Why do we Dry Grain?

Suggested moisture content for stored grain at various temperatures

•The longer we wish to store grain the more important it is to get the grain to a moisture level that will prevent spoilage.

Grain Structure

• Drying involves driving out the moisture trapped
inside the “seed coat” or skin. Moisture can
migrate in both directions.
• Over drying or drying to quickly will cause stress
cracks in the seed coat. These cracks can lead to
broken kernels and more fines in storage. Cracks
can also lead to easier access for molds and
insects to the seed interior.
• Rice drying small increments (2% at a time) then
steep and temper. Repeat.

Drying Types

• Aeration – While not a form of drying is
essential to the drying process. It is simply
forcing air through the grain mass to maintain
condition.
• Natural Air Drying – Is the use of outside
ambient air without the addition of heat.
• Mechanical dryers (screen type, tower dryers
etc.)
• In-Bin or In-silo drying.

Airflow

• Airflow is the amount of air passing through
the grain mass. It is a volume of air crossing a
volume of grain in a given time period.
Airflow is usually measured in cubic feet per
minute per bushel or cubic meters per minute
of air per cubic meter of grain.

• Airflow is the amount of air passing through the
grain mass. It is a volume of air crossing a volume
of grain in a given time period. Airflow is usually
measured in cubic feet per minute per bushel or
cubic meters per minute per cubic meter.

• Airflow requirements depend on silo usage.
Aeration can run from 0.10 cfm/bu [0.08 cu m/cu
m/min] in large commercial silos to 4 cfm/bu [3.2
cu m/cu m/min] in natural air drying.

• Airflow is dependent on the “static pressure”
of the grain. Air traveling in the voids
between kernels of grain lose energy due to
friction and turbulence. This energy drop or
pressure drop is what we relate as static
pressure.

• In general, the taller the silo and the smaller
the grain kernel, the larger the static pressure.

Fans

• Fans force air through the grain mass.
• There are two (2) basic types of fans. Axial flow
fans and centrifugal fans.
• Axial fans produce more air flow at lower static
pressures than centrifugal fans.
• Centrifugal fans work better at higher static
pressures and are much quieter than axial.
• Fans can work either forced air or suction. We
only recommend forced air systems in silos as
suction systems can cause damage.

Natural Air Drying

• Natural Air Drying is a process used to dry
grain by forcing unheated air through the
grain mass until the grain comes into
equilibrium moisture with the outside air.

• Natural Air Drying is a process used to dry
grain by forcing unheated air through the
grain mass until the grain comes into
equilibrium moisture with the outside air.
• Drying with natural air can be accomplished
only if the air temperature and relative
humidity conditions allow a net moisture
transfer from the grain to the air.

• Generally natural air drying requires large
volumes of air and a long periods of time.
That is 1-5 cfm/bu [0.8 -4 cu m/cu m*min] and
in most cases more than 30 days.
• Natural air drying is not something that will
work in commercial silos or situations. It is
restricted to farm silos 48’ [15 m] and smaller.

• Relative humidity is one of the major factors in
drying. Air at 100% RH will not hold anymore
water.
• The warmer the air the more water it will carry. If
the air temperature is raised about 10 degrees F
[6 C] the RH is halved or the warmer air will carry
twice as much water.
• As air passes the grain and absorbs moisture it
will become saturated (100% RH). The drying air
will take this moisture from a narrow band in a
silo. We call this a “drying front”.

Photo of drying front.

Cooling & Warming Grain

• In temperate climates with cool to cold winters
the grain needs to be cooled with the decreasing
temperatures of winter.
• As the average outside temperature cools by 10
degrees F [6 degrees C] the temperature of grain
needs to be dropped by starting the fans and
cooling the grain inside.
• The length of time (in hours) the fans need to run
can be calculated by the equation 15/(cfm/bu)
(produced by the fan(s)) [12/(cubic
meters/minute*cubic meters)].

