Peace Corps appropriate community technology/Solar Agricultural Dryers

Phase V: Solar Agricultural Dryers[edit | edit source]

Health and Nutrition

The Role of the Volunteer in Development

Phase V Calendar[edit | edit source]

	DAY 1	DAY 2	DAY 3
A.M.	SESSION 1: Introduction to Agricultural Dryers (Skill Area I)	SESSION 5:. Rewiew of Existing Solar Dryer Plans (I)	SESSION 8: Design of Solar Agricultural Dryers (III)
	SESSION 2: Tour of Solar Dryers (I)	SESSION: 6. Smoke Testing Solar Dryers (IV & V)
P.M.	SESSION 3: Solar Agricultural Dryer Design Procedures and Rules of Thumb (III)	SESSION 7: New Technologies: Introducing Solar Dryers (I & II)	SESSION 9: Site Selection and Preparation (III)
	SESSION 4: Two-Hour Dryer Construction (IV)	Independent Study	SESSION 10: Construction of Solar Agricutural Dryers (IV)
	DAY 4	DAY 5	DAY 6
A.M.	Construction (continued)	Construction (continued)	SESSION 12: Natural Cooling (III)
			SESSION 13: Approaches to Health Systems (III)
P.M.	Construction (continued)	SESSION 11: Issues and Methods in Development and Diffusion of Appropriate Technology (IV)
	DAY 7	DAY 8	DAY 9
A.M.	SESSION 14: Nutritional Gardening (I)	SESSION 16 CPR (I)	Independent Study Presentation Time
			SESSION 18: Introduction to Cost Benefit Analysis (IV & V)
P.M.	Construction (continued)	SESSION 17: Dryer Assessment and Modification (V)	SESSION 19: Presentation of Solar Dryers (III)
	SESSION 15: Practical Drying Tips (IV)		SESSION 20: Introduction to the Final Phase of the Training Program (III)

Session 1. Introduction To Agricultural Dryers[edit | edit source]

Total time:	3 hours
Objectives:	To discuss the phase schedule To compare and contrast food storage techniques To discuss solar agricultural food drying as a potentially appropriate technology To discuss relationships between food storage and culture
Resources:	Attachment V-1, "The Potential of Solar Agricultural Dryers in Developing Areas" Farallones, "Solar Agricultural Dryers Slide Show" Brace Research Institute, A Survey of Solar Agricultural Dryers ISES, "Sunworld," 1980 Vol. IV, No. 6, pp. 180, 181
Materials:	Slide projector, screen, notebooks, pens, pencils, newsprint and felt-tip pens

Procedures:

Trainer Notes This session requires considerable preparation in setting up the slide show, copying the attachment and putting the phase schedule on newsprint.

Step 1. (5 minutes)

Present the objectives and outline the session activities.

Step 2. (15 minutes)

Present and discuss the phase schedule.

Trainer Notes Make changes in the schedule to meet the participants needs. Be flexible.

Step 3. (20 minutes)

Have the participants form small groups and brainstorm a list .of global food storage and preservation techniques.

Trainer Notes Circulate among the groups to offer suggestions.

Step 4. (40 minutes)

Reconvene the groups. Post, review and discuss the list of food storage and preservation techniques.

Perception of food as a commodity, instead of a nutrient

Step 5. (10 minutes)

Distribute Attachment V-1 and have the participants read it.

Step 6. (20 minutes)

Briefly discuss the history, development and use of agricultural drying throughout the world and the possible use of solar agricultural dryers as potentially appropriate technologies.

Trainer Notes Consult the resources listed in this session and those in the bibliography for background information.

Step 7. (10 minutes)

Explain solar agricultural dryer nomenclature and the food drying microclimate.

Trainer Notes Sketch section and perspective views of a generic dryer and ask people to help label the parts: drying chamber, trays, solar pre-heater, solar chimney, glazing, insulation, etc. Sketch a typical seed to demonstrate how drying occurs. (The participants have already discussed the three types of heat transfer (Phase II: Session 7 and Phase II: Session 16) and can help note where each type of heat transfer occurs in the seed.)

Step 8. (10 minutes)

Break.

Step 9. (45 minutes)

Present the Farallones Solar Agricultural Dryer slide show.

