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. 1. Fix 2 of 2.5*1.25*.01m plywood to 2 of 5*.1*.02m batons. Fix 3 of 1.25*.04*.01m cross straps at the following places:

   • On either end of the drier across the free ends of the 5m straps

   • At 1.5 from the downward end of the drier across the free ends of the 5m straps

   2. Line area on plywood between batons with foil-backed foam and cover with polythene., held in place by 3*.01*0.005m straps tacked to either side to hold polythene cover in place. 

   3. Attach to 5m side batons 4 of .4*.04*.015 as tray supports 0.05m from top edge of side batons.

   4. Make 4 trays for fruit drying using .04*..015 staps. Mesh is glued and stapled to base of rectangular frame, and .015*.01 strap nailed over glued edge of mesh.

   • 2 of 1.2*.04*.15 for long sides of tray

   • 2 of .84*.04*.15 for short sides of tray

   • 1 of .84*.04*.15 placed horizontally in centre of try on opposite side from mesh to keep long sides from bowing.

   5. 8mm holes drilled in side batons at 70cm intervals, starting 3 cm from ends. 6mm black painted wire hoops of 1.53m length slotted into holes to give vertical height above fruit tray of .35m. .04m of each end of loop is bent vertically so it can slot into the holes. 

   
   

   6. 2 of 1.67*.06cm black-painted tin sheet loops are placed over the wire loops at each end and fastened to sides with screws. This ensures plastic cover fits smoothly to drier ends without any gap for insects.

   
   

   7. Insect-proof net curtain mesh fitted to both ends of tunnel. Mesh of dimensions 1.5m .4m has 6cm seam sewn on upper long side. Tin sheet loop and supporting wire are threaded through the seam and put back in place. Mesh curtain then hangs from loop. Free end of curtain is rolled on 1 of 1.25*.015*.015 strap and when the mesh is taught, this is secured under the base with screws into side rails. This is repeated at the other ends to create insect-proof ends. 

   8. Place strip of black-painted tin metal (tunuka in Russian) of dimensions 1.5*1.2m on downwind (lower) end of drier, supported on 3 straps of dimensions 1.5*.04*.015m placed equally part in the direction of airflow to create a space under the sheet metal for airflow.

   
   

   9. Over-tunnel polythene, double layer, of dimensions 5.1*1.5m placed over wire loops. On windward side, 2 of 2.5*.015*.01m straps placed inside double layer, and edge fixed with tacks to top of side rail. 5*.04*.015 baton placed inside other edge of polythene to cover drier. String loop placed over this side baton to hold the drier closed. Poly extends over the tunnel.  On each end of polythene cover, a 5cm extension is folded over tin sheet ends and pegged with clothpegs. Elastic tape of approx 2m used to keep polythene securely closed to tin sheet loops to prevent entry of insects.

   10. Build 2 trestles as per design:

   
   

   Use .04*.04 straps of dimensions shown. The extension of the trestles can be varied so that the downwind end fo the drier is lower than the opposite end, allowing for an air flow through thr drier.

   
   

   11. A simple curtain can be made to place over the downwind end of the drier. This reduces the airflow through the drier and increases the drying temperature. A polythene square of dimensions 1.2*.7m has a strip of dimensions 1.2*.015*.01 stapled to each long end. One of the strips with polythene attached is hooked under the cross strap inside the drier, and the other long end with strip attached, hangs outside the drier.

Materials List

: