Although the moisture migrates slowly, it continues to do so, as long as temperature differences exist in the grain. If allowed to continue for months, or even a few weeks, the accumulated moisture may promote insect activity, microbial growth, and spoilage in the upper layers of stored grain, particularly in large silos.
The direction of migration reverses itself when the weather changes from winter to springtime conditions, with potential moisture condensation occurring near the bottom center of the grain mass.
Aeration can control moisture accumulation by creating a uniform grain temperature throughout the grain mass. An effective method is to move small quantities of air (1/10 CFM/ bushel [about 6.25 M3/Hour/metric ton]) through the grain more or less continuously until temperatures are equalized to within a range of the average ambient air temperatures. In this regard, the goal is to maintain the grain temperature within 4° – 5°C of the average daily ambient air temperature.
Operation of the aeration system in a four-season climate is relatively simple. During harvest, or at the initial filling of the storage in the case of grain just transported to the site, the aeration fan should begin operation as soon as the aeration ducts are completely covered. Aeration should operate continuously for about ten days during this period, with the goal of lowering the grain temperatures to about 15° C. Obviously, this is dependent upon the ambient temperatures available for cooling.
In some locations, night-time-only aeration operation may provide the necessary cooling potential. Keep in mind that it will require about 150-to-200 hours of fan operation to lower grain temperatures 2° – 3° C. Use of a temperature monitoring system will provide finite results of the effects of aeration and provide the operator with another management tool in deciding how long to operate the aeration system.
During the late fall, when ambient temperatures begin to stay cool, the aeration system should operate for another week, with the goal of lowering the grain temperatures to as low as 4° – 5° C. Although air with ambient temperatures below freezing can be used for aeration, caution is advised to avoid freezing any portion of the grain mass and frosting over of the roof vents, which can lead to roof damage or structural failure. Warming grain in the spring is recommended, in some situations, to avoid moisture migration to the lower center of the grain mass. To accomplish this, when temperatures begin to stay warm, operate the aeration system for about one week.
The goal is to raise grain temperatures to within 2° – 3° C of the average ambient air temperatures.Operation of the aeration system in subtropical climates presents a completely different management philosophy than operation in four-season fall crop conditions.
In subtropical climates, the goal is to maintain the lowest possible grain temperatures without increasing the moisture content of the stored grain. In this case, the use of some type of grain temperature monitoring system is indispensable, to provide the operator with timely information from which to make fan operation decisions.