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1.2. Solar radiation – the energy source for solar drying................................................3
1.3. Classification of solar drying...............................................................................................8
1.3.1. Passive solar dryer...................................................................................................10
1.3.2. Active solar dryer......................................................................................................12
2.1. Non Technical Ascepts…………………………………………………………………………..13
2.2. Grains specification considerations……………………………………………………….16
2.3 Operation conditions……………………………………………………………………………...19
2.4 Design of solar dryer……………………………………………………………………………...22
3.1. Process used............................................................................................................................28
3.2. Materials and their properties.........................................................................................37
4.1. Cost analysis...........................................................................................................................42
5.1. Future scope............................................................................................................................43
In the majority of countries, agriculture represents the biggest part of
the economy. 80-90% of the working population is employed in agri-culture.
Despite these large numbers, national food production still
does not meet the needs of the population. The lack of appropriate
preservation and storage systems caused considerable losses, thus reduc-ing
the food supply significantly. The dent in food production caused
by crop-failures as well as significant seasonal fluctuations in availa-bility
can be ironed out by food conservation, e.g., by drying.
Sun drying of crops is the most widespread method of food preserva-tion
in a lot of countries due solar irradiance being very high for the
most of the year. There are some drawbacks relating to the traditional
method of drying, i.e., spreading the crop in thin layers on mats, trays or
paved grounds and exposing the product to the sun and wind.
These include poorer quality of food caused by contamination by
dust, insect attack, enzymatic reactions and infection by micro-organisms.
Also this system is labour and time intensive, as crops have to be
covered at night and during bad weather, and the crops continually have
to be protected from attack by domestic Animals. Non-uniform and insuffi-cient
drying also leads to deterioration of the crop during storage. Serious
drying problems occur especially in humid tropical regions where some
crops have to be dried during the rainy season. Traditional sun drying of
sweet pepper and coffee.
In order to ensure continuous food supply to the growing population
and to enable the farmers to produce high quality marketable
products, efficient and at the same time affordable drying methods
are necessary. Studies have shown that even small and most simple
oil-fired batch dryers are not applicable for the most farmers, due to
lack of capital and insufficient supply of energy for the operation of the
The high temperature dryers used in industrialized countries are
found to be economically viable in developing countries only on large
plantations or big commercial establishments. Therefore the introduc-tion
of low cost and locally manufactured solar dryers offers a promising
alternative to reduce the tremendous post harvest losses. The oppor-tunity
to produce high quality marketable products seems to be a
chance to improve the economic situation of the farmers. However, tak-ing
into account the low income of the rural population in developing
countries, the relatively high initial investment for solar dryers still
remains a barrier to a wide application.
1.2 Solar radiation- The Energy Source For Solar Dry-ing
The sun is the central energy producer of our solar system. It has the
form of a ball and nuclear fusion take place continuously in its centre. A
small fraction of the energy produced in the sun hits the earth and
makes life possible on our planet. Solar radiation drives all natural cy-cles
and processes such as rain, wind, photosynthesis, ocean currents and
several other which are important for life. The whole world energy
need has been based from the very beginning on solar energy. All fossil
fuels (oil, gas, coal) are converted solar energy. The earth's atmosphere is
being changed at an unprecedented rate by pollutants resulting from
wasteful fossil fuel use. These changes represent a major threat to interna-tional
security and are already having harmful consequences over many
parts of the globe. It is imperative to act now.
So it’s the time that
we have to make
that will be helpful
for overcoming the
shortage and need of
today. That is why
there are alterna-tives
sources that we
are using like solar
energy, wind energy,
Most of the energy to the earth is being supplied by the sun only. Total
amount of energy that is supplied is 172 pw, of which 30% is reflected
back, rest of which is absorbed by clouds and landmasses. This is a huge
amount of energy and it can be harnessed. Solar energy is one of the best
sources because its is clean energy, versatile , renewable source ,non pol-luting
The Earth receives 174 petawatts (PW) of incoming solar radiation at the
upper atmosphere. Approximately 30% is reflected back to space while the
rest is absorbed by clouds, oceans and land masses. The spectrum of solar
light at the Earth's surface is mostly spread across the visible and near-infrared
ranges with a small part in the near-ultraviolet.
Earth's land surface, oceans and atmosphere absorb solar radiation, and
this raises their temperature. Warm air containing evaporated water from
the oceans rises, causing atmospheric circulation or convection. When the
air reaches a high altitude, where the temperature is low, water vapour
condenses into clouds, which rain onto the Earth's surface, completing the
water cycle. The latent heat of water condensation amplifies convection,
producing atmospheric phenomena such as wind, cyclones and anti-cyclones.
Sunlight absorbed by the oceans and land masses keeps the sur-face
at an average temperature of 14 °C. By photosynthesis green plants
convert solar energy into chemical energy, which produces food, wood and
the biomass from which fossil fuels are derived.
The total solar energy absorbed by Earth's atmosphere, oceans and land
masses is approximately 3,850,000 exajoules (EJ) per year. In 2002, this
was more energy in one hour than the world used in one year. Photosyn-thesis
captures approximately 3,000 EJ per year in biomass. The technical
potential available from biomass is from 100–300 EJ/year. The amount of
solar energy reaching the surface of the planet is so vast that in one year it
is about twice as much as will ever be obtained from all of the Earth's non-renewable
resources of coal, oil, natural gas, and mined uranium combined.
Solar energy can be harnessed at different levels around the world, mostly
depending on distance from the equator.
Solar power for a home used to be an outlandish energy idea that was
fraught with more issues in terms of getting it to work than results. How-ever,
those were the very early days of the industry. Unfortunately, until
people actually see the results and potential benefits of today's solar power
options for a home, many still refer back to those images from the 1980s
when solar-powered homes were still on the drawing board, so to speak.
Today's solar photovoltaic power systems are light years ahead of those
early designs and hap hazardous setups. The most modern systems use a
method of sun exposure to generate electricity via semiconductors. Simple,
direct exposure to the sun and its heat generate electrons that are then cap-tured
into the system and translated into electricity. The design can be used
for a variety of things as small as powering a mobile phone to as large of a
system as that needed to power your home.
The sun gives us energy in two forms: light and heat. For many years, peo-ple
have been using the sun’s energy to make their homes brighter and
warmer. Today, we use special equipment and specially designed homes to
capture solar energy for lighting and heating
Solar energy can be harnessed by various devices like solar panel, solar col-lectors
A solar collector is a device which captures as much sunlight as possible, in
order to either redistribute (focus) or absorb it into a transport medium.
Solar collectors are generally used to generate heat, although in some cases
a parabolic dish is used to focus sunlight on a special high-temperature
solar cell. The heat generated by a solar collector can be di-
rectly used to heat another object (e.g. a kettle or a body of water) or can be
indirectly used to generate electricity by driving a steam turbine (Stirling
engine). Solar collectors come in a large variety of shapes, sizes and pur-poses.
