Compression Ratio
Compression ratio is defined
as the discharge pressure divided by the suction pressure of a given stage
of compression. Compression
ratio is used in the equations for discharge temperature, volumetric efficiency,
discharge event, power and rod load.
Compression ratios are typically
in the range of 2 to 4 for gathering applications and typically below
2 for pipeline applications.
Higher compression ratios will
result in higher discharge
temperatures, lower volumetric
efficiencies and lower valve discharge
events. Typically,
the upper end of the compression ratio is limited by reaching a maximum
discharge temperature or low volumetric efficiency. Some
gasses with lower ratio of specific heat values (k or N) can reach higher
compression ratios before being limited by discharge temperature. Some
cylinders with lower volumetric clearance can reach higher compression
ratios before being limited by volumetric efficiency. This
makes limiting the upper end of compression ratio subjective.
The main difficulty with higher
compression ratios is the loss of flexibility of operating conditions.
Higher
compression ratios result in a greater sensitivity to changes in suction
and discharge pressure. This
can limit the useful operating range of an application. Higher
ratios also have an adverse effect on compressor valve performance and
reliability.
Rod load is a function of the
suction and discharge pressures within a cylinder, so are also impacted
by the compression ratio. High
compression ratios generally result in higher rod load values. Small
changes in pressures at higher ratios can result in larger changes in
rod load.
When compression ratios are in
the higher range, any clearance pocket devices should be checked for full
range use. With
higher ratios, clearance has a much greater effect on the volumetric efficiency.
If the
clearance pockets cannot be used fully at the higher compression ratios,
considerations should be made to either limit the use of the pockets,
or to omit clearance pocket devices from the cylinders.
When no other flags exist, a good
practical limit for the higher end of the compression ratio range is 4.5
ratios. A
warning has been placed in the performance software to flag this ratio
to help draw attention to the possible difficulties. When
this warning flag occurs, the specific application must be reviewed for
the applicability of the compression ratio for the specific operating
range.
Ariel
Corporation Application Manual
29 July 2008