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Adding heat to dry out grain in the bin

aerated grain bins

Supplemental heat can turn a poor drying day in November into a beautiful drying day, says Joy Agnew, who manages ag research services at Prairie Agricultural Machinery Institute (PAMI).

“Even if it’s 100 per cent humidity outside and it’s cold, if you warm the air by 10 degrees, you cut the humidity by half. Now it’s 50 per cent humidity and warm air, so it’s perfect drying conditions.”

Agnew thinks supplemental heat has been under-used, although last year’s fall gave it a boost. “But I think it can be utilized almost yearly. And that’s going to widen the harvest window.”

PAMI plans to analyze whether higher grades will make the investment in a supplemental heating system worthwhile. At interview time PAMI was lining up the last bit of funding. Agnew says they hope to have the economic analysis done within a year.

The first thing to know about supplemental heat is that the warmer the air, the more water it can hold, says Agnew. The more water the air can hold, the greater the drying capacity.

Farmers should set a target of 15 C to 25 C for air going into the grain. Anything lower than 15 C won’t have much drying potential, Agnew says. Air hotter than 25 C turns the bin into a heated air dryer, and the bin’s air flow rate can’t handle those high temps, she says.

Whether or not a farmer can dry grain when it’s -10 C outside depends on the system’s heating capacity. If it can only handle a 10 C increase, it’s unlikely to be useful when the outside air temperature is below 5 C.

Agnew also warns that if farmers are adding heat when it’s really cold out, they should keep the air temperature below 25 C as it enters the bin. With hot air inside the bin and cold air outside against the bin’s steel surface, “you’re guaranteed to have condensation and freezing in those situations. So when it’s freezing cold outside, kind of dial back the target temperature going in.”

Equipment options are, Agnew says, “becoming really interesting because there are so many new players in the equipment option game. For example, the oil and gas industry is realizing the potential of grain drying as another opportunity for their equipment.”

The oil and gas industry’s high-capacity fans and heaters are perfectly suited for grain drying, Agnew says. Most bin fan manufacturers have heater options as well, she adds.

Natural gas is by far the most cost-effective way to fuel supplemental heat. Propane, electric, and diesel are also options.

Hydronic systems, which use things such as flax straw as fuel, can also work and can be a low-cost option to produce heat. “The biggest drawback of supplemental heat is the cost to fuel it, basically.”

Heaters can be upstream or downstream of the fan, depending on what works for that situation, Agnew says. Portable heaters are also an option.

Farmers can also put together their own systems. “I’ve heard of all kinds of farmer-rigged systems that work great. But not many of them are commercially available.”

For example, Agnew says a group of Humboldt-area farmers have retrofitted a swimming pool heater system into a hydronic system. Agnew says it’s “extremely robust” and easy to maintain.

The drawback to most heating systems is that they’re either on or off. “You can’t dial it in to have a specific temperature going in to the grain.”

The swimming pool system comes with controls that create a consistent air temperature going into the grain, which is an advantage.

“The energy to pull moisture out of the grain comes from both the airflow rate and the temperature difference between the air and the grain,” says Agnew.

Farmers adding heat to the air will need at least one CFM per bushel. Anything lower than that risks heating the grain without removing moisture, Agnew says.

Agnew says there’s also a rule of thumb to figure out how much BTU one needs: airflow rate in CFM/bu. x target temperature increase (in Fahrenheit) x 0.8. Agnew says the target temperature increase is usually around 18 F.

For example, a farmer wanting to add heat to a 5,000 bushel bin would use the formula as follows: 5,000 CFM x 18 F x 0.8 = 72,000 BTU per hour.

Agnew notes that this formula assumes ideal heat transfer. If the system is losing heat in the transfer, the farmer might have to increase the BTU to offset the loss.

Fan capacity and other available equipment will determine whether a farmer has to fill the bin part way and dry grain in batches. The CFM per bushel might dictate that the bin only be filled halfway, for example.

“The issue with that is now you have a huge headspace in the bin. And that’s where moist air is going to be collecting and accumulating.”

Agnew says in those cases, more ventilation is needed to move out the moist air.

“It’s probably better just to size a fan properly for the size of bin you have so that you don’t have to have a ton of head space in there. But with these big bins, it’s becoming more and more difficult to get a fan big enough to get one CFM per bushel.”

“The big hundred-horsepower fans coming out of the oil and gas industry might change that,” she adds.

It’s hard to estimate how much moisture one can pull from the grain with supplemental heat. It depends on grain type, air flow rate, and other factors.

“But basically it’s what you can expect from a good drying day,” says Agnew. A half a per cent or a per cent a day is a reasonable expectation. Farmers will need to turn the grain to get a uniform moisture and temperature profile, she adds.

Farmers should also keep a close eye on the grain so it doesn’t over-dry. Agnew has talked to farmers who saw two to three per cent moisture drops a day.

“It goes extremely fast if you have the right airflow and temperature combination.”

About the author

Field Editor

Lisa Guenther

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.



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