Grain Aeration

Agdex#:	110/717
Publication Date:	01/84
Order#:	85-003
Last Reviewed:	01/84
History:	Revision of Factsheet "Grain Aeration," March, 1976
Written by:	D. Hilborn - Engineer (By-Products)/OMAF

Table of Contents

1. Introduction
2. When to Aerate
3. Time Required Per Aeration
4. Aeration Airflow Rates
5. Direction of Air Movement
6. Conditioning of Grain for Adequate Aeration
7. Monitoring Stored Grain
8. Available Aerations Systems
9. Design of Aeration Duct Systems
10. Selection of Aeration Fans
11. Metric Conversion Factors
12. Credits

Introduction

Aeration is the practice of forcing small quantities of air through stored grain to maintain quality. To accomplish this, aeration must reduce or eliminate the following conditions.

Temperature And Moisture Variations within the Grain Mass
As the storage is being filled, the incoming grain will probably vary in temperature and moisture content, because of changes in grain maturity, weather conditions and drying fluctuations. Hot, spoiled zones may be created in the bin even though the average condition of the grain may be good.

Warm or Hot Grain
If the storage is filled with a warm or hot grain, problems can occur even if the moisture content is low (Figure 1). High field heat or inadequately cooled grain from a dryer are the two sources of warm or hot grain.

Figure 1. Relationship of storage temperature and grain moisture content to insect heating, fall in germination (to 95% in 35 weeks' storage) and damp grain (fungal heating).

Temperature Variations between Grain Mass and Ambient (Outside) Conditions

Temperature variations between the grain mass and ambient conditions will cause moisture migration problems. Figure 2 shows a typical fall migration caused by lower ambient temperatures.

Figure 2. Fall Moisture Migration

The differences in temperature will create an air convection current with air dropping through the cold grain along the outside walls and rising through the warm grain in the centre of the bin. As the air rises it will warm, increase in moisture holding capacity and pick up moisture from the corn. However, when the air nears the cool upper surface it will cool, lose moisture holding capacity and drop moisture picked up earlier. Thus a high moisture area is formed in the top centre of the bin creating a potential for spoilage.

In the spring, the opposite air flow will occur (Figure 3) because of the higher ambient temperature. Condensation with a potential for spoilage will occur at the bottom centre of the bin.

Figure 3. Spring Moisture Migration

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When to Aerate

Grain should be aerated as soon as possible after placing it into storage. This is especially important if large moisture and/or temperature variations exist or if the whole mass is warm to begin with. During the fall, the grain should be cooled to match average weather conditions. Aeration should begin whenever average outside air temperatures are at least 5°C cooler than the warmest grain in the bin. Continue cooling in steps until the overall grain temperature is between 0 to -5 °C . During the spring, warming of the grain is necessary if extended storage (post June) is required or if the grain mass is below 0°C. If required, the aeration fans should be started as soon as the average outside temperature is 5-7°C higher than the grain temperature.

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Time Required per Aeration

Aeration cools or warms grain similar to the way a bin dryer dries grain through a "drying front". Essentially a similar front, which could be called "Aeration Front", moves up or down (depending on direction of airflow) through the grain mass. Table 1 shows the time it takes for the "aeration front" to move through the grain. This time depends on the airflow rate per unit of grain and the time required to cool or warm the grain.

Once aeration is started, it should continue (even through high humidity periods) until the aeration front has moved completely through the grain mass. However, once the front is through, the grain will be relatively uniform and continued aeration especially during high humidity periods will cause a new front.

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Aeration Airflow Rates

Normal aeration airflow rates range from 1-2 litres of air per second per cubic meter of grain (1-2 L/s-m3) (0.08-0.16 cfm/bu.). Some larger systems use lower airflow rates (0.3--0.5 L/s-m³), however, excellent management and facilities will be required. Higher rates should be used (2-6 L/s-m³) if grain is stored at higher moisture levels or if a large variance in incoming moisture levels exist.

An aeration system can't be expected to be a drying system since natural air drying rates are at least 10 times longer.

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Direction of Air Movement

Aeration air can move either up or down through the grain mass. Most fans have the ability to either push or pull, however, airflow volumes and power requirements will often change due to direction.

There are advantages and disadvantages to both systems as shown in Figures 4 & 5.

Figure 4. Airflow up through the Grain Mass

Figure 5. Airflow down through the Grain Mass

In summary, both air directions will work. The most important factor is to understand a method and to know where to monitor each system.

