On-Farm Corn Drying and Storage

Air is typically being used as a tool for corn drying by forcing large quantities of unheated or heated air through the grain bulk. Air temperature and relative humidity (RH) are the two key characteristics that determine corn drying rate and final moisture content. For a given air temperature and relative humidity, corn will only loose moisture until a certain moisture and eventually achieve a state of equilibrium with the environment. This property is called "Equilibrium Moisture Content” (EMC). Thus, temperature and relative humidity properties of the drying air determine corn moisture level. The following table present the EMC of corn in equilibrium with air at various temperatures and relative humidity levels. To illustrate the use of the EMC table, assume that air at 70% relative humidity and 60^oF temperature is being forced through corn kernels in the bulk, this corn will not dry below 15.7%. Careful monitoring of the EMC and management of drying times will provide the most economical drying. In many cases, particularly at night, the addition of heat may be needed to condition the air to correct its EMC. To determine the value of EMC in your area now, click the link Determination of the current temperature and relative humidity based on your zip code.

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 moisture contents.

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"

Most buyers use the corn moisture content of 15.5% as the base moisture. When any grains are delivered to the elevator above its base MC, buyers use a factor called “shrink factor” in order to adjust the quantity for the excess moisture. This is because corn buyers will not pay for excess water. Applying a shrink factor approximates the equivalent number of bushels that would be in the load if the corn were dried to the base MC. Conversely, some farmers often deliver corn to the grain terminal at moisture levels below the base MC. These factors clearly demonstrate how sensitive the corn production economics are to the moisture content of the corn sold. A good example that estimates the potential loss due to corn shrinkage under the assumption that these calculations are based on a 1,000 bu, a normal truck would be hauling, of wet corn as shown in the following table. It is clear that marketing corn at any moisture level greater than 15.5% will decrease the total profit.

 

Effects of shrinkage factor on total loss.

• In-Bin Drying

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.

Grain bin.

Number of corn bushels in grain bins

• Batch and Continuous Flow Drying

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.

• Layer drying

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

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.

• Combination drying

In this configuration, high-temperature drying and in-bin drying processes are integrated making a combination system. The high-temperature drying method is an initial drying step used to reduce the moisture content of wet corn so that in-bin drying can be used to finish the drying process. Combination drying needs an auger or bucket elevator, a wet holding bin, a high temperature dryer, a drying bin, a drying fan, a heater, a perforated floor, unloading and sweep augers, and a grain spreader. Combination drying allows corn producers to begin harvesting whenever the grains mature instead of waiting for sufficient field drying. Furthermore, this process is more energy efficient than high-temperature drying systems alone. On the other hand, combination drying requires a relatively high capital investment and management if purchased as a unit because it incorporates two complete drying systems.

 

• Dryeration

This process combines drying and aeration, and is known for maintaining the quality of corn kernels during drying. In this process, wet corn is taken directly from the dryer, at a high temperature, to a tempering bin. Then, grain is allowed to steep in their vapors, for about 4 hours, before air-cooling (aeration) is initiated. This approach facilitates moisture removal without the use of additional energy. Heat is removed from grain through two processes, sensible heat transfer and latent heat transfer. Sensible heat transfer between the kernel and the surrounding air depends on the temperature differences. Latent heat transfer, on the other hand, occurs when the excess heat stored in the grain evaporates the kernel moisture. The gradual grain cooling in this process minimizes the prospects of thermal stresses or cracks. Therefore, dryeration is an economical drying method that maintains corn quality at a high drying rate with higher energy efficiency.

