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This website will briefly discuss the fundamentals of on-farm corn drying and drying
methods. Additionally, it provides guidance to select the optimum drying fan size.
Moreover, it introduces the simple tips that guide producers during drying, storing
and handling their corn in a safely manner.
To learn more about Equilibrium Moisture Content, see the fact sheet titled: "Grain Drying Tools: Equilibrium Moisture Content Tables and Psychrometric Charts"
To determine the values of EMC for various grains, download the Excel sheet by clicking
the link: "Equilibrium Moisture Content"
As mentioned earlier, air is the medium used to carry moisture away from grain during
drying and conditioning. Air is typically forced into the bottom section of a bin
under a perforated floor supporting the grain using one or more fans. Even air distribution
is critical to allow complete drying of the corn and avoiding “hot spots” where sections
of corn do not dry properly resulting in spoilage. Grain producers should select the
manufactured fan that best fits their drying needs. Over sizing, the fan leads to
unnecessary energy consumption in the form of electricity from the fan motor and gas
or electricity from the air heater. On the other hand, under sizing the fan size will
cause too little airflow resulting in drying being too slow. The higher the airflow
rate and temperature accelerate the drying rate and increase the cost.
Grain drying fans are classified as either axial-flow or centrifugal flow. Each type
of classification could be used to optimize the airflow rate and minimize the energy
consumption for maintaining grain quality. In both types, air is forced into the bin
by the fan. Axial-flow fans move air parallel to the axis or impeller shaft. This
type of fan is suitable for grains that create low static pressure, less than 4 inches
of water. Axial flow fans are also typically create much more noise during operation
than centrifugal fans, which should be considered when locating drying facilities
near residences. The second type of grain drying fans is the centrifugal fan. In the
centrifugal fans, air enters one end of the impeller parallel to the shaft and exits
perpendicular to the shaft. Centrifugal fans used for grain drying and storage generally
have backward-curved blades. They are usually the most efficient type of fans when
static pressure is greater than 4 inches of water and are typically capable of generating
much greater pressure than axial fans. Centrifugal fans are the ideal fans to use
for drying operations, which generally require moving air flow rates of 1 to 4 cfm/bu.
Centrifugal fans also operate with less noise than axial fans.The actual amount of
air needed to dry corn depends on its initial moisture content. The following table
shows the minimum recommended airflow rate to dry grain at various levels of initial
Recommended Minimum Airflow Rates for Drying
To select and maintain your fan, please see the fact sheet titled "Selection, Performance and Maintenance of Grain Bin Fans"
Effects of shrinkage factor on total loss.
In-bin drying processes can utilize either natural air (unheated) or low temperature
air (slightly heated usually less than 10 °F) to dry grain in bins (see figure below).
The air is forced up through the grain with fans until the grain moisture content
is sufficiently reduced. This is typically done in bins with a raised perforated floor
to ensure even airflow, but can also be done using air ducts laid on the concrete
bin floor prior to adding grain.
Bin capacity, measured in bushels of grain, increases by increasing the bin diameter
and/or the grain depth (shown in the following table). For example, a grain bin with
28 ft diameter filled to a level height of 16 ft height can hold up to 7,882 bushels
of corn. Increasing the grain depth increases the static pressure that the fan has
to overcome to provide the same cfm/bu. Stirring devices, re-circulators, or automatic
unloading augers can be used to increase drying rate. After drying corn to 17%, use
unheated air to dry it to about 15.5%. During this period, run the fan continuously
to provide uniform drying and moisture distribution within the corn. Operate drying
fans only during low humidity hours to finish drying. This management scheme will
minimize the amount of corn over dried in the bottom of the bin. It should be mentioned
that excess heat could cause severe over drying.
Number of corn bushels in grain bins
In this process, corn is added to the drying bin in daily batches, usually between
2.5 and 4 feet deep, then dried and cooled. The dried corn batch is then moved to
storage bins as a new batch of wet corn is added to the drying bin. The main idea
of the batch-in-bin dryer is to pass relatively large quantities of air through a
shallow corn depth to achieve drying rapidly. This allows corn producers to accommodate
larger harvest rates than with other in-bin drying methods. No storage of wet corn
is necessary in batch-in-bin drying since the batch size is adjusted to accommodate
the day’s harvest. The batch-in-bin drying technique requires a perforated floor,
a fan, a heating unit, a grain spreader, a sweep auger, and an under-bin unloading
auger. In this process, it should be noted that the diameter of the drying bin must
be large enough so that the recommended maximum grain depth of 4 feet is not exceeded.
