The Beggs and Brill correlation was designed to calculate the compressibility of multiphase flow lines, and is essentially a best-fit equation to the standard Standing and Katz Z-factor chart. These are the equations as implemented in the Excel spreadsheet.
Pr and Tr are the critical temperatures and pressures and are calcualted from the mole fractions of N2, CO2 and H2S specified in the spreadsheet. The correlation can't be used if the reduced temperature is below 0.92.
The compressibility factor models non-ideality in real gasses and is important when sizing long-distance natural gas pipelines.
Download Excel spreadsheet to calculate the Compressibility Factor.