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IN THE SOUTHEAST
Marketing Research Report No. 488
Transportation and Facilities Research Division
Agricultural Marketing Service",
U.S. Department of Agriculture
in cooperation with the
University of Georgia
College of Agriculture Experiment Stations
This report is based on information from tests using aeration systems that were designed and in-
stalled for research studies and from commercially installed aeration systems. It supplements the
information in Marketing Research Report No. 178 (revised November 1960), "Aeration of Grain in
Commercial Storages," which emphasizes the design and selection of equipment and installation of
grain aeration systems.
The purpose of this publication is to present information on suitable systems to effectively aerate
grain in the Southeast. Although tests were conducted on wheat, corn and oats, the same guiding
principles can be applied when aerating other grains. Information is included on the most advan-
tageous time for operating aeration systems, when the desirable weather conditions are most likely to
occur, and how much aeration is needed for certain grain storage conditions.
Dean W. Winter, agricultural engineer, and Leo E. Holman, supervisory project leader, Agricul-
tural Marketing Service, assisted in preparing this report. Grain storage operators made their fa-
cilities available for the tests. Research was conducted in cooperation with the Georgia Agricultural
Operating problems 1
Description of aeration tests 2
Athens, Ga 2
Waynesboro, Ga 3
Perrv, Ga 3
Orchard Hill, Ga 3
Fitzgerald, Ga 4
Midville, Ga 4
Essentials of aerating grain 4
Airflow rates 4
Time required to cool grain 5
Direction of airflow 5
Issued November 1961
Applications of aeration 6
Cooling grain in storage 6
Equalizing stored grain temperatures 6
Removing storage odors 6
Preventing mold growth and insect activity. _ 7
Applying f umigants to stored grain 7
Storing moist grain for short periods 7
Fan controls 7
Ways to control fans 7
Care of controls 8
Schedules for fan operation 8
Relating aeration to weather 8
Seasonal operating schedules 9
Cost of operating an aeration system 10
OPERATING GRAIN AERATION SYSTEMS
IN THE SOUTHEAST^
T :, Beowj
riy Lloyd L.l Smith and Ralph W. 1 Bkownj Agricultural engineers, Transportation and Facilities Research Division,
U- I — - Agricultural Marketing Service
Aeration is an accepted practice for maintain-
ing the quality of stored grain in the Southeast.
However, studies show that properly selected
operating schedules are necessary to obtain the
greatest efficiency from an aeration system.
Operating schedules should be varied to suit
daily and seasonal atmospheric conditions.
Other factors influencing the selection of a sched-
ule are: (1) Direction of airflow through the
grain; (2) airflow rate used; (3) kind of grain
being aerated; and (4) method of controlling fan
Manual and automatic control of fans is dis-
cussed. Two types of automatic control systems
suggested are: (1) a thermostat and humidistat
in series; and (2) a timeclock, sometimes supple-
mented with a humidistat. A humidistat pre-
vents fan operation during high-humidity
weather. A thermostat, adjusted at least monthly,
permits fan operation only at times when cooling
the grain is possible. Timeclocks can be used in
the summer to permit fan operation during the
cooler portion of the day.
Some advantages of aerating stored grain are:
cooling the grain, equalizing grain temperatures,
and preventing moisture migration. When grain
is cooled below 50° F., insect activity is nearly
stopped. Mold growth is retarded, and grain can
be held in storage longer without deterioration.
About the same amount of power is required
for upward or downward air movement through
the grain. Downward airflow is generally rec-
ommended in the Southeast for deep storage. Air
moving downward through the grain counteracts
the tendency of heat to rise and cause moisture
accumulation in the top layers of the grain. Up-
ward airflow in deep storage is acceptable in the
spring when the grain begins to warm up. Only
a small amount of aeration is necessary with up-
ward airflow to remove the warm air above the
grain and reduce the grain temperatures in the
At certain times of the year, air is warmed in
passing upward through the stored grain, causing
moisture accumulation in the cooler upper layers.
This accumulation can be prevented by using a
properly designed aeration system and a recom-
mended operating schedule.
Airflow rates ranging from one-hundredth
to one-fifth cubic foot of air per minute (cfm)
per bushel were tested in both flat and upright
storages in the Southeast. Suitable airflow rates
for intermittent operation are one-fifth to one-
tenth cfm per bushel in flat storages and one-tenth
to one-twentieth cfm per bushel in upright stor-
ages. The lower airflow rates usually are used
when aerating grain such as wheat; the higher
rates are for corn and oats.
The amount of time required to cool grain in
the fall and winter varies with the airflow rate
used. Tests in the Southeast indicate that when
the airflow rate is halved, about 50 percent more
time is required.
