Grain Drying (cont.)
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
Introduction
Drying Advantages and Disadvantages
Recommended
Storage Moisture Contents and
Estimated
Allowable Storage Times
Influence
of Drying Conditions
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
Dryers can be categorized in different ways. There are
natural air, low temperature, and high temperature
dryers; there are batch, automatic batch and continuous
flow dryers; and there are in-bin and column or
self-contained dryers. Dryers can also be classified
according to the direction of airflow through the grain;
cross-flow, counter-flow, and concurrent-flow.
Advantages:
- No harvest bottle neck. The bins can be filled at
the harvest rate.
- A properly sized system may dry the crop more
economically than a high temperature dryer.
Disadvantages:
- There is a limit on initial moisture content that
can be effectively dried.
- Electrical power must be available at each bin
for dryer fan motors.
Natural air/low temperature drying refers to drying
grain using little or no additional heat. Drying takes
place in a drying zone which advances upward through the
grain (Figure 1).
Figure 1.
A typical bin dryer utilizing natural air/flow
temperature drying.
(10KB b&w diagram)
Grain above this drying zone remains at the initial
moisture content or slightly above, while grain below the
drying zone is at a moisture content in equilibrium with
the drying air. The equilibrium moisture content of three
grains is shown in Table 3.
Table 3. Equilibrium Moisture Contents
of Three Grains (% W.B.).
--------------------------------------------------
Oil
Relative Wheat Corn Sunflower
Humidity ---------- ---------- -----------
% 40�F 70�F 40�F 70�F 40�F 70�F
--------------------------------------------------
20 8.5 7.7 7.4 6.4 4.6 4.2
30 10.2 9.2 9.3 8.1 5.6 5.0
40 11.7 10.7 11.0 9.7 6.5 5.9
50 13.2 12.0 12.7 11.2 7.4 6.6
60 14.6 13.3 14.5 12.8 8.3 7.4
70 16.2 14.8 16.4 14.5 9.2 8.3
80 18.0 16.5 18.7 16.6 10.3 9.3
90 20.4 18.7 21.7 19.4 11.9 10.7
--------------------------------------------------
Drying may take several weeks depending on the airflow
rate, climatic conditions and the amount of water to be
removed. Natural air/low temperature drying requires
enough airflow to complete drying within the allowable
storage time. Minimum airflow rates for natural air/low
temperature drying of wheat, corn and sunflower are shown
in Table 4.
Table 4. Minimum Airflow Rates for
Natural AirlLow Temperature Drying of
Wheat, Corn and Sunflower.
----------------------------------------------
Maximum Initial Moisture Content (% WB)
Airflow Rate -------------------------------
(cfm/bu) Sunflower Wheat Corn
----------------------------------------------
1/2 15 16 18
1 17 18 21
2 21 20 23
----------------------------------------------
A perforated floor is recommended for all in-bin
drying. Since air does the drying, it is imperative that
air reaches all the grain. Provide one square foot of
perforated surface area for each 30 cubic feet per minute
(cfm) of airflow. One square foot of bin exhaust opening
should be provided for each 1000 cfm of airflow.
The uniform airflow distribution required for drying
is more difficult to achieve with ducts than with
perforated floors. However, drying can be done
successfully with proper duct spacing and careful
attention to detail.
Perforated ducts should be placed on the floor with a
maximum centerline spacing equal to one-half the grain
depth or the shortest distance to the grain surface, and
the distance from the duct to the wall must not exceed
one-fourth the grain depth at the duct next to the wall.
Provide at least one square foot of duct cross-sectional
area for each 2000 cfm of airflow. Provide at least one
square foot of perforated surface for each 30 cfm of
airflow. If the duct is longer than 100 feet, it is
better to place a fan at each end of the duct.
Example: A rectangular building 36 by 72
feet is being used to dry wheat. The wheat is spread
to a depth of 10 feet (20,800 bushels). At an airflow
rate of 1 cfm/bu, a total of 20,800 cfm of air is
required. The ducts must not be spaced more than 5
feet apart to be spaced at one-half the grain depth.
The distance from the ducts to the wall must not
exceed 2.5 feet to be spaced at one-fourth the grain
depth. Eight ducts are needed, with the first duct
placed 2.5 feet from the wall and the remainder
placed 4.5 feet apart. Each duct must handle 2600 cfm
of airflow (20,800 � 8). With a velocity of 2000
ft/minute, a duct area of 1.3 square feet is needed
(2600 � 2000). This is an 14-inch square duct, a
semi-circular duct with a diameter of 25 inches, or a
round duct 16 inches in diameter.
