Page 116 - Air and gas Drilling Field Guide 3rd Edition

5.4 Intermittent (Positive Displacement) Compressors 107

Screw compressors operate at rather high speeds. Thus, they are rather high
volumetric flow rate compressors with relatively small exterior dimensions (i.e.,
small location footprint).
Most helical lobe rotary compressors use lubricating oil within the compres-
sion space. This oil is injected into the compression space and is recovered,
cooled, and recirculated. The lubricating oil has several functions:
n Seal the internal clearances
n Cool the gas (usually air) during compression
n Lubricate the rotors
n Eliminate the need for timing gears
There are versions of the helical lobe rotary compressor that utilize water
injection (rather than oil). The water accomplishes the same purposes as the
oil, but the air delivered by these machines can be oil free.
Some helical lobe rotary compressors have been designed to operate with an
entirely liquid-free compression space. Because the rotating elements of the com-
pressor need not touch each other or the housing, lubrication can be eliminated.
However, such helical lobe rotary compressor designs require timing gears. These
machines can deliver totally oil-free, water-free dry gas.
The helical lobe rotary compressor can be staged very much like the sliding
vane compressor. However, these compressors are restricted to two-stage sys-
tems. In general, this is due to the fact that the helical lobe rotary compressor
has some characteristics for the dynamic compressor type. A helical lobe rotary
compressor stage can be altered from their original fixed design by adjusting
the cross-sectional area of the flow at the exit from that stage. This is essentially
converting the kinetic energy of the flow to pressure.
The helical lobe rotary compressor has one serious disadvantage. It is very sen-
sitive to dust particles in the input air. This dust degrades the seals on the helical
surfaces. Once the seals are degraded, the stage begins to take on more character-
istics of a dynamic compressor. This problem is evidenced by the fact that nearly
all drilling operation primary helical lobe compressors will have a measured volu-
metric flow rate output that is approximately 7 to 8% less than the volumetric
flow rate specified by manufacturers (even when accounting for altitude).
Detailed calculations regarding the design of the helical lobe rotary compres-
sor are beyond the scope of this book. Additional details can be found elsewhere
[1, 7].

Liquid Piston Compressors
The liquid piston (or liquid ring) rotary compressor utilizes a liquid ring as a pis-
ton to perform gas compression within the compression space. The liquid piston
compressor stage uses a single rotating element that is located eccentric to the
center of the housing (see Figure 5-12)[2]. The rotor has a series of vanes extend-
ing radial from it with a slight curvature toward the direction of rotation. A liquid,
such as oil, partially fills the compression space between the rotor and the