To cool a bin of canola, the aeration fan needs to move 0.1 to 0.2 cubic feet of air per bushel per minute. To remove moisture, airflow should be about 10 times that. Does your fan achieve those rates? How do you know?
A five-horsepower axial fan can blow more air per minute than a five-horsepower centrifugal fan, but axial fans fail under high static pressure. While an axial fan might be able to blow enough air to cool a full bin of canola, it probably won’t be able to move enough air to remove moisture.
The 0.1 to 0.2 cubic feet per minute per bushel (cfm/bu.) needed to cool a bin of canola should not be a problem as long as fan size matches bin size. (A small fan on a big, tall bin might not be able to get over the static pressure barrier from that volume of canola — and in that “dead head” situation, it just won’t blow much air at all.)
The more challenging situation for many fans is to blow hard enough to remove moisture. Consistent moisture removal requires minimum airflow of 0.75 cfm/bu., and 1.0 to 2.0 is preferred.
Why does moisture removal require so much more airflow? Joy Agnew, storage research project manager with Prairie Agricultural Machinery Institute (PAMI) in Humboldt, Sask., says the reason is likely because water is heavy. “You need more momentum and energy to actually carry water vapour through the grain mass and out the top of the bin,” she says. “With cooling, you don’t need as much energy to move the ‘heat.’”
How much air does my fan blow?
The answer depends on many factors, starting with fan type and size. Step one is to find the fan’s manual and look for a table that shows airflow as it relates to static pressure. If you can’t find the manual or if the manual doesn’t include this information, get the table from the manufacturer. (See Table 1 below for an example)
The next step is to measure or estimate the static pressure, which varies by crop type, amount of airflow, aeration ducting and depth of grain. Static pressure is given as inches of water in a water column, which is in reference to the simple manometers used to measure pressure.
Crop type. Large-seeded crops like peas have very low static pressure compared to small-seeded crops like canola. Wheat is somewhere in between. For example, if a bin is filled to 15 feet and the target airflow is the drying rate of 1.0 cfm/bu., static pressures are one inch for peas, 5.5 inches for wheat and 7.5 inches for canola.
Ductwork. Aeration ductwork can make a big difference. According to Agnew, full-floor perforations in a flat-bottom bin will add about one inch of static pressure, partial-floor perforations will add one to two inches, a “rocket” in a hopper-bottom bin could add up to two inches and other systems may add more than that.
Imre Varro, aeration engineering manager with AGI in Nobleford, Alta., says that if any ductwork is providing more than one inch of static pressure, the system is flawed. “We are careful to recommend rocket sizes that will result in less than one inch of static pressure based on the airflow required,” he says. “In a majority of cases when producers are getting static pressures in their systems of two to three inches or more, the system has been incorrectly sized for the desired airflow the system is to provide.”
Airflow. The more air a fan blows, the more back-pressure it creates. For example, in a bin of canola filled to 15 feet, airflow of 1.0 cfm/bu. creates 7.5 inches of static pressure while airflow of 0.5 cfm/bu. creates only four inches of static pressure. If a fan doesn’t have capacity to maintain airflow in the face of this back-pressure, the fan ceases to do its job.
Depth of grain. This can be a major factor in the airflow and drying capacity of a fan. Reducing the amount of canola in a bin can make the difference between properly moving moisture through the grain and having a moisture front stalled in the middle of the grain mass. The fan might sound like it’s working, but a moisture front stalled for weeks is bound to cause crusting and heating.
With an estimate of the static pressure, insert that number into the airflow tables for the specific fan model, then do the math to figure out airflow per bushel. Using the three-h.p. high-speed centrifugal model GCF-80311 in Table 1, if static pressure is six inches, airflow will be 2,740 cfm. When the bin contains 2,500 bushels, airflow per bushel will be right around 1.0 cfm/bu.
Take out the guesswork
A pressure gauge installed in the duct between the fan and the bin provides an immediate reading of static pressure. Many new aeration systems include these gauges, but they are a fairly easy retrofit.
Glenn Wilde farms at Cudworth, Sask., and runs a grain storage solutions company, Wilde Ag Ventures, in partnership with his brother Michael. They make and sell gauges specific to fan models. The Wildes take information from airflow tables for a particular model and overlay that onto the gauge dial, providing a quick estimate of airflow based on the static pressure reading.
