The paper deals with some of the results obtained during experimental research of fluid dynamic behavior of cascade columns. The three most commonly used types of trays (tunnel, sieve, and valve) occupied our attention and 595 experimental runs were performed on a 314 mm inside diameter column with an air-water system. Analysis of measured values of froth porosity has confirmed that Azbel＇s equation should be used, despite its theoretical origin. For clear liquid height, a new correlation was established, because statistical parameters of equations taken from literature were not satisfactory. Both equations were tested on experimental data obtained on two rectangular columns, showing good statistical parameters.