EQUIPMENT-SPECIFIC LUBE SERIES
Fig. 2. Typical centrifugal compressor design
Fig. 3. The basic operating principle behind centrifugal compressors
Centrifugal compressors Centrifugal compressors, like that illustrated in Fig. 2, deliver gases at higher flow rates than positive displacement compressors, but at lower pressures. They’re used extensively in the refining and petrochemical industry because there is no contact between the gas and the lubricant— meaning that centrifugal compressors produce oil-free gas. Reactive gases can also be compressed without coming in contact with the lubricant. Operation… The typical centrifugal compressor relies on the rotating blades (or vanes) of an impeller to accelerate a gas and, thus, create pressure. The impeller sits in a volute—a widening chamber connected to a gas discharge line. Gas that enters the compressor is swept up by the impeller vanes and moved from the center to the outside by centrifugal force, causing an increase in gas velocity. When the gas leaves the impeller and enters the volute, it slows down as the chamber widens. This slowdown converts velocity or kinetic energy to pressure. (Not shown in Fig. 3 are stationary diffuser plates that initially slow down the gas from the impeller and direct it into the volute.) 10 | LUBRICATION MANAGEMENT & TEChNOLOGy
To achieve desired pressures, most centrifugal compressors are designed as multi-stage units. (The cutaway in Fig. 2 reflects a five-stage design.) In a multi-stage unit, the impellers are all mounted on the same shaft along with a volute for each impeller and one suction- and one discharge-line. Gases aren’t cooled between stages, as they would be in a reciprocating compressor. Typical multi-stage operating pressures are in the range of 100-150 psi. SEPTEMBER/OCTOBER 2012