fruits of the silk road - facilitating east west trade

Solar tunnel drier

This is a modified version of the drier found at the following site:

http://www.efn.org/~itech/

Side of tunnel can be raised to inspect and sort fruit

Tunnel drying: A 6m solar tunnel dryer

The tunnel is simple and cheap to build and can be made from locally available materials. The tunnel entrance is set to face the prevailing wind, and then the tunnel is angled upwards at an angle of 3 degrees, to ensure a slow air flow through the tunnel. For a 6m tunnel, this means that the upwind exit should be 30cm higher than the downwind entrance.

A 2m length of black painted roofing tin placed at the entrance to the dryer acts as a solar collector, and ensures a flow of solar heated hot air over the drying fruit. The the whole tunnel is encased in clear polythene. Each end of the tunnel is netted to ensure that no moths can enter the dryer. After 2 days of drying, in the evening once the fruit is hard, the fruit is turned on the trays using a small hard plastic sheet. In the event of a dust storm, the entry to the tunnel can be covered by a layer of polythene. Black polythene can be placed over the part of the tunnel containing drying fruit on day 3 to protect from over­drying. The tunnel can be adjusted to ensure optimum drying conditions.

Optimisation of Drying:

The tunnel temperature can be increased by reducing the air­flow through the tunnel. This can be done by lowering a polythene curtain over the lower, down­wind entrance to the tunnel. This raises the temperature by up to 20 Celcius. The curtain should not be closed completely as this would increase the humidity and prevent drying. The situation is monitored by a hydrometer placed over the trays. Clothes pegs are used to regulate the in­flow curtain height. 2. By adjusting the peak height of the polyethylene cover, (25cm recommended)3. 3. Tilt angle of the dryer, (3 degrees recommended) and amount of produce placed in the dryer. (40kg in 6m dryer)

View from lower end of tunnel, showing reduced aperture to increase temperature.

The height of the polyethylene cover adjusts the volume of heated air, resistance to airflow the path of airflow, traversing high above the trays or closer to the trays. The tilt angle of the dryer adjusts the airflow rate. The amount of produce placed in the dryer affects the humidity in the dryer and the rate of drying.

Keeping the drier insect proof:

This is achieved firstly by spacing 2 of the wire rings 10cm apart at each end. They are held equidistant by a 10cm wire spacer. An arched mesh piece with the arched edge hemmed is prepared. The inner wire loop is threaded through the hem. The mesh is drawn over the outer hoop and fixed below the drier by means of a cross strap. The feet of the support tressels should be placed in water pots to prevent ants from entering the drier.

Steps to Construction:

1. Fix 2 of 4.88*1.22 6mm plywood to 2 of 4.88*.1*.025m batons. Fix 2 of .1*.25*1.22 batons across drier on under side at 1.22m from each end to support tressels.

2. Line area on plywood between batons with foil backed foam and cover with polythene.

3. Attach .03*.01*4.88 strap down sides of staps, .03m from top edge of strap.

4. Make 5 trays for fruit drying using .02*.03 staps. Mesh is glued and stapled to base of rectangular frame, and .02*.005 strap nailed over glued edge of mesh.

5. 6mm holes drilled in side batons at 40cm intervals, except for end where holes are 10cm apart. 6mm black painted wire hoops of approx 2m length slotted into holes to give vertical height above fruit tray of 35cm.

6. Insect proof mesh fitted to ends of tunnel. Arched mesh of approx dimensions 1.25m *1m has seam sewn on arched end. Inner of end wire loop threaded through the seam. Wire spacer used to keep two end loops 10cm equidistant apart. Mesh drawn over end wire loop, and sealed with .02*.02*1.22 strap under each end.

7. Over tunnel polythene, double layer, strapped to one side of baton with .005*2*4.88m strap. Poly extends over tunnel. Free end rolled around .02*.03*4.88m strap and fixed with .0.5*.02*4.88 strap.

8. Build 2 trestles as per design:

Materials List

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