Natural Air Corn Drying Systems

Factsheet - ISSN 1198-712X   -   Copyright Queen's Printer for Ontario
Agdex#: 111/736
Publication Date: 08/86
Order#: 86-066
Last Reviewed: 02/97
History: Original Factsheet
Written by: R.P. Stone - Engineer, Soil Management/OMAFRA; P.S. Plue - Resources and Planning/OMAFRA

Table of Contents

  1. Introduction
  2. Advantages
  3. Disadvantages
  4. How Does It Work?
  5. Dryer Components
  6. Sizing the System
  7. Operating the System
  8. Solar Corn Drying
  9. Costs
  10. Drying Other Crops


Natural air drying uses the limited, but free, drying potential of unheated air to lower the moisture content of corn to an acceptable level for storage. It is a relatively slow method of drying - often stretching over several weeks. The drying concept is similar to the cribbing of cob corn. However, in this case the corn is shelled and we rely on fans rather than wind to move air through the material.

Natural air drying evolved from low temperature systems first used in the U.S. Mid-West during the late sixties. The typical low temperature system used electric resistance heaters to provide a 60°F to 100 °F (30 °C to 50°C) rise in ambient air temperature. After several years of operating these systems, farmers discovered that by shutting off the heaters they could afford to run the fans for a much longer period and still save on energy costs. As a result, new systems omitted the heater, preferring to spend the money saved on increased fan capacity. This is the natural air system as we know it today.

Natural air drying is particularly suited to small cash crop and livestock operations.

Natural Air Drying System used on a dairy farm.

Figure 1. This Natural Air Drying System is used on a dairy farm.


  1. High quality corn, free of stress cracks, will result.
  2. A minimum of corn handling takes place at the dryer with no bottlenecks encountered in the system.
  3. A lower operational cost per bushel of corn for the natural air drying system is possible.
  4. With few moving parts, the system requires little supervision during drying.


  1. The corn moisture content should be 25% or less at time of harvest.
  2. The drying process is very slow, often requiring as long a period as 60 days. Occasionally, corn is partially dried during the fall season with the completion of drying taking place in the spring.
  3. Corn must be clean to allow adequate air movement.
  4. The final moisture content of the corn is very dependent upon weather conditions and may not reach the 15.5% moisture content during fall drying. This higher moisture level may be acceptable for storage and feeding to livestock but further spring drying will be necessary to reduce the moisture content to an acceptable level for the cash crop market.
  5. Good management and monitoring of the bin is necessary to be sure that the corn retains it's quality and is drying according to schedule.

How Does It Work?

A. Equilibrium Moisture Content

Natural air drying works on the principle that the vapour pressure within a kernel of corn will attempt to equalize with that of the air moving past it. In high temperature drying there is an enormous difference between the vapour pressure of the hot air and the vapour pressure within the kernel. As a result, drying occurs rapidly. With natural air drying, the difference in vapour pressure is minimal, and drying proceeds slowly.

Researchers have developed a chart of Equilibrium Moisture Content which allows us to predict the final moisture content of corn when exposed to particular conditions of temperature and humidity. For instance, from Figure 2 we see that air at 40°F (4.5 °C) and 75% relative humidity will lower corn moisture content to about 16.5%.

Figure 2. Equilibrium Moisture Content of Shelled Corn at Various Temperatures and Relative Humidities
Air temp.
Relative humidity, %
50 55 60 65 70 75 80 85 90
Equilibrium percent moisture, wet basis
30 13.0 13.5 14.5 15.5 16.5 17.4 18.7 20.3 22.5
40 12.5 13.0 13.8 14.7 15.5 16.5 17.6 19.4 21.5
50 12.0 12.5 13.3 14.0 14.8 15.8 16.9 18.6 20.5
60 11.4 12.0 12.6 13.4 14.0 15.0 16.0 17.7 19.5

Note that for axial fans, air moving over the motor will often increase in temperature by about 2°F (1.1°C). This temperature rise effectively lowers the relative humidity by about 5%. Thus with ambient conditions of, say 38 °F (3°C) and 80% relative humidity, we would be correct using the equilibrium moisture content chart assuming plenum conditions of 40 °F (4°C) and 75% relative humidity.

The above could also apply to centrifugal fans if shrouding is placed to draw air past the motor.

B. Allowable Storage Time

A second concept which is important is the fact that grain temperature and moisture content determine the allowable storage time of corn. Figure 3 provides guidelines concerning the number of days corn can be held at various conditions before deterioration becomes significant.

Figure 3. Allowable Storage Time for Shelled Corn
Grain temp.
Corn moisture, percent
30 648 321 190 127 94 74 61
35 432 214 126 85 62 49 40
40 288 142 84 56 41 32 27
45 192 95 56 37 27 21 18
50 128 63 37 25 18 14 12
55 85 42 25 16 12 9 8
60 56 28 17 11 8 7 5
65 42 21 13 8 6 5 4
70 31 16 9 6 5 4 3
75 23 12 7 5 4 3 2
80 17 9 5 4 3 2 2

C. Weather Conditions

In light of the above, it is obvious that fall weather conditions are much more important to natural air drying than to high temperature systems. Figure 4 compares long-term averages of temperature and humidity for several Ontario locations as well as for Rockford, Illinois. These averages suggest that Central and Eastern Ontario are every bit as acceptable for natural air drying as southwestern Ontario. Even though lower harvest moisture contents might be expected in the southwest, their relatively higher fall temperatures suggest that allowable storage times will not differ significantly.

Figure 4. Thirty-year Weather Averages
Temp. (°F) R.H.
Ottawa 58 74 47 73 35 77 19 77
Trenton 60 75 49 74 38 77 24 79
Toronto 60 74 49 71 38 80 26 81
London 60 79 49 80 38 83 26 84
Windsor 63 75 52 75 40 79 29 81
Rockfield, Illinois 63 74 53 71 38 75 25 78

In addition, the higher relative humidities of southwestern Ontario will result in higher Equilibrium Moisture Contents than for other parts of the province.

Dryer Components

A natural air corn drying system will include the following components:

1. Storage Bin

The bin serves as the drying bin as well as the storage for the corn. Corn depths are to be kept as shallow as practical to maintain a minimum static pressure. The preferred depth range is 10 to 12 ft (3.0-3.6 m) with a maximum depth of 14 ft (4.2 m). Large diameter, shallow bins should be constructed for natural air drying systems.

Shallow-depth storage bins reduce static pressure, fan horsepower requirements and energy use.

Figure 5. Shallow-depth storage bins reduce static pressure, fan horsepower requirements and energy use.

The installation of 2" (5 cm) diameter, capped pipes into the sides of the storage bin at approximately 2 ft (60 cm) vertical intervals will be beneficial in obtaining corn samples and following the drying front.

Corn Moisture sampling from horizontal pipes in bin wall.

Figure 6. Corn Moisture Sampling From Horizontal Pipes in Bin Wall.

2. Full Aeration Storage Bin Floor

The fully perforated floor is the most desirable flooring for a storage bin using the natural air drying system. Uniform air distribution is necessary when drying high moisture corn and this can be achieved most effectively with the use of a fully perforated floor.

3. Fan

The fan is to be selected to deliver the desired amount of air at the expected static pressure. Either an axial or centrifugal fan may be chosen. Electrical requirements for the fan motor can be substantial and should be considered in the original planning process. Fans must always push air through the bin.

4. Distributor

The use of a grain spreading device in natural air drying bins in recommended. Distributors will assist in spreading the corn, including fines, evenly in the bin. The corn surface should be kept level to maintain a uniform air flow through the corn mass.

5. Roof Vents

Sufficient exhaust air capacity is to be provided through roof vents and hatches to avoid back-pressure and to assure that no condensation forms on the roof and sidewalls. Bin roof openings must be sufficient in size to provide one square foot (930 sq. cm.) of opening for each 1000 cubic feet of air per minute (470 l/s) of fan capacity.

6. Grain Cleaner

Fine material and broken grain increases air flow resistance, thus reducing total air flow. Screening this material out with a grain cleaner is important for successful natural air drying. Since fines tend to accumulate in the centre of the bin, unloading some corn from the centre when the bin is full can be beneficial for air circulation.

A grain cleaner must be used in a Natural Air System.

Figure 7. A grain cleaner must be used in a natural air system.

7. Other Management Related Equipment

The management practice of frequent checks on the corn drying process gives early warning signs of possible equipment or drying process failures. An operator should have the following equipment in order to properly manage the system.
  1. Sling Psychrometer - measures wet bulb-dry bulb temperature readings for determining relative humidity of the air.
  2. Manometer - measures static pressure that the fan is working against and will give indications of problems developing in the corn.
  3. Moisture Tester - allows the corn moisture to be checked during filling and as the drying front moves through the corn.
  4. Grain Thermometer Probe - provides temperature readings of the corn mass at probed locations.

Sizing the System

A. Airflow

As indicated, the success of a natural air system is most dependent on adequate delivery of air. A general recommendation in Ontario is to provide 2 cubic feet of air per minute (c.f.m.) per bushel of corn. This figure should be adjusted upward if harvest moisture contents are expected to be consistently at 30%. It could also be adjusted downward slightly if hybrids are grown which consistently are harvested at, say 22% moisture.

If a farmer encounters a situation where corn must be harvested wetter than the system was designed for, he should limit the depth of corn placed in the bin in order to effectively increase the c.f.m./bu.

B. Fan Selection

Farmers have the choice of two basic types of fans: axial (propeller), or centrifugal. The latter is a more expensive fan but is better suited to situations where static pressures of 4" (1000 Pa.) or greater are expected. In addition, a centrifugal fan runs much more quietly than an axial fan.

Static pressure is estimated using Figure 8. With an air flow rate of 2 c.f.m./bu. (26 l/s/m3) it suggests that grain depths must be limited to less than 14' (4.2 m) if we wish to keep static pressures below 4" of water column (1000 Pa.).

Static pressure requirements for airflow in shelled corn.

Figure 8. Static Pressure Requirements for Airflow in Shelled Corn.

