Grain Handler's "mixed flow" drying system offers reduced drying cost as depicted by the following graph.

Drying Cost Comparison

The figures below extracted from the text "Drying and Storage of Grains and Oilseeds", depict the relative directional flow patterns of grain and air mixed flow and cross flow type dryers. The figures also illustrate the characteristics of the variation in grain moisture, grain temperature and air temperature vs distance that the grain travels downward in the grain columns of mixed flow Grain Handler and cross flow Screen dryers.



It is seen that in a cross flow dryer the hot air flows in one direction only, inside to outside which is perpendicular to the direction that the grain is traveling, (thus, the cross flow designation). Because of this air/grain flow pattern, a drying front exists and grain is over and under dried at the inside and the outside of the grain columns respectively. This is illustrated in the "Grain Moisture Content vs Distance" graph for screen dryers. This graph has a moisture curve for the inside of the column, one for the outside, and their average as a function of the distance traveled down the grain column. According to the text, the moisture differential between the inside and the outside of the columns can be 8% after cooling. While mixing does occur in the unload auger, the moisture variation does not equalize and grain that is non-uniform in moisture quality is the result.

Some screen dryer manufacturers have realized this problem and have designed an optional grain turner for their columns. This device is normally positioned about midway along the vertical extent of the dryer heating zone and simply shifts grain from the inside of the column to the outside and vice-versa. According to the text authors and others, this reduces the uneven drying problem, but does not eliminate it. The authors go on to say that the grain exchanger actually causes a problem in that it makes the screen dryer somewhat less energy efficient because the drying air below the exchanger exits the system with unused humidity absorption potential. Screen dryers have a reputation in the industry as being energy expensive compared to mixed flow design dryers.

The screen dryer graphs also show the grain near the inside of the column can reach nearly the same temperature as the drying air, (even if exchanged across the column). If heat above 160 to 180 Fahrenheit is used, this leads to kernel stress cracking, a significant increase in breakage potential, and loss of test weight. If a user decreases drying temperature, then dryer throughput is sacrificed.



The above figures illustrate the air/grain flow pattern, grain moisture, grain temperature, and air temperature vs distance traveled down the column for a mixed flow dryer. It is seen that air flows across, concurrent with and opposite to the flow of grain, (thus, the mixed flow designation). In this dryer design, Grain Handler, the grain stream is actually cascaded, gently tumbled and continuously mixed as it flows around a "honeycombed" arrangement of ducts which are delivering heated air and removing moisture uniformly across the width of the grain columns. Thus, there is no drying front in a mixed flow dryer and uniformly dried grain is the result.

This fact is illustrated in the graph which shows only a single grain moisture content vs distance curve (compare this with the multiple moisture curves for the screen type dryer). The text authors significantly reduces kernel stress and breakage potential. We at Grain Handler believe that this is an important mechanism for the maintenance of grain test weight/quality. It should be noted that the stair-stepping moisture curve in the mixed flow design results from the grain coming alternately near heat inlet ducts followed by exhaust ducts. Another important observation from the graphs is that the grain temperature in a mixed flow dryer stays well below the air temperature of the dryer. This has a significant influence on grain quality as discussed above.

These data show that the mixed flow design is superior to the cross flow design for grain drying. Mixed flow dryers produce higher quality grain, higher test weight grain, and use less fuel and electrical energy than screen dryers. The Grain Handler mixed flow dryer also performs at its rated throughput.

We hope this discussion clarifies the differences between mixed flow and cross flow dryers. Please contact Grain Handler USA with any additional questions you may have.