Grain aeration systems are now used success-
fully in commercial storages in all grain areas of
the United States. They were first installed in
the Southeast about 1955. Procedures for operat-
ing the systems vary considerably between sections
of the country, because aeration usually depends
on the use of natural air. The cooler, drier cli-
mate in most of the Midwest allows more time
for aeration than the warm, humid conditions of
the Southeast. Thus, a schedule for operating a
system should be tailormade for the weather
conditions of a particular area.
The purpose of aeration is not the same for all
areas or even within an area. Aeration systems
are used for cooling grain, equalizing grain tem-
peratures, removing storage odors, and holding
high-moisture grain for short periods. The type
of operating schedule depends on the time of year
and the condition of the grain. No one method
of operating an aeration system is best for all
Grain is usually harvested in the Southeast
when the weather is warm. The available time
for aeration is limited, especially after the late
MARKETING RESEARCH REPORT 488, U.S. DEPT. OF AGRICULTURE
spring 1 harvest, because the average air tempera-
tures are already increasing. Corn harvesting
and storage starts in August, at least 2 months
before the weather turns cooler. Therefore, the
grain cannot be cooled sufficiently. However, re-
moving harvest and respiration heat generally
helps to minimize deterioration of the grain.
Both manual and automatic methods of con-
trolling the fan operation are used. Automatic
controls, such as thermostats, humidistats, time-
clocks, and interval timers, are used singly or in I
various combinations. Storable grain can be |
aerated satisfactorily by several methods. Select-
ing the proper method is important when grain
with more than 12 percent moisture content is
placed in storage.
Description of Aeration Tests
This report is based on aeration tests and obser-
vations made in the Southeast. Only the tests
conducted in Georgia are discussed in this report.
The tests determined the best operating pro-
cedures for aeration systems and the most eco-
nomical aeration methods, and developed
information on design of the aeration ducts and
size of the fan and the motor. Only those results
that pertain to the operating procedures are re-
ferred to in this report.
The tests were made in deep bins varying from
40 to 100 feet high, with diameters varying from
16 to 30 feet. Bins that were less than 60 feet
high had flat bottoms ; the others had hopper bot-
toms. The capacity of each bin ranged from
10,000 to 30,000 bushels. Both steel and concrete
bins were studied to check differences in grain
quality because of the structural materials. Tests
were made in flat storages with 24,000- to 90,000-
bushel capacity; observations were made on a
500,000-bushel flat storage.
The grain was sampled by using a standard
grain probe or by cutting the stream as the grain
was turned out of the bin. Grain moisture content
or official grade was determined from each sample
and used to measure the effectiveness of aeration.
Recording instruments or official U.S. Weather
Bureau records were used to provide records of
the atmospheric temperatures and relative humidi-
ties at each test location.
Except for one location, one kind of grain was
stored for part of the year and another kind the
rest. Usually wheat and corn were aerated in
the same bin with the same equipment but at
different seasons. Due to the different densities
of various grains, the airflow rates varied with
the type of grain under aeration.
Tests were conducted in three concrete bins.
Each bin was 16 feet in diameter and 80 feet
high ( 12,000-bushel capacity) and had a hopper
bottom (fig. 1).
Aeration equipment for one bin was a centrifu-
gal fan driven by a 3-horsepower motor and a
rectangular duct, 12 by 24 inches in cross section
and 12 feet long. The duct was on the hopper
bottom, in line with the bin drawoff. Fan oper-
ation was controlled by a thermostat and humid-
- J m
^!^ r t T*"-"
» ^JlPWf "^f '
Figure 1. — Upright concrete storage, Athens, Ga.
istat electrically connected in series. During the
summer aeration tests, a timeclock was used to
control fan operation.
Aeration equipment for the other two bins con-
sisted of ducts connected by a manifold to a
centrifugal fan powered by a 5-horsepower motor.
One bin had a circular duct, shaped as a partial
cone, around the inside of the bin against the
vertical wall. The other bin used a rectangular
duct, 12 by 24 inches in cross section and 12 feet
long. Air was directed down through the stored
corn. The airflow rate was one-eighth cubic foot
per minute (cfm) per bushel when both bins were
m use and one-seventh cfm per bushel when one
OPERATING GRAIN AERATION SYSTEMS IN THE SOUTHEAST
bin was sealed off. The fan controls were a ther-
mostat and humidistat electrically connected in
series. Summer fan operation was controlled by a
Both corn and wheat were aerated at different
times of the year in one concrete bin of an ele-
vator. The bin was 16 feet in diameter and 100
feet high and had a 15,000-bushel capacity. Air-
flow was downward at a rate ranging from one-
fortieth to one-seventeenth cfm per bushel for
corn. A 15-inch circular duct was first used, then
replaced by a 30-inch semicircular duct in later
tests. The centrifugal fan was powered by a
3-horsepower motor, controlled automatically by
thermostat and humidistat connected electrically
in series. During the summer the fan was
controlled by a timeclock.
Tests were conducted in three flat-bottomed
bins, each 20 feet in diameter and 40 feet high,
with a capacity of 10,000 bushels (fig. 2). Test
variations included type and size of ducts, air-
flow rates, and fan controls. These controls
consisted of either a thermostat and humidistat
connected in series or a timeclock during the sum-
mer. Continuous operation with no controls was
Figure 2. — Upright storage, Perry, Ga.
One bin was equipped with an aeration system
using a centrifugal fan powered by a 1-horse-
power motor. A rectangular duct, 12 by 12
inches and 12 feet long, was used. The fan de-
livered air through corn at the rate of one-eighth
cfm per bushel and through wheat at one-twelfth
cfm per bushel.
The second bin had a duct, 12 inches in diameter
and 8 feet long, of spiral wire covered with wire
screen cloth. A small utility blower driven by a
% -horsepower motor delivered air at a rate of
one-twentieth cfm per bushel through corn and
one-fortieth cfm per bushel through wheat.
The third bin was equipped with a duct,
8 inches in diameter and 12 feet long, of spiral
wire covered with wire screen cloth. A small
utility blower powered by a i/3-horsepower motor
aerated corn at a rate of about one-thirtieth cfm
per bushel and aerated wheat at a rate of about
one-fiftieth cfm per bushel.
Orchard Hill, Ga.
Tests were conducted in one hopper-bottomed
concrete bin, 16 feet in diameter and 80 feet high,
with a capacity of 12,000 bushels (fig. 3). A
-Upright reinforced-conerete storage, Orchard
rectangular duct, 12 by 24 inches and 12 feet long,
was used. Corn was aerated in this bin at an air-
flow rate of one-ninth cfm per bushel and wheat
at a rate of one-eighteenth cfm per bushel, with
downward air movement the first year. The air-
flow was reversed for the remainder of the tests
the following 2 years. A 3-horsepower motor
provided the power for the centrifugal fan. Fan
operation was controlled by a thermostat and
humidistat electrically connected in series.
MARKETING RESEARCH REPORT 488, U.S. DEPT. OF AGRICULTURE
Six flat-bottomed bins, each 30 feet in diameter
and 56 feet high, with a capacity of 30,000 bushels,
were aerated individually with a portable cen-
trifugal fan and a 3-horsepower motor (fig. 4).
The fan operation was controlled by a thermostat
and humidistat in series. A 13-inch-diameter
duct, 20 feet long, was installed in each bin. The
airflow was downward at a rate of one-eighth cfm
per bushel through the corn.
Figure 4. — Upright steel storage, Fitzgerald, Ga.
Tests were made in one flat storage 80 feet by
100 feet holding 90,000 bushels of oats (fig. 5) . A
Figuee 5. — Flat storage, Midville, Ga. Supply pipe outside
the building connects to crosswise ducts.
24-inch propeller fan and a 2-horsepower motor
furnished an airflow rate of one-tenth to one-fifth
cfm per bushel. The direction of airflow was
upward through the grain. Fan operation w T as
controlled by a thermostat and humidistat in
series. Triangular ducts, 30 inches on each side,
were placed on the floor crosswise, to the building.
One flat storage, 50 by 60 feet, holding 24,000
bushels of wheat, was aerated with a 20-inch pro-
peller fan driven by a ^-horsepower motor. The
airflow rate ranged from one-tenth to one-fifth
cfm per bushel, with air moving upward through
the grain. Triangular ducts, 30 inches on each
side, w r ere used. The fan operation was controlled
automatically by a thermostat and a humidistat
connected in series.
Essentials of Aerating Grain
Satisfactory aeration of grain depends pri-
marily on the airflow rate. The aeration system
is designed and fan operating time established
only after the airflow rate is selected. Since most
aeration systems use ducts, the airflow through
the grain is not uniform. The selected rate of air-
flow then is an average rate and must be high
enough for an adequate air supply to reach grain
in all areas of the storage. This small amount of
air reaching some areas must be able to cool the
grain before deterioration begins. Air distribu-
tion usually is more even in upright than in flat
storages. Thus, recommended airflow rates are
higher for flat storages (table 1).
At low rates of airflow (below one-twentieth
cfm per bushel), even continuous fan operation
adds little moisture to the grain. This was illus-
trated by one test on a bin of shelled corn. The
fan was operated continuously through February
and March for a total of 1,104 hours. The airflow-
rate was one twenty-seventh cfm per bushel. The
initial moisture content of the corn was 11.5 per-
Table 1. — Suitable airflow rates for
Type of storage
Airflow rates '
Cfm per bushel
}i to Yio.
Ko to }io-
Ho to y i0 .
1 Lower rate is for grain 12 percent or under in moisture
cent and the highest moisture content recorded was
12.4 percent in the top 3 feet. The average tem-
perature of the corn was reduced from 77° to
55° F. during the aeration period. Weekly sam-
pling of surface layers of corn showed a fluctu-
ation in moisture content from 11.5 to 12.4 percent.
Neither the weekly sampling by probe nor the
final sampling as the corn was turned showed an
increase in total damage.
At higher rates of airflow (one-tenth cfm per
bushel or over), the time to operate fans should
be carefully selected. Weather conditions may
OPERATING GRAIN AERATION SYSTEMS IN THE SOUTHEAST
be such that, at these airflow rates, moisture is
added to the grain or warming instead of cooling
Air usually travels a shorter distance through
the grain in a flat storage as compared to a deep
bin. Higher airflow rates are thus possible, re-
sulting in less time needed for cooling.
Time Required To Cool Grain
The time required to cool a bin of grain to about
the atmospheric temperature or to the control set-
ting of the high thermostat will depend mainly
on the airflow rate used. The uniformity of the
airflow through the grain and the drying effect
will also affect the number of hours needed.
Usually, about 160 hours of fan operation is
required to cool grain to near the atmospheric
temperature at one-tenth cfm per bushel (fig. 6).
In the Southeast, this time applies only to fall
and winter aeration. During the summer, the
primary objective is to keep the grain in good
condition and equalize the temperature in the
bin, rather than to cool the grain. Weather con-
ditions in the summer limit the amount of aera-
tion that is available to cool the grain.
The number of hours necessary for cooling in-
creases when airflow rates below one-tenth cfm
per bushel are used. Generally, when the airflow
rate is halved, the hours of fan operation increase
by 50 percent.
In the early fall, more total hours of fan oper-
ation are required to cool the grain with intermit-
tent operation for 3 to 4 hours per day than with
CORN TEMPERATURE ZONES
AVG. CORN TEMPERATURE
BEFORE COOLING -89°
AFTER COOLING — 57°
16' DIAM. X 80' HIGH
AFTER 154 HOURS
Figure 6. — Corn temperature zones during aeration.
constant operation for 8 to 10 hours per day.
Normally 40 to 50 days are required to cool the
grain in the early fall, but only 16 to 20 days in
Direction of Airflow
All aeration tests were conducted in bins having
the duct and fan at floor level. Air moving in
both an upward and downward direction through
the grain mass was studied. Horsepower require-
ments were about the same for either direction of
air movement. Systems having the fan located
on top of the bin and systems where the air move-
ment is horizontal through the grain have been
studied in other areas, but not in the Southeast.
Downward movement of the air through the
grain in deep storage is generally recommended
for the Southeast. This downward movement
counteracts the natural tendency of warm air to
rise. When the warm air rises and contacts cooler
grain, some condensation may take place. When
the air is moved down through the grain and ex-
hausted from the bottom of the bin, condensation
does not occur. Moving the air downward cools
the upper layers of grain first and reduces the
possibility of moisture migration and subsequent
condensation. Tests extending over a 5-year
period indicate that little damage from moisture
accumulation occurred in any portion of the bin.
Rather, the moisture in the grain equalized
throughout the bin.
Limited tests were conducted with air moving
upward through deep bins. This method was
satisfactory except for a few cases where some
condensation collected on the roof and dripped
onto the grain. A few spots of caked grain were
reported. No determination was made as to
whether all the caking was caused by the dripping
of condensate. There is a possibility that the up-
ward air movement caused moisture to accumu-
late in the surface layers of grain. Moisture
accumulation due to air movement can be greatly
reduced when the differential in temperature be-
tween the outside air and the grain is 10° F. or less.
In a steel bin the roof and the sections of wall
above the grain react quickly to atmospheric tem-
perature changes. Warm air rising from the grain
causes moisture to collect on these surfaces at night
and during cooler portions of the day. The cooler
the outdoor temperature, the more condensation
occurs; the cooler the grain, the less condensa-
tion occurs. Concrete bins have a larger wall and
roof mass than steel bins. Changes in outdoor
temperatures affect concrete less; therefore con-
densation problems usually are minor.
Where grain is kept in storage from one storage
year to the next and into the summer, upward air
movement through the grain is advantageous. In
the summer, the upper part of the grain and the
air above it within the bin tend to warm up first.
During this time, short, intermittent periods of
MARKETING RESEARCH REPORT 488, U.S. DEPT. OF AGRICULTURE
aeration, with the air movement upward, will ex-
haust the heated air to the outside. The disad-
vantage of downward air movement is that warm
air from the bin overspace is drawn into the grain.
To retain the lower grain temperatures, obtained
by winter aeration, through the summer, the fan
must be operated only long enough to refresh the
storage air and remove the stored grain odors.
If upward airflow is used during the summer, the
flow of air must be reversed when the weather be-
comes cooler. Continued aeration with cooler air
upward through the grain can cause moisture ac-
cumulation and damage the grain in the surface
Airflow distribution in deep bins was studied
by installing thermocouple cables in the bins, re-
cording the grain temperatures, and then plot-
ting the temperature patterns. The uniformity
of the patterns and the uniform progression of
cool temperatures indicated that the air distribu-
tion was satisfactory with either upward or down-
ward air movement.
Upward airflow through the grain in flat stor-
age is recommended for the Southeast. Tests indi-
cated that, in a flat storage, a fan blowing into a
long duct forces air more uniformly through the
perforated walls for the entire length of the duct.
Since most ducts in flat storages are on the floor,
air is forced upward through the grain. The large
airspace above the grain lessens the amount of
condensation and the possibility of moisture drip-
ping onto the grain. Screened ventilators should
be large enough for the air to be exhausted with-
Applications of Aeration
Aeration offers many benefits in commercial
grain storage and is not costly when compared to
the savings provided by the maintenance of grain
quality and the extended in-storage time. Moving
air through the stored grain costs less than turning
the grain through the air. Aeration can be used
for many purposes.
Cooling Grain in Storage
Cool grain maintains its original market quality
longer than warm grain. In the Southeast grain
retains the heat from the harvesttime long after
it is placed in storage (fig. 7). Only a portion
of this heat can be removed during the summer;
fall aeration cools the grain more, thus substan-
tially extending the storage period. Cooling also
removes heat due to grain respiration and heat
left from artificial drying.
Desirable storage temperatures are determined
CORN TEMPERATURES IN AERATED
AND UNAERATED BINS
Unoorated bin _
- x_^- — ■ — ■ — '
^ ^ Aerated bin
L_ — I—— -J L
2 3 4 5 6
WEEKS IN STORAGE
153 224 284
CUMULATIVE HOURS OF FAN OPERATION
Figure 7. — Corn temperatures in aerated and unaerated
by the moisture content of the grain, its possible
time in storage, and its intended use.
Studies indicate that grain can be cooled to be-
tween 45° and 50° F., which are satisfactory stor-
age temperatures for the Southeast. Stored grain
at 50° can be moved during the summer with little
danger of moisture condensation and subsequent
Equalizing Stored Grain Temperatures
One advantage of aeration, particularly sum-
mer aeration, is the equalizing of temperatures of
stored grain. Tests indicated that little cooling is
accomplished during the summer (the first 2 or 3
months of storage for small grains). Compari-
sons of aerated and unaerated wheat showed that
temperatures in the aerated bins were equalized
and the wheat maintained nearly the same initial
average temperature. Unaerated wheat, after
30 to 60 days of storage, not only had tempera-
ture differentials of 20° F. but also had average
increases of 15°.
Warm air not only has a natural tendency to
rise but also has the capacity to hold more mois-
ture than cool air. If air moves from a warm area
to a cool area, moisture may condense in the grain
in the cool area and cause spoilage. Little or no
moisture migration occurs when stored grain tem-
peratures are equalized or nearly equalized
throughout the bin.
Removing Storage Odors
Storage odors are common to most grains after
they have been in storage for more than a few
weeks. Any removal of these odors is an added
dividend. The fresh grain smell of properly aer-
ated grain is one of its most striking character-
istics. Some odors can be readily removed with
only a few air changes, others require longer
OPERATING GRAIN AERATION SYSTEMS IN THE SOUTHEAST
periods of aeration. Some odors are only tem-
porarily removed or reduced in intensity by aera-
tion, while sour or fermented odors are seldom
Preventing Mold Growth and Insect
Warm, moist conditions are associated with
grain mold growth and insect activity and repro-
duction. Only a few grain molds will grow at
temperatures below 70° F. if the grain has a
moisture content recommended for safe storage.
Some odors of stored grain are caused by mold
growth. Aeration and cooling of grain can be a
deterrent to mold growth and the formation of
some storage odors.
