Grain Drying
AE-701
(Revised), November 1994
Dr.
Kenneth J. Hellevang, PE
Extension Agricultural Engineer
Introduction
 Drying
Advantages and Disadvantages
Recommended
Storage Moisture Contents and
Estimated Allowable
Storage times
Influence
of Drying Conditions
Types
of Dryers and Drying
Natural Air/Low
Temperature Drying
Layer Drying
High Temperature Bin Drying
Column Dryers
Combination Drying
Dryeration and In Storage
Cooling
Energy,
Quality, Fire, Moisture and Fans
Energy
Efficiency
Selecting a Drying System
Maintaining Quality During
Drying
Drying Fire Hazard
Moisture Determination
Moisture Shrink
Selecting Fans
Heaters,
Costs, Safety and Managing Stored Grain
Selecting a Heater
Drying System Cost
Grain Handling Systems
Safety Considerations
Managing Stored Grain
Other Drying and Storage
Information Available
Grain drying, as used in this publication,
refers to the removal of some of the moisture from grain by mechanically
moving air through the grain after it has been harvested. Grain in the
field dries naturally as the crop matures, giving up moisture to the air
until the grain moisture is in equilibrium with the moisture in the air
(equilibrium moisture content). Conditions become less favorable for grain
to dry to moisture contents considered safe for storage as the harvest
is delayed into late fall.
Drying Advantages and Disadvantages
Grain
drying has several advantages and disadvantages.
Advantages
include:
- Increases quality of harvested
grain by reducing crop exposure to weather.
- Reduces harvesting losses,
including head shattering and cracked kernels.
- Reduces dependency on weather
conditions for harvest.
- Allows use of straight combining
for small grains.
- Reduces size and/or number
of combines and other harvest-related equipment and labor required due
to extending harvest time.
- Allows more time for post-harvest
field work.
Disadvantages
include:
- Original investment for
drying equipment and annual cost of ownership.
- Operating costs for fuel,
electricity and labor.
- Extra grain handling required
may result in further investment for equipment.
Recommended Storage Moisture Contents and Estimated
Allowable Storage Times
The
length of time grain can be stored without significant deterioration is
determined by temperature and the moisture content at which it is stored.
Table 1 shows the maximum recommended moisture content for storage with
aeration of some typical North Dakota grains. Short-term storage generally
refers to storage under winter conditions while long-term storage considers
the effect of summer conditions. Grain with damaged kernels or with significant
amounts of foreign material needs to be stored at a 1 to 2 percentage
points lower moisture content than sound, clean grain.
Table 1. Maximum Recommended Moisture
Contents of Selected Clean, Sound
Grains for Storage with Aeration in
North Dakota.
-----------------------------------------
Short term Long term
(less than (more than
6 months) 6 months)
-----------------------------------------
Barley 14 % 12 %
Corn 15.5 13
Edible Beans 16 13
Flax seed 9 7
Millet 10 9
Oats 14 12
Rye 13 12
Sorghum 13.5 13
Soybeans 13 11
Non-Oil Sunflower 11 10
Oil Sunflower 10 8
Wheat 14 13
-----------------------------------------
Grain
can be stored at a higher moisture content without significant fungus
development when stored at colder temperatures. Table 2 shows the relationship
between moisture and temperature and its effect on allowable storage time
for cereal grains.
Table 2. "Approximate" Allowable Storage Time
(days) For Cereal Grains.
----------------------------------------------------
Temperature (�F)
M.C. --------------------------------------------
(%) 30� 40� 50� 60� 70� 80�
----------------------------------------------------
14 * * * * 200 140
15 * * * 240 125 70
16 * * 230 120 70 40
17 * 280 130 75 45 20
18 * 200 90 50 30 15
19 * 140 70 35 20 10
20 * 90 50 25 14 7
22 190 60 30 15 8 3
24 130 40 15 10 6 2
26 90 35 12 8 5 2
28 70 30 10 7 4 2
30 60 25 5 5 3 1
----------------------------------------------------
Based on composite of 0.5 percent maximum dry matter
loss calculated on the basis of USDA research at lowa
State University; Transactions of ASAE 333-337, 1972;
and "Unheated Air Drying," Manitoba Agriculture Agdex
732-1, rev. 1986.
*Approximate allowable storage time exceeds 300 days.
The
allowable storage time for corn has been established to be the time until
a 0.5 percent dry matter reduction is reached. At that point there will
be a reduction of one grade. Storage life is cumulative. If half of the
storage life is used before the grain is dried, only half of the indicated
storage time at the lower moisture content is available after the grain
has been dried.
A
rough estimate of storage life for oil crops might also be made based
on the values for corn using an adjusted moisture content calculated
using the equation:
Comparable Corn Moisture Content =
Oil Seed Moisture Content x 100.
--------------------------------
100 - Seed Oil Content
For
example, oil sunflower at 12.0 percent moisture content is comparable
to corn at 20 percent moisture content.
Comparable Corn Moisture Content =
12
--------- x 100 = 20%
100 - 40
Influence of Drying Conditions
Airflow
rate, air temperature and air relative humidity influence drying speed.
In general, higher airflow rates, higher air temperatures and lower relative
humidities increase drying speed.
Raising
the temperature of the drying air increases the moisture-carrying capacity
of the air and decreases the relative humidity. As a general rule of thumb,
increasing the air temperature by 20 degrees Fahrenheit (F) doubles the
moisture-holding capacity of air and cuts the relative humidity in half.
The
drying rate depends on the difference in moisture content between the
drying air and the grain kernel. The rate of moisture movement from high
moisture grain to low relative humidity air is rapid. However, the moisture
movement from wet grain to moist air may be very small or nonexistent.
At high relative humidities, dry grain may pick up moisture from the air.
The
airflow rate also affects drying rate. Air carries moisture away from
the grain, and higher airflow rates give higher drying rates. Airflow
is determined by fan design and speed, fan motor size and the resistance
of the grain to airflow. Deeper grain depths and higher airflow rates
cause higher static pressures against the fan. Higher static pressures
decrease fan output.
As
air enters the grain, it picks up some moisture, which cools the air slightly.
As air moves through a deep grain mass, the air temperature is gradually
lowered and relative humidity increased until the air approaches equilibrium
with the grain. If the air reaches equilibrium with the grain, it passes
through the remaining grain without any additional drying. If high relative
humidity air enters dry grain, some moisture is removed from the air and
enters the grain. This slightly dried air will begin to pick up moisture
when it reaches wetter grain. Air in a 12 to 16 inch grain column does
not reach equilibrium with the grain.
N
E X T
Types
of Dryers and Drying
Energy, Quality, Fire, Moisture and Fans
Heaters, Costs, Safety and Managing
Stored Grain
AE-701
(Revised), November 1994
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