Fill Drying Bins in Layers to Reduce Drying Time and Energy Cost

Fill Drying Bins in Layers to Reduce Drying Time and Energy Cost

September 12, 2008

Many grain producers have on-farm grain drying and storage facilities. Most have more than one bin equipped with high capacity aeration fans able to deliver over one cubic foot of air per minute per bushel of grain (cfm/bu) when the bin is filled to the eave. Many also have the ability to add supplemental heat to the air before it is passed through the grain, reducing the time required to dry the grain.

I did a mathematical analysis to study the advantage of filling dryer bins in layers as an alternative to the normal practice of filling one bin completely before filling the next bin. Four scenarios were studied under each system:

  1. natural air (assumed to be 60°F and 50% relative humidity)
  2. heating the ambient air to 80°F,
  3. heating the ambient air to 95°F, and
  4. heating the ambient air to110°F.
Table 1. Aeration fan parameters for conditions specified in this study.
Grain
Depth
(feet)

Bushels
in Bin

Airflow
(cfm)

Airflow
(cfm/bu)

Static
Pressure
(inches)

18.0

8,245

9,194

1.12

3.65

13.5

6,183

10,551

1.71

3.34

9.0

4,122

11,997

2.91

2.69

4.5

2,061

13,265

6.44

1.56


Table 2. Comparison of Drying Full Bin vs. Layers 1
Scenarios
Days to dry when starting with a full bin
Days to dry in 4 layers
Reduction
in drying time
Natural Air 2
14.5
9.2
36.6%
Heated to 80°F 3
6.12
4.5
26.5%
Heated to 95°F 3
5.08
3.8
25.2%
Heated to 110°F 3
3.74
2.9
22.4%

1Values generated by the FANS computer program available for free download from the University of Minnesota Post Harvest Handling of Crops Web site. Fan model: Caldwell F24-10, 10.5 hp axial flow fan. Bin: 27 ft diameter with a full mesh drying floor.
2Ambient air was assumed to be 60°F and 50% relative humidity.
3The simulation assumed a stirring device was used when adding heat to the incoming air to prevent over-drying the grain on the bottom of the bin.

It was assumed harvest would begin when the corn reached 20% moisture content. In the simulation where the bin was filled the first day, the entire grain mass was assumed to be 20% moisture. For the simulations where the bin was filled in layers, one-quarter of the bin capacity (2,061 bushels) was assumed to be added at a time. The next layer would be added as soon as sufficient time had elapsed for the air passing through the grain to carry away the excess moisture in the preceding layer. The moisture content of the standing corn was assumed to drop one-quarter percentage point per day while waiting for the preceding layer to reach 15.5% moisture.

The time required to dry grain is a function of how much moisture must be removed from the grain to bring it down to the desired moisture content, the airflow through the grain (cubic feet per minute per bushel, cfm/bu) and the properties of the incoming air (temperature and relative humidity). The airflow in a bin is a result of the interaction of the fan performance curve (cfm vs static pressure) and the airflow resistance curve for various depths of corn.

Table 1 shows the expected airflow (cfm and cfm/bu) for the specified fan model when the specified bin is filled in four equal layers.

Table 2 shows the results of simulations computed to estimate the time needed to remove the excess moisture in the new grain each time more grain was added to the bin. The target average moisture was 15.5% in all scenarios.

Tom Dorn
Extension Educator, Lancaster County