Here, we will introduce you to the most commonly used types of so-lar
collectors. These include the solar collectors commonly seen in solar
Evacuated tube collector
Evacuated tube collectors consist of a parallel row of evacuated glass tubes.
Within each tube, another glass tube is placed, which is covered in a strong-ly
absorbing material. Since the evacuated space blocks both convection
and conduction, the absorbed heat has little means of escape. The tempera-ture
within the tube itself can therefore reach extreme values, with tem-peratures
of 170 °F to 350 °F commonly achieved. An inherent advantage
of the evacuated tubes, is that their cylindrical form means that the collec-tor
is always perpendicular the sun. A disadvantage to this system is that
sunlight shining in between the tube sis not captured. This can be partly
countered by adding a reflective film to the back of the collector. Another
disadvantage is cost: evacuated tube collectors are approximately twice as
expensive as their flat-plate counterparts.
Parabolic through collector
A parabolic through system consists of a long curved mirror, which focuses
the sunlight on an insulated tube. This tube contains a heat transfer fluid,
which transports the heat to either a generator or a water reservoir. In the
former case, which is most commonly encountered, the heated transfer flu-id
is used to boil water. The acquired steam is passed through a sterling en-gine,
which in turn is used to generate electricity. The advantages of a par-abolic
through system are efficiency (20%), ease of tracking (only one axis
needs tracking) and scalability. These advantages make the system highly
suitable for use in large power plants like Nevada solar one. A large number
of existing and planned solar power plants are based on parabolic through
There is however one major drawback to the parabolic through: it is ex-tremely
sensitive to weather. A decrease in incident solar energy will cause
dramatic decreases in the system’s yield. Basically, no direct sunshine
means next to no power. A parabolic through is therefore only beneficial in
areas that receive plenty of sunshine. In order to bridge hours of limited
sunshine, it is possible to direct the heated fluid through a tank of molten
nitrate salt. Nitrate salts have a tremendously high thermal inertia and are
thus very suitable for heat storage. So, when the sun gets clouded or sets
under the horizon, the still hot salts will make sure the system remains op-erative.
Parabolic dish collector
A parabolic dish (see title image) consists of a single parabolically shaped
mirror, which focuses the incident sunlight on a single point (the focal
point). Temperatures in the focal point can easily reach extreme values,
although performance is highly dependant on weather conditions. In the
focal point, one can place any object that requires heating. Parabolic dishes
are commonly used in solar cooking, but can also be used in solar photovol-taic.
In such a case, a special high-yield high-temperature solar cell is
placed in the focal point. The main advantage of a parabolic dish is its ex-treme
power. Disadvantages are the requirement of a dual axis tracker and
the sensitivity to weather conditions.
Towers and chimneys
More exotic approaches to solar power generation include the so-called
solar tower and the solar chimney. In a solar tower system, an array
of concentric mirrors (heliostats) concentrates the sunlight on a single re-ceiving
station, which is located high in a central tower.
The solar chimney is different in that it doesn’t directly use the heat, but ra-ther
makes use of air movement as a result of solar heating. The air is heat-ed
in a huge circular collector area. In the center of this collector area, a
very high chimney is placed. Since hot air has the tendency to rise, it will
forcefully expel itself through the chimney. By installing a turbine in the
chimney, the air currents can be converted to electricity. An obvious disad-vantage
is that the system’s efficiency is strongly limited by the efficiency of
the installed wind turbine. Despite poor efficiency, a solar chimney is rela-tively
cheap to install and keep going. Note that solar chimneys are also
commonly called solar updraft towers.
Flat plate solar collector
The most basic and most common type of solar collector is the flat plate so-lar
collector. At the heart of this collector you will find a sheet of thermally
conductive dark material (usually metal) which absorbs as much sunlight
as possible. Directly below this sheet a series of water conduits is found;
the heat collected by the absorber is absorbed into the water and subse-
quently carried away by water flow. The collector is housed in an insulating
box, with a glass plate on top to further insulate and heat the system. Due
to their flexibility, relatively low costs and ease of installation, flat-plate col-lectors
are often used in solar water heating systems.
Another type of solar collector is the flat-plate collector. Flat-plate collec-tors
look like large flat boxes with glass covers and dark-colour metal
plates inside that absorb heat. Flat-plate collectors are usually placed on
roofs of houses where no trees or tall buildings will block the sun’s rays. Air
or a liquid, such as water, flows through flat-plate collectors and is warmed
by the heat stored in the absorber plates. The air or water heated inside the
solar collector . The heated air or water inside the house. In an active solar
air heater, a fan pushes the air heated inside the collector into a large bin
full of rocks under the house. The heat is stored there so it can be used lat-er.
In an active solar water heater, the water heated inside the collector is
pumped through pipes into a hot water tank. The first flat-plate collectors
were installed on the roof of a house in Los Angeles in 1909. Since then,
millions of solar water and space heaters have been installed in homes and
other buildings all over the world.
1.3 Classification of solar dryers
All drying systems can be classified primarily according to their operating
temperature ranges into two main groups of high temperature dryers
and low temperature dryers. However, dryers are more commonly
classified broadly according to their heating sources into fossil fuel
dryers (more commonly known as conventional dryers) and solar-energy
dryers. Strictly, all practically-realized designs of high temperature dry-ers
are fossil fuel powered, while the low temperature dryers are either
fossil fuel or solar-energy based systems. To classify the various types
of solar dryers, it is necessary to simplify the complex constructions
and various modes of operation to the basic principles. Solar dryers
can be classified based on the following criteria:
• Mode of air movement
• Exposure to insulation
• Direction of air flow
• Arrangement of the dryer
• Status of solar contribution
Solar dryers can be classified primarily according to their heating
modes and the manner in which the solar heat is utilized. In broad
terms, they can be classified into two major groups, namely:
• Active solar-energy drying systems (most types of which are often
termed hybrid solar dryers)
• Passive solar-energy drying systems (conventionally termed natu-ral-
circulation solar drying systems).
Three distinct sub-classes of either the active or passive solar drying
systems can be identified (which vary mainly in the design arrangement of
system components and the mode of utilization of the solar heat, namely
• Integral-type solar dryers;
• Distributed-type solar dryers; and
• Mixed-mode solar dryers.
Natural convection is used on the diminution of the specific weight of the
air due to heating And vapour uptake. The difference in specific weight be-tween
the drying air and the ambient air promotes a vertical air flow.
Natural convection dryers therefore can be used independent from elec-tricity
supply. However, the airflow in this type of dryer is not sufficient to
penetrate higher crop bulks. Furthermore the air flow comes to a
standstill during night and adverse weather conditions. The risk of
product deterioration due to mould attack and enzymatic reactions is
Using integral (direct) mode of drying, is should be noted, that sun-light
may affect certain essential components in the product e.g. chlo-rophyll
is quickly decomposed. Due to the limitation of the bulk
depth, such dryers need large ground surface areas. If grounds are
scarce, indirect mode type of dryers are preferred for drying larger quanti-ties.
