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Group Title: Circular
Title: Moisture migration in stored grains
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Permanent Link: http://ufdc.ufl.edu/UF00014454/00001
 Material Information
Title: Moisture migration in stored grains
Series Title: Circular
Physical Description: 5 p. : ill. ; 28 cm.
Language: English
Creator: Talbot, Michael T ( Michael Thomas ), 1948-
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1992
 Subjects
Subject: Grain elevators   ( lcsh )
Grain -- Storage   ( lcsh )
Grain aeration   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: Michael T. Talbot.
General Note: Title from cover.
General Note: "April 1992."
 Record Information
Bibliographic ID: UF00014454
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 - AAA6903
ltuf - AJF7216
oclc - 26248290
alephbibnum - 001744459

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April 1992 Circular 1045


Moisture Migration


in Stored Grains



Michael T. Talbot


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Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida
John T. Woeste, Dean


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1025


Circular 1045


April 1992















































































Michael T. Talbot, Associate Professor, Agricultural Engineering Department, Institute of Food and Agricultural Sciences, University
of Florida.









Introduction
A grain farmer may have the returns from much
of the year's work stored on the farm for future
feeding or sale. To minimize the risk of post-har-
vest losses, the grain must be placed in storage at
the proper moisture content and temperature. It
must be aerated, and a regular and accurate
method of inspection and sampling followed to
maintain the stored grain quality. For additional
information concerning sampling procedures refer
to the extension publication entitled Grain Sam-
pling. Potential problems exist when: 1) damaged
and/or high moisture grain is stored; 2) the aera-
tion system is inadequate or improperly used; or 3)
the grain bin is incorrectly filled or unloaded. For
additional information concerning principles of
grain storage, refer to the extension publication en-
titled Grain Drying and Storage on Florida Farms.

Grain is a good insulator; heat loss from grain is
relatively slow in comparison to other materials.
For this reason, when grain is placed in a bin in the
fall, the grain near the center tends to maintain the
temperature at which it came from the dryer or
field. The grain near the bin wall tends to cool near
the average outside temperature. As the outside
temperature decreases, the difference in tempera-
ture between the grain at the center of the bin and
that near the bin wall produces air currents inside
the grain mass. The cool air near the bin wall falls
since it is more dense, forcing the warmer air up
through the center of the grain mass (Figure 1). As
the cold air passes through the center of the grain
mass, it warms and picks up more moisture. As


WINTER CONDITIONS
z





COLD COLD
AIR AIR


\ UNLOADING AUGER
PERFORATED FLOOR
AERATION FAN OR DRYING FAN
Figure 1. Natural air currents created in bins with inadequate
aeration during winter conditions.


this air nears the top center surface of grain, it
cools to a point where it can no longer hold the
moisture it has picked up. This moisture condenses
on the surface of the grain, increasing the surface
grain's moisture content and creating a local envi-
ronment that enhances mold or insect growth. This
surface moisture change can occur even though the
average grain moisture content is at or below rec-
ommended levels. The reverse situation occurs
during the summer months (Figure 2). In this case,
the moisture condenses near the bottom center of
the grain mass.
SUMMER CONDITIONS


z JW



-.-2


WARM COD E / WARM
SZONAIR


Figure 2. Natural air currents created in bins with inadequate
aeration during summer conditions.


Generally, the problem of natural air currents
developing within a bin may be minimized by cov-
ering fan outlets when not in use and by keeping
the grain temperature in the center of the bin
within 10OF of the average grain temperature near
the bin wall. Temperatures can be maintained in
most farm structures by using aeration fans that
pull air down through the grain at airflow rates of
at least 0.25 cubic feet per minute (cfm) for each
bushel of grain in the bin until the temperature of
the grain mass is within 10OF of the average
monthly temperature. A slightly lower airflow rate
may be used in very large farm or commercial
structures. However, it is not necessary to lower
the temperature of the grain mass below 40F be-
cause fungi that attack stored grain cannot develop
below this temperature. Also, the aeration system
should not be used to raise the temperature above
60oF because mold and insect growth occur at a
much faster rate above this temperature. (For ad-
ditional information concerning insects refer to the
extension publication entitled Pest Management
Strategies for Storing Grain in Florida.) It takes








approximately 120 hours (five days) for the entire
grain mass to cool or warm when air is supplied at
the rate of 0.1 cfm per bushel. This time is reduced
to 12 hours when the airflow rate is increased to 1
cfm per bushel, which would be typical of the per-
formance of a drying fan used for aeration.

