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Extruder technology: A view from the wear side Katharina Diehl, Carl Aug. Picard GmbH, Albert Keun, Pro-Consult Kolding Aps
Extrusion of aquatic feeds is a very broad and highly complex process considering the number of different aquatic species being farmed in the world today. Feeds for shrimp, eel, trout, salmon, catfish, carp, tilapia, milkfish, yellowtail, frog, etc. are some of the major aquatic feeds manufactured on either a single-screw or twin-screw extruder. Aquatic feeds can be defined as floating feed, suspended feed and sinking feed. Cold-water fish, like salmonids and trout, normally are top feeders and require a highly expanded floating feed that must be stable in water for at least 10-15 minutes – the time required for top feeders to consume their feed. Seabass and seabream require suspended feed – they are not top or bottom feeders. Usually, warm-water fishes like carp, eel, tilapia, milkfish, shrimp, etc. are bottom feeders and require a denser sinking feed that must be stable in water for at least 30-60 minutes – the time required for bottom feeders to consume their feed. Extruded aquatic feeds with good water stability are important for maintaining water quality and good conversion ratios.
Figure 1. Material portfolio wear parts for single screw extruders.
Extrusion The pre-set screw and barrel configurations represent many years of analytical design, research and comprehensive testing and have their own specifications to produce floating aquatic feeds, semimoist aquatic feeds, slowly sinking aquatic feeds and sinking aquatic feeds. The first 25% of the extruder length normally is the transport section which utilizes twin flight transport screw elements. Then, the mixing and kneading zones come, utilizing cut flight reverse screw elements and both single and twin flight screw elements. The single flight screw elements serve the need to hold the pressure generated within the mixing,
kneading and cooking zones constant and stable. The last 25% of the extruder length, called the cooking zone, utilizes single-flight feeding screw elements just before the die plate. Moisture levels of 25-30% in the form of steam and water are a catalyst in extrusion. This moisture combined with the accumulated pressure from the cooking, mixing and kneading zones results in extraordinary wear of the screw elements starting at the outlet. Therefore, the choice of metallurgy used for both screw elements and barrel liners is extremely important. C.A.PICARD® does not only offer a wide range of metallurgies for aquatic feeds, pet food, plastics, etc., but have also optimized its
Aquafeed: Advances in Processing & Formulation Vol 14 Issue 2 2022
