CropWatch, Jan. 22, 2010: Q&As on Grain Drying and Buying a Bin
January 22, 2010
When should I start warming up corn at 16-18% moisture and a temperature of 25-30 °F to finish drying it. How long will it take to get it dried to 15% moisture?
Background: None of my corn dried very well in the field last fall. I still had corn fields with 20-22% moisture when I harvested in mid-November. The elevator would not take corn that was above 17% moisture so I had no choice but to put high moisture corn in the bin and try to dry it. I have two bins equipped with full mesh drying floors, stirring systems, and fans intended for drying grain. One bin is now down to 16% moisture and the other is down to 17.9%.
I took your advice and cooled the grain to 25-30°F in December. I ran the stirring system to even out the moisture in the bin before I quit running the aeration fans except to check the grain for heating a couple of times a month.
Since you described these as dryer bins, I will assume you can push at least one cubic foot of air per minute per bushel (1 cfm/bu). Given the grain moisture content you described, you could start drying again when air temperature is above 40°F and the humidity is below 60%. However, if you don't detect any signs of heating in the bin, you could reduce hours of fan operation to dry the grain if you wait until 24-hour average temperatures are predicted to be above 50°F to start drying.
Caution: Once you warm the grain above 45°F, it will be more susceptible to mold growth, so warm the grain only if the forecast calls for several days of good drying conditions based on the Equilibrium Moisture Content Table.
If the weather turns cold again and is predicted to stay cold, run the fan to cool the grain down to 30-35°F to arrest mold growth until the next good drying opportunity.
Check the estimated drying time for corn in the CropWatch article, Drying Time for Binned Corn Using Natural Air. You said your two bins of corn are 16% and 17.9% moisture. The bin with 16% moisture should dry in about five days if the 24-hour average air temperature is 40°F and humidity is 50%. The time to dry from about 18% to 15% moisture is about 14.5 days under the same average temperature and humidity conditions.
I have a 48-foot diameter bin with a full mesh drying floor and a 15 horsepower centrifugal fan. The last week in November I filled this bin to the eave with 17-19% moisture corn. I have run the fan off and on when I thought I could do some good, but the grain that I can reach with my 6-foot grain probe is as wet as ever. What advice can you give me?
When you don't have a stirring system in the bin and are pushing the air through from the bottom, a drying front forms in the grain and moves upward over time. The grain moisture above the drying zone remains unchanged or may actually be slightly wetted by the saturated air passing out of the drying zone. That is why you have not been able to detect any change in the moisture content in the top of the bin.
I would add, large bins such as yours are intended for storing dry corn when they are filled to the eave and are equipped with a single 15 horsepower fan.
The minimum recommended rate of airflow for drying grain is one cubic foot per minute per bushel (1 cfm/bu). According to the FANS program, you have only one-third cubic foot per minute per bushel (0.32 cfm/bu) airflow when this bin is filled to the eave. Drying time is proportional to the airflow (assuming air properties don't change). One-third of a cfm/bu is adequate for regulating grain temperature, but is not adequate for drying grain. Warming the high moisture grain you have could hasten the rate of deterioration. My advice is to continue to run your fan as necessary keep the corn at 35-40°F until you can act on the remainder of my advice.
To use the bin and fan you have for drying, the bin must be only partially full. Move some of this grain to another facility to get it dried. Before the end of February I would recommend you find a neighbor who has a drying bin or high speed dryer they are no longer using and get the grain dried to safe moisture content .
The fan you have on this bin can produce the minimum recommended 1 cfm/bu airflow for drying when the grain depth is 12 feet. Therefore, you should move 27,500 bushels out of this bin and retain 17,400 bushels to dry using your fan. You should be able to dry it in place when the weather conditions are conducive to drying. (See the Equilibrium Moisture Content Table.)
Air takes the path of least resistance through grain. You will need to level the grain in your bin after unloading the excess so the airflow is uniform across the entire grain mass.
We ran short of bin space this year. I want to build more on-farm storage. I am thinking about buying a 48-foot diameter bin that is 32 feet to the eave (grain depth will be 31 feet). Can you give me some guidance on the type of fan to buy and the horsepower you would recommend?
I looked at two alternatives to manage a bin of this size, assuming you plan to use the bin for drying corn.
Alternative 1. Plan to fill the bin completely full when drying.
Using the FANS* program, I found that to push 1cfm/bu through this bin when it is full (31 feet of grain depth) would require three 40-horsepower centrifugal fans in parallel. If only one of these fans were operating for controlling grain temperature and the other two were capped off, the single fan would produce 0.54 cfm/bu.
Alternative 2. Plan to partially fill the bin and dry the corn in smaller batches. I recommend you keep grain depth between 15 and 17 feet when you are drying.
To maximize your options, I decided to recommend you install two smaller fans in parallel (each with its own transition duct).
Using the FANS program, I found a 12.5 horsepower axial flow fan model (Caldwell F28-12) that would deliver just over one cubic foot of air per minute per bushel (cfm/bu) when both fans are operating and the grain depth is 17 feet. One cfm/bu is the minimum airflow I recommend for drying corn. If you want 1.25 cfm/bu, keep grain depth to 15 feet. (Note: Mention of brand name and model is included for clarification only and is not intended as an endorsement.)
The reason I am recommending that you buy fans that dry only 15 to 17 feet of grain is because of the huge difference it makes in the horsepower and therefore the electricity required for that level versus drying when the bin is filled to the full 31 foot grain depth (25 horsepower vs. 120 horsepower).
When drying in batches, once you have dried the initial batch of corn, you have two choices:
- Move the first load out and load another batch of corn in the bin to be dried.
- Fill the remainder of the bin with corn that is at (or very near) safe storage moisture content.
When the bin is filled with dry corn and you want to aerate the bin to control grain temperature, you could cap one of the fan openings and run only one fan. One fan would push 1/3 cfm/bu through 31 feet of corn. This would be more than adequate for aerating to control grain temperature and should be able to push a temperature front through the bin in about 50 hours. If you run both fans when the bin is full, they would produce nearly 1/2 cfm/bu. That is not adequate to do much drying, but it would push a temperature front through the bin in about 30 hours.
Extension Educator, Lancaster County