SolarDrying.ppt

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Importance of solar drying

• _{The world population is more than 6 billion and}_{The world population is more than 6 billion and} about 800-900 million people do not have enough

about 800-900 million people do not have enough

food to eat.

• _{There are three methods to solve hunger}_{There are three methods to solve hunger} problem:

problem:

– Increase food production_{Increase food production} – Reduce population growth_{Reduce population growth}

– _{Reduce loss of food during and after harvesting}_{Reduce loss of food during and after harvesting}

• It has been estimated that world as a whole more _{It has been estimated that world as a whole more}

than 20-30 percent food grains and 30-50 percent

than 20-30 percent food grains and 30-50 percent

vegetables, fruits/fish etc. are lost before it

reaches to the consumers.

• Drying is a traditional method for preserving _{Drying is a traditional method for preserving}

food. Solar drying is an effective method to

food. Solar drying is an effective method to

preserve food.

• Solar energy is diffuse in nature and thus _{Solar energy is diffuse in nature and thus}

suitable for crop drying, locally available and

thus saves transportation, solar dryers can be

made locally of any size and capacity and solar

made locally of any size and capacity and solar

dryers are economical if cash crops are dried.

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Advantages in Favour of Crop

Drying

•

It permits early harvesting and reduces the

field losses of the products,

•

Reduces the risk of field losses caused by wild

animals,

•

It permits better planning of harvesting

season,

•

It reduces spoilage in storage drastically,

•

It permits the farmer to sell his product at

better price during early period of harvesting

season,

•

Quality of the product gets enhanced

significantly and hence farmer gets more

money for his product, and

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OPEN SUN DRYING

• It is the simplest method of drying used in most _{It is the simplest method of drying used in most}

developing countries.

developing countries.

• The food / crop is spread out in the open under _{The food / crop is spread out in the open under}

the sun.

Advantages

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(1) No technology involved No technology involved (2)

(2) Very low cost (cost of labour only) Very low cost (cost of labour only)

Disadvantages

(1) Contamination of the product due to dirt and

insects.

insects.

(2) Wastage by birds / mice.

(3) Spoilage due to sudden and unpredicted rain.

(4) There is no control of temperature over crop

drying.

(5) Overdrying may cause loss of germination

(5) Overdrying may cause loss of germination

power, nutritional changes, sometimes complete

damage.

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PARAMETERS FOR SOLAR

DRYING

• The drying of product depends on external variables _{The drying of product depends on external variables}

like temperature, humidity and velocity of air stream

like temperature, humidity and velocity of air stream

and internal variables which is a function of drying

material and depends on parameters like surface

characteristics (rough or smooth surface), Chemical

characteristics (rough or smooth surface), Chemical

composition (sugar, starch, etc.), physical structure

(porosity, density, etc.). and size and shape of the

product. The rate of moisture movement from the

product inside to the air outside differ from one

product inside to the air outside differ from one

product to another and very much depends weather

the material is hygroscopic or non-hygroscopic. Non-

hygroscopic materials can be dried to zero moisture

level while the hygroscopic materials like most of

level while the hygroscopic materials like most of

the food products will always have a residual

moisture content.

• The design of a solar dryer depends on : solar _{The design of a solar dryer depends on : solar}

radiation, temperature of air, relative humidity of

radiation, temperature of air, relative humidity of

air, moisture content of the product, amount of

product to be dried, time required for drying,

availability of auxiliary energy, material of

construction of dryer and the resource availability.

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PHYSICS OF DRYING

•

Heat by convection and radiation to Surface of

product



Goes to interior of product

•

Increase in temperature

•

Formation of water vapour



Evaporation of moisture from Surface

Drying can be accelerated by:

•

Increasing flow rate of air

•

Increasing temperature of drying air

•

Initial Drying

- Surface drying, later on drying

depends on type of materials.

•

Non hygroscopic

- drying possible upto zero

moisture content.

•

Hygroscopic

- grains, fruit, food stuff have

residual moisture.

