Page 114 - Air and gas Drilling Field Guide 3rd Edition

5.4 Intermittent (Positive Displacement) Compressors 105

the compressor. As each set of vanes reaches the outlet port, the gas trapped
between the vanes is discharged. The clearance between the rotating cylinder
and the stationary cylindrical housing is fixed, and thus the pressure ratio of com-
pression for the stage is fixed. The geometry (e.g., cylinder length, diameter, the
inside housing diameter, the inlet area, and the outlet area) of each compressor
stage determines the stage displacement volume and compression ratio.
The principal seals within the sliding vane compressor are provided by the
interface force between the end of the vane and the inside surface of the cylindri-
cal housing. The sliding vanes must be made of a material that will not damage
the inside surface of the housing and slide easily on that surface. Therefore, most
vane materials are composites such as phenolic resin-impregnated laminated fab-
rics. Usually vane compressors require oil lubricants to be injected into the gas
entering the compression cavity. This lubricant allows smooth action of the slid-
ing vanes against the inside of the housing. There are, however, some sliding vane
compressors that may be operated nearly oil free. These utilize bronze or carbon/
graphite vanes [7].
The volumetric flow rate for a sliding vane compression stage, q s ,is
approximately

q s ¼ 2:0 al ðd 2 mtÞ RPM (5-2)
and
d 2 d 1
a ¼ ; (5-3)
2
3
where q s is the volumetric flow rate (cfm, m /minute), a is the eccentricity (ft,
m), l is the length of the cylinder (ft, m), d 1 is the outer diameter of the rotary
cylinder (ft, m), d 2 is the inside diameter of the cylindrical housing (ft, m), t is
the vane thickness (ft, m), m is the number of vanes, and RPM is the speed of
the rotating cylinder (rpm).
Some typical values of a vane compressor stage geometry are d 1 /d 2 ¼ 0.88,
a ¼ 0.06 d 2 , and l/d 2 ¼ 2.00 to 3.00. Typical vane tip speed usually should not
exceed 50 ft/sec (15 m/sec).

Helical Lobe (Screw) Compressors
A typical helical lobe (screw) rotary compressor stage is made up of two rotating
helical-shaped shafts, or screws. One is a female rotor and the other is a male
rotor. These two rotating components turn counter to one another (counterrotat-
ing)(see Figure 5-11)[1]. As with all rotary compressors, there are no valves. The
gas flows (due to negative pressure conditions at the inlet) into the inlet port and
is squeezed between the male and the female portions of the rotating intermesh-
ing screw elements and the housing. The compression ratio of the stage and its
volumetric flow rate are determined by the fixed geometry of the two rotating
screw elements and the rotation speed. Thus, the rotary screw compressor is a
fixed ratio machine.