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A Regional Research and Dissemination Programme

Phase III

SOLAR-BIOMASS HYBRID TUNNEL DRYER

DESIGN, CONSTRUCTION AND OPERATION MANUAL

Energy Field of Study School of Environment, Resources and Development

Asian Institute of Technology P.O. Box 4, Klong Luang

Pathumthani 12120, Thailand

February 2003

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Preface

The ‘AIT Solar Tunnel Dryer Construction Manual’ has been specially prepared for those keen on fabricating the dryer and using it to dry various agricultural products. The reader can go through the first chapter to gain more understanding on the theoretical aspects of solar drying, or go directly to the second chapter which describes the construction of the AIT Solar Tunnel Dryer. The procedure for constructing the dryer is explained in a step-by-step do-it-yourself format. We thank the Swedish International Development Cooperation Agency (Sida) for their financial support in the actual construction of the dryer and publication of this manual undertaken under the Project ‘Renewable Energy Technologies in Asia: A Regional Research and Dissemination Programme’. It is hoped that further dissemination and usage of the solar tunnel dryer in the developing countries can be promoted by this manual.

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Table of Contents

1. Solar Drying of Agricultural Products 1.1 Introduction 7 1.2 Drying Process 1.2.1 Types of Drying Processes 7

1.3 Advantages of Drying 8 1.4 Natural Sun Drying 8 1.5 Solar Drying 9 1.6 Factors Governing Solar Drying 9 1.7 Classification of Solar Dryers 1.7.1 Natural Open-Air Dryers 9 1.7.2 Direct Solar Dryers 10 1.7.3 Indirect Solar Dryers 10 1.7.4 Mixed Mode Dryers 10 1.7.5 Hybrid Systems 10 1.8 Solar Tunnel Dryer 10 1.9 Outstanding Features of a Solar Tunnel Dryer 11

1.10 Current Status of Solar Tunnel Dryers 11

2. Construction of AIT Solar Tunnel Dryer 2.1 Introduction 12 2.2 Technical Specification of AIT Solar Tunnel Dryer 12 2.3 Construction Materials 13 2.4 Dryer Cost 13 2.5 Products and Quantities that can be dried 13 2.6 Tools and Equipment 14 2.7 Construction of the Dryer 2.7.1 Brickwork for the Footing 14 2.7.2 Module 17 2.7.3 Side Support 20 2.7.4 Roof Support 22 2.7.5 Crank 22 2.7.6 Final Assembly 23

2.7.7 Electrical Connection 24 2.7.8 Construction of Biomass Stove-Heat Exchanger assembly 30

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3. Design drawings 33 4. Operation and Maintenance of the Tunnel Dryer 3.1 Introduction 49 3.2 Disinfecting of the Products 49 3.3 Health and Hygiene of Personnel 49 3.4 Cleaning and Disinfecting of the Solar Dryer 3.4.1 Washing 50 3.4.2 Disinfecting 50 3.5 Procedure for Drying Fruits and Vegetables 50 3.6 Drying of Various Products 3.6.1 Fish Drying 51 3.6.2 Vegetable Drying 51 3.6.2.1 Chilli 52 3.6.3 Fruit Drying 52 3.6.3.1 Banana Chips 52 3.6.3.2 Banana Fruits 53 References 54

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Solar Drying of Agricultural Products

1.1 Introduction Small farmers in developing countries produce vegetables, fruits and fish as major agricultural and aquatic products. These products are intended mainly for domestic consumption and sale in the local market. However, in case of overproduction, tremendous losses occur because farmers have neither access to markets in big cities nor to the international market due to poor product quality and absence of good marketing and distribution system. As an alternative to the marketing of fresh fruits and vegetables, small farmers can produce dried products. In spite of the abundance of fresh fruits throughout the year, dried fruits are becoming popular as snack items. Besides the traditional dried fruits such as apples, raisins, apricots and figs, the demand for dried tropical fruits such as mango, banana, papaya and pineapple is increasing due to their exotic aroma and appearance for the use as basic ingredients in breakfast food production or as snacks.

1.2 Drying Process The process which involves deliberate removal of moisture from a product is termed as drying. Various agricultural products such as vegetables, fruits, fish, coffee, cocoa and tobacco are being dried using open-air drying technique. But in open-air drying, since there is little control over drying rates, crops can either be over-dried or under-dried. Over-drying the crops in the sun can cause discoloration, bleaching and scorching, loss of germination power, and reduction in nutritional value.

On the contrary, under-drying leads to the development of bacteria and fungi. Thus, drying under controlled conditions of temperature and humidity helps the crop to dry reasonably rapidly to a safe moisture content level and ensure a superior quality of the product .

The maximum temperatures recommended for drying different kinds of vegetables and fruits are given in Table I, along with their initial and final moisture contents.

