Differences Between Dry Screws and Oil-Flooded Screws: Dry Screw Compressors

A synchronizing gear (often called timing gear) is used to avoid contact between the rotors. In oil and gas service the bearings are typically hydrodynamic journal and thrust bearings. At each shaft end a shaft seal is placed between the compression chamber and the journal bearing (sometimes called conveying chamber seals). Depending on the type of coupling and coupling guard a labyrinth seal for sealing the driveshaft may be necessary.

An important value for the characterization of screw compressors is the male-rotor tip speed uM. For dry screw compressors the typical tip speed range for all compressor sizes is between 50 and 150m/s. In some cases, even lower or higher tip speeds are realized. The rotor diameters are in the range of approximately 100mm up to >800mm. Therefore, the largest types have speeds of 1000 to 3600rpm while the smallest sizes may have speeds up to 25,000rpm with medium-size compressors in between. Due to the high speeds a gearbox between driver and compressor is needed for small- to medium-sized machines. For large machines a direct drive with a steam turbine is often used but an electric drive with or without gearbox is also common.

The built-in volume ratio vi is defined by the casing geometry and is a fixed value for a dry screw compressor. Dry screw compressors perform well with many gases (e.g., corrosive, toxic, flammable), and with changing molecular weights. If required a liquid injection for cooling or washing purposes is possible.

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