Group Title: Circular
Title: Management of stored grains with aeration
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 Material Information
Title: Management of stored grains with aeration
Series Title: Circular
Physical Description: 8 p. : ill. ; 28 cm.
Language: English
Creator: Talbot, Michael T ( Michael Thomas ), 1948-
Florida Cooperative Extension Service
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1993
Subject: Grain aeration   ( lcsh )
Grain -- Storage   ( lcsh )
Grain -- Losses   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: Michael T. Talbot.
General Note: Title from caption.
General Note: "May 1993."
 Record Information
Bibliographic ID: UF00008573
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6837
ltuf - AJQ6972
oclc - 28531972
alephbibnum - 001832866

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Circular 1104
May 1993

Management of Stored Grains with Aeration'
Michael T. Talbot2


Estimated annual grain loss from harvest to
consumption is approximately 10 percent of total
production. About half the loss occurs during
harvest; the remainder, in storage. Grain loss can be
reduced, and, in the case of storage, eliminated if
proper storage procedures are followed. This
publication discusses methods of using aeration to
properly manage grain stores.

Aeration is a process of moving small volumes of
air through grain or seed to cool and ventilate the
material and maintain quality. The moisture content
of the seed or grain is changed very little by aeration
due to the low volume of air. Therefore, aeration
should not be confused with drying, which reduces
moisture to a level acceptable for safe storage or
commercial sale. Drying can be rapid if heated air
and high air flow rates are used, thus changing the
moisture content considerably in a short period. So
these two terms are used in referring to moisture
control and preservation of grain and seed; the two
should not be confused. For information concerning
principles of grain drying, refer to Extension
publication Circular 673, Grain Drying and Storage on
Florida Farms.

Aeration conditions grain and seed by lowering
the temperature of the material and equalizing the
temperature within the storage structure. This
prevents moisture migration and condensation.

All organisms responsible for losses in stored
grain and seed are affected by the temperature and
moisture of the material. Such organisms include
bacteria, insects, molds and mites. Therefore, cool,
dry grain and seed keep longer if these deteriorating
conditions are prevented or retarded.

These organisms are greatly inhibited at
temperatures below 40 F. Little insect reproduction
occurs in grain below 60 F. It should be pointed out,
however, that aeration is an aid to insect control, not
a substitute for proper pest management. For
additional information concerning grain pests, refer to
Extension publication Circular 873, Pest Management
Strategies for Storing Grains in Florida.


Moisture often condenses in the top of stored
grain even if it was dry at time of storage and was
stored in a weather-tight bin. Grain and seed are
normally placed in bins in the fall of the year and lose
heat as winter progresses. The material near the
walls and surface cools faster than the material in the
center. This temperature differential within the bin
produces air currents, as shown in Figure 1.

The air near the bin walls cools, becomes denser
and settles downward, producing a downward motion
near the walls. The air in the center portion of the
bin is heated, expands and becomes lighter, causing it
to rise. The warmer air has greater moisture-holding
capacity and, therefore, absorbs moisture from the
grain near the center of the bin. As this warm, moist

1. This document is Circular 1104, Agricultural Engineering Department, Florida Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida. Publication date: May 1993.
2. Michael T. Talbot, Associate Professor, Agricultural Engineering Department, Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research,
educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap,
or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean


L'-.-.;_,.l.. w, :.;ad Grains with Aeration

Figure 1.

Air currents in stored grain produced by differential

air rises through the top layer of cooler grain, the air
is cooled, loses some of its water-holding capacity, and
moisture condenses on the grain.

Also, as air from the center continues to rise to
the underside of the bin roof, further condensation
occurs if the roof surface is cool. Thus, water
accumulates in the top layer of grain because of these
air currents moving through the grain, although the
bin is weather tight. The moisture accumulation in
the top layers of grain produces spoilage. For
additional information on moisture migration refer to
Extension publication Circular 1045, Moisture
Migration in Stored Grains.


Adequate air must reach all areas of the stored
grain to cool it before condensation begins.
Satisfactory aeration depends primarily upon air flow
rate. The air flow rate through the grain will not be
uniform where ducts are used. Thus, the air flow rate
is an average value, and must be high enough for
adequate air supply to reach the grain in all areas of

Air distribution usually is more uniform in upright
bins than in flat storage. For this reason, higher air
flows are recommended for flat storage bins. For
additional information on the design of flat storage
aeration, refer to Extension publication Circular 861,



AERATE: Aeration System Design Software for Flat
Grain Storages.

Recommended air flow rates for intermittent
operation in the Southeast are as follows:

upright storage 1/10 to 1/20 cubic feet minute
(cfm) per bushel (bu)

flat storage 1/5 to 1/10 cfm per bu

Lower rates should not be used unless moisture
content is less than 12 percent (wet basis).

