STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Compressors 493
Summary of Rotary Compressors
The main advantage of rotary compressors is that most are easy to maintain in
field conditions and in industrial settings. Also, they can be constructed to be rather
portable since they have rather small exterior dimensions. Also, many versions of the
rotary compressor can produce oil-free compressed gases.
The main disadvantage are that these machines operate at a fued pressure ratio.
Thus, the cost of operating the compressor does not basically change with reduced
back pressure in the discharge line. As long as the back pressure is less than the
pressure output of the rotary, the rotary will continue to operate at a fixed power
level. Also, since the pressure ratio is built into the rotary compressor, discharging
the compressor into a back pressure near or greater than the pressure output of the
machine will significantly reduce the volumetic flowrate produced by the machine.
Centrifugal Compressors
The centrifugal compressor is the earliest developed dynamic, or continuous flow,
compressor. This type of compressor has no distinct volume in which compression
takes place. The main concept of the centrifugal compressor is use of centrifugal
force to convert kinetic energy into pressure energy. Figure 3-81 shows a diagram of
a single-stage centrifugal compressor. The gas to be compressed is sucked into the
center of the rotating impeller. The impeller throws the gas out to the periphery by
means of its radial blades and high-speed rotation. The gas is then guided through
the diffuser where the high-velocity gas is slowed, which results in a high pressure. In
multistage centrifugal compressors, the gas is passed to the next impeller after the
diffuser of the previous impeller. In this manner, the compressor may be staged to
OUT
Figure 3-81. Single-stage centrifugal compressor [23].