The drying process dictates that water be removed from the surface of the seed. Since most moisture testers test surface moisture, it seems logical that the moisture content of the grain will test lower right after drying then it will a day later when the moisture inside the kernel has had a chance to become even throughout the kernel. This does in fact take place - a phenomenon known as "moisture rebound". For this reason it is advisable to dry the grain to a moisture content about 1 percent less than the suggested storage moisture content.
It is the responsibility of the owner of the machine to see that the operator in charge of the dryer is knowledgeable in the operation of the machine.
How Grain Dries
There are three basic reasons for heating with air:
1) Hot air carries more water.
The water holding capacity is roughly doubled every time the temperature of air is increased 16 degrees Fahrenheit.
2) Water evaporates more readily into hot air.
The Equilibrium Moisture Content of the grain is reduced.
3) Warm kernels move moisture to the surface more quickly.
The hot air warms the kernel and increases the rate of Moisture Migration within the kernel. Watch the Maximum Safe Kernal Temperature. Understand Moisture Rebound.
Equilibrium Moisture Content is moisture content a kernel of grain will reach if air of a certain temperature and relative humidity blows across it. If the kernel is wetter to begin with, it will dry to the equilibrium moisture content. If it is drier to begin with it will take on moisture.
Moisture Migration is the process where moisture near the center of a kernel moves to the surface. The rate of moisture migration is very slow when the kernel is cool. It can take many weeks to remove even a few percent of water. The rate increases as the temperature of the kernel increases.
Maximum Safe Kernel Temperature, the rate of moisture migration increases as the temperature of the kernel increases. However, be careful never to exceed the maximum safe kernel temperature or you can reduce the quality of grain by reducing germination, reducing the quality of baking etc. If a kernel is giving up moisture it will not get as hot as the air around it. To determine kernel temperature you CANNOT just stick a thermometer into the side of the operating grain dryer. To determine Grain Temperature see next page.
Moisture Rebound happens as grain is drying, the surface of the kernel will be slightly drier than the inside of the kernel. If this grain is put in a moisture tester immediately the tester might be fooled into indicating that the sample is drier than it is because it can't see or feel the moisture near the center of the kernel. If the same sample is tested several hours later, the moisture tester may read higher because some of the moisture has migrated closer to the surface where the tester can see or feel it. Moisture rebound is commonly in the range of ¼% to 1%.
Determining Grain Temperature
The temperature of the grain can not be determined simply be inserting a thermometer into the side of the Grain Dryer because you will only read the temperature of the air passing between the kernels.
Remove a sample of grain from the dryer. It is convenient to reach a duct. Usually you will want to take grain from near the bottom row of heating ducts, as this is where the material will be hottest.
The sample of grain should then be placed in a covered container for 15 minutes. Then a thermometer is installed to be certain that the grain and air between the kernels are more nearly the same temperature.
Dry moisture content is 10.5%, but due to the above average moisture rebound, sunflowers should be dried down to 9 or 9.5 % moisture. After 48 hours, the sunflowers should be rechecked for moisture to make sure that they do not rebound above 10.5%. The moisture rebound will be greater when more moisture is taken out. As a rule for sunflowers, for every 10 points of moisture removed from the grain dryer, there may be as much as 2 points of moisture rebound as they leave the drier. Moisture rebound will vary depending on the tester used, variety of grain, moisture removed, and retention time. Drying temperature should also be kept down while drying sunflowers. This is because sunflowers are an oil seed and have a relatively large surface area; they can spontaneously ignite if subjected to temperatures in excess of 150 degrees Fahrenheit for long periods of time. A safe temperature for drying would be 150 degrees Fahrenheit for continuous flow drying and 130 degrees Fahrenheit for batch drying.
The inside of the plenum may be caked with fuzz if operating under dirty conditions and it is recommended that this be cleaned out periodically to eliminate the spontaneous ignition of fuzz.
There are a few details to consider concerning the operation of fuel system. There should not be more than 5 psi pressure difference on the two pressure gauges. If the dryer is equipped with a changeable orifice as small a one as possible should be used. These guidelines will help to keep temperature fluctuations to a minimum and also prevent fuel surges by restricting pressure and orifice. The high temperature limit should be set to shut the flame off, should the temperature in the plenum get 20 degrees above the desired drying temperature.
