Two-stage compressors usually do not have
exactly equal loading on all cylinder ends. In some
cases, reciprication compressors operate on two or
more suction systems operating at different
temperatures and at different pressures. Various
arrangements to secure part-load operation such as
lifting suction valves, opening clearance pockets,
cutting in partial by-pass systems, etc., may
introduce various degrees of unbalance. Most
types of compressors are covered by a NEMA table
in which each common application is assigned an
application number and “Compressor Factor.”
Figure 45 illustrates percent current pulsation for
NEMA application No. 5 covering a single stage,
two cylinder, double acting compressor with 90°
cranks, plotted against “Compressor Factor” “X” as
the abscissa. This curve shows that with “X”
values of 2.0 to 6.0 or above 12.0 the current
pulsation will not exceed 66%.