Machines used for drying farm crops are made to a wide variety of designs and many sizes to cover the diverse requirements of different crops. The function of these machines called dryers is simply to remove excess moisture without loss of the quality of the product.
Batch or Bin dryers
Flat bed type
Bin type
Continuous flow type
Mixing type (L.S.U.Baffle)
Non mixing type, Recirculatory, RPEC
Rotary dryers
Batch type - Columnar
Bin type - Horizontal
Tray type dryers
Spray dryers
Freeze dryers
Electrical dryers
Solar dryers
Sack dryers
Batch or Bin driers
The material to be dried is placed in a bin or container and air is forced through the mass until dry. Design, utilizes the angle of repose of the grain and is nearly or completely self emptying. Drying of mass takes place progressively in the direction of air flow.
Continuous Flow Dryers
- Columnar
The grain flows between vertical screens which may vary in shape and design according to make. They may be cylindrical or prismoid (sometimes inverted) or like a truncated informs and in other cases the double screens are parallel. The wet material is placed in the hopper and flows by gravity between the perforated retaining walls and is discharged at the bottom by continuously operating melting valve. Heated-air is forced through the column at right angles to the direction of motion. The column may be inclined to simply construction or fitted with baffles to stir the mass as it progresses through the drier. The capacity is directly proportional to the column width and material movement rate through the column. Since the retention time is fixed the each individual situation drier capacity is proportional to height. Column drier performance may be estimated by the deep bed procedure.
Recirculatory Batch Dryer
- Construction
This dryer consists of two centric circular cylinders made of perforated (2mm) mild steel sheet of 20 gauge. The two cylinders are set 15 to 20 cm apart. These two cylinders are supported on four channels sections. The whole frame can be supported by a suitable foundation or may be bolted to a frame made of channel section. A bucket elevator of suitable capacity is used to feed and recirculated the grain into dryer. A centrifugal blower blows the hot air into the inner cylinder, which acts as a plenum. The hot air from the plenum passes through the grain moving downward by gravity and comes out of the another perforated cylinder. A torch burner is employed to supply the necessary heat with kerosene oil as fuel.
- Operation
The grain is fed to the top of the inside cyclinder while descending through the annular space from the feed end by gravity the grain comes in contact with a crossflow hot air. The exhaust air comes out through the outlet of the hopper. The feed rate of the grain is controlled by closing or opening the gate provided with the outlet pipe of discharge hopper. The grain is recirculated till it is dried to the desired moisture level.
Louisiana State University Dryer
- Construction
- Rectangular bin
Usually the following square sections of the bin ae used for the design of L.S.U. dryer:
- 1.2 m x 1.2 m
- 1.5 m x 1.5 m
- 1.8 m x 1.8 m and
- 2.1 m x 2.1 m
- Air distribution system
- Grain discharge mechanism
- Air heating system
Layers of inverted V shaped channels are installed in the drying chamber. Heated air is introduced at many points through the descending grain bulk through these channels.
Alternate layers are inlet and air outlet channels. In the inlet layers the channel opening face the air inlet plenum chamber but they are sealed at the opposite wall whereas in the outlet layers the channel opening face the exhaust but ae sealed other side. The inlet and outlet ports are arranged one below the other is an offset pattern. Thus air is forced through the descending grain while moving from the feed end to the discharge end.
Grain discharging mechanism consists of three or more ribbed rollers provided at the bottom of the drying chamber, which can be rotated at different low speeds for different discharged rates of grains. The grain is discharged through a hopper fixed at the bottom of the drying chamber.
The air is heated by burning fuels such as natural gas, butane gas, etc or liquid fuels such as kerosene furnace oil, fuel oil etc or solid fuels like coal, husk etc. Heat can be supplied directly by the use of gas burner or oil burner or husk fired furnace and indirectly by the use of heat exchangers. The heated air is introduced at many points in the dryer so as to be distributed uniformly through the inlet ports and the descending grain bulk. It escapes through the outlet ports. In general the capacity of the dryer varies from 2 to 12 tonnes of grain but sometimes dryers of higher capacities are also installed.
- Baffle dryers
Baffle dryers are same as columnar dryer, but fitted with baffles which conduct the grain downward in zig-zag path as heated air is forced through the descending grain. Air is blown in the centre of the dryer.
Construction
The baffle dryer consists of :
- Grain receiving bin.
- Drying chamber fitted with baffles.