Trainer Notes During the slide show, comment on both the technical details and the socio-economic aspects of the solar dryers.

Step 10. (20 minutes)

Discuss the potential impact of a new preservation and storage technology, such as solar agricultural dryers, on people in developing countries.

Trainer Notes Points to consider are: Who is likely to build the dryer? Who is likely to use the dryer? Will there be any conflict between the builder and the user? How can the technology be readily accepted? Should dryers be small- or large-scale? Who is likely to pay for a dryer?

Step 11. (10 minutes)

Review and evaluate the session.

Trainer Notes Distribute a file card (3" x 5" or 7.5 cm x 12.5 cm) to each participant and ask for an evaluation of the session: What went well What didn't go well How the session could have been better When the cards are filled out, ask participants to share their comments.

THE POTENTIAL OF SOLAR AGRICULTURAL DRYERS IN DEVELOPING AREAS

Presented to the UNIDO Conference

Vienna, Austria

February 14-18, 1977

by

T. A. Lawand

Brace Research Institute

Macdonald College of McGill University

Ste. Anne de Bellevue, HOA 1 CO

Quebec, Canada

Introduction

One of the oldest uses of solar energy since the dawn of civilization has been the drying and preservation of agricultural surpluses. The methods used are simple and often crude but reasonably effective. Basically, crops are spread on the ground or platforms, often with no pre-treatment, and are turned regularly until sufficiently dried so that they can be stored for later consumption. Generally little capital is required on the expenditure of equipment but the process is labor intensive.

There is probably no accurate estimate of the vast amounts of material dried using these traditional techniques. Suffice it to say it is a widespread technology practiced in almost every country of the globe and at nearly every latitude. Diverse products such as fruit, vegetables, cereals and grains, skins, hides, meat and fish and tobacco are dried using these simple techniques.

These technologies have originated in many of the developing countries so there is no major social problem in their acceptance or in the use by the local popoulations of dehydrated foods for consumption. There are several technical problems, however, with the process. They are:

Intermittent, affected by cloudiness and rain

Subject to insect infestation

Affected by high levels of dust and atmospheric pollution

Affected by the intrusion from animals and man

In the more advanced segment of the society, whether in developing areas or in industrialized regions, artificial drying has in many cases surplanted traditional sun drying in order to achieve better quality control, reduce spoilage and in general cut down on the losses and inefficiencies engendered by the above difficulties.

The relatively high cost of labor in most industrialized areas and the hitherto, until recently, inexpensive costs of fossil fuels permitted the development of the artificial, generally large scale, drying processes to be evolved. The cost of dehydration was added to the cost of selling the process materials. The advent of higher charges for fossil fuels as well as the danger of depletion and scarcity of these fuels has stimulated renewed interest in solar agricultural dryers.

It is estimated according to the FAO World Book that the amount of agricultural produce dehydrated in 1968 using solar energy amounted to 225 million tons. In that year alone, Australia exported over 72 thousand tons of sun-dried foods worth over 27 million dollars. If all this drying, or even part of it, were to be done using fossil fuels, it would put an even greater strain on our already limited reserves. Over the past three decades, increasing interest has been paid to the development of solar agricultural dryers which make use of known principles of heliotechnology in order to combat some of the principal disadvantages of classical sun drying.

In evaluating technologies which might be amenable to applications in developing areas, one should distinguish between small and large scale operations. In general, small scale systems would be used in those areas where land holdings are not large, with the result that individual farmers, fishermen and herdsman only produce modest amounts of surplus products. The objective is to dehydrate these surpluses for use often only by the family of the producer or for sale in the local market in the immediate vicinity. At times, small scale surpluses of certain products such as peanuts or rice are delivered to central facilities for processing, dehydration and eventual marketing. These systems are generally well established and require a fair degree of organization in the industry. In many instances, these amalgamated handling facilities do not exist. Therefore, in providing an overview of some of the technologies, one must differentiate between the existence of commercial and physical infra-structures within a given locality.

