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Natural air drying and supplemental heat

Managing risk and defining best management practices.

September 15, 2020  By Donna Fleury

Smaller- to medium-sized bins are more suitable for drying efficiently based on the effect of grain depth on airflow rate and commercially available fan sizes. Photo by Bruce Barker.

Under certain harvest conditions, producers may have to consider drying grain on-farm to minimize the risk of spoilage during harvest. With the right management and operational practices, natural air drying (NAD) systems with additional supplement heat can be a good intermediate system between heated air drying, which has higher capital and energy costs, and NAD systems.

Researchers at PAMI conducted a two-year project to define best management practices and provide practical information to help producers make management and operational decisions related to using supplemental heating. In particular, researchers wanted to determine how the use of supplemental heat affected the drying rate and storage conditions of two common crops, wheat and canola. In the first year, researchers used bench-scale drying trials and bench-scale test bins to evaluate the effect of air flow rate on supplemental heating with NAD compared to NAD alone. In the second year, researchers evaluated the rate of drying with supplemental heat at three different temperature increases (ambient, 5 C and 10 C above ambient). The trials were conducted in mid-late fall to ensure the ambient conditions were representative of conditions where supplemental heating is typically used. Researchers also completed an economic assessment of using supplemental heating systems with NAD and with various fuel types.

“The results from the project reaffirmed that supplemental heat can be used when conditions aren’t favourable for drying without heat,” says Charley Sprenger, project leader with PAMI at Portage la Prairie, Man. “Conditions that are cool, rainy and later into the fall is when supplemental heat will be useful. We were able to provide some good information based on the project, and to develop some best management practices for producers. Supplemental heat can work great, but it has to be managed to be effective and efficient.”


One of the key considerations for using NAD systems, with or without heat, is how damp the starting grain is; a heated air dryer is recommended for moisture contents more than three per cent above “dry” for any commodity. This rule of thumb is based on the achievable drying rates when implementing NAD and the risk of spoilage. Careful monitoring or reducing the grain bed depth to increase airflow rate can help to mitigate risk if a heated-air dryer is not available. Equilibrium moisture condition charts (EMC) for grain should be used to help guide when to add heat. Generally, a 10 C increase in temperature of the air going into the bin cuts the relative humidity (RH) of the air in half, which increases the air’s capacity to dry based on the EMC equations. The project results showed that a 10 C increase in temperature is adequate if that achieves a plenum temperature of greater than 5 C. Greater energy requirements and higher temperature increases would be required if sub-zero ambient conditions are being experienced for prolonged periods of time.

“One of the most important factors when managing grain drying in a bin is that the fan being used is large enough to get sufficient airflow through the bin,” Sprenger explains. “Look at the size of the bin, and more importantly the depth of the grain, and then compare the fan size. The airflow rate (cfm) from the fan should be at least one cfm per bushel in order to get any drying in the bin. Smaller to medium size bins are more suitable for drying efficiently based on the effect of grain depth on airflow rate and commercially available fan sizes. If sufficient airflows can’t be achieved in a large bin and that is all that is available, it is recommended to dry grain in smaller batches and move to another bin for long-term safe storage. Grain must be cooled to less than 15 C after drying. It is also important to have at least one moisture-temperature cable monitor in the bin for monitoring both during drying and after, during grain storage.”

There are several fan manufacturers and increasingly more heaters available for grain drying, although the efficiencies of these systems are not all known. Producers are recommended to always use a certified CSA heater designed for use with grain storage fans for safety and grain quality reasons. There are various alternatives for implementing direct or indirect systems either upstream or downstream of the aeration fan; however, all come with advantages and disadvantages unique to individual operations. “There can also be concerns about condensation and water dripping or freezing on the bin roof when adding heat to NAD systems in colder months,” she adds. “Make sure there is enough air flow to get the damp moist air moved right out of the bin. There are some new ventilation systems or strategies, such as raising the roof on the bins, to avoid this problem. This is one of the projects we are starting to investigate and work with manufacturers to find out more information.”

The project also included an economic assessment of using supplemental heating systems and various fuel types with NAD to summarize the capital and operating costs. The economic assessment showed that fuel type has the greatest impact on operating costs, with natural gas being the most inexpensive fuel, compared to diesel and propane. However, access to natural gas can be capitally hindering in certain regions. Drying system efficiency can also vary greatly. The efficiency of NAD systems with supplemental heat range from 50 per cent to 75 per cent, compared to efficiencies of 40 per cent to 55 per cent for dedicated heated-air drying systems.

“One of our key conclusions is that NAD with supplemental heat systems can work very well, but requires careful management and attention to the ambient conditions, otherwise it could be more inefficient and less effective than expected,” Sprenger notes. “It is really all about managing risk, and coming up with the management decision that makes sense for individual operations. If conditions outside are not favourable, then producers have to decide whether an in-bin with supplemental heat system makes sense or if a dedicated dryer system is required. Where it makes sense, supplemental heat can be a good in-between option if you don’t have access to a dryer, and can add a heater to existing aeration fans and turn poor drying days into good drying days. However, careful management is required to keep operating costs of a NAD system with supplemental heat comparable to that of a dedicated dryer system.”

General recommendations for implementing supplemental heating were developed and are available online at


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