What’s better, mixed flow or conventional dryer?

Cross flow 

Mixed flow dryers are up to 30 percent more fuel efficient than conventional screen dryers. That efficiency relates to how air flows through the dryer, and that depends on dryer design.

“In a conventional cross-flow, the air flows perpendicular to the grain. The grain column is vertical and the air flow is horizontal,” said Lee Bonn, senior product specialist with GSI Group.

Bonn explained that in a counter-flow dryer, grain is augured in at the top, where it’s continuously spread in a thin uniform layer. Heated air is pushed up from the perforated floor through the grain column while dry grain at the bottom of the column is augured out to the cooling bin.

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In a mixed-flow dryer, the grain column is continually mixed as it passes over alternating rows of heat ducts, which allows for slower, gentler drying. Unlike cross-flow dryers, mixed-flow dryers have no screens, which is why they’re considered all-purpose dryers. The more a screen plugs, the more fuel is required to overcome the restricted air flow. Downtime for screen cleaning, while necessary, takes up a lot of time.

“Heavier test weights are also a benefit of mixed-flow dryers,” Bonn said. “Grain in the dryer is heated much more evenly. This reduces or even eliminates heat damage and hot spots. As a result, our mixed-flow dryers consistently deliver test weights up to two pounds heavier than those through conventional screen dryers.”

Bonn advised that when buying a continuous-flow dryer, make sure it’s designed for the crop you’ll handle most. In Canada, most farmers prefer a dryer with 050 screens because these are needed to hold canola even though they restrict airflow. Eighty percent of the dryers on the Canadian Prairies are conventional 050 screens which need to be cleaned regularly to work properly.

“But there’s a drawback to mixed flow also,” Bonn said. “They just have those triangles where the heat comes in through the plenum and then turns and mixes and goes back out through the little triangle. You mix and dry the grain as it goes through the triangles.”

Mixed flow is more efficient in some situations while conventional offers better performance in others. Bonn said it depends on what is being dried. The screen dryer is the most versatile, able to handle everything from canola to cereals to corn.

“When you dry grain, the maximum capacity is determined by how much grain you can hold in that dryer in the heat cycle,” he said. “If you want to dry 1,000 per hour, you need to hold 1,000 bushels in the heat cycle for that one-hour period. When you go to a mixed flow dryer, it has to be taller. It needs the extra capacity to hold grain in the heat cycle that long.”

Given the disparity in dryer efficiency between cereals and canola, it would only be a matter of time before large farmers would run two machines, each dedicated to a specific crop type. Bonn said this is already happening with some of his clients in northern Alberta who have enough acres to justify two separate bin yards.

“One client grows 15,000 acres of canola, so he has a dedicated canola site,” he said. “That facility is set up to handle only canola. His second facility handles only corn, wheat and barley. With that volume of crop, he needs two dryers anyway, so two specific dryers make sense.

“We’re seeing more of these dual systems as farm size grows. Harvest is the bottleneck. Drying is the bottleneck. The premise of drying is to harvest faster and sooner. You don’t want your trucks sitting in line waiting to dump at the elevator. You are in control of your crop for more days out of the year.”