At the Michelin plant in Homburg, Germany, an oil-free variable speed drive ZR air compressor from Atlas Copco is at the core of a new compressor concept that has already saved the company a six-figure sum in energy costs over the first year. The system functions so efficiently that the specific energy requirement for the compressed air supply fell from 0.143 to 0.108 kWh per standard cubic meter.
ZR315VSD compressor at Michelin
The Homberg plant predominantly produces 22.5-inch front truck tires. Homberg is home to the entire production process, right through to the finished tire: from the rubber mixing process to the tire substrate and shell construction, through to vulcanization and quality inspection.
100% availability a must
Seven compressors are available, which can feed compressed air into two pipe networks, each of which provides air of a different quality. One network supplies cold air at 3°C as industrial air, while the other network supplies control air, which must be completely dry and is therefore cooled to –28°C. “This air is used in the laboratory and for sensitive units,” explains Winfried Schäfer, Technical Manager at Michelin.
If the four turbo and three screw compressors ran at the same time, they could produce around 18 400 standard cubic meters of compressed air per hour. Current average demand is only 10 000 Nm 3/h. Redundancy ensures that production can continue around the clock, even if one or more compressors were to fail.
Speed regulation and master control system save energy costs
Some time ago, the company attempted to implement retroactive regulation of the turbo compressors (which were only able to run at full load), so that the compressed air supply could be better adapted to demand. A 20% success rate was achieved with this system of turbo regulation – although the compressors no longer worked so efficiently.
It was established that the existing compressors and their manufacturer were unable to deliver a satisfactory solution, and a master control system and an oil-free ZR 315 VSD FF screw compressor supplied by Atlas Copco now form the core of a new compressed air concept. In the designation ZR 315 VSD FF, “VSD” stands for “Variable Speed Drive”, the number 315 refers to the installed power in kilowatts, and the “FF” is for “Full-Feature”: full specification with integrated adsorption dryer.
The ZR 315 VSD FF works at a variable, regulated speed. Thus, instead of constantly generating more than the required amount of compressed air, the exact quantity required can now be produced. According to Schäfer: “We never look for the cheapest or quickest solution, but always the most reliable – and it must be efficient.”
System runs 25% more efficiently
The solution with the master control system and the new compressor has already provided significant savings: the specific energy demand was previously 0.143 kWh per standard cubic meter, but now reaches an average of 0.108 kWh. “This means the system now runs almost 25 percent more efficiently,” reports Winfried Schäfer. “Four compressors are usually in operation at the same time – three turbo compressors and the Atlas Copco VSD compressor, which adjusts to supplement the ‘missing’ air up to the target pressure of 7.0 bar and compensates for pressure fluctuations.”
Michelin Tire assembly
Not a drop of oil that could affect quality
The new compressor is water-cooled and compresses with absolutely no oil – and therefore absolutely no risk; another argument that impresses Michelin. Since oil in the production process could impact product quality, this is an absolute no-go for the tire manufacturer. If a single drop of oil were to enter the rubber mixture, the material would become unusable. The MD adsorption dryer integrated in the compressor, which is also speed-regulated, helps to dry the air and cool it to a pressure dew point of –30°C. The integration of the dryer also keeps pressure drop to a minimum, and the energy demand of the MD is extremely low due to its speed regulation system and the use of compression heat for regeneration.
Increases in efficiency save EUR 300 000 per year
“With the new system and the new compressor, we have made savings of at least EUR 300 000, projected over a year, based on 10 000 cubic meters of air per hour and the specified increases in efficiency,” explains Schäfer.
Closed circular pipes are used wherever possible in order to prevent loss of pressure. This has allowed the working pressure to be reduced from 8 to the current level of 7 bar, achieving an additional energy saving of approximately 7%.
Additional VSD compressor could pay for itself within two years
The fact that the integrated MD adsorption dryer in the ZR uses its compression heat to directly regenerate the drying agent aligns well with the Michelin philosophy, which focuses not only on quality but also on sustainability. In Homburg, the tire manufacturer uses the steam generated during the vulcanization process to recover energy for its own factory, and feeds any excess into the district heating network in the town of Homburg.
“Based on our positive experience, a new compressor could pay for itself after just two years,” says Schäfer.