The following are some tips that may help corn producers achieve better grain quality while minimizing the drying cost:

• Harvest corn at 20% or less moisture content.
• Load corn into clean bins immediately after harvest. Bins should be cleaned and sanitized prior to harvest to minimize insect problems. Move corn from the field to grain bins as soon as possible. The amount of time before spoilage begins depends on grain moisture content and air temperature. A safe rule of thumb is to hold freshly harvested corn in carts or trucks no longer than 12 hours. Warm air temperatures > 80^oF, and higher grain moisture levels are the most critical factors for decreasing the time required for the grain to spoil.
• Check the moisture content of each load of grain as it is placed in the drying bin. There can be some variation in moisture content, but you need to know the average moisture content of the bin to determine the minimum necessary air flow needed and the allowable depth of grain in the bin.
• Open air exits and start the fan as soon as the grain depth is about 1 foot deep on the perforated floor. Be sure to use spreading devices or some other means to keep the grain leveled as the bin is being filled. If the grain is allowed to cone, there will be an increase of small particles in the center of the cone/ or central portion of the bin resulting in the air not being able to reach this grain because of increased resistance to flow. This makes it very hard to dry and control moisture uniformly in the grain bin and may cause spoilage.
• Add corn to drying bin in shallow layers until the moisture content decreases. High moisture corn (18 to 20%) can be added in 4 feet layers on top of dry grain if the fan can provide at least 3 to 4 cfm/bu through the total depth in the bin.
• Level corn inside each drying bin continuously – never allow coning to occur. Some manual work may be required to maintain a level surface on the top when the maximum depth is reached. This will ensure uniform airflow through all the grain assuming it has been placed in the bin with a good spreader.
• Monitor the moisture content of corn daily. Corn must be cooled to avoid nighttime condensation on the inner walls. If the heat has been on long enough for the complete mass of wheat to be warmed and the weather is clear and dry with humidity below 60%, turn the heat off when the moisture content of the grain drops to within 1% of the target moisture content. Continue running the fans, and the residual heat in the grain will finish the drying process.
• Probe the bin periodically to check for insect infestation and grain temperature increase. Corn temperature increase usually means moisture migration. Aerate whenever this is detected. If the problem is in the center of the bin and aeration is not effective, move the grain to another bin to solve this problem. Problems in the center of the bin usually indicate that a lot of fines and/or trash accumulated in this area during filling.
• Never add more heat than necessary to adjust the humidity of the drying air down to about 55%. The maximum heat needed, even in rain or 100% humidity will be about 15-17^oF above the outdoor temperature.

• Cool grain as soon as possible in the fall. Target temperatures should be initially around 60°F.
• Continue to aerate and uniformly cool grain to between 30°F to 40°F if possible. This will help avoid internal moisture migration and insect activity.
• Monitor grain and aerate monthly to maintain uniform temperature and moisture levels throughout. Aerate more often if moisture or temperatures increase.
• Keep the grain cool as long as possible into the early spring.
• Do not aerate in early summer unless problems develop.
• Cover fans and openings when not in use to help avoid air, moisture, and potential insect movement.
• Monitor carefully and fumigate if needed.
• Inspect corn surface at least every week throughout the storage period.

As mentioned in the previous sections, that grain drying could be accomplished by utilization of high-pressure fans without heat. Other drying equipment utilizes natural gas or propane for heat and electricity to power fans to move the heated air through the drying system. Both types of systems dry grain effectively (depending on weather conditions) they contribute to the energy bill. As a result, reducing the grain drying costs is the goal of all producers. Following are some tips that might help producers reducing the drying costs.

• Perform annual maintains on grain dryers. This can reduce the energy cost and increase the lifespan of equipment. Dirt, poor bearings lubrication, and inaccurately calibration of thermostats reduce the unit's efficiency.
• Some researchers estimated that proper maintenance of grain drying equipment could save 10% on the energy bill.
• Clean drying bins and make sure that the floors, columns, screens, fan housings, and fan blades are dirt free.
• Check the drain holes to make sure that they are open and not clogged with dirt.
• Tighten the belt drives to ensure they are at appropriate tension levels. Review the alignment of the pulleys.
• Lubricate all bearings. Check all mounting bolts and secure locking collars.
• Calibrate grain moisture sensors and thermostats. Normally compare them to a certified unit.
• Check the gas pressure regulators. You may need to call the Gas Company or liquid propane supplier to perform this check.
• Check the color of the burner flame for proper combustion. Blue color indicates complete combustion, while yellow color indicates poor combustion.

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 arises.

• 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 the exit.

• 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 reactions.

• 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.