The concept of system limit or ultimate capacity has been around for over 40 years. During the period 1959-1961 Tek [F.R.I. Topical Report 21, Special Collections Section, Oklahoma State University Library, 1959; F.R.I. Topical Report 25, Special Collections Section, Oklahoma State University Library, 1961] combined the entrainment flood approach of Souders and Brown [I&EC 26 (1) (1934) 98] with the drop stability work of Hinze [Appl. Sci. Res. Al (1949) 273; AIChE J. 1 (3) (1955) 289] and came up with the concept that in a turbulent field there is a maximum capacity of counter flow devices (trays or packings) that is a function of vapor velocity and system properties and is independent of the hardware parameters. He developed a model using data obtained from what was believed to be the highest capacity device known-high open area dualflow (DF) trays (29% hole area) at spacings of 24 to 96 in. A revised model was developed by Stupin [F.R.I. Topical Report 34, Special Collections Section, Oklahoma State University Library, 1965] in 1965 based on additional data, primarily at high pressure, and concepts developed by Levich [Physiochemical Hydrodynamics, Prentice-Hall, Englewood Cliffs, NJ, 1962]. At that time it was recognized that the relatively few experimental data points at low liquid loads tended to fall below model projections. This was not considered to be a problem since the vapor rates in question are normally only encountered in vacuum operations and in 1965 tray pressure drop was the governing factor in vacuum. Recent experimental work with modem, high capacity, low pressure drop devices confirm that at low liquid rates there appears to be a capacity ceiling which is independent of liquid loading. It should be noted that the liquid loading effect was empirical in all versions of the model. This was necessary since the theoretical development was based on the stability and/or deformation of a single drop.