Horsepower requirements and static pressure in
inches of water for aeration fan operation are shown
in Table 1. The table is valid only for clean grain
without excessive fines or chaff.

To aerate 5,000 bushels of wheat stored at a
depth of 15 feet with an air flow rate of 1/10 cfm per
bushel, Table 1 can be used to determine that the
horsepower (hp) required to drive the fan is 5 x 0.04
= 0.2 hp, or less than 1/4 hp. The air flow is 500 cfm
and the static pressure against the fan is 1.25 inches.


Normally, air should be drawn downward through
stored grain to counteract the tendency of the warm
air to rise. Some condensation may occur when
warm, moist air rises through a cooler top layer.
Moving the air downward cools the upper layers of
grain first and reduces the possibility of moisture

If heat is trapped above the grain in partially
filled bins, the downward motion of air can raise the
grain temperature, which is undesirable. Under this
condition, open the bin top and allow the hot air to
rise before aeration begins.

It may be necessary to reverse the air flow in
grain containing fines which accumulate near
perforated ducts and block air movement. If the air
flow rate is high, the direction of air flow is not
critical. Therefore, it is normally not necessary to
reverse the direction of rotation of a drying fan used
for aeration.


Page 2

Management of Stored Grains with Aeration


Perforated floors distribute air uniformly, are well
suited for aeration, and are frequently used with
various drying systems. It is often not feasible to
install a perforated floor for aeration only. Ducts are
less expensive and satisfactory for aeration. However,
ducts above the floor make it impossible to use sweep
augers. Vertical aerators, as shown in Figure 2, are
effective in round storage bins and normally can be
used with sweep augers for unloading the bins. Some
vertical aerators are portable and can be installed
after the bins are filled. One aerator can serve bins
up to 18 feet in diameter if air flow is adequate.

Aeration ducts, as shown in Figure 3, can be used
to distribute air for aeration. Short ducts (length over
diameter less than 50) should have openings or

Page 3

perforations equally spaced over their surface area for
air passage into the grain. For uniform air
distribution in long ducts, the percentage of
perforation in relation to the duct surface must be
lowest at the fan end. The perforation in long ducts
(length over diameter greater than 50) can be as low
as one percent near the fan and as high as 20 percent
at the farthest point.

Perforation distribution in long ducts needs to be
calculated by an engineer. It is desirable for ducts to
have enough perforated surface area to limit air
velocity through the grain near the duct to 20 feet per
minute (fpm) or less even though the velocity through
the perforation may be 10 times this value.

Friction losses in ducts increase as air velocity
increases. Ducts must be large enough to prevent

Table 1. Horsepower requirements and static pressure for aeration fan operation.

Depth Horsepower per 1000 bushels at Static pressure (inches of water) at various
of various air flow rates and air flow rates and grain depths
Grain grain depths

1/5 1/10 1/20 1/5 1/10 1/20
cfm/bu cfm/bu cfm/bu cfm/bu cfm/bu cfm/bu

Shelled Corn
10-15 0.04 0.02 0.01 0.60 0.55 0.51
20 0.05 0.02 0.01 0.70 0.65 0.57
25 0.06 0.03 0.01 1.00 0.77 0.63
10-15 0.04 0.01 0.01 0.50 0.50 0.50
20 0.05 0.02 0.01 0.70 0.55 0.50
25 0.10 0.02 0.01 0.90 0.65 0.50

10 0.05 0.03 0.02 1.05 1.00 0.95
15 0.08 0.04 0.02 1.45 1.25 1.05
20 0.10 0.05 0.02 2.00 1.60 1.20
25 0.15 0.07 0.03 270 2.05 1.45

10 0.05 0.03 0.01 0.90 0.80 0.70
15 0.07 0.03 0.02 1.25 0.95 0.80
20 0.09 0.04 0.02 1.70 1.20 0.92
25 0.15 0.05 0.02 2.50 1.50 1.07

Grain Sorghum
10 0.05 0.03 0.02 1.05 1.00 0.95
15 0.08 0.04 0.02 1.45 1.25 1.05
20 0.10 0.05 0.02 2.00 1.60 1.20
25 0.15 0.07 0.03 2.70 2.05 1.45

Management of Stored Grains with Aeration

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Page 4

excessive static pressure losses from duct friction. Air
velocities in ducts of 1,000 to 1,500 fpm are desirable.
The cross-sectional area of supply ducts can be
determined as follows:

Duct cross-section area =

Total air volume (cfm)

Air velocity (fpm)

To determine the supply duct size for aerating the
wheat in the previous example, use an air velocity in
the duct of 1,000 fpm. The required duct size is:
500 cfm
50 -= 0.5 square foot (sq ft)
1000 fpm

A 10-inch circular supply duct or a 6 x 12 inch
rectangular supply duct would be sufficient (Table 2).
The total cross-sectional area of the collector ducts
should have at least as much area as the supply duct.

Figure 2. Vertical aerators are satisfactory for smaller bins of
5000 bushels or less. If used in square or rectangular bins, To prevent excessive friction losses as air enters
the distance between aerators should not much exceed the the perforated duct and leaves the grain, the duct
grain depth. needs enough perforated surface area to limit air

I ii

Plan View Duct System


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Figure 3. Duct system for round or rectangular bins.

r - - -

Management of Stored Grains with Aeration

Table 2. Cross-sectional area of ducts.

Diameter Area
or depth of circle Area of rectangle in square feet
(inches) in square feet when top width in inches is:

9 12 15 18

4 0.08 0.25 0.33 0.41 0.50
6 0.20 0.38 0.50 0.62 0.75
8 0.35 0.50 0.67 0.83 1.00
10 0.55 0.62 0.83 1.04 1.25
12 0.75 0.75 1.00 1.25 1.50
14 1.05 0.88 1.16 1.46 1.75
16 1.40 1.00 1.33 1.67 2.00

velocity through the grain near the duct to 20 fpm or 25 sq ft duct surface area 9.61minimumengthperforated
less. This determines the minimum length of the 2.62 (Table 3, 10' pipe) pipe required
duct. The area of the duct perforations should be at
least 10 percent of the total duct surface. GRAIN PRESSURE ON DUCTS
Distribution of these perforations is discussed in the Ducts can be of any shape or configuration. They
section Air Distribution Methods. The total required o c
surface area of a perforated duct can be determined ae normal ma o oo, st, rete, ceramic,
plastic or a combination of these materials. For
as olows example, ducts can be made of two rows of concrete

Total air volume (cfm) blocks and boards by placing short boards over the
Total surface area (sq ft) = blocks and leaving cracks for air movement. Wire
Air Surface Velocity (fpm) screen placed over such a duct will keep grain out of
it. The duct must be strong enough to support the
In the previous example: grain regardless of its shape or material used.

500 cfm 25 sq ft of duct surface area The vertical and lateral loads on ducts can be
20 fpm determined from Table 4 if the depth and type of
grain are known. The table shows the maximum
The surface area (square feet) per foot of length weight of common grain in pounds per cubic foot for
for the ducts given in Table 2 is given in Table 3. calculating vertical loads, equivalent fluid weight for
calculating lateral loads, and the angle of repose for
For the 10-inch diameter round pipe previously calculating the filling or emptying configurations.

Table 3. Surface area of round and rectangular ducts.

or depth round duct Rectangular duct (square feet)
(inches) (square feet) when top width is:

9 12 15 18

4 1.05 1.42 1.67 1.92 2.17
6 1.57 1.75 2.00 2.25 2.50
8 2.09 2.08 2.33 2.58 2.83
10 2.62 2.42 2.67 2.92 3.17
12 3.14 2.75 3.00 3.25 3.50
14 3.67 3.08 3.33 3.58 3.83
16 4.19 3.42 3.67 3.92 4.17
18 4.71 3.75 4.00 4.25 4.50

Page 5

Management of Stored Grains with Aeration

To determine the vertical and lateral loads on a
duct covered with soybeans to a depth of 15 feet, the
load in pounds per square foot is determined by
multiplying the grain depth in feet by the densities in
pounds per cubic foot given in Table 4.

Vertical load on top of duct = 46.4 Ibs per ft x 15 ft deep
(Ibs per sq ft) = 696 Ibs per sq ft

Lateral load on vertical surface = 16.1 Ibs perftx 15ft deep
(Ibs per sq ft) = 242 Ibs per sq ft

The lateral load acts on a vertical surface, such as
on walls or sides of ducts. The angle of repose is the
angle measured from the horizontal which the
granular material takes if allowed to pile or funnel.


Aeration should begin when outside air
temperature is approximately 100 F below the grain
temperature. The grain temperature should be
measured at several points with a thermometer
encased in a pipe which is inserted into the grain.
Measuring the temperature of exhaust air will give an
indication of the average grain temperature.
However, the exhaust air temperature will not detect
"hot spots." For additional information on grain
sampling and temperature monitoring, refer to
Extension publication Circular 1046, Grain Sampling.

Table 4. Design data and storage pressures for grain.

