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Content

Chapter 1

1.1. Introduction...............................................................................................................................2

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

Chapter 2

2.1. Non Technical Ascepts…………………………………………………………………………..13 2.2. Grains specification considerations……………………………………………………….16

2.3 Operation conditions……………………………………………………………………………...19

2.4 Design of solar dryer……………………………………………………………………………...22

Chapter 3

3.1. Process used............................................................................................................................28

3.2. Materials and their properties.........................................................................................37

Chapter 4

4.1. Cost analysis...........................................................................................................................42

Chapter 5

5.1. Future scope............................................................................................................................43

5.2. References................................................................................................................................43

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CHAPTER 1

1.1 INTRODUCTION

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 dryers.

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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 some alternatives 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, geothermal energy etc.

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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 and cheap.

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

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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 etc.

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-

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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 water heaters.

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 technology.

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

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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-

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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

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• 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.

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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 high. 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 combination.

• 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 strong winds.

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• 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 dryer.

• 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 bottom. 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 wooden supports. 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

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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 types. 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.

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CHAPTER 2

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. Drying behavior

Apart from weather conditions the drying behavior of agricultural crops during dryingdepends on the • Product • Size and shape

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• 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 Weather conditions

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 Storage

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

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• Too risky (given the risk of losses and unpredictability of future pric-es), or

• 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 nations. Capacity

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 jobs. 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) 5. Drying 6. Packaging 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-

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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

• Wheat

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.

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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.

• Rice

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.

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• 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 flow dryers. 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.

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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.

Product

Moisture content %

Maximum temperature allowable

For the drying(oC)

Initial Final

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

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Fish, raw 75 15 30

Fish, water 75 15 50

Onion rings 80 10 55

Apples 80 24 70

Apricots 85 18 65

Grapes 80 15-20 70

Bananas 80 15 70

Pineapple 80 10 65

Coffee 50 11 -

Coffee beans 55 12 -

Guavas 80 7 65

Mulberries 80 10 65

Cocoa beans 50 7 -

Cotton 50 9 75

Cotton seed 50 8 75

Copra 30 5 -

Groundnuts 40 9 50

Silk cocoons 68-70 10-12 80

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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)."

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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

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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. Flat-plate collectors

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 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-

24

net. The warm air rises by natural convection & passes through the

drying chamber.

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 properties.

Uses

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

Manufacturing process

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

25

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.

4. Ventilation

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

collector.

• The electrical energy for the fan increases with the mass flow

• rate.

• The effect of leakages increases with the air flow rate.

• For drying purposes a certain temperature level is often need-

ed.

Electrical Fan

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.

26

Drawing of solar dryer

27

LIST OF COMPONENT

• Glass thickness 3.2mm

• Mild steel Sq pipe

• CRC sheet

• Glass wool insulation

• Pop rivets

• Screws

• Welding

• Aluminium channel

• Hinges

• Silicon sealant

• Black paint

• Tower bolt

• Glass thermometer

28

CHAPTER 3

3.1 PROCESSES USED IN THE FABRICATION

• DRILLING

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

29

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

bushings

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

microinches.

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

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.

30

High spindle speeds that exceed 10,000 RPM also require the use of bal-

anced tool holders.

• WELDING

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.

31

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

and vehicles.

Power supplies

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-

32

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

the problem.

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

33

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.

34

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.

Corrosion issues

35

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.

36

Safety issues

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.

Eye damage

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

arc.

Inhaled matter

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

37

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

the workplace.

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

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

Density

38

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.

Compressive strength

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.

Tensile strength

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 high compression.

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

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.

Thermal stress

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

Atomic Structure

39

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.

Crystal Structure

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.

Density

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-

40

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.

Thermal Conductivity

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.

Corrosion Resistance

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-

41

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

out.

Melting Temperature

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.

42

CHAPTER 4

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

43

Chapter 5

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.

5.2 References

• A book by H.P Garg in advances of solar energy tech-

nology, vol.3.

• E.D Howe, ”principles of drying and evaporating”

• Internet resources:-

Wikipedia.com

Google.com