Direct heating is achieved primarily by convection. A warm or hot gas, usually air (which may be mixed with the combustion gases of the fuel) or steam ( see Section 3.11.3.4) is passed through, over or around the material(s) to be dried, which may be in e.g. a rotating drum, on racks or jigs.
Typical direct drying systems are:
●with a flowing gas:
○ e.g. rotating drum, drying oven or kiln, tunnel dryers, spiral belt dryers, tray dryers
●with aerated solids:
○ e.g. through circulator, batch dryers, stationary rack dryers
●with large scale agitation of solids:
○ e.g fluidised bed, spin flash drying.
直接加熱處理基本上是應用熱對流原理。通常是以熱空氣或蒸汽通過或包圍原物料將之烘乾，熱空氣可能是和燃料的燃燒氣體混合的氣體，所使用的機具可能是迴轉筒、開架床或篩選機。
典型的烘乾機系統有下列幾種：
●利用流動氣體：如旋轉筒、烘乾窯爐、隧道式烘乾爐、迴旋帶式烘乾機、盤式烘乾機等
●利用充氣固體：如通過循環機、分批烘乾機、固定式開架烘乾機
●利用大型固體攪動機器：如流體床、旋轉閃蒸烘乾技術等
