Keeping your stored grain cool and dry

With increased storage depth comes increased risk. Are we moving enough air through today’s larger grain bins?

For anyone visiting the rural Prairies for the first time in a few years, one of the first things they might notice is that those are not their dad’s grain bins — 20,000- to 50,000-bushel bins are now a common sight.

But bigger bins mean bigger challenges in managing moisture, and bigger losses if things go wrong. Aeration is the key, but fan technology has not caught up with the need to keep air moving through larger volumes of grain, say researchers at the Prairie Agricultural Machinery Institute (PAMI) — an applied research organization headquartered in Humboldt, Sask., with a branch office in Portage la Prairie, Man.

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PAMI researchers Joy Agnew and Lorne Grieger have been investigating the best ways to manage grain in large bins for several years, and say much of their research has provided more questions than answers. They’ve identified supplementary heating, fan size, layering and ventilation as subjects for further research.

“There is a combination of things you need to understand to try to get all the pieces to the puzzle,” Grieger says. “The risk is that much higher if you have a quarter of a million dollars in the bin, so you want to be sure you use the right method.”

That’s all the more reason to install monitoring systems. A growing number on the market monitor grain in different ways. All have their upsides and downsides, but at the end of the day producers should have one, Grieger says.

“Any monitoring system is better than nothing.”

NAD is not aeration

The three main methods of managing grain in bins include aeration, natural air drying (NAD) and supplemental heating. The first two are often referred to interchangeably even though they are quite different, so a quick refresher is useful.

Aeration controls the temperature of the grain, cooling it and evening the temperature profile in the bin. The equipment is basic and widely available — some kind of ducting or air distribution at the bottom of the bin and a fan to blow air through the grain. The method requires low airflow rates, usually 0.1 cubic foot per minute per bushel (cfm/bu.).

NAD controls both moisture and temperature. This method relies a lot on the temperature outside the bin and is typically used mid-season when the air has the greatest capacity to dry. It requires mid-airflow rates of around one cfm/bu. “Natural air drying needs to have sufficient airflow rate through the grain using the natural ability of warm air to extract moisture from the grain,” Grieger says.

Supplemental heating is used for moisture control, typically later in the growing season when the outside temperature is too cool for NAD. It uses high airflow rates of around 20 cfm/bu., with a cooling cycle used to bring temperatures down after grain drying.

A major challenge with supplemental heating is the high airflow rate required — the higher the airflow, the greater the resistance. “You are extracting more moisture at a faster rate so you really have to have the right combination of temperature and airflow rate to do supplemental heating effectively,” Grieger says.

Complicating factors

Resistance to airflow is one of the biggest challenges for any moisture- or temperature-control tactic. It can be caused by several factors. For example, smaller seeds create smaller voids and lead to greater airflow resistance. High fan speeds also create resistance.

PAMI research on airflow rates in large bins has found that fan technology is not keeping up with the increased grain depth.

“Average bin sizes in the 1980s were 2,000 to 4,000 bushels with typical grain depths of 10 to 20 feet,” Grieger says. “Meanwhile, average bin sizes today average 20,000 to 50,000 bushels with grain depths of 20 to 40 feet.”

The risk is that much higher if you have a quarter of a million dollars in the bin, so you want to be sure you use the right method.” – Lorne Grieger, PAMI.
photo: Prairie Agricultural Machinery Institute

He says one of the key questions PAMI researchers have been asking is how lower-speed fans perform compared to their high-speed counterparts when using NAD in large bins. “One of the things we’re trying to understand is the static pressure capacity, or the ability of a fan — measured in inches of water — to force air through the grain.”

PAMI tested a 10-horsepower low-speed centrifugal fan — a small fan that is used on many Prairie farms — on a 25,000-bushel bin of canola. “The bin could only hold 17,000 bushels — or 22 feet — before the fan stalled out at a static pressure of seven inches H²0,” Grieger says.

The expected static pressure in a full 25,000-bushel bin (32 ft.) of canola is up to 16 inches H²0 when blowing 0.25 cfm/bu. of air through the grain. However, a 10-hp fan will stall out at 14 inches H²0.

So do producers need bigger fans (which require three-phase power), more fans, or different fan setups such as power takeoff? It’s still not an easy question to answer because sometimes a solution can create new challenges. Grieger says larger or supplementary fans come with an efficiency trade-off. For example, PAMI discovered that adding a 10-hp fan to 15-hp fan increased airflow rate by a moderate 16 per cent but increased operating costs (mostly electrical) by 66 per cent.

The layering effect

One area PAMI researchers would like to look at is the effect of layering in bins. “It’s just a fact that bins are large now, and as you are taking grain off and filling the bins throughout the day with multiple truckloads, you’ll have greater quantities of grain in various conditions. Grain with different properties such as moisture content and dockage will affect airflow uniformity,” Grieger says.

“There is a layering effect as you fill the bin that’s not something you can really avoid. The question is how you manage the risk that can come with having grain at various moisture contents and temperatures inside the bin.”

PAMI would also like to identify best practices for ventilation in large bin headspaces. Exhaust vents help expel moist air from the top of the bin, preventing condensation under the roof or on top of grain. The current literature recommends one foot of vent per 1,000 cfm. However, Grieger wonders if this is enough in large bins.

“If you’re natural air drying or something that will add supplemental heat while extracting moisture from the grain, you want to make sure you move that moisture to the top of the bin,” he says.

Comparing levelled versus spread surfaces of grain and their effect on voidage and airflow in bins is another opportunity for research. Although initial research found airflow uniformity wasn’t very different between peaked and levelled bins, that’s based on a single trial with one type of spreader — in this case a gravity spreader.

“There’s a call here for additional grain spreader evaluations,” Grieger says.

Ducting, which is essential for temperature and moisture control in bins, also requires further evaluation to see if some systems move air more efficiently, Grieger says. Commonly used systems today include cross-duct, Y-duct, fully and partially perforated floors as well as “rocket” systems.

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