Page 113 - Reciprocating Compressors Operation Maintenance

1OO Reciprocating Compressors: Operation and Maintenance

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FIGURE 2-56. Pressure wave generated by single-acting cylinder.

Figure 2-57 shows a double-acting compressor cylinder with the com-
posite wave form and the multiples that make it up. The dominant frequen-
cy in this wave form occurs twice per revolution. Therefore, the dominant
frequency from a double-acting cylinder at 300 rpm would be 10 cps.

The pressure waves generated by reciprocating action move through the
gas with a speed referred to as the acoustic velocity. This velocity depends
upon the molecular weight of the gas, temperature, and gas constants.
Depending on gas composition and temperature, it varies from a low
of about 700 fps for propane, up to a maximum of about 1,500 fps for
methane. This range covers most gases encountered. Hydrogen-rich
gases can have acoustic velocities well over 2000 fps.
Lengths of piping elements, such as reduced diameter and expanded
diameter sections, will have a pressure build-up or tend to resonate
when excited by a characteristic frequency called the resonant frequen-
cy. The length corresponding to this resonant frequency is called the
acoustic length.