Two correlations of measurements of entrainment for annular flow in horizontal pipes are presented for liquids with viscosities close to that of water. Entrainment is considered to result from a balance between the rate of atomization of the liquid layer flowing along the pipe wall and the rate of deposition of drops. At low gas velocities, gravitational settling controls the rate of deposition. At high gas velocities, droplet turbulence controls deposition. The first approach uses an empirical relation for entrainment in horizontal pipes at low gas velocities and an empirical relation for entrainment in vertical pipes at high gas velocities. The second uses a theoretical analysis for the rate of deposition that is valid both for high and low gas velocities. Present practice has been to use correlations that have been developed for vertical annular flows. They are faulty because they do not directly include the effects of gravitational settling. (C) 2002 Elsevier Science Ltd. All rights reserved.