Comparison Of Single And Twin Screw Extruders

Twin screw extruders are 1.5 to 2.5 times more expensive at a given throughput than single screw extruders. This is due primarily to the complexity of the screw, drive, and heat transfer jackets (Harper, 1989). The extra cost of a twin screw extruder can be partially offset by the ability to run at lower moistures which would require less energy for drying (Harper, 1989).

In single screw extrusion, the use of steam for heating provides about one half of the energy necessary for heating. The other half comes from mechanical energy. Twin screw extrusion heat is provided primarily with mechanical energy inputs. The use of steam will lower energy costs as it is a inexpensive source of energy giving single screw extruders the advantage. However, single screw extruders run at higher moisture contents than twin screw extruders and with dryer effeciencies only at 40%, the production of low moisture products are becoming more attractive to processors. Twin screw extruders have a increased capability for the production of low moisture products (Dziezak, 1986; Harper, 1989).

Mechanically, twin screw extruders are more complex and have limited operating ranges than their counterpart single screw extruders. Twin screw extruders have limitations on torque, pressure, and thrust that are necessary to prevent excessive damage to the drive train (Harper, 1989).

Twin screw extruders are easier to control in terms of processing variables. This makes them easier to control and operate. Twin screw extruders are also capable of handling a wider range of ingredients due to the greater conveying angle and the self-wiping feature of the screws (Dziezak, 1986; Harper, 1989). Twin screw extruders also improve processing by increasing the amount of mixing in the channel, narrowing the residence time distribution which refers to the amount of time the product spends in the barrel, and the uniform shear rate across the channel depth (Harper, 1989). Pitch angles in the feed section are approximately 15° and produce an effective conveying angle in excess of 30°, or about three times the conveying angle of single screw extruders with similar pitch. The conveying capability of twin-screw extruders allows them to handle sticky and/or otherwise difficult-to-convey food ingredients.

Both twin screw extruders and single screw extruders have many applications in the food industry. Twin screw extruders offer a increased range of cooking applications and allow for better control and operating flexibility. If this is not a requirement for your food application, than the more economical single screw extruder is suitable for your processing.

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