An air-cooled condenser system for steam condensing applications in a power plant Rankine cycle includes an air cooled condenser having a plurality of interconnected modular cooling cells. Each cell comprises a frame-supported fan, inlet steam headers, outlet condensate headers, and tube bundle assemblies having extending between the headers. The tube bundle assemblies may be arranged in a V-shaped tube structure. A plurality of deflection limiter beams are arranged coplanar with the tube bundles. Top ends of each deflection limiter beam are slideably inserted in an associated floating end cap affixed to an upper tubesheet which moves vertically relative to the beams via thermal expansion/contraction concomitantly with the tubes. The deflection limiter beams provides guided restraint system for expansion/contraction of the tube bundles which prevents out of plane tube bowing.