Sukup Manufacturing is dedicated to producing top quality grain drying and grain handling equipment to make farming operations more efficient. One of the most important factors in any grain drying system is a basic understanding of grain drying principles. The information contained on this page will explain the basics of grain drying and grain storage.

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Principles of Drying Corn

Any kernel of grain is constantly trying to equalize with the moisture content of the surrounding air. Table 1 shows the relationship of corn at various temperature and relative humidity situations. Similar charts showing this relationship are available for wheat, sorghum, soybeans and rice. Contact your local extension office for information on grains not listed here.

Relative Humidity, Percent

 
10
20
30
40
50
60
70
80
90
F

Equilibrium Moisture Content, Percent

20
9.4 11.1 12.4 13.6 14.8 16.1 17.6 19.4 22.2
25
9.8 10.5 11.9 13.1 14.3 15.6 17.1 19.0 21.8
30
8.3 10.1 11.4 12.7 13.9 15.2 16.7 18.6 21.1
35
7.9 9.6 11.0 12.3 13.5 14.8 16.3 18.2 20.8
40
7.4 9.2 10.6 11.9 13.1 14.5 16.0 17.9 20.5
45
7.1 8.8 10.2 11.5 12.8 14.1 15.7 17.6 20.5
50
6.7 8.5 9.9 11.2 12.5 13.8 15.4 17.3 20.2
55
6.3 8.2 9.6 10.9 12.2 13.5 15.1 17.0 20.0
60
6.0 7.9 9.3 10.6 11.9 13.3 14.8 16.8 19.7
65
5.7 7.6 9.0 10.3 11.6 13.0 14.6 16.5 19.5
70
5.4 7.3 8.7 10.0 11.4 12.7 14.3 16.3 19.3
75
5.1 7.0 8.5 9.8 11.1 12.5 14.1 16.1 19.1
80
4.9 6.7 8.2 9.6 10.9 12.3 13.9 15.9 18.9
85
4.6 6.5 8.0 9.3 10.7 12.1 13.7 15.7 18.7
90
4.4 6.3 7.7 9.1 10.4 11.9 13.5 15.5 18.5
95
4.1 6.0 7.5 8.9 10.2 11.7 13.3 15.3 18.4
100
3.9 5.8 7.3 8.7 10.0 11.5 13.1 15.1 18.2

Table 1 Equilibrium Moisture Content for Corn

As you can see, under certain conditions, no matter how long the fan is operated, the grain may not reach the desired moisture content that will allow it to be stored without spoilage. In addition, at certain other conditions, the grain will be too dry and the producer has lost money two ways: (1) An excessive amount of energy has been used to dry the grain. (2) The crop is worth less because of shrinkage. Keep in mind, the air temperature and relative humidity are not constant; however, the average during the drying period determines the final moisture content.

Air can hold more moisture when it is warm than when it is cool. The amount of moisture in the air as opposed to the amount it could hold if it were fully saturated is referred to as a percentage of "Relative Humidity". As a rule of thumb, heating air 20 F (11 C), reduces the relative humidity by one-half. Table 2 illustrates how heating air decreases drying time and is expressed by the "Drying Ratio" column.

For Every 20 F Heat Rise, Relative Humidity Is Cut By 1/2
Outside Air Heated Air To RH Dries Grain To Drying Ratio
70 F
60% RH
No Heat 60% 13% 1.0
70 F
60% RH
90 F 31% 8% 2.6
70 F
60% RH
110 F 17% 5% 4.3

Table 2 Drying Basics

Unfortunately, 8% or 5% moisture content grains are not desirable. By incorporating a stirring machine to mix the dry grain at the bottom of the bin with the upper wet grain, a desired average moisture may be obtained. In fact, Iowa State University tests show that a properly stirred bin will have less than 1% variation of moisture content from the top to the bottom.

Grain Storage

More grain is lost because of improper storage than for any other reason. The most common problems are as follows:

  1. Mold and insects in improperly dried, damaged or improperly managed grain.
  2. Pockets of fines (broken kernels, weed seeds and trash) restrict airflow and provide food for insects and mold.
  3. Grain began to deteriorate because it was held too long without adequate aeration prior to drying.
  4. Improper cooling of grain after drying.
  5. Inadequate observation of grain during storage.
  6. Poor initial grain quality or not dried to a safe moisture content.
  7. Improper or lack of insect control.

