Analysis of solar dryers is key to understanding its performance parameters. Depending on existing conditions, a moist product in a stream of air may lose or gain moisture, this depends on the partial pressure vapors in the air and the gradient of vapor pressure of water that is present in the product. Assuming partial pressure of vapor in the air remains constant in a sample, if we pass a stream of air for a sufficiently long time, we will reach an equilibrium condition referred to as equilibrium moisture content (EMC).
EMC depends on temperature, relative humidity and the nature of the product.
Water Activity; the more water there is in the product the better it is for the growth of micro-organism, this determines the shelf stability and safety, the water activity of apple is considered to be 0.95. Equation of water activity 7.
Usually, the drying rate is proportional to the difference in the moisture content between materials to be dried and the equilibrium moisture content. Average drying rate Adr can be determined from the mass of moisture removed by solar heat and drying time(td) by the eqn. (17) 11;
Efficiency of solar dryer in relation to total heat energy of evaporation and irradiation relates as 14;
Where E is the total useful energy received by the drying air in kJ as obtained from eqn. (5). The energy falling on the collector from the sun can be converted to units of energy (kWh) (Eu) as;
The pressure difference across the drying product bed is solely due to the density difference between the hot air inside the drying chamber and the ambient air. Air pressure can be determined by the equation given by Jindal and Gunasekaran (1982) 7;
P 0.00308 (g T Ti am)H (23)
Where H is the pressure head (height of the hot air column from the base of the dryer to the point of air discharge from the dryer), P is the air pressure in Pascals, g is acceleration due to gravity and Tam is the ambient temperature(?C)
Satisfactory drying depends upon both the airflow rate supplied and the ability of the air to hold water. The ability of a fan to move air through the grain will depend upon the fan design, and the resistance to the airflow. The pressure a fan must develop to over¬come the resistance of grain to airflow is referred to as static pressure. The unit most commonly used for measuring resistance to airflow is inches of water as measured by a manometer (Figure 13). One inch of water is equal to 0.036 pounds per square inch (psi). Figure 14 shows typical resistances to airflow of some clean grains commonly grown in North Dakota. This resistance to airflow is technically referred to as static pressure drop through the grain. Multiply this pressure drop for clean grain by 1.3 to 1.5 to adjust for packing and foreign material in the grain. The value varies depending on the cleanliness and physical properties of the grain. A value of 1.3 is commonly used for wheat and 1.5 for other crops. Tables 7 and 8 list the estimated static pressure for various airflow rates and grain depths for bin drying clean grain.
There are several different types of fans. Each has specific operating characteristics and applications. The common types of fans used for grain drying applications are the axial-flow, low speed centrifugal, high speed centrifugal, and the in-line centrifugal (Figure 15).
Grain Handling Systems
The addition of grain drying to a farm operation will normally increase the grain handling required. Increased handling makes hopper type storage structures and good handling equipment more important.
Handling the grain can be done by portable equipment, permanent equipment or a combination. When using portable conveyors, circular bin arrangements are satisfactory, enabling one conveyor to reach all the bins from the center of the bin circle. However, if permanently installed handling equipment is planned, a straight-line arrangement is more suitable. Straight-line bin arrangements lend themselves more readily to drive through unloading facilities, ease of expansion, and efficient use of permanently installed loading and unloading equipment.
In planning, it is necessary to consider the grain flow pattern. This needs to include the time factor as well as where the grain goes. For example, when using a portable batch or continuous flow dryer, both wet and dried grain holding bins are necessary to enable efficient use of the dryer. Generally speaking, the continuous flow drying process requires a more complex grain handling system than the batch system. The batch drying system requires larger equipment since large quantities of grain are handled in fairly short periods of time.
Several factors need to be considered in selecting equipment. Table 11 lists some information on bins and Table 12 lists information on conveyors.
Four types of grain handling systems for drying are shown in Figures 16, 17, 18 and 19. Figure 16 uses portable augers for conveying grain into a dry¬ing bin. The batch-in-bin dryer in Figure 17 is con¬tained in a circular bin arrangement. Figure 18 shows a system for a continuous flow dryer, and Figure 19 shows a completely mechanized grain handling-drying system using permanently installed equipment. Table 13 shows the dimensions for a cir¬cular bin arrangement for some bin diameters.
Safety should be a part of all grain drying and handling operations. Use proper shielding of all mov¬ing parts. Avoid high voltage power lines when locating or moving equipment. Contact your electric power supplier before constructing a grain drying system for assistance with electric service.
Bottom unloading equipment and grain bins have a special suffocation hazard. As this equipment is unloaded, the grain flows off the top and down the center withdrawal cone. Anyone entering the withdrawal cone will be pulled down and may suffocate if they become covered. The only way to pre¬vent this type of accident is to be absolutely sure no one is inside a grain bin, hopper bottom bin, truck box, or wagon when unloading takes place.
Managing Stored Grain
For best results in storing dried grain, an accurate moisture test is needed to determine that the grain is dry and an aeration system is necessary for controlling grain temperature. The drying fan can be us¬ed for cooling if the grain is stored in the bin in which it is dried. If the grain is placed into a different bin, it should be equipped with an aeration system to control grain temperature during storage. It is im¬perative that the grain be cooled during storage to control insects and reduce moisture migration.
Other Drying and Storage Information Available
Other information on grain drying and storage is available from the county extension offices and from Extension Agricultural Engineering at North Dakota State University.