AE-90
Right `actions' taken as grain first goes into on-farm storage and then throughout the storage period will minimize the chance of problems that necessitate expensive `reactions'.
Grain has a limited storage life. If most of this life is used up during the fall and winter, the grain may not make it through the following summer. There are basic management practices, however, that can prolong this storage life. Attention to these practices should help insure successful grain storage the year around.
This publication discusses the types of `actions' needed for year-round storage. They involve the proper condition of the grain at bin-filling, correct design and operation of the aeration system, and other suggestions to prevent trouble. Although the discussion here deals specifically with corn, the principles apply to most feed and cereal grain, unless otherwise noted.
Grain stores best if it is dry, cool and clean. Grain must be dry to hold it through the summer months. Cooling can sometimes replace drying, as when moist grain is held through the winter. And grain that is clean will better resist mold growth and insect infestation even if stored at 1-2 percent higher moisture content than will grain that is `dirty' (i.e., containing a lot of broken kernels, chaff and foreign material).
Clean, undamaged shelled corn, properly cooled and managed, will keep for one year at 14 percent moisture. Corn mechanically damaged or dried at temperatures above 140° F or containing more than a trace of broken kernels and fines, should be dried down 1-2 percent tower than clean corn. Corn properly cooled can be held at up to 15-16 percent moisture through the winter if it is to be sold, fed or dried down further before April 1 (March 15 in southern Indiana).
To maximize storage life and prevent moisture migration and buildup, grain should be held at near-average outdoor temperatures. The technique now used almost exclusively to control and maintain these desired grain temperatures is aeration, which merely mechanical ventilation of grain in storage.
Aeration dates back only to the post World War II years. It developed at a time when grain surpluses were increasing, storage of these surpluses for periods of more than a year was common, and larger bins or flat storages were coming into use to cut costs.
Aeration received a further boost with the advent of field shelling and increased use of heated-air dryers for corn. This artificially-dried corn was much more brittle than that dried naturally in cribs or bins. So when it was `turned' (moved from bin to bin to equalize temperatures), excessive breakage resulted. To eliminate this moving or handling of the grain, turning gave way to aeration.
Aeration itself is no guarantee that storage problems are over. But if properly practiced, it can overcome most of the potential difficulties.
In bins over 2000 bushels capacity, the grain bulk or mass is so large that it fails to cool uniformly enough to avoid storage problems as outdoor temperatures change with the seasons. The unequal temperature in the grain mass then causes air current to circulate from warm to cold grain.
Since warm air holds more moisture than cold, the air moving up through the warm grain center picks up a full load of moisture, depositing some as it moves through the cold grain in the top layer. This causes moisture buildup, molding and crusting. These minute `convection currents' in the. grain cause moisture migration and accumulation that can only be prevented by reducing temperature difference in the grain bulk.
Basically uniform temperatures can be maintained in aerated grain storage if the aeration system has been well designed and is properly operated.
The first essential to successful grain aeration is an adequate, well-designed system. There are four basic requirements, and these are reviewed briefly in the following paragraphs.
This means a 10,000-bushel bin requires a minimum of 1000 cfm of air. Tables 1 and 2 provide the necessary data for calculating fan horsepower to aerate shelled corn in bins of various diameters and depths of fill. Aeration fans sized for shelled corn are also adequate for soybeans but not for wheat.
Aeration airflow rates per bushel
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corn depth 1/10 cfm 1/4 cfm 1/2 cfm 3/4 cfm 1 cfm
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ft. horsepower/100 sq. ft.*
10 .015 .04 .10 .19 .31
15 .025 .06 .24 .54 1.00
20 .034 .10 .48 1.20 2.30
25 .050 .21 1.00 2.30 4.30
30 .070 .36 1.70 4.30
40 .140 .83 4.30
50 .260 1.60 8.80
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*Fan horsepower per 100 sq. ft. of bin floor or cross-sectional area.
Hp value
Bin di- Floor Multi- from Fan hp
ameter area plier Table 1 for bin
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ft. sq.ft.
18 250 2.5 x ______ = ______
21 346 3.5 x ______ = ______
24 452 4.5 x ______ = ______
27 572 5.7 x ______ = ______
30 706 7.1 x ______ = ______
33 855 8.6 x ______ = ______
36 1017 10.2 x ______ = ______
40 1256 12.5 x ______ = ______
42 1385 13.9 x ______ = ______
48 1808 18.1 x ______ = ______
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If a perforated duct System or partial aeration floor (strips, squares or circles of perforated surface) is used to distribute the air under a grain mass, the air passage area from the fan into these ducts of floor segments must be sized to keep air velocity at or below 1500 fpm.
To provide the 1000 cfm airflow rate in the example given above, a duct cross section of 0.67 square feet is required (1000 cfm ÷ 1500 fpm). This would be achieved using a 12-inch round duct or a 10-inch by 10-inch (or equivalent) square duct.
This is one of the most important, yet most frequently overlooked requirements in aeration system design. To determine needed perforated duct surface area to maintain entering air velocity at or below 30 fpm, divide total bin cfm (total bushels x cfm per bushel) by 30 fpm. Using the previous example, 10,000 bu. x 1/10 cfm/bu. = 1000 cfm ÷ 30 fpm = 33 sq. ft. of perforated duct surface required.
