What are D_b and D_a in the Generalized Fugacity Coefficients table?
$D_b$ and $D_a$ are deviation terms which must be used in the place of $D$ to obtain the fugacity coefficients for fluids whose critical compressibility factor is not 0.27. Replace $D$ with $D_a$ if $z_c &gt; 0.27$ and with $D_b$ if $z_c &lt; 0.27$.