STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

For a reciprocating piston compressor, Equation 3-70 becomes
Compressors 487
(3-77)
Rotary Compressors
Another important positive displacement compressor is the rotary compressor.
This type of compressor is usually of rather simple construction, having no valves
and being lightweight. These compressors are constructed to handle volumetric
flowrates up to around 2,000 actual cfm and pressure ratios up to around 15 (see
Figure 3-69). Rotary compressors are available in a variety of designs. The most widely
used rotary compressors are sliding vane, rotary screw, rotary lobe, and liquid-piston.
The most important characteristic of this type of compressors is that all have a
fixed built-in pressure compression ratio for each stage of compression (as well as a
fixed built-in volume displacement) [25]. Thus, at a given rate of rotational speed
provided by the prime mover, there will be a predetermined volumetric flowrate
through the compressor, and the pressure exiting the machine at the outlet will be
equal to the design pressure ratio times the inlet pressure.
If the back pressure on the outlet side of the compressor is below the fixed output
pressure, the compressed gas will simply expand in an expansion tank or in the
initial portion of the pipeline attached to the outlet side of the compressor. Figure
3-74 shows the pressure versus volume plot for a typical rotary compressor
operating against a back pressure below the design pressure of the compressor.
If the back pressure on the outlet side of the compressor is equal to the fixed
output pressure, then there is no expansion of the output gas in the initial portion of
the expansion tank or the initial portion of the pipeline.
Figure 3-75 shows the pressure versus volume plot for a typical rotary compressor
operating against a back pressure equal to the design's pressure of the compressor.
If the back pressure in the outlet side of the compressor is above the fixed output
pressure, then the compressor must match this higher pressure at the outlet. In so
doing the compressor cannot expel the compressed volume within the compressor
efficiently. Thus, the fixed volumetric flowrate (at a given rotation speed) will be
reduced from what it would be if the back pressure were equal to or less than the
fixed output pressure. Figure 3-76 shows the pressure versus volume plot for a typical
rotary compressor operating against a back pressure greater than the design pressure
of the compressor.
operabion below
desion oresswe
Operation at
design pressure
J- design pressure
(discharge)
a
Volume
Volume
Figure 3-74. Rotary compressor with Figure 3-75. Rotary compressor with
back pressure less than fixed
back pressure equal to fixed pressure
pressure output [4]. output [4].