Page 268 - Compression Machinery for Oil and Gas

Chapter 6

Screw Compressors

†
Eugene “Buddy” Broerman*, Tim Manthey ,J€ urgen Wennemar ‡
and Justin Hollingsworth*
* †
Southwest Research Institute, San Antonio, TX, United States, Aerzen USA, Coatesville, PA,
‡
United States, MAN Energy Solutions SE, Oberhausen, Germany

Two Types of Screw Compressors

Screw compressors generally come in two basic designs: dry screw compressors
or wet/oil-flooded screw compressors. Both compressor types consist of rotors
that are closely mated. Dry screw compressors do not have any oil in-between
the screws. A dry screw compressor consists of two rotors, a closely mating pair
that are installed in a tight clearance cylindrical bore. Wet or oil-flooded screw
compressors have oil in-between the screws. An oil-flooded screw compressor
also has a pair of closely mating rotors that are installed in a tight clearance
cylinder bore; however, oil-flooded screw compressors do not require clear-
ances to be as tight as those that are required for the dry screw compressors.
The subsequent sections of this chapter describe the various aspects (similarities
and differences) of these two types of screw compressors.

Working Principle of Screw Compressors
Screw compressors consist of two rotors in a common casing. Both rotors carry
intermeshing helical lobes and rotate against each other with tight clearances
between the rotors, and between the rotors and casing. During rotation, the lobes
and casing form compression chambers that steadily decrease in volume as the
rotors turn, changing cyclically from maximum volume to zero and back to
maximum again. Thus, the working principle is similar to other positive dis-
placement machines like reciprocating compressors.
In an oil-flooded screw compressor, a slide valve is available for capacity
control. This slide valve moves axially beneath the rotors and changes the effec-
tive rotor length, and also opens an internal recycle volume on the suction side
of the compressor. This is explained in more detail in the sections further.
Fig. 6.1 shows the rotors in different positions and with a varying working
chamber volume.

Compression Machinery for Oil and Gas. https://doi.org/10.1016/B978-0-12-814683-5.00006-7
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