Reliable pneumatic conveying of fish feed

Fish feed pellets are susceptible to damage and valuable feed can be lost during pneumatic conveying of feed pellets from the feed barge to the sea cages. Experiments were designed to determine the lowest airflow rate necessary for reliable conveying without risk of pipe blockage and damage to the product at different product flow rates. The technical quality of the feed was assessed by use of three different methods: (1) the Holmen durability tester; (2) pellet strengths in diametrical compression and Weibull analysis; and (3) in an impact tester with use of high-velocity photography. Reliable conveying conditions were determined for product flow rates in the range 0.17-0.83 kg s(-1) and air mass flow rates in the range 0.18-0.23 kg s(-1). A plot of the feed pellet flow rate that ensured safe conveying as a function of the rpm of the air mover (roots type blower) showed a linear relation. Only 0.7% of the feed were lost when transported at the highest inlet air velocity tested. The Holmen durability was high, 98.9%, and the feed pellet strengths could adequately be represented with the Weibull distribution with a value for the modulus, m of 10.1 and the 37% Weibull strength at failure was 355 kPa. Impact fracture was not seen at a particle velocity of 15 m s(-1), but fracture (chipping and cleavage) was triggered off at a particle velocity of 20 m s(-1), and destructive fracture was observed at 30 m s(-1). The value for the Weibull modulus, m is the highest reported in feed pellets, good technical quality of the feed is one reason for the low product loss during transportation. In addition, the result of the impact experiment suggests that the particle velocities during transportation were less than that required for causing fracture in the feed particles. Different meal sizes transported to the fish requires specific levels of air mass flow rates, and it is suggested that optimum air supply can be provided by controlling the rpm of the blower. (c) 2005 Elsevier B.V. All rights reserved.