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Extension > Agriculture > Crops > Corn Production > Harvest > Natural-air corn drying in the upper Midwest

Natural-air corn drying in the upper Midwest

William Wilcke and R. Vance Morey

This publication is for corn producers, educators, consultants, and equipment dealers who are interested in natural-air corn drying in Minnesota and neighboring states. Answers to some common questions provide a description of natural-air drying, its advantages and disadvantages compared to other types of drying, equipment requirements, management recommendations, and expected energy use.

Does natural-air corn drying really work in northern states?

Yes! Natural-air corn drying has been used successfully for many years by researchers at a number of agricultural experiment stations and by thousands of corn producers. The process works best under cool (40 to 60 degrees F), dry (55 to 75% relative humidity) conditions. Since average fall temperature and humidity are in these ranges in the Upper Midwest, natural-air drying usually works quite well.

How does it work?

Figure 1. Natural-air drying bin equipped with grain spreader, exhaust vents, fan, and full-perforated floor.

Natural-air drying, also called ambient-air drying, near-ambient drying, unheated-air drying, or just air drying, is an in-storage drying method that uses unheated, outdoor air to dry corn to a safe storage moisture (13 to 15%). Instead of using heat energy from fossil fuels to remove moisture, natural-air drying uses electricity to operate fans, with energy for removing moisture coming primarily from the drying potential of outdoor air. Natural-air drying of shelled corn is similar in principle to the drying that takes place in cribs of ear corn, except that, because the airflow resistance for shelled corn is greater than for ear corn, fans rather than wind pressure move air through the bin.

Natural-air drying is basically a race between drying progress and growth of the fungi (commonly called molds) that cause grain spoilage. The bin is usually filled in a few days and the fan is started as soon as bin filling begins. Drying takes place in a one- to two-foot thick drying zone (also called a drying front) that moves slowly up through the bin (Figure 1). Grain below the zone is generally dry enough to be safe from spoilage, while grain above the zone remains at its initial moisture until the zone passes. (Note that positive pressure, or upward airflow, is recommended for natural-air drying so that wet grain is at the top of the bin. There it is easier to watch for signs of mold and to move moldy corn out of the bin if necessary.)

How long does drying take?

Drying time depends on initial grain moisture, airflow per bushel provided by the fan, and weather. If corn moistures and airflows recommended in Table 1 are used, drying usually takes four to eight weeks depending on the weather. In cool, damp falls, the drying zone doesn't reach the top of the bin before winter and drying is completed in spring.

Table 1. Airflow and moisture recommendations for natural-air corn drying in the Upper Midwest.

Airflow (cfm/bu)* Initial corn moisture (% wet basis)
Northern Minnesota, North Dakota, South Dakota
1.0 22.0
1.25 23.0
1.5 23.0
2.0 24.0
3.0 25.5
Southern Minnesota, Iowa, Wisconsin
1.0 21.0
1.25 21.5
1.5 22.5
2.0 23.5
3.0 24.5
Values are for bins filled rapidly (in a day or two) in mid-October. Use of these airflows should result in drying without spoilage, or need to move corn to prevent spoilage, at least 90% of the time.

*cfm/bu = cubic feet of air per minute per bushel of grain

Does grain ever spoil before it dries?

There is some risk of grain spoilage, but if airflow is matched to grain moisture and bins are monitored closely, spoilage can be avoided.

Table 2 shows the allowable storage time for shelled corn. This is the approximate amount of time that corn can be held at different temperatures and moisture contents before there is enough mold damage to cause price discounts or possible animal feeding problems. At temperatures higher than 60 degrees F, corn dries fast, but mold grows faster – especially if corn moisture is higher than 22%. Mold growth is very slow at temperatures lower than 40 degrees F, but drying is also slow. Thus, the best temperatures for natural-air drying are about 40 to 60 degrees F. Drying is slow and expensive when fall weather is colder than normal, but the greatest risk of spoilage comes during unusually warm falls.

Table 2. Allowable storage time (days) for shelled corn. This is the approximate number of days corn can be held before there is enough mold growth to cause price discounts or feeding problems.