The equations to calculate duct cross-sectional area
are:
Square or Rectangle
Area (ft2) = Width (in.) x Depth
(in.) � 144
Round
Area (ft2) = 3.14 x Diameter
(in.) x
Diameter (in.) � 576
Semi-Circle
Area (ft2) = 3.14 x Diameter
(in.) x
Diameter (in.) � 1152
Drying 20,300 bushels of wheat using the same airflow
rate in the same building with the wheat 6 feet deep on
the sidewall and peaked to 15 feet in the center would
require nine ducts as shown in Figure 2. The ducts are
spaced apart no more than one-half the shortest air path
out of the grain. The shortest path is different than the
grain depth as shown in the figure. The distance between
the wall and the first duct must not exceed one-fourth
the grain depth. The duct size varies because the
quantity of grain that receives air from the duct varies.
Figure 2. Duct
size and spacing for natural air drying in a 36'x72'
building with grain 6 feet deep next to the walls and
15 feet deep in the center. Perforated duct diameter
varies due to different amounts of air required.
(8KB b&w diagram)
The addition of supplemental heat to the air decreases
the final moisture content of the grain. The airflow rate
affects the drying rate. Using the temperature and
relative humidity of the air after it has been heated,
the grain equilibrium moisture content can be determined
from Table 3. Heating the air 10F
will reduce the relative humidity about one-fourth, and
heating the air 5F will reduce the relative humidity
about one-eighth. With air at 40F and 80 percent relative
humidity, heating it 10F will reduce the relative
humidity to about 60 percent. Grain harvested at or near
freezing temperatures may be held over winter at
acceptable natural air/low temperature drying moisture
contents and dried in the spring. The grain should be
cooled to about 25F for storage during the winter and
monitored regularly. Start drying corn and sunflower in
the spring as soon as daily temperatures average above
freezing (April) and wheat about May 1.
The greatest risk involved with natural air/low
temperature drying occurs if an abnormally warm, damp
period of weather occurs after the grain has been placed
in the drying bin. This permits rapid mold development
while drying speed is increased very little.
Layer drying or combination drying, to be described
later, are options used with natural air/low temperature
drying when the grain is wetter than the system is
designed to handle.
For more information refer to NDSU Extension Bulletin
35, "Natural Air/Low Temperature Crop Drying."
Advantage:
- Grain with a higher initial moisture content can
be harvested as compared to the maximum initial
moisture content used in full-bin drying.
Disadvantage:
- The harvesting schedule may be restricted.
Layer drying is very similar to natural air/low
temperature drying except the grain is placed into the
drying bin in layers normally about 4 to 5 feet deep. An
initial batch or layer of grain is placed in the bin and
drying is begun. A drying zone is established and begins
to move through the grain. Other layers of grain are
periodically added so that a depth of wet grain exists
ahead of the drying zone. Limiting grain depth to get a
higher airflow rate allows drying a crop at higher
moisture contents than the system can handle on a
full-bin basis. In a bin designed for 1 cfm/bushel on a
full-bin basis, the air flow rate is estimated to be
about 4 cfm/bushel if the bin is one-fourth full, Figure
3. The actual airflow rate will vary due to individual
fan performance.
Figure 3. Example
of layer drying. The higher airflow rates on a per
bushel basis early in the filling permit a higher
initial moisture content to be loaded.
(7KB b&w diagram)
The drying front may be found by probing and measuring
the moisture content at various levels. Several points
should be checked, since progress of the drying front
will not be uniform throughout the bin because of fines
accumulation. A common problem with layered drying
systems is adding additional wet grain too rapidly,
resulting in spoilage of the upper layers.
Advantages:
- The bin can be used for storage at the end of the
drying season.
- Wetter grain can be dried than can be dried with
a natural air or low temperature dryer.
Disadvantages:
- A large moisture variation between grain kernels
is possible.
- Grain damage may occur from stirring.
Batch-in-Bin Drying
The batch-in-bin drying process involves using a
bin as a batch dryer. A 3 to 4-foot deep layer of
grain is placed in the bin and the fan and heater
started. Typical drying air temperatures are 120 to
160F with airflow rates of 8 to 15 cfm/bushel. Drying
begins at the floor and progresses upward. Grain at
the floor of the bin becomes excessively dry while
the top layer of the batch remains fairly wet. The
grain is cooled in the bin after it is dried. Some
batch-in-bin dryers hold the grain being dried in a
layer near the roof. After the grain is dried it is
dropped to the bin floor where it is cooled. As the
grain is moved from the bin, the grain is mixed, and
the average moisture content going into final storage
should be low enough that mold growth will not be a
problem.
