Machinery & Capital

Heated air convection type dryers are commonly used for drying paddy. Following types of dryers are in common use for drying paddy :-

1. Bag dryers

2. Bin or batch dryers

3. Continuous flow dryers

4. Rotary drum dryers

1. Bag Dryers :

This type of dryer is most suitable for drying smaller quantity of grains. It requires large number of unskilled labour for handling the bags. The dryer consists of a large floor area in the building with the opening over which the grain bags are placed. The openings are covered with a perforated material to support the bags and allow air movement through the bags. The air is blown in the holes by a burner and air blower assembly.

In this type dryer, air is heated up to 45⁰C and is blown in the dryer at the rate of 4 cubic meter per minute per bag of 45 kg of dried grains. The bag should be turned once during operation for exposing both sides to heated air.

2. Batch Dryer:

A dryer consists of a bin or a container to hold grain with an air blower and a burner for providing hot air. The bin or the container is fitted with a perforated bottom in case of most of the bin or batch dryers. However godown or a room in a building with a perforated floor may also be used. The godown or room is equipped with air dust system for air distribution throughout the room. A fan with heater unit is then connected to the duct system to supply the required air to dry the grains.

In all batch type dryers the depth of grain is limited due to high power requirement to force air through deeper grain mass.

3. Continuous flow dryers:-

Continuous flow dryers can be divided into two types (I) mixing and (ii) non-mixing type. Both types of dryers are used for commercial drying of paddy and hence high drying capacities. Grain is usually fed in the top and flows through the dryer by gravity. Rate of discharge is mechanically regulated at the bottom. Drying is accomplished by forcing heated air through the grain as it flows downwards.

In non-mixing columnar dryer the paddy descends between two parallel screens set 15 to 20 cms apart while heated air is blown through the screens. No appreciable mixing occurs and effect is similar to drying in static bed with a depth equal to distance between the screens.

Mixing type are of many designs, two of the most popular being baffle design and Louisiana State University design. In a baffle dryer paddy flows downward in zig-zag path through the baffles while heated air is forced through the grains. The Louisiana State University dryer consists of a bin in which layers of inverted shape air channels are installed. Alternating layers are air inlet and air exhaust channels. Each layer is offset so that the top of each inverted channel split in the stream of paddy coming on it. Paddy flows between channels taking a zig-zag path as it descends. Drying air passes from air inlet the air exhaust channels thus removing moisture from the paddy.

4. Rotating Drum dryer:

This consists of a large drum 1 to 2 meter in diameter and 3 to 6 meters in length placed on a slight inclination and the drum rotates about its axis. The inside of the drum is equipped with the helical straight edge ribs to provide the mixing and the forward movement of the paddy during drying operation. The hot air blown at the lower end of the drum and the paddy flows counter current to direction of air flow. These types of dryers are normally used for drying parboiled paddy.

Improved parboiling technique of hot water soaking and atmospheric steaming (CFTRI) and pressure parboiling (RPEC) have been developed. The Cuttack centre has developed a mini parboiling unit for on farm/domestic parboiling of paddy. Traditionally soaking and steaming are done separately with parboiling unit. Both the operations can be done without any further handling in the same unit. A hot soaking for 3-5 hours in 75⁰C water and steaming and subsequent sun drying results in about 2 percent more head rice than achieved traditionally

Specification for Mini Parboiling Unit

It is a relatively small unit with a throughout capacity of 100 to 150 kg per hour operated by oil or steam engine or an electric motor. In small villages single hullers are used while in bigger mills of 1 to 2 tonnes per hour capacity batteries of 3 to 4 hullers are used. Most of the other operations, excepting milling proper, are done invariably. The huller does the job of the husk removal and bran removal simultaneously. The working element of this machine is made of cast iron, which bears very heavily on rice grains thereby breaking them to pieces which are liable to be lost alongwith husk. The hullers even though cheaper in cost are wastage and have high labour requirement.

2. Centrifugal shellers

In this machine the grains are subjected to centrifugal force by means of a rotating impeller. Significant characteristics of these centrifugal shellers are: Their high capacity and simple constitutional features as there is only one moving part i.e. an impeller. Its casing is lined with rubber. The paddy is fed to the centre of the rotor from where it is thrown towards the casing with great force where it is shelled. This machine has a very good potential, as there is a possibility of combining several units and obtain a compact multistage mill to affect complete shelling. Another advantage, in case of this type of machine is that heating of grains does not take place as there is no grinding action involved. Polishing of brown rice obtained is done by a separate polisher. The centrifugal shellers are available which have the built-in winnower to remove the husk.

