Energy Consumption Analysis for a Single Screw Extruder - Intelligent Computing for Sustainable Energy and Environment - page 535

used. In such process, the plastic granules is pushed by the screw from the feed zone to
the die, and the heat generated by the shear stress and barrel heating helps to melt the
granules.
Melt pressure near the screw tip is known to be proportional to the screw speed, it
is also slightly affected by the melt temperature and screw geometry, as well as the
material being processed. In industry, this pressure is use as the main indicator of melt
quality. Unstable melt pressure would cause fluctuations on the throughput, which re-
sults in variations on the quality of final product. However, previous research has shown
that the melt viscosity is probably the best indicator of melt quality [6].
The melt viscosity can be described as the resistance of material to flow, and is
derived from the ratio of shear stress and shear rate of the flow as shown in equation (1)
η = τ
γ

(1)

where η represents the viscosity, τ is the shear stress, and γ denotes the shear rate.
Additionally, shear stress is determined by the pressure drop in a slit die or capillary
die, while the shear rate is proportional to the volumetric flow rate through the die. For
a slit die, the viscosity can be calculated by [7]

η = ΔPWH 2
12 LQ

3 n
2 n +1

(2)

where n is the power law index under the operating conditions, ΔP denotes the pressure
drop along the slit die, W and H are the slit width and height, L is the length between
the two pressure points, and finally Q represents the volumetric flow rate. According
to [8], throughput Q is related to the melt pressure and screw speed. It shows that for a
low density polyethylene (LDPE), the throughput can be approximated by a polynomial
model with the order up to 2, and the model fit error can be less than 3%.

3 Online Energy Monitoring

The real-time monitoring of power consumption at each component is desirable for op-
timizing the overall energy efficiency. The usage of power meter is the easiest way to
achieve this purpose, but the cost is sometimes too high. Mathematical models based on
process settings can provide satisfactory accuracy in power monitoring, but it is difficult
to apply the same model to other extruder with different geometry and processing ma-
terials. In this section, a simple method based on the controllers of thermal heating and
motor drive will be presented. All methods were developed and verified on a Killion
KTS-100 single screw extruder located at Queen's University Belfast.

3.1 Monitoring of Heating and Cooling Energy Consumption

The Killion KTS-100 extruder has separate temperature controllers (Eurotherm 808) for
each heating zone. The displacement contactors AFM215-303 are used to regulate the
heating and cooling. As a result, the pulse width modulation (PWM) is incorporated to
implement the PID (proportional-integral-derivative) control. Moreover, zone 1-3 (solid