Objectives
- Better define management practices for on-farm storage of flax seed.
- Verify EMC equations for flax seed grown in Saskatchewan.
- Assess the effect of airflow rate on cooling rate, moisture removal and to determine the capacity of using cool air to remove moisture from flax seed.
- Collect data on resistance to airflow of flax seed to assist better fan selection for aeration.
- Develop tools for effective dissemination of information to producers.
Project Description
Grain aeration and natural air drying (NAD) of grain crops are commonly used on-farm to minimize the risk of grain spoilage in the bin. Grain aeration (or cooling) typically requires lower airflow rates of 1.4 to 2.7 L/s per m3 (0.1 to 0.2 cfm/bu) and is used to control grain temperatures. NAD requires higher airflow rates of 10.1 to 27 L/s per m3 (0.75 to 2 cfm/bu). However, the airflow rates required to achieve desired cooling and drying rates have not been extensively evaluated, specifically for flaxseed. Flaxseed, compared to most crops, has a very high airflow resistance and requires higher static pressures to achieve the same airflow rate. This, coupled with the fact that flaxseed has a high oil content, makes safe storage practices even more important. Grain temperature, moisture content, and resistance to airflow (static pressure) need to be measured to better define airflow rates and fan running times for optimum storage practices for flaxseed.
The objective of this project was to conduct replicated bench-scale drying trials to determine the effect of airflow rates on moisture content, temperature, and resistance to airflow of flaxseed. As well, bin-scale data collection provided a further understanding of resistance to airflow of flaxseed at a larger scale. In 2017 (Year 1), a bench-scale drying trial was performed and bin-scale data was collected with the cooperation of local producers. A literature review of existing flaxseed drying characteristics was also performed and included in this report. In Year 2, bench-scale trials and bin-scale data collection was repeated to validate Year 1 findings.
The bench-scale apparatus consists of six bins, each with a 530 L (15 bu) capacity. Each bin is outfitted with a fan, plenum, sampling ports, and instrumentation to continuously monitor the temperature and relative humidity (RH) of the air at the inlet, outlet, and within the grain. Each bin is also suspended on a load cell to log the weight gain and weight loss of the grain throughout the trial. The Year 1 bench-scale trial for flaxseed (average moisture content of 14.2%) began on October 10, 2017. Airflow rates of 2.7, 6.7, and 13.4 L/s per m3 (0.2, 0.5, and 1.0 cfm/bu) were applied to two bins each. The fans ran continuously for 38 days (data was also recorded for an additional three days after fans were shut off), and the data was analysed to determine the effect of airflow rate on moisture removal and cooling rates. In Year 2 (2018), the bins were loaded with flaxseed at 11.4% moisture on October 30, 2018. The same airflow rates as in Year 1 were applied randomly to the replicate bins in Year 2; the fans ran continuously for 22 days. An additional eight days of NAD with supplemental heat was examined at the end of the Year 2 trial to form a hypothesis for continued research; NAD with supplemental heat may be an option for late harvested flax, but an understanding of the effect of adding heat on seed quality and uniformity of moisture removal is required.
In Year 1 (2017), the moisture content (MC) decreased over the 38-day drying period for all three airflow rates. The high airflow rate (13.4 L/s per m3 or 1.0 cfm/bu) resulted in the fastest moisture removal and the overall highest moisture loss. At the end of the trial, the average MC of the grain inside the bins with low, medium, and high airflow rates was 12.9%, 12.2%, and 10.4%, respectively, down from an average of 14.2% at the start of the trial. This resulted in a reduction in MC of 1.5%, 1.9%, and 3.0% for the low, medium, and high flow rates, respectively. Flaxseed is considered dry at 10%, so only the high airflow rate came close to achieving a “safe to store” MC within 38 days. The minimum grain MC in the bins was the same as the final MC and occurred at the end of the trial for all fan flow rates.
For the Year 2 (2018) flax seed drying trial, the ambient conditions were not favourable for drying as the equilibrium moisture content (EMC) of the air was greater than the starting grain MC for most of the trial duration. The high airflow-rate treatment achieved a MC reduction of 1% by the end of the trial; however, none of the bins reached a “safe-storage” moisture. The 6.8 L/s per m³ (0.5 cfm/bu) and 2.7 L/s per m³ (0.2 cfm/bu) rates both resulted in moisture losses of 0.2%. An additional 1% reduction in average bin moisture was measured during the additional eight days of NAD with supplemental heat (+10°C) in the high airflow rate bins. The mid and low airflow rates showed little success at moving additional moisture through and out of the grain bulk over this period.
EMC charts are available in literature for flaxseed using the modified Henderson model. The in-grain temperature and humidity data were used to calculate the MC of the grain using these EMC models and correlated with the actual MC of the samples collected from those bin locations. The MC calculated by the modified Henderson and modified Chung-Pfost models ranged from 4% under to 3% over the actual MC; however, both were a good fit for the data as the majority of sample points were predicted within a 1% MC difference. Therefore, these EMC charts may be used as a general guideline to understand how air conditions may affect grain MC, but they are not exact.
The temperature data indicated that airflow rate had little effect on grain temperature, particularly in the bottom part of the bin. The grain temperature at the top did not fluctuate as much with the low airflow rates as it did with the higher airflow rate. Grain temperatures generally followed ambient temperature trends; up to one day of lag was observed at the top of each bin.
Resistance-to-airflow data was recorded from both the bench- and bin-scale trials. The measured values were higher than the predicted airflow resistance (static pressure) for flaxseed, based on Shedd’s model, in all cases. This was expected, as the resistance to airflow models only account for the resistance of the grain itself whereas the measured values include the resistance due to the ducting. Once these additional static pressures were accounted for, the airflow resistance model proved to be an adequate predictor of airflow resistance of flaxseed.
