Herbs in Africa
Dehydration Apparatus
Part ~ 4
Part ~1      Part ~ 2    
Part ~ 3     Part ~ 5    Part ~ 6
Ivor Hughes

Part four in a series of articles which outline the basic requirements, for small scale, sustainable cultivation, and processing techniques, for rural communities continues.


The techniques for the preservation of foods, along with the manufacture of  stone and bone tools, must rank amongst the very earliest of humankind's  technological achievements.

Combination smoking and sun drying, was a common technique of hunter-gatherer cultures, which still finds widespread usage today. Strips of lean meat (jerky or biltong) may be quickly and efficiently preserved in situ, by smoking and sun  drying. The process of haymaking in temperate climates, is sun drying on a large  scale.

The sun drying of medicinal plants, or parts, is an extremely destructive  process, during which, up to 85% of the plants bio-activity may be lost in a 12  hour period. Compare that with warm air drying, where the economic loss may be  reduced to 35%, or with skill, to as little as 15% within the same period of  time.

The Production of Warm Air - Simple Drying Shed.

In the early days of large scale herbal cultivation, such sheds were  commonplace. The introduced innovations were the provision of heated air and  drying racks which replaced the bunching and hanging of the herbs undercover.  For the first time the commercial herb grower had a modicum of control over his  main production process. The gains were considerable. The grower could plan harvesting and dehydration schedules and operate on a 24 hour basis at peak  harvesting times. Temperature control was rudimentary and relied upon the opening and closing of convection vents and doors, and the damping controls on  the combustion stoves. Ergonomically they left much to be desired and the crop dried in an uneven manner. This necessitated much turning and moving of the harvested material. Today the technology has advanced considerably and many high-tech solutions are available. However many of those solutions are not appropriate to the needs of small scale community production.

Appropriate Dehydrator Technology.

Solar Vertical Stack Dryer. New Zealand 1984. The  photograph is of my first, owner built solar dehydrator, which I operated across 3 seasons. The sheathing was weather treated plywood. The solar air heating  panel to the front of the apparatus was constructed from scavenged materials.  The solar heat collecting plate was made from corrugated roofing iron, painted  matt black. It operated on the principle of warm air convection. I was able,  during sunny weather, to heat the drying chamber to 40�C. That was usually, on  average, 15�C above ambient temperature. Crude temperature control was achieved  by    means of an adjustable, ambient air flap, opposite, and at the same height as the solar air inlet. The stack, which stands above the drying chamber, houses  two cheap plastic domestic bathroom convection fans to assist the warm moisture  laden air to be vented to the atmosphere. The fans are powered by warm air convection and not electricity. The shape of the drying chamber roof and the narrowing of the stack was intended to increase the air velocity needed, to carry the vaporised water into the atmosphere

Observations and Comments

The sheathing was three ply. So that in those months that it was used  operationally. Considerable solar heat gain was garnered. It was also light in  weight. The apparatus was constructed so that it could be easily stripped down, and reassembled at a new site. Therefore it would have potential to be  manufactured in kit sets. Now that sounds like a good community project to me!  
The dryer is simple. However some thought must be given to the size of the  drying chamber, which in turn influences the dehydrator herb loading. The size  of the chamber must also bear some relationship to the size of the cultivation.  This in order to prepare a proper harvesting and dehydration schedule. The size  of the drying chamber will influence the size of the solar panel. The panel must  be sized to give the required heat gain needed for successful operation. It produced herb of good aroma and cosmetic quality. The negatives were, it was  only operational from sunrise to sunset. There was always a risk of dew point  being reached in the dehydrator during the hours of darkness. I would close the dehydrator down at dusk. This I did by closing off the air inlet from the solar panel vent and opening the dehydrator door in order to cool the heated herb down  to ambient temperature. This decreased the risk of dew point in the dehydrator.  The door was then closed and the ambient air inlet flap opened slightly.  Operations commenced again 45 minutes after dawn. 

Different Types of Dehydrators

The Hybrid System

Considerable savings in energy may be achieved by including a solar air  heating panel as the air inlet for the heating plenum.

The horizontal stack and tunnel dryer are fuel burners and use  electricity powered axial flow fans. The horizontal stack is side loaded. Pride  of place

must go to the tunnel dryer, for use on the larger commercial cultivation. It  can be constructed from adobe or mud bricks, soil cement bricks or concrete  blocks. It has a Solar hot air panel which forms the roof. The furnace is  constructed from a suitably modified 200L oil drum, which makes it economic in  operation. The major expense is the power required for the fans. A system of  vents allows for precise temperature adjustments and the recirculation of air.  The dehydrator climate may be modified at will to allow the operator precise air  and temperature control through the various drying phases. The moisture laden warm air on exit may be suitably condensed to produce single distilled water which bears a relationship to the herb from which it has been stripped. As such,  after a further single distillation, it may be used as part of a hydro-alcoholic solvent, to extract the herb from which it was recovered.

Useful Data

Enthalpy of Evaporation.

Solar Gain Chart

Conversion Technology

Total per
G/W hour
 Coal Fired


1 962 964
 Oil Fired ? ? 726 726++
 Gas Fired ? ? 484 484++
 Geothermal N/A 3.7 300.3 340
 Hydro power N/A 10 N/A 10
 Wind N/A 7.4 N/A 7.4
 Photo voltaic N/A 5.4 N/A 5.4
 Solar Thermal N/A 3.6 N/A 3.6
 Wood on a basis of
 sustainable harvest
2.9 1346.3 Minus

Combustion and Fuel Values

Combustion, or burning, is a chemical process involving carbon, hydrogen and  oxygen. Oxygen reacts with the fuel and produces combustion products, some of which contribute to ozone layer damage. The reaction is sensed as heat and light.

Combustibles may be solid, liquid or gaseous; and the fuel energy values that  follow should be read as mean global values, because the hydrocarbon chemical content of fossil fuels, eg,. coal, oil or gas, vary according to the  geographical source. The same situation applies to bio-mass fuels, eg., wood or ethanol.

Fuel Energy Values

Fuel Type Source State Energy kJ/kg
 Carbon Elemental Solid 33,000
 Coal Fossil Solid 30,000
 Coke Coal Solid 28,000
 Fuel Oil Fossil Liquid 42,000
 Kerosene Oil Liquid 45,000
 Petrol Oil Liquid 45,000
 Coal Gas Coal Gaseous 20,000
 Methane Bio Gaseous 42,000
 Natural Gas Oil Gaseous 38,000
 Charcoal Bio Solid 33,000
 Ethanol Bio Liquid 28,000
Wood Bio Solid 20,000
Solar Energy Sun Radiant 1.025 kW/m2


Part ~1      Part ~ 2       Part ~ 3      Part ~ 5     Part ~ 6

The Pharmageddon Herbal
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These articles have been published in
Science In Africa
Next in this series: Extraction apparatus