- Plenum fitted with hot air inlet.
- Grain discharge control device.
- Hopper bottom.
A number of baffles are fitted with the drying chamber to divert the flow and effect certain degree of mixing of grain. The two baffle plates with outer and inner sides are set 20 cm apart, for the passage of grain in the drying chamber. The dryer is made of mild steel sheet.
Operation
Grain is fed at the top of receiving bin allowed to move downward in a zig-zag path through the drying chamber where it encounters a cross flow of hot air. On account of zig-zag movement a certain degree of mixture of grain takes place. The partially dried grain discharged from the hopper bottom is recirculated by a bucket elevator until it is desired moisture level.
Rotary Drier
It consists of large drum of 2 m in diam, 3 to 6 m in length, placed on slight inclined. Grain is fed into the upper end and as unit rotates, the grain moves downward and mixes with the air flowing around the grain and grain and air may move in same or opposite direction. Agricultural rotary driers are direct fired and single, double and triple drum types. The multiple drum types are preferred since the overall length can be short and heat losses by conduction and radiation is small. The inside of the drum may be fitted with flights that little lift the material and shower it down through the heated air. Flight designs varies with the material to be dried. Chains or other dividing devices may be fitted to the inside the drum to divide materials that tend to clump as they pass through the drier. The rate of material movement through the drum is controlled by flight design.
Horizontal Rotary Dryer
Horizontal rotary driers of various designs have been developed by different countries for the drying of parboiled paddy. Some of them are fitted with external steam Jacket an internal steam tubes as well. As parboiled paddy can stand high temperature without significant increase of cracks in grains. These dryers can be employed for rapid drying of parboiled paddy using temperatures as high as 100 to 1100 C.
- Construction
It consists of a cylindrical shell 9.15 m long and 1.22 m in diam with 48 paris of 5 cm and 3.75 cm size steam pipes in two concentric rows inside the shell in combination with common steam inlet and condensate outlet fittings. The shell is equipped with longitudinal flights of 9.15 m long and 15.24 cm wide for lifting and forward movement of the parboiled paddy towards the discharge and while it is being dried. Over the feed and breaching box and there are feed hopers and screw conveyers with an adjustable sliding gate. The dryer is equipped with an air blower and a small steam tube heat exchange for supplying heated air at the entrance of the feed and breaching box.
- Operation
The soaked and steamed paddy is fed to the dryer by the screw feeder. Heated air at about
600 C is blown through the dryer in the same direction as the paddy moves and exhausted through the exhaust pipe. The parboiled paddy comes in contact with the steam heated pipes for a very short time in each rotation and is gradually dried to about 16% moisture content in a single pass.
Tray driers
Agricultural materials that can not be dried by any of the previously discussed methods are dried on trays e.g.Fruits and vegetables. The material is placed in shallow trays, which are stacked on cars. The trays are spaced to permit air to circulate between them. The car of trays is dried in cabinet or in a tunnel. Cabinet drying is a batch process with the principle applicable for low rate installations. The cars are moved through the tunnel by a flow by moving drag chain, a ratched ram or manually. Parallel airflow gives a fast initial drying rate. Counter flow gives faster drying at the dry end at the tunnel. The air rate must be high enough so that the relative humidity of the discharge air is below the equilibrium relative humidity of the material at the point where the material discharges.
Spray Driers
Spray driers are used to remove water from solutions or suspensions and dry the resulting powder to a moisture content that approaches equilibrium with the exhaust drying air. Spray driers are used extensively in the food, chemical, and pharmaceutical industries.
- High pressure atomization
The liquid is forced through a nozzle under high pressure. Mixing with the drying air and the spray pattern can be controlled.
- Centrifugal
The liquid is fed at low pressure on to a horizontal disc or cup turning at speeds up to 20,000 rpm or more. The material breaks up in to small drops as of leaves the edge of the rotor. The drops are of uniform size and materials not suitable for nozzles can be dried.
- Two-fluid Atomization
Air or streams breaks the liquid into fine drops by a mechanism comparable to that of point sprayers. This system is used only for breaking the material into fine drops.
Operation
The grain is placed on the supporting screen and the heated air is forced through the deep bed of grain. After drying of grains to the desired moisture level, they are discharged manually. The temperature of the heated air should be limited to 450 C. The drying rate varies with the drying temperature.
Freeze Dryers
These dryers use the principle of freeze drying. The rate of respiration of the food product and the microorganisms present is reduced in a low temperature atmosphere. By reducing the respiration rate the storage life of the product is increased. The value of refrigeration in reducing the body activity has recently been provided during successful heart operations by placing the vital heat organ in a cold environment.
A typical portable unit consists of a cylindrical drying chamber 40 cm diameter by 60 cm mounted horizontally on top of a cabinet (1.07 m long, 0.8 m wide and 0.6 m high) which can be moved about on rubber tyred swivel casters. One end of the chamber has a flagged opening, to which is fitted a swing cover. Similarly flouged but with a self retaining 0-ring vacuum seal and incorporating a viewing window.
Heating and cooling systems, essential to the freeze-drying process, are located inside the chamber. The heating system consists of a bank of five blackened, electrically heated radiant heaters (55 cm long, 25 cm wide) located in the central region of chamber. Four metal trays to hold the product slide between the heaters, which are spaced about 6 cm apart. The cooling system consists of 9 m of 18 mm diameter, cooling coil running the length of the chamber in zigzag fashion, with nearest surface of the oil 12 mm from the inside wall. The space between the heating and cooling systems is shielded in such a way as to reduce heat exchange between heaters and coil yet allow water vapours to pass through.
There are the vacuum pumps and mechanical refrigeration compressor, pipelines I the cabinet beneath the chamber from which run to the cooing coil through specia scales in the back of the chamber. The vacuum pump is a single stage air ballast type having a displacement of 8.5 m3 h-1 and growing a pressure of 26.7 pa. The refrigeration compressor is a 1.1 kW (R 502 refrigerant) single stage unit with sufficient capacity to reduce the coil temperature to 400 C under full load. Pirani gauage is used to measure the vacuum pressure and the heater temperature is controlled by means of a transistorized electronic device.
Solar Dryers
To avoid risks involved in the traditional practice of grain drying by direct exposure of sunshine, use of mechanical dryer is recommended for controlled drying and better quality of grains. Development of suitable solar energy collectors with forced air circulation provides potential means for effective utilization of solar radiation in grain drying operations. Different crop like paddy wheat, maize, groundnut etc are harvested at high moisture content. The moisture content must be suitably reduced for proper storage.
- Solar Collector
Solar collectors are objects that convert solar energy into thermal energy or heat. One such simple device is a blackened metal sheet kept in a closed box. Covered by glass cover on one side and a thermal insulating material on the other side. The temperature of this black plate can be raised to 120 to 1400 C on a clear sunny day. Such a system is called a flat plate collector. The black plate heats up and turns heat to a fluid mostly air and water flowing under, through or over it.
- Solar Concentrations
To produce high temperature for industrial used agricultural drying and other applications solar concentrations employing a reflect or system are used. A parabolic reflector gives a high concentration of sunrays and to keep the concentration of sunrays and to keep the concentration of a solar energy constant all through the day, auxiliary mirrors are used. Heating temperature upto 4000 C are achieved by such solar furnaces.
- Solar Cookers
Several solar cookers have been developed to cook certain food items through slow and low heating in the range of 120 to 3000 C temperature. Simple hot box type cookers can cook rice, vegetable, meat etc at low temperature while the concentrating type cookers attain higher temperatures. These cookers can save 30 to 50% of commonly used fuels like wood, coal, LPG, kerosene etc.
- Solar Air Heaters
Air heaters based on solar energy can be used for better drying of fruits, foodgrain, vegetables, wood etc. That is possible through traditional method of exposure to the sun. These heaters are similar in principle to flat plate solar collectors. Air is blown through tubes or allowed to go over the blackened metal sheet. As a temperature of about 1300 C can be attained in such systems, the hot air may be used to dry any material like wood or agricultural products.
Sack Dryer
A sack dryer consists of a larger floor in building with opening over which the sacks of grain are placed. The openings are covered with perforated material to support the sacks. A blower is connected to an air chamber beneath the floor to supply air. Suitable for drying seed grains to prevent mixing of varieties and to conserve strain purity and viability. The type of air distribution system may be main and lateral duct system, single central perforated duct, perforated false floor. When the amount of grain is small the sack dryer is useful. In this drying maximum air temperature should be used only 43.30C (1100F). Drying rate is the faster for 50 kg sack 4m3/m. During drying process sacks must be overturned at least once. For supply of air below fan related to air chamber there is a blower.
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