Page 111 - Air and gas Drilling Field Guide 3rd Edition

102 CHAPTER 5 Compressors and Nitrogen Generators

compressors are rather large and bulky and generally require more maintenance
than similar capacity rotary compressors. In any positive displacement compres-
sor, such as a liquid positive displacement pump, the real volume flow rate is
slightly smaller than the mechanical displacement volume. This is due to the fol-
lowing factors:
n Pressure drop on the suction side
n Heating up of the intake air
n Internal and external leakage
n Expansion of the gas trapped in the clearance volume (reciprocating piston
compressors only)
Reciprocating compressors can be designed with multiple stages. Such multi-
stage 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, because 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.
These compressors can be used as either primary or booster compressors in
drilling operations.

5.4.2 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 2000 actual cfm (actual 944 liters/sec) and pressure
ratios up to around 15 (see Figure 5-2). Rotary compressors are available in a vari-
ety 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 top pressure versus volumetric flow rate plot in Figure 5-9 shows the typ-
ical 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, passes through the expan-
sion tank and continues into the initial portion of the pipeline [1].
The middle pressure versus volumetric flow rate plot in Figure 5-9 shows the
typical situation when the back pressure on the outlet side of the compressor is
above the built-in design output pressure. Under these conditions, the compressor