E- Compendium

for the course

Protected Cultivation and

Secondary Agriculture
Course No. : AGENGG 321
(B. Sc. Ag. III Year)

Department of Processing and Food Engineering,

College of Technology and Food Engineering,
Maharana Pratap University of Agriculture and Technology,
Udaipur (Rajasthan) 313001

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 Content

S. No. Chapter Page No.

1. Green house technology : Introduction 1-6

2. Types of Greenhouse 7-16

3. Planning and design of greenhouse 17-22

4. Greenhouse equipments 23-26

5. Greenhouse cooling system 27-29

6. Irrigation system used in greenhouse 30-37

7. Material for construction of traditional and low 38-46

cost greenhouse

8. Engineering properties of cereals, pulses and 47-52

oil seeds

9. Introduction of cleaning, grading and sorting 53-72

10. Milling process of grain 73-86

11. Drying process 87-105

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 Chapter 1
Green house technology: Introduction
Protected Cultivation
 Protected cultivation practices can be defined as a cropping technique
wherein the micro environment surrounding the plant body is controlled
partially/ fully as per plant need during their period of growth to maximize
the yield and resource saving.
 Greenhouse is the most practical method of achieving the objectives of
protected agriculture, where natural environment is modified by the use of
sound engineering principles to achieve optimum plant growth and yield
(more produce per unit area) with increased input use efficiency.
 The green house is generally covered by transparent or trans-lucent material
such as glass or plastic.
 The greenhouse covered with simple plastic sheet is termed as poly house.
 The green house generally reflects back about 43% of the net solar radiation
incident upon it allowing the transmittance of the “photosynthetically active
solar radiation” in the range of 400-700 Nm wave length.
 The sunlight admitted to the protected environment is absorbed by the crops,
floor, and other objects.
 These objects in turn emit long wave thermal radiation in the infra red region
for which the glazing mate-rial has lower transparency.
 As a result the solar energy re-mains trapped in the protected environment,
thus raising its temperature. This phenomenon is called the “Green house
Effect”.
 Greenhouse is the most practical method of accomplishing the objectives of
protected cultivation.
 Tomato, Capsicum and cucumber are the most exten-sively grown
vegetables under green houses and give higher returns.
 Growing of cucumber using cost effective plastic greenhouses provides an
alternative for raising crop in the period of scarcity in Himachal Pradesh.
 This also ensures to meet year round supply of fresh produce with more
efficient resource utilization.

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  New features added to these structures have cut down the requirement of
water and energy in such cultivation through novel means like micro
irrigation-cum-fertilization (fertigation) and rainwater harvesting.
Greenhouse effect
• In general, the percentage of carbon dioxide in the atmosphere is 0.0345%
(345 ppm). But due to the emission of pollutants and exhaust gases into the
atmosphere, the percentage of carbon dioxide increases which forms a
blanket in the outer atmosphere.
• This causes the entrapping of the reflected solar radiation from the earth
surface.
• Due to this, the atmospheric temperature increases, causing global warming,
melting of ice caps and rise in the ocean levels which result in the
submergence of coastal lines.
• This phenomenon of increase in the ambient temperature, due to the
formation of the blanket of carbon dioxide is known as greenhouse effect.
• The greenhouse covering material acts in a similar way, as it is transparent
to short wave radiation and opaque to long wave radiation.
• During the daytime the short wave radiation enters into the greenhouse and
gets reflected from the ground surface.
• This reflected radiation becomes long wave radiation and is entrapped inside
the greenhouse by the covering material.
• This causes the increase in- the greenhouse temperature.
• It is a desirable effect from point of view of crop growth in the cold regions.

Fig 1.1 Greenhouse effect

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Advantages of Greenhouse:
• Throughout the year four to five crops can be grown in a greenhouse due to
the availability of required plant environmental conditions.
• The productivity of the crop is increased considerably.
• Superior quality produce can be obtained as they are grown under suitably
controlled environment.
• Gadgets for efficient use of various inputs like water, fertilizers, seeds and
plant protection chemicals can be well maintained in a greenhouse.
• Effective control of pests and diseases is possible as the growing area is
enclosed.
• Percentage of germination of seeds is high in greenhouses.
• Different types of growing medium like peat mass, vermiculate, rice hulls
and compost that are used in intensive agriculture can be effectively utilized
in the greenhouse.
• Export quality produce meeting international standards can be produced in a
greenhouse.
• When the crops are not grown, drying and related operations of the
harvested produce can be taken up utilizing the entrapped heat.
• Greenhouses are suitable for automation of irrigation, application of other
inputs, and environmental controls by using computers and artificial
intelligence techniques.
• Self-employment for educated youth on farm can be increased.
Limitations
• Manual or hand pollination in cross pollinated vegetables like cucurbits or
development of their parthenocarpic hy-brids/varieties.
• Expensive, short life and non-availability of cladding materials.
• Lack of appropriate tools and machinery.
• Structure cost initially looks unaffordable. Farmers with zero risk
affordability do not come forward to adopt it.
Causes for Green House Failure / Damage
 The profile used in the GH frame, trusses and other member too light which
deformed by
 Strong Winds.
 Cladding material some time appeared to be stronger than structure.

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  Poly film tearing because of rough and sharp edge of the frame.
 The foundation not sufficiently secured against uplift forces.
 Damage of polyfilm often started from the ventilation openings.
Government Assistance under National Horticulture Mission (NHM)
Broad Activity Sub Activity Pattern of Assistance
Horticulture Protected Cultivation Green House, Fan & 50% of cost (15% higher for hilly
Infrastructure Pad System (limited to 4000 sq m per areas), Rs. 700/- to 825/- per sqm.
(Green, Poly House, beneiciary)
Structure etc.) Naturally Ventilated System (Maximum 50% of cost (15% higher for hilly
4000 sq m per beneiciary) areas),
(i) Rs. 422/- to Rs. 530/- per sq m.
Tubular structure
(ii) Rs. 270/- per sq m. wooden
structure,
(iii) Rs. 225/- per sq m. Bamboo
structure
Shade Net House: Tubular structure 50% of cost (15% higher for hilly
(Maximum 1000 sq m per beneiciary) areas) upto Rs. 355/- per sq m
Bamboo & Wooden Structure (Maximum 50% of cost (15% higher for hilly
200 sq m per beneiciary limited to 5 units) areas), Rs. 180/- and Rs. 246 per
sq m. for bamboo and wooden
structures respectively.
Plastic Mulch 50% of cost (15% higher for hilly
areas) upto Rs. 16,000/- ha.
Plastic Tunnel: (Maximum 1000 sq m/ 50% of cost (15% higher for hilly
beneiciary) areas) upto Rs. 300/- per sq m

Review questions
Q. 1 Write the role of greenhouse in protected cultivation.
Q. 2 What is greenhouse effect and how does it helpful in crop cultivation.
Q. 3 Write down the advantages and limitations of greenhouse.
Q. 4 Write down the causes of failure of greenhouse structure.

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 Chapter 2
Types of greenhouse

Greenhouse structures of various types are used successfully for crop production.
Although there are specific advantages in each type for a particular application, in
general there is no single type greenhouse, which can be considered as the best.
Different types of greenhouse are designed to meet the specific needs. In this
chapter, different types of greenhouses based on shape, utility, construction and
covering materials are briefly described.
Types of Greenhouse:
• Different types of the greenhouse are designed to according to match
specific needs.
• There are different types of greenhouses available
– based on shape
– Based on construction
– Based on covering material
A) Greenhouse type based on Shape
1. Lean-to type Greenhouse
• It is used when the greenhouse is placed against the side of an existing
building.
• This design makes the best use of sunlight and minimizes the requirement
of roof supports.
• The roof of the building is extended with appropriate greenhouse covering
material and the area is properly enclosed.

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 Fig. 2.1 Lean-to type Greenhouse

2. Even Span type Greenhouse

• In this type, the two roof slopes are of equal pitch and width.
• This design is used for the greenhouse of small size, and it is constructed on
leveled ground.
• Several single and multiple span types are available for use in various
regions of India.
• For single span type, the span in general varies from 5 to 9 m, whereas the
length is around 24 m. The height varies from 2.5 to 4.3 m.

Fig. 2.2 Even Span type Greenhouse

3. Uneven Span Type Greenhouse
• This type of greenhouse is constructed on hilly terrain.

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 • The roofs are of unequal width, which make the structure adaptable to the
side slopes of hill.
• Seldom used. Now-a-days as it is not adaptable for automation.

Fig. 2.3 Uneven Span Type Greenhouse

4. Quonset Greenhouse
• The pipe arches or trusses are supported by pipe purlins running along the
length of the greenhouse.
• In general, the covering material used is polyethylene.
• Less expensive than the gutter connected greenhouses and are useful when a
small isolated cultural area is required.
• Either in free standing style or arranged in an interlocking ridge and furrow.

Fig. 2.4 Quonset Greenhouse

5. Ridge and Furrow Type Greenhouse

• Designs of this type use two or more A-frame greenhouses connected to one
another along the length of the eave.
• The eave serves as a furrow or gutter to carry rain and melted snow away.

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 • The side walls are eliminated between the greenhouses, which results in a
structure with a single large interior.
• Reduces labour, lowers the cost of automation, improves personal
management and reduces fuel consumption, as there is less exposed wall
area through which heat escapes.

Fig. 2.5 Ridge and Furrow Type Greenhouse

6. Saw Tooth Type Greenhouse
• These are also similar to the ridge and furrow type greenhouses except that,
there is provision for natural ventilation in this type.
• Specific natural ventilation flow path develops in a saw tooth type
greenhouse.

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 Fig. 2.6 Saw Tooth Type Greenhouse
B) Greenhouse type based on construction
1. Wooden Framed Structures
• In general, for greenhouses with span less than 6 m, only wooden framed
structures are used.
• Side posts and columns are constructed of wood without the use of a truss.
• Pine wood is commonly used as it is inexpensive and possesses the required
strength.
• Timber locally available, with good strength, durability and machinability
also can be used for the construction.

Fig. 2.7 Wooden Framed Structures

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2. Pipe Framed Structures
• When the clear span is around 12 m, pipes are used for the construction of
greenhouses.
• In general, the side posts, columns, cross-ties and purlins are constructed
using pipes.

Fig. 2.8 Pipe Framed Structures

3. Truss Framed Structures

• If the greenhouse span is greater than or equal to 15 m, truss frames are
used.
• Flat steel, tubular steel or angle iron is welded together to form a truss
encompassing rafters, chords and struts.
• Angle iron purlins running throughout the length of greenhouse are bolted to
each truss.
• Most of the glass houses are of truss frame type.

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 Fig. 2.9 Truss Framed Structures
C) Greenhouse type based on covering materials
1. Glass Greenhouses
• Only glass greenhouses with glass as the covering material existed prior to
1950.
• Glass as covering material has the advantage of greater interior light
intensity.
• These greenhouses have higher air infIltration rate, which leads to lower
interior humidity and better disease prevention.
• Lean-to type, even span, ridge and furrow type of designs are used for
construction of glass greenhouse.

Fig. 2.10 Glass Greenhouses

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2. Plastic Film Greenhouses
• Flexible plastic films including polyethylene, polyester and polyvinyl
chloride are used as covering material in this type of greenhouses.
• Become popular, as they are cheap and the cost of heating is less when
compared to glass greenhouses.
• The main disadvantage with plastic films is its short life as the covering
material.
• For example, the best quality ultraviolet (UY) stabilized film can last for
four years only.
• Quonset design as well as gutter-connected design is suitable for using this
covering material.

Fig. 2.11 Plastic Film Greenhouses

Component of greenhouse

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• Roof: transparent cover of a green house.
• Gable: transparent wall of a green house
• Cladding material: transparent material mounted on the walls and roof of a
green house.
• Rigid cladding material: cladding material with such a degree of rigidity
that any deformation of the structure may result in damage to it. Ex. Glass
• Flexible cladding material: cladding material with such a degree of
flexibility that any deformation of the structure will not result in damage to
it. Ex. Plastic film
• Gutter: collects and drains rain water and snow which is place at an
elevated level between two spans.
• Column: vertical structure member carrying the green house structure
• Purlin: a member who connects cladding supporting bars to the columns
• Ridge: highest horizontal section in top of the roof
• Girder: horizontal structure member, connecting columns on gutter height
• Bracings: To support the structure against wind
• Arches: Member supporting covering materials
• Foundation pipe: Connection between the structure and ground
• Span width: Center to center distance of the gutters in multispan houses

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• Green house length: dimension of the green house in the direction of gable
• Green house width: dimension of the green house in the direction of the
gutter

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 Chapter 3
Planning for greenhouse

 A greenhouse has basically one purpose of providing and maintaining a

growing environment that will result in optimum production at maximum
yield.
 The agriculture in the controlled environment is possible in all the regions
irrespective of climate and weather.
 As an enclosing structure for growing plants, greenhouse must admit the
visible light portion of solar radiation for plant photosynthesis and,
therefore, must- be transparent.
 At the same time, to protect the plants, a greenhouse must be ventilated or
cooled during the day because of the heat load from the radiation.
 The structure must also be heated or insulated during cold nights. A
greenhouse acts as a barrier between the plant production areas and the
external or ambient environment.
 Production is protected from external stresses such as weather and pollution
from industrial and other sources. Hence, while planning for greenhouse
facility care must be taken in the selection of site and its orientation, in
choosing the type of structural design and in the choice of covering material
for better functioning and operation of greenhouses.
1. Site Selection and Orientation
• The building site should be as level as possible to reduce the cost of grading
• The site should be well aerated and should receive good solar radiation.
• Provision of a drainage system is always advisable, because of the extensive
use of water in greenhouse operations.
• It is also advisable to select a site with a natural windbreak such as a tree
line or hill, on the north and northwest sides.
• In regions where snow is expected, trees should be 30.5 m away in order to
keep drifts back from the greenhouses.
• To prevent shadows on the crop, trees located on the east, south, or west
sides should be at a distance of 2.5 times their height. Owing to thee limited
availability of the agricultural labourers, high wages are to be paid to attract
them.

