This paper presents a dynamic simulator of water-in-tube boilers with natural recirculation, the kind of equipment widely used in industries for steam generation either as a source of power or for providing heating capabilities in process plants. The development is based on a combination of two non-linear models, one for the evaporation in the vertical tubes and the other for the phase separation in the steam drum. An application is made to the boiler of a 30 MW thermoelectric power plant and the results are discussed. The dynamic responses of all variables show the consistency of the model representation with the expected behavior, including the effects of a PI level control adjusted using classic Ziegler-Nichols tuning rules.