Page 124 - Air and Gas Drilling Manual

4-10 Air and Gas Drilling Manual
The middle pressure versus volume plot in Figure 4-9 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 cannot
expel the gas volume within it efficiently. Thus, the fixed volumetric flow rate (at a
given rotation speed) will be reduced from the volumetric flow rate when the back
pressures are equal to, or less than the built-in design output pressure [1].
The lower pressure versus volume plot in Figure 4-9 shows the typical situation
when the back-pressure on the outlet side of the compressor is less than the built-in
design output pressure. Under these conditions, the gas exiting the compressor
expands in the expansion tank and the initial portion of the pipeline until the
pressure is equal to the pipeline back pressure [1].
Rotary compressors can also be designed with multiple stages. Such multistage
compressors are designed with nearly equal compression ratios for each stage (i.e.,
minimum input power requirements). Thus, since 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.
Sliding Vane Compressors
The typical sliding vane compressor stage is a rotating cylinder located eccentric
to the center-line of a cylindrical housing (see Figure 4-10) [1, 2]. The vanes are in
slots in the rotating cylinder, and are allowed to move in and out in these slots to
adjust to the changing clearance between the outside surface of the rotating cylinder
and the inside bore surface of the housing. The vanes are always in contact with the
inside bore due to either pressured gas under the vane (in the slots), or spring forces
under the vane. The top of the vanes slide over the inside surface of the bore of the
housing as the inside cylinder rotates. Gas is brought into the compression stage
through the inlet suction port. The gas is then trapped between the vanes, and as the
inside cylinder rotates the gas is compressed to a smaller volume as the clearance is
reduced. When the clearance is the smallest, the gas has rotated to the outlet port.
At the outlet port, the compressed gas is discharged to a surge tank or pipeline
system connected to the outlet side of 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 compression for the stage is fixed. The geometry, e.g.,
cylinder length, diameter, the inside housing diameter, the inlet area, 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 cylindrical
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 fabrics.
Usually vane compressors require oil lubricants to be injected into the gas entering
the compression cavity. This lubricant allows smooth action of the sliding 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].