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RATIONALE FOR CONTROLLED

DRYING

1. Grain 1. Grain

• Improves product quality, _{Improves product quality,}

• _{Improves storage capability,}_{Improves storage capability,}

• _{Reduces time and space requirement for drying,}_{Reduces time and space requirement for drying,}

• Facilitates quick preparation of fields for next cropping, _{Facilitates quick preparation of fields for next cropping,}

• _{Facilitates wet season harvesting and storage,}_{Facilitates wet season harvesting and storage,} • Improves drying hygene. _{Improves drying hygene.}

2. Timber 2. Timber

• _{Improves product quality,}_{Improves product quality,}

• Reduces period capitoltied up in drying stock, _{Reduces period capitoltied up in drying stock,}

• _{Improves low expertise, low capital, improved drying}_{Improves low expertise, low capital, improved drying}

options, options,

• Expands range of usable timber species, _{Expands range of usable timber species,}

• Improves attainable drying level. _{Improves attainable drying level.} 3. Fruits, Vegetables & Fish

3. Fruits, Vegetables & Fish

• Reduces product seasonability, _{Reduces product seasonability,}

• _{Improves marketing control of farmer,}_{Improves marketing control of farmer,} • Reduces spoilage, _{Reduces spoilage,}

• Improves drying hygene, _{Improves drying hygene,}

• _{Improves storage capability,}_{Improves storage capability,}

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EARLY WORK ON SOLAR DRYING IN

INDIA

1.

1. In NPL, New Delhi in 1954 solar heated air was used In NPL, New Delhi in 1954 solar heated air was used for drying of coal fines.

for drying of coal fines.

2.

2. In NPL, New Delhi in 1955, plane glass mirror In NPL, New Delhi in 1955, plane glass mirror

concentrators with overall dimensions of 1.83m x 0.91

m were used for making jaggery from sugar cane and

palm juice.

palm juice.

3.

3. Khadi & Village Industries Commission (KVIC), Khadi & Village Industries Commission (KVIC),

Ahmedabad in 1968, using similar mirrors erected a

pilot plant which dehydrates palm nira and turns it

into gur or syrup.

into gur or syrup.

4.

4. Forest Research Institute (FRI), Dehradun, Developed Forest Research Institute (FRI), Dehradun, Developed a 7.1 cu.m, capacity timber seasoning kiln in 1972.

a 7.1 cu.m, capacity timber seasoning kiln in 1972.

5.

5. Central Arid Zone Research Institute (CAZRI), Jodhpur Central Arid Zone Research Institute (CAZRI), Jodhpur developed a solar cabinet dryer in 1972 and tested it

developed a solar cabinet dryer in 1972 and tested it

for several years for drying fruits and vegetables.

for several years for drying fruits and vegetables.

6.

6. Annamalai University, in the year 1978 developed one Annamalai University, in the year 1978 developed one ton per day solar paddy dryer.

ton per day solar paddy dryer.

7.

7. National Industrial Development Corporation (NIDC) National Industrial Development Corporation (NIDC) of India developed several solar grain dryer in 1980

of India developed several solar grain dryer in 1980

and put to use in few cities in India.

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REQUIREMENTS FOR SOLAR ENERGY

AS AN ALTERNATIVE OF HIGH

ENERGY DRYERS

•

Capacity of solar dryers must be

equivalent to fossil fuel based dryer

•

The labour input for solar dryer

operation should not increase

•

Solar dryer operation must be

independent of weather conditions

•

With solar dryer the quality of dried

product should not be lowered

•

The operating conditions should be

reliable

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Crops

Crops Initial moisture Initial moisture content (%) wb

content (%) wb

Final moisture

content (%) wb

Water removed

(kg/t of dried

product) product) Energy Energy requirement requirement (10

(1066_kJ/t)_kJ/t)

GRAIN

GRAIN

Wheat, barley, rye,

Wheat, barley, rye,

oats, paddy Corn

20-25 20-25 25-45 25-45 14-16 14-16 12-14 12-14 50-147 50-147 147-600 147-600 0.30-0.88 0.30-0.88 0.80-3.60 0.80-3.60 VEGETABLES VEGETABLES

Peas, beans Potatoes

Peas, beans Potatoes

Onion, Garlic, Onion, Garlic, Carrot, beets, Carrot, beets, Cabbage, Tamatoes Cabbage, Tamatoes 60-70 60-70 65-85 65-85 70-80 70-80 80-90 80-90 90-95 90-95 5-10 5-10 14 14 5-10 5-10 5-10 5-10 5-10 5-10 1250-2157 1250-2157 1458-4733 1458-4733 2000-3750 2000-3750 3500-8500 3500-8500 8000-3300 8000-3300 7.50-13.00 7.50-13.00 8.75-28.40 8.75-28.40 12.00-22.50 12.00-22.50 21.00-51.00 21.00-51.00 48.00-108.00 48.00-108.00 FRUITS FRUITS Apples, Apples, Apricots, peaches, Apricots, peaches, prunces, grapes, prunces, grapes, figs, banana figs, banana 75-80

75-80 14-2314-23 2080-33002080-3300 12.48-19.8012.48-19.80

FORAGES

FORAGES

Hay

Grass, alfala

Grass, alfala 40-60 40-60 80-90 80-90 10-14 10-14 10-14 10-14 433-1250 433-1250 3300-8000 3300-8000 2.60-7.50 2.60-7.50 19.80-48.00 19.80-48.00

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CLASSIFICATION OF SOLAR

DRYERS

• DIRECT TYPE DRYERSDIRECT TYPE DRYERS : In direct or natural convection : In direct or natural convection type dryers, the agricultural product is placed in shallow type dryers, the agricultural product is placed in shallow layers in a blackened enclosure with a transparent cover. layers in a blackened enclosure with a transparent cover. The solar radiations are directly absorbed by the product The solar radiations are directly absorbed by the product itself. The food product is heated up and the moisture itself. The food product is heated up and the moisture from the product evaporates and goes out by the natural from the product evaporates and goes out by the natural convection/circulation.

convection/circulation.

• INDIRECT TYPE DRYERSINDIRECT TYPE DRYERS : In these dryers the food : In these dryers the food

product is placed in a drying chamber. The air is heated in product is placed in a drying chamber. The air is heated in solar air heaters and then blown through the drying

solar air heaters and then blown through the drying chamber. In some of the designs, dryers receive direct chamber. In some of the designs, dryers receive direct

solar radiations and also heated air from solar air heaters. solar radiations and also heated air from solar air heaters. In these dryers manipulation of temperature, humidity and In these dryers manipulation of temperature, humidity and drying rate is possible to some extent.

drying rate is possible to some extent.

• FORCED CIRCULATION TYPE DRYERS_{FORCED CIRCULATION TYPE DRYERS} : In these dryers, : In these dryers, hot air is continuously blown over the food product. The hot air is continuously blown over the food product. The food product itself is loaded or unload continuously or food product itself is loaded or unload continuously or periodically. These kind of dryers are comparatively periodically. These kind of dryers are comparatively

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(c) Forced circulation type solar dryers (a) Direct type solar

dryers

(b) Indirect type solar dryers

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Drying terminology

• Percent moisture content, dry basis :

The mass ratio of water to dry solid multiplied by 100.

%

100

)

(







d

W

dry

M

where, W is the wet mass and d is dry mass of the product. • Percent moisture content, wet basis:

The mass fraction of water in the commercially dry solid multiplied by 100.

%

100

)

(







W

d

W

wet

M

• Bound moisture :

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• _{Unbound moisture}_{Unbound moisture}_:_:

The moisture contained by a material which

The moisture contained by a material which

exerts an equilibrium vapour pressure equal to

that of pure liquid at the same temperature.

• _{Equilibrium moisture}_{Equilibrium moisture}_:_:

The amount of moisture in the material that is

The amount of moisture in the material that is

in the equilibrium with its vapour in the gas

phase. For a given temperature and humidity

conditions, the material cannot be dried below

its corresponding equilibrium moisture

its corresponding equilibrium moisture

content.

• Hydrogroscopic material _{Hydrogroscopic material}::

A material that may contain bound moisture in

small capillaries.

• _{Non-hydroscopic material}_{Non-hydroscopic material}_:_:

The material which cannot hold moisture in the

The material which cannot hold moisture in the

bound form.

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• Constant rate periodConstant rate period: :

The part of the drying process during which the drying rate is constant and is

controlled by external rather than internal conditions.

• Falling rate periodFalling rate period: :

The part of the drying during which the drying rate varies with time. Internal

The part of the drying during which the drying rate varies with time. Internal

factors, i.e., physical and transport properties of the material, control the

drying process.

• External drying factors_{External drying factors}: :

The independent variables associated with the conditions and flow of gas

The independent variables associated with the conditions and flow of gas

phase.

• Internal drying factorsInternal drying factors: :

The properties of the material that influence the transport of heat and mass

within the phase.

within the phase.

• Critical moisture content_{Critical moisture content}: :

The moisture content of a material at the end of constant rate period. The

critical moisture content is not a unique moisture property of a material but

critical moisture content is not a unique moisture property of a material but

is influenced by its physical shape as well as the conditions of the drying

process.

• Batch dryingBatch drying: :

The type of drying operation in which the material is fed to and discharged

from drying chambers in batches at definite intervals of time.

from drying chambers in batches at definite intervals of time.

• Specific volumeSpecific volume: :

The volume per unit weight of dry air.

• EnthalpyEnthalpy: :

The total energy contents of the substrate. For moist air, it is equal to the

The total energy contents of the substrate. For moist air, it is equal to the

heat content of the moist air per unit weight of dry air above a certain

reference temperature.

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Basics of Solar Drying

• Drying or dehydration of material means removal of moisture Drying or dehydration of material means removal of moisture from the interior of the material to the surface and then to

from the interior of the material to the surface and then to

remove this moisture from the surface of the drying material.

• The drying of product is a complex heat and mass transfer The drying of product is a complex heat and mass transfer process which depends on external parameters such as

process which depends on external parameters such as

temperature, humidity and velocity of the air stream; drying

temperature, humidity and velocity of the air stream; drying

material properties like surface characteristics (rough or smooth

surface), chemical composition (sugar, starches, etc) physical

structure (porosity. density. etc.); size and shape of the product.

structure (porosity. density. etc.); size and shape of the product.

• The rate of moisture movement from the product inside to the air The rate of moisture movement from the product inside to the air outside differs from one product to another and very much

outside differs from one product to another and very much

depends on whether the material is hygroscopic or

non-hygroscopic. Non-hygroscopic material can be dried to zero

hygroscopic. Non-hygroscopic material can be dried to zero

moisture level while the hygroscopic materials like most of the

food products will always have a residual moisture content. This

food products will always have a residual moisture content. This

moisture in hygroscopic material may be a bound moisture

(remains) in the material due to closed capillaries or due to

surface forces) or unbound moisture which remains in the

material due to surface tension of water. When the hygroscopic

material due to surface tension of water. When the hygroscopic

material is exposed to air, it will either absorb moisture or desorb

moisture depending on the relative humidity of air. The

equilibrium moisture content (EMC) will soon be reached when

equilibrium moisture content (EMC) will soon be reached when

the vapour pressure of water in the material becomes equal to

the partial pressure of water in the surrounding air. The

equilibrium moisture content is, therefore, important in the

equilibrium moisture content is, therefore, important in the

drying since this is the minimum moisture to which the material

can be dried under a given set of drying conditions.

