A dynamic model was formulated for a new type of steam generator that will be used in a number of pressurized water plants currently on order. This type of steam generator, called an integral economizer or preheat steam generator, introduces part of the feedwater directly into the riser section of a U-tube recirculation-type system. A dynamic model provides a tool for simulation and control studies of this new design. The model is a 24th-order, linearized, moving boundary representation of the system. It includes nodal mass and energy balances for primary fluid, tube metal, and secondary fluid sections. Flows are determined by quasi-static momentum balances. Transients were simulated using the models for various perturbations of interest. Confirmation must await operation of plants with the new steam generator tests, but the model gives physically plausible results. The model provides a tool with adequate detail for control and simulation studies, but with moderate complexity and computer cost.