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Factoring the Heat From Your Fans

Thursday, August 1, 1996
filed under: Harvest/Storage

Does that fan on your natural air/low-temperature drying system generate more heat than traditionally assumed? Ken Hellevang believes so — and some recent research backs him up.

The standard assumption, says the North Dakota State University extension ag engineer, has been that incoming air is warmed by two to three degrees as it moves over the fan motor, through the fan and into the bin. His own observations, however, led him to question that assumption — especially as it applies to small grains and sunflower.

“Most of the ‘two- to three-degree’ comments were based on corn, where we’re looking at fans operating at perhaps three to four inches of static pressure and moving a lot of air flow,” Hellevang says. “With the way these systems are set up for small grains, we’re commonly seeing wheat being dried at operating static pressures of six or seven inches. You don’t move as much air; so you’d suspect the temperature rise would be different.

“For those farmers who have any fan — but particularly an in-line centrifugal that they’re using on wheat, barley and then sunflower — that fan typically will operate at this kind of a static pressure (i.e., six or seven inches).” The probable effect, Hellevang says, is a corresponding air temperature increase of four to five degrees, not two or three.

The result can be an over-drying of the grain or sunflower in the lower portion of the bin. “So instead of ending up with 10-percent moisture sunflower, we might have nine- or even eight-percent ’flowers,” Hellevang observes, “because as we warm the air, we drop the relative humidity — which is what regulates that final moisture content.”

Hellevang began to investigate his theory by contacting several fan manufacturers to learn whether they had researched the temperature increase that occurs as air passes through their fans. Of the companies contacted, only one could supply such data — and those numbers were for just one type of fan.

So the NDSU engineer set up a laboratory experiment to measure the temperature increase as air passed through seven tested fans. Among them were: three 3.0-horsepower high-speed centri-fugal fans (one with a 13-inch impeller, another with a 15.25-inch impeller and the third with a 15.5-inch impeller); a 5.0-hp high-speed centrifugal with 15.5-inch impeller; two 3.0-hp in-line centrifugal fans (one with an 18-inch impeller and another with a 24-inch impeller); and a 1.5-hp in-line centrifugal with an 18-inch impeller.

The test apparatus consisted of a plywood duct about 10.5 inches square, 16 feet long and sanded smooth inside. There also was a transition attachment at the front of the duct, with the fan then mounted at the transition opening. (The purpose of the transition attachment was to create a gradual change in dimensions from the fan to the duct.) At the other end of the duct was an adjustable cover whose opening/closing could be regulated to simulate different static pressure levels.

Devices placed at various points inside the duct measured static pressure (all four sides of the duct); air velocity (nine locations); temperature (at the fan air intake and at three other locations around the duct); and power consumption.

Hellevang found a 1.9- to 6.4-degree temperature rise range among the 3.0-hp high-speed centrifugal fans (table, next page). With one (15.25-inch impeller), for example, when static pressure was at four inches, the temperature increase registered 2.7 degrees. With the same fan, at seven inches of static pressure, the temperature rose by 4.8 degrees.

On the 3.0-hp in-line centrifugal with 18-inch impeller, the NDSU investigation found a 3.0-degree rise in temperature at a static pressure level of three inches and a 5.9-degree increase at six inches of static pressure. For the 3.0-hp in-line with 24-inch impeller, the temperature increase registered 3.7 degrees at the three-inch static pressure level and 4.4 degrees at six inches. Finally, for the 1.5-hp in-line fan, the temperature increase measured just 2.1 degrees at the three-inch level but then jumped to 7.1 degrees at five inches of static pressure (table). At five inches of static pressure with the 1.5-hp in-line centrifugal, the adjustable cover was nearly closed and the fan was operating beyond its efficient range.

What do these numbers imply for the sunflower producer? The bottom line is that the fan on that natural air/low-temp drying system likely is helping pull down the moisture content of seeds more than what you’d expect. So unless you’re adequately measuring seed moisture content throughout the bin — and not just at or above the drying zone — you could be over-drying your sunflower, thereby costing you pounds (and dollars) when the seeds are marketed.

Hellevang uses the example of “typical” North Dakota mid-October conditions of a 47-degree outdoor temperature and 65-percent relative humidity. If using an air flow rate of one cfm per bushel, 20 days would be required to take a bin filled with 15-percent moisture oil-type sunflower down to an equilibrium moisture content (EMC) of 8.5 percent. However, if a fan-induced five-degree temperature rise is factored into the equation, it would have the effect of dropping that 65-percent relative humidity reading down to 53 percent. The result as of 16 days later, according to Hellevang, would be a final sunflower EMC of 7.3 percent, not 8.5.

The situation in November — a time when natural air drying without supple-mental heat borders on being ineffective under North Dakota conditions — would be even more dramatic. The same 15-percent moisture oil sunflower would require 32 days to be brought down to 10.1-percent moisture (based on an outdoor temperature of 27 degrees and relative humidity of 73 percent). If a five-degree temperature rise from the fan was factored in, however, it would drop the relative humidity down to 58 percent — and place the seed moisture level at 8.3 percent as of about 22 days later.

Under a natural air/low-temp system, drying zones will move from the bottom

of the bin toward the top, Hellevang reminds. “So if you’ve put in 15-percent moisture sunflower and the seeds at the bottom are drying down to nine percent, those at the top are going to stay at 15 until that drying zone comes all the way through.

“If we start adding supplemental heat, now perhaps we’re drying the bottom seeds down to seven or eight percent while those at the top still sit at 15. The fan is already giving us four or five degrees of temperature rise, and then we add on an additional five degrees for a heater. All of a sudden we’re warming the air a total of nine or 10 degrees.” If moisture levels throughout the bin are not closely monitored, “we can get to the point where we’re doing some serious over-drying,” Hellevang cautions.

“This needs to be part of a grower’s management,” the NDSU drying/storage specialist emphasizes. “He has to realize there is a temperature rise occurring that will affect the moisture content of the grain in the bin.

“A lot of people have the idea that adding supplemental heat speeds up the drying process. It does a little; but its main effect is to change the final moisture content of the grain,” Hellevang

concludes. “So if we’re talking typical September or October conditions, we may actually need some higher-humidity nighttime air to prevent over-drying.” The only way to know for sure what’s going on throughout that bin, he reiterates, is to be pulling representative samples on a regular basis throughout the drying process. — Don Lilleboe
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