Allowable Storage Time for Shelled Corn

 

Corn Moisture, Percent

Grain

Temp F

18
20
22
24
26
28
30
Days
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

Table 3 Allowable Storage Time for Shelled Corn with Aeration

Holding Wet Grain

Grain will begin to deteriorate depending upon temperature and moisture content. Using corn as an example, Table 3 illustrates how fast grain can spoil even with proper aeration.

Table 4 shows the recommended aeration when storing grain in a wet holding tank at various moisture contents.

Moisture Content

CFM/BU

Corn
Soybeans
Wheat
Rice
Airflow
14% 10-11% 12-13% 10% 1/10-1/8
15-17% 12-13% 14-15% 11-12% 1/7-1/5
18-20% 14%-Max 16-17% 13-14% 1/4-1/2

Table 4 Wet Holding Tank Airflow Requirements

To ensure grain is dried to the point that it is considered safe to store, use the percentages shown in Table 5.

Grain Max. Safe Moisture Content
Shelled Corn & Sorghum  
  To be sold as #2 grain or equivalent by spring 15 1/2%
  To be stored up to 1 year 14%
  To be stored more than 1 year 13%
Soybeans  
  To be sold by spring 14%
To be stored up to 1 year 12%
Wheat 13%
Small Grains (oats, barley, etc.) 13%
Sunflowers  
  To be stored up to 6 months 10%
  To be stored up to 1 year 8%
Rice 12 1/2%

Table 5 Safe Storage Moisture Control of Grain

Aeration

Cooling Grain for Winter Storage

Begin cooling grain whenever it is first placed in the bin. Also, whenever the average day-night temperature is 10 - 15 F cooler than the grain, begin aeration until the cooling front moves completely through the grain. Repeat this cycle as often as necessary until the grain is cooled to 35 to 45 F. Be sure to continue each aeration cycle until the cooling front has moved completely through the grain. This minimizes the chance for a moisture front within the grain mass to cause spoilage. Table 6 shows the length of time required to change grain temperature; however, taking the temperature of the grain is the only way to ensure the cycle is completely through the grain.

Airflow Rate

CFM/BU

Fall Cooling Hours

Winter Cooling Hours

Spring Cooling Hours

1/20 300 400 240
1/10 150 200 120
1/5 75 100 60
1/4 60 80 48
1/3 45 61 36
1/2 30 40 24
3/4 20 27 16
1 15 20 12
1 1/4 12 16 10
1 1/2 10 13 8

Table 6 Approximate Grain Cooling or Warming Times

Managing Grain In The Spring

Start the fan when the average outdoor temperature is 10 - 15 F above the grain temperature. Once the warm-up cycle is started, do not turn the fan off. Stopping the front before the cycle is completed generally encourages condensation of moisture and spoilage. As outside temperatures continue to warm, repeat this cycle as often as needed until the average grain temperature is 50 - 60 F.

Summer Management of Grain

Do not warm the grain to high summer temperatures (80 - 90 F). Be sure grain moisture content is within the limits shown in Table 5.

Fan Selection

Axial fans are the least expensive and provide excellent airflows at low static pressures (up to 3" of water). The 1750 RPM centrifugal fans are best for medium to high static pressures or if noise is a factor. When aeration is required for tall bins or small grains which create high static pressures, 3500 RPM fans are recommended. In-Line centrifugal fans are used to dry small grains such as canola or for high static pressure aeration.

Airflow Requirements

The use of Sukup Software with a personal computer is recommended to ensure the proper fan is selected for the application. The software is available for Windows 95, Windows 98 or WindowsNT operating systems. Below are some general guidelines for airflow requirements.

Aeration (storage) - 1/20 to 1/5 CFM/Bu (usually 1/10 is used)
Cooling - 12 CFM/Bu/Hr
Dryeration - 1/2 to 1 CFM/Bu (usually 1/2)
Wet Holding Tank - 1/4 to 1/2 CFM/Bu
Drying:    Natural Air - 1 to 3 CFM/Bu
              Low Temp - 1 to 3 CFM/Bu
              High Temp - 1 1/2 to 5 CFM/Bu
              Roof Dryer - 12 to 22 CFM/Bu

Roof Vents

Without adequate opening area to let air and moisture out of the bin, the aeration or drying system will not work efficiently. Provide one square foot for every 1000 CFM the fan can produce.

