Natural Air Drying of Beans

A very common request today is “I want to put enough air on the new bin I am putting up to take a couple points of moisture out of Beans”. Today’s larger bins make that unlikely. It will be easier to understand if we go over some air and grain issues that affect this situation.
There are three types of processes involving air in grain bins, Aeration, Cooling and Drying.

Each has a set of parameters that you must work within to successfully complete each process.

Aeration involves a relatively low amount of air, usually 1/10 to 1/4 cfm/bu., when the bin is full being pushed through grain already at safe storage moisture. This is the norm, but any amount of air pushed through already dry grain is really aeration. The primary goal with aeration is equalizing the moisture in the individual kernels which can take up to a week depending on airflow, stabilizing the grain and to bring the temperature of the grain down as outside temperatures decrease. This is normally taken at least down to 50 degrees which is considered the safe storage temperature and at times down into the 30’s. Aeration also can optionally be used to bring the temperature of the grain back up in the spring. Speed is not a bad thing with aeration, but it is not an imperative.

Cooling What we call Cooling is when grain at or very near safe storage moisture has been heated between 100 and 140 degrees and more air then normal aeration is necessary to bring the temperature down quickly before the grain can go out of condition. Corn is the primary grain that is dried this way, beans and wheat usually have too low a kernel temperature after drying to gain much from this process and beans have issues with oil quality problems. It is also very difficult to get adequate airflow through wheat. Corn goes out of condition much quicker when hot and it is important that the grain is brought down to the outside temperature quickly. Cooling air rates normally fall between 1/3 cfm/bu. and 1 cfm/bu. depending on how fast the bin is filled. Hot Grain out of a high temperature Grain Dryer is often over 130 degrees. Cooling does remove a lot of heat, but it does not move much moisture through the bin. 17.5% at the back of a dryer at 130 degrees is not accurate. Actual moisture is around 15.5%.

Drying grain is different than the above two processes in several ways. It involves grain above safe storage moisture. It causes the movement of a significant amount of moisture. It takes a lot longer and often flirts with the maximum safe storage time of grain at a particular moisture and temperature before the process is complete. (See Chart A below) You should never attempt to Dry grain with less the 1 cfm of airflow. Using less airflow may seem to work, but unnoticed condition problems and damage almost always occur. The more moisture you try to remove, the poorer the conditions and the larger the bin the more likely you will fail, and the consequences are often serious and expensive.

But I am just trying to take a couple of points out of Soybeans early in the fall you say. My Father did it, heck I have done it, why can’t I do it in the new larger bin I am building?

Several things have changed in today’s farming practices that make things different. One is the earlier start time we now have for harvest. In the past though, Soybean harvest took place before Corn harvest it didn’t start until the average outside temperatures were in the 60’s. Take a look at Chart B below. The equilibrium point for 13% moisture in soybeans at 60 degrees is around 70% relative humidity. It also happens that the fall average for relative humidity throughout the Corn Belt is close to 70%. Today Soybean harvest can happen when the outside temperature is in the 60’s, but it is more likely going to be in the 80’s or even 90’s. Note that pushing that temperature of air through Soybeans will get you closer to 11% then 13%. This is only the first and not the most serious problem.

The more serious issue is in the past you were likely storing your Soybeans in a 24’ 7 2.66” ring bin which held 7,500 bushels and only took a 3/4 H.P. 12” fan to have aeration airflow of close to 1/4 cfm. Even if you were using a 30’ 7 ring bin it only took a 1 H.P. fan to approach the same airflow. If you wanted to crank it up and take a few points out of Soybeans you could just throw on a 10 H.P. 24” or at the most a 15 H.P. 28” fan and 2 you had yourself a drying bin. You had enough air that in a matter of a few days in the right window of conditions and you took out those couple points easily. Even if you didn’t have quite 1 cfm of air the relatively shallow depth of grain meant at least most of the drying bin requirements were still in place.

The average size bin sold today is 48’ in diameter and right at 14 wide rings tall. This bin holds 60,000 bushel and takes a minimum of a 20 H.P. 1750 Centrifugal fan to get 1/5 of a cfm of airflow. You can potentially take moisture out of Soybeans even at this airflow rate, but it had better be perfect conditions, meaning temperature and humidity, and those conditions had better last and stay constant for weeks if not months if you want to arrive at the goal before grain condition becomes an issue. There is another problem with grain of that depth. Yes, low airflow can remove moisture. But, it has a long way to travel before it leaves the bin. That moisture will probably just move to and be redeposited in the upper portion of the bin before actually leaving the bin. Those Soybeans at the top of the bin which are the wettest and at the highest temperature are not a good place to move and leave that moisture.

You can’t fix that by simply moving up the air flow. Moving it up to the drying airflow rate of 1 cfm will
reduce the time it takes to dry, but you will still be moving a lot of moisture up into the upper portions of the bin due to the greater depth of grain. There is a real good reason why drying bins are best at 7 rings of height and it isn’t only because it takes less horsepower to get good airflow. With 20’ or under grain depths moisture leaves the grain mass quickly and is not redeposited back into the upper portions.
But, horsepower requirement cannot be ignored here either. That 48’ 14 ring bin, which isn’t all that big of a bin anymore, takes (4) 50 H.P. 3450 rpm high speed centrifugal fans to hit .6 cfm/bu. of airflow. We don’t have a fan option that will hit 1 cfm/bu. We all know that is unreasonable and it still might take more than a week to get the 2 points out of the Soybeans. Two 40 H.P. 1750 rpm centrifugal fans get you to .4 of a cfm/bu. which is too little to make a difference.

So, can you “Take a couple points out of soybeans in a new grain bin?”

Yes, you can, that is if it is the right bin with the right airflows. Bumping up the fans a size or two will not turn today’s larger “Storage” bin into a “Drying” bin. Even with the smaller bins our harvest conditions have changed such that the conditions are not as conducive to getting final moisture of 13% at the time of harvest. Trying to hold 90 degree 15% Soybeans for a couple months to hit the right conditions is probably not a wise thing to do. You could invest in an Aeration Control system like those from IntelliFarms or OPI (Integris) that only lets air that was the right humidity pass through the Soybeans right from the start and even add a low temp heater to the centrifugal fan to lower the humidity, but that doesn’t completely eliminate the issue of moisture deposited in the upper part of the bin. Can you say “Crusting”?

The only real reasonable answer I have been able to come up if you cannot use your high temperature Grain Dryer to take the two points out is to stick with an eave height of 20’ or under, add a centrifugal fan that can deliver 1 cfm of air and a low temp burner that is controlled with a humidistat cycling the burner so that no air enters the bin that is above 70% relative humidity. IntelliFarms or OPI with their complete management and control using equilibrium technology to dry or even bring a point or two back is the best way to control this process. You also need a good spreader to get the grain as level as possible. The main point is that you need a drying bin to take a couple points out of Soybeans, not a storage bin with a little extra air. J

Gary Woodruff
The GSI Group 

 
Chart A: This chart is for Corn not Soybeans, but the principle is the same.

Chart B: