An end cap configured for use with a recirculation heat exchanger of an aircraft environmental control system includes a body having an inlet section adjacent an inlet end, an outlet section adjacent an outlet end, and a transition section fluidly coupling the inlet section and the outlet section. The inlet section includes a generally rounded portion having a radius of about 0.870 inches (2.21 cm) and at least one flange formed at a side of the rounded portion. An exterior of the flange is positioned at a distance of about 1.012 from an origin of the rounded portion.