Energy-efficient extrusion: How can we lower energy costs?
Polymer extrusion is an energy-intensive process. In the last decades, many efforts have been directed towards improving the energy efficiency of SML extrusion lines. Especially in times of rising energy prices and a growing environmental awareness, the topic of saving energy is now attracting more attention than ever.
“A major contribution towards energy savings can be made when configuring an extrusion line. Most of the energy is consumed in the extrusion process when melting the polymer and so the choice of the extruder size and the design is crucial”, Hans-Jürgen Luger, Head of Research & Development at SML, clarifies. Extruders are generally efficient when operating with ideal drive and screw configurations. This applies in particular to SML´s High Speed Extruders with diameters of 75 mm and 90 mm, which are designed to deliver up to 1,200 kg/h for PP, LDPE, LLDPE and PS. Compared to extruders with a larger diameter, a HSE requires up to 80 % less heating power with the same throughput. Another example is SML’s HO-LT extruder for sensitive polymers like EVOH or tie materials, which enables high output rates with low melt temperatures. By using a 35 mm HO-LT extruder instead of a 75 mm standard extruder for EVOH with the same maximum throughput, the specific energy input (SEI) is reduced from 250 Wh/kg to 200 Wh/kg.
Factory: efficient cooling water treatment
Apart from the heating and melting of the polymer, the cooling process is also something which deserves consideration. Firstly, the water quality in the cooling circuits is crucial to achieve a high cooling efficiency and process reliability. The heat transfer deteriorates as a result of deposits and cooling circuit contamination and, thus, the cooling water temperature has to be lowered to achieve the same cooling capacity. In general, the temperature level required for the cooling water should not be lower as necessary. Secondly, the system for cooling water generation offers the opportunity to save energy. For example, SML uses an advanced system of water wells on the premises, geothermal pumps (which can also be solar-powered) and a rooftop chiller in the headquarters in Redlham. Additionally, heat dissipated from the extrusion lines in operation is used to heat the building in winter.
Line configuration: efficient reduction of edge trims
Another key-topic is the amount of edge trims ground, refed into the process and re-melted. In principle, the portion of refeeding decreases as the film width of an extrusion line increases. Thus, wide extrusion lines with a larger film width can be assumed to be more efficient. Comparing SML´s MiniCast stretch film line (1,500 mm net width) with SML’s PowerCast XL line (4,500 mm net width) as exhibited at K'2022, the portion of edge trim is decreased from 27 % to 17 %. This means, that the additional power consumption due to refeeding per kg/h net output of the line is 50 Wh/kg for the PowerCast XL line compared to 90 Wh/kg for the MiniCast.
Decreasing heat emissions
Insulating hot surfaces, which are subject to heat loss due to radiation, is a very effective and inexpensive method to cut energy costs. Furthermore, insulation, for example on melt adapters, can be easily retrofitted. Measurements have shown that the power consumption per 1 m length to maintain the set temperature of an adapter can be decreased from 8 kWh without any insulation to 6 kWh with insulation (- 25 %). SML has also modified the heating/cooling-unit for the extruder barrel in order to reduce radiation losses in the heating mode by up to 30 %.
Detecting hidden energy consumption in peripheral equipment
Pump drives or blowers frequently operate at full speed, regardless of the line speed and output. By installing frequency converters and modifying the machine control, the drive speeds and, thus, the power required can be adjusted to demand without affecting the output of the line. A further step is to use bitWise, SML's data generation and analysing tool to optimise the machine parameters to exploit the full savings potential.