In developing the control system for a process it is desirable to know
precisely the number of process variables which the system designer i
s entitled to attempt to regulate, commonly known as the ''degrees of
freedom'' of the process. An extremely simple procedure is described;
the maximum degrees of freedom for any process is shown to be the tota
l number of process streams, including streams of heating and cooling
media. If inventory in processing units is excluded unless, it affects
some more significant variable, such as pressure or composition, then
the effective degrees of freedom are generally determined by subtract
ing one from the total for every additional phase contained in a unit.