1.2.1 Types of Drying Processes Several types of dryers and drying methods, each suited for a particular product and situation, are commercially used to remove moisture from a variety of food products including fruits and vegetables.

There are three basic types of drying process:

• solar drying

• atmospheric drying including batch (kiln, tower, and cabinet dryers) and continuous (tunnel, belt, belt-trough, fluidized bed, explosion puff, foam-mat, spray, drum and microwave) drying

• sub-atmospheric dehydration (vacuum shelf/belt/drum and freeze dryers)

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While sun drying is practised for certain fruits such as prunes, apricots, grapes and dates, atmospheric dehydration processes are used for apples, prunes and several vegetables. Continuos processes such as tunnel, trough, fluidized bed and foam mat drying are mainly used for vegetable drying. Factors for selection of a particular dryer/drying method include:

• forms of raw materials and their properties

• desired physical forms and characteristics of the dried product

• operating costs

Produce Moisture Content (% w.b.) Max. Permissible Initial Final Temperature (°°°°C)

Green Peas 80 5 65 Green beans 70 5 75 Cauliflower 80 6 65

Cabbage 80 4 55 Onions 80 4 55 Chilies 80 5 65

Potatoes 75 13 75 Tomatoes 96 10 60

Brinjal 95 6 60 Apple 80 24 70

Apricots 85 18 65 Peaches 85 18 65 Grapes 80 15-20 70

Table I: Max. permissible temperature for drying agricultural produce

and their initial and final moisture contents (Sharma V.K. et al., 1993)

1.3 Advantages of Drying The major advantages of drying fruits and vegetables are given below: • Drying hardly affects the main calorie-providing constituents. • Dried fruits and vegetables have a longer shelf-life under proper storage

conditions. • Transportation, handling and storage costs are substantially lowered. • They provide a consistent product, an important modern marketing requirement. • They utilise the most economical and disposable form of packaging.

1.4 Natural Sun Drying Traditionally, sun-drying is carried out by spreading the product out on the ground and exposing it to the sun during the day and covering it at night to protect it from rain, dust and other damaging elements. Though open-air (natural) sun drying has been practiced for a very long time, there are many disadvantages associated with it:

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• since the drying temperatures cannot be controlled, quality products cannot be obtained.

• the products cannot be safe from scavenging animals and birds. • contamination by dust and other foreign particles. • alternate drying and wetting of the product (during day and night) can cause

cracking of the kernel, which in turn reduces the quality of the crop.

1.5 Solar Drying To have a valuable quality product and to minimize the wastage, artificial drying of the product should be employed. The earlier dryers used conventional fuels like electricity, coal and fossil fuel to heat ambient air for drying purpose. Drying of agricultural product is an energy intensive operation. High cost of fossil fuel and gradual depletion of its reserve has incited the use of solar energy as an alternative energy source for dryers. Solar dryers can find wide-spread applications in tropical and sub-tropical countries because of the following reasons:

• the amount of solar energy available in most cases is high enough to cover the heat requirement of small dryer units as well as the temperature level required for optimal drying of the crops.

• the technology is much easier to adapt than fully mechanised technologies. • solar dryers can be easily constructed using local and cheap raw materials.

1.6 Factors Governing Solar Drying Solar drying is a continuos process where moisture content, air and product temperature, and the humidity of air all change simultaneously along with the two basic inputs to the system: the solar insolation and the ambient temperature. The drying rate is affected by ambient climatic conditions. These include:

ß temperature ß relative humidity ß available solar insolation ß wind velocity ß frequency and duration of rain-showers during the drying period 1.7 Classification of Solar Dryers Various types of solar dryers have been developed in many countries over a long period. The solar dryers can be classified according to their heating modes, or the manner in which the heat derived from the solar radiation is utilized :

1.7.1 Natural Open-Air Dryers These types of dryers are the simplest in construction. The agricultural product to be dried is placed on a tray or a mat and left to dry by ambient sunshine and wind. They have no protection against rain, dust and scavenging animals.

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1.7.2 Direct Solar Dryers In this type of dryers, the product to be dried is placed on a tray or a mat and covered by a transparent plastic sheet or glass. The transparent material reduces the loss of heat. A thin layer of the material to be dried is placed inside the enclosure and exposed to direct

solar radiation. Heat is generated by the absorption of solar radiation by the product itself as well as the heat build-up caused by the enclosure. The moist air is removed by

natural convection. Covering the product to be dried reduces drying time and gives protection from dust and rain. Insect infestation and losses caused by birds and rodents cannot be prevented in this type of dryer. 1.7.3 Indirect Solar Dryers The collector heats up the drying air which flows through the dryer. The main advantage of using indirect solar dryers is that the crop is not directly exposed to direct solar radiation. This prevents the undesired discolouring while drying products such as apricots, apples and grapes. The crop is also protected from rain, dust, animals and insects. Therefore, indirect solar drying accounts for the production of high quality products.

