Grain aeration basics

by Sonja Heyck-Merlin

At the 2022 Grain Growers Conference, Don Hostetler discussed the role of aeration in improving grain conditioning outcomes. Hostetler is a product specialist with Brock Grain Systems in Indiana. The conference was put on by the Northern Grain Growers Association and University of Vermont Extension.

Grain conditioning refers to the process of increasing the storage life of grain. Variables impacting the success of conditioning include moisture, temperature and the amount of airflow into the grain bin. Aeration – the process of moving ambient air through stored grain to reduce grain deterioration and prevent storage losses – is a key component of conditioning.

Hostetler said, “If you have an 80° grain temperature and 15% moisture, the storage time is only around a month. If you have a grain temperature of 40° and the moisture is 15%, we have about 500 days of storage available.”

Grain growers typically use fans and supplemental heat to achieve conditions conducive to long-term storage. If the grain is dry enough when it enters the bin, the general rule of aeration is 0.1 of a cubic foot per minute (cfm) of airflow per bushel of grain. When drying grain, however, more airflow is needed. Hostetler suggested going up to 0.5 to 1 cfm per bushel.

“When we talk about aeration in a grain bin, we also need to talk about static pressure and how it is related to the airflow,” Hostetler said. Static pressure is the measure of the pressure required to overcome the resistance of the grain for a particular airflow, grain species and grain depth. The amounts of fines and foreign material in the grain will also impact static pressure.

For example, it’s easier to push air through soybeans than small grains like rye or barley, which lay tightly together. Because of this, it takes higher static pressure to move air through the small grains. Also, the deeper the grain is the more static pressure it takes to push air through it.

Static pressure is measured with a manometer. Hostetler suggested testing static pressure in at least two places – anywhere around the bin and in the transition zone or ductwork between the fan and bin. “The measure in the transition will really determine your true airflow from the fan. It’s also really good to know if that transition is contributing much to the static pressure,” he said.

As static pressure increases, airflow decreases and the more difficult it becomes to push air through the grain. Knowing the static pressure will help growers determine what style of fan is called for. There are four types: axial, low-speed centrifugal, high-speed centrifugal and inline centrifugal. Hostetler said that the axial fan, which has a medium airflow capability and 3,450 rpm, is the most common fan on smaller operations. The low-speed centrifugal, typically seen in larger bins, has a lower rpm and does not have a lot of static pressure capability. High-speed centrifugal fans generate higher static pressure and are good for taller grain bins. Their airflow is lower, so it may take multiple fans to get the necessary air. Finally, the inline centrifugal fan runs quietly compared to the others and is designed to perform in medium static pressure applications.

Grain aeration basics

According to Hostetler, fans should only be running when it’s dry and not raining. “If it’s raining and we run fans, we don’t get any drying, and it’s a waste of electricity,” he said. Temperature and humidity levels also dictate how long and when the fans should be on. If the grain enters the bin with fairly high moisture, it may be necessary to run fans for several days straight to prevent deterioration and mold growth.

Weather conditions will also determine whether supplemental heat is required. If it’s really cold outside, it’s not as essential to quickly dry the grain. “If your falls are anything like ours, you can still get 80º weather during harvest,” Hostetler said. “Grain at that temperature doesn’t have much storage time.”

Heaters work by drying the air that is entering the bin. Either upstream or downstream heaters can be added to the fans. Downstream heaters are installed between the fan and the grain bin – air flows from the fan, through the heater and then into the grain. Upstream heaters are installed on the inlet side of the fan. The fan draws air through the heater and then blows the air into the grain bin.

There are low temperature and high temperature heaters. Which type is required depends on what is being dried, how fast it needs to be dried and whether or not it will be used for seed. “We generally recommend low temperature heat in places where there’s storage of seed that will be planted. If it gets too hot, it probably won’t germinate. Any grains that tend to crack, we want to use low temperature heat,” Hostetler said.

Another factor that impacts aeration is whether or not it’s a negative or positive system. Positive aeration, when the fans push air into the bottom of the bin under an aeration floor, is the more common of the two. The air enters up into the grain and exits through roof vents, and the fans are located on the ground for easy maintenance. A disadvantage of this system is that as air is pushed through the grain, the air can warm up. Because of this, grain in the top of the bin may not dry, allowing mold to form.

A negative aeration system works opposite from the positive system; air is pushed at the top of the system. This is accomplished by installing fans at the eave of the bin or by ducting air up the side of the bin. “Or we can actually reverse the fans and connect the inlet or suction side of the fan to the bottom of the grain bin and pull air out. That will draw fresh air in at the top,” Hostetler explained. The advantage of a negative system is that it’s easier to monitor the condition of the grain. Grain that begins to deteriorate will have an odor to it.

Regardless of system type, the goal is to use aeration strategies to prevent grain crops from spoiling and to increase their storage time. “If we can’t store it, we’ll probably lose it,” Hostetler said.

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