Various embodiments of the present invention provide a heat exchanger having a collection tank having an internal chamber, and a header connected to the collection tank and receiving a number of flat tubes through apertures in the header. In some embodiments, a reinforcing member received within the collection tank and/or the header functions to reinforce the header and/or the collection tank in applications where these elements are subject to internal pressures that would otherwise deform the header and/or collection tank to an unacceptable extent. Also, some heat exchanger embodiments utilized a curved gasket on each flat tube to match the shape of the flat tubes and a curved header. Also, another gasket can be located between a metal or plastic collection tank and a plastic or metal header, respectively, to prevent leakage therebetween.