C. Example System

Example: Size a bin to dry 5,000 bushels (125 tonnes) of 25% corn using natural air.

  1. From Figure 9, a 27' (8.1 m) bin will hold bushels in about 11' (3.3 m) of depth.
  2. From Figure 8, with 11' (3.3 m) of corn and air flow of 2 c.f.m (26 l/s/m3) static pressure is about 2.2" (550 Pa.).
  3. Choose a fan which will move 5,000 bushels x 2 c.f.m./bu. = 10,000 c.f.m. @ 2.2" static pressure.
  4. Check with your dealer for an appropriate fan. Manufacturer's data may suggest a 24" (30 cm), 5-7 hp. (6.7-9.4 Kw.) axial fan.
Figure 9. Approximate Storage Capacity of Steel Bins*
Diameter (ft.) bu./ft. bu./10 ft. bu./12 ft. bu./14 ft.
18 204 2040 2440 2850
21 277 2770 3320 3880
24 362 3620 4340 5070
27 458 4580 5500 6410
30 565 5650 6790 7920
33 684 6840 8210 9580
36 814 8140 9770 11,400

(*) Note: A perforated floor will use 12" to 16" of sidewall height

Operating the System

Management Guidelines for natural air drying are as follows:

  1. Harvest corn at moisture content in mid to low 20's if possible.
  2. Clean the corn before filling the storage in order to assist airflow.
  3. Start the fan as soon as enough corn has been placed in the bin to provide a seal over the fully perforated floor.
  4. Use a distributor to spread fines and provide a reasonably level surface for uniform airflow. Final leveling by hand may be necessary.
  5. Allow fan to run continuously. Do not shut off in damp weather.
  6. Allow fan to run for 2 days after drying front has reached the grain surface. Check the entire surface of corn for any 'tough' spots which may still remain.
  7. If extremely cold weather is encountered, the fan may be shut off as minimal further drying will occur. When milder weather returns, re-start the fan to attempt to push the drying front through before winter arrival.
  8. If the drying front has not reached the surface of the corn before winter arrives, shut the fan off and complete the drying in March and early April. Relative humidities will typically average 65-70% at that time.

Solar Corn Drying

The addition of a solar panel to a corn bin for the purpose of increasing ambient temperatures and reducing the relative humidity of the incoming air has been practiced to a limited degree. Solar panels constructed primarily of fibreglass, have been either attached to the side of the storage bin or constructed as self-standing units. The intake air is drawn through the collector to pick up the solar energy. Maximum temperature increases through the collector have been found to be in the range of 10°F (5.5°C), however, average temperature increases over a 24 hour period are in the order of 1-2°F (0.5-1.1°C). The addition of a collector cannot be economically justified unless it can be constructed for a very low cost. i.e. less than $0.20 per bushel of corn dried (0.7c/m3). With the addition of a collector there will be some minimal gains shown in reducing the corn drying period and in having more impact during a year when consistently high relative humidities are encountered.


A. Capital Outlay

The 5,000 bushel (125 tonne) system which was referred to previously involves additional costs over a basic storage bin of that capacity. These result from:

  1. the increased fan size required over a conventional aeration fan, and
  2. a larger diameter, low profile bin which is slightly more expensive than a taller, narrow bin of the same capacity.

Fully perforated floors are almost a standard recommendation for any new bin being erected, so it may not be correct to include that cost as an extra for the natural air drying system.

Based on a March, 1986 estimate, the system recommended in our example would have a list price of about $14,000. This figure includes $3,000 for installing the bin foundation and erecting the bin. Site preparation and wiring would be extra.

B. Drying Costs

Depending on initial moisture content, and on weather conditions, hydro consumption may be anywhere in the range of 50-100 kilowatt-hours per tonne. At 4¢ per kilowatt-hour, this equals $2 to $4 per tonne of corn. (5¢-10¢ per bushel of corn).

Drying Other Crops

Natural air drying may be used for other crops grown on the farm.

(a) Soybeans

Soybean field losses can be significantly reduced by combining the crop at 16-18% moisture and using a natural air drying system to complete the drying process. Since soybeans are harvested at only 3-5% moisture above their safe storage moisture content, they are very adaptable to natural air drying.

The same natural air drying system designed for corn may be used for soybeans. An airflow rate of 2 cubic feet of air per minute per bushel (26 l/s/m3) of soybeans is recommended for moisture contents in the 16-18% range. The resistance to airflow through soybeans is approximately 25% less than corn, thus, approximately 25% more soybeans than corn can be dried at one time with an equivalent system.

(b) Wheat

Wheat can be harvested up to 18-20% moisture and the drying can be completed in a natural air drying system.

An airflow rate of 2 cubic feet of air per minute per bushel of wheat (26 1/s/m3) is recommended for moisture contents in the 18-20% range. However, wheat offers considerably more resistance to airflow than corn or soybeans. The static pressures will be about 60% higher with wheat than that of corn of the same depth. A rule of thumb is to dry only one half the quantity or depth of wheat as that of corn at one time for which the system was designed. Since allowable storage time for high moisture wheat is very short, air must be moved through the grain immediately after harvesting.


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