A mathematical model that describes the evaporated flux from high rate sources like those used in the deposition of semiconductor films were developed from mass balances with suitable constitutive equations describing the molecular beam distribution. The model was experimentally verified in laboratory scale equipment. Good agreement between the measured distributions and model results were obtained using a single fitting parameter. The fitting parameter is material specific and was correlated to the Knudsen number at a fixed location in the nozzle. A generalized procedure to evaluate this parameter and estimate the incident flux from a molecular beam source is outlined.