What are D_b and D_a in the Generalized Fugacity Coefficients table? I've been referring the book: Chemical Process Principles: Part-II Thermodynamics. I found a table that gives me fugacity coefficients in a tabular form corresponding to various reduced pressures and reduced temperatures. This is it:



I dont know what the $D_b$ and $D_a$ in the table. In the bottom of the table, I see:

$$\left(\frac{f}{p}\right)^{'} = \left(\frac{f}{p}\right)(10^{D({z_c}-0.27)})$$

Is the $D$ in this equation related to the $D_b$ and $D_a$ in the 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 $z_c &lt; 0.27$.