1.7.4 Direct and Indirect Solar Dryers (Mixed Mode) The combined action of the solar radiation incident directly on the material to be dried and of air pre-heated in a solar collector can also be used for drying agricultural products. This mixed mode dryer consists of a solar air heating collector and a drying chamber holding the trays for the crops. 1.7.5 Hybrid Systems These are solar dryers which use another energy source such as a fossil or biomass fuel or electricity, to supplement solar energy in the drying process; e.g. for additional air heating during cloudy period. Hybrid dryers find useful applications in developing countries where the conventional energy sources are either scarce or expensive and where heat generating capacity of the solar system alone is insufficient. 1.8 Solar Tunnel Dryer The solar tunnel dryer consists of a solar collector, drying tunnel, and a small radial flow fan. The crop to be dried is placed in a thin layer inside the drying tunnel. Heat is generated by absorption of solar energy on the absorber of the collector as well as on the crop itself. Air entering the solar collector is heated and this air is forced on the crop placed in the drying tunnel. This type of forced air circulation is made possible by using fans at the air inlet of the solar collector.

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1.9 Outstanding Features of a Solar Tunnel Dryer a) easy to build with simple tools and local materials; relatively low labour

requirements. b) easy to handle; maintenance can be done by the farmers themselves. c) distribution of drying air inside the drying chamber is leading to uniform drying of

the agricultural products. d) provision for integration with a biomass furnace (hybrid system) allowing the use

of dryers during cloudy, rainy seasons and also during night. e) improved product quality as the crop is permanently protected from rain, dust,

insects and other small animals. f) operation of fan incurs low power requirement. g) short pay-back period.

1.10 Current Status of Solar Tunnel Dryers Various activities have already been carried out to disseminate solar tunnel drying technology. Around 150 dryers in 28 different tropical and subtropical countries are now in operation. Table II lists the countries with solar tunnel dryers already in operation.

Brazil Greece Peru Turkey Chile China Indonesia Portugal Thailand India Egypt Kenya South Korea Tunisia Philippines Ethiopia Yugoslavia Spain Uruguay Saudi Arabia France Mali Sri Lanka Argentina Ghana Morocco Sudan Costa Rica

Table II: List of countries with commercial operation of solar tunnel dryer

(Esper A. et al., 1996)

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Construction of AIT Solar Tunnel Dryer

2.1 Introduction Considering the land-holding capacities of marginalized rural farmers in developing countries, large quantities of agricultural production cannot be anticipated. Hence a solar tunnel dryer has been designed for drying small quantities of agricultural products that are harvested in the region . This dryer is based on the design of University of Hohenheim, Germany, and is modified suitably. The prototype described here is designed to dry 115 kg of fruits (banana) and is fabricated at AIT. One of the ways to minimize the cost is to keep wastage (metal sheets, etc.) to a minimum. Hence, the designer should be prudent in selecting standard-sized materials available in the market. 2.2 Technical Specifications of AIT Solar Tunnel Dryer The AIT solar tunnel drier consists of a solar collector, drying tunnel, and five radial flow fans to drive the moist air out of the drier. The product to be dried is placed as a single layer inside the drying tunnel. Air entering the solar collector is heated and is forced on the products placed in the drying tunnel using five fans at the air inlet of the solar collector. The fans, with a rating of 14W, have an air handling capacity of 130m3/hour each. Glass wool insulation provided at the back side of the collector and drying chamber minimises heat losses. For hygienic and ergonomic reasons, the drier stands on a 75 cm high brick plinth. Both the collector and tunnel are covered with a 0.2 mm thick, UV-stabilised polythene sheet. The collector is painted matt black to act as an absorber. Products to be dried are placed in perforated aluminium trays and loaded inside the drier. Easy loading and unloading of the materials is facilitated by rolling one side of the plastic cover up or down using a hand-operated pipe and crank arrangement. The dimensions and other design parameters of the AIT dryer are presented below:

Width : 1.8 m Length (collector) : 4.00 m Length (dryer) : 4.25 m Length (total) : 8.25 m Collector Area : 7.2 m2 Drying area : 7.65 m2

Total number of fans : 5 Approx. air flow rate (single fan) : 130 m3/h Power consumption (single fan) : 14 Watts Top cover : 0.2 mm thick UV-treated PE Bottom insulation : 4 cm-two layers of 25mm glass wool

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2.3 Construction Materials

S.No Item Description Unit Quantity

Unit Price (Baht)

Total Price (Baht)

