Rotary-screw compressors are generally used to supply compressed air for larger industrial applications. They are best applied in applications that have a continuous air demand such as food packaging plants and automated manufacturing systems. In larger facilities, that may have only intermittent applications, the average usage among the many work stations will place a continuous demand on the compressor. In addition to fixed units, rotary-screw compressors are commonly mounted on tow-behind trailers and powered with small diesel engines. These portable compression systems are typically referred to as construction compressors. Construction compressors are used to provide compressed air to jack hammers, riveting tools, pneumatic pumps, sand blasting operations and industrial paint systems. They are commonly seen at construction sites and on duty with road repair crews throughout the world.
In an oil-free compressor, the air is compressed entirely through the action of the screws, without the assistance of an oil seal. They usually have lower maximal discharge pressure capability as a result. However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psi (10 atm) and output volume of over 2,000 cubic feet per minute (57 m3/min).
Oil-free compressors are used in applications where entrained oil carry-over is not acceptable, such as medical research and semiconductor manufacturing. However, this does not preclude the need for filtration, as hydrocarbons and other contaminants ingested from the ambient air must also be removed prior to the point of use. Consequently, air treatment identical to that used for an oil-flooded screw compressor is frequently still required to ensure a given quality of compressed air.
In an oil-injected rotary-screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling sink for the gas charge. The oil is separated from the discharge stream, then cooled, filtered and recycled. The oil captures non-polar particulates from the incoming air, effectively reducing the particle loading of compressed-air particulate filtration. It is usual for some entrained compressor oil to carry over into the compressed-gas stream downstream of the compressor. In many applications, this is rectified by coalescer/filter vessels. Refrigerated compressed air dryers with internal cold coalescing filters are rated to remove more oil and water than coalescing filters that are downstream of air dryers, because after the air is cooled and the moisture is removed, the cold air is used to pre-cool the hot entering air, which warms the leaving air. In other applications, this is rectified by the use of receiver tanks that reduce the local velocity of compressed air, allowing oil to condense and drop out of the air stream to be removed from the compressed-air system by condensate-management equipment.
Oil-injected rotary-screw compressors are used in applications that tolerate a low level of oil contamination, such as pneumatic tool operation, crack sealing, and mobile tire service. New oil flooded screw air compressors release <5mg/m3 of oil carryover. PAG oil is polyalkylene glycol which is also called polyglycol. PAG lubricants are used by the two largest U.S. air compressor OEMs in rotary screw air compressors. PAG oil-injected compressors are not used to spray paint, because PAG oil dissolve paints. Reaction-hardening two-component epoxy resin paints are resistant to PAG oil. PAG compressors are not ideal for applications that have mineral oil greases coated seals, such as 4-way valves and air cylinders that operate without mineral oiler lubricators, because the PAG washes away the mineral grease and degrades Buna-N rubber.
Post time: Nov-14-2019