Cooperative Extension Service · The University of Georgia College of Agriculture & Environmental Sciences
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by

Paul E. Sumner and E. Jay Williams

Extension Engineers

PRINCIPLES OF GRAIN DRYING

Drying is one of the oldest methods of preserving food and feedstock. It is simply the removal of moisture from a product, usually by forcing dry air through the material.

Air serves two basic functions in grain drying. First, the air supplies the necessary heat for moisture evaporation; second, the air serves as a carrier of the evaporated moisture. Both functions are essential, regardless of the type drier you use. The amount of moisture which can be removed from grain depends on the moisture content of the grain, and the relative humidity and temperature of the drying air.

Air temperature determines to a large extent the total water-carrying capacity of the drying air. Hot air can hold more moisture than cold air. For example, a pound of air at 40°F can hold only 40 grains of moisture (7000 grains = 1 pound) while a pound of 80 o F air can hold 155 grains - almost a four fold increase.

The temperature of the drying air also affects the dried grain quality. Grain to be fed or milled can be dried at 150°F or higher, while grain for seed should not be heated above 110°F or reduced germination occurs. High heat often cracks the seed coat leading to grain breakage in handling.

Relative humidity also plays an important part in the drying process. Air at 100°F and 50 percent relative humidity can absorb 60 more grains of moisture per pound of air than it can at 75 percent humidity.

When grain is placed in a drier and air is forced through the grain, a drying zone is established at the point where the air enters the facility (Figure 1). The drying zone moves uniformly through the grain in the direction of air flow at a rate depending on the volume, temperature and relative humidity of the air and the moisture content of the grain.

Figure 1 . Grain is dried from the point of air entry with the drying front moving in the direction of air flow. The wetter grain occurs where the air leaves the grain layer.

Forcing air through deep layers of grain is more difficult and requires more fan capacity and horsepower than forcing air through thin layers of grain. The pressure built up by the fan due to the resistance of air flow through grain is called static pressure and is normally measured in inches of water. The pressure increases as grain depth and air flow rate increases. Grain such as wheat or grain sorghum has less void space than corn. Less void space for air to move through requires more static pressure.

Drying Methods

Layer-in-Bin Drying

This method is frequently called layer drying; it involves drying deep layers of grain (3-6 feet deep) with low heat, usually less than 110°F. When the first layer is dry, another layer is added on top of the first dry layer and the next layer is dried. When the second layer is dry, a third layer is added and dried. This process is continued until the bin is filled and dried. The filling process may require up to two weeks with up to three weeks required for the drying process. Fan requirements: medium (25 CFM/sq. ft. @ 2 inches static pressure). Heat requirements: low (90 – 110°F).

Batch-in-Bin Drying

In this method a two to four foot layer of grain is placed in a drying bin. The layer (batch) is rapidly dried then cooled and removed. A new batch is then placed in the bin and the process repeated. Fan requirements: medium to high (40 CFM/sq. ft. @ 3 inches static pressure). Heat requirements: medium (120 – 140°F.).

Batch Drying

Batch drying involves special drying equipment which holds a relatively thin layer of grain (1-2 feet).

Some models recirculate the grain during drying for uniform moisture removal. Grain is normally dried, cooled and then removed. Fan requirements: very high (50 - 100 CFM/sq. ft.). Heat requirements: medium high (160 - 180°F).

Continuous Flow Drying

A thin layer of grain ( 2 / 3 - 1 ½ ft.) moves continuously through the drier; first through a drying section then through a cooling section. Continuous loading and unloading is required. Fan requirements: very high (75 - 125 CFM/sq. ft.). Heat requirements: very high (180 - 200°F).

Low Temperature Drying

This method is similar to layer-in-bin drying but with less heat and extended drying time. The heater provides only 5 – 7° heat rise. Drying time is normally 35 - 40 days depending upon air flow rate. The extended drying time limits this method to cold climates which prevents spoilage during drying. The bin is normally filled to capacity in only a few days. This is not recommended for Georgia conditions.

