Earth Air Fire and Water
The Pharmageddon Herbal
Chapter 5

Dehydration. The Nuts, Bolts and Spanners.

Although written from the perspective of a medicinal herb grower, it must be understood that the ownership of a dehydration apparatus is of great economic benefit for the grower, irrespective of the type of crop grown.

The usage may be extended to timber, fish, fruit, vegetables flowers and herbs. The operation of such apparatus immediately removes the grower from the vagaries of the fresh market treadmill, and promotes them to the level of wholesaler from the primary production level.

Herbology, in the correct sense is the natural medicines equivalent, of the old Galenic disciplines of Pharmacognosy and Pharmacy. No natural healer may ignore the foundations of the medicines that they use, without a corresponding lack of knowledge, in terms of the efficacy or otherwise of the herbal medicine employed by that healer.

Traditionally the knowledge of such matters was in the hands of the monasteries of old. Not only did the monks provide hospital and medical services, but they grew and prepared the medicines, which they dispensed
Heat and Air Convection 5.1
For the purpose of this chapter, the following table should be taken as
�Standard Values� (StV)
Ambient air temperature 15� Celsius
Atmospheric pressure 101 kPa (760mm Hg)
Water temperature 12� Celsius
Herb at water temperature 12� Celsius

Two variables are required to successfully dehydrate or evaporate a substance, ie, heat and air movement.

It should be now understood that the quality and potency of dried herb is a function not only of the soil that produced the crop, but also a function of the time taken to kill and stabilize the herb. Therefore, it is of prime importance that the correct balance between heat and air movement is attained for an energy efficient and quality crop.

Air Convection 5.2
Air convection falls into two categories;

1. Natural Convection; i.e. the tendency for warm air to rise; for example the convective forces set in motion by the energy of the sun falling upon the earth.

By the early 1800�s, comparatively large scale herb production units were well established and able to supply distant as well as local markets, eg., tea, coffee, tobacco and hops. Because of the unreliability of solar energy, artificial heat was resorted to for the production of convective forces to enhance the drying process.

The drying apparatus was either a shed or a kiln; and the convected or displaced air was made good either by natural leakage through gaps and cracks or by windows and vents.

Kilns were usually employed where a two stage drying process was required, e.g., tobacco drying, where the leaf was first sweated and yellowed to destroy starches and sugars before the leaf was finally killed by dehydration.

The drying shed was favored by the medicinal and culinary herb growers, whose aim was to produce the herb in as near a whole state as possible, within the limits of available technology.

The use of drying sheds removed many of the problems associated with sun or shade drying. The sheds were usually longer than they were wide. The heat was provided by combustion stoves placed at intervals along the length of the shed.

Typical Drying Shed Figure 5.2A

Hitherto, unheated sheds or buildings were the norm; so the introduction of an independent heat source was a great improvement, which provided better quality herbs for market. Subsequent improvements in combustion control and the provision of adjustable convection vents, enabled the grower to reduce the overall drying period to between 36 and 48 hours.

However, large areas of drying surface were required and frequent handling of the herb was necessary to eliminate wet spots, with consequent damage from leaf shatter. Nonetheless, the drying shed allowed the herb grower a modicum of production planning and economic security that was previously out of reach. However, within a few short years the introduction of forced convection revolutionized the practice of dehydration so that what was previously an art, became both art and science.

2. Forced Convection, or the directed movement of air from one place to another is achieved by the use of a fan, which when coupled to circulation ducts, give the herb grower almost complete control over the drying process. The method required a further energy input, but the gains in efficiency and planning far outweigh capital and running costs. During peak harvesting periods the grower was able to undertake continuous day and night operations, irrespective of weather, with drying times greatly reduced and predictable to within an hour.

Types of Air Convection 5.3
For the purpose of dehydration, forced air circulation may be placed in three categories;

1. Air Push - one fan mounted at the air inlet.

2. Air Pull - one fan mounted at the air outlet.

3. Balanced - two fans, one at inlet and one at outlet.

The Single Fan System 5.4
In practice, single fan systems are usually employed on small cultivations of 5000 m3 or less. Fan size is determined by the size of the dehydrator, which in turn is determined by the size of the area under cultivation. For practical reasons, single fan systems are inefficient, irrespective of the area under cultivation, and are not recommended for areas of over 5000 m3.

When a fan moves air in an enclosed space a small drop in air pressure is created on the air inlet side of the fan, thus increasing air throughput. This creates increased static pressure which the fan must overcome, i.e. the air moving through the fan must start to push against the static air on its outlet side and set it in motion.