With Wilde Ag Ventures’ low-cost gauge, farmers know how much to fill a bin and still meet the target cfm/bu. For example, if the farmer puts 1,000 bushels of tough canola into a bin and the airflow on the gauge reads 3,000 cfm, the farmer can keep adding canola. If at 2,000 bushels the gauge dips down to 2,000 cfm, airflow is at the target 1.0 cfm/bu. This would be a good time to stop filling.
“Basically the farmer can make sure they have at least one cfm/bu. no matter what type of grain it is,” says Wilde.
As noted earlier, the simplest way to relieve static pressure and improve airflow is to reduce the amount of grain in a bin. If bin space is severely underutilized, the farmer could consider a bigger fan with capacity to move more air at higher static pressure.
The key is to recognize that cooling and drying are distinctly different jobs requiring significantly different airflow. A fan, duct and bin system just right for cooling will not necessarily provide the airflow needed for drying.
Drying grain with a swimming pool heater
Glenn and Michael Wilde, who run Wilde Ag Ventures and farm at Cudworth, Sask., use a swimming pool heater to add supplemental heat to their aeration systems. Warm water runs from the pool heater through hoses to a radiator on the outside of the bin’s aeration fan.
“Swimming pool heaters have low upfront and operating costs and are simple to operate,” says Glenn Wilde. They also have thermostat control, which is “extremely valuable,” says Joy Agnew of PAMI. “Controlling the temperature increase from supplemental heating is critical to improve the efficiency of the system and prevent issues related to overheating and condensation.”
Wilde says that with water temperature set at 40 C, they can warm air at the fan inlet to 28 C or so.
The Wildes put the rad on the air-inlet side of the fan rather than between the fan and the bin. While this is not advised with open-flame heaters, it works well for hot-water rads. This also makes it easy to move the rad from fan to fan.
But in this sequence, Wilde says the obstruction before the fan will produce lower airflow even if static pressure stays the same. That means their pressure gauge will read inaccurately.
“To address this, we typically set our airflow requirements at 1.25-1.5 cubic feet per minute per bushel (cfm/bu.) to ensure we have enough flow,” Wilde says. Meanwhile, they are working on a new version of their gauge to read accurately even if the inflow is restricted.
As for the pool heater system, Wilde says, “If someone is interested in this, we’d definitely put together a system for them.”
This is the Wildes’ pool heater system, which attaches by hose to a rad that delivers heated air to the inlet side of an aeration fan.
Know your fans
What is the right fan for the job? Here are four fan types, with benefits and limitations for each.
Axial: These are packaged in round cylinders and the fan itself is like a table fan or airplane propeller.
Benefits and limitations: Axial fans blow more air than other fan types, but they choke out at about six inches of static pressure. For moisture removal, axial fans work better for large-seeded crops like corn and peas or in bins where grain depth is shallow. Axial fans are lower cost. They are also louder than centrifugal fans.
Inline centrifugal: All centrifugal fans are shaped like a paddlewheel or water wheel. They have more blades and push air more aggressively. Inline centrifugal fans have cylindrical housings just like axial fans.
Benefits and limitations: Inline centrifugal fans perform under higher static pressure than axial fans. Like all centrifugal fans, these are quieter than axial fans. Inline centrifugal fans tend to cost less than other centrifugal fans.
High-speed centrifugal: With this design style, air comes in the side and turns 90-degrees as the fan blows it into the bin. Fan speed is 3,500 rpm.
Benefits and limitations: Compared to a similar-sized inline centrifugal, a high-speed centrifugal will keep blowing at higher static pressures, making it best-suited for removing moisture from a small-seeded crop like canola. Airflow is the lowest of all fans, so it could be unnecessarily slow at removing moisture and temperature from large-seeded crops.
Low-speed centrifugal: Shape and style is similar to high-speed centrifugal fans, but these use size rather than speed to push more air. Fan speed is 1,750 rpm.
Benefits and limitations: While low-speed, these large fans can move a lot more air through large-seeded crops. Like axial fans, they choke out at moderate static pressure and are not well-suited to drying small-seeded crops. This is the quietest fan.