Previous airflow/static pressure recommendations developed from small bins still seem to be applicable for safe long term canola storage and recommended practices are effective in conditioning canola for storage over 5 months. However, the fan requirements and grain monitoring practices may need to be adapted for larger bins.
One of the significant changes in on-farm storage in recent years has been the large increase in the average size of grain bins. Large bin sizes increase the airflow resistance and the mass of grain that needs to be conditioned in the same amount of time compared to smaller bins. This project was conducted to determine whether existing recommendations for safe canola storage developed 20-30 years ago still apply from small bins when the average bin size in the prairies has increased to 25,000 bu. For longer term storage over five months, the canola storage target is a maximum of 8% moisture and cooled to 15°C or lower throughout the entire bin.
The specific objectives of this project were to compare the effects of a “peaked” vs. “spread” filling system on the grain distribution, to measure airflow rates and static pressures delivered from commonly recommended fans, to measure airflow uniformity, and to measure grain pressure distribution on the bin floor. The initial trial was to monitor the canola conditioning with a peaked center, which was created by loading canola into the center of the bin directly from an auger. The second trial emptied the partially conditioned canola into a second 25,000 bu bin to equalize the grain temperature and re-filled the first bin using a gravity driven spreader to produce a more level grain surface.
The gravity-driven spreader resulted in a more level surface than if discharged directly from an auger (“peaked”). However, the ‘spread’ grain surface was not completely level as the centre remained somewhat higher than the side and generated high spots several feet from the center. In these tests, actual airflow rates were slightly higher from the peaked test compared to the spread test at an average grain depth of about 20 to 22 feet. Measured static pressures were similar and actually slightly less than the predicted static pressure developed previously from smaller bins.
This project determined that the typical 10 hp centrifugal fans available for single phase power were not adequate to provide the required static pressure and airflow to condition canola in a 25,000 bu bin. After filling to about 17,000 bu or 70% full in a 25,000 bu bin, two 10 hp centrifugal fans operating in parallel had reached their ‘stall’ static pressure and a single 10 hp centrifugal fan was nearing its stall static pressure.
The conventional method of comparing air distribution by observing the relative temperature ‘front’ movement was not suitable in this trial due to the many variable conditions including the ambient temperature change, partial established temperature fronts before the bin was full, and the unexpected grain distribution from the gravity driven spreader. Therefore, an alternate approach was used to obtain an indication of relative airflow distribution by using temperature change ratios between the ambient temperature, and the initial and final grain temperatures. These ratios showed no clear indication that a gravity spreader provided any airflow benefits.
Although the grain pressure measurements did not provide a mathematical relationship between the five sensors at various distances from the bin center, some interesting results occurred. The vertical load steadily increased with grain depth as expected, but the sensors indicated a higher vertical force near the center for the peaked trial and a higher vertical force about 10 ft away from the center for the spread trial. The average load per grain depth suggested that the spreader increased the overall grain density.
In summary, the previous airflow/static pressure recommendations developed from small bins still seem applicable for safe long term canola storage. The recommended practices appear to be effective in conditioning canola for long term storage over 5 months. However, the fan requirements and grain monitoring practices may need to be adapted for larger bins. When comparing the two trials, the gravity spreader provided a marginal benefit in grain surface distribution, a small decrease in airflow rate, no differences in the airflow uniformity, and an increased grain density when storing canola. However, these conclusions are only applicable to a gravity spreader since the grain distribution did not produce a level grain surface as intended and were only measured on a partially filled bin.
Future work is recommended to adapt the previous grain storage research to larger bins. Because the fan limitation prevented filling the bin for these tests, another similar project to determine the suitable fan requirements should be conducted to determine the important aspects of conditioning canola for safe storage. In addition, a program of joint physical testing and computer simulation will provide the most accurate and cost effective approach to provide solutions to the outstanding producer questions that have arisen from the recent trend to large grain bins on farms.