1.3.1 Passive solar dryers
Passive solar dryers are also called natural circulation or natural convec-tion
systems. They are generally of a size appropriate for on-farm use. They
can be either direct (e.g. tent and box dryer) or indirect (e.g. cabinet
dryer). Natural-circulation solar dryers depend for their operation en-tirely
on solar-energy. In such systems, solar-heated air is circulated
through the crop by buoyancy forces or as a result of wind pres-sure,
acting either singly or in
• Tent dryers
Tent solar dryers, are cheap and simple to build and consist of a frame of
wood poles covered with plastic sheet. Black plastic should be used on the
wall facing away from the sun. The food to be dried is placed on a rack
above the ground. Drying times are however not always much lower
than for open-air drying (-25 %). (Probably, insufficient attention has
so far been paid to utilizing natural convection.) The main purpose of the
dryers may be to provide protection from dust, dirt, rain, wind or predators
and they are usually used for fruit, fish, coffee or other products for which
wastage is otherwise high. Tent dryers can also be taken down and stored
when not in use. They have the disadvantage of being easily damaged by
• Box dryers
The box-type solar dryer has been widely used for small scale food
drying. It consists of a wooden box with a hinged transparent lid. The in-side
is painted black and the food supported on a mesh tray above the
dryer floor. Air flows into the chamber through holes in the front and
exits from vents at the top of the back wall. The fundamental features of
the standard Brace Institute solar cabinet dryer. Brace type dryers
achieve higher temperatures, and thus shorter drying times, than tent dry-ers.
Drying temperatures in excess of about 80 °C were reported for the
• Seesaw dryer
The traditional seesaw dryer has a rigid, rectangular frame, the length of
which being 3 times the width' resting on a support with an axis. This sup-port
is oriented north-south and is sufficiently high to allow the frame to be
tilted 30° - towards east in the morning and towards west in the afternoon
.The material for drying is placed on a number of trays, which have a
wooden frame and a mesh bottom, which can be made of a variety of ma-terials,
such as wire netting, old fishing nets, bamboo lattice or any other
material that will allow vertical air circulation and maximum evaporation.
The bottom of the improved seesaw dryer is made of galvanized cor-rugated
iron sheets reinforced crosswise by wooden planks and length-wise
by two wooden planks, about 15 cm high. The upper surface of the
bottom is painted black. Good thermal insulation can be provided by
attaching insulation plates made of lignified wood fibre, expanded pol-ystyrene
various layers of corrugated cardboard etc. to the underside of the
The removable trays are placed on top of the corrugated iron bottom either
in a continuous row or with space between them, which will result in
better heating of the air above the blackened surface of the corrugated
iron bottom. In this case the edges of the trays should be propped up with
A greenhouse effect is obtained by placing a transparent plastic sheet
over the filled trays. This sheet rests on the raised edges of the trays
and is kept stretched by the weight of bamboo canes fixed to the
sides of the plastic sheet. When not in use the sheet is rolled around
the bamboo canes.
• Cabinet solar dryers
The crop is located in trays or shelves inside a drying chamber. If the
chamber is transparent, the dryer is termed an integral-type or direct
solar dryer. If the chamber is opaque, the dryer is termed distributed-type
or indirect solar dryer. Mixed-mode dryers combine the features
of the integral (direct) type and the distributed (indirect) type solar
dryers. Here the combined action of solar radiation incident directly on the
product to be dried and pre-heated in a solar air heater furnishes the
necessary heat required for the drying process.
1.3.2 Active solar dryers
Active solar dryers are also called forced convection or hybrid solar dryers.
Optimum air flow can be provided in the dryer throughout the drying pro-cess
to control temperature and moisture in wide ranges independent of
the weather conditions. Furthermore the bulk depth is less restricted and
the air flow rate can be controlled. Hence, the capacity and the reliability of
the dryers are increased considerably compared to natural convection dry-ers.
It is generally agreed that well designed forced-convection distributed so-lar
dryers are more effective and more controllable than the natural-circulation
The use of forced convection can reduce drying time by three times and de-crease
the required collector area by 50 %. Consequently, dryer using fans
may achieve the same throughput as a natural convection dryer with a col-lector
six times as large. Fans may be powered with utility electricity if it is
available, or with a solar photovoltaic panel. Almost all types of natural
convection dryers can be operated by forced convection as well.
2.1 Non-technical aspects
A huge advantage of solar dryers is the fact that different types of fruits and
vegetables can be dried. The quality of products dried in this way is excel-lent,
due to the fact that the food is not in direct sunlight (cabinet or in-house
dryer), and due to a shorter drying process - up to a 1/3 of the time
in comparison to traditional sun drying.
The drying operation must not be considered as merely the removal of
moisture since there are many quality factors that can be adversely af-fected
by incorrect selection of drying conditions an equipment. The de-sirable
properties of high-quality, e.g. for grains, include:
• low and uniform moisture content
• minimal proportion of broken and damaged grains
• low susceptibility to subsequent breakage
• high viability
• low mould counts
• high nutritive value
• consumer acceptability of appearance and organoleptic properties.
Even where there is a demand for loss reducing technical changes,
farmers may find it difficult to adopt recommended technologies,
because of cash flow problems, labour constraints, or lack of mate-rials.
Small farmers and traders often find it difficult to obtain credit
at reasonable interest rates, since formal financial institutions consider
loans to them be too risky.
Apart from weather conditions the drying behavior of agricultural
crops during dryingdepends on the
• Size and shape
• Initial moisture content
• Final moisture content
• Bulk density
• Thickness of the layer
• Mechanical or chemical pre-treatment
• Turning intervals
• Temperature of grain
• Temperature, humidity of air in contact with the grain
• Velocity of air in contact with the grain
The performance of solar dryers is significantly dependent on the
weather conditions. Both the heat required for removing the moisture as
well as the electricity necessary for driving the fans are generated in the
most cases by solar energy only. In addition to the pre-treatment of
the product, the weather conditions have the biggest influence on the ca-pacity
of product that can be dried within a certain time period.
The drying time is short under sunny conditions and accordingly ex-tended
during adverse weather conditions. The difference in drying
capacity between dry and rainy season has to be taken into considera-tion
for the calculation of the yearly capacity of the dryer.
The utilization of solar energy as the only energy source is recom-mended
for small-scale dryers where the risk of spoilage of big quantities
of crops due to bad weather is low. If large-scale solar dryers are used
for commercial purposes it is strongly recommended to equip the dryer
with a back-up heater to bridge periods with bad weather
For small farmers the main purpose in storing grains is to ensure house-hold
food supplies. Farm storage also provides a form of saving, to cover
future cash need through sale, or for barter exchange or gift-giving. Grain
is also stored for seed and as inputs into house hold enterprises such
as beer brewing, or the preparation of cooked food.
There is an ongoing debate about whether farmers are forced to sell
because of debt and economic dependence on others, or whether they sell
because they regard storage as
• Too costly (in terms of time), or
• Too risky (given the risk of losses and unpredictability of future pric-es),
• Unprofitable in relation to other investments such as cattle.