Research studies have not provided conclusive
evidence on the effects of aeration on insect activ-
ity in stored grain. Little or no insect reproduc-
tion takes place in grain at temperatures below
60° F. ; most activity stops below 50°. Many in-
sects die from starvation when temperatures drop
to 40° and below for any length of time.
Tests in an 80- by 100-foot flat storage con-
taining 90,000 bushels of grain indicated that in-
sects are less active in or tend to move out of grain
being aerated. During the first year, with little
or no aeration, heavy insect infestations occurred
and complete fumigation was necessary. Fans
were installed to provide adequate aeration the
second year. Insects moved to the grain near the
surface and were easily controlled with surface
Although studies indicate that aeration is val-
uable in insect control, by providing lower grain
temperatures and possibly moving insects out of
the grain mass, aeration will not entirely replace
fumigation and other chemical means of insect
Applying Fumigants to Stored Grain
Aeration systems provide a practical method of
applying fumigants to stored grain. The fumi-
gant is usually more uniformly distributed and
the dosage requirement is somewhat less for aera-
tion than for other methods. Fumigants can be
purged from the grain, after a prescribed ex-
posure period, by operating the fan for a few
The time necessary to introduce the required
amount of fumigant into the airstream depends
on the rate of airflow. Usually 10 to 20 minutes
are desirable to meter the fumigant into the air-
stream at airflow rates of from one-fortieth to
one- twentieth cfm per bushel.
Storing Moist Grain for Short Periods
Aeration can be used advantageously during
periods of heavy receipts of moist grain. Spon-
taneous heating of the grain can be reduced, mak-
ing short time storage possible. Cooling the grain
by aeration removes any heat generated by mold
growth, the principal source of heat, and helps to
slow down mold growth and other deterioration.
Upper limits of moisture content at which grain
can be held for brief periods with aeration have
not been established. Corn with moisture contents
of 15 to 16 percent has been held satisfactorily for
2 to 3 weeks with aeration when airflow rates were
one-tenth cfm per bushel or more. Fans should
be operated continuously, since the grain will pick
up little or no moisture from the air during the
short storage period.
The operation of fans may be controlled either
automatically or manually. Some aeration sys-
tems designed for low airflow rates are operated
continuously with no controls. When automatic
controls are used, the elevator operator need not
constantly check the weather conditions. Also
more fan operation is possible since controls oper-
ate full time.
Two accessories, although not used to control
fan operation, may be used as an aid to aeration.
One is an elapsed-time meter which records the
accumulated hours of fan operation. Knowing
the total hours, an operator can judge the progress
of aeration better. Another accessory is a thermo-
couple system used to indicate temperatures of the
grain in a bin. When temperatures are read at
regular intervals, hot spots can be detected and
the aeration system operated accordingly. The
operator can use these grain temperatures as
guides for setting controls to provide the greatest
amount of cooling.
Ways to Control Fans
Manual control of fans requires a close check
of the outdoor temperature and relative humidity
during aeration. A hygrometer is desirable to de-
termine the relative humidity of the air.
A disadvantage of manual control is that many
hours of suitable weather occur at night when no
one is available to turn on the fan. This method
also requires the operator's attention many times
a day. Aeration is needed most when the grain
is first put into the bin. The harvest season usu-
ally is the busiest time around an elevator ; because
of the demands on the operator's time, the aera-
tion system may be neglected.
MARKETING RESEARCH REPORT 488, U.S. DEPT. OF AGRICULTURE
Thermostat and Humidistat
Thermostats and humidistats, when used for
controlling fan operation, should be wired in series
in the control electrical circuit (fig. 8). Thus,
both the atmospheric temperature and relative
humidity must be lower than the control settings
to allow the fan to operate. Only a high-limit
thermostat and humidistat are recommended for
the Southeast, because atmospheric conditions sel-
dom are below freezing for long periods. This
method of controlling the fan is the most common
and generally accepted.
ARRANGEMENT I. SUITABLE
WHEN LINE VOLTAGE DOES
NOT EXCEED 230 VOLTS.
POWER SOURCE .
M - ELAPSED TIME METER
H - HUMIDISTAT
T,-HIGH LIMIT THERMOSTAT
T 2 - LOW LIMIT THERMOSTAT
S- SWITCH FOR MANUAL CONTROL
ARRANGEMENT 2. RELAY
SHOULD BE USED IF
VOLTAGE EXCEEDS 230
OR MORE THAN ONE
STARTER IS ACTUATED.
Figure 8. — Control circuits for an aeration fan.
When used for controlling fan operation, a time-
clock is the main control in the electrical circuit.
