For FTL Seals Technology, using air actuators is the answer to the difficult ambient conditions found inside a coal pulverizer. The company, which is based in Leeds, England, manufactures pressure cylinders which press the grinding gears of the pulverizer against the material to be ground. The cylinders ensure that the set grinding pressure remains steady, and also balance out the abrasion on the grinding balls. A certain degree of compliance is also required when the grinding balls roll over larger pieces of coal.
Until recently, these pressure cylinders in coal pulverizers have been fitted with traditional metal cylinders with hydraulically-actuated pistons. The manufacturer documented an average service life of three months. The cylinders could not stand up to the coal dust and the oily environment for any longer. Nitrogenoperated air actuators for coal pulverizers, on the other hand, have proven themselves under adverse conditions: they function maintenance-and oil-free and without sliding gaskets.
The expected service life of the new pressure cylinders equipped with air actuators is ten times as long. In one of their first places of application, in a coal-fired power station in South Africa, they even worked for more than five and a half years. The much increased expected service life and oil-free operation mean less strain not only on the coal pulverizers, but also on the environment. Air actuators are now in use in coal pulverizers and other pulverizers in England, Mexico, Holland, India and South Africa.
Air actuators are dimensionally stable elements made of superior elastomer qualities with vulcanized textile tensile members. Two basic designs are the convolution air spring and the rolling lobe air spring. On air actuators designed for pneumatic functions the movement is generated by the flexing of a highly flexible shell. Piston rods and the guiding and sealing elements that come with them are therefore not required in an air actuator. Single, double and triple convolution air springs have a very low minimum height and low return forces. Compared to convolution air springs, rolling lobe air springs have a smaller diameter but the same maximum force, and emit an almost constant level of force via the stroke.
The load, is raised using channeled air, just as it would be with a pneumatic cylinder. The stroke can be realized in a number of different ways: with oil-free compressed air, with compressed air containing oil or also with other gaseous media, e.g. nitrogen. In low-pressure areas, hydraulic media such as water and glycol can also be used.
For application in adverse ambient conditions, air actuators are practically irreplaceable: as they function without mechanically moving parts, they are wear and maintenance-free. Dirt, dust, granulated material and sludge cannot damage the air actuator, and it is also completely resistant to many chemicals.
The standard actuators are designed for temperatures from -30°Cto 70°C. There are also designs for other operating conditions: e.g. alow-temperature cylinder (-60°C) in the wood industry and a hightemperature cylinder for the glass and paper industry (up to 115°C constant temperature and 130°C peak temperature). Special elastomers are also available for specialist requirements.
The maximum pressure of the standard air actuator is 8bar. Reinforced designs are available for higher demands. Even with just 8bar internal pressure, pressing and load-bearing capacities from 0.5 to 440kN can be achieved – meaning standard products with a diameter of 1000mm can lift masses of 44t. Depending on the type used, the stroke can reach up to 455mm without a stick-slip effect: even with low changes in pressure, the stroke is performed with zero pressure. In addition to this, air actuators can be tilted. Angles from 5° to 30° and a lateral off set of up to 20mm are possible. This means that certain connection elements and joint structures are not necessary.