all these systems a working fluid is heated by the concentrated

sunlight and is then used for power generation or energy storage.

Thermal mass systems can store energy in form of heat at domestically useful temperatures for

daily or seasonal durations. Thermal storage systems generally use readily available materials

with high specific heat capacities such as water, earth and stone. Well-designed systems can

lower peak demand, shift time of use to off-peak hours and reduce overall heating goal

requirement. Phase change materials such as paraffin wax and Glavber’s salt are another thermal

storage media. These materials are inexpensive, readily available and can deliver domestically

useful temperatures (approximately 640C). The “Dover House” (in Dover, Massachusetts) was

the first to use a Glauber’s Salt heating system in 1948.


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Solar energy can be stored at high temperatures, using molten salts. Salts are effective storage

mediums because they are low-cost, have a high specific heat capacity and can deliver heat at

temperatures compatible with conventional power systems. The solar too used this method of

energy storage, allowing it to store 1.44 TJ in it 68m3 storage tanks with an annual storage

efficiency of about 99%.

Off-grid PV systems have traditionally used rechargeable batteries to store excess electricity.

With grid-tied systems, excess electricity can be sent to the transmission grid. Net metering

programs give these systems credit offsets electricity provided from the grid when the system

cannot meet demand, effectively using the grid as a storage mechanism.

Pumped-storage hydro-electricity stores energy in form of water pumped when energy is

available from a lower elevation reservoir to a higher elevation one. The energy is recovered

when demand is high by releasing the water to run through a hydroelectric power generator.

1.3 Chalk Production Processes

Chalk used in school classrooms comes in slender sticks, approximately .35 of an inch (9 mm) in

diameter and 3.15 inches (80 mm) long. Lessons are often presented to entire classes on

chalkboards (or blackboards, as they were originally called) using sticks of chalk because this

method has proven cheap and easy.

1.3.1 Raw Materials

The main component of chalk is calcium carbonate (CaCO3), a form of limestone. Limestone

deposits develop as coccoliths (minute calcareous plates created by the decomposition of

plankton skeletons) and accumulate to form sedimentary layers. Plankton, a tiny marine

organism, concentrates the calcium found naturally in seawater from .04 percent to 40 percent,


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which is then precipitated when the plankton dies.

1.3.2 The Manufacturing Process

To make chalk, limestone is first quarried, generally by an open pit quarry method. Next, the

limestone must be crushed. Primary crushing, such as in a jaw crusher, breaks down large

boulders; secondary crushing pulverizes smaller chunks into pebbles. The limestone is then wet-

milled with water in a ball mill — a rotating steel drum with steel balls inside to further pulverize

the chalk. This step washes away impurities and leaves a fine powder. The base of pastel chalks

is calcium sulfate (CaSO4), which is derived from gypsum (CaSO4 .2H2O), an evaporite mineral

formed by the deposition of ocean brine; it also occurs disseminated in limestone. Chalk and

dehydrated gypsum thus have similar origins and properties. Pastels also contain clays and oils

for binding, and strong pigments. This mixture produces sticks that write smoothly without

smearing and draw better on paper than on chalkboards. Although great care is taken to eliminate

contaminants when chalk is manufactured, some impurities inherent to the mineral remain. Chief

among these are silica, alumina, iron, phosphorus, and sulfur. In less significant, amounts,

manganese, copper, titanium, sodium oxide, potassium oxide, fluorine, arsenic and strontium

may also occur.

1.3.3 Dehydrating Gypsum

Gypsum, like limestone, is also quarried and pulverized. The major difference in processing

gypsum is that it must be dehydrated to form calcium sulphate, the major component of coloured

chalk. This is done in a kettle, a large combustion chamber in which the gypsum is heated to

between 244 and 253 degrees Fahrenheit (116-121 degrees Celsius). It is allowed to boil until it

has been reduced by twelve to fifteen percent, at which point its water content will have been

reduced from 20.9 percent to between 5 and 6 percent. To further reduce the water, the gypsum


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is reheated to about 402 degrees Fahrenheit (204 degrees Celsius), at which point it is removed

from the kettle. By now, almost all of the water has evaporated, leaving calcium sulphate.

Ca2SO4 .2H2O + heat = Ca2SO4 .1/2H2O + 1

1/2H2O

The particles of chalk or calcium sulphate are now conveyed to vibrating screens that sift out the

finer material. The ensuing fine chalk is then washed, dried, packed in bags, and shipped to the

manufacturer. Upon receiving chalk or calcium sulphate, the chalk factory usually grinds the

materials again to render them smooth and uniformly fine.

1.3.4 Making White Classroom Chalk

To make white classroom chalk, the manufacturer adds water to form thick slurry with the

consistency of clay. The slurry is then placed into and extruded from a die — an orifice of the

desired long thin shape. Cut into lengths of approximately 24.43 inches (62 cm), the sticks are

next placed on a sheet that contains places for five such sticks. The sheet is then placed in an

oven, where the chalk cures for four days at 1880F (85

0C). After it has cured, the sticks are cut

into 80 millimeters lengths.

1.4 Types of Chalk Dryer

Having known that the most common type of chalk drying technique is sun drying, various types

of dryer have been built which use electricity as their source of power. Solar technologies that

can be broadly characterized as either passive or active are also being developed, depending on

the way they capture, convert and distribute sunlight. Active solar technologies use photovoltaic

panels, pumps and fans to convert sunlight into useful outputs. Passive solar technologies include

selecting materials with favourable thermal properties, designing space that naturally circulate

air, and referencing the position of a building to the sun.


