Page 275 - Compression Machinery for Oil and Gas

260 SECTION II Types of Equipment

Screw compressors do not have any contacting parts other than shaft seals.
Oil-injected screw compressors are typically direct driven by electric
drivers or gas engines. Therefore, the typical speeds are between 1500
and 3600rpm.
The speeds of dry screw compressors can range between approximately
1000rpm and up to 25,000rpm depending on the machine size.
The tip speeds for both screw compressor types are lower than those of
centrifugal or axial compressors. Therefore, screw compressors are less sus-
ceptible to erosion by droplets or contaminants. With oil-flooded screw com-
pressors, erosion is not an issue due to the relatively low rotor tip speeds.
Oil-free screw compressors with continuous liquid injection at the suction
side can, due to higher rotor tip speeds, be subject to erosion of the rotors
and—to a lesser extent—the casing, if carbon steel is used. Stainless steel
rotors are often used in this case, and a stainless steel casing is sometimes
used as well.

Differences Between Dry Screws and Oil-Flooded Screws

Dry Screw Compressors
Fig. 6.6 shows sectional views of a dry screw compressor. 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 drive-
shaft may be necessary.
An important value for the characterization of screw compressors is the
male-rotor tip speed u M . 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 tur-
bine is often used but an electric drive with or without gearbox is also
common.
The built-in volume ratio v i 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.