Static pressure is increased by obstructions, e.g. sharp angles in air ducts and the presence of herb and dehydrator furniture, (trolleys or racks). To overcome static pressure, an increase in velocity pressure must be supplied by increasing power input. If a fan power rating is too low, or if its blades are of the wrong type, the net result is to stir the air instead of pushing it. In such circumstance, the air pull system would be better than the air push system.

Single fan systems are unsatisfactory if air recirculation is needed and wet spots are a common occurrence.

The Balanced System 5.5
The balanced two fan system will give precise control of air flow and can be relied upon to give good results under most conditions. The extra power required is offset by increased efficiency and fuel economy.

Air Movement 5.6
Moving air has velocity or speed which, in the context of fan technology, is measured in meters per second. The actual volume of air moved is calculated in cubic meters of air per second, or, better still, for our purpose, in liters of air per second. 1000 liters has a volume of 1 m3.

Manufacturers rate their fans in accordance with the ability to deliver a given volume of air against a given static pressure. Therefore, when specifying requirements the following terms should be used; cubic meters per second (m3/s) or liters per second (l/s).

Types of Fans 5.7
The ability of a fan to �deliver� is not solely a function of power. How the air is moved, i.e. stirred or pushed, is of more importance for herbal dehydration than just brute force. Fan blades, that merely stir the air are counter productive and can lead to dew point occurring in the drying chamber. Fans may be placed in the following categories;

A. Disc Fans.

B. Axial Flow Fans.

C. Centrifugal Fans.

Disc Fans are cheap to buy. Typical of the type is the portable domestic or office fan, and also the ceiling mounted type. The blade configurations are designed to stir, rather than push the air; and would have an average power rating of 250 watt. They are not suitable for dehydration purposes.

Centrifugal Fans may be encountered where there are large volumes of air required for industrial ventilation such as mines and tunnels. They are similar in operation to the domestic cross flow fan, which are usually found as a component of the so called fan heater. Second hand industrial types are easily obtained, and in terms of air flow may be considered satisfactory. The major drawback is the size and the amount of power required to run them.

The Axial Flow Fan must take pride of place for herbal dehydration purposes. They are efficient and able to deliver against the type of static pressure that is encountered in a loaded dehydrator; and if an air recirculation system is incorporated in a dehydrator, then the axial flow fan is essential. They are widely available, moderately priced, and are supplied with control gear, so that air volume may be varied at will. They are rated from 250 liters per second and upwards by convenient increments. A vehicle radiator fan is a good example and with a bit of ingenuity could be utilized for small dehydration units.

Factors in Fan Selection 5.8
In terms of energy use, static pressure costs money. Bad dehydrator design will considerably increase energy requirements. Long narrow and sharply angled bends in ducting can more than double the frictional drag, which is engendered by dehydrator loading and furniture.

The amount of air delivered by a fan will decrease with an increase of static pressure. Therefore, an increase of power is needed to overcome static pressure. The static pressure within the system will rise as a square of the velocity pressure, this occurs because the air gets compressed on the outlet side of the fan, consequently, a large input of energy is needed to produce a modest increase in velocity.

You will now understand why two fans are better than one, and in the right circumstance will use less power than a single fan system. When purchasing fans it would be wise to increase your estimated air volume requirements by 50%. The aerodynamics of a given fan may suit your requirements, but thought must be given to its operating environment.

Remember the drying ratios for herbal material, e.g. 1 tonne of fresh herb contains around 750 kg/liters of water. In addition to the fans internally generated heat, it must pass heated air at up to 60�C on a continuous basis. The outlet fan will also need to cope with atmospheric conditions; so it is essential that the supplier be given full details of expected operating conditions. If you fail to do that, and there are subsequent problems, you may forfeit guarantee or consumer protection rights.

Do not forget that �cheapest� may turn out to be the most expensive. The means of calculating the air volume requirements for a dehydrator will be dealt with later on in the text.

The Production of Heated Air 5.9
The production of heated air is a relatively simple matter and commercial systems of many types are widely available.Typical of the portable variety is the �hot air blower�, which is commonly found in the form of domestic fan heaters. For obvious reasons, they are not suitable for the dehydration of fresh herb.

However, they may serve the purpose of a cabinet type conditioning dryer. Such dryers are used to condition previously dried herb, which has been in storage. The herb will be at equilibrium moisture; usually 10 to 12% and will need to be conditioned to around 8% moisture for onward processing. The maximum drying chamber is around 2 m3, and will hold around 15 kg of dried herb, ie., 90 grams of moisture to be removed.