Larger scale systems invariably require the use of an external power source. Where conventional electric power supplies are available, reliable and not excessive in cost, it is logical to utilize these external sources for the operation of fans and blowers, vents and duct baffles in order to increase the efficiency and operating performance of a solar agricultural system. Some dryers are of the portable, powered type, wherein solar air heater collectors are fitted with electrically powered fans (this could be tone using gasoline or diesel engines as well) and are taken directly to the areas of production for in-situ drying. Traditionally, this process was used with fossil fuel, often butane or propane gas, as the energy source. As the price of these systems increases, there has been a tendency to develop systems of this nature relying on solar energy to provide the bulk of the energy required for dehydration. In fact, in some instances, fossil fuels are used to supplement these solar collectors in order to maintain optimum operating conditions in a system partially operated by solar energy.

The other major category applicable for dehydration in the industrialized sectors of developed and developing nations, is to use the roof area of existing buildings as the solar collector, fitting the buildings with suitable blowers, ducts, collectors and often storage mechanisms. In the United States of America, a number of activities along these lines have been developed and interest has been generated in some of the prestigious industrial and academic institutions in the country. An example of this is the project funded by the United States government where solar energy is used as a substitute in dehydration for natural gas. This project is being undertaken by California Polytechnic University and TRW Systems. They indicate that the State of California alone produces annually over 450 million dollars in dried fruit and vegetables. Their system will no doubt become increasingly cost-effective as the cost of fossil fuels and the electricity generated by them continue to escalate. (Ref. Solar EM: -1976, October.) Another system receiving increasing interest in this field both in developed and the developing regions is the use of greenhouses to dehydrate surplus produce. This combined effect of drying and greenhouse operations has much validity and has to be examined for each particular set of circumstances. A number of studies have been undertaken in this regard for specialized crops. Finally, an older but certainly no less valid system has been the use of heat extracted from the underside of roofs. This has proven quite satisfactory in providing some dehydration potential in a number of applications. This is one of the oldest applications in solar agriculture drying.

Solar Energy Magazine

Technical Characteristics of Solar Agricultural Dryers

There are two principal aspects of this process:

The solar heating of the working fluid (generally air).

The drying chamber wherein the heated air extracts moisture from the material to be dried.

The solar heating aspect can in turn be subdivided into two categories:

Separate solar air heater collectors using natural or forced convection to preheat the ambient air and reduce its relative humidity.

Direct, in situ heating of air which in turn directly dehydrates the produce.

The sun drying principles have been well described by Lof and others in earlier literature and in some instances are less well understood than commercial dehydration.

A discussion of drying theory is beyond the scope of this paper but a few principles may be advantageously outlined here. These are particularly applicable to direct radiation drying, inasmuch as the principles involved in the drying of materials in various types of opaque enclosures by means of hot air, whether from a solar heater or some other type of heating unit, are well outlined in the drying literature. The first requirement is a transfer of heat to the surface of the moist material by conduction from heated surfaces in contact with the material, or by conduction and convection from adjacent air at temperatures substantially above that of the material being dried, or by radiation from surrounding hot surfaces or from the sun. Absorption of heat by the material supplies the energy necessary for vaporization of water from it, 590 calories per gram water evaporated. Water starts to vaporize from the surface of the moist material when the absorbed energy has increased the temperature enough for the water vapor pressure to exceed the partial pressure in the surrounding air. Steady state is achieved when the heat required for vaporization becomes equal to the rate of heat absorption from the surroundings.

To replenish the moisture removed from the surface, diffusion of water from the center to the surface of the drying material must take place. This may be a rapid or a slow process, depending upon the nature of the material being dried and upon its moisture content at any time. It may thus be the limiting rate in the drying operation, or if moisture diffusion is rapid, the rate of heat absorption on the surface or the rate of vaporization may be the controlling factor. In some very porous materials, vaporization may take place even below the apparent surface of the material, vapor then diffusing through pores in the solid.

In the case of direct radiation drying, part of the radiation may penetrate the material and be absorbed within the solid itself. Under such conditions, heat is generated inside the material as well as at the surface and thermal transfer in the solid is facilitated.

For economic reasons, maximum drying rates are usually desired. Product quality must be considered, however, and excessive temperatures must be avoided in many materials. In addition, because drying occurs at the surface, those materials which have a tendency to form hard, dry surfaces relatively impervious to liquid and vapor transfer must be dried at a rate sufficiently low to avoid this crust formation. Close control of heat transfer and vaporization rates, either by limiting the heat supply or by control of the humidity of the surrounding air, must be provided.

