STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

494 Auxiliary Equipment
increase the pressure of the ultimate discharge (see Figure 3-82). Since the compression
pressure ratio at each stage is usually rather low, of the order 2 or so, the need for
intercooling is not important after each stage. Figure 3-82 shows a typical multistage
centrifugal compressor configuration with an intercooler after the first three stages
of compression.
The centrifugal compressor must operate at rather high rotation speeds to be
effective. Most commercial centrifugal compressors operate at speeds of the order of
20,000-30,000 rpm. With such rotation speeds very large volumes of gas can be compressed
with equipment having rather modest external dimensions. Commercial
centrifugal compressors can operate with volumetric flowrates up to around 10.4
actual cfm and with overall compression ratios up to about 20.
Centrifugal compressors are usually used in large processing plants and in some
pipeline applications. They can be operated with any lubricant or other contaminant
in the gas stream, or they can be operated with some small percentage of
liquid in the gas stream.
These machines are used principally to compress large volumetric f lowrates to
rather modest pressures. Thus, their use is more applicable to the petroleum refining
and chemical processing industries.
More details regarding the centrifugal compressor may be found in other references
[4,231.
Axial-Flow Compressors
The axial compressor is a very high-speed, large volumetric flowrate machine.
This is another dynamics, or continuous flow machine. This type of compressor
sucks in gas at the intake port and propels the gas axially through the compression
space via a series of radially arranged rotor blades and stator (diffuser) blades (see
Figure 3-83). As in the centrifugal compressor, the kinetic energy of the high-velocity
flow exiting each rotor stage is converted to pressure energy in the follow-on stator
(diffuser) stage. Axial-flow compressors have a volumetric flowrate range of about
3 x 104-106 actual cfm. Their compression ratio is typically around 10 to 20. Because
OUT
Figure 3-82. Multistage centrifugal compressor with intercooling [23].