Corn temperature
(degrees F)
Moisture content (% wet basis)
16 18 20 22 24 26
3820 1459 722 427 287 212
1700 648 321 190 127 94
756 288 142 84 56 41
336 128 63 37 25 18
149 57 28 16 11 8
83 31 16 9 6 5

The key to success is to provide enough airflow to move the drying zone all the way through the bin before any spoilage occurs. Because wet grain spoils faster, it is important to use more airflow per bushel for wetter corn.

Regardless of airflow, check the condition of grain at the top of the bin every few days during drying. If heating, musty or sour odors, or moldy kernels are detected, move some or all of the wet grain out of the bin and feed, sell, or dry it in another dryer.

What equipment is necessary for natural-air drying?

A fan, a bin, and an air distribution system are required, and exhaust vents and a grain spreader are desirable. Sometimes grain stirrers, heaters, or both, are added, but they aren't normally necessary for successful drying.

What size fan is necessary?

Figure 2. Probability of success, or percentage of years that corn is expected to dry without spoilage for different harvest moistures and airflows. (Mid-October harvest near St. Paul, MN, 16 ft corn depth.)

Figure 3. Average energy use per bushel for natural-air corn drying with different harvest moistures and different airflows. (Mid-October harvest near St. Paul, MN, 16 ft corn depth.)

Required fan size depends on corn moisture, corn depth in the bin, and the desired probability of success. Probability of success is the percentage of drying seasons, or number of years out of 100, that the drying zone is expected to move all the way through the bin before spoilage occurs, or before you need to move corn out of the bin to prevent spoilage.

One way to increase probability of success for any corn moisture is to use a higher airflow per bushel (Figure 2). Greater airflow means faster drying and less time for spoilage to occur. Disproportionately larger fans, which draw more electrical power, are necessary to deliver higher airflow, and energy use per bushel increases (Figure 3). Use Figures 2 and 3 to compare expected probability of success and energy consumption for different airflows and grain moistures. Although weather data for St. Paul, MN were used to prepare Figures 2 and 3, the results apply to much of the Upper Midwest.

Information like that in Figure 2 was used to develop Table 1. If moisture and airflow recommendations in Table 1 are followed, grain should dry without spoilage at least 90% of the time. This means that moving grain to prevent spoilage shouldn't be necessary more than about 10 years out of 100.

To select a fan, use the desired airflow per bushel and the normal drying depth to determine the expected static pressure (Table 3). Next, multiply airflow per bushel by the number of bushels in the bin to get total airflow (cfm). Finally, use fan manufacturers' catalogs to select a fan that will provide the desired airflow (cfm) at the expected static pressure (inches of water, or in. water).

Table 3. Static pressure (inches of water) for airflow through shelled corn.*

Corn depth (ft)
Airflow (cfm/bu)
2 0.1 0.1 0.1 0.1 0.1
4 0.1 0.1 0.2 0.2 0.4
6 0.3 0.3 0.4 0.6 1.0
8 0.5 0.6 0.8 1.2 2.1
10 0.8 1.1 1.4 2.0 3.7
12 1.2 1.6 2.1 3.2 5.8
14 1.7 2.3 3.0 4.6 8.5
16 2.4 3.2 4.2 6.4 12.0
18 3.1 4.3 5.6 8.7 16.2
20 4.0 5.6 7.3 11.3 21.3
*Airflow resistance values have been multiplied by 1.5 to give table values. This accounts for fines and packing in the bin. If corn is stirred, airflow resistance is reduced, so divide table values by 1.5.

There are three basic types of grain drying fans – axial-flow, centrifugal, and in-line centrifugal. Any of the three types can be used, but axial-flow fans are most common for natural-air corn dryers because they are the least expensive and the most efficient type at the low static pressures encountered in corn drying. Also, heat given off by the motor is captured by the drying air.

Table 4 provides a rough estimate of fan power requirements (horsepower, or hp) for different airflows and corn depths. Power requirements increase drastically as depth and airflow per bushel increase. High-power fans are expensive to install and operate. Thus, natural-air drying is most economical if corn depth is less than 18 ft and corn moisture is less than 23% (full bin airflow is less than 1.25 cfm/bu). Short, large-diameter bins are more expensive to build than tall, slender ones, but the energy savings for short drying bins makes the extra initial investment worthwhile.

Table 4. Approximate fan power requirements (hp per 1000 bu) for natural-air corn drying.

Corn depth (ft)
12 14 16 18 20
Airflow (cfm/bu)
hp per 1000 bu
1.0 0.4 0.5 0.7 1.0 1.3
1.25 0.6 0.9 1.3 1.7 2.2
1.5 1.0 1.4 2.0 2.7 3.5
2.0 2.0 2.9 4.1 5.5 7.1
3.0 5.5 8.1 11.3 15.3 20.1

For more information on fan selection, get a copy of the bulletin Selecting Fans and Determining Airflow for Crop Drying, Cooling, and Storage, FO-5716, or the FANS computer program from the Minnesota Extension.

Should full perforated floors be installed in natural-air drying bins?

Yes. Drying is much more uniform when air is distributed through a full perforated floor that is set at least a foot above the concrete pad. It might be possible to dry relatively low moisture corn using a duct system to distribute air, but airflow and drying in the grain above the ducts are not uniform – especially at the higher airflows necessary for higher moisture corn.

Are roof vents necessary?

An exhaust area of about one square foot (sq ft) per 1000 cfm of airflow is needed above the grain to prevent excessive condensation under the bin roof and to prevent back pressure that would reduce airflow delivery by the fan. Calculate the total area that is likely to be open when the fan is operating, and if it is less than 1 sq ft/1000 cfm, install extra exhaust vents. When calculating exhaust area, include openings such as the gap at the eave, which in some cases can provide sizable exhaust area.

Area of eave gap (sq ft) = [gap width (in.) / 12 in./ft] x 3.14 x bin diameter (ft)

Should corn be cleaned before it goes into the bins?

Fines (small pieces of broken grain, dirt, chaff, and weed seeds) cause problems in drying and storage bins because they restrict airflow, and they are more susceptible to spoilage than whole kernels. Worse yet, fines tend to concentrate in areas directly under the spout used to fill the bin. Running corn through a cleaner to remove fines during bin filling is one of several options for managing fines.

Figure 4. Periodically withdrawing grain during bin-filling to reduce fines from bin center.

First try to minimize production of fines. Set combines for minimum damage and maximum cleaning. Handle grain gently by using grain conveyors that are easy on grain (bucket elevators, drag conveyors) and by reducing drop heights and number of times grain is handled. Augers and pneumatic conveyors have the potential to cause a lot of grain damage. Reduce damage potential by operating augers slowly and full, and by using gentle curves in tubing and the proper air/grain ratio in pneumatic conveyors.

Cleaning grain to remove fines before natural-air drying is the best management choice. Fines removal reduces spoilage risk, and it reduces drying cost by allowing the fan to deliver greater airflow. An Iowa State University study indicated that it takes more than three times as much fan power to move the airflow needed for natural-air corn drying through non-cleaned vs. cleaned corn.

If a cleaner isn't available, or if cleaning creates too great a bottleneck in grain handling, periodically remove some grain from the bin center during bin filling to remove fines (Figure 4) . If you can't feed or sell fines, or choose not to remove them, then at least use a grain spreader to distribute them more uniformly throughout the bin.

Should the grain surface in natural-air drying bins be leveled?

Yes. The grain surface should always be leveled after grain is put into or removed from a bin in which natural-air drying is in progress. If grain depth over the drying floor is not uniform, airflow will be greater and drying will be faster in areas that have shallower grain depths. But the fan must be kept running until the drying front moves through areas with greater grain depth, which increases drying cost per bushel for the whole bin.

The situation is even worse if grain is peaked under the spoutline. Drying is very slow in grain peaks due to the low airflow caused by the greater grain depth and greater concentration of fines.

Would adding heat reduce drying time?

Adding heat speeds drying slightly, and it slightly increases the chance of completing drying in fall. The main effects of adding heat, however, are to increase drying cost and to overdry corn at the bottom of the bin.

Heaters are not generally needed on natural-air corn dryers in the Upper Midwest. Most years, corn dries to a safe storage moisture in fall or the following spring, without the use of supplemental heat. Table 5 gives corn equilibrium moisture values for typical drying season air conditions. Values in the table are moisture contents that corn approaches when exposed to the different combinations of temperature and humidity.

Table 5. Equilibrium moisture content (% wet basis) of shelled corn. These are the moisture contents that corn would reach if exposed to the listed combinations of temperature and humidity for very long periods of time.

(degrees F)
Relative humidity (%)
50 60 70 80 90

Because heat increases temperature and decreases relative humidity of drying air, the effect on final corn moisture is surprisingly large (Table 6). Even small amounts of heat will almost always dry corn to well under 15% moisture. Drying corn to less than 15% is expensive, not only because of the extra energy required to get it that dry, but also because the extra water removal results in less weight of corn available for sale.

Table 6. Example of the effect of supplemental heat on air relative humidity and final corn moisture.

Supplemental heat (degrees F) Drying air temperature (degrees F) Drying air relative humidity (%) Corn moisture content (%)
*Axial-flow drying fans heat the air 2 to 3 degrees F.

Another reason that supplemental heat is not usually needed is that heat from the motor and impeller of axial-flow fans increases drying air temperature at least 2 to 3 degrees F, which is enough to reduce corn moisture about 0.8 percentage points more than would be expected based on outdoor air conditions.

People often ask about the practicality of using solar collectors to heat air for grain drying. Research in the late 1970s and early 1980s showed that properly sized solar collectors had about the same effect as electric or gas heaters. That is, adding solar collectors increased the drying rate slightly, but they also increased total drying cost and the amount of overdrying compared with natural-air drying.

If reducing drying time is the objective, a larger fan is usually a better investment than a heater. But if for some reason it is necessary to add supplemental heat, size the heater to increase the air temperature no more than about 5 degrees F, and use a humidistat to turn the heater off during dry weather. It can be difficult to find gas heaters in this size range. Portable gas space heaters might be one option. Avoid kerosene or fuel-oil heaters because they could leave an oily or smoky odor on the grain. Electric heaters are more common, but they are expensive to install and operate. Consult your local power supplier before installing an electric heater.

Calculate heater size as follows:
Gas heater (Btu/hr) = 1.1 x (temperature rise, degrees F) x (airflow, cfm)
Electric heater (kW) = 1.1 x (temperature rise, degrees F) x (airflow, cfm) / 3412

Are there advantages to stirring corn when drying without heat?

Grain stirrers are an essential component in some types of heated-air dryers, but the situation is different for natural-air drying and drying where the air is heated less than 10 degrees F. Stirring grain provides advantages for natural-air drying although the value is probably not enough to justify the cost of buying new stirring equipment. If you have a natural-air dryer that already contains stirring equipment, don't operate the stirrers continuously. Stirring too much will sift fines to the floor where they could restrict airflow. Also, stirring too frequently will reduce drying efficiency. Natural-air drying is most efficient when there is a layer of wet grain at the top of the bin and drying air is nearly saturated when it leaves the bin. The drier the top layer is, the less saturated the air and the less efficient the drying process.

If grain stirrers are available, operate them as follows:

  • During bin filling, stir to loosen grain and boost airflow provided by the fan. Stop stirring within 24 to 48 hours after the bin is full.
  • If harvest moisture was greater than 20%, stir again when the average moisture in the bin is 18 to 20%. Stirring blends overdry corn from the bottom of the bin with wet corn at risk of spoilage from the top. This reduces overdrying and spoilage risk, but still leaves wet enough corn on top to maintain reasonable drying efficiency.
  • Finally, stir again when average moisture in the bin reaches the desired value (usually 14 or 15%). Toward the end of drying, the top layer is still wetter than the average bin moisture and the bottom is usually drier than average. Stirring allows you to turn off the fan sooner because it blends corn uniformly, top to bottom, to attain the recommended average moisture.

Should the fan be stopped at night or during humid weather?

Leave the fan running if the bin contains corn wetter than about 16% and the temperature is warmer than about 40 degrees F. If the corn is warm and wet and the fan is off for very long, mold growth might cause the corn to heat. Also, some operation during humid weather is needed to rewet corn at the bottom of the bin that overdried during dry weather. If the fan only operates during the driest weather, corn will be badly overdried. Remember, fan heat reduces air relative humidity and allows corn drying even under fairly humid conditions.

If corn is nearly dry and the temperature is low, there is little risk of corn spoilage and it is safe to stop the fan during humid weather. Stopping the fan will save some energy. Regardless of corn moisture, it is usually best to stop the fan during heavy snowfall to avoid plugging holes in the perforated drying floor.

How can I tell when drying is finished?

Grain at the top of the bin remains near its initial moisture content until the drying zone moves all the way up through the bin (Figure 1). The top grain will finally start to dry when the drying zone reaches the grain surface. Continue drying until the top grain reaches the desired final moisture content (usually 14 to 15%). Check the moisture at several locations on the surface because movement of the drying front might not be completely uniform due to irregularities in airflow. By the time the drying zone has reached the grain surface, corn below the surface has normally dried to a safe storage moisture.

Locate the drying front and check drying progress every week or so. This can be done by using a sampling probe to pull grain samples from various depths and by measuring the moisture content of the samples. It is also possible to locate the drying front by pushing a small diameter rod down into the grain. The rod will push hard through the wet grain above the drying zone, but suddenly push more easily when it reaches dry grain in the drying zone. Watch out for overhead electrical powerlines when handling long metal rods at the top of grain bins.

When the condition of the air entering a natural-air drying bin is constant for long periods of time, it might be possible to use temperature measurements to locate the drying zone. Grain cools as it gives up moisture, so a transition from warm to cool grain can indicate the location of the drying zone. Because outdoor air temperature changes frequently, however, drying bins can have warming and cooling zones moving through them in addition to the drying zone. Thus, in practice, it is difficult to use grain temperature to track drying fronts.

What if drying isn't completed before winter?

Just turn off the fan, restart it as needed during winter to keep grain cooled to about 30 degrees F, and then resume drying in spring. It is not economical to continue natural-air drying in winter because drying is very slow and the equilibrium moisture is such that corn wouldn't dry any further than about 17% moisture anyway. It is safe to turn the fan off during winter, because mold growth is very slow at temperatures less than 30 degrees F. Grain temperature can be checked by using permanently mounted temperature cables that hang from the bin roof, by using thermometers or temperature sensors mounted on the ends of probes, by quickly measuring the temperature of grain samples pulled to the surface of bins, or by measuring the temperature of exhaust air. Measure exhaust temperature by placing a thermometer about a foot below the grain surface while the fan is running.

Use the following criteria to decide when to stop drying in fall:

  • Turn the fan off when moisture of corn at the top of the bin is less than 15.5%. Restart the fan to cool the corn to about 30 degrees F as soon as the weather gets cold enough.
  • After November 1, turn off the fan when corn moisture at the top of the bin is less than 17% and corn temperature is less than 30 degrees F. If the weather forecast calls for a period of warm weather, resume drying until average temperatures drop below 30 degrees F again.
  • After November 15, turn the fan off when corn moisture at the top of the bin is less than 18% and corn temperature is less than 30 degrees F.
  • After December 1, turn the fan off when corn moisture at the top of the bin is less than 19% and corn temperature is less than 25 degrees F.
  • After December 15, turn the fan off when corn temperature at the top of the bin drops below 25 degrees F, regardless of corn moisture.

People often refer to operating the fan at air temperatures lower than 32 degrees F as "freezing" corn and ask if that practice causes any problems. Although free water freezes at 32 degrees F, corn does not. Drying is quite slow at temperatures lower than 32 degrees F, but as long as there is no condensed water in the bin, running the fan at these temperatures does no harm.

If drying isn't completed in fall, and the drying zone is at least halfway through the bin, and corn moisture at the top of the bin is less than about 23%, drying can usually be completed in spring. If there is too much wet corn going into spring, or if the corn is wetter than 23%, spoilage is likely during spring drying. In these cases, feed or sell at least part of the wet corn, or dry it in another type of dryer before spring.

When should I resume drying in the spring?

Use the following information to determine how to manage natural-air dryers in spring:

Notice that the wetter the corn at the top of the bin is, the earlier you need to start drying. This is to make sure the drying front reaches the top of the bin before outdoor temperatures get high enough that the corn would mold. Also, corn at the bottom of the bin will be badly overdried (less than 13% moisture) if drying continues into late spring.

What if the corn is too wet at harvest to fill natural-air bins?

Some years, weather conditions are such that corn just doesn't dry in the field to safe levels for full-bin natural-air drying. Also, some producers have so many acres to harvest that they can't afford to wait until corn dries in the field to levels that are safe for natural-air drying. In both cases, layer filling or combination drying allows harvest at corn moistures greater than those recommended for full-bin drying.

Figure 5. Airflow produced by a typical 10-hp axial-flow fan on a 30-ft diameter bin for different depths of corn.

Layer filling is filling a natural-air bin slowly over a period of several weeks, instead of in a day or two. This can be done by filling on a regular schedule (for example, one quarter of the bin depth per week), or by putting in a few feet of grain at a time and waiting for the drying zone to reach the top layer before adding more grain. Regardless of filling schedule, make sure the top surface is level after each layer is added to the bin.

Layer filling works because airflow per bushel is much higher when a bin is only partly full. For one thing, the airflow provided by the fan serves fewer bushels when the bin isn't full. For another, fans, especially axial-flow fans, deliver much greater total airflow when grain depth is shallow. The fan in Figure 5 , for example, delivers about 1 cfm/bu when the bin contains 18 ft. of corn and almost 7 cfm/bu when the bin contains 4 ft. of corn. When airflow per bushel is high, drying is fast and reliable even for corn in the 24 to 26% moisture range.

Layer filling works best for producers who have several natural-air drying bins and can conveniently switch filling from one bin to another. It also works well for producers who are not in a big hurry to finish harvest.

Combination drying uses a gas-fired dryer to dry corn to 20 to 22% moisture before starting the natural-air drying process. Almost any kind of gas-fired dryer can be used for the first drying stage – including a bin-type dryer where the burner is simply switched off when corn moisture reaches 20 to 22%. The natural-air portion of combination drying is managed just like normal natural-air drying.

Advantages of combination drying include:

  • Producers can start harvesting corn at any moisture. This allows starting natural-air drying earlier in the season and makes natural-air drying possible in cool, wet falls.
  • Final grain quality is much better than that of grain dried completely in a gas-fired dryer.
  • Switching to combination drying greatly increases the capacity (bushels per hour) of the gas-fired dryer. If an existing gas-fired dryer can't keep up with the current harvest rate, switching to combination drying might eliminate this harvest bottleneck.

What about using grain preservatives or mold inhibitors in natural-air dryers?

Preservatives or mold inhibitors slow mold growth and allow more time for drying corn. Propionic acid is an example of a preservative that can be used on shelled corn. It is most commonly used to store wet corn temporarily for animal feeding in situations where drying is not feasible. It could also be used to reduce mold growth in natural-air dryers that have lower than recommended airflow. But use of propionic acid has some disadvantages. These include cost of application, corrosion of metal equipment, and the fact that treated corn can only be used for animal feed.

Anhydrous ammonia also inhibits mold growth on shelled corn. It can be used in a trickle ammonia process, where small amounts of ammonia are periodically injected into the drying air downstream from the fan (between the fan and the bin) on natural-air dryers. The ammonia slows mold growth and slightly increases the corn's protein content. Disadvantages of ammonia include corrosion of electrical components, handling of a potentially hazardous material, and treated corn that can only be fed to animals. Contact the Minnesota Extension for more information on the trickle ammonia process.

Other preservatives or mold inhibitors, either chemical or biological, might become available for use in natural-air corn dryers. Before using any of these products, though, make sure they have been approved for use on corn. Consider cost, safety, corrosion, and whether potential buyers will accept treated corn.

Is natural-air drying cheaper than heated-air drying?

Energy cost is what usually comes to mind when drying cost is mentioned. But energy is only part of drying cost; total cost includes labor, equipment (the dryer plus auxiliary holding bins and handling equipment), repairs, maintenance, taxes, and insurance.

Electrical energy use for natural-air drying depends on initial grain moisture, weather, airflow per bushel, and fan efficiency. For average weather conditions in the Upper Midwest, mid-October harvest, typical fan and motor efficiency, and about 16 ft of 20 to 22% moisture corn, average electrical energy use is about one kilowatt hour per bushel of corn dried (1 kWh/bu). Energy use is lower for earlier harvest, more efficient fans, shallower depths, or lower moisture. It is higher for later harvest, less efficient fans, deeper bins, or higher harvest moisture. Energy use can easily be 0.5 times the long-time average value in warm, dry years and 1.5 times the average value in cool, wet years. Electric heaters often draw as much power as the fan, so use of electric supplemental heat can easily double energy cost.

To calculate energy cost, multiply energy use (kWh/bu) times electricity cost per kilowatt-hour ($/kWh). To estimate how much it costs per day to operate a natural-air dryer, multiply the electrical demand of the fan in kilowatts (kW), times 24 hours per day, times the cost of electricity ($/kWh). As a rough approximation, fans draw about one kilowatt of electrical power per rated horsepower (1 kWh/hp).

  • $/day = fan hp x 1 kW/hp x 24 h/day x $/kWh

For comparison, gas-fired dryers use 0.015 to 0.025 gal of propane per bushel per percentage point of moisture removed. Gas-fired dryers also use some electrical energy per bushel, but this is small compared to the gas cost.

  • Gas cost for gas-fired dryer ($/bu) = points of moisture removed x gal/bu/point x gas cost ($/gal)

If grain is going to be stored on farm anyway, the only extra equipment costs for natural-air drying are a larger fan, drying floor, and perhaps a spreader and extra roof vents. Labor requirements during harvest are low, but the bins need to be checked every day or two while the fan is operating. Equipment for gas-fired drying includes the dryer and perhaps extra holding bins and handling equipment. Labor requirements can be high during drying, but once grain is dried, storage bins only need to be checked every week or two. So which drying method is cheaper? Because the answer depends on local gas and electricity costs, equipment costs, storage needs, and availability of farm labor, producers need to do cost calculations for their own situations.

Are there ways to reduce natural-air drying costs?

Here are some possible ways to reduce costs:

Are there safety hazards involved in natural-air grain drying?

Yes. Dangers include falls while climbing bins, suffocation in grain, and breathing mold spores.

Management of natural-air dryers includes frequent (every day or two) climbs to the top of the bin to inspect grain. Install safety cages around ladders and guardrails around the opening into the bin. To make climbing safer and easier, consider installing stairs instead of ladders on bins.

Every year, a number of people die from suffocation in grain bins. This happens when they are pulled under flowing grain, when steep piles of moldy, caked grain collapse on them, or when they fall through bridges of moldy grain that sometimes remain at the top of partially emptied bins. To avoid these hazards, stay out of bins when grain unloading equipment is operating, and use long poles to knock down grain piles or bridges from a safe distance.

Mold spores can cause short- and long-term health problems. To work safely around moldy grain, wear a dust mask or respirator that is capable of filtering mold spores. Single-strap, disposable dust masks will not filter mold spores; at a minimum, tight-fitting, two-strap masks are necessary to be safe.

Get a copy of Safe Storage and Handling of Grain, FO-0830, from the Minnesota Extension for more complete safety recommendations.

What are the advantages of natural-air drying?

Compared with higher-temperature, gas-fired drying methods, natural-air drying:

Are there any disadvantages?

Natural-air drying has disadvantages that limit its usefulness for some producers.


Airflow resistance: pressure required to force air through grain; usually measured in inches of water.

Allowable storage time: amount of time that grain can be stored at a specified temperature and moisture content before there is enough mold growth to reduce the grain's value.

Axial-flow fan: a crop drying fan that has the motor and a multibladed impeller inside a barrel-shaped housing. Air flows over the motor and through the impeller in line with the motor shaft. These fans are very noisy, but they are usually the most efficient type for natural-air corn drying.