With extremely wet sunflowers (above 20%), the operator must be careful not to enter the top of the garner and step on them causing them to pack tightly between ducts and possibly bridge. If the sunflowers are allowed to bridge in this manner, problems may exist in getting that portion of the grain to move downward freely. Therefore, when drying under these conditions it is a good idea to empty the dryer every other day or so to check and make sure that the grain column is clear.
When using an electric motor to drive the blower, it is necessary to restrict the air to the blower using blower air reduction slides. These slides should be set so as to restrict as little air as possible or until the seeds start lifting in the exhaust ducts. if a problem exists with a lot of hulls or debris blowing from the ducts the slides can be closed further. The slides should be set so that both blower inlets are restricted the same amount, otherwise problems may occur concerning temperature distribution throughout the plenum.
Dry moisture content is 15.5%. Wet corn can withstand somewhat higher drying temperatures than most cereal grains, because of its large size. In a Grain Handler Dryer, the corn is near the hottest air for a short period of time. This allows the operator to increase his plenum temperature to over 200 degrees Fahrenheit and have no stress cracking.
Dry aeration also becomes quite economical with corn. With dry aeration, the corn is dumped hot from the dryer at 16-18% into a cooling bin, where it is allowed to temper for 4 to 12 hours and then cooled. From here, the grain is transferred to a storage bin. The Grain Handler Dryer is designed to allow the entire unit to be heated, if required, by simply closing the cooling flap in front and adjusting the rear divider door to the batch position.
Over drying of corn should be kept to a minimum by utilizing proper storage techniques. If corn is dried from 24% to 13% rather than 15.5% it will require 40% more fuel plus up to a 30% reduction in drying capacity.
Utilizing a high as possible drying temperature is the best way to increase fuel efficiency as long as grain quality is not negated. In a Grain Handler Dryer, temperatures up to 240 degrees can be used for all or part of the grain column.
When drying soybeans, the lowest possible drying temperature that will get the necessary drying accomplished is best. Too much heat will cause the seed coats to crack. Again since the soybeans will be in contact with the hottest air for a short period of time in Grain Handler Dryer, the highest possible temperature can be utilized, in comparison to most screen type dryers. Temperatures in the neighborhood of 150 to 170 degrees Fahrenheit should be possible without any seed coat cracking.
When using a dryer, it will be possible to harvest soybeans 10 days earlier and have higher crop quality, yield and the ultimate market price. An early harvest can also help to minimize crop loss by shatter .
Lentils can be dried in a Grain Handler Dryer. Engineers have recorded temperatures between 120 and 130 degrees Fahrenheit in the surface of swaths on good warm days, this means that as long as the kernel temperatures are kept below this level in the dryer, no damage will occur. In the batch mode of operation, it is suggested that the dryer not be set above 120 degrees Fahrenheit. In the continuous flow mode, the temperatures can be increased dramatically up to 150 to 170 degrees Fahrenheit, without deterring seed quality.
Pinto and Navy Beans:
These beans are quite particular about temperature, but the same rules can be applied as per lentils. An indication that too high a temperature is being used can be noticed by a color change in the seed coat. If the seed coat is starting to wrinkle or turn brown, the temperature should be decreased. Pinto beans dry relatively slowly. In the batch mode of operation, it is not uncommon to have drying rates of as low as 1% moisture removed per hour. If you use continuous flow, this rate can be dramatically increased to 5% per hour of retention at a time.
With most varieties of beans, splitting can be a problem if they are run through a screw conveyor. To overcome this problem, screw conveyors should be run as full and slow as possible. Correct clearances should also be maintained between the flighting and the sidewalls of the screw conveyor. As a general rule, 3 kernel diameters of clearance will be adequate. These clearances are possible in a Grain Handler Dryer by simply loosening the cables to the clean out doors or by operating the dryer with the clean out door handle slightly open.
A good method for checking the maximum temperature of any grain kernels would be to take a small pail and fill it with some grain or seeds and leave it sitting in the plenum with a thermometer in the pail. After 10 minutes, with the dryer stopped and the container in the plenum, so it does not cool off, the operator can get a reasonably accurate estimate of the temperature of the kernels. By taking the sample from the inside ducts, they would surely be getting the warmest possible kernels.