1. G.I. sheet, #18, 4’*8’ sheet 3 728.17 2,184.51 2. G.I. sheet, #28, 3’*6’ sheet 18 214.08 3,853.50 3. G.I. pipe, φ = 1” meter 6 25.85 155.10 4. M.S. pipe (square), 3/4” meter 12 11.59 139.08 5. M.S. pipe (square), 1.5” meter 36 29.42 1,059.12 6. Al. equal angle, 1”*1”,t= 3 mm meter 24 28.97 695.28 7. Al. split bar, 1/2”* 1mm thick. meter 24 5.35 128.40 Al. flat bar, 1/2” meter 6 9.80 58.85

8. Al. flat bar, 1”, meter 24 24.96 599.04 9. Al. wire mesh, 1mm sieve, meter 10 69.55 695.55

10. Al. washers pcs. 50.00 11. Al. rivets, 1/8”*1/2” pcs. 1,000 0.30 300.00 12. ACSR wire, 1/4” meter 12 10.00 120.00 13. Super Glue tube 4 200.00 14. Silicone sealant, 330 ml tube 1 155.15 155.15 15. Glasswool,

1.22m*30.50m*2mm(blanket) Kg 22.3 112.10 2,500.00

16. Electric Fan, 220 V/AC, 14 W Pcs. 5 220.00 1,100.00 17. Car window/door seal meter 18 30.00 540.00 18. Rubber gasket (flat) meter 22 7.00 154.00 19. Rubber gasket (curved) meter 5 20.00 100.00 20. U.V-treated polythene sheet,

3m wide, 0.2 mm thick meter 10 67.41 674.00

21. Misc. electrical items, such as wire, switches and connector

929.00

22. Black paint (flat) and thinner oil 400.00 Costs as of June, 1997

Table III: Construction materials required for the fabrication of solar tunnel dryer 2.4 Cost of Dryer Cost of construction materials Baht 16,725.00 Sheet metal bending Baht 1,000.00 Masonry and other fabrication work Baht 5,000.00 TOTAL Baht 22,725.00 [Costs as of June 1997 Exchange rate as of June 1997 1 US$.....................Baht 26.00] 2.5 Products and Quantities that can be dried Various products and the quantity that can be dried using the AIT solar tunnel dryer is given in Table IV.

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Agricultural Product Amount (Kg) Load Density (Kg/m2) Apricots 115 15 Banana 115 15 Chili 77 10 Cocoa 153 20 Coffee 191 25 Coconut 77 10 Fish 77 10 Ginger 153 20 Herbs 38 5 Onion 77 10 Papaya 96 12.5 Paprika 77 10 Paddy 191 25 Spices 77 10

Table IV: Products and quantities that can be dried using tunnel dryer fabricated at AIT

(By the time of publication of this manual, only bananas and chillies were tested for drying). 2.6 Tools and Equipment The tools and equipment used in the construction of the dryer are as follows: 1. Drilling machine and drill bits (HSS, 1/8”) 2. Riveting machine 3. Sheet Cutter for cutting GI sheets 4. Sheet metal bending machine (available in a local workshop) 5. Paint brush 6. Sand paper 7. Metal hammer 8. Screw driver 9. Knife 10. Hack saw 11. Measuring tape 12. Vernier caliper 2.7 Construction of the Dryer 2.7.1 Brickwork for the footings Both the dryer and collector rest on these footings. The exact size of the brick column depends on the size of locally available bricks and may vary from place to place. Steps: 1. Mark the center points for each column on the ground (Picture 1). Refer to

Drawing 1. The location should be free from shadows of trees, buildings and other obstructions.

2. Dig the ground and lay suitable concrete foundation for each column (Picture 2).

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3. After the foundation has been set, bricks are laid to make the column (Picture 3). The approximate dimension of each column is given in Drawing 2, but it may vary slightly according to the size of locally available bricks.

4. All the columns should be on the same level. This can be done by making use of a long flexible hose pipe filled with water, which acts as a simple leveling instrument.

Picture 1 Centre point marking for each column of the footing

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Picture 2 Laying of concrete foundations for the footings

Picture 3 Completed footing work for the dryer

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2.7.2 Module Glass wool or any material with a good thermal insulation property is sandwiched between two GI sheets to form a module. Both the dryer and the collector base are made from these modules. A square steel tube is also sandwiched in between, to avoid sagging. Steps: 1. Take a GI sheet with the following specifications : 6’ long and 3 ’ wide, 28 gauge.

2. Mark the GI sheets (length-wise) 40 mm and 10 mm from the two ends. Refer to Drawing 3.

3. The GI sheet is bent at 90 degrees along the markings by a bending machine. In case a bending machine is not available, bending can also be done manually by using a metal hammer.

4. This piece is labeled as Part A in the drawings (Picture 4). Make eighteen such pieces.

5. Take a square tube (M.S) with the following specifications : 38mm* 38mm.

6. Use a hack saw to cut the tube into a 1770 mm long piece (Drawing 4).

7. This piece is labeled as Part B in the drawings. Make eighteen such pieces.

(Picture 5).

