Canola harvested tough or damp can be quite volatile in storage. While cool temperature can reduce the risk significantly, keeping the whole bin consistently cool until delivery can be a challenge and a risk.
These steps will help improve conditioning results and reduce risk:
1. If possible, under-fill available bin space. The less depth to aerate, the better, because it is possible that spoilage could occur before the aeration front even reaches the peak of the bin. If using natural air drying for canola, fill the bin 1/4 to 1/2 full.
2. Level the cone. A flat surface to the grain will improve airflow. Levellers built into bins can also help to distribute chaff and fines.
3. Keep aeration fans on. Even if the air doesn’t have capacity to dry at this point, aerate to cool and create uniform temperature conditions in the bin. While some recommend turning fans off at night or off during the day, this might increase the risk, depending on the situation. The mass transfer of water between air and grain is complex. As Joy Agnew says: “We can predict how air will affect grain moisture content assuming the temperature of the air doesn’t change. But we know air changes all the time.”
4. Use supplemental heat, if possible. Air that is less than 10°C has very limited drying potential. Adding heat to cool air will increase its water holding capacity, and therefore capacity to dry. (See below for more points on this.)
5. Turn the bulk frequently. Turning the bulk can break up potential hot spots that have started to form, and will also help to even out the conditions in the bin. This also provides a chance for additional inspection.
6. Monitor diligently.
Use of supplemental heat
Natural gas, electric or propane heaters added before or after (after is better) the aeration fan can greatly improve air’s capacity to dry. Target a heater with output around 17,000 BTUs. To increase air temp by 10°C, that heater in combination with a 5,000 cfm fan will cost 50¢ to $3 per hour, depending on fuel type and efficiency of heat transfer system
Here are some key tips for that process:
1. Air needs to be heated to more than 10°C to have good drying potential. Air below 5°C will have very little if any capacity to dry.
2. Only heat the air up to 15-20°C. With relatively low air-flow rates of aeration systems, it is important to only use warm air, not hot, to avoid “baking” seeds closest to the fan.
3. Airflow of at least 0.75 cubic feet per minute per bushel is recommended for natural air drying with supplemental heat. (See below for more on how to estimate your fan’s airflow.) This is important. While airflow of 0.1 to 0.2 cfm/bu is adequate to cool a bin of canola, this airflow rate could actually put grain at greater risk if using supplemental heat. At low airflow rates, it could take a week or more to push the drying front all the way through a bin. That means a band of very high moisture air will take a day or so to work through each few feet of grain. Spoilage of tough or damp canola in warm temperatures can start within two days.
4. To estimate the required heater capacity for supplemental heating, multiply the desired temperature rise in degrees Fahrenheit by the air-flow rate provided by the fan in cfm. Then multiply by 0.8. The result will be the heater capacity in btu/hr. For example, for a target temperature rise of 10°C (18°F) and an airflow of 5,000 cfm, the required heater capacity is 18 x 5,000 x 0.8 = 72,000 btu/hr.
Although it is potentially possible to stabilize a bulk if the temperature is uniformly dropped to less than 5°C, drying canola now is better than waiting. Otherwise the bin will be unstable as soon as temperatures warm up, whether next spring or sometime this fall, and will need to be dried immediately. More: Tips for drying tough and damp canola
What if you have a plan for drying, but it means holding onto the grain in the meantime? The graph below is often referenced when trying to determine the number of “safe days” producers have before spoilage begins. Be very cautious using these charts. This work was performed in small tubes, where the temperature and moisture conditions were uniform. Realistically, conditions are never totally uniform in a bin, and this is what precipitates the spoilage process.
These charts should only act as a very rough guide. Assume much fewer safe days in the “real world.”
How to estimate airflow rate (cfm/bu) through a bin
The airflow rate from the fan depends on the fan type (axial, centrifugal, etc.), fan size (hp) and the resistance to airflow (grain type, grain depth, type of ducting, etc.)
If the fan already has a static pressure gauge, read the pressure (usually given in inches of water) when the fan is running and the bin is full (or is holding the amount of grain you want to condition or dry). Then find the fan curve chart for the make/model/size of the fan. Most fan manufacturers list these specs online. An example chart for low-speed centrifugal Chief fans is below. The last digits in the model number indicate the fan size in hp.
Using this sample table, if the static pressure (or resistance to airflow) the fan is pushing against is 6″ of water and you have a 5 hp fan, the airflow rate will be 3,700 cubic feet per minute (cfm). Then divide that number by the number of bushels in the bin to estimate the cfm/bu airflow rate.
If you don’t have a pressure gauge (after-market gauges can be installed), estimate the resistance to airflow based on grain depth using the graph below. Joy Agnew from PAMI has a video that explains in more detail why airflow matters and how to use this graph. (To see the video, click the link or scroll down.)