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Using a Lubricant
Injected Rotary Screw Compressor
There are many situations in industry and
manufacturing that utilize a lubricant-injected rotary screw compressor
controlled by an electric motor. In fact, these types of compressors are
the most popular for a variety of uses.
The process known as the compression principle is described as follows:
Two intermeshing rotors that are contained in a stator housing with an
inlet port at one end and a discharge port at the other end is contained
in the lubricant-injected, rotary-screw compressor.
The lobes of the male rotor are put together helically lengthwise; and
the female rotor contains helical grooves or flutes that correspond with
the male rotor. The designs of the rotors are similar, but the amount of
helical lobes and grooves may vary slightly. This design makes it
possible for air to flow in through the inlet port which fills in the
spaces between the lobes on each of the rotors. The air is trapped
between the lobes and the stator due to the rotation because the
inter-lobe spaces pass by the inlet port. A lobe on one rotor rolls into
another groove on the other rotor as rotation occurs. This process
allows the intermeshing to move along the axial length of the rotors
which reduces the space occupied by the air. This results in enhanced
pressure.
Compression will continue up until the time that the inter-lobe spaces
are exposed to the discharge port as the compressed air is discharged.
Three essential functions are served as a lubricant is inserted into the
compression chamber. These functions are taking away most of the heat
caused by compression; it acts as a seal in the clearances between the
meshing rotors and between the rotors and stator; and it lubricates the
intermeshing rotors and associated bearings.
Lubricant is merely a generic term. The lubricant used may actually be a
hydrocarbon product, although most compressors currently use cleaner and
longer lasting synthetic lubricants. These lubricants include diesters,
polyglycols, polyalphaolefins, polyol esters and silicon-based
lubricants. The newer lubricants are able to withstand a broader range
of temperatures.
Compressed air mixed with the lubricant that has been inserted exits the
air end and is then passed to a sump/separator. At that point, the
lubricant is taken from the compressed air. Most of the liquid is
separated by directional and velocity changes. A coalescing filter
separates the remaining aerosols in the compressed air. As a result,
merely a few parts per million (ppm) of lubricant will carry-over. This
will most often be in the range of up to 5 ppm. Until a normal system
pressure can be achieved at start-up a minimum pressure device usually
combined with a discharged check valve, will prevent excess speed
through the separator element. Although some designs may utilize a
lubricant pump, most of the lubricant-injected rotary screw compressors
use the air pressure contained in the lubricant sump/separator after the
discharge of the air end to circulate the lubricant through a filter and
cooler before re-injection into the compression chamber.
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