Page 123 - Air and Gas Drilling Manual

Chapter 4: Compressors 4-9
The main advantage of the multistage reciprocating piston compressor is the positive
control of both the volumetric flow rate, which can be put through the machine, and
the pressure of the output. Many reciprocating piston compressors allow for the
rotation to be adjusted, thus, changing the throughput of air or gas. Also, provided
there is adequate input power from the prime mover, reciprocating piston
compressors can adjust to any back pressure changes and maintain proper rotation
speed (which in turn maintains a given volumetric flow rate).
The main advantage of this subclass of compressor is their extremely high
pressure output capability and reliable volumetric flow rates (see Figures 4-2 and 4-
3). The main disadvantage to multistage reciprocating piston compressors is that
they can not be practically constructed in machines capable of volumetric flow rates
much beyond 1,000 actual cfm. Also, the higher-capacity compressors are rather
large and bulky and generally require more maintenance than similar capacity rotary
compressors. In any positive displacement compressor, like a liquid positive
displacement pump, the real volume flow rate is slightly smaller than the mechanical
displacement volume. This is due to the following factors:
• Pressure drop on the suction side
• Heating up of the intake air
• Internal and external leakage
• Expansion of the gas trapped in the clearance volume (reciprocating piston
compressors only)
Reciprocating compressors can be designed with multiple stages. Such
multistage compressors are designed with nearly equal compression ratios for each
stage (it can be shown that equal stages of compression lead to minimum input
power requirements). Thus, since the volumetric flow rate (in actual cfm) is reduced
from one stage to the next, the volume displacement of each stage (its geometric
size) is progressively smaller.

4.4.2 Rotary Compressors
Another important positive displacement compressor is the rotary compressor.
This type of compressor is usually of rather simple construction, having no valves
and being lightweight. These compressors are constructed to handle volumetric flow
rates up to around 2,000 actual cfm and pressure ratios up to around 15 (see Figure
4-2). Rotary compressors are available in a variety of designs. The most widely
used rotary compressors are the sliding vane, helical lobe (screw), and liquid piston.
The most important characteristic of this type of compressor is that all have a
fixed, built-in, compression ratio for each stage of compression (as well as a fixed,
built-in volume displacement) [1]. Thus, at a given rotational speed (provided by
the prime mover), there will be a predetermined volumetric flow rate through the
compressor (the geometry of the compressor inlet is fixed), and the pressure at the
outlet will be equal to the built-in design pressure ratio of the machine multiplied by
the inlet pressure.
The upper pressure versus volume plot in Figure 4-9 shows the typical situation
when the back pressure on the outlet side of the compressor is equal to the built-in
design output pressure. Under these conditions, there is no expansion of the output
gas as it exits the compressor and passes through the expansion tank and continues
into the initial portion of the pipeline [1].