A stirring device can be added to provide more
uniform drying and moisture content and to increase
the capacity of the bin dryer. Research conducted at
Iowa State University indicates that with a stirring
device there is less than 1 percentage point moisture
variation between upper and lower layers of a batch
of grain. This research also indicates there is some
reduction in resistance to airflow, permitting an
increase batch size in the typical bin. Stirring
allows depths of up to 7 or 8 feet for corn. There is
a tendency for fine materials to migrate to the bin
floor as the stirring device is in operation.
Condensation is likely to form on the bin walls.
If the last batch of grain to be dried is to be left
in the bin for the winter months, air tubes and bin
liners have been used to help reduce the problems of
mold growth next to the bin wall. Another technique
that has shown some benefit is to operate the
stirring device next to the wall to provide extra
stirring.
A disadvantage of batch-in-bin drying is that
additional storage for wet grain holding is required.
Recirculating Bin Dryer
The recirculating bin dryer incorporates a tapered
sweep auger which removes grain from the bottom of
the bin as it dries (Figure 4). The sweep auger may
be controlled by temperature or moisture sensors.
When the desired condition is reached, sensors start
the sweep auger, which removes a layer of grain.
After one complete revolution around the bin, the
sweep auger stops until the sensor determines that
another layer is dry. This dried grain is
redistributed on top of the grain surface. The dried
grain will be partially rewet by the moist air coming
through the grain, which reduces drying efficiency.
After all the grain has been dried, the grain is
cooled in the bin. The dried and cooled grain is then
moved to storage or may be left in the bin. It is
common to dry the last bin full of grain using a
continuous flow bin dryer as a recirculating bin
dryer.
Figure 4. Grain
recirculators convert a bin dryer to a high speed
recirculating batch or continuous flow dryer.
(11KB b&w diagram)
Continuous Flow Bin Dryer
The continuous flow bin dryer also incorporates a
tapered sweep auger which removes grain from the
bottom of the bin as it dries, but the grain is moved
to a second bin for cooling (Figure
4). Up to 2 points of moisture may be removed in
the cooling bin if dryeration is used. (Dryeration is
described later in this publication.) Increasing the
grain depth will reduce the airflow rate, cfm, and
the drying rate of a continuous flow bin dryer. In a
recirculating batch or continuous flow bin dryer, it
is the total airflow capacity, cfm, that determines
the drying rate, not the airflow rate, cfm/bu.
Advantages:
- Dryer does not occupy grain storage space.
- Portable units can be moved from one location to
another.
Disadvantages:
- The heat available in the dryer is not used as
efficiently as in deep bed drying.
Column Batch Dryers
Column batch dryers are completely filled at one
time. A common batch dryer configuration is two
columns surrounding a plenum chamber (Figure 5).
Several circular-shaped batch dryers are also
available. Hot air forced into the plenum from a
fan-heater unit passes through the grain-filled
columns and dries the grain. Common batch capacity of
batch dryers varies from 80 to 1,000 bushels. Column
widths are normally from 10 to 20 inches. High
temperatures and high airflow rates characterize
batch dryers. The typical operating sequence is
fill-dry-cool-unload. Time for one batch varies, but
an average may be two to three hours per batch.
Control of the drying sequence can be either manual
or automatic.
Figure 5. Cross-section
of a column batch dryer.
(7KB b&w diagram)
A recirculating device may be added to some batch
dryers (Figure 6). This has the effect of reducing
the moisture variation across the column of the
dryer. For some crops, a higher temperature may be
used with a recirculating batch dryer since a kernel
of grain will not be next to the heated air for the
entire drying cycle and as a result should not get as
hot.
Figure 6. Recirculating
batch dryer.
(19KB b&w diagram)
Continuous Flow Drying
Wet grain constantly feeds in the top and is dried
and cooled in a continuous flow dryer. Dry grain is
drawn off the bottom and placed into storage. These
dryers are similar to batch dryers in configuration
but have a divided plenum chamber. Hot drying air is
pushed into the top chamber, and unheated air for
cooling is pushed into the lower chamber. Column
widths on continuous flow dryers vary from 8 to 20
inches. A sensor controls the discharge rate and
consequently the moisture content of the dried grain.