The drying of a product simply by permitting relatively dry air to circulate around it, without the use of any direct or indirect heat source, is known as adiabatic drying. The heat required for vaporizing the moisture is supplied by the air to the solid material, thereby reducing the air temperature while increasing its absolute and relative humidity Because of the low heat capacity of air, in comparison with the high latent heat of vaporization of water, large volumes of air at reasonably low relative humidity must be used in this type of drying process. Air leaving the drier is nearly saturated with water at the wet-bulb temperature. The air supply, at its initial dry-bulb temperature, and humidity is thus cooled and humidified toward its wet-bulb temperature, while the moist solids in contact with this air approach the wet-bulb temperature also.

The foregoing generalization must be somewhat modified if the materials being dried are at all soluble in the water present. Fruits and other agricultural products contain salts and sugars which cause a lowering of the vapor pressure. The surface temperatures of these materials must therefore be higher than the wet-bulb temperature of the air in order for vaporization to take place. This means that the adiabatic drying of these solids requires air at lower relative humidities than do the materials having no solutes in the aqueous phase.

An important property of materials processed by direct radiation drying is their absorptivity for radiation. Fortunately, most solids have relatively high absorptivities but they may change as drying proceeds, the surfaces of the materials becoming less or sometimes more "black" during the process. Also, there may be changes in opacity of the surface of the materials which are partially transparent to some of the wave lengths in the spectrum of the radiant source.

The thermal conductivity of the material is also an important property, particularly if the solids are dried in a layer of sufficient depth to require conduction of heat from particle to particle. If the thermal conductivity is poor, circulation of heated air through and between the particles of moist solid would permit better heat transfer than direct radiation on the surface of a relatively deep bed of particles.

In larger scaled dehydration systems, forced convection, generally powered with an external, non-renewable power source, increases the diffusion transfer of moisture and, if properly applied, increases the rate of dehydration and the quality of the produce. These systems are well documented in the literature.

Session 2. Tour of Solar Dryers[edit | edit source]

Total time:	1 hour
Objective:	To examine and discuss existing solar dryers
Materials:	A variety of solar dryers, a bee smoker or other smoke source and dried fruit or vegetables

Trainer Notes The following preparation will be necessary for this session: Construct a variety of solar dryers to be used as samples. If there are dryers already available, arrange and repair them so that they can be demonstrated efficiently. Put some food into the dryers one or two days in advance so that participants can see how the dryers work.

Procedures:

Step 1. (5 minutes)

Review the session objective and activity.

Step 2. (55 minutes)

Demonstrate and explain each of the sample solar agricultural dryers that are available.

If it is a sunny day, blow smoke into the dryers and have participants note the air flow pattern and rate.

Session 3. Solar Agricultural Dryer Design Procedures and Rules of Thumb[edit | edit source]

Total time:	2 hours
Objectives:	To examine and discuss the technical design procedures for solar dryers
	To review and discuss the rules of thumb for solar dryer design
Resources:	Attachment V-3-A, "Technical Design Information for Solar Dryers"
	Attachment V-3-B, "The Psychrometric Chart"
	Attachment V-3-C, "Design Rules of Thumb for Solar Dryers"
	ISES, "Sunworld," 1980, Vol. 1/No. 6, pp. l8081
Materials:	Thermometer, gauze, rubberbands, string, newsprint and felt-tip pens

Procedures:

Step 1. (5 minutes)

Present the objectives and outline the activities.

Step 2. (1 hour)

Distribute Attachment V-3-A. Review-and discuss the attachment and the key variables in dryer design.

Trainer Notes Post and review the following key variables in solar dryer design: Vent Area - the area, in square centimeters, of the lower (intake) or upper (exhaust) vent, whichever is smaller Solar Gain - the amount of solar heat being absorbed by the collector, in Kgcal/hr (found by multiplying the hourly insolation rate, given in Kgcal/m² hr. by the aperture or area of the solar collector, given in m ) Height - the distance between the top of the lower vent and the bottom of the upper vent Change in Temperature (At) - the difference, in degrees centigrade, between -the exhaust air temperature (or the maximum allowable temperature for the agricultural product) and the ambient, or inlet, air temperature Guide the participants through the formulas in the attachment, encouraging their questions and comments. Ask how each formula is applied to dryer design. Explain to those people who are having trouble with the mathematics that you will be discussing more general rules of thumb for these same mathematical formulas and that it is not necessary to understand mathematics to design successful solar dryers.

