Optimizing grain dryer operations
July 01, 1997
by Teresa Acklin
An agricultural engineer explains how to obtain the best performance from grain drying equipment.
By Dirk E. Maier
Unusually high harvest moisture, late crop maturity and poor harvest weather conditions can expose many weaknesses in dryer operations, and management and equipment shortcomings can create significant bottlenecks and frustration. But observing a few operational guidelines can help optimize grain dryer performance even under the most difficult conditions.
Before loading a drying bin, cleaning the grain should be considered. Although fines are more difficult to remove from wet grain than from dry grain, wet grain should be cleaned if it is to remain in the same bin for storage.
A spreader is generally the only feasible way to coming close to a level fill, which is critical in assuring even air distribution for drying. Although undesirable, some hand leveling may be needed at times to compensate for uneven spreading.
Using a spreader to distribute fines when filling a bin with dry grain can be less effective than combining a grain cleaner and the drawing of a core. Drawing out dry maize multiple times while filling a bin is even more effective than drawing a core one time after filling is complete.Low Temperature In-Bin Systems
The biggest mistake in operating low temperature drying systems, which are defined as those using natural air or air heated up to about 38°C, is filling them too quickly. For example, if moisture content in maize is higher than 20%, a grain bin should not be filled in a single batch, but in layers. Better yet, if several bin dryers are available, layers of grain should be spread into all of them. Distributing the drying load over more than one bin maximizes the drying capacity.
The drying front must be monitored closely. Often, layers of higher and lower moisture content grain are added on top of each other. As the drying front moves up through the grain, moisture is added to some layers and removed from others.
Monitoring the top layer of the pile has to occur over several days. A reading of 18% moisture on one day may increase to a reading of 20% the next as the drying front pushes through the different moisture layers. When the moisture readings remain consistently below 16% to 17% for several days, drying is nearing completion.
Finally, in climates where the weather turns colder after harvest, drying cannot always be completed before the weather changes because the drying potential of the air becomes too low. In these situations, the drying fan may be turned off after the top layer drops below 18% moisture and the grain temperature falls below about 0°C.
Fan operation subsequently should occur about once a week for four to six hours on dry, cool days until the weather turns warmer again. Drying to 15% then can then be completed by running the fan continuously.
To assure that the top layer is below 18% moisture by the time cold weather arrives, supplemental heat may be needed in a poor drying year. But avoid using too much heat; studies have shown that many operators use too much heat too often, resulting in significant water losses from overdrying and higher-than-necessary energy bills.High Temperature In-Bin Systems
Any bin dryer operated above 38°C is considered a high temperature in-bin dryer. Whether a batch is dried and cooled in a shallow bed before moving it into the final storage bin or dried and cooled using stirrators, the biggest mistake in these systems is filling them too deep.
If stirrators are not used, the optimum bed depth is 75 to 120 centimeters; if stirrators are used the optimum depth ranges from 1.8 to 3 meters. Drying capacity decreases disproportionately as depth is increased. For example, drying 26% moisture maize to 15% at a 2.25-meter depth may take 41 hours, while doubling the depth increases the drying time to as long as 95 hours.
Additionally, initial moisture content has a significant effect on drying time. For example, drying 22% moisture content maize to 15% at a 2.25-meter depth cuts the drying time from 41 hours to about 27 hours.
Another key item to monitor while operating high temperature bin dryers is condensation on the walls and roof. As the weather turns colder, water running or dripping back into the grain from condensation can create significant spoilage problems during the storage season.
When a batch is dried and moved to a final storage bin, condensation is less of a problem because rewetted kernels mix with drier kernels and tend to equilibrate. However, deep batches that remain in the bin are more susceptible to spoilage induced by condensation. Because most stirrators do not reach all the way to the wall, adding air tubes that pipe warm air up along the inside walls of a bin helps to dry out wall condensation.
Adding additional vent space and elevating the roof eave opening helps to allow moisture-laden air to exhaust more readily from the bin. The recommended design is 0.09 square meters of vent surface area per 30 cubic meters of air per minute at a minimum more is better.
Opening the center hatch of the bin can create real problems, especially when a leg spout unloads into the bin and the support structure of the stirrator also is anchored there. As the air exhausts from the center hatch, it has plenty of opportunity to condense moisture on the cold steel of the hanging support structure. Keeping the center hatch closed and adding additional vents to the roof reduces this problem.Column Dryers
Only a few batch column dryers are sold new today because batch drying is inefficient, slow and takes too much supervision. Most of the new high speed, high temperature column dryers available are continuous flow dryers that allow for a batch mode if needed. Because batch dryers in operation today are generally quite old, the primary operational problem is with maintenance.
One common oversight is improperly locating the temperature sensor. It's a good idea to install more than one sensor, or at least to move the one built-in sensor to several locations in the plenum to determine the coldest and hottest spots.
Installing the sensor at the cold spot will create significantly hotter temperatures in the rest of the dryer, which causes overdrying and stress cracking of maize. Placing the sensor in the hotter part of the dryer will assure that overdrying is minimized. Converting a column batch dryer to a combination dryer by eliminating the cooling step and transferring hot grain can significantly improve drying capacity and grain quality.
For continuous flow column dryers, operations are more efficient with a heat recovery system, which reduces fuel costs by 20-30%. Whether some air is recirculated back into the dryer through an external compartment or whether air is preheated by drawing it through the cooling section into the dryer, heat recovery always increases energy efficiency.
Although the first reaction to handling higher moisture grain and maintaining drying capacity is to increase the drying air temperature, quality deterioration results from increased stress cracks and breakage susceptibility. Using a range of drying air temperatures such as 94° to 105°C in the top of the dryer for the wettest grain and 77° to 88°C in the bottom of the dryer for the driest grain helps reduce kernel stress. Operating a high temperature column dryer in the hot-and-cool mode, as well as the full heat mode, provides the greatest flexibility for optimizing quality, capacity and fuel costs.
Reducing the drying air temperature significantly below 77° to 88°C does not improve energy efficiency. As a matter of fact, total energy consumption increases sharply for high speed dryers, which generally operate with an airflow of more than about 75 cubic meters per minute per tonne.
A critical parameter is to maintain kernel temperatures as low as possible. Rapid cooling of the kernels without tempering causes most of the stress-crack formation, and equipping high speed column dryers with tempering sections before the cooling stage should be a standard modification. Tempering allows the moisture gradient that develops inside each kernel to equilibrate and the built-up internal kernel stresses to relax. Turning a grain column inside out along the drying section helps to minimize the inherent moisture gradient that develops across the column.Combination Dryers
Combination drying systems combine high-temperature drying with slow cooling and drying in bins. High temperature drying can occur in a continuous flow, automatic batch or bin dryer. Potential advantages of combination drying are better grain quality, higher drying capacity, reduced fuel costs and more operational flexibility.
Because operating these drying systems involves transferring hot grain, one of the most common mistakes involves inaccurate measurement of grain moisture content. Transferring maize at too high a moisture content can lead to spoilage problems during the storage season.
High temperature drying followed by transferring hot grain at 17% to 18% moisture content into a steeping bin is known as dryeration. The critical operational procedure involves steeping the hot maize for six to 12 hours, followed by cooling with ambient air at roughly 0.5 to 1.0 cubic meters per minute per tonne.
After a cooling time of 10 to 20 hours, the maize is moved into a final storage bin equipped with regular aeration fans. Cooling the hot maize too quickly because of insufficient steeping time or too much airflow will not reduce the moisture content below the 15% safe storage level, and condensation in the steeping bin is significant. However, because the cooled maize is transferred to a final storage bin, wetter kernels are mixed and will tend to equilibrate during aeration in storage.
High temperature drying followed by transferring hot grain at 16% to 17% moisture content into a storage bin is known as in-bin cooling. Because no steeping takes place, the fan is turned on as soon as a few meters of hot maize cover the perforated floor. Cooling occurs over about 48 hours with ambient air at a full bin airflow rate of 0.5 to 1.0 cubic meters per minute per tonne. Cooling the hot maize too slowly because of undersized fan capacity can lead to spoilage. Also, condensation can become a storage problem because the maize remains in the bin and wetter kernels are not remixed.
High temperature drying followed by transferring hot grain at 19% to 23% moisture content into a natural air drying bin appears to be a little known practice. This two-stage drying operation reduces fuel costs and increases drying capacity more than any other combination drying operation. According to research conducted in the U.S. state of Minnesota, total fuel and energy consumption can be reduced by 40% to 60% for maize with initial moisture contents of 24% to 28%.
The hot maize is cooled in the bin as the low temperature drying front is started. Cooling will remove about 1 point of moisture. But sizing the fans to the proper airflow and operating the fans continuously until the top layer drops below 18% moisture is critical.
Choosing between in-bin cooling and high-low two-stage drying can be based on the break-even costs between heating fuel and electricity. Because in-bin cooling requires high-temperature drying to 16% to17% as a first step, more heating fuel than electricity is used in drying. Conversely, more electricity than heating fuel is used during high-low two-stage drying.
Finally, specialty grains such as food-grade white and yellow maize or high-amylose maize must be handled much more delicately than regular commercial maize. A maximum kernel temperature of 49° to 60°C is recommended for these heat-sensitive grains, and operating conventional drying equipment with the same temperature settings assures poor product quality. Field drying below 20% moisture and applying as little heat as possible are musts to minimize stress-cracking and/or denaturization of proteins.
In addition, combination drying and multistage drying should be implemented as the preferred drying methods.
Dirk E. Maier is assistant professor and extension agricultural engineer at Purdue University, West Lafayette, Indiana, U.S. He specializes in post harvest engineering, including grain and feed handling, drying, storage and processing.