Industrial size hot air blowers are commonly used as space heaters in workshops and are often found in horticultural operations, eg., for the heating of tunnel houses for out of season crops. The industrial versions are dual power units, i.e. electricity is used for ignition and operation of the fan blower, whilst the combustion is provided by a fossil fuel such as kerosene or gas. The noxious and carcinogenic combustion products are moisture laden, and are deposited onto the growing or drying crops.

The situation is analogous to venting a vehicle exhaust into an enclosed space and is not an acceptable procedure for herbal dehydration. The common alternative is to combust a fuel of choice within an enclosed chamber. The hot gas is then led away by means of a flue pipe and vented to the atmosphere, (see drying shed), the stove and flue pipes then radiate heat energy to the ambient air.

Fuel Energy politics 5.10
Queen Beatrix, in her 1988 Christmas broadcast to the people of Holland, had this to say;

"The Earth is slowly dying, and the inconceivable,the end of life itself, is actually becoming conceivable. 
We have become a threat to our planet".

To simply say that "they must do something", is to suffer from a crisis of perception, because �they�, is �us�, as individuals. Each of us must exercise the options that are open to us if we wish to weather the global climate change process.

To quote Buckminster Fuller, "If you are not part of the solution, then you are part of the problem".

We already have the alternative and appropriate energy solutions; what is needed is that we loosen the grip that the corporate energy providers have upon us, and select in a conscious and responsible manner the energy appropriate to need. I am not advocating hair shirts and flagellation, because if you need a cell phone, then it is not appropriate to launch your own satellite, or lay your own fibre optic cable. If we are to have any degree of freedom then the individual must be able to select from the menu what is �appropriate� to need, without being dictated to.

High and Low Grade Energy 5.11
The definition of high and low grade energy is somewhat hazy. It is normally taken to mean; �that threshold, below which a given process cannot take place; and that threshold above which a given process can take place�; therefore, the threshold will depend upon the resources available. A passive solar system in the terms of industrial production is low grade energy. However, with people and land, low grade energy can be converted to a sustainable high energy catalyst.

Figure 5.11A

To reach this level you start from where you are and tap into available corporate energy supplies, and slowly break the hold that they have on you.

Combustion and Fuel Values 5.12
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.

Table 5.12A

Fuel Type



Energy kJ/kg.













Fuel Oil












Coal Gas








Natural Gas
















Solar Energy



1.025 kW/m2

The Table may be read in many different ways, and the overall interpretation depends on which bias is used, e.g. cost per kg related to energy values; or cost per kg related to global values; or cost per kg related to local availability.

However, for many people there is no luxury of choice for an energy source except, take it or leave it. Those of us who live in the affluent nations, have a choice which is only limited by personal financial constraints. Therefore, it is up to each of us to accept and take up the challenge to create a sustainable future for our children. Sustainable is the key.

Solar Energy 5.13
Sun worship was the crowning facet of many cultures and it needs no great breadth of imagination to understand why. All substance known to mankind are nothing more, or nothing less, than solar matter; either from our own sun or more distant ones.

Everything is energy and the primary difference between one substance and another, at a sub-atomic level is the degree of condensation of the solar matter. Helio-technology is not new, it has been with us for thousands of years in one form or another; as a science it is around 110 years old.

Solar energy is the sub-strata of all other energy forms; and when we combust a fuel, we release stored solar energy. The energy may be ancient, as in fossil fuels which are finite. Old, as in mature stands of forest, or recent, as in fast growing softwood trees, methane or ethanol.

So sustainability depends on what level we tap into the energy tree, and how efficient our conversion systems are, and most important, how reliable they are. Accordingly, if we tap into the energy tree at the solar, hydro or wind levels, then a storage system is necessary. Remember that wind and water are manifestations of solar radiation.

The Solar Constant 5.14
Solar radiation is not constant; it is in a state of flux. However, for the purpose of scientific calculation, it has been given a mean value, which is called the Solar Constant. The constant is a measure of energy radiated from the sun, per unit of area, perpendicular to the suns rays, as measured at the outer limits of the earths atmosphere, and corresponds to the following measurement;

1.360 kW/m2.

The value of the 1.025 kW/m2, which is given in Table 5.12A, is the mean value of the solar radiation upon the earths surface from the outer atmosphere and a sky vault clear of cloud. The solar constant does not represent the amount of energy available to an earthbound collector. The amount collected would depend on a combination of variables;

1. Collector latitude and angle.

2. Efficiency of the collector.

3. Hours of sunlight and seasonal variations.

4. The amount of pollution and cloud cover.

Solar collectors that track the sun across the sky are available; but there are limiting factors such as size and cost, therefore, for practical and economic reasons, most collectors are fixed azimuth and angle. The azimuth is meridian, and the angle between 45� and 60� according to latitude.

