One of the more popular continuous flow dryers is called a cross flow dryer. In this type of dryer the air flow is generally perpendicular to the grain flow. The grain flows by gravity down a 10 to 16″ column as the heated air blows across the grain column. The energy efficiency is a function of the plenum air temperature and the air flow rate with the highest efficiency occurring at high plenum temperatures and low air flow rates. In practice high efficiency is not achieved due to grain quality issues. The lower the air flow rate the higher the grain moisture content difference between the grain kernels on the inside versus the outside of the column. Many manufacturers have developed methods to mix the grain as it flows down the column to lessen the moisture variation at the dryer discharge.
In a counter-flow dryer the grain and air flow are in opposite directions. Typically a round bin is used that has a perforated floor and a sweep auger. Heated air is pushed up through the grain while the sweep auger removes grain as the control system indicates the grain has reached the desired moisture. Hot grain is moved to a cooling bin when it is discharged from the dryer. One advantage of this dryer type is that wet grain can be piled on top of the drying grain, eliminating the need for a separate wet holding bin. Fines can affect airflow from the perforated floor so it is recommended that the grain be screened before drying and it is usually necessary to shut down the dryer every 3-4 days to remove fines from the floor to maintain proper air flow. These are one of the more efficient continuous flow type dryers being about 40% more efficient than a cross-flow dryer without heat recovery. This type of dryer can be retrofitted to an existing bin, reducing the initial dryer cost and can be used to store grain by drying the last grain operating the dryer in a re-circulating batch dryer mode.
Mixed flow dryers are column dryers that have air flow in both counter and concurrent directions. Theses dryers often have multiple zones and can use higher air temperatures without crop damage than cross-flow dryers because like the concurrent flow dryers, all grain kernels are exposed to the same air temperatures. The dryers are usually self cleaning and have energy efficiency similar to a counter-flow dryer, using about 40% less energy than a cross-flow dryer without heat recovery. One major advantage compared to a dryer with a screen column like a cross-flow dryer is the ability to use the dryer for a wide variety of different grains; from small rape seed to corn or soybeans. This type of dryer is popular outside of the US but has not been adapted as readily in the corn growing regions possibly due to the higher capital cost.
Bulletin on Continuous Flow Dryers.
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