Primary

• Drying is a method of preservation of an agricultural crop that involves the removal of moisture to a level considered safe for storage.
• Drying is accomplished by vaporizing the water that is contained in the food, and to do this the latent heat of vaporization must be supplied.
• The property of air most concerned with this phenomenon is vapor pressure.
• Temperature and humidity should be monitored in the store, together with grain temperature and moisture.
• During storage, grain constantly interacts with the surrounding air space.
• If conditions in the store are too humid, the grain moisture will increase and damage might occur due to bacteria, mould or insects.
• To adequately understand the operation of drying, the reader has to be acquainted with the basics of psychrometrics, equilibrium moisture content, airflow, and drying rate.
• Drying of grain involves exposing grain to ambient air with low relative humidity or to heated air, which will evaporate the moisture from the grain.
• Rice is a hygroscopic material, and when dry rice is exposed to air with high relative humidity (RH) the rice grains will absorb water from the air (re-wetting).
• If fine particles are present, there may be considerable solids carried over with the exit gas, and cyclones and bag filters are needed for their recovery.
• The rate of drying and the final water content in the product are affected by the rotational spread of the roller, steam pressure, thickness of the film, and properties of the product.
• In a dehydrator dryer, food is dried using a controlled heat temperature, with air circulated from the top of the unit to each of the five trays and base.
• In spotlight drying method, there is an inverse relationship between air temperature and drying time, with the air temperature as radiation intensity being an influential external parameter.
• In a solar tunnel dryer, food is dried using solar energy, with different types of solar dryers used based on the quantity and time.
• Roller Dryer consists of a cylinder, heated from the inside by steam, with a thin film of the product sprayed on the outside and heated while the cylinder rotates.
• In OYSD, products are simply dried in open atmospheric space, with the chances of products getting affected due to rain and dust being more.
• Rotary Dryer is heated by hot gases passing through an external jacket on the shell, steam condensing in a set of longitudinal tubes mounted on the inner surface of the shell, or by direct contact of air or gas with the solids.
• In a fluidized-bed dryer, the particles are fluidized by air or gas in a boiling bed unit, with the average time a particle stays in a bed usually between 30 and 60s.
• Oven drying is the simplest way to dry food because you need almost no special equipment, but it can be used only on a small scale.
• When wet rice is exposed to air with low RH the rice grains will release water to the air (drying).
• Drying permits the farmer to secure a greater economic return for the following reasons: early harvest (at high MC) minimizes field damage and shatter loss and facilities for tillage operations, long period storage without product deterioration is possible, viability of seeds is maintained over long periods, production operations are facilitated for such products as cotton and corn, products with greater economic value are produced (tobacco, dried fruits and vegetables), and waste products can be converted to useful products (livestock feed from fruit pulp and almond hulls).
• Indirect-fired burner is safer but less efficient in heat utilization and more expensive to acquire and operate.
• Band dryer is preferable if the particles to be dried are rather coarse (i.e between 5 to 10 mm), with the particles spreading evenly into a slowly moving, perforated belt.
• A government financed economic trade-off program may be necessary to encourage greater use of solar energy.
• Active solar systems have been used for decades and proven feasible in many parts of the world.
• Cabinet or Tray Dryer is classified as batch type and band dryer and can dry almost everything, but due to the labor required for loading and unloading, they are expensive to operate.
• Propeller type blower is used for large air volume at low static pressure, with air moving parallel to the fan axis and at a right angle to the field of rotation of the blades.
• Mechanical drying consists of a burner, blower, plenum and drying bin, with the burner or furnace burning the fuel to heat the drying air, the blower forcing the heated air through the mass of grains, and the plenum equalizing the air pressure in the drying bin which holds the grains.
• Different grain crops are harvested at different seasons and have different physical properties that affect the drying rate.
• During drying, two processes are occurring at the same time: heat transfer to the solid to provide heat for evaporation of moisture and mass transfer of moisture (liquid or vapor form) from within the solid to surface, and water vapor from the surface to the drying medium.
• The size of the drying operation must accommodate the size of the crop so that the harvest can proceed in an orderly manner.
• The AIT solar dryer design concept involves a clear plastic sheet covering the grain bed, a black plastic sheet covering the ground, and a construction cost of 4,000 - 5,000 baht dependent on the availability of the materials in each location.
• The availability of solar radiation at harvest time and the relative humidity at a specific location will affect the effectiveness of solar energy for grain and crop drying.
• The AIT solar dryer is capable of drying one metric ton of paddy, with the drying period depending on the availability of sunlight and a life expectancy of one year.
• Direct-fired burner is efficient in the use of heat but there is the possibility of damage or undesirable odor, color and taste imparted on the grains because of smoke and unburned fuel coming in contact with the grains being dried.
• The rotary dryer consists of a cylindrical shell, horizontal or slightly inclined toward the outlet.
• In paddy grain, moisture is present at two places: at the surface of the grain (surface moisture) and inside the kernel (internal moisture).
• Surface moisture readily evaporates when grain is exposed to hot air.
• Internal moisture evaporates much slower because it first has to move from the kernel to the outside surface.
• Surface moisture and internal moisture evaporate at a different rate.