In this type the two operations of dehusking and whitening were intended to be carried out separately in separate machines. The disc sheller mills may be called unit mills as all the operations involved in milling, cleaning, dehusking paddy separation, polishing and grinding are done mechanically and by separate machines. The basic machine i.e. under runner disc sheller consists of two discs with their inner faces lined with emery bottom one rotating and the other stationary. The paddy passes in between the two discs and gets shelled or dehusked. Therefore the breaking is less and the white rice yield is also more of the order of 66-68 percent with 10 to 15 percent brokens. There are adjustments provided on the dehusker so that the clearance between the discs could be adjusted for most of the variety of paddy. It gives a marked increase (2 to 4 percent) in rice output over conventional huller in almost all milling conditions

The greatest advantage of rubber roller over cast iron cylinder or the emery roller is that due to their compressible nature the grain handled gently during the process of dehulling. The functional coefficient between paddy grain surface and rubber is lower than that of paddy and steel. This facilities easy dehulling. The two rubber rollers rotate in opposite direction at differential speed to take off the aspiration. Since all the paddy is not dehusked paddy must be separated. This is accomplished by a paddy separator. The brown rice (rice with bran coating on) is then fed to rice whitener where the bran is removed.The rubber roller mill gives better quality rice and bran which fetch more price in the market.

Paddy Drying

Paddy is required to be dried to 12 to 14 per cent of moisture wet basis for safe storage and milling. Most of the paddy crop in India is sundried upto about 16 to 18 percent moisture on the plant and from these by allowing the bundles in the field. The threshed paddy is sundried upto about 13 to 14 per cent on the floor.

The multicropping practice of paddy has enough the essential requirement to harvest the crop at higher moisture content after the crop has attained maturity. The earlier harvest of paddy at higher moisture content provide following advantages:

1. Due to shattering, lodging birds, rats, and etc.there are minimum field losses and maximum grain yield obtained per unit area.

2. More head rice and improved milling quality of the paddy are achieved by harvesting paddy at higher moisture content and dried by mechanical means.

In general two methods of paddy drying in practice: -

1. Sundrying

2. Mechanical drying

Sundrying is a traditional process of drying, a major quantity of paddy grown in India is dried by sun-energy.

Sun drying has the following features: -

1. There is uncontrollable non-uniform drying which result in sun checks or cracks in kernels. Due to these checks and cracks when dried grains milled give larger quantity of brokens.

2. This process is dependent on availability of sun energy normally not available in monsoon season.

3. Large numbers of unskilled labours are required with no mechanical energy.

4. During the process quantity of 1 to 2 per cent of paddy is lost due to birds, insects, rats and other rodents.

5. The cost of drying per tonne of paddy is relatively low as compared to mechanical drying as it requires no fuel or mechanical energy.

6. It is not suitable for the rice mills located in cities having scarcity of land as it requires very large area.

Raw paddy and parboiled paddy both are sun dried. There are various methods of drying raw paddy as: -

1. Drying of standing crop: Grains are dried on the plant till proper moisture content is attained. Then the crop is harvested and threshed, considerable quantity (12 percent) is lost due to shattering while crop is standing in the field. The drying process is very slow and takes about 2 to 3 weeks after grains have attained maturity.

2. Drying of grains on stalk: This is an improvement over the previous method. The crop is harvested at higher moisture content and is left in the field or on bounds of field till it has dried to proper moisture content.

Another improved method followed in Japan is to dry harvested crop on racks. There the harvested plants are found in a bundle near the ears and are hung on a rope exposing the ears to the sun.

It is felt that since the grains attached to the plant the moisture and temperature stresses causing the sun cracks are not as severe as in case of drying the threshed grain in the sun. However, there are losses of grain due to shattering, birds and rodents.

3. Drying of threshed grain: The harvested crop of higher moisture content is threshed and the grains are dried on a drying yard or public road where grains are on the floor in 13 mm thin layer and is continuously stirred by human labour till it has attained desirable moisture content. In this process the grains are subjected to uncontrolled drying causing cracks in the kernels. Hence, paddy dried by this method has extremely poor milling quality.

Sundrying of parboiled paddy: Most of paddy parboiled in the traditional mills is sundried on a drying floor. The drying floor is normally cemented floor consisting of layer of brick bats with cement plaster on it. The parboiled paddy is spread on the floor 10 to 30 mm thick layer by labour with the help of a plank then it is continuously stirred by a spiked plank or by feet. After four hours of drying in sun, paddy is heaped and covered by mats for tempering for a period of 2 to 3 hours. Then the paddy is spread again for 2 hours in the sun to dry it to desirable moisture content. Therefore drying of parboiled paddy from 30 percent to 13 percent moisture content takes about 6 hours during summer and relatively long period about 8 to 10 hours during winter.

Mechanical drying of paddy: It is the process utilising mechanical means for drying grain by ventilating natural air or heated air through the grains mass to accomplish removal of moisture from it. In other words mechanical drying of paddy refers to removal of moisture from wet paddy by unheated and/or heated forced air at a predetermined rate of flow, air temperature and air relative humidity. Rate of moisture removal could be regulated. It is observed that if paddy is harvested at about 24 percent moisture and dried mechanically in stages by 3 or 4 passes of heated air the milling losses are minimum.