This method is used mainly for summer aeration
and can be effectively used to allow fan operation
for predetermined periods. Thus, full advantage
is taken of the coolest portions of the day when
the relative humidity may be changing rapidly. A
humidistat may be wired in series with the time-
clock to prevent the fan from operating during
foggy or rainy weather.
The continuous operation of the fan without
controls is most effective for airflow rates ranging
from one-fiftieth to one-thirtieth cfm per bushel.
When such low airflow rates are used, the grain
should not be put into the bin at moisture contents
in excess of 12 percent. By operating the fan
continuously, a much smaller fan, motor, and duct
system can be used. However, from three to five
times as many hours of fan operation are needed
than when larger fans are used. This method is
not practical for year-round operation, since
moisture may be added to the grain during long
periods of foggy or rainy weather.
Care of Controls
The controls for the aeration system require a
minimum of attention but, like other mechanical
instruments, perform better when cared for prop-
erly. Insects, dust, and dirt accumulate on the
controls ; insects are particularly injurious to any
hair element in a humidistat. A systematic spray-
ing around the weather shelter will help to keep
the insects under control. Removing dust from
the hair element requires caution to prevent dam-
age. The sensing element of the thermostat should
be wiped off regularly.
The proper location of the control shelter is es-
sential to reflect the true atmospheric conditions.
The shelter should protect the controls from rain,
yet allow a free flow of the air so that the fans
will operate only during satisfactory atmospheric
conditions. Figure 9 illustrates a suitable shelter
for the controls.
Figure 9. — Weather shelter for humidistat and thermostat.
Most controls are calibrated at the factory and
are fairly accurate. However, all controls should
be checked before they are installed in the control
system. Most thermostats will remain in calibra-
tion for some time without any readjustment;
humidistats tend to get out of calibration within
a year or two. Since field adjustment is difficult,
the humidistats should be either returned to the
factory for repair or replaced with the newest
Schedules for Fan Operation
Relating Aeration to Weather
Natural air is used for the aeration of grain.
Any effect of weather on grain storage is thus am-
plified when air is forced through the grain.
Two properties of air that directly affect the
success of aeration are relative humidity * and
temperature. Both are related not only to each
other but also to grain moisture content. Natural
1 An expression of the degree of wetness of air, spe-
cifically the ratio of the quantity of vapor present to the
greatest amount possible at a given temperature.
OPERATING GRAIN AERATION SYSTEMS IN THE SOUTHEAST
forces tend to balance the moisture content of grain
with the relative humidity of air. Air tempera-
tures also affect this balance to some extent. For
example, fully exposed shelled corn will reach 14
percent moisture in air at 85° F. and 80 percent
relative humidity. The corn would reach a
moisture content of 10 percent, with air at 85°
and relative humidity lowered to 50 percent. But
the grain moisture would increase to 17 percent
if the air temperature was lowered to 40° with
the relative humidity still 80 percent.
Therefore, aeration in adverse weather could
be hazardous. However, any changes in grain
moisture are slow because of the low airflow rates
used in aeration. Also, weather is constantly
changing, and changes in moisture content of the
grain depend on the average daily relative humid-
ity and temperature. The average daily weather
must be considered when storing grain in any
The amount of time suitable for aeration may
vary from only a few hours to over 400 hours a
month, depending on the selection of an upper
temperature limit (table 2). Computed from
weather data for Macon, Ga., in 1958, the infor-
mation in table 2 should serve as a guide in select-
ing operating times in the Southeast. More pre-
Table 2. — Total hours each month suitable for
aeration, in the Macon, Ga., area, 1958; relative
humidity of the air less than 80 percent and air
temperatures less than those shown
cise information for other locations can be
obtained from a nearby U.S. Weather Bureau
Seasonal Operating Schedules
Becommended schedules for aerating grain in
the Southeast are based on climatic conditions, air-
flow rates, and cooling time. Schedules differ for
each season to permit fan operation during cool
weather in order to cool the grain and minimize
deterioration. Some suitable monthly high-limit
thermostat and humidistat settings are given in
table 3. Thermostat settings can be lowered as
the grain becomes cooler.
Table 3. — Suitable monthly high-limit thermo-
stat and humidistat settings for fan operation
in the Southeast 1
55 to 60-.
65 to 70_ -
75 to 80__
1 Normally these settings allow 200 hours of fan opera-
tion per month.
In the Southeast, most of the small grain is
harvested in June, when outside temperatures are
relatively high. Often, grain is put into storage
at temperatures up to 90° F.