Step 3. (20 minutes)

Distribute Attachment V-3-B and have the participants read and discuss it. Fashion a simple sling psychrometer and demonstrate its use.

Trainer Notes Explain wet and dry bulb temperatures and the psychrometric chart. To fashion the sling psychrometer, fasten wet gauze to the bulb of a thermometer, tie it to a cord, and twirl the thermometer at the end of the cord. Review the psychrometric chart, Attachment V-3B. Explain that the chart can be used anywhere in the world. Demonstrate how the chart can be used to diagram what happens during the drying process (See Section F. Attachment V-3-A).

Step 4. (20 minutes)

Distribute and review Attachment V-3-C, "Design Rules of Thumb for Solar Dryers."

Trainer Notes Explain that in order to design a successful solar dryer, you only need to understand the rules of thumb and the interaction of the key variables in dryer design.

Step 5. (15 minutes)

Review and discuss the session activities and objectives.

Trainer Notes Explain that the participants now have the necessary technical information for solar dryer design. Explain that they will have an opportunity to use this technical information when they design their solar dryers. Encourage the participants to think how this information might be communicated to people with little or no formal education.

TECHNICAL DESIGN INFORMATION FOR SOLAR DRYERS

A. How to find Percent moisture content (wet basis) Where:

M = percent moisture

w = weight of wet sample

d = weight of dry sample*

dry = oven dried, 222°C (450°F) for 48 hours

Example:

10 kg of fresh fruit which weigh 6 kg when dry.

w = 10 kg

d = 6 kg B. Energy Balance for Drying

The Energy Balance is an equation which expresses the following idea mathematically:

The energy available from the quantity and temperature of air going through the dryer should be equal to the energy needed to evaporate the amount of water to be removed from the crop.

The formula is: m_ac_p(T_i-T_f)=m_wL

Where:

m_a = mass (or weigh) of drying air

c_p = specific heat capacity of the air (i.e., how much heat it holds per degree of temperature rise)

T_i = initial temperature

T_f = final temperature

L = Latent heat of vaporization of water from grain (amount of heat needed to vaporize each unit of water)

m_w = mass (or weight) of water to be removed by evaporation

NOTE: The task in solar dryer design is to figure and then achieve high enough temperatures (Tf) and air flow to remove the specified amount of water (mw).

C. How to figure how much water (m_w) must be removed from your crop:

The formula: M_w<nowiki>= mass (weight of water to be removed

w_i = initial mass (weight) of crop to be dried

M_i & M_f = initial and desired final % moisture of the crop

Example:

How much water must be removed from 100 kg of groundnuts in reducing from initial moisture of 26% to final moisture of 14%?

Substituting: D. Two Constants:

Latent heat of vaporization of water (L):

Amount of energy needed to vaporize (evaporate) each unit (gram, pound, etc.) of water from the crop.

For free water (in open pan), it's about 2,400 KJ* /kg For water from crops, it's more

and varies a bit with temperature

and moisture content: 2,800 KJ/kg.

KJ = kilo joules

1 KJ = 1 BTU or 1/4 Kcal

Specific heat capacity of air (C_p):

Amount of heat air can hold per degree of its temperature rise.

Varies a bit with humidity and temperature.

For this, use: 1.02 KJ/Kg° C

Example:	How much heat is given up if the temperature of 3 kg of air drops from 40 to 35° C?
	<nowiki>=1.02 KJ x 3 kg x (40-35°)
	<nowiki>=1.O2 (3x5)
	<nowiki>=15.3 KJ

E. How to figure volume (V) of air from weight:

Air is usually quantified as volume at atmospheric pressure (P) and temperature (t).

The formula: PV = m_aR_t

Where:

P = Pressure (in kilopascals - kPa)

V = Volume (m³)

t = temperature (degrees kilvin)

m_a = the mass (weight of air)

R = A constant factor, it equals about 0.291 kPa m³/kgk under dryer conditions

The Rule of Thumb is:

1 kg air at 35°C and normal pressure = 0.9 m³ or use psychrometric chart Useful Solar Dryer Formulae:

THE PSYCHROMETRIC CHART

The upper curve of the chart is for saturated air and is label led wet-bulb and dewpoint temperature. (The word "dewpoint" arose from the observation that dew forms on grass when the grass cools, by radiaiton to the sky, to a temperature equal to or less than the wet-bulb temperature of the air above it.)

