An important issue arising from prior studies of thermosyphon heat exchangers for use in solar water heaters is the need for heat transfer and pressure drop correlations for the laminar, mired-convection regime in which these many of these heat exchangers operate. In this paper, we present empirical correlations for tube-in-shell heat exchangers with the thermosyphon flow on the shell side. The correlations are determined for uniform heatflux on the tube walls. Ranges of Reynolds and Grashof numbers are 130 to 2,000 and 4 x 10(5) to 8 x 10(7), respectively. Nusselt number correlations are presented in a form that combines the contributions of forced and natural convection. Mixed convection dominates forced convection heat transfer in these geometries. Pressure drop is not significantly affected by mixed convection.