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2. Selection of structural design:
• The structural parts that can cast shadows in the greenhouse should be
minimized. So the covering materials should have the largest possible
unsupported area, and consequently offer the highest possible light
transmittance.
• At the same time, greenhouse structures and structural components,
including the covering, should be strong enough to resist loads from snow,
wind, crops and installations, to provide adequate margins of safety to
prevent structural damage or serviceability problems.
Therefore, following aspects should be considered
• Overall structural design and the properties of the individual structural
components.
• Specific mechanical and physical properties, which determine the structural
behaviour of the covering materials.
• Specific sensitivity of the crop to light and temperature to be grown in the
greenhouse.
• Specific requirements relevant to the physical properties of the covering
material.
• Agronomic requirements of the crop.
3. Selection of covering Materials:
Covering material should have following characters.
• It should transmit the visible light portion of the solar radiation, which is
utilized by plants for photosynthesis.
• It should absorb the small amount of UV in the radiation and convert a
portion of it to fluoresce into visible light, useful for plants.
• It should reflect or absorb IR radiation which are not useful to plants and
causes greenhouse interiors to overheat.
• It should be of low cost.
• It should have usable life of 10 to 20 years.
Designing criteria of greenhouse
• A piece of land larger than the grower’s immediate need is required
• Ultimate size of the greenhouse range should be estimated
• Area should then be added to this estimated figure to accommodate service
buildings, storage, access drives and a parking lot etc.
• Floor area of service buildings = 13% of the
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 • Service building is centrally located in a nearly square design of the firm
• Doors between the service buildings and the greenhouse should be wide
enough (3.1 m wide and 2.7 m) to facilitate full use of the corridor width
• Preferred direction for length = East-West
• Preferred direction for width = North-South= Gutter direction
• Greenhouse area= length x width
Construction of glass greenhouse
• Advantage of greater interior light intensity
• Higher air infiltration rate lower interior humidity disease
prevention
• Higher initial cost
• Several types of glass greenhouses are designed to meet specific needs
• 3 frame types used in glass greenhouses: Wood frames (6.1 m in width),
Pipe frames (12.2 m in width) and Truss frames (15.2 m in width)
• Truss frame greenhouses are best suited for prefabrication
• Structural members of the glass greenhouse cast shadows that reduce plant
growth during the dark months of the year
• Glass greenhouse construction: High profile or low profile
• Low profile greenhouses: Single panels of glass extend from eave to ridge
• High profile greenhouses: Require more than single panel to cover the eave
to ridge
Construction of pipe greenhouse
• Low initial investment and relatively long life
• Structural member: Galvanized mild steel
pipe in association with wide width UV stabilized LDPE film
• The structural members of greenhouse are:
• (a) Hoops
• (b) Foundation
• (c) Lateral supports
• (d) Polygrip assembly
• (e) End frame

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 Fig. 3.1 constructional details of pipe framed greenhouse
Materials required for a pipe greenhouse
The following materials are required for a greenhouse having 4m x20 m floor area:
• GI pipe class A (25 mm diameter, 85 cm ong, 30 m total length)
• GI pipe class B (15 mm diameter, 6.0 m long, 21 No.s)
• GI sheet (20 gauges, size 90*24 cm, 4 sheets)
• MS flat (25*3 mm size, 4 m length)
• Lateral support to end frames (10 mm diameter rod, 10 m length)
• Cement concrete (1: 3: 6 mix, 1.0 m3)
• UV- stabilized LDPE film (single layer 800 gauge, 5.4 m2/ kg, 154 m2)
• Polygrip (channel 2000 *3.5*4 cm, 2 No.s; Angle 2000 *2*2 cm, 2 No.s;
both made from the procured 20 gauge GI sheet, key 6 mm diameter and 56
mm length)
• Wooden end frames (5*5 cm wood, 0.15 m3)
• Nuts and bolts 96 mm diameter, 35 mm long, 70 sets)

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 • Miscellaneous items like nails, hinges and latches as per requirement
Installation procedure
• A 4m x 20m rectangular area is marked on the site, orienting the longer
dimension in eastwest direction. This rectangle will act as the floor plan of
the greenhouse
• Mark four points on the four corners of the rectangle.
• Start from one corner point and move along the length of marked rectangle,
marking a point every 1.25 m distance until reaching the other corner (16
bays; 17 points). The same procedure is repeated on the other side of the
rectangle.
• Dig 10 cm diameter holes up to 70 cm depth on all marked points with the
help of bucket auger (or) a crowbar. This way a total of 34 holes on both the
parallel sides of the greenhouse floor are obtained
• Polygrip sections formed according to the drawing into two 20m length.
• Fix the prefabricated polygrip channels to the foundation pipes on 1.25 m
spacing with the help of 6 mm diameter bolts
• Set these assemblies on temporary supports between the holes with the
foundation pipes hanging vertically in the holes.
• Pour cement concrete mix of 1: 3: 6 around foundation pipes in such a way
that the lower 15 cm to 20 cm ends are covered in concrete. The concrete is
compacted around the foundation pipes with the help of the crowbar and is
allowed to cure for 2-3 days.
• After curing, fill the soil around the foundation pipes to the ground level and
compact it well.
• Position end frames on the two ends. Mark the position of legs and dug holes
for fixing of legs. Now install both the end frames.
• Put the ringside of lateral support members on adjacent foundation pipe to
the corner, and other side is hooked to the end frame
• Put all the hoops in the foundation pipes in such a way that straight portion
of hoop is inserted into the foundation and rests on the bolt used for fixing of
polygrip channel.
• Take a 20 m long ridge line by spacing 15 mm diameter pipes together. Put
the 20m long pipe at the ridge line of the hoops.
• Use cross connectors on the ridge line pipe, in such a way that one half of it
remains on the one side of the hoop and the other half on the other Side
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• Put two bolts of 6 mm diameter in the holes provided in the ends of cross-
connector. Tie a few of them with the help of nuts.
• Repeat the same procedure for joining all the hoops with ridge line pipe
• While forming cross-connectors, the distance between the cross-connectors
or hoops should be maintained 1.25 m center to center.
• This poly grip mechanism will provide a firm grip of the ridge line pipe and
hoops at right angles without allowing for slippage

Review Questions
Q. 1 List the characters to select greenhouse covering material.
Q. 2 How to select the greenhouse structure?
Q. 3 list out the materials required to construct greenhouse.

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 Chapter 4
Greenhouse equipments
Screens
• It helps control the amount of light, humidity and temperature inside the
facility, which turns to an improvement of the crop conditions and a
reduction of the energy costs.

Extractor fans
 The extractor fans allow forcing the ventilation inside greenhouses when the
natural ventilation using roof and/or perimeter vents, does not allow
reaching the desired rate of air renewal, which is an innate need for
producing crops as well as livestock farms.

Air circulation fans

• The air circulation fans or recirculation fans help obtain a suitable air
movement contributing to maintain a homogeneous interior climate,
avoiding hot air accumulation at the upper section of the greenhouse,
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 reducing substantially the degree of water condensation and favouring the
crops’ transpiration and CO2 absorption.

Cooling system
• This water evaporation cooling system is comprised of extractor fans and
cooling panels installed on opposite walls of the greenhouse to create a
negative pressure area inside the greenhouse. This forces the outside air
flowing through the dampened panels becoming charged with water
molecules and cooled down and thus decreases the temperature inside the
greenhouse.

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Fertigation system
• It consists of applying simultaneously water and fertilizers through the
irrigation system, supplying the nutrients required by the crops to the soil or
substrate.
• Fertigation is especially useful in the case of drip irrigation. By means of
automatic high technology fertigation equipments the water and the nutrients
are perfectly placed in the absorption area of the roots, improving the rate of
growth and quality of the crops.
• This system allows carrying out a more rational use of the water and
fertilizers, respecting the environment and minimizing the environmental
impact.

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Fog system
• Consists of incorporating a large number of micro-particles of water to the
ambient air, which remain suspended in the air inside the greenhouse long
enough to evaporate without wetting the crops. The water is added in the
form of fog using special nozzles distributed uniformly all over the surface
of the greenhouse.
• The Fog system is very useful for humidifying and cooling down the
greenhouse in a controlled manner and carrying out disinfection treatments
using soluble plant protection products.
Irrigation system
• The objective of a Greenhouse Irrigation System is to deliver optimum
water and nutrient levels directly into the root zone and reduce wastage and
evaporation.
• Drip system dispenses water through a network of valves, pipes, tubing, and
emitters.
• It is more efficient than other types of irrigation systems, such as surface
irrigation or sprinkler irrigation, depending on how well the system is
outlined, placed, maintained, and operated.

Other equipments:
Soil moisture meter
Temperature and Humidity meter

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 Chapter 5
Greenhouse cooling system

 Active summer cooling is achieved by evaporative cooling process.

 The evaporative cooling systems are developed to reduce the problem of excess
heat in greenhouse.
 In this process, cooling takes place when the heat required for moisture
evaporation is derived from the greenhouse surrounding environment causing a
depression in its temperature.
 The 'two active summer cooling systems in use presently are fan-and-pad and
fog systems. In the evaporative cooling process, the cooling is possible only up
to the wet bulb temperature of the incoming air.

1. Fan-and-Pad Cooling System

 The fan-and-pad evaporative cooling system has been available since 1954 and
is still the most common summer cooling system in greenhouses.
 Along one wall of the greenhouse, water is passed through a pad that is usually
placed vertically in the wall.
 Traditionally, the pad was composed of excelsior (wood shreds), but today it is
usually made of a cross-fluted cellulose material somewhat similar in
appearance to corrugated cardboard.

Fig. 5.1 Components of greenhouse fan and pad cooling system

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 Exhaust fans are placed on the opposite wall.
 Warm outside air is drawn in through the pad.
 The supplied water in the pad, through the process of evaporation, absorbs heat
from the greenhouse air passing through the pad as well as from the
surroundings of the pad and frame, thus causing the cooling effect.
 Khus-khus grass mats can also be used as cooling pads.

2. Fog Cooling System

 The fog evaporative cooling system, introduced in greenhouses in 1980,

operates on the same cooling principle as the fan-and-pad system but uses quite
a different arrangement.
 A high-pressure pumping apparatus generates fog containing water droplets
with a mean size of less than 10 microns using suitable nozzles.
 These droplets are sufficiently small to stay suspended in air while they are
evaporating and utilize the heat of greenhouse air.
 Fog is dispersed throughout the greenhouse, cooling the air everywhere. As this
system does not wet the foliage there is less scope for disease and pest attack.
People and plants stay dry throughout the process.
 This system is equally useful for seed germination and cutting propagation,
since it eliminates the need for a mist system.
 Both types of summer evaporative cooling systems can reduce the greenhouse
air temperature well below the outside temperature.

Fig 5.2 Nozzle of greenhouse fog cooling system

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 The fan-and-pad system can lower the temperature of incoming air by about
80% of the difference between the dry and wet bulb temperatures, while the fog
cooling system can lower the temperature by nearly the 100% of difference.
 This is due to the fact that complete evaporation of the water is not taking place
because of bigger aroplet size in fan-and-pad, whereas in the fog cooling
system, there will be complete evaporation because of the minute size of the
water droplets. Thus, lesser the dryness of the air, greater evaporative cooling is
possible.

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 Chapter 6
Irrigation system used in greenhouse

 A well-designed irrigation system will supply the precise amount of water

needed each day throughout the year.
 The quantity of water needed would depend on the growing area, the crop,
weather conditions, the time of year and whether the heating or ventilation
system is operating.
 Water needs are also dependent on the type of soil or soil mix and the size and
type of the container or bed.
 Watering in the greenhouse most frequently accounts for loss in crop quality.
Though the operation appears to be the simple, proper decision should be taken
on how, when and what quantity to be given to the plants after continuous
inspection and assessment.
 Since underwatering (less frequent) and overwatering (more frequent) will be
injurious to the crops, the rules of watering should be strictly adhered to.
 Several irrigation water application systems, such as hand watering, perimeter
watering, overhead sprinklers, boom watering, and drip irrigation which are
currently in use will be discussed in this chapter.

Rules of Watering

The following are the three important rules of application of irrigation water.

Rule 1: Use a well-drained substrate with good structure

 If the root substrate is not well drained and aerated, proper watering
cannot be achieved. Hence substrates with ample moisture retention
along with good aeration are indispensable for proper growth of the I
plants.
 The desired combination of coarse texture and highly stable structure can
be obtained from the formulated substrates and not from field soil alone.

Rule 2: Water thoroughly each time

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  Partial watering of the substrates should be avoided; the supplied water
should flow from the bottom in case of containers, and the root zone is
wetted thoroughly in case of beds.
 As a rule, 10 to 15% excess of water is supplied. In general, the water
requirement for soil based substrates is at a rate of 20 11m2 of bench, 0.3
to 0.35 I per 0.165 m diameter pot.

Rule 3: Water just be/ore initial moisture stress occurs

 Since overwatering reduces the aeration and root development, water

should be applied just before the plant enters the early symptoms of water
stress.
 The foliar symptoms, such as texture, colour, and turgidity can be used to
determine the moisture stresses, but they vary with crops.
 For crops that do not show any symptoms, colour. Feel and weight of the
substrates are used for assessment.

1. Hand Watering

 The most traditional method of irrigation is hand watering and in present

days is uneconomical. Growers can afford hand watering only where a
crop is still at a high density, such as in seed beds, or when they are
watered at a few selected pots or areas that have dried sooner than others.
In all cases, the labour saved will pay for the automatic system in less
than one year. It soon will become apparent that this cost is too high.
 In addition to this deterrent to hand watering, there is a great risk of
applying too little water or of waiting too long between waterings.
 Hand watering requires considerable time and is very boring. It is usually
performed by inexperienced employees, who may be tempted to speed up
the job.

2. Perimeter Watering

 Perimeter watering system can be used for crop production in benches or

beds.

31
  A typical system consists of a plastic pipe around the perimeter of a
bench with nozzles that spray water over the substrate surface below the
foliage.

