Many authors suggest to use a process model to find the degrees of freedom. However
this approach will be error prone, it is easy to write too many or too few equations. Fortunately, it is in most cases relatively straightforward to establish these numbers from process insight.
Ponton (1994) propose a method for finding Nss by counting the number of streams and subtracting the number of “extra” phases (i.e. if there are more than one phase present in that unit). It is easy to construct really easy examples where the method fails. Consider a simple liquid storage tank with one inflow and one outflow. According to the above, we would have Nss = 2, which is clearly wrong. Maybe we should have subtracted the vapor phase which probably exists above the liquid. This gives Nss = 2-1 = 1, which gives the correct answer. However, if we add a reaction in the tank, then conversion depends on the holdup in the tank and Nss should be equal to 2. A better approach is needed.