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1.5 Statement of Problem

A lot of work has been done on improvement of drying methods, harnessing solar energy and

solar powered dryers for other processes apart from chalk drying (Ogunremi and Orimolade

2006). Also, research and practical work on the chalk dryer has utilized electricity as its source

of power supply . To the best of our knowledge, little or no research has been done on the use of

solar energy in powering chalk dryers. This project thus extends knowledge by adapting solar-

powered dryers for drying chalk instead of food and also using solar energy source for the chalk

dryer instead of electricity.

1.6 Aim and Objectives of the Project

The main aim of this project work is to develop a mechanism of powering of a chalk dryer using

solar energy. The specific objectives are to

(i). design and fabricate a dryer for chalk drying;

(ii). extract energy from the sun, using appropriate devices;

(iii). design a mechanism that will convert the solar energy extracted to heat to dry the chalk;

and

(iv). test the drying equipment for its performance.

1.7 Scope of the project

Although different types of driers exist, for example, driers for food conservation and

preparation, this study is limited to the design of chalk dryer. The energy source is also limited to

solar radiation out of several other sources that may exist. In this study, the box type dryer

among different types will be concentrated on. Locally available materials will be employed in

construction and fabrication.


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1.8 Justification of the Project

Literature reveals that over 50% of energy used in the sun drying process is being wasted and

also that materials or things to be dried are subjected to contamination. Hence, this work is set to

develop a system which will efficiently utilize solar radiation from the sun and at the same time

provide enough protection.


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CHAPTER TWO

LITERATURE REVIEW

2.0 Introduction

This chapter focuses on the review of works that are germane to this study. It opens with an

examination of drying and the solar energy that is to be used. The latter part of the chapter is a

survey of works done by scholars on solar energy collection, conversion and utilization in

various fields and the chapter ends with a summary of our position on the respective outcomes.

2.1 Solar Energy Drying Systems

In many countries of the world, the use of solar thermal systems in the agricultural area to

conserve vegetables, fruits, coffee and other crops has proved to be relevant, economical and

environmental friendly. Solar heating systems used to dry food and other crops can improve the

quality of the product, while reducing waste products and traditional fuel, thus improving the

quality of life. However, the availability of good information is lacking in many of the countries

where solar food processing systems are most needed. Solar food dryers are available in a range

of sizes and designs and are used for drying various food products. It is found that various types

of driers are available to suit the needs of farmers. Therefore, selection of dryers for a particular

application is largely a decision based on what is available and the types of dryers currently used

widely.

A comprehensive review of the various designs, details of construction and operational

principles of the wide variety of practically realized designs of solar-energy drying systems

reported previously is presented. A systematic approach for the classification of solar-energy

dryers has evolved. Two generic groups of solar-energy dryers can be identified, viz. passive or

natural-circulation solar-energy dryers and active or forced-convection solar-energy dryers.


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Some very recent developments in solar drying technology are highlighted. Open sun drying of

various agricultural produce is the most common application of solar energy. With the objective

of increasing the drying rate and improving quality of the products, natural convection and

forced convection type solar dryers have been developed for various commodities.

2.2 Solar Energy Collection and Conversion

In the invention of Glaser (1973), solar radiation is collected and converted to microwave energy

by a means on a satellite system maintained in outer space. The microwave energy is then

transmitted to earth and converted to electrical power for distribution.

Rom (1975), in his work on solar energy, invented an apparatus for converting solar radiation to

heat energy for heating gaseous stream, such as air, to be used for heating or drying purposes.

The air or other gaseous mixture is flowed through an elongated heating passage defined by

plastic film formed of solar radiation absorbing or opaque (black) material and the film is

inflated by the fluid pressure of the gaseous stream passing through. The heating passage is

surrounded on top by an insulating space, defined by the outer surface of the top portion of the

opaque film and by an outer sheet of clear (solar radiation transparent) plastic film that is inflated

into an expanded position by the gaseous insulating medium therein. Air or another gaseous

mixture is flowed through the heating passage wherein solar radiation absorbed by the film is

converted to thermal energy that is transferred to the gaseous stream.

Coleman (1977) reported a solar energy apparatus for gathering and transmitting solar radiation

to an energy storage area. Wide-angle lens apparatus is used to focus solar radiation on an end of

an optical fiber bundle. The other end of the optical fiber bundle is placed in the energy storage

area and has a radiating device attached thereto to more efficiently remove the solar energy from

the optical fibre bundle. The other end of the optical fibre bundle is placed in the energy storage


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area and has a radiating device attached thereto to more efficiently remove the solar energy from

the optical fibre bundles. A heat sink is advantageously utilized as a storage means for the solar

energy thus gathered and transmitted.

Also, Keyes (1978) presents a self-contained apparatus for collecting, storing and transmitting

solar heat which includes an elongated rectangular insulated housing in which a quantity of heat

retaining material is confined and a collector on the horizontal and vertical faces of the housing,

which has only one layer of glass for each face through which solar heat may pass and be

collected upon a heat-collecting surface. A conditioning pump is provided within the apparatus

to circulate conditioning air through both the collector and the heat retaining material within the

housing, so that heat is transferred from the collector to the heat retaining material. Specially

designed and positioned ducts connect the collector to the interior of the housing in a manner

such that air interchange between the collector and the interior of the housing is prevented except

during operation of the conditioning pump. Both the collector and the interior of the housing are

provided with appropriately positioned baffles to expose the conditioning air to all of the heat

retaining material. Utility pump means are also provided in the apparatus for withdrawing heat

from the heat retaining material and circulating it through a remote building structure.

