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Is It Too Cold for Wheat, Sunflower or Corn Drying?
The ability to dry grain when it is cold outside depends on the moisture-holding capability of the air, grain equilibrium moisture content (EMC), and the type of drying.
The final moisture content (EMC) of grain dried by natural air is related to the air temperature, relative humidity and type of grain. Average climatic conditions for North Dakota in November are 27 degrees and 73 percent relative humidity. Under these conditions wheat will dry to 17.5 percent moisture, oil sunflower to 10.1 percent, and corn to 18.2 percent.
Warming the air reduces its relative humidity, which reduces the grain moisture content. Warming the air by 10 degrees in November is required to dry wheat to the recommended storage moisture content. Corn and sunflower, however, are dried to acceptable levels by warming the air by 5 degrees.
Moisture-holding capacity of air, which affects drying speed, is related to the air temperature. The moisture-holding capacity of air is reduced by approximately one-fourth for each 10 degrees that the air is cooled. That means as outdoor temperatures approach freezing, the drying efficiency and speed are greatly reduced.
WHEAT
Heating the air increased the moisture-holding capacity and reduces the relative humidity. Wheat can be dried from 18 percent to 13 percent in 25 days using an airflow rate of 1 cubic foot per minute per bushel, assuming average August conditions.
In November it takes 35 days with the air heated 10 degrees. Adding this amount of heat will approximately triple the cost of drying.
SUNFLOWER
An airflow rate of 1 cubic foot per minute per bushel will dry 15 percent moisture oil sunflower to about 10.2 percent in about 37 days, assuming average November conditions. Because drying rate is directly related to the airflow rate, increasing the airflow rate reduces the drying time.
Remember that fan horsepower requirements increase rapidly with increasing airflow rates, so sunflower depth in the bin may need to be reduced to achieve the desired airflow rate. Some heating of the air will occur as air passes through the fan. The amount will vary depending on the fan and operating static pressure, but a 4 or 5 degree increase is expected at about 6 inches of pressure. This needs to be considered when deciding how much heat to add.
Warming the air by 5 degrees reduced the drying time from 37 days to about 22 days, but also reduces the sunflower moisture content to about 8.3 percent. Adding more heat will greatly over-dry the sunflower unless a stirring device is used.
CORN
An airflow rate of 1.25 cubic foot per minute per bushel will dry 21 percent moisture corn to 15 percent moisture in about 52 days if the air is heated by 5 degrees. Additional heat cannot be used without overdrying the corn, unless the corn is being thoroughly mixed by sing a stirring device. Increasing the airflow rate to 2 cubic foot per minute per bushel reduces the drying time to about 22 days. This, however, increases the fan horsepower required on an 18 feet corn depth in a 21-foot diameter bin from 5 horsepower to 20 horsepower. The airflow rate of 2 cubic foot per minute per bushel could be achieved with the 5 horsepower centrifugal fan by reducing the corn depth to 12 feet.
It will be more economical to dry the grain using a natural air system during April than to dry with a low temperature system in November. Corn that is 21 percent moisture can be dried to about 15 percent in 41 days under normal April conditions and to 13 percent in 35 days in May with an airflow rate of 1.25 cubic foot per minute per bushel using a natural air drying system. The grain should be cooled to about 25 degrees for storage over winter.
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