CLEAN AIR COMPRESSORS
Pouringwater on troubled oil
Andy Pye investigates a brand new design of air compressor for use in sterile environments
T
he total globalmarket for industrial air compressors is expected to reachmore than $17.5bn (£14.3bn) this year, with compressed air accounting for 10 per cent of industrial electricity demand. Air
compressors are used in all sectors, but there is a critical requirement for oil-free air in sterile environments including aerospace,medical theatres, dental surgeries, pharmaceutical labs and food production plants. Oil is themost efficient lubricant needed for
compressors to operate atmaximumefficiency; however, oil particles can contaminate the air produced. Alternatives are water injection or the dry-running screw. Water-injection has been used for decades in
otherwise dry-running twin-screw compressors, for example as a wash (to clear particulate fromthe rotors) in applications such as lime-kiln gas and coke-oven gas
recovery.More recently, water- injection has become regarded as an attractive alternative to dry-running, because compression with liquid injection ismore energy efficient than dry, or adiabatic, compression. A water-injected
The conical-shaped screws – one inner, one outer – are the key to the efficiency of the Vert.80
screw is not noisy, and does not produce the uncomfortable high-pitch “screaming” noise, which is typical of dry-running screw compressors. Injection of water instead of oil removes the risk
of contamination, producing high quality air, compliant with ISO 8753-1 Class 0.Water can serve multiple functions: as a coolant, providing practically isothermal compression within the compressor, thus enabling it to achieve far greater pressure ratios and differential pressures within acceptable discharge temperature limits; as a lubricant, to avoid rotor-to-casing rubs under higher differential pressures and rotor deflections; as a sealant, substantially reducing the clearance radially, axially and through the blowhole; and as a sound-damper, both by absorbing some of the noise generated and also significantly lowering tip speeds and rotor passing frequencies. But amajor difficulty has been that, to operate
with water, twin-screw compressors have to be re- engineered fromcorrosion-freematerials – the screw rotors in twin-screw compressors are typically manufactured fromcarbon steel. Because of the
April 2017 /// Environmental Engineering /// 19
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