Figure 5.14A shows the theoretical intensity of solar radiation falling on a horizontal surface, with corrections for altitude of the collector. Practical experimentation, and allowing for the variables, indicates that a 30% reduction of the figures derived would be more accurate. The chart may be applied North or South of the equator.

Figure 5.14A

Remember that the chart depicts the suns rays falling onto a flat horizontal surface. Accordingly, as the surface rises in altitude, then the sun angle is being decreased, which increases the distance that the radiation had to travel, however, the increasing clarity of the atmosphere produces solar gain.

Therefore, irrespective of the altitude, the angle of a solar collector must match as near as possible the solar altitude. For a fixed collector, the best angle is the average sun angle for a 12 month period, as logged in your region.

Solar Energy as Sole Heat Source 5.15
It may be readily understood from Figure 5.14A, that for practical reasons, on a small scale, solar energy, unless fed through a conversion system, is low grade energy. It is excellent for water or space heating in living accommodation, workshops or store rooms.

Conversion Systems 5.16
There are many sophisticated conversion systems, most of which are beyond the tooling and economic capacity of the average person. (For example, focusing collectors in conjunction with various forms of heat exchangers, or the high technology photo-voltaic panels with storage batteries and current converters.) From the �do-it-yourself� angle there are wind, hydro and bio-gas converters, so the options open are many. However, caution is required, because the technology you choose can free you or enslave you. It can accelerate or help the planet to maintain equilibrium.

Conversion Systems and Global Warming 5.17
In May 1990, the UN IPCC. Working group on global warming, completed a detailed doomsday report; the figures given in this section are abstracted from that report.

To produce high grade energy a conversion system is essential, and all energy conversion systems contribute to greenhouse gas emissions. The greenhouse gases are essential to the life of the planet, but it is all a question of balance, and because of the overwhelming complexity of the planetary cycles, we do not know where the critical balance point is.

The increasing incidence of freak weather conditions are the first symptoms of fever. If the planet starts to sweat and shiver, then our support structures, i.e. food, water, sewage, power, medical care, and communications will collapse Choose your conversion and conservation wisely.

Table 5.17A

Greenhouse Gas

Main Source

Rate of Increase

% of Total

Carbon Dioxide (CO2)

Fossil fuel burning, Deforestation

0.5 % per year



Industrial & domestic Refrigerants

4 % per year


Methane (CH4)

Wetlands, rice paddies & animal Flatulence

0.9 % per year


Nitrous oxide (N2O)

Biomass burning & fossil fuel combustion

0.8 % per Year


In 1989, the United States Department of Energy issued the data upon which the following Table is based.
The figures represent metric tonnes of CO2 emitted per GW/hour (Giga Watt), i.e., one thousand million watts.

Table 5.17B



Construction Technology

Actual Conversion

Total per
G/W hour.

Coal Fired





Oil Fired




726 ++

Gas Fired




484 ++






Hydro power










Photo voltaic





Solar Thermal





Wood on a basis of sustainable harvest.

Minus 1509.1



Minus 159.9

Dehydration and Solar Energy 5.18
Solar energy, as a stand alone system for herbal dehydration, presents several problems;

1. Solar radiation received on a daily basis is not predictable with any degree of accuracy.

2. Unless some form of a heat storage device, that can provide sustained release, is included in a dehydrator that operates on solar energy, then it is possible that dew point will occur during the nocturnal hours.

3. Under operational conditions, a solar dehydrator with natural convection, at a latitude of 36� South, could not better a two day drying period during the summer months. The higher the humidity the longer the process will take.

4. The operator has no effective control over the process.

The problems may be summarized as lack of quality and production efficiency. The degree of catabolism that occurs in the drying herb is a function of time.

Generally, extended drying times, are detrimental to quality. The time of harvest is not under the growers control. An estimate of when the crop is at peak condition can be made, thereafter, around three days either side of that date, in which to harvest and process, to obtain a quality crop.

A cultivation of 0.25 ha would yield on average 2000 kg of fresh herb. A 1 x 1 metre drying tray will hold on average 3.5 kg of chopped fresh herb, therefore with a seasonal average drying period of 3 days, 300 m� of drying space would be required if losses were not to occur. The energy needed to dehydrate 2000 kg of fresh herb in a 3 x 8 hour day, approximates 42 kW/hr i.e. the solar air panel would need to be 42 m� in size, and this is on the assumption of 24 hours of brilliant sunshine per day. If the herb were to be dried in the correct manner then a solar air panel would need to be circa 160 m� in size.