The fan should be operated as the grain is being
put into the bin, since the grain will cool faster
with the higher airflow. If the weather is not
foggy or rainy, the fan should operate continu-
ously for the first few days to remove harvest heat
and to even the temperatures in the grain. Later,
automatic controls can be used to permit max-
imum fan operation during periods of suitable
A timeclock also may be used as a fan control
in the summer. The clock is set to allow the fan
to operate during the cooler portions of the day,
such as late in the evening and early in the morn-
ing (table 4). Aeration during these periods,
when the humidity is 90 percent or less, will ac-
complish some cooling of the grain. The best
way to set the timeclock for fan operation is to
check the temperature of the grain regularly and
then set the clock so the fan will operate during
those periods late in the evening and early in the
morning when grain temperatures would normally
be higher than atmospheric temperatures.
MARKETING RESEARCH REPORT 488, U.S. DEPT. OF AGRICULTURE
Table 4. — Recommended daily settings for each
month when timeclock is used to control fans in
the Southeast x
June - _ _ _
7:15 a.m. to 11:30 a.m.
August _ -
6:45 p.m. to 1:00 a.m.
8:15 a.m. to 10:45 a.m.
6:00 p.m. to 9:30 p.m.
7:15 a.m. to 10:30 a.m.
7:30 p.m. to 1:30 a.m.
7:15 a.m. to 12:00 m.
6:00 p.m. to 3:30 a.m.
1 A high-limit humidistat, set to allow fan operation at
less than 95 percent RH, may be included to prevent fan
operation on rainy days.
Figure 10 shows the hours in the morning and
evening of an average day in each of 4 months —
June, July, August, and September — at Macon,
Ga., that are suitable for aeration.
Grain stored through the summer will reach av-
erage temperatures ranging from 75° to 80° F.
If storage is continued through the fall and win-
ter, the grain can be cooled progressively as the
outside air cools.
Grain harvested in the fall may be put into the
elevator at temperatures up to 90° F. It should
be aerated in stages because the atmospheric con-
ditions in the Southeast fluctuate widely until
about the first of December. The grain will heat
unless the fan is operated on a regular schedule.
The high-limit thermostat and humidistat settings
given in table 3 can be used as guides when auto-
matic controls are used.
After the grain has been cooled to approxi-
mately 55° F., the thermostat setting can be low-
ered to 40°. When the grain has reached this
HOURS SUITABLE FOR GRAIN AERATION
4 Typical Days, Macon, Go., 1958
~' / ^ r ~\*' -kssT^
: — "'
12 M |12 PM
ULTUDAL MARKETING SERVICE
Figure 10. — Hours suitable for grain aeration, 4 typical
days, Macon, Ga., 1958.
average temperature, aeration should be stopped
to prevent overcooling the grain. If hot spots
reappear in the grain mass, it should be aerated
more in order to move these hot spots out of the
bin. Normally, no further fan operation for cool-
ing is necessary until the following May or June.
However, grain temperatures should be checked
weekly to determine the condition of the grain. To
prevent odors from building up in the grain, the
fan may be operated 4 to 5 hours per week when
the temperature of the air is below that of the
Most aerated grain will have been cooled during
the winter and should store well into the spring.
At this time of year, the air temperature increases
faster than the grain temperature. This normal
increase cannot be prevented. However, the grain
should be checked regularly for sudden tempera-
ture changes, hot spots, and insect activity. Fans
need to be operated only long enough to remove
any storage odors or hot spots.
Cost of Operating an Aeration System
The cost of electric power is a major cost item
in operating an aeration system. The kind and
depth of grain and the rate of airflow affect this
cost because they are factors used in determining
the size of fan and motor needed. The number
of times that grain must be cooled during the
year and the uniformity of airflow also affect this
cost because they determine the amount of time
that the motor must be operated.
Table 5 shows some typical electric power costs
for operating aeration systems in the Southeast.
These costs were computed from the horsepower
rating of the motor and total hours of operation
during the year.
Table 5. — Cost of electric power for operating
aeration systems during 1 cooling stage
Cents per bushel
% to Ha--
Yn to V25 - -
1 Based on a rate of 3 cents per kilowatt-hour.
OPERATING GRAIN AERATION SYSTEMS EST THE SOUTHEAST 11
When grain must be cooled two or three times ating properly. This requires about 5 to 10 min-
during the year, the cost also multiplies two or u tes of one man's time each week,
three times. Projecting the power costs to a yearly Maintenance is a minor expense when the motor,
basis gives a yearly cost or about one-halt cent „ , . / »
r bushel J J fan, and controls are cared for regularly, lest
Very little labor is required to check and set installations in Georgia have been in operation for
the aeration controls. The system should be 4 years without any major repair or replacement
checked at least once a week to see that it is oper- costs.
U.S. GOVERNMENT PRINTING OFFICE : 1961 — 600253
Growth Through Agricultural Progress