The types of aeration systems and methods of
bin filling determine the probable storage problem
locations within the bin.


Aeration Systems
Most modern grain storage bins are equipped
with either subfloor aeration ducts or perforated
floors. Subfloor duct systems may be of several
types usually resembling an "X" "Y," or "I" type sys-
tem (Figures 3, 4, and 5).

Air flows along the path of least resistance;
hence, there may be "dead space" areas through
which very little air passes when using a duct type
aeration system (Figures 6 and 7). Likewise,
overfilling of a bin may create "dead space" zones
(Figure 8). When inspecting a bin for possible
trouble spots, be sure to probe into these "dead
space" zones if possible.

The best method for distributing air evenly
through the grain mass is to use a perforated floor
(Figure 9). However, if improper filling procedures
are used, possible trouble areas could still occur if
there is an accumulation of fine material and for-
eign matter in the top and bottom centers of the bin


(Figure 10). Likewise, overfilling may present the
same problem for bins equipped with perforated
floors as for those with duct systems (Figure 8).


Filling and unloading grain bins
Storage problems may result from factors other
than inadequate aeration. For example, when
grain bins are filled, the foreign and light materi-
als, such as trash, weed seed and broken parts of
kernels, tend to accumulate in the center of the bin
and may form a "core" of material from top to bot-
tom (Figure 11). This core may be so tightly packed
that aeration of drying air will go around it through
the surrounding loose, clean grain. Consequently,
this zone may not dry properly, and in the case of
in-bin drying systems, it provides an excellent envi-
ronment for mold and insect problems. This poten-
tial problem may be reduced by using a grain
spreader that evenly distributes the fines. It is also
possible to remove the center material by unloading
the bin with a center draw unloading auger, and
then uniformly spreading this material over the top
surface of the grain after leveling. Other options
would include feeding or selling the core material
separately.

One method of determining when the central
core has been removed is to place tissue paper on
the grain surface and observe when it passes
through the unloading auger. Core removal may
involve some risk to workers. No one should be in-
side the bin when it is being unloaded. The pre-
ferred procedure is to clean the grain before it is



















AIR INLET


AIR INLET



Figure 3. An "X" type aeration subfloor
duct system.


Figure 4. A "Y" type aeration subfloor
duct system.


v-- -AIR INLETS
Figure 5. A parallel "I" type aeration
subfloor duct system.






















SPACE


DEAD SPACI


Figure 6. Typical air flow distribution for a parallel "I" type
aeration duct system (side view).


L-AERATION FANS-

- TROUBLE SPOTS DUE TO INADEQUATE AERATION.


Figure 7. Top view of possible damage areas near the bin floor
associated with "dead spots" in the air distribution system.


Figure 9. Aeration system using a perforated floor.


Figure 10. Storage location where spoilage generally occurs
with inadequate aeration when using perforated floors.


-CORE OF FORIEGN
MATERIAL


NATION FAN


Figure 8. A dead space zone created by overfilling the grain
bin.


Figure 11. A core of foreign material may be formed in the
center of the bin if a grain spreader is not used, or if the grain
is not cleaned prior to filling the bin.





















S- UNLOADING
PERFORATED FLOC
AERATION FAN OR DRYING FAN


Figure 12. A load of wet grain placed into a bin of dry grain
may spoil unless dried, even though the "average" moisture
content of the grain is considered safe for storage.

placed in the bin. Probing the center of the bin
should indicate the extent to which a center core
has formed.

"Hot spots" may be found in any part of the grain
mass. These trouble zones usually occur around
accumulations of trash or foreign material. How-
ever, if a load of relatively wet grain is placed into a
bin of dry grain, then wet grain may begin to spoil
regardless of the average moisture content of the
entire grain mass (Figure 12). When probing a bin,
investigate points where the probe has relative dif-
ficulty in penetrating. Generally, wet grain or
trash offers more resistance to probe penetration
than does dry grain. Again, the safety aspects asso-
ciated with entering a bin of grain are important in
that the grain may have bridged across the bin.