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• A series of drying characteristic curves can be plotted. _{A series of drying characteristic curves can be plotted.}

The best is, if the moisture content M of the material

The best is, if the moisture content M of the material

is plotted versus time as shown in Fig. Another curve

can be plotted between drying rate i.e. dM/dt versus

time t as shown in Fig. But more information can be

time t as shown in Fig. But more information can be

obtained if a curve is plotted between drying rate

dM/dt versus moisture content M as shown in Fig. As is

seen from this figure for both hygroscopic and

seen from this figure for both hygroscopic and

non-hygroscopic materials, there is a constant drying rate

hygroscopic materials, there is a constant drying rate

terminating at the critical moisture content followed

by falling drying rate. The constant drying rate for both

by falling drying rate. The constant drying rate for both

non-hygroscopic and hygroscopic materials is the same

while the period of falling rate is little different. For

non-hygroscopic materials, in the period of falling rate,

non-hygroscopic materials, in the period of falling rate,

the drying rate goes on decreasing till the moisture

content becomes zero. In the hygroscopic materials,

the period of falling rate is similar until the unbound

the period of falling rate is similar until the unbound

moisture is completely removed. then it further

decreases and some bound moisture is removed; this

continues till the vapour pressure of material becomes

continues till the vapour pressure of material becomes

equal to the vapour pressure of drying air. When this

equilibrium reaches then the drying rate becomes zero.

Basics of Solar Drying

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Rate of moisture

loss

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Eight thermodynamic properties of moist air, generally used in Eight thermodynamic properties of moist air, generally used in drying are vapor pressure, relative humidity, humidity ratio, drying are vapor pressure, relative humidity, humidity ratio, dry bulb temperature, dew point temperature, web bulb dry bulb temperature, dew point temperature, web bulb temperature, enthalpy, and specific volume. These parameters temperature, enthalpy, and specific volume. These parameters are correlated and the effect of one on another can be seen on are correlated and the effect of one on another can be seen on psychometric chart.

psychometric chart.

The drying process can be explained with the help of the The drying process can be explained with the help of the psychometric chart of Fig. If the air is not saturated (say dry psychometric chart of Fig. If the air is not saturated (say dry bulb temperature is 30

bulb temperature is 30C and wet bulb temperature is 20C and wet bulb temperature is 20C) C) and is allowed to pass over the material and if no external heat and is allowed to pass over the material and if no external heat is applied, then the sensible heat of air and material is is applied, then the sensible heat of air and material is exchanged for latent heat of vaporization of water. The path exchanged for latent heat of vaporization of water. The path travelled on psychometric chart will be 20

travelled on psychometric chart will be 20C wet bulb line C wet bulb line shown by line AB. During this process the humidity ratio shown by line AB. During this process the humidity ratio changes from 0.0140 to 0.0104 i.e. about 0.0036 kg of vapour changes from 0.0140 to 0.0104 i.e. about 0.0036 kg of vapour per kg of dry air is absorbed. Now by using solar energy, the per kg of dry air is absorbed. Now by using solar energy, the air is heated to 45

air is heated to 45C with a relative humidity of 17 per cent and C with a relative humidity of 17 per cent and is passed over the drying material. During the drying process, is passed over the drying material. During the drying process, this air is cooled-adiabatically along the 24

this air is cooled-adiabatically along the 24 C wet bulb line, C wet bulb line, then the final humidity ratio will be 0.0189. Thus the moisture then the final humidity ratio will be 0.0189. Thus the moisture evaporated with the. heated air will be 0.0075 kg of vapour per evaporated with the. heated air will be 0.0075 kg of vapour per kg of dry air which is almost double the water evaporated kg of dry air which is almost double the water evaporated compared to when air was not heated.

compared to when air was not heated.

Basics of Solar Drying

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Natural Convection or

Direct type Solar Dryer

• These dryers appear to be more attractive for use These dryers appear to be more attractive for use in developing countries since these do not use

in developing countries since these do not use fan or blower to be operated by electrical energy. fan or blower to be operated by electrical energy.

• These dryers are low in cost and easy to operate._{These dryers are low in cost and easy to operate.}

• Some of the problems with these dryers are: slow Some of the problems with these dryers are: slow drying, no control on temperature and humidity, drying, no control on temperature and humidity, small quantity can be dried, and some products small quantity can be dried, and some products change colour and flavour due to direct exposure change colour and flavour due to direct exposure to sun.

to sun.

• Several direct type dryers are fabricated, tested, Several direct type dryers are fabricated, tested, and analysed in many countries.

and analysed in many countries.

