Apply Wet Screw Compressors Wisely

The machines suit many applications but pose a number of specific issues.

By Amin Almasi, rotating equipment consultant

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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.

Because of the high volume ratio, the internal pressure ratio can be high, which is a great advantage for medium-pressure applications. For best efficiency, the volume ratio should be set so the machine's internal compression ratio matches the system compression ratio. For optimum efficiency, internal clearances within the machine should be kept as small as possible. The presence of a large quantity of oil during the compression process lessens the chance of contact between the screw rotors.

Oil injection in the compressor results in simpler, less-expensive and more-reliable operation. The oil enables control of the compressed gas temperature, which permits relatively large pressure rises across the compressor. It serves as sealant, partially filling the clearance between rotors, and the rotor and casing. The oil, which covers all the metal surfaces in the compression chamber, also acts as a significant barrier to corrosion and thus allows the compressor to handle difficult gases. (Selecting the proper oil is critical for tough services.) Another important feature is that the oil dampens noise.

The oil is injected in the region where gas compression is taking place (the oil can also enter from the bearing). It absorbs around 70–85% of the heat generated by gas compression. Commonly, a pumped oil circuit serves the bearing and seal while a pump-less circuit supplies the suction injection. For a pump-less oil-injected machine, gas discharge temperature could remain relatively constant over a wide range of operation (for example, within 10–15°C at varying pressure ratios). With a pumped system (forced-oil injection), gas outlet temperature usually can be maintained more tightly. The forced-feed oil-injection system could be a reliable option for large (critical) screw compressors.

Always check the compatibility of the injected oil with the process gas — to avoid risks of process system deterioration or oil degradation. Examine the entire downstream system; the oil shouldn't cause problems in exchangers, reactors, etc.

Recovery of oil from the discharge gas is an important consideration. The oil separator usually uses coalescing filter technology. If vapor-phase oil carryover is a significant concern, put in two oil separators and install an after-cooler (to condense vaporized oil) between the primary and secondary oil-separator vessels. Usually, oil content (including vapor, aerosols, etc.) ranges from 1 to 5 ppm. Services requiring lower residual oil content (for example, 0.1–0.5 ppm or even less) may need a three-level oil-separator system; these systems require more-than-usual attention.

For special applications, a liquid other than oil may be injected. For instance, water-injected two-stage screw compressors are popular where gas tends to polymerize. Other types of compressors such as centrifugal ones wouldn't work in these difficult services because of rapid polymerization.

The screw rotor length-to-diameter (L/D) ratio usually falls in range of 1.1–2.2/1, with 1.5–1.9/1 most common. Using larger ratios can increase the capacity at a given speed but might reduce the permissible differential pressure — with long, slender rotors, this may be as low as 4 bar. Short (stubby) rotors can accept differential pressures of 24 bar. A single stage of compression can handle most chemical processing duties. However, for pressure ratios between 15 and 24, a two-stage screw compressor will afford better efficiency.

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