Thus, grain aerated twice can lose 1/2 – 2/3 % moisture while grain aerated three times may lose 3/4 % or more from aeration.  A 100,000 bu bin of 12% wheat (or grain of any kind) aerated three times between August after harvest and the following March, may shrink to about 99,200 bu of 11.2% wheat (an 800 bu weight shrinkage or 0.8% weight loss) when shipped  due to aeration moisture losses.

 With uniform airflow, at the commercial grain aeration recommended rate of 1/10 (0.1) cfm/bu, a cooling cycle of about 90-100 hours are required to cool grain.  During the fall (October through mid-December), with reduced evaporative cooling one complete aeration cycle requires about 120-150  hours, while winter aeration, cooling grain at lower temperatures than in the summer and fall, a cooling cycle requires about 175-200 hours.  Cooling with other airflow rates is proportional. Cooling grain in concrete silos with 1/20 (0.05) cfm/bu aeration requires twice as long as steel bin or warehouse cooling at 1/10 (0.1) cfm/bu.

 Grain cooling is one of the most beneficial and sustainable integrated pest management (IPM) practices for insect control.  As grain cools, insect feeding and reproduction slows.  A 15 F (8.3C) reduction in grain temperature from 90F to 75F (32 C to 24 C) will reduce grain insect activity by 50 to 75 %.  Reducing grain temperatures below 55-60 F (15.5 C) will stop most insect activity.  Insects get moisture from the grain they eat.  Insects in grain at 40-45F or below die due to dehydration. 

 Aeration is an excellent IPM tool, especially important to organic grain and seed producers and elevators and mills marketing organic grain.  It is also an excellent treatment for overseas markets which require grain without pesticide residuals. 

 Aeration is often used in conjunction with recirculation fumigation of phosphine gas, as aeration ducts make distribution of phosphine much more uniform.  Recirculation fumigation, called closed loop fumigation (CLF) can be adapted to steel bins, flat storage warehouses and concrete silo annexes that have been well sealed with minimal costs as the blower requirements are relatively small (typically 1/3 to 1.5 HP centrifugal fans),schedule 40 PVC pipe, flexible non-perforated black drainage tubing, and duct tape are primary equipment and materials required.  When CLF systems are installed by farmers or elevator maintenance workers, total costs run between 0.5 and 1.0 cent ($0.005-$0.01) per bushel.  When commercial contractors install CLF systems on larger facilities, costs range from about 1.0 to 2.5 cents ($0.01- $0.025) per bushel.  Sealing is often a primary cost of installing CLF.    

Many grain specialists used to recommend aerating grain at 10 F (5.5 C) reductions in ambient air temperature.  This was not only a very difficult recommendation to achieve, but it resulted in excessive aeration drying of grain.  The theory in the 60s through late 80s was that grain could only cool 10-15 degrees during a cooling cycle, regardless of the temperature differential between the grain and ambient air.  A USDA engineering study conducted in the early 1960s cooling wheat in concrete silos showed that with sufficient ambient temperature and airflow rates, grain will closely follow air temperatures.  This study demonstrated cooling wheat at airflow rates of 1/20 cfm/bu from about 90 F (32 C) to about 35 F (2 C) in two aeration cycles.  The time required to aerate the second time was significantly longer than during the first aeration, illustrating that summer and early fall cooling was faster than winter cooling. 

 Grain absorbs moisture during grain warming.  Grain warming for the purpose of adding moisture back to grain should be avoided as surface moisture accumulation on cold grain during the warming process may trigger uncontrolled rapid mold fungi development.  Warming also reduces the resistance of grain to invasion by store grain insects such as lesser grain borer, rice weevil and Indian meal moth (the three most damaging grain insects) as well as red flour beetles, saw-tooth grain beetles and confused flour beetles. 

 USDA Agricultural Research Service (ARS) engineers also documented commercial recommendations for grain system facility designs.  Their recommendations for airflow velocity through transition ducts and in main perforated ducts was 2000-2500 ft/min.  Recommendations for air passing through perforated ducts as it entered or exited the grain bulk was 30 ft/minute or 0.5 ft/second.  Commercial recommendations for roof vents have been about 1000 ft/minute velocity.  These are excellent design guidelines, but I consider them to be conservative. 

There are times when space is limited where adding fan power to overcome slightly higher airflow resistance through ducts is good design.  In pneumatic conveying, typical air velocities range between 3,500 and 5,000 feet/minute.  Aeration duct velocities of 3,000 to 4,000 ft/min do not create major friction losses. 

 Exhaust or inlet air velocities through perforated duct surfaces of 45 – 60 feet/minute are acceptable designs.  Under certain conditions when space is a premium, higher velocities may be justified.  Example:  Retrofitting suction aeration systems into concrete silo systems where perforated screened manifolds are installed on grain discharge spouts under concrete elevator silos may limit area so that 90-100 ft/minute are required.  The alternative – conventional ducts installed inside concrete silo hoppers -- may be prohibitive in cost. 

 Two prototype suction aeration systems were installed in two Oklahoma concrete elevators (at Yukon, OK and Ponca City, OK) during this USDA SARE study.  Both systems incorporated CLF as part of the overall design.  Results indicate that lateral ducting to the down spout suction plenum manifolds needs to be larger, but the operational data were encouraging.  Satisfactory cooling of heating zones in silos were controlled by the new suction aeration system. 

 Aeration System Recommendations:

 Aeration should be considered as Crop Insurance.  Cooling grain as soon as possible is the best means of maintaining high grain quality.  Aeration airflow rate recommendations of 0.1 cfm/bu are too low for high risk southern and central U.S. regions.  Airflow rates of 0.2 cfm/bu should be a base design, with 0.25 or 0.3 cfm or higher used where cost can be justified.  Being able to cool two to four times faster will allow fast cooling of hot grain with during summer weather conditions. 

 TV weather forecasts now provide adequate lead time for excellent manual aeration management of grain.  Timers can be installed to turn the fans on and off at selected times.  Simple thermostats can also be integrated into the starter circuits with hour meters that record total fan motor operating time.

 Some grain can be cooled from the 90s or 100s to mid-70s in southern states during the summer if producers and elevator grain managers have high speed aeration.  If an aeration system has airflow rates of 0.3 cfm per bushel, a bin of grain can often be cooled in 30-35 hours of night time cooling to 75-80F in late June, July or early August, which will reduce insect activity by a major amount.  Cooling a second time in early to mid-September into the 60s can prevent the need to fumigate and eliminate insect population buildup and costly grain damage or seed grain damage losses.