• The simplest direct type solar dryer is solar _{The simplest direct type solar dryer is solar} cabinet dryer.

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Solar Cabinet Dryer

• The solar cabinet dryer in its simple form _{The solar cabinet dryer in its simple form}

consists of a wooden (or of any other material) consists of a wooden (or of any other material)

box of certain width and length (length is box of certain width and length (length is

generally kept as three times its width), insulated generally kept as three times its width), insulated

at its base and preferably at the sides and at its base and preferably at the sides and

covered with a transparent roof. covered with a transparent roof.

• The inside surfaces of the box are coated with _{The inside surfaces of the box are coated with}

black paint and the product to be dried is kept in black paint and the product to be dried is kept in the trays made of wire mesh bottom. These trays the trays made of wire mesh bottom. These trays

loaded with product are kept through an loaded with product are kept through an

openable door provided on the rear side of the openable door provided on the rear side of the

drier. drier.

• Ventilation holes are made in the bottom through _{Ventilation holes are made in the bottom through} which fresh outside air is sucked automatically. which fresh outside air is sucked automatically. Holes are also provided on the upper sides of the Holes are also provided on the upper sides of the

dryer through which moist warm air escapes. dryer through which moist warm air escapes.

• This dryer has given encouraging results and _{This dryer has given encouraging results and}

reduced the drying time by one third compared to reduced the drying time by one third compared to

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Mixed Mode Type Solar Dryer

• In the mixed mode type of solar dryers, the solar _{In the mixed mode type of solar dryers, the solar}

air heater with or without any electric fan along

air heater with or without any electric fan along

with a drying bin is used.

• Such simple mixed mode type solar dryer was _{Such simple mixed mode type solar dryer was}

developed at AIT Bangkok for drying paddy and

developed at AIT Bangkok for drying paddy and

therefore named as rice dryer.

• It consists of a solar air heater made of a frame of _{It consists of a solar air heater made of a frame of}

bamboo poles and wire covered with 0.15 mm thick

transparent PVC sheet. The ground is covered with

transparent PVC sheet. The ground is covered with

burnt rice husk which absorbs the solar radiation

and heats the air in contact.

• The hot air in this air heater rises to the drying _{The hot air in this air heater rises to the drying}

chamber which either consists of transparent PVC

chamber which either consists of transparent PVC

sheets on bamboo frame absorbing directly the

solar radiation or a bamboo frame covered from all

the four side with some opaque material.

the four side with some opaque material.

• The drying material (rice etc.) is kept on the nylon _{The drying material (rice etc.) is kept on the nylon}

net tray in thin layer through which hot air heated

from air heaters enters its bottom and goes up into

from air heaters enters its bottom and goes up into

the chimney.

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•

The chimney is a long cylinder made of

bamboo frame covered with black PVC to

keep the inside air warm. There is a cap at

the top of the chimney, leaving some space in

between chimney top and cap to allow warm

humid air to go out and protecting the

product from rain and other foreign

materials.

•

The height of the chimney and the hot air

inside it creates a pressure difference

between its top and bottom thereby creating

forced movement of air through the rice bed

to the top of the chimney.

•

The drying rate will depend on the depth of

the bed, initial moisture content of the

material, solar insolation, ambient

temperature, and the design of the dryer.

Mixed Mode Type Solar Dryer

(contd.)

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Forced Circulation Type Solar

Dryer

• As the name implies, such dryers use some kind of _{As the name implies, such dryers use some kind of} one or several electric operated blower/exhaust fan one or several electric operated blower/exhaust fan to circulate air between air heater/storage

to circulate air between air heater/storage bin/drying chamber.

bin/drying chamber.

• Such dryers are more efficient, faster, reliable, _{Such dryers are more efficient, faster, reliable,} preferred and can be used for drying large

preferred and can be used for drying large quantities of agricultural products.

quantities of agricultural products.

• These dryters can be used at low as well as at high _{These dryters can be used at low as well as at high} temperatures and used for drying large quantities temperatures and used for drying large quantities of product.

of product.

