An air-cooled condenser comprising: a longitudinal axis; a longitudinally-extending steam header configured for receiving steam from a source of steam; a pair of longitudinally-extending circular condensate headers positioned below the steam header and spaced laterally apart; a pair of inclined tube bundles each comprising a plurality of tubes connected to an upper tubesheet and a lower tubesheet, the tube bundles disposed at an acute angle to each other; each tube bundle extending between and fluidly coupled to the steam header at top and a different one of the condensate headers at bottom forming an A-shaped tube structure; a fan mounted to a fan support frame and positioned below the tube bundles; wherein the tube structure is self-supporting such that the tube bundles are unsupported by the fan support frame between the upper and lower tubesheets (70, 71); an A-frame thermal restraint unit fixedly mounted to the fan support frame and spaced apart from the tube bundles, the thermal restraint unit including a fixation member fixedly attached to each of the upper tubesheets (70, 71), the fixation member configured and operable to restrain the upper tubesheets from thermal growth and movement along the longitudinal axis; wherein the fixation member is a T-shaped vertically oriented keel plate projecting upwardly from an apex of the thermal restraint unit, the keel plate received in a downwardly open receptacle of a seal box attached between the upper tubesheets; and wherein the keel plate is coupled to the thermal restraint unit by a sliding expansion joint formed between the keel plate and the thermal restraint unit, wherein the keel plate is movable vertically upwards with the upper tubesheets when the tube bundles grow due to thermal expansion; wherein the sliding expansion joint comprises a vertical slot in the keel plate which slideably receives a guide bolt fixedly mounted to the thermal restraint unit, the slot operable to limit the vertical movement of the keel plate; and wherein the keel plate comprises a horizontal flange fixedly seal welded to each of the upper tubesheets and operable to arrest thermal growth of the tube bundles in a vertical direction when the air-cooled condenser is heated by steam. a longitudinal axis; a longitudinally-extending steam header configured for receiving steam from a source of steam; a pair of longitudinally-extending circular condensate headers positioned below the steam header and spaced laterally apart; a pair of inclined tube bundles each comprising a plurality of tubes connected to an upper tubesheet and a lower tubesheet, the tube bundles disposed at an acute angle to each other; each tube bundle extending between and fluidly coupled to the steam header at top and a different one of the condensate headers at bottom forming an A-shaped tube structure; a fan mounted to a fan support frame and positioned below the tube bundles; wherein the tube structure is self-supporting such that the tube bundles are unsupported by the fan support frame between the upper and lower tubesheets (70, 71); an A-frame thermal restraint unit fixedly mounted to the fan support frame and spaced apart from the tube bundles, the thermal restraint unit including a fixation member fixedly attached to each of the upper tubesheets (70, 71), the fixation member configured and operable to restrain the upper tubesheets from thermal growth and movement along the longitudinal axis; wherein the fixation member is a T-shaped vertically oriented keel plate projecting upwardly from an apex of the thermal restraint unit, the keel plate received in a downwardly open receptacle of a seal box attached between the upper tubesheets; and wherein the keel plate is coupled to the thermal restraint unit by a sliding expansion joint formed between the keel plate and the thermal restraint unit, wherein the keel plate is movable vertically upwards with the upper tubesheets when the tube bundles grow due to thermal expansion; wherein the sliding expansion joint comprises a vertical slot in the keel plate which slideably receives a guide bolt fixedly mounted to the thermal restraint unit, the slot operable to limit the vertical movement of the keel plate; and wherein the keel plate comprises a horizontal flange fixedly seal welded to each of the upper tubesheets and operable to arrest thermal growth of the tube bundles in a vertical direction when the air-cooled condenser is heated by steam.