Many IP varieties should not be artificially dried, especially with heat. Producers should contact the buyer concerning acceptable moisture levels and possible drying of IP soybeans.
Grain Dryers
The three basic general types of grain dryers used on farm are:
- In-bin.
- Batch.
- Continuous flow.
No single drying system is superior to others in every respect. System selection is dependent on desired features. These features include:
- Drying capacity.
- Grain quality.
- Fuel/drying efficiency (KJ/kg or BTU/lb of water removed).
- Convenience and labour required.
- Ability to dry a variety of crops.
- Maintenance required and capital cost.
Dryers move heated air past the grain which evaporates moisture from the grain and carries the water vapour away. Heat is added to the air to reduce its relative humidity which improves the air’s capacity to hold moisture. Wet grain can be dried at higher temperatures since it will be cooled as the moisture evaporates from the seed. As the grain dries, it will approach the temperature of the drying air. The longer the grain is in contact with the heated air, the drier and hotter the seed will get.
Drying Soybeans with Heated and Unheated Air
Sometimes, because of rainy weather, soybeans are harvested at a higher moisture content, or they are harvested earlier to minimize anticipated combine losses. All drying methods are adaptable to soybeans with some restrictions on the use of heat and handling practices.
Caution is required when using heated air to dry soybeans. To prevent seed coats from splitting, the drying air’s relative humidity must be kept above 40%. Experience has shown it is possible to cause 100% of the soybeans to crack with just a few minutes exposure to high heat. To dry commercial soybeans, a maximum temperature of 60ºC is suggested. In good drying weather, this drying temperature may need to be reduced to control seed coat cracking. To gauge the drying effect, check the number of split seeds before and after drying.
Seed soybeans should be dried at temperatures below 40ºC, although, some seed companies disallow of the use of any heat in drying seed soybeans. Enquire with your seed company what kind of conditioning it accepts or recommends for seed beans.
With bin dryers, use caution with any systems that involves moving the soybeans in the bin with re-circulators or stirring devices. Damage from handling can be severe, especially as the moisture content drops below 12%.
Natural-Air Drying
If soybeans are above storage moisture, natural air can be used to dry them in good weather conditions. Natural air drying requires careful management by the operator, since soybeans absorb and lose moisture easily. The fan must only run when the outside conditions aid in the drying progress. If the fan is left on throughout the night, re-wetting will occur, reversing any progress made during the day.
Minimum requirements for natural-air drying soybeans are:
- Full aeration floor in the bin.
- Level soybean surface across the whole bin.
- Minimum airflow of more than 6.5 L/sec/m3 (0.5 CFM/bu).
- Clean beans with no pods or fines accumulation.
- Accurate moisture reading of the beans in the bin.
- Accurate outside air temperature and relative humidity measurement.
To move air uniformly throughout the bin’s content, a full aeration floor is required. Spoilage problems can occur with a partial aeration floor, air duct system, or dead areas. Airflow can be restricted or diverted by bean pods, trash, and fine accumulations. Air moving through the bean mass will take the path of least resistance.
Determining Airflow
To move drying air through the whole bean mass, sufficient airflow is required. To remove moisture, the minimum airflow required is 6.5 L/sec/m3 (0.5 CFM/bu). Anything less will only change the temperature, but not the moisture content of soybeans. Airflow rates of 26 L/sec/m3 (2 CFM/bu) or higher, only gets the job done more quickly. To determine the (L/sec/m3 or CFM/bu) value for a bin, determine the number of bushels in the bin and the static pressure that the fan is operating against. A manometer can be used to measure the static pressure in the air plenum, between the perforated floor and the concrete pad under a grain bin. The static pressure is displayed in centimeters or inches of water column. To determine the fan output at the measured static pressure by using the fan performance curve. Divide the L/sec/m3 (CFM) output of the fan by the number of bushels in the bin to give the L/sec/m3 (CFM/bu) airflow. One strategy to get adequate airflow is to only fill the bin partially. This way, the fan will be operating at less static pressure and deliver higher airflow rates per bushel.
Equilibrium Moisture Content
To help predict the final moisture content of soybeans when exposed to air at a specific temperature and relative humidity, equilibrium moisture content tables have been created. See Table 1, Equilibrium Moisture Content (% Wet Basis) for Soybeans Exposed to Air. For example, to determine the equilibrium content of soybeans exposed to outside air at 10ºC and 70% relative humidity, find the point at which the 10ºC row and the 70% relative humidity column intersect. This point is the equilibrium moisture content for soybeans. In this example, the soybeans will dry to 13.2% moisture content.
Table 1. Equilibrium Moisture Content (% Wet Basis) for Soybeans Exposed to Air
| Temperature | Relative Humidity | ||||
| 50% | 60% | 70% | 80% | 90% | |
| 0°C | 10.0 | 11.8 | 13.7 | 16.2 | 19.8 |
| 5°C | 9.8 | 11.5 | 13.5 | 15.9 | 19.6 |
| 10°C | 9.5 | 11.2 | 13.2 | 15.7 | 19.4 |
| 15°C | 9.2 | 11.0 | 13.0 | 15.5 | 19.2 |
| 20°C | 9.0 | 10.7 | 12.8 | 15.2 | 19.0 |
| 25°C | 8.7 | 10.5 | 12.5 | 15.0 | 18.8 |
Measuring Relative Humidity
It can be difficult to measure the precise relative humidity of the outdoor air, but it is necessary to air-dry soybeans. This reading can be obtained from a nearby weather station; however, conditions can vary from one location to another. Household hygrometers tend to be inaccurate and are not suggested for measuring relative humidity when air-drying soybeans. A sling psychrometer or a good quality hygrometer is ideal for this purpose.
When to Run the Fan
Air temperature and relative humidity levels determine when a fan can operate, not time of day. On some days, drying can be accomplished from 9 a.m. until midnight, while on other days, it may only be from 9 a.m. until 6 p.m. Throughout the day, repeatedly check the air’s temperature and relative humidity level. To make drying progress, the outside air must be drier than the inside air. If the equilibrium moisture content on a given day is less than the moisture content of the wettest beans, drying is possible. Humidistats can activate the fan at pre-set humidity levels. The operator can adjust the relative humidity level at which the fan is activated.
The beans at the top of the bin will be the last to dry. Each day of fan operation will push a drying front up through the bin. The top of the bin may not dry out as quickly as anticipated. Take moisture samples at the same depth each time to know how the moisture content is changing at that depth. Bins with stirring devices – stirrators – will have fairly uniform moisture levels throughout the whole bin.