The other curves on the psychrometric chart that are similar in shape to the wet-bulb line are lines of constant relative humidity (in X). By definition, relative humidity is a ratio: the partial pressure of the water vapor at a given temperature - the saturation pressure of the water vapor at the same temperature. The scale at the left side of the chart gives the pressures. Graphic

The straight lines sloping gently downward to the right are lines of constant wet-bulb temperatures. The intersection of a dry-bulb and a wet-bulb line gives the state of the air for a given moisture content and relative humidity. The lines of constant wet-bulb temperature also give values of constant enthalpy (total heat content), measured in heat units per unit weight of dry air.

Other lines sloping more steeply to the right give the specific volume of dry air, the volume occupied by one kilogram of dry air under the indicated conditions.

In examining a psychrometric chart, note that:

Processes in which air is heated or cooled without change in moisture content give horizontal lines. Heating along such lines will decrease the relative humidity, while cooling will increase it.

The wet-bulb temperature lines, sloping downward to the right, are lines of adiabatic cooling (where there is no change in heat content). These lines typify drying processes in which air is passed over the surface of wet material and is cooled by evaporation of water from the material. Lines of constant total heat parallel these wet-bulb tines.

Although no processes follow the lines giving the specific volume of dry air, these lines show that at any given dry-bulb temperature, the density of air decreases as either the temperature or the relative humidity rises.

DESIGN RULES OF THUMB FOR SOLAR DRYERS

A. Assorted considerations for solar agricultural dryer designs:

1 kg of air at 35°C @ 0.9 m³

For grain drying, make beds no more than l5 cm thick, giving a maximum loading rate of 9Okg/M² (requires stirring).

Tropical-monsoon insolation of 5-25 MJ* /M² per day. Use 15 MJ/M² per day for estimate (approximately 14,000 BTUs or 3,500 kcal).

Typical conservative day long efficiency of stationary collection: -25%

(That is, the energy delivered as heated air to the drying crop is 25% of the energy in the sunlight striking a horizontal surface of equal area to the dryer's collector.)

MJ = Mega Joule or 1 million joules.

B. Collector size:

Making the collector equal to three times the tray area gives a high drying rate dryer.

C. Dryer capacity:

In the tropics, figure on about 180 M of air to remove 1 kg of water.

Figure about 3/4 M² of collector area to remove 1 kg of water per day (i.e., dry 1.5 kg fresh fruits or 5.25 kg grain per day).

D. Dryer temperature:

1. Depends upon insolation, collection area and vent size.
2. Is very sensitive to vent size (cutting vent size by one half increases delta t by about three times (up to some limit).
3. Doubling area of collector increases it by about one half.
4. Raising temperature from 20 to 35°C can triple the water capacity of the air.

E. Dryer air flow rate:

1. Doubling vent area doubles the air flow rate (but drops delta by about 3/4).
2. Doubling height increases air flow by 0.4.
3. Doubling collector area increases air flow by about 40% (also increases At by 1/2).

F. Required moisture contents of crops/approximate values:

	For Storage	Fresh
Fruits	10%	70 - 85%
Vegetables	18%	70 - 85%
Grains	14%	25 - 35

G. Figuring how much air you need for drying:

There are two methods: using the psychrometric chart or using the energy balance equation.

Method #1. Using the Psychrometric Chart

Example:

You want to dry 1 kg. of rice from initial moisture of 22X to final moisture of 14%. Assume ambient air temperature is 30°C at 80X humidity and you pre-heat the air to 45 for drying.

The path A-B represents the heating process. Note that in moving the temperature to 8, the humidity drops to 35%.

The path B-C represents the change in the air as it passes through the dryer, cooling and picking up moisture from the rice. Initially (because rice is quite wet), air gets to C. At the end of the process, it only reaches D. As the rice gets dryer, you find points C & D from the table* of equiibrium moisture contents (it's similar for all crops). In this case, the air's humidity ratio rose by about 0.005. (That's how much water the air carried away.)