Page 6

A grain temperature of 50 F is generally
satisfactory, particularly if the grain is to be moved
the following summer. For grain that is to be stored
more than a year, lowering the temperature below
50 F will give better insect and mold control. If cold
grain is removed from storage on a warm, humid day,
condensation may occur. Therefore, it is desirable to
operate the fan and warm the grain to within 15 to
20 F of the air temperature before removal.

It is best to aerate the grain with air that does not
change the grain moisture content. Air flow is usually
low enough to allow only gradual moisture changes.
Continuous aeration with air flows of 1/20 to 1/50 cfm
per bushel, even in rainy weather, will not appreciably
change grain moisture content.

Crop drying fans, if used for aeration, have
considerable air flow capacity and should not be
operated when humidity is extremely high or low.
The higher air flow cools the grain rapidly. It is
desirable to aerate only when the humidity is below
60 percent and the grain 10 to 150F warmer than
outside air temperature. During warm periods, aerate
when it is cooler, during evening or early morning. It
is advisable to operate the fan continuously for a few
days to remove harvest heat.

Equivalent fluid Maximum weight
weight per cubic* per cubic foot
Angle of repose foot (pounds per (pounds per
Grain (degrees) cubic foot) cubic foot)

Emptying Filling
or or
Funneling Piling

Shelled Corn 27 16 18.0 48.0
Soybeans 29 16 16.1 46.4
Oats 32 18 10.8 35.2
Barley 28 16 15.6 43.2
Wheat 27 16 21.5 55.0
Rye 26 17 18.1 46.2
Flaxseed 25 14 17.5 43.2

*Fluid pressure method is not recommended for estimating lateral loads in bins deeper than their width on diameter.

Management of Stored Grains with Aeration

Table 5. Approximate equilibrium moisture content of grain.

Grain moisture 400 F 600 F 750 F
percent (wet basis) Relative humidity

13 54 61 65
12 47 53 58
11 40 45 51
10 33 37 44



Grain moisture content is related to the
temperature and relative humidity of the air which
surrounds it. Under certain conditions, grain will

Page 7

absorb moisture; under other conditions it releases
moisture. Under proper conditions of temperature
and humidity, grain will neither lose nor gain
moisture. This condition, called "equilibrium moisture
content," gives an indication of desirable temperature
and humidity conditions for aerating grain.

Table 5 shows the equilibrium moisture content
of grain to be 13 percent with air at 60 F and 61
percent relative humidity. At the same air
temperature and 75 percent relative humidity, the
grain would pick up moisture. At 50 percent relative
humidity and 60*F, it would lose moisture. As a
general rule, little wetting of the grain will occur 1) if
outside air is 10 to 150 F cooler than the grain, and 2)
if no aeration occurs during periods of high humidity.
Aerating the grain a few hours each week after the
desired temperature has been reached eliminates
objectionable odors and prevents the musty smell.

Humiistat Thermostat....................
Humidstat Thermostat

IMotor Controller
Figure 4. Automatic control wiring diagram.

Management of Stored Grains with Aeration


Studies show an air flow rate of 1/10 cfm per
bushel will cool grain by aerating about 80 hours in
summer, 120 hours in the fall and 160 hours in the
winter. Of course, the air flow rate and degree of
uniformity affects cooling time. When the air flow
rate is doubled, the aeration time should be reduced
about one-half or vice versa.


The electricity required to power aerating fans is
influenced by air flow rate, type and depth of grain,
the degree of uniformity, and the number of seasons
when cooling is needed. Small grain, such as wheat,
may be aerated during fall and winter. Using average
cost of electricity and considering the above facts, the
cost of electricity for aerating grain varies from 1/5 to
3/4 cent per bushel.


Automatic controls which take full advantage of
favorable weather are normally used for larger
amounts of stored grain; manual controls may be
practical for relatively small amounts of grain.
Manual controls require a close check of temperature
and humidity to determine when to aerate. A
hygrometer which contains a wet and dry bulb
thermometer can be used to determine the relative
humidity of the air.

Page 8

A humidistat and a thermostat in the wiring circuit,
as shown in Figure 4, provide automatic control.
Both the humidistat and thermostat are adjustable.
The humidistat prevents operation of the fan during
periods of high humidity. The thermostat prevents
aeration when the air temperature is too high to cool
the grain. With these controls, the fan will not
operate unless both the humidity and temperature are
below the control settings.


This publication has discussed methods and use of
aeration to properly manage grain stores. Successful
storage of grain begins with preharvest management
practices to insure proper grain bin sanitation and
insect control, followed by proper drying of the grain.
The grain must be monitored during storage to head
off storage problems. Aeration should be employed
to help maintain grain quality in storage to insure
uniform temperature throughout the grain mass and
to prevent moisture migration.

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