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Types of
Lubricant-free
Compressors
Lubricant-free rotary screw compressors
are much like the lubricant-injected counterparts. The main difference
is that no lubricant is injected into the compression chamber. The two
types of lubricant-free rotary screw compressors are the dry type and
the water-injected type.
The dry-type of compressor contains intermeshing rotors that cannot
touch; their positions are sustained by lubricating the timing gears
outside of the compression chamber. To remove compression heat, most
devices utilize two phases of compression with an intercooler between
the phases and an after-cooler after the second phase. Higher rotation
speed is needed with these types than with the lubricant injected types,
since there is no sealing fluid. Dry types have a range of 25 to 4,000
hp or 90 to 20,000 cfm. The single-stage types can operate as high as 50
psig and the two-stage can operate up to 150 psig.
With the water-injected types of compressors, there is a comparable
timing gear in which water is injected into the compression chamber.
This forms a seal in the inner clearances and removes the compression
heat. Pressures can then be accomplished with just one stage in the 100
to 150 psig range.
An uncomplicated moisture separator is used to remove the injected water
and the air's condensed moisture from the discharged compressed air. The
lubricant-free rotary screw compressors are usually equipped with the
accessories you need.
Lubricants are still needed to maintain the bearings and gears on the
lubricant-free rotary screw compressors. This is because the bearings
and gears are separate from the compression chamber. In the air-cooled
units the lubricant might be used for stator jacket cooling. The flow of
lubricant can be guaranteed at the start-up and run-down by a lubricant
pump that is driven from a shaft in the gearbox if there is a power
failure. Bearings, gears and the lubricant pump are protected by a
lubricant filter with a 10 micron rating.
An air cooler after each phase is contained in the cooling system for
the dry, lubricant-free rotary screw compressor. The radiator contained
on these units may be water-cooled or air-cooled. Some of the older
models may need an extra heat exchanger so that some of the compressed
air can be recycled to the compressor inlet throughout the unloading.
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