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Seed Drying

Seed Drying means Lowering down the seed moisture content to safe moisture limits

Seed drying is very important in order to maintain seed viability and vigor

Moisture in seeds may deteriorate fast due to mold growth and increased micro-organism activity

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Methods of seed drying

1. Sun drying

2. Forced air drying –

1. Natural air drying

2. Drying with supplemental heat

3. Heated Air drying

Sundrying

Seed drying

The system involves harvesting of crops when they are fully dried in the field,

The moisture of seed is generally reduced in the field before harvest and later by sun drying on the threshing floor

Harvested produce is left in field for a couple of days for sun drying and later threshed and winnowed produce is spread in thin layer on threshing floor for sun drying

Advantages of sun drying

The main advantages of sun drying are

1. No additional expenditure

2. No special equipment is required

Disadvantages of sun drying

1. delayed harvesting

2. risk of weather damage and

3. increased possibilities of mechanical admixtures

Precautions to be taken while sun drying

1. Do not spread the produce on wet, dirty and kaccha, threshing floors.

2. Only one crop variety should be handled at a time and care should be taken to avoid mechanical mixtures

Forced Air Drying

In this system natural air is forced into seeds

The air passing through damp seeds pick up the water

The evaporation cools the air and the seed

The heat necessary for evaporating the water comes from the temperature drop of the air

Principle of Forced Air-Drying

Seeds are highly hygroscopic living material and their moisture content depends upon temperature and relative humidity of surrounding air

Whenever the vapour pressure in the seed is greater than that of the surrounding air vapour pressure will move out of the seeds i.e. the seeds will loose moisture

If however, the vapour pressure gradient is reversed, the moisture moves into the seeds and the seed will gain the moisture

When the two vapour pressures are equal the moisture content of seed is in a state of equilibrium with the surrounding atmosphere

Seed drying takes place when there is a net movement of water from the seed into surrounding air

The rate of seed drying depends on rate of moisture migration from the centre of the seeds to surface and by the speed at which surface moisture is evaporated in the surrounding air

The difference in moisture content of the air entering and leaving the seed is known as stratification

The amount of stratification and width of drying zone depends upon the volume of air flowing through the seed and its relative humidity.

At high airflow rates or with air of low relative humidity the drying zone may extend the entire bin except at bottom dried zone there will be reduced stratification (ie. Difference between moisture content of upper most and lower most layers)

The outlet should be twice the size of the inlet so that backpressure is not exerted.

Factors influencing the rate of moisture migration

1. The temperature of the seed

2. physical structure of the seed

3. chemical composition of the seed

4. seed coat permeability

influence the rate of moisture migration from the centre to the surface of the seed.

Factors influencing the removal of moisture

1. Surface saturation

2. relative humidity

3. temperature of drying air

influence removal of the moisture from the surface

Zones of drying of seeds

When air is forced through the seed for drying, all the seeds do not dry uniformly at the same time

Actually all the seeds in the drying bin may be considered to be in the three zones

1. The dried zone

2. The drying zone and

3. The wet zone

1. The Dried Zone

As the air enters the seeds, the zone nearest to the inlet gets dried first with either natural air or heated air

The seeds will dry below the desired level to some degree

The dried zone will gradually move upwards as drying proceeds

2. Drying Zone

The air passing through the dried zone picks up moisture in the next region, the drying zone, until it reaches moisture equilibrium or saturation in case of very wet seeds

How much moisture it can pick up before it reaches equilibrium is determined by the width of the drying zone

The lower edge of the drying zone at the interface with dried zone is called as drying front

3. Wet Zone

Refers to the region above the drying zone i.e. the seed in-between top of drying zone and the top surface of the seed, which is wet 16-20 % moisture

The top most layer will be wettest and last to dry. The drying front will not always be a parallel plane except when there is parallel airflow from all parts of the perforated floor below the seed

Generally the ducts are very commonly used and hence a covered drying front will be observed surrounding each inlet

Methods of Forced Air drying

There are three major drying methods for drying with forced air

1. Natural air drying

Natural air is used in this type of drying method

Requires 2 to 3 weeks for reducing the moistuee form seeds

Mostly used in western countries

2. Drying with supplemental heat

In this method temperature of the air is raised to about 10 to 20 oF for reducing relative humidity of the air

Requires 2 to 3 weeks for reducing the moistuee form seeds

Mostly used in western countries

3. Heated Air drying

In this method the drying air is heated to 110 degree F

Heated air-drying is mostly favored and used for seed drying

In this method the seed is dried in special drying bins or wagons using heated air

After drying the seed is moved into processing assembly or storage bins, if processing is not done immediately

Heated Air drying systems

Building of Air drying systems

This involves construction of bins/storage structures for drying and air distribution system

The building for seed drying system depends upon

1. size of operation

2. Number of different kinds of seeds to be dried

3. Level of mechanization desirable and

4. Future expansion

Different types of structures can be used for storage of seeds to be dried with forced air -drying

The storage structures are made of steel, wood, concrete or plywood and they may be in cylindrical or rectangular in shape.

