Abstract: A hot air drier system for breaking up and drying particles or aggregates of moist material includes a preliminary drier unit with a moist solids inlet, a hot air inlet, and a dried solids outlet. A pneumatic conveying conduit receives solids from the outlet and subjects them to further drying while they are conveyed to a suitable separator. Hot air is supplied to the drier inlet through a supply conduit which has a bypass conduit for delivering fully heated air to the pneumatic conveying conduit close to the solids outlet and which also has a cooling air conduit or bypass for selectively feeding heated air of somewhat lower temperature to the drier inlet.