Hunt (1982) in his studies on energy collection and conversion, presented an apparatus for

collecting radiant energy and converting same to alternate energy form includes a housing

having an interior space and a radiation transparent window allowing, for example, solar

radiation to be received in the interior space of the housing. Means are provided for passing a

stream of fluid past the said window and for injecting radiation absorbent particles in the fluid

stream. The particles absorb the radiation and, because of their very large surface area, quickly

release the heat to the surrounding fluid stream. The fluid stream particle mixture is heated until


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the particles vaporize. The fluid stream is then allowed to expand in, for example, a gas turbine

to produce mechanical energy. In an aspect of the present invention properly-sized particles need

not be vaporized prior to the entrance of the fluid stream into the turbine, as the particles will not

damage the turbine blades.

Constantine et al. (1981), in their studies on utilization of solar energy, presented a process and

system for economic utilization of solar energy. Solar energy was absorbed and converted to

thermal energy by means of, at least, two systems, operating in different temperature ranges, for

circulating a primary fluid heat transfer medium through separate collector sections of a solar

receiver to recover solar heat; and through separate output heat exchangers to supply heat to a

second heat transfer medium functioning as a working medium, with heat storage means being

associated with each system for the purpose of satisfying the heat requirements of the working

fluid and also to prevent cooling down of the collector during the time that little or no solar

radiation is available.

Lenz (1989) provided a solar heat collecting apparatus, which comprises a solar collector panel

rotatably supported about a horizontal and a vertical axis and drive means for rotating the panel

simultaneously about the two axes. The collector panel comprises a battery of individual

collector units, wherein each collector unit comprises an elongated trough member, a concave

reflective surface forming at least a portion of the interior surface of the trough and a fluid

carrying pipe extending longitudinally within the trough member, the longitudinal axis of the

pipe being substantially coincident with the focal line of the concave reflective surface. The pipe

was formed of a material that is substantially transparent to solar radiation. In addition, there was

a linear focusing lens, substantially covering and extending over the concave reflective surface.

The focal line for the linear lens should be substantially coincident also with the longitudinal axis


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of the fluid carrying pipe.

Stirbl (1993) describes a system for collecting solar radiation by generating waveform energy,

directing the energy to a pre-determined region of the atmosphere located at a pre-established

distance above a surface of the earth; controlling the energy directed to modulate an index of

refraction of air in the predetermined region of the atmosphere to produce a pre-determined

refraction index pattern in the region; modifying the distribution of solar radiation passing

through a region to thereby concentrate the solar radiation at a pre-determined location on the

surface; and absorbing at the location, a substantial quantity of the concentrated solar radiation to

produce heat energy.

2.3 Solar Energy Applications

Corazizini (1995), in his work on solar energy application, presents a solar-powered window

shade which consists of a venetian blind mounted within an interior of a frame of a window in a

wall of a building. An apparatus was carried by the venetian blind, for converting solar radiation

of sunlight into electrical energy. Also, a mechanism was placed on the venetian blind for

utilizing the electrical energy to open and close it. At sunrise and all through the day, the

venetian blind will remain open to allow sunlight to enter through the window, to help heat up

the building. At sunset and all through the night, the venetian blind remained closed to produce a

thermal barrier, to help retain the heat within the building.

Ampratwum and Dorvlo (1998) develop a solar collector in the form of a prototype solar cabinet

dryer which was evaluated at no load as an air-heating system. The dryer was operated for 28

days from mid-April to the end of May 1996. For the period of operation, the dryer attained an

average temperature of 81.30C with a standard deviation of 8.6

0C within a 7-h period from 8:00h

to 15:00h. From hourly temperature considerations, it was determined that the rate of solar


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energy absorbed by the dryer ranged from 0.51kWsqm-1

to 0.93kW sqm-1

. The peak was reached

at 11:00h and a high solar-energy rate of capture of 0.90kW sqm-1

and over was maintained for

about 6 hours.

Gikuru and Njoroge (2004) develop a small solar dryer with limited sun tracking capabilities and

was tested. The dryer had a mild steel absorber plate and a polyvinyl chloride (pvc) transparent

cover and could be adjusted to track the sun in increments of 150. The performance was tested by

adjusting the angle the dryer made with the horizontal either once, three, five or nine times a day

when either loaded with coffee beans or under no load conditions. The temperature distribution

in the plenum and also the drying rate of parchment coffee were determined. The temperature

inside the plenum chamber could reach a maximum of 70.4 _C and the dryer could lower the

moisture content of coffee beans from 54.8% to below 13% (w.b.) in 2 days as opposed to the 5 –

7 days required in sun drying. Tracking the sun, though allowing a faster rate of drying, did not

offer a significant advantage in terms of length of drying duration.

