Page 128 - Air and Gas Drilling Manual

4-14 Air and Gas Drilling Manual
Some helical lobe rotary compressors have been designed to operate with an
entirely liquid-free compression space. Since the rotating elements of the
compressor 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. Often helical lobe rotary compressors are utilized in two-stage, three-
stage, and four-stage versions.
Detailed calculations regarding the design of the helical lobe rotary compressor
are beyond the scope of this book. Additional details can be found in other
references [1, 7].
Liquid Piston Compressors
The liquid piston (or liquid ring) rotary compressor utilizes a liquid ring as a
piston 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 4-12) [2]. The rotor has a series of vanes extending 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 housing walls.
As rotation takes place, the liquid forms a ring as centrifugal forces and the vane
geometry force the liquid to the outer boundary of the housing. Since the element is
located eccentric to the center of the cylindrical housing, the liquid ring (or piston)
moves in an oscillatory manner. The compression space in the center of the stage
communicates with the gas inlet and outlet parts and allows a gas pocket. The
liquid ring alternately uncovers the inlet part and the outlet part. As the system
rotates, gas is brought into the pocket, compressed, and released to the outlet port.

Figure 4-12: Liquid piston rotary compressor, 1) impeller, 2) housing, 3) intake port, 4)
working liquid, and 5) discharge port [1].
The liquid piston compressor has rather low overall efficiency, about 50 percent.
The main advantage to this type of compressor is that it can be used to compress
gases with significant liquid content in the stream.

4.4.3 Summary of Positive Displacement Compressors
The main advantages of the reciprocating piston compressors are: 1) dependable
near constant volumetric flow rate and, 2) variable pressure capability (up to the