Absolutely oil-free

Armin Salzmann

The Schwan pencil factory, founded in 1865, has developed into a globally successful corporation with an annual turnover of 340 million euros (business year 2006/2007). Although lead pencils are still produced today, the Schwan-Stabilo Cosmetics subsidiary, with a turnover share of approximately 54 %, has meanwhile evolved into an independent cosmetics provider that operates worldwide, supplying leading cosmetics brands with cosmetic pencils as a private label partner. This admirable success on the market was only possible with particularly high-quality products. The quality demanded of prod-uction, and hence also of the compressed air, is correspondingly strict.

“Our products have a lot in common with the pharmaceuticals industry. This is why absolutely oil-free compressed air is an imperative prerequisite for our production processes. It not only serves as control and drive air, it also comes into contact with the cosmetic masses and the final product“, Maintenance Manager Klaus Krieger emphasises. “We guarantee to all Schwan-Stabilo Cosmetics customers that we only use absolutely oil-free compressed air. This represents a crucial additional quality argument. Today, we produce this absolutely oil-free compressed air exclusively with water-injected screw compressors in the Almig Lento series.”

Until January 2007, oil-free compressed air was produced by one oil-free compressing piston compressor as the base load plant (delivery rate 17.6 m³/min) and two oil-injected, air-cooled screw compressors as reserve machines (delivery rate 2.63 and 4.17 m³/min; oil-free treatment by means of activated carbon adsorbers). If this piston machine failed, only a very modest emer-gency supply was feasible. Furthermore, there was only one refrigerant dryer, which meant that if it broke down, there was no dried compressed air available. This lack of redundancy in the production and treatment processes significantly impaired the company’s production reliability. This was not the only problem: the policy at Schwan-Stabilo Cosmetics was also “away from oil”. In January 2007, therefore, two water-injected Almig screw compressors were put into service to replace the two oil-injected screw compressors (all delivery rates referred to 7 bar operating pressure), one variable-speed Lento machine (Lento 55, motor rated output 55 kW, delivery rate from 2.04 to 8.19 m³/min) and one fixed speed Lento (Lento 37 D, motor rated output 37 kW, delivery rate 4.9 m³/min). The old oil-free compressing piston compressor remained as a standby machine.

“We took a conscious decision not to purchase an oil-free compressing piston compressor”, Werner Reiß, Head of Mechanical Plant Maintenance, comments. “These compressor systems work with oil-flooded gears in the neighbouring compression chamber. Leaks at this interface have already caused problems in other production areas. We wanted to make absolutely sure. This is why there was no alternative to the water-injected type.”

The simple and clear design of the water-injected Lento machines is very convincing. They compress in a single stage at very low speeds of only 3000 to 4000 min-1. Compression is almost isothermal and therefore with very little heat generation. And heat that is not generated during compression has to be neither paid for nor dissipated. The compressor stages are manufactured with utmost precision and achieve a service life that is comparable to their oil-injected counterparts. The rotors, which are made from a high-strength plastic compound, work in a housing manufactured from a non-corrosive bronze alloy. They are mounted in greased cylindrical roller and angular ball bearings for maximum operating reliability. Almig invested considerable effort in the design of the compressor stages with their hermetically sealed bearings. For example, seals on the rotor shafts prevent grease from the bearings from penetrating the compression chamber: Mechanical, water-lubricated shaft seals provide an additional grease barrier.

The grease-lubricated bearing chamber is moreover equipped with atmospheric apertures, so that excess grease can escape if need be and is prevented from entering the compression chamber owing to a natural pressure differential. The maximum quantity of grease in the bearings is 12 g.

