Its compact design, relatively low cost, pressure ratio capability per stage, high efficiency and good reliability make a wet screw compressor the best choice for numerous small- and medium-size applications. Such machines, which also are called "oil-flooded" or "oil-injected," offer the same performance advantages as reciprocating compressors, mainly a constant (adjusted) capacity under varying pressures and a high efficiency. In addition, screw compressors boast the same advantages as centrifugal compressors with respect to reliability, availability and small footprint. Wet screw compressors don't have a surge limitation (which is the main restriction of centrifugal and axial compressors). Also, they don't present high pulsation amplitudes and cylinder valve issues (which can pose major problems in reciprocating machines).
A wet screw compressor contains male and female screw rotors, with the 4/6 combination (4 male lobes and 6 female flutes) traditionally popular at process plants. The compressor uses a slide-valve capacity control system to regulate the volume flow; this system can offer step-less control (usually in a 20–100% range) and excellent energy efficiency.
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