The higher the moisture level, the greater the time needed to reach the desired dry condition. The drying rate of seed also affects this time for drying. Most seeds have a fast drying rate at high contents of moisture, but have an increasingly lower rate as they approach the final, desired moisture content. Considerable time may be required to remove the last 1 percent of moisture.

The final moisture content for safe storage of seed is 4 percent to 14 percent, depending on the kind of seed, the type of storage, and the anticipated storage period. Lower moisture levels generally are desirable for longer storage times and confined storage conditions.

Airflow rate, too, can influence drying time. If an extremely low airflow rate is used, the drying air can become saturated before completing its passage through the seed, and no further drying with that air can occur. Drying then is limited by the airflow rate through the seeds, and greater airflow will lower the drying time.

On the other hand, as airflow increases, a point is reached at which the air is absorbing all the moisture that is made available to it. Beyond that point, additional airflow does not reduce drying time, because rates at which moisture diffuses from the inner parts of the seed to the surface are the main controlling factors.

A rule of thumb that can be used to determine drying time is that about 0.3 percent of the moisture can be removed per hour with an airflow rate of 5 c.f.m./bu. cubic feet per minute per bushel at 110 . This drying rate varies with different seed, temperature, and the initial moisture. The hourly rate will be less if the initial moisture content is low and if the drying air is unheated or at temperatures below 110 .

Recommended minimum airflow rates for drying seed with unheated air under average conditions are: For seed with 25 percent moisture, 8 c.f.m./bu.; 22 percent moisture, 6.5 c.f.m./bu.; 18 percent moisture, 4.5 c.f.m./bu.; and 15 percent moisture, 3 c.f.m./bu. Generally, airflow rates greater than 8 c.f.m./bu. are not economical with unheated air because more power is required at the higher rates and no appreciable increase in drying rate can be obtained.

When seed is dried with heated air in bin-type barn driers, the airflow rate for light seed should be at least 3 c.f.m./bu. and at least 5 c.f.m./bu. for heavy seed. Uneven drying between bottom and top of the batch will be minimized if the seed depth in the bin is not excessive.

Most commercial driers use greater airflow rates and heated air. Rates usually vary between 20 and 40 c.f.m./ bu. A uniform drying of seed is possible with those rates and the seed depths often used in commercial units.

THE DEPTH of seed and the resulting static pressure resistance to airflow through seed alters the power needed to produce a given airflow rate in bin-type driers. All seed offers some resistance to airflow according to its size, shape, moisture content, and weight.

To illustrate: One foot of seed of alfalfa or sericea lespedeza in a bin offers a resistance to airflow of about 0.66 inch of water (static pressure) at 5 c.f.m./bu., but 4 feet of soybeans or shelled corn have the same static pressure. Another example: Shelled corn at that airflow rate exhibits a static pressure of 0.35 inch of water for a depth of 2 feet and 3.37 inches of water for a depth of 8 feet.

Static pressures also vary with the airflow. Shelled corn, again, has a static pressure of 0.35 inch of water for a depth of 2 feet at 5 c.f.m./bu., but the same depth at 20 c.f.m./bu. has a static pressure of 1.01 inches of water, or nearly three times as much.

Static pressures have less importance in commercial continuous-column or tower-type driers. Usually the seed depth that air must penetrate is only 6 to 16 inches in this type of drier. Also, the total air delivery of a commercial drier cannot be changed in many instances. About the only variation in the rate of airflow is the one due to small differences in static pressures of various seed types.