Sreekumar et al. (2006) give the development and testing of another type of efficient solar dryer,

particularly meant for drying vegetables and fruit. The dryer has two compartments: one for

collecting solar radiation and producing thermal energy and the other for spreading the product

to be dried. This arrangement was made to absorb maximum solar radiation by the absorber

plate. In this dryer, the product was loaded beneath the absorber plate, which prevented the

problem of discoloration due to irradiation by direct sunlight. Two axial flow fans, provided in

the air inlet, can accelerate the drying rate. The dryer had six perforated trays for loading the

material. The absorber plate of the dryer attained a temperature of 97.20C when it was studied

under no load conditions. The maximum air temperature in the dryer under this condition was

78.10C. The dryer was loaded with 4 kg of bitter gourd having an initial moisture content of


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95%, and the final desired moisture content of 5% was achieved within 6 h without losing the

product colour, while it was 11 h for open sun drying. The collector glazing was inclined at a

particular angle, suitable to the location, for absorption of maximum solar radiation. A detailed

performance analysis was done by three methods, namely ‘annualized cost method’, ‘present

worth of annual savings’ and ‘present worth of cumulative savings’. The drying cost for 1 kg of

bitter gourd was calculated as Rs. 17.52, and it was Rs. 41.35, in the case of an electric dryer.

The life span of the solar dryer was assumed to be 20 years. The cumulative present worth of

annual savings over the life of the solar dryer was calculated for bitter gourd drying, and it turned

out be Rs. 31659.26, which was much higher than the capital cost of the dryer (Rs. 6500). The

payback period was calculated as 3.26 years, which was also very small considering the life of

the system (20 years). So, the dryer would dry products free of cost during almost its entire life

span. The quality of the product dried in the solar dryer was competitive with the branded

products available in the market.

Sitompul et al. (2001) are concerned with heterogeneous modeling of deep-bed grain dryers

based on a two-phase model by taking into account coupled heat and mass transfer within grains.

This model also considered axial mass and heat dispersion in the fluid phase. The dynamic two-

phase equations are solved numerically by finite difference with alternating direction implicit

method algorithm; then applied to simulate humidity and temperature profile of drying gas

across dryers, together with moisture content and temperature of grains. The capabilities of these

models were compared with experimental data obtained from available literature, under drying

conditions such as temperature and absolute humidity of drying gas and moisture content of

grains. The simulation results showed that the dynamic of corn drying within the bed is well

predicted by the two-phase model.


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The recent developments by engineers have unfolded the facts that, the chalk dryer can be

utilized with cost effective power supply. This can be achieved by changing the means of

powering available chalk dryers that are mostly electric to solar-powered. The solar energy is in

abundance and is easier to tap than some other sources of power supply like gas, electricity, bio-

diesel and some others. Also, the environmental benefits of the solar energy in comparison to

petroleum based fuel and other sources should also be taken into consideration.

2.4 Solar radiation

Olopade and Sanusi (2008) studied the performance of PV modules under tropical sky

conditions. Two modules were used, each of 0.92m x 0.30m comprising 27 Solar cells. While

one of the modules was positioned flat, the other was tilted at the latitude of the site. The study

was carried out in June 2004, on the roof top (350m above sea level) of the Department of

Physics, University of Ilorin, Ilorin (8.320N, 4.340E), Nigeria. Results and analyses show that,

under very cloudy sky of 0.070 average clearness index (KT), the photovoltaic modules receive

approximately 0.7-kWh/m2 average daily insolation of mainly diffuse radiation. The tilted

module performs better by about 47% than the flat module. This is confirmed by the R-square

values of the linear least square regression analyses between the efficiencies of the modules and

the cloudy sky radiation parameters. The R-square value of the tilted module was 0.996014,

while that of the flat module was 0.617168. However, the diurnal variations of net radiation have

been studied by analysing one year data measured at a tropical station, Osu (7.43 ° N, 4.58 ° E),

in Nigeria (Jegede, 1996). The maximum net daytime flux (which occurs around 14h local time)

varies in the course of the year from 382.6±136.7Wm -2 in the wet season (April-October) to

480.3±61.8Wm -2 in the dry season (November-March). The low values (and large fluctuations)

of the hourly means recorded during the wet (monsoon) season are attributed to the important


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roles that the convective clouds and water vapour play in the atmospheric radiation budget,

which is very pronounced in the tropical areas of West Africa. The daily amplitude of the net

radiation is larger for the dry season (maximum in November) than it is for the wet season

(minimum in July). A lag of about 2 hours is observed between the times when the maxima of

the air temperature and the net radiation courses occur over the area.

2.5 The sun

The sun is a giant nuclear fusion reactor combining hydrogen to form helium, it generates a great

amount of energy. Radiation from the sun catalyzes or directly supplies most of the natural

energy systems on earth. The sun causes the air in the atmosphere to warm up, setting

temperature differentials that generate wind. Major patterns of winds are due to the earth's

rotation about its axis and the day-night cycles of solar heating and subsequent cooling.

Figure 2.5: The Sun and the Earth (Crowther, Richard L., Sustainable Design, Sun-Earth)


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A great amount of energy is absorbed by the earth's surface itself, as well as by the oceans, plants

and buildings. The energy absorbed by the plants is converted into food energy and from the

oceans, evaporation causes the hydrologic conversion cycle.

2.6 Sun's Apparent Movement

The earth rotates around the sun. This causes the sun to "rise" and "set". The angle of the sun and

its intensity on earth is affected by location of the place on the surface of the earth. The length of

the atmosphere that the solar radiation has to pass through determines the amount of radiation

that reaches the earth's surface. During the day, the sun is directly overhead and radiation travels

through least amount of atmosphere enroute to the earth's surface. As the sun moves closer to the

horizon (sunset), the path of the radiation through the horizon lengthens and the intensity of the

radiation decreases. Also, at a high elevation, the amount of atmosphere that the solar rays have

to travel through is lesser and therefore the energy content is somewhat higher.

