The performances of a Flexible Manufacturing System with multiple part classes are evaluated by closed queueing network analysis. The problem considered here refers specifically to the presence into the system of a real time intelligent router. It is well known that this condition sharply contrasts with the classical hypothesis needed for an exact solution of the steady state condition of the network (product form solution). Heuristic load-dependent coefficients have been introduced in the Reiser's recursive algorithm of Mean Value Analysis to model a dispatcher that routes parts to a set of equivalent machines according to the unfinished workload. The method has been extended to the case of unequal machine loads. Numerical results are given to show the achievable accurancy of the approximation.