Fig. 6.1 Schematic diagram of perimeter watering system

 Either polyethylene or PVC pipe can be used. While PVC pipe has the
advantage of being very stationary, polyethylene pipe tends to roll if it is not
anchored firmly to the side of the bench.
 This causes nozzles to rise or fall from proper orientation to the substrate
surface. Nozzles are made of nylon or a hard plastic and are available to put
out a spray arc of 180°,90 or 45°.
 Regardless of the types of nozzles used, they are staggered across the
benches so that each nozzle projects out between two other nozzles on the
opposite side.

32
  Perimeter watering systems with 180° nozzles require one water valve for
benches up to 30.5 m in length. For benches over 30.5 m and up to 61.0 m, a
water main should be installed on either side, one to serve each half of the
bench. This system applies 1.25 V/min/m of pipe. Where 180° and 90° or
45° nozzles are alternated, the length of a bench serviced by one water valve
should not exceed 23 m.

3. Overhead Sprinklers

 While the foliage on the majority of crops should be kept dry for disease
control purposes, a few crops do tolerate wet foliage.
 These crops can most easily and cheaply be irrigated from overhead.
Bedding plants, azalea liners, and some green plants are commonly watered
from overhead.
 A pipe is installed along the middle of the bed. Riser pipes are installed
periodically to a height well above the final height of the crop. A total height
of 0.6 m is sufficient for bedding plants flats and 1.8 m for fresh flowers.
 A nozzle is installed at the top of each riser. Nozzles vary from those that
throw a 360° pattern continuously to types that rotate around a 360° circle.
 Trays are sometimes placed under pots to collect water that would otherwise
fall on the ground between pots and is wasted. Each tray is square and meets
the adjacent tray.
 In this way nearly all water is intercepted. Each tray has a depression to
accommodate the pot and is then angled upward from the pot toward the tray
perimeter.
 The trays also have drain holes, which allow drainage of excess water and
store certain quantity, which is subsequently absorbed by the substrate.

33
 Fig 6.2 Schematic diagram of overhead sprinkle watering system.

4. Boom Watering

 Boom watering can function either as open or a closed system, and is

used often for the production of seedlings grown in plug trays.
 Plug trays are plastic trays that have width and length dimensions of
approximately 0.30 x 0.61 m, a depth of 13 to 38 mm, and contain about
100 to 800 cells.
 Each seedling grows in its own individual cell. Precision of watering is
extremely important during the 2 to 8 week production time of plug
seedlings.
 A boom watering system generally consists of a water pipe boom that
extends from one side of a greenhouse bay to the other.
 The pipe is fitted with nozzles that can spray either water or fertilizer
solution down onto the crop.
 The boom is attached at its center point to a carriage that rides along
rails, often suspended above the center walk of the greenhouse bay.

34
  In this way, the boom can pass from one end of the bay to the other. The
boom is propelled by an electrical motor.
 The quantity of water delivered per unit area of plants is adjusted by the
speed at which the boom travels.

5. Drip Irrigation

 Drip irrigation, often referred to as trickle irrigation, consists of laying

plastic tubes of small diameter on the surface or subsurface o the field or
greenhouse beside or beneath the plants.
 Water delivered to the plants at frequent intervals through small holes or
emitters located along the tube.
 Drip irrigation systems are commonly used in combination with
protected agriculture, as an integral and essential part of the
comprehensive design. When using plastic mulches, row covers, or
greenhouses, drip irrigation is the only means of applying uniform water
and fertilizer to the plants.
 Drip irrigation provides maximum control over environmental
variability; it assures optimum production with minimal uses of water,
while conserving soil and fertilizer nutrients; and controls water,
fertilizer, labour, and machinery costs.
 Drip irrigation is the best means of water conservation. In general, the
application efficiency is 90 to 95%, compared with sprinkler at 70% and
furrow irrigation at 60 to 80%, depending on soil type, level of field and
how water is applied to the furrows.
 Drip irrigation is not only recommended for protected agriculture but also
for open field crop production, especially in arid and semi-arid regions of
the world.
 Drip irrigation is replacing surface irrigation where water is scarce or
expensive, when the soil is too porous or too impervious for gravity
irrigation, land leveling is impossible or very costly, water quality is
poor, the climate is too windy for sprinkler irrigation, and where trained
irrigation labour is not available or is expensive.
 In drip irrigation weed growth is reduced, since irrigation water is applied
directly to the plant row and not to the entire field as with sprinkler,

35
 furrow, or flood irrigation. Placing the water in the plant row increases
the fertilizer efficiency since it is injected into the irrigation water and
applied directly to the root zone.
 Plant foliage diseases may be reduced since the foliage is not wetted
during irrigation.
 One of the disadvantages of drip irrigation is the initial cost of equipment
per acre, which may be higher than other systems of irrigation.
 However, these costs must be evaluated through comparison with the
expense of land preparation and maintenance often required by surface
irrigation.
 Basic equipment for drip irrigation consists of a pump, a main line,
delivery pipes, manifold, and drip tape laterals or emitters.
 The head, between the pump and the pipeline network, usually consists
of control valves, couplings, filters, time clocks, fertilizer injectors,
pressure regulators, flow meters, and gauges.
 Since the water passes through very small outlets in emitters, it is an
absolute necessity that it should be screened, filtered, or both, before it is
distributed in the pipe system.
 The initial field positioning and layout of a drip system is influenced by
the topography of the land and the cost of various system configurations.
Design considerations should also include the relationship between the
various system components and the farm equipment required to plant,
cultivate, maintain and harvest the crop.

36
1. Pump 9. Mainline
2. Pressure relief valve 10. Submain secondary filter
3. Air vents (at all high points) 11. Field control valves
4. Check valve 12. Submains
5. Filter injector/tank 13. Drip tape laterals
6. Mainline valve or gate 14. Lateral hook up
7. Pressure gauges 15. Drain / flush valves
8. Filter 16. System controller
Fig 6.4 A typical layout of drip irrigation system.

37
 Chapter 7
Material for construction of traditional and low cost greenhouse

 Materials that are commonly used to build frames for greenhouses

are wood, bamboo, steel, galvanized iron pipe, aluminium and
reinforced concrete.
 Frames often incorporate a combination and alloys of these
materials. The selection of these materials was based on their
specific physical properties and requirements of design strength.
 Life expectancy and the cost of the construction materials also
decide the selection of materials.
 The general characteristics of the materials of greenhouse
construction, such as wood, galvanized iron and glass and their
suitability in specific components of construction are discussed in
this chapter.
1. Wood
 Wood and bamboo are -generally used for low cost polyhouses.
 In these houses, the wood is used for making frames consisting of
side posts and columns over which the polyethylene sheet is fixed.
 The commonly used woods are pine and casuarina, which are strong
and less expensive. In pipe-framed polyhouses, wooden battens can
be used in the end frames for fixing the covering material.
 In tropical areas, bamboo is often used to form the gable roof of a
greenhouse structure.
 Wood must be painted white to improve light conditions within the
greenhouse, but care should be taken to select a paint that will
inhibit the growth of mold.
 Wood must also be treated for protection against decay. Special
treatment should be given to the wood that may come into contact
with the soil Chromated copper arsenate and ammonical copper
arsenate are water based preservatives that are safe to use where
plants are grown.
38
 Even natural decay resistance woods, such as redwood or cypress
should be treated, in desert or tropical regions, but they are
expensive.

Fig 7.1 Wooden scissors-truss type film plastic greenhouse.

39
2. Galvanized Iron, Aluminium, Steel and Reinforced Cement Concrete

Galvanized iron (GI) and steel members are generally used in different frame
works of greenhouse structure designs. In galvanizing operation the surface
of iron or steel is coated with a thin layer of zinc to protect it against
corrosion. Among many others, the two commonly followed processes are:

1. Hot dip galvanizing (hot process) where the cleaned member is dipped in
molten zinc, which produces a skin of zinc alloy to the steel.

2. Electro-galvanizing (cold process) where the member is zinc plated

similar to the process of electro-plating.

As the wood is becoming scarce and more expensive, GI pipes, tubular steel and
angle iron are used for side Posts, columns and purlins. The galvanization
process makes the iron rust proof; hence the common problem of rusting of
iron structural members is eliminated. In the pipe frames, GI pipes are
mostly used as side posts, columns, cross ties and purlins. All the pipe
components are not interconnected but depend on the attachment to the sash
bars for support.

Often the frames may be all aluminium or steel or a combination of the two
materials. Aluminium and hot dipped GI are comparatively maintenance
free. In tropical areas, it is advisable to double dip the steel, especially when
the single dip galvanizing process does not give a complete zinc cover of
even thickness to the steel. Aluminium and steel must be protected from
direct contact with the ground to prevent corrosion. If there is a risk of any
part of the aluminium or steel coming into contact with the ground it must be
tlforoughly painted with bitumen tar.

For truss frames, flat steel, tubular steel or angle iron is welded together to form
a truss consisting of rafters, chords and struts. The angle iron purlins running
throughout the length of the greenhouses are bolted to each truss. Now-a-
days, the greenhouse construction is of metal type that is more permanent.
While use of reinforced cement concrete is generally limited to foundations
and low walls, concrete is sometimes used as support posts for frames made

40
 of bamboo. In permanent bigger greenhouses, floors and benches for
growing the crops are also made of concrete.

3. Glass

 Glass has been the traditional glazing material all over the world. The most
widely used glass for greenhouse is the "single drawn" or "float glass",
secondly the "hammered" and "tempered glass".
 Single drawn glass is made in the traditional way by simply pulling the
molten glass either by hand or by mechanical equipment.
 Float glass is made in modern way by allowing the molten glass to float on
the molten tin. The single drawn or float glass has a uniform thickness of 3
to 4 mm.
 The term "hammered glass" defines a cast glass with one face (exterior)
smooth and the other one (interior) rough, designed so as to enhance light
diffusion. Therefore, this glass is not transparent but translucent.
 The thickness of this type of glass is usually 4 mm.
 The tempered glass is the glass, which is quickly cooled after manufacture,
adopting a procedure similar to that used for steel.
 This kind of processing gives the glass higher resistance to impact which is
generally caused by hail.
 Coatings for glass, such as metal oxide with a low emissivity are used for
saving of energy with adequate light transmittance. Such coatings are
applied only to float glasses.
 Glass though fragile, is a strong material when it is used properly and loaded
in the correct way. Glass, used as a covering material of greenhouses, is
expected to be subjected to rather severe wind loading, snow and hail
loading conditions.
 To limit the chances of breakage of glass coverings, design rules should be
applied to the supporting structure of the glass panels.
 Such rules are aimed, in general, at limiting the deformation of the structural
components which are supporting the glass panels.
 Thus, the standard properties of the glass to be used as construction material
are as follows (Briassoulis, et ai, 1997). The maximum bending deflection
(ugut) of gutters, purl ins in the roof and ridge profiles is given by:

41
where, Lm is the distance between trusses and n is the number of panels within this
distance.
The maximum bending deflection (Ugab) of gable supporting parts is given by:

where, Lgab is the length of gable column or gable purlin.

The maximum bending deflection (ug/a) of glazing' bars for support of rigid
materials is given by:

where, Lg/a is the span of the glazing bar.

 Also, to provide the proper supporting conditions for glass panels, rules are
given for the grooves in glazing bars.
 Different calculation methods are given for glass panels depending on
whether they are supported on 2, 3 or 4 sides.
 The strength mainly depends on the length/width ratio of the panel and on
the thickness of the panel, but the most widely used thickness is 4 mm.

Greenhouse covering materials

 Flexible plastic films, including polyethylene, polyester and polyvinyl
chloride, have been used for greenhouse coverings.
 Polyethylene is principally used today for two reasons. Firstly, film plastic
greenhouses with permanent metal frames cost less than glass greenhouses.
 Even greater savings can be realized when film plastic is applied to less
permanent frames, such as quonset greenhouses.
 Secondly, film plastic greenhouses are popular because the cost of heating
them is approximately 40% lower compared to single-layer glass or
fiberglass-reinforced plastic greenhouses.
 A thermal screen is installed inside a glass greenhouse that will lower the
heat requirement to approximately that of a double-layer film plastic
greenhouse, but this increases the cost of the glass greenhouse.

42
  Polyethylene film was developed in the late 1930s in England, and its use as
a greenhouse covering was pioneered around the middle of this century.
 The use of polyethylene for greenhouses has increased rapidly and continues
to do so.
 Some disadvantages exist along with the advantages of film plastic. These
covering materials are short lived compared to glass and plastic panels. UV
light from the sun causes the plastic to darken, thereby lowering
transmission of light, also making it brittle, which leads to its breakage due
to wind.
 However, under proper management, the savings in fuel as well as the lower
initial purchase price make the film plastic greenhouse less costly than a
glass greenhouse. In this chapter, salient features of greenhouse covering
films, such as polyethylene, polyvinyl chloride, polyester, Tefzel T and rigid
panels like fiberglass-reinforced plastic, polycarbonate are discussed.
1. Polyethylene Film
 From the large variety of plastics available today in the market, those
commonly used for greenhouse coverings are the thermoplastics. The basic
characteristic of thermoplastics is that they consist of individual long chain
molecules.
 They soften with heating and harden with cooling and this process is
reversible. Thermoplastics constitute a group of materials that are attractive
to the designer for two main reasons:
1. Their basic physical properties can be exploited in a wide range of properly
designed articles that have the stiffness, robustness and resilience to resist
loads and deformations imposed during normal use.
2. They can readily be processed using efficient mass production techniques
which result in low labour charge.
 Polyethylenes used for covering year-round production greenhouses have a
UV -inhibitor in it. Otherwise, it lasts for only one heating season.
 UV -grade polyethylene is available in widths up to 15.2 m in flat sheets and
up to 7.6 m in tubes. Standard lengths include 30.5, 33.5, 45.7, 61.0 and 67.0
m.
 Several companies provide custom lengths up to a maximum of 91.5 m.
 A polyethylene covering is colder than the air inside the greenhouse during
winter. When warm and moist greenhouse air comes in contact with the cold
polyethylene, the air gets cooled.
 As a result, water vapour condenses on the polyethylene surface. Since the
surface is repellent to water, the water forms into beads and with time the

43
 water beads increase in size to a point where they drop off to the plants
below.
 The wet foliage fosters disease development, while the constantly wetted
soil becomes waterlogged and oxygen deficient.
 With the antifog surfactant (a chemical discouraging condensation) built into
the film or panel, it is advisable to use them because in addition to the water
dripping problems, this condensation also reduces light intensity within the
greenhouse.
 Warm objects, such as plants, the greenhouse frame and soil radiate IR
energy to colder bodies, such as the sky at night. This condition results in
loss of heat in greenhouses. Since polyethylene is a poor barrier to radiant
heat, polyethylene formulated with IR-blocking chemicals into it during
manufacture will stop about half of the radiant heat loss.
 On cold and clear nights, as much as 25% of the total heat loss of a
greenhouse can be prevented in this way and on cloudy nights only 15% is
prevented.
 UV -stabilized polyethylene, on an average, transmits about 87% of
photosynthetically active radiation (PAR) into the greenhouse.
 IR-absorbing polyethylene, which reduces radiant heat loss, transmits about
82% of PAR.
2. Polyvinyl Chloride Film
 Polyvinyl chloride films are UV light resistant vinyl films of 0.2 and 0.3 mm
(8 and 12 mil) thicknesses (1 mil = 111000 in) and are guaranteed for four
and five years respectively.
 This extended life period was a definite advantage in the recent past, when
polyethylene lasted for only one or two years. With the recent advent of
four-year polyethylene, this advantage is nearly gone.
 The cost of 0.3 mm (12 mil) vinyl is three times that of 0.15 mm (6 mil)
polyethylene.
 Although vinyl film is produced in rolls up to 1.27 m wide, any width strip
can be purchased, since the strips of vinyl can be sealed together.
 The vinyl films tend to hold a static electrical charge, which attracts and
holds dust. This in turn reduces light transmittance unless the dust is washed
off.
 Vinyl films are seldom used in. the United States. In Japan, 95% of
greenhouses are covered with film plastic and within this group 90% are
covered with vinyl film.
3. Polyester Film
 Polyester films offer long life and are strong.

