PETpla.net Insider 04 / 2011

BOTTLE MAKING 36 PET planet insider Vol. 12 No. 04/11 www.petpla.net Compressor coordination The central unit is made up of a battery of compressor packages, each one usually with its motor-com- pressor, air receiver and dryer. ABC Compressors’ normal practice is to form self-contained and centralised packages on a skid (Fig. 1). Outlets are taken from each of the compres- sors to a common pipe, thus forming a central air production unit, run by PLC-controlled management systems to select the optimum combination of compressors. Installation of check valves on the compressor’s outlet will help ensure that pressurised air from the line does not affect the final-stage exhaust valves of an idling compressor. Fit- ting a cut-off valve on the unit outlet makes isolation from the high-pres- sure line easier when, for example, swapping compressor units or under- taking maintenance. Where compres- sor units are cooled by common water systems, installing a solenoid valve in the water inlet pipe will prevent cold water from circulating when the compressor has stopped, thus avoid- ing unnecessary costs, losses and internal condensations. The compres- sor unit, made up of a complete and automated package, possibly built on to the chassis, will simplify entry or exit from the central unit. Compressor regulation Each compressor in the central- ised system will be working at full load or stopped, with the exception of a single, regulating unit. This will minimise energy losses and also help to minimise energy consumed in blowing air. A further important factor should be considered in order to reduce energy consump- tion by the regulating compres- sor. This motor-compressor should have a VSD (variable speed drive or frequency converter), in order to Fig. 5: ABC project with a bottler in 2010 ensure that it idles for the shortest time possible. ABC Compressors’ units claim speed reductiond of up to 50% without compromising lubrica- tion and smooth operation. (Fig. 2) Distribution pipes As compressed air travels through the pipe to the blower, it loses pressure. As the pipe diam- eter gets smaller, the pressure loss increases. If several blowers are connected along the pipe, the high demand for air from any of them could destabilise the line pressure and have a harmful affect on the blowing pressure for downstream machines. The pipe distribution design is fundamental to prevent pressure losses and consequent energy cost. Pipes optimised for the flow and working pressure, with a pressure drop target of 0.5 bar, should be fitted. Future extensions to the cir- culating flow should be anticipated by incorporating new blowers and new compressors. The pipe should be arranged into a closed ring, from which blowers are hung. This arrangement allows more air flow to be distributed with the same diam- eter of pipes, while stabilising the pressure. Best practice experience In 2010 ABC Compressors and a world-class bottler developed and installed a very large compressor for use in a PET bottle production plant. The factory’s high-pressure system was redesigned according to the key aspects previously outlined. An ABC 4HP-6-LT compressor works as a base compressor unit, with two ABC 4HP-4-LT machines of 3,000m 3 /h each, totalling 12,000 m 3 /h 40 bar high-pressure air. The four-stage com- pressors have horizontally-opposed, double acting cylinders and have long running times (around 8.000 hours) between services. The machines are PLC coordinated and regulated with a VSD in one of them. The fac- tory’s high pressure line is in a closed loop and provides air to more than five blowing lines, producing differ- ent container sizes and with highly variable air demand throughout the year. This installation demonstrated that it is possible to make progres- sive improvements, as time and resources become available, and to obtain immediate savings when imple- mented. Adapted from a speech on energy saving in central compressor units, by Ignacio Azcuna. www.abc-compressors.com Fig. 2: VSD. 50% of regulation Fig. 4: Three compressors produce 12,000m³/h 40 bar