To figure the total area that your system presently provides, simply total up the square feet of each area where the air moves from duct into grain (if a full round, perforated duct that lies directly on the bin floor is used, only 80 percent of the perforated surface can be considered for air discharge.). This should include both perforated duct surface and any open areas formed by blocking the duct up off the floor or by leaving the end of the duct open.
In addition, the distance from the nearest duct to the wall should not exceed half the depth of the grain.
Proper operation (or management) of grain aeration systems is as important as good design. Here are the basic management practices that, if applied, should minimize grain storage problems.
There are two important things to remember relative to grain aeration: (1) it takes a lot of time to cool (or warm) grain at the low airflow rates used for aeration; and (2) grain cools by layers or zones progressing in the direction of air flow.
In other words, a few hours of fan operation on a cold day will not cool all the grain a few degrees. Rather, the layer of grain near the air entrance will be cooled to a temperature close to that of the cold air. But if aeration is not continued, temperature differences between the cooled layer and the rest of the grain may be greater than if the fan had not been run at all.
At the minimum recommended airflow rate of 1/10 cfm per bushel, about 160 hours (nearly a week!) of continuous fan operation is required to uniformly cool the grain. At higher airflow rates, the fan operation time can be proportionately shorter; at lower rates, the time is longer.
Immediate aeration is especially important under two circumstances: (1) if the grain was binned during hot weather and its temperature is above the average day-night outdoor temperature for that time of year; or (2) if it was artificially dried at high temperature.
Even though the dried grain is cooled before binning, it is still probably 10-15 degrees above the air temperature. On a sunny October afternoon, that may be 85-90°F, which is much too warm for storage.
Since one objective of aeration is to keep the grain within a few degrees of normal average outdoor temperatures, aerate in late November or early December to reduce its temperature to near wintertime norms. But be careful not to freeze the grain, especially if it's to be carried into the next summer.
It is true that cold grain keeps better than warm grain; but frozen grain can cause problems when aeration is used to warm it. Also, very cold grain will sweat and may become musty when unloaded in warm, humid weather if not first warmed in the bin.
Some operators run their aeration fans for an hour or so every other week during the winter to check on grain condition. By smelling the air and feeling its temperature, trouble can be spotted early.
This is a controversial practice. Some operators prefer to leave the grain at 30-40° F as long as possible. However, if the grain is stored into late summer, reverse moisture migration may cause moisture to accumulate between the warm grain on the surface and the cold grain toward the center of the bin. The trouble spots usually develop 2-3 feet below the grain surface.
Moisture migration and accumulation in summer usually proceeds slower than that which occurs on the grain surface during cold weather. Thus, it may not cause molding and heating until July or early August. If the grain is likely to be stored beyond July 1, then it should be warmed in mid-spring to near outdoor temperatures.
Once begun, if fan operation is stopped for any length of time, there will be a zone of condensed moisture in the cold grain at the point where the warming front had reached. Conditions in this zone would be ideal for rapid mold development. If aeration is continued, however, the wet grain will be warmed and the moisture evaporated before the grain can deteriorate.
Summer aeration is not recommended unless `hot spots' or other trouble develops in the grain. Aeration fans can be run occasionally in the summer to check grain condition, but for less than an hour at a time.
It is a good idea to cover the fan when not operating. This prevents the possibility of excessive grain cooling in winter or rapid warming in summer from air that will move through the fan opening.
Outside wind pressure combined with a `chimney effect' in the bin can cause substantial air movement through the grain. Covers will eliminate this as well as help keep rodents and insects out of the bin plenum and duct systems.
If insect problems show up in the grain prior to mid-August, the bin should be fumigated. Light infestations appearing after mid-August can probably be controlled by aeration cooling.
Insects develop very slowly at temperatures below 60° F, and will become dormant or die out in extended storage periods of 35°F or below. But remember, if insects are only dormant, they will be in the marketing sample and become active as soon as the sample warms. Fumigation may be require d to market grain free of live insects.
An emptied storage bin should always be swept clean of dust and old grain before refilling. It is also recommended that both the inside surface and surrounding ground area be treated with an approved residual insecticide spray.
It is not a good practice to add new grain on top of old in a storage. If absolutely necessary, then fumigate the entire storage immediately following the addition.
Do not fill a bin to a peak or until the grain touches the roof. This will interfere with uniform air flow and prevent moisture movement out of the grain surface. Level fills work best!
The grain surface should be inspected at least every other week throughout the storage period. To make an inspection, walk over the grain and poke around with your arm or a rod, smelling, feeling and looking for indications of trouble.
Evidences of hot spots, insect infestations or other problems that start in the grain mass soon migrate to the surface. Hot spots will be seen as damp, warm, musty areas. Insects and mold growth are more likely to show up where broken corn has accumulated.
Wear a surgical-type disposable mask when working around moldy or spoiled grain. Exposure to and inhaling mold can cause severe allergic reactions.
Never enter a bin when grain is being unloaded. In flowing grain, you have only 2-4 seconds before you are helpless, even at modest 6-inch auger flow rates.
Also, beware of crusted grain. It not only is likely to be moldy, but it also may be concealing large cavities where grain has been removed below the crust. Anyone Stepping on that crust may plunge through and be covered with grain, even if the unloading equipment is not operating.
Grain suffocation accidents do happen, and all too often! Think before you move. Safety must be an action, not a reaction!
New 7/79
Cooperative Extension work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture cooperating: H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an equal opportunity/equal access institution.