Continuous flow dryers use high temperatures and high
airflow rates. Airflow rates of 50 to 100 cfm/bushel
of grain are common. Continuous flow dryers are
available in a large range of sizes. Portable units
are available in sizes up to about 1000 bushel per
hour capacity, and stationary units of larger
capacity are available. The first grain through a
continuous-flow dryer generally will need to be
cycled through the dryer again for drying to be
completed. A continuous flow dryer with cross-flow
airflow is shown in Figure 7.
Figure 7. Cross-flow
dryer with forced-air drying and cooling.
(10KB b&w diagram)
Some cross-flow models reverse the airflow through
the dryer as the grain progresses down the column to
reduce overdrying. Some reverse the air flow in the
cooling section to increase energy efficiency (Figure
8). A concurrent flow dryer with counter-flow cooling
is shown in Figure 9. The con-current-airflow in the
drying section and counterflow in the cooling section
improves energy efficiency and reduces stress
cracking in corn. With this system, the heated air
enters the grain near the top of the dryer and moves
downward in the same direction as the grain. The
cooling air moves in the opposite direction as the
grain.
Figure 8. Cross-flow
dryer with reverse-flow cooling.
(16KB b&w diagram)
Figure 9. Schematic
of a concurrent-flow dryer with counter-flow cooling.
(20KB b&w diagram)
Another type of dryer is the mixed flow dryer
shown in Figure 10. In this type, the grain flows
over alternating rows of heated air supply ducts and
air exhaust ducts. This action provides mixing of the
grain and alternate exposure to drying air that is
relatively hot and air which has been cooled by
previous contact with the grain. It promotes moisture
uniformity and nearly equal exposure of the grain to
the drying air.
Figure 10. Mixed-flow
dryer.
(19KB b&w diagram)
Advantages:
- Increases drying rate of high temperature dryer
by about 300 percent.
- Increases energy efficiency.
Disadvantages:
- Requires natural air/low temperature drying to
complete drying.
- Requires more grain handling.
Combination drying is a process using a high
temperature dryer to dry the crop to a certain level,
then a natural air/low temperature drying system
completes the drying process. This system may be used to
increase the capacity of the high temperature drying
equipment, for increased energy efficiency, or when
conditions are not suitable to start drying with a
natural air/low temperature drying system. The crop is
dried from the harvest moisture content to a level
acceptable for natural air/low temperature drying, then
it is moved to the natural air/low temperature dryer and
drying is completed.
Dryeration and In Storage
Cooling
Advantages:
- Increases drying rate by about 60 percent for
dryeration and 30 percent for in-storage cooling.
- Increases energy efficiency.
Disadvantages:
- Requires cooling fan and bin.
- Requires more grain handling.
Dryeration is a process where hot grain is removed
from the dryer with a moisture content 1 or 2 percentage
points above that desired for storage. The hot grain is
placed in a dryeration bin where it is allowed to temper
without airflow for at least four to six hours. The
moisture content equalizes in the kernel during
tempering. After the first hot grain delivered to the bin
has tempered, the cooling fan is turned on while
additional hot grain is delivered to the bin. The grain
is cooled and 1 to 2 percent moisture content is removed
by the airflow before it is moved to final storage.
Cooling is normally completed about six hours after the
last hot grain is added if the cooling rate equals the
filling rate.
In-storage cooling eliminates tempering. Grain is
dried to the desired moisture content for storage in the
dryer, then moved to storage where it is cooled. Quality
of the grain is improved with both in-storage cooling and
dryeration because the final drying and cooling are done
at a slower rate than in a conventional high temperature
drying system.
Refer to NDSU Extension Circular AE-808, "Crop
Dryeration and In-Storage Cooling," for detailed
information on dryeration and in-storage cooling.
B
A C K | N E X T
Introduction
Energy, Quality, Fire, Moisture and
Fans
Heaters, Costs,
Safety and Managing Stored Grain
AE-701 (Revised), November 1994
NDSU Extension Service, North Dakota State University
of Agriculture and Applied Science, and U.S. Department
of Agriculture cooperating. Sharon D. Anderson, Director,
Fargo, North Dakota. Distributed in furtherance of the
Acts of Congress of May 8 and June 30, 1914. We offer our
programs and facilities to all persons regardless of
race, color, national origin, religion, sex, disability,
age, Vietnam era veterans status, or sexual orientation;
and are an equal opportunity employer.
This publication will be made in alternative
format upon request 701/231-7881.
North Dakota State University
NDSU Extension Service
|