For steps 8 to 11, please refer to Picture 6 8. Place Part A on the working table. This is referred to as Part A (lower).

9. Cut the glasswool blanket so that it has the same dimensions as that of unbent portion of Part A.

10. Place a layer of glasswool blanket on top of Part A(lower).

11. Place two pieces of Part B on top of the glasswool so that it is 20 mm from the two ends of Part A (lower). Please refer to drawing 5.

12. Cover Part B with a layer of glasswool (Picture 7).

13. Take another piece of Part A, which will be referred to as Part A (upper) and place it over Part A (lower) such that the 10 mm bend of this Part A(lower) comes over the 40 mm bend of the other Part A(upper). Refer to Drawing 6 and Picture 8.

14. Keep the 10 mm bend over the 40 mm bend of both the Part A s, drill a hole through the two plates and rivet the two G.I. sheets by an Aluminium rivet.

15. After riveting the sides of the module, the square tubular stiffener will also have to be riveted with both the Part A s.

16. Drill holes on the square tubes through the G.I. sheets of both the Part A s.

17. Rivet the square tubes to the G.I. sheets (Upper and Lower Part A).

18. Make nine pieces of such modules.

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Picture 4 Part A of the module

Picture 5 Part B of the module

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Picture 6 Glasswool and Part B placed over Part A (lower)

Picture 7 Part B covered with glass wool

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Picture 8 A complete module

2.7.3 Side Support The side support is made to hold both the dryer/collector modules and the top plastic cover. The long flat base (Drawing 7) rests on the footings and the curved upper portion attaches the polythene cover with reinforced plastic clamps (car washers). The dryer/collector module slides through section AA’ (Drawing 7). Steps: 1. Take a GI sheet with the following specifications : 8’ long and 4’ wide, 18 gauge.

2. Have the G.I. sheet cut and bent as shown in Drawing 7.

3. Make six such pieces (Picture 9 and 10).

4. Make two pieces of side supports as per Drawing 7, but with a total length of 920 mm.

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Picture 9 Side support

Picture 10 Side support from another angle

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2.7.4 Roof Support The polythene cover rests on the roof supports. Each roof support has a hole drilled in the middle. The steel wire that provides extra support for the roof passes through this hole. Steps (type A): 1. Take a GI sheet with the following specifications : 8’ long and 4’ wide, 18 gauge

2. Have the G.I. sheet cut and bent as shown in Drawing 8.

3. Make seven such pieces.

Type B roof supports are also made according to Drawing 8, except that the width of the strips is 110 mm instead of 60 mm. Also in type B roof support, one of the 20 mm bent end is made to face upwards, unlike downwards in type A. Two such type B roof supports are made. Adhesive (for example, Loctite) is applied to the 20 mm protruding edge of type B roof support. Rubber gasket (curved) is inserted so that the sharp edges are covered. This prevents the PE sheet from getting damaged. 2.7.5 Crank for rolling cover A crank for rolling the PE cover on the dryer side for loading and unloading the trays is made from a G.I. pipe (φ = 1”). Steps: 1. Cut a G.I. pipe (φ = 1”), 4.35 m long, and make a suitable handle for the crank, also

with G.I. pipe.

2. Join the handle of the crank to the 4.35 m long pipe, by welding.

3. Cut the PE sheet to 4.35 m in length (Drawing 9).

4. Roll the PE sheet a few times over the pipe.

5. Cut the aluminium strip to 4.35 m in length.

6. Attach the sheet now to the pipe by placing rubber washer (flat) in between the rolled PE sheet and aluminium strip. Rivet together the rolled PE sheet, rubber washer (flat) and the Aluminum strip, with the GI pipe.

2.7.6 Final assembly Steps: 1. The side support pieces are joined together by inserting small GI sheet pieces

between the joining ends of the side support and riveting (Picture 11). The side supports are then placed on the footings (Picture 12).

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2. Each individual module is also joined with another module by inserting GI strips on both the surfaces (top and bottom) and riveting. These modules are then made to fit inside the side support (Picture 13 and 14).

3. The zinc coating on the GI sheet (collector side) is abraded by making use of sand paper. This surface is painted with matt black paint.

4. The roof supports are fixed to the side support by riveting (Picture 15). One of the type A roof support is placed at the end of the dryer and fixed with the side support by riveting. Two other type A roof supports are placed in between the dryer section. The last two pieces of type A roof support is placed in between the collector and fixed with the side support by riveting.

5. Type B roof supports are placed at the two ends of the collector (Picture 15). They are similarly fixed with the side support by riveting.

6. Square pipe is attached to the module by using nuts and bolts on the two ends of the solar dryer. Steel wire is inserted inside the holes made on the roof support and attached to the square pipe by using clamps (Picture 15).

7. Rubber gasket (curved) is fixed with the help of superglue (or Loctite) to all the sharp edges of the side support (top edge) that comes in contact with the polythene cover (Picture 16).

8. End plate on the collector side with appropriate holes are made to hold the fans (Picture 17). The fans are then fitted on this end plate by means of nuts and bolts.

9. The dryer end is fixed with a wire net of appropriate size.

10. The PE sheet attached to the handle is made to cover the whole dryer (Picture 18). Plastic clamps (car window washers) are used to press the polythene sheet against the sealed edge of the side support, on the other side (Picture 19). Finally, the ends of the polythene are stretched and pressed against the edge of the type B roof support and sealed with the plastic clamp.

11. Trays to hold the fruits and vegetables are made from Aluminum bars and wire mesh. The frame of one such tray is shown in Picture 20. This frame is covered by an aluminium wire mesh to hold the products that are to be dried.

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Picture 11 Joining two side supports by inserting a metal sheet and riveting

2.7.7 Electrical connection A weather-proof distribution box has the following components: • MCB in between the supply and connector. • Five individual switches on the live wire for each fan after the connector. The neutral wire from the connector and the five live wires from each switch is connected to the individual fans (Drawing 10).

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Picture 12 Side supports resting on the footings

Picture 13 Joining of individual modules and inserting inside the side support

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Picture 14 Joining of individual modules and inserting inside the side support

Picture 15 Roof support with steel wire

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Picture 16 Rubber gasket (curved) sealing the sharp edges of the side support

Picture 17 End plate on the collector side for fixing fans

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Picture 18 PE sheet attached to the handle, covering the dryer section

Picture 19 Plastic clamps attaching the PE sheet with the side support

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Picture 20

Frame for a dryer tray

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2.7.7 Construction of Biomass Stove-Heat Exchanger assembly

A combustion type biomass stove was incorporated in the tunnel dryer to improve the reliability of the dryer and to enhance the quality of the dried product. A cross flow shell and tube type heat exchanger was incorporated at the ambient air inlet to the collector. Five fans, each of 14W capacity, were used to force ambient air into the dryer, through the heat exchanger ‘shell’. The fans had an air handling capacity of 130m3/hr each. The ‘tubes’ of the heat exchanger were connected to a biomass stove at one end and a chimney at the other. Noting that the maximum permissible temperature for fruits and vegetable drying is about 60oC, the temperature at the outlet of heat exchanger was to be not less than this value. During the combustion process, as volatiles burn at about 600oC, this was taken as the design inlet temperature of flue gas to the heat exchanger. The flue gas outlet temperature was assumed to be about 300oC. A channel stove of rectangular shape was chosen, and the main dimensions of the stove were designed. The thermal energy required to supply hot air (by the heat exchanger) at 70°C for drying was found to be 12.7 kW. From the calorific value of briquettes (18.8MJ/kg), the rate of fuel consumption was estimated as 2.44 kg/hr. A mild steel stove was fabricated with a width of 0.3 m, length of 0.275 m, and height of 0.4 m. A grate punched with 44 holes of 1.5 cm diameter was used to increase the efficiency and quality of combustion. The stove used a high chimney to produce hot flue gas in natural draught. The chimney was designed for a flue gas flow rate of about 139 m3/hr. A rectangular shape was chosen for the chimney design with a cross section of 0.275 m× 0.16 m and the height of chimney was calculated to be about 1m.

A rectangular shape was decided for the shell side, to connect the heat exchanger with the solar tunnel dryer, with the following dimensions: length: 1.72m, width: 0.6m and height: 016m. To provide an estimated total required heat transfer area of 1.74m2, eight galvanised iron (GI) pipes with outer diameter 50mm and inner diameter 44mm were used. Tubes were arranged in a staggered manner, as shown in Figure 21.

Figure 21: Arrangement of tubes in the heat exchanger The biomass stove-heat exchanger unit, with attached chimney is shown in Figure 22. The heat exchanger was insulated with 100mm thick rockwool and clad with 1mm thick aluminium sheet, to reduce thermal losses. The biomass stove was insulated with castable refractory mortar, along the inside walls.

120mm

70mm

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Figure 22: Biomass stove-heat exchanger unit, before insulation The completed hybrid dryer is shown in Figure 23.

Figure 23: Solar-biomass hybrid dryer To reduce the uneven temperature distribution across the tunnel, mixing fans were used to mix the hot air so that the air temperature is uniform across the tunnel, before it enters the dryer. Two 12V/6W DC fans are used at the collector inlet, just after the heat exchanger, to facilitate effective mixing of air. The fans, which are operated using an automobile battery, have an air handling capacity of 80m3/hour each.

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Design drawings

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Operation and Maintenance of the Tunnel Dryer 3.1 Introduction Agricultural products dried in any dryer should be of the highest hygienic standards as they are made for human consumption. The regular maintenance will therefore focus on cleaning and disinfection of the trays, dryer and the polythene cover. To avoid the leakage of heated air, the plastic clamps attaching the polythene cover to the side supports should be checked regularly. Apart from this, general maintenance required for this type of dryer are: (i) Re-painting of the collector surface with matt black paint

The collector surface has to be checked periodically for any peeling of the black coating. If peeling is noted, then the surface is thoroughly cleaned using emery sheets/sand papers to remove all traces of the peeled coating and a fresh coating of matt black paint is applied. (ii) Replacing/repairing damaged polythene covers

The polythene cover sheets are to be checked regularly for any damage such as holes and tearing which could have occurred during loading, unloading or cleaning the dryer. In such cases, the cover sheets should be repaired suitably or replaced with new ones. This will minimise the risks of rain water entering the dryer as well as loss of dryer efficiency. The following maintenance information is applicable to all types of solar dryers and products dried, and has been collected from various literature. 3.2 Disinfection of the Products In order to guarantee the micro-biological quality of the final product, the majority of the product, after being washed in potable water, should be disinfected by immersion in chlorinated water (15 to 30 ppm of free chlorine or 30 to 60 grams of chlorine in 1,000 liters of water) for 3 to 5 minutes before going on with further processes. Disinfecting with chlorinated water is effective for food products such as fruit, legumes and vegetables (Vergas, 1996). 3.3 Health and Hygiene of Personnel The persons or dryer operators, who directly come in contact with the dryer and food products should be healthy and clean in terms of personal hygiene. Dryer operators not complying with sanitary norms put the health of the consumers to risk. All the personnel, including the supervisors, should wear some kind of a head covering and should wash their hands before entering the processing area. Removal of unsecured jewelry is mandatory. It is also necessary to wear effective hair restraints such as caps and hair-nets.

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All necessary steps have to be taken by the supervisors to prevent operators from contaminating the food with micro-organisms or foreign substances such as perspiration, hair, cosmetics, tobacco and mendicants (Dauthy, 1995). 3.4 Cleaning and Disinfecting of the Solar Dryer (Vergas, 1996) Cleaning the equipment consists of eliminating residues and other impurities. Disinfection consists of destroying pathogenic germs and other micro-organisms which could deteriorate the quality of the product. Cleaning and disinfecting are the two consecutive operations. Disinfection should be carried out minutes before using the equipment. The dryer should be washed routinely after each processing. It should be disinfected with chlorinated water after the dryer is unloaded. Gloves and aprons should be used during the entire process. 3.4.1 Washing The trays should be removed from the dryer and then washed using brushes with nylon bristles together with soap and potable water. Care should be taken to scrub all the tray area, including the tops and bottoms of the trays. Afterwards the trays should be rinsed thoroughly with enough potable water so that no traces of soap and detergents are left. The inside portion of the dryer, and the inside and outside surfaces of the polythene cover should also be washed, but using only hose pipe, sponge and potable water, taking care not to damage the stabilized plastic cover. The formation of a dust layer on the plastic should always be removed, as this impedes the passage of solar rays into the dryer. 3.4.2 Disinfecting The dryer trays should be disinfected with chlorinated water before the products to be dried are placed on them. This is done to eliminate any undesirable micro-organisms and thus guarantee the final quality of the dried product. Put the chlorinated water in a plastic container and use a sponge to rinse the entire surface of the trays with it. 3.5 Procedure for Drying Fruits and Vegetables The general processes involved in solar drying of fruits and vegetables are as follows: • Reception of raw materials • Weighing • Classification and selection • Storage • Washing • Disinfection • Peeling

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• Cutting • Pre-treatment • Solar Drying • Packing and storing Since dried banana is widely consumed in Thailand, methods for preparing banana chips and banana fruits is described in one of the following sections. 3.6 Drying of Various Products

Various kinds of aquatic and agricultural products produced by farmers needs to be preserved. Drying of aquatic products like fish and agricultural products like fruits and vegetables are described here.

3.6.1 Fish Drying Unless fish is preserved or processed in some way, within a few hours of being caught, it will start to decay at the high ambient temperatures in tropical countries. Spoilage is caused by the action of enzyme (autolysis) and bacteria in the fish, and also by chemical oxidation of fat, which causes rancidity. Salting and drying are the traditional methods of preserving fish. Reducing the moisture content of fresh fish to around 25% by drying will stop bacterial growth and reduce autolytic activity, but the moisture content must be reduced to 15% to prevent mold growth. Salt retards the bacterial action and aids the removal of water by osmosis. When fish is salted prior to drying, a final moisture content of between 35% and 45% in the flesh, depending on the salt concentration, is often sufficient to inhibit bacteria.

