Positive Displacement Rotary Compressors

The helical-lobe compressor is the more robust of the rotary compressor types. Its positive reliability factor is the absence of wearing parts if the timing gears are neglected. Timing gears are not used on the flooded type. However, on the negative side, is the parts count. There are two rotors, four bearings, four seals, and, on the nonlubricated machine, there are timing gears. Another negative aspect of the helical-lobe compressor, particularly the nonlubricated version, is the presence of high frequency pulsations. If untreated, these pulsations can contribute significantly to failures of system components. While this problem may be solved, ihe solution adds to the system complexity, detracting from the overall reliability. To add a side note, the location of the silencer, used to treat the acoustic pulsations, should be directly attached to the discharge flange it at all possible. There are reported cases of acoustic problems when a spool piece was used between the silencer and the compressor flange. As mentioned before, the nature of the application may alter the overall reliability picture, as the nonlubricated helical-lobe compressor will tolerate fouling gas. It tends to be unique in that arena,

Taking the balance of the lobe-type machines as a group, the biggest single factor leading to poor reliability is excessive rotor deflection. If the rotors are allowed to touch each other, generally a failure occurs. Excessive rotor deflection is caused by a higher differential pressure across the compressor than the design limit. Since most of the rotary compressors, other than the helical lobe are not intended for continuous service, their use in continuous service applications may cause the application to experience reliability problems. Sparing and monitoring for maintenance intervals commensurate with true expected run time will certainly keep the system's reliability up, if the unplanned shutdowns are kept low. Maintenance cost, included in an evaluation of life cycle cost, may well direct usage to a more suitable compressor selection.

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