Qualities of Storage bins for seed drying

1. Adequate strength

Seeds of small grains in bulk exert large pressure against the sidewalls

A sound foundation is necessary since, the side pressure of the seed is converted into a vertical load on the foundation

2. Weather tight

The roof and walls must keep out rain and snow, which are important causes for the damage of stored seed

For drying the seeds satisfactorily the walls must be airtight

3. Easy to fill and empty

The openings for filling and removal of seed should be large enough and so situated that minimum time is lost in filling and unloading the seed

A full size entrance door is desirable

4. Convenient to inspect, fumigate and clean

For easy inspection there should be 60 to 120 cms of headspace above the seed

Cleaning and spraying are made comparatively easy if sharp corners are avoided

For fumigation the structure should be airtight, with provisions for temporary sealings of all openings

5. Multiple Use

The structure should be usable for drying and storage of more than one kind of crop

6. Good air distribution system

The air distribution system should be able to carry adequate quantities of air for the drying of seed, and distribute it as uniformly as possible through all portions of the seed bulk

7. Adequate air venting

Flow of air to the outside, after it leaves the seed should proceed rapidly enough so that back pressure do not hinder flow of drying air into the seed

For this the size of the outlet should be more than twice the cross section area of the main duct of air distribution system

Types of Air distribution systems for seed Drying

There are three main types of air distribution systems

1. Main and lateral duct system

2. Single central perforated duct and

3. Perforated false floor system

1. Main and lateral duct air distribution system

In this system the main duct can be located in the centre of the bin, or it can be located at one side of the bin

When the central duct is located outside the bin under the floor it can also serve to empty the bin

When the main duct is located on the side of the bin it can be located inside the bin or on outer wall of the bin

2. Single central perforated duct system

For this air distribution system there must be equal thickness of seed not exceeding 6 feet, around the duct, which is made of perforated metal

For drying the air should be forced upwards through the seed

The sidewalls of the bin must be perforated so that air can flow laterally through the seed

This type of air distribution system is more commonly used for drying maize cobs

3. Perforated false floor air distribution system

This is most commonly used air distribution system for heated air-drying

In this method the air is introduced under the perforated false floor, the air passes up through the perforations and through the seed

The false floor can be made of hardware cloth, screen or perforated metal sheet

The metal false floors are more durable and convenient to use

It is recommended that this type of flooring must be supported on concrete blocks placed at every 3 to 4 ft. interval

It is followed the floor will support load upto 500 lbs/square ft

The channels and openings for the flow of air must be carefully designed too carry the air stream satisfactorily

When perforated metal flooring is used the tota l area of all the openings in the steel sheet should not be less than 8-10 % of the storage floor area

This is important when the drying floor does not extend completely to the sidewalls

4. Multiple Storage Bins

These are used to dry several types of seeds simultaneously using the same drying fan or fans

In this method sliding air gates are there for controlling the flow of air to the respective bins

Multiple bin arrangements are advantageous when 2 or more kind of seeds are grown

Selection of crop dryers and system of heated air-drying

Dryers for heated air drying unit consists of a heater unit where the fuel is burned and a fan to force the heated air through a canvas connecting duct into the air distribution system of the drying bin

The drying bin is connected to an automatic thermostat, which controls the temperature at higher limit and cuts off the burner flame if the air temperature exceeds the safe limit

There are two types of dryers according to the manner in which heat is supplied to the air.

1. Direct fired

2. Indirect fired

1. Direct fired

In this the fuel is burned and the hot combustion gasses are thrown directly into the air stream which goes into the air distribution system

The fuel used is liquid propane gas, butane gas or natural gas

Advantages of this system are it is highly heat efficient

The disadvantages are there is possibility of blowing soot entering into the air distribution system

Unburnt fuel and objectionable fumes may enter the seed bin

With some fuels there is also danger of blowing small sparks into the seed, leading to fire hazards

2. Indirect Fired

The hot combustion gasses pass into a chamber

The drying air circulates around this chamber and picks up the heat and enters the air distribution system

The fuel used is kerosine oil or rarely coal. The fan may be driven by either an electric motor of oil engine

The advantages of this system are, there is no possibility of combustion gasses or soot entering the bin and it safe with respect to fire hazards

One of the disadvantage is it is less efficient in use of heat

Types of seed dryers for heated air-drying

There are four types of seed dryers.

1. Layer in Bin Dryer

In this method the bin is filled to a specific depth depending upon seed moisture, the drying unit and bin sizes

After drying this seed to safe moisture level for storage, next level is added

The diameter of the bin will range from 21 to 40 ft. and requires 5 to 20 HP motors

It is most efficient but slow drying method. The seed is uniformly dried between the top and bottom of the bin

2. Batch in bin dryer

In this type the high moisture seed is loaded in the drying bin

The seed is dried to safe moisture level, cooled and removed to storage bin

The drying equipment used is similar to that of layer drying but requires high capacity of heater and fan

Seed depths are typically 2.5 to 4.0 ft. the deeper the seed depth lower is airflow and slow is the drying process

3. Batch Dryer

These are bins with inner air chamber (plenum) surrounded by two parallel perforated steel walls to contain a desired thickness of seed

The fan heater unit is connected to one end or side of the plenum as heated air for drying and natural air for cooling can be forced through the seed

Batch dryers are generally rectangular or cylindrical

Fan power ranges from 3 to 40 HP

The number of batches per day may be 8-10 for small dryers and 2-3 for large units

4. Continuous Dryers

In this method there is a continuous flow of seed through heating and cooling sections

The flow of the seed can be regulated

Heated air is forced through the upper 2/3 or ¾ of the seed column

The dried seed is removed for storage continuously

Procedure for heated air drying in bins

Put the seed into the bin to the recommended depth and there should be uniform distribution of trash and broken seeds

2. Operate the dryer at recommended temperature for that seed using a thermostat

3. When drying is completed, continue blowing air through seed without heat to bring the seed temperature down to air temperature or to 50oF if air temperature is lower

4. This may require around 30 minutes to 2 hours depending on the quantity being dried and the air temperature

5. The seed must be dried to safe moisture levels as given below. Wheat, sorghum and rice to 12 % Oats, barley and corn to 13 % Soybeans to 11%

Wagon Drying

It is a special type of batch drying with heated air

The seed is directly loaded from a combine into a wagon that is specially built for drying

The wagon is drawn to the dryer and connected to the canvass distribution duct

Three to four wagons can be dried at a time

The heated air is forced through the perforations of the wagon floor for drying the seed

After drying is over it is disconnected from the heating system and the seed is cooled with a small fan of half to three and half HP as required

After cooling the wagons are taken to storage bins

Advantages of wagon drying are

1. Drying is continuous

2. It is versatile

3. Low initial cost

4. Saves on seed handling and

5. Can be used for other purpose

Bag Drying

The drying is carried out in bags when many varieties are to be handled simultaneously or when seed lots are small in size and when the seed is received from the field in jute bags

The drying depth is one sack deep in a typical design of 25-40 cu.m. of air per minute per cu.m. of seed at a static pressure of 3 cm or even less

Box Drying

It is a modified bag drier

The identity of small seed lots can be maintained despite bulk handling

The boxes are made locally with perforated bottoms

Hot air is forced through the bottoms

After drying the boxes are shifted to storage area

Management of seed drying operations

1. Dry the seed soon after it is received. If there is any delay aerate the bin by fitting with a fan. Aeration prevents heating of the seed.

2. Care may be taken not to accumulate trash at one place. This problem is more when the seed is discharged through conveyor. Using a spreader can solve it. Small trash has high resistance to sir flow.

3. Observe the temperature in different drying zones. When the temperature of the top layer is equal to incoming air, drying of the entire bin is completed. Moisture content should be tested at random through out the bin to ensure that no wet spots are present. If germination percentage falls 1-2 % during drying check for the following

1. Excessive holding time before drying commences

2. Insufficient air flow

3. Excessive static pressure

4. High relative humidity of drying air

5. Drying air temperature may be more than 43 degree C

6. Excessive seed depth

7. Uneven air flow through the seeds