There is no single answer to the debate, since there is much variation in the
circumstances under which individual farmers operate, both within and be-tween
The capacity of a solar dryer mainly depends on the crop itself and the
shape. On the one hand, it should not be too big to ensure that the prepara-tion
(washing, slicing and pre-drying processing) of the product to be dried
can be completed within a certain time period. On the other hand it should
be big enough to enable the user to generate income and thus to create new
Selection, cleaning and pre-treatment
A process similar to the following seven steps is usually used when drying
fruits and vegetables (and fish, with some modifications)
1. Selection (fresh, undamaged produce)
2. Cleaning (washing & disinfection)
3. Preparation (peeling, slicing, etc.)
4. Pre-treatment (e.g. sulfurizing, blanching, salting)
7. Storage or sale
Only fresh, undamaged food should be selected for drying to reduce the
chances of spoilage and to help to ensure a quality product. After selection,
it is important to clean the produce. This is because drying does not al-ways
destroy micro organisms, but only inhibits their growth. Fruits,
vegetables, and meats generally require a pre-treatment before drying.
The quality of dried fruits and vegetables is generally improved with one or
more of the following pre-treatments: anti-discoloration by coating with
vitamin C, de-waxing by briefly boiling and quenching, and sulfurization by
soaking or fumigating. Fish is often salted. A small amount of chemical will
treat a large amount of produce, and thus the cost for these supplies is usu-ally
small. However, potential problems with availability and the complexi-ty
of the process should be considered. After selection, cleaning, and pre-
treatment, produce is ready to place in the dryer trays. Solar dryers
are usually designed to dry a batch every three to five days. Fast dry-ing
minimizes the chances of food spoilage. However, excessively fast
drying can result in the formation of a hard, dry skin a problem
known as case hardening. Case hardened foods appear dry outside, but
inside remain moist and susceptible to spoiling. It is also important
not to exceed the maximum temperature recommended, which ranges
from 35 to 45°Cdepending upon the produce. Learning to properly so-lar
dry foods in a specific location usually requires experimentation. For
strict quality control, the drying rate may be monitored and correlated to
the food moisture content to help determine the proper drying parameters.
After drying is complete, the dried produce often requires packaging to
prevent insect losses and to avoid re-gaining moisture. It should cool
first, and then be packaged in sanitary conditions. Sufficient drying and
airtight storage will keep produce fresh for six to twelvemonths. If
possible, the packaged product should be stored in a dry, dark location un-til
use or sale. If produce is to be exported, it must meet the quality stand-ards
of the target country. In some cases this will require a chemical
and microbiological analysis of dried samples in a laboratory.
Food drying requires significant labour for pre-treatment (except for
grains), and minimal involvement during the drying process such as
shifting food to insure even drying. Solar drying equipment generally
requires some maintenance.
2.2 Grain specific considerations
The most critical decision in harvesting is not the degree of mechanisation
but the timing of the harvest. If the harvest starts late, the grain becomes
too dry and rate of grain shattering is high. The longer a ripe crop is left in
the field or on the threshing floor, the higher will be the loss from natural
calamities including hailstorm, fire, birds, or rodents. The moisture content
of the grain will be high, making drying difficult if the harvest start too ear-ly.
The moisture content of wheat grain is a crucial factor from harvest
until milling. Moisture content of 25 % is not uncommon in newly
harvested grain in humid areas but it must be dried immediately to
protect it against mould. At 14 % moisture grain can be safely stored for 2
to 3 months. For longer periods of storage from 4-12 months, the moisture
content must be reduced to 13 % or below.
Field drying of the harvested paddy (rice), if it is not a shattering variety,
should be practiced moderately during the dry season only. If hand-harvested
by sickle the grip size bundles are better laid out separated ra-ther
than stacked to achieve greater aeration rather than stacked. Stacking
of moist paddy will cause heating up of the paddy, increasing the activity of
micro- organisms and initiate a major deterioration in quality. A safe
way is to thresh the paddy immediately after harvesting.
Two-stage drying consisting of flash or high-temperature short-exposure
or fast drying to 18 % during the first stage and low-temperature and
slow drying or sun drying to 14 % during the second stage is another
technique to save a large volume of wet grain. Paddy at 18 % moisture
content can be stored for two weeks. However, re-wetting of the grain
should be avoided to prevent cracking or fissuring which will have
telling effects in milling.
• Drying of Seed Grain
If grain is destined for use as seed then it must be dried in a manner
that preserves the viability of the seed. Seed embryos are killed by
temperatures higher than 40-42°C and therefore low temperature dry-ing
regimes must be used. Seed grain may be dried in any type of dryer
provided that it is operated at a low temperature and preferably with
higher air flow rates than generally used. It is essential that batches of
grain of different varieties are not mixed in any way and therefore the dry-ers
and associated equipment used must be designed for easy cleaning. In
this respect simple flat-bed dryers are more suitable than continuous
Noted that seed paddy can be sun dried at depths of up to 30 mm but that
the final stages of drying to 12 % moisture should be conducted
in the shade to avoid overheating and kernel cracking. Flat-bed dryers
can be used with bed depths of up to 0.3 m, air temperatures not exceeding
40 °C, and airflows of 1.3 - 1.7 m³/s per tonne of grain.
Cross-mixing between batches of different varieties can be avoided by
drying in sacks in a flat-bed dryer although care must be taken in packing
the loaded sacks in the dryer to ensure reasonably even distribution of air-flow.
Specialized tunnel dryers in which sacks or portable bins are individ-ually
placed over openings in the top of the tunnel have been developed.
2.3 OPERATIONAL CONDITIONS
During the drying process the humidity ratio changes from 0.0104 to
0.0140 i.e. about 0.0036 kg of vapour per kg of dry air is absorbed. Now by
using solar energy, the air is heated to 45oC with a relative humidity of 17
per cent and is passed over drying material. During the drying process, this
air is cooled adiabatically along the 24oC wet bulb line, and then the final
humidity ratio will be 0.0189. thus the moisture evaporated with the heat-ed
air will be 0.0075 kg of vapour per kg of dry air which is almost double
the water evaporated compared to when air was too heated.
The initial moisture content, the final moisture content and the maximum
temperature at which product should be dried are very important and the
values for a variety of products are given in Table below.
Table for maximum temperature allowable for drying and the initial and final
Contents of various products.
Moisture content %
Maximum temperature allowable
For the drying(oC)
Paddy, raw 22-24 11 50
Paddy, par-boiled 30-35 13 50
Maize 35 15 60
Wheat 20 16 45
Corn 24 14 50
Rice 24 11 50
Green peas 80 5 65
Cauliflower 80 6 65
Carrots 70 5 75
Green beans 70 5 75
Onions 80 4 55
Garlic 80 4 55
Cabbage 80 4 55
Sweet potato 75 7 75
Potatoes 75 13 75
Spinach 80 10 -
Cassava 62 17
Chillies 80 5 65
BENEFITS OF SOLAR DRIED FOOD
Dried foods are tasty, nutritious, lightweight, easy-to-prepare, and easy-to-store
and use. The energy input is less than what is needed to freeze or can,
and the storage space is minimal compared with that needed for canning
jars and freezer containers.