Equilibrium moisture content of rough rice, per cent wet basis.*
Temperature	Relative humidity of air (%)
(°C)	20	30	40	50	60	70	80	90
10	8	9	11	12	13	14	16	19
20	7	9	10	11	13	14	15	18
30	7	8	9	11	12	13	15	17
40	6	7	8	10	11	12	14	17

Moisture level at which rice will stabilize if exposed to the specified temperatures and humidity conditions.

The amount of water to be extracted from 1 kg of rice in this case can be figured using the equation found in Part C of Attachment A. From the definition of humidity ratio (weight of water vapor in the air - the weight of dry gases in the same air), it follows that the mass of air needed (ma) in this case, where humidity ratio rose by 0.005, is: We can transform this weight to volume with the equation from Part E of Attachment A:

PV = m_a RT

When

P = 101.3 (normal sea level)

T = 308 (35°C)

Then Method #2. Usinq Energy Balance Equation (See Example)

We have calculated above that the amount of water to be removed (M_w) = 0.093 kg.

We know the two constants:

1. Latent heat of vaporization (L) = 2,800 KJ/kg
2. Specific heat of air (C_p) = 1.02 KJ/kg°C

Assuming initial temperature (T_i) = 45°C and final temperature is a mean value of 32°C, we can substitute in the energy balance equation to get m_a:

[[File:]]

We can transfer that to M³ using our rule of thumb (1 kg @ 0.9 M³) or PV = M_aRT and we get about 17.3 M³ of air.

You will notice that this result is not identical to the 16.5 M³ calculated above using the psychrometric chart. However, the result is close enough for design work.

H. To figure air flow rate:

Example:

Say we want to dry 1,000 kg of rice. We've figured it takes 17 M³/kg, so that's 17,000 M total. If we want this to flow in 30 hours (say, four 7-1/2 hour solar days), that's:

17,000/30 or 566-2/3 M³/hr. or 9.44 M³/min.

I. To figure area of solar collector needed:

You must determine:

1. Mass of water to be evaporated (M_w)
2. Specific latent heat of vaporization of water from the crops (L) = 2,800 KJ/kg
3. The quantity (Q) of insolation per unit horizontal area per day
4. The efficiency at the collector (e )

Example:

For 1,000 kgs. of rice, we calculated that we must remove 93 kg. of water. We know L = 2,800 KJ/kg. So the heat required is 93 x 2,800 = 260,400 KJ (260.4 MJ)*

J = Megajoule (1,000,000 joules or 1,000 kilojoules)

This heat must come from the available solar energy.

Tropical monsoon insolation is highly variable, depending upon cloudiness: from 5 to 25 MJ/M² per day.

Use 15 MJ/M² per day as a conservative average in absence of data.

Assuming 15 MJ/M² per day and 25% efficiency of the collector yields 3.75 MJ/M² per day or 15 MJ/M² in four days.

So, the total area of collector required is: J. How to figure vent area. using two methods:

Method #1:

If you have the required flow rate figured (See Section H.), use this formula: Example:

Assume air flow calculations showed a flow rate of 9.4 M³/min required to dry our 1,000 kg or rice in four days (review Section H.). Then checking data sources, assume that 'he desired temperature of the drying rice is 62 C and that the ambient temperature is 30°C. So delta temperature (change in air temperature in the dryer) is 62° - 30° = 32°. Assume a height Of 4m for the dryer. Substitute in the equation: Method #2:

If you have an aperture (collector) area and some idea of solar intensity, use this formula: Assume that a maximum of 15% of the total daily radiation falls in the hottest mid-day hour. This is 0.15 x 25 MJ = 3.75 MJ/hr M² = 896 Kcal/ hr. m² *

1 MJ/m² = 239 Kcal/m² = 88 BTu/m²

Using the aperture area found in Section I, it's 17.5 m². Let D t = 32ºC and h = 4M as above.

Always assume a high insolation rate so your vents will be large enough to prevent over-heating, even under the most intense sun conditions. You can always close the vent to some degree, if necessary.

Then, substituting the formula: Note: This is the maximum vent area you would ever need. With a lower insolation rate of 15 MJ/day, 2 the vent area could be cut down to about 2,600 CM².