Steam tube dryer is a variation of the rotary dryer in which a number of tubes are supported longitudinally within the rotating cylinder and supply the required heat. Only a small flow of air through the cylinder is needed to sweep out the vapor. The tubes are connected to a header into which steam (or other heat transfer fluid) is introduced to provide the energy for the drying process. The feed is introduced into the unit and is brought into contact with the steam tube bundle. In stationary units, the motive force is air and will create a fluid bed around the tube bundle. Rotary units will lift the material and cascade it over the steam tubes multiple times while the feed dries. The dry product is discharged from the unit through an airlock device. The evolved moisture is exhausted from the system through an induced draft fan. The unit may be designed for vacuum operation or installed inside a vacuum chamber for vacuum drying operation.
Steam humidifiers create water vapor directly. Steam is already a gas and it is easily injected into an air stream, instantly adding water vapor. Steam humidifier use heating elements to boil the water in a pan. They provide higher humidification output than other humidifier types. Steam humidifiers should not be used where the steam is treated. Where a central steam or high-temperature water supply is provided and a steam humidifier is to be used, a small heat exchanger can be used to generate steam from demineralized water.