The nutritional value of food is only minimally affected by drying. Vitamin
A is retained during drying; however, because vitamin A is light sensitive,
food containing it should be stored in dark places. Yellow and dark green
vegetables, such as peppers, carrots, winter squash, and sweet potatoes,
have high vitamin A content. Vitamin C is destroyed by exposure to heat,
although pretreating foods with lemon, orange, or pineapple juice increases
vitamin C content.
Dried foods are high in fiber and carbohydrates and low in fat, making
them healthy food choices. Dried foods that are not completely dried are
susceptible to mold. "Microorganisms are effectively killed when the inter-nal
temperature of food reaches 145 degrees Fahrenheit (F)."
2.4 Design of solar dryer
3D view of solar dryer
Components of solar dryer
1. Flat solar collector
2. Drying chamber.
3. Insulated panel(main body).
4. Ventilation system.
1. Flat Solar collector
The most basic and most common type of solar collector is the flat
plate solar collector. At the heart of this collector you will find a
sheet of thermally conductive dark material (usually metal) which
absorbs as much sunlight as possible. Directly below this sheet a se-ries
of water conduits is found; the heat collected by the absorber is
absorbed into the water and subsequently carried away by water
flow. The collector is housed in an insulating box, with a glass plate
on top to further insulate and heat the system. Due to their flexibility,
relatively low costs and ease of installation, flat-plate collectors are
often used in solar water heating systems.
Another type of solar col-lector
is the flat-plate col-lector.
look like large flat boxes
with glass covers and dark-colour
metal plates inside
that absorb heat. Flat-plate
collectors are usually
placed on roofs of houses
where no trees or tall
buildings will block the
Air or a liquid, such as water, flows through flat-plate collectors and
is warmed by the heat stored in the absorber plates. The air or water
heated inside the solar collector . The heated air or water inside the
house. In an active solar air heater, a fan pushes the air heated inside
the collector into a large bin full of rocks under the house. The heat is
stored there so it can be used later. In an active solar water heater,
the water heated inside the collector is pumped through pipes into a
hot water tank. The first flat-plate collectors were installed on the
roof of a house in Los Angeles in 1909. Since then, millions of solar
water and space heaters have been installed in homes and other
buildings all over the world.
2. Drying chamber
Operation principle of solar dryer is that isolation passes through
the clear cover & is absorbed on the blackened interior surface which
are thereby heated and subsequently warm the air within the cabi-
net. The warm air rises by natural convection & passes through the
3. Insulated panel (main body) and insulation
Insulated panel is the outer body of drying chamber which resist the
flow of heat from inside to outside.
Glass wool insulation
Glass wool or fiberglass insulation is an insulating material made
from fibers of glass arranged into a texture similar to wool. Glass
wool is produced in rolls or in slabs, with different thermal and me-chanical
Glass wool is a thermal insulation that consists of intertwined and
flexible glass fibers, which causes it to "package" air, resulting in a
low density that can be varied through compression and binder con-tent.
It can be a loose fill material, blown into attics, or, together with
an active binder sprayed on the underside of structures, sheets and
panels that can be used to insulate flat surfaces such as cavity wall in-sulation,
ceiling tiles, curtain walls as well as ducting. It is also used
to insulate piping and for soundproofing
After the mixture of natural sand and recycled glass at 1,450 °C, the
glass that is produced is converted into fibers. It is typically produced
in a method similar to making cotton candy, forced through a fine
mesh by centripetal force, cooling on contact with the air. The cohe-sion
and mechanical strength of the product is obtained by the pres-ence
of a binder that “cements” the fibers together. Ideally, a drop of
bonder is placed at each fiber intersection. This fibers mat is then
heated to around 200 °C to polymerize resin is calendared to give it
strength and stability. The final stage involves cutting the wool and
packing it in rolls or panels under very high pressure before palletiz-ing
the finished product in order to facilitate transport and storage.
The air flow rate is crucial to the over all system performance. Too
high air flow consumers excessive fan power and too low rates caus-es
poor thermal performance of the system. In summary
• The higher the mass flow rates, the higher the efficiency of the
• The electrical energy for the fan increases with the mass flow
• The effect of leakages increases with the air flow rate.
• For drying purposes a certain temperature level is often need-ed.
Fans are flow machines designed to convey a certain air volume and
to increases the pressure in order to overcome the resistance of the
system. They should work with the best possible efficiency and at
lowest possible noise level.
Fans can be divided and classified according to the air flow direction
through the fan.
The major types are axial flow, radial flow and mixed flow.
3.1 PROCESSES USED IN THE FABRICATION
Drilling is a cutting process that uses a drill bit to cut or enlarge a hole of
circular cross-section in solid materials. The drill bit is a rotary cutting tool,
often multipoint. The bit is pressed against the workpiece and rotated at
rates from hundreds to thousands of revolutions per minute. This forces
the cutting edge against the workpiece, cutting off chips (swarf) from the
hole as it is drilled.
Exceptionally, specially-shaped bits can cut holes of non-circular cross-section;
a square cross-section is possible.
Drilled holes are characterized by their sharp edge on the entrance side
and the presence of burrs on the exit side (unless they have been removed).
Also, the inside of the hole usually has helical feed marks.
Drilling may affect the mechanical properties of the workpiece by creating
low residual stresses around the hole opening and a very thin layer of high-ly
stressed and disturbed material on the newly formed surface. This caus-es
the workpiece to become more susceptible to corrosion at the stressed
surface. A finish operation may be done to avoid the corrosion. Zinc plating
or any other standard finish operation of 14 to 20 μm can be done which
helps to avoid any sort of corrosion.
For fluted drill bits, any chips are removed via the flutes. Chips may be long
spirals or small flakes, depending on the material, and process parameters.
The type of chips formed can be an indicator of the machinability of the ma-terial,
with long gummy chips reducing machinability.
When possible drilled holes should be located perpendicular to the work-piece
surface. This minimizes the drill bit's tendency to "walk", that is, to be
deflected, which causes the hole to be misplaced. The higher the length-to-diameter
ratio of the drill bit, the higher the tendency to walk. The tenden-cy
to walk is also pre-empted in various other ways, which include:
• Establishing a cantering mark or feature before drilling, such as by:
• Casting, moulding, or forging a mark into the workpiece
• Center punching
• Spot drilling (i.e., center drilling)
• Spot facing, which is facing a certain area on a rough casting or forg-ing
to establish, essentially, an island of precisely known surface in a sea of
imprecisely known surface
• Constraining the position of the drill bit using a drill jig with drill
Surface finish in drilling may range from 32 to 500 microinches. Finish cuts
will generate surfaces near 32 microinches, and roughing will be near 500
Cutting fluid is commonly used to cool the drill bit, increase tool life, in-crease
speeds and feeds, increase the surface finish, and aid in ejecting
chips. Application of these fluids is usually done by flooding the workpiece
or by applying a spray mist.
