STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Compressors 491
portion of the rotating intermeshing screw elements and their housing. The
compression ratio of the stage and its volumetric flowrate are determined by the
geometry of the two rotating screw elements and the speed at which they are rotated.
Screw compressors operate at rather high speeds. Thus, they are rather high
volumetric f lowrate compressors with relatively small exterior dimensions.
Most rotary screw compressors use lubricating oil within the compression space.
This oil is injected into the compression space and recovered, cooled, and recirculated.
The lubricating oil has several functions
seal the internal clearances
cool the gas (usually air) during compression
lubricate the rotors
eliminate the need for timing gears
There are versions of the rotary screw compressor that utilize water injection (rather
than oil). The water accomplishes the same purposes as the oil, but the air delivered
in these machines is oil-free.
Some screw compressors have been designed to operate with an entirely oil-free
compression space. Since the rotating elements of the compressor need not touch
each other or the housing, lubrication can be eliminated. However, such rotary screw
compressor designs require timing gears. These machines can deliver totally oil-free,
water-free dry air (or gas).
The screw compressor can be staged. Often screw compressors are utilized in threeor
four-stage versions.
Detailed calculations regarding the design of the rotary screw compressor are
beyond the scope of this handbook. Additional details can be found in other
references [4,25,26,27].
Rotary Lobe Compressor
The rotary lobe compressor stage is a rather low-pressure machine. These
compressors do not compress gas internally in a fixed sealed volume as in other
rotaries. The straight lobe compressor uses two rotors that intermesh as they rotate
(see Figure 3-79). The rotors are timed by a set of timing gears. The lobe shapes may
be involute or cycloidal in form. The rotors may also have two or three lobes. As the
rotors turn and pass the intake port, a volume of gas is trapped and carried between
the lobes and the housing of the compressor. When the lobe pushes the gas toward
the outlet port, the gas is compressed by the back pressure in the gas discharge line.
Volumetric efficiency is determined by the leakage at tips of the lobes. The leakage
is referred to as slip. Slippage is a function of rotor diameter, differential pressure,
and the gas being compressed.
For details concerning this low pressure compressor see other references [4,25,26,27].
Liquid Piston Compressor
The liquid piston 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 eccentrically inside a housing (see
Figure 3-80). The rotor has a series of vanes extending radially 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 vanes force the liquid to the
outer boundary of the housing. Since the element is located eccentrically in the