Plant Services - August 2007 - (Page 49) T he growing demand for higher-quality compressed air constantly fuels the discussion on how to produce it. Slogans and one-sided arguments are less helpful than objective and balanced views. No single process or product is the panacea for producing high-quality compressed air, often referred to as oil-free air. Ultimately, the key factor is whether the technical solution fulfi ls the requirements of maximum reliability and efficiency. of products. e question arises: Why do some applications use oil-lubricated systems while others use oil-free? No-go without air treatment De nition e term “oil-free compressor” refers only to the compression chamber, not the full system or the air quality. “Oilfree” is a vague phrase used by some marketing departments solely to stir up emotion. Stay on safe ground and adhere to the quality classes specified in standard ISO 8571-1, which provides the precision needed for reliable comparison. Compressor manufacturers don’t restrict product ranges to ‘oil-free’ or oil-cooled systems. Many produce a variety Oily air 12 Gear milling 10 8 mg/m3 6 Turning 4 2 Other manufacturing 0 Time Drilling Saying it’s a compromise to introduce oil only to remove it later 12 ignores several facts. Precisely defined air quality can Gear milling be achieved only with efficient treatment systems, irrespective of the compression method. Not even the most confi10 dent advocates of oil-free compression would deny this. In 1996, 8 manager described oil-free systems as follows: one Drilling “An oil-free compressor produces identical compressed air quality to that of the intake air. erefore, the air inside 6 the compressor system should also remain oil-free. HowTurning ever this is not guaranteed to be the case if oil vapors escape 4 when ventilating the gear casing. is is a problem that is so far unresolved for large compressors.” [1] 2 Furthermore, industrial compressors often see higher levOther manufacturing els of oil aerosols in the ambient air (Figure 1). “Manufacturing0 facilities with inadequate or no contaminant extracTime tion systems can even have air oil concentration levels as high as 300 mg/m3.”[2] mg/m3 Cranking it up Theoretical speci c power requirement for compression from 1 bar (a) Isentropic Pspec (kWm3/min) N = 1, 4=k (for air) n = 1, 3 n = 1, 1 n = 1, 0 Isothermic Figure 1. Analysis of oil mist concentration in various elds of manufacturing reveals values that exceed 10 mg/m3. The value remained more or less constant in some areas, where others showed signi cant hourly uctuations. Pressure ratio Figure 2. Theoretical speci c power requirement for compression from 1 bar absolute pressure. A Theoretical speci c power requirement for compression from 1 bar (a) Isentropic N = 1, 4=k .PLANTSERVICES. (for air) ∆ρ (bar) n = 1, 3 Filter element pressure di erential with time 49 http://www.plantservices.com
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