MACHINERY | EXTRUDER DEVELOPMENTS
Typical cost breakdown of extruded products Other
Depreciation interest
Labour Materials
Energy Scrap
Chiller, water cooling
Energy breakdown on a pipe extrusion line Vacuum/cooling tanks Pumps
Main drive
Drying Source: Tecnomatic Source: Tecnomatic The energy input is obtained from the electrical
energy supplied to devices such as drive motor, motor cooling fan. The energy losses – which may occur in these devices and other mechanical or functional systems such as transmission and forced or natural cooling – come under E_losses. Of the energy consuming devices, drive motor and barrel/ die heaters are likely to consume more than 90% of the total energy supply while they are also responsi- ble for the highest energy losses. In extrusion, there is little potential of useful recovery of rejected energy as it is largely released to air or water.
Thermodynamic efficiency The thermodynamic efficiency of an extruder can be determined by comparing the actual energy consumed by the extruder to the theoretical energy required to transform the polymer from initial (input) stage to the desired/output stage. Therefore, the extruder energy efficiency (ηextruder
) is given by: (ηextruder = (_ − _)/_ x 100%
The thermal efficiency (ηextruder, thermo extruder is given by:
) of an Pipe head
Energy savings Pipe extrusion is highly dependent on electricity and a large difference is seen between the energy required and the energy actually consumed during the process – due to power losses in the system related to the drive, transmission, barrel heating and control. Improving both energy and thermal efficiency is a priority for leading manufacturers of extrusion machines. Tecnomatic, for instance, has developed its Zephyr series, which use several design elements to minimise energy losses. One crucial factor for reducing energy consump-
tion is screw geometry. It is very important that the energy input in the system is used with the highest efficiency to transport and plasticise the material. The most energy-efficient extrusion operations are those where most of the heat is supplied by the extruder screw and only a small amount of heating by the barrel and die heaters, without any need for cooling of the extruder. Optimising and enhancing the torque and shearing elements has improved output while allowing the material to be processed at lower melt temperatures. The high throughput and linearity of the output
ηextruder, thermo = _/_ = (m × ∆)/_ = ∆/ = (m × ∆)/ x 100% where:
The machine total energy consumption _ [] in a given interval time ∆ is defined as: _ = P x ∆ and P is the main effective power supplied to the machine in the time interval ∆; and,
The machine-related specific energy consumption is given by the total machine energy consumption divided by the extruder mass m in the same time interval ∆ e = _/ [h/]
32 PIPE & PROFILE EXTRUSION | July/August 2018
at different pressures depends on a properly adjusted intake geometry of the screw. The advantage of guiding the melt in grooves – and increasing the barrel friction coefficient – can also be a problem, as it causes thermal stress and increases the danger of melting material within the grooves. This can be improved by using a spiral grooved feed bush instead of axial grooves. The design of such a grooved feed bush is calculated with regard to the material and the screw geom- etry, and based on a mixture of friction-driven and form-fitting conveying. Energy-efficient direct drives reduce the costs of
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