Type of Heater and Controls

Major considerations in heater selection are:

  1. Temperature rise required
  2. Type of fuel
  3. Upstream vs. downstream
  4. Controls

Temperature Rise Required - Temperature rise is defined as the difference between ambient (surrounding air) temperature and plenum temperature. Electric heaters are rated in KW/Hr (Kilowatt Hours) and LP-gas/natural gas in BTU/Hr (British Thermal Unit Hours). With these ratings, temperature rise can be calculated by using the following. Electric: KW/Hr x 3000 divided by CFM equals temperature rise. LP/NG: 0.93 x BTU/Hr divided by CFM equals temperature rise.

Very little temperature rise is desired for rice or soybeans whereas corn is often dried with quite high temperatures.

AS A RULE OF THUMB, IF A STIRRING MACHINE OR CONTINUOUS COUNTER-FLOW SYSTEM IS NOT USED, NO MORE THAN 10 DEGREES F TEMPERATURE RISE SHOULD BE APPLIED TO AVOID OVERDRYING GRAIN, ESPECIALLY AT THE BOTTOM OF THE BIN. IMPORTANT: TO MAINTAIN GRAIN QUALITY AND AVOID RISK OF BIN FIRE, MAXIMUM DRYING TEMPERATURE MUST NOT EXCEED 130F FOR IN-BIN, STIR-DRYING SYSTEMS OR 180F FOR IN-BIN CONTINUOUS FLOW SYSTEMS. 

When using LP-gas as a fuel, either liquid or vapor may be used. Generally, a heater must include a vaporizer for liquid fuel when ambient temperature is below 32F and one million BTU/Hr is required.

Type of Fuel - Electric heaters provide 2 to 10 degrees of temperature rise. Usually, use of LP-gas or natural gas burners provides greater flexibility at a lower operating cost.

Use of either LP-gas or natural gas is based on availability, as burners for both fuels can be sized to provide the heat required. When using natural gas, contact your gas company to determine if adequate line pressure is available for your operation.

Upstream vs. Downstream - These terms refer to the placement of the heater in the fan airstream. By placing a heater downstream, between the fan and the transition, air goes through the fan and then is heated. Because air expands as it is heated, additional drying capacity is obtained.

Normally, all axial fan heaters are placed downstream. Air straightening vanes must be included in the axial fan or heater to provide proper burner operation and even heat distribution in plenum of bin.

Centrifugal fans may be equipped with either upstream or downstream heaters. With low temperature heaters, either location may be used. For high temperature operation, downstream is preferred to provide greater drying capacity. Sukup and Chicago downstream heaters are equipped with adjustable air deflecting devices so that heat distribution in plenum can be altered if necessary.

Controls - Heater efficiency and cost of operation can be improved through proper selection of controls. Following is a list of types of controls that are commonly available, along with principles of operation.

Thermostat-This unit cycles the heater competely on or off to maintain plenum temperature. It is the least expensive control along with being the least efficient. When the burner is shut off, the fan is blowing air at the outside temperature into the plenum, cooling it off. The on and off action provides an average temperature corresponding to the setting, but with extreme high and low temperatures at the point the thermostat cycles the burner on or off.

High-Low Burner-The high-low burner control cycles the burner from a high setting to a 20 - 30F lower setting to maintain a constant, predetermined plenum temperature. This eliminates the extremes in plenum temperatures and provides economy of operation. Use the High-Low Control with a dual burner control when more than one fan and heater are used on the bin.

Modulating Valve-The modulating valve provides the most precise temperature control by regulating the burner flame to the exact size to maintain a constant temperature. The modulating valve works through a capillary tube filled with gas that expands and contracts with changes in plenum temperature. This gas moves a diaphragm controlling LP-gas or natural gas pressure to the burner.

Humidistat-The humidistat is used with a low temperature burner and is located in the plenum of the bin. This cycles the burner on and off to control the humidity of the drying air.

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