In deciding which drill(s) to use it is important to consider the task at hand
and evaluate which drill would best accomplish the task. There are a varie-ty
of drill styles that each serve a different purpose. The sub land drill is ca-pable
of drilling more than one diameter. The spade drill is used to drill
larger hole sizes. The indexable drill is useful in managing chips.
Microdrilling refers to the drilling of holes less than 0.5 mm (0.020 in).
Drilling of holes at this small diameter presents greater problems since
coolant fed drills cannot be used and high spindle speeds are required.
High spindle speeds that exceed 10,000 RPM also require the use of bal-anced
Welding is a fabrication or sculptural process that joins materials, usually
metals or thermoplastics, by causing coalescence. This is often done by
melting the workpieces and adding a filler material to form a pool of mol-ten
material (the weld pool) that cools to become a strong joint, with pres-sure
sometimes used in conjunction with heat, or by itself, to produce the
weld. This is in contrast with soldering and brazing, which involve melting
a lower-melting-point material between the workpieces to form a bond be-tween
them, without melting the work pieces.
There are several different ways to weld, such as: Shielded Metal Arc Weld-ing,
Gas Tungsten Arc Welding, Tungsten Inert Gas and Metallic Inert Gas.
MIG or Metallic Inert Gas involves a wire fed "gun" that feeds wire at an ad-justable
speed and sprays a shielding gas (generally pure Argon or a mix of
Argon and CO2) over the weld puddle to protect it from the outside world.
TIG or Tungsten Inert Gas involves a much smaller hand-held gun that has
a tungsten rod inside of it. With most, you use a pedal to adjust your
amount of heat and hold a filler metal with your other hand and slowly feed
it. Stick welding or Shielded Metal Arc Welding has an electrode that has
flux, the protectant for the puddle, around it. The electrode holder holds the
electrode as it slowly melts away. Slag protects the weld puddle from the
outside world. Flux-Core is almost identical to stick welding except once
again you have a wire feeding gun, the wire has a thin flux coating around it
that protects the weld puddle.
Many different energy sources can be used for welding, including a gas
flame, an electric arc, a laser, an electron beam, friction, and ultrasound.
While often an industrial process, welding may be performed in many dif-ferent
environments, including open air, under water and in outer space.
Welding is a potentially hazardous undertaking and precautions are re-quired
to avoid burns, electric shock, vision damage, inhalation of poison-ous
gases and fumes, and exposure to intense ultraviolet radiation.
Arc welding is a type of welding that uses a welding power supply to create
an electric arc between an electrode and the base material to melt the met-als
at the welding point. They can use either direct (DC) or alternating (AC)
current, and consumable or non-consumable electrodes. The welding re-gion
is usually protected by some type of shielding gas, vapour, or slag. Arc
welding processes may be manual, semi-automatic, or fully automated.
First developed in the late part of the 19th century, arc welding became
commercially important in shipbuilding during the Second World War. To-day
it remains an important process for the fabrication of steel structures
Engine driven welder capable of AC/DC welding.
To supply the electrical energy necessary for arc welding processes, a
number of different power supplies can be used. The most common classi-fication
is constant current power supplies and constant voltage power
supplies. In arc welding, the voltage is directly related to the length of the
arc, and the current is related to the amount of heat input. Constant current
power supplies are most often used for manual welding processes such as
gas tungsten arc welding and shielded metal arc welding, because they
maintain a relatively constant current even as the voltage varies. This is
important because in manual welding, it can be difficult to hold the elec-trode
perfectly steady, and as a result, the arc length and thus voltage tend
to fluctuate. Constant voltage power supplies hold the voltage constant and
vary the current, and as a result, are most often used for automated weld-ing
processes such as gas metal arc welding, flux cored arc welding, and
submerged arc welding. In these processes, arc length is kept constant,
since any fluctuation in the distance between the wire and the base materi-al
is quickly rectified by a large change in current. For example, if the wire
and the base material get too close, the current will rapidly increase, which
in turn causes the heat to increase and the tip of the wire to melt, returning
it to its original separation distance.
The direction of current used in arc welding also plays an important role in
welding. Consumable electrode processes such as shielded metal arc weld-ing
and gas metal arc welding generally use direct current, but the elec-
trode can be charged either positively or negatively. In welding, the posi-tively
charged anode will have a greater heat concentration and, as a result,
changing the polarity of the electrode has an impact on weld properties. If
the electrode is positively charged, it will melt more quickly, increasing
weld penetration and welding speed. Alternatively, a negatively charged
electrode results in more shallow welds. Non-consumable electrode pro-cesses,
such as gas tungsten arc welding, can use either type of direct cur-rent
(DC), as well as alternating current (AC). With direct current however,
because the electrode only creates the arc and does not provide filler mate-rial,
a positively charged electrode causes shallow welds, while a negatively
charged electrode makes deeper welds. Alternating current rapidly moves
between these two, resulting in medium-penetration welds. One disad-vantage
of AC, the fact that the arc must be re-ignited after every zero
crossing, has been addressed with the invention of special power units that
produce a square wave pattern instead of the normal sine wave, eliminat-ing
low-voltage time after the zero crossings and minimizing the effects of
Duty cycle is a welding equipment specification which defines the number
of minutes, within a 10 minute period, during which a given arc welder can
safely be used. For example, an 80 A welder with a 60% duty cycle must be
"rested" for at least 4 minutes after 6 minutes of continuous welding. Fail-ure
to observe duty cycle limitations could damage the welder. Commer-cial-
or professional-grade welders typically have a 100% duty cycle.
Consumable electrode methods
o Shielded metal arc welding
One of the most common types of arc welding is shielded metal arc welding
(SMAW), which is also known as manual metal arc welding (MMAW) or
stick welding. An electric current is used to strike an arc between the base
material and a consumable electrode rod orstick. The electrode rod is made
of a material that is compatible with the base material being welded and is
covered with a flux that gives off vapours that serve as a shielding gas and
provide a layer of slag, both of which protect the weld area from atmos-pheric
contamination. The electrode core itself acts as filler material, mak-ing
a separate filler unnecessary. The process is very versatile, requiring
little operator training and inexpensive equipment. However, weld times
are rather slow, since the consumable electrodes must be frequently re-placed
and because slag, the residue from the flux, must be chipped away
after welding. Furthermore, the process is generally limited to welding fer-rous
materials, though specialty electrodes have made possible the welding
of cast iron, nickel, aluminium, copper and other metals. The versatility of
the method makes it popular in a number of applications including repair
work and construction.
o Gas metal arc welding (GMAW)
Gas metal arc welding (GMAW), Commonly called MIG (for metal/inert-gas),
is a semi-automatic or automatic welding process with a continuously
fed consumable wire acting as both electrode and filler metal, along with an
inert or semi-inert shielding gas flowed around the wire to protect the weld
site from contamination. Constant voltage, direct current power source is
most commonly used with GMAW, but constant current alternating current
are used as well. With continuously fed filler electrodes, GMAW offers rela-tively
high welding speeds, however the more complicated equipment re-duces
convenience and versatility in comparison to the SMAW process.