44
  Films of 0.13 mm (5 mil) thickness are used for roofs and will last for four
years, while 0.08 mm (3 mil) films are used on vertical walls and have a life
expectancy of seven years.
 Although the cost of polyester is higher than that of polyethylene, it was
offset by the extra life expectancy.
 Other advantages include light transmittance equal to that of glass and
freedom from static electrical charges, which collect dust.
 Polyester is still used frequently, in heat retention screens because of its high
capacity to block radiant energy.
4. Tefzel T Film
 The most recent addition of greenhouse film plastic covering is 2 Tefzel T
film (ethylene tetrafluoroethylene).
 Actually, this film was earlier used as the transparent covering on solar
collectors.
 The anticipated life expectancy is 20 years or more.
 The light transmission is 95% and is greater than that of any other
greenhouse covering material.
 A double layer has a light transmission of 90% (0.95 x 0.95).
 Tefzel T film is more transparent to IR radiation than other film plastics.
Hence less heat is trapped inside the greenhouse during hot weather.
 As a result less cooling energy is required. On the negative side, the film is
available only in 1.27 m wide rolls.
 This requires clamping rails on the greenhouse for every 1.2 m. Efforts are
underway to produce wider sheets of the film.
 If reasonable width strips become available, the price will not be excessive
because a double layer covering will still cost less than a polycarbonate
panel covering with its aluminium extrusions, and will last longer, and will
have much higher light intensity inside the greenhouse.
5. Polyvinyl Chloride Rigid-Panel
 Initially, polyvinyl chloride (PVC) rigid panels showed promise as an
inexpensive covering material (about 40% of cost of fiberglass r~inforced
plastics).
 They had a life expectancy of five years or more, wqen polyethylene lasted
one year. Commercial use of these panels soon indicated that this life
expectancy was much short, sometimes as little as two years.
 This was unacceptable as the cost of PVC panels was four to five times that
of polyethylene film and they required much more time to install. Now-a-
days, PVC rigid panels are not in use.

45
6. Fiberglass-Reinforced Plastic Rigid Panel
 Fiberglass-reinforced plastic (FRP) was more popular as a greenhouse
covering material in the recent past (Fig. 8.1). Based on the grade, the usable
life period of FRP panel varies.
 Some grades give five to ten years while better grades can last up to 20
years.
 Corrugated panels were used because of their greater strength.
 Flat panels are occasionally used for the end and side walls where the load is
not great. Panels are available in 1.3 m widths, lengths up to 7.3 m, and in a
variety of colours.
 The panels are flexible enough to conform to the shape of quonset
greenhouses, which make FRP a very versatile covering material. FRP
panels can be applied to the inexpensive frames of film plastic greenhouses
or to the more elaborate frames of glass type greenhouses.
 In the former case, the price of the FRP panel greenhouse lies between that
of a film plastic greenhouse and that of a glass greenhouse, but the cost is
compensated by the elimination of the need for replacement of film plastic.
 In the latter case, the FRP panel greenhouse costs about the same as the glass
greenhouse.
7. Acrylic and Polycarbonate Rigid Panel
 Acrylic and poly carbonate double-layer rigid panels have been available for
about 15 years for greenhouse use.
 The panels have been used for glazing the side and end walls of film plastic
greenhouses and for retrofitting old glass greenhouse, while the acrylic
panels are highly inflammable, the polycarbonate panels are non flammable.
 The acrylic panels are popular due to their higher light transmission and
longer life. Polycarbonate panels are preferred for commercial greenhouses
due to lower price, flame resistance, and greater resistance to hail damage.
 Acrylic panels are available in thicknesses of 16 and 18 mm, and have 83%
of PAR light transmission. The thicker panels cannot be bent, but the thinner
panels. It can be bent to fit curved-roof greenhouses.
 These panels are also available with a coating to prevent condensation drip.
Polycarbonate panels are available in thicknesses of 4, 6, 8, 10 and 16 mm.
 These panels are also available with a coating to prevent condensation drip
and also with an acrylic coating for extra protection from UV light.

46
 Chapter 8
Engineering properties of cereals, pulses and oil seeds

The knowledge of physical properties such as shape, size, volume, surface

area, test weight, density, porosity, angle of repose, etc., of different grains is
necessary for the design of different equipment for handling, processing and
storage of grains.
1. Physical Properties
Shape and size
 Shape of the grain is connected with the geometrical form of the grain. Size
of the grain refers to the characteristics of an object which in term determine
how much space it occupies and, within limits, can be described in terms of
length, width, and thickness.
 The Shape and size together with other characteristics of the grains is
important in the design of the seed grader.
 These factors determine the free flowing or bridging tendencies of the seed
mass, and therefore, determine the suitable handling and feeding equipment.
i) Roundness: It is measure of the sharpness of the solid material. The most
widely accepts method for determining the roundness of irregular particle is
given below.

Fig. Diagram for roundness

ii) Sphericity: It is defined as the ratio of the diameter of a sphere of the
same volume as that of the particle and the diameter of the diameter of a
smallest circumscribing sphere or generally the largest diameter of the
particle. If De is the diameter of a sphere having same volume as that of the
particle and Dc is the diameter of smallest circumscribing sphere, the the
sphericity can be expressed as:

47
  The sphericity (φ) of the fruits can be calculated using the following
formula.

where, (a) = major diameter ; (b) = intermediate diameter; (c) = minor

diameter
Bulk density
 The bulk density (ρb) considered as the ratio of the weight of the grain in kg
to its total volume in m3.
 The bulk density of grains is measured using 1 liter measuring cylinder and
electronic balance.
 The bulk density of the food grains changes with the change in the moisture
content.
 Hence, the moisture content of the food grains at which the bulk density was
measured also to be reported. The bulk density can be calculated using the
following formula

where, ρb= bulk density, kg/m3,

Ws = weight of sample, kg and
Vs = volume of the sample i.e., 1000 cc or 10-3
True density
 The true density (ρt) defined as the ratio of mass of the sample (W) to its
true volume. The true density (ρt) is determined using a Multivolume
Pycnometer (Helium gas displacement method).
 Multivolume Pycnometer‘s Helium displacement method provides a rapid
means for precisely determining the true volume of pores, porous materials,
and irregularly shaped food grains.
 The true density of the grains is found to be decreased with an increase in
moisture content as the increase in true volume of the grains is higher
compared to the increase in moisture content of the grains.
 Since, the true density varies with the moisture content of the food grains,
the moisture content of the food grains also to be reported.
 True density can be calculated using following formula.

48
Porosity
 Properties such as bulk density, true density and porosity of grains are useful
in design of various separating, handling, storing and drying systems.
 Resistance of bulk grain to airflow is a function of the porosity and the
kernel size.
 The porosity (ε) defined as the percentage of void space in the bulk grain
which is not occupied by the grain can be calculated from the following
relationship:

where, ε = porosity
ρb= bulk density, kg/m3
ρt = true density, kg/m3
2. Aero and hydrodynamic properties:
 The aero and hydrodynamic properties such as terminal velocity of
agricultural products are important and required for designing of air and
water conveying systems and separation equipments.
 For example, in pneumatic conveying and separation processes the material
is lifted only when the air velocity os greater that its terminal velocity.
Drag coefficient
 When fluid flow occurs about immersed objects, the action of the forces
involved can be illustrated by diagram as shown in figure. The pressure on
the upper side of the object is less than that on the lower side is greater than
the pressure P in the undisturbed fluid stream. This results in a decrease of
pressure, - P, on the upper side, and an increase of pressure, +P. In addition
to these forces normal to the surface of the object, there are shear stresses, ,
acting tangential to the surface in the direction of flow and resulting from
frictional effects.

Fig. The Forces acting on a body immersed in a fluid current

49
 The resultant force F may be resolved into components, Fh, the drag and FL
the lift. In most agricultural engineering applications the moving object is
usually free to assume its own random orientation. For this reason the net
resistance force Fr can be given in terms of an overall drag coefficient C as
follows

Where, FR = resistance drag force or weight of particle at terminal velocity,

kg
C = Overall drag coefficient
Ρf = mass density of fluid, kg s2/m4
Ap = projected area of the particle normal to direction of motion, m2
V = relative velocity between main body of fluid and material, m/s

Terminal velocity:
 The terminal velocity of a particle may be defined as equal to the air velocity
at which a particle remains in suspended state in a vertical pipe.
 In the steady state conditions, after attaining the terminal velocity, if the
density of the particle is greater than the density of the fluid, the particle will
move downward.
 If the density of particle is lesser than the density of the fluid, the particle
will rise upward.
 If the separation of mixture of wheat and foreign matters is to be removed by
air stream, the terminal velocities of each component of mixture decide the
range of air velocity to be used for a definite extent of separation.
3. Frictional properties:
 The frictional properties such as coefficient of friction and angle of repose
are important in designing of storage bins, hoppers, chutes, pneumetic
conveying system, screw conveyors, forage harvesters, threshers etc.
Static friction: the friction may be defined as the frictional force acting
between surfaces of contact at rest with respect to each other.
Kinetic friction: It may be defined as friction forces existing between the
surfaces in relative motion.
 If F is the force of friction and W is the force normal to the surface of
contact, then the coefficient of friction ‘f’ is given by:

50
  The coefficient of friction may also be gien as the tangent of the angle of the
inclined surface upon which the friction force tangential to the surface and
the component of the weight normal to the surfaces are acting.
Angle of repose:
 The angle of repose is the angle between the base and the slope of the cone
formed on a free vertical fall of the granullar material to a horizontal plane.
 The size, shape, moisture content and orientation of the grains affect the
angle of repose.
 There are two angles of repose, (1) static angle of repose, and (2) dynamic
angle of repose.
 Static angle of repose: It is the angle of friction taken up by granular
material to just slide upon itself.
 Dynamic angle of repose: It comes in picture when bulk of the grain
material is in motion like discharge of grain from bins and hoppers. The
dynamic angle of repose is more important that static angle.
Angle of repose of some grains
Grain Angle of repose
Wheat 23-28
Paddy 30-45
Maize 30-40
Barley 28-40
Millets 20-25
Rye 23-28
 It has been found that increase in moisture content in grain, angle of repose
also increases.
Thermal Properties:
The thermal properties like specific heat, thermal conductivity, thermal diffusivity,
enthalpy, surface heat transfer coeffiecint etc. are important for the development of
any thermal processing system.
The themral processing system may include heating, cooling, freezing, drying etc.
The heat treatment to cereals and some of the pulses is given for stimulating
germination. The heat treatment given to cereals like wheat, maize, sorghum and
few millets for thermal killing of insect pest in storage has been proved to be a
promising technology.
Heat treatment is also given to the fruits for fruit fly quarantine disinfection.
To design a dryer, the calculation of heat requirment is the most important step.
Specific heat: the specific heat may be defined as amount of heat in that must be
added to or removed from 1 kg of substance to change its temperature by 1oC.
The specific heat of bone dry grain varies from 0.35-0.45 kcal/kg oC.

51
ii) Thermal conductivity: The thermal conductivity may be defined as the rate of
heat flow through unit thickness of material per unit area normal to the direction of
heat flow and per unit time for unit temerature difference.
It is a measure of ability of the material to conduct heat.
The thermal conductivity can be expressed by the following equation:
Q = KA∆T
Where: Q = heat flow rate
A = Area
∆T = temperature difference in the direction of heat flow
K = thermal conductivity
The thermal conductivity of single grain ranges from 0.3-0.6 kcal/m.hr.oC and bulk
grain varies from 0.10-0.15 kcal/m.hr.oC. The difference is due to the air spaces
present in the bulh grain. The thermal conductivity of air is 0.02 kcal/m.hr.oC
iii) Enthalpy: Enthalpy is the total heat content or energy of a material.
The enthalpy data are required for frozen foods that freeze over a range of
temperature below 0oC and not for those substances that freeze in a narrow
temperature limits, as the case of pure substance like water.
The enthalpy of moist material can beestimated by using following expression.
h2-h1 – mcp(T2-T1) + mXwL
Where, h2-h1 = enthalpy difference
M = mass of the product
cp = specific heat of the product
T2-T1 = temperature difference
L = latent heat of fusion for water.
iv) Thermal diffusivity: The thermal diffusivity may be calculated by dividing the
thermal conductivity with the product of specific heat and mass density.
It may be expressed as:

µ = thermal diffusivity
ρ = density
K = thermal conductivity
Cp = specific heat

Thermal diffusivity is important in determination of heat transfer rates in solid

food materials of any shape.
Physically it shows the relationship between the ability of a material to conduct
heat to its ability to store heat.

52
 Chapter 9
Introduction of cleaning, grading and sorting

Cleaning and grading:

Cleaning and grading are the first and most Important post harvest
operations undertaken to remove foreign and undesirable materials from the
threshed crops/grains and to separate the grains/products into various fractions.
The comparative commercial value of agricultural products is dependent on their
grade factors. These grade factors further depend upon. 1) Physical characteristics
like size, shape, moisture content, colour etc., 2) Chemical characteristics like
odour, freefatty acid content and 3) Biological factors like germination, insect
damage.
A mixture of seeds can be separated on the basis of difference in length,
width/thickness, specific gravity, surface texture, drag in moving air, colour, shape,
electrical conductivity and magnetic properties.
Cleaning in agricultural processing generally means the removal of foreign
and undesirable matters from the desirable grains/products. This may be
accomplished by washing, screening, hand picking etc.
Grading refers to the classification of cleaned products into various quality
fractions depending upon the various commercial values and other usage.
Sorting refers to the separation of cleaned products into various quality
fractions that may be defined on the basis of size, shape, density, texture and
colour.
Scalping refers to the removal of few large particles in an initial process.
Screening:
Screening is a method of separating grain/seed into two or more fractions
according to size alone. For cleaning and separation of seeds, the most widely
used device is screen. When solid particles are dropped over a screen, the particles
smaller than the size of screen openings pass through it, whereas larger particles
are retained over the screen or sieve. A single screen can thus make separation
into two fractions. When the feed is passed through a set of different sizes of
sieves, it is separated into different fractions according to the size of openings of
sieves. Screens along with an air blast (air screen) can satisfactorily clean and sort

53
 most of the granular materials. The screens are generally suspended by hangers,
and when this unit is oscillated by an eccentric unit they have a horizontal
oscillating motion and at the same time a smaller vertical motion. These two
motions cause grains to travel downward to the screen and at the same time the
grains are thoroughly stirred during the passage.
Screen motions:
The purpose of screen motion are,
1) to spread the material over the surface of screen.
2) to cause the fine particles to settle at the sub-surface.
3) to discharge the oversize particles.
Types of screens :
In most screens the grain/seed drops through the screen opening by gravity.
Coarse grains quickly and easily through large opening in a stationary surface.
With finer particles, the screening surface must be agitated in some way. The
common ways are,
1) revolving a cylindrical screen about a horizontal axis and
2) shaking, gyrating or vibrating the flat screens.
i) Grizzly :
The grizzly is a simple device consisting of a grid made up of metal bars,
usually built on a slope, across which the material is passed. The path of material
flow is parallel to the length of bars. The bars are usually so shaped that the top is
wider than the bottom. The grizzly is often constructed in the form of a short
endless belt so that the oversize is dumped over the end while the sized material
passes through. In this case bar length is transverse to the path of materials. The
grizzly is used for coarsest and rough separations.
ii) Revolving screen / cylinder sorter:
Trammel or revolving screen is a cylinder that rotates about its longitudinal
axis. The wall of the cylinder is made of perforated steel plate or sometime the
cloth wire on a frame, through which the material falls as the screen rotates. The
axis of cylinder is inclined along with the feed end to the discharge end. Sizing is
achieved by having smallest opening screen at the feed end with progressively
larger opening screens towards the discharge end. This type of sorter is simple and
compact with no vibration problem. But the capacity of cylinder sorter is lesser
than the vibrating screen of same size. Although it is an accurate sizer, it does not
perform well with friable material or in cases where particle degradation is
54
undesirable because tumbling produces some autogeneous grinding. The speed of
rotation of the trammel be kept within the limit at which the material is carried
from bottom to a distance equal to the radius of cylinder before it starts tumbling.
The inclination of cylinder sorter for dry granular materials is kept upto 125
mm/m. the capacity, bed depth and efficiency of these screens can be changed by
changing the speed of operation and the inclination of cylinder by multiplying the
length of cylinder by 1/3 of the diameter.
iii) Shaking screen :
Like the vibrating screen, shaker is a rectangular surface over which material
moves down on an inclined plane. Motion of the screen is back and forth in a
straight line. Although in some cases vibration is also given to the screen. Unlike
the vibrating screen, the shaker does not tumble or turn material enroute except
that some shaking screens have a step-off between surface having different size
openings, so that there may be two or three tumbles over the full length of the
screen. The shaker is widely used as combined screen and conveyor for many
types of bulk material.
iv) Rotary screen:
Rotary and gyratory screens are either circular or rectangular decked. Their
motion is almost circular and affects sifting action. These are capable of accurate
and complete separation of very fine sizes but their capacity is limited. These
screens are further classified into two categories.
a) Gyratory screens:
This is generally a single decked machine. It has horizontal plane motion,
which is circular at feed end and reciprocating at the discharge end. The drive
mechanism is at the feed end and is either a V-belt or direct coupling.
The shaft that imparts motion to the screen is a counter balanced eccentric. The
shaft moves about a vertical axis. At the discharge end most rotary screens operate
with screening surface nearly horizontal.
b) Circular screens:
These are also rotary screens but their motion in horizontal plane is circular
over the entire surface. Similar to the gyratory screens, the screening surface of
circular screens are also little bit tilted for allowing the material to move over
them.

55
v) Vibratory screen:

The vibratory screens are agitated by an eccentric unit. When materials to be

separated are put on a vibratory screen, because of its vibration, materials are also
agitated and separated during their transit over the screen. The eccentricity is
usually of two types,
1) a shaft to which off centre weights are attached, and
2) a shaft that itself is eccentric or off centered.
In the later case the eccentricity is balanced by a fly wheel for providing
uniform vibration. Most vibrating screens are inclined downward from the feed
end. Vibration is provided to the screen assembly only, and the body and other
surrounding structure are isolated from vibration. Generally, upto three decks are
used in vibrating screens. The capacity of vibrating screen is higher than any other
similar sized screen and is very popular for cleaning and grading of granular
agricultural products.
vi) Horizontal screen:
Horizontal screens are special case of vibrating screen. These are designed
for operation with low head room. These operate absolutely flat without the aid of
gravity. All sorting, stratification and material transportation take place on the
strength of a sharp forward thrust stroke pulls the deck out from underneath the
bed. Effectiveness of these screens is higher because material is kept on the screen
for a longer period in comparison to inclined screens.

Various other types of screens used for cleaning and separation are listed below:
1) Rotex screens
2) Hummer screens
3) Circular screens
4) Symon’s rod deck screens
5) Resonant screens
6) Centrifugal screens

Screen openings:
Screens are generally constructed by perforated sheet metal or woven
wiremesh. The openings in perforated metal sheets may be round, oblong or
triangular as shown in Fig. The openings in wiremesh are square or rectangular.

56
The size and shape and their combination of the screens available in market are
identified by some trade numbers.

Perforated metal screens:

i) Round openings: The round openings in a perforated sheet metal screen
measured by the diameter (mm or in.) of the openings. For example, 1/18 screen
has round perforation of 1/18 in. in diameter or 2 mm.
ii) Oblong openings: The oblong or slotted openings in a perforated sheet metal
screen are designed by two dimensions, the width and length of the opening. While
mentioning oblong openings the dimension of width is listed first then the length as
1.8x20 mm. generally, the direction of the oblong opening is kept in the direction
of the grain flow over the screen.
iii) Triangular openings: There are two different systems used to measure
triangular perforations. The most commonly used system is to mention the length
of each side of the triangle in mm, it means, 9 mm triangle has 3 equal sides each 9
mm long. The second system is to mention openings according to the diameter in
mm that can be inscribed inside the triangle. This system is identified by the letter
Vas 9V, 10V etc.
Wiremesh screens:
i) Square mesh: The square openings in wire mesh are measured by the number
of openings per inch in each direction. A 9x9 screen has 9 openings per inch.
ii) Rectangular mesh : The rectangular openings in wiremesh screens are
measured in the same way as square wiremesh screen. A 3x6 rectangular
wiremsh screen will have 3 openings per inch in one direction and 6 openings

57
 per inch in the other direction. The rectangles formed by the wiremesh are
parallel to the direction of grain flow.

Fig. Wiremesh screens

Equipment for Cleaning and Grading:

It is very difficult to clearly differentiate among the processes of cleaning,
grading and separation because all of these are carried out simultaneously with the
common procedures.
The operation of cleaning, grading and separation of the products are
performed by exploiting the difference in Engineering properties of the materials.
These products may be used either for food or seed purposes. Various types of
cleaning, grading and separation equipment have been designed and developed on
the basis of properties of product to be handled. Thus, these equipment can be
classified based upon following characteristics of the material.
1) Size
2) Shape
3) Specific gravity or weight
4) Surface roughness
5) Aerodynamic properties
6) Ferro-magnetic properties
7) Colour
8) Electrical properties

58
1) Separation based upon size:
Screen cleaners / graders :
It performs the separation according to size alone. The mixture of grain and
foreign matter is dropped on a screening surface which is vibrated either manually
or mechanically.
A single screen can make the separation into two fractions. The screening unit may
be composed of two or more screens as per the cleaning requirement.
A hand operated screen cleaner is shown in Fig. This equipment is made of
mild steel. The separation takes place due to difference in size of grain and foreign
matter . the cleaner is operate by hanging on an elevated point with the help of four
ropes. Grain is fed on the screening surface in batches. The screens can be changed
as per the grain to be handled. The cleaner is swung to and fro till all the grain is
screened. The cleaned grain is retained by the bottom sieve which can be
discharged by pulling a spring loaded shutter. Impurities of larger size, stubbles,
chaff etc. are retained on the top sieve and can be removed easily. Down stream
from the bottom sieve consists of dust, dirt, brokens, shrivelled grain etc. drop
down during the operation.
A typical seed grader is shown in Fig. It consists of a hopper, seed, roller for
controlling the feed rate, set of three sieves, pulley, eccentric system, outlets, frame
and electric motor. The sieves are detachable and can be replaced by suitable
sieves if other round grains are to be graded. The seed is put into the hopper and it
is dropped onto the sieve through feed rollers. Sieves are vibrated through an
eccentric system. Graded seeds are collected through three different spouts. The
machine is suitable for grading of food grains.

59
The schematic diagram of a groundnut grader is shown in Fig. It consists of a
feeding hopper, two oscillating sieves, brushes below the sieves to avoid clogging,
frame and an electric motor. There are two oscillating sieves which are oscillated
by the eccentric mechanism. At the head end of the sorting sieve, a spreader is
provided to create uniform layer for efficiency in grading. The machine grades
groundnut pods/kernels into three distinct grades according to size e.g., Grade-I.
Grade-II and rejects. Oscillating sieves are replaceable by different grades
depending upon the groundnut varieties to be graded.

60
Air-screen cleaners:
The screens used in combination with air blast performs satisfactory
cleaning and separation operations for most of the granular materials. The air-
screen cleaner uses three cleaning systems; blowing or aspiration, scalping screens
and grading lower screens. The air-screen grain cleaner can be classified in two
distinct types:
i) Vibratory air-screen cleaner:
The screening unit is composed of double or multiple (upto 8 number)
screens. These screens are tightened together and suspended by hangers in such a
manner that these have horizontal oscillating motion and slightly vertical motion.
These two motions in combination move the grain down the screen and at the same
time toss sufficiently above the screen so that the bed of grain is properly stirred.
The slope of the screen is adjustable to control the rate of downward travel of the
grain. The screens are available in various shapes like; round, triangular or slotted
holes as discussed earlier. Sometimes the holes of the screen are clogged when fine
degree3 of sorting is made by the machine. To avoid the clogging, the screens are
generally fitted with a brush which moves under the screen and pushes the clogged
material back through the screen . other such devices can also be used for this
purpose.
A simple vibratory type air-screen cleaner is shown in Fig. it is a two screen
machine fitted aspirator to suck the lighter materials. The grain passes from the
61
feed-hopper over baffle plates to the upper screen. Light particles are sucked away
by aspirator during the operation. Coarse impurities such as stones, straw particles
etc. are screened off by upper screen and discharged out through an outlet. The
grain falls through on to the lower screen, where the sand and the dust particles are
screened off. The grains leave the machine through discharge funnel. While
passing through the funnel the grains are again cleaned by upward draft of air in
the ascending separator. During this process the remaining light impurities and
shriveled grains are sucked away and the light impurities are removed by a
cyclone separator to which dust bags are attached for the collection of the
impurities.

Such cleaners can also be operated manually either using hand or pedal system.
But the capacity of cleaners is lower than the power operated machines. A pedal-
cum-power operated cleaner is shown in Fig. The machine is made up of mild steel
and consists of a grain hopper, with feeding mechanism, sieve box on hanging
shoes, blower unit, driving and eccentric unit. The machine can be operated either
manually(pedal) or electric motor.

62
i) Rotary screen cleaner :
The rotary screen cleaner has normally circular decks. Their motion is
circular in horizontal plane. These have either single or double drum. A single
drum rotary screen cleaner is shown in Fig.

The machine consists of a rotary screen, aspirator and hopper and equipped
with an electric motor which gives drive to the rotary screen and the aspirator. The
mixture is fed into the hopper. The sound grains pas through the screen perforation
into the centre of the screen drum , where as oversized material is retained above
and pass out though an outlet. The sound grains come out at the centre side of the
63
screen drum rotating at low speed and fall onto the vibratory screen which remove
the dirt particles. The light particles like straw and dust are sucked away by the
aspirator and discharged through the aspirator outlet. The cleaned grains are
delivered through the discharge chute.
A double drum rotary screen cleaner is shown in Fig. It has two rotary
screens and other components are same as in the case of single drum rotary screen
cleaner. The two screens rotate in opposite direction to each other.
Disc separator :
The disc separator separates materials on the basis of difference in length of
various constituents. The separator has pockets or indentations on its surfaces.
When the machine is operated, the smaller sized materials are caught in the
pockets while the larger ones are rejected. It is used especially for removing
dissimilar material like wheat, rye, mustard, barley from oats.
The indent disc separator consists of a number of disks in series fitted on a
shaft inside a close housing which revolves on a horizontal axis. The pockets are
slightly undercut on each disk as shown in Fig. As the disks revolve through a
mixture of grains, the pockets pick up short grains and drop them in a trough at the
side of the machine. The desirable or undesirable materials not lifted by the disks
are conveyed through the disk spokes to the end of the machine and pass out
through the tailing opening.
A number of distinct separations or grading of the grain of varying length
can be made in a single machine by installing combination of disks having pockets
of different sizes. The mixture first passes through the disks with small pockets and
then disks with pockets progressively larger from inlet to discharge. If only one
separation has to be performed, combination of disks of same size of pockets is
used in the machine to increase its handling capacity. The removable vanes are
provided with the spokes of the disks which serve the purpose to move grains
through the machine, agitate them and also bring them in contact with the pockets.

