Grain drying

by Susan Reidy
Share This:

Grain moisture impacts the length of time grain can be stored, and its overall condition. In some cases post-harvest grain drying is required.

How long grain can be stored without deterioration depends on the temperature and moisture content at which it is stored, according to a paper written by Kenneth J. Hellevang, extension agricultural engineer, with North Dakota State University Extension, Fargo, North Dakota, U.S.

The allowable storage time for corn has been established to be the time until a 0.5% 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.

Grain with damaged kernels or with significant amount of foreign material needs to be stored at a 1 to 2 percentage points lower moisture content than clean grain, Hellevang said. Grain can be stored at a higher moisture content without significant fungus development when stored at colder temperatures.

Drying types

Several types of grain dryers are available and can be categorized in different ways, Hellevang said. Varieties include: natural air, low temperature, high temperature, batch, automatic batch, continuous flow, in-bin and column and self-contained. Dryers also can be classified based on the direction of airflow through the grain: cross-flow, counter-flow and concurrent-flow.

Natural air/low temperature drying. This type of drying eliminates any possible harvest bottleneck and a properly sized system may dry the crop more economically than a high temperature dryer. However, there is a limit on initial moisture content that can be effectively dried and electrical power must be available at each bin for dryer fan motors, Hellevang said.

This type of drying uses little or no additional heat with drying taking place in a drying zone which advances upward through the grain. Grain above the drying zone remains at the initial moisture content or slightly above, while grain below is at a moisture content in equilibrium with the drying air.

Drying may take several weeks depending on the airflow rate, climatic conditions and the amount of water to be removed. A perforated floor is recommended so that air reaches all of the grain. One square foot of bin exhaust opening should be provided for each 1,000 cfm of airflow.

Perforated ducts can be used but uniform airflow distribution is more difficult, Hellevang said. 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.

One square foot of duct cross-sectional area is needed for each 2,000 cfm of airflow and one square foot of perforated surface for each 30 cfm of airflow.

Layer drying. This type of drying is 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 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 for a higher airflow rate allows for drying of a crop at higher moisture contents than the system can handle on a full-bin basis, Hellevang said. However, a common problem with this system is adding additional wet grain too rapidly, resulting in spoilage of the upper layers.

High temperature bin drying. Wetter grain can be dried with this system compared to natural air or low temperature drying. However, a large moisture variation between grain kernels is possible and grain damage may occur from stirring.

One high temperature method is batch-in-bin drying which uses 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 are started. Drying begins at the floor and progresses upward.

Grain at the bin floor becomes excessively dry while the top layer of the batch remains fairly wet. The grain is cooled in the bin after it is dried. 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.

Research at Iowa State University found 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.

A recirculating bin dryer includes a tapered sweep auger that removes grain from the bottom of the bin as it dries. It may be controlled by temperature or moisture sensors. When the desired condition is met, sensors start the sweep auger which removes a layer of grain.

After one complete revolution, the sweep auger stops until the sensor determines that another layer is dry. The dried grain is redistributed on top of the grain surface. The dried grain is 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 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. Up to 2 points of moisture may be removed in the cooling bin if dryeration is used. 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.

Column dryers. These type of dryers do not occupy grain storage space and portable units can be moved from one location to another. But the heat available in the dryer is not used as efficiently as deep bed drying.

Column batch dryers are completely filled at one time. The configuration includes two columns surrounding a plenum chamber. Hot air forced into the plenum from a fan-heat unit passes through the grain-filled columns and dries the grain, Hellevang said. Common batch capacity varies from 80 to 1,000 bushels. Column widths are normally from 10 to 20 inches.

High temperatures and high airflow rates are common characteristics of batch dryers with an operating sequence of fill-dry-cool-unload. Time for one batch varies, but an average is two to three hours per batch.

A recirculating device can be added which reduces the moisture variation across the column. For some crops, a higher temperature may be used since a kernel of grain will not be next to the heated air for the entire drying cycle and should not get as hot.

In a continuous flow drying column system, wet grain is constantly fed in the top and is dried and cooled. 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 vary from 8 to 20 inches, and a sensor controls the discharge rate and the moisture content of the dried grain. The first grain through a continuous-flow dryer generally will need to be cycled through the dryer again for the drying to be completed.

Choosing a system

Each drying system has advantages and limitations, Hellevang said, and it is important to weigh all the factors before selecting a dryer.

For example, a continuous flow column dryer needs holding facilities for wet and dry grain. Large electric motors that need adequate electrical service are likely involved. Fuel must be stored and supplied to the dryer. The system needs to be located to allow for good traffic flow and drainage, he said.

Purchase the dryer from a reputable dealer who will be able to service the dryer and provide other assistance. Check with people who have dealt with that type of dryer and examine existing systems or dryer installations.

Remember to leave space for future facilities and equipment. Plan for the foreseeable future and then double it. This allows an orderly development as the system expands, Hellevang said.

Cost is another factor when selecting a drying system, and it can vary greatly depending on the type of dryer and the energy requirements. Generally, the less supplemental heat required, the less expensive the system will be, Hellevang said.

When calculating the cost, consider interest, depreciation and insurance, and operating expenses such as fuel and electricity. The labor required and the amount of grain to be dried are also important factors, he said.


Any dryer that uses an open flame to heat the air poses a constant fire hazard, especially when drying sunflower and sorghum, Hellevang said. Fine fibers from sunflower seed or other plant materials may be ignited by the burner and carried to the seeds, causing them to ignite.

The fire hazard can be reduced by turning portable dryers into the wind so airborne fibers are blown away from the dryer intake, and by pointing permanent dryers into the prevailing wind, he said. A moveable air intake duct may be placed on the burner intake to draw clean air away from the dryer. The duct has to be large enough that it doesn’t restrict airflow.

Make sure to clean the dryer, air ducts and the area around the dryer at least daily. Remove the collection of lint on the dryer column and in the plenum chamber because it can become extremely dry and ignite during dryer operation. Make sure the dryer is cleaned out completely after each batch, and check a continuous flow dryer hourly to ensure that the grain is moving.

Fires can be controlled if they are noticed immediately, which means constant monitoring is necessary, Hellevang said. Many fires can be put out by shutting off the fan to cut off the oxygen. A small amount of water directly on the fire at the early stages may put it out if shutting off the fan doesn’t work.