LITTLE CAN be done about temperature when seed is dried with natural or unheated air. Relative humidity of the air therefore becomes a key point. The most economical drying with unheated air is accomplished when the atmospheric relative humidity is below 70 percent.

An example of the importance of relative humidity: If the drying air reaches 100 percent saturation during its passage through the seed, air at 80 and 40 percent relative humidity will have about twice the drying potential of air at 80 and 70 percent relative humidity.

Temperature, although not controlled, also can influence natural-air drying because the maximum moisture that air can hold depends on its temperature. Using the similar assumption as above that air becomes completely saturated, air at 80 and 50 percent relative humidity again will have about twice the drying potential of air at 60 and 50 percent relative humidity.

When heated air is used for drying seeds, the drying temperature usually should not exceed 110 . Some seeds, such as peanuts and certain vegetables, are dried at 90 to 100 . If the temperature sensitivity of a given type of seed is not known, it is wise to select the drying temperature according to the moisture level of the seed.

This schedule is safe for a number of field and vegetable seeds: 18 to 30 percent moisture content, 90 ; 10 to 18, 100 ; under 10 percent, 110 .

FORCING AIR in one direction only through the seed mass is practiced in nearly all drying with unheated air and in most drying with heated air.

Often much usable heat is lost in heated-air drying because the heated air travels through the seed mass once and is then exhausted to the atmosphere. This is particularly true in commercial driers, in which high rates of airflow are used to obtain fast drying and a uniform, final moisture content. Simple construction of the seed-holding bin and of the air distribution system is an advantage possible with this method of air handling.

Two-way or reverse-airflow drying is practiced by reversing the direction of air movement through the seed mass during part of the drying operation. It makes for a more uniform moisture in the seed mass, and high airflow rates are unnecessary. The drying time and operating costs are reduced slightly.

The chief disadvantage is that the fan must be detached, turned around, and reconnected, or extra ducts and tight construction must be provided at the top of the seedbin. An extensive practice in drying seed corn in the ear is to pass the same air successively through several bins. Drying efficiency is greater because better use is made of the heat.

BOTH HEATED and unheated air have advantages and disadvantages.

When one plans for drying facilities, he should determine first if atmospheric conditions in his locality permit satisfactory drying with unheated air. He can get this information from agricultural colleges.

If atmospheric conditions are satisfactory for unheated-air drying, the final choice will rest mainly on the amount of seed to be dried and the time available for drying.

Drying by heated air can be done regardless of weather, and the drying time is short, usually from a few hours to less than a day. A high drying capacity per fan horsepower may be obtained, because of the faster drying. Among the disadvantages are higher costs of equipment and maintenance, fuel expense, some fire hazard, and the need for close supervision.

Equipment and maintenance costs are lower when unheated air is used; there is no fuel expense and no fire hazard. Little supervision is required. Success, however, depends on weather conditions. The time required for drying usually is several days to several weeks, and more space is needed to dry the same amount of seed than with heated air.

SOME SEED is dried and stored in the same bin. It is common when drying is done with unheated air, but heated air sometimes is used.

Drying in n storage is commonly called a batch-type operation, as the entire batch is dried without moving the seed. A simple bin with perforated floor can be used for supporting the seed in the bin. The perforated floor is elevated above the floor of the building to provide a fan or drier-outlet connection to the space between these floors. Air forced into this space flows upward through the perforated floor and the seed mass. The floor of the building and foundation walls of the bin must be reasonably airtight to prevent excessive loss of drying air. The perforated floor may be supported on concrete blocks, laid flat side up. One should allow one block for each 50 bushels of storage capacity.

A system of lateral ducts branching from the main or central duct often is used in place of a perforated floor. The ducts rest on the floor of the building, and the main duct is connected to the fan or drier. The duct systems usually are less expensive than the perforated floor with supports. Bins with perforated floors are somewhat easier to unload, although the ducts are made in lengths that can be easily removed as the bin is emptied. Factory made metal ducts are available, or ducts may be built of lumber.

WHEN ATMOSPHERIC AIR is used for drying, a slight increase in air temperature can improve drying, especially during damp or cold weather.

The use of supplemental heat at times may mean the difference between success and failure in maintaining high quality of seeds.

Drying in Bin with Unheated Air

Special units supplemental heaters are available. They are usually designed to provide a maximum rise in air temperature of 10 . They often have humidistats that shut them off when the relative humidity of air is below 60 or 70 percent. Many of the heaters burn liquefied petroleum or natural gas, weigh little, and are relatively inexpensive.