Moisture Levels For Safe Storage

Crops which are to be sold or used for seed should be dried to a safe storage level. The moisture content recommended for safe storage of various crops is shown below. This table assumes storage through the warm months with aeration to cool the grain during fall and winter to prevent moisture migration.

Seed crops should be dried at temperatures at or below 110 o F to prevent seed damage and reduction of germination.

Grain which is to be stored a short time and then marketed can be stored at higher moisture levels. The safe storage level is dependent upon moisture content and temperature as shown in Table 2. The time interval in days indicates the time required for the corn to drop one grade.

Storage time exceeding those given in Table 2 will lead to loss in corn quality and will result in a lowering of grade. It should not be inferred that corn within these limits will suffer no loss in quality.

Other grains are similar to corn in storage time. For example, corn held at 24 percent moisture and 80°F can be stored only four days before deteriorating to the next lower grade. If this 24 percent corn is held for two of the four days at these conditions, 50 percent of the allowed storage time will be consumed even if the crop is then dried and cooled.

The relationship between moisture and temperature as it affects the storage life of soybeans is shown in Figure 2.

Figure 2 . Storage time with respect to moisture and temperature for short term soybean storage.

Equilibrium Moisture Content

Grain can be dried in many areas (except along the coast) of our state using natural air if the drying layer is limited to three to four feet and a sufficient volume of air with the proper relative humidity and temperature is circulated through the grain. If, for example, corn is to be dried to 12 percent moisture, air must be circulated which will remove moisture from the corn rather than adding moisture. When the air circulating through the corn neither absorbs moisture nor adds moisture, the air and corn are said to be at the equilibrium moisture content. Table 3 shows 12 percent moisture corn to be in equilibrium with air at 50°F and 50 percent humidity. If the humidity increases to 60 percent and the air temperature remains at 50°F, it is not possible to dry the corn below 13.3 percent. If the relative humidity dropped below 50 percent and remained at 50°F, drying to 12 percent or below would be possible.

A small amount of heat raises the drying air temperature and reduces the humidity which increases the drying capability of the air. A 20°F temperature rise reduces the relative humidity by 50 percent. For example, air at 60°F and 70 percent relative humidity heated to 80°F. (20°F temperature rise) reduces the relative humidity to 35 percent (50 percent of the 70 percent). With shelled corn, the original air (60°F and 70 percent humidity) would reach equilibrium at 14.2 percent while the 80 o F and 35 percent relative humidity would reach equilibrium at 8.3 percent. This would result in more drying capability (Table 3). If the air were heated 10°F (one half the 20°F above), the relative humidity would drop only 25 percent or one half the above value to about 50 percent.

Equilibrium moisture content of soybeans is given in Table 4.

Increasing air temperature increases the drying capability often making drying possible when it is not possible to dry grain with natural air.

Field drying can be costly in terms of field losses and weather hazard, but it is an alternative, and fuel savings are possible if one is willing to take the risk.

Air Temperature

Drying temperatures should be kept below 110°F for seed, 140°F for market corn, and 200°F for feed. Temperatures higher than 110°F reduce germination while exceeding 140°F for market corn makes it difficult to separate the constituents of the corn such as sucrose and starch. Temperatures exceeding 200°F, also causes stress in the grain kernels, a condition which produces cracking and splitting and increases damage potential from insects.

Grain should be cooled after drying because high temperatures can spoil the grain. To cool, the heater should be shut off and the fan allowed to operate until the grain is cooled to within 10°F of outside air. Some drying will occur during cooling and should be included in the desired drying time.

Higher air temperatures produce greater spread in moisture between top and bottom grain layers in bins. No more heat should be used in bin driers than necessary. The heat should be adjusted to dry the batch in the available time before spoilage. Stirrers are sometimes used in bins to allow uniform drying from bottom to top and allow the use of higher temperatures without excessively drying the bottom layer. Moving the grain after drying also aids in evenly spreading the moisture throughout the grain.