3.6.2 Vegetable Drying Different types of vegetables are produced seasonally. It is therefore likely that there are both periods with a surplus and deficit of vegetables. During the deficit periods, shortage of essential nutrients in the farmer’s diet can be anticipated. Drying is a particularly suitable method of preserving vegetables. Certain vegetables are not advisable to be dried, considering the taste of the vegetable after the drying process. Vegetables such as lettuce, melons, cucumber, radishes and asparagus are not suitable for drying. Vegetables are not subject to attack by the same range of pathogenic organisms which thrive on flesh food such as fish. Poor drying techniques are more likely to lead to spoiled produce rather than dried foods which are dangerous to the health. Care should be taken while drying vegetables as mold growth on incorrectly dried vegetables can contain toxins which are poisonous.

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Chilli: Chilli is an important agricultural product in Asia, and is popular in fresh form as well as dried and powdered form. It plays a vital role in the traditional food habits of the region and imparts a sharp hot taste to food. Chillies are dried as whole vegetables and do not need any pre-treatment. It usually takes two to three days for drying chilli in solar tunnel dryers. Ripe chilli fruits are spread on aluminium netted trays and the trays placed inside the dryer, after opening the polythene cover using the crank. The crank is then lowered, to close the dryer with the polythene cover, and the fans are switched on. At the end of the day, chillies are collected and stored in a plastic bag, to facilitate uniform diffusion of moisture in the chillies. Drying is continued the next day, by spreading the chillies once again, in the dryer. The process will be continued until the required moisture content is achieved. 3.6.3 Fruit Drying Fruits are valued for containing ascorbic acid (vitamin C) and beta carotene (pro-vitamin A). Like vegetables, fruits are also seasonal and needs to be properly preserved. Two forms of dried fruits exist: semi-moist fruits and dried fruits. Semi-moist fruits such as dried grapes contain a high level of natural sugar, allowing it to be preserved at a higher moisture content than other dried fruits. Typically, semi-moist products can have a moisture content of about 25%. This gives these products the advantage that they can be eaten directly in their preserved state without any need to re-hydrate them. Most varieties of fruits are suitable for drying with the exception of citrus fruits like oranges and lemons. (i) Banana Chips: It has been observed that matured bananas are best suited for the production of dried banana chips which are then ground and used for preparing snacks. The following steps are involved in the preparation of banana flour: 1. Bananas are cut to single pieces from the bunches. 2. They are put into a basket and the basket immersed into hot water at 65oC for 5

minutes (in order to kill the bacteria). 3. Bananas are then put into cold water (to cool them to ambient temperature). 4. The skin is removed and the raw bananas are sliced into 3 mm thick chips. 5. Immediately after slicing to chips, they are put into acid in order to prevent oxidation

which will change the color. Peeled whole bananas may also be put into the acid for 2 minutes to prevent oxidation.

6. Then banana chips are arranged on aluminium netted trays. They should be arranged in such a way that there is only a single layer of chips. One chip over the other would affect drying time and quality.

7. The trays containing the chips are then placed inside the dryer and left for drying. 8. At the end of the day, the trays are to be removed from the dryer and stored indoors,

to avoid possible re-wetting during the night.

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9. Drying has to be continued for next days, until the required moisture content is attained.

(ii) Banana fruit: Ripe bananas are peeled and then spread in one layer on aluminium netted trays inside the drying tunnel. Bananas are dried as whole fruits without any chemical pre-treatments. At the end of each day, bananas are collected and put in plastic boxes for uniform diffusion of moisture. At the end of the third day, bananas are flattened and packed for further processing. The procedure adopted for drying in the tunnel drier are as follows: Pre-drying: Water on top of the plastic cover due to condensation of mist in the night should be wiped out and the fans switched on for some time to remove any condensed water from inside the collector and dryer. Arrangement: Once the pre-drying was over, the peeled whole banana fruits are to be arranged on the trays, forming a single layer. Then the trays should be placed inside the dryer and the dryer covered with the polythene sheet by operating the crank. Drying: The drying process is started by switching on the fans. At the end of the first day, the trays with the products should be removed from the dryer and stored indoors, to avoid any re-wetting during the night. On the second day, again the water on the top of the plastic sheet and inside the collector are to be dried and the trays placed inside the dryer and drying started. Drying is continued until the required moisture content is attained.

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of a multi-purpose solar tunnel dryer for use in humid tropics, Renewable Energy, 1(2), p 167-176, 1991.

Brenndorfer B., L. Kennedy , C.O. Oswin Bateman, D.S. Trim, G.C. Mrema and C.W. Brobby, Solar Dryers: their role in post-harvesting processing, Commonwealth Science Council, Commonwealth Secretariat, London SW1Y 5HX, 1987.

Dauthy M.E., Fruit and Vegetable Processing, FAO Agricultural Services Bulletin, Rome, 1995.

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