Batch-in-bin drying is popular due to its flexibility when selecting the drying system
equipment. In addition, the management of batch-in-bin system is less intensive than
that required for other in-bin drying systems.
This process involves drying the newly harvested corn in layers. It is typically accomplished
by placing an initial corn layer in the drying bin. The drying air starts the drying
front that moves through the corn. Then, additional layers of wet grain are added
periodically so that the depth of wet grain always precedes the drying front. Layer
drying requires a bin, a perforated drying floor, a fan, and a heating unit with a
transition, a grain spreader, a sweep auger, a stirring device, and an unloading auger.
Layer drying offers the advantage of low heat input, making it one of the most energy-efficient
drying techniques in terms of heat required for drying. In addition, corn stays in
the storage bin after drying thus minimizing the handling and labor costs. Conversely,
the drying rate of the system is relatively slow which necessitates greater system
management. The slowness of the system may also affect the harvesting rate, and eliminate
the possibility of multiple uses of the same bin during the drying season.
Portable batch and continuous-flow drying are considered fast drying techniques. They
share similarities in configuration and operation. The basic idea in both processes
is to pass large volumes of air, i.e., 50 - 125 CFM/bu through relatively thin corn
column (12 to 24 in.) to attain high drying rates. The portable batch units typically
dry, cool then unload a fixed amount of corn into storage at set intervals. Drying
temperatures range from 160°F to 200°F and heater capacities are from 2 to 5 MMBtu/h.
The main advantage of these units is their large drying capacity that enables corn
producers to dry large volumes of harvested grain rapidly. Most portable drying units
are fully automated, thus reducing labor requirements for loading and unloading. Their
movability allows for easy replacement or capacity expansion. The main disadvantage
of these drying units, however, is their relatively low efficiency in terms of energy
consumption. Heat recapture devices may improve the energy efficiency of these dryers.
Drying cost may be relatively higher than other drying systems.
Working with either moving and/or stationary grain requires special precautions and
training. Therefore, it is necessary to keep in mind the following tips while working
with rough rice as well as other grain:
Do not inspect grain bin alone. Always request help from coworkers when entering a bin. Inspection requires at least
one worker inside the bin with a safety harness and one outside to assist if needed.
When entering a questionable bin, ask two workers to stay outside for assistance if
needed. A safety rope should be attached to the worker who is entering the bin. The
workers standing outside must be capable of pulling the person inside if emergency
Avoid entering into a grain bin or gravity unload vehicle when grain is flowing. It should be mentioned that several accidental deaths occurred during handling and
unloading grain. Therefore, lock out the control circuit on automatic unloading equipment
before entering or cleaning a bin or repairing conveyors. Flag the switch on manual
equipment so someone else does not start it. Do not enter a bin unless you know the
nature of previous grain removal, especially if any crusting is evident.
Avoid walking on any surface crust. Crusted or bridged grain can collapse and could bury workers. Do not depend on a
second person-on the bin roof, on the ground, or at some remote point-to start or
stop equipment on your shouted instructions. If a grain bin is peaked close to the
roof, be extremely cautious. Crawling between roof and peak can cave grain and block
Wear appropriate masks when working around dusts.Always wear a respirator mask capable of filtering fine dust to work in obviously
dusty-moldy grain. Never work in such conditions, even with protection, without a
second person on safety standby. Exposure to and inhaling mold can cause severe allergic
Be alert while working with out - of condition grain. Grain that has gone out of condition may contain molds, cavities, cave-ins, or crusting.
Exercise caution while working with flowing grain. Flowing grain can trap and suffocate a worker in seconds. Moreover, the noise coming
out of the equipment further blocks the shouts for assistance. Even with moderate
flow rates of a 6" auger, a worker is trapped only 2-4 seconds after stepping into
the cone of flowing grain. This worker would be totally submerged within 20 seconds
at a grain flow rate of 1,000 bu/h.
Electrocution from grain augers. Grain augurs are usually as long as 40 to 50 feet, and are used to place grains in
bins. Sometime while moving the auger from one bin to other, there is a chance that
the upper end of it touches overhead power cable. Caution should be exercised while
maneuvering the grain augers. Or, in the first place consider installing underground
power cables or make an arrangement to move the wires too close to metal bins.