Bin Batch Drying

In the bin batch drying system, grain is dried and cooled in a layer usually less than 4 feet deep before being transferred into final storage. In operation, the fan and heater are turned on as soon as the floor of the drying bin is covered with grain. Additional grain is added and leveled throughout the day as harvesting progresses while the drying continues, usually in a 24 hour cycle.

Drying starts at the bottom of the grain bin where the drying air enters the grain (Figure 1). As the flow of air continues, drying progresses upward in the direction of air travel which is from the bottom upward. As the drying air enters the grain, the air picks up moisture from the bottom layer and the air comes into equilibrium with the grain above this layer without picking up additional moisture from the layers above. Thus, a drying zone moves up through the grain from the bottom upward. The rate at which this drying zone moves upward is dependent upon the moisture content of the grain, the condition (temperature and humidity) of the air and volume of drying air. The greater the air flow, the faster drying progresses.

The depth of material to be dried in a batch-in-bin system should not exceed 4 feet and a minimum air flow of 6 cubic feet of air per minute per bushel of grain to be dried should be maintained. Static air pressure will range from 1 to 1 ½ inches of water when drying shelled corn or beans and 1 ½ to 2 ½ inches when drying sorghum and small grains at a depth of 4 feet. Table 5 gives the capacity of different diameter bins for each foot of depth.

The grain should be leveled after each load is placed in the drying bin for uniform drying. Grain distributors available for leveling grain will save considerable labor as well as distribute fine materials more evenly. Fine material should be screened out as soon as possible since these materials encourage spoilage and slow drying by restricting air flow and can cause channeling if not uniformly distributed in the grain.

Match Drier To Harvesting Rate

In selecting drying equipment with capacity to meet the harvest rate, one must know the amount of moisture to be removed, the time allowed for drying, the volume of grain and type of grain, temperature allowable, air flow rate, heat required, fan motor size and the size of the drier or drying bin.

As an example, assume 2700 bushels of soybeans at 20 percent moisture are to be dried to 12 percent in 20 hours in the fall of the year using a batch-in-bin drier with a maximum soybean depth of 4 feet and 110°F drying air. What size bin is required? Note that in Table 5, a bin 33 feet in diameter will hold 685 bushels per foot of depth or 2740 bushels when 4 feet deep which is the bin size necessary in this example.

The weight of water per bushel of grain is shown in Table 6. To obtain the water loss of a bushel of grain or soybeans drying from 20 to 12 percent, take the difference in weight of these moisture contents.

In the soybean example, the difference in weight of beans at 20 to 12 percent (Table 6) is (12.9 - 7.0) = 5.9 pounds of water per bushel to be removed. The total moisture removed per hour must be the total moisture removed (2700 bushels X 5.9 pounds per bushel in the example) divided by the time in hours which is 20, in this case, yielding about 800 pounds per hour.

Air Volume Required For Moisture Removal

The amount of moisture removed by the drying air at various drying temperatures and humidity is given in Table 7.

Air flowing at a rate of 1000 CFM and heated from an average design temperature of 60°F and 65 percent humidity heated to 110°F will remove 38 pounds of moisture per hour (Table 7). The air flow rate required to dry a given quantity of grain is given by the expression below where:

CFM = capacity of drying fan in cubic feet of air per minute

Q = pounds of moisture to be removed from wet grain in one hour

H = pounds of moisture removed each hour by 1000 CFM of drying air, Table 7.

E = efficiency of drying air in removing moisture (0.75 for average fall condition)

The efficiency of drying depends upon the efficiency of heat utilization which drops as the outside temperature drops. For average design conditions of 60°F and 65 percent relative humidity, the drying efficiency can be assumed to be 0.75 and would be typical for fall conditions in Georgia . If harvest is delayed into cold weather, the efficiency could go to 0.6. In summer the drying efficiency may be as high as 0.85.

In the soybean example problem, Q = 800 pounds per hour as discussed earlier, E = 0.75, and H is 38 pounds (Table 7) when the drying air is 110 o F. So the air volume needed in this example is:

The bin has 882 square feet of floor area (Table 5, 33 feet diameter bin) or 32 CFM of air per square foot of floor (28,070/882 = 32).

