A system and the equipment for economically enhancing recovery of hydrocarbon fluids from a subterranean reservoir in which a plurality of production and injection wells are formed. High grade steam from a common source is mixed with relatively cold water at each injection well to be treated. To avoid, or substantially reduce condensation shock in the water/steam mixer, the high velocity cold water is initially heated in a steam compartment with minimal contact between the two fluids. Thereafter, the steam and the water are introduced to a heat exchange chamber in which the temperature of the water is raised prior to the two streams being merged for injection into a well as either heated water or as low quality steam.