processing feature | Improving energy effi ciency
production rate increases, then the SEC (kWh/kg) decreases. This is the inevitable result of amortising the ‘fi xed’ loads over the ‘process’ loads at both the site and at the machine.
The wonderful thing about this is that there is no confl ict at all between the desire to improve energy effi ciency and the desire to improve production output. Unlike in cars, where the energy effi ciency gets worse as the speed increases, the energy effi ciency of extruders gets better as the production rate increases. Being green can also be profi table.
Figure 2: Site SEC for extrusion
Amortising the ‘fi xed’ loads over the ‘process’ loads to reduce the total load (per kg) works not only when the ‘load’ is the power used but also when the load is the cost of the product. Increasing the production rate (kg/h) will decrease the relative cost (£/kg) for most costs and a general curve for this is shown in Figure 4 overleaf on page 71.
Figure 3: Machine SEC for extrusion
extrusion companies on where they should be looking for energy savings and the main extruder motor is the obvious fi rst choice.
There is no confl ict! In our work on reducing energy use at extrusion companies, we have measured the energy use of many sites and many machines. The most common (and often badly misused) index of energy use for both sites and machines is the Specifi c Energy Consumption or SEC. This is the familiar kWh/kg value that many sites commonly use to measure their energy effi ciency at either the site or machine level. At the site level, the fi eld results for SEC versus
production rate give the curve shown in Figure 2 and this can be used as a benchmark curve to check the effi ciency of the site. This can be compared with similar actual fi eld
results at the machine level (shown in Figure 3) which can be used as a benchmark curve to check the effi ciency of an individual extrusion machine: Both Figure 2 and Figure 3 show that as the
68 COMPOUNDING WORLD | March 2014
At any level and for almost any load, running plastics processing machinery harder increases the effi ciency. However, this is not a linear relationship, it is a reciprocal relationship (y = Ax-1+B) and the improve- ments are also not linear. When the production rate is already high, then increasing the production rate further may give only marginal improvements to the energy effi ciency or the production costs; the real benefi ts are when the process is already operating at low production rates. As production rates increase to high levels, the energy effi ciency or cost approaches the value of B.
Working the motor hard Working the extruder hard will increase the relative energy effi ciency of the process but the largest energy use in the process will still be the main extruder motor (see Figure 1b). The relative percentage used in the motor will vary with the process but the main variations in sites will occur after the actual extruder, for example operating the calibrators for profi les, the rollers for sheet, the ovens for BOPP or the pelletizers for compounding. The extruder motor uses approximately 66% of the
power input to the extruder and it is essential to get the right motor for the job. There are now a range of motor options, but whatever the motor type, the fi rst essential is to get the right size of motor and extruder for the job. The extruder and screw design should always be checked to make sure they are right for the polymer and product.
Extruders run most effi ciently (not only in energy
terms) when operating at the design conditions. As far as is possible, the extruder should be set to run at the maximum design speed, as this is usually the most
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