PETpla.net Insider 05 / 2019

COMPRESSORS PET planet Insider Vol. 20 No. 05/19 www.petpla.net 18 COMPRESSORS A review of PET compressor developments A large amount of air is needed to blow PET preforms into bottles. The compressors that compress atmospheric air into compressed air must withstand extreme conditions such as high pressures, heat and continuous operation. But the success of the blowing process depends on a consistent and clean flow of air regulated at a specific pressure. The first models of compressors constructed for the bottle-making process appeared on the market when the PET bottle industry took off in the late 1970s and early 1980s. Reciprocating piston compressors are the most common type in the stretch blow moulding field. Since the early 2000s, centrifugal compressors have also been found on the PET bottling market. Centrifugal compressors pro- vide 7-13 bar air, which is compressed to the desired pressure by a booster. This type of compressor requires less maintenance than equivalent models of reciprocating compressors as there are fewer inbuilt sliding parts. Dis- advantages are the amount of floor space and the need for two units to do the work. With piston compressors, the air becomes ever more compressed in different cylinders and then is cooled again. After opening the suctions valves, atmospheric air is fed into the cylinder through an air filter and is compressed for example to 4-5 bar when drawing back the piston to the cylinder head, and this warms it up. The compressed air is pushed out of the cylinder through a deliv- ery valve and is cooled in an inter cooler. Moisture is removed. In the next step, compression, for example, to 15-17 bar is achieved. The air is cooled again and dehumidified before compression, for example, to 40 bar is achieved. The required pressure depends to a certain extent on the container’s design and its wall thickness. Thick walls and sharp edges require higher pressures than simpler shapes and thinner walls. As a general rule, the stretch blow moulding process works with a pressure of 10-25 bar for pre- blowing and a maximum of 40 bar for the bottle blowing process. A pressure of 7-10 bar is needed to actuate pneu- matic equipment. For most water or CSD lines, a maximum output of around 3,000m 3 /h is sufficient; for plants with particularly high compressed air consumption, for example for hot filling, models with an output of around 6,000m3/h are available. Depending on the layout of the cylinders, designs include L-shape; Y-shape; V-shape; or M/W-shape, horizontally opposed design. Is the air clean? The purity of the air is essential for the stretch blow moulding process. Water, dust and oil are the typical particles that must be cleaned from the air before bottle production by a separate air purifier system. Some compressor manufactur- ers rely on oil-free systems. However, because of the lack of lubricant, these can more easily generate abrasion particles, making it necessary to filter out these typically larger particles. In order to recycle the compressed air after blowing the bottle, it can be captured using an annular pressure vessel and is fed back into the net- work using an air recovery system. A bundle of energy Manufacturing PET bottles is an energy-intensive process. As the PET market has matured, plant engineers have come to realise that their choice of certain compressor models affects the profitability of the overall system. Ultimately, the compressors are the largest individual energy user in the system, taking around 50% of the total energy expenditure. To save costs and also because of rising environ- mental awareness, more and more companies are paying close attention to the calibration of the air pressure they actually require. As mentioned above, a pressure of 40 bar is the maximum required to manufacture standard PET bottles. Each reduction brings energy and cost savings, which explains the trend towards lower pres- sure settings. Depending on bottle volume and production performance, analysis is performed to establish which blowing pressure is needed for which product. Compressors for PET bottle manufacturing – some highlights 1999 to 2019 as reported in PETplanet 2 0 0 0 The very first article on compressors in PETplanet Insider came from LMF Austria (issue 1/2000). The company had developed and brought onto the market a new compressor design, delivering from 353-1,413 cfm (600-2,400m³/h FAD). The basic aggregate consists of either a 3-stage VC type compressor for 353 cfm (600m³/h) or a VCL type for 706 cfm (1,200m³/h). Its total capac- ity can be doubled by adding a Roots blower as a booster. It is possible to adjust the blower motor capacity via a PLC control system to produce exactly the amount of air required at any one time (e.g. when the size and/ or number of bottles per hour is not constant). Belliss & Morcom supplied their first PET compressor system in 1978, when PET bottle manufacture was still in its infancy. In 1995 the company merged with Hamworthy and became part of the Powell Duffryn group. The merger provided the newly-named company, Hamworthy Belliss and by Gabriele Kosmehl