When a typical farm grain bin is unloaded, grain
from the top portion of the bin is removed first.
The grain will continue to flow until it reaches a
natural angle with the bin floor called the angle of
repose (Figure 13). The angle of repose usually
ranges from 25 to 35 degrees, depending in part on
the grain moisture content. A bin may continue to
be filled and unloaded without ever removing that
portion of the grain in the stagnant areas (Figure
13), as would be the case of a wet holding bin for a
dryer. This "stagnant" grain should be carefully
examined because it may have a higher moisture
content or contain different levels of foreign materi-
als compared to the rest of the grain.

When grain is drying in a bin, a drying front
moves in the direction of the airflow. When air is
forced from the bottom to the top of the bin (Figure
14), the grain above the drying front remains at es-
sentially the same moisture content as when it en-
tered the bin. The grain below the drying front will
have a lower moisture content and will be in equi-
librium with the drying air. The level of the drying
front may be found by locating the point where the
temperature of the grain increases significantly.
This level will also be where a probe begins to move
more freely through the grain indicating a drier
portion of the grain mass has been reached. When
examining grain dried in a bin, compare the mois-
ture content a few inches from the surface with
that a few feet lower to be sure that the system was
allowed to operate until all the grain was dried
(Figure 15).


UNLOADING AUGER
FLOOR


Figure 13. A portion of the grain in a bin (stagnant area) may
not be unloaded by gravity flow and forms a natural hopper for
incoming grain.


Figure 14. A drying front moves up through the grain in non-
stirred, in-bin drying systems.























I R / LE-UNLOADING AUGER
D- PERFORATED FLOOR
DRYING FAN
Figure 15. When checking grain previously dried in the bin,
examine points near the surface to make sure all the grain has
dried.


Stirring devices break up the drying zone so that
the entire mass of grain tends to dry more uni-
formly, thereby reducing the problem near the sur-
face of the grain. However, grain near the floor of
the bin where the stirring auger cannot reach may
be severely overdried. A sample taken from this
"dead zone" area may lead to a faulty conclusion
concerning average moisture content (Figure 16).

When bin floors are constructed, a layer of plas-
tic should be placed under the concrete floor to
serve as a vapor barrier to prevent water from con-
densing on the floor and wetting the grain. Like-
wise, sealing the side wall and roof and the bottom


ring on the concrete slab will prevent rain from
wetting the grain. Spoilage will generally be found
near any point where wetting occurs (Figure 17).

The unloading auger should be cleaned before
grain is placed in a bin or after a partial unloading.
Otherwise, when the grain is unloaded, the sample
taken from the truck may indicate contamination
by mold or insects at a level higher than that actu-
ally present inside the bin (Figure 18). Also, water
may collect inside an auger and wet the grain left
from a previous unloading.










/-FAILURE TO SEAL THE
BOTTOM RING


L--SPOILAGE AREA RESULTING FROM CONDENSATION
OR RAIN ENTERING THE BIN NEAR THE FLOOR

Figure 17. A sheet of plastic should be placed under the con-
crete floor during bin construction to serve as a vapor and
water barrier and prevent the rewetting of grain.


UNLOADING AUGER


POSSIBLE SOURCES OF
CONTAMINATION


AERATION FAN OR DRYING FAN


Figure 16. A "dead zone" may be found under the stirring au-
gers which may lead to faulty conclusions concerning the av-
erage moisture content of grain in the bin.


Figure 18. The unloading auger should be cleaned before the
bin is filled to reduce chances of mold or insect infestation.


DRYING FAN











Summary
This publication has discussed moisture migra-
tion in grain storage structures, problems caused
by moisture migration, and management steps to
prevent these problems. Successful storage of grain
begins with preharvest management practices to
insure proper grain bin sanitation and insect con-
trol 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.








































COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, John T. Woeste,
Director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June
30,1914 Acts of Congress; and is 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. Single copies of extension publications (excluding 4-H and youth
publications) are available free to Florida residents from county extension offices. Information on bulk rates or copies for out-of-state purchasers
is available from C.M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida32611. Before publicizing
this publication, editors should contact this address to determine availability. Printed 3/92.




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