Figure 2.6: The tilt of the earth remains constant at 23.47o as it revolves around the Sun.

(The Passive Solar Energy Book by Mazria Edward)

Because of the earth's tilt and rotation, the length of atmosphere that solar radiation passes

through varies with the time of day and month of the year. The path of the earth around the sun is


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a slight ellipse. As the earth orbits around the sun, it rotates on its axis that extends from the

North pole to the South Pole every 24 hours. The axis is tilted 23.470 from the vertical to the

plane of the earth's orbit around the sun. The solar rays reach the Earth's surface after being

attenuated by the atmosphere. Made up of elementary particles called photons, they are

characterized by a wavelength inversely proportional to their energy - the shorter the wavelength,

the greater the energy. The availability of solar radiation for technological application is

determined, to a large extent, by clearness of the sky. The solar flux is attenuated by a number of

factors before its final reception on earth. Clouds, consisting of water droplets or ice crystals

constitute a major factor, which attenuate the solar flux, mainly by scattering (Fagbenle, 1990).

Depending on the depth and number of layers of the cloud, radiation is scattered either forward

or backward. When the depth is substantial, back-scattering predominates, and thick stratus can

reflect up to 70% incident radiation (Monteith and Unsworth, 1990; Meinel and Meinel, 1976).

Beyond the earth's atmosphere the intensity of sunlight is about 1,350 watts per square meter

(429 British thermal units [Btu] per hour per square foot). Passage through the atmosphere

depletes the intensity due to absorption by the various gases and vapors in the air and by

scattering from these gases and vapors and from particles of dust and ice also in the air. Thus,

sunlight reaching the earth is a mixture of direct (unscattered) and diffuse (scattered) radiation.

At sea level the intensity is reduced to approximately 1,000 watts/square meter (295 Btu/hour/

square foot) on a bright clear day (Kaplan 1985). The intensity is further reduced on overcast

days. From literatures that have been reviewed (Olopade and Sanusi (2008), Kuku and Salau

(1985)) there is an average value of 700wh/m2solar radiation in Ile-Ife.


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3.0 Methodology

3.1. Experimental set up

The experimental set up, shown in Plate. 3.1, consists of the solar-aided chalk drier provided

with mirror reflector tilted at 00

and 150 to the dryer (Appendix 1 and 2). Basically, the

developed dryer consists of three parts: the reflectors, top collector and bottom drying chamber.

The top section of the solar dryer was the reflector area. The body of the dryer was made of

65x55x1.6 mm thick ply wood. The reflector used was made with mirrors gently positioned on

plywoods of equal sizes and positioned at 300

to the vertical with each side of 55 cm at the

bottom. All the sidewalls and bottom wall has plywood as insulators. A 4 mm doubled clear

glass which was glued together and allowed to get dried for 24 hours was mounted on the top of

the dryer as a glazing plate. The reflector was designed in a way such that the angl at which it is

to the dryer can be varied for Ile-Ife (latitude 7028

1 ). The height of the absorber region was 65.5

cm and that of the back portion was 80cm when the reflector is tilted at 150.

Plate 3.1: Solar-aided chalk dryer with the thermocouple


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Air holes of area 0.125m2 was provided in the front and under the dryer for air flow through the

drying chamber.

The total volume of the drying chamber was 0.197 m3, and it was located underneath the glazing

plate. It consists of two trays to load the material to be dried.

The material used to make the tray was wood and chicken wire. The size of the single tray was

0.5 m2. And the trays were arranged one above the other. The distance between the consecutive

bottom and top of trays was 15cm as shown in Fig 3.1. Four rows of holes, having 5 holes per

row, facilitated the exhaust of warm moist air from the dryer. The diameter of an individual hole

was 5 mm. The individual rows were separated a distance of 6 cm, and the distance between

holes was 4 cm. See template 3 for details.

Fig 3.1: Side sketch of chalk dryer showing distance between trays

Air outflow

Air inflow

Tray 1

Tray 2

Reflectors

Stand

15cm


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3.2. Experimental procedure

The experiments were conducted in the Solar Laboratory of the Department of Mechanical

Engineering, Obafemi Awolowo University Ile-Ife Nigeria (70

281 latitude). The performance of

the system was continuously monitored during the mid periods of the month of March 2010. All

the experiments were conducted from 10:00 am to 3:00 pm. Basically, two types of studies were

carried out on the solar dryer. First was when reflector was tilted at 00 and the other when at 15

0.

Various measuring devices were used to investigate the effects of the environmental and

operating parameters on the performance of the dryer. The ambient temperature, tray

temperatures in the drying chamber, absorber plate temperature, etc. were monitored using the

thermocouple. The temperature readings were taken every 30 minutes. The relative humidity of

the atmosphere was obtained by using the Sling psychrometer to get the wet and dry bulb

temperature and checking the psychrometric chart. Samples of products in the dryer were

weighed at 1 hr intervals using a a weighing balance (±0.001 g). At the end of the drying

process, the moisture content of the sample was determined by comparing the weight of an

already dried sample with a sample that has just been moulded.