The disk pockets are made of three basic shapes of various sizes. The ‘R’
pocket derives its name from ‘rice’ and was designed to remove broken rice grains
from tubular or elongated grains. It has a round lifting edge and round leading
edge. It rejects round grains but lifts out cross-broken or flat grains.
The ‘V’ pockets derives its name from ‘Vetch’ and was designed to pick up
and remove round shaped grains. It has a round lifting edge which tends to rejects
64
tubular or elongated grains. Disks with other letter designations are designed to
perform specific separators.

Fig: Disc separator

Indented cylinder separator:
The indented cylinder separators also separate the materials on the basis of
relative lengths like disk separators. It consists of a horizontal rotating cylinder
which has indents are closely spaced and hemispherical in shape. When the
mixture of grain is fed into one end of the cylinder, short grains are picked up by
the combined effect of fitting into the indents and centrifugal force. These grains
are dropped into an adjustable trough inside the cylinder near the top of rotation. A
screw conveyor is provided in the bottom of the trough which conveys the
material. Generally, the cylinder is kept at slight inclination to facilitate gravity
flow of grains in the cylinder.
The cylinders with indents of different sizes are available, but the size of all
important adjustments for obtaining the desired level of separation. Since the
centrifugal force helps to handle the grain in the pocket, it affects the distance
traveled by grains before they fall.
The excessive speed will not allow grains to drop from the indents. Too slow
speed will not lift short grains from the mixture. The position of separation edge of
the adjustable trough should be such that it can catch the desired fraction of the
dropping grain.

65
Spiral separator:
The spiral separator separates the grains as per their roundness. The main
component of the separator is a stationary, open screw conveyor standing on one
end (Fig.). The mixture is fed at the top of the unit. The round materials of the
mixture pick up speed as they slide or roll down the inclined surface until their
centrifugal force becomes sufficient enough to throw them in the outer helix.
While the non- round materials are caught in the inner helix and are discharged
through a separate spout.
There is no moving part in the spiral separator. The rate of feeding is the
only adjustable component. The feeding should be such that each grain / particle
roll independently for effective separation. The main limitation of the spiral is lack
of flexibility.
Separation of mustard, rape, soybean, wild peas or other round seeds can be
performed from wheat, flax, oats etc. This device is less versatile as compared to
other mechanical cleaners, but it is simple, inexpensive and quite useful for seed
cleaning purposes.

66
Inclined draper :
The separation by inclined belt draper takes place due to difference in shape
and surface texture of the material. This technique of separation is used when all
other methods fail.
The mixture to be separated is fed over the centre of an inclined draper belt
moving in upward direction. The round and smooth grains roll or slide down the
draper at faster rate than the upward motion of the belt, and these are discharged in
a hopper. The flat shape or rough surfaced particles are carried to the top of the
inclined draper and dropped off into another hopper. (Fig.).

The belts of different degrees of roughness may be used as a draper for

separate materials. If rolling tendencies of the grain are predominant, the rough
canvas belt may be used. The smooth, plastic belt may be used in case sliding
action is desired for the lower fraction. Feed rate, speed of draper and angle of
inclination are other important variables for effective separation of dissimilar
materials.
The feed rate is kept low enough to give opportunity to each grain for
separation. The speed of the draper may be varied to simulate with the length of

67
incline. The angle of inclination is adjusted to assure rolling or sliding of the
desired lower fraction.
To increase the capacity of the separator, number of belts may be used one
above the another in a single machine.
Velvet roll separator:
The velvet roll separator or roll mill separates grains on the basis of
difference in shape and surface texture. It is a finishing machine and should be
used only after cleaning and separation of grain from the chaff and trash. It is
effective in separation grain with a rough seed coat or sharp angles from smooth
surface grain.
The separator consists of two parallel inclined rolls covered with velvet cloth
and placed side by side in contact with each other. The rollers rotate in opposite
directions. An adjustable curved shield is provided just above the rollers (Fig.).

The mixture to be separated is fed onto the upper end of the rollers. As the
rollers rotate, the smooth grains bounce down the inclined trough and are
discharged at the lowr end of the machine. The rough surfaced grains or the grains
having sharp or broken edges are caught in the velvet. These grains are thrown up
against the shield and take a bouncing path between shield and rollers and are
finally thrown over the sides.

68
 For achieving desired separation, adjustments can be made in feed rate,
speed of rollers, characteristics of cylinder roughness and roll inclination. To
increase the capacity of machine, the number of roll pairs one above the other can
be increased.
Pneumatic and aspirator separators:
The pneumatic separation is based on the difference in aerodynamic
properties of various constituents of the mixture. The aerodynamic properties of a
particle depends upon its shape, size, density, surface and orientation with respect
to air current. Both the aspirator and the pneumatic separator use terminal velocity
of th grain to separate different fractions. This refers to the velocity of air required
to suspend particles in a rising air current.
In a pneumatic separator, the fan is placed at the intake end of the machine
which creates higher pressure than the atmospheric pressure. The high pressure air
blast separates the materials. The mixture of products is introduced into a confined
rising air stream, the particles with low terminal velocities than air lifted by the air
current whereas the particles with higher terminal velocities than air velocity fall
down. The air velocity can be adjusted by altering the speed of air inlet.
The aspirator has a fan at the air discharge point which creates a vacuum or
negative pressure within the machine. The scalping separator is a type of aspirator
separator in which rough separation is performed. The mixture of the grain is
dropped into a rising air column which has a velocity slightly lower than the
terminal velocity of the heavier grains. The leaves, trash and lighter particles rise
with the air and are deposited in an enlarged settling chamber. The denser, plumper
grains fall through the incoming air into a container.
The fractioning aspirator is another type of separator. The mixture of grains
is fed into the lower end of an expanding air column, the heavier grains fall against
the air flow while the lighter particles are lifted. The grains with high terminal
velocity are dropped in the expanding column. The lighter fractions of grains
discharged as per the relative weight through different outlets positioned in the
column. Thus the mixture is separated into various fractions.

69
Magnetic separator:
The magnetic separator performs separation on the basis of surface texture
and stickiness properties of the grain. Since the grains do not contain any free iron,
therefore, are not attracted by the magnet. A selective pretreatment of mixing
finely ground iron powder to feed mass is given. The grain mixture is fed to a
screw conveyor or other mixing device that tumbles and mixes the grain with a
proportioned amount of water .due to moisture, iron powder adheres to rough,
cracked, broken and sticky seed coats. Moisture dos not remain on smooth grains
so no iron powder adheres to smooth surfaced grains.
The grain mixture is fed onto the top of a horizontal revolving magnetic
drum, the smooth grains that are relatively free of powder fall along the drum
simply by gravity. The materials with iron powder are attached by the magnetic
drum and stick to it and are removed by rotary brush or break in the magnetic field
as shown in Fig.
Most magnetic separators have two or three revolving magnetic drums
operating in a series. The grain mixture is passed over these magnetic drums to
increase the efficiency of operation. The extent of difference in seed coats, amount
of water mixed, amount of iron powder and thoroughness of powder-water mixing
operation affect the degree of successful separation by magnetic separators.

70
Cyclone separator:
The cyclone separator is a device for collecting the end product in
processing operations. It is most commonly used for collection of dust and wastes
during processing of grains. It can also be used with air screen cleaners to collect
light particles which could be carried by air stream. The application of cyclone
separator is also made to separate out air borne material from the discharge of
pneumatic conveyor. In operation of the separator, the air and material both enter
the cyclone tangentially at the top of the separator where pressure drop occurs and
air forms a vortex around the centre of the chamber. The whirling air being lighter
gets collected at the centre and is delivered out through the top opening as shown
in Fig. The heavier materials slide down along the walls of the cyclone and is
discharged at the bottom.
In a cyclone separator a particle is acted upon by two forces, the centrifugal
force and the weight of the particle. The centrifugal force can be described as
under.

71
 WV 2
cf 
gR
where, cf = centrifugal force, kg
W = weight of particle, kg
g = acceleration due to gravity, 9.81 m/s2
R = radius of rotation, m
V = linear or tangential velocity, m/s
The separating force can be given as follows:

V2
F W 1
g 2G 2

72
The performance or separation factor of cyclone can be given by the following
equation:

cfV2
S 
Performance factor, W gR

It has been found that as ‘S’ increases the separation becomes more effective.
Colour separator:
The colour separator separates the fruits, vegetables or grains due to
difference in colour or brightness. The colour separators are generally used for
larger crop seeds like peas and beans. These seeds differ in colour because of
varietal differences and also due to immaturity or disease. The mud balls and
discoloured or defective seeds can be removed with the help of electronic
separator.
The grain mixture is fed uniformly into the optical camber of the separator.
Two photo cells are fixed at a particular angle which direct both beams to one
point of the parabolic trajectory of the grains. A needle is placed on the other side
which is connected to a high voltage source (Fig.). When a beam falls on a dark
object through photoelectric cells, current is generated on the needle. The needle
end receives a charge and imparts it to the dark seeds. The grains are then passed
between two electrodes with a high potential difference between them. The seed is
compared with a selected background or colour range, and is separated into two
fractions according to difference in colour. Since each grain is viewed individually
by this machine, the capacity is low.

73
 Chapter 10
Milling Process of grains

Milling is a general trade name which normally means reduction of food

grain into various end products like meal, flour, splitted products etc. Milling
includes; cleaning, grading, separating, mixing, pearling, polishing, dehusking, size
reduction etc. The meaning of the term milling varies with the crop. For example,
milling of wheat refers to a grinding operation to produce flour, whereas in rice
industry, milling refers to overall operations in a rice mill i.e., cleaning, dehusking,
paddy separation, bran removal and grading of milled rice. Milling also refers to
extraction of juice, oil or separation of fibre etc.
Most of the agricultural products products are in solid form and which is
generally difficult to handle, compared to liquid and gases. In processing, solids
appear in many forms as large irregular pieces or finely divide powders. These
particles may be hard and abrasive, soft, brittle, dusty or sticky and plastic.
According to the forms of solids, means are to be found to manipulate them into
end products and possibly to improve their handling characteristics.
Principles of size reduction
Crushers and grinders are the equipment mostly used for size reduction of
agricultural products. An ideal size reducer should fulfill the following conditions,
namely
(1) large capacity,
(2) should yield a predesired sized product or range of size,
(3) small power input requirement per unit of product handled
and
(4) easy and trouble free operation.
Usually the performance of any milling equipment is compared with respect to an
ideal operation as standard. The characteristics of the actual equipment are
compared with those of the ideal unit.
Size reduction results in the production of small particles which may be
required either for larger surface area or because of their definite shape, size and
number. Amount of power required to create smallest particles is one of the
parameters of the efficiency of operation. Second parameter is the desired
uniformity of size. The actual unit seldom yield a uniform sized product.

74
Irrespective of uniformity of feed size the ground product consists of a mixture of
various particle sizes. In some equipment there is a provision to control the
magnitude of the largest particles like the hammer mill, but the fine size is beyond
control. In some size reducing machine fines are minimized but they ca not be
eliminated altogether.
It has been found that the diameter of largest particle in comminuted product
may be as large as 1000 times the diameter of smallest particle.
The major expenditure involved in crushing and grinding operation is the
power requirement. Therefore, the factors controlling the power requirement are
important. Work required to strain the material is temporarily stored in it in the
form of mechanical energy, the material is disturbed beyond its strength and finally
broken into fragments. It results in generation of new surface. Solids have a certain
amount of surface energy, thus for creation of new surface, work is required and it
is supplied by the release of stress energy when material breaks. In size reduction
process, the stress energy excess of the new surface created is converted into heat
energy.
Energy requirements
The energy absorbed by a unit mass of the material is given by the following
equation.

e Ap  A f 
Ea c

Where, e = surface energy per unit area

Ap = area per unit mass of product
Af = area per unit mass of feed
The input energy (E) requirement for size reducing machine is greater than
the energy absorbed by the solid (Ea). Some part of the input energy is used to
overcome friction in the moving parts and bearing s of machine, rest is used for
crushing. The ratio of the energy absorbed to the input energy is known as the
mechanical efficiency ‘ηm’.

e Ap  A f 
Then E  Ema   m c

The power required by the machine can be calculated by the following equation.

feA p  A f 
P  E f 
 m c

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Or,
Where, Dp and Df = volume surface mean diameter of the product and feed,
respectively
 and  = sphericity of product and feed, respectively
p = particle density
f = feed rate
When a feed is reduced to symmetrical particles of a smaller size as shown
in Fig., the energy requirements must be related to some function of the size of the
feed and ground product. As per assumption both the particles are symmetrical, a
common dimension is used to calculate energy requirement

Fig. Reduction of feed to symmetrical smaller particle

E  X
X

E  c XXn
Therefore, the necessary energy required for size reduction is,
E  c  dx
Xn

Rittinger’s and Kick’s laws

A crushing law proposed by Rittinger states that the work required in
crushing is proportional to the new surface created. Rittinger assumed that size
reduction is essentially a shearing procedure. Therefore, energy requirement is
proportional to the square of the common linear dimension and thus the values of
‘n’ becomes 2. The energy requirement is given by the following equation

