Effect of ambient conditions on grain drying-part 2

The goal is to keep grain temperatures as low as possible, i.e. within 4 – 5°C of the average ambient temperature Do not aerate stored grain in the middle of the day. Relative humidity is lowest in the middle of the day, but temperatures are highest .

Late afternoon represents an ideal time for aeration of stored grain. Ambient air is cooling, and the relative humidity is still low. Plan for starting the fans about 17.00 and operating them until 20.00 Early morning is also a good time to aerate stored grain. Begin fan operation about 5.00 and operate the fans until 8.00 If the grain temperatures are near your pre-determined goals, do not operate the aeration fans. Remember, pumping ambient air into the storage means that the grain will eventually reach the same temperature and relative humidity as the ambient air Check the grain temperatures often, perhaps even daily, to help decide if grain aeration is necessary.
Cooling air can be provided to aeration systems using commercially available portable refrigeration units. These units can effectively lower grain temperatures, even in tropical conditions, without risking the addition of moisture to stored grain.
Distribution of air to the grain mass is the third component in aeration system design. Computer modelling indicates that air distribution becomes relatively uniform at about 60-to-70 percent of the grain depth in a silo, regardless of the geometry of the floor duct air distribution system. The arrangement of this distribution system, however, affects how much air reaches the grain mass in the lower 30 percent of the silo. Fans can be connected to the aeration ducts either in the pressure mode or the suction mode (positive or negative pressure systems). Either system is acceptable and offers unique advantages. A consideration when choosing between these alternatives is that positive pressure systems add heat to the air that enters the grain mass because of the inefficiency of the fan.
This inefficiency results in a temperature increase of up to 1° C. A negative pressure system is preferable if dust control at the surface of the grain is a priority. Negative system operation prevents condensation on the silo or building roof as the grain is cooled and allows use of solar heat in the roof when warming the grain. One disadvantage of this system is that, because the bottom layers of the grain are the last to cool, it may be difficult to know when cooling is complete, and any moisture movement in the grain is drawn downward, where moisture or quality problems become harder to detect.
Positive pressure systems allow the addition of grain layers on top of the existing grain mass without rewarming or cooling grain. It is also easier to determine when the grain is in proper condition, because the top layer is the last to cool and there is usually personnel access to this level. There is some evidence that positive systems provide better air distribution than negative systems.
In either case, it is necessary to provide adequate entry or exit areas for the aeration air. This is usually accomplished in silos by the use of roof vents. The number of roof vents correlates directly with the size of the fan(s). Lack of roof vents, or inadequate numbers of vents, can result in structural failure or roof damage.
Final Discussion
Grain aeration systems are an efficient management tool for storage operators. They can be used to maintain and occasionally change the condition of stored grain, regardless of climatic conditions. Aeration systems can prevent the growth of moulds, fungi, and insect populations. Properly designed, equipped and operated, aeration systems are an economical tool that can maintain grain quality and increase profits for the storage operator.