Fan Capability and Horsepower

Pressure is required to force air through grain. This pressure is normally expressed as static water pressure. The term 'static' comes from the fact that the pressure is measured perpendicular to the direction of air flow and gets none of the dynamic or velocity effect from moving air. The static pressure developed by air flowing through grain or beans can be determined if the air flow in CFM per square foot of drier surface (or bin floor) and grain depth is known. Figure 3 shows the relationship between air flow and static pressure in inches of water for beans and common grains.

Figure 3. Resistance of grains to air flow, as shown by pressure drop in inches of water per foot of grain (if fines are present in the crop, multiply this data by 1.5).

In the soybean example, 32 CFM of air was required for each square foot of floor and the soybean depth was 4 feet. From Figure 3, note static pressure to be 0.25 inch of water per foot of soybean depth if the 32 CFM per square foot is forced through soybeans. This gives a static pressure of 1 inch for the 4 feet depth.

Fans must be sized based upon volume of air delivery and static pressure. Fan motors must be sized on the same criteria as fans. The expression below gives the fan motor horsepower required if it is assumed that most fans are about 50 percent efficient in moving air. The constant 3,178 is a conversion factor.

In the example, the total air flow was 28,070 CFM and the total pressure is 1 inch of water.

Heat Required

The heat required to raise the temperature of the drying air depends upon the volume of air and the temperature rise as given by the expression:

The temperature rise is the drying air temperature minus the initial design condition which is 60°F (see footnote under Table 7).

In the example, the total heat required to heat the 28,070 CFM of air from 60 to 110°F is given as follows:

Natural Air Drying

Many small farmers prefer to dry crops using unheated or natural air drying in bins in layers 3 to 6 feet deep. This process works for crops such as soybeans which are not prone to have aflatoxin buildup and when good management practices are followed. However, it is desirable to have heat available on standby which will allow safe drying .in any weather. Significant aflatoxin buildup can occur in 48 hours in grain if the air leaving the grain is between 55 and 105°F and relative humidity is over 85 percent.

Table 8 gives the maximum quantities of grain that can be dried per batch per fan horsepower for minimum air flow rates and maximum depths using natural air under favorable conditions.

The air flow rate in Table 8 is the minimum flow rate with grain depth being the maximum for clean grain with little or no fines using unheated air. (Heat should be available on standby). The static pressure on the fan is given in inches of water.

The approximate drying time for grains and seeds at different initial moisture contents and seasons (in Georgia ) is shown in Table 9 for unheated air. Notice drying with natural air is quite slow. For this reason, many farmers will not find natural air drying acceptable since faster methods will usually be needed. Drying rate can be increased by adding heat (limited to 110°F. for seed) or increasing air flow rate, or both. Slow drying is not desirable to maintain grain quality and reduce risk from aflatoxin (See Aflatoxin section).

Adding low levels of heat (10 – 15 o F) allows faster drying and the drying process becomes less sensitive to the weather. A good method for controlling the supplemental heat is to install a humidistat in the blower discharge air stream or near the top of the storage bin. The humidistat, set at the desired humidity level, provides almost perfect control if three conditions are met. First, the heating unit should have a modulating valve so heat output changes will be gradual. Second, the humidistat must be able to operate in a dusty environment. Third, reaction time of the humidistat and heater should be such that there is little if any overrun of the heater.

Aflatoxin

The mold aspergillus flavus grows on most grains, particularly corn, in a temperature range of 55 to 105°F. in an environment of high humidity (85 percent and higher). The mold is present throughout the southeast and is commonly found on grain at harvest.

To minimize the potential for aflatoxin buildup, harvest early and dry grain which is greater than 17 percent moisture within 48 hours or less. Also, store only clean dry grain keeping seed coat damage to a minimum and aerate.

For further information on aflatoxin, see Extension Bulletin 1231 entitled “Reducing Aflatoxin in Corn During Harvest and Storage.”

Shrinkage

  Over-drying grain results in loss of weight (water) that could have been sold since grain is usually sold on a weight basis. If the initial weight, initial moisture