Larger Diameter Bins Save Grain Drying Energy and Time

Larger Diameter Bins Save Grain Drying Energy and Time

image1

August 24, 2007

If you're thinking about building more on-farm grain storage, consider the efficiency of different sized bins.

The airflow produced by an aeration fan depends on the static pressure the fan must overcome. Figure 1 shows a typical axial flow fan curve. The greater the static pressure, the lower the volume of air produced. Table 1 shows airflow resistance for shelled corn. More static pressure is required to push a given rate of airflow, cubic feet per minute per bushel (cfm/bu) through grain as the depth of grain increases. Static pressure also must increase to push increasing rates of airflow (cfm/bu) through any given depth of grain.

The time required to dry grain in a bin is a function of the amount of water removed, the air properties and the rate of airflow through the grain (cfm/bu).

Since drying time is directly related to the rate of airflow, cfm/bu, we want airflow rates as high as practical when drying grain. By keeping grain depth as shallow as possible, resulting in higher airflow rates, we can reduce total drying time and save energy cost for drying grain.

Building larger diameter bins and then partially filling them when drying, keeps static pressure low while not sacrificing the number of bushels dried per batch. Consider the difference in static pressure when a 27-foot diameter bin and a 33-foot diameter bin are each used to dry 8,000 bushels of corn at one time. Grain depth in the 27-foot bin would be 17.5 feet, whereas grain depth in the 33-foot bin would be only 11.7 feet.

Using the FANS computer program from the University of Minnesota to compare these scenarios provides some interesting results.

Smaller Fan — Same Bushels

It would take 4.0 inches of static pressure and an estimated 10.6 horsepower (hp) to push 10,000 cfm (1.25 cfm/bu through 8,000 bushels) in a 27-foot diameter bin. To push 1.25 cfm/bu through 8,000 bushels in a 33-foot diameter bin would only take 1.5 inches of static pressure and an estimated 4.0 hp. This scenario assumes a smaller fan is installed on the larger bin but would still produce 10,000 cfm when overcoming 1.5 inches of static pressure.

Table 1. Airflow resistance data for shelled corn.

Grain
Depth
(feet)

Airflow (cfm/bushel)
0.5
0.75
1.0
1.23
1.5
2.0
Expected Static Pressure (inch of water)
8
0.2
0.3
0.5
0.6
0.8
1.2
10
0.3
0.5
0.8
1.1
1.4
2.0
12
0.5
0.8
1.3
1.6
2.1
3.2
14
0.7
1.2
1.7
2.3
3.0
4.6
16
0.9
1.6
2.4
3.2
4.2
6.4
18
1.2
2.1
3.1
4.3
5.6
8.7
20
1.6
2.7
4.0
5.6
7.3
11.3

Assuming the fan motor is 70% efficient and electricity cost is $0.09/kWh, drying shelled corn using natural air in mid- to late-October (assuming 20 days drying time), the energy cost for drying in the 27-foot diameter bin would be $0.06 per bushel and the drying cost in the 33-foot diameter bin would only be $0.023 per bushel — just 38% of the energy cost for the smaller bin.

Same Fan — More Bushels

A management alternative would be to fill the larger diameter bin to the point the same fan model used on the 27-foot bin would be delivering the same 1.25 cfm/bu airflow and would be using the same horsepower as when installed on the smaller diameter bin. The FANS program shows it takes the same 10.6 horsepower to push the 1.25 cfm/bu through 15.6 feet (10,674 bushels) in the 33-foot diameter bin. The fan would be producing 13,343 cfm and 3.04 inches of static pressure. The drying time would be the same as drying 8,000 bushels in the 27-foot diameter bin because the airflow rate is the same (1.25 cfm/bu). Increasing the bin diameter, and reducing grain depth and static pressure, results in the ability to dry one-third more grain in the same time and for the same energy cost as when using the smaller bin.

Tom Dorn
Extension Educator

Online Master of Science in Agronomy

With a focus on industry applications and research, the online program is designed with maximum flexibility for today's working professionals.

A field of corn.