November - December 2000
With relatively high yields and low prices, Missouri farmers are storing more grain this season. In some areas and on some farms, storage space is limited. For this reason some farmers may build additional on-farm grain bins. There are many different sizes and options available in grain drying and storage systems. Each has its advantages, disadvantages, and associated costs.
Costs are a major consideration, and may be categorized as either fixed or variable. Fixed costs occur regardless of the extent of utilization of a grain drying and storage system. Examples are taxes, depreciation, interest on investment, etc. Variable costs on the other hand are a function of level of use. Examples include labor, repairs, drying and handling fuel, etc.
Fuel costs can account for a significant portion of the drying and storage costs for grain and careful planning should go into a system to reduce this cost as much as possible. Grain depth is an important factor in grain drying and therefore in the resulting fuel costs. Extra depth increases airflow resistance, decreasing the drying rate. This extra depth also increases fan power requirements and the cost per bushel of drying grain. Reducing the depth in a low temperature drying bin can speed drying, decrease the risk of grain spoilage, and reduce drying cost. Typically, reducing drying depth by 1/4 will reduce the energy cost per bushel by 1/3.
When buying "deep" versus "shallow" or "low-profile" bins, it makes sense that the initial bin cost per bushel will be lower for a deeper bin. After paying for the concrete floor and roof, the addition of an extra ring or two results in cheap storage. However, this doesn't take into consideration the purchase cost of a drying fan and motor or the cost of operating the drying fan.
According to agricultural engineer, David Williams, in University of Missouri Guide 1300, Low-Profile Bins for Grain Drying, a low-profile bin has a maximum grain depth of 12 to 13 feet, rather than the more typical depth of 17 to 18 feet. In order to hold the same amount of grain, the low-profile bin must have a larger diameter. Due to the larger diameter, the low-profile bin will have a higher initial cost per bushel of capacity due to the extra concrete, larger perforated floor, larger roof, etc. With the low-profile bin, a smaller, less expensive, fan may be used. This may offset the extra cost of the concrete and bin structure is some cases.
With fixed versus variable costs in mind, Williams examined some of the costs associated with low-profile bins relative to deeper bins commonly used for low-temperature drying. Fans were sized for bins from 27 to 36 feet in diameter to provide an airflow of 1.5 cubic feet of air per bushel (cfm/bu.). This airflow is recommended for drying corn to 21% moisture or lower in the central half of Missouri on October 1 if filling in one or two days.
Costs for bins, equipment, concrete, and labor were provided by bin suppliers. Energy costs were based on an electricity rate of 6¢ per kilowatt-hour. Bin costs included the concrete slab, perforated floor, fans, motor, inside and outside ladders, power grain spreader, and 8-inch unloading augers. Costs do not include roof vents, heaters, thermostats or humidistats. Medium-profile 8,000 and 10,000 bushel bins were also studied, but the costs were similar to the deep bins.
With the values and assumptions used by Williams, the additional initial cost for a low- profile bin was 5.4¢ per bushel. However, the annual energy saved with the system was 3.1¢ per bushel. This resulted in a payback period of less than two years.
For the 10,000-bushel bins, the low-profile bin construction costs totaled $1.262 per bushel, 9.4¢ per bushel more than the deep bin. For the 8,000-bushel bins, the low-profile bin totaled $1.323 per bushel, 5.4¢ per bushel more than the deep bin.
The difference in cost per bushel between the deep and low-profile systems is small primarily due to fan costs. The 27-foot bin needs a 15-hp centrifugal fan at a cost of about $1,500, while the 33-foot bin can provide the needed airflow with a 10-hp axial fan at an approximate cost of $1,000. Likewise, for the 10,000-bushel bins, the 30-foot bin needs a 20-hp centrifugal fan at a cost of more than $1,600, while the 36-foot bin uses a 10-hp axial fan at a cost of approximately $1,000.
The electrical costs of the fans were calculated on the basis of fan operation for 30 days per year at a cost of 6¢ per kilowatt-hour. At these rates, the low-profile, 8,000-bushel bin would pay the difference in initial cost after 1.7 years of operation, and the low-profile, 10,000-bushel bin would pay back the difference in 3.2 years of operation.
The time required for the low-profile bin to pay for itself depends on many factors including bin capacity, electricity rates, and airflow requirements. Some may not pay back as soon as these examples. For smaller diameter bins in the 18- to 30-foot diameter range, low-profile bins may actually have a lower initial cost than their deep counterparts when fans and motors are considered. In the latter case, the savings in energy costs add to the cost savings from the very first day of operation.
William's comparison was done in the '90s when both fixed and variable costs were lower. But, with major increases in electricity and heating fuel prices in the past year, the payback period is likely less than those he calculated. It should be noted that the costs shown in his guide are not the total costs for drying a bushel of corn. It is assumed that depreciation, interest, insurance, labor, and repair/maintenance costs will be nearly equal for systems of equal capacity.
For producers who already have the higher profile bins, Williams points out they can also get some of the benefit discussed from an existing deep bin if they manage it as a low-profile bin. Simply limiting the grain depth in the bin to 12 or 13 feet will result in a higher airflow rate, faster drying time, and lower energy cost per bushel. Risk of grain spoilage in the top layers of grain will also be reduced.