The Lento machines function with a closed cooling water system. To a large extent, compressed air and water are separated in a stainless steel cyclone pre-separator. The 100 % saturated compressed air is effectively cooled down in the integrated refrigerant dryer; precipitating condensate is returned to the internal water system as fresh water. Excess condensate is discharged by an electronic steam trap and can be introduced into the sewage system without any further treatment. Thanks to the permanent generation of condensation, the continuous exchange of water in the system guarantees optimal water quality and makes it unnecessary to add any water from the drinking water supply. A galvanic anode in the water system rules out system damage while a water filter filters out suspended matter. The variable-speed direct drive adjusts the delivery rate of the controlled plant precisely to fluctuating compressed air requirements, prevents switching and avoids the costly no-load times that result. The energy-efficient start-up phase takes place without power surges. Furthermore, the operating pressure can be freely selected in 0.1 bar steps between 5 and 10 bar. By adapting flexibly to each specific application, Lento machines save valuable electricity and hence hard cash. When work begins, the variable-speed Lento 55 starts up first of all. As soon as it reaches around 90 % of its output (approximately 7.4 m³/min), the smaller Lento automatically starts up at a fixed speed (delivery rate 4.9 m³/min) as the base load plant; the controlled plant cuts back accordingly and handles varying peak requirements. The station works completely autonomously at a constant high pressure of 7.8 bar. Supplies from other stations (e.g. with oil-injected compressors and subsequent oil-free treatment) are non-existent, even in an emergency. The oil-free compressing piston plant is currently only kept as a reserve machine.

“Since our machines were commissioned eighteen months ago, our experience with the water-injected screw compressors has been altogether positive. They deliver immediately without any problems, apart from minor readjustments at the outset. We have therefore already ordered a third water-cooled machine. This screw compressor will be accompanied by a revision of the general concept of our oil-free compressed air station for the cosmetics sector“, Maintenance Manager Klaus Krieger explains. Once again, the variable-speed version was selected when ordering the new machine. It is identical in design and output to the Lento 55 supplied back in January 2007. After the third compressor has been installed, the two variable-speed screw compressors will be able to work with the fixed-speed Lento compressor, providing mutual redundancy. The company also hopes to reduce the pressure from 7.8 to 7.3 bar, which would bring a 3 % electricity saving.

When this third plant is installed, all three machines will be arranged in series. In addition, the waste heat that is produced by the water-cooled compressors will be discharged from the station via a special duct system. A thermostat-controlled bypass valve will ensure optimal room temperatures, even in winter. This function is pres-ently performed by a thermostat-controlled exhaust air fan. The waste heat duct system will further reduce the temperature in the station and hence also the temperature of the air that is sucked out of the chamber for compression, thus improving the thermal situation in the compressors. The supply air required for compression will continue to enter the station from the outside and be sucked out of the chamber by the compressors.

At Schwan-Stabilo Cosmetics, the compressed air is initially guided via a manifold into a compressed air receiver (volume 5 m³) and then treated in a refrigerant dryer. This external dryer was already installed in the old station. It works at a pres-sure dew point of approx. 3 °C and will be active in the new station too, whereas the standard dryers integrated in the Lento machine pre-dry the air at a pressure dew point of approximately 3 to 7 °C. The external dryer also has an integrated dew point gauge that immediately reports any deviations. “Despite absolutely oil-free compression, the compressed air flows through an activated carbon adsorber, which acts as a police filter, before entering the network. The adsorber serves to remove all oil particles from the compressed air which – like those from the exhaust system of a motor-ised vehicle, for example – could potentially get into the system“, Gerhard Bottner, deputy to Werner Reiß and Mechanical Plant Maintenance Coordinator, explains.

Each individual compressor in the station is currently connected to the building automation system via a group fault signal. Faults are reported to the factory gate for 24-hour monitoring by means of a fault sensor printer. In addition to the group fault signal, the serial interface of the machines permits detailed data to be forwarded, not only to the automation system but also to an external service station, for instance, or to Almig. In the final configuration, all detailed operating data and fault messages supplied by the compressors will be reported to the building automation system. A maintenance contract has been concluded for the station that provides the owner with a two-year warranty on the equipment as well as fixed, and hence calculable, maintenance costs. Only spare parts are charged on top of the agreed fixed price. The guaranteed use of original spare parts extends the service life of the compressors. The owner and Almig have agreed on a maximum response time of 24 hours for service.

“For about eighteen months, we have been gathering experience with the water-injected screw compressors. This trial period has been entirely positive. The concept has utterly convinced us. If a company wishes to exclude the risk of residual oil in its compressed air, the only option in our opinion is the direct method using water-injected compressed air production. At the same time, these compressors improve the quality of the compressed air because the dust particles contained in the intake air are reduced to around one third by the injected water”, says Reiß, summing up.

Almig Kompressoren GmbH sales@almig.de www.almig.de