• Average Air Temperature
• AAT = (HT+LT)/2
• HT = Daily High Temperature
• LT = Daily Low Temperature

• Example: We have a 20 Hp NECO centrifugal
fan used on a 60’ [18.3 m] diameter silo by 12
rings tall [9.8 meters tall].
• Silo Capacity = 89,057 bu [2511 MT wheat].
That fan will produce 15,362 cfm [435 cu
m/min] at 6.51” [165 mm] of static pressure.
Cfm/bu = 15362/(89057/1.06) = 0.18

• Example: We have a 20 Hp NECO centrifugal
fan used on a 60’ [18.3 m] diameter silo by 12
rings tall [9.8 meters tall].
• Silo Capacity = 89,057 bu [2511 MT wheat].
That fan will produce 15,362 cfm [435 cu
m/min] at 6.51” [165 mm] of static pressure.
Cfm/bu = 15362/(89057/1.06) = 0.18
• Time to move cooling front = 15/0.18 = 83
hours or 3.5 days.

• The fans need to be run continuously to drive
the “cooling front” all the way through the
grain.
• This only an estimate of the time required.
Using a thermometer on a stick, that can be
driven into the grain pile, the actual
temperatures can be checked at different
levels to determine if the front has moved all
the way through.

• This process will need to be repeated each time
the outside air temperature drops 10-15 degrees
F [6-8 C] until the grain mass reaches 32-35
degrees F [0-3 C]. Insect activities and mold
growth slow or stop below 40 F [5 C]. Freezing of
the grain is not recommended.
• The entire process needs to be run in reverse in
the spring as outside temperatures begin to
increase. The grain in the silo should be no more
than 15 F [8 C] different than ambient.

Tropical Climates

• Cooling grain is probably not an option in
tropical climates and other measures must be
taken.
• Moisture control is the only choice to keep
your grain in condition.
• Grain must be dried to a lower moisture
content in tropical conditions than in
temperate climates (corn 12-13%).

Requirements for “Hot Spots”

Allowable Storage Time

• If ambient air conditions will not allow the
removal of moisture, it may be necessary to
add heat.
• In cases where a “Hot Spot” cannot be
eliminated by aeration it will be necessary to
recirculate the grain. Moving the grain from
one silo to another will break-up the spot and
can allow for the introduction of pesticides or
fungicides during the moving process.

Mechanical Dryers

• There are many different styles of mechanical
dryer. Some like the screen (tower or stack) are
cross flow design.
• Some dryers are counter-flow where the material
is moving one direction and the air is moving in
the opposite direction.
• Most times we can dry to within 2 points
moisture and draw the rest with the cooling
process or aeration.
• NECO produces a mixed flow commercial dryer.

NECO Dryer

In-Bin or In-Silo Drying

• Can be done with just a fan for natural air
drying.
• Add a heater to the fan and dry. Best if done
with low heat and/or layered drying.
• Mechanical systems added to silo; such as a
stirring machine, recirculator or a continuous
flow unloading system.

Continuous Flow System

Recirculating System

Stirring Machine

In-Bin or In-Silo Drying

• Can be done with just a fan for natural air
drying.
• Add a heater to the fan and dry. Best if done
with low heat and/or layered drying.
• Mechanical systems added to silo; such as a
stirring machine, recirculator or a continuous
flow unloading system.
• EZ-DRY silo drying system.

In-Bin or In-Silo Drying
EZ-DRY

Stirring in Silos

• Provides increased airflow. Less energy
consumed by the fan to get the same results.
• Prevents over-drying of the grain by mixing.
• Mixes the fines in the grain mass to help
promote even air flow. (Prolonged use can
push fines to the bottom.)
• Moisture content of grain is consistent
through the entire silo.

Wall Loads

• Drying makes two (2) major changes in the grain
during the drying process.
• Shrinkage –causes vertical forces on walls.
5% removal = 10% shrink
10% removal = 16 % shrink
• Lower angle of repose and Internal friction.
Creates more wall tension and less vertical
load. (Water has no internal friction and
relates all to horizontal wall loads.)

• In some cases part of the silo is filled and dried
and there is a break before any more grain is
added. This dried grain has shrunk and the grain
is putting the wall into tension.
• The new grain needs to be dried and has a higher
internal friction angle. This creates a point of
discrepancy where these two layers meet.
• This location may be where we see some of the
recorrugations take place.