Pneumatic transport in horizontal pipes takes place under turbulent pipe flow conditions, which means that the influence of the fluid density on the state of flow is significant. The aerodynamic resistance of coarse grained particles also depends on the density of the carrier fluid. Experimental evidence for the role of fluid mechanics is thus obtained only through the vast variation of the static pressure. Therefore, the theoretical considerations presented in this paper start with our own experimental results, obtained at static pressures in the range of 1 to 20 bars. The breakdown of the steady-state conveying at low fluid velocities (plugging limit) is identified as a specific indirect interaction between the fluid and the particles. The theory, as well as our own experimental results, reveal the decisive role of the slip velocity between the two phases for both types of pneumatic transport, namely the strand flow type and the fully suspended type. It shows that the pressure gradient due to the fluid manifests the most peculiar feature of the problem in the form of the self-sustained particle transport. A unified representation of both types of particle transport allows the reliable prediction of the type of transport observed in plant practice, and a prediction of the pressure drop that is to be envisaged.