Grain drying and energy efficiency: The basics

An article published Oct. 4, 2021, Drying Tech Q&A with James Dyck, created some confusion around grain dryer energy efficiency and how to make improvements. I am indebted to an astute reader who brought this to my attention. This article has been written to provide clarity.

Grain drying is a simple concept in theory, but as any dryer operator will tell you, getting it right is as much an art as a science, and takes a lot of experience and knowledge of your own system. Energy efficiency in particular can be hard to determine.

In grain drying, energy efficiency measures the amount of energy it takes to remove water from grain. This is normally measured in British Thermal Units per pound of water (BTU/lb). The metric equivalent is kilojoules per kilogram of water (kJ/kg). 

In theory, it takes 970 BTU/lb. (2,260 kJ/kg) to evaporate water. In the real world, because of inefficiencies and losses, most grain dryers use much more energy than this. 

It’s easy to determine the total amount of energy your dryer uses by looking at your fuel bills. However, the amount of water being removed depends on the amount of grain, and the starting and ending moisture levels. 

For reference, drying one bushel of corn from 25 per cent to 15 per cent moisture will evaporate 3.4 kg (7.5 lb.) of water (3.4 litres or 0.9 US gallons). 

Natural-air or low-temperature drying, where just a few degrees of heat are added to reduce humidity, is generally the most energy-efficient drying method, because it takes advantage of “free” heat in ambient air. 

These dryers can run at efficiencies of 1,200-1,500 BTU/lb. However, this drying method can take days or weeks instead of hours and may not be well-suited to high-capacity drying needs. Natural air drying also requires much more careful management based on relative humidity, or the energy use can increase quickly.

High-temperature dryers are much more time-efficient and easier to control but tend to use more energy. The operating temperature, outdoor air temperature, grain moisture, dryer control system and other options (including whether a heat recovery system is being used) all influence energy use. 

Most high-temperature grain dryers operate at efficiencies between 2,000 and 3,000 BTU/lb. 

It’s difficult to make recommendations that apply to every situation. However, research has found the following to be generally true for high temperature drying:

• Lower temperatures and higher airflow rates generally result in lower variation in moisture levels across a drying column.
• Higher temperatures and lower airflow rates generally result in better energy efficiency.
• Test-weight generally increases during drying, with higher drying temperature resulting in lower test-weight gains. 
• Higher drying temperatures generally lead to higher breakage during drying. 
• The type of dryer has a big impact on energy use, although similar dryers can still have large differences in energy use.
• A 2011 study of 10 grain drying systems in Wisconsin by Brinker and Anderly found efficiencies ranging from 1,800 to 3,700 BTU/lb. 
• An ongoing three-year study in Alberta by 3D Energy Ltd. is monitoring 36 in-bin and five continuous dryer systems. The efficiencies range from 1700 to more than 6,000 BTU/lb. for continuous dryers, and from 1,300 to nearly 8,000 BTU/lb. for fuel-fired in-bin dryers. 

So, what does this mean for the average grain dryer operator? How can you get closer to that magical “minimum” amount of energy to dry your grain? 

• Monitor your dryer’s energy use by keeping track of your fuel bills during drying season. 
• Get up-to-date information by taking daily readings of your gas meter.
• Keep daily records of incoming and outgoing moisture levels and number of tonnes or bushels dried.
• Keep dryers clean and well-maintained. Keep screens clean to reduce static pressure.
• Clean grain before entering the dryer to remove fines and small particles that restrict airflow.
• Be aware of the maximum allowable grain temperature for the end use of your grain and set your dryer accordingly. Livestock feed, flour milling and seed all have different heat limits.
• Try adjusting dryer temperature (up or down) by 5 degrees C and run through a drying cycle. Monitor any change in energy use by checking the gas meter before and after the new cycle. Keep a close eye on grain quality coming out of the dryer, as higher temperatures can increase cracking and heat damage.
• If test weight is a concern, lower the dryer temperature in 5 C increments, and monitor how test weight changes after drying.
• Consider heat recovery to improve efficiency. The Wisconsin study found an average of 12 per cent energy savings when heat recovery was installed on dryers.
• Consider combination drying (high-temperature followed by low-temperature drying) or dryeration (high-temperature drying followed by tempering and in-bin cooling). A study by Montross and Maier in 2000 found dryeration and combination drying can save up to eight per cent and 28 per cent, respectively, in energy, and increase dryer throughput by more than 70 to 150 per cent. However, these methods require more handling and careful management.
• Consider newer control systems, particularly for low temperature drying to automate fan operation based on relative humidity. 
• Alternative heat sources such as solar or heat pumps can substantially improve efficiency of low temperature drying.

The most important goal is to get the grain dry. Without that, your stored crop won’t last in the bin and its value will decrease quickly. Energy efficiency shouldn’t take precedence over grain quality, but keeping an eye on energy use might help save a few dollars while you dry.