3.2.1. Test of the dryer with reflectors at 00 to the dryer

The experiment was conducted for 5 hrs to measure the maximum temperature achieved by the

absorber plate of the dryer and the drying cabinet. The two trays were loaded with chalk having

an initial moisture content on 95%. The total quantity loaded was 8.4kg, 4.2 kg in each tray.

During the study, the outlet holes were kept. The temperatures of the ambient, absorber plate and

dryer trays were recorded every 30 mins during the study period. The weight of the chalk being

dried was recorded every 1 h. Chalk, used for the drying tests, is the most popular blackboard


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writing materials in Nigeria, especially in rural area institutions. It is manufactured from

Limestone. These are abundantly available in Ogun State. It can also be used in Snail rearing as

it is known to contain calcium which is good for Snail shells. For the drying tests, pieces

weighing 8.4 kg of chalk was loaded in the dryer. The freshly moulded chalk was placed in the

dryer after being allowed to solidify for 30 minutes in the mould.

3.2.2. Test with reflectors tilted at 150to the dryer

All parameters were monitored as described above except that the reflectors were tilted at 150

(Fig 3.2) for the same number of hrs.

15

Air inflow

Tray 1

Tray 2

Reflectors

Stand

15cm

Fig 3.2: Side sketch of chalk dryer showing reflectors tilted at 15


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4. RESULTS AND DISCUSSION

4.1. Test of the dryer with reflector tilted at 00

The parameters monitored, such as ambient temperature and temperature in the solar dryer are

shown in Fig. 4.1a the experimental time period was from 10:00 am to 3:00 pm. In this study, the

reflectors were placed directly on the dryer and not tilted throughout the experimental period.

The maximum temperature monitored in the inner wall was 650C, which was at 2:30 pm. The

average relative humidity was 75% which ranges from 80% at 10am to 55% at around 3pm. The

high temperature of the inner wall was due to the coating of a black gloss paint, which has an

absorptivity of 0.91 (Reflective Insulation Manufactures Association April 1999). The maximum

tray temperature in the dryer was monitored as 550C at 2.30 pm.

Time

10 10.3 11 11.3 12 .00 12.3 1 1.3 2 2.3 3

Tempera

ture

25

30

35

40

45

50

55

60

65

Open sun drying temp. against time

Tray 1 temp. against time


Fig. 4.1a. Variation of temperature in the solar dryer and ambient temperature with Time

when reflector is are at 00 to the dryer.


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Table 4.1. Temperature and mass properties (reflector at 00

to the dryer)

Avg RH =75% Temperature Mass of chalk

Time Ambient

temp

Tray 1 Tray2 Wall(Tw) Tw -TA Tray1 Tray2 Sundrying

temp

10am 32 45 41 55 23 4.20 4.20 4.2

10.30am 31 43 40 54 23 4.15 4.20 4.18

11.00am 33 45 43 57 24 4.13 4.15 4.16

11.30am 32 46 45 59 27 4.13 4.13 4.14

12 noon 32 49 47 61 29 4.12 4.10 4.14

12.30pm 34 52 47 60 26 4.10 4.05 4.12

1.00pm 34 55 48 62 28 4.05 4.03 4.12

1.30pm 33 58 50 60 27 4.02 4.02 4.10

2.00pm 34 59 52 62 28 4.00 4.02 4.10

2.30pm 36 58 55 65 29 3.99 4.00 4.08

3.00pm 35 57 54 63 28 3.98 4.00 4.06

The temperature reading of the chalk under open sun drying tends to fluctuate within the period

of experiment within a range of 31 to 360C and the peak which is 36

0C was obtained at 2.30pm.

The fluctuation in temperature could have been as a result of periodic cloud cover on the sun and

wind effects which tend to reduce chalk temperature. A slight difference of tray temperature in

the drier was noticed even though they follow the same pattern. Tray 1 reached its peak

temperature at 2.00pm while Tray 2 reached its peak at2.30pm. This could have been as a result

closeness of tray 1 to the top of the dryer which is the entry point of solar rays than tray2. We

suspect that it takes some little period of time for tray 2 to attain the chamber temperature than

tray 1. It was observed that the temperature rise of the dryer wall above ambient air was in the

range of 23 – 290C during the study period.

The solar energy required for removal of the moisture content from 8.4 kg of the chalks was

calculated as 2043 kJ/batch. The samples were collected from the different trays to analyze the

uniformity of drying, and it was found that the reduction in moisture content of Tray 1 is slightly

higher than that of tray 2. Drying was very fast in the initial hours of operation, and then it

started to decrease. This might have been because of the evaporation of surface moisture at the


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beginning of the process. It was known that as the moisture content of the product is reduced,

more energy might be required to evaporate the same amount of moisture from the product. The

result obtained was compared with chalk dried in the open sun. The moisture content of the chalk

was reduced from an initial moisture content of 21.43% to the final value of 17.09% for tray 1,

17.50% for tray 2 and 19% for open sun drying within an effective drying time of 5 hrs as shown

in Fig. 4.1b. The moisture content at the of end of the experiment when the reflectors are placed

at 00 to the dryer was 17.09% for Tray 1and 17.50% for tray 2, but the entire moisture was

removed from the product loaded in the solar dryer in the second day itself. We suspect that the

dryer continued the removal of moisture content during the late periods of the day.

Time

10 10.3 11 11.3 12.00 12.3 1 1.3 2 2.3 3

Mass(kg)

3.95

4.00

4.05

4.10

4.15

4.20

4.25

Mass of Open sun drying against Time

Mass of Chalk in tray1 against time

Mass of Chalk in tray2 against time

Fig. 4.1b. Reduction of moisture content of chalk with Time when reflector is at 00 to the

dryer


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4.2. Test with reflector tilted at 150to the dryer

In this experiment, the dryer was loaded with 8.4 kg of freshly cast chalk to study its drying

behaviour. The dryer, with solar radiation when the reflector was tilted at 150 to the dryer,

radiation incident on inner wall of the dryer, ambient temperature and rise in tray temperatures

inside the dryer above the ambient temperature are shown in Fig. 4.2a. The rise in tray

temperature varied from 90C to 19

0C during 10 am to 3pm. The average relative humidity of the

air at the time of experiment was 75%.

Time

10 10.3 11 11.3 12 .00 12.3 1 1.3 2 2.3 3

Temperat

ure

25

30

35

40

45

50

55

Open sun drying temp. against time


Tray 2 temp against time

Fig. 4.2a. Variation of temperature in the solar dryer and Open air sun drying with Time

when reflector is at 150 to the dryer.


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during the study period. The average relative humidity was 52% which ranges from 70% at 10am

to 43% at around 3pm.The moisture content of the chalk was reduced from an initial moisture

content of 21.43% to the final value of 18.30% for tray 1 and 19.12% for both tray 2 and

ambient, within one day with an effective drying time of 5 hrs as shown in Fig. 4.2b. The solar

energy required for removal of the moisture from 8.4 kg of product was calculated as 2043

kJ/batch. The samples were collected from the different trays to analyze the uniformity of drying,

and it was found that the reduction in moisture in Tray 1 is slightly higher than that of tray 2.

Drying was very fast in the initial hours of operation, and then it started to decrease as also in the

case of 4.1. The result obtained was compared with chalk dried in the open sun and it was

discovered that at the end of the period of experiment, chalks from tray 2 has the same moisture

content as that of the ambient. This could be as a result of low temperature values when

compared to reading with reflectors at 00 or as a result of low relative humidity.

Time

10 10.3 11 11.3 12 12.3 1 1.3 2 2.3 3

Mass(kg)

4.02

4.04

4.06

4.08

4.10

4.12

4.14

4.16

4.18

4.20

4.22

Mass of chalk in open sun drying against Time

Mass of Chalk in Tray 1 against time

Mass of Chalk in Tray 2 against time

Fig. 4.2b. Reduction of moisture content of chalk with Time when reflector is at 150 to the

Dryer


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5. Conclusions

An efficient and economic solar-aided chalk dryer has been developed using locally available

material in Nigeria and characterized. The experiments were conducted for drying chalk. The

product was loaded in the trays inside the dryer. The drying duration of the product was reduced

considerably in comparison with traditional sun drying. Moreover, the chalk could retain its

original colour after the drying process, which helps the writing quality of the chalk. The dryer

can be enlarged for large-scale drying and commercial purposes by increasing the reflector and

collector size. There is no need of carrying the chalks inside during the nights in order to avoid

re-wetting since the dryer is sealed with glass and wood to protect the samples from dew and

rain.

5.1 Recommendations

-The mirror can be replaced with stainless steel to avoid the risk of breakage and maintenance.

-The chalk dryer can be made single tray equipment or each tray should be used individually i.e.

one at a time, to maximize its operating efficiency for chalk drying.


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Notes

1. Interview with Mr Adaranijo of Dara chalk

2. Interview with Mr Aderemi Awojobi, on solar powered dryers

3. National Aeronautics and Space Administration (NASA)

4. Reflective Insulation Manufacturers Association (RIMA)


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References

Glaser E. P. (1973) Method and apparatus for converting solar radiation to electrical power United states Patent 19.

Rom F. E. (1975) Method and apparatus for converting solar radiation to heat energy United states Patent.

Coleman R. F. (1977) Solar Energy Apparatus United states Patent.

Keyes J. H.( 1978) Method and apparatus for collecting, storing and transmitting solar heat.

united states patent.

Hunt A. J. (1982) R adiant energy collection and conversion apparatus and methodunited states Patent.

Constantine D. M., Arlington, Albert G. L., Walter L. and Guetlhuber F. (1981). Method andapparatus for utilizing solar energy United states Patent, 4,265,223.

Lenz E. (1989) Apparatus and method for extracting focused solar radiant energy, united states

patent.

Stirbl R. C. (1993) Method and apparatus for generating atmospheric solar energy concentrator,

united states patent.

Corazzini W.( 1995) Development of a solar powered window shade, united states patent.

Ampratwum B. D. and Dorvlo A. S.S. (1998) Evaluation of a solar cabinet Dryer as an air- heating system. Journal of Applied energy, Vol. 59, No. 1, pp. 63-71.

Gikuru M. and Njoroge K. S. (2004) Performance of a solar dryer with limited sun trackingcapability, Journal of Food Engineering, 74 (2006), pp. 247 – 252.

Sitompul J. P., Istadi, Widiasa I.N. (2001) Drying Technology, Volume 19, Issue 2 , pages 269 – 280

Wikipedia free encyclopedia, www.wikipedia.com

Ezekoye B.A. and Enebe O.M.(2006) Development and performance evaluation of modified

integrated passive solar Grain Dryer , The Pacific Jour. of Science and Technology, vol.

