An improved air-cooled vacuum steam condenser in a steam turbine cycle comprising a steam powered system comprising a turbine for converting steam energy into mechanical energy upon expansion of steam therein, a boiler for generating steam to be fed to the turbine, and a conduit arrangement coupling the boiler to the turbine and then recoupling the turbine exhaust to the boiler through steam condensing mechanisms, the condensing mechanisms include a plurality of finned tubes through which the expanded exhaust steam flows and is condensed; a plurality of bundle front headers at the input ends of the condensing tubes for receiving exhaust steam from the turbine; a plurality of bundle rear headers at the output ends of the condensing tubes for receiving condensate and non-condensible gasses; a plurality of vertically oriented water leg pipes, one for each bundle rear header of each bundle, coupling each rear header with a water leg manifold; a hydraulic balance device in the condensate drain system coupling the water leg manifold and a condensate storage tank, the hydraulic balance device providing the means to maintain a predetermined hydraulic pressure therein for thereby sustaining the heights of the condensate in the water leg pipes within predetermined limits for fluid sealing purposes; and means to remove non-condensible gasses from the rear headers.