Accordingly solar energy as a stand alone system for herbal dehydration may be seen as �right idea� but �inadequate technology. In arid regions where the availability of bio-fuels is a problem, then the matter of dew point in the dehydrator during the hours of darkness, may be addressed by the provision of a mud brick or concrete block apron, to act as a heat storage device for a slow sustained release of heat to the dehydrator during the hours of darkness.

Biomass Conversion to Energy 5.19

Energy Out � Energy In = Efficiency %

A deciduous tree is a sublime statement at any level of approach. As a solar tracking and conversion system it has no peer. The leaves represent many thousands of individual solar trackers and diffuse radiation scavengers, i.e. a solar panel that covers hundreds of square meters with infinite combinations for solar altitude and azimuth. It switches off when net energy gain reaches zero, and switches on again when energy levels are at break even.

The earth�s landmass represents about 30% of the total area of the planet. Trees and plants in dry mass terms, produce about 115 thousand million tons of biomass, of which, we harvest about 1.2 billion tons for food. Figures of that magnitude are meaningless unless they are reduced to human scale. The current practice of clear felling forests, for no other reason than shareholders short term gain, only serves to illustrate the insanity of an economic system founded on usury. Such practices threaten the long term viability of the planet to sustain life.

Table 5.19A

Nett Ecosystem Productivity in g/m�/year and its relationship to precipitation.



Cultivated Land

650 gm

Desert and Scrub

90 gm

Temp. Deciduous Forest

1200 gm

Temp. Evergreen Forest

1300 gm

Tropical Rain Forest

2200 gm

Given some thought, it will be understood that we have the basis for sustainable high grade energy production. A 1 hA wood-lot    harvested on a sustainable basis, would produce around 8 m� of solid timber per year. 1m� of mixed timber would average 500kg in weight. The calorific value of wood averages 20,000 kJ/kg (table 5.12A) which equates to 106 kJ/m�.




That is sufficient energy to dehydrate 30 tonnes of fresh herb. When timber is burnt to provide high grade energy, in addition to the heat energy produced, there, is also a large volume of various gases released. The actual amount produced, per kg of wood, depends on the efficiency of the burner, that means more heat, less gas, or less heat, more gas.

The gas contains valuable by products that may be recovered by using home workshop technology. For example, tar and creosote, which are a timber preservatives. Current timber preservative methods and substances are destructive of the biosphere.

Biomass may be converted to ethanol by fermentation and distillation. The alcohol may then be used as fuel and for the solvent extraction of medicinal herbs. The technology and methods will be covered in Module 6.


Components of a Dehydrator 5.20
The requirements of dehydration are simple;

1. Drying Chamber.

2. Air Heating Device.

3. Air Moving Device.

These basic components may be combined in various ways. The drying methods described here may be adapted to match cultivations from 100 m� to 4 ha. Such apparatus may also be fabricated and moved from site to site.

Fig 5.20A The Vertical Stack


The vertical or gravity stack is usually employed as a conditioning dryer.

In field conditions it has severe ergonomic and capacity problems and is not recommended for a cultivation in excess of 400 m�.





Fig 5.20B The Horizontal Stack.

The horizontal stack may operate as a single or twin fan dryer depending on its size The heater and plenum may be situated according to the drying method employed.

Ergonomically suitable for up to 10,000m�.


Fig 5.20C. The Tunnel Dryer

The tunnel dryer was a development of the horizontal stack .It may be scaled to meet the needs of cultivations ranging from 1 to 5 ha.

Basically it is a tunnel, through which hot air is blown. The herbal material is progressed through on trayed trolleys. This type of dryer will meet all the requirements. It may be used for parallel and counter flow drying, and the air may be re circulated as needed.

Air Heating Components 5.21

Herbal material should, under no circumstances, be exposed to direct radiation as a method of drying. Air must be heated first and then passed to the drying chamber. This is done by first passing the air across a heat radiating surface. The most convenient method is to incorporate a heating device into the heat plenum as shown below.

Fig. 5.21A

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

Fig 5.21B The Hybrid System

The solar panel may be mounted to suit, i.e. wall, roof or free standing. If flexible ducting is used, the panel angle can be adjusted to match the seasonal solar angle; that would give a significant heat gain. Depending on the air flow through the panel, a temperature boost in the range of 5� to 15�C could be expected. Air flow through the panel can be modified by inserting baffles and introducing adjustable air vents to the heat plenum.

Chapter 5 Part 2.

Pharmageddon Herbal Block Index