Originally developed for welding aluminium and other non-ferrous materi-als
in the 1940s, GMAW was soon economically applied tosteels. Today,
GMAW is commonly used in industries such as the automobile industry for
its quality, versatility and speed. Because of the need to maintain a stable
shroud of shielding gas around the weld site, it can be problematic to use
the GMAW process in areas of high air movement such as outdoors.
o Flux-cored arc welding
Flux-cored arc welding (FCAW) is a variation of the GMAW technique.
FCAW wire is actually a fine metal tube filled with powdered flux materials.
An externally supplied shielding gas is sometimes used, but often the flux
itself is relied upon to generate the necessary protection from the atmos-phere.
The process is widely used in construction because of its high weld-ing
speed and portability.
Submerged arc welding (SAW) is a high-productivity welding process in
which the arc is struck beneath a covering layer of granular flux. This in-creases
arc quality, since contaminants in the atmosphere are blocked by
the flux. The slag that forms on the weld generally comes off by itself and,
combined with the use of a continuous wire feed, the weld deposition rate
is high. Working conditions are much improved over other arc welding
processes since the flux hides the arc and no smoke is produced. The pro-cess
is commonly used in industry, especially for large products. As the arc
is not visible, it is typically automated. SAW is only possible in the 1F (flat
fillet), 2F (horizontal fillet), and 1G (flat groove) positions.
Non-consumable electrode methods
Gas tungsten arc welding (GTAW), or tungsten/inert-gas (TIG) welding, is a
manual welding process that uses a non-consumable electrode made of
tungsten, an inert or semi-inert gas mixture, and a separate filler material.
Especially useful for welding thin materials, this method is characterized by
a stable arc and high quality welds, but it requires significant operator skill
and can only be accomplished at relatively low speeds. It can be used on
nearly all weldable metals, though it is most often applied to stainless steel
and light metals. It is often used when quality welds are extremely im-portant,
such as in bicycle, aircraft and naval applications. A related pro-cess,
plasma arc welding, also uses a tungsten electrode but uses plasma
gas to make the arc. The arc is more concentrated than the GTAW arc, mak-ing
transverse control more critical and thus generally restricting the tech-nique
to a mechanized process. Because of its stable current, the method
can be used on a wider range of material thicknesses than can the GTAW
process and is much faster. It can be applied to all of the same materials as
GTAW except magnesium; automated welding of stainless steel is one im-portant
application of the process. A variation of the process is plasma cut-ting,
an efficient steel cutting process.
Other arc welding processes include atomic hydrogen welding, carbon arc
welding, electroslag welding, electrogas welding, and stud arc welding.
Some materials, notably high-strength steels, aluminium, and titanium al-loys,
are susceptible to hydrogen embrittlement. If the electrodes used for
welding contain traces of moisture, the water decomposes in the heat of the
arc and the liberated hydrogen enters the lattice of the material, causing its
brittleness. Stick electrodes for such materials, with special low-hydrogen
coating, are delivered in sealed moisture-proof packaging. New electrodes
can be used straight from the can, but when moisture absorption may be
suspected, they have to be dried by baking (usually at 800 to 1,000 °F or
427 to 538 °C) in a drying oven. Flux used has to be kept dry as well.
Some austenitic stainless steels and nickel-based alloys are prone to inter-granular
corrosion. When subjected to temperatures around 700 °C (1,300
°F) for too long a time,chromium reacts with carbon in the material, form-ing
chromium carbide and depleting the crystal edges of chromium, impair-ing
their corrosion resistance in a process calledsensitization. Such sensi-tized
steel undergoes corrosion in the areas near the welds where the tem-perature-
time was favorable for forming the carbide. This kind of corrosion
is often termed weld decay.
Knifeline attack (KLA) is another kind of corrosion affecting welds, impact-ing
steels stabilized by niobium. Niobium and niobium carbide dissolves in
steel at very high temperatures. At some cooling regimes, niobium carbide
does not precipitate, and the steel then behaves like unstabilized steel,
forming chromium carbide instead. This affects only a thin zone several
millimeters wide in the very vicinity of the weld, making it difficult to spot
and increasing the corrosion speed. Structures made of such steels have to
be heated in a whole to about 1,950 °F (1,070 °C), when the chromium car-bide
dissolves and niobium carbide forms. The cooling rate after this
treatment is not important.
Filler metal (electrode material) improperly chosen for the environmental
conditions can make them corrosion-sensitive as well. There are also issues
of galvanic corrosion if the electrode composition is sufficiently dissimilar
to the materials welded, or the materials are dissimilar themselves. Even
between different grades of nickel-based stainless steels, corrosion of
welded joints can be severe, despite that they rarely undergo galvanic cor-rosion
when mechanically joined.
Welding safety checklist
Welding can be a dangerous and unhealthy practice without the proper
precautions; however, with the use of new technology and proper protec-tion
the risks of injury or death associated with welding can be greatly re-duced.
Heat and sparks
Because many common welding procedures involve an open electric arc or
flame, the risk of burns from heat and sparks is significant. To prevent
them, welders wear protective clothing in the form of heavy leather gloves
and protective long sleeve jackets to avoid exposure to extreme heat,
flames, and sparks.
Exposure to the brightness of the weld area leads to a condition called arc
eye in which ultraviolet light causes inflammation of the corneaand can
burn the retinas of the eyes. Welding goggles and helmets with dark face
plates - much darker than those in sunglasses or oxy-fuel goggles - are
worn to prevent this exposure. In recent years, new helmet models have
been produced featuring a face plate that automatically self-darkens elec-tronically.
To protect bystanders, transparent welding curtains often sur-round
the welding area. These curtains, made of a polyvinyl chloride plastic
film, shield nearby workers from exposure to the UV light from the electric
Welders are also often exposed to dangerous gases and particulate matter.
Processes like flux-cored arc welding and shielded metal arc welding pro-duce
smoke containing particles of various types of oxides. The size of the
particles in question tends to influence the toxicity of the fumes, with
smaller particles presenting a greater danger. Additionally, many processes
produce various gases (most commonly carbon dioxide and ozone, but oth-ers
as well) that can prove dangerous if ventilation is inadequate.The use of
compressed gases and flames in many welding processes also pose an ex-plosion
and fire risk; some common precautions include limiting the
amount of oxygen in the air and keeping combustible materials away from
Interference with pacemakers
Certain welding machines which use a high frequency alternating current
component have been found to affect pacemaker operation when within 2
meters of the power unit and 1 meter of the weld site.
3.2 MATERIALS USED
Glass is one of three basic types of ceramics; Glass is distinguished by its
amorphous (non-crystalline) structure solid material that exhibits a glass
transition. Glasses are typically brittle and can be optically transparent.
Structure: Network formers
Molecules that link up with each other to form long chains and network.Hot
glass cools, chains unable to organize into a pattern. Solidification has
short-range order only.
Amorphous structure occurs by adding impurities (Na+,Mg2+,Ca2+, Al3+). Im-purities:
interfere with formation of crystalline structure
The density of glass is 2.5, which gives flat glass a mass of 2.5 kg per m2 per
mm of thickness, or 2500 kg per m3.
The compressive strength of glass is extremely high: 1 000 N/mm2 = 1 000
MPa. This means that to shatter a 1 cm cube of glass, it requires a load of
some 10 tonnes.
When glass is deflected, it has one face under compression and the other in
tension. Whilst the resistance of glass to compressive stress is extremely
high, its resistance to tensile stress is significantly lower. The resistance to
breakage on deflection is in the order of:- 40 MPa (N/mm2) for annealed
glass- 120 to 200 MPa for toughened glass (depending on thickness, edge-work,
holes, notches etc). The increased strength of SGG SECURIT tough-ened
glass is the result of the toughening process putting both faces under
Young’s modulus, E
This modulus expresses the tensile force that would theoretically have to
be applied to a glass sample to stretch it by an amount equal to its original
length. It is expressed as a force per unit area.
Linear expansion is expressed by a coefficient measuring the stretch per
unit length for a variation of 1 °C. This coefficient is generally given for a
temperature range of 20 to 300 °C. The coefficient of linear expansion for
glass is 9 x 10-6 m/mk.
Due to the low thermal conductivity of glass, partially heating or cooling a
sheet of glass creates stresses, which may cause thermal breakage. When
glass is framed, the edges are encased in the rebate, which protects them
from direct solar radiant heat. This can cause temperature differentials suf-ficient
to cause thermal breakage. This risk is increased where heat absor-bent
solar control glasses are used.
Aluminium is the third most plentiful element known to man, only oxygen
and silicon exist in greater quantities. The element aluminium, chemical
symbol Al, has the atomic number 13. According to present concepts, this
means that an aluminium atom is composed of 13 electrons, each having a
unit negative electrical charge, arranged in three orbits around a highly
concentrated nucleus having a positive charge of 13. The three electrons in
the outer orbit give the aluminium atom a valence or chemical combining
power of 3.
When metals change from the molten to the solid state, they assume crys-talline
structures. The atoms arrange themselves in definite ordered sym-metrical
patterns which metallurgists speak of as "lattice" structures. Alu-minium,
like copper, silver and gold, crystallizes with the face-centred-cubic
arrangement of atoms, common to most of the ductile metals. This
means that the atoms form the corners of a cube, with one atom in the cen-tre
of each face . The length of the sides of the cube for high purity alumini-um
has been determined as 4.049 x 10-8 cm, the shortest distance between
two atoms in the aluminium structure is 2 divided by 2 x 4.049. The face
centred cubic structure is one of the arrangements assumed by close
packed spheres, in this case with a diameter of 4.049 x 10-8 cm, the corners
of the cube being at the centre of each sphere.
Lightness is the outstanding and best known characteristic of aluminium.
The metal has an atomic weight of 26.98 and a specific gravity of 2.70, ap-proximately
one-third the weight of other commonly used metals; with the
exception of titanium and magnesium
Electrical Conductivity and Resistivity
The electrical conductivity of 99.99% pure aluminium at 200OC is 63.8% of
the International Annealed Copper Standard (IACS). Because of its low
specific gravity, the mass electrical conductivity of pure aluminium is more
than twice that of annealed copper and greater than that of any other met-al
. The resistivity at 200 °C is 2.69 microohm cm. The electrical conductivi-ty
which is the reciprocal of resistivity, is one of the more sensitive proper-
ties of aluminium being affected by both, changes in composition and
thermal treatment. The addition of other metals in aluminium alloys low-ers
the electrical conductivity of the aluminium therefore this must be off-set
against any additional benefits which may be gained, such as an in-crease
in strength. Heat treatment also affects the conductivity since ele-ments
in solid solution produce greater resistance than undissolved con-stituents.
The thermal conductivity, κ, of 99.99% pure aluminium is 244 W/mK for
the temperature range 0-1000 °C which is 61.9% of the IACS, and again be-cause
of its low specific gravity its mass thermal conductivity is twice that
of copper . The combined properties of high thermal conductivity, low
weight and good formability make aluminium an obvious choice for use in
heat exchangers, car radiators and cooking utensils while in the cast form it
is extensively used for I/C engine cylinder heads.
Reflectance and Emissivity
Emissivity, the ease with which a substance radiates its own thermal ener-gy,
is closely allied to reflectivity; the best reflecting surface being the
poorest emitter, and conversely the worst reflecting surface being the best
emitter. Plain aluminium reflects about 75% of the light and 90% of the
heat radiation that falls on it. The emissivity of the same piece of alumini-um
is, however, low (< 10% of that of a black body at the same temperature
and with the same surroundings). The combined properties of high reflec-tivity
and low emissivity give rise to the use of aluminium foil as a reflective
insulating medium, either in dead air spaces or as a surface laminate com-bined
with other insulating materials where it can also be arranged to pro-vide
the added benefit of an effective vapour barrier.
Aluminium has a higher resistance to corrosion than many other metals
owing to the protection conferred by the thin but tenacious film of oxide.
This oxide layer is always present on the surface of aluminium in oxygen
atmospheres. The degree of corrosion and its effect on strength in two dif-ferent
environments. The famous statue of Eros in London's Piccadilly Cir-
cus is an example of the corrosion resistance; after an inspection following
eighty years of exposure to the London atmosphere, the statue showed only
surface corrosion. The formation of the oxide is so rapid in the presence of
oxygen that special measures have to be taken in thermal joining processes
to prevent the oxide instantly forming while the process is being carried
The melting point of aluminium is sensitive to purity, e.g. for 99.99% pure
aluminium at atmospheric pressure it is 660 °C but this reduces to 635 °C
for 99.5% commercial pure aluminium. The addition of alloying elements
reduces this still further down to 500 °C for some magnesium alloys under
certain conditions. The melting point increases with pressure in a straight
line relationship to 980 °C at 50 kilo bar.
4.1 Cost Analysis
Total cost of mild steel sheet = Rs 1500
Cost of carbon black paint and brush = Rs 150
Cost of iron frame = Rs 500
Cost of glass sheet (3.2mm) = Rs 700 (including cutting cost)
Cost of pop rivet = Rs 90
Cost of hinges (4 pieces) = Rs 350
Cost of glass wool insulation = Rs 500
Cost of support pipes (10ft) =Rs 350
Cost of support wheels (4piceses) =Rs 250
Cost of tools and equipment =Rs 550
Cost of electronic equipment =Rs 300
Cost of Silicon Sealant=RS 150
Net cost of the device =Rs 5400
5.1 Future scope of the study
The project is carried out in order to get outside knowledge
and involve in practical applications beyond in our day-to-day
academic studies under in the module of “Advanced Topics in
Mechanical Engineering”. Designing of the solar dryer mini-mizing
shortcomings associated with than low efficiency, cost
not portable solar dryer.
• A book by H.P Garg in advances of solar energy tech-nology,
• E.D Howe, ”principles of drying and evaporating”
• Internet resources:-