Btu: British thermal unit; a unit of energy.

Bu: bushel. Bushels can be calculated using volume or weight. Using the volume definition, shelled grains occupy 1.25 cubic feet. Using the weight definition, one bushel of shelled corn weighs 56 lb at 15.5% moisture (wet basis).

Centrifugal fan: a type of fan that has a wheel-type (sometimes called squirrel cage) impeller. Air enters the side of the wheel and then turns 90 degrees. The motor is usually outside the air stream. These fans are quiet and work well at high static pressures.

Cfm: cubic feet of air per minute; total amount of air flowing through a bin of grain.

Cfm/bu: cubic feet of air per minute per bushel of grain in the bin; calculated by dividing total airflow by bushels.

Combination drying: using a gas-fired dryer to quickly dry wet corn to a moisture content that is safe for natural-air drying.

Equilibrium moisture content: moisture content that grain reaches if exposed to air of constant temperature and relative humidity for a long period of time.

Exhaust vent: screened opening that allows air to exhaust from the top of grain bins. Enough vents are installed to provide a total of 1 sq ft exhaust area per 1000 cfm.

Fines: small pieces of broken grain, chaff, dirt, and weed seeds that are mixed with whole grain.

Fungi: scientific name for a group of chlorophyll-free plants that includes grain storage molds.

Grain spreader: a gravity- or motor-powered device mounted in grain bins just until the fill hatch; purpose is to prevent formation of cone-shaped grain piles and the concentration of fines that normally develops in the center of cone-shaped piles.

Grain stirrers: devices that vertically mix grain in bins. They consist of one or more vertical, bare screws mounted on a horizontal arm that moves along a track attached just below the top of the bin wall. The rotating vertical screws pull grain up from the bin floor as they move around the bin.

Hp: horsepower; used to indicate the output power of fan motors.

In-line centrifugal: a fan that uses a wheel-shaped (sometimes called squirrel cage) impeller inside a barrel-shaped housing; for a given horsepower, performance is usually between that of axial-flow and ordinary centrifugal fans.

In. water: inches of water; unit used to measure pressure developed by fans, or the amount of pressure needed to force air through grain.

kW: kilowatt; unit of electrical power; rate of use of electrical energy.

kWh: kilowatt-hour; unit of electrical energy.

kWh/bu: electrical energy used to dry a bushel of grain.

Layer filling: a natural-air drying method where the bin is filled slowly over a period of several weeks. This speeds drying and allows drying of wetter corn because airflow per bushel is much greater when the bin is only partly full.

Off-peak rates: reduced electrical rates offered in an attempt to shift electricity use from busy times of the day when power suppliers experience peak demand on their system to times when there is less demand on the system.

Pneumatic conveyors: systems that use air pressure to move grain; such systems usually include a high-pressure blower, air lock, cyclone separator, and a number of pipes or tubes.

Probability of success: This is the number of years out of 100 that you could expect successful drying (no spoilage or need to move grain to prevent spoilage) for a given grain moisture and airflow per bushel.

Static pressure: in grain systems, the pressure measured at the wall of a duct or plenum, or perpendicular to a moving stream of air. Static pressure indicates airflow resistance of grain.

Temperature cables: cables that hang from bin roof to floor and are used to measure temperature of stored grain; consist of steel cables that have electronic temperature sensors attached every few feet along their length and electronic read-out boxes that can be plugged into the cable to read temperature of each sensor.

Wet basis (moisture content): moisture content calculated by dividing weight of water in a sample by the total, or wet weight, of the sample. Grain moisture is usually expressed on a wet basis.

The authors, William F. Wilcke and R. Vance Morey , are associate professor and extension engineer, and professor and head of the University of Minnesota's Department of Biosystems and Agricultural Engineering, College of Agricultural, Food, and Environmental Sciences.

Thanks to Extension Plant Pathologist Richard Meronuck, Extension Engineers Fred Bergsrud and John Shutske, and Meeker County Extension Educator David Schwartz, for reviewing this publication, and Diedre Nagy, John Molstad, and Michael White of the Educational Development System for producing it.

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