Equilibrium moisture content of selected grains at 25◦c (percentage wet basis)
Table 1.14.
|
20% Relative Humidity |
40% Relative Humidity |
60% Relative Humidity |
Maize |
7.1 |
10.0 |
12.4 |
Rough rice |
6.5 |
9.4 |
12.2 |
Soybeans |
5.3 |
6.9 |
9.7 |
Wheat (hard red) |
7.2 |
9.9 |
12.1 |
Airflow. As grain kernels lose moisture during the drying process, the evaporated water is carried by a flow of air from the grain bed in the form of water vapor. To properly design and operate a grain-drying system, the principles of air movement must be understood, especially as it relates to static pressure, fan characteristics, and system operating condition.
Product constants for the equilibrium moisture content equation of selected grains
Table 1.15.
|
a |
b |
c |
Maize |
0.339 |
0.059 |
30.205 |
Rough rice |
0.294 |
0.046 |
35.703 |
Soybeans |
0.416 |
0.072 |
100.288 |
Wheat (hard red) |
0.356 |
0.057 |
50.998 |
Static Pressure. The static pressure of a grain drying system is mainly the resistance to the flow of air though the grain. The static pressure (AP) is expressed in Pascals per meter thickness of grain and can be calculated for a particular grain species by
where a and b are product constants, and Q is the airflow rate in cubic meters per second per square meter of grain-layer area.
Values for the product constants a and b are tabulated for selected grains in Table 1.16. The relationship between A P and Q is shown in graphical form for grains in Fig. 1.14. In a well-designed grain-drying system, over 90% of the resistance to the airflow occurs in the grain, and less than 10% in the airducts, perforated floors, and grain-column screens [1]. The system curve of a grain-drying system is a plot of the total static pressure of the system versus the airflow rate.
Product constants for the static pressure equation of selected grains
Table 1.16.
Maize |
2.07 > |
104 |
30.4 |
Rough rice |
2.57 > |
104 |
13.2 |
Soybeans |
1.02 > |
104 |
16.0 |
Wheat (hard red) |
2.70 > |
104 |
8.8 |
1.14. Airflow resistance of grains. Equilibrium Moisture Content (% Wet Basis) of Various Grains (Adapted from asae (2005b) Based on a Temperature of 77°f)
Table 6.1
Grain |
Equilibrium Relative Humidity (%) |
||
60 |
65 |
70 |
|
Corn, shelled |
12.4 |
13.2 |
14.0 |
Soybeans |
10.5 |
11.5 |
12.5 |
Rough rice |
12.0 |
12.6 |
13.2 |
Wheat, hard |
12.9 |
13.6 |
14.4 |
Wheat, soft |
11.8 |
12.3 |
13.0 |