7, No. 2, November.

Sreekumar A., Manikantan P.E, Vijayakumar K.P. (2006), Performance of indirect solar cabinet

dryer Journal of Energy Conversion and Management 49 (2008) 1388 – 1395.

Ndukwe I. C. (1998) Measurement of solar energy radiation at Okigwe using silicon solar cell, Department of Physics, Federal University of Technology, Owerri, Imo State, Nigeria.

http://www.informaworld.com/smpp/950766202-33263183/title~content=t713597247~db=allhttp://www.informaworld.com/smpp/950766202-33263183/title~content=t713597247~db=allhttp://www.informaworld.com/smpp/950766202-33263183/title~content=t713597247~db=allhttp://www.informaworld.com/smpp/950766202-33263183/title~content=t713597247~db=all~tab=issueslist~branches=19#v19http://www.informaworld.com/smpp/950766202-33263183/title~content=g713628026~db=allhttp://www.wikipedia.com/http://www.wikipedia.com/http://www.informaworld.com/smpp/950766202-33263183/title~content=g713628026~db=allhttp://www.informaworld.com/smpp/950766202-33263183/title~content=t713597247~db=all~tab=issueslist~branches=19#v19http://www.informaworld.com/smpp/950766202-33263183/title~content=t713597247~db=all


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35

Olopade M.A. and Sanusi Y.K. Solar Radiation Characteristics and the performance of

Photovoltaic (PV) Modules in a Tropical Station, Journal of Sci. Res. Dev., 2008, Vol.

11, 100 – 109.

Wengert and Oliveira Improvements in solar dry kiln design U.S. department of agriculture

forest service 1996.

www.azom.com, Retrieved on 26th of May, 2009

Duffie J. A. and Beckman W. A. (1991). Solar Engineering of Thermal Processes, Second

Edition, pp 296 - 301. Wiley-Interscience, New York.

Kuku T.A. and Salau A.A.M. (1985) Field performance of a polycrystalline silicon module,

Department of Electronic and Electrical Engineering, OAU, Ile-Ife, Nigeria.

Ogunremi A.R. and Orimolade A.P. (2006) Design, construction and evaluation of a solar ice

block making machine Obafemi Awolowo University, Ile-Ife.

William Becker, www.williamgbecker.com/MakeSolarOven.html

Monteith, J.L. and M.H. Unsworth, 1990, Principles of Envirionmental Physics, 2nd Ed.,

Edward Arnold, New York (p 53-54)

Meinel A.B. and Meinel M.P., Applied Solar Energy: An Introduction, Addison-Wesley, 1976,

650 pp.

RIMA (1999), Radiant Barriers and Radiant Control Coatings, Reflective Insulation.

Manufacturers Association, April 1999, p.25.

http://www.azom.com/http://www.azom.com/http://www.azom.com/


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APPENDIX I

Table of Relative humidity, Temperature and mass (reflector at 00

to the dryer)

Temperature Mass of chalk Avg RH =75%

Time Open(TA) Tray

1

Tray2 Wall(Tw) Tw -

TA

Tray1 Tray2 Open Rel. Humidity

10am 32 45 41 55 23 4.20 4.20 4.2 80

10.30am 31 43 40 54 23 4.15 4.20 4.18 78

11.00am 33 45 43 57 24 4.13 4.15 4.16 72

11.30am 32 46 45 59 27 4.13 4.13 4.14 73

12 noon 32 49 47 61 29 4.12 4.10 4.14 74

12.30pm 34 52 47 60 26 4.10 4.05 4.12 73

1.00pm 34 55 48 62 28 4.05 4.03 4.12 73

1.30pm 33 58 50 60 27 4.02 4.02 4.10 73

2.00pm 34 59 52 62 28 4.00 4.02 4.10 73

2.30pm 36 58 55 65 29 3.99 4.00 4.08 68

3.00pm 35 57 54 63 28 3.98 4.00 4.06 55


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Table of Relative humidity, Temperature and mass (reflector at 150

to the dryer)

Temperature Mass of chalk Avg RH = 52%

Time Open Tray1

Tray2 Wall Tw -TA Tray1 Tray2 Open Rel. Humidity

10am 29 39 38 39 10 4.20 4.20 4.2 70

10.30am 30 40 38 40 10 4.20 4.20 4.2 65

11.00am 30 39 38 40 10 4.15 4.16 4.19 55

11.30am 32 41 39 42 10 4.13 4.14 4.17 54

12 noon 31 42 42 42 9 4.12 4.14 4.16 52

12.30pm 32 45 43 46 14 4.10 4.12 4.15 52

1.00pm 30 47 45 50 20 4.10 4.12 4.13 52

1.30pm 31 49 44 50 19 4.08 4.10 4.13 45

2.00pm 32 50 43 56 24 4.06 4.10 4.10 45

2.30pm 33 52 47 58 25 4.06 4.08 4.10 43

3.00pm 33 50 45 56 13 4.04 4.08 4.08 43


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Appendix II

Template 1: Solar-aided dryer (Reflector is at 00)


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S

Template 2: Solar-aided dryer (Reflectors tilted at 150)


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Template 3: Solar-aided dryer when loaded with chalk


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Template 4: The Thermocouple


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Template 5: Psychrometric chart


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Template 6: Wet and Dry Bulb Thermometer


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Template 7: Drying Tray


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Template 8: Weighing Scale

solar powered chalk dryer

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