Extrusion equipment can be classified thermodynamically or by pressure development in the extruder (15). According to the former classification, there are (1) autogenous extruders, which convert mechanical energy into heat energy during the flow process; (2) isothermal extruders where constant temperature is maintained throughout the extruder; and (3) polytropic extruders that operate between the extreme conditions of (1) and (2). If classified by the manner in which pressure is developed in the extruder, there are (1) direct or positive displacement types, include the ram or piston-type extruders and the intermeshing coun-terrotating twin-screw extruders; and (2) indirect or vis cous drag types, which include roller extruders, singlescrew extruders, intermeshing corotating twin-screw extruders, and nonintermeshing multiple-screw extruders.
The components of an extruder have been well described (4,16). They consist of drive, feed assembly, screw, barrel, die head assembly, cutters, and take-away system. Figure 1 shows a twin-screw extruder system manufactured by Werner & Pfleiderer with various components. Either AC or DC types of motors can be used to rotate the extruder screw in the barrel. Depending on the capacity, the size of the motors can be as small as a few horsepower for a laboratory-type extruder or several hundred horsepower for a commercial extruder used for full-scale production of extruded foods. Except for simple extruders such as the collet type, most extruder drive motors have a transmission and a speed variation device for controlling the screw rotation speed. Since the screw pushes food forward against the die, which builds pressure that could exceed hundreds of atmospheric pressure, a thrust bearing is required to support and center the screw and absorb the thrust exerted by the screw.
The feed assembly consists of dry ingredient and liquid ingredient feeders. Preblended dry ingredients are held in hoppers or bins above feeders, which can be vibratory, variable speed auger, or loss-in-weight type. Liquid ingredients are metered through positive displacement pumps, variable orifice, variable head, or water wheels. If a precondi-tioner is used, dry ingredients are mixed with water, steam, or other ingredients in a closed vessel, which can be operated under pressure if needed. The uniform feeding of dry and liquid ingredients is imperative for the consistent operation of an extruder.
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It is a well known fact that homemade food is always a healthier option for pets when compared to the market packed food. The increasing hazards to the health of the pets have made pet owners stick to containment of commercial pet food. The basic fundamentals of health for human beings are applicable for pets also.