An air-cooled heat exchanger is connectable between an upstream fluid source and a downstream fluid destination by a plurality of external pipes having connection ends arranged in one or more external pipe connection configurations. The air-cooled heat exchanger has a cooling tubes assembly that includes a plurality of headers, a plurality of intermediate pipes connected to the plurality of headers, and a modular manifold system that connects the plurality of intermediate pipes to the plurality of external pipes. The modular manifold system includes a plurality of pipe elbows each connected to a corresponding one of the plurality of intermediate pipes, and one or more trays. Each of the plurality of pipe elbows is retained within the one or more trays in a configuration that matches the one or more external pipe connection configurations.