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Conditioning of Grain for Adequate Aeration

Fines, weed seeds and other foreign materials will adversely affect aeration especially if these materials are concentrated in one location. Since aeration uses small air flow rates, any increased resistance will have a large effect on airflow patterns. As a result, it will take a much longer time to move the drying front through the fine area (Figure 6).

Figure 6. Chronological movement of Aeration Front up (or down) through Grain Mass.

To avoid this problem one or more of the following steps can be taken:

1. Clean grain before placing it in storage. 
2. Prevent excessive velocity of grain entering the storage to avoid breakage. 
3. Spread grain (including fines) throughout the bin. However, this will cause a higher overall density in the bin increasing the overall resistance to airflow.
4. Unload some grain from the centre which could remove some accumulated fines in centre. The removed centre core is again filled with clean grain. 
5. Make sure aeration front makes it through all the grain. This could mean aerating for a longer period of time, increasing fan size or decreasing grain depth.
   

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Monitoring Stored Grain

To properly manage stored grain, the operator should be able to obtain temperatures of the grain throughout the bin especially at the last location of the aeration front. On smaller bins, probes can often work quite effectively, however, with larger bins, a remote monitoring system using thermocouples is often necessary.

If the operator has to enter the bin to monitor it, adequate safety measures must be taken.

1. Never enter the bin while the unloader is running.
2. If part of the bin has been unloaded, beware of bridged grain which could hide a cavity. 
3. Ladders should be installed in the bins to allow easy and safe access. A safety rope should be used if the operator leaves the ladder. 
   

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Available Aerations Systems

Small grain storages can be aerated using a commercially available portable system with a perforated pipe which screws into the grain mass. A small aeration fan unit is attached to the top of the pipe. For larger storages, a more complex system is required. Round storages use one of the following systems:

1. A single perforated duct.
2. A cross duct arrangement.
3. A V shaped duct.
4. A rectangular perforated area located in the middle of the floor.
5. A fully perforated floor.

The first three systems can be located above or below floor level. Above the floor systems are normally easier to clean and inexpensive. The below the floor systems, allow a sweep auger to rotate without obstruction greatly reducing labour requirements.

Rectangular storages use one or more parallel ducts located above or below floor level.

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Design of Aeration Duct Systems

Table 2 is a system sizing guideline. The following steps should be taken to use the chart to design the system.

1. Determine the amount of bin storage in m³.
2. Choose a desired aeration airflow rate in L/s-m³.
3. Determine total airflow (1) x (2) L/s.
4. Go to Table 2. Find Minimum Duct area (m³), Minimum Floor Area (m²) and Minimum Roof Vent Area (m²).
5. Design your system using components which meet or exceed the above minimums.

Another factor to be looked at is variation in the distance aeration air must travel through the grain. This is especially true with rectangular storages. A good guideline to follow is to keep the longest air path from the duct to the grain surface no more than 1 1/2 times the shortest air path. Figure 7 shows the number and location of ducts required in a rectangular building.

Figure 7. Lengthwise Ducts Required for Rectangular Buildings.

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Selection of Aeration Fans

Tube-axial (Figure 8) and centrifugal fans are used although tube-axial are the most common. Fans should be selected on the basis of airflow required and static pressure.

Figure 8. Typical Aeration Fan

Static pressure is dependent on the desired aeration rate, grain depth and grain type (Table 3).

From airflow (determined in Step 3 of Duct System Design) Figure 9 can be used to determine the model and power of fan required.

Figure 9. Fan Performance Graph for Typical Tue-Axial Fans

Assistance Available

Further assistance will be available from the Ontario Ministry of Agriculture and Food, Agricultural Engineering Service. Many manufacturers and dealers have excellent personnel and information available to assist you with the problem.

Metric Conversion Factors

1 inch of water = 250 Pa (pascals)
1 cubic foot per minute = 0.47 L/s (litres per second)
100 bushels = 3.6 m³ (cubic metres)
1 cubic foot per minute per bushel = 13 L/s m³ (litres per second per cubic metre)
1 foot = 0.3 m (metres)
1 square foot = 0.1 m² (square metres)
1 horsepower = 0.75 kW (kilowatts)

Credits

Table 1 - Managing an Aeration System in Your Grain Storage; Farm Information Service, United Grain Growers, Winnipeg.
Table 2, Figures 2, 3, 7 - Movement of Natural Air Through Grain; O.H. Friesen & H.P. Harnes, Engineering Section, Manitoba Agriculture.

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