• These dryers are of bin type, tunnel type, belt type, _{These dryers are of bin type, tunnel type, belt type,} column type, or rotary type.

column type, or rotary type.

• Some forced circulation type solar dryers use some _{Some forced circulation type solar dryers use some} kind of thermal storage unit, heat recovery wheel kind of thermal storage unit, heat recovery wheel and auxiliary heating arrangement.

and auxiliary heating arrangement.

• Auxiliary energy may be supplied either by electric _{Auxiliary energy may be supplied either by electric} heating or oil or gas burners and used only when heating or oil or gas burners and used only when solar air heaters or the heat from the thermal

solar air heaters or the heat from the thermal

storage device is not sufficient to supply necessary storage device is not sufficient to supply necessary energy for drying the product.

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• Several storage systems are proposed but the most _{Several storage systems are proposed but the most} preferred one is the rock bed storage system which preferred one is the rock bed storage system which

stores the heat in the form of sensible heat and stores the heat in the form of sensible heat and

performs the dual function of storing the heat and that performs the dual function of storing the heat and that

of a heat exchanger. of a heat exchanger.

• A hybrid solar dryer (solar assisted) was developed at _{A hybrid solar dryer (solar assisted) was developed at} Fresno, California for drying large amount of fruits and Fresno, California for drying large amount of fruits and

vegetables. vegetables.

• It consists of several solar air heaters with a total area _{It consists of several solar air heaters with a total area} of 1350 m

of 1350 m22_{, a thermal storage (rock bed type) of 350m}_{, a thermal storage (rock bed type) of 350m}33

volume, a rotary wheel type heat recovery whell and a volume, a rotary wheel type heat recovery whell and a

tunnel dehydration in which 14 trucks loaded with tunnel dehydration in which 14 trucks loaded with

prepared food move at a rate of 24 hours per truck in prepared food move at a rate of 24 hours per truck in

one direction and the heated air is sent from the other one direction and the heated air is sent from the other

direction. direction.

• The system is designed for a fixed air flow rate of _{The system is designed for a fixed air flow rate of} 9.5m

9.5m33/s to the dehydrator 24 hours a day. The drying _{/s to the dehydrator 24 hours a day. The drying}

temperature varies from 60

temperature varies from 60C in the beginning of June C in the beginning of June to 66

to 66C in August to September.C in August to September.

• The solar contribution in this hybrid systems is 1582 _{The solar contribution in this hybrid systems is 1582} MJ/hr which is about 60 per cent of the total heat

MJ/hr which is about 60 per cent of the total heat requirement of drying.

requirement of drying.

Forced Circulation Type Solar Dryer

(contd.)

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Photograph of Forced

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Important Conclusions

• Experience over the past four decades has shown that _{Experience over the past four decades has shown that} inspite of high potential of solar drying it has not

inspite of high potential of solar drying it has not taken off. Some of the reasons are;

taken off. Some of the reasons are;

• Systematic work on solar dryer has been done only in _{Systematic work on solar dryer has been done only in} few countries.

few countries.

• Solar dryer has not been developed as a system. _{Solar dryer has not been developed as a system.}

• In industralized countries, there is great interest _{In industralized countries, there is great interest} towards solar drying. However, neither the

towards solar drying. However, neither the

temperature nor the heat requirement can be temperature nor the heat requirement can be achieved with solar collector.

achieved with solar collector.

• Solar drying is considered more applicable to low _{Solar drying is considered more applicable to low} temperature in-storage type drying in tropical and temperature in-storage type drying in tropical and subtropical countries.

subtropical countries.

• Pre-healing of drying air in batch dryers has been _{Pre-healing of drying air in batch dryers has been} demonstrated to be techno-economically viable. demonstrated to be techno-economically viable.

• Solar drying should be disseminated for medium and _{Solar drying should be disseminated for medium and} low scale farmers for drying cash crops.

low scale farmers for drying cash crops.

• To popularise solar drying, pilot demonstration _{To popularise solar drying, pilot demonstration} followed by training and workshop will have to be followed by training and workshop will have to be intensified.

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