Application Notes Dry Compressors

Screw compressors of the dry type generate high frequency pulsations that move into the system piping and can cause acoustic vibration problems. These would be similar to the type of problems experienced in reciprocating compressor applications, except that the frequency is higher. While volume bottles will work with the reciprocator, the dry type screw compressor would require a manufacturer-supplied proprietary silencer that should take care of the problem rather nicely.

There is one problem the dry compressor can handle quite well. Unlike most other compressors, this one will tolerate a moderate amount of liquid. Injection of liquids for auxiliary cooling can be used, normally at a lower level than would be used in the flooded compressor. The compressor also takes reasonably well to fouling service, if the material is not abrasive. The foulant tends to help seal the compressor and, in time, may improve performance. One other application for which the dry machine is particularly well-suited is for hydrogen-rich service, where the molecular weight is low, with a resulting high adiabatic head. For larger flow streams, within the centrifugal compressor's flow range, the screw compressor is a good alternative. While the high adiabatic head requires expensive, multiple centrifugal casings, the positive displacement characteristic of the screw compressor is not compromised by the low molecular weight. For very low molecular weight gas, such as pure hydrogen or helium, a good seal is important to keep the slip in control. This can be tedious and, in extreme cases, a liquid injection is used for leakage control to maintain performance.

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