The disclosed apparatus and control system produces a single, on demand, energetic gaseous working fluid from any heat source. Working fluid in a liquid phase is released into a heat exchange tube in the form of very fine droplets or atomized mist, where it is rapidly heated to its gaseous phase. The gaseous working fluid can continue to absorb heat before exiting the heat exchange tube to perform work. The disclosed system controls the release of working fluid into the heat exchange tube and/or the heat energy to which the tube is exposed, resulting in a flow of energetic gaseous working fluid that can be quickly adjusted in response to changing conditions without a large pressure vessel.