Segmented screwbarrel singlescrew wet extruders

A typical drawing of the single segmented wet extruder is shown in Fig. 3.1. Segmented screw/barrel single-screw extruders are the most widely applied cooking extrusion design in the food, pet foods and feeds processing industries. 'Wet' means that steam and water can be injected into the barrel during processing. Typically, the barrels of these machines are also equipped with heating and cooling jackets. They process more tonnage of extruded products than any other extruder design. The products produced range from fully cooked, light density corn snacks, to dense, partially cooked and formed pastas (Rokey, 2000). They are the focus of discussion in this section.

A typical single-screw extruder consists of a live bin, feeding screw, preconditioning cylinder, extruder barrel, die and knife. The live bin provides a

Fig. 3.1 Single-segmented wet extruder.

buffer of raw material so the extruder can operate without interruption. Typically, the height of raw material in the bin is maintained within defined limits by high and low sensors which activate a conveyor supplying the bin. The bin is designed to prevent bridging of its contents and blocking the feed screw leading to the preconditioner. Speed of the feed screw to the conditioner or extruder must be variable to ensure a continuous uniform supply of raw material, which, in turn, leads to consistent and uniform operation of the extruder.

Because single-screw extruders have relatively poor mixing ability, they are usually supplied with premixed material which often has been preconditioned with added steam and water. Generally, preconditioning prior to extrusion enhances extrusion processes which benefit from higher moisture content and longer equilibration time. Preconditioning of the raw material typically improves the life of wearing components in the extruder by several fold. Although the weight of ingredients in the extrusion system is increased, preconditioners are relatively inexpensive to build for the volume they hold and time added to the process for preconditioning. Product quality can be improved greatly by preconditioning the raw ingredients.

The single-screw extruder barrel assembly is composed of a jacketed head, a rotating extruder shaft which carries screws and shearlocks, a stationary barrel housing, a die, and the product cut-off knife. The screws are the key element of the single-screw extruder and their geometry influences performance of the extruder. The barrel bore may be uniform in diameter from inlet to discharge; it can be tapered, decreasing in a bore diameter from inlet to discharge; or it can be of uniform diameter with the final segment of the barrel being tapered or decreasing in diameter. A screw configuration consisting of a variable pitch, constant depth, increasing root diameter, increasing number of flights, shear-locks, and decreasing end diameter is most frequently used in the food industry.

A single-screw barrel can be divided into three processing zones: feeding zone, kneading zone and the final cooking zone (Mercier etal., 1989). The feeding zone generally has deep channels which receive the feed. The preconditioned or dry material entering this zone is conveyed to the kneading zone. Water may be injected at this point to help develop a dough and improve heat transfer in the extruder barrel. As the material is conveyed into the kneading zone, its density increases because of water and steam addition. Screw pitch in this zone decreases and the flight angle also decreases to facilitate mixing and a higher degree of barrel fill. This zone applies compression, mild shear and thermal energy to the feedstock, and the extrudate begins to lose some of its granular definition. By the end of this zone, the feed material is a viscoamorphic mass at or above 100°C (212°F) (Faubion et al., 1982). The reduced slip at the barrel wall prevents the food material from turning with the screw, referred to as 'drag flow' (Miller, 1990). A continuous screw channel serves as a path for 'pressure-induced flow' because the pressure behind the die is much higher than at the extruder inlet. 'Leakage flow' also occurs in the clearance between the screw tip and the barrel wall. The flight of the screw may be interrupted in this area to further increase mixing via leakage flow (Rokey, 2000). The mechanism of shear begins to play a dominant role because of the barrel fill in this zone. Steam and water can be injected in the early part of this zone. Steam injection increases thermal energy and the moisture content of the extrudate. As the extrudate moves through the kneading zone, it begins to form an increasingly cohesive flowing dough mass, which typically reaches its maximum compaction. The material exhibits a rubbery texture similar to a very warm dough. At this stage, the material enters the extruder final cooking zone. Screw flights in this zone are typically shallow, and have a short pitch. The function of this zone is to compress and pump the material in the form of a plasticized mass to the die. Temperature and pressure typically increase very rapidly in this region because of the extruder screw configuration. Shear is highest in this zone, and product temperature reaches its maximum and is held for less than five seconds before the product is forced through the die (Harper, 1978). The product expands as a result of moisture vaporization as it exits through the die into a region of lower pressure. The extruded material can then be cut into desired lengths by the knife attachment.

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  • Jo
    Is high moisture extrusion possible with singlescrew extruder?
    4 months ago