Grain Journal - July/August 2008 - (Page 8) Aeration Management EFFICIENT AND EFFECTIVE TECHNIQUES FOR DELIVERING AIR TO STORED GRAIN to compensate for 10 hours of rain or fog. It’s better to keep air moving at a steady pace through the grain mass. • The chart (to the right) gives basic recommendations on air volume according to commodity and moisture content. Note that above a certain maximum moisture content, aeration won’t work to protect the commodity. • Static pressure varies according to the commodity, moisture content, level of cfm delivered, and the depth of the grain. Types of Aeration Systems Pull-down or negative systems. These are the oldest type of aeration system. This type of system eliminates the need for roof exhausters and is less expensive than a push system. This type of system is less commonly used today, because it is associated with a number of problems. It tends to pull problems in the tank such as high moisture, fines, and foreign material (FM) down to the bottom of the tank. This can cause the flooring to deteriorate and plug openings. Another problem is that hot air tends to rise, and when you switch the fans on, it is pulled down through the grain mass again, reducing efficiency. It also means that bad air is pulled through grain that otherwise is in good condition. Push or positive systems. These are more expensive than a pull type of system, but it’s also more efficient and effective. Part of the additional expense is the required roof exhausters and air vents. A rule of thumb is that you need to pull 1.5 times as much air out of the top of the tank as you put into the bottom. Also, the roof systems should be wired separately from the groundlevel aeration fans. This takes extra power. The push-type system works with the natural tendency of warm air to rise, which does save some energy at the bottom of the tank. The flooring This article is based on a presentation by Kevin Miles, president of Rolfes@Boone, Boone, IA (800-265-2010). He spoke in March at a Grain Operations Conference sponsored by the Corn Belt Chapter of the Grain Elevator and Processing Society and the Grain and Feed Association of Illinois in Bloomington, IL. Aeration is a nearly universal procedure for maintaining the condition of stored grain. It’s a slow process to keep commodities in condition. Air always goes where it’s easiest for it to go, and that’s a key concept for aeration system design. • The main concern with aeration is to keep grain temperature constant while ambient air temperature fluctuates, generally 10 to 15 degrees difference between the daily high and low. • It’s nice to store grain in the winter at 40 degrees and at 65 degrees during the summer months, if the grain will be stored that long. Run your fans any time you can lower the grain temperature 10 8 GJ J/A to 15 degrees with outside average ambient temperature. If the high for the day is 70, and the low is 40, your average ambient temperature is 55 degrees. If the stored grain is at 75 degrees, run an aeration front through the grain. • Aeration volume often is expressed as a fraction of a cubic foot per minute of air per bushel of grain – for example, 1/10 cfm per bushel. As a rule of thumb, each fraction represents 12 hours of fan time. Thus, 1/10 cfm per bushel requires 120 hours to move air completely through the tank; 1/7 cfm takes 84 hours. A 1/7 system delivers more air more quickly to the grain than a 1/10 system, but it also requires more powerful fans and more energy usage. • A start-stop type of operation can produce too many air fronts working their way through the tank, thus producing a crusting effect on the grain surface. This can be a safety issue, as well as blocking air flow through the tank. • Rain or fog generally isn’t a problem. It takes one hour of fan operation
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