This paper expounds a simple, fundamental theory for predicting sedimentation particulate fouling thresholds for horizontal flows inside heat exchanger tubes. The velocities and shear stresses at the tube wall predicted by this theory for keeping particulate matter suspended compare favorably with industrial experience and proprietary Chevron data. This theory is also not inconsistent with the literature in that the shear stress required for sedimentation fouling mitigation is roughly 4-6 Pa. However, the situation where small particles become encapsulated in the viscous sublayer cannot be resolved with a simple sedimentation particulate fouling threshold theory at this time, necessitating future research.