Page 121 - Air and Gas Drilling Manual

Chapter 4: Compressors 4-7
gas axially through the compression space via a series of radial arranged rotating
rotor blades and stationary stators (or diffuser) blades (see Figure 4-6) [1]. 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 flow rate range of approximately 3 x 10 4
6
to 10 actual cfm. Their compression ratios are typically around 10 to 20. Because
of their small diameter, these machines are the principal compressor design for jet
engine applications. There are also applications for axial-flow compressors in large
process plant operations where very large constant volumetric flow rates at low
compression ratios are needed.
More detail regarding axial-flow compressors can be found in other references [2,
7, 8, 9].

in out
Figure 4-6: Multistage axial-flow compressor [1].

4.4 Positive Displacement Compressors
In general, only the reciprocating compressor allows for rather complete reliable
flexibility in applications requiring variable volumetric flow rates and variable
pressure ratios. The rotary compressor (which has a fixed pressure ratio built into
the compressor design) does not allow for much variation in either.
4.4.1 Reciprocating Compressors
The reciprocating compressor is the simplest example of the positive
displacement class of compressors. This type of compressor was also the earliest
designed. Like reciprocating incompressible fluid pumps, reciprocating compressors
can also be either single acting or double acting. Single-acting compressors are
usually of the trunk type (see Figure 4-7) [1]. Double-acting compressors are usually
of the crosshead type (see Figure 4-8) [1].
Reciprocating compressors are available in both lubricated and non-lubricated
versions. The lubricated versions provide lubrication for the moving pistons (in the
cylinder) either through an oil lubricated intake gas stream, or via an oil pump and
injection of oil to the piston sleeve. There are some applications where oil must be
completely omitted from the compressed air or gas exiting the machine. For such
applications where a reciprocating piston type of compressor is required, there are