76
Where, Xp and Xf = length of product and feed, respectively.
Kicks proposed another law which based on stress analysis deformation
within the elastic limit. He assumed that the energy requirements for size reduction
is a function of a common dimension of the material, therefore, the value of ‘n’
becomes 1, and the energy requirements can be given by the following equation
X 
E  c ln 
f
X 
 p 
Kicks law can also be expressed as within the elastic limit the work required
for crushing a given quantity of material is constant for the same reduction ratio
irrespective of the original sizes. The reduction ratio of crushers is often expressed
as the ratio of the feed opening to the discharge opening. These openings
determined the maximum diameters of feed and product. To be more informative it
is necessary to specify the size distribution of feed and product.
Bond’s law
Bond reported a method for estimating the power required for crushing and
grinding operation. According to this law the work required to form particles of
size ‘Dp’ from very large feed is perpendicular to the square root of the surface-to-
volume ratio of the product
Sp 6

vp D p
Since,
P K

f Dp
Where, k = constant, depends on machine type and material being handled.
For use of above equation the work index (wi) for the material being reduced is
defined. The work index is the gross energy requirement in kilowatt-hour per tonne
of feed needed to reduce a very large feed to such a size that 80% of the product
passes through a 100µm screen.

where, P = power in kW
f = feed rate, t/hr

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 Dp = 80% of product passes through mesh of dia. Dp, mm
Df = 80% feed passes through mesh of dia. Df,, mm
The work index may be found experimentally from laboratory crushing and
grinding tests.
Size reduction procedures
The size of agricultural products may be reduced by several ways, but
mainly the following four methods are used in size reduction machines, (1)
compression or crushing, (2) impact, (3) shearing and (4) cutting.
Crushing
When an external force applied on a material excess of its strength, the
material fails because of its rupture in many directions. The particles produced
after crushing are irregular in shape and size. The type of material and method of
force application affects the characteristics of new surfaces and particles. Food
grain flour, grits and meal, ground feed for livestock are made by crushing process.
Crushing is also used to extract oil from oilseeds and juice from sugarcane.
Impact
When a material is subjected to sudden blow of force in excess of its
strength, it fails like cracking of nut with the help of a hammer. Operation of
hammer mill is an example of dynamic force application by impact method.
Shearing
It is a process of size reduction which combines cutting and crushing. The
shearing units consists of a knife and a bar. If the edge of knife or shearing edge is
thin enough and sharp, the size reduction process nears to that of cutting, whereas a
thick and dull shearing edge performs like a crusher. In a good shearing unit the
knife is usually thick enough to overcome the shock resulting from material hitting.
In an ideal shearing unit the clearance between the bar and the knife should be as
small as practicable and the knife as sharp and thin as possible.
Cutting
In this method, size reduction is accomplished by forcing a sharp and thin
knife through the material. In the process minimum deformation and rupture of the
material results and the new surface created is more or less undamaged. An ideal
cutting device is a knife of excellent sharpness and it should be as thin as
practicable. The size of vegetables and the knife as sharp and thin as possible.
Cereal grinding

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 All the cereal grains are plant seeds and contain a large centrally located
starchy endosperm which is also rich in protein. Almost all the cereal grains are
covered with protective outer layers such as husk and bran. The germ or embryo is
located near the bottom of the seed.
For general food uses we remove hulls which are largely cellulose and
indigestible to human being, the coloured bran and germ which contain most of the
oil. Since the oil can be attacked by enzymes and may produce rancid condition in
grain, the germ is removed. For various food purposes starchy and proteinaceous
endosperm is used. Sometimes the ‘B’ vitamins and minerals are added to ground
meal, which is known as enrichment.
Grinding of cereal may be broadly classified into two; 1) plain grinding and
2) selective grinding. In the first case grains are milled to a free flowing meal
consisting of sufficiently uniform particle size, whereas in the second process the
grinding operation is carried out in various stages depending upon the differences
in structural and mechanical properties of components of grain. Hardness of seed
effects the power requirements of the grinding.
Size reduction machinery
Size reduction devices are grouped as follows:
1. Crushers
2. Grinders
3. Fine grinders
4. Cutting machines
Crushers
These type of reducing machines squeeze or press the material until it
breaks. Crushers are mostly used to break large pieces of solid materials into small
lumps. Crushers are used in industrial operations, like mines etc. Use of crushers in
agricultural operations is limited. The crushers in use are, 1) jaw crushers, 2)
gyratory crushers and 3) crushing rolls.
Lime and other stones are first reduced by the jaw or gyratory crushers. In a jaw
crusher feed is admitted between two jaws, which are open at the top like ‘V’, one
of the jaws is fixed and somewhat vertical (Fig.) while the other is the swinging
jaw. This jaw reciprocates in a horizontal plane, and makes and angle of 20 to 30°
with the fixed jaw. The movable jaw is operated by an eccentric unit so as to
impart great compressive force. The solids which has to be broken is caught
between the two jaws. Large lumps of solid material are caught between the upper
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parts of the jaws and subsequently broken and dropped into the narrower space
below. The broken pieces are further reduced next time when jaws come closer.
The number of strokes given to the movable jaw by eccentric unit ranges between
250 to 400 times per minute.

Fig. Jaw crusher

In a gyratory crusher the jaws between which the solid materials fed, are
circular. In such crushers the material is being at all times at some point. Solids
are caught between ‘V’ shaped space between the head and casing (Fig). The
material is repeatedly broken in sufficiently small pieces to pass out from the
bottom. The crushing head is rotated by an eccentric unit. The speed of crushing
head ranges between 125 to 425 gyrations per minute. The discharge from the
gyratory crusher is continuous, hence the driving motor is uniformly loaded.
Less maintenance is required as compared to the jaw crusher, also the power
requirements is low.

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 Fig. Gyratory crusher
1. Feed 2. discharge 3. maximum opening 4. minimum opening 5. eccentric unit

Crushing rolls
In agricultural operations crushing rolls are mainly used for extraction of juice
from sugarcane. The crushing rolls are of two broad types; 1) smooth roll crusher
and 2) serrated or toothed- roll crushers.

Fig. Schematic diagram of a smooth roll crusher

Two heavy smooth – faced metal rolls rotating towards each other at same speed
on parallel horizontal axes are the working elements of the smooth-roll crusher. It
is shown in Fig. The size of the material / particles that can be caught by the rolls
depends upon the coefficient of friction between the material and the roll surface
and can be estimated by the following equation.
dp = 0.04 R + g
where, dp = maximum size of particle
R = roll radius
G = half of the width of gap between the rolls
The rolls exert great force and to avoid any damage to roll surface because
of some unbreakable material coming with the feed, at least one of the rolls should
be spring loaded.
Apart from extraction of juice the smooth- roll crushers are used to make
grits or meal from food grains. These are also extensively used for making food
grains flakes.
Serrated or toothed- roll crushers
In such crushers the rolls are serrated as per need (Fig.) . Toothed roll
crushers are much more versatile than smooth-roll crushers. The best examples of
such type are the break and reduction rolls of a wheat flour milling plant.

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Disintegrators are the toothed roll crushers in which the corrugated rolls are
rotating at different speeds. These

Fig. Serrated or toothed-roll crusher

machine tear the feed apart. The size3 reduction in serrated=roll crushers is by
compression, impact and shear and not by compression alone, as in the case of
smooth roll crushers. The serrated-roll crushers can also accommodate larger
particles than smooth-roll crushers.
Grinders
The grinders are used to mill the grains into powder. The grinder comprises
a variety of size-reduction machines like attrition mills, hammer mills, impactors
and rolling compression mills.
Attrition mills
In an attrition mill the grains are rubbed between the grooved flat faces of
rotating circular discs. These mills are also known as burr or plate mills. The axis
of the roughened discs may be horizontal or vertical (Fig.). In attrition mill one
plate is stationary and fixed with the body of the mill, while other one is rotating
disk. The material is fed between the plates and is reduced by crushing and shear.
Mills with different patterns of grooves, corrugations on the plates perform a
variety of operations. In attrition mills the materials are slowly fed, overfeeding
lowers the grinder’s performance, also heat generation during milling increases.

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The disks of burr mills are usually 20 to 137 cm in diameter and are operated at
350 to 700rpm.

Fig. A vertical disk attrition mill

These mills are used for making whole grain and dehusked grain flour, but their
use in spices grinding is limited. Double runner disks type attrition mills are also
available. These are used for grinding of soft materials. In these mills both disks
are driven at high speed in opposite directions.
The fineness of grinding in burr mills is controlled by the type of plates and
the gap between them. The spacing between the plates is adjustable and usually the
arrangement is spring loaded to avoid damage to plates in case of over loading or
to overcome the damage to plates by foreign material coming along with the feed.
The salient features of burr mill are its lower initial cost and lower power
requirements. But foreign matter may cause damage/ breakage, and operation
without feed may result in burr wear.
Hammer mills
Hammer mills are used for various types of size reduction jobs. These mills
contain a high-speed rotor, rotating inside a cylindrical casing. The shaft is usually
kept horizontal. Materials are fed into the mill from the top of the casing and is
broken by the rotating hammers and fall out through a screen at the bottom. The
material or feed is broken by fixed or swinging hammers which are pinned to a
rotor. The hammers are rotated between 1500 to 4000rpm, strike and grind the
83
material until it becomes small enough to pass the bottom screen (Fig.).Fineness
of grinding is controlled by the screen size.

Fig. Hammer mill

There are several designs of striking edge of the hammers. Hammer mill can
grind almost anything- like tough fibrous solids, steel chips, food grains, sticky
clay, hard rock etc. commercial mills are reduce solids between 60 to 240 kg/
kWhr of energy consumption. Hammer mills are used for poultry feed grinding. It
was also found suitable for grinding of wet sorghum and millets and also for
potato, tapioca, banana and similar flour making.
Ball mills
The ball mill is a cylindrical or conical shell slowly rotating about a
horizontal axis. Half of its volume is filled with solid grinding balls (Fig.). The
shell is usually

Fig. Ball mill

84
 made of steel lined with high carbon steel plate, porcelain or silica rock. For
medium and fine reduction of abrasive materials ball mills are used. In a ball mill
size reduction is achieved by impact of the balls when they drop from near the top
of the shell. The balls are carried up the side of the shell nearly to the top. By
gravity the balls drop on the feed underneath. The energy consumed in lifting the
balls is utilized for grinding job.
When the ball mill is rotated, the balls are carried out by the mill wall nearly
to the top, where they are released by gravitational pull and drop to the bottom and
picked up again. Centrifugal force keeps the ball in contact with the mill wall.
Most of the grinding is done by the impact of balls. Due to centrifugal force, if the
speed of rotation of mill is faster, the balls are carried to more distance. In case of
too high speed, balls stick to mill wall and are not released. This is a stage of
centrifuging. The rotational speed at which centrifuging occurs is known as critical
speed, at this speed as the balls are released from the top, no impact occurs hence
little or no grinding results. The speed at which the outermost ball released from
the mill wall, depends on the interaction of gravitational and centrifugal forces.
The critical speed can be determined by the following equation
1 g
nc 
2 Rr
where, nc = critical speed, revolution/ s
g = acceleration due to gravity, 9.80 m/ s2
R = radius of the mill, m
R = radius of the ball, m
Rotational speeds of the ball mills are kept at 65 to 80% of the critical speed,
with the lower values for wet grinding in viscous suspension.
Cutting machines
Size reduction of fruits and vegetables are mostly performed by cutting
operation. To make thin slices of fruits and vegetables knife cutters are used. Few
types of knife cutters have been developed for cutting slices/ chips of potatoes,
cassava, banana etc. several knives are fixed to the rotor. Feed enters the chamber
from the top and are cut by the rotating knives afterwards discharged from the
bottom of the equipment.
Rietz mill or disintegrator

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 Rietz mill consists of a rotor inside a circular screen enclosure. The rotating
shaft is usually vertical. The rotor includes a number of hammers running at a
fairly close clearance. The hammers are generally rigidly fixed to the shaft, but in
sum cases swing hammers are also used. The product is discharged radially out
through a perforated sizing screen which surrounds the rotor.(Fig.). Rietz mill’s
many applications are on wet materials. The advantages of this mill was found in
those cases where solid content is in the range of 40 to 80 per cent. This keeps
running because the close hammer clearance keeps the sizing screen open.
Therefore, more fine and uniform grinding is possible. Rietz mill is able to grind
materials 15 micron size.

Fig. Rietz mill

Rietz machines are normally supplied in rotor diameters from 10 to 60 cm.

the hammer tip speeds are in the range of 5.2 to 111m/ s and horse power ranges
from 0.5 to 200.
Dispersion and colloid mills
Colloid mills are used for fine grinding. Where very little breakdown of
individual particles and disruption of weekly bonded clusters are required,
colloid mills are used.
Concentric cylinder abrasive mills

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 Such type of mills are mostly used for scouring of husk or seed coverings of
pulses and cereals. These work on frictional properties principle. Inside a larger
drum a d abrasive roller rotates. The outer cylinder can be made by a perforated
pipe or from metal sheet. The outer metal cylinder may be fabricated as bottom
half perforated, whereas the upper half portion is made from plain m.s. sheet.

87
 Chapter 11
Drying process

• Dehydration is termed as removal of moisture from grain to very low levels

usually to bone dry condition.
• Drying refers to removal of moisture from grains and other products to a
predetermined level which provides a safe level to store the grain for longer
time period.
• The predetermine level of moisture removal retains the nutritional quality of
food material otherwise over drying may deteriorate the nutritional
composition of food material.
• Drying is one of the oldest methods of food preservation.
• Drying is a thermo-physical and physio-chemical operation by which excess
moisture from a product is removed.
• Drying makes the food grains and other products suitable for safe storage
and protects them against attack of insects, mold and other micro-organisms
during storage.
Importance of drying process
• It minimizes microbial spoilage and chemical deteriorative reactions greatly
which provides safe storage facility for long time.
• It Permits continuous supply of product throw-out the year
• Permits early harvest which reduces field damage and shattering loss
• Permits the farmers to have better quality product
• Makes products available during off season
• The reduced weight of dried products decreases packaging, handling and
transportation costs.
• Most food products are dried for improved milling or mixing characteristics
in further processing.
• It helps in preparing different food product development.
Moisture migration during drying process
• During drying heat flows over the product and goes in to the product.
• This heat increases the temperature of product and moisture which converts
the moisture in to water vapor which results in to increase in the vapour
pressure that moves moisture towards the surface

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 • Hot air delivers two type of heat to the product moisture
– 1. sensible heat to raise the temperature of water within food product.
– 2. latent heat of vaporization to convert moisture into water vapour.

Fig. 11.1 moisture removal from food product during drying process

Fig. 11.2 Removal of moisture from capillaries of grains

A: Distribution of moisture in crystalline solids in first stage
B: Distribution of moisture in solids in middle
C: Distribution of moisture in solids at the end of drying
Moisture content
• The water content of agricultural product is given in terms of moisture
content.
• It is the percentage of water content present in product. It is designated by
two methods
1. wet basis

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 2. dry basis

Where,
M is moisture content on a percent basis,
w is total weight (also called as wet weight) and
d is dry weight.
• The value of dry basis MC is more than the wet basis MC.
• Wet basis MC is generally used by farmers at farm level.
• Dry basis MC can be more than 100% during calculation.
Determination of moisture content
• The methods of determining moisture content can be divided into two broad
categories:
1. Direct measurement: water content is determined by removing moisture and
then by measuring weight loss
– Air oven drying
– Vacuum oven drying
– Infra-red method
– Distillation method
2. Indirect measurement: an intermediate variable is measured and then
converted into moisture content. Building up calibration charts before applying
indirect measurements is a prerequisite.
– Electrical resistance method
– Dielectric method
– Chemical method

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1.1 Air oven method:
• Applied when moisture content of grain is upto 13%.
• Samples are heated at 130oC for 1-2 hours in oven till constant weight gain.
• Afterwards, samples are placed in desiccator to cool down.
• Drop in weight of grain is measured based on initial weight.
• Time and temperature are variable factor according to type of grain.

Fig 11.3 Air oven for determination of moisture content

1.2 vacuum oven drying:
• Vacuum facilitates the drying process at lower temperature.
• For fruits and vegetables where heat sensitivity is problem, vacuum is
applied in the oven to decrease the boiling point of moisture.
• The product temperature generally varies in vacuum oven between 60-70°C
and vacuum is maintained at <450 mm Hg.

.
Fig. 11.4 Vacuum oven drying method

91
1.3 Infra-red method:
• Moisture is directly measured by evaporation of water from sample.
• Instrument consists of a balance, a pan counter balanced by fixed weight and
a variable length of weighting chain.
• Infrared lamp is mounted on an arm above the pan.
• Lamp produces heat which evaporates moisture from sample and direct
moisture content of sample is shown on the instrument

Fig. 11.5 Infra-red drying apparatus

1.4 Brown-Duvel fractional distillation method:
• 100 gm whole grain along with 150 ml of mineral oil is taken in a flask and
sample is boiled. Evaporated moisture is collected in graduated flask.
• Millilitre of moisture shows the % of moisture content.
• Wet basis moisture content.

Fig. 11.6 Brown-Duvel fractional distillation drying method

2. Indirect method
2.1 Electrical resistance method
• The electrical resistance or conductivity of product depends upon its
moisture content.

92
 • This principle is used in résistance measuring devices.
• The moisture meter measures the electrical résistance of the material and is
calibrated against moisture determination from oven or other primary
methods.
• The universal moisture meter gives fairly accurate readings of moisture
content on wet basis.

Fig. 11.7 universal moisture meter

2.2 Dielectric method
• Such devices measure the dielectric constant of grains.
• Dielectric constant is a quantity measuring the ability of a substance to store
electrical energy in an electric field.
• Grains are filled in chamber. The sides of the chamber are formed by the
plates of a condenser between which a high frequency current is passed to
measure capacitance of the sample.
• The capacitance varies as per the water present in the sample, the degree of
compaction and grain temperature.

93
 Fig. 11.8 moisture meter based on dielectric properties
2.3 Chemical method
• In this method, the water is removed by adding a chemical, which
decompose or combines with water.
• From the chemical reaction a gas is produced which can be measured
volumetrically or which decreases the original weight of sample.
• This method is used for determining the m. c. of forages and grains by
shaking an excess of calcium carbide (Ca C2) with 30 grams of material.
• About 10 to 25 minutes are required to reaction.
Equilibrium moisture content
• It is the moisture content of a product in equilibrium with the surrounding air
at given temperature and humidity conditions.
• It is the minimum moisture content to which a material can be dried under
given conditions.
• The equilibrium moisture content for biological materials generally
increases rapidly with a relative humidity above 60 to 80% because of
capillary and dissolution effects.
• If a dry food is placed in environment with a constant humidity and
temperature, it will take up moisture by adsorption until it reaches its
equilibrium moisture content (where the net moisture exchange is zero)
which is called as adsorption EMC.
• If, however, a wet food with the same properties is placed in the same
environment, it will loose moisture by desorption and reach to equilibrium
moisture content which is called as desorption EMC.

94
 • If these values are plotted on a graph a loop is obtained which is called
hysteresis.
• The hysteresis effect is observed due to shrinkage effect during desorption
which changes the water binding properties of the food product. Therefore,
during adsorption same path of EMC is not observed.

1. Adsorption 2. Desorption
Fig. 11.9 Hysteresis effect
Moisture content can be classified according to its availability in the food matrix in
following types:
• 1. Bound moisture: Bound moisture is the amount of water tightly bound to
the food matrix, mainly by physical adsorption on active sites of hydrophilic
macromolecular materials such as proteins and polysaccharides, with
properties significantly different from those of bulk water.
• 2. Free moisture content: Free moisture content is the amount of water
mechanically entrapped in the void spaces of the system.
• Free water is not in the same thermodynamic state as liquid water because
energy is required to overcome the capillary forces.
• Furthermore, free water may contain chemicals, especially dissolved sugars,
acids, and salts, altering the drying characteristics.
Mode of heat transfer
There are three modes of heat transfer
1. Conduction

95
 2. Convection
3. Radiation

Fig. 11.10 Different modes of heat transfer

Conduction
• Conduction is the method of transfer of heat within a body or from one body
to the other due to the transfer of heat by molecules vibrating at their mean
positions.
• There is no actual movement of matter while transferring heat from one
location to the other.
• Conduction occurs usually in solids where molecules in the structure are
held together strongly by intermolecular forces of attraction amongst them
• Example: heat transfer through one layer of solid mater to another layer of
the material, heating of utensils etc.
• Determination of conductive heat by Fourier's law

Convection
• Convection is the mode of heat transfer which occurs mostly in liquids.
• In this method, heat transfer takes place with the actual motion of matter
from one place within the body to the other.

96
 • Example: boiling of liquid, drying of grain is mainly due to convective heat
transfer.
• Determination of convective heat transfer by Newton’s Law:

Radiation
• It does not require any medium and can be used for transfer of heat in a
vacuum as well.
• This method uses electromagnetic waves which transfer heat from one place
to the other.
• The heat and light from the sun in our solar system reach our planet using
radiation only.
• Example: heating of grain by sun’s radiation.
• Determination of radiation heat by Stefan-Boltzmann equation
Drying methods
• Sun drying
• Conduction drying
• Convection drying
• Radiation drying
• Freeze drying
• Osmotic drying
• Fluidized bed drying
• Dielectric drying
• Thin layer drying
• Deep bed drying
Sun drying:
• Traditional method of drying.
• Drying takes place through radiation mode of sun’s electromagnetic waves.

97
Conduction or contact drying:
• Heat is transferred to wet material mainly by conduction mode through
solid surface.
• Surface temperatures may vary widely.
• Dryers can be operated under low pressure and in inert atmosphere.
• Dust and dusty material can be removed easily.

Fig. 11.12 Contact drying

1. Wet material 2. Dry material 3. Hot cylinder
Convection drying:
• Drying agent is hot gas or hot air and supplies heat to the wet grain.
• Steam heated air, direct flue gases of agricultural waste etc. can be used as
drying agents.
• Drying temperature varies widely.
• If atmospheric humidity is high, natural air drying needs dehumidification.
• Fuel consumption is high as compare to conduction drying for same
capacity.

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 • Ex: fluidized bed dryer, hot air oven dryer

Fig. 11.13 Convective drying

1. Ambient air 2. Heater 3. wet material 4. Dry material 5. Exit air 6. Drying
chamber
Radiation drying:
• Based on absorption of radiant energy of sun and transformation into heat
energy by grain.
• Moisture movement and evaporation is caused by temperature difference
and partial pressure of water vapor b/w surrounding air.
• Ex: sun drying

Fig. 11.14 Radiation dryer

1. wet material 2. Radiator 3. Exit air 4. Dry material 5. Air inlet
Freeze drying:
• Drying is based on the sublimation (solid to gas) of frozen moisture from
wet product placed in a drying chamber.
• Works at low pressure.
• Heat is supplied by radiation or conduction mode.

99
Osmotic drying:
• Moisture is removed using osmo-active substance like, 60% aqueous
solution of saccharose or 25% aqueous NaCl.
• Concentration difference generated on both sides of the semi permeable
membrane.

Fig. 11.16 Concept of osmotic drying

Fluidized bed drying:
• Products are being dried under fluidized condition.
• Fluidization occurs by drying air with sufficient high velocity to cause
suspension.
• High rate of moisture migration takes place.
• Uniform drying.
• Used for high initial moisture content and lighter material.

Fig. 11.15 Fluidized bed drying

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Dielectric drying:
• Heat is generated within the solid by placing it in fixed high frequency
current.
• Polar molecules of substance get polarized and begin to oscillate in
accordance with the frequency.
• Oscillation causes friction which generates heat within the food stuff.
• Ex: microwave heating.
Thin layer drying
• Process in which all grains are fully exposed to the drying air under constant
drying conditions i.e. at constant air temp. & humidity.
• Drying rate is independent of air velocity.
• Up to 20 cm thickness of grain bed is taken as thin layer
• All commercial dryers are designed based on thin layer drying principles.
Deep bed drying
• All grains are not fully exposed to the same condition of drying air.
• Condition of drying air changes with time and depth of grain bed
• Rate of airflow per unit mass of grain is small
• Drying of grain in deep bin can be taken as sum of several thin layers.
• Humidity & temperature of air entering & leaving each layer vary with time
• Volume of drying zone varies with temp & humidity of entering air,
moisture content of grain & velocity of air

Fig. 11.18 deep bed drying characteristics at different depths

Type of dryers
Flat-bed type batch dryer
• This is a static, deep bed, batch dryer.
• Simple in design and most popular on farm.

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Construction:
• It is a rectangular box type dryer.
• Size of dryer depends on area of the supporting perforated screen on which
grain is placed.
• Holding capacity- 0.25-1 tonn/batch.
• Motor capacity- 0.25-1 hp.
Operation:
• Grain is placed on supporting screen and heated air is forced through the
deep bed of grain.
• After desired moisture content observed, grain is discharged mannualy.
• Temperature of heated air should be limited upto 45oC.
• Air flow rate- 20 to 40 m3/min/1000kg of raw paddy on initial moisture
content.
Advantages of flat-bed dryer:
• Reasonable price.
• Intermittent drying can also be used.
• Simple operation.
• Can be manufactured locally .
• Can be used for seed drying and for storage purpose also after drying.
Disadvantages:
• Slow rate of drying.
• Uneven drying.
• Small holding capacity.
Applications:
• For granular product such as cereals, pluses etc.

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 Fig 11.19 Flat-bed dryer
1. Exit air 2. Plenum chamber 3. blower

Fig 11.20 Flat-bed dryer

Tray dryer
• In tray dryers, the food is spread out, generally quite thinly, on trays in
which the drying takes place.
• Heating may be by an air current sweeping across the trays, or heated
shelves on which the trays lie, or by radiation from heated surfaces.
• Most tray dryers are heated by air, which also removes the moist vapors.
• Many shallow trays are kept one above other with a gap between.
• Tray may or may not have perforated bottoms.

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 • Generally used for vegetables and similar perishables.

Fig 11.21 Diagram of a tray dryer

1. exit air 2. blower 3. heater 4. inter space b/w trays 5. trays 6. plenum chamber
Advantages of tray dryer
• No loss of substance during handling.
• It is a batch dryer so that small amount of wet solid mixture can also be
dried.
• Easier to operate and repair.
• Good control on heat and humidity.
• It may be operated under vacuum.
Disadvantages of tray dryer:
• It is not suitable for large scale production.
• High labor requirement.
• Time consuming method.
Application of tray dryer:
• For drying of sticky material, granular material or crystalline material,
precipitates and pastes can be dried.
Solar drying
• This is traditional method of drying of crops and grains.
• Using the energy of the sun to remove moisture from the product.
• Traditional sun drying
• Mechanical solar drying
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Advantages of traditional sun drying:
• No fuel or mechanical energy is required.
• Unskillful labor requirement.
• No pollution.
Disadvantages of traditional sun drying
• Uncontrolled, and non-uniform drying, results in sun cracks in kernels.
• Completely weather dependent, not possible round the clock and round the
year.
• More losses occur due to shattering, birds, rodents.
• Require large drying floor and large number of labor.
Solar cabinet dryer
 The cabinet is a large wooden or metal box and the product is located in
trays or shelves inside a drying cabinet.
 If the chamber is transparent, the dryer is named as integral-type or direct
solar dryer. If the chamber is opaque, the dryer is named as Distributed type
or indirect solar dryer.
 Mixed-mode dryers combine the features of the integral (direct) type and the
distributed (indirect) type solar dryers.
 The combined action of solar radiation incident directly on the product to be
dried and hot air provides the necessary heat required for the drying process.
In most cases, the air is warmed during its flow through a low pressure drop
solar collector and passes through air ducts into the drying chamber and over
drying trays containing the crops.
 The moist air is then discharged through air vents or a chimney at the top of
the chamber. It should be insulated properly to minimise heat losses and
made durable (within economically justifiable limits).
 Construction from metal sheets or water resistant cladding, e.g. paint or
resin, is recommended.
 Heated air flows through the stack of trays until the entire product